Process and apparatus for dehydrogenation of paraffins

Abstract

The invention is entitled "Paraffin Dehydrogenation Process and Apparatus". A process for recovering hydrogen from a dehydrogenation reactor effluent is disclosed. A feed stream comprising hydrocarbons and hydrogen is passed to a dehydrogenation reactor maintained at dehydrogenation conditions to provide a dehydrogenation reactor effluent. The dehydrogenation reactor effluent is passed to a cold box separation unit to provide a liquid hydrocarbon product stream and a recycle hydrogen stream. A return portion of the recycle hydrogen stream is passed to a reactor effluent compressor. The subject discloses improved processes and apparatus that enable a paraffin dehydrogenation reactor to be operated at a reduced H2 / HC ratio without the need for an investment in a re-sized compressor or a re-sized turbo-expander or separator in the cold box.

Description

[0001] CLAIM

[0002] This application claims the benefit of U.S. Patent Application Serial No. 17 / 131,453, filed December 22, 2020, which is incorporated by reference herein in its entirety. TECHNICAL FIELD

[0003] The field relates to paraffin dehydrogenation. More specifically, the field relates to hydrogen management in paraffin dehydrogenation. BACKGROUND

[0004] Hydrocarbon dehydrogenation is an important commercial hydrocarbon conversion process due to the existing and growing demand for dehydrogenated hydrocarbons used to manufacture various chemical products such as detergents, high octane gasoline, oxygenate gasoline blending components, pharmaceutical products, plastics, synthetic rubbers, and other products well known to those skilled in the art. Specifically, the demand for propylene in the petrochemical industry has grown significantly due to its use as a precursor in the production of polypropylene for packaging materials and other commercial products. Other downstream uses of propylene include the manufacture of acrylonitrile, acrylic acid, acrolein, propylene oxide and glycol, plasticizer oxoalcohols, cumene, isopropyl alcohol, and acetone. One pathway for producing propylene is propane dehydrogenation.

[0005] Processes for converting paraffins to olefins involve passing a paraffin feed stream over a high selectivity catalyst in which the paraffins are dehydrogenated to the corresponding olefins and produce a dehydrogenation reactor effluent. The dehydrogenation reactor effluent is provided for cooling and separation into a hydrocarbon-rich fraction and a hydrogen-rich vapor fraction in a separation zone, where a portion is a net off-gas that is not recycled. The separation zone typically includes a reactor effluent compressor (REC) and a series of turbo expanders and separation vessels, commonly referred to as cold boxes.

[0006] In paraffin dehydrogenation reactors, hydrogen is commonly co-fed to minimize the amount of carbon-containing material deposited on the catalyst and to improve catalyst stability. In practice, the amount of hydrogen co-feed is expressed as a ratio of hydrogen to hydrocarbon (H2 / HC), which is calculated by dividing the hydrogen molar flow rate by the hydrocarbon molar flow rate. While hydrogen reduces coking on the catalyst, it also changes the equilibrium conversion of paraffins to the desired olefins at a given temperature and pressure. Thus, there is a trade-off between minimizing catalyst coking and maximizing conversion.

[0007] Based on recent developments, operating with a reduced H2 / HC ratio enables the reactor inlet temperature (RIT) to be lowered, which in turn reduces fouling and increases the time between reactor turnarounds. While reducing the H2 / HC ratio in the dehydrogenation reactor results in improved product olefin yield, it presents challenges to existing REC and cold box designs that rely on hydrogen present within the dehydrogenation reactor effluent stream. Accordingly, there is a need for an improved process with improved hydrogen management. SUMMARY

[0008] An improved process for managing the H2 / HC ratio in a paraffin dehydrogenation process has been discovered that enables the reactor effluent compressor and cold box turboexpander to operate at the same H2 / HC ratio as the original design, and at the same time enables the paraffin dehydrogenation reactor to be operated at a reduced H2 / HC ratio. A returned hydrogen stream is recovered from the reactor effluent and returned to the reactor effluent compressor and cold box turboexpander to enable them to be operated at the same H2 / HC ratio, despite the dehydrogenation reactor being operated at a reduced ratio.

