METHODS AND SYSTEMS FOR PERFORMING CHEMICAL SEPARATIONS
The present disclosure provides a method for generating higher hydrocarbon(s) from a stream comprising compounds with two or more carbon atoms (C2+), comprising introducing methane and an oxidant (e.g., O2) into an oxidative coupling of methane (OCM) reactor. The OCM reactor reacts the methane with the oxidant to generate a first product stream comprising the C2+ compounds. The first product stream can then be directed to a separations unit that recovers at least a portion of the C2+ compounds from the first product stream to yield a second product stream comprising the at least the portion of the C2+ compounds.
This application claims priority to U.S. Provisional Patent Application Ser. No. 62/436,312, filed Dec. 19, 2016, which is entirely incorporated herein by reference for all purposes.
STATEMENT OF GOVERNMENT SUPPORTThis invention was made with government support under grant numbers DE-EE0005769 awarded by the United States Department of Energy (DOE). The government has certain rights in the invention.
BACKGROUNDThe modern refining and petrochemical industry may make extensive use of fractionation technology to produce and separate various potentially desirable compounds from crude oil. The conventional fractionation technology may be energy intensive and costly to install and operate. Cryogenic distillation has been used to separate and recover hydrocarbon products in various refining and petrochemical industries.
SUMMARYRecognized herein is a need for non-cryogenic separation methods and systems, such as for oxidative coupling of methane (OCM) processes.
Aspects of the present disclosure provide processes for recovering olefins from a stream containing mix of hydrocarbons by utilizing techniques based on the use of adsorbents. In some embodiments, systems and methods enable the separation, pre-separation, purification and/or recovery of hydrocarbons, including, but not limited to, olefins such as ethylene and propylene, paraffins such as methane and ethane, and CO2, from a multicomponent hydrocarbon stream such as an effluent stream from an oxidative coupling of methane (OCM) reactor or an ethylene-to-liquids (ETL) reactor. The hydrocarbon stream can also be a feed to the OCM or ETL reactor in certain cases. In certain cases, the feed to the ETL reactor is an effluent from OCM reactor. In some cases, a separation process utilizing adsorbents can be used to purify and pre-treat existing hydrocarbon streams (such as refinery off-gases, cracker off-gas, streams from NGL plants, and others), followed by use of the resulting olefin rich stream (e.g., pressure swing adsorption tail gas) as the ETL feed.
The present disclosure provides various improvements in OCM and ETL processes, such as, without limitation, a separation and pre-separation process to recover desired or predetermined components from an OCM reactor effluent, CO2 recovery and capture techniques, enhanced heat recovery methods to utilize the OCM reaction heat more efficiently, and techniques and technologies to further reduce the carbon footprint of the OCM process.
An aspect of the present disclosure provides a method for separating a product from a gas mixture, the method comprising: (a) at a first total pressure, directing a gas mixture comprising at least one impurity and a product gas into a pressure swing adsorption (PSA) vessel containing an adsorbent to adsorb the product gas on the adsorbent, wherein the product gas has a first partial pressure; (b) at a second total pressure, directing a sweep gas into the PSA vessel to adsorb the sweep gas on the adsorbent and displace the product from the adsorbent to yield a displaced product, wherein the sweep gas has a second partial pressure that is greater than the first partial pressure of the product in the gas mixture; and (c) desorbing the sweep gas from the adsorbent such that additional product is capable of adsorbing on the adsorbent.
In some embodiments, (c) is performed at substantially the first total pressure. In some embodiments, an amount of the product that adsorbs on the adsorbent at the first partial pressure in (a) is substantially equivalent to the amount of sweep gas that adsorbs on the adsorbent at the second partial pressure in (b). In some embodiments, an amount of heat released by the adsorption of the product in (a) is substantially equivalent to an amount of heat released by the adsorption of the sweep gas in (b). In some embodiments, the displaced product is enriched relative to the concentration of the product in the gas mixture by a factor of at least about 2, 3, 4, 5, 10, 15, 20 or more. In some embodiments, the displaced product includes at least some of the sweep gas. In some embodiments, the method further comprises, following displacing the product from the adsorbent, separating the product from the sweep gas (e.g., by distillation). In some embodiments, the product is ethylene. In some embodiments, the sweep gas is ethane. In some embodiments, the gas mixture comprises at least 3, 4, 5, 6, 7, 8, 9, 10 or more impurities. In some embodiments, the at least one impurity comprise carbon monoxide (CO), carbon dioxide (CO2), methane, ethane, hydrogen (H2), or any combination thereof. In some embodiments, the product has a concentration of less than about 30%, 25%, 20%, 15%, 10%, 5%, 1% or less in the gas mixture. In some embodiments, the adsorbent is a metal organic framework (MOF). In some embodiments, the MOF is M2(dobdc). In some embodiments, at the first pressure, a selectivity of the adsorbent for adsorbing the product as compared to the at least one impurity is at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 or more. In some embodiments, the at least one impurity is methane. In some embodiments, the gas mixture is derived from an effluent from an oxidative coupling of methane (OCM) reactor. In some embodiments, the method further comprises recycling the at least one impurity to the OCM reactor following the adsorption of the product from the gas mixture. In some embodiments, the method further comprises, prior to recycling the at least one impurity to the OCM reactor, converting carbon dioxide (CO2), carbon monoxide (CO), and/or hydrogen (H2) components of the at least one impurity to methane (CH4). In some embodiments, following the adsorption of the product from the gas mixture, the gas mixture comprises a predetermined amount of a given impurity. In some embodiments, the given impurity is CO2. In some embodiments, the adsorbent comprises (i) a first material that adsorbs the product and the given impurity, and (ii) a second material that adsorbs the product but does not substantially adsorb the given impurity. In some embodiments, an amount of the first material relative to an amount of the second material is selected to achieve the desired amount of the desired impurity. In some embodiments, the first material is a CaX zeolite and the second material is a metal organic framework.
