Title of Invention	"ADSORBENTS AND A METHOD FOR MANUFACTURE OF THE SAME"
Abstract	Adsorbents useful in the selective adsorption of unsaturated hydrocarbons, the manufacture of the adsorbents, and processes for the separation of unsaturated hydrocarbons using the adsorbents.

Full Text

ADSORBENTS AND METHODS FOR THE SEPARATION OF ETHYLENE AND PROPYLENE AND/OR UNSATURATED HYDROCARBONS FROM MIXED GASES BACKGROUND OF THE INVENTION Technical field of the invention This invention relates to a new adsorbent useful in selective adsorption of unsaturated hydrocarbons and a process for the manufacture of such adsorbent. More specifically, this invention relates to an adsorbent having a high degree of selectivity and affinity for olefin molecules and a high adsorption capacity for olefins, a process for producing such adsorbent and a process for separating olefins from a gas mixture. More particularly, this invention relates to an ethylene and/or propylene separation process employing a specially prepared adsorbent to effectively separate ethylene and/or propylene from a mixed gas containing ethylene and/or propylene together with a component selected from the group consisting of H2, He, CH4, C2H5, C3H6 and mixtures thereof, in an efficient manner using the novel adsorbent of the present invention having a high adsorptive capacity for unsaturated hydrocarbons such as ethylene and propylene. Description of Related Art Unsaturated hydrocarbons such as ethylene and propylene are basic raw materials in synthetic chemistry. These are produced by naphtha/natural gas cracking or by dehydrogenation of paraffin. Invariably these are obtained as mixtures necessitating separation before their use. Prior art processes conventionally employed for the separation of ethylene from ethane and propylene from propane involve low temperature and/or high-pressure distillation. These processes are highly energy intensive and difficult to achieve and therefore, are not commercially very attractive. A common conventional process for the separation of mixture of ethane-ethylene is carried out at -25°C and 320 psig in a distillation column containing over 160 trays and propane-propylene at -30°C and 30 psig pressure in a distillation column containing over 200 trays [Keller, G. E; Marcinkowsky, A. E; Verma, S. K; Williamson, K. D.; Olefin Recovery and Purification via silver complexation; Li, N..N.; Calo, J. M.; In Separation and Purification Technology, Eds, Marcel Dekker, New York, 1992)]. It is now generally accepted that separations of ethane-ethylene and propane-propylene by distillation are some of the largest energy consuming separation processes in the petrochemical industry. Therefore, there is a large demand of an efficient and low cost, low energy process for the separation of olefins, particularly, ethane and propane from gas mixtures containing them, especially in view of the fact that demand for ethylene and propylene is ever increasing. For instance, world wide ethylene capacity of 80.8 million metric tons (mmt) is expected to grow to 122.1mmt by 2005. For the past several years, various attempts have been made to develop alternative technologies such as adsorption, chemical absorption and membrane separation processes. Of the various alternate technologies, adsorption appears to be the most promising [Eldridge, R. B., Olefin/Paraffin Separation Technology: A Review., Ind. Eng.Chem. Res., 32, 2208, 1993]. Conventional adsorbents such as activated alumina, activated carbon, silica gel and zeolites do not show good selectivity for olefins over paraffins. Hence, development of a suitable adsorbent has become a key factor for the successful development of adsorption process. Some of the adsorbents that have been reported for paraffin-olefm separation are CuCl [Gilliland, E. R., Bliss, H/ L., Kip, C. E., Reaction of olefins with solid cuprous halide, J. Am. Chem. Soc., 63, 2088, 1941; Gilliland, E. R., Concentration of Olefins, U. S Patent No. 2,369, 559, 1945, Long, R. B. Separation of Unsaturates by Complexing with solid copper salts., In Recent Development in Separation Science; Li, N.M. Ed., CRC Press, Cleveland, 1972) ion exchanged zeolites [Rosback, D. H., Olefin Separation Process Using Copper-Exchanged Type X Zeolites., U. S Patent No. 3, 755, 153, 