APPARATUS AND METHOD FOR SEPARATING OLEFINS FROM AN OLEFINS-RICH PRODUCT STREAM

Abstract: The present invention provides a method and apparatus for separating olefins from an olefins-rich product stream comprising producing a product stream comprising at least about 60 % olefins passing said product stream through an olefin selective facilitated transfer membrane, and immediately thereafter collecting a permeate stream comprising about 0.2 wt.% saturates or less. A key advantage of the invention is that a demethanizer or a deethanizer is not required to recover the second olefins-rich product stream.

Description: APPARATUS AND METHOD FOR SEPARATING OLEFINS FROM AN OLEFINS-RICH PRODUCT STREAM This application claims priority to U.S. Provisional Patent Application Number 60/033,949 filed December 31 , 1996. Field of the Invention The present invention relates to a simplified system for separating olefins from non-olefins in a product stream. More particularly, the invention relates to the use of a facilitated transport membrane to separate olefins from an olefins-rich product stream. Background of the Invention Olefins, such as ethylene and propylene, are important petrochemical products. Olefins and a myriad of byproducts, such as ethane, propane, hydrogen, and carbon monoxide, among others, are produced in a single stream obtained from an olefins generation/preparation process. Before the olefins can be sold, the olefins must be separated from these myriad byproducts. Olefin products generally are required to have greater than about 98 wt% purity, and to contain substantially inert components, such as methane and ethane, in proportions less than about 2000 molar ppm each. Potentially reactive components, such as hydrogen, carbon monoxide, carbon dioxide, and others, can be present only in proportions less than about 20 molar ppm each. There are advantages to employing processes that are highly selective in producing light olefins, i.e., olefins with carbon numbers of 2 to 4 and most preferably of 2 and 3. One example of such a process is known as the methanol to olefins process, an example being taught in Kaiser, US-A- 4,499,327, incorporated herein by reference. This process has promise as an effective and efficient alternative to conventional hydrocarbon pyrolysis, or steam cracking, to produce olefins. However, both pyrolytic product streams and methanol to olefin product streams still must be separated into olefins-rich product streams from which byproducts are largely removed. Because ethylene has a very low normal boiling point, very low temperature vapor-liquid flash and fractional distillation techniques are required to obtain a high recovery of ethylene in a product stream. The equipment required to perform these very low temperature processes- particularly to separate ethylene from components which have even lower boiling points, such as hydrogen, carbon monoxide, and methane-is very expensive. Many ethylene recovery and separation processes require temperatures as low as -215 °F in certain equipment. Special metallurgies and refrigeration systems, known as the chill train, the demethanizer tower, and a substantially dedicated refrigeration system, are required to operate equipment at such low temperatures. The cost of purchasing and running the chill train, the demethanizer tower, and related low temperature equipment, accounts for much of the total capital cost and energy consumption of ethylene manufacture. More cost efficient methods are needed to economically and efficiently separate olefins from non-olefins in such product streams. Facilitated transport membranes have been used successfully to separate saturated from unsaturated hydrocarbons in pyrolytically processed hydrocarbon streams. However, the preferential passage of unsaturated hydrocarbons through such membranes is facilitated by metal ions or salts, which are likely to also preferentially bind with components found in a methanol to olefin stream, such as carbon monoxide, which are not found in a pyrolytically cracked stream. It previously has not been established whether facilitated transport membranes can be used to separate olefins-rich product streams, such as methanol to olefin product streams. SUMMARY OF THE INVENTION The present invention provides a method and apparatus for separating olefins from an olefins-rich product stream comprising producing a product stream comprising at least about 60% olefins, passing said product stream through an olefins selective facilitated transfer membrane, and immediately thereafter collecting a permeate stream comprising about 0.2 wt% saturates or less. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a system incorporating facilitated membrane separation of an olefins-rich product stream according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a simplified separation scheme using a selective, facilitated transport membrane to reject saturated hydrocarbons from unsaturated hydrocarbons in an olefins-rich product stream. In a preferred embodiment, the olefins-rich product stream comprises at least about 60% olefins. Many methods are under development as alternative methods for producing olefins. The present invention should be useful in conjunction with any of such methods. A preferred olefins-rich product stream for use in the present invention is a methanol to olefin product stream. A preferred embodiment of an "apparatus" for practicing the invention is illustrated in Figure 1. The term "apparatus" typically is used in the context of a single device; however, for purposes of the present invention, the term "apparatus" is defined to encompass several systems. Referring to Figure 1, an olefins-rich product stream 11, preferably produced by a methanol to olefin system 10, is fed into the apparatus of the present invention. Preferably, the product stream 11 is first subjected to a water removal system 12. Suitable water removal systems include, but are not necessarily limited to compression followed by expansion or adsorption on alumina or molecular sieve drying agents. An additional consideration is that the membrane performance is enhanced by the presence of some amount of water vapor in the stream to be separated. Thus, in this water removal step, it may be advantageous to remove only part of the water while removing substantially the remaining water subsequent to the membrane separation step. Water removal is helpful, but is not absolutely essential to the operation of the invention. The product stream A is passed to an olefins selective membrane module 14. In a preferred embodiment, the olefins selective membrane module 14 comprises two chambers separated by an olefins selective facilitated transfer membrane 15 such as that described in US-A-5,062,866, incorporated herein by reference. The olefins-rich product stream 11 flows into a retentate chamber 13 in contact with the membrane 15. The membrane 15 preferably is oriented so that the greatest surface area possible is exposed to the product stream. The olefins in the product stream held in the retentate chamber 13 preferentially cross the membrane into the permeate chamber 14 producing an olefins-rich permeate stream comprising about 0.2 wt% saturates or less, preferably about 0.1 wt% saturates or less. This leaves a saturate rich retentate in the retentate chamber 13, which preferably is sent for suitable disposition, such as to a burner or steam cracker or to an additional separation stage. Laboratory tests demonstrated that carbon dioxide and carbon monoxide were transferred through the membrane somewhat faster than alkanes, but that such transport was 5 to 80 times slower than the preferential transport of olefins. The olefins-rich permeate stream 16 is sent through an adsorber 18 or a series of adsorbers to remove residual carbon monoxide, water, and other byproducts. Suitable adsorption agents include, but are not necessarily limited to molecular sieves and alumina. Alternately, the permeate stream 16 may be treated in a reactive separation step to effect the removal of these by-products. Thereafter, the olefins-rich permeate stream 16 is sent to a cryogenic separator 20. This separation actually may be a single cryogenic separation device such as a C2-C3 splitter or a series of separation steps depending upon the particular make up of stream 16. One example of a series of devices is a deethylenizer followed by a C3 splitter to cryogenically separate the C3= from heavier olefins by distillation. Depending upon the specific economics, further separation and purification of the heavy fraction E from this second cryogenic tower 24 may or may not be advisable. A key feature of the invention is that neither a demethanizer nor a deethanizer is required in order to recover the final olefins product from the olefins-rich product stream. The separation of ethane from ethylene and of propane from propylene is thus much easier and much less expensive than when a deethanizer and/or a demethanizer is required. For puφoses of the present invention, the term "olefins selective facilitated transfer membrane" is defined as follows. The olefins selected facilitated transfer membrane is prepared from at least one hydrophilic polymer associated with a complexing metal ion or salt and a hydrophilic salt of a Group I metal. Suitable polymers for preparing the membranes include, but are not necessarily limited to polyvinyl alcohol, polyvinyl acetate, sulfonyl- containing polymers, polyvinylpyrrolidone, polyethylene oxide, and polyacrylamide, as well as blends of two or more of these polymers, and copolymers of these polymers. A preferred polymer is polyvinyl alcohol. Preferably, the polymers should be capable of undergoing cross- linking in the