Another aspect of the present disclosure provides a system for recovering compounds with two or more carbon atoms (C2+ compounds) from an oxidative coupling of methane (OCM) process, the system comprising: (a) an OCM reactor configured to receive oxygen (O2) and methane (CH4) and react the O2 and CH4 to produce an OCM product stream, which OCM product stream comprises C2+ compounds including ethylene (C2H4) and ethane (C2H6) and (ii) C1 compounds including carbon dioxide (CO2) and un-reacted CH4; (b) a first separations unit fluidically coupled to the OCM reactor and configured to receive the OCM product stream, wherein the first separations unit is configured to separate the OCM product stream into (i) a first light stream comprising the un-reacted CH4 and a first portion of the CO2 (ii) a first heavy stream comprising the C2H4 and the C2H6 and a second portion of the CO2; (c) a methanation reactor fluidically coupled to the first separation unit and to the OCM reactor, wherein the methanation reactor is configured to receive the first light stream and convert the first portion of the CO2 into additional CH4, which additional CH4 is recycled to the OCM reactor; and (d) a second separations unit fluidically coupled to the first separations unit and configured to receive the first heavy stream, wherein the second separations unit is configured to separate the first heavy stream into (i) a second light stream comprising the C2H6 and (ii) a second heavy stream comprising the C2H4.
In some embodiments, the OCM reactor has a catalytic section and a cracking section, which catalytic section reacts the O2 and CH4 to yield ethylene (C2H4), ethane (C2H6) and heat, which cracking section uses the heat to convert C2H6 into C2H4. In some embodiments, the second light stream is recycled to the cracking section of the OCM reactor. In some embodiments, the second portion of the CO2 is vented. In some embodiments, the system further comprises a CO2 removal unit fluidically coupled to the first separations unit and the second separations unit and configured to receive the first heavy stream, which CO2 removal unit is configured to separate the second portion of the CO2 from the first heavy stream before passing the first heavy stream on to the second separations unit. In some embodiments, the first separations unit contains CaX zeolite. In some embodiments, the second separations unit contains a metal organic framework (MOF). In some embodiments, the MOF is M2(dobdc). In some embodiments, the MOF is M2(m-dobdc). In some embodiments, the first separations unit is a pressure swing adsorption (PSA) unit. In some embodiments, the second separations unit is a pressure swing adsorption (PSA) unit. In some embodiments, the first separations unit is configured to be purged with vacuum, with ethane, with propane, or with any combination thereof. In some embodiments, the second separations unit is configured to be purged with vacuum, with ethane, with propane, or with any combination thereof. In some embodiments, the first separations unit and the second separations unit are configured to be purged with propane. In some embodiments, the first separations unit and the second separations unit are layers of a separation bed. In some embodiments, the first separations unit and the second separations unit are combined in a single separations unit having M2(m-dobdc).
Another aspect of the present disclosure provides a method for separating oxygen (O2) from nitrogen (N2), the method comprising: (a) at a first pressure, directing a mixture of O2 and N2 into a pressure swing adsorption (PSA) vessel containing an adsorbent to adsorb the O2 on the adsorbent, wherein the adsorbent is a metal organic framework (MOF) that is selective for O2; and (b) at a second pressure that is less than the first pressure, desorbing the O2 from the adsorbent with a purge gas.
In some embodiments, the method further comprises depressurizing the PSA vessel prior to (b). In some embodiments, the second pressure is less than atmospheric pressure. In some embodiments, the second pressure is less than about 1 bar, 0.9 bar, 0.8 bar, 0.7 bar, 0.6 bar, 0.5 bar, 0.4 bar, 0.3 bar, 0.2 bar, 0.1 bar or less. In some embodiments, the purge gas displaces the O2 from the adsorbent, whereby the purge gas becomes adsorbed on the adsorbent. In some embodiments, the purge gas is CO2 or CH4. In some embodiments, the purge gas is air. In some embodiments, the purge gas is the N2 that is not adsorbed in (a). In some embodiments, the MOF is M2(dobdc). In some embodiments, the MOF is Fe2(dobdc). In some embodiments, the mixture of O2 and N2 is air. In some embodiments, the first pressure is a pressure such that the adsorbent is at least about 70%, 80%, 90%, 95%, 99% saturated with O2. In some embodiments, (a) is performed for less than about 1 minute. In some embodiments, (a) and (b) are performed at different temperatures.
Another aspect of the present disclosure provides a system for separating oxygen (O2) from nitrogen (N2), the system comprising: a pressure swing adsorption (PSA) vessel containing an adsorbent, wherein the PSA vessel is configured to receive a mixture of O2 and N2 at a first pressure and adsorb the O2 on the adsorbent, wherein the adsorbent is a metal organic framework (MOF) that is selective for O2; a source of a purge gas in fluid communication with the PSA vessel; and a controller operatively coupled to the source of the purge gas, wherein the controller is programmed to subject the purge gas to flow from the source of the purge gas to the PSA vessel to desorb the O2 from the adsorbent at a second pressure that is less than the first pressure.
In some embodiments, the controller is programmed to depressurize the PSA vessel prior to subjecting the purge gas to flow from the source of the purge gas to the PSA vessel. In some embodiments, the second pressure is less than atmospheric pressure. In some embodiments, the second pressure is less than about 1 bar, 0.9 bar, 0.8 bar, 0.7 bar, 0.6 bar, 0.5 bar, 0.4 bar, 0.3 bar, 0.2 bar, 0.1 bar or less. In some embodiments, the purge gas is CO2 or CH4. In some embodiments, the purge gas is air. In some embodiments, the MOF is M2(dobdc). In some embodiments, the MOF is Fe2(dobdc). In some embodiments, the system further comprises an oxidative coupling of methane (OCM) reactor downstream of the PSA vessel, wherein the OCM reactor is configured to (i) receive methane (CH4) and at least a portion of the O2 and (ii) react the at least the portion of the O2 and the CH4 in an OCM process to yield a product stream comprising compounds with two or more carbon atoms (C2+ compounds).
Another aspect of the present disclosure provides a method for generating compounds with two or more carbon atoms (C2+ compounds), comprising: (a) directing oxygen (O2) and methane (CH4) into an oxidative coupling of methane (OCM) reactor that reacts the O2 and the CH4 in an OCM process to yield a product stream comprising (i) C2+ compounds including olefins and paraffins and (ii) carbon monoxide (CO) and/or carbon dioxide (CO2); (b) directing the product stream from the OCM reactor into a separations unit that selectively adsorbs the olefins from the paraffins, wherein the separations unit comprises (i) a pressure swing adsorption (PSA) unit, (ii) a temperature swing adsorption (TSA) unit, or (iii) a membrane unit, and wherein the PSA unit, the TSA unit or the membrane unit comprises a sorbent that selectively adsorbs the olefins; and (c) desorbing the olefins from the sorbent.
