Systems and methods for producing high-purity hydrogen

Abstract

A system for producing high-purity hydrogen fuel gas may include a chilling train configured to receive a purified, compressed cracked gas and produce one or more condensed liquid streams and a tail gas stream; and a demethanizer tower downstream of the chilling train configured to receive at least a portion of the one or more condensed liquid streams and produce a demethanizer overhead stream and a demethanizer bottoms stream. The system may be configured such that the tail gas and demethanizer overhead streams are not mixed. Rather, the system may be configured to process the tail gas stream in a multi-stage hydrogen expansion unit to produce a hydrogen stream for the high-purity hydrogen fuel gas and process the demethanizer overhead stream in a methane expander and drum unit to produce a methane stream. The hydrogen stream and methane stream may be used as a refrigerant in the chilling train.

Description

Technical Field

[0001] The present disclosure relates generally to producing hydrogen. More specifically, but not by way of limitation, this disclosure relates to the production of high-purity hydrogen from the fuel gas generated from a cracking furnace effluent.Background

[0002] Hydrogen is an important chemical that may be used in a variety of industrial and transportation applications. Depending on the application, the purity of the hydrogen may be important. One method of producing high-purity hydrogen is separating the hydrogen from the fuel gas produced when steam cracking hydrocarbons. For example, effluent from a steam cracker utilizing ethane as the feedstock contains hydrogen, methane, ethylene, ethane and hydrocarbons of higher molecular weight. This effluent from the cracking furnace is referred to as cracked gas. The primary products of steam cracking unit are olefins (e.g., ethylene and some propylene). When most of ethylene, ethane and hydrocarbons of higher molecular weight are separated out of the cracked gas, the reminder will be a hydrogen-rich stream containing about 82 mole% hydrogen and the rest predominantly methane. This stream is referred to as fuel gas as it is normally used as a fuel. The term fuel gas will be used to refer to this stream in the rest of this document.

[0003] Production of a high-purity hydrogen stream having hydrogen content greater than 98 mole% from the fuel gas stream requires an additional separation unit like a membrane module or a pressure swing adsorber. For example, the fuel gas is compressed and separated into a hydrogen stream and a methane stream using a membrane module or a pressure swing adsorber. The purity of the hydrogen product may be as high as 98% hydrogen after the membrane separation. However, the process to compress the hydrogen and methane mixture to the high pressure required for membrane separation or pressure swing adsorption requires high compression power. Additionally, the membrane module and pressure swing adsorber are expensive. Thus, production of high-purity hydrogen from the fuel gas stream is both capital and energy intensive.Summary

[0004] One or more embodiments includes a system comprising: an acetylene hydrogenation unit configured to receive a compressed cracked gas and produce a purified, compressed cracked gas; a chilling train downstream of the acetylene hydrogenation unit and configured to receive the purified, compressed cracked gas and produce one or more condensed liquid streams and a tail gas stream, the chilling train comprising one or more demethanizer feed drums and one or more heat exchangers; a demethanizer tower downstream of the chilling train configured to receive at least a portion of the one or more condensed liquid streams and produce a demethanizer overhead stream and a demethanizer bottoms stream; a multi-stage hydrogen expansion unit configured to receive the tail gas stream and produce two or more liquid hydrocarbon bottoms streams and a hydrogen stream, wherein the multi-stage hydrogen expansion unit comprises two or more hydrogen expander / drum combinations in series; and wherein at least one of the one or more heat exchangers in the chilling train are configured to receive the hydrogen stream as a refrigerant.

[0005] One or more embodiments include the system of any previous paragraph, wherein the system is configured to not mix the tail gas stream and the demethanizer overhead stream.

[0006] One or more embodiments include the system of any previous paragraph, wherein the one or more demethanizer feed drums are 2 to 10 demethanizer feed drums in series, wherein each of the 2 to 10 demethanizer feed drums except for a first demethanizer feed drum are configured to receive an overhead stream from an upstream demethanizer feed drum after the overhead stream passes through at least one of the one or more heat exchangers, and wherein along a flow path of the overhead streams, each heat exchanger is at a lower temperature than an upstream heat exchanger.

[0007] One or more embodiments include the system of any previous paragraph, wherein the two or more hydrogen expander / drum combinations in series are 2 to 10 hydrogen expander / drum combinations in series.

[0008] One or more embodiments include the system of any previous paragraph, wherein each of the hydrogen expander / drum combinations comprises a hydrogen expander upstream of a hydrogen drum, and wherein, except for a first expander / drum combination, each hydrogen expander is configured to receive an overhead stream from an upstream hydrogen drum. In one or more embodiments, each hydrogen drum produces a liquid hydrocarbon bottoms stream of the two or more liquid hydrocarbon streams; each of the liquid bottoms streams independently have a flow path configured to become a portion of a recycle stream or a portion of a methane produce stream; and at least one of the one or more heat exchangers in the chilling train are configured to receive the recycle stream and / or the methane produce stream as a refrigerant.