1973; Rosback, D. H., Adsorbing Olefins with a Copper-Exchanged TyplTX""2eoiite., U.S Patent No. 3, 720, 604, 1973); polymer supported aluminium silver chloride [Hirai, H., Kurima, K., Wada, K., Komiyama, M., Selective Ethylene Adsorbents Composed of Copper (I) Chloride and Polystyrene Resins having Amino Groups., Chem Lett., 1513, 1985; Hira, H., Hara, S., Komiyama, M., Polystyrene-Supported Aluminium Silver Chloride as selective Ethylene Adsorbent., Angew. Makromol. Chem., 130, 207, 1985; Hirai, H., Polymer Complex for the Separation of Carbon Monoxide and Ethylene., In Polymers For Gas Separation; Toshima, N. Ed., VCH Publishers, Inc., New York, Chapter 7, 1992); and copper containing resins [Dielacher, M.; Hansen, U. Separation of H Unsaturated Compounds from Liquid Hydrocarbon Mixtures Containing the v\Same, U.S Patent No. 3,979, 280,1976). Most of these adsorbents suffer from one or the other drawbacks such as slow adsorption kinetics, poor adsorption capacity, and/or selectivity. More recently, Yang and Kikkinides [New Sorbents for Olefins/Paraffin Separations by Adsorption via O-complexation. AIChE J 41, 509, 1995) and Cho and coworkers [Wu, Z., Han, S. S., Cho, S. H., Kim, J. N., Chue, K. T., Yang, R.T., Modification of Resin-Type Adsorbents for Ethane/Ethylene Separation, Ind Eng. Chem. Res., 36, 2749, 1997] have reported more promising adsorbents. Among the adsorbents reported by them, Ag+ resin and CuCl/A12O3 showed high olefin adsorption capacity and good selectivity. However, ethylene and propylene sorption kinetics on Ag+ resin are slow. CuCl/A12O3 is CuCl dispersed on γ- Al2O3 by monolayer dispersion technique and hence, is obtained in powder form. For commercial use this adsorbent needs to be formed into pellets, which leads to reduction in adsorption capacity and selectivity. Further, adsorbent formulations prepared using Cu[I] compounds are unstable and easily get oxidised to Cu(II) leading to a loss in adsorption capacity and selectivity of the adsorbent. Prior art also reports a series of adsorbents containing Cu(I). [Xie, Y. C., Tang Y. Q., Spontaneous Monolayer Dispersion of Oxides and Salts onto Surfaces Supports: Applications to Heterogenous Catalysis, Advances in Catalysis, 1, 37, 1990; Xie, Y. C., Bu, N. Y., Liu, J. Yang, G., Qiu, J. G., Yang, N. F., Tang,. Y. C., Adsorbents for Use in the Separation of Carbon Monoxide and/or Unsaturated Hydrocarbons from Mixed Gases. U. S. Patent No. 4,917, 711, 1990). These were also prepared in powder form. Hence, these adsorbents also" suffer from the above-mentioned drawbacks. Objects of the invention It is therefore, the primary object of the present invention to provide an adsorbent having a high degree of selectivity and a high capacity for adsorbing olefins. It is a further object of the present invention to provide an adsorbent which has a high selectivity and a high capacity for adsorbing olefms like ethylene and propylene. It is a further object of the present invention to provide an adsorbent, which avoids the drawbacks of prior art, which is stable and does not lose its adsorbing capacity when pelletised. It is yet an important object of the present invention to provide a low cost, low energy process for the separation olefins, particularly, ethylene and propylene from gas mixtures. Finally, it is an important object of the present invention to provide an effective and commercially viable process for the preparation of an adsorbent for use in the above-mentioned separation process. SUMMARY OF THE INVENTION The above mentioned and other objects are achieved by the novel adsorbents of the present invention. It has now been found that a group of solid adsorbents in the form of pellets/beads have high adsorptive capacity and selectivity for ethylene and/or propylene not known in the prior art and that they can be produced by a simple process as described below. These adsorbents comprise (i) a silver compound and (ii) a support having a sufficiently high surface area on which said silver compound is supported. These adsorbents are highly stable and are capable of reversibly adsorbing substantial quantity of ethylene and/or propylene at room temperature. The rates of adsorption of ethylene and/or propylene are also very fast in these adsorbents. Thus, the present