presence of an effective amount of cross-linking agent under suitable conditions. The polymer matrix also comprises metal ions or salts which can reversibly complex with aliphatically unsaturated hydrocarbons. The metal ion or salt preferably should be non-reactive, or at least not significantly more reactive than other components in a methanol to olefin product stream than with unsaturated hydrocarbons in order to maintain selectivity of the membrane for the unsaturated hydrocarbons, as well as to avoid fouling of the membrane with undesired reaction by-products. Suitable metals, which may be used alone or in various combinations, in the presence or absence of non-metal or non-complexing metal ions, comprise the transition metals of the Periodic Table of the Elements having atomic numbers above 20. For example, useful metals are those of the first transition series having atomic numbers from 21 to 29, such as chromium, copper, manganese, and the iron group metals, such as nickel and iron. Other useful complex-forming metals are in the second and third transition series, i.e., having atomic numbers from 39 to 47 or 57 to 79, such as molybdenum, tungsten, and rhenium, as well as mercury. The noble metals such as silver, gold, and platinum group, among which are platinum, palladium, rhodium, ruthenium, and osmium, are also suitable. Preferred metals are the noble metals, particularly silver, which may be present in the polymer as the ion (preferably in the +1 state, Ag ) or as a metal salt, such as AgN03, Ag(NH3)2 + , or Ag(-pyridine)2 + . The amount of complex-forming metal present in the polymer may vary, but should be sufficient to accomplish the desired separation, and preferably sufficient to provide an adequate complexing rate with minimal membrane surface area. The membranes may be made by first forming a polymer solution comprising one or more of the foregoing polymers and at least one of the foregoing metal ions or salts in a solvent, preferably water. In a preferred embodiment, the polymer solution further comprises a cross-linking agent in an amount effective to promote cross-linking of the polymer under suitable conditions. Suitable cross-linking agents include, but are not necessarily limited to, formaldehyde, divinyl sulfone, toluene diisocyanate, glyoxal, trimethylol melamine, terephthalaldehyde, epichlorohydrin, vinyl acrylate, and maleic anhydride. Formaldehyde, divinyl sulfone and toluene diisocyanate are preferred. Also in a preferred embodiment, the polymer solution comprises an effective amount of at least one compound selected from hydrophilic salts of metals of Group I of the Periodic Table of the Elements. Suitable salts include, but are not necessarily limited to, sodium nitrate, sodium methyl sulfonate, potassium nitrate, potassium methyl sulfonate, and lithium nitrate. Preferably, the same anion should be present in both the transition metal complex and in the Group I metal salt. In a fabricated membrane, such salts preferably comprise between about 1 to 8 wt%. The presence of salts of Group I metals in the membrane is believed to increase hydrated water content in the membrane, which may enhance diffusion of the transition metal-olefin complex and accompanying complexing and decomplexing with the olefin. In particular, Group I metal nitrates are believed to assist in maintaining the transition metal complex in an oxidation state which is effective for complexing with the olefin. The polymer solution should be cast onto a solid support using casting techniques known in the art, for example, "knife-casting" or "dip-casting." When a cross-linking agent is present in the polymer solution, the membrane after casting is treated under temperature conditions sufficient to effect cross- linking of the polymer but insufficient to reduce the hydrocarbon-complexing metal ion or salt. A temperature in the range of about 60-80°C maintained for about one to five days, preferably a temperature of about 75 C C maintained for about three days, should be sufficient. The resulting membranes are "solid, homogenous" membranes in the sense that they comprise a single polymeric phase in the substantial absence of a liquid phase. The membranes may be as thin as 0.1 μm, but preferably should be at least about 0.5 μm, and most preferably between about 1 μm and about 15 μm. Olefins having from 2 to about 20 carbon atoms per molecule, preferably 2 to 8 carbon atoms, and most preferably 2 to 3 carbon atoms per molecule may be separated from methanol to olefin product streams according to the present invention. The feedstream should be contacted with a first side of the membrane of the invention and a permeate collected at a second side of the membrane comprising the aliphatically unsaturated hydrocarbon in increased concentration relative to the feedstream. By "permeate" is meant that portion of the feedstream