FIELD OF THE INVENTION 
     The present invention relates to manufacture of higher olefins by oligomerization of a lower olefin, and in particular to oligomerization of ethylene, with simultaneous separation of the higher olefins, using catalytic distillation conditions. The process is operated without the need for added solvent, in contrast to the prior art. 
     BACKGROUND OF THE INVENTION 
     Several catalytic processes have been developed for oligomerization of olefins to higher olefins, and in particular for oligomerization of ethylene to a series of higher olefins (C 2 H 4 ) n  (Equation 1).
 
C 2 H 4 &lt;==&gt;C 4 H 8 , C 6 H 12 , C 8 H 16 ,   (1)
         i. [Catalyst]       

     The higher olefins initially so formed normally are terminal(alpha) olefins ie. olefins having a single double bond at the first carbon atom. The terminal olefins may then isomerize to one or more internal olefins ie olefins having a double bond on an interior carbon atom. However, usually the terminal olefins have higher commercial utility and value than the internal olefins. For example, it is desirable to use terminal olefins in combination with ethylene to form partially branched polyolefin co-monomers, biodegradable detergents, lubricants, or plasticizers. 
     Thus it is desirable to operate the catalytic reaction of the process under conditions where the isomerisation reaction is minimized, thus ensuring a higher selectivity to terminal olefins. Operation at low temperatures minimizes the rate of the isomerisation reaction. However, it also is desirable to have a high reaction rate. Operation of the process at high temperatures provides a higher reaction rate than low temperatures. However, this requires high reactor pressures to allow for high olefin concentrations in the liquid phase. 
     There are at present three major commercial processes in use for oligomerization of olefins, each of which has a relatively high degree of complexity and less than a desirable efficiency. Both Chevron and Ethyl Corporation use Ziegler type catalysts in a homogeneous catalyst system. The Shell Higher Olefins Process (SHOP) uses a complex of nickel as the catalyst. Each of these systems uses a solvent and a catalyst in a liquid-phase reactor necessarily equipped with an intercooler. The mixture in the product stream is then purified in a series of separation columns. 
     Solid state catalyst processes used in slurry reactor systems allow for easier separation of the catalyst from the reaction mixtures but present several challenges. There is strong adsorption of the products on the catalyst surfaces, as well as on the reactant. Also, there is a negative thermodynamic influence on selectivity to the desired terminal olefin products at high reaction temperatures, with internal olefins being formed. There is a need for more active catalysts. Each of these factors including catalyst deactivation by the formation of decomposition and isomerisation products, must be overcome. 
     There are several bases for potential beneficial changes that would improve oligomerization processes, including use of milder conditions, thereby maximizing selectivity, and development of more active and selective catalysts, thereby enhancing yield and production rate. 
     Among the many catalysts known to catalyze the oligomerization of olefins, it has been found that highly acidic heterogeneous catalysts comprising, for example, finely divided nickel supported on sulfated alumina are particularly active for dimerization of propylene, as described in French Patent 2641 477 issued in 1990. The Ni/sulphated Al 2 O 3  catalyst used in &#39;477 is active at room temperature for dimerization of propylene in a slurry with an inert hydrocarbon solvent. Further, a similar catalyst comprising Ni/sulfonated non-porous Al 2 O 3  (commercially available ALON) was shown to be active for oligomerization of ethylene, as described by Zhang et al. in “Oligomerization of Ethylene in a Slurry Reactor Using a Nickel/Sulfonated Alumina Catalyst,” Ind. Eng. Chem. Res., 36, 3433-3438 (1997), the disclosure of which is incorporated herein by reference. 
     Several additional processes for oligomerization of olefins have been described in patents and the open literature. Among these are descriptions of catalyst systems for oligomerization of ethylene using either homogeneous or heterogeneous catalysts. However, a characteristic of all prior art is the use of a solvent that is necessary for conducting the process, in contrast to the process of the present invention. Examples of other prior art from which the present invention is so distinguished include: Krug et al. in U.S. Pat. No. 6,841,711; Gildert et al. in U.S. Pat. 6,274,783, Vora et al. in U.S. Pat. No. 6,025,53 and Townsend et al. in U.S. Pat. No. 6,004,256. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process for oligomerization of lower olefins, and in particular for oligomerization of ethylene, without use of an added solvent, so that there is no need for separation of the higher olefins product from a fluid such as a hydrocarbon solvent. The process operates under catalytic distillation conditions such that the product higher olefins are in liquid form, so that the product is easily separated from the reaction mixture as liquid. 