In some embodiments, the separations unit selectively separates ethylene from the paraffins. In some embodiments, the sorbent has dispersed metal ions that are capable of complexing with the olefins. In some embodiments, the sorbent is selected from a zeolite, a molecular sieve sorbent, a carbon molecular sieve, an activated carbon, a carbon nanotube, a metal-organic framework (MOF), and a polymeric resin. In some embodiments, the method further comprises separating the CO and/or CO2 from the C2+ compounds. In some embodiments, the sorbent is a MOF, the olefin is ethylene, and the ethylene is desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a method for generating compounds with two or more carbon atoms (C2+ compounds), comprising: (a) directing oxygen (O2) and methane (CH4) into an oxidative coupling of methane (OCM) reactor that reacts the O2 and CH4 in an OCM process to yield a product stream comprising (i) C2+ compounds including ethylene (C2H4) and (ii) C1 compounds including un-reacted CH4; and (b) directing the product stream into a separations unit that separates the C2+ compounds from the C1 compounds, which separations unit does not contain a de-methanizer.
In some embodiments, the separations unit contains a distillation column and an oil absorber. In some embodiments, the distillation column does not condense methane. In some embodiments, the separations unit comprises a MOF. In some embodiments, the C2+ compounds are desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a method for generating compounds with two or more carbon atoms (C2+ compounds), comprising: (a) directing oxygen (O2) and methane (CH4) into an oxidative coupling of methane (OCM) reactor that reacts the O2 and CH4 in an OCM process to yield a product stream comprising (i) C2+ compounds including ethylene (C2H4) and (ii) C1 compounds including un-reacted CH4; and (b) directing the product stream into a separations unit containing a metal organic framework (MOF) that produces (i) a bottoms stream comprising the C2+ compounds and (ii) an overhead stream comprising the C1 compounds.
In some embodiments, the method further comprises (c) directing the overhead stream to a methanation unit for converting carbon dioxide (CO2) and/or carbon monoxide (CO) into methane (CH4); and (d) directing the CH4 into the OCM reactor. In some embodiments, the method further comprises (e) directing the bottoms stream to a second separations unit containing a metal organic framework (MOF) that separates olefins from paraffins. In some embodiments, the separations unit comprises a pressure swing absorber (PSA) that contains the MOF. In some embodiments, the separations unit comprises a temperature swing absorber (TSA) that contains the MOF. In some embodiments, the overhead stream includes hydrogen (H2). In some embodiments, the C2+ compounds are desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a method for generating compounds with two or more carbon atoms (C2+ compounds), comprising: (a) directing oxygen (O2) and methane (CH4) into an oxidative coupling of methane (OCM) reactor having a catalytic section and a cracking section to produce an OCM product stream, which catalytic section reacts the O2 and CH4 to yield ethylene (C2H4), ethane (C2H6) and heat, which cracking section uses the heat to convert C2H6 into C2H4, and which product stream comprises (i) C2+ compounds including ethylene (C2H4) and ethane (C2H6) and (ii) C1 compounds including un-reacted CH4; (b) directing the product stream into a separations unit containing a metal organic framework (MOF) that produces (i) a first stream comprising the C2H4, (ii) a second stream comprising the C2H6 and (iii) a third stream comprising the C1 compounds; (c) directing the second stream into the cracking section; and (d) directing the third stream into the catalytic section.
In some embodiments, the third stream is directed to a methanation unit prior to directing to the catalytic section, which methanation unit converts carbon dioxide (CO2) and/or carbon monoxide (CO) into methane (CH4). In some embodiments, the separations unit comprises a pressure swing absorber (PSA) that contains the MOF. In some embodiments, the separations unit comprises a temperature swing absorber (TSA) that contains the MOF. In some embodiments, the third stream includes hydrogen (H2). In some embodiments, the ethylene is desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a system for generating compounds with two or more carbon atoms (C2+ compounds), comprising: an oxidative coupling of methane (OCM) reactor configured to receive oxygen (O2) and methane (CH4) and react the O2 and CH4 in an OCM process to yield a product stream comprising (i) C2+ compounds including olefins and paraffins and (ii) carbon monoxide (CO) and/or carbon dioxide (CO2); and a separations unit fluidically coupled to the OCM reactor and configured to receive the product stream from the OCM reactor, wherein the separations unit selectively adsorbs the olefins from the paraffins, wherein the separations unit comprises (i) a pressure swing adsorption (PSA) unit, (ii) a temperature swing adsorption (TSA) unit, or (iii) a membrane unit, and wherein the PSA unit, the TSA unit or the membrane unit comprises a sorbent that selectively adsorbs the olefins, which olefins can be desorbed from the sorbent.
In some embodiments, the separations unit selectively separates ethylene from the paraffins. In some embodiments, the sorbent has dispersed metal ions that are capable of complexing with the olefins. In some embodiments, the sorbent is selected from a zeolite, a molecular sieve sorbent, a carbon molecular sieve, an activated carbon, a carbon nanotube, a metal-organic framework (MOF), and a polymeric resin. In some embodiments, the system further comprises a module capable of separating the CO and/or CO2 from the C2+ compounds. In some embodiments, the sorbent is a MOF, the olefin is ethylene, and the ethylene is desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a system for generating compounds with two or more carbon atoms (C2+ compounds), comprising: an oxidative coupling of methane (OCM) reactor configured to receive oxygen (O2) and methane (CH4) and react the O2 and CH4 in an OCM process to yield a product stream comprising (i) C2+ compounds including ethylene (C2H4) and (ii) C1 compounds including un-reacted CH4; and a separations unit fluidically coupled to the OCM reactor and configured to receive the product stream from the OCM reactor, wherein the separations unit separates the C2+ compounds from the C1 compounds, and wherein the separations unit does not contain a de-methanizer.
In some embodiments, the separations unit contains a distillation column and an oil absorber. In some embodiments, the distillation column does not condense methane. In some embodiments, the separations unit comprises a MOF. In some embodiments, the C2+ compounds are desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a system for generating compounds with two or more carbon atoms (C2+ compounds), comprising: an oxidative coupling of methane (OCM) reactor configured to receive oxygen (O2) and methane (CH4) and react the O2 and CH4 in an OCM process to yield a product stream comprising (i) C2+ compounds including ethylene (C2H4) and (ii) C1 compounds including un-reacted CH4; and a separations unit fluidically coupled to the OCM reactor and configured to receive the product stream from the OCM reactor, wherein the separations unit contains a metal organic framework (MOF) that produces (i) a bottoms stream comprising the C2+ compounds and (ii) an overhead stream comprising the C1 compounds.
