Separation of ethane from methane by pressure swing adsorption

Abstract: Ethane is separated from a methane- and ethane-containing feedstream using pressure-swing adsorption wherein the adsorbent is a faujasite type of zeolitic aluminosilicate containing (1) at least 20 equivalent % of a cation selected from zinc, a rare earth metal, hydrogen, ammonium or calcium, and (2) not more than 80 equivalent percent of an alkali metal or alkaline earth metal cation other than calcium.

Description: The present invention relates to processes for the separation of ethane from feedstreams containing mixtures thereof with methane. More particularly, the present invention relates to the use of particular cation types of zeolitic molecular sieve having the faujasite type of crystal structure as selective adsorbents in pressure swing adsorption processes for methane purification BACKGROUND OF THE INVENTION Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene, from a feedstream to produce an ethylene-rich product. In pressure swing adsorption, a multi component gas is typically fed to at least one of a plurality of adsorption zones at an elevated or upper pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined intermediate level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure in the adsorption zone is further reduced to a lower level by withdrawing desorbed gas countercurrently to the direction of feedstream. Finally, the adsorption zone is purged and repressurized. The final stage of repressurization is typically with product gas and is often referred to as product repressurization. In multi-zone systems there are typically additional steps, and those noted above may be done in stages. U.S. Patent Nos. 3,176,444; 3,986,849; 3,430,418 and 3,703,068 describe multi-zone, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization. Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon and activated alumina. When PSA processes are used to purify hydrogen-containing streams, the hydrogen is essentially not adsorbed on the adsorbent. However, when purifying methane-containing streams, methane is often adsorbed on the adsorbent along with the impurity. The phenomenon is known in the PSA art as coadsorption. The coadsorption of methane often causes a temperature rise in the adsorption zone due to the exothermic heat of adsorption which can be substantial, e.g., 22°C (40°F) or more. The degree of temperature rise depends, in part, upon the amount of methane present and the particular adsorbent employed. As is well known in the art, such a temperature rise can be undesirable since the equilibrium loading of many adsorbates, e.g., ethane, is reduced at higher temperatures. Hence it would be desirable to reduce such temperature rises during adsorption. Similarly, during PSA regeneration, the desorption of the coadsorbed methane along with the impurity often causes a temperature decrease of about the same magnitude as the previously mentioned temperature rise due to the endothermic heat of desorption. Also, as known in the art, such a temperature decrease during desorption can be undesirable since as noted above, the equilibrium loading of many adsorbates, e.g., ethane, is reduced at higher temperatures. Accordingly it would be desirable to reduce the temperature decreases often observed during desorption. However, as can be discerned from the above, the actual thermal behavior of the PSA system runs counter to what is desired Nevertheless, a variety of processes have been proposed for purifying methane that utilize pressure swing adsorption. U.S. Patent No. 3,594,983, discloses a process for the removal of ethane and other hydrocarbons in a natural gas feedstream, i.e., methane, using a process that employs both pressure swing and thermal swing adsorption. The patentees recognized that zeolitic molecular sieves, e.g., 13X and 4A, were suitable for use in the process due to their high affinity for the impurity components. The solution to the above-described thermal problem associated with methane coadsorption was to incorporate the thermal swing step into the adsorption process. U.S. Patent No. 4,775,396 discloses a PSA process for the bulk separation of CO₂ from methane, e.g., landfill gas. The patent discloses that for purification processes, CO₂ can be effectively removed from gas mixtures containing same using the calcium ion-exchanged form of zeolite A, but because of the strong affinity between the sorbent and adsorbate, thermal energy is required for effective desorption of the CO₂. However, for the bulk removal of CO₂ from methane, the patent discloses that PSA can be effective when using faujasite type of zeolitic aluminosilicate containing at least 20 equivalent percent of at least one cation species selected from the group consisting of zinc, rare earth metal, hydrogen and ammonium and containing not more than 80 equivalent percent of alkali metal or alkaline earth metal cations. Japanese Patent No. 1039163 discloses a process for the purification of methane by the removal of ethane from a methane-containing feedstream that does not require the use of a thermal swing regeneration step. The patent discloses