     According to the invention, a process is provided for making higher olefins of formula C n H 2n , wherein n is an integer greater than two through catalytic oligomerization of lower olefins wherein n is an integer from 2 to 5, and in particular to oligomerization of ethylene, and simultaneous separation of the higher olefins as liquid using catalytic distillation conditions e.g. in a catalytic distillation column. There is no need for added solvent. The process, which can be continuous, is operated at a temperature and a pressure such that the higher olefins are primarily in the liquid phase and the ethylene is present both as gas and dissolved phase, to form a solution with the liquid higher olefins. Suitable catalysts include the homogeneous and heterogeneous catalysts described above e.g. a catalyst comprising nickel dispersed on a non-porous alumina support is highly active and has good selectivity to terminal olefins at low temperatures. For example, the above-described catalyst known as ALON has been found to be useful 
     If the catalyst is solid, it is called heterogeneous (gas-solid or liquid solid). If the solid or liquid catalyst is dissolved in the liquid reaction mixture, there is only one phase (liquid), thus called homogeneous catalyst. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and for further objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram of a catalytic distillation for concurrent oligomerization of ethylene and separation of higher olefins. 
         FIG. 2  shows a first profile of composition and temperature from modeling of the process of ethylene oligomerization using a catalytic distillation column. P=40 atm, RR=12, D/F=0.45, RXN at Stage 3, Equilibrium conversion=0.05, Total conversion of Ethylene=54.36%. Stage 1 is at the top of the column. 
         FIG. 3  shows a second profile of composition and temperature from modeling of the process of ethylene oligomerization using a catalytic distillation column. P=40 atm, RR=12, D/F=0.45, RXN at Stage 3, Equilibrium conversion=0.05, Total conversion of Ethylene=44.03%. 
         FIG. 4  shows a third profile of composition and temperature from modeling of the process of ethylene oligomerization using a catalytic distillation column. P=40 atm, RR=12, D/F=0.45, RXN at Stage 3, Equilibrium conversion=0.05, Total conversion of Ethylene=44.03%. 
         FIG. 5  shows a fourth profile of composition and temperature from modeling of the process of ethylene oligomerization using a catalytic distillation column. P=40 atm, RR=15, D/F=0.35, RXN=Stage 3, Equilibrium conversion=0.10, Total conversion of Ethylene=61.27%. 
         FIG. 6  shows a fifth profile of composition and temperature from modeling of the process of ethylene oligomerization using a catalytic distillation column. P=40 atm, RR=79, D/F=0.05, RXN=Stage 3, Equilibrium conversion=0.50, Total conversion of Ethylene=96.88%. 
         FIG. 7  shows a sixth profile of composition and temperature from modeling of the process of ethylene oligomerization using a catalytic distillation column. P=40 atm, RR=79, D/F=0.05, RXN=Stage 3, Equilibrium conversion=0.50, Total conversion of Ethylene=96.88% 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description comprises data obtained through laboratory experiments and simulations using ASPEN PLUS. 
     Referring to  FIG. 1 , an apparatus  10  having a catalytic distillation column  12  is provided for the present process. Column  12  has an upper first portion  14 , a middle second portion  16  and a lower third portion  18 . A condenser  20  is provided at first portion  14  for condensation of condensable components of a gas phase reaction mixture  27  for their return as liquids to first portion  14  of column  12 . A reboiler  22  is provided at third portion  18  of column  12  for vaporization of the more volatile components of a liquid reaction mixture  28  there collected for return as volatiles to third portion  18  of column  12 . 
     Second portion  16  of column  12  includes a catalyst bed  24  having therein an oligomerization catalyst  26 . It has been found through experimental testing that preferably catalyst  26  is an acidic catalyst. The catalyst is considered to be acidic if it consumes a significant amount of base during titration. For example, the sulfated nickel/alumina(ALON) will consume NH 3  during a titration experiment. This finding is in agreement with that for a different system by Espinoza et al. in “Catalytic Oligomerization of Ethylene over Nickel-exchanged Amorphous Silica-alumina: Effect of the Acid Strength of the Support”  Appl. Catal.  29, 295 (1987). 