In some embodiments, the system further comprises a methanation unit fluidically coupled to the separations unit and configured receive the overhead stream from the separations unit, wherein the methanation unit converts carbon dioxide (CO2) and/or carbon monoxide (CO) into methane (CH4), and wherein the CH4 is directed into the OCM reactor. In some embodiments, the system further comprises a second separations unit fluidically coupled to the separations unit and configured receive the bottoms stream from the separations unit, wherein the second separations unit contains a metal organic framework (MOF) that separates olefins from paraffins. In some embodiments, the separations unit comprises a pressure swing absorber (PSA) that contains the MOF. In some embodiments, the separations unit comprises a temperature swing absorber (TSA) that contains the MOF. In some embodiments, the overhead stream includes hydrogen (H2). In some embodiments, the C2+ compounds are desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides a system for generating compounds with two or more carbon atoms (C2+ compounds), comprising: an oxidative coupling of methane (OCM) reactor configured to receive oxygen (O2) and methane (CH4) and react the O2 and CH4 to produce an OCM product stream, the OCM reactor having a catalytic section and a cracking section, which catalytic section reacts the O2 and CH4 to yield ethylene (C2H4), ethane (C2H6) and heat, which cracking section uses the heat to convert C2H6 into C2H4, and which product stream comprises (i) C2+ compounds including ethylene (C2H4) and ethane (C2H6) and (ii) C1 compounds including un-reacted CH4; and a separations unit fluidically coupled to the OCM reactor and configured to receive the product stream from the OCM reactor, wherein the separations unit contains a metal organic framework (MOF) that produces (i) a first stream comprising the C2H4, (ii) a second stream comprising the C2H6 and (iii) a third stream comprising the C1 compounds, and wherein the second stream and the third stream are directed into the cracking section and the catalytic section respectively.
In some embodiments, the third stream is directed to a methanation unit prior to directing to the catalytic section, which methanation unit converts carbon dioxide (CO2) and/or carbon monoxide (CO) into methane (CH4). In some embodiments, the separations unit comprises a pressure swing absorber (PSA) that contains the MOF. In some embodiments, the separations unit comprises a temperature swing absorber (TSA) that contains the MOF. In some embodiments, the third stream includes hydrogen (H2). In some embodiments, the ethylene is desorbed from the MOF using ethane, propane or any combination thereof.
Another aspect of the present disclosure provides an adsorbent, comprising: (a) a first material that adsorbs (i) a product gas at a first heat of adsorption and (ii) a sweep gas at a second heat of adsorption; and (b) a second material that adsorbs the sweep gas at a third heat of adsorption, wherein the first material and the second material are included in a mixed material in a relative proportion such that the mixed material has an average heat of adsorption for the sweep gas, and wherein an absolute value of a difference between the first heat of adsorption and the average heat of adsorption is less than an absolute value of a difference between the first heat of adsorption and the second heat of adsorption.
In some embodiments, the first material is a metal-organic framework. In some embodiments, the second material is a metal-organic framework. In some embodiments, the first and second materials are different metal-organic frameworks. In some embodiments, the product gas is ethylene. In some embodiments, the sweep gas is propane. In some embodiments, at least one of the first material and the second material is a zeolite. In some embodiments, the first heat of adsorption, the second heat of adsorption and the third heat of adsorption have negative values. In some embodiments, the absolute value of the difference between the first heat of adsorption and the average heat of adsorption for the sweep gas is less than or equal to about 50% of the absolute value of the difference between the first heat of adsorption and the second heat of adsorption. In some embodiments, the mixed material comprises regions of the first material and of the second material having an average diameter of less than or equal to about 10 millimeters (mm). In some embodiments, the mixed material has a three dimensional shape that is adapted for use in a pressure swing adsorption unit.
Another aspect of the present disclosure provides an adsorbent, comprising: (a) a first material that adsorbs (i) a product gas at a first heat of adsorption and (ii) a sweep gas at a second heat of adsorption; and (b) a second material that adsorbs (i) the product gas at a third heat of adsorption and (ii) the sweep gas at a fourth heat of adsorption, wherein the first material and the second material are included in a mixed material in a relative proportion such that the mixed material has a first average heat of adsorption for the product gas and a second average heat of adsorption for the sweep gas, and wherein an absolute value of a difference between the first average heat of adsorption and the second average heat of adsorption is less than an absolute value of a difference between the first heat of adsorption and the second heat of adsorption.
In some embodiments, the second average heat of adsorption is substantially equal to the first average heat of adsorption. In some embodiments, the second average heat of adsorption is less than or equal to about 50% of the first average heat of adsorption. In some embodiments, an absolute value of a difference between the third heat of adsorption and the fourth heat of adsorption is less than an absolute value of a difference between the first heat of adsorption and the second heat of adsorption. In some embodiments, the first material is a metal-organic framework. In some embodiments, the second material is a metal-organic framework. In some embodiments, the first and second materials are different metal-organic frameworks. In some embodiments, the product gas is ethylene. In some embodiments, the sweep gas is propane. In some embodiments, at least one of the first material and the second material is a zeolite. In some embodiments, the first heat of adsorption, the second heat of adsorption, the third heat of adsorption and the fourth heat of adsorption have negative values. In some embodiments, the mixed material comprises regions of the first material and of the second material having an average diameter of less than or equal to about 10 millimeters (mm). In some embodiments, the mixed material has a three dimensional shape that is adapted for use in a pressure swing adsorption unit.
Additional aspects and advantages of the present disclosure will become readily apparent to those skilled in this art from the following detailed description, wherein only illustrative embodiments of the present disclosure are shown and described. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
INCORPORATION BY REFERENCEAll publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.
The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings or figures (also “FIG.” and “FIGS.” herein), of which:
While various embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions may occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed.
