Membrane processes cover a group of separation processes in which the characteristics of a membrane (porosity, selectivity, electric charge) are used to separate the components of a solution or a suspension. In these processes the feed stream is separated into two: the fraction that permeates through the membrane, called the permeate, and the fraction containing the components that have not been transported through the membrane, usually called the retentate The size of the components to be separated and the nature and magnitude of the driving force provide criteria for a classification of the membrane separation processes, as shown in the table. It should be noted that the boundaries between some of the processes, such as reverse osmosis and ultrafiltration, are arbitrary.
Of the processes listed in Table 1, reverse osmosis and ultafiltration are the most widely used industrially. Abundant examples of their application can be found in water Desalinization, the dairy industries and the separation of organic solutes from aqueous solutions. Membrane processes do not require heating, which makes the process suitable for the treatment of thermolabile products. In addition the relatively low capital and operating costs involved make membrane processes an appealing alternative to more conventional separation processes, particularly when dealing with dilute solutions. In the next sections the principles and characteristics of the processes listed in Table 1 will be described briefly.
Osmosis is the process of transfer of the solvent of a solution across a membrane separating two liquid solutions of different concentrations. The transfer takes place from the phase in which the chemical potential solvent is higher. A classical example is the transfer of water to a sugar solution. Figure 1(a) shows two compartments separated by a water permeable membrane. One contains the solution, while the other holds pure water. Initially the level of the solution is placed at the same level as that of the water. Since the chemical potential of pure water is higher than that of water in the solution, water transfers across the membrane to the solution, increasing the level on the solution side and thus creating a hydrostatic head (Figure 1b). When the process is at equilibrium the difference in the hydrostatic pressure between the two phases is that required to make the chemical potential of water in both phases the same. In the special case of this example, when the pure solvent is on one side of the membrane this pressure difference is the osmotic pressure, P. If solutions are at both sides of the membrane, the pressure difference gives the osmotic pressure difference ΔП. For very dilute solutions the osmotic pressure is given by
where β is a constant that takes into account the dissociation of the solute, ci is the molar concentration of solute i, R the gas constant and T the absolute temperature. The constant (β is in general equal to the number of ions produced by the dissolved solute; thus for electrolytes that dissociate fully such as Nalco and CAC2 it is equal to 2 and 3, respectively, while β = 1 for molecules that do not dissociate. When the solute has a large molecular mass, the osmotic pressure of the solvent ceases to depend linearly on the concentration of the solute and a polynomial approximation must be used. For concentrated solutions as well as nonideal solutions the osmotic pressure must be determined experimentally or estimated using activities instead of concentrations.
The term reverse osmosis is applied to the separation of water from solutions by transfer though a semipermeable membrane that ideally should only be permeable to water. If as shown in Figure 1(c) a mechanical pressure is applied to the compartment where the level is higher, the chemical potential of the solvent on the solution side will increase, forcing the transfer of the solvent through the membrane into the pure solvent phase. This pressure difference between the phases, ΔP, creates the driving force for the transfer and is the principle on which the reverse osmosis and ultrafltration processes are based. In order to achieve the transfer, ΔP must be greater than the difference between the osmotic pressures ΔП. The rate of solvent permeation across the membrane is measured as its linear velocity normal to the membrane, J1. Ideally the membrane should be only permeable to the solvent. However this is very difficult to achieve and membranes have some permeability to the solute. The rate of permeation of the solvent is given by:
where K1 is the permeability coefficient of the membrane and ΔP is the transmembrane pressure. The permeability coefficient depends on the membrane resistance, the resistance to transfer caused by deposits on the membrane and the diffusion resistance in the boundary layer at the retentate side of the membrane. Depending on the thickness of the solid deposits and the hydrodynamic conditions of the retentate the two latter resistances may be negligible. In fact membrane equipment are operated under conditions that tend to minimize these two resistances in order to increase J.
The difference in solute concentration between the retentate and the permeate (in practice membranes have some permeability to the solute) creates a driving force for solute transfer across the membrane. The flux of solute, J2, is given by:
where K2 is a constant that includes the characteristics of the membrane, and cR and cP are the concentration of the solute in the retentate and permeate respectively.
