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Approval of the Graduate School of Natural and Applied Sciences
Prof. Dr. Canan Özgen Director I certify that this thesis satisfies all the requirements as a thesis for the degree of Master of Science.
Prof. Dr. Nurcan Baç Head of Department This is to certify that we have read this thesis and that in our opinion it is fully adequate, in scope and quality, as a thesis and for the degree of Master of Science.
Prof. Dr. N. Suzan Kıncal Co-Supervisor
Prof. Dr. Hayrettin Yücel Supervisor
Examining Committee Members Prof. Dr. Timur Doğu Prof. Dr. Hayrettin Yücel Prof. Dr. N. Suzan Kıncal Dr. Cevdet Öztin Prof. Dr. Zeki Aktaş (METU, CHE) (METU, CHE) (METU, CHE) (METU, CHE) (AÜ, CHE)
I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work.
Çiğdem ÇUHADAR
Dr. were investigated.Sc. Hayrettin YÜCEL Co-Supervisor: Prof. Raw materials were impregnated with different H3PO4 solutions of 30%. Dr. acid impregnated samples were heated. 50% and 60% by weight. 400 and 500oC). The volume and surface areas of mesopores (2-50 nm) and BET surface areas of the samples were determined by N2 gas adsorption technique at -195. The pore volume and the area of the micropores with diameters less than 2 nm were determined by CO2 adsorption measurements at 0oC by the application of Dubinin Radushkevich equation. Department of Chemical Engineering Supervisor: Prof. 110 pages In this study. Suzan KINCAL June 2005.ABSTRACT PRODUCTION AND CHARACTERIZATION OF ACTIVATED CARBON FROM HAZELNUT SHELL AND HAZELNUT HUSK ÇUHADAR. To produce activated carbon. at relatively low temperatures (300. the pore structures and surface areas of activated carbons produced from hazelnut shell and hazelnut husk by chemical activation technique using phosphoric acid (H3PO4). Çiğdem M. iv . at a heating rate of 20 oC/min to the final carbonization temperature and held at that temperature for 2 hours. N.6oC. 40%.
husk impregnated with 40 wt. respectively. shell impregnated with 60 wt. carbonized at 400 ºC) and as 1565 m2/g among hazelnut husk based samples (HH 40. carbonized at 400 ºC). The highest BET surface area was obtained as 596 m2/g among hazelnut shell based samples (HS 60.% H3PO4. Hazelnut Husk.4. respectively. CO2 (D-R) surface areas of the hazelnut shell and hazelnut husk based activated carbons were in the range of 433-576 m2/g and 376-724 m2/g. Hazelnut Shell. Keywords: Activated Carbon. Pore Structure.% H3PO4. Hazelnut shell based activated carbons were mainly microporous while hazelnut husk based ones were mesoporous.4.N2 (BET) surface areas of the hazelnut shell and hazelnut husk based activated carbons were in the range of 242-596 m2/g and 705-1565 m2/g. H3PO4 Activation v .
çapları 2 nm den küçük olan mikrogözeneklerin gözenek hacmi ve yüzey alanı 0oC de CO2 adsorpsiyon ölçümleriyle Dubinin-Radushkevich denkleminin kullanılmasıyla elde edilmiştir. asit emdirilmiş örnekler aktif karbon üretmek için 20oC/dak. 400 ve 500oC) fındık kabuğu ve fındık çotanağından üretilen aktif karbonların gözenek yapıları ve yüzey alanları incelenmiştir. 40%. N. Suzan KINCAL Haziran 2005. 50% ve 60% ağırlıksal derişimdeki H3PO4 çözeltileri emdirilmiş. 110 sayfa Bu çalışmada. vi . Ürünlerin mezogözenek (2-50 nm) yüzey alanları ve hacimleri ve BET yüzey alanları -195.ÖZ FINDIK KABUĞU VE FINDIK ÇOTANAĞINDAN AKTİF KARBON ÜRETİMİ VE KARAKTERİZASYONU Çuhadar. Hammaddelere 30%. Çiğdem Yüksek Lisans. kimyasal aktivasyon tekniği ile aktive edici madde olarak fosforik asit (H3PO4) kullanılmak suretiyle oldukça düşük sıcaklıklarda (300. Dr. Kimya Mühendisliği Bölümü Tez Yöneticisi: Prof. ısıtma hızıyla son karbonizasyon sıcaklıklarına kadar ısıtılıp 2 saat süreyle sabit sıcaklıkta tutulmuştur. Dr. Hayrettin YÜCEL Ortak Tez Yöneticisi: Prof.6oC de N2 gaz adsorbsiyon tekniği kullanılarak.
Fındık kabuğundan üretilen aktif karbonlar içinde en yüksek BET yüzey alanını HS 60. Anahtar Kelimeler: Aktif Karbon. fındık çotanağından üretilenler içinde ise HH 40. Fındık kabuğundan üretilen aktif karbonların çoğunlukla mikrogözenekli. Fındık Kabuğu.4 örneği (ağırlıkça 60% asit derişimi ve 400 oC karbonizasyon sıcaklığı) 596 m2/g olarak. Fındık Çotanağı. Ürünlerin CO2 (D-R) yüzey alanları fındık kabuğundan üretilenler için 433-576 m2/g ve fındık çotanağından üretilenler için 376-724 m2/g aralıklarında bulunmuştur. H3PO4 Aktivasyonu vii . Gözenek Yapısı. fındık çotanağından üretilen aktif karbonların ise mezogözenekli oldukları tespit edilmiştir.Fındık kabuğu ve fındık çotanağından üretilen aktif karbonların BET yüzey alanlarının sırasıyla 242-596 m2/g ve 705-1565 m2/g aralıklarda olduğu ölçülmüştür.4 örneği (ağırlıkça 40% asit derişimi ve 400 oC karbonizasyon sıcaklığı) 1565 m2/g olarak vermiştir.
DEDICATION viii .To My Family.
Deniz Akkaş. Thanks are due to all the staff of the Chemical Engineering Department. Suzan Kıncal for all their understanding. Yalçın Yılmaz. N. Hayrettin Yücel and Prof. Özge Yazıcıoğlu. Ms. my dear brother Özgür Çuhadar and many others that I could not mention here. Kemal Behlülgil for their help in the chemical and physical analysis of the samples. Ms. I would like to thank Ms. Salih Obut. support and sound advice in all aspects of my research work. Kerime Güney. Mr. İsmail Doğan. Dr. Emrah Irmak. Selahattin Uysal.ACKNOWLEDGEMENTS I wish to express my deepest gratitude to my thesis supervisors Prof. Gülten Orakcı. Mihrican Açıkgöz. Açelya Gökalp. my laboratory mates Wisam Abdallah and Ural Yağşi. ix . Oluş Özbek. who gave me helpful suggestions for the improvement of the document and moral support. creating very pleasant working conditions. I should mention. Dr. I am very much obliged for their objective and tolerant attitude. Nahide Özkan. Dr. Necati Özkan and Dr.
........................................................................ ACTIVATED CARBON ................... ix TABLE OF CONTENTS ............................................................... vi DEDICATION............................. x LIST OF TABLES .. 9 x ....................... viii ACKNOWLEDGEMENTS ............................................................................................................ Principle of Activation Process ...... Production Methods ................. iii ABSTRACT .................... iv ÖZ............................................................................................................................................. 7 2.............................. xviii CHAPTER 1............... xv LIST OF SYMBOLS ........................................... Raw Materials.............................................................................. 1 2... INTRODUCTION ....TABLE OF CONTENTS PLAGIARISM .......................... Definition and Properties ........... 5 2.............. 7 2......................................................................................xiv LIST OF FIGURES ...........................................1..............................2................................................................2.2...................................................................1...........................................................................................................................................2................... 5 2.....................................................
........ Porosity .... Experimental Set-Up .....................2......................... 25 3............... Chemical Activation of Samples..................... 45 5............................................................. 47 5.................. 17 2.....2.............1............................................................ General ..2.. 15 2.3.....................1.......... 45 5...........................................1.......... 22 3....................................... 35 4......... True Density Determinations ...........2............2. Properties of Raw Materials ..........2............................................. Chemical Activation.............2.................... 47 5...........................2........ 10 2......................2..............2............................... PREVIOUS WORK ON PREPARATION OF ACTIVATED CARBON ..1.........3........ Chemical Properties of Activated Carbon .........4.. Gas Adsorption Phenomena and Standard Isotherms....... Physical Structure of Activated Carbon ................ 12 2............ Carbonization Experiments ............................... 40 5.................. Physical Activation .....1......1....................... Characterization of Microporosity..........................................................2.... 46 5... Emmett and Teller (BET) Theory......... 44 5........... METHODS USED IN PHYSICAL CHARACTERIZATION OF ACTIVATED CARBON .. 30 3.......... Properties and Preparation of Raw Materials..........................3........................................... 33 4.......2.........5.................. The Brunauer. Physical Activation ....2.......... 27 3................... 48 xi .......... 22 3....... 19 3.. Pore Characterization by Adsorption / Desorption .......... EXPERIMENTAL WORK ....................................................................2..1............................2...1.................................2............. Preparation of Raw Materials ...1.... 36 4.....2................................ Chemical Activation............ 24 3................................
.3..................................................2.....1 Preliminary Experiments .2......................3......................................... 80 6...... Oxygen and Hydrogen Content ........................................ 81 7...................................... Physical Characterization of the Products.... Carbon Content ..1.......3...............2........... ANALYSIS OF N2 SORPTION DATA . Carbon Dioxide Gas Adsorption Results .1...........2.............. 51 5............................... Analysis of Mesopores ............................. CONCLUSIONS .3............ 77 6.......................... 85 8................3......................3............ TGA of Impregnated Hazelnut Shells and Hazelnut Husks .................................4....................... 76 6............................ 51 5.............................. Nitrogen Gas Adsorption Measurements . RESULTS AND DISCUSSION ....................................................... 50 5.......2....... 55 6................. 79 6............................................. 93 xii ...................................... True Density Determinations ......2.............................3 Characterization of the Products...............................3. Ash content........1....................2...... Nitrogen Content ......................................................................... Chemical Analysis of Products .......... Results Based on Nitrogen Gas Adsorption.............. 57 6. 79 6........ CO2 Gas Adsorption Measurements ............4........ True Density Determinations ........3............... 54 6.. 87 REFERENCES .............................. RECOMMENDATIONS ............................... 68 6.. 81 6.3....... 55 6..............................3......... 93 A....................................................... 88 APPENDICES A...................................... 58 6..5..............1................................3.................................
.......3..... Sample Calculation . 107 xiii .............................................. Sample Calculation .................................3........ 101 B........................ 99 A.....1........2.3.. TABULATED FORM OF CHEMICAL COMPOSITIONS OF ACTIVATED CARBONS ................................................. 104 D................................. TGA RESULTS....................... 98 A......................... ANALYSIS OF CO2 ADSORPTION DATA ......................2..... 100 B.... Analysis of Micropores .......................... 106 E........ 101 B................................................................................1........................... Determination of BET Surface Area...........................................2...................................... ANALYSIS OF HELIUM PYCNOMETER DATA .............................. 102 C...... Calculation of BET Surface Area ..........A....... 99 A.................... Calculation of Mesopore Volume and Area ........
.........................................LIST OF TABLES Table 1... 45 5........ 83 D.. 47 5..................1 Chemical Compositions of Activated Carbons and Raw Materials . 8 5.............................................2 Impregnation Ratios and Weight Increases of the Raw Materials ............. 49 5... 106 xiv ............. 82 6... 3 2.1 Pore Sizes of Activated Carbon ... .............. 50 6........2 Material Balance on Activated Carbon Production ...............................................................1 Chemical Composition of Raw Materials ..1 Fixed Carbon Contents of Raw Materials Employed in Activated Carbon Manufacture................3 Experimental Conditions and Sample Codes for Hazelnut Shell...4 Experimental Conditions and Sample Codes for Hazelnut Husk.....1 Yield Values (%) of Selected Products and Raw Materials ....
..... 23 3.................. 44 5.................................... 53 6.6 BET Surface Areas of Hazelnut Husk Based Products ............1 BET Surface Areas of Hazelnut Shell Based Samples............................... 60 6..... 61 6.. 34 5.................... 17 3........... 63 6........... 6 2. 59 6.................................... 15 2..... 28 3............3 Carbon Atom Arrangements in Graphite Crystal .7 Comparison of BET Surface Areas of HS and HH Based Products ........2 Schematic Representations of Different Types of Adsorption Isotherm 25 3..................LIST OF FIGURES Figure 2....2 BET Surface Areas of Hazelnut Shell Based Samples.... 48 5................................................1 Micropore..............................5 N2 Adsorption/Desorption Isotherms of Hazelnut Shell Based Carbons 62 6........................... 63 xv ........................4 Block-Diagram of the Overall Experimental Approach ...............3 BJH Mesopore Areas of Hazelnut Shell Based Samples .2 Experimental Set-up .4 Mesopore Volumes of Hazelnut Shell Based Samples .................. 58 6.........2 Schematic Representation of (a) Nongraphitizing and (b) Graphitizing Structure of Carbon.3 Schematic Representation of Surface Analyzer..1 Pore Structure of Activated Carbon (TEM) ...... Mesopore and Macropore Regions of Activated Carbon .......1 Procedure Followed in Experiments ..............3 Types of Adsorption-Desorption Hysteresis Loops..........................................
. 67 6.....18 Micropore Volume Values of Hazelnut Husk Based Products ........ 108 xvi ............... Micro...17 Micropore Area Values of Hazelnut Husk Based Products ......2 TGA Result of 50% H3PO4 Impregnated Hazelnut Shell at 400 ºC .. D-R and BJH Results of Hazelnut Shell Based Products ......... 71 6.............13 N2 Adsorption/Desorption Isotherms of Hazelnut Husk Samples Impregnated with 30.....12 N2 Adsorption/Desorption Isotherms of 60% H3PO4 Impregnated Hazelnut Husk Samples.............. 74 6....... 75 6.......................... ............... D-R and BJH Results for Hazelnut Husk Based Products ................. 69 6........and Mesopore Areas...............................................11 BJH Mesopore Volumes of Hazelnut Husk Based Samples ............. 40 and 50% H3PO4 and Carbonized at 400 ºC... 73 6................. 66 6..1 TGA Result of 40% H3PO4 Impregnated Hazelnut Shell at 400 ºC ...........................19 Comparisons of BET... 107 E.... 69 6........21 True Density Values of the Activated Carbons .................. 78 E...........8 BET Surface Areas of Hazelnut Husk Samples Impregnated with 60% H3PO4 solution........14 Micropore Area Values of Hazelnut Shell Based Products .... 67 6..................15 Micropore Volume Values of Hazelnut Shell Based Products .22 Elemental Analysis Results of Activated Carbons and Raw Materials ....9 BET Surface Areas of Hazelnut Husk Samples at Carbonization Temperature of 400 ºC.............................................. 65 6....................................................10 BJH Mesopore Areas of Hazelnut Husk Based Samples ...............6..................16 Comparisons of BET........ 64 6................ 108 E....... ..................... 72 6..3 TGA Result of 60% H3PO4 Impregnated Hazelnut Shell at 500 ºC ........................................................... ..... 65 6............... 77 6................................20 Comparison of HH and HS Series in terms of BET....
.6 TGA Result of 60% H3PO4 Impregnated Hazelnut Husk at 500 ºC .5 TGA Result of 60% H3PO4 Impregnated Hazelnut Husk at 400 ºC ..E. 109 E.. 109 E....... 110 xvii .4 TGA Result of 50% H3PO4 Impregnated Hazelnut Husk at 400 ºC .
m2 /g xviii . nm : Slope of the BET plot : BET surface area. 6. g/mol : Avagadro's constant.LIST OF SYMBOLS Am C : Cross-sectional area of the adsorbate.1 CSAN2 : Cross sectional area of nitrogen molecule. kJ/mol : Heat of adsorption of the second and subsequent layers.023*1023 molecules/mol : Partial pressure of the gas i. kJ/mol : The gas constant. 8.314x10-3 KJ/mole-K : Actual pore radius. Pa. kJ/mol : Gibbs free energy change of the reaction. Pa. atm. nm or µm : Kelvin radius. 8. mmHg : Heat of adsorption of the first layer. nm2 Dp Ei ∆G I K M NA Pi P P0 q1 q2 R rp rK rKAVE rPAVE S SBET Scum : Pore diameter. µm : Adsorption potential. mmHg : Saturation pressure. psia. nm : Average Kelvin radius. mean radius of the liquid meniscus.314x107erg/mole-K. nm2 : A constant in equation 3. kJ/mol : Intercept of the BET plot : A constant in equation B. m2/g : Cumulative pore surface area. nm : Average pore radius. psia. atm.1 : Adsorbate molecular weight. mmHg : Pressure.
nm : Temperature.Smeso t tm T Ta Tc V V0 Vcum Vm Vmeso Vmol ∆Vgas ∆VLiq W0 Wa Wm : Mesopore surface area m2 /g : Thickness of the adsorbate layer. 34. cm3/g : Micropore Volume. °C : Volume adsorbed. cm3/g STP : Mesopore volume. °C : Critical temperature of the adsorption. cm3/g : Molar volume of the nitrogen. cm3 : The quantity of adsorbed at a particular relative pressure. cm3 : The quantity of adsorbed at correspond to BET monolayer. g/cm3 : Contact angle of liquid meeting the pore wall : Cross sectional area of a CO2 molecule. cm3/g : Limiting adsorption space volume. cm3/g : Monolayer volume. g/cm3 : Helium (True Density). erg/cm2 γ xix . °C : Ambient temperature.6oC : Incremental molar adsorbed gas volume. nm2 : Surface tension. cm3 Greek Letters µm Å ρ ρHe θ σ : Micrometer (10-6 meter) : Angstrom (10-10 meter) : Liquid density. cm3/g : Incremental molar adsorbed liquid volume. cm3/g : Cumulative volume.6x1024 A3/mol at -195. nm : Thickness of the monolayer.
