HK1034277A - Process for recovering olefins - Google Patents

Description

Process for recovering olefins

Technical Field

The present invention relates generally to the separation of olefins from olefin-containing gases. More particularly, the present invention relates to an improved process for separating olefins from a gas containing olefins and hydrogen by removing hydrogen from the gas using a combination of membrane separation and pressure swing adsorption techniques.

Background

Olefins such as ethylene, propylene and butylene can be produced by heating saturated hydrocarbons such as ethane, propylene or butane at elevated temperatures. Likewise, naphtha, gas oil and other heavy hydrocarbon feeds may also be thermally cracked in a cracking furnace in the presence of steam to produce olefins.

The cracked effluent produced by heating a saturated hydrocarbon, naphtha or gas oil feed typically contains hydrogen, steam, carbon dioxide, carbon monoxide, methane, ethane, ethylene, propane, propylene and minor amounts of other components, such as heavy hydrocarbons. The cracked effluent is then sent to the product recovery section of the olefin plant.

In the product recovery section, the cracked effluent is compressed in one or more compression stages to partially liquefy the hydrocarbon components for cryogenic distillation separation. Carbon dioxide, water vapor and heavy hydrocarbons must be removed prior to freezing the cracked effluent to prevent freezing and plugging of the equipment. After removing these components from the cracking effluent, the effluent is passed to a cryogenic section (commonly referred to as a "cold box") where the temperature of the effluent is reduced so that the separation of the hydrocarbon components can be carried out by distillation. Passing the hotter portion of the cold box through an ethylene refrigeration cycle; while the colder portion of the cold box provides the refrigeration balance of the cold box by expansion of the exhaust stream.

The distillation section typically has three columns, a demethanizer to remove light components, a deethanizer to remove heavy components, and an ethane/ethylene splitter to separate ethylene products from the ethane recycle stream. The ethylene refrigeration cycle also provides reboiling and condensing duty to the distillation section.

The cold end of the cryogenic section is partially equilibrated with the hydrogen contained in the cracked gas. However, its presence requires cooler temperatures in the distillation section in order to separate the product. In the distillation section, the hydrogen also acts as a ballast, which avoids processing additional quantities of product.

In view of the disadvantages associated with the presence of hydrogen in the cracked effluent, various methods have been proposed for removing hydrogen from the cracked effluent. See, for example, US 5082481, 5452581, and 5634354; the disclosures of which are hereby incorporated by reference. The processes described in these patents include the use of membrane separators to remove hydrogen from the cracked effluent.

However, there are several disadvantages associated with these approaches. For example, unless the disclosed process uses very selective membranes, an indefinite amount of product is lost in the permeate stream. Even when highly selective membranes are used, the hydrogen removal rate may not be high enough to make the process economically viable.

Accordingly, there is a need in the art for a process that minimizes or eliminates product loss in the permeate stream without the need for highly selective membranes. In addition, there is a need in the art for a process that can employ higher hydrogen removal rates without concomitant product loss.

Light olefins may also be produced by the catalytic conversion of feedstocks containing methanol, ethanol, dimethyl ether, diethyl ether or mixtures thereof. See, for example, US 4499327, the entire disclosure of which is hereby incorporated by reference. Such processes are commonly referred to as Methanol To Olefin (MTO) or Gas To Olefin (GTO) processes. In these processes, hydrogen is sometimes used as a diluent, which must be removed from the desired olefin product.

Thus, there is also a need in the art for an economical and efficient process for separating hydrogen from the olefin product streams resulting from these processes.

Summary of The Invention

The present invention meets the above-described need in the art by providing an improved process for recovering olefins from a cracking effluent containing olefins and hydrogen. The process comprises compressing the cracked effluent in at least one compression stage to obtain a compressed cracked effluent; contacting the compressed cracked effluent with a membrane under conditions effective to produce a hydrogen-rich permeate stream and a hydrogen-depleted retentate stream; and feeding the permeate stream to a pressure swing adsorption system under conditions effective to produce a hydrogen-rich non-adsorbed stream and a desorbed stream comprising olefins.