The term “higher hydrocarbon,” as used herein, generally refers to a higher molecular weight and/or higher chain hydrocarbon. A higher hydrocarbon can have a higher molecular weight and/or carbon content that is higher or larger relative to a starting material in a given process (e.g., OCM or ETL). A higher hydrocarbon can be a higher molecular weight and/or chain hydrocarbon product that is generated in an OCM or ETL process. For example, ethylene is a higher hydrocarbon product relative to methane in an OCM process. As another example, a C3+ hydrocarbon is a higher hydrocarbon relative to ethylene in an ETL process. As another example, a C5+ hydrocarbon is a higher hydrocarbon relative to ethylene in an ETL process. In some cases, a higher hydrocarbon is a higher molecular weight hydrocarbon.
The term “OCM process,” as used herein, generally refers to a process that employs or substantially employs an oxidative coupling of methane (OCM) reaction. An OCM reaction can include the oxidation of methane to a higher hydrocarbon and water, and involves an exothermic reaction. In an OCM reaction, methane can be partially oxidized and coupled to form one or more C2+ compounds, such as ethylene. In an example, an OCM reaction is 2CH4+O2→C2H4+2H2O. An OCM reaction can yield C2+ compounds. An OCM reaction can be facilitated by a catalyst, such as a heterogeneous catalyst. Additional by-products of OCM reactions can include CO, CO2, H2, as well as hydrocarbons, such as, for example, ethane, propane, propene, butane, butene, and the like.
The term “non-OCM process,” as used herein, generally refers to a process that does not employ or substantially employ an oxidative coupling of methane reaction. Examples of processes that may be non-OCM processes include non-OCM hydrocarbon processes, such as, for example, non-OCM processes employed in hydrocarbon processing in oil refineries, a natural gas liquids separations processes, steam cracking of ethane, steam cracking or naphtha, Fischer-Tropsch processes, and the like.
The terms “C2+” and “C2+ compound,” as used herein, generally refer to a compound comprising two or more carbon atoms. For example, C2+ compounds may include, without limitation, alkanes, alkenes, alkynes and aromatics containing two or more carbon atoms. C2+ compounds can include aldehydes, ketones, esters and carboxylic acids. Examples of C2+ compounds may include ethane, ethene, acetylene, propane, propene, butane, and butene.
The term “non-C2+ impurities,” as used herein, generally refers to material that does not include C2+ compounds. Examples of non-C2+ impurities, which may be found in certain OCM reaction product streams, may include nitrogen (N2), oxygen (O2), water (H2O), argon (Ar), hydrogen (H2) carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4).
The term “small scale,” as used herein, generally refers to a system that generates less than or equal to about 250 kilotons per annum (KTA) of a given product, such as an olefin (e.g., ethylene).
The term “world scale,” as used herein, generally refers to a system that generates greater than about 250 KTA of a given product, such as an olefin (e.g., ethylene). In some examples, a world scale olefin system generates at least about 1,000, 1,100, 1,200, 1,300, 1,400, 1,500, 1,600, or more KTA of an olefin.
The term “item of value,” as used herein, generally refers to money, credit, a good or commodity (e.g., hydrocarbon). An item of value can be traded for another item of value.
The term “carbon efficiency,” as used herein, generally refers to the ratio of the number of moles of carbon present in all process input streams (in some cases including all hydrocarbon feedstocks, such as, natural gas and ethane and fuel streams) to the number of moles of carbon present in all commercially (or industrially) usable or marketable products of the process. Such products can include hydrocarbons that can be employed for various downstream uses, such as petrochemical or for use as commodity chemicals. Such products can exclude CO and CO2. The products of the process can be marketable products, such as C2+ hydrocarbon products containing at least about 99% C2+ hydrocarbons and all sales gas or pipeline gas products containing at least about 90% methane. Process input streams can include input streams providing power for the operation of the process. In some cases, power for the operation of the process can be provided by heat liberated by an OCM reaction. In some cases, the systems or methods of the present disclosure have a carbon efficiency of at least about 50%, at least about 55%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, or at least about 90%. In some cases, the systems or methods of the present disclosure have a carbon efficiency of between about 50% and about 85%, between about 55% and about 80%, between about 60% and about 80%, between about 65% and about 85%, between about 65% and about 80%, or between about 70% and about 80%.
The term “C2+ selectivity,” as used herein, generally refers to the percentage of the moles of methane that are converted into C2+ compounds.
The term “specific oxygen consumption,” as used herein, generally refers to the mass (or weight) of oxygen consumed by a process divided by the mass of C2+ compounds produced by the process.
The term “specific CO2 emission,” as used herein, generally refers to the mass of CO2 emitted from the process divided by the mass of C2+ compounds produced by the process.
The term “unit,” as used herein, generally refers to a unit operation. A unit operation may be one or more basic operations in a process. A unit may have one or more sub-units (or sub-systems). Unit operations may involve a physical change or chemical transformation, such as separation, crystallization, evaporation, filtration, polymerization, isomerization, other reactions, or combinations thereof. A unit may include one or more individual components. For example, a separations unit may include one or more separations columns or an amine unit may include one or more amine columns.
The term “sorbent,” as used herein, generally refers to a material that may be used to adsorb or absorb another material, such as from a liquid or gas phase. A sorbent may be an adsorbent or an absorbent.
Separations Including Pressure Swing Adsorption (PSA)Various non-cryogenic separation techniques have been employed for gas separations, purifications and recovery of hydrocarbons. Membrane based processes and adsorbents have been studied for large scale applications for olefins recovery. Since the development of synthetic adsorbents and pressure swing adsorption (PSA) cycles, adsorption has been playing an important role in gas separation and purification.
Processes of the present disclosure can employ a variety of different separations techniques, alone or in combination. For example, OCM processes can employ amine and caustic systems for CO2 removal, molecular sieve guard beds for water removal, and cryogenic distillation or other separation techniques for recovery and purification of hydrocarbon components. Cryogenic separation can refer to separations using temperature levels below 120 K or about −153° C. Other techniques may include Selexol™ and Rectisol™ processes for CO2 removal.
OCM product effluent can comprise a mixture of hydrocarbons including but not limited to methane, ethane, ethylene, propane, propylene, butanes, butenes, and/or higher hydrocarbons. OCM product effluent can also comprise varying amounts of other components such as hydrogen (H2), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2) and water (H2O). The product of an OCM reaction can include ethylene. The ethylene product can be polymer grade, refinery grade or chemical grade. Depending on the purity level required, different separation and/or purification techniques can be employed with the OCM process. To recover high purity ethylene, separation methods such as those discussed herein can be used to remove a wide range of components.