These unique characteristics are responsible for its adsorptive properties. remaining amorphous. nut shells. X-ray diffraction patterns of activated carbon reveal that it is nongraphitic. Commercial activated carbon products are produced from organic materials that are rich in carbon. Through choice of precursor.and gas-phase applications. the highly developed and accessible internal pore structure. Activated carbon is generally considered to exhibit a low affinity for water. pitches.CHAPTER I INTRODUCTION Activated carbon is a predominantly amorphous solid that has an extraordinarily large internal surface area and pore volume. which are exploited in many different liquid. which is an important property with respect to the adsorption of gases in the presence of moisture. The structure of activated carbon is best described as a twisted network of defective carbon layer planes. method of activation. particularly coal. cross-linked by aliphatic bridging groups. and cokes. 2003) The pyrolysis of starting material with the exclusion of air and without addition of chemical agent usually results in an inactive material with a specific surface area of the order of several m2/g and low adsorption capacity. This property of activated carbon contributes to its most unique feature. (Kirk-Othmer. The majority of activated carbon used throughout the world 1 . peat. wood. lignite. One can prepare a carbon with a large adsorption capacity by activating the carbonized products with a reactive gas. the adsorptive properties of products are tailored for applications as diverse as the purification of potable water and the control of gasoline emissions from motor vehicles. namely. and control of processing conditions.
(Ruthven. 2 . 1984) Activated carbon may not be pure carbon but also contain some impurities depending on the type of starting material. the carbonized product is reacted with steam over 900oC. The production at lower temperatures promotes the development of a porous structure. the adsorption characteristics of activated carbon for certain uses (such as adsorption of electrolytes and non-electrolytes from solutions) are significantly influenced by even small amounts of ash. The most commonly used activating agents are phosphoric acid. However. Another procedure used in the production of activated carbon involves the use of chemical activating agents before the carbonization step. the adsorption of gases is also influenced by the ash content. activated carbons are produced and classified as granular. On the other hand. Chemical activation is usually carried out at lower temperatures than the simple pyrolysis and the activation process with steam or carbon dioxide. of activated carbon is affected by industry regeneration capacity. the raw material should contain as small ash as possible. It must be noted that. Therefore. Landfill disposal is becoming more restrictive environmentally and more costly (Kirk-Othmer. To meet the engineering requirements of specific applications. 1970) Most of the available surface area of activated carbon is nonpolar in nature. (Smisek and Cerny. or shaped products. Activated carbon is a recyclable material that can be regenerated. during production the interaction of surface with oxygen produces specific active sites giving the surface of slightly polar nature. 1992). the raw material must have relatively low cost (Balcı. powdered. 2003). especially the market growth. because under these conditions elementary crystallites of smaller dimensions are formed. Moreover. zinc chloride and salts of sodium and magnesium.is produced by steam activation (physical activation). In this process. carbon adsorbents tend to be hydrophobic and organophilic. As a result. Thus the economics. Chemical agents act as dehydration agents and they may restrict the formation of tar during carbonization.
beverages. and catalysis. Applications include solvent recovery. cooking oil. transitional mesopores and macropores.According to the IUPAC definition. Liquid-phase activated carbon can be applied either as a powder. Granular and shaped carbons are used generally in continuous systems where the liquid to be treated is passed through a fixed bed. and pharmaceuticals. groundwater remediation.500 Å) Pores with diameters less than 2 nm (20 Å) Most activated carbons contain pores of different sizes. among others. granular. Most of the activated carbon used in gas-phase applications is granular or shaped. automotive/gasoline recovery. sweetener decolourization. industrial and municipal wastewater treatment. industrial off-gas control. Liquid-phase carbons have significantly more pore volume in the macropore range. 3 . Liquid-phase applications of activated carbon include potable water treatment. and miscellaneous uses including chemical processing. Table 1. Activated carbons for use in liquid-phase applications differ from those used in gas-phase applications primarily in pore size distribution.1 Pore Sizes of Activated Carbon Macropores Mesopores Micropores Pores with diameters larger than 50 nm (500 Å) Pores with diameters between 2 nm and 50 nm (20. and catalysis. Separation processes comprise the main gas-phase applications of activated carbon. pores can be distinguished in three groups with respect to their dimensions (Reinoso. Gas-phase applications of activated carbon include separation. mining and the production of food. Therefore they are considered as adsorbents with wide variety of applications. gas storage. 1985). which permits liquids to diffuse more rapidly into the mesopores and micropores. or shaped form. micropores.
Emmet. are agricultural by-products. Although they have very high volatile content and hence give low yields of activated carbon. Hazelnut shell is mainly used for heating purposes in the Black Sea Region but hazelnut husk is mostly left to decay because of moist climate. About 60 percent of the crop is produced in the Eastern Black Sea Region. 2000). 15 percent is produced in the Central Black Sea Region and the remaining 25 percent is produced in the Western Black Sea Region. Joyner. 140. hazelnut shell and hazelnut husk by chemical activation using phosphoric acid and characterize these products in terms of pore volume and surface area by BET (Braunauer. agricultural by products including hazelnut shell and husk are abundantly available. The major novelty of this work is the production of activated carbons from phosphoric acid impregnated hazelnut shell and hazelnut husk samples by chemical activation technique in a temperature range of 300oC to 500oC and with different acid concentrations. In Turkey. DR (Dubinin.000 tons of hazelnut husks are obtained as waste. this value has reached up to 500 000 t in recent years (Köksal.Some of the major raw materials used. 1992). It covers approximately 70 percent and 82 percent of the world's production and export respectively.000 tons of hazelnut shell and 110. Turkey's average in-shell production is 350-400 000 t. 4 . Hallenda) Method. The applicability of hazelnut husk for the production of activated carbon was investigated for the first time in this study. The objective of this study is to produce activated carbon from agricultural wastes. Turkey is the largest hazelnut producer and exporter in the world. Radushkevich) Method and Helium Pycnometer. Most of the studies have shown that. Accordingly. BJH (Barrett. Teller) method. they are relatively inexpensive and economical starting materials. activated carbons obtained from agricultural by-products can be favorably compared with other activated carbons used in industry with respect to their adsorptive properties (Balcı.
The removal of impurities from gases and liquids by activated carbon takes place by adsorption.CHAPTER II ACTIVATED CARBON 2. Adsorption process can be considered as either physical adsorption or chemisorption. 1974). purification of exhausted air. Carbonaceous adsorbents found greater use in the solution of environmental problems related to water purification and removal of air pollutants (Smisek and Cerny.1. Definition and Properties Activated carbon is a microcrystalline. Hassler. removal of heavy metals. 1970). the separation of gas mixtures. 5 . especially the recovery of solvents. Because of these properties. they have been used for many decades for the purification of gases. Its chemical structure allows it to preferentially adsorb organic materials and other nonpolar compounds from the gas or liquid streams. X-ray analysis of activated carbons shows a structure which is much more disordered than that of graphite. nongraphitic form of carbon. which describes the existence of a higher concentration of a substance at the interface between a fluid and a solid than is present in the fluid. decolourization of solutions and purification of water. 1970. The spaces between the crystallites of activated carbon constitute the microporous structure with a large internal surface area of 250 m2/g-2500 m2/g. Adsorption is a term. having crystallites only a few layers in thickness and less than 10 nm in width (Smisek and Cerny.
Figure 2. The porous structure and chemical nature of an activated carbon is a function of the raw materials used in its preparation and the activation method used.In physical adsorption the impurities are held on the surface of the carbon by weak van der Waals forces while in chemisorption. but there is no guarantee that the entire surface is available for the adsorption of organic compounds.1. in chemisorption the efficiency of carbon will depend upon its accessible surface area and also upon the presence of active sites on the surface where chemisorption may occur (Balcı. Therefore. 1992).1 Pore Structure of Activated Carbon (TEM) (Yağşi. the forces are relatively strong and adsorption occurs at active sites on the surface. Pore structure of activated carbon (from TEM) is shown in Figure 2. 2004) The internal surface area of activated carbons can be determined by the adsorption of nitrogen. Since organic molecules are 6 . This is the reason why surface area or pore volume of activated carbons can vary widely from one kind to another.
The atoms of elementary carbon thus released are grouped into organized crystallographic formations known as elementary graphitic crystallites.1.much larger than a nitrogen molecule and if the pores are the same size as molecular dimensions.e. most of the noncarbon elements (hydrogen. so that free interstices remain between them. and. 1992). Activation of the carbonized material is possible via two routes: chemical activation and physical activation (Wigmans. Raw materials for the production of activated carbon include number of carbonaceous 7 . these become filled or blocked by disorganized carbon. it is essential for an adsorptive carbon to have a large accessible surface area which in turn is directly related to pore size distribution (Balcı. traces of sulphur and nitrogen) are first removed in a gaseous form by pyrolytic decomposition. Although the activation procedure employed mainly determines the chemical nature of the surface oxides and the surface area of the resultant product. Raw Materials The quality of the resulting activated carbon is considerably influenced by the raw material. it is possible to have a sieving action.2. apparently as the result of deposition and decomposition of tarry substances. The mutual arrangements of these crystallites is irregular. Any cheap substance with a high carbon and low ash content can be used as a raw material. Thus..2. 1989). the structure of the pores and the pore size distributions are largely predetermined by the nature of the starting material. Principle of Activation Process During carbonization of a carbonaceous material (i. 2. 2. oxygen. The resulting carbonized product generally has only a small adsorption capacity. pyrolysis in the absence of air and without addition of chemical agents).
especially wood. almond shells. Holden. (Smisek and Cerny. Since the manufacturing process involves the removal of volatile matter. almond shell. 1982) Table 2. 1992). 1970.1. the younger fossil materials-wood.materials. bituminous coal. coconut shells.1 Fixed Carbon Contents of Raw Materials Employed in Activated Carbon Manufacture (Holden. availability and quality of raw materials on one side and volatile content on the other side. petroleum-based residues and pulp mill residues (Balcı. peat. coconut shell. Fixed carbon contents of some raw materials are given in Table 2. brown coal. Material Soft Wood Hard Wood Coconut Shell Lignite Bituminous Coal Anthracite Percent Carbon Content 40 40 40 60 75 90 8 . lignite. is an important one. pits from peaches and other fruit. the economic relationship between price. 1981. (mainly birch wood and beech wood) peat and wastes of vegetable origin (such as fruit stones. Martin. On the other hand. The most important advantages of using bituminous coal and anthracite are their relatively low volatile content and hence high yield of product. saw dust) can be activated easily and give high quality products. 1982).
" • Partial gasification. Production Methods Activated carbon can be prepared by one of the following two methods: • Carbonization after addition of substances. For example." The activation agents most often used are steam. During activation of the intermediate product. whereas for 9 .2. generally called "physical activation. Although high quality products can be obtained by both procedures. 1989). KOH. sometimes. Sometimes the chemically activated carbon is subjected to additional activation with gaseous environment in order to change the pore structure of the final product. for the recovery of solvent. The further development of the porosity on increasing burn-off depends on the mechanism of carbon removal via active site formation and the relative rate of reaction in the direction parallel with the plane of the graphitic layers compared to carbon removal in the direction perpendicular to this plane (Wigmans.2. chemical and physical are widely used. chemically activated carbons are preferred. The basic production processes can be combined in different ways. carbon dioxide. which restrict the formation of tar. In this way. a carbonized product with the properties of a good activated carbon can be obtained in a single operation. Yet. K2S or KCNS that has to be applied in large quantities has to be removed via washing in order to reveal the porous structure and to make the material practically applicable. In contemporary technologies both types of activation. and air or a combination of these. they are not equally good for all purposes.2. the activation agent like ZnCl2. 10-20% burn-off) is first removed to expose the surface of the elementary crystallites to the action of the activation agent. the disorganized carbon (depending on the carbonization procedure. H3PO4. This route is generally called "chemical activation.
An important advantage of chemical activation is that the process normally takes place at a lower temperature and for a shorter time than those used in physical activation. Moreover. Other disadvantages of chemical activation process are the need of an important washing step because of the incorporation of impurities coming from the activating agent. 10 . and the corrosiveness of the chemical activation process (Lozano-Castello. Several activating agents have been reported for the chemical activation process: phosphoric acid. On calcinations.water treatment.2. are used for activation of coal precursors or chars. Chemical Activation In chemical activation the precursor is impregnated with a given chemical agent and pyrolyzed after that. and once the chemical agent is eliminated after the heat treatment. alkaline metal compounds. the general mechanism for the chemical activation is not so well understood as for the physical activation. As a result of the pyrolysis process. 2001. the impregnated chemicals dehydrate the raw materials.1. usually KOH. a much richer carbon content material with a much more ordered structure is produced. (Smisek and Cerny. while. 1970) 2. the porosity is highly developed. which may affect the chemical properties of the activated carbon. which results in changing and aromatization of the carbon skeleton by the creation of a porous structure and surface area. 1992). In addition. However. carbon activated with steam appears to be preferable. zinc chloride and alkaline metal compounds.2. very high surface area activated carbons can be obtained. Balcı. the yields of carbon in chemical activation are usually higher than those in physical activation because the chemical agents used are substances with dehydrogenation properties that inhibit formation of tar and reduce the production of other volatile products. Phosphoric acid and zinc chloride are activating agents usually used for the activation of lignocellulosic materials which have not been previously carbonized.
Simultaneously other reactions. by which the macromolecules are gradually depolymerized. 1992). 1963. The electrolytic action of the activation agent causes the cellulose to undergo a change known as swelling. When the degree of impregnation is further raised. the number of larger diameter pores increases and the volume of the smallest pores decreases (Balcı. the action of the chemical agent on cellulose must first be considered. One advantage of using phosphoric acid in chemical activation is that. 1970). must be cleaned from the chemical agent before their commercial use. The effect of the degree of impregnation on the porosity of the resulting product is apparent from the fact that the volume of salt in the carbonized material is equal to the volume of pores. Smisek and Cerny. especially when ZnCl2 is used. which are freed by its extraction. orientated in the direction of their longitudinal axes. 11 . up to 1800-2000 nm long. 2004). which form agglomerates known as micelles. hydrolytic or oxidative. it can be cleaned from the activated carbon by rinsing with boiling pure water (Yağşi. a small increase in impregnation amount causes an increase in the total pore volume of the product showing an increase in the volume of smaller pores. but the lateral bonds are broken down with the result that the inter-and-intra micelle voids increase until finally the cellulose is dispersed. and therefore in a discussion of the mechanism of chemical activation. Cellulose is composed of elongated macromolecules. during which the arrangement of the molecules in the direction of the longitudinal axis remain unchanged. take place. An important factor in chemical activation is the degree of impregnation. For small degrees of impregnation. This weight ratio of the anhydrous activation agent to the dry materials is defined as the coefficient of impregnation. The processes lead to the formation of a homogeneous plastic mass consisting of the partially depolymerized substance uniformly saturated with the activation agent (Browning. The orientated chains of molecules are laterally bounded by bonds of different types of strength.The material mainly used in the production of activated carbon consists predominantly of cellulose. The activated carbons produced through chemical activation.
Carbonization of lignocellulosic material starts above 170oC and it is nearly completed around 500oC. The main aim of carbonization is to reduce the volatile content of the source material in order to convert it to a suitable form for activation. Carbonization The method of production of the carbonized intermediate product has a marked effect on the quality of the final activated carbon product. 1992). it is desirable to carry out its pyrolysis sufficiently quickly. In the production of charcoal. During the phase of the carbonization.2. hydrogen and oxygen are first removed in gaseous form by pyrolytic decomposition of the starting material and the freed atoms of elementary carbon are grouped into organized crystallographic formation known as elementary graphitic crystallites (Balcı. or by a chemical reaction (for 12 . carbon content of the product attains a value of about 80 percent. Such carbonized materials can then be partially activated by removing the tarry products by heating them in a stream of an inert gas. Further important factors are uniform heating of the retort and the temperature of carbonization. 2004). In the simple carbonization product. at least for carbonization at lower temperatures. or by extracting them with a suitable solvent. part of the formed tar remains in the pores between the crystallites and on their surface. as a result of deposition and decomposition of tarry substances.2. which must not be very high (Yağşi. the mutual arrangement of the crystallites is irregular. However. Presumably.600oC. Physical Activation a. so that free interstices remain between them. these become filled or at least blocked by disorganized (amorphous) carbon. in order to reduce the time of contact of the carbon formed with the decomposition products.2. The resulting carbonized product has small adsorption capacity. The rate of pyrolysis is significantly influenced by the moisture content of the starting material.2. By carbonization most of the non-carbon elements.