invention provides new highly stable solid adsorbent in pellet/bead form for selective adsorption of unsaturated hydrocarbons, which adsorbent is a composite comprising (a) a silver compound and (b) a suitable support having a sufficiently high surface area, at least a portion of said silver being supported on said support. The adsorbent is obtainable by impregnation of silver in said support, and heat treatment of the thus obtained adsorbent. Optionally the adsorbent may also contain a promoter compound. Accordingly, the present invention provides a process for the manufacture of an adsorbent for use in selective adsorption of unsaturateu nyarocarbons or mixtures thereof from a mixed gas, comprising (a) providing a suspension of a silver compound of the kind such as herein described in a suitable solvent, (b) contacting a support material of the kind such as herein described, having a sufficiently high surface area with said suspension of silver compound and impregnating said suspension on said support material, r (c) drying off the excess solvent to produce a solid material, and / (d) heat treating solid material to obtain said adsorbent.. The present invention also provides process for the separation of olefin molecules or mixtures thereof from a mixed gas containing said olefin molecules or a mixture thereof, which comprises: (a) passing said mixed gas through the adsorbent comprising of a (i) Suitable" support having a sufficiently nigh surface area and (ii) a 0 silver compound impregnated thereon, at a temperature in the range of from 0 to 100°C and a pressure in the range of 1 to 100 atmospheres, to effect adsorption of said olefin, and releasing in any conventional manner the adsorbed olefin. In particular, the present invention provides a process for the separation of ethylene and/or propylene from a mixed gas containing ethylene and/or propylene together with another component selected from such as H2, He, CH4, C2H5 C3H6 and mixtures thereof, which process comprises passing a stream of said mixed gas through a mass of adsorbent at a temperature from 0°C to 100°C and a pressure from 1 to 100 atmospheres, and releasing the adsorbed ethylene and/or propylene by lowering pressure and/or increasing temperature. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS Figure 1 shows the adsorption breakthrough curve for ethylene and ethane. Figure 2 shows the semi-continuous adsorption-desorption cycles for ethylene-ethane mixture. DETAILED DESCRIPTION OF THE INVENTION »-v The adsorbents of this invention are obtained by dispersion of silver compound and/or promoter on the surface of a suitable support having a high surface area by the action of a surface reaction between silver compound and the support which have been brought into contact with each other. Many silver(I) compounds or silver (II) compounds or their mixtures can be used as silver compound. When silver (II) compounds are used as silver compound, silver (II) needs to be reduced to silver (I) in a reducing atmosphere. Some of the representative examples of the silver compound which can be suitably utilised in the practice of this invention include, for example, silver nitrate, silver halides such as silver chloride, silver bromide and silver iodide, silver carboxylates such as silver formate and silver acetate, and silver oxide. Preferred silver compounds are silver nitrate and silver acetate. As promoters, rare earth metallic compounds such as lanthanum compounds and cerium compounds or mixtures thereof can be used. Some of the representative examples of promoter compounds are lanthanum(III) nitrate, lanthanum(III) chloride, cerium(III) chloride and cerium(III) nitrate. As the support used to produce the adsorbents of this invention, a fairly large class of solid materials can be utilised provided they have a sufficiently high surface area and have an affinity to the silver compound. It is desirable in the preparation of the adsorbents of the invention that the surface area of the materials used as a support is greater than 100 m2/g, preferably greater than 400 m2/g. Some representative examples of those materials that can be used as the support for the adsorbent of this invention include aluminium oxide, natural or synthetic zeolites such as zeolite A, zeolite Y, and