Advantages of the advanced OCM processes described herein can include reducing the cost, reducing the number of unit operations (“units”) used, and hence improving the overall process for producing high purity polymer grade ethylene. Overall conversion and carbon efficiency can also be improved. The separation methods disclosed herein can also improve the overall conversion and carbon efficiency.
Various separation and purification techniques discussed herein can be used to separate the OCM product effluent (e.g., process gas) into a plurality of streams, including but not limited to a first stream comprising methane, hydrogen, carbon monoxide and other lighter inerts and a second stream comprising ethane, ethylene, propylene, and higher hydrocarbons. Separation systems or subsystems employed can include those discussed herein, such as a cryogenic demethanizer, a membrane separation system, or a PSA based system.
The separation techniques discussed herein can be employed to remove CO2, such as from an OCM product effluent stream. One or more separations techniques can be used to remove CO2 including but not limited to absorption, adsorption, CO2 distillation, membrane separation and combinations thereof. The separation technique can be cryogenic or non-cryogenic.
Cryogenic separation (e.g., distillation) can be used for recovering ethylene, propylene, and/or other components from olefin plants, refinery gas streams, and/or other sources. These separations in some instances may be difficult to accomplish due to e.g., proximity of relative volatilities, and significant temperature and pressure requirements for operation. The ethane/ethylene distillation can be performed at about −25° C. and 320 pounds per square inch gauge (psig) in a column containing over 100 trays. Distillation of propane and propylene can be performed at about −30° C. and 30 psig. These can be some of the most energy intensive distillations in the chemical and petrochemical industry. In general, the use of distillation towers to separate recover and purify components can be energy intensive.
Also provided in the present disclosure are adsorbents that may be employed for separation and purification of olefin rich streams. The adsorbents may include PSA-based adsorbent systems to separate, purify, and recover olefins like ethylene and propylene from streams containing one or more impurities such as methane, hydrogen, carbon monoxide, carbon dioxide, ethane, or others. The streams, or parts of the streams, can be generated in an OCM process, an ETL process, or combinations thereof. The streams can be final product streams where PSA is used to recover and purify the final product. The streams can be intermediate streams which are purified prior to use as a feed in a subsequent process, such as an ETL process, an ethylene cracker (steam cracker), a refining unit, a fuel gas system, a natural gas recovery plant or any other product fractionation or product treatment unit.
A pressure swing adsorption (PSA) process cycle is a process in which adsorption and desorption take place at different (higher or lower) conditions including such as temperatures, pressures, or combinations thereof. For example, reduction of pressure can be used to shift the adsorption equilibrium and affect regeneration of the adsorbent. Low pressure may not be as effective as temperature elevation in totally reversing adsorption, unless very high feed to purge pressure ratios are applied. Therefore, some PSA cycles can have high residual loadings and thus low operating loadings. These low capacities at high concentration may require that cycle times be short for reasonably sized beds (e.g., seconds to minutes). These short cycle times may be attainable because particles of adsorbent respond quickly to changes in pressure. Example uses for PSA processes include purification as well as applications where contaminants are present at high concentrations.
As shown in
PSA cycles can be used for purification of wet gases and of hydrogen. High pressure hydrogen which may be employed in processes such as hydrogenation, hydrocracking, and ammonia and methanol production can be produced by PSA beds compounded of activated carbon, zeolites and carbon molecular sieves. Other example applications include: air separation, methane enrichment, iso/normal separations, and recovery of CO and CO2.
Adsorbents Including Metal Organic Frameworks (MOF)Adsorbents can be natural or synthetic materials, such as those having amorphous or microcrystalline structure. Example adsorbents useful for large scale operation include but are not limited to activated carbon, molecular sieves, silica gels, and activated alumina. Other useful adsorbents may include pi complexation sorbents, silver and copper complexation adsorbents, zeolites, synthetic zeolites, mesoporous materials, activated carbons, high surface area coordination polymers, molecular sieves, carbon molecular sieves (CMS), silica gels, MCM, activated alumina, carbon nanotubes, pillared clays, polymeric resins, and combinations thereof.
For systems where the incoming stream is a multi-component mixture of gases and the number of compounds to be separated cannot be removed by a single adsorbent, different layers of adsorbents can be used. For example, hydrogen purification from a methane stream in a reforming operation, where H2 is contaminated with H2O, CO2, CO, and unconverted CH4, can employ activated carbon to remove H2O and CO2 in combination with additional layers of different adsorbents used to increase the loading of CO.
Zeolites, molecular sieves, and carbon molecular sieves (CMS) can be used for industrial separations employing PSA. Inorganic materials, like special kinds of titanosilicates, can be used for kinetic separations.
For systems configured to separate ethane/ethylene and propane/propylene, example types of adsorbents include zeolites/molecular sieves and pi complexation sorbents. Zeolites/molecular sieves can be used for kinetic separation, such as separation based on higher diffusivity of olefins over that of paraffins. The use of 4A zeolite is one such example. For example, a three-bed system can be used to recover olefins from a stream containing 80-85% olefins and 10-15% paraffins, using a 4A type zeolite at elevated temperatures (e.g., the Petrofin process). Pi complexation sorbents, such as AgNO3/SiO2, can give excellent results as compared to 4A zeolite. PSA units as discussed herein can employ a range of different sorbents, including but not limited to a zeolite/molecular sieve sorbent, a pi complexation based sorbent, a carbon molecular sieve sorbent or any other form of activated carbon, carbon nanotubes, polymeric resin based sorbents, other sorbents or combinations thereof.
Adsorbents can be selected based on a number of different criteria. Adsorbent selection criteria can include capacity for the target components (e.g., affinity for the desired components to be separated from the multi-component feed stream), selectivity between components competing for same adsorption sites, regenerability of the adsorbent, (e.g., the ability of the adsorbent to release the adsorbed target components at a reasonable pressure rate of gas diffusion into the adsorbent—this can also affect the size of the bead that is chosen and consequently the pressure drop across the bed; an insufficient diffusion rate can require smaller diameter beads that can result in higher pressure drop and hence increased operating costs), and chemical compatibility (e.g., selecting an adsorbent resistant to chemical attack that may poison or destroy the adsorbent, such as liquid hydrocarbons causing physical breakdown of the adsorbent resulting in loss of efficiency and back pressure).