The reactions of carbon with molecular oxygen are favored thermodynamically at all temperatures up to 4000 K. when the burn off is not higher than 10 percent.example. (Smisek and Cerny. 1985) b. The most common activation agents are steam. In the initial stage. Activation of Carbonized Intermediate Product with Gaseous Agents A carbon with a large adsorption capacity can also be produced by activating the carbonized material under such conditions that the activating agent reacts with the carbon. Activation takes place in two stages. The activation step is generally conducted at temperatures between 800oC and 1100oC. for a given carbon in the absence of a catalyst. or the 13 . The endothermic reaction between carbon and water vapor (steam) is favored by elevated temperature and reduced pressure and occurs slowly at temperatures below 1200 K. carbon dioxide and oxygen (air). 1970. disorganized carbon is burnt out preferential and the closed and clogged pores between the crystallites are freed. The effect which becomes increasingly significant is the widening of existing pores. depending on the gaseous agent employed. The removal of nonorganized carbon and the non-uniform burn out of elementary crystallites leads to the formation of new pores. and to the development of the macroporous structures. The reaction of carbon with carbon dioxide is endothermic and. The burning out of the crystallites must proceed at different rates on different parts of the surface exposed to reaction. Wigmans. The active oxygen in the activating agent basically burns away the more reactive portion of the carbon skeleton as carbon monoxide and carbon dioxide. By the removal of disorganized carbon. takes place at a rate several orders of magnitude slower than the carbon-oxygen reaction at the same temperature: it proceeds very slowly at temperatures below 1000 K (727 oC). otherwise new pores could not be formed. the surface of the elementary crystallites became exposed to the action of the activation agent. heating in an atmosphere of sulphur vapor at temperatures lower than those at which reactions with carbon take place).
Rodriguez-Reinoso. ii) the crystallinity and structure of the carbon. In kinetics control region. carbons activated with oxygen have a large amount of surface oxides. Therefore. the temperature must be carefully chosen to make the chemical reaction between carbon and gaseous agent the rate determining factor. However. carbonization and activation steps are carried out separately.formation of larger size pores by the complete burn out of walls between adjacent micropores. 1992). 1985. reactions are too slow. At lower temperatures. Generally. steam and oxygen are: i) the concentration of the active sites on the carbon surface. Hassler. oxygen (air) activation is rarely used (Balcı. However at much higher temperatures reactions become diffusion controlled and occurs on the outside of the carbon particle (Balcı. Hence the removing of carbon from the pore walls causes the enlargement of the pores. 1974. The reaction of oxygen with carbon is exothermic. Activation with steam and carbon dioxide are carried out at temperatures between 800 ºC and 1100 ºC. It must be noted that. due to the very aggressive action of oxygen. iii) the presence of inorganic impurities and iv) the diffusion of reactive gases to the active sites. reactions take place at the interior surface of the carbon. some difficulties may arise. Due to the difficulties and disadvantages explained. Furthermore. The main factors that influence the rates of the reactions of carbon with carbon dioxide. but recently there is an increasing tendency to conduct the two processes together. 1991) 14 . for the activation of the carbonized product with oxygen it may be difficult to maintain the correct temperature in the oven. burn out is not limited to pores but also occur on the external surface of the grain by causing great loss. (Smisek and Cerny. Possible local overheating prevents uniform activation. According to the type of the gaseous activation agent. 1970. 1992). Wigmans.
Figure 2.3. nongraphitizing carbons and graphitizing carbon. (Figure 2.2) In general.2. 1992). forming a porous mass. The graphitizing carbons are prepared from substances containing more hydrogen. 1970) 15 . Such substances develop a strong system of cross linking of crystallites on heating at low temperature. softer and less porous carbon is obtained (Balcı. As a result. The crystallites remain relatively mobile during the early stages of carbonization and cross linking is much weaker. Physical Structure of Activated Carbon The structure studies of Franklin (1951) on carbonized materials showed two distinct well defined classes.2 Schematic Representation of (a) Nongraphitizing and (b) Graphitizing Structure of Carbon (Smisek and Cerny. the nongraphitizing carbons are formed from substances containing little hydrogen or more oxygen.
Microcrystallites are interconnected by interaction of functional groups terminating the graphitic planes. 1959). 1970).3. the structure of activated carbon differs somewhat from that of graphite. The diameter of the planes forming the microcrystallites. form a group of carbonaceous materials of which the structure and the properties depending on it are more or less similar to the structure and properties of graphite.142 nm. However.36 nm. Planar separation distance these in carbon is approximately been 0. as well as the stacking height. The layer planes are in parallel array with an interlayer spacing of 0. has been estimated at 2 -5 nm indicating that each microcrystallite consists of about 5-15 layers of graphite planes (Wolff. structures have interpreted microcrystallite consisting of fused hexagonal rings of carbon atoms structurally. as From X-ray spectrograph. The interatomic distance between the carbon atoms in the individual layer planes is 0.335 nm. Each microcrystallite comprises a stack of graphite planes (Smisek and Cerny. having a structure similar to that of graphite are formed. similar to those in the rings of aromatic organic compounds. shown in Figure 2. 16 . together with other types of chars. carbon can therefore be considered to consist of rigid interlinked cluster of microcrytallites. During carbonization process.Activated carbons. Graphite is composed of layer planes formed by carbon atoms ordered in regular hexagons. several aromatic nuclei.
4. the spaces between the elementary crystallites become cleared of various carbonaceous compounds and nonorganized carbon. and other forms (Smisek and Cerny. 17 . 1970). Results seem to indicate that.3 Carbon Atom Arrangements in Graphite Crystal. v-shaped. Carbon is also removed partially from the graphitic layers of the elementary crystallites. there are pores with a contracted entrance (ink-bottle shaped). pores in the shape of capillaries open at both ends or with one end closed. pores in the shape of more or less regular slits between two planes.Figure 2. 2. The resulting voids are termed as pores. tapered pores. Porosity During the process of activation.
1989. each group having a certain range of values for the effective dimensions. However. Pores of an effective diameter larger than about 50 nm. 1967. the percentages of the transitional pores and the micropores could be adjusted employing special production procedures. Pores with an effective diameter of less than about 2 nm are called micropores.5 m2/g to 2 m2/g. Transitional pores are those in which capillary condensation with the formation of a meniscus of the liquefied adsorbate can take place. Their specific surface area is generally around 5 % of the total surface area of the activated carbon. This phenomenon usually produces a hysteresis loop on the adsorption isotherm. The micropore volume is generally around 0. 1970). Rodriguez Reinoso. Smisek and Cerny.5 cm3/g and their surface area is 0. Their volume in the activated carbon is generally between 0. Thus. are classified as macropores. According to the type of application. 18 . 1992). Balcı.2 cm3/g and 0. the calculation of diameters of pores assuming cylindrical capillary shapes yields values which approach the actual dimensions of the pores.15 cm3/g to 0.In most cases. The effective diameters of transitional pores are in the range of 2 nm to 50 nm. it is difficult to determine the pore shapes reliably. for the adsorption of vapors and gases from mixtures in which they are present in small concentrations.50 cm3/g. (Gregg and Sing. Activated carbon usually has pores belonging to several groups. 1970. Each of these three groups of pores has its specific function in the process of adsorption on activated carbon. Usually the specific surface area of micropores amounts to over 90 % of the total specific surface area. markedly microporous active carbons are the most suitable. But active carbons used for the recovery of vapors of industrial organic solvents from waste gases and removal of heavy metals from solutions should contain a certain fraction of transitional pores (Smisek and Cerny. The suitability of active carbon for a particular application depends on the ratio in which pores of different sizes are present.
which is not an organic part of the product. for example. especially for polar substances. these values can not serve as specification for activated carbon's quality or properties (Faust and Aly. One of them is represented by chemically bonded elements. The decisive component of the adsorption forces on a highly ordered carbonaceous surface is the dispersion component of the van der Waals forces.2. by the presence of imperfect graphitic layers in the crystallites. As a result incompletely saturated valences or unpaired electrons appear. The elemental composition of activated carbon typically comprises 85-90 % C. 0. obviously change the arrangement of the electron clouds in the carbon skeleton. Activated carbon contains two types of admixtures. 1983). and this influences the adsorptive properties of the active carbon.5 % H. However. Disturbances in the elementary microcrystalline structure as.5. In adsorption of electrolytes and non-electrolytes from solutions. and 1 % S. 1970). Surface areas generated by the more reactive edges of the microcrystallite contain a wide variety of functional groups and are accordingly quite heterogeneous in nature.5 % N. Chemical Properties of Activated Carbon The adsorptive properties of activated carbon are determined not only by its pore structure but also by its chemical composition. These are derived from the starting material and remain in the structure of activated carbon as a result of imperfect carbonization or become chemically bonded to the surface during activation. The ash content and its composition vary widely with the kind of active carbon. 5 % O. The nature of the relevant functional groups is determined to a large extent by the method of activation as well as by the type 19 . The other type of admixture consists of ash. 0. the adsorption characteristics of active carbon are significantly influenced even by small amounts of ash (Smisek and Cerny. the balance of 5-6 % representing inorganic (ash) constituents. in the first place oxygen and hydrogen.
Adsorption of electrolytes is affected by the presence of basic or acidic surface oxides. if oxidizing gases possess the oxygen. The presence of chemisorbed oxygen on the surface of activated carbon has important effects on its capacity to adsorb water vapor and other polar adsorbate vapors. hydrogen peroxide and nitric acid. The oxygen content of activated carbon ranges between 1 % and 25 % and has been shown to vary considerably with the activation temperatures. Materials prior to activation contain hydrogen in the form of hydrocarbon chains and rings attached to border atoms of the hexagon planes. In adsorbents prepared from materials of high oxygen content. 1992). the distance between the parallel graphitic layers is appreciably smaller. this oxygen also be chemisorbed and bound as surface oxides on the edge of the layer planes. 200oC-500oC. The presence of surface oxygen complexes will also impart a polar character to the activated carbon surface. Carbons activated at lower temperatures. It is to be noted that. generally will develop acidic surface oxides. The acidic surface oxides could mainly include phenolic hydroxyl groups. persulfate. which should result in preferential adsorption of comparatively polar organic compounds (Balcı. termed as H-carbons. The oxygen content of starting material has a considerable influence on the arrangement and size of the elementary crystallites formed in carbonaceous adsorbents.of raw material from which the activated carbon is produced (Cookson. permanganate. 1992). The carbons activated at higher temperatures. Wigmans. The oxygen content generally decreases with an increase in the activation temperature (Balcı. Most of this hydrogen is removed during activation at temperatures below 950oC. the evolution of this latter portion of 20 . will develop basic surface oxides. develop the same characteristics as L-carbon. The carbons activated by chemical treatment in aqueous solutions with such oxidizing agents as chloride. 800oC–1000oC. 1980. termed as L-carbons. the course of carbonization and the required carbonization temperature depend strongly on the oxygen content of raw material. On the other hand. Moreover. but some hydrogen is still held after activation and is not released unless much higher temperatures are reached. 1985).
chlorine and other elements can also be present in active carbon (Balcı. nitrogen. In addition to hydrogen and oxygen. Hydrogen is more strongly chemisorbed than oxygen. 21 . The aliphatic hydrogen was suggested to be present in the form of aliphatic chains and alicyclic rings attached to the peripheral aromatic rings. calcined sulphur.hydrogen at very high temperatures is paralleled by a simultaneous decrease in adsorptive power. Infrared studies showed that hydrogen was present in aromatic and aliphatic form. The aromatic hydrogen was suggested to be bonded covalently to the carbon atoms at the periphery of the aromatic basal planes. 1992).
1 Pore Sizes of Activated Carbon Macropores Mesopores Micropores Pores with diameters larger than 50 nm (500 Å) Pores with diameters between 2 nm and 50 nm (20. (IUPAC) is mostly used. meso. originally proposed by Dubinin and now officially adopted by the International Union of Pure and Applied Chemistry. (This classification is given in Table 1. (e.1.) Table 1.g. General Conventional classification of pores according to their diameters. micro) there is not single technique to provide information in 22 . Since the size of the pores vary in a wide range. macro.1 in Chapter I.CHAPTER III METHODS USED IN PHYSICAL CHARACTERIZATION OF ACTIVATED CARBON 3.500 Å) Pores with diameters less than 2 nm (20 Å) There are numerous techniques and methods for the characterization of pore structure of activated carbon.
For the quantitative characterization of the pore structures. the main theory and methods involved in these characterization tests are given. 23 .1 shows micro.1 Micropore. are also used for the characterization of pores. small angle scattering (X-rays or Neutrons). meso and macropore regions of activated carbon. For this purpose. 1994) Figure 3. estimation of pore surface area. pore volume. transmission electron microscopy. Therefore. Complementary techniques. In the following sections of this chapter. such as.all ranges of pores. etc. Mesopore and Macropore Regions of Activated Carbon Figure 3. in most cases a combination of different methods are used. and pore size distribution together with the true and apparent density determinations are needed. (Şenel. the adsorption of gases and vapors by standard gravimetric or volumetric techniques helium pycnometry and mercury porosimetry are still classical and convenient approaches to the general characterization of porosity in activated carbon.
The gas molecules are transferred and accumulated on and in the solid material as a result of the forces between the solid surface and the adsorbate. The slope increases at higher pressures as it is true for the Type II. found that most of the adsorption isotherms fit into one of the five types shown in Figure 3.2. Type II isotherms are most frequently encountered when adsorption occurs on nonporous powders or on powders with pore diameters larger than micropores. Physical adsorption includes attractive dispersion forces. (1940). Brunauer et al.2. at very short distances well as the contribution from the polarization and electrostatic forces between the permanent electric moment and the electric field of the solid. an adsorption process begins. The inflection point of the isotherm usually occurs near the completion of the first adsorbed monolayer and with increasing relative pressure. they are also associated with the 24 . the knee generally occurs near the completion of the first monolayer.3. pressure and the interaction potential between the vapor and the surface. repulsive forces. based upon an extensive literature survey. The amount adsorbed on a solid surface will depend upon the temperature. Type V isotherms result from small adsorbate-adsorbent interaction potentials similar to the Type III isotherms. Type I isotherm indicates a microporous adsorbent. Type III isotherms are observed when the adsorbate interaction with an adsorbed layer is greater than the interaction with the adsorbent surface. at some equilibrium pressure and temperature. Type IV isotherms occur on porous adsorbents possessing pores mainly in the mesopore range. However. a plot of weight of gas adsorbed per unit weight of adsorbent versus relative pressure (P/P0) is referred as the sorption isotherm of a particular vapor-solid interface. Gas Adsorption Phenomena and Standard Isotherms When a gas (adsorbate) is confined in a closed space in the presence of an outgassed solid (adsorbent). Therefore. second and higher layers are completed until at saturation the number of adsorbed layers becomes infinite.
pores in the same range as those of the Type IV isotherms. A new rare type of isotherm, Type VI recently has been found which exhibits a series of steps (Şenel, 1994).
Figure 3.2 Schematic Representations of Different Types of Adsorption Isotherm
3.2.1. The Brunauer, Emmett and Teller (BET) Theory
Although derived over sixty years ago, the BET theory continues to be almost universally used because of its simplicity, and its ability to accommodate each of the five isotherm types. The BET model extends the monolayer Langmuir model to multilayer adsorption. It assumes that the surface is homogeneous and that
the different layers of molecules do not interact. Each adsorbed molecule in the monolayer is assumed to be adsorption site for second layer of molecules, and so on as the relative pressure increases, until bulk condensation occurs. In the region of relative pressures near the completion of monolayer, the BET theory and experimental isotherms do agree very well leading to a powerful and extremely useful method for the estimation of surface areas of various materials including activated carbon, coal and coal chars. In the final form it is given as;
P 1 = V [PO − P ] VmC
C −1 P V mC P O
where; 'V" and 'Vm" are the volume adsorbed, at the relative equilibrium pressure P/Po, and the monolayer capacity respectively, "C" is a constant, which is related exponentially to the heat of adsorption at the first and subsequent layers by the equation:
C = exp
[(q 1
− q
where; "q1" is the heat of adsorption of the first layer, "q2", is the heat of adsorption of the second and subsequent layers. The determination of surface areas from the BET theory is a straightforward application of equation (3.1). A plot of P/V(P-Po) versus P/Po, will yield a straight line usually in the range of 0.05 < P/Po < 0.35. The slope "S" and the intercept of "I" of a BET plot will give
V mC
1 V mC
Solving the preceding equations for "Vm" and "C" gives;
Vm =
The BET equation usually gives a good representation of the frequently appearing Type II and IV isotherms within the range of relative pressures 0.05-0.3, and this range is generally used in practice for measurement of the surface area. At higher relative pressures, the BET equation is usually inaccurate because of capillary condensation effect, while at P/Po values below about 0.05, the amount of adsorbed gas is too small to be measured with sufficient accuracy. A poorer description is obtained for the type I, III and V isotherms, but in practice, they too are often analyzed by the BET method. In order to calculate the surface area, it is necessary to know the mean cross-sectional area Am occupied by one molecule of adsorbate gas. The specific surface area is calculated from the equation,
V mN AA V mol
in which; "Vm" is volume of monolayer, "NA" is the Avagadro's constant and Vmol is the molar volume of the gas. The cross-sectional area of any adsorbed gas molecule can be estimated from the density of the condensed phase of the gas. For surface area determinations, nitrogen as being the ideal adsorbate, exhibits the unusual property that on almost all surfaces its "C" value is sufficiently small to prevent localized adsorption and yet adequately large to prevent the adsorbed layer from behaving as a two dimensional gas. Thus, the unique properties of nitrogen have led to its acceptance as a universal, standard adsorbate with an assigned cross sectional area of 0.162 nm2 at its boiling point of -195.6oC (Şenel, 1994). Using BET it is possible to measure pores down to 10 Å.