ZSM-5, microporous aluminium phosphates, clay minerals and the like. In the process for the preparation of the adsorbent according to the present invention, a mixture containing the silver compound, an optional promoter, and a support is used. The above mixture can be obtained by adding to the support a solution or suspension of the silver compound in a suitable solvent and optionally promoter compound, equilibrating for a period of 0.1 to 24 hours, preferably 1 to 4 hours, and thereafter removing the solvent from the resultant mixture by heating and/or purging with air/inert gas. Representative examples of the solvent that can be suitably used include, for example, water, hydrochloric acid containing aqueous solution, primary or secondary alcohols having 1 to 7 carbon atoms, acetone, ethyl acetate, hydrocarbons having 4 to 7 carbon atoms, proprionitrile, and acetonitrile. The mixture can also be obtained by physically mixing silver compound and optionally promoter compound in solid form to solid support. In the above-described mixture containing the silver compound and the support, the amount of silver in the form of the compound is preferably from 1 to 150 %, more preferably from 10 to 80 %, by weight of the support. Thereafter, the prepared mixture containing the silver compound, the optional promoter compound and the support is subjected to heating. The heating step can be performed at a temperature in the range of 30 to 500°C, preferably 100 to 250°C for a period of time from about 0.1 to 48 hours, preferably from about 1 to 10 hours. The heating step can be conducted in a suitable atmosphere such as nitrogen and helium. The adsorbents of the invention described above can be used to separate ethylene or propylene from mixed gas. The separation process comprises passing a stream of the mixed gas through an adsorber bed charged with the adsorbent(s) of the invention. The adsorbed ethylene and/or propylene can be readily desorbed either by lowering the pressure or by increasing the temperature of the adsorber bed resulting in a regenerated adsorbent. The adsorbent so regenerated can be reused as an adsorbent for the separation of ethylene and/or propylene from mixed gases. Raw material gases wherein ethylene and/or propylene are present as impurities can be purified by this separation process. The invention is hereafter illustrated by the following examples in detail. All of the given examples are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. The adsorption capacity and selectivity data involved in these examples were obtained by measuring adsorption isotherms in Cahn 1100 microbalance system. In a typical adsorption isotherm measurement, a known quantity of the adsorbent was loaded in the reactor tube and activated under the flow of helium gas at 200°C for several hours. The adsorbent was then cooled to the desired adsorption temperature under helium flow. The reactor tube was then evacuated to 10'4 mm Hg using a two-stage turbo molecular pump. Isotherm was then measured by admitting pulses of pure hydrocarbon gas into the reactor tube. After each adsorption isotherm measurement, desorption experiment was also carried out to check the reversibility of the adsorption isotherm. Example 1 Silver nitrate solution prepared by dissolving 2.014g of silver nitrate in 2.5 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 5.479 g of γ-A12O3 (gamma alumina) and allowed to equilibrate for 2 hours at room temperature. The said γ-A12O3 had a surface area of 460 m2/g and is commercially available. The solution was just enough to wet the solid. The wet adsorbent was dried at room temperature by purging with helium gas. The resultant adsorbent was dried at 110°C for 6 hours followed by calcination at 250°C for 6 hours under helium flow. The adsorbent of 1 g adsorbed 0.76 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. Under the same experimental conditions the adsorbent was able to adsorb only 0.11 mmol of ethane. The adsorption selectivity ratio of the adsorbent for ethylene to ethane was 6.9. Ethylene adsorption selectivity over ethane on the alumina support was 1.2. Example 2 Silver nitrate solution prepared by dissolving 2.0056 g of silver nitrate in 4.5 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 5.0054 g of γ-Al2O3 (gamma alumina) and allowed to equilibrate for 1 hours at room temperature. The said γ-Al2O3 had a surface area of 460 m2/g and is commercially available. The excess solvent was dried at room temperature by purging with helium gas. The resultant adsorbent was dried at 110°C for 6 hours followed by calcination at 250°C for 4 hours under helium flow. The adsorbent of 1 g adsorbed 1.00 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. Under the same experimental conditions the adsorbent was able to adsorb only 0.16 mmol of ethane. The adsorption selectivity ratio of the adsorbent for ethylene to ethane was 6.3. The same adsorbent after activating at 200°C for 4 hours adsorbed 1.22 mmol/g and 0.90 mmol/g of propylene at 760 mm Hg pressure and 25°C and 60°C respectively. The adsorbed propylene was completely desorbed under vacuum at 100°C. Under the same experimental conditions the adsorbent adsorbed only 0.43 mmol/g and 0.25 mmol/g of propane at 25°C and 60°C respectively. Example 3 Silver nitrate solution prepared by dissolving 2.2524 g of silver nitrate in 4.2 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 5.0139 g of γ-Al2O3 (garnma alumina) and allowed to equilibrate for 1 hours at room temperature. The said γ-Al2O3 had a surface area of 460 m2/g. The excess solvent was dried at room temperature by purging with helium gas. The resultant adsorbent was dried at 110°C for 4 hours followed by calcination at 250°C for 6 hours under helium flow. The adsorbent of 1 g adsorbed 0.85 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. Under the same experimental conditions the adsorbent was able to adsorb only 0.16 mmol of ethane. The adsorption selectivity ratio of the adsorbent for ethylene to ethane was 5.3. Example 4 Silver nitrate solution prepared by dissolving 6.2152 g of silver nitrate in 5.2 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 5.479 g of γ-A12O3 (gamma alumina) and allowed to equilibrate for 2 hours at room temperature. The said y-AbOa had a surface area of 360 m2/g. The solution was just enough to wet the entire solid. The wet adsorbent was dried at room temperature by purging with nitrogen for 30 minutes. The resultant adsorbent was dried at 110°C for 2 hours followed by calcination at 250°C for 2 hours under nitrogen atmosphere. The adsorbent of 1 g adsorbed 0.52 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. Under the same experimental conditions the adsorbent was able to adsorb only 0.10 mmol of ethane. The adsorption selectivity ratio of the adsorbent for ethylene to ethane was 5.2. Example 5 Silver nitrate solution prepared by dissolving 3.148 g of silver nitrate and 0.6662 g of lanthanum nitrate hexahydrate in 2.75 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 5.0161 g of γ-Al2O3 and allowed to equilibrate for 2 hours at room temperature. The said γ-Al2O3 had a surface area of 360 m2/g. The wet adsorbent was dried at room temperature by purging with helium gas for 30 minutes. The resultant adsorbent was further dried at 110°C for 4 hours followed by calcination at 350°C for 6 hours under helium flow. The adsorbent of 1 g adsorbed 0.57 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. Example 6 Lanthanum nitrate hexahydrate solution prepared by dissolving 3.3343 g of lanthanum nitrate hexahydrate in 18 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 30.0092 g of said y-AlaOa and allowed to equilibrate for 1 hour at room temperature. The said y-AbOa had a surface area of 460 m2/g. the wet adsorbent was dried at room temperature by purging with helium for 1 hour. The resultant adsorbent was further dried at 110°C for 4 hours followed by calcination at 250°C for 6 hours under helium atmosphere. Example 7 Silver nitrate solution prepared by dissolving 2.2540 g of silver nitrate in 4.2 ml demineralised water was thoroughly mixed with 5.0178 g of the adsorbent obtained in Example 6 and equilibrated for 1 hour. The wet adsorbent was dried at room temperature with helium purge for 30 minutes. The resultant adsorbent was dried at 110°C for 4 hours followed by calcination at 250°C for 6 hours under helium atmosphere. The adsorbent of 1 g adsorbed 0.92 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. The adsorption selectivity ratio of the adsorbent for ethylene to ethane was 