a PSA process that employs the use of silica gel as the adsorbent. WO-A-8002558 discloses a process for separating ethane from methane by passing a gaseous mixture of the ethane and methane under a pressure of at least 1 bar and temperature 270-300K through a bed of an adsorber having an effective molecular sieve size from 4-10Å until the methane concentration per unit volume of gas leaving the bed is at least as great as the methane concentration per unit volume of the mixture passing into the bed, reducing to zero the pressure of the mixture and collecting the gas which is released from the bed next after a complete cessation of methane release. It is desirable to provide both high methane recoveries, i.e., minimum loss of product component, and high purity, i.e., low ethane content. Generally, however, there is an inverse relationship between purity and recovery and as such, processes have been operated to provide a high purity product at low recoveries or a low purity product at high recoveries. For example, it is not uncommon to obtain less than 50% methane recovery when purities are maintained at about 300 ppm ethane or less. Even at about 1000 ppm ethane in the product methane, typical processes may only achieve about 55% methane recovery. Accordingly, processes for the purification of methane are sought which can provide a high purity product at higher recoveries than heretofore possible. SUMMARY OF THE INVENTION By the present invention, a PSA process is provided for methane purification that can yield a high purity product at higher recoveries than heretofore possible. The process employs the use of a zeolitic molecular sieve adsorbent that can effectively control the thermal problems associated with methane coadsorption yet provide suitable separation of impurity adsorbates. In a broad aspect of the present invention there is provided a PSA process for the separation of ethane from a feedstream ethane and methane. The process includes the steps of a) passing the feedstream at an upper adsorption pressure to an adsorption zone containing a zeolitic molecular sieve of the faujasite type having a framework SiO₂/Al₂O₃ molar ratio of from 2:1 to 100:1 and containing (1) at least 20 equivalent percent of a cation species selected from zinc, rare earth metal, hydrogen, ammonium, calcium or mixtures thereof, and (2) not more than 80 equivalent percent of an alkali or alkaline earth metal cation other than calcium or mixtures thereof, wherein ethane is adsorbed and recovering an adsorption effluent stream enriched in methane relative to the feedstream; b) regenerating the adsorption zone by depressurizing the adsorption zone to a desorption pressure that is lower than the adsorption pressure, and recovering a desorption effluent stream containing ethane; and c)repressurizing the adsorption zone to the adsorption pressure and returning to step a). In another specific aspect of the present invention there is provided a pressure swing adsorption process for the separation of ethane from a feedstream containing ethane and methane. The process comprises the steps of a) passing the feedstream at an adsorption pressure of from 690 to 3448 kPa (100 to 500 psia) to one of at least four adsorption zones containing a zeolitic molecular sieve of the faujasite type having a framework SiO₂/Al₂O₃ molar ratio of from 2:1 to 100:1 and containing (1) at least 20 equivalent percent of a cation species selected from zinc, rare earth metal, hydrogen, ammonium, calcium or mixtures thereof, and (2) not more than 80 equivalent percent of an alkali or alkaline earth metal cation other than calcium or mixtures thereof; wherein ethane and methane are adsorbed and recovering an adsorption effluent stream enriched in methane relative to the feedstream, b) cocurrently depressurizing the adsorption zone and passing the effluent stream therefrom to another adsorption zone undergoing a repressurizing step as hereinafter described and equalizing the pressures therebetween; c) further cocurrently depressurizing the adsorption zone and passing the effluent stream therefrom as a purge gas stream to another adsorption zone undergoing a purging step as hereinafter described; d) countercurrently depressurizing the adsorption zone to a lower pressure of from about 101 to 345 kPa (14.7 to 50 psia) and recovering a desorption effluent stream comprising ethane and methane; e) countercurrently purging the adsorption zone with the purge gas stream comprising the cocurrent depressurization effluent stream of step (c) from another adsorption zone and recovering a countercurrent purge effluent stream comprising ethane; f) repressurizing the adsorption zone by passing the cocurrent depressurization effluent stream of step (b) from another adsorption zone to the adsorption zone to equalize the pressures therebetween; and, g) further repressurizing the adsorption zone by passing a portion of the adsorption effluent stream from another adsorption zone thereto. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the recovery and purity for the separation of ethane from methane. Figure 2 is a schematic flowsheet of a four bed PSA system. DETAILED DESCRIPTION OF THE INVENTION The process of the present invention is useful for purifying methane-containing streams. In general, typical feedstreams will contain water up to saturation levels, less than 25 mol.% C₂-C₅ hydrocarbons, less than 30 mol.% carbon dioxide and less than 2 mol.% C₆ and higher hydrocarbons. Natural gas is a common source of such impurity-containing methane, and in natural gas the hydrocarbon impurities are primarily the saturated type such as ethane and propane. Additionally, natural gas can contain other components such as nitrogen, helium and argon, although such other components are not appreciably adsorbed in the process of the present invention. Preferably, the feedstream will contain from about 70 to 98 mol.% methane, from about 0.1 to 10 mol.% ethane, from about 0.1 to 10 mol.% C₃+ hydrocarbons and from about 0.1 to 20 mol.% CO₂. More preferably, the feedstream will comprise from about 75 to 95 mol.% methane, from about 0.5 to 10 mol.% ethane, from about 0.1 to 10 mol.% CO₂, and from about 0.1 to 5 mol.% C₃+ hydrocarbons. The process is of the general PSA type wherein the methane feedstream is introduced at the highest pressure to the inlet end of an adsorption zone and the impurities are selectively adsorbed in each of at least two sequentially operated adsorption zones. Impurity-depleted methane is discharged from the adsorption zone so that impurity adsorption fronts are formed in the zone at the feedstream inlet end and progressively move toward the purified methane discharge end. Preferably, the impurity-depleted methane product recovered as an adsorption effluent contains less than about 2000 ppm of ethane and more preferably less than 300 ppm. The feedstream flow is terminated when the impurity adsorption fronts are at a predetermined point between the zone inlet and discharge ends. Impurity-depleted methane gas is then preferably released from the adsorption zone discharge end thereby cocurrently depressurizing the adsorption zone to lower pressure in one or more stages. The adsorption zone is then countercurrently depressurized to the lowest desorption pressure to remove the adsorbed impurities and adsorbed methane. The depressurized zone is purged of the impurities by flowing one part of the impurity-depleted methane gas from another adsorption zone countercurrently therethrough from the discharge end to the inlet end. The purged zone is at least partially repressurized by another adsorption zone prior to the introduction of the feedstream to the inlet end. The term "cocurrent" denotes that the direction of gas flow is cocurrent to the direction of gas flow during the adsorption step. Similarly, the term "countercurrent" denotes that the gas flow is countercurrent to the directon of gas flow during the adsorption step. The faujasite type of zeolite employed in the practice of the present invention can be either of the type X or the type Y. Zeolite X and the method for its preparation is described in detail in U.S. Patent No. 2,882,244. The SiO₂/Al₂O₃ molar ratio of zeolite X is from 2:1 up to 3:1. In the as synthesized form, zeolite Y has a SiO₂/Al₂O₃ molar ratio of from greater than 3:1 up to 6:1. The method for synthesizing zeolite Y is disclosed in detail in U.S. Patent 3,130,007. The forms of zeolite Y which contain molar SiO₂/Al₂O₃ values greater than 6:1 can be prepared by several dealumination techniques well known in the art. For example, high temperature steaming treatments which result in dealumination are reported by P. K. Maher et al. in MOLECULAR SIEVE ZEOLITES, Advan. Chem. Ser. 101, American Chemical Society, Washington, D.C., 1971, p. 266. A more recently reported procedure, especially useful for increasing SiO₂/Al₂O₃ of zeolite Y, involves dealumination and the substitution of silicon into the dealuminated lattice sites. This process is disclosed in U.S. Patent No. 4,503,023. As used herein, the term "faujasite type of structure" means the framework structure, irrespective of chemical composition, distribution of the different T-atoms, cell dimensions and symmetry, designated as "FAU" in the ATLAS OF ZEOLITE STRUCTURES TYPES, W. M. Meier and D. H. Olsen, Published by the Structure Commission of the International Zeolite Association (1978). In order to be useful in the process of the present invention, the faujasite zeolite must be treated in order to have a framework SiO₂/Al₂O₃ molar ratio of from 2:1 to 100:1 and containing (1) at least 20 equivalent percent of cation species selected from zinc, rare earth metal, hydrogen, ammonium, calcium or mixtures thereof, and (2) not more than 80 equivalent percent of an alkali or alkaline earth metal cation other than calcium or mixtures thereof. Preferably, the framework SiO₂/Al₂O₃ ratio of the zeolitic molecular sieve is from 2:1 to 20:1 and the zeolitic molecular sieve contains at least 20 equivalent percent of one or a mixture of two or more of zinc, rare earth metal, hydrogen, ammonium and calcium cations. It is further preferred that the zeolitic molecular sieve contains at least 40 equivalent percent of a zinc, rare earth metal, hydrogen, ammonium, calcium cation or mixtures thereof, and less than 