[0009] One or more embodiments include the system of any previous paragraph, further comprising: a methane expander and drum unit configured to receive the demethanizer overhead stream and to produce a second liquid hydrocarbon bottoms stream and a methane overhead stream. In one or more embodiments, at least one of the one or more heat exchangers in the chilling train are configured to receive a recycle stream as a refrigerant; and the second liquid hydrocarbon bottoms stream has a flow path configured to become a portion of the recycle stream. In one or more embodiments, the recycle stream further comprises a portion of a final condensed liquid stream from a final demethanizer feed drum of the one or more demethanizer feed drums.

[0010] One or more embodiments include the system of any previous paragraph, wherein the hydrogen stream (i) does not pass through a membrane separation unit or a pressure swing adsorption unit and (ii) is optionally not passed through a compressor downstream of the chilling train.

[0011] One or more embodiments include a method comprising: hydrogenating at least a portion of acetylene from a compressed cracked gas stream to produce a purified, compressed cracked gas stream; chilling and separating the purified, compressed cracked gas stream into one or more condensed liquid streams and a tail gas stream; demethanizing the one or more condensed liquid streams to produce a demethanizer overhead stream and a demethanizer bottom stream; expanding then separating the tail gas stream at least twice to produce two or more liquid hydrocarbon bottoms streams and a hydrogen stream, wherein the hydrogen stream comprises 98 mole% or greater hydrogen; and using the hydrogen stream as a refrigerant in one or more heat exchangers used for the chilling of the purified, compressed cracked gas stream.

[0012] One or more embodiments include the method of any previous paragraph, wherein the tail gas stream is not mixed with the demethanizer overhead stream.

[0013] One or more embodiments include the method of any previous paragraph, wherein the chilling and separating step comprises passing the purified, compressed cracked gas stream through a series of two or more heat exchangers and two or more demethanizer feed drums, wherein at least one heat exchanger is present between each of the two or more demethanizer feed drums, and wherein a final demethanizer feed drum of the two or more demethanizer feed drums operates at temperature ranging from -265°F to -240°F. In one or more embodiments, the method further comprises: using a portion of a condensed liquid stream from the final demethanizer feed drum as another refrigerant or as a portion of another refrigerant in one or more heat exchangers.

[0014] One or more embodiments include the method of any previous paragraph, wherein the two or more liquid hydrocarbon bottoms streams comprise a first liquid hydrocarbon bottoms stream having a C2+ concentration lower than a threshold concentration and a second liquid hydrocarbon bottoms stream having a C2+ concentration greater than or equal to the threshold concentration; wherein the threshold concentration is from 1 mole% to 10 mole%; and wherein the method further comprises: expanding then separating the demethanizer overhead stream to produce a methane overhead stream and a third liquid hydrocarbon bottoms stream; combining the first liquid hydrocarbon bottoms stream and the methane overhead stream to produce a methane product stream; and combining the second liquid hydrocarbon bottoms stream and the third liquid hydrocarbon bottoms stream to produce a recycle stream. In one or more embodiments, the method further comprises: using the methane product stream and / or the recycle stream as another refrigerant in at least one of the one or more heat exchangers. In one or more embodiments, the methane overhead stream has a concentration of C2+ hydrocarbons of 1 mole% or less.Brief Description of the Drawings

[0015] FIG. 1 illustrates a nonlimiting example of a system of the present disclosure for producing a high-purity hydrogen stream from fuel gas. FIG. 2 illustrates a nonlimiting example for portions of a system of the present disclosure including a multi-stage hydrogen expansion unit and a methane expander and drum unit. FIG. 3 illustrates a nonlimiting example for a portion of a system of the present disclosure including a chilling train. Detailed Description

[0016] The present disclosure relates to the production of high-purity hydrogen from fuel gas. The methods and systems of the present disclosure include a chilling train configured to generate a tail gas stream that contains hydrogen in excess of 91 mole%. Said hydrogen may be used as a feedstock for generating high-purity hydrogen with 98 mole% hydrogen or greater without the use of a membrane separation unit or pressure swing adsorption unit. More specifically, the systems and methods use series of two or more hydrogen expander / drum combinations in a multi-stage hydrogen expansion unit to purify the tail gas stream. Advantageously, the configuration of the series of two or more hydrogen expander / drum combinations results in a high-purity hydrogen stream at a useful pressure and temperature to recover refrigeration credits and mitigate the need for additional compression steps. Thus, capital and operating costs are reduced.