3.2.2. Pore Characterization by Adsorption / Desorption
Another way to get information on the porous texture of the adsorbent is to look at the shape of the desorption isotherm. It is commonly found for porous solids
the release of condensate is limited by the neck radius. Type II corresponds to spheroidal cavities or voids as well as to "ink.that the adsorption and desorption branches are not coincident over the whole pressure range. At relative pressures above 0. The formation of a cylindrical meniscus occurs at a higher P/Po than the emptying process.3 Types of Adsorption-Desorption Hysteresis Loops Type I hysteresis is often associated with capillary condensation in open-ended cylindrical-shaped pores. which proceeds through the evaporation from a hemispherical meniscus. Figure 3. that is with microporous 28 . therefore. The liquid trapped in the body of the pore until P/Po is reduced to allow evaporation from the neck. Type III hysteresis exhibits no limiting adsorption at P/Po = 1 and is indicative of slit shaped pores. 1982). Figure 3. which are correlated with various pore shapes.3 shows idealization of the four types of hysteresis (Gregg and Sing.3.bottle" pores. De Boer (1958) has identified five types of hysteresis loops. Type IV hysteresis is associated with Type I isotherms.
Alternatively. Then. ⎡ ⎤ 5 t = tm ⎢ ⎥ ⎣ ln (P / P o ) ⎦ 1/3 (3. the actual pore radius "rp" is given by.6oC. an adsorbed film remains on the pore wall when evaporation of the center core takes place. called the Kelvin radius or the critical radius.6) and (3. with "tm" as 0. "R" is the gas constant . Thus. 1967) as. is not the actual pore radius since some adsorption has already occurred on the pore wall prior to condensation. These can be described by Kelvin's capillary condensation equation (Gregg and Sing.6) where.7) for nitrogen as the adsorbate at its normal boiling point of -195."T" is the absolute temperature. r p = r k + t (3. leaving a center core or radius. Halsey (1948) set up a useful analytical expression for the thickness of the layer "t" as a function of the relative pressure. "σ" is the surface tension.adsorbents.8) Here. "tm" is the thickness of the monolayer.7) The term "rk" indicates the radius into which condensation occurs at the required relative pressure. "(θ)" is the angle of contact between the condensed phase and the surface of the solid. r P = − 2σ V RT ln mol Cos θ [P / P o ] (3. the equation: 29 . due to condensation and evaporation phenomena. In finding the pore radius by the Kelvin equation it is necessary to take into consideration the thickness "t" of the adsorbate layer. replacing equation (3. rk.8) into (3. This radius. "rp" is the mean radius of the liquid meniscus. The hysteresis part of the isotherms contains information about the mesopores. There is a relationship between shape and position of the isotherm and the pore geometry.354 nm. during desorption.
Pore sizes of similar order of magnitude as the sizes of the adsorbate molecules lead neither to the progressive completion of a monolayer nor to multilayer adsorption but to the filling up of the micropore volume with the adsorbate in a liquid-like condition. 15 (P O / P ) ⎡ ⎤ 5 + 3 . 1970). calculation of the size distribution of the mesopores from the adsorption or desorption data permits a simple determination of the mesopore surface area. 3.9) is obtained. Broekhoff.9) is used to calculate the corresponding mesopore radius. 1959. Characterization of Microporosity Adsorption on microporous solids is not very well understood in comparison with that on non-porous or mesoporous solids. 303 log (P O / P ) ⎦ (3. One computational method (BJH). proposed by Barrett.3. For a symmetrical pore geometry. 54 ⎢ ⎥ ⎣ 2 . 1. Stepwise computational methods for finding the pore radius distribution and the mesopore surface area and volume are described by several investigators (Orr.2. Derivations of the related equations are given in Appendix A. The gas volumes adsorbed or desorbed upon a change of the relative pressure are taken from the isotherms.r P = log 4 . Here. is frequently used in practice. Assuming certain pore geometry. A major development in understanding adsorption of gases and vapors on microporous carbons was provided by the potential theory of adsorption of Polanyi (1932). 30 . Joyner and Halenda (1951). and Equation (3. the contribution to the surface area from the pores of various sizes can be found from the pore radius distribution. a closely packed hexagonal liquid structure is assumed for the nitrogen molecules. The question of whether or not the adsorption or desorption branch is better suited for calculation of the mesopore size has not yet been answered definitely.
Tc. (i) when the temperature is well below the critical temperature of the adsorbate. the adsorbed vapor may be considered as liquid-like. Polanyi expressed the adsorption potential for volume filling as the amount of work necessary to compress the adsorbate from its equilibrium vapor pressure P1 to the compressed adsorbate pressure.11. W=O and Ei=Emax. The first case is. the adsorbate will be as compressed gas. P2. the adsorption potential at constant volume filling is also temperature independent. 31 . (ii) when the temperature is just below the TC most of the adsorbate will be as liquid like but also the adsorbate may be as compressed gas. "E" is equal to the ∆G "equivalent free energy change". The state of the compressed adsorbate in the adsorption space depends on the temperature. As one moves away from the surface the values of adsorption potential decrease until it falls to zero and the adsorption space increases up to a limiting value Wo (zero potential). by far. Therefore the adsorption potential will take the form in Equation 3. Polanyi distinguished three different cases. Polanyi described the adsorption space as a series of equipotential surfaces.Potential theory assumes that at the adsorbent surface the molecules of gases are compressed by attractive forces acting between the surface and the molecules and these forces of attraction decrease with increasing distance from the surface. Polanyi assumed that since dispersion and electrostatic forces are independent of temperature. and each enclosing a volume Wi. P 2 E = P1 ∫ RT P dP = RT ln P P 2 1 (3. The building up of the volume enclosed within the adsorption space may be described by the function of the type E = f(W). This means that the curve E = f(W) should be the same for a given gas and a given adsorbent at all temperatures. (iii) when the temperature is above the Tc.10) Thus. the most common one. This relationship between "E" and "W" is called the characteristic curve. each with the adsorption potential Ei. At the surface.
W = nM ρ = nV mol (3. (D-A)" equation. Dubinin's treatment has been modified by Kaganer to yield a method for calculation of specific surface from the isotherm. then it is possible to obtain the volume filled adsorption space by Equation 3. A plot of log W against log (Po/P) will be a straight line having an intercept equal to micropore volume. log W = log W O − D log 2 ⎛ PO ⎞ ⎜ ⎟ ⎝ P ⎠ (3.Astakhov. The temperature-invariance of the adsorption potential which is the fundamental postulate of the Polanyi's theory. obtained the following "Dubinin .12) where. "β". They stated that for an identical degree of filling of the volume of adsorption space. Dubinin and Astakhov.12. "M" is the molecular weight of the adsorbate and "ρ" is the liquid density. Dubinin and Radushkevich. arrived at an expression which is known as "Dubinin Radushkevich". (D-R) equation.303 K (RT / β )2. "Wo".11) In this equation it is assumed that the liquefied adsorbate is incompressible and has the normal density of the liquid at the given adsorption temperature.14) 32 . log W = log W O − D ' log n ⎛ PO ⎞ ⎜ ⎟ ⎝ P ⎠ (3. Using the experimental data and assuming that pore size distribution is Gaussian. "n" is the amount adsorbed in moles.13) where D is 2. has been demonstrated (1966). the ratio of adsorption potentials for any two vapors is constant which is called the affinity coefficient.E = RT ln PO P (3. rather than a Gaussian. mainly by Dubinin and co-workers and they have added a second postulate. assuming a Weibull distribution of pore sizes.
The physical characterization of activated carbon samples were characterized according to this scheme. when n=2 (Şenel. contribution to the volume made by pores or internal voids must be excluded when measuring the true density. no fluid completely fills the pore volume of activated carbons.14). that "DR" equation is a special case of "D-A" equation (3. one needs to have a non interacting fluid which completely fills all the pores. Figure 3.303(n-1) (RT/E)n.13).7Å. True Density Determinations True density of a porous solid is defined as the ratio of the mass to the volume occupied by that mass. 33 . It follows from the equation (3. Helium is the smallest molecule available with an atomic diameter of 1. true density should be treated in this way. To determine the true density of a solid. In reality. it has the best chance of penetrating the entire porosity of activated carbon.4 depicts a block-diagram of the overall experimental approach.3. Therefore. 1994). 3. Therefore. the term.where D'= 2. Therefore.
/DES.4 Block-Diagram of the Overall Experimental Approach 34 . (-195.BET N2 ADS.6oC) BJH (KELVIN HALSEY EQN'S) SURFACE AREA MESOPORES 50 nm>Dp>2 nm CO2 ADS. (0oC) D-R EQ'N MICROPORES Dp<2 nm TRUE DENSITY He PYCNOMETER Figure 3./DES.
they decompose to various pyrolysis products. Roberts. etc. Gas phase products include all lower molecular weight products (CO.and gas-phase applications in the industry for its adsorptive properties. chars. CO2. production of activated carbon from hazelnut shell and hazelnut husk and the quality of the products have been investigated. For manufacture of activated carbon. lignin. CH4. In order to 35 . usually constituting approximately 15-20 percent yield. Agrawal and Mc Cluskey 1983). 1957. 1970. H2. these products can be grouped into three classes. is extremely complex. The major components. Pyrolysis of lignocellulosic materials such as shells or stones of fruits etc. cellulose and hemicellulose mainly react independently and the pyrolysis of the lignocellulosic materials is the result of hundreds of cocurrent and consecutive reactions. Tars are any of several high molecular weight products that are volatile at carbonization temperatures but condense onto any surface near room temperature. In this study.) including water. As lignocellulosic materials are heated in an inert atmosphere. Any one of the pyrolysis products could be the most desirable product. Char is a carbon-rich nonvolatile solid residue. Depending on their volatility. Usually gas phase products constitute 20-25 percent of the total products of pyrolysis. Tar comprises approximately 60-65 percent of the products (Schwenker and Pascu. gases and tars. the char is the desirable product.CHAPTER IV PREVIOUS WORK ON PREPARATION OF ACTIVATED CARBON Activated carbon is one of the most commonly used adsorbents in many different liquid. Most commercial scale pyrolysis plants are designed to have only one class as the principal product with at least one of the other classes serving as a fuel source.
At temperatures below about 300oC. the ring opening reaction is favored over depolymerization. %) 36 .4 glucosidic bonds results in depolymerization of cellulose and is responsible for the formation of tars. So investigation of cellulose pyrolysis mechanism gives an idea about the pyrolysis of lignin and hemicellulose (Balci. Higher heating rates cause temperatures favorable to depolymerization to be reached more quickly. and hence give higher tar yield. Cellulose is the major components of plant cells and hemicelluloses have approximately the similar molecular structure with cellulose.investigate mechanism of pyrolysis. molecular bonds will be broken.4 C-O-C glucosidic and the 1. As discussed by Agrawal (1988 a. 1983) The production of activated carbon is a typical gas-solid reaction. 1992). (Agrawal and Mc Cluskey. During heating in an inert atmosphere. it is better to study pyrolysis of each component separately. The influence of the experimental conditions such as temperature. The adsorptive capacities of activated carbon are mainly associated with its internal pore properties such as pore surface area.5 acetal bonds leads to ring opening and results in formation of gases and chars. heating rate.1. 1992).b) the breakage of 1. hazelnut shell. Trace amount of impurities are thought to catalyze the ring opening reactions.5 C-O-C acetal linkages in the cellulose macromolecule. walnut shell and apricot stone through chemical activation using ZnCl2 (30wt. Since C-O bonds are weaker than the C-C bonds the principal candidates for scission are the 1. residence time at high temperature can be interpreted in terms of competition between these two types of cellulose degradation. (2003) produced activated carbon from almond shell. which develop during the activation of chars (Balcı. pore volume and pore size distribution. Chemical Activation Aygün et al. Pyrolysis of pure cellulose has been widely studied. The breakage of 1. 4.
the higher the micropore volume. V-DR N2 as 1. the lower the heating rate during the pyrolysis process. In this study the activation agent/coal ratio was found to be the most important parameter in a chemical activation process. Balcı et al. As activation agent/coal ratio increases micropore volume and surface area increases reaching a maximum ratio of 4:1 giving micropore volume of 1. Activated carbon from the hazelnut shells had the highest surface area (793 m2/g) and iodine number (965 mg/g). hazelnut shell and walnut shell were found with 10 h activation at 750 °C but for apricot stones it was obtained with 18 h activation at 800 °C. In a next study they used NaOH to produce activated carbon from Spanish anthracite. (2001) produced activated carbons from Spanish anthracite by using KOH. However. chemical activation carried out at 350oC gave products with surface area values above 500 m2/g.as the activating agent. The order of suitability of raw materials for activated carbon production was established as hazelnut shell > walnut shell ≈ apricot stone > almond shell. Lozano-Castello et al. (1994) carbonized ammonium chloride-impregnated and untreated almond shell and hazelnut shell samples in a flow of nitrogen at relatively low temperatures. Iodine molecule gives information on the surface area contributed by pores larger than 10 Å. It was also observed that. The higher the temperature and the time of pyrolysis.04 cc/g and V-DR CO2 as 0. Also. Nitrogen flow and the activating agent/coal ratio were found to be the most important parameters to achieve the best development of porosity. the surface area of 37 . The highest result was obtained for 960 ml/min nitrogen flow rate and activating agent/coal ratio of 3:1 at 730 °C as BET surface area of 2669 m2/g. the higher the microporosity development. It was observed that.45 cc/g and a BET surface area of 3290 m2/g. In other words.64 cc/g. It was concluded that physical mixing of NaOH and coal rendered the best results. Highest surface area of activated carbons from the almond shell. the surface area values observed for the products obtained from untreated raw materials were about half of this value. it was concluded that an acid washing after carbonization could be avoided because the results were not very different from those obtained with water washing.
500 or 700 oC.products obtained from NH4Cl-impregnated samples reached values of over 700 m2/g when the carbonization temperature was increased to 700oC. cadmium 38 . Hazelnut shell showed a good efficiency in removing from aqueous solution toxic ions such as three and hexavalent chromium. (2002) obtained activated carbon from date pits by using phosphoric acid. 40% and 50% phosphoric acid by weight. Demirbaş et al. Girgis et al. although with high capacity for the uptake of probe molecules from solution. Adsorption increased with increasing temperature. Cimino et al. as the temperature increased the BET surface area increased from 700 m2/g up to 1400 m2/g. 400 and 500oC. they used 20%. %) followed by pyrolysis at 300. Carbons obtained at 300 oC were very poorly porous. % H3PO4 impregnated sample having 945 m2/g of BET surface area. and decreasing particle size. The highest BET surface area was obtained from the sample which was impregnated with 30% phosphoric acid and carbonized at 500oC. (2000) studied on removal of toxic cations and Cr(VI) from aqueous solution by hazelnut shell as biosorbent substrate. It was suggested that adsorption data of hazelnut shell based activated carbons fitted best to the Langmuir adsorption isotherm. In this study.5/1 was employed and it was observed that. Activated carbons used in the batch mode adsorption studies had a BET surface area of 441 m2/g. The best porosity development was found at 700 o C with 50 wt. a ratio of acid volume: weight of raw precursor of 1. For impregnation. (2002) studied the removal of Ni(II) from aqueous solution by adsorption onto hazelnut shell activated carbon. Girgis et al. Activated carbons were obtained by impregnation of the raw material with concentrated H2SO4 and activation in a hot air oven at 150 oC for 24 h. The raw material was impregnated with increasing concentrations of H3PO4 (30-70 wt. pH and agitation speed. 30%. Activated hazelnut shells were prepared by using H2SO4 as the activating agent and drying in an air oven at 40 oC for 2 h. Carbons obtained at 500 and 700 oC were found to be good to excellent adsorbents. (1998) carbonized phosphoric acid impregnated apricot stones at 300.