[0009] The process includes passing a feed stream comprising hydrocarbons and hydrogen to a dehydrogenation reactor under dehydrogenation conditions to provide a dehydrogenation reactor effluent. The amount of hydrogen co-feed is expressed as a hydrogen to hydrocarbon (H2 / HC) ratio, which is calculated by dividing the hydrogen molar flow by the hydrocarbon molar flow. The dehydrogenation reactor effluent is passed to a reactor effluent compressor to provide a compressed hydrocarbon stream. The compressed hydrocarbon stream is passed to a cold box separation unit to provide an olefin stream and a recycle hydrogen stream. A returned portion of the recycle hydrogen stream is passed to the reactor effluent compressor.

[0010] The cold box separation unit consists of a cold combined feed exchanger, a plurality of separation vessels, an expander, and a cooler. The recycle hydrogen stream is split upstream of the cold combined feed exchanger into a returned portion of the recycle hydrogen stream and a feed portion, wherein the returned portion is 30-50 wt% of the recycle hydrogen stream and the feed portion is 50-70 wt% of the recycle hydrogen stream.

[0011] These and other features, aspects, and advantages of the present disclosure are further explained in the following detailed description, along with the appended claims and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a schematic diagram of a paraffin dehydrogenation process using the process of the present disclosure.

[0013] Those skilled in the art will appreciate that the elements in the figures are illustrated for simplicity and clarity and that the actual implementation can be provided with additional or different elements, as is readily apparent to one of ordinary skill in the art. For example, the sizes and relative positions of some of the elements in the figures can be exaggerated or not drawn to scale for purposes of illustration.

[0014] DEFINITIONS

[0015] The following detailed description is merely exemplary in nature and is not intended to limit the application and use of described implementations. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

[0016] The term "in communication" means operable to allow the flow of matter between enumerated components.

[0017] The term "downstream communication" means that at least a portion of the matter flowing to the subject in the downstream communication can operably flow from the object in communication therewith.

[0018] The term "upstream communication" means that at least a portion of the matter flowing from the subject in the upstream communication can operably flow to the object in communication therewith.

[0019] The term "direct communication" means flowing from an upstream component into a downstream component without a change in composition due to physical fractionation or chemical transformation.

[0020] The term "bypass" means that the object loses downstream communication with the bypass subject at least in the range of the bypass.

[0021] As used herein, the term "stream" can include various hydrocarbon molecules such as straight, branched, or cyclic alkanes, alkenes, alkadienes, and alkynes, and optionally other substances such as gases, e.g., hydrogen, or impurities such as heavy metals, as well as sulfur and nitrogen compounds. The stream can also include aromatic and non-aromatic hydrocarbons. Further, the hydrocarbon molecules can be abbreviated as C1, C2, C3... C n where "n" represents the number of carbon atoms in one or more hydrocarbon molecules. Further, a superscript "+" or "-" can be used with one or more hydrocarbon symbols in the abbreviation; for example, C 3+ or C 3- which includes one or more hydrocarbons in the abbreviation. As one example, the abbreviation "C 3+ " means one or more hydrocarbon molecules having three carbon atoms and / or more. Further, the term "stream" can apply to other fluids such as aqueous and non-aqueous solutions of alkaline / basic compounds such as sodium hydroxide.

[0022] The term "column" means one or more distillation columns for separating one or more components of different volatile materials. Unless otherwise indicated, each column includes a condenser on the top of the column for condensing a portion of the overhead stream and refluxing it back to the top of the column, and a reboiler at the bottom of the column for vaporizing a portion of the bottoms stream and sending it back to the bottom of the column. The feed to the column can be preheated. The top pressure is the pressure of the overhead vapor at the exit of the column. The bottom temperature is the liquid bottoms exit temperature. The overhead and bottoms lines refer to the net lines from the column downstream to the column. Alternatively, a stripping stream can be used for heat input near the bottom of the column.

[0023] As used herein, the term "enrich" can mean that the outlet stream has a greater concentration of a specified component than in the inlet stream to the vessel.