[0017] Further, the methods and systems of the present disclosure also include subsystems, such as a multi-stage hydrogen expansion unit and a methane expander and drum unit to separate hydrogen from other components in cracked gas. These various subsystems are an alternative to the costly membranes and other components of a membrane module or a pressure swing adsorber. By not including the membrane module or the pressure swing adsorber, the compression requirements may be reduced which reduces the overall energy requirements of the system.

[0018] Further, the subsystems may be integrated to recover refrigeration credits from the hydrogen stream and the methane stream, which may further reduce the energy requirements of the overall system. For example, in a simulation, use of the systems described herein compared to a system with a membrane module, the power consumption was reduced from about 20 MW for the system with the membrane module to about 5 MW for the system of the present disclosure. Separation using a membrane module or pressure swing adsorber requires fuel gas to be compressed to a high pressure in excess of 500 psig, which is energy intensive. By adopting the methods outlined in the present disclosure, only the methane separated out of the total stream needs to be compressed. This results in the savings mentioned above. In the simulation, the purities of the hydrogen produced for each system were similar.

[0019] It is noted that the figures show merely preferred embodiments according to the invention, and that the figures are provided by way of examples only and should be understood as such. In the figures, the same or similar reference signs or numbers refer to equal or corresponding parts.

[0020] FIG. 1 illustrates a nonlimiting example of a system of the present disclosure for producing a high-purity hydrogen stream from cracked gas. The cracked gas stream 100 from a cracking furnace is preprocessed in a multistage cracked gas compression unit 102. The multistage cracked gas compression unit 102 may include a multistage compressor, a deethanizer tower which could be heat pumped, intercoolers, suction drums, vapor and liquid dryers, a caustic tower, the like, and any combination thereof. The multistage cracked gas compression unit 102 may produce a plurality of effluent streams. In the illustrated example, only a C3+ stream 104 and a compressed cracked gas stream 106 are shown.

[0021] The compressed cracked gas stream 106 may include hydrogen, carbon monoxide, methane, ethane, ethylene, and acetylene.

[0022] The compressed cracked gas stream 106 may have a pressure ranging from 425 psi absolute (psia) to 525 psia (e.g., 425 psia to 500 psia, 450 psia to 525 psia, or 450 psia to 500 psia).

[0023] The compressed cracked gas stream 106 from the multistage cracked gas compression unit 102 may be conveyed to an acetylene hydrogenation unit 108. The acetylene hydrogenation unit 108 may be configured to selectively hydrogenate the acetylene to produce cracked gas stream containing acetylene at 0.15 mole% or less based on the inlet composition of a compressed cracked gas stream 106. The concentration of acetylene in the purified, compressed cracked gas stream 110 may be 100 ppm or less (e.g., 0 ppm to 100 ppm, 0 ppm to 50 ppm, or 0 ppm to 25 ppm).

[0024] The purified, compressed cracked gas stream 110 may be conveyed to a chilling train 112. The chilling train 112 may include one or more heat exchangers 114 to condense ethylene, ethane, and heavier hydrocarbons and one or more demethanizer feed drums to separate the condensed liquid. Generally, the purified, compressed cracked gas stream 110 may be condensed and separated multiple times using combinations of heat exchangers and demethanizer feed drums at progressively lower temperatures to produce one or more condensed liquid streams and a tail gas stream 122. Various configurations for the demethanizer feed drums and heat exchangers 114 are discussed in more detail below relative to FIG. 3. The present example illustrates three condensed liquid streams 116, 118, 120. The number of condensed liquid streams may range from 1 to 10 (e.g., 1 to 8, 1 to 6, 1 to 4, 1 to 3, 2 to 10, 2 to 8, or 2 to 5).

[0025] Each of the condensed liquid streams 116, 118, 120 may have a different composition and include one or more of the C2+ hydrocarbons in the purified, compressed cracked gas stream 110. The tail gas stream 122 may be composed primarily of hydrogen. The concentration of hydrogen in the tail gas stream 122 may range from 90 mole% to 96 mole% (e.g., from 90 mole% to 96 mole%, from 92 mole% to 96 mole%, or from 94 mole% to 96 mole%), based on the total composition of the tail gas stream 122. To achieve high hydrogen concentrations, the final demethanizer drum may be operated at a temperature ranging from -265°F to -240°F (e.g., -250°F to -245°F, or -255°F to -250°F) and / or at a pressure ranging from 380 psia to 415 psia (e.g., 390 psia to 405 psia, or 395 psia to 400 psia).

[0026] The condensed liquid streams 116, 118, 120 may contain predominantly C2+ hydrocarbons, dissolved methane, and a small amount of hydrogen. At least a portion of each of the condensed liquid streams 116, 118, 120 may be conveyed to a demethanizer tower 124 for removal of dissolved methane and hydrogen. Each of the condensed liquid streams 116, 118, 120 may be introduced to the demethanizer tower 124 at appropriate feed locations based on the temperature, pressure, and composition of each of the condensed liquid streams 116, 118, 120.

