PROCESS FOR MAXIMIZING HYDROGEN RECOVERY

Abstract

The process can be used in any hydrocarbon process in which it is desirable to recover hydrogen. The process can include catalytically reforming a hydrocarbon feed, a paraffin dehydrogenation to produce light olefins or a synthesis gas generating process. There is an effluent stream having hydrogen and hydrocarbons that is first sent to an adsorption zone to produce a pure hydrogen stream and a tail gas stream. The tail gas stream is then sent across a feed side of a membrane having the feed side and a permeate side. The membrane that is selected is selective for hydrogen over one or more C1-C6 hydrocarbons and light ends including CO, CO2, N2 and O2, and withdrawing from the permeate side a permeate stream enriched in hydrogen compared with a residue stream withdrawn from the feed side. The permeate stream is then recycled to be sent through the adsorption zone.

Claims

1. A process for recovery of hydrogen, comprising: A) obtaining a stream comprising hydrogen, hydrocarbons, carbon monoxide, carbon dioxide, oxygen and nitrogen from a reaction zone; B) sending said stream through an adsorption zone to produce a hydrogen stream and a tail gas stream; C) passing at least a portion of the tail gas stream across a feed side of a membrane having the feed side and a permeate side, and being selective for hydrogen over one or more C1-C6 hydrocarbons, carbon monoxide, carbon dioxide, oxygen and nitrogen; D) withdrawing from the permeate side a permeate stream enriched in hydrogen compared with a residue stream withdrawn from the feed side; and E) recycling said permeate stream into said stream from said reaction zone.

2. The process according to claim 1 wherein said residue stream is sent to a product recovery section of an upstream process for additional recovery of feed and product.

3. The process according to claim 1, wherein the permeate stream comprises no more than about 8 percent, by mole, of nitrogen and no more than about 0.3 percent, by mole, of carbon monoxide.

4. The process according to claim 1, wherein the permeate stream comprises no more than about 100 ppm, by mole, of nitrogen and no more than about 100 ppm, by mole, of carbon monoxide.

5. The process according to claim 1, wherein the permeate stream comprises no more than about 20 ppm, by mole, of nitrogen and no more than about 40 ppm, by mole, of carbon monoxide.

6. The process according to claim 1, wherein an amount of hydrogen by mole percent in the permeate stream is substantially the same as a gas stream obtained from the effluent stream and upstream of the waste hydrocarbon stream.

7. The process according to claim 5, wherein the amount of hydrogen composition in the permeate stream is within about 15%, by mole, of the gas stream.

8. The process according to claim 5, wherein the amount of hydrogen composition in the permeate stream is within about 2%, by mole, of the gas stream.

9. The process according to claim 5, wherein the amount of hydrogen composition in the permeate stream is within about 1%, by mole, of the gas stream.

10. The process according to claim 1, wherein the membrane comprises a hollow fiber membrane.

11. The process according to claim 1, wherein the membrane comprises a spiral wound membrane.

12. The process according to claim 1, wherein the permeate comprises from about 60 to 97%, by mole, hydrogen.

13. The process according to claim 1, wherein the permeate comprises at least about 98%, by mole, hydrogen.

14. The process according to claim 9, wherein the hollow fiber membrane comprises at least one polymer selected from the group consisting of polyimide, cellulose acetate, cellulose triacetate, and polysulfone.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0016] The FIGURE is a schematic depiction of an exemplary hydrogen recovery process flow scheme.

DETAILED DESCRIPTION

[0017] Referring to the FIGURE, a feed 10 comprising a mixture of hydrocarbon and hydrogen 10 can be from a reforming process, a paraffin dehydrogenation process or a synthesis gas producing process. Feed 10 may then proceed to a mix point 20 and continue as PSA feed 30 to a PSA unit 40 to produce a hydrogen product stream 50 that may comprise over 99% hydrogen and a tail gas 60 that still has an appreciable content of hydrogen such as about 35% hydrogen. Tail gas 60 is compressed in compressor 70 to produce compressed tail gas stream 80. Then tail gas stream 80 is sent through membrane unit 90 which produces a non-permeate stream 100 that contains a small amount of hydrogen that can be further processed to recover hydrocarbons or used as fuel and permeate 110 from membrane unit 90 is recycled to mix point 20.

[0018] PSA unit (also referred to as PSA zone, herein) 40 is a pressure swing adsorption (which may be abbreviated PSA) zone which produces hydrogen product stream 50 with a purity of about 90.0-about 99.9999%, by mole, hydrogen, or preferably of about 95.0-about 99.99%, by volume, hydrogen. The hydrogen product stream 50 may recover about 80-about 95%, by mole, or even about 85-about 90%, by mole, of the hydrogen feed stream 30. Additionally, tail gas stream 60 that is produced by the PSA unit 40 during a desorption or purge step at a desorption pressure ranging from about 30-about 550 KPa.