[0024] As used herein, the term "separator" means a vessel having one inlet and at least one overhead vapor outlet and one bottoms liquid outlet, and can also have an aqueous stream outlet from a boot. A flash tank is a separator that can be in communication downstream from the separator, which can be operated at a higher pressure. DETAILED DESCRIPTION

[0025] The methods described herein are particularly useful for maintaining the H2 / HC ratio in the REC and the series of expanders and separation vessels commonly referred to as a cold box separation unit, while enabling the dehydrogenation reactor to operate at a lower H2 / HC ratio. Applicants have developed an improved method of passing a portion of the recycle hydrogen stream obtained from the cold box separation unit located downstream of the dehydrogenation reactor to the reactor effluent compressor.

[0026] Conventionally, propane dehydrogenation reactors are operated at a relatively high H2 / HC ratio of 0.4 or higher. Following developments in the field of dehydrogenation, it was observed that greater selectivity to propylene could be achieved by reducing the H2 / HC ratio in the dehydrogenation reactor. As a result, refineries are moving towards operating the dehydrogenation reactor at a lower H2 / HC ratio. This results in a reduction in the H2 / HC ratio of the dehydrogenation reactor effluent stream. If there is less hydrogen present in the dehydrogenation reactor effluent stream, the existing REC and cold box design that relies on the hydrogen present within the dehydrogenation reactor effluent stream needs to be modified in order to achieve the desired product specifications. The improved method requires passing a portion of the recycle hydrogen stream to the REC. This increases the H2 / HC ratio of the stream being processed in the compressor and the subsequent cold box separation unit, allowing the REC and cold box turbo expanders to operate at the same H2 / HC ratio as the original design, while enabling the paraffin dehydrogenation reactor to be run at a reduced H2 / HC ratio.

[0027] The method includes passing a feed stream comprising hydrocarbons and hydrogen to a dehydrogenation reactor under dehydrogenation conditions to provide a dehydrogenation reactor effluent. The dehydrogenation reactor effluent is passed to a reactor effluent compressor to provide a compressed hydrocarbon stream. The compressed hydrocarbon stream is passed to a cold box separation unit to provide an olefin stream and a recycle hydrogen stream. A return portion of the recycle hydrogen stream is passed to the reactor effluent compressor. This improved method allows the reactor effluent compressor and the cold box turboexpander to operate at the same H2 / HC ratio as the original design, while enabling the paraffin dehydrogenation reactor to be run at a reduced H2 / HC ratio.

[0028] An improved paraffin dehydrogenation method has been developed. In particular, the method is used to convert propane to propylene. FIG. 1 An example of the method 100 is shown, which includes a dehydrogenation step, a compression step, a series of cryogenic separation steps. As shown in FIG. 1 The integrated process and apparatus 100 includes a dehydrogenation reactor 105, a reactor effluent compressor 110, and a cold box separation unit 170, which includes a cold combined feed exchanger 115, a series of separators 120, 130, 140, and 150, a series of turboexpanders 125, 135, and 145.

[0029] As shown in FIG. 1 The fresh hydrocarbon stream in line 5 is passed to the dehydrogenation reactor 105. In one embodiment, the fresh hydrocarbon stream 5 is mixed with a feed portion stream in line 50, which comprises hydrogen, to provide a combined feed stream in line 8. The combined feed stream 8 can be preheated in the cold combined feed exchanger 115 to provide a preheated feed stream 10, which is then passed to the dehydrogenation reactor 105. The preheated feed stream comprises hydrogen and paraffins.

[0030] The fresh hydrocarbon stream comprises propane. In some embodiments, the fresh hydrocarbon stream comprises other light paraffins, such as butane, isobutane, isopentane, or pentane. In some embodiments, the fresh hydrocarbon stream comprises at least one paraffin having from 2 to 30 carbon atoms. The molar ratio of hydrogen to hydrocarbons of the feed stream is in the range of 0.01 to 0.4.

[0031] The preheated feed stream is contacted with a dehydrogenation catalyst in the dehydrogenation reactor 105, which is maintained at dehydrogenation conditions, to produce a dehydrogenation reactor effluent stream comprising hydrogen, unconverted paraffins, and olefins in line 12. The dehydrogenation reactor 105 can be a reaction zone comprising multiple stages or reactors, typically in series.