Equations (2) and (3) indicate that the driving forces for solvent and solute transport are independent. Therefore by increasing transmembrane pressure it is possible to increase the rate of solvent transfer without increasing the concentration of solute in the retentate. This in fact leads to an increase in the solute partition coefficient between retentate and permeate. However the increase in solute concentration in the retentate, and in particular at the membrane wall, may lead to an increase in osmotic pressure important enough to reduce J1. This effect is called membrane polarization and will be discussed in connection with ultrafiltration.
Ultrafiltration is also a membrane separation process driven by pressure. Its main difference from reverse osmosis is the type of membrane used. While reverse osmosis membranes are nonporous and therefore, permeable only to very small molecules, ultrafiltration membranes are microporous and permeable to small solute species in addition to the solvent. Ultrafiltration is therefore used to keep large solute species in the retentate. The effect of osmotic pressure is therefore less important in this process so that, the transmembrane pressures required are not as high as for reverse osmosis. The solvent flux obtained with uitrafiltration is also given by Eq. (2), but the membrane permeability K1 now depends on the characteristics of the ultrafiltration membrane and the diffusional resistances specific to this process. As a result of solvent transfer the retentate becomes more concentrated and a concentration gradient of solute builds up in the direction of transfer with the higher concentration at the membrane wall. This phenomenon, called membrane polarization, creates a concentration driving force for solute diffusion in the direction opposite to solvent transfer. At equilibrium the diffusional flux of solute back to the retentate phase equals the convective flux towards the membrane:
where J is the velocity of the permeate phase through the membrane, cp is the concentration of solute in the permeate, D is the diffusivity of the solute in the retentate and y the distance in the direction normal to the membrane. As the separation proceeds the concentration gradient becomes steeper due to solute accumulation next to the membrane. As the solutes retained in ultrafiltration are larger than in reverse osmosis, their diffusivities are lower and thus accumulation near the membrane wall is more likely in ultrafiltration than in reverse osmosis.
Equation (2) allows comparison of the response of reverse osmosis and ultrafiltration to transmembrane pressure. As ΔP is increased, solute accumulation at the membrane wall on the retentate side is higher in ultrafiltration than in reverse osmosis due to the lower diffusivity of the solute. This accumulation of solute leads to membrane polarization as well as solute precipitation on the membrane (gelification), which in turn decrease K1 and increase ΔП. The process reaches a region in which the permeate flow rate becomes independent of pressure.
The separation of particles of micron and submicron levels can effectively be performed using membrane filters. The suspended particles for which the process is industrially used include colloids, microorganisms and emulsion droplets. There are two different configurations for the microfiltration operation: (1) dead-end microfiltration, and (2) crossflow microfiltration. In the first, the membrane plane is normal to the feed flux, while in the second, it is tangential. The advantages and disadvantages of the type of configuration depend on the characteristics of the feed. Highly concentrated suspensions are not suitable for dead-end treatment since the separated particles rapidly accumulate on the membrane, increasing the resistance to filtration by forming a cake and/or clogging the membrane. In crossflow filtration, the flow is parallel to the membrane and the drag forces close to the membrane wall reduce the amounts of particulate material deposited on the membrane. Therefore the first is used for dilute suspensions while the latter can deal with concentrated ones, such as slurries.
Microfiltration is a pressure driven process and as such the flux of filtrate is given by Equation (2). In this case ΔП can be neglected since it is not relevant to this process.
Dialysis is a process in which the solute, usually an electrolyte, transfers across the membrane driven by the difference in concentration between the two sides of the membrane. Two conditions have to be fulfilled for the process to be effective: (1) the concentration on the permeate side has to be kept low so that the driving force remains as high as possible, and (2) the osmotic pressure must be low, and remain low during the process, so that a counterflux of solvent does not result in feed dilution. Since the combination of these two conditions is not likely to occur, the use of selective membranes has become general practise so that the partition coefficient of the solute between the two phases can be substantially increased by using the Donnen effect. This is achieved by adding to the feed a salt with one of its ions in common with the solute and the other one rejected by the membrane. If, for example, NaCl is the solute to be separated, the addition of NaX (where X is the ion rejected by the membrane) will lead to the following expression for the NaCl partition coefficient at equilibrium:
where the square brackets indicate concentrations. This equation show that the larger the concentration of NaX, the greater the separation enhancement. (See also Dialysis.)