Toles et al. K2CO3 and Na2CO3).25 to 2 and reached a maximum of 2450 m2/g. with CO2. ZnCl2 activation gave the best products with high adsorption capacities. Hu et al. CO2 flow resulted in larger mesopore and total pore volumes especially for long-time activated samples.78 g Kg maximum sorption capacity at pHadj 2. respectively. Raw material particle size.7 g Kg –1 -1 .42. or physically.25 and 3. It was observed that.5 and the . Total yield was obtained as 33-41% for chemical activation and 7-22 for physical activation of the raw materials.b) produced activated carbon from almond shells through chemical activation. Chemically produced activated carbons had BET surface areas between 955-1267 m2/g while surface areas of physically produced activated carbons were in the range of 104-485 m2/g. 3. (1984 a. (1997) prepared activated carbon from almond and pecan shells.08 and 1. many micropores were enlarged to mesopores. Also. Cr3+ and Zn2+ ions the maximum sorption capacities were 5. (2001) prepared activated carbons from ZnCl2 impregnated coconut shells by pyrolysis under nitrogen flow until temperature reached 800 oC. Among several activating agents (H3PO4. Bevla et al. 39 . under a variety of conditions followed by oxidation at 300 ºC for 4 hours.5-3. lignocellulosic precursors for the production of granular activated carbon (GACs) in order to create carbons for the adsorption of both organic compounds and metals.0 is 17. A maximum adsorption capacity had been observed at 500 oC. The BET surface area increased with increasing ratio of ZnCl2 to shell from 0. They activated samples either chemically. It was indicated that as the activating agent/shell ratio was higher than 2.0 for Cd2+. which were hard.and zinc. at pHadj 4. activation time and impregnation ratio were studied. ZnCl2. with H3PO4. activating reagent recovery and adsorptive capacity increased as the raw material particle size decreased. For hexavalent chromium removal pH interval is 2. and then switching the gas flow to carbon dioxide. Activating agent/shell ratio (w/w) was between 0. Based on the Langmuir isotherm model. showing the impregnation ratio to be the most effective variable in increasing the adsorptive power of the products up to 2111 m2/g.
or by activation after carbonization under nitrogen atmosphere. sugarcane bagasse and pecan shells. Carbonization of pecan shells was performed at 450 oC under the flow of nitrogen. at low temperatures direct activation with air developed meso and macroporosity to a larger extent than the activation which was preceded by carbonization. They compared the carbons to each other and to two commercial carbons in an effort to ascertain the relative value of the carbons in terms of yield. rice hulls and sugarcane bagasse based activated carbons were obtained by activating with a mixture of nitrogen and carbon dioxide at 900 oC after the pyrolysis at 750 oC. (1980) produced activated carbon from direct activation of almond shells with CO2 or air. In chemical activation of pecan shells 25% and 50% H3PO4 solutions were used as activating agent. sugarcane bagasse showed a better potential than rice straw or rice hulls as precursor of granular activated carbons with the desirable properties of a sugar decolorizing carbon. Toles et al. It was observed that. Pecan shell based activated carbons were produced by both physical and chemical activation methods. The products obtained by the activation of air in the temperature range from 300 to 400oC. It was also observed that. Physical Activation Solano et al. surface 40 . did not show large surface areas. direct activation with carbon dioxide in the temperature range of 750oC and 900oC gave activated carbons with similar or larger surface area and micropore volumes than those obtained by carbonization followed by activation. (2000) prepared a series of steam or CO2 activated carbons made from almond shells using six different activation or activation/oxidation methods. air or a mixture of both. 4. The products had surface areas ranging from 150 m2/g to about 2000 m2/ g for different degrees of burn off and types of production. rice hulls. (2000) produced activated carbon from rice straw.2. Also. Chemically activated pecan shells found to have higher surface area values (1200 m2/g) than physically activated ones. the reverse case was observed for the high temperature products. Rice straw.Ahmedna et al. However.
Arol et al. metal uptake. attrition. (2001) investigated the pre-carbonization effect on the porosity development in activated carbons. It was found that the inclusion of pre-carbonization process contributed to the formation of activated carbons with higher values of the BET surface area and 41 . Activation with carbon dioxide led to the appearance of micropores and to a considerable increase in surface area. peach stones had the largest surface area of 923 m2/g followed by apricot stones (850 m2/g) and hazelnut shells (825 m2/g). Carbonization was performed at 700. In pre-carbonization step the raw material was heated until the temperature reached 900 oC under nitrogen flow and kept at that temperature for 1 h. apricot and peach stones to determine their suitability for gold metallurgy. Two types of activated carbons were manufactured from rice straws. as well as estimated cost of production. Rodriguez-Reinoso et al. surface functional groups. It was concluded that active carbons produced from apricot and peach stone can be employed in the recovery of gold from gold cyanide bearing solutions. Peach stone was found to result in hardest carbon and hazelnut shell the weakest. the method that produced the best overall performing almond shell carbon and least expensive carbon in terms of production cost was the "Air-Activation" method. 800 and 900 o C under nitrogen flow and samples were activated at the same temperatures in CO2 for 3 h. This method involved the simultaneous activation and oxidation of almond shells under an air atmosphere. The raw materials were carbonized under a flow of nitrogen at 650 oC and then activated with steam at 800 or 900 oC. In terms of BET surface area. (2002) tested activated carbons made of hazelnut shells. The highest BET surface area (adsorption at 77 K) was obtained as 1597 m2/g for the material carbonized at 850 oC and activated at 825 oC for 24 h with 5 oC/min heating rate. Of the six methods investigated.area. (1982) prepared activated carbons from almond shells by means of carbonization in a flow of nitrogen at 750-900 activation in a flow of carbon dioxide at 650-800 o o C. organic uptake. Yun et al. by the one-stage and two-stage processes. followed by C. respectively.
Wartelle et al. 3 h were the optimum activation conditions within the experimental range. paranitrophenol and methylene blue had been studied to investigate the microporosity. A gas chromatographic analysis revealed that macadamia nut shell granular activated carbons seem to be an excellent adsorbent for polar and non-polar compounds from C3 and C6-C10 tested. It was found that 800 o C. i-butane. respectively. They used three activation temperatures. Activation was performed at 800 oC in a mixture of CO2-N2 or at 850 oC with steam. resulting in (790 m2/g). CO2. The adsorption of N2. evolution and 42 . They obtained activated carbons with a well-developed porous structure. BET surface areas of the products were found as 583. They found no direct relationship between exposed BET surface areas (the surface where activation reaction takes place). (2000) used macadamia nut shells. 840 and 880oC. 800. especially macro porosity to larger extents. Carbonization of raw materials was done at 700-750 o C under nitrogen flow.23 cm3/g to 0. hazelnut shells and black walnut shells. hazelnut shells and black walnut shells as the precursors for activated carbon production. (1985) prepared activated carbons from plum and peach stones by carbonization followed by carbon dioxide activation with activation time ranging from 8 hours to 16 hours. Porosimeter results showed that. predominantly microporous with high BET surface areas. N2 adsorption studies gave the micropore volumes ranging from 0.27 cm3/g to 0.77 cm3/g for products from plum stones and 0. Direct activation led to development of these two ranges of porosity. (2001) prepared three series of activated carbons from Quercus agrifolia wood using a two-step process.microporosity than those of activated carbons without pre-carbonization. covering the 18-85 wt % yield range by variation of residence time. 572 and 829 m2/g for macadamia nut shells. Sanchez et al. macro and meso porosities were more developed in carbons prepared from plum stones. They characterized samples by N2 and CO2 adsorption. carbonization followed by physical activation with CO2. Reinoso et al.38 cm3/g for those obtained from peach stones for various production methods and conditions.
CO2 (0oC) and mercury porosimetry experiments to determine the characteristics of all carbons prepared.gasification rate variation.217-0. They carried out adsorption of N2 (-195. They exploited the results obtained activation using response surface most methodology. 0. They produced activated carbons with BET surface areas in the range of 400-1200 m2/g. 115–490 mg/g for the adsorption of methylene blue. 741–1495 mg/g for the adsorption of iodine.557 cm3/g for the volume of pores with a diameter greater than 3.377 cm3/g for the micropore volume.225–0.6oC). Baçaoui et al.3–132 m2/g for the external surface area. They obtained the optimal activated carbon when using 68 min as activation time and 822oC as activation temperature.7 nm and 31. They found experimental response varying between: 13–27% for the total yield. 514–1271 m2/g for the BET surface area. (2001) prepared a series of activated carbons from olive-waste cakes by physical activation with steam. 43 . influential They in represented activated these carbon responses and studied in all experimental regions of activation time and temperature. the factors preparation. 0.
1.1 Procedure Followed in Experiments 44 . Raw Material Crushing S i evi ng Impregnation Drying Carbonization C ool i ng Crushing Washing With Distilled Water Drying Product Figure 5.CHAPTER V EXPERIMENTAL WORK The main aim of this study was to produce activated carbon from agricultural wastes. hazelnut shell and hazelnut husk and to characterize these products. Chemical activation method was applied by using phosphoric acid as the activating agent. A summary of the experimental procedure is shown in Figure 5.
3 0. in the literature raw materials generally have ash content of 3-5 %. also reported ash content of hazelnut shell as 0.) 0.) 49.4 Ash% (wt.8 45 .9 O %( wt. and Balci et al.) 6. Properties of Raw Materials Hazelnut shells and hazelnut husks used throughout the experiments were obtained from Trabzon and Giresun.4 5.) (by difference) 43. Ash content of raw hazelnut shell is surprisingly low compared to the husk. Table 5.1.5. Aygün et al.2 N% (wt.49% and 1.1 Chemical Composition of Raw Materials Raw materials Hazelnut Shell Hazelnut Husk C% (wt.6 45. respectively.3 5.) 0. Elemental analysis of raw materials and activated carbons were carried out using a "Leco CHN Elemental Analyzer". Ash contents of the products were determined by Thermogravimetric Analyzer (TGA) and ash contents of the raw materials were determined by following TS 6879 (Turkish Standards Institution (TSE)).1.7 H% (wt.1.1.4 42. Also. This low ash content might be a specific property for hazelnut shell.33%. Properties and Preparation of Raw Materials 5. The chemical compositions of raw materials are given in Table 5. respectively.
) H3PO4 solutions separately at 25oC for 24 hours. So.7 mm) and hazelnut husk particles having sizes 20-50 mesh (0. of hazelnut husk was mixed with 5 ml of acid solution.3-0. 40% and 30% (wt. Hazelnut shells were crushed by a home blender and hazelnut husks were ground by a laboratory type grinding machine. In Table 5. in the impregnation of hazelnut shells 1 g. shell was mixed with 2 ml acid solution while 1 g.2. Through the experiments the effect of activating agent concentration was investigated. 50%.8 mm) were used in the rest of the experimental work.1. The resulting particles were sieved and the hazelnut shell particles having sizes 12-18 mesh (1-1. To have uniform size of hazelnut husk. The choice of using smaller particle size for hazelnut husks is because of heterogeneous physical structure of the material. solution was filtered to remove the residual acid. The ratio of weight of dried samples (impregnated) to the weight of raw material before the impregnation step indicates the weight increase. grinding was required.2. 46 . After impregnation.5. the dried samples were ready for the carbonization experiments. After this procedure. Subsequently. In the literature particle sizes of raw materials used in activated carbon production is generally in the range of 0. the impregnated samples were air dried at room temperature for 3 days. As a proportion.2-2 mm. the impregnation ratios and weight increases of the raw materials are given. and allow all particles contact with the acid. Preparation of Raw Materials Raw materials were dried at room temperature. raw materials with the desired particle sizes were treated with 60%. The reason for choosing these amounts of acid volumes was to obtain homogenous mixtures of raw materials and acid solutions.
25 6. %) 30 40 50 60 30 40 50 60 Impregnation ratio (acid : raw material) (w : w) 2. After cooling. 47 .2.32 2. a 20 mm inside (24 mm outside) diameter quartz tube of 90 cm length was placed horizontally into the furnace.50 2.Table 5. To measure the N2 flow rate passing through the system a N2 flow meter was connected to inlet of furnace.2.65 7. the outlet gases were purged to the hood by a heat resistant hose.2. (wt. Carbonization Experiments 5. Experimental set up used in the experiments is shown in Figure 5.1. to avoid the escape of N2 from the system and to avoid the entrance of air to the system.2 Impregnation Ratios and Weight Increases of the Raw Materials H3PO4 conc.80 6. The inlet and the outlet of quartz tube were connected with quartz fittings.82 5. % 21 23 35 40 133 212 275 348 5. Experimental Set-Up Carbonization experiments were carried out in a horizontal "Lenton Unit C2" furnace.66 2. A bubbler in a cooling bath was used at the outlet of the system to cool the outlet gases and to indicate the N2 flow by bubbling. To ensure the inert atmosphere in the furnace by N2 gas flow.05 Raw material Hazelnut Shell (2 ml acid: 1 g shell) Hazelnut Husk (5 ml acid: 1 g husk) Weight inc.
Activation of phosphoric acid impregnated raw materials was carried out by carbonizing the materials under nitrogen flow (200 cc/min or 64 cm/min) at a heating rate of 20oC/min. Chemical Activation of Samples In this study. in other words.) H3PO4 solutions were carbonized for 2 hours at carbonization temperatures of 300. A faster removal of the gases evolved during carbonization process favors the creation of micropores or. on the properties of the products was investigated.valve quartz hood sample N2 cooling bath furnace flowmeter Figure 5.2 Experimental Set-up 5. 50% and 60% (wt. In the literature high nitrogen velocities (30 cm/min) were recommended. and 500 ºC.2. the effect of activating agent concentration and carbonization temperature. a lower concentration of these gases produces higher micropore development 48 . 400. 40%. raw materials. For this purpose.2. impregnated with 30%.
and the temperature was kept constant by the temperature controller of the furnace. Carbonization time was measured from the point when the selected temperature was reached.5 Variable Parameters Carbonization Temp.3 HS 30.3 HS 40.%) N2 flow rate 200 cc/min (velocity: 63 cm/min) Heating Rate 20 ºC/min 40 Carbonization Time Mixing Proportion 2:1 (ml acid : g.3 HS 60.5 HS 60.5 HS 40. 2001).3 Experimental Conditions and Sample Codes for Hazelnut Shell Fixed Parameters Particle size 12-18 mesh H3PO4 conc.5 HS 50. the furnace was cooled using a hair dryer. the furnace was started to heat till the chosen carbonization temperature was achieved. After 2 hours of carbonization time. (wt. Lastly. product was taken to a flask and distilled water was added to prevent interaction with the air.(Lozano-Castello. and continued for 30 minutes. shell) 60 2 hours 50 30 300 400 500 300 400 500 300 400 500 300 400 500 HS 30. After purging.4 HS 30. Experimental conditions and samples codes are given in Table 5. respectively.4 HS 50.4 HS 40. Samples were placed in the furnace and the N2 flow started to purge the air in the quartz tube.3 and Table 5. when the temperature of furnace decreased down to 100oC.4 HS 60. (ºC) Sample Code 49 .4 for hazelnut shell and hazelnut husk. Table 5.3 HS 50.
50 . results in higher removal of the acid and more effectively opening of pores filled by tarry substances. Decreasing particle size of the products before washing step.4 Variable Parameters Carbonization Temp.4 Experimental Conditions and Sample Codes for Hazelnut Husk Fixed Parameters Particle size 20-50 mesh H3PO4 conc. Prior to making all characterization tests. products were crushed by a mortar in distilled water to obtain smaller particles and rinsed with boiling distilled water until the pH values of the activated carbons reached to 5-6. (wt. the effect of activating agent concentration and carbonization temperature to the pore structure.%) N2 flow rate Heating Rate Carbonization Time Mixing Proportion 5:1 (ml acid : g. Through the characterization of the products. ºC Sample Code 5. total surface area and true densities of the samples were examined.5 200 cc/min (velocity: 63 cm/min) 20 ºC/min 2 hours 40 50 400 400 HH 40.3 Characterization of the Products Before the characterization. Suspension pH values of the activated carbons were determined by following TS 5896 (Turkish Standards Institution (TSE)). husk) 60 300 400 500 HH 60.4 HH 60.4 30 400 HH 30.3 HH 60.Table 5.4 HH 50. the samples were oven dried at 110oC overnight under vacuum.
To measure the mesopore surface area and the mesopore volume of the samples nitrogen gas adsorption and desorption isotherms were obtained at -195. an equilibration time of about 30 minutes was allowed.181 g/cm3.. Surface area of the samples was determined by using BET equation. Joyner and Halenda (BJH) method (as indicated in section 3. USA was used to measure the surface area and to determine the pore size distributions of mesopores using N2 adsorption data at -195. 2004).2.5. Analysis of the micropores was also determined by this unit by CO2 adsorption at 0oC.2.3. respectively. A minimum of 30 minutes was allowed for equilibrium to be established at each point of the CO2 isotherm. 1951) considering the IUPAC mesopore range definition which ranges from 50 nm down to 2 nm in terms of the pore diameters of the cylindrical pores. the details of which are explained in the following section..6oC. CO2 Gas Adsorption Measurements The micropore volume of the samples was estimated by application of the Dubinin-Radushkevich equation to carbon dioxide adsorption at 0oC. (Barrett et al. Accelerated Surface Area and Porosimetry System manufactured by Micromeritics Co.162 nm2 (Şenel. 5. The cross sectional area of the nitrogen molecule was taken as 0.3 (Yağşi.2). The data were automatically collected and evaluated by using the software/ computer system. Nitrogen Gas Adsorption Measurements A commercial volumetric gas adsorption apparatus "ASAP 2000". The 51 .1.17 nm2 and 1. For each experimental point. Micropore surface area of the samples were calculated from the DR micropore volume. The area and the volumes of the pores as well as their distributions were evaluated from the nitrogen adsorption isotherms using the Barrett. 1994). taking the cross sectional area and the density of a CO2 molecule as 0.6oC. Schematic diagram of this apparatus is given in Figure 5.3.
User Manual. Appendix C. 52 .saturation vapor pressure was taken as 26142.000 mmHg at the analysis temperature of 0oC (Micromeritics ASAP 2000. 1993).
Figure 5.3 Schematic Representation of Surface Analyzer .3 Schematic Representation of Surface Analyzer 53 Figure 5.
1-0. the variable volume chamber. The He Pycnometer instrument is consisted essentially of a sampleholding vessel.5. True Density Determinations True (Helium) densities of samples were determined by using a commercial Pycnometer. After obtaining three values. This indicates where the reading should be taken. the relative position of which is indicated on the front panel dial to five decimal places and a pressure detector. the valve was turned to gauge position for sealing the helium in the system at atmospheric reference pressure. both were evacuated in the pycnometer chamber for a sufficient time.3. By this way. Once the valve is in gauge position. sample chamber and pressure detector are connected and sealed off as a closed system. After some time. for standard volume and for sample as "VCELL". the pressure increase to the point where contact is broken between the bellows of the pressure detector and an electric contact in the detector. respectively. In a typical run. chamber was opened to atmosphere by means of a 4 position valve. A dial light reveals whether the pressure in the system is the same as that in the detector. for empty cup. "VEXP" and "VSAMP" values. enough helium is allowed to escape into the system in order to reduce the pressure in the system to the atmospheric reference pressure. After filling the system with helium. 54 . 0.3. When the variable volume is changed so as to decrease the volume of the system.2 g degassed sample was placed into a propylene cup and then. true density of the sample was determined using the known weight of sample according to procedure given in Appendix C. a cylinder fitted with a movable piston.