In a preferred embodiment, the present invention relates to a process for recovering olefins and high purity hydrogen from a cracking effluent. The process comprises compressing the cracked effluent in at least one compression stage to obtain a compressed cracked effluent; contacting the compressed cracked effluent with a membrane under conditions effective to produce a hydrogen-rich permeate stream and a hydrogen-depleted retentate stream; compressing the permeate stream in at least one further compression stage to obtain a compressed permeate stream; feeding the compressed permeate stream to a pressure swing adsorption system under conditions effective to produce a non-adsorbed stream comprising high purity hydrogen and a desorbed stream comprising olefins; and recycling the desorbed stream to the at least one compression stage.

More generally, the process of the present invention can be used to separate olefins from an olefin and hydrogen containing gas obtained from any source, including a methanol to olefin process (MTO) or a gas to olefin process (GTO). In this case, the process comprises compressing the gas in at least one compression stage to obtain a compressed gas; contacting the compressed gas with a membrane under conditions effective to produce a hydrogen-rich permeate stream and a hydrogen-depleted retentate stream; and feeding the permeate stream to a pressure swing adsorption system under conditions effective to produce a hydrogen-rich non-adsorbed stream and a desorbed stream comprising olefins.

In a preferred embodiment, the present invention relates to a process for recovering olefins and high purity hydrogen from a gas containing olefins and hydrogen. The method comprises compressing a gas in at least one compression section to obtain a compressed gas; contacting the compressed gas with a membrane under conditions effective to produce a hydrogen-rich permeate stream and a hydrogen-depleted retentate stream; compressing the permeate stream in at least one further compression stage to obtain a compressed permeate stream; feeding the compressed permeate stream to a pressure swing adsorption system under conditions effective to produce a non-adsorbed stream comprising high purity hydrogen and a desorbed stream comprising olefins; and recycling the desorbed stream to at least one compression stage.

Brief description of the drawings

FIG. 1 is a schematic flow diagram of one embodiment of the present invention.

FIG. 2 is a schematic flow diagram of another embodiment of the present invention.

Figure 3 is a schematic flow diagram of a typical ethylene plant using PSA.

Figure 4 is a schematic flow diagram of a typical ethylene plant using a separate PSA and membrane separation system.

FIG. 5 is a schematic flow diagram of an ethylene plant according to the present invention.

Detailed description of the preferred embodiments

In the present invention, it is preferable to use a membrane separator and a Pressure Swing Adsorption (PSA) system in combination. In particular, the permeate stream from the membrane separator, which contains primarily hydrogen and some valuable products such as olefins, is optionally recompressed in one or more compressors and then sent to the PAS system. The PSA system preferentially adsorbs products present in the permeate stream to produce a non-adsorbed stream rich in hydrogen. The adsorbed product is desorbed at low pressure to produce a desorbed stream containing product. The desorbed stream may be recycled to the suction side of the at least one compression stage. Alternatively, the desorbed stream may be compressed in one or more additional compressors and then recycled to the feed side of the membrane separator.

Any membrane can be used in the process of the present invention so long as it is completely permeable to hydrogen and substantially impermeable to hydrocarbons such as ethylene. In addition, the membranes should have good compatibility with the gas to be separated, high structural strength to withstand high transmembrane pressure differences, and sufficiently high flux for certain separation parameters, among other things. Such membranes may be made of polymeric materials such as vitamin derivatives, polysulfones, polyamides, polyaramides and polyimides. Such membranes can also be made of ceramics, glass and metals. Preferred membranes for use in the present invention include those disclosed in EP219878 and US 5085774; the disclosure of which is hereby incorporated by reference.