[0027] The demethanizer tower 124 may separate the condensed liquid streams 116, 118, 120 to produce a demethanizer bottoms stream 126 and a demethanizer overhead stream 128. The demethanizer bottoms stream 126 may include C2+ hydrocarbons from the condensed liquid streams 116, 118, 120. The demethanizer bottoms stream 126 may have a low concentration of methane (e.g., 250 ppm or less, 150 ppm or less, 100 ppm or less, or 50 ppm or less). The demethanizer overhead stream 128 may contain hydrogen, methane, and C2+ hydrocarbons. The demethanizer overhead stream 128 may have a low concentration of C2+ hydrocarbons (e.g., 10 mole% or less, 7 mole% or less, 5 mole% or less, 3 mole% or less, or 1 mole% or less).

[0028] The demethanizer tower 124 may be operated at an overhead pressure ranging from 325 psia to 370 psia (e.g., 325 psia to 355 psia, 335 psia to 350 psia, or 340 psia to 345 psia). Ethylene (e.g., at about -150°F) may be used as a refrigerant to condense gross overhead vapors from the tower to generate the demethanizer overhead stream 128 and a reflux (not illustrated) that is returned back to the demethanizer tower 124.

[0029] The demethanizer overhead stream 128 may include methane, ethylene, carbon monoxide, and hydrogen. For example, the demethanizer overhead stream 128 may comprise from 80 mole% to 85 mole% methane, from 2 mole% to 5 mole% ethylene, from 0.01 mole% to 0.2 mole% carbon monoxide, impurities, and a balance of hydrogen, each based on a total composition of the demethanizer overhead stream 128. The composition of the demethanizer overhead stream 128 may depend on the feedstock and conditions of the upstream cracking process.

[0030] From the process description herein, the demethanizer overhead stream 128 and the tail gas stream 122 may have a low concentration of C2+ hydrocarbons (e.g., each having 10 mole% or less, 7 mole% or less, 5 mole% or less, 3 mole% or less, or 1 mole% or less). However, these streams may have vastly different hydrogen content. For example, the tail gas stream 122 may contain 90 mole% or greater hydrogen, while the demethanizer overhead stream 128 may contain 10-18 mole% hydrogen and 80 mole% or greater methane. In conventional processing, these streams were combined and expanded through a single / multi-stage turbo expander system. The resulting condensed liquid was returned to the multistage cracked gas compression unit 102 at a suitable location. The vapor from the single / multi-stage turbo expander system in conventional processing formed the fuel gas stream composed of 82 mole% hydrogen. Accordingly, in conventional processing, the production of high-purity hydrogen stream (>98 mole%) requires installation of an additional membrane module and / or a pressure swing adsorption unit. To effectively use either of the membrane module or pressure swing adsorption unit, the fuel gas stream needs to be compressed to high pressure.

[0031] Advantageously, the present disclosure does not combine the demethanizer overhead stream 128 and the tail gas stream 122 but rather processes the streams separately. More specifically, the tail gas stream 122 may be processed through a series of two or more hydrogen expander / drum combinations in a multi-stage hydrogen expansion unit 136, discussed in more detail in FIG. 2, to increase the concentration of hydrogen therein. Further, the demethanizer overhead stream 128 may be processed through a series of one or more methane expander / drum combinations in a methane expander and drum unit 130, discussed in more detail in FIG. 2, to increase the concentration of methane therein. As a result, the systems and methods of the present disclosure produce both a high purity hydrogen-stream and a high-purity methane stream.

[0032] Referring back to FIG. 1, the demethanizer overhead stream 128 may be processed by a methane expander and drum unit 130, discussed in more detail at FIG. 2, to produce a liquid hydrocarbon bottoms stream 132 and a methane overhead stream 134.

[0033] The tail gas stream 122 from the chilling train 112 may be processed by a multi-stage hydrogen expansion unit 136, discussed in more detail at FIG. 2, to produce a plurality of streams including a hydrogen stream 142. The multi-stage hydrogen expansion unit 136 may also produce one or more methane-rich liquid streams that may or may not contain appreciable concentrations of C2+ hydrocarbons. In the illustrated example, two such streams are shown: a methane product stream 140 that may be essentially free of C2+ hydrocarbons (e.g., having less than 3 mole% C2+ hydrocarbons, preferably having less than 1 mole% C2+ hydrocarbons) and a recycle stream 138 that contains C2+ hydrocarbons.