[0032] The light paraffin dehydrogenation process utilizes a high selectivity platinum-based catalyst system. One example of a suitable catalyst light paraffin dehydrogenation process can be, for example, a catalyst composite comprising a Group VIII noble metal component, a Group IA or Group IIA metal component, and a component selected from the group consisting of tin, germanium, lead, indium, gallium, thallium, or mixtures thereof, all supported on an alumina support. The heavy paraffin dehydrogenation process uses a selective platinum catalyst.

[0033] Dehydrogenation conditions include a temperature of 400°C to 900°C, a pressure of 0.01 to 10 absolute atmospheres, and a liquid hourly space velocity (LHSV) of 0.1 hr -1 to 100 hr -1 The pressure in the dehydrogenation reactor 105 is kept as low as practicable consistent with equipment limitations to maximize the chemical equilibrium advantage. The light paraffin dehydrogenation process will generally be conducted at a lower pressure than the heavy paraffin dehydrogenation process.

[0034] The dehydrogenation reactor effluent stream exiting the dehydrogenation reactor in line 12 is compressed in REC 110 to provide a compressed hydrocarbon stream in line 18. The reactor effluent compressor can have two or three stages of compression. The dehydrogenation reactor effluent stream is compressed at a pressure of 1 MPa(a) to 2 MPa(a) and at a temperature of 120°C to 150°C. The compressor increases the pressure of the dehydrogenation reactor effluent stream exiting the reactor, which facilitates separation of the hydrogen gas-entrained gas phase and the unconverted hydrocarbons and reaction products-entrained liquid phase in the cold box separation unit 170.

[0035] The compressed hydrocarbon stream 18 is passed to the cold box separation unit 170 to provide a liquid hydrocarbon product stream comprising olefins in line 64, a recycle hydrogen gas stream in line 48, and a net gas hydrogen stream in line 52. The cold box separation unit 170 includes a cold combined feed exchanger 115, a high pressure separator 120, a medium pressure separator 130, a low pressure separator 140, a net gas separator 150, and a series of turbo expanders 125, 135, and 145. The liquid hydrocarbon product stream is separated from hydrogen gas and byproduct methane by condensation in the cold box separation unit 170.

[0036] The compressed hydrocarbon stream 18 is cooled in a cold combined feed exchanger 115 of the cold box separation unit 170. In the cold combined feed exchanger 115, heat exchange occurs between the compressed hydrocarbon stream 18 and the combined feed stream 8, such that the compressed hydrocarbon stream is cooled and the combined feed stream is preheated. The cooled hydrocarbon stream in line 20 is then passed to a series of separators and turbo expanders present in the cold box separation unit 170 to provide a liquid hydrocarbon product stream in line 64 comprising olefins, a recycle hydrogen stream 48 and a net gas hydrogen stream 52. The net gas hydrogen stream can comprise methane.

[0037] The cooled hydrocarbon stream 20 is passed to a high pressure separator 120 to separate a high pressure separator overhead stream in line 22 comprising hydrogen and methane from a high pressure separator bottoms stream in line 24 comprising hydrocarbons. The high pressure separator 120 can operate at a pressure of 1.1 MPa to 1.4 MPa and at a temperature of -80°C to -100°C.

[0038] The high pressure separator overhead stream 22 is passed to a turbo expander 125 for expansion and cooling. The expanded and cooled stream is then passed to a medium pressure separator 130 to further separate hydrocarbons in a medium separator bottoms stream in line 34. The medium pressure separator 130 can operate at a pressure of 0.7 MPa to 1.1 MPa and at a temperature of -100°C to -130°C.

[0039] A first portion of the medium separator overhead stream in line 32 in line 36 is passed to a net gas cooler 145. The cooled first portion is then passed to a net gas separator 150 to provide a net gas hydrogen stream in line 52 and a net gas separator bottoms stream comprising a hydrocarbon fraction in line 54. The net gas separator 150 can operate at a pressure of 0.4 MPa to 0.7 MPa and at a temperature of -100°C to -130°C.