The membranes used in the present invention may be contained in one or more membrane separation stages, which may be in the form of membrane separators. The membrane separator may contain a series of alternating membrane and spacer layers that are wound in a "spiral wound" fashion around the collection tube. The gas enters the separator and the permeate passes through the wound membrane into the collection tube. The permeate flows through the collection tube and exits the separator through the outlet. The impermeable gas, i.e. retentate or residue, is discharged from the separator through a further outlet.

In another alternative method, the membrane may be in the form of a hollow fiber. In such separators, the gas entering the separator contacts the fiber membranes. The permeate enters the hollow fibers while the impermeable gas, i.e., retentate or residue, remains outside the fibers. The permeate is conveyed under reduced pressure within the fibers to a header and then to a permeate outlet. The retentate is delivered to the separator outlet at substantially the same pressure as the incoming feed gas.

Some examples of such membrane separators are further described in the following documents: spillman, "economics of gas separation membranes", progress in chemical engineering, 1 month 1989, pages 41-62; haggin, a new generation membrane developed for industrial separation, chemical and engineering news, 6.6.6.1988, pages 7-16; and "MEDAL-Membrane separation System, Du Pont/AirLiguid".

PSA systems suitable for use in the process of the present invention are well known in the art and are available from industrial gas companies in the united states. Briefly, PSA systems employ one or more adsorbent beds to selectively adsorb and desorb gas components from a gas mixture through a combination of pressure cycling and valve sequencing.

As a preferred application in the present invention, the PSA system produces a high purity hydrogen product which is substantially free of more strongly adsorbed hydrocarbons and which contains at least 98% by volume hydrogen. The PSA system may also produce a desorbed stream containing methane, ethane, ethylene and higher hydrocarbons, as well as some hydrogen, which is typically lost during the depressurization and purge steps.

By using a combination of membrane separation and PSA separation systems in accordance with the present invention, it is possible to use a lower selectivity membrane and/or higher hydrogen removal rates without losing valuable product in the permeate stream as in prior art processes. In the present invention, valuable products are collected in the PSA system, optionally recycled and recovered. By operating at a higher hydrogen removal rate, the capacity of the distillation section of the separated product can be increased, and the cryogenic section can also be operated at a higher temperature. In addition, the process of the present invention allows for greater amounts of pure hydrogen to be recovered in the PSA system, making the overall plant more economical.

The process of the present invention may preferably be used for separating olefins from hydrogen in any gas stream containing olefins and hydrogen. Such gas streams include, but are not limited to, those resulting from cracking processes and GTO or MTO processes. Of course, the gas may contain other components normally associated with these streams.

Various preferred embodiments of the present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like parts throughout.

Referring to fig. 1, a naphtha feed 10 is fed to a cracking furnace 102. Naphtha feed 101 is thermally cracked in the presence of steam in a cracking furnace 102 to produce a cracked effluent 103. The cracked effluent typically contains hydrogen, steam, carbon monoxide, carbon dioxide, and various hydrocarbons including ethylene, propylene, and other olefins. In the quench unit 105, the pyrolysis effluent 103 is quenched with water 104. Water vapor is removed from the quench unit 105 via line 106. A quenched cracking effluent 107 is withdrawn from the quench unit 105 and sent to the main separation section, where heavy ends 108 are removed and a steam condensate 109 is withdrawn. The main separation section comprises a distillation column 110 and a condenser 111. Product vapor 112 is withdrawn from condenser 111 and sent to a series of compressors 113a, 113b, 113c and 113d, where product vapor 112 is compressed to a pressure suitable for subsequent cryogenic olefin recovery. Before the final compression stage 113d, the compressed stream 114 is treated with soda 116 in a scrubber 115 for removing CO₂. Spent soda 117 is removed from scrubber 115. The scrubbed gas 118 is then sent from the scrubber 115 to a final compressor 113d and then to a dryer 119 to remove residual water therefrom. The preconditioned cleavage effluent 120 is withdrawn from dryer 119.