Electrodialysis is a process in which ion-selective membranes are used together with an electric field normal to the membrane phases. The basic principles of electrodialysis are better explained with reference to Figure 2. An electrodialysis stack consists of parallel compartments separated alternately by cation-exchange and anion-exchange membranes. The feed is introduced into each compartment, so that aided by the electric field and selected by the membranes, the cations and the anions are transferred in opposite directions to the neighboring compartments. In this way a demineralized solution leaves compartments 2 and 4 and the concentrated one compartments 1, 3 and 5.
Depending on the type of membrane used, ions can be selected according to their valence. Therefore this process can be used for fractionating ions of different valence. The main application of electrodialysis is in demineralization in general and in the desalinization of brackish water. (See also Electrodialysis.)
Pervaporation is the combination of the selective separation and transfer of a component across the membrane and its evaporation on the permeate side. In order to achieve evaporation the pressure on the permeate side must be such that the partial pressure of this component is lower than its saturation vapor pressure. Therefore although the driving force for transfer is the difference in activity of the transferred species, this is the result of applying a vacuum on the permeate side.
Pervaporation is more expensive than other membrane processes due to the required supply of heat to produce the evaporation. It is, therefore, used in the separation of components from mixtures that are difficult to treat, such as azeotopic solutions and isomer mixtures, for which conventional processes would be more costly.
A liquid membrane is a liquid phase that separates two fluid phases of different composition. In most applications the liquid membrane is an organic phase placed between two aqueous phases. The principles involved in liquid membrane separations are those of solvent extraction, rather than membrane separation. The solute is first selectively extracted by the liquid membrane, it then transfers across the liquid film driven by its concentration gradient and when it reaches the other side of the liquid membrane it is stripped by the third phase.
There are two types of mechanisms of transfer to and from the membrane: (1) Physical transfer, and (2) carrier mediated transfer. The first is based on the solubility of the solute in the membrane, while the second requires the presence of a selective reactant in the organic phase called the carrier. In this case the process is based on the reversibility of the chemical reaction. The extraction of Zn2+ from an aqueous feed with a carrier RH dissolved in the organic membrane, will be used as an example of carrier mediated extraction. The overall reaction between the cation and the carrier can be represented by:
where bars indicate species in the organic phase. Reaction 6 is reversible. At pH 3 the forward reaction takes place while at low pH values the reverse reaction predominates. A suitable liquid membrane system for the separation of a membrane containing RH should have a high pH value in the feed phase and a low one in the stripping phase. Figure 3 shows a schematic drawing of the concentration gradients in the feed, the membrane and the stripping phase with the corresponding concentration profiles.
The main advantage of liquid membranes over conventional solvent extraction is that by contacting the three phases simultaneously, the solute partition coefficient between the two aqueous phases may be several orders of magnitude greater than the one that can be obtained with conventional solvent extraction.
There are two ways in which a liquid membrane can be formed leading to two configurations: (1) emulsion liquid membranes and (2) supported liquid membranes. These are illustrated in Figures 4a and 4b. The emulsion liquid membrane is obtained by adding a surfactant into the organic phase in order to stabilize a water in oil emulsion. The emulsion is then dispersed in the feed so that the organic phase, which is the continuous phase of the dispersed globules, becomes the membrane. Supported liquid membranes are obtained by impregnation of porous fibers of hollow fiber modules.
Bowen, W. R. (1991) Membrane separation processes, in Chemical Engineering, Vol. 2, 4th edn., Coulson, J. M. and Richardson, J. R, Pergamon Press, Oxford.
Rautenbach, R. and Albrecht, R. (1989) Membrane Processes, John Wiley and Sons, Chichester.