that is withdrawn at the second side of the membrane, exclusive of sweep gas and other fluids. In a preferred embodiment, the olefins of the feedstream are in the vapor phase, and a driving force for permeation is maintained by a partial pressure differential across the membrane. The partial pressure differential preferably should be in the range of about 0.01 atm (0.15 psi) to 55 atm (809 psi), and more preferably about 0.5 atm (7.4 psi) to 30 atm (441 psi). The partial pressure differential is preferentially established by compression of the feedstream to an elevated pressure and drawing the olefins rich product stream off at a reduced pressure. Alternatively, the differential partial pressure may be aided by the application on the permeate side of vacuum, sweep gas or sweep liquid. If a sweep gas or liquid is employed, the gas or liquid is preferably essentially inert with respect to the metal ions in the membrane and is easily removed from the olefin rich permeate stream. It may be preferable to introduce the sweep gas or liquid in a countercurrent fashion. Suitable sweep flluids include but are not necessarily limited to C4 + olefins or paraffins or mixtures thereof. It will be appreciated by persons of skill in the art that the inclusion of a sweep fluid will necessitate the inclusion of an additional separation step or increased duty on an already included separation. In another, non-preferred embodiment of the invention, the feed is maintained under conditions of temperature and pressure such that substantially all of the olefins in the feedstream are in the liquid phase, and the unsaturated hydrocarbon is recovered under vacuum in the vapor phase (i.e., pervaporated) at the permeate side of the membrane. The vacuum on the permeate side of the membrane can range between about 1 mm Hg to about 750 mm Hg (0.99 atm or 14.5 psi) at room temperature. This embodiment is not preferred because of the low temperatures, and concomitant cost, that would be required to maintain ethylene and/or propylene-preferred products of the present process-in a liquid phase. The membrane may be in the form of hollow fibers, tubes, films, sheets, etc. The process is conveniently carried out in a permeation cell divided into compartments by a membrane, with the compartments having means for removing the contents therefrom. The specific design and configuration of the permeation cell may vary according to individual requirements of capacity, flow rate, etc. The process may be carried out continuously or batchwise, in a single or multiple stages, but preferably in a continuous manner. The invention will be better understood with reference to the following examples: EXAMPLE I A gas mixture containing components representative of a methanol to olefin product stream after water removal, as shown in the following Table, was applied to a first side of a 5 micron thick, cross linked polyvinyl alcohol membrane saturated with AgNθ3 (Ag/PVA). The following were the relative rates of transfer of the various components: The relative rate of transport of the alkenes examined was roughly 1000 times faster than for the alkanes. While CO2 and CO were transported somewhat faster than the alkanes, they were still, respectively, 5 or 80 times slower than ethylene or propylene. The net result was that the concentration of methane, ethane, and propane was greatly reduced in the stream passing through the membrane ("permeate"). Given the foregoing, estimated concentrations of the various species on both the olefins-rich side and the olefins-depleted retentate side of the membrane were calculated as follows for three equivalent residence times, based on the flow rate that would be required to separate 100 Ib/hr C2 = from the product stream: The ratio of (methane+ethane)/ethylene is considered to be a "figure of merit" for the separation. In order to achieve very high purity olefins, a higher fraction of the olefins should be retained in the retentate stream. The amount of olefins that will be lost because of this will depend upon both the membrane selectivity, and the initial saturates content of the inlet stream. Based on the particular membrane tested, and based on methanol to olefin yields, only a small fraction of olefins should be lost in this way. EXAMPLE II A methanol to olefin product stream is processed by the system schematically diagramed in Fig. 1 and described in detail above. At a flow rate of 100 Ib hr ethylene, the following are the flow rates of the other products of the system: Flow Rates, LB/HR Persons of ordinary skill in the art will recognize that many modifications may be made to the present invention without departing from the spirit and scope of the present invention. The embodiments described herein are meant to be illustrative only and should not be taken as limiting the invention, which is defined in the following claims.