The preconditioned cracked effluent 120 is sent to a membrane separator 121 under conditions effective to produce a hydrogen-enriched permeate stream 122 and a hydrogen-depleted retentate stream 123. The permeate stream 122 is compressed in one or more additional compressors (not shown) and, if desired, then fed to a PSA system under conditions effective to produce a hydrogen-enriched non-adsorbed stream 125 and a desorbed stream 126 containing hydrocarbonaceous products from the permeate stream 122. The desorbed stream 126 is compressed in compressor 127 and recycled via line 128 to the feed side of the membrane separator 121. Optionally, at least a portion of the compressed desorbent stream 128 is recycled to the suction side of the compressor 113a as indicated by the dashed line 129.

The hydrogen-depleted retentate stream 123 comprising hydrocarbons is separated in a cryogenic separation section into various components (not shown). The cryogenic section includes a demethanizer 130 that separates methane 131 from heavier hydrocarbon products 132. The heavier hydrocarbon products 132 containing ethylene are then sent to an additional fractionation column (not shown) to produce the desired products. Optionally, at least a portion of the methane overhead stream 131 is recycled to the PSA system 124, as indicated by the dashed line 133.

Referring to fig. 2, the method shown here is the same as that shown in fig. 1 up to the PSA system 124. In the process of fig. 2, the desorbed stream 126 containing the hydrocarbonaceous product is simply recycled back to the suction side of the compressor 113 a. In addition, a membrane separator 134 is used to separate hydrogen from the off-gas 135 in the overhead stream 131 of the demethanizer 130. The membrane separator 134 may use the same or different membrane as in the membrane separator 121. The membrane separator 134 is operated under conditions effective to produce a permeate stream 135 that is rich in hydrogen and a retentate stream 136 that is lean in hydrogen. At least a portion of the permeate from the membrane separator 134 is recycled to the PSA system 124 as shown by line 137.

The invention will now be described with reference to the following examples.

Examples

Computer simulations were performed on the basis of the process flow diagrams shown in fig. 3-5. Simulating the conditions shown here at about 500 lbs/inch²An ethylene plant operating under pressure, and a cold box simulated to operate at about-105 ℃. C given below₂/C₂The loss does not include the loss in distillation.

Comparative example 1

Figure 3 shows a typical ethylene plant using PSA. Briefly, the apparatus includes a cracking section where fresh feed, recycle stream and steam are mixed and reacted in a cracking furnace at near atmospheric pressure and elevated temperature. The effluent 1 of this section, commonly referred to as "cracked gas", is compressed in an effluent compressor and then dried in one or more dryers (not shown). The compressed effluent is then sent to a cryogenic section (cold box) where its temperature is reduced to such a level that separation of the components in the effluent can be carried out by distillation. The hotter portion of the cold box is passed through an ethylene refrigeration cycle and the colder portion of the cold box is passed through an expander to provide the refrigeration balance to the cold box. The off-gas 6 providing the refrigeration duty to the expander is a mixture of methane and hydrogen.

The frozen effluent is then sent to a demethanizer to yield an overhead stream 4 containing methane and hydrogen and a distillation feed stream 5 containing heavier hydrocarbons. As noted above, a portion 6 of overhead stream 4 is used to provide refrigeration duty to the expander duty. The remainder 7 of the overhead stream 4 is sent to the PSA system which produces a nonadsorbed stream 8 containing high purity hydrogen and a desorbed tail gas stream 9. Distillation feed stream 5 is sent to a distillation section to produce an ethylene product stream.

The results of the simulation of the process flow shown in FIG. 3 are shown in Table 1 below.