Way, J. D., Noble, R. D., Flynn, T. M., and Sloan, E. D. (1982) Liquid membrane transport: a survey, J. Memb. Sci., 12, 239-259. DOI: 10.1016/S0376-7388(00)80185-4
- Bowen, W. R. (1991) Membrane separation processes, in Chemical Engineering, Vol. 2, 4th edn., Coulson, J. M. and Richardson, J. R, Pergamon Press, Oxford.
- Rautenbach, R. and Albrecht, R. (1989) Membrane Processes, John Wiley and Sons, Chichester.
- Way, J. D., Noble, R. D., Flynn, T. M., and Sloan, E. D. (1982) Liquid membrane transport: a survey, J. Memb. Sci., 12, 239-259. DOI: 10.1016/S0376-7388(00)80185-4
Mchakato wa membrane ni kundi la michakato ya kutenganisha ambayo sifa za membrane (porosity, selectivity, umeme) hutumiwa kutenganisha vipengele vya ufumbuzi au kusimamishwa. Katika mchakato huu, mkondo wa kulisha umegawanywa katika sehemu mbili: sehemu ambayo hupita kwenye utando, inayoitwa permeate, na sehemu inayojumuisha vipengele ambavyo havijaendeshwa kupitia utando, kwa kawaida huitwa retentate. ukubwa wa vipengele vinavyotenganishwa na asili na ukubwa wa nguvu ya kuendesha hutoa vigezo vya uainishaji wa mchakato wa kutenganisha utando. Ni lazima ieleweke kwamba mipaka kati ya baadhi ya michakato, kama vile osmosis reverse na ultrafiltration, ni arbitrary. Katika mchakato wa kutengeneza, osmosis ya nyuma na ultrafiltration ni njia za kawaida za kutengeneza. Mifano ya wingi wa matumizi yao inaweza kupatikana katika maji Desalination, viwanda maziwa na kutenganishwa kwa solutes kikaboni kutoka ufumbuzi aqueous. Utaratibu wa utando hauhitaji joto, ambayo inafanya mchakato huo uwe mzuri kwa matibabu ya bidhaa za thermolabile. Kwa kuongezea, gharama za chini za mtaji na za uendeshaji zinazohusika hufanya michakato ya utando kuwa mbadala wa kuvutia kwa michakato ya kutenganisha ya kawaida, haswa wakati wa kushughulika na suluhisho la kuyeyusha. Katika sehemu zifuatazo, kanuni na sifa za michakato iliyoorodheshwa katika Jedwali 1 itaelezwa kwa ufupi. Osmosis ni mchakato wa uhamisho wa mchanganyiko wa suluhisho kupitia utando kutenganisha suluhisho mbili za kioevu za mkusanyiko tofauti. Uhamisho hufanyika kutoka awamu ambayo kemikali uwezekano solvent ni ya juu. Mfano wa kawaida ni uhamisho wa maji kwenye suluhisho la sukari. Picha ya 1 inaonyesha sehemu mbili zilizotenganishwa na utando unaoweza kupenya maji. Moja ina suluhisho, huku ile nyingine ikiwa na maji safi. Mwanzoni kiwango cha ufumbuzi ni kuwekwa katika ngazi sawa na ile ya maji. Kwa kuwa uwezo wa kemikali wa maji safi ni kubwa kuliko ile ya maji katika suluhisho, maji hupita kwenye utando na kuhamia kwenye suluhisho, na kuongeza kiwango cha maji katika upande wa suluhisho na hivyo kuunda kichwa cha hydrostatic. Wakati mchakato ni katika usawa, tofauti katika shinikizo hydrostatic kati ya awamu mbili ni kwamba inahitajika kufanya uwezo wa kemikali ya maji katika awamu zote mbili sawa. Katika kesi hii, wakati solvent safi iko upande mmoja wa utando, tofauti hii ya shinikizo ni shinikizo la osmotic, P. Ikiwa suluhisho ziko pande zote mbili za utando, tofauti ya shinikizo hutoa tofauti ya shinikizo la osmotic. Kwa suluhisho nyingi, shinikizo la osmotic linapatikana kwa ambapo β ni mara kwa mara ambayo inachukua