600 ºC activation temperatures).CHAPTER VI RESULTS AND DISCUSSION 6. These preliminary results led to the conclusion that hazelnut husk was not a proper precursor for activated carbon production by physical activation with steam. A comparison was made with hazelnut shell in order to have an idea on applicability of hazelnut husk in activated carbon production. The reason for this result could be the choice of lower activating temperatures with respect to the literature values (900 ºC. 55 . But. In the physical activation of hazelnut husk. it was also observed that the sample was not resistant to 800 ºC activation temperature.1 Preliminary Experiments At the beginning of the experimental work the plan was to obtain activated carbons from hazelnut husk by physical activation with steam and from hazelnut shell by chemical activation using potassium hydroxide as the activating agent. carbonization temperature was 400 ºC and activation temperatures were 400. resulting in only 13% yield. BET surface area results showed that. BET surface area results showed that samples activated at 400 and 600 ºC had very low surface areas (2-5 m2/g). steam). 600 and 800 ºC.334 m2/g) with respect to hazelnut husk. Physical activation with steam was applied to hazelnut shell using the same parameters with hazelnut husk experiments (400 ºC carbonization. hazelnut shell gave higher surface areas (313. under the same conditions with hazelnut husk.
two activated carbon samples were obtained having BET surface areas of 399 and 512 m2/g. another chemical. the average of which was 398. At carbonization temperature of 400 ºC and under 200 cc/min N2 flow rate. % H3PO4 solution was mixed with hazelnut shells in the ratio of 2 ml acid: 1 g shell. 60 wt. Surface area results were around 380-420 m2/g. resulting in the failure of the experiment. the reason probably being the incompatibility of KOH with the structure of hazelnut shell. showing that the experiments were reproducible. the tarry substances formed during carbonization might 56 . input weight of raw material did not solve this problem. Changes in experimental parameters like. gas flow rate. This was attributed to either blockage of the quartz tube or dragging of the sample in the tube. The less the input weights of the raw material. the experiments of KOH impregnated samples could not be completed and no product was obtained. this difference in BET surface areas was because of the difference in the amount of raw material placed in the tube. which was 4% of the average of BET surface areas. the higher the surface area of the products. while for the production of the sample having 512 m2/g of BET surface area 6 gram of raw material was used. exactly under the same experimental conditions. six experiments were performed.. potassium hydroxide (KOH) was tried as the activating agent. it was observed that gas evolved during carbonization was very dense and the system pressure became unstable.3. at the same temperature but 130 cc/min gas flow rate. phosphoric acid. Solid KOH was dissolved in 10 ml distilled water and shells were mixed with these solutions in the shell/KOH ratio of 1:1 and 2:1. When the sample in the tube was not loose. For these data the standard deviation was found as 16.5 m2/g. So. In the production of the activated carbon having 399 m2/g of BET surface area. 18 gram of raw material was used. was tried in the impregnation step for hazelnut shell. It was noticed that. KOH might have changed the structure of the raw material during impregnation in such a way that the layers of carbon matrix collapsed when carbonization started and the material became nonporous and sticky. Since no product could be obtained by using potassium hydroxide as the activating agent.For the production of activated carbon from hazelnut shell. On the other hand. BET surface areas of the products were obtained by using N2 adsorption data. But. carbonization temperature.
Under identical experimental conditions. The samples were characterized by determination of solid (true) density. 57 . micropore area.6oC and carbon dioxide adsorption at 0oC. Helium pycnometry and D-R method (CO2) were used to determine these values.2. TGA experiments were carried out to supply information in terms of yields of acid impregnated samples. 50% and 60% wt. 400 and 500oC) and for different H3PO4 concentrations (30%. to provide information on elemental composition. nitrogen adsorption at -195. The rest of the experiments were performed by placing 6-7 grams of impregnated hazelnut shells in the furnace. The objective of this study was to investigate the pore structures and surface areas of activated carbons produced from hazelnut shell and hazelnut husk at relatively low temperatures (300. mesopore volume.4. Physical Characterization of the Products The physical characterization of the products was carried out according to the experimental procedures given in Figure 3. N. In terms of chemical characterization. When phosphoric acid impregnation was applied to hazelnut husks.). The pore structure of the activated carbons was characterized by different physical techniques.cover the reaction surface of the material and lower the reaction rate. 6. H. 40%. micropore volume and surface area. O elements and ash analysis. even higher surface area activated carbons were obtained. mesopore area. the analyses were limited to the determination of C. By using helium pycnometry true densities of the products were obtained. BET (N2) surface area measurement.
5 HS 40.2. Samples impregnated with 30% H3PO4 solution (HS 30.3 HS 50.6.4 HS 60.1.5) had very low values of surface area. In Figure 6.5 84 4 3 252 270 134 94 242 426 380 596 2 Figure 6. 596 m2/g.5 HS 60.2 the effect of carbonization temperature and phosphoric acid concentration to BET surface areas can be seen more clearly.3. Hazelnut Shell Based Activated Carbons BET surface area values of hazelnut shell based activated carbons are shown in the Figure 6. which was impregnated with 60% H3PO4 and carbonized at 400 ºC.5 HS 50. had the highest BET area. (ii) volume of mesopores.3 HS 60. and HS 30. Moreover. Results Based on Nitrogen Gas Adsorption The nitrogen adsorption measurements of the products include the determination of the (i) BET surface area. samples carbonized at 300 ºC gave relatively low results. HS 30.4 HS 40. i.1.4 HS 50. and (iii) cumulative surface area of mesopores.3 HS 40.3 HS 30. BET Surface Area (m /g) 700 600 500 400 300 200 100 1 0 HS 30.4.4 HS 30.1 BET Surface Areas of Hazelnut Shell Based Samples 58 .4. Sample HS 60.
So.700 600 BET Surface Area (m /g) 2 500 400 300 200 100 0 200 300 400 Carbonizaiton Temp. increasing the impregnation ratio results in mainly formation of micropores and higher surface areas but higher impregnation ratios can result in widening of micropores and formation of meso. to some extent. In the literature the impregnation ratio generally shows a maximum point.and macropores giving lower surface areas. BET surface area values increased. using higher concentrations of phosphoric acid in impregnation can result in higher BET surface areas but probably reaches a maximum. then tends to decrease.2 the appropriate carbonization temperature can be seen to be 400 ºC. therefore. In the literature it was shown that in chemical activation there are two different mechanisms: the first one is the micropore formation which starts with the addition of chemicals to the raw materials and the second one is the pore widening which is the result of the chemical effect inside the opened pores. ( C) o 30% H3PO4 40% H3PO4 50% H3PO4 60% H3PO4 500 600 Figure 6. Pore widening normally begins when there are a number of opened pores in the structure. 300 ºC was not enough for the formation of pores and obtaining 59 .2 BET Surface Areas of Hazelnut Shell Based Samples As the H3PO4 concentration increased. it becomes significant when the impregnation ratio is reasonably high (Lozano-Castello. So. In Figure 6. 2001).
Also.3 BJH Mesopore Areas of Hazelnut Shell Based Samples 60 .3 the BJH mesopore areas of the selected products were shown. Ring opening results in formation of char. which is the desirable product in our case.5 27 15 47 34 58 Figure 6. were chosen and other characterization methods were applied. At 500 ºC depolymerization can be favored resulting in the formation of tarry substances hence lower surface areas.4 HS 40. Six activated carbons. In the literature at lower temperatures the ring opening reaction is said to be favored over depolymerization. which had the highest BET surface areas among twelve hazelnut shell based products.5 HS 50.4 HS 50.4 HS 30. Activated carbons produced at 500 ºC had lower mesopore areas then the ones carbonized at 400 ºC. Increasing the carbonization temperature to 500 ºC resulted in relatively lower surface areas. In Figure 6. BJH Mesopore Area (m2/g) 70 60 50 40 30 30 20 10 0 HS 30. as acid concentration increased mesopore areas increased.acceptable areas.5 HS 40.
04 0.037 Figure 6.034 0.009-0. (1943) and classified into 6 well-known groups.022 0. But.4 HS 60.03 0.4 HS 50.2) one may say that all products show similar isotherms to the type I.02 0. For all samples mesopore areas are around 9-12 % of the BET surface areas.5 0.4 Mesopore Volumes of Hazelnut Shell Based Samples Figures 6. (as it is shown in Figure 3.005 0 HS 40.5 shows the N2 adsorption / desorption isotherms of the samples.5 HS 50. this type of isotherm is observed in the case of microporous solids. another way of obtaining information on the porous texture of the solids is to compare the shape of the hysteresis loop (Figure 3. Mesopore volume values are directly related with the mesopore area values.020 0.009 0.023 0.4 HS 40. Isotherm shapes in the literature which were originally defined by Brunauer et al.3) with the shape of adsorption and desorption branches of the standard shapes which were originally classified by De Boer (1958).01 0. As discussed in section 3.2. As it is shown in the Figure 6.037 cm3/g.035 0. as it is shown in the 61 . BJH Mesopore Volume (cc/g) 0.2.Mesopore areas of the samples are in the range of 15 to 58 m2/g.4 mesopore volumes of the samples are in the range of 0.5 HS 60.025 0.015 0. According to Brunauer.
had the lowest surface area 705 m2/g.8 0. impregnated with 60% H3PO4 solution and carbonized at 300 ºC.4 HS 50.5 HS 60.6 0.3 sample. there is no distinct hysteresis loops at adsorption / desorption isotherms.9 1 Relative Pressure (P/P0) HS 40.1 0.4 HS 50.3 0. In Figure 6.8 the appropriate carbonization temperature was seen as 400 ºC for 60 wt.5 0. Similar to hazelnut shell results. The highest BET surface area was obtained for the sample impregnated with 40% H3PO4 solution and carbonized at 400 ºC as 1565 m2/g (HH 40. carbonization at 300 ºC and 500 ºC resulted in lower surface areas.5 HS 60.4 Figure 6. (Figure 6.5 HS 40. when compared to the values of hazelnut shell based products.4). N2 Adsorption/Desorption 200 Volume Adsorbed (cc/g STP) 180 160 140 120 100 80 60 40 20 0 0 0. Hazelnut Husk Based Activated Carbons In Figure 6.5 N2 Adsorption/Desorption Isotherms of Hazelnut Shell Based Carbons ii.4 0.2 0. But. hazelnut husk gave much higher values.isotherm figures of the samples.% H3PO4 treatment.7) 62 . HH 60.6 BET surface areas of hazelnut husk based activated carbons are shown.7 0.
4 HS 40.3 HH 60.5 HH 60.5 HH 50.3 HH 60.4 HH 60.7 Comparison of BET Surface Areas of HS and HH Based Products 63 .5 HH 50.4 705 1305 1565 1419 1429 2 Figure 6.6 BET Surface Areas of Hazelnut Husk Based Products BET Surface Area (m /g) 1800 1600 1400 1200 1000 800 600 400 200 0 HS 40.4 HH 60.4 252 270 596 426 242 380 705 1305 1016 1565 1419 1429 2 Figure 6.4 HH 30.5 HS 50.5 HS 60.4 HS 50.4 HH 30.4 HH 40.4 HH 40.BET Surface Area (m /g) 1800 1600 1400 1200 1016 1000 800 600 400 200 0 HH 60.4 HS 60.
8 BET Surface Areas of Hazelnut Husk Samples Impregnated with 60% H3PO4 solution. In Figure 6. As discussed earlier for hazelnut shell based carbons. BJH mesopores areas of the products are shown in Figure 6.10. the effect of acid concentration to BET surface areas of the samples carbonized at 400 ºC was shown. ( C) o 2 60% H3PO4 500 Figure 6.3 and HH 30. For the other samples mesopore areas are around 62-82% of the BET surface areas.4 the mesopores areas correspond to 43% and 49% of BET surface areas. 64 . respectively. Mesopore areas of the samples are in the range of 302-1042 m2/g.9. BET surface area showed a maximum for 40% H3PO4 impregnated product and decreased as the acid concentration increased. For samples HH 60.1400 1300 BET Surface Area (m /g) 1200 1100 1000 900 800 700 600 300 400 Carbonization Temp.
4 HH 30.4 HH 60.3 HH 60.4 996 836 696 1042 978 2 302 Figure 6.9 BET Surface Areas of Hazelnut Husk Samples at Carbonization Temperature of 400 ºC.4 HH 40.5 HH 50.1600 BET Surface Area (m /g) 1550 1500 1450 2 400 ºC 1400 1350 1300 1250 30% 40% H 3PO4 concentration 50% 60% Figure 6. BJH Mesopore Area (m /g) 1200 1000 800 600 400 200 0 HH 60.10 BJH Mesopore Areas of Hazelnut Husk Based Samples 65 .
83 1.01 cc/g).11.83 Figure 6. mesopore volumes of the samples are in the range of 0. except that of HH 60.4 HH 40.01 0.6 0. Sample HH 60. 66 .01 cc/g. Also. mesopore area and volume seem to be mainly microporous (Type I isotherm) when the adsorption/desorption isotherm is considered.58 0.2 0 HH 60.4 0.5 HH 50.3 HH 60.4 0.As it is shown in Figure 6.12 and 6.3 which has the lowest BET area.98 0.13 show the N2 adsorption/desorption isotherms of hazelnut husk based activated carbons.3. BJH Mesopore Volume (cc/g) 1. Among the samples carbonized at 400 ºC. The isotherms of the other products indicate that they are mainly mesoporous materials (Type II isotherm). higher carbonization temperatures resulted in higher mesoporosity.22 0.8 0. the product impregnated with 50% H3PO4 solution gave the highest mesopore area and volume (1042 m2/g and 1.4 HH 30. For the samples impregnated with 60% H3PO4 solution.4 HH 60. show that they have slit-shaped pores.11 BJH Mesopore Volumes of Hazelnut Husk Based Samples Figures 6. As the acid concentration decreased to 40% and 30 % mesopore area and volume decreased. type III hysteresis loops of the products.22-1.2 1 0.
4 HH 30.9 1 Relative Pressure (P/P0) HH 60.4 0.5 0.4 0.1 0. 40 and 50% H3PO4 and Carbonized at 400 ºC.3 0.5 0.3 0.1 0.2 0.6 0.8 0.4 HH 40.N2 Adsorption/Desorption 800 HH 60.2 0. N2 Adsorption/Desorption 900 Volume Adsorbed (cc/g STP) 800 700 600 500 400 300 200 100 0 0 0.8 HH 50.9 1 Relative Pressure (P/P0) Figure 6. 67 .13 N2 Adsorption/Desorption Isotherms of Hazelnut Husk Samples Impregnated with 30.12 N2 Adsorption/Desorption Isotherms of 60% H3PO4 Impregnated Hazelnut Husk Samples.7 0.7 0.5 HH 60.6 0.4 0.4 Volume Adsorbed (cc/g STP) 700 600 500 400 300 200 100 0 0 0.3 Figure 6.
the micropore volumes were calculated from intercept of the volume of CO2 adsorbed. increase in acid concentration impregnated.15. 68 .4 sample.14. similar to BET area.2. micropore area decreased for samples impregnated with 50% and 60% acid whereas micropore area increased for 40% acid impregnated samples. As carbonization temperature increased.3.15-0.2.4 sample. Micropore volumes of the samples are shown in Figure 6. For this purpose. Micropore area values of the samples are shown in the Figure 6. was obtained from the HS 40. 576 m2/g.24 cc/g. using the Dubinin-Radushkevich (D-R) equation. Carbon Dioxide Gas Adsorption Results i. versus (P/Po)2 in log-log plot. The lowest micropore area 433 m2/g.2. Similar to BET area. Also. the highest micropore area. Hazelnut Shell Based Activated Carbons Micropore analysis of the samples was carried out by CO2 adsorption studies as described in section 5. was obtained from the HS 60. As it is shown in the figure. resulted in higher micropore areas. This method was applied because CO2 adsorption at 0oC is widely used in literature for reporting the micropore volume and the surface area of the activated carbons. micropore volume graph is similar to micropore area graph.6. Micropore volume values are in the range of 0.
3 0.5 HS 50.4 HS 50.4 HS 50.2 0.5 433 455 464 576 541 2 Figure 6.1 0.05 0 HS 30.5 0.4 HS 60.17 Figure 6.22 0.Micropore Area (m /g) 700 600 503 500 400 300 200 100 0 HS 40.15 Micropore Volume Values of Hazelnut Shell Based Products 69 .15 0.15 0.4 HS 30.24 0.16 0.5 HS 40.4 HS 40.5 HS 60.25 0.18 0.5 HS 50.14 Micropore Area Values of Hazelnut Shell Based Products Micropore Volum e (cc/g) 0.4 HS 40.