[0040] A second portion of the medium separator overhead stream 32 in line 38 is passed to a turbo expander 135 for further expansion and cooling. The cooled second portion is then passed to a low pressure separator 140 to further remove hydrocarbons in a low pressure separator bottoms stream in line 44 and recover a hydrogen rich stream in overhead line 42. The low pressure separator 140 can operate at a pressure of 0.3 MPa to 0.4 MPa and at a temperature of -130°C to -145°C. The hydrogen rich stream 42 can be cooled in the net gas cooler 145 to provide a recycle hydrogen stream 48.

[0041] The hydrocarbon fractions in the separator bottoms streams in lines 24, 34, 44, and 54 are collected in a liquid hydrocarbon product stream in line 64. The liquid hydrocarbon product stream in line 64 can be further separated using a suitable fractionation scheme, not shown, to recover unconverted hydrocarbons and product olefins including propylene. The unconverted hydrocarbons can be recycled to the dehydrogenation reactor 105. The net gas hydrogen stream in line 52 can be optionally purified and recycled, or output as a hydrogen product, or can be used as a fuel source for the refinery complex.

[0042] The recycle hydrogen stream in line 48 is split into a return portion stream in line 56 and a feed portion stream in line 50. The split is made upstream of the cold combined feed exchanger 115 so that the fresh hydrocarbon stream in line 5 is mixed with the feed portion hydrogen stream 50 to provide a combined feed stream in line 8. The combined feed stream in line 8 can be preheated in the cold combined feed exchanger 115 before it is passed to the dehydrogenation reactor 105 in line 10.

[0043] The return portion stream in line 56 is combined with the dehydrogenation reactor effluent stream in line 12 to provide an enriched stream in line 16. The enriched stream in line 16, enriched with the return portion stream in line 56 of recycle hydrogen stream, has a H2 / HC ratio that is greater than the H2 / HC ratio of the dehydrogenation reactor effluent in line 12. The enriched stream in line 16 is then passed to the REC 110. In one embodiment, the return portion stream 56 can be preheated in the cold combined feed exchanger 115. The preheated return portion stream in line 58 is combined with the dehydrogenation reactor effluent stream 12 to provide the enriched stream 16.

[0044] EMBODIMENTS

[0045] Conventionally, dehydrogenation reactors are operated at H2 / HC ratios of 0.4 or higher. Applicants simulated a dehydrogenation process in which the reactor was operated at a H2 / HC ratio of 0.5. In this simulated base case, a propane feed was passed to a dehydrogenation reactor operated at a H2 / HC ratio of 0.5 to obtain a dehydrogenation reactor effluent comprising 42-46 mole% hydrogen and 54-58 mole% hydrocarbons. The H2 / HC ratio of the dehydrogenation reactor effluent entering the REC would vary between 0.72-0.8. This dehydrogenation reactor effluent was passed directly to the REC.

[0046] Applicant also simulated Scenario 1 and Scenario 2 for the dehydrogenation process operating at a H2 / HC ratio less than 0.4 to demonstrate the capability of the apparatus and method. In Scenario 1, the dehydrogenation reactor is operated at a H2 / HC ratio less than 0.4 with no hydrogen recycle to the dehydrogenation reactor effluent. In Scenario 2, the dehydrogenation reactor is operated at a H2 / HC ratio less than 0.4 with hydrogen recycle to the dehydrogenation reactor effluent.

[0047] In the simulated Scenario 1, the propane feed is passed to the dehydrogenation reactor operating at a low H2 / HC ratio less than 0.4 to obtain a dehydrogenation reactor effluent comprising 34-38 mole% H2 and 62-66 mole% hydrocarbons. The dehydrogenation reactor effluent is passed directly to the REC. The H2 / HC ratio of the dehydrogenation reactor effluent entering the REC varies between 0.5-0.6, which is relatively low compared to the original design when the dehydrogenation reactor is operated at a high H2 / HC ratio greater than 0.4 with a dehydrogenation reactor effluent entering the REC having a H2 / HC ratio of 0.72-0.8.