kuunganishwa kwa dissolution ya solute, ci ni mkusanyiko wa molar ya solute i, R ni mara kwa mara ya gesi na T ni joto la kabisa. Kiwango cha kawaida cha b ni sawa na idadi ya ioni zinazozalishwa na solutute iliyomwagika, kwa hivyo kwa electrolytes ambazo hutenganisha kabisa kama vile NALCO na CAC2 ni sawa na 2 na 3, wakati b = 1 kwa molekuli ambazo hazitenganishi. Wakati solvent ina molekuli kubwa, osmotic shinikizo la solvent huacha kutegemea linearly juu ya mkusanyiko wa solvent na polynomial approximation lazima kutumika. Kwa ajili ya ufumbuzi mkali kama vile ufumbuzi nonideal shinikizo osmotic lazima kuamua majaribio au makadirio kutumia shughuli badala ya viwango. "Utafiti wa ""reverse osmosis"" unaonyesha jinsi maji yanavyotenganishwa na maji kwa njia ya membrane ya semipermeable ambayo inapaswa kuwa na maji tu." Kama inavyoonyeshwa katika Picha 1 (c), shinikizo la mitambo linatumika kwa chumba ambacho kiwango ni cha juu, uwezo wa kemikali wa mchanganyiko upande wa suluhisho utaongezeka, na kusababisha uhamisho wa mchanganyiko kupitia utando katika awamu safi ya mchanganyiko. Tofauti hii ya shinikizo kati ya awamu, ΔP, huunda nguvu ya kuendesha kwa ajili ya uhamisho na ni kanuni ambayo mchakato wa osmosis ya nyuma na ultrafltration ni msingi. Ili kufikia uhamisho, ΔP lazima kuwa kubwa kuliko tofauti kati ya osmotic shinikizo ΔP. Kiwango cha upenyezaji wa solvent katika utando hupimwa kama kasi yake ya linear ya kawaida kwa utando, J1. Kwa kweli utando unapaswa kuwa tu permeable kwa solvent. Hata hivyo hii ni vigumu sana kufikia na utando kuwa na baadhi ya permeability kwa solute. Kiwango cha upenyezaji wa solvent ni kutokana na: ambapo K1 ni coefficient upenyezaji wa utando na ΔP ni shinikizo transmembrane. Permeability coefficient inategemea upinzani wa utando, upinzani wa uhamisho unaosababishwa na amana kwenye utando na upinzani wa kuenea katika tabaka la mpaka upande wa utando. Kulingana na unene wa amana imara na hali hydrodynamic ya retentate upinzani mbili za mwisho inaweza kuwa negligible. Kwa kweli vifaa vya utando ni kuendeshwa chini ya hali ambayo huwa na kupunguza upinzani hizi mbili ili kuongeza J. Tofauti katika mtiririko wa solute kati ya retentate na permeate (katika mazoezi utando kuwa na baadhi ya permeability kwa solute) inajenga nguvu ya kuendesha kwa ajili ya uhamisho solute katika utando. Mtiririko wa solutes, J2, ni kutokana na: ambapo K2 ni mara kwa mara ambayo inajumuisha sifa za utando, na cR na cP ni mkusanyiko wa solutes katika retentate na permeate. Kwa mfano, katika equation ya 2 na 3 kuna nguvu za kuendesha kwa ajili ya usafirishaji wa solvent na solvent. Kwa hiyo kwa kuongeza shinikizo transmembrane inawezekana kuongeza kasi ya uhamisho wa solvent bila kuongeza mkusanyiko wa solute katika retentate. Hii kwa kweli inaongoza kwa ongezeko la solute mgawanyiko coefficient kati ya retentate na permeate. Hata hivyo, kuongezeka kwa mkusanyiko wa solutes katika retentate, na hasa katika ukuta membrane, inaweza kusababisha ongezeko la shinikizo osmotic kiasi muhimu ya kupunguza J1. Athari hii inaitwa membrane polarization na itazungumziwa kuhusiana na ultrafiltration. Ultrafiltration pia ni mchakato wa