[0034] The hydrogen stream 142 may contain hydrogen at a concentration typically 98 mole% or greater. The hydrogen stream 142 may contain hydrogen at a concentration ranging from 96 mole% to 99.5 mole% (e.g., 97 mole% to 99.5 mole%, 98 mole% to 99.5 mole%, 98.1 mole% to 99.5 mole%, 98.2 mole% to 99.5 mole%, 98.3 mole% to 99.5 mole%, 98.4 mole% to 99.5 mole%, or 98.5 mole% to 99.5 mole%), based on a total composition of the hydrogen stream 142. The hydrogen stream 142 coming from the multi-stage hydrogen expansion unit 136 may be at a temperature ranging from -310°F to -250°F (e.g., -310°F to -260°F, -310°F to -270°F, or -300°F to -280°F) and / or may be at a pressure ranging from 95 psia to 320 psia (e.g., 95 psia to 200 psia, 150 psia to 250 psia, or 200 psia to 320 psia). The hydrogen stream 142 may be directly used without compression when the required hydrogen pressure may be lower than 320 psia. Refrigeration credits may be recovered at different temperatures by warming the hydrogen stream 142, for example, to about ambient temperature.

[0035] The methane overhead stream 134 from the methane expander and drum unit 130 may be combined with one or more streams having little to no C2+ hydrocarbon produced in the multistage hydrogen expansion unit (detailed in FIG. 2) to yield the methane product stream 140. The methane product stream 140 may include methane at a concentration ranging from 80 mole% to 90 mole% (e.g., 82 mole% to 90 mole%, 84 mole% to 90 mole%, 86 mole% to 90 mole%, or 87 mole% to 90 mole%), based on a total composition of the methane product stream 140. Refrigeration credits may be recovered by warming the methane product stream 140, for example, to about -57°F.

[0036] The liquid hydrocarbon bottoms stream 132 from the methane expander and drum unit 130 may be combined with one or more C2+ hydrocarbon containing streams produced in the multistage hydrogen expansion unit (detailed in FIG. 2) to yield the recycle stream 138. The recycle stream 138 may include methane and C2+ hydrocarbons.

[0037] Each of the recycle stream 138, the methane product stream 140, and the hydrogen stream 142 may be used to derive refrigeration credits in one or more of the heat exchangers 114. The recycle stream 138 may then be recycled to an appropriate location within the multistage cracked gas compression unit 102.

[0038] The methane product stream 140 may be used for any suitable purpose. As illustrated, the methane stream may be compressed in a compressor 144 before use in a downstream unit (e.g., a steam methane reformer, an autothermal reformer, and the like). For example, the methane product stream 140 may by compressed to a pressure ranging from 500 psi gauge (psig) to 600 psig (e.g., 525 psig to 575 psig) in a reformer feed compressor.

[0039] The hydrogen stream 142 may be used in applications where high-purity hydrogen is required. Such applications may include make-up hydrogen gas to petrochemical processes, hydrogen-rich fuel for reducing carbon footprint, use in fuel cells, and the like.

[0040] FIG. 2 illustrates a nonlimiting example for portions of a system of the present disclosure including a multi-stage hydrogen expansion unit and a methane expander and drum unit. In the multi-stage hydrogen expansion unit, the tail gas stream 122 may be expanded in a first hydrogen expander 200. A portion of the tail gas stream 122 may condense as it is expanded through the first hydrogen expander 200. The resulting stream 202, which may be a mixture of vapor and liquid, may be conveyed to a first hydrogen drum 204 to produce a liquid hydrocarbon bottoms stream 206 and a vapor overhead stream 208. The liquid hydrocarbon bottoms stream 206 may be enriched in hydrocarbons, and the vapor overhead stream 208 may be enriched in hydrogen.

[0041] The vapor overhead stream 208 may be expanded in a second hydrogen expander 210, which may result in similar partial condensation. The resulting stream 212 may be conveyed to a second hydrogen drum 214 to produce a liquid hydrocarbon bottoms stream 216 and a vapor overhead stream 218. The liquid hydrocarbon bottoms stream 216 may be enriched in hydrocarbons, and the vapor overhead stream 218 may be enriched in hydrogen.

[0042] The process of hydrogen expansion and separation to produce vapor overhead and liquid bottoms may be performed at least twice (e.g., 2 to 10 times, 2 to 8 times, 2 to 6 times, or 2 to 4 times). Accordingly, the number of hydrogen expander and drum combinations in the multi-stage hydrogen expansion unit may be at least 2 (e.g., 2 to 10, 2 to 8, 2 to 6, or 2 to 4).

[0043] In a final combination, a vapor overhead stream may be expanded in a final hydrogen expander 220. The resulting stream 222 may be conveyed to a final hydrogen drum 224 to produce a liquid hydrocarbon bottoms stream 226 and a hydrogen stream 142 as the vapor overhead stream. The liquid hydrocarbon bottoms stream 226 may be enriched in hydrocarbons. The hydrogen stream 142 is described in FIG. 1.