6.4. Example 8 Silver nitrate solution prepared by dissolving 1.4104 g of silver nitrate in 3.04 ml demineralised water was thoroughly mixed with 3.556 g of the adsorbent obtained in Example 6 and equilibrated for 1 hour. The wet adsorbent was dried at room temperature with helium purge for 30 minutes. The resultant adsorbent was dried at 110°C for 4 hours followed by calcination at 250°C for 6 hours under helium atmosphere. The adsorbent of 1 g adsorbed 0.97 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 110°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. The adsorption selectivity ratio of the adsorbent for ethylene to ethane was 3.2. Example 9 1.0 g of 2.0 mm diameter beads of y-AbOa was finely ground (lOOum) in a mortar and pestle. This powder was mixed with 0.45 g of silver nitrate and heated at 250°C for 6 hours at helium atmosphere. The product was adsorbent in powder form which adsorbed 0.86 mmol of ethylene per gram of the adsorbent at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. Example 10 Silver nitrate solution prepared by dissolving 2.0045 g of silver nitrate in 5.1 ml demineralised water was thoroughly mixed with 5-10 mesh diameter beads of 5.1054g of silica gel and allowed to for 1 hour at room temperature. The said silica gel had a surface area of 425 m2/g. The excess solvent was dried at room temperature by purging with helium gas. The resultant adsorbent was further dried at 110°C for 6 hours followed by calcination at 250°C for 4 hours under helium flow. The adsorbent of 1 g adsorbed 0.50 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. Example 11 1.0 g of zeolite X was mixed with 0.45 g of silver nitrate and then heated at 250°C for 24 hours in helium atmosphere. The product adsorbent adsorbed 2.5 mmol of ethylene per gram of the adsorbent at 25°C and 760 mm Hg of ethylene. The adsorbent had ethylene selectivity ratio over ethane of 2.1. Example 12 1.0 g of zeolite Y with Si/Al=20 was mixed with 0.5 g of silver nitrate and then heated at 250°C for 24 hours in helium atmosphere. The product adsorbent adsorbed 1.8 mmol of ethylene per gram of the adsorbent at 25°C and 760 mm Hg of ethylene. The adsorbent had ethylene selectivity ratio over ethane of 2.3. Example 13 Silver acetate solution prepared by dissolving 2.2500 g of silver acetate in 5.0 ml demineralised water was thoroughly mixed with 2.0 mm diameter beads of 5.0036 g of y-AbOa and allowed to equilibrate for 1 hour at room temperature. The said y-AlaOs had a surface area of 460 m2/g. The excess solvent was dried at room temperature by purging with helium gas. The resultant adsorbent was further dried at 110°C for 4 hours followed by calcination at 250°C for 6 hours under helium flow. The adsorbent of 1 g adsorbed 0.88 mmol ethylene at 25°C and 760 mm Hg pressure of ethylene within 10 minutes. The adsorbed ethylene was completely desorbed by evacuation at 100°C. The adsorbent was able to adsorb the same amount of ethylene under the same conditions. Example 14 100 g of the adsorbent was prepared in the same manner as described in (Example 5 3\684 g of this adsorbent on an ambient basis was packed in a stainless steel column of 250 mm height and 20 mm diameter (78.5 ml internal volume). The adsorbent was activated at 200°C for 3 hours under helium flow, and then the column was filled with helium up to 770 mm Hg with temperature reduction to 24°C. Feed mixture consisting of 70.5% of ethylene and 29.5% of ethane by volume was allowed to pass through the adsorbent bed with feed flow rate of 150 sccrn at 800 mm Hg. The adsorption breakthrough curve for ethylene and ethane is shown in Figure 1. In the first 540 seconds the concentration of ethylene in the off-gas was lower than 0.01% by volume. The dynamic adsorption capacity of ethylene on the above adsorbent as obtained by the breakthrough curve was 0.76 mmol/g (at 560 mm Hg). The adsorbed ethylene could be completely desorbed by evacuation to 30 mm Hg at 100°C. Example 15 Semi-continuous adsorption-desorption cycles for said ethylene-ethane mixture were performed at 60° C on the adsorption column described in Example 14. It was carried out in a four step semi-continuous cycles consisting of (a) ethane re-pressurisation, (b) adsorption with feed, (c) ethylene rinse, and (d) vacuum