kutenganisha utando unaoendeshwa na shinikizo. Tofauti yake kuu kutoka osmosis reverse ni aina ya utando kutumika. Wakati membrane za osmosis ya nyuma hazina pores na kwa hivyo, zinaweza kupenya tu molekuli ndogo sana, membrane za ultrafiltration ni microporous na zinaweza kupenya spishi ndogo za solute pamoja na kioevu. Ultrafiltration ni kwa hiyo kutumika kuweka kubwa solute spishi katika retentate. Matokeo ya shinikizo osmotic ni hivyo chini ya muhimu katika mchakato huu hivyo kwamba, shinikizo transmembrane required si kama juu kama kwa osmosis reverse. Mtiririko wa solvent kupatikana na uitrafiltration pia ni kupewa na Eq. Uwezo wa kupenya wa membrane (K1) hutegemea sifa za membrane ya ultrafiltration na upinzani wa kuenea kwa mchakato huu. Kama matokeo ya uhamisho wa solvent, retentate inakuwa zaidi ya umakini na gradient ya umakini wa solvent hujenga katika mwelekeo wa uhamisho na mkusanyiko wa juu kwenye ukuta wa utando. Jambo hili, linaloitwa membrane polarization, huunda nguvu ya kuendesha mkusanyiko kwa kuenea kwa solute katika mwelekeo kinyume na uhamisho wa mchanganyiko. Katika hali ya usawa, mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiriririko wa mtiriririko wa mtiririririko wa mtiririririko wa mtiririririko wa mtiririko wa mtiririko. Kama kutenganisha inaendelea gradient mkusanyiko inakuwa steeper kutokana na mkusanyiko solute karibu na utando. Kwa kuwa solutes zilizochukuliwa katika ultrafiltration ni kubwa kuliko katika osmosis ya nyuma, diffusivities yao ni chini na hivyo mkusanyiko karibu na ukuta wa utando ni uwezekano zaidi katika ultrafiltration kuliko katika osmosis ya nyuma. Kwa mfano, katika mfano wa 2 (mfano wa 2), unaweza kuona jinsi osmosis ya nyuma na ultrafiltration ya mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko wa mtiririko. Kama ΔP ni kuongezeka, solvent mkusanyiko katika ukuta utando juu ya retentate upande ni juu katika ultrafiltration kuliko katika osmosis reverse kutokana na chini diffusivity ya solvent. Kujipanga kwa solutes huongoza kwa polarization ya utando na pia mvua ya solutes kwenye utando (gelification), ambayo kwa upande hupunguza K1 na kuongeza ΔP. Mchakato hufikia eneo ambalo kiwango cha mtiririko wa permeate inakuwa huru kutoka kwa shinikizo. Kutenganisha chembe za viwango vya micron na submicron inaweza kwa ufanisi kufanyika kwa kutumia filters utando. Chembechembe kusimamishwa kwa ajili ya ambayo mchakato ni viwandani kutumika ni pamoja na colloids, vijidudu na matone emulsion. Kuna aina mbili za microfiltration: (a) Microfiltration ya mwisho, na (b) Microfiltration ya mtiririko. Katika ya kwanza, safu ya utando ni kawaida kwa mtiririko wa kulisha, wakati katika ya pili, ni tangential. Faida na hasara za aina ya Configuration inategemea sifa ya kulisha. Suspensions high-concentrated si yanafaa kwa ajili ya matibabu dead-end tangu chembe kutenganishwa haraka kukusanya juu ya utando, kuongeza upinzani kwa filtration kwa kuunda keki na au clogging utando. Katika crossflow filtration, mtiririko ni sambamba na utando na drag nguvu karibu na ukuta utando kupunguza kiasi cha chembechembe vifaa deposited juu ya utando. Kwa hiyo ya kwanza ni kutumika kwa ajili ya dilute