TABLE 1

Membrane recovery rate		N.A.
								L/V		70％
PSA recovery		87％
								C2/C2Rate of loss		1.5％
	Outlet of cracking reactor	Separator gas									Expander feed	PSA feed	PSA hydrogen	PSA tail gas	De-distillation
			Logistics	1	4	6	7	8	9	5
H2	2500.0	2400.0									480.0	1920.0	1670.4	249.6	100.0
			C1	2000.0	540.0	108.0	432.0	0.0	432.0	1460.0
C2＝	2700.0	48.0									9.6	38.4	0.0	38.4	2652.0
			C2	1300.0	12.0	2.4	9.6	0.0	9.6	1288.0
C3＝	1100.0	0.0									0.0	0.0	0.0	0.0	1100.0
			C3	400.0	0.0	0.0	0.0	0.0	0.0	400.0
Total up to	10000.0	3000.0									600.0	2400.0	1670.4	729.6	7000.0
			H2(％)	25.0	80.0	80.0	80.0	100.0	34.2	1.4
C1(％)	20.0	18.0									18.0	18.0	0.0	59.2	20.9
			C2＝(％)	27.0	1.6	1.6	1.6	0.0	5.3	37.9
C2(％)	13.0	0.4									0.4	0.4	0.0	1.3	18.4
			C3＝(％)	11.0	0.0	0.0	0.0	0.0	0.0	16
C3(％)	4.0	0.0									0.0	0.0	0.0	0.0	5.7
			Total up to	100.0	100.0	100.0	100.0	100.0	100.0	100.0

Comparative example 2

Figure 4 shows a typical ethylene plant using separate PSA and membrane separation systems. The process flow in fig. 4 is the same as that in fig. 3, except that the membrane separation system is inserted after the feed compressor. The off-gas stream 3, which contains mainly hydrogen in the permeate stream, is removed and not sent to the cold box and distillation section. Retentate stream 2 is processed in the same manner as the compressed cracked effluent described above. The results of the simulation of the process flow shown in fig. 4 are shown in table 2 below.

TABLE 2

Membrane recovery rate		40％
										L/V		80％
PSA recovery		84％
										C2/C2Rate of loss		2.8％
	Outlet of cracking reactor	Permeate flow											Separator feed	Separator gas	Expander feed	PSA feed	PSA hydrogen	PSA tail gas	De-distillation
			Logistics	1	3	2	4	6	7	8	9	5
H2	2500.0	1000.0											1500.0	1387.0	480.0	907.0	761.9	145.1	113.0
			C1	2000.0	46.0	1954.0	346.8	108.0	238.8	0.0	238.8	1607.2
C2＝	2700.0	54.0											2646.0	28.3	9.6	18.7	0.0	18.7	2617.7
			C2	1300.0	22.1	1277.9	7.1	2.4	4.7	0.0	4.7	1270.8
C3＝	1100.0	16.5											1083.5	0.0	0.0	0.0	0.0	0.0	1083.5
			C3	400.0	15.6	384.4	0.0	0.0	0.0	0.0	0.0	384.4
Total up to	10000.0	1154.2											8845.8	1769.2	600.0	1169.2	761.9	407.3	7076.6
			H2(％)	26.0	86.6	17.0	78.4	78.4	77.6	100.0	35.6	1.6
C1(％)	20.0	4.0											22.1	19.6	19.6	20.4	0.0	58.6	22.7
			C2＝(％)	27.0	4.7	29.9	1.6	1.6	1.6	0.0	4.6	37.0
C2(％)	13.0	1.9											14.4	0.4	0.4	0.4	0.0	1.1	18.0
			C3＝(％)	11.0	1.4	12.2	0.0	0.0	0.0	0.0	0.0	15.3
C3(％)	4.0	1.4											4.3	0.0	0.0	0.0	0.0	0.0	5.4
			Total up to	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0

Example 1

FIG. 5 shows an ethylene plant according to the invention. It preferably uses a combination of PSA and membrane separation systems. The compressed effluent 10 is sent to a membrane separator as in figure 4. Unlike the flow scheme in fig. 4, permeate stream 3 is recompressed in an additional compressor and fed as feed to the PSA system. The tail gas 9 of the PSA system is sent to the suction side of the feed compressor and back to the distillation section, the tail gas 9 now containing valuable products present in the permeate stream 3. Pure hydrogen is recovered as non-adsorbed stream 8 of the PSA system.