40 equivalent percent of an alkali or alkaline earth metal cation other than calcium or mixtures thereof. When Zeolite X is employed as the adsorbent, it is preferred that zinc cations be used as an ion-exchange cation When Zeolite Y is employed as the adsorbent, it is preferred that calcium cations be used as an ion-exchange cation. The various ion-exchange techniques and the like which can be used to prepare the faujasite zeolites of the present invention are well known to those skilled in the art and need not be further described herein. In addition to the faujasite zeolite, it may be desirable in certain instances to employ another adsorbent in the adsorption zone in order to remove some of the feedstream impurities prior to separating ethane from methane with the faujasite zeolites. For instance, it may be desirable to utilize an adsorbent bed of silica gel, activated carbon or alumina, or combinations thereof, to adsorb the C₃+ hydrocarbon fraction or water. The use of such adsorbents and appropriate selection thereof can be determined by those skilled in the art. Those skilled in the art will further recognize that other adsorbents in addition to those specifically enumerated above can be used in accordance with the present invention in combination with the faujasite zeolite adsorbent. In accordance with the present invention, the adsorption pressure is generally from 345 to 6895 kPa (50 to 1000 psia) and preferably from 690 to 3448 kPa (100 to 500 psia). The desorption pressure is generally from 6.9 to 690 kPa (1 to 100 psia) and preferably from 101 to 345 kPa (14.7 to 50 psia). Suitable operating temperatures are generally within the range of from 10 to 66°C (50 to 150°F). There can be a variety of cocurrent depressurization steps to intermediate pressures, cocurrent purging steps and countercurrent purging steps, all of which are well known to those skilled in the art and described in the previously cited patents relating to PSA processes. For example, three or more of such cocurrent depressurization steps can be employed for pressure equalization to further improve product recovery such as disclosed in U.S. Patent No. 3,986,849. In addition, the purge step can be performed either prior to, simultaneously with, or subsequently to the regenerating or desorption step by passing a methane-containing stream countercurrently through the adsorption zone and recovering a countercurrent purge effluent stream comprising ethane. The methane-containing stream can comprise a portion of the adsorption effluent product stream However, it is generally preferred that at least a portion of one or more of the cocurrent depressurization effluent streams be employed as the methane-containing stream, i.e., purge gas. Moreover, the process of the present invention may be practiced with essentially any adiabatic pressure swing process such as the multi zone adsorption systems described in U.S. Patent No. 3,986,849, the four adsorption zone systems described in U.S. Patent Nos. 3,430,418 and 3,564,816, the three adsorption zone systems described in U.S. Patent No. 3,636,679, and the two adsorption zone systems described in U.S. Patent No. 3,738,087. The selection of the appropriate PSA process configuration and operating conditions can be determined by those skilled in the art. For instance, those skilled in the art will recognize that performing the adsorption step at an excessively high pressures, i.e., which depend upon the particular system and can be determined by experimentation, can produce non-self-cleaning conditions and a large, abrupt increase in C₂+ content in the product. As used herein, the term "self cleaning" is used in connection with impurity enrichment. Self-cleaning is evidenced by a stable impurity front which does not progressively encroach upon the product end of the adsorption zone as numerous repetitive cycles proceed. If the adsorption zone is not self-cleaning with respect to an impurity, then the impurity will eventually appear in the effluent stream from the product end of the adsorption zone and upon continued cycling, its concentration in the product will gradually increase. In general, operating at lower pressures during adsorption favors a product of higher purity (lower C₂+ content). However, as the adsorption pressure is reduced relative to the purge pressure, differential loading on the adsorbent decreases and adsorption zone size tends to increase (or cycle time must be shorter). This increases the fraction of product lost in blowdown and required for purging, and recovery of methane is reduced. The process of the present invention will hereinafter be described with reference to the drawings. Figure 1 illustrates the recovery and purity relationship for the separation of ethane from methane. The shaded area, designated curve A, was reproduced from Figure 5 of Japanese Patent No. 1039163 and represents the typical variation of product purity with methane recovery observed by the patentees using silica gel adsorbent. The data represented by the round data point show typical performance obtained with the prior art silica gel adsorbent when separating a feedstream containing about 6 mol.