desorption. The time gaps between each step were 1-5 minutes. Step time, flow rate and bed pressure during each step are given below. Step Time/sec Flow rate/seem Pressure/mm Hg Ethane 35 250 up to 772 re-pressurisation Adsorption 330 150 760 to 780 Ethylene rinse 90 100 770 Vacuum desorption 480 — 30 The results are shown in Figure 2. The data by this cyclic adsorption-desorption experiments was reproducible, which ensured the existence of steady state conditions. The purity of the effluent ethane gas was 97.5-99.0%. The breakthrough time was 220 seconds compared to 540 seconds of a fresh adsorbent. Purity of ethylene product obtained was at least 95 %. We claim: 1. A process for the manufacture of an adsorbent for use in selective adsorption of unsaturated hydrocarbons or mixtures thereof from a mixed gas comprising: a) providing a suspension of a silver compound of the kind such as hereinbefore described in a suitable solvent; b) contacting a support material of the kind such as herein described, having a sufficiently high surface area of at least 100 m2/g with said suspension of silver compound and impregnating said suspension on said support material, optionally in the presence of a promoter such as herein described; c) drying off the excess solvent to produce a solid material, and d) heat treating solid material at a temperature in the range of 30 to 500°C to obtain said adsorbent. 2. The process as claimed in claim 1 wherein said support material is selected from the group consisting of alumina, a zeolite such as herein descrbed and a silica gel in powder or pellet/bead form, and wherein said zeolite is selected from natural or synthetic zeolites such as zeolite A, zeolite Y, and ZSM-5. 3. The process as claimed in claim 1 or 2, wherein said silver compound is selected from the group consisting of silver nitrate, silver carboxylate, silver halide, silver oxide, and any mixtures thereof. 4. The process as claimed in any preceding claim wherein the solvent is selected from the group consisting of water, alcohols, hydrocarbons, aqueous hydrochloric acid, acetone, ethyl acetate, propionitrile and acetonitrile. 5. The process as claimed in claim 4 wherein said alcohol is selected from primary or secondary alcohol having from 1 to 7 carbon atoms and said hydrocarbons are selected from hydrocarbons having from 4 to 7 carbon atoms. 6. The process as claimed in claim 1 wherein said promoter compound is selected from the group consisting of lanthanum nitrate, lanthanum chloride, cerium chloride, cerium nitrate, and any mixtures thereof, and is present in an amount of from about 2 to 205 by weight of said support. 7. The process as claimed in any preceding claim wherein after contacting said suspension of the silver compound with said support material, the reaction mixture is equilibrated for a period of 0.1 to 24 hours, preferably 1 to 4 hours. 8. The process as claimed in any preceding claim wherein the amount of silver in the form of compound is preferably from 1 to 150%, more preferably from 10 to 80% by weight of said support material. 9. The process as claimed in any preceding claim wherein said heat treatment is carried out at a temperature of 30 to 350°C, preferably in the range of 100 to 250°C, and for a period of preferably 0.1 to 48 hours, more preferably for 1 to 10 hours, and in the optional presence of nitrogen or helium. 10. The process as claimed in any preceding claim wherein the surface area of the said support material is greater than 360 m2/g, preferably about 400 m2/g, more preferably about 460 m2/g. 11. A process for the separation of olefin molecules or mixtures thereof from a mixed gas containing olefin molecules or a mixture thereof using an adsorbent of any preceding claim. 12. A process for the manufacture of an adsorbent for use in selective adsorption of unsaturated hydrocarbons or mixtures thereof from a mixed gas, substantially as described herein and with reference to and as illustrated in the foregoing examples. 13. A process for the separation of olefin molecules or mixtures thereof from a mixed gas containing said olefin molecules or a mixture thereof, substantially as herein described and with reference to and as illustrated in the foregoing examples.

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