Separations, such as of ethylene and propylene, can be conducted using an amorphous fluoropolymer based membrane. Facilitated transport using silver ions can selectively transport ethylene and/or propylene. The membrane can be a part of a membrane contactor system. The feed to the system can be of a low to moderate olefin concentration. The feed to the system can contain other hydrocarbons, including, but not limited to, methane, ethane, propane, butane, butenes, C5 components and higher hydrocarbons. The feed can also contain CO2, CO, H2, and inert components, such as nitrogen.
The separation section of OCM unit can employ cryogenic distillation systems. In some cases, the distillation section can be partially or completely replaced by efficient advanced separation technologies that operate at higher/room temperatures, such as membranes or PSA. This can result in energy savings.
Among the materials used for membranes and adsorption beds, metal-organic frameworks (MOFs) can be beneficial. MOFs can comprise metal ions and organic linkers. MOFs can be highly porous sponge-like materials. The choice of metal ion and linker can define the structure and hence the properties of MOFs. MOFs can exhibit advantages of both organic and inorganic moieties. They can be more advantageous than zeolites due to their higher surface areas and higher flexibility in pore sizes (e.g., based on their synthesis). They can be better suited than typical membranes for separation since they can be more robust, more mechanically and thermally stable, and can avoid issues such as carrier poisoning or reduction of complexing agents.
The process effluent from OCM can comprise light gases, such as methane, hydrogen, carbon dioxide, ethylene, ethane, acetylene, propane, propene and C4+ compounds. MOFs can be used to separate C2+ compound streams from the bulk CH4 and H2 in effluent. MOFs can also be used to recover ethylene from a mixed stream of C2 compounds, C3 compounds and C4+ compounds, remove CO2, and recover hydrogen for further processing.
Different combinations of MOFs can be synthesized to provide different separation properties. MOFs can be useful in hydrocarbon separation due to their capability of separating component gases by mechanisms such as molecular sieving, characteristic gate opening pressures for different penetrant molecules or other changes in the structure of the MOFs due to adsorbent/adsorbate interactions. Without being limited by theory, adsorption selectivity can arise from interactions using π-complexation between the double bond in ethylene molecules and partial positive charges of co-ordinatively unsaturated metal ions (e.g., Cu(II)). MOFs such as HKUST-1 can be used to separate ethylene from ethane. Other MOFs capable of separating ethylene over ethane include Ag+ based MOFs, Co2(2,5-dihydroxyterephthalate, or “dhtp”), and Mg2(dhtp). MOFs such as ZIF-7, ZIF-8, and ZIF-4 can be used for selective adsorption of paraffins (e.g., ethane) over ethylene due to the gate-opening effect or the breathing behavior of the MOF. ZIF-8 can adsorb alkanes (e.g., methane) over alkenes (e.g., ethylene). The selectivity of this separation can be controlled by adjusting the hydration level of the MOF. MOFs such as ZIF-67, SBMOF-1, SBMOF-2, Cu-TDPAT, USTA-33a, ZJU-61, USTA-33, USTA-10a can be used for selective separation of methane from other hydrocarbons such as C2 compounds. The MOF M2(dobdc) can be used to effectively separate acetylene, ethylene, ethane, and methane collectively or individually from their mixtures. The M2(dobdc) can be in the meta form M2(m-dobdc) or the para form M2(p-dobdc). The metal can be any suitable metal such as iron (Fe), nickel (Ni), manganese (Mn) or cobalt (Co). Further information on these MOFs can be found in PCT Publication No. WO 2015/066693A1, which is incorporated herein by reference in its entirety. IRMOFs, such as MOF-5, can be used for separation of hydrogen from hydrogen/methane and hydrogen/C2 mixtures. RPM3-Zn can be used to separate C1-C4 paraffins. MOFs such as UTSA-100, SIF SIX, ZJU-5 can be utilized for acetylene removal from the olefins stream where back-end acetylene removal is used rather than acetylene hydrogenation. MOFs such as M-(dobdc) can be modified with amines to selectively remove CO2. Several MOFs such as ZIF-68-70, 78, 79, 81 82, MOF-11, MOF-508b, PCN-60, 61, MIL-100, MIL-101, ZIF-8, SNU-9, MIL-102(Cr), MIL-53(Cr) have been studied for removal of CO2 from methane and nitrogen and can be utilized for, e.g., a front end CO2 removal system. MOFs such as M2(dobpdc) can be used to remove CO2 from other gases and can be used for CO2 removal front or back of the OCM process described herein. MOFs such as Fe-BTTri can be used for CO removal from various components such as CO2, N2, CH4 and can be used for back end CO removal in the OCM unit.
MOFs can be used in the adsorbent beds of PSA/TSA system or as a part of membrane based applications. As part of membrane systems, they can be incorporated in thin film membranes or mixed matrix membranes (MMMs). With MMMs, MOFs have shown improved gas separation qualities, with increased permeability and selectivity using MMMs. Mixed matrix membranes can combine the advantages of easy and cheap fabrication of polymer membranes with the improved gas separation properties of different MOFs.
OCM and/or ETL Systems with Adsorptive Separation
PSA technology can be applied to processes including those involving a hydrocarbon stream containing a mix of the following hydrogen, carbon dioxide, carbon monoxide, methane, ethane, ethylene, propane, propylene, butanes, butenes and/or other higher hydrocarbons needing to be purified or separated into desirable products (e.g., ethylene, methane, hydrogen, or propylene).
Hydrocarbon streams can be produced via traditional refining and petrochemical processes. Hydrocarbon streams can be produced from OCM or ETL reactor systems.
The present disclosure provides the use of PSA in processes and systems for oxidative coupling of methane (OCM) and ethylene-to-liquids (ETL) operations, and the application of adsorbent based processes used in conjunction with OCM and ETL processes to generate significant process improvements and enhance the economic value of the processes. OCM systems are described in, for example, U.S. Patent Publication No. US 2015/0210610, which is entirely incorporated herein by reference. ETL systems are described in, for example, U.S. Patent Publication No. 2015/0232395, which is entirely incorporated herein by reference.
An OCM system, such as that shown in
Stand-alone OCM processes can use adsorptive separation. In certain cases, the separations section of the OCM system can be eliminated, or partially eliminated, by utilizing an advanced separations method as discussed in this application. The advanced separation method can be a PSA unit or a membrane based method, or a cryogenic system.