% ethane, balance methane, using silica gel adsorbent in a PSA test cycle having the steps of; adsorption at a pressure of 896 kPa (130 psia), cocurrent depressurization to a pressure of 483 kPa (70 psia), cocurrent depressurization to a pressure of 228 kPa (33 psia), cocurrent depressurizaton to a pressure of 179 kPa (26 psia), countercurrent depressurization to a pressure of (110 kPa (16 psia), countercurrent purge with product methane and repressurization to 896 kPa (130 psia). A single bed adsorption zone was used for the testing and bad a length of 1.8 m (5.9 ft.) and an internal volume of 0.03 ft³, i.e., about 850 cc. The adsorption zone was insulated to approximate adiatatic operation. The adsorption zone was loaded with commercially available silica gel adsorbent. The data represented by the square data point illustrates the unexpectedly enhanced results by replacing the 850 cc silica gel adsorption zone with a zinc-exchanged zeolite X containing about 84 equivalent percent Zn⁺² cations in accordance with the present invention. The PSA test cycle and testing unit were the same as described above. It can be seen that for an ethane level of about 300 ppm in the product gas, the methane recovery was about 29% higher, i.e., 66% versus 51%, than with the silica gel. Enhanced results were also observed when about 25 vol.%, i.e., about 212 cc, of the silica gel adsorbent from the effluent end of the adsorption zone was replaced with zinc-exchanged Zeolite X. The PSA test cycle and testing unit were the same as described above. At 300 ppm ethane in the product gas, the recovery of methane was about 60% for the combined silica gel - ZnX adsorption zone. Similarly, enhanced results were also observed when about 25 vol.% of the silica gel adsorbent from the effluent end of adsorption zone was replaced with calcium-exchanged zeolite Y containing about 90 equivalent percent calcium cations. At 300 ppm ethane in the product gas, the recovery of methane was about 57% for the combined silica gel - CaY adsorption zone. By extrapolation of the data obtained from the test wherein a combined adsorption zone of silica gel and zinc-exchanged zeolite X was used, it is expected that an adsorption zone containing 100 vol.% calcium-exchanged zeolite Y would have between about 61-63% methane recovery. Hence, it is possible to obtain a substantial improvement in performance even when only a portion of the adsorption zone contains the adsorbent material of the present invention. The data represented by the triangular data point is presented for comparison purposes and illustrates the expected results using a faujasite that is not cation exchanged in accordance with the present invention. As determined by mathematical simulation of a PSA cycle as described above, the expected performance using a sodium-exchanged zeolite X, i.e., 13X, is about the same or worse than that of silica gel. Figure 2 is a schematic flowsheet of a four-adsorption zone PSA system With reference to Figure 2, the four adsorption zones are loaded with a zinc cation-exchanged zeolite X adsorbent The four adsorption zones are arranged in parallel between feedstream manifold 10 and product manifold 11. Feed to, and product from the adsorption zones are controlled respectively by automatic valves 1A-D and 2A-D. A high P pressure equalization step is accommodated by connecting conduits 15 and 16 containing respectively automatic valves 4AB and 4CD. Cocurrent depressurization for purge is accomplished through manifolds 17 and 18 containing respectively automatic valves 5A-B and 5C-D together with cross-over conduit 20 containing manual trim valve 22. A low P pressure equalization step is accomplished through cross-over conduit 21 which also connects manifolds 17 and 18 and contains automatic valve 19. Flow from the adsorption zones to waste manifold 12 is controlled by automatic valves 3A-D. Product for final repressurization of the adsorption zones returns through conduit 27 containing regulating valves 23 and 28, then flows through repressurization manifold 29 containing check valves 30 and 31, and finally passes through one of manifolds 32 and 33 containing respectively automatic valves 6A-B and 6C-D. The full cycle will be described for Adsorption zone A and is typical for all adsorption zones. Pressures and times are illustrative. Assume adsorption zone A is pressurized and that all its associated valves are initially closed. Valves 1A and 2A open and feedstream at 157 psia flows from feedstream manifold 10 through heater 40 and manifold 41 to the adsorption zone, and product flows from the adsorption zone into manifold 11. Flow continues at steady feedstream pressure for 5 minutes. Now valves 1A and 2A close and valve 4AB opens, thereby establishing flow between adsorption zone A and adsorption zone B which has been partially repressurized and is initially at 30 psia pressure. The adsorption zones equalize in pressure at 97 psia in 0.75 minute. It is important to note that due to pressure drop through the apparatus, the actual pressure in the two adsorption zones may, in fact, not be exactly equal. Differences in the actual pressures are expected