     One preferred catalyst  26  comprises nickel (Ni) well-dispersed and supported on a non-porous alumina support, for example the commercial product ALON. We have found that such a catalyst has highly active Ni sites that enable the oligomerization process to be conducted under very mild conditions (about −10° C. to about 8° C.) for oligomerization of substantially pure ethylene(see below) as feed  30  when operated at, for example, about 40 to 50 atmospheres pressure. The desirable surface acidity is achieved via surface sulphation. Since ALON has only external surfaces and the reaction products are large molecules, the desorption of reaction products is enhanced(less products adsorbed on catalyst surface) resulting in higher reaction rate and improved catalyst stability. 
     In a second embodiment of apparatus  10  (not illustrated), there a plurality of catalyst beds  24  at different heights within column  12 . When there are more than one catalyst beds  24 , the temperature gradient within column  12  is smoothed out, and the relative concentration of feed (e.g. ethylene)  30  within column  12  is more readily controllable. 
     No added solvent is required in the reaction mixture. 
     It has been found that the oligomerization reactions of the present process (Equation 1 above) take place within catalyst bed  24 . When the temperature and pressure are sufficiently high that ethylene  30  is present primarily as liquid (the critical temperature—boiling point—for ethylene is −8.9° C.), and the acidic catalyst  26  comprises Ni supported on non-porous alumina, contact between products  36  and ethylene  30  facilitates desorption of said products  36 . The liquid ethylene  30  dissolves higher olefins  36  adsorbed on the catalyst surface active sites, so minimizing further catalytic reactions. Consequently there is minimization of olefin isomer or other by-product formation, thus enhancing selectivity to desirable terminal linear olefins (alpha-olefins). 
     Feed  30  is more volatile than products  36 . When the process is operated at a sufficiently high temperature and pressure, products  36  are present primarily in liquid phase  28 . Preferably, feed  30  is fed as liquid, and it is present as gas and liquid in equilibrium within the refluxing reaction mixture. 
     We will now summarize the process using ethylene as an example of feed  30 . The ethylene feed  30  may be selected from substantially pure ethylene, typically 99.9% ethylene with 0.1% ethane as used in polyethylene manufacture, or a mixture rich in ethylene, for example an unfractionated industrial ethylene stream comprising, typically, about 80.5% ethylene, 18.2% ethane and 1.3% acetylene. Optionally, the acetylene may be removed or converted before being fed to the oligomerization reactor. It will be appreciated by those skilled in the art that the reaction parameters to provide the low olefin feed in liquid form at its boiling point will vary somewhat for different compositions of the feed mixture rich in ethylene. For example, when feed  30  comprises the above unfractionated industrial ethylene stream, the mixture flashes between 16° C. and 17.5° C. at 50 atm. It will also be appreciated that when the low olefin feed composition includes C3, C4, C5 etc., the temperature and pressure required to provide the feed in the requisite liquid form, will be different ie different boiling points. 
     Liquid ethylene  30  is fed via an inlet line  32  to upper portion  14  of column at a position above an upper surface  34  of catalyst bed  24 . Ethylene  30  is oligomerized to a series of higher olefins CnH 2 n  36  which mix with ethylene to form liquid phase reaction mixture  28  that descends via a bottom surface  38  of catalyst bed  24  to collect in third portion  18  of column  12 . Liquid ethylene  30  supplied via line  32  washes liquid phase products  36  off the catalyst surface as liquid mixture  28 . Thus ethylene  30  is continuously supplied, reacts within catalyst bed  24 , and with products  36  descends as liquid mixture  28 . 
     The position of inlet line  32  as shown in  FIG. 1  is above catalyst bed  24 . It will be recognized by those skilled in the art that inlet line  32  may be positioned above, below, or at some point within the vertical extent of catalyst bed  24 . Further, there may be more than one feed line  32  positioned at different heights on column  12 . The product distribution is affected by the positioning of inlet line  32 . The distribution within the slate of products can be controllably varied by amending the position of inlet line  32 , and controlling the reflux rate and the reboiler duty. It should be noted that line  42  is optional. It is required only if there present impurities in the gas phase at the condenser. 