6oC. micropore areas are greater than BET areas. N2 molecules at -195. First. Therefore. Thus. it cannot be claimed that the decrease in surface area is because of the enlargement of the micropores to mesopores for samples impregnated with 50% and 60% acid. First. Secondly. 70 . For increasing acid concentrations. BET. So. impractically long periods are required for equilibrium to establish. The 40% acid impregnated sample gave higher results for 500 ºC carbonization temperature. 1994). Second. kinetic energy of CO2 molecules at the adsorption temperatures used far exceeds that of N2 molecules at -195. rate of diffusion of CO2 into the activated carbon micropores will be significantly higher than that of N2 (Senel. for all the samples except for HS 60. It is now generally agreed that N2 adsorption at -195. in some cases.BET equation has been used extensively to determine surface areas of activated carbons from nitrogen adsorption isotherms measured at -195. 1994). For low acid concentration impregnated samples the difference between BET and micropore areas was more obvious whereas for 60% acid impregnated samples these values were closer.6oC for two reasons.6oC do not posses enough kinetic energy to readily penetrate into the micropores. the micropores undergo some decrease in their size at low temperatures. As it is shown in the Figure 6. minimum dimension of a CO2 is smaller than that of N2 molecule. due to the activated diffusion limitations. Consequently. Mesopore areas of all the samples decreased with increasing carbonization temperature from 400 ºC to 500 ºC. micropore and mesopore areas of samples carbonized at 400 ºC were all increased.16. the CO2 areas greatly exceed those determined from N2 adsorption at -195.4.6oC.6oC does not measure the total surface area of activated carbons for two important reasons. CO2 can measure pores down to 4-5 Å while BET can measure pores down to 10 Å. Other interpretation have also been found in literature that CO2 adsorption may be influenced by the quadropole moment of CO2 molecule interacting with the oxygen functionalities present on the carbon surface and that higher surface area may be caused by a CO2 induced swelling effect (Senel.
5 HS 60. 71 .4 34 HS 60. in terms of micropore area HH 30. Samples impregnated with 60% acid. D-R and BJH Results of Hazelnut Shell Based Products ii.5 HS 50. the samples carbonized at 400 ºC had minimum micropore area for 50% acid impregnated sample and showed more increase as the acid concentration decreased to 40% and 30%. Although the highest BET area was obtained for HH 40. Hazelnut Husk Based Activated Carbons Micropore areas of the hazelnut husk based activated carbons are shown in Figure 6.BET Area 700 600 503 500 433 400 300 200 100 30 0 HS 40.17.4 Figure 6. showed similar behavior to BET results.16 Comparisons of BET.5 27 252 270 426 Micropore Area Mesopore Area 596 541 455 576 464 380 242 47 58 15 HS 50.4 HS 40.4).4 was found to have the highest area 724 m2/g.4. at 400 ºC carbonization temperature gave the highest area (HH 60. In terms of acid concentration effect.
Micropore volumes are in the range of 0.4 HH 40. HH 40.3 HH 60.4 HH 30.53 cc/g. Samples impregnated with 60% acid showed the same behavior like in micropore area graph.4 was the second in micropore area values.53 cc/g. the lowest micropore volume was found for 50% acid impregnated sample again as 0.4 376 445 2 724 Figure 6. Although. it had the highest micropore volume 0.5 HH 50.4 HH 60.17 Micropore Area Values of Hazelnut Husk Based Products Micropore volumes of the samples are shown in Figure 6.20 cc/g.Micropore Area (m /g) 800 710 700 602 600 511 500 400 300 200 100 0 HH 60.18. In terms of acid concentration effect. 72 .18-0.
26 Figure 6. for sample HH 60.41 0.6 0.4 0.18 Micropore Volume Values of Hazelnut Husk Based Products In Figure 6. Except for samples HH 30.38 0.3 HH 60.18 0.19 BET.3 0. 73 . the mesopore areas increased while micropore areas decreased but.1 0 HH 60.4 HH 30.Micropore Volume (cc/g) 0. BET areas were higher than micropore areas.4 HH 40.4 an increase in micropore area and a decrease in mesopore area was observed.53 0.4 and HH 60.47 0. mesopore and micropore areas of the samples were compared.3.4 0. mesopore areas were higher than micropore areas for all samples. Unlike the hazelnut shell based activated carbon results.4 HH 60.5 HH 50. As acid concentration increased from 30% to 50%.2 0.5 0.
4 HH 30.4 376 302 705 602 445 511 996 1016 836 710 724696 2 1565 1429 1305 1042 978 Figure 6.19 Comparisons of BET. D-R and BJH Results for Hazelnut Husk Based Products Figure 6.5 HH 50. micropore and mesopore areas of all the products to make a comparison between hazelnut shell and hazelnut husk based samples.4.4 HH 60. 74 .4 HH 40.4 and HH 40.BET Area Micropore Area Mesopore Area 1800 1600 1419 1400 1200 Area (m /g) 1000 800 600 400 200 0 HH 60. micropore areas were found as 433 m2/g and 710 m2/g for HS 40. Although activated carbons produced from hazelnut husk were mainly mesoporous.3 HH 60. BET surface areas and mesopore areas of HH based samples were very much higher than those of HS based ones. the micropore areas exceeded that of hazelnut shell based samples for most of the products.20 shows BET. % phosphoric acid and carbonized at 400 ºC. respectively. For samples impregnated with 40 wt.
20 can be compared with the literature survey results also. but hazelnut husk sample gave a higher result of 1565 m2/g.and Mesopore Areas. (1998) produced activated carbon from apricot stone by using 40 wt.5 HH 30.20 Comparison of HH and HS Series in terms of BET.4 HH 40. Girgis et al. (2002) produced activated carbons from date pits using phosphoric acid. Girgis et al.4 HH 60.4 HH 60. resulting in 1303 m2/g surface area.BET Area 1800 1600 1400 Area (m 2/g) 1200 1000 800 600 400 200 0 Micropore Area Mesopore Area HS 40. impregnation with 50 wt.5 HS 50.4 HS 40. Under similar conditions hazelnut shell sample resulted in 252 m2/g surface area.% phosphoric acid and 400 ºC carbonization temperature.5 BET Area Micropore Area Mesopore Area 252 433 30 270 503 27 426 541 47 242 455 15 596 576 58 380 464 34 1429 724 696 1565 710 978 1419 511 1042 705 376 302 1305 602 996 1016 445 836 Figure 6. % phosphoric acid and carbonization at 400 ºC gave 1372 m2/g surface area for apricot stone while HS and HH based samples had 426 and 1419 m2/g.3 HH 60.5 HS 60. % acid and carbonization at 500 ºC resulted in BET surface area of 556 m2/g and micropore area of 464 m2/g for date pits. Hazelnut shell sample had 242 m2/g of BET surface area and 455 m2/g of micropore area under similar conditions. Results in Figure 6. Additionally. Impregnation with 50 wt. respectively.4 HS 60. Hazelnut shell samples had lower surface areas than apricot stones while hazelnut husks had higher results under similar conditions.4 HH 50.4 HS 50. Considering the micropore areas of the 75 . Micro.
As it is shown in figure true density values of hazelnut shell and hazelnut husk based samples are in the range of 1.55 g/cc. generally the structure of the starting material very much influences the adsorptive properties and surface chemistry of the products as in the case of apricot stones and hazelnut shell and husk.74-2.3 and are shown in Figure 6. By increasing carbonization temperature.21. 6.samples it can be stated that hazelnut shells and date pits give close results under similar experimental conditions. the volatile matter (low density matter) content of the activated carbon might decrease resulting in the increase in true density.2.3. True Density Determinations True densities values of the activated carbons are determined at room temperature by Helium Pycnometry as explained in section 5. True densities of all the products are increasing with the increasing carbonization temperature. But.75 g/cc and 1. Acid concentration has no significant effect on true density of hazelnut shell based products.52-1. 76 .3. but true densities of hazelnut husk based activated carbons are increasing with increasing acid concentration. respectively.
57 1.4 HS 50.1.4 HH 60.1. which was carried out according to the procedure explained in section 5. 77 .3 HH 60. trace amount of N and 0.67 1.6 % hydrogen.06 Density (g/cc) 2 1.4 % ash.5 2.4-22.54 1.74 1.75 1.5 0 1.61 1. activated carbons produced in this study contain about 70.3-83.52 1. 12.6 % oxygen. As it can be seen from this figure.5 HS 60.5 1 0.4 Figure 6.Pycnometer Results (True Density) 3 2.4 HS 60. Chemical Analysis of Products Elemental (C.5 HS 50.4 HH 40.8-3. 1.3.1 in Appendix D. is given in Figure 6.55 2. O) analysis of activated carbons and raw materials.86 1.4 HS 40.22 and Table D.62 HS 40.4 HH 50.21 True Density Values of the Activated Carbons 6. H.5 HH 30.7-2.1 % carbon.
6 14.2 77.22 Elemental Analysis Results of Activated Carbons and Raw Materials .9 70.3 80.2 82.4 77.7 C H O ash 60 50 40 22.0 16.5 60.4 60.4 60.1 80.4 30.3 10 HS HS HS HS HH 50.1 Figure 6.4 40.4 raw HS raw HH 0 HS HS 40.5 Figure 6.5 80 79.4 50.5 60.4 40.0 12.3 70 49.6 81.5 50.8 83.2 78.7 76.3 HH HH HH HH HH 60.22 Elemental Analysis Results of Activated Carbons and Raw Materials 78 30 20 15.4 42.4 16.90 80.
1. The oxygen and hydrogen functional groups provide sites where molecules of water and other polar substances or easily polarizable gases and vapors are adsorbed (Smisek and Cerny.6. 1983). indicates that. 50% and 60% acid impregnated samples have higher carbon content at 400 ºC than 500 ºC. As discussed in Section 2. impregnated with 60% acid and carbonized at 500 ºC was found to have the lowest carbon content of all the activated carbons as 70. Faust and Aly. carbon content of 40% acid impregnated sample was increased with increasing carbonization temperature. The highest carbon content was found for the hazelnut shell. Carbon content of hazelnut husk.3. 1974). impregnated with 60% H3PO4 and carbonized at 400 ºC as 83.5 these elements are combined with the atoms of carbon by chemical bonds.3.22.4% for activated carbons produced from hazelnut husk. 1970.3%. carbon content is reported to be around 85-90 % (Section 2. respectively. High carbon content value is desired to achieve high BET surface area.2. Hassler. 6. Raw hazelnut shell and hazelnut husk have 49% and 43% carbon content. Oxygen and Hydrogen Content The presence of oxygen and hydrogen influence the adsorptive properties of activated carbon. for hazelnut shell based activated carbons.4 as 80. 79 .5. As carbon content of the activated carbon increase it is expected that the BET surface area value increase. For a typical activated carbon. Carbon Content Data reported in Figure 6. But. Carbonization of the samples impregnated with H3PO4 resulted in higher carbon contents. BET area value is the most important parameter for activated carbons.1%. The highest carbon content was found for sample HH 60.
However, O and H contents of the samples were slightly higher than typical values. For hazelnut shell based products oxygen content values were in the range of 14-16%. O content of hazelnut husk based activated carbons was in the range of 12.4-22.6 %. As acid concentration increased oxygen content of the products was decreased. As carbonization temperature increased O contents of the products increased. The lowest O value was obtained from HH 60.4 as 12.4%. The hydrogen content of all the products was decreased with increasing carbonization temperature. For hazelnut shell based activated carbons H content decreased with increasing acid concentration. For hazelnut husk based ones the highest H content was for HH 60.4 and the lowest value was found for HH 30.4 as 2.6% and 1.8%, respectively. Since phosphoric acid (H3PO4) was used as activating agent and it contained hydrogen and oxygen groups in its structure, these high O and H content values could be obtained. Most probably, the high oxygen content of the products was also because of phosphor content of the acid. In the stated percentages of oxygen, phosphor content might be hidden since oxygen contents were found by adding C, H, N and ash contents and subtracting from 100%. For a typical activated carbon the recommended oxygen content is around 5 % and the hydrogen content is 1 % (Faust and Aly, 1983).
6.3.3. Nitrogen Content
For a typical activated carbon, nitrogen content is less than 1 %. For the products obtained from hazelnut shell by chemical activation have nitrogen contents in small amounts which are less than 1 percent (Figure 6.22). On the other hand, hazelnut husk based products have nitrogen slightly higher than the typical value. During carbonization in nitrogen atmosphere, small amount of nitrogen can be chemisorbed (Smisek and Cerny, 1970;).
6.3.4. Ash content
A good activated carbon must have low ash content. A small increase in ash content causes a decrease in adsorptive properties of activated carbon. The raw hazelnut shell and hazelnut husk have ash content of about 0.4% and 5.8%, respectively (Table 5.1). The typical ash content of activated carbons is around 5-6 %. However, all the products had lower ash content than the typical activated carbons in the range of 0.8-3.4 %. Moreover, the ash content of the carbonized materials is expected to be high compared with the raw material values.
6.4. TGA of Impregnated Hazelnut Shells and Hazelnut Husks
TGA experiments were carried out to obtain the yield values for selected products. It was not possible to obtain the yield values from the experiments which were done with the experimental set-up since after the carbonization was completed products were immediately taken to a flask and mixed with distilled water to prevent interaction with the air. Yield values of raw hazelnut shell and husk were obtained during preliminary experiments by carbonization at 400 ºC. TGA experiments were done at same experimental conditions like carbonization time and temperature, particle size, N2 flow rate, impregnation ratio and heating rate. Yield values of samples for different carbonization temperatures and acid concentrations are given in Table 6.1.
Table 6.1 Yield Values (%) of Selected Products and Raw Materials Name HS 40.4 HS 50.4 HS 60.4 HS 60.5 HH 40.4 HH 50.4 HH 60.4 HH 60.5 Raw HS (400 ºC) Raw HH (400 ºC) Yield% 51.5 49.4 47.0 43.9 54.3 56.5 56.8 43.2 31 34
As it is seen from the table, yield values of phosphoric acid impregnated samples are higher than that of raw materials for the same temperature. Yield values are decreasing with the increasing carbonization temperature. Hazelnut shell based products gave yield values of 47-51.5% at 400 ºC while yields of samples produced from hazelnut husk were higher, in the range of 54.3-56.8%. As the carbonization temperature was increased to 500 ºC yields were decreased to 43.2-43.9% for both kinds of products. For HS series, increasing acid concentration decreased the yield values while in HH series yield values were increased with increasing acid concentration. Compared with the literature values, products have acceptable yields. Addition of phosphoric acid to the raw materials effects the depolymerization reactions (as indicated in Chapter 4 and Section 18.104.22.168) during the carbonization. Phosphoric acid (H3PO4) restricts the formation of tar (Section 184.108.40.206) by its dehydration effect and volatile materials do not evacuate from the structure. Therefore, yield values of acid impregnated samples increase with respect to raw samples. TGA figures of the acid impregnated samples are given in Appendix E.
%).4 and HH 40. since hazelnut shell is used mainly for heating purposes in The Black Sea Region. a material balance was done. considering the availability of the raw materials.25 g H3PO4 (40 wt.69 Comparison of HS 40. when the decomposition of the material during carbonization resulting in leaving of gases and tarry substances from the carbonized sample is considered.5 % (TGA method). On the other hand.5 6. the output weight was found as 0.4.4 was obtained as 51. 1 gram of hazelnut husk resulted in 1.4 and HH 40. Table 6.12 Carbonization output (g) 0.69 gram of activated carbon by using 6. raw material. showed that hazelnut husk gave higher amounts of activated carbon from 1 gram of husk.4 samples.2 shows the data for HS 40. hazelnut husk might be cheaply obtained in great amounts. But. Obtaining product weight less than the raw material weight consumed for that process can be expected. 1 gram of hazelnut shell was mixed with 2 ml phosphoric acid solution (40 wt. on the basis of 1 g. The weight increase of HS 40 series (impregnated and subsequently dried samples) was 23% (Table 5.23 g.25 Carbonization input (g) 1. Since the yield of HS 40.%) solution. based on raw materials and phosphoric acid used during the experiments. weight of phosphoric acid 83 .633 1.5 g.2 Material Balance on Activated Carbon Production (basis: 1 g raw material) Raw material Hazelnut shell Hazelnut Husk H3PO4 (40%) volume (ml) 2 5 H3PO4 (40%) weight (g) 2. Also. obtaining a product weighting higher than the amount of raw material is interesting as in the case of hazelnut husk. Weight of input sample was calculated as 1. Similarly. weighting 2.2). Table 6.For a comparison of probable feasibility of hazelnut shell and husk.23 3.633 g.
This high consumption of phosphoric acid might increase the unit cost of HH 40.5 times higher than that of used for HS 40.4. although hazelnut husk based activated carbon production consumed higher amounts of acid. So. 84 . Although the material balance was mainly based on the carbonization step. after washing step the product weight might not change significantly with respect to the carbonization output amount indicating that the unexpected high amount of carbonization output for husk was not a cause of free phosphoric acid. But during the experiments final activated carbon amounts could not be measured after the removal of acid.4. It might be more meaningful to determine yield considering the whole production process instead of only carbonization.4 production was 2. On the other hand. since this step caused loss of products to some extent.used for HH 40. the importance of washing step after carbonization should not be ruled out. washing step might remove the impregnated acid and lower the unit cost. At that point the phosphor content of the products appears to be important.