The simulation results for the process flow shown in fig. 5 are listed in table 3 below.

TABLE 3

Membrane recovery rate		40％
											L/V		78％
PSA recovery		84％
											C2/C2Rate of loss		0.3％
	Outlet of cracking reactor	Membrane feeding												Permeate flow	Separator feed	Separator gas	Expander feed	PSA feed	PSA hydrogen	PSA tail gas	De-distillation
			Logistics	1	10	3	2	4	6	7	8	9	5
H2	2500.0	2677.5												1071.0	1606.5	1571.5	462.0	1109.5	932.0	177.5	35.0
			C1	2000.0	2302.6	48.1	2254.5	428.6	126.0	302.6	0.0	302.6	1825.9
C2＝	2700.0	2723.1												56.4	2666.7	32.7	9.6	13.1	0.0	23.1	2634.0
			C2	1300.0	1305.8	23.1	1282.7	8.2	2.4	5.8	0.0	5.8	1274.5
C3＝	1100.0	1100.0												17.2	1082.8	0.0	0.0	0.0	0.0	0.0	1082.8
			C3	400.0	400.0	16.3	383.7	0.0	0.0	0.0	0.0	0.0	383.7
Total up to	10000.0	10509.0												1232.1	9276.9	2040.9	600.0	1440.9	932.0	506.9	7236.0
			H2(％)	25.0	25.5	86.9	17.3	77.0	77.0	77.0	100.0	34.9	0.5
C1(％)	20.0	21.9												3.9	24.3	21.0	21.0	21.0	0	59.5	25.2
			C2＝(％)	27.0	25.9	4.6	28.7	1.6	1.6	1.6	0.0	4.5	36.4
C2(％)	13.0	12.4												1.9	13.8	0.4	0.4	0.4	0.0	1.1	17.6
			C3＝(％)	11.0	10.5	1.4	11.7	0.0	0.0	0.0	0.0	0.0	15.0
C3(％)	4.0	3.8												1.3	4.1	0.0	0.0	0.0	0.0	0.0	5.3
			Total up to	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0

From a comparison of example 1 with comparative examples 1 and 2, it can be seen that the process of the present invention can combine C₂/C₂The loss of product is reduced to below 0.5%.

While the invention has been described with reference to the drawings, examples and preferred embodiments, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Such modifications and improvements are within the purview and scope of the claims appended hereto.

Claims (30)

1. A process for recovering olefins from a gas stream comprising olefins and hydrogen, said process comprising the steps of: (a) compressing said gas in at least one compression stage to produce a compressed gas stream; (b) contacting said compressed gas stream with a membrane under conditions effective to produce a permeate stream enriched in hydrogen and a retentate stream depleted in hydrogen; and (c) feeding said permeate stream to a pressure swing adsorption system under conditions effective to produce a hydrogen-rich non-adsorbed stream and a desorbed stream comprising olefins.

2. The process according to claim 1, further comprising recycling at least a portion of said desorbed stream to said at least one compression stage.

3. The process according to claim 1, further comprising passing at least a portion of said desorbed stream to at least one additional compression stage to obtain a compressed desorbed stream, and recycling at least a portion of said compressed desorbed stream to said membrane contacting step.

4. The process of claim 1 wherein said olefin comprises ethylene.

5. The method of claim 1 wherein said non-adsorbed species is substantially pure hydrogen.

6. The process according to claim 1, further comprising passing said retentate stream to a cryogenic separation section.

7. The process according to claim 6, wherein said cryogenic separation section comprises a demethanizer having an overhead stream comprising methane and a bottoms stream comprising said olefins.

8. The process according to claim 7 further comprising recycling at least a portion of said overhead stream to said pressure swing adsorption system.

9. The process according to claim 1, wherein said gas stream is from a Methanol To Olefins (MTO) unit or a Gas To Olefins (GTO) unit.