     Liquid product mixture  28  is removed via reboiler  22 , from which the more volatile components, and in particular ethylene, are returned as volatiles to column  12 . The remaining portion is liquid products  36  that are removed via line  40 . 
     The rate of feed of ethylene  30 , the process operating conditions, and the rate of removal of liquid products determines the composition of the product liquid removed from column  12 . Preferably, the reaction is operated at elevated pressure, for example 40 atmospheres, so as to maintain ethylene  30  at its boiling point. The process operates at low temperatures, preferably from about −20° C. to about 8° C., and more preferably at −10° C. to 8° C., when operated at 40 to 50 atmospheres pressure. Under these conditions ethylene is present primarily as liquid at its boiling point in first portion  14  of column  12 , and as a solution with products  36  as a condensed phase  28  within catalyst bed  24  and in third portion  18  of column  12 . It is desirable to run the CD column at the highest possible temperature where ethylene is a liquid at its boiling point (both gas and liquid are present). At the top of the column, there are no products and so it is preferable to operate at the boiling point of ethylene (about 8° C. at 50 atm) in this zone. The temperature of the bed increases once products are formed, or when higher boiling components such as ethane are present. 
     It is well known that industrial ethylene contains impurities, including ethane. Further, ethane or other light hydrocarbons may accumulate in the reaction mixture and, as they are volatile, primarily in first portion  14  of column  12 . Thus it will be necessary to remove these volatile materials  44 , from time to time when operating in batch mode or continuously when operating a continuous process. A stream containing the undesirable volatiles  44  is removed via outlet line  42 . 
     The new process for oligomerization of olefins, and in particular oligomerization of ethylene, has the following beneficial characteristics. Several catalysts are active for oligomerization of ethylene, including homogeneous catalysts and heterogeneous catalysts. One preferred catalyst has highly active Ni sites that enable oligomerization process at very mild conditions. The desirable surface acidity is achieved via surface sulphation. This preferred catalyst comprises Ni well dispersed and supported on a non-porous alumina support, thus facilitating product desorption, consequently minimizing isomer formation, and so enhancing selectivity to desirable terminal linear olefins (alpha-olefins). Both liquid feed and higher olefins formed through oligomerization of the feed also serve as the liquid medium, without added solvent, thus facilitating product desorption from the catalyst surface. 
     Use of catalytic distillation column  12  provides further advantages. The heat of exothermic reaction (22 kcal/mol) is used to reduce energy requirement in the distillation step. There are no hot spots, and so there is no need for an inter-cooler. Solvent is not required as the liquid feed and product higher olefins  36  serve as solvent, and only feed olefin is fed to the column. The resulting high reactant concentration results in low mass transfer resistance and high reaction rate. The acidic Ni/Al 2 O 3  catalyst, details of which are described in Example 1 below, has superior selectivity and stability. At least one fixed catalyst bed  24  is used as the reactor in catalytic distillation column  12 , and there is no need to provide another column for separation of catalyst from the reaction mixtures, in contrast to possible highly acidic homogeneous reactive distillation systems that may be used without solvent. Thus, while the heterogeneous reaction is substantially similar to the liquid phase reaction used in several present commercial processes, the catalyst and catalytic distillation process described herein confer significant additional benefits. 
     EXAMPLES 
     Example 1 
     Activity of Catalyst Comprising Ni Supported on Alumina 
     We have shown that the data reported by Zhang et al. in “Oligomerization of Ethylene in a Slurry Reactor Using a Nickel/Sulfated Alumina Catalyst”  Ind. Eng. Chem. Res.  36, 3433-3438 (1997) are reproducible, and that the catalyst is useful for the present invention under catalytic distillation conditions. 
     Zhang et al. conducted a series of experimental runs using a batch reactor and under mild operating conditions when using heptane as solvent. The catalyst Ni/ALON, prepared as described by them, was shown to be highly active when used under the following operating conditions:
         Reaction temperatures: 278, 298, 308, 323 K   Pressure: 170.26 kPa.   Run duration in a Parr reactor: 3 h   n-Heptane (solvent) charge: 120 mL   Stirring speed: 450 rpm   Catalyst: 1.7 wt % Ni and 5.0 wt % SO 4   2−     Catalyst charge: 0.2, 0.3, 0.4, 0.5 g       