[0044] In a methane expander and drum unit, the demethanizer overhead stream 128 from the demethanizer tower 124 may be expanded in a methane expander 232. A portion of the demethanizer overhead stream 128 may condense as it passes through the methane expander 232. The resulting stream 234 may be a mixture of vapor and liquid. The resulting stream 234 may be conveyed to a methane drum 236 to produce the liquid hydrocarbon bottoms stream 132 and the methane overhead stream 134.

[0045] The liquid hydrocarbon bottoms stream 132 may contain significant amounts of C2+ hydrocarbons with the balance being methane and impurities (e.g., 1 mole% or less impurities). For example, the concentration of C2+ hydrocarbons in the liquid hydrocarbon bottoms stream 132 may be up to 40 mole% (e.g., up to 35 mole%, or up to 30 mole%).

[0046] The methane overhead stream 134 may be depleted in C2+ hydrocarbons. The concentration of C2+ hydrocarbons in the methane overhead stream 134 may be 1 mole% or less (e.g., 0.75 mole% or less, 0.5 mole% or less, or 0.25 mole% or less).

[0047] The methane expander 232 may reduce the pressure of the demethanizer overhead stream 128 to a pressure ranging from 45 psia to 60 psia (e.g., 50 psia to 60 psia, or 55 psia to 60 psia). Lower discharge pressure may reduce the C2+ hydrocarbons carried out in the methane overhead stream 134. The discharge pressure of the methane expander 232 may be set to achieve a desired concentration of C2+ hydrocarbons in methane overhead stream 134.

[0048] Returning to the multistage hydrogen expansion unit, the liquid hydrocarbon bottoms streams 206, 216, 226 (and any additional liquid bottoms streams if additional hydrogen expander / drum combinations are included) obtained from the hydrogen drums 204, 214, 224 may be rich in methane and may or may not contain C2+ hydrocarbons. Each of the liquid hydrocarbon bottoms stream 206, 216, 226 may independently be combined with (i) methane overhead stream 134 or (ii) the liquid hydrocarbon bottoms stream 132. A liquid bottoms stream from a hydrogen drum containing high amounts of C2+ hydrocarbons may be combined with the liquid hydrocarbon bottoms stream 132 obtained from the methane drum 236 to generate the recycle stream 138. A liquid bottoms stream from a hydrogen drum containing low to negligible amounts of C2+ hydrocarbons may be combined with methane overhead stream 134 from the methane drum 236 to generate the methane product stream 140. The amount of C2+ hydrocarbons present in a liquid bottoms stream from a hydrogen drum to determine if said stream should be used to produce the recycle stream 138 or the methane product stream 140 may depend on the desired purity of the methane product stream 140 and desired overall ethylene recovery from the steam cracker unit. The threshold concentration for C2+ hydrocarbons present in a liquid bottoms stream from a hydrogen drum may be a value from 1 mole% to 10 mole% (e.g., 10 mole%, 8 mole%, 6 mole%, 5 mole%, 4 mole%, 3 mole%, 2 mole%, or 1 mole%, or any value therebetween), where a liquid bottoms stream from a hydrogen drum with a concentration lower than the threshold concentration may be used for producing the methane product stream 140 and a liquid bottoms stream from a hydrogen drum with concentration greater than equal to the threshold concentration may be used for producing the recycle stream 138.

[0049] For example, the liquid hydrocarbon bottoms stream 206 may contain more than 3 mole% C2+ hydrocarbons and, as illustrated, be mixed with liquid hydrocarbon bottoms stream 132. Further, while liquid hydrocarbon bottoms streams 216, 226 that contain lesser amounts of C2+ hydrocarbons and, as illustrated, be mixed with methane overhead stream 134. Recycling streams containing C2+ hydrocarbons may enhance ethylene recovery at the expense of compressor power.

[0050] The power obtained from the methane expander 232 and each of the hydrogen expanders (e.g., 200, 210, 220, and more, if included) may be used to compress process fluids (e.g., recycle ethane vapors, residue gas, and the like) or may be used to generate electricity.

[0051] FIG. 3 illustrates a nonlimiting example for a portion of a system of the present disclosure including a chilling train. The chilling train may include one or more combinations of a heat exchanger and a demethanizer feed drum where the liquid condensed out in the heat exchanger due to chilling is separated in a demethanizer feed drum. This liquid bottoms stream from the demethanizer feed drum may be used as demethanizer feed, and the vapor overhead stream separated in the demethanizer feed drum may be chilled further.

[0052] In the illustrated example, the purified, compressed cracked gas stream 110 may be conveyed through a first heat exchanger 114a and then a first demethanizer feed drum 300 to produce the condensed liquid stream 120. The overhead stream 302 may be passed through one or more heat exchangers, illustrated as one heat exchanger 114b. The heat exchanger 114b chills the overhead stream 302, which may condense a portion of the overhead stream 302, before introduction to a second demethanizer feed drum 304. The second demethanizer feed drum 304 separates the overhead stream 302, which upon introduction to the second demethanizer feed drum 304 may be a mixture of liquid and vapor, to produce the condensed liquid stream 118 and an overhead stream 306. The overhead stream 306 may be passed through one or more heat exchangers, illustrated as one heat exchanger 114c, to chill the overhead stream 306.