and are within the scope of the present invention. Next, valve 4AB closes and valves 5A, 5C, 3C and 26 open to establish flow between adsorption zones A and C through manifold 20. Adsorption zone C has just completed countercurrent blowdown and is now purged by gas from adsorption zone A which is throttled by valve 22 to about 23 psia. Purge continues for 3.5 minutes when adsorption zone A pressure drops to 37 psia. The terminal pressure is sensed by a pressure switch PS-C which is actuated to close valve 3C and open valve 19. Pressure switches PS-A, FS-B and FS-C are also provided to be operated in conjunction with adsorption zones A, B and D respectively. Flow continues from adsorption zone A to adsorption zone C but now adsorption zone C is dead-ended so that pressures equalize at 30 psia in 0.75 minute. Valves 5A, 5C, 19 and 26 close and valve 3A opens, thus releasing residual pressure in adsorption zone A countercurrently through waste manifold 12. The final pressure, 23 psia, is set by a regulating valve (not shown) downstream in waste conduit 12, which delivers the gas to a fuel header. Valve 26 is a resistance valve rather than a shut-off valve and when closed, imposes a flow restriction which prevents excess flow velocities in adsorption zone A. Countercurrent depressurization is complete in 0.75 minute after which, valve 26, 5A and 5D open. This allows purge gas to flow from adsorption zone D through manifold 20 to adsorption zone A at about 23 psia, then through waste manifold 12. Purge continues for 3.5 minutes. Adsorption zone A has completed its adsorption phase, its product recovery phase and its desorption phase. It is now ready to begin a three-step repressurization sequence. Valve 3A closes and flow from adsorption zone D continues but with adsorption zone A dead-ended so that pressures equalize at 30 psia in 0.75 minute. Valve 6A also opens to simultaneously permit product from manifold 11 to return through valves 23 and 28 to adsorption zone A. Now valves 5A and 5D close and valve 4AB opens. This establishes communication with adsorption zone B which initially is at feedstream pressure (157 psia), and the two adsorption zones equalize at 97 psia in 0.75 minute. Finally, valve 4AB closes and only product from manifold 11 continues to flow to adsorption zone A. The pressure in adsorption zone A reaches substantially feedstream pressure (157 psia) in 4.25 minute. This completes a full 20-minute cycle for adsorption zone A which is now ready to receive the feedstream for the adsorption stroke by closing valve 6A and opening valves 1A and 2A. The cycle for adsorption zone A is typical for all adsorption zones A-D, and the adsorption zones are placed on adsorption sequentially in 1/4-cycle phase relationship such that the feedstream and product flows are continuous.

Claims: A pressure swing adsorption process for the separation of ethane from a feedstream comprising ethane and methane, said process comprising the steps of: a) passing the feedstream into an adsorption zone maintained at adsorption conditions including an upper adsorption pressure and containing a zeolitic molecular sieve adsorbent of the faujasite type having a framework SiO₂/Al₂O₃ molar ratio of from 2:1 to 100:1 and containing at least 20 equivalent % of a cation selected from zinc, rare earth metal, hydrogen, ammonium, calcium or mixtures thereof, and containing not more than 80 equivalent % of an alkali or alkaline earth metal cation other than calcium or mixtures thereof, wherein ethane is adsorbed and recovering an adsorption effluent stream enriched in methane relative to the feedstream; b) regenerating the adsorption zone periodically by depressurizing the adsorption zone to a lower desorption pressure and recovering a desorption effluent stream comprising ethane; and thereafter, c) repressurizing the adsorption zone to the upper adsorption pressure; and returning to step a). The process of Claim 1 wherein said depressurizing in step b) is performed in a direction countercurrent to the flow of feedstream in step a). The process of Claim 1 or 2 where step b) includes one or more cocurrent depressurization steps. The process of Claim 1, 2 or 3 where a countercurrent purge step is performed either prior to, simultaneously with, or subsequently to said regenerating step and where the purge step comprises passing a methane-containing purge gas countercurrently through said adsorption zone and recovering a countercurrent purge effluent stream comprising ethane. The process of any one of Claims 1 to 4 where the zeolitic molecular sieve contains at least 40 equivalent % of zinc or calcium cations. The process any one of Claims 1 to 5 where the adsorption zone further comprises a second adsorbent selected from the group consisting of silica gel, alumina, and activated carbon. The process of any one of Claims 1 to 6 wherein the upper adsorption pressure is from 690 to 3448 kPa (100 to 500 psia) and wherein the lower desorption pressure is from 101 to 345 kPa (14.7 to 50 psia).

Inventors: MAURER RICHARD T, MAURER, RICHARD T.
Applicants: UOP INC, UOP
Application year: 1991
Main classification: C07C9
IPC codes: B01D53, C07C7