kusimamishwa wakati wa mwisho inaweza kushughulika na wale umakini, kama vile slurries. Microfiltration ni mchakato shinikizo kuendeshwa na kama vile mtiririko wa filtrate ni kutokana na Equation (2). Katika kesi hii ΔΠ inaweza kupuuzwa tangu si muhimu kwa mchakato huu. Dialysis ni mchakato ambapo solutes, kawaida electrolytes, uhamisho katika utando kuendeshwa na tofauti katika mkusanyiko kati ya pande mbili za utando. Masharti mawili yanapaswa kutimizwa ili mchakato uwe na ufanisi: (1) mkusanyiko wa upande wa permeate lazima uhifadhiwe chini ili nguvu ya kuendesha iweze kuwa ya juu iwezekanavyo, na (2) shinikizo la osmotic lazima liwe la chini, na liwe la chini wakati wa mchakato, ili mtiririko wa kinyume cha kioevu usisababisha kuyeyusha. Kwa kuwa mchanganyiko wa hali hizi mbili hauwezi kutokea, matumizi ya utando wa kuchagua imekuwa mazoezi ya kawaida, ili kuwa na uwiano wa kugawanyika kwa solutes kati ya awamu mbili inaweza kuongezeka kwa kiasi kikubwa kwa kutumia athari ya Donnen. Hii ni kupatikana kwa kuongeza kwa kulisha chumvi na moja ya ions yake katika kawaida na solute na nyingine moja kukataliwa na utando. Kwa mfano, ikiwa NaCl ni solvent ya kutenganishwa, kuongeza NaX (ambapo X ni ion iliyokataliwa na utando) itasababisha usemi ufuatao kwa sababu ya kugawanyika kwa NaCl katika usawa: ambapo mabano ya mraba yanaonyesha mkusanyiko. Hii equation kuonyesha kwamba kubwa mkusanyiko wa NaX, kubwa kugawanyika kuimarishwa. (Ona pia Dialysis.) Electrodialysis ni mchakato ambapo ion-chaguo utando ni kutumika pamoja na uwanja wa umeme kawaida kwa awamu utando. Kanuni za msingi za electrodialysis zinaelezwa vizuri zaidi kwa kumbukumbu ya Picha 2. electrodialysis stack inajumuisha compartments sambamba kutenganishwa alternately na cation-kubadilishana na anion-kubadilishana utando. Mazao huingizwa katika kila chumba, ili kwa msaada wa uwanja wa umeme na kuchaguliwa na utando, cations na anions ni kuhamishwa katika mwelekeo kinyume na vyumba jirani. Kwa njia hii, suluhisho la demineralized huondoka katika sehemu za 2 na 4 na ile iliyoimarishwa katika sehemu za 1, 3 na 5. Kulingana na aina ya utando kutumika, ions inaweza kuchaguliwa kulingana na valence yao. Kwa hiyo mchakato huu unaweza kutumika kwa ajili ya fractionating ions ya valence tofauti. Maombi kuu ya electrodialysis ni katika demineralization kwa ujumla na katika desalinization ya maji ya brakish. (Ona pia Electrodialysis.) Pervaporation ni mchanganyiko wa kutenganishwa na uhamisho wa kuchagua wa sehemu katika utando na evaporation yake juu ya upande permeate. Ili kufikia evaporation shinikizo juu ya upande permeate lazima kuwa kama kwamba shinikizo sehemu ya kipengele hiki ni chini ya shinikizo yake saturation mvuke. Kwa hiyo, ingawa nguvu ya kuendesha kwa ajili ya uhamisho ni tofauti katika shughuli ya spishi kuhamishwa, hii ni matokeo ya kutumia utupu juu ya upande permeate. Pervaporation ni ghali zaidi kuliko taratibu nyingine utando kutokana na mahitaji ya ugavi wa joto kuzalisha evaporation. Kwa hiyo, hutumiwa katika kutenganisha vipengele kutoka kwa mchanganyiko ambao ni vigumu kutibu, kama vile suluhisho za azeotopic na mchanganyiko wa isomer, ambayo mchakato wa kawaida