Claims: We Claim: 1. A method for separating olefins from an olefins-rich product stream comprising: (a) producing a product stream comprising at least about 60% olefins; (b) passing said product stream through an olefins selective facilitated transfer membrane; and (c) immediately thereafter collecting a permeate stream comprising about 0.2 wt% saturates or less. 2. The method of Claim 1 wherein said product stream comprises a methanol to olefin product stream. 3. The method of Claim 1 wherein said permeate stream comprises about 0.1 wt% saturates or less. 4. The method of Claim 1 wherein said permeate stream primarily comprises olefins selected from the group consisting of between about 2 to 20 carbon atoms. 5. The method of Claim 1 wherein said permeate stream primarily comprises olefins selected from the group consisting of between about to 3 carbon atoms. 6. The method of Claim 2 wherein said permeate stream primarily comprises olefins selected from the group consisting of between about 2 to 3 carbon atoms. 7. The method of Claim 3 wherein said permeate stream primarily comprises olefins selected from the group consisting of between about 2 to 3 carbon atoms. 8. A method for separating olefins from an olefins-rich product stream comprising: (a) producing an olefins-rich product stream comprising at least about 60% olefins; (b) passing said product stream through an olefins selective facilitated transfer membrane; and (c) collecting a permeate stream comprising about 0.2 wt% saturates or less in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer. 9. The method of Claim 2 wherein said permeate stream is collected in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer. 10. The method of Claim 3 wherein said permeate stream is collected in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer. 11. The method of Claim 5 wherein said permeate stream is collected in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer. 12. A separation system for separating olefins from an olefins-rich product stream comprising: (a) a first system for producing a first olefins-rich product stream comprising at least about 60% olefins; and (b) an olefins selective facilitated transfer membrane for collecting a second olefins-rich product stream comprising about 0.2 wt% saturates or less. 13. The separation system of Claim 12 wherein said first system comprises a methanol to olefin system. 14. The separation system of Claim 12 devoid of a device selected from the group consisting of a demethanizer and a deethanizer. 15. The separation system of Claim 13 devoid a device selected from the group consisting of a demethanizer and a deethanizer. 16. A method for separating olefins from an olefin product stream comprising: (a) producing a product stream comprising a first concentration of carbon oxides and first concentration of olefins; (b) passing said product stream through an olefins selective facilitated transfer membrane; and (c) immediately thereafter collecting a permeate stream comprising a second reduced concentration of carbon oxides and a second increased concentration of olefins. 17. The method of Claim 16 wherein said second concentration of olefins is about 0.2 wt% or less. 18. The method of Claim 16 wherein said first concentration of olefins is about 60 wt%. 19. The method of Claim 17 wherein said first concentration of olefins is about 60 wt%. 20. The method of Claim 16 wherein said product stream comprises a methanol to olefin product stream. 21. The method of Claim 17 wherein said product stream comprises a methanol to olefin product stream. 22. The method of Claim 16 wherein said permeate stream comprises about 0.1 wt% saturates or less. 23. The method of Claim 16 wherein said permeate stream primarily comprises olefins selected from the group consisting of between about 2 to 3 carbon atoms. 24. The method of Claim 17 wherein said permeate stream primarily comprises olefins selected from the group consisting of between about 2 to 3 carbon atoms. 25. The method of Claim 16 wherein said permeate stream is collected in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer. 26. The method of Claim 17 wherein said permeate stream is collected in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer. 27. The method of Claim 22 wherein said second olefins-rich product stream is collected in the absence of an apparatus selected from the group consisting of a deethanizer and a demethanizer.

Inventors: HALL RICHARD B, HO W S WINSTON, SUN HSIANG-NING, VAUGHN STEPHEN N, HALL, RICHARD, B., HO, W., S., WINSTON, SUN, HSIANG-NING, VAUGHN, STEPHEN, N.
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1997
Main classification: C07C7
IPC codes: C07C7, C07C11