The product obtained from 60% H3PO4 impregnated hazelnut shell gave the highest surface area value 596 m2/g at carbonization temperature of 400oC. Novelties of this work are the use of phosphoric acid (H3PO4) as the activating agent for hazelnut shell during carbonization and using hazelnut husk for activated carbon production as the starting material. On the other hand. pretreatment of the raw materials with H3PO4 solution before carbonization increased the values of surface area of the activated carbons produced at relatively low temperatures. it is concluded that hazelnut shells and hazelnut husks can be used as raw materials for the production of activated carbon by using H3PO4 as the activating agent.CHAPTER VII CONCLUSIONS From the results of this work. For activated carbons produced from hazelnut husk. hazelnut husk based products gave much higher surface areas (in the range of 705-1565 m2/g) for 300. Results showed that. By increasing acid concentration from 30% to 60% higher surface area activated carbons were obtained from hazelnut shell. N2 and CO2 adsorption / desorption isotherms show that activated carbons obtained from hazelnut shell have a certain degree of porosity. Phosphoric acid concentration showed appreciable effects on the properties of activated carbons. 40% phosphoric acid impregnated sample 85 . 400 and 500 ºC carbonization temperatures. located mainly in micro size of pores while hazelnut husk based activated carbons are mainly mesoporous.
86 .gave the highest surface area 1565 m2/g while further increasing the acid concentration decreased the surface areas of the products.
Some of the most critical parameters are the impregnation ratio. Also. To increase the BET surface area value of the activated carbons produced from hazelnut shell and hazelnut husk. Also. it is recommended to carry out experiments with different experimental parameters. Increasing phosphoric acid concentration to 70% or 80% for hazelnut shell might give higher BET surface areas but probably tend to decrease after a maximum point. To obtain microporous activated carbon from hazelnut husk the carbonization temperature should be kept low (300 ºC). The preliminary experiments showed that a decrease in the weight of raw material placed in the tube in furnace resulted in higher surface area products.CHAPTER VIII RECOMMENDATIONS Activated carbon has many industrial applications depending on its porous structure and adsorption capacity. carbonization temperature and carbonization time. For further characterization of the products phosphor contents should be investigated to have an idea on surface chemistry. to have idea about the effective surface areas of the products adsorption of organic materials from solutions may be performed. The experiments should be performed in a fluidized bed furnace. particle size distribution of the products should be determined. heating rate. Using fluidized bed will probably result in a more effective carbonization. 87 .
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1 which relates the equilibrium vapor pressure of a curved surface such as that of a liquid in a capillary or pore. The terms liquid. to the equilibrium pressure of the same liquid on a plane surface (Gregg and Sing. ln P Po = − 2 γ VmolCos r p RT θ (A.1) where P is the equilibrium vapor pressure of the liquid contained in a narrow pore of radius rp and Po is the equilibrium pressure of the same liquid at a plane surface. this process consists of three steps: evaporation from the bulk liquid.1. the pore wall. 1982). If the transfer of dn moles of vapor in equilibrium with the bulk liquid at pressure Po into a pore where the equilibrium pressure P is considered. The first and third of these steps γ θ and Vmol are surface tension and molar volume of the is the contact (wetting) angle with which the liquid meets 93 . Analysis of Mesopores Adsorption studies reading to measurements of pore sizes and pore size distributions generally make use of the Kelvin equation A.APPENDIX A ANALYSIS OF N2 SORPTION DATA A. expansion of the vapor from Po to P and condensation into the pore. respectively.
are equilibrium processes and are therefore accompanied by a zero free energy change.2) dG = − (γ Cos θ )dS θ (A. the volume to area ratio can be too complex to express mathematically.4) The volume of liquid adsorbate which condenses in a pore of volume Vp is given by dVp = Vmoldn Substituting equation A.3) where dS is the change in the film-vapor interfacial area and is the wetting angle which is taken to be zero since the liquid is assumed to wet completely the adsorbed film.6) The ratio of volume to area within a pore depends upon the geometry.3. when combined −γ dn = dS RT ln (P / Po ) (A.4 into A. whereas the free energy change for the second step is described by ⎛ ⎛ P ⎞⎞ ln dG = ⎜ RT ⎜ ⎟⎟ ⎜ ⎟ dn ⎝ Po ⎠ ⎠ ⎝ When the adsorbate condenses in the pore (A.5 gives (A. When the shapes of the pores are highly irregular or consisting of a mixture of regular geometries.2 and A. Equations A. In these cases.5) dVp dS = − γ Vmol RT ln ( P / Po ) (A. or in the absence of specific knowledge of the 94 .
pore geometry. 85 34 . Alternatively. Since the ratio of volume to area for cylinders is r/2. This radius. is not the actual pore radius since some adsorption has already occurred on the pore wall prior to condensation.6oC. 314 x10 ⎟ (77 K )(2 . 303 ) log (Po / P ) Kmol ⎠ ⎝ (A. 6 ⎟ ⎟ ⎜ cm ⎟ ⎟⎜ mol cm 2 ⎠ ⎜ ⎝ ⎠ ⎠⎝ ⎝ rk = erg ⎞ ⎛ 7 ⎜ 8 .8) where 8.9 can be found as rk = 4.6 cm3 is the molar volume of liquid nitrogen at -195. If the depth of the film when condensation or evaporation occurs is t.6 gives the Kelvin equation. leaving a center core or radius rk. − 2 γ Vmol ⎛ P ⎞ ln ⎜ ⎟ = rRT ⎝ Po ⎠ (A. during desorption.6oC.15 log(Po / P ) (A. the Kelvin equation can be written as erg ⎞ ⎛ cm 3 ⎞ ⎛ 10 8 o A ⎞ ⎛ ⎟ ⎜ 2 ⎜ 8 .85 erg/cm2 is the surface tension and 34.10) 95 .7) For nitrogen as the adsorbate at its normal boiling point of -195. then the actual pore radius rp is given by rp=rk+t (A. the assumption of cylindrical pores is usually made. called the Kelvin radius or the critical radius. the equation A. Then equation A.9) The term rk indicates the radius into which condensation occurs at the required relative pressure. an adsorbed film remains on the pore wall when evaporation of the center core takes place.
Essentially equation A.12) On nonporous surfaces it has been shown that when the quantity Wa/Wm is plotted versus P/Po the data all approximately fit a common type II curve above a relative pressure of 0.11 asserts that the thickness of the adsorbed film is simply the number of layers times the thickness τ of one layer regardless of whether the film is in a pore or on a plane surface. the quantity adsorbed at a particular relative pressure and the weight corresponding to the BET monolayer. 02 x 10 ⎠⎝ ) 3 23 1 mol ⎞ ⎟ ⎠ = 3 . The common curve is described closely by Halsey. 54 A (A. respectively. 6 x 10 24 A ⎞⎛ ⎟ ⎟ ⎜ 6 . 2 ⎜ mol ⎝ .11) Where Wa and Wm are.303 log (P / P ) ⎟ ⎟ o ⎝ ⎠ 1/ 3 (A. The t value of τ can be calculated by considering the area S and volume Vmol occupied by one mole of liquid nitrogen if it were spread over a surface to the depth of one molecular layer τ = V mol S = (34 ⎛ A 2 ⎜ 16 . Using the assumption that the adsorbed film depth in a pore is the same as that on a plane surface for any value of relative pressure.54 ⎜ ⎜ 2.This equation can be used to calculate rp but some means of evaluating t is required if the pore radius is to be determined. one can write ⎛ Wa t =⎜ ⎜W ⎝ m ⎞ ⎟ ⎟τ ⎠ (A.13) The thickness of the adsorbed layer which is calculated for a particular relative 96 . (1948) equation which for nitrogen can be written as ⎛ ⎞ 5 t = 3.3.
V p = πr 2 PAVE L (A.pressure from the above equation which becomes thicker and thicker with successive increase in pressure.17) The surface area of the pore walls can be calculated from the pore volume by 97 . Barrett Joyner and Halenda developed the method (BJH) which incorporates these ideas. and since. This is given by ∆ V liq = 22414 (cc / mol − STP ) ∆ V gas (cc / g ) (34 .16 V p ⎛ r PAVE = ⎜ ⎜ r ⎝ KAVE ⎞ ⎟ ⎟ ⎠ 2 [∆ V liq − (∆ t ∑ S )(10 −4 )] (cc) (A.6(cc / mol )) = ∆ V gas (1 . The algorithm used on the ASAP 2000. so that the measured quantity of gas adsorbed in a step is composed of a quantity equivalent to the liquid cores formed in that step plus the quantity adsorbed by the pore walls of pores whose cores have been formed in that and previous steps. The ∆Vgas then can be converted to ∆Vliq by multiplying by the liquid molar volume for nitrogen at standard temperature and pressure. A. by combining the equations. the incremental volume: the change in adsorbed volume between two successive P/Po values can be determined by subtracting the successive values. the N2 adsorption apparatus used in the present work.14) The actual pore volume was evaluated by ∆ V liq = π r 2 KAVE + ∆ t ∑ S (A. According to this method.15) In this equation rKAVE is the average Kelvin radius and the term ∆t ΣS is the product of the film area and the increase in the film depth. ∆Vgas.16) where L is the pore length.15 and A. is an implementation of the BJH method.54 x10 − 3 ) (A.
002 µ m d p = 0 . using the following relation ( Brunauer et al.20) A. P 1 = V [PO − P ] VmC + C − 1 P V mC P O (A.2.17 and A. 1938). in the relative pressure range 0-0.05 µm ] ] (cc/g) (A.19) [∑ S d p = 0 .18: Vmeso = S meso = [∑V p d p = 0. Determination of BET Surface Area BET surface areas of the samples can be obtained from the plot of P/V(Po-P) versus P/Po plot.18) Then. 05 µ m (m2/g) (A.2.21) The slope and intercept of this plot which are given as S= C − 1 V mC and I= 1 VmC (A..002 µm − ∑Vp −∑S d p = 0. mesopore volume and surface areas were calculated by using the incremental pore volume and surface area values evaluated from the equations A.22) can be used to evaluate BET surface area by the equation 98 .S = 2V r PAVE p (10 ) 4 (m2) (A.
3.0.825024 cc/g Cumulative Pore Surface Area up to dp=20 oA (2 nm)= 979.20: Smeso= 979.020593 cc/g Replacing these quantities into equation B.23) where CSAN2 is the cross sectional area of a nitrogen molecule.896396 = 978.020593=0. Hazelnut Husk impregnated with 40% H3PO4 solution) A. Sample Calculation Sample Code HH 40.023x1023 (1 / mol) 3 / mol − STP 1018 nm2 / m2 ))( ( )( ))((S + I )(g / cc − STP))] )] (A.2653 .S BET = [(22414(cm [(CSA N2 nm2 6.3689 m2/g 99 .4 (Carbonization Temperature=400oC.896396 m2/g Replacing these quantities into equation A.2653 m2/g Cumulative Pore Surface Area up to dp=500 oA (50 nm)= 0.845617.3.1. A. Calculation of Mesopore Volume and Area Cumulative Mesopore Volume up to dp=20 oA (2 nm)=0.0.19: Vmeso= 0.845617 cc/g Cumulative Mesopore Volume up to dp=500 oA (50 nm)=0.
A.162 nm2 (Walker et al.5037 m2/g 100 .002768 I=Intercept= 0.2.000014 CSA N 2 =0.3. 1968) Replacing these quantities into equation A..23: SBET= 1564. Calculation of BET Surface Area S=Slope= 0.
303K ⎜ ⎜ β ⎟ ⎟ ⎝ ⎠ 2 ⎡ Po ⎤ ⎢log P ⎥ (cc / gSTP) ⎦ ⎣ 2 (B. Analysis of Micropores The micropore volume of the samples calculated by applying the Dubinin Radushkevich (D-R) equation to the CO2 adsorption data in the relative pressure range 1x10-4 – 1x10-2 ⎛ RT ⎞ logV = logVo − 2. In this equation.1) A plot of log V versus log (P/Po)2 gives straight line with an intercept of log Vo from which Vo. K is also a constant determined by the shape of the pore distribution curve. The micropore volume in the unit of (cc/g STP) which refers to the adsorbate state based on the ideal gas behavior at STP was also converted into the unit of (cc/g) which is based on the specific volume of the adsorbate in the adsorbed state at 0oC by the following equation V o ( cc / g ) = V o ( cc / g − STP ) MW CO 2 ( g / mol ) ( 22414 ( cc / mol − STP ))( ρ CO 2 ( g / cc )) (B.1. β is a constant which is the ratio of the adsorption potentials. the micropore volume (cc/g STP) could be calculated.2) 101 .APPENDIX B ANALYSIS OF CO2 ADSORPTION DATA B.
1993) Absolute Pressure Range= 7.4 (Carbonization Temperature=400oC. Micropore Analysis unit which processed the collected CO2 adsorption data. In these calculations. Appendix C.472 cc/g (Micromeritics ASAP 102 .181 g/cc (Micromeritics ASAP 2000.000 mm Hg at 0oC 2000. User Manual.2: Vmicro= 0.2.3) where σ is the cross sectional area of a CO2 molecule.44x10-2 Vmicro= 158. Sample Calculation Sample Code: HH 30. 02 x 10 (22414 cc ) 10 18 nm 2 / m 2 o ( ( 23 ) ) (B.901. User Manual.3.24243.6431 cc/g STP Corresponding micropore volume in the units of cc/g was evaluated by taking Density of CO2 at 0oC=1. B. Saturation pressure of CO2 =26142.43604 mm Hg Corresponding Relative Pressure Range= 2.where ρ is the density of the CO2 molecule. 1993) and replacing into equation B. Hazelnut Husk impregnated with 30% H3PO4 solution) Micropore volume (cc/g STP) directly obtained from the ASAP 2000M. Micropore surface area or so called D-R surface area was then evaluated by S DR = (σ nm )V 2 ( cc / g − STP ) 6 .77x10-4. Appendix C.
1993) Replacing into equation B.3 SD-R= 724 m2/g 103 .Micropore surface area (D-R surface area) obtained by taking Cross Sectional Area of CO2 molecule =0.17 nm2 (Micromeritics ASAP 2000. Appendix C. User Manual.
APPENDIX C ANALYSIS OF HELIUM PYCNOMETER DATA True densities of the samples were determined by helium displacement method. and that the valve is then closed. VCELL is P 1 (VCELL − VSAMP ) = nC RTa where nC= the number of moles of gas in the sample cell. The mass balance equation across the sample cell. are at ambient temperature Ta.2) nE= the number of moles of gas in the expansion volume. and Ta= the ambient temperature The mass equation for the expansion volume is (C. 104 .1) P aV where. R= the gas constant. VCELL is then charged to an elevated pressure P1. Assume that both VCELL and VEXP are at ambient pressure Pa. EXP = n E RT a (C. A commercial He Pycnometer apparatus was used to measure true densities of the activated carbons.
2) into (C.8) This equation (C.3) V CELL − V SAMP = Pa − P2 V EXP P2 − P1 (C. P2 and Pa are expressed in equations (C.When the valve is opened.8) then becomes the working equation for the pycnometer.5) Since P1. P2. and the mass balance equation becomes P2 (VCELL − V SAMP + V EXP ) = n C RTa + n E RTa Substituting from equations (C. 105 .6) (C.5) is arranged so that Pa is subtracted from both P1 and P2 before use.7) V SAMP = V CELL − V EXP P1 g −1 P2 g (C.1) and (C. V SAMP = V CELL − V EXP (P1 − P a (P 2 − P a ) −1 ) (C.5) as absolute pressures and equation (C.1) through (C.4) If we rearrange this equation. new P1g and P2g may be redefined as gauge pressures P1g = P1-Pa P2g = P2-Pa And equation (C.3) (C. the pressure will fall to an intermediate value.5) rewritten as (C.
4 1.7 1.3 0.7 1.9 3.5 HH 60.3 HH 60.9 O(%) 15.5 2.6 1.8 83.4 HH 60.7 2.4 22.9 2.2 1.7 H(%) 2.8 0.4 13.2 N(%) 0.APPENDIX D TABULATED FORM OF CHEMICAL COMPOSITIONS OF ACTIVATED CARBONS Table D.8 2.4 HH 30.8 12.1 3.8 0.4 42.4 1.8 0.1 2.6 45.3 80.3 5.3 1.1 80.4 HS 60.5 49.2 77.6 16.5 HH 50.2 78.4 5.4 43.9 0.5 HS 50.6 2.5 1.9 80.6 16.8 0.9 1.3 77.4 raw HS raw HH C(%) 79.8 6.4 HH 40.8 106 .2 ash(%) 2.4 HS 50.4 HS 40.0 1.4 1.5 HS 60.1 0.1 Chemical Compositions of Activated Carbons and Raw Materials name HS 40.9 1.2 82.7 76.8 0.9 2.6 81.4 70.4 0.
4) 107 .1 TGA Result of 40% H3PO4 Impregnated Hazelnut Shell at 400 ºC (HS 40.APPENDIX E TGA RESULTS Figure E.
4) Figure E.2 TGA Result of 50% H3PO4 Impregnated Hazelnut Shell at 400 ºC (HS 50.Figure E.3 TGA Result of 60% H3PO4 Impregnated Hazelnut Shell at 500 ºC (HS 60.5) 108 .
5 TGA Result of 60% H3PO4 Impregnated Hazelnut Husk at 400 ºC (HH 60.Figure E.4) Figure E.4) 109 .4 TGA Result of 50% H3PO4 Impregnated Hazelnut Husk at 400 ºC (HH 50.
5) 110 .Figure E.6 TGA Result of 60% H3PO4 Impregnated Hazelnut Husk at 500 ºC (HH 60.
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