10. A process for recovering olefins and high purity hydrogen from a gas containing olefins and hydrogen, said process comprising the steps of:

(a) compressing said gas stream in at least one compression stage to obtain a compressed gas stream;

(b) contacting said compressed gas stream with a membrane under conditions effective to produce a permeate stream enriched in hydrogen and a retentate stream depleted in hydrogen;

(c) compressing said permeate stream in at least one additional compression stage to obtain a compressed permeate stream;

(d) feeding said compressed permeate stream to a pressure swing adsorption system under conditions effective to produce a non-adsorbed stream comprising high purity hydrogen and a desorbed stream comprising olefins;

(e) recycling said desorbed stream to said at least one compression stage.

11. The process of claim 10 wherein said olefin comprises ethylene.

12. The process according to claim 10, further comprising passing said retentate stream to a cryogenic separation section.

13. The process according to claim 12, wherein said cryogenic separation section comprises a demethanizer having an overhead stream comprising methane and a bottoms stream comprising said olefins.

14. The process of claim 13 further comprising contacting said overhead stream with a second membrane under conditions effective to produce a permeate stream enriched in hydrogen and a retentate stream depleted in hydrogen.

15. The process according to claim 14, further comprising recycling at least a portion of said permeate stream to said pressure swing adsorption system.

16. The process according to claim 10, wherein said gas stream is from a Methanol To Olefins (MTO) unit or a Gas To Olefins (GTO) unit.

17. A process for recovering olefins from a cracking effluent containing olefins and hydrogen, said process comprising the steps of:

(a) compressing said cracked effluent in at least one compression stage to obtain a compressed cracked effluent;

(b) contacting said compressed cracking effluent with a membrane under conditions effective to produce a permeate stream enriched in hydrogen and a retentate stream depleted in hydrogen; and

(c) the permeate stream is fed to a pressure swing adsorption system under conditions effective to produce a hydrogen-rich non-adsorbed stream and a desorbed stream comprising olefins.

18. The process according to claim 17, further comprising recycling at least a portion of said desorbed stream to said at least one compression stage.

19. The process according to claim 17, further comprising passing at least a portion of said desorbed stream to at least one additional compression stage to provide a compressed desorbed stream, and recycling at least a portion of said compressed desorbed stream to said membrane contacting step.

20. The process of claim 17 wherein said olefin comprises ethylene.

21. The process according to claim 17 wherein said non-adsorbed stream is substantially pure hydrogen.

22. The process according to claim 17, further comprising passing said retentate stream to a cryogenic separation section.

23. The process according to claim 22, wherein said cryogenic separation section comprises a demethanizer having an overhead stream comprising methane and a bottoms stream comprising said olefins.

24. The process according to claim 23 further comprising recycling at least a portion of said overhead stream to said pressure swing adsorption system.

25. A process for recovering olefins and high purity gases from a cracking effluent, said process comprising the steps of:

(a) compressing said cracked effluent in at least one compression stage to obtain a compressed cracked effluent;

(b) contacting said compressed cracking effluent with a membrane under conditions effective to produce an enriched permeate stream and a hydrogen-depleted retentate stream;

(c) compressing said permeate stream in at least one additional compression stage to obtain a compressed permeate stream;

(d) feeding said compressed permeate stream to a pressure swing adsorption system under conditions effective to produce a non-adsorbed stream comprising high purity hydrogen and a desorbed stream comprising olefins; and

(e) recycling said desorbed stream to said at least one compression stage.

26. The process according to claim 25 wherein said olefin comprises ethylene.

27. The process according to claim 25, further comprising passing said retentate stream to a cryogenic separation section.

28. The process according to claim 27, wherein said cryogenic separation section comprises a demethanizer having an overhead stream comprising methane and a bottoms stream comprising said olefins.

29. The process of claim 28 further comprising contacting said overhead stream with a second membrane under conditions effective to produce a permeate stream enriched in hydrogen and a retentate stream depleted in hydrogen.

30. The process according to claim 29, further comprising recycling at least a portion of said permeate stream to said pressure swing adsorption system.