[0019] The PSA zone 40 can include a plurality of adsorption beds containing an adsorbent selective for the separation of hydrogen from the hydrocarbons. Often, each adsorption bed within the adsorption zone undergoes, on a cyclic basis, high pressure adsorption, optional cocurrent depressurization to intermediate pressure levels with the release of product from void spaces, countercurrent depressurization to lower desorption pressure with the release of desorbed gas from the feed end of the adsorption bed, with or without purge of the bed, and repressurization to higher adsorption pressure. This process may also include an addition to this basic cycle sequence, such as a cocurrent displacement step, or co-purge step in the adsorption zone following the adsorption step in which the less readily adsorbable component, or hydrogen, is essentially completely removed therefrom by displacement with an external displacement gas introduced at the feed end of the adsorption bed. The adsorption zone may then be countercurrently depressurized to a desorption pressure that is at or above atmospheric pressure with the more adsorbable component being discharged from the feed end thereof.

[0020] In a multibed adsorption system, the displacement gas used for each bed may be obtained by using at least a portion of the debutanizer overhead vapor stream, although other suitable displacement gas, such as an external stream including one or more C1-C4 hydrocarbons, may also be employed. Usually, the high pressure adsorption includes introducing the feedstream or hydrogen-rich gas stream to the feed end of the adsorption bed at a high adsorption pressure. The hydrogen passes through the bed and is discharged from the product end thereof. An adsorption front or fronts are established in the bed with the fronts likewise moving through the bed from the feed end toward the product end thereof. Preferably, the PSA zone 40 can include pressures of about 300-about 6,890 KPa.

[0021] The PSA zone 40 can be carried out using any adsorbent material selective for the separation of hydrogen from hydrocarbons in the adsorbent beds. Suitable adsorbents can include one or more crystalline molecular sieves, activated carbons, activated clays, silica gels, activated aluminas, and combinations thereof. Preferably, the adsorbents are one or more of an activated carbon, an alumina, an activated alumina, and a silica gel. An exemplary PSA zone is disclosed in, e.g., U.S. Pat. No. 5,332,492.

[0022] The PSA zone 40 can provide the hydrogen product stream 50 that can be provided to a reaction zone that requires hydrogen, and the tail gas stream 60. The membrane unit 90 forms a feed side 120 and a permeate side 130. The membrane unit 90 may be a hollow fiber membrane, a spiral wound membrane or other suitable type of membrane. The hollow fiber membrane can be made of at least one of a polyimide, cellulose acetate, cellulose triacetate, and polysulfone. Typically, the polyimide may be formed by reacting a dianhydride and a diamine or a dianhydride and a diisocyanate. Such membranes are disclosed in, e.g., U.S. Pat. No. 4,863,492.

[0023] A residue or non-permeate stream 100 can be withdrawn from the feed side 120 of the membrane 90. The residue stream 100 may include nitrogen; one or more carbon oxides, typically carbon monoxide although carbon dioxide may be present instead or additionally; and one or more C1-C6 hydrocarbons. Often, the residue stream 100 is enriched in nitrogen, the one or more carbon oxides, typically carbon monoxide, and the one more C1-C6 hydrocarbons compared with the tail gas stream 60. The residue stream 100 can be provided to a fuel header, provided as a feed to another process such as a steam-methane reforming unit, or optionally recontacted in a vessel to recover liquefied petroleum gases prior to being sent to the fuel header.

[0024] Generally, the compressed tail gas stream 80 contacts the membrane 90 with the smaller hydrogen molecules passing through the membrane 90 and other molecules, such as C1-C6 hydrocarbons, nitrogen, carbon dioxide, and carbon monoxide, are blocked. Particularly, the membrane 90 can block nitrogen and carbon oxides, such as carbon monoxide.