     The catalyst is highly active for oligomerization. It was found that the process has first order kinetics with respect to ethylene (Eq. 2) and the activation energy is 16.3 kJ/mol. Neither inter-nor intra-particle resistances may be ignored with this catalyst. The combined resistance to external diffusion, internal diffusion and reaction, expressed as in Eq. 3, is the controlling step. 
     
       
         
           
             
               
                 
                   
                     
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     At low temperatures (&lt;298 K) and near-atmospheric pressure, high selectivities to 1-butene and 1-hexene can be attained when using n-heptane as solvent. No apparent deactivation occurs under these conditions. 
     Example 2 
     Simulation of the Process with Different Operating Parameters 
     The following are data obtained through simulations using ASPEN PLUS® software. The design was based on a column equipped with 20 equilibrium stages. The simulation results show the temperature and concentration profiles along the column. At the top of the column (stage 1), it can be seen from the concentration profile that its composition is similar to the feed (mostly C2=). At the bottom of the column (stage 20), the product contains C4= and C6= and residual C2=. In the simulation it is assumed that the oligomerization produces only C4= and C6= to a different fraction of equilibrium concentrations. The results on product distribution are shown in  FIGS. 2-6 .  FIG. 7  is another simulation run with only 10 equilibrium stages in the column, all other conditions are identical to those used in  FIG. 6 . The temperature profile provides the design data for choosing the reboiler and the condenser. 
     It will be appreciated by those skilled in the art that various forms of device can be used for presentation of catalysts within at least one catalyst bed of a catalytic distillation column. 
       FIGS. 2 through 7  show the concentration and temperature profiles throughout column  12  using different sets of process operating parameters for oligomerization of ethylene to higher olefins. In each case, the reactions occurred over catalyst  26  within catalyst bed  24 . 
     Under each set of conditions, reaction occurs sufficiently rapidly that there is little ethylene present in third portion  18  of column, and ethylene dissolved in liquid phase  28  is returned to column  12  as volatiles from reboiler  22 . The small proportion of olefin products  36  present in the vapor phase at first portion  14  of column  12  are returned as liquid from condenser  20 . 
     It should be noted that the catalytic distillation column can be operated with a homogeneous catalyst. In this case the catalyst is mixed with feed ethylene and introduced at the top of the column. 
     REFERENCES CITED 
     U.S. Patent Documents 
     
         
         
           
             U.S. Pat. No. 6,841,711 Krug et al. Process for making a lube base stock from a lower molecular weight feedstock in a catalytic distillation unit 
             U.S. Pat. No. 6,274,783 Gildert et al. Catalytic distillation process for the production of C8 alkanes 
             U.S. Pat. No. 6,025,533 Vora et al. Oligomer production with catalytic distillation 
             U.S. Pat. No. 6,004,256 Townsend et al. Catalytic distillation oligomerization of vinyl monomers to mke polymerizable vinyl monomer oligomers, uses thereof and methods for same
 
U.S. Patent Applications
 
           
         
       
    
     2007/0123743A1 Ng et al. Composite catalyst for the selective oligomerization of lower alkenes and the production of high octane products 
     Foreign Patent Documents 
     French Patent 2641 477 C. Yves and C. Dominique “Process for the preparation and use, in the dimerisation of olefins, of a catalyst containing nickel, sulphur and alumina 
     Other References 
     Q. Zhang, M. Kantcheva, I. G. Dalla Lana, “Oligomerization of Ethylene in a Slurry Reactor Using a Nickel/Sulfated Alumina Catalyst”  Ind. Eng. Chem. Res.  36, 3433-3438 (1997). 
     R. L. Espinoza, R. Snel, C. J. Corf, C. P. Nicolaide, “Catalytic Oligomerization of Ethylene over Nickel-exchanged Amorphous Silica-alumina: Effect of the Acid Strength of the Support”  Appl. Catal.  29, 295 (1987).
A process is disclosed for making higher olefins by oligomerization of a lower olefin e.g ethylene, to higher olefins, using catalytic distillation conditions. Simultaneously and interdependently, the lower olefin is catalytically oligomerized to higher olefins, and said higher olefins are separated and recovered as liquid.