ungekuwa na gharama kubwa zaidi. Mzunguko wa kioevu ni awamu ya kioevu ambayo hutenganisha awamu mbili za kioevu zenye muundo tofauti. Katika maombi mengi membrane kioevu ni awamu ya kikaboni kuwekwa kati ya awamu mbili aqueous. Kanuni zinazohusika katika kutenganisha utando wa kioevu ni zile za uchimbaji wa mchanganyiko, badala ya kutenganisha utando. Sulfur ni kwanza selectively kuchukuliwa na utando kioevu, kisha ni kuhamishwa katika filamu kioevu inaendeshwa na gradient yake mkusanyiko na wakati inafikia upande mwingine wa utando kioevu ni stripped na awamu ya tatu. Kuna aina mbili za uhamisho wa kimwili na kutoka kwa utando: (1) Uhamisho wa kimwili, na (2) Uhamisho wa uhamisho wa kubeba. Njia ya kwanza inategemea mumunyifu wa kioevu katika utando, wakati ya pili inahitaji uwepo wa reagent ya kuchagua katika awamu ya kikaboni inayoitwa carrier. Katika kesi hii mchakato ni msingi juu ya reversibility ya athari ya kemikali. Utoaji wa Zn2+ kutoka kwa chakula cha maji na RH ya kubeba iliyofumuliwa katika utando wa kikaboni, itatumiwa kama mfano wa uchimbaji wa kubeba. Mmenyuko wa jumla kati ya cation na carrier inaweza kuwakilishwa na: ambapo baa zinaonyesha spishi katika awamu ya kikaboni. Mmenyuko wa 6 ni reversible. Katika pH 3 athari ya mbele hufanyika wakati katika maadili ya pH ya chini athari ya nyuma inatawala. Mfumo wa membrane ya kioevu inayofaa kwa ajili ya kutenganisha membrane yenye RH inapaswa kuwa na thamani ya pH ya juu katika awamu ya kulisha na moja ya chini katika awamu ya kuondoa. Picha 3 inaonyesha mchoro wa gradients ya mkusanyiko katika chakula, utando na awamu ya kuondoa na maelezo ya mkusanyiko yanayolingana. Faida kuu ya utengenezaji wa utupu wa kioevu juu ya utengenezaji wa kawaida wa solvent ni kwamba kwa kuwasiliana na awamu tatu wakati huo huo, coefficient ya mgawanyiko wa solvent kati ya awamu mbili za maji inaweza kuwa na amri kadhaa za ukubwa kuliko ile ambayo inaweza kupatikana na utengenezaji wa kawaida wa solvent. Kuna njia mbili ambazo membrane ya kioevu inaweza kufanyizwa na kuongoza kwa miundo miwili: (1) membrane ya kioevu ya emulsion na (2) membrane ya kioevu iliyoungwa mkono. Hii inaonyeshwa katika picha 4a na 4b. Emulsion kioevu utando ni kupatikana kwa kuongeza surfactant katika awamu ya kikaboni ili kuimarisha maji katika emulsion mafuta. Emulsion ni kisha kutawanyika katika kulisha ili awamu ya kikaboni, ambayo ni awamu ya kuendelea ya globules kutawanyika, inakuwa utando. Msaada membrane kioevu ni kupatikana kwa impregnation ya nyuzi porous ya modules nyuzi tupu. Bowen, W. R. (1991) Membrane kutenganisha michakato, katika Chemical Engineering, Vol. "Kama ilivyoelezwa katika kitabu cha 4th Edition cha Coulson, J. M. na Richardson, J. R., ""Pergamon Press,"" Oxford." Rautenbach, R. na Albrecht, R. (1989) Mchakato wa Membrane, John Wiley na Wana, Chichester. Way, J. D. Noble, R. D. Flynn, T. M. na Sloan, E. D. (1982) Usafiri wa membrane ya kioevu: uchunguzi, J. Memb. Sci., 12, 239-259. (Sehemu ya 10106 (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) - (S) (S) - (S) (S) "Kama ilivyoelezwa katika kitabu cha 4th Edition cha Coulson, J. 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