[0025] The membrane 90 can be operated in such a way that the permeate stream 110 can include no more than about 8 mol %, or about 20 ppm, by mole, of nitrogen and no more than about 0.3 mol %, or about 40 ppm, by mole, of carbon monoxide. Generally, the amount of hydrogen by mole percent in the permeate stream 110 is substantially the same as the feed stream 10 and typically the amount of hydrogen composed in the permeate stream 110 is within about ?15%, about ?5%, about ?2%, or about ?1%, by mole, of the feed stream 10. Additionally, the permeate stream 110 can include at least about 60%, or even at least about 95% or about 97%, by mole, hydrogen. The permeate stream 110 may recover at least about 90%, by mole, of the hydrogen in the compressed tail gas stream 80. Meanwhile, the membrane 90 rejects at least about 70%, by mole, carbon monoxide; about 76%, by mole, nitrogen; about 79%, by mole, methane; about 91%, by mole, ethane; and almost 100%, by mole, of the one or more C3+ hydrocarbons in the vapor stream 30. The non-permeate or residue stream 100 may be sent to a product recovery zone 140. The process and system of the invention can achieve an overall hydrogen recovery of at least about 98%, by mole, of the hydrogen present in the feed stream 10.

Example

[0026] The following example demonstrates the specifics and benefits of this invention:

Net Gas Conditions

[0027]

TABLE-US-00001 Flow Rate, lb-mol/hr 8102.13 Temperature, ? F. 97 Pressure, psig 50 Composition, mol % Hydrogen 93.03 Carbon Monoxide 0.08 Methane 6.44 Ethylene 0.03 Ethane 0.30 Propene 0.06 Propane 0.06

PSA Product Specifications

[0028]

TABLE-US-00002 Pressure, psig 340 Composition Hydrogen 99.999 mol % Carbon Monoxide <1 ppmv Methane <10 ppmv

Tail Gas Specifications

[0029]

TABLE-US-00003 PSA tail gas pressure 5 psig Fuel header pressure 85 psig
Process Schemes: The following process schemes were evaluated: [0030] Scheme #1: Once-through PSA with the tail gas being compressed to fuel header pressure. The PSA feed pressure is set at 350 psig. The hydrogen recovery is 90%. [0031] Scheme #2: PSA with approximately 72% tail gas recycle. The compressed, recycled tail gas gets mixed with the raw net gas upstream of the net gas compressor. The overall hydrogen recovery is 97%. [0032] Scheme #3: The PSA tail gas is compressed to 204 psig (instead of 85 psig) and is fed to a membrane unit. The membrane permeate gas is recycled and mixed with the raw net gas upstream of the net gas compressor. The overall hydrogen recovery is 99%. [0033] Scheme #4: The PSA tail gas is compressed to 467 psig (instead of 85 psig) and is fed to a membrane unit. The membrane permeate gas is recycled and mixed with the raw net gas upstream of the net gas compressor. The overall hydrogen recovery is 99%.
Results: The table below summarizes the four process schemes described above.

TABLE-US-00004 Scheme # 1 2 3 4 Net gas Compressed to 356 psig (hp) 7,861 9,847 9,147 9,052 Tail Gas Compressed to 85 psig (hp) 1,237 2,626 Tail Gas Compressed to 204 psig (hp) 2,141 Tail Gas Compressed to 467 psig (hp) 2,653 Total Compressed (hp) 9,098 12,473 11,288 11,705 Overall H2 Rec (%) 90 97 99 99 # Membrane Modules 12 4

[0034] With a once-through PSA operating at 350 psig (Scheme #1), the maximum hydrogen recovery is 90%. A tail gas compressor is required to boost the tail gas pressure to fuel header pressure (85 psig). The total power consumption over the netgas and tail gas compressors is approximately 9,098 hp.

[0035] By employing the tail gas recycle concept (Scheme #2), the overall hydrogen recovery can be increased to 96%-97%, max. In doing so, the total power consumption increases to approximately 12,473 hp (or plus 37% compared to Scheme #1).

[0036] The overall hydrogen recovery and total power consumption of Scheme #2 are dictated by the tail gas impurities that build up in the recycle loop. Compressing the PSA tail gas to at least 204 psig and passing it through a membrane unit (Scheme #3) eliminates this impurity bottleneck since the bulk of the impurities get rejected in the membrane's non-permeate (residue) stream. The membrane's permeate stream can be recycled in its entirety and mixed with the raw Oleflex netgas upstream of the netgas compressor. As a result, the overall hydrogen recovery can improve to 99%+ while the total power consumption increases to about 11,288 hp (or plus 24% compared to Scheme #1). Looking at the netgas compressor alone, the increase in power consumption compared to Scheme #1 is approximately 16%.

[0037] Scheme #4 shows that by compressing the PSA tail gas to 467 psig instead of 204 psig cuts the number of membrane modules (housings) by more than half while the total power consumption increases by less than 4%. In this process, a membrane non-permeate or residue stream may be recycled to a product recovery section, such as a de-ethanizer for recovery of propane and propylene.

[0038] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0039] In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by volume, unless otherwise indicated.

[0040] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.