[0053] Each of the heat exchangers along the flow path chills to cool and at least partially condense the purified, compressed cracked gas stream 110. The heat exchangers may get increasingly cold along the flow path from the first demethanizer feed drum 300 to a final demethanizer feed drum 308. That is, a downstream heat exchange may be at a lower temperature than a correspondingly upstream heat exchanger.

[0054] The process of chilling and separating may be performed at least once (e.g., 1 to 10 times, 1 to 8 times, 1 to 6 times, 1 to 4 times, 2 to 10 times, 2 to 8 times, 2 to 6 times, or 2 to 4 times). The number of heat exchangers an individual overhead stream passes through before separation in the next demethanizer feed drum may range from 1 to 3 (e.g., 1 to 2). Each overhead stream does not have to pass through the same number of heat exchangers. Accordingly, the number of combinations of a demethanizer feed drum with 1 to 3 heat exchangers in the chilling train may be at least 1 (e.g., 1 to 10, 1 to 8, 1 to 6, 1 to 4, 2 to 10, 2 to 8, 2 to 6, or 2 to 4).

[0055] In each of the heat exchangers 114b, 114c (and any additional heat exchangers), refrigeration credits may be recovered from cold process streams that need to be warmed before leaving the chilling train 112. Refrigeration credits may be recovered from the hydrogen stream 142, the methane product stream 140, the recycle stream 138, and any other cold streams that need to be warmed before leaving the chilling train 112.

[0056] The streams 110, 302, 306 are chilled in the heat exchangers 114 against the cold process streams as described above. The total refrigeration duty available in the cold process streams (e.g., streams 142, 140, 138) may not match the chilling requirements of streams 110, 302, 306 being chilled. Accordingly, the residual refrigeration duty may be supplied by refrigerant streams at suitable temperatures.

[0057] The coldest level of refrigeration used may be ethylene refrigeration at a temperature of -150°F. Preferably, the final demethanizer feed drum 308 operates at a temperature ranging from -265°F to -240°F. The final heat exchanger in the flow path from the first demethanizer feed drum 300 to a final demethanizer feed drum 308 is the coldest heat exchanger and may not need an external refrigerant to supply residual duty. Refrigeration credits may be attained from the hydrogen stream 142, the methane product stream 140, the recycle stream 138, or any combination thereof. Refrigeration credits in the last (coldest) heat exchanger may also be derived from the liquid bottoms 116 (which is the condensed liquid stream 116) from the final demethanizer feed drum 308, which is shown as stream 116a. As illustrated, the stream 116a is combined with the recycle stream 138. Other configurations, including directly passing the stream 116a through the last (coldest) heat exchanger, are contemplated.

[0058] Compared to conventional processes for producing hydrogen, the final demethanizer feed drum 308 of the systems and methods of the present disclosure may operate at colder temperatures. The additional refrigeration duty needed to attain such temperatures may be obtained by combining a portion 116a of the liquid bottoms 116 from the final demethanizer feed drum 308 with the recycle stream 138. The combined stream may be passed through one or more of the heat exchangers 114 in chilling train 112 before passing on to the cracked gas compression unit 102.

[0059] In another alternate system, a cryogenic feed stock if available may be integrated into heat exchanger 114 to reduce duty supplied by refrigerant or to eliminate the need to recycle the portion 116a of the liquid bottoms 116 from the final demethanizer feed drum 308 to the recycle stream 138.

[0060] In yet another alternate system, some of C3+ hydrocarbons may be carried over through the compressed cracked gas stream 106 from the cracked gas compression unit 102 into the chilling train. The condensed liquid (e.g., the condensed liquid streams 116, 118, 120 ) may be processed in a combination of two demethanizer towers in series.

[0061] The foregoing description of certain examples, including illustrated examples, has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Numerous modifications, adaptations, and uses thereof will be apparent to those skilled in the art without departing from the scope of the disclosure. For instance, any examples described herein can be combined with any other examples to yield further examples.

Claims

1. A system comprising: an acetylene hydrogenation unit configured to receive a compressed cracked gas and produce a purified, compressed cracked gas; a chilling train downstream of the acetylene hydrogenation unit and configured to receive the purified, compressed cracked gas and produce one or more condensed liquid streams and a tail gas stream, the chilling train comprising one or more demethanizer feed drums and one or more heat exchangers; a demethanizer tower downstream of the chilling train configured to receive at least a portion of the one or more condensed liquid streams and produce a demethanizer overhead stream and a demethanizer bottoms stream; a multi-stage hydrogen expansion unit configured to receive the tail gas stream and produce two or more liquid hydrocarbon bottoms streams and a hydrogen stream, wherein the multi-stage hydrogen expansion unit comprises two or more hydrogen expander / drum combinations in series; and wherein at least one of the one or more heat exchangers in the chilling train are configured to receive the hydrogen stream as a refrigerant.