[0048] Scenario 2 differs from the operation in Scenario 1 in that a portion of the recycle hydrogen stream is combined with the dehydrogenation reactor effluent to obtain an enriched stream. The enriched stream is then passed to the REC. The enriched stream comprises 42-46% H2 and 54-58% hydrocarbons. The H2 / HC ratio of the enriched stream entering the REC varies between 0.72-0.8, which is higher than Scenario 1 and the same as the base case. The results are shown in the table below.

[0049] TABLE

[0050]

[0051] Applicant has discovered that by passing a portion of the recycle hydrogen stream to the REC, it is possible to operate the dehydrogenation reactor at a lower H2 / HC ratio in order to achieve improved selectivity to olefin production, while allowing the REC and cold box turboexpander to operate at the same H2 / HC ratio as the original design without the need for capital investment to re-size the REC or turboexpander or separator in the cold box.

[0052] SPECIFIC EMBODIMENTS

[0053] While the following is described in conjunction with the specific embodiments, it will be understood that it is intended to cover not only this description but also equivalents and alternatives and modifications thereof.

[0054] A first embodiment of the invention is a process for recovering hydrogen from a dehydrogenation reactor effluent, the process comprising passing a feed stream comprising hydrocarbons and hydrogen to a dehydrogenation reactor maintained at dehydrogenation conditions to provide a dehydrogenation reactor effluent; passing the dehydrogenation reactor effluent to a reactor effluent compressor to provide a compressed hydrocarbon stream; passing the compressed hydrocarbon stream to a cold box separation unit to provide a liquid hydrocarbon product stream and a recycle hydrogen stream; and passing a return portion of the recycle hydrogen stream to the reactor effluent compressor. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, further comprising mixing the return portion of the recycle hydrogen stream with the dehydrogenation reactor effluent to provide an enriched stream and passing the enriched stream to the reactor effluent compressor. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, wherein the hydrogen to hydrocarbon ratio of the enriched stream is higher than the hydrogen to hydrocarbon ratio of the dehydrogenation reactor effluent. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, further comprising splitting the recycle hydrogen stream into the return portion of the recycle hydrogen stream and a feed portion. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, wherein the return portion is 20-99 wt% of the recycle hydrogen stream and the feed portion is 1-80 wt% of the recycle hydrogen stream. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, further comprising splitting the recycle hydrogen stream upstream of a cold combined feed exchanger located in the separation unit. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, further comprising mixing the feed portion of the recycle hydrogen stream with a hydrocarbon stream upstream of the cold combined feed exchanger to provide the feed stream for the dehydrogenation reactor. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, further comprising exchanging heat between the feed stream and the compressed hydrocarbon stream in the cold combined feed exchanger. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, wherein the hydrogen to hydrocarbon ratio of the feed stream is in the range of 0.01 to 0.4.

[0055] A second embodiment of the invention is a process for recovering hydrogen from a dehydrogenation reactor effluent, the process comprising passing a feed stream comprising hydrocarbons and hydrogen to a dehydrogenation reactor maintained at dehydrogenation conditions to provide a dehydrogenation reactor effluent; passing the dehydrogenation reactor effluent to a reactor effluent compressor to provide a compressed hydrocarbon stream; passing the compressed hydrocarbon stream to a cold box separation unit to provide a liquid hydrocarbon product stream and a recycle hydrogen stream; and mixing a return portion of the recycle hydrogen stream with the dehydrogenation reactor effluent to provide an enriched stream and passing the enriched stream to the reactor effluent compressor. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the second embodiment of this paragraph, wherein the hydrogen to hydrocarbon ratio of the enriched stream is higher than the hydrogen to hydrocarbon ratio of the dehydrogenation reactor effluent. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the second embodiment of this paragraph, further comprising splitting the recycle hydrogen stream into the return portion of the recycle hydrogen stream and a feed portion upstream of a cold combined feed exchanger located in the separation unit. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the second embodiment of this paragraph, further comprising mixing the feed portion of the recycle hydrogen stream with a hydrocarbon stream upstream of the cold combined feed exchanger to provide the feed stream to the dehydrogenation reactor. One embodiment of the invention is one, any or all of the preceding embodiments of this paragraph through the second embodiment of this paragraph, wherein the hydrogen to hydrocarbon ratio of the feed stream is in the range of 0.01 to 0.4.

[0056] A third embodiment of the present invention is an apparatus for recovering hydrogen from a dehydrogenation reactor effluent, comprising a dehydrogenation reactor operating under dehydrogenation conditions configured to selectively dehydrogenate a feed stream comprising hydrocarbons and hydrogen and provide a dehydrogenation reactor effluent; a reactor effluent compressor in fluid communication with the dehydrogenation reactor via the dehydrogenation reactor effluent and configured to compress the dehydrogenation reactor effluent to provide a compressed hydrocarbon stream; a cold box separation unit in fluid communication with the reactor effluent compressor via the compressed hydrocarbon stream and configured to provide a liquid hydrocarbon product stream and a recycle hydrogen stream; and the reactor effluent compressor in fluid communication with the cold box separation unit via a return portion of the recycle hydrogen stream. One embodiment of the present invention is one, any or all of the preceding embodiments of this paragraph through the third embodiment of this paragraph, wherein the cold box separation unit consists of a heat exchanger comprising the cold combined feed exchanger, a plurality of separation vessels, an expander. One embodiment of the present invention is one, any or all of the preceding embodiments of this paragraph through the third embodiment of this paragraph, wherein the cold combined feed exchanger is in fluid communication with the reactor effluent compressor via the compressed hydrocarbon stream. One embodiment of the present invention is one, any or all of the preceding embodiments of this paragraph through the third embodiment of this paragraph, wherein a split is present upstream of the cold combined feed exchanger to divide the recycle hydrogen stream into the return portion and a feed portion. One embodiment of the present invention is one, any or all of the preceding embodiments of this paragraph through the third embodiment of this paragraph, wherein the reactor effluent compressor is in fluid communication with the cold combined feed exchanger via the return portion of the recycle hydrogen stream. One embodiment of the present invention is one, any or all of the preceding embodiments of this paragraph through the third embodiment of this paragraph, wherein the dehydrogenation reactor is in fluid communication with the cold combined feed exchanger via a feed stream obtained by mixing a hydrocarbon stream with a feed portion of the recycle hydrogen stream upstream of the cold combined feed exchanger.

[0057] While the application has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. It is therefore intended that whatever is described herein is to be considered merely as illustrative and non-limiting examples and that all changes and modifications that come within the scope of the present disclosure are therefore intended to be protected.

[0058] In the foregoing, all temperatures are in degrees Celsius, and all parts and percentages are by weight, unless otherwise indicated.

Claims

1. A process for recovering hydrogen from a dehydrogenation reactor effluent, the process comprising: passing a feed stream comprising hydrocarbons and hydrogen to a dehydrogenation reactor maintained at dehydrogenation conditions to provide a dehydrogenation reactor effluent, wherein the hydrogen to hydrocarbon ratio of the feed stream is in the range of 0.01 to 0.4; passing the dehydrogenation reactor effluent to a reactor effluent compressor to provide a compressed hydrocarbon stream; passing the compressed hydrocarbon stream to a cold box separation unit to provide a liquid hydrocarbon product stream and a recycle hydrogen stream; splitting the recycle hydrogen stream upstream of a cold combined feed exchanger located in the separation unit into a return portion of the recycle hydrogen stream and a feed portion; and passing the return portion of the recycle hydrogen stream to the reactor effluent compressor.

2. The method of claim 1, further comprising mixing the return portion of the recycle hydrogen stream with the dehydrogenation reactor effluent to provide an enriched stream and passing the enriched stream to the reactor effluent compressor.

3. The method of claim 2, wherein the hydrogen to hydrocarbon ratio of the enriched stream is higher than the hydrogen to hydrocarbon ratio of the dehydrogenation reactor effluent.

4. The method of claim 1, wherein the return portion is 20-99 wt% of the recycle hydrogen stream and the feed portion is 1-80 wt% of the recycle hydrogen stream.

5. The method of claim 1, further comprising mixing the feed portion of the recycle hydrogen stream with a hydrocarbon stream upstream of the cold combined feed exchanger to provide the feed stream to the dehydrogenation reactor.

6. The method of claim 5, further comprising exchanging heat in the cold combined feed exchanger between the feed stream and the compressed hydrocarbon stream.

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