2. The system of claim 1, wherein the system is configured to not mix the tail gas stream and the demethanizer overhead stream.

3. The system of any preceding claim, wherein the one or more demethanizer feed drums are 2 to 10 demethanizer feed drums in series, wherein each of the 2 to 10 demethanizer feed drums except for a first demethanizer feed drum are configured to receive an overhead stream from an upstream demethanizer feed drum after the overhead stream passes through at least one of the one or more heat exchangers, and wherein along a flow path of the overhead streams, each heat exchanger is at a lower temperature than an upstream heat exchanger.

4. The system of any preceding claim, wherein the two or more hydrogen expander / drum combinations in series are 2 to 10 hydrogen expander / drum combinations in series.

5. The system of any preceding claim, wherein each of the hydrogen expander / drum combinations comprises a hydrogen expander upstream of a hydrogen drum, and wherein, except for a first expander / drum combination, each hydrogen expander is configured to receive an overhead stream from an upstream hydrogen drum.

6. The system of claim 5, wherein each hydrogen drum produces a liquid hydrocarbon bottoms stream of the two or more liquid hydrocarbon streams; wherein each of the liquid bottoms streams independently have a flow path configured to become a portion of a recycle stream or a portion of a methane produce stream; and wherein at least one of the one or more heat exchangers in the chilling train are configured to receive the recycle stream and / or the methane produce stream as a refrigerant.

7. The system of any preceding claim further comprising: a methane expander and drum unit configured to receive the demethanizer overhead stream and to produce a second liquid hydrocarbon bottoms stream and a methane overhead stream.

8. The system of claim 7, wherein at least one of the one or more heat exchangers in the chilling train are configured to receive a recycle stream as a refrigerant; and wherein the second liquid hydrocarbon bottoms stream has a flow path configured to become a portion of the recycle stream; optionally, wherein the recycle stream further comprises a portion of a final condensed liquid stream from a final demethanizer feed drum of the one or more demethanizer feed drums.

9. The system of any preceding claim, wherein the hydrogen stream (i) does not pass through a membrane separation unit or a pressure swing adsorption unit and (ii) is optionally not passed through a compressor downstream of the chilling train.

10. A method comprising: hydrogenating at least a portion of acetylene from a compressed cracked gas stream to produce a purified, compressed cracked gas stream; chilling and separating the purified, compressed cracked gas stream into one or more condensed liquid streams and a tail gas stream; demethanizing the one or more condensed liquid streams to produce a demethanizer overhead stream and a demethanizer bottom stream; expanding then separating the tail gas stream at least twice to produce two or more liquid hydrocarbon bottoms streams and a hydrogen stream, wherein the hydrogen stream comprises 98 mole% or greater hydrogen; and using the hydrogen stream as a refrigerant in one or more heat exchangers used for the chilling of the purified, compressed cracked gas stream.

11. The method of claim 10, wherein the tail gas stream is not mixed with the demethanizer overhead stream.

12. The method of claim 10 or claim 11, wherein the chilling and separating step comprises passing the purified, compressed cracked gas stream through a series of two or more heat exchangers and two or more demethanizer feed drums, wherein at least one heat exchanger is present between each of the two or more demethanizer feed drums, and wherein a final demethanizer feed drum of the two or more demethanizer feed drums operates at temperature ranging from -265°F to -240°F.

13. The method of claim 12, further comprising: using a portion of a condensed liquid stream from the final demethanizer feed drum as another refrigerant or as a portion of another refrigerant in one or more heat exchangers.

14. The method of any one of claim 10-13 wherein the two or more liquid hydrocarbon bottoms streams comprise a first liquid hydrocarbon bottoms stream having a C2+ concentration lower than a threshold concentration and a second liquid hydrocarbon bottoms stream having a C2+ concentration greater than or equal to the threshold concentration; wherein the threshold concentration is from 1 mole% to 10 mole%; and wherein the method further comprises: expanding then separating the demethanizer overhead stream to produce a methane overhead stream and a third liquid hydrocarbon bottoms stream; combining the first liquid hydrocarbon bottoms stream and the methane overhead stream to produce a methane product stream; and combining the second liquid hydrocarbon bottoms stream and the third liquid hydrocarbon bottoms stream to produce a recycle stream.

15. The method of claim 14, further comprising: using the methane product stream and / or the recycle stream as another refrigerant in at least one of the one or more heat exchangers; optionally, wherein the methane overhead stream has a concentration of C2+ hydrocarbons of 1 mole% or less.

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