As discussed above, the PSA unit can comprise one or more adsorbent materials that can be suitable to achieve the component recoveries. The sorbent can be a zeolite/molecular sieve based be a mesoporous material, a carbon based sorbent, or a π-complexation sorbent. In some cases the sorbent material can be a polymeric resin, carbon nanotubes, and carbon fibers. The PSA unit can be configured to have layers of different sorbents so as to result in high recoveries from the multi-component feed streams to the desired products.
In certain cases the PSA can be a multi stage unit (see, e.g.,
In certain cases, the process requirements can dictate that only a limited amount of recovery is required in the PSA unit and subsequent recovery and purification is performed in a fractionation column or the gas is a feed for a downstream process unit. The downstream process unit can be an ETL system, an ethylene steam cracker system, a gas processing plant, NGL extraction plant, a refinery off-gas separations system, or other process unit.
OCM process retrofits can use adsorptive separation. OCM can be employed to convert a feedstock comprising methane to ethylene and other olefins. Historically, ethylene has been produced via steam cracking of gaseous or liquid hydrocarbon feedstocks like ethane, propane, LPG, or naphtha. As in most of the refining and petrochemical operations, a steam cracking operation can involve a cryogenic fractionation or a separations section that consists of a series of fractionation columns to successively recover various components at high product purity.
The present disclosure includes the application of PSA processes to an OCM retrofit of an existing ethylene cracker (e.g., steam cracker).
An example application for OCM combined with a PSA unit involves an existing petrochemical plant such as a steam cracker is considering low cost ways to add ethylene capacity. An example revamp to add capacity may include addition of, or debottlenecking of, the existing fractionation towers for the entire flow addition for the revamp. However, as shown in
The combination of an OCM unit and an existing ethylene cracker can provide synergistic benefits. It can provide for a low cost alternative to add ethylene capacity to the existing cracker. In some cases, prior to retrofit of an ethylene cracker with OCM, the entire overhead from the existing demethanizer is used as fuel gas, and can now be available as one of the feeds to the methanation unit. In some cases, the demethanizer overhead off-gas comprises up to about 80%, 85%, 90%, 95%, or more methane, which can be converted to ethylene in the OCM reactor, hence increasing the total ethylene capacity. In some cases, the hydrogen content in the existing demethanizer overhead is substantial, and may be enough to meet the hydrogen requirement of the methanation unit.
In some cases, retrofitting an ethylene cracker with OCM reduces (or allows for reduction of) the severity of cracking in the existing cracker, enabling value addition by increasing the production of pyrolysis gasoline components in the cracker effluent, as the OCM reactor produces the ethylene that may be needed to achieve the total system capacity. The cracker can then be operated on high propylene mode to produce more propylene and at the same time meeting the ethylene production rate by the new OCM unit. This retrofit can result in greater flexibility for the ethylene producer with respect to the existing cracker operation.
In some instances, the overall carbon efficiency can be increased as the methane and hydrogen from the existing demethanizer off-gases can be utilized to convert the carbon dioxide and carbon monoxide to methane, which is fed to the OCM reactor.
In some instances, ethane and/or propane recycle streams from the existing cracker can be routed to the OCM unit (e.g., instead of the cracking furnaces). These recycle streams are typically routed to the cracking furnaces where they are cracked to extinction. This can provide an advantage over routing the recycle streams to OCM over the cracking furnace, such as higher selectivity to ethylene in the OCM process.
In certain cases, more than one stages or PSA columns may be employed to achieve higher recovery and higher product purity. As in shown
The application of a PSA unit to OCM systems, standalone or retrofits to existing facilities exhibits immense potential in terms of cost savings and ease of integration and retrofit to existing facilities.
ETL processes can use adsorptive separation.
The ETL reactor can be a tubular, packed bed, moving bed, fluidized bed, or other reactor type. An ETL reactor can be an isothermal or adiabatic reactor. The ETL system can benefit from a feed concentrated in olefins. The ETL reactor system can use a recycle stream to control and moderate the temperature increase in the reactor bed due to the highly exothermic nature of the ETL reactions. ETL systems are described in, for example, U.S. Patent Publication No. 2015/0232395, which is entirely incorporated herein by reference.
In some cases, one or more of the fractionation towers can be deemed redundant if using the PSA, as an example, a demethanizer may not be required and the sales gas or purge gas to fuel can be sent from the PSA itself.
Retrofit applications for midstream and refining processes can use adsorptive separation. Systems, such as those of
OCM-ETL systems of the present disclosure can be integrated into and combined into conventional natural gas liquids (NGL) extraction and NGL fractionation sections of a midstream gas plant. Where NGLs in the gas stream are declining (or gas is dry), the deployment of OCM-ETL can utilize an existing facility to produce additional liquid streams. The implementation of OCM-ETL can allow for the generation of on specification “pipeline gas.” The products from the facility can be suitable for use (or on specification or “spec”) as pipeline gas, gasoline product, hydrocarbon (HC) streams with high aromatic content, and mixed C4 products. The PSA systems discussed above can be employed to separate, pre-separate or purify the hydrocarbon feed streams in the integrated NGL OCM-ETL system.
Refinery gas typically contains valuable components like hydrogen, methane, ethane, ethylene, propane, propylene, and butane. Most commonly, refinery off-gases (ROG) are exported to the fuel gas system, thereby losing the value of the components contained therein. The OCM-ETL process can be used to improve the value of products as the OCM converts the methane to ethylene and the ETL converts olefins (e.g., those existing in the ROG and those generated by OCM) to higher value liquids as C4 components, gasoline blends, or aromatic components.
As shown in
Adsorbent Based Systems for Hydrocarbon Separation with High Olefin Selectivity
An OCM process configuration that can reduce the overall capital costs can utilize non-cryogenic separations that do not require high process gas compressions and refrigeration. The cryogenic separation can operate at pressures in the order of 40-50 bar, whereas the PSA system described herein can be operated at much lower pressure range. A multi-stage adsorbent bed system can be used that is capable of removing the bulk of lighter components (e.g., methane, hydrogen, ethane, and carbon monoxide) in the first stage, and then removing the remaining of the lighter components in a subsequent stage. The materials used for the beds can be Calcium-X (CaX) zeolite (or CaLSX zeolite), or metal organic framework such as M2(para or meta dobdc) (where dobdc is 2,5-dioxido-1,4-benzenedicarboxylate and M can be, without limitation Fe, Co, Ni, Mg, Mn), or a layered combination.
Since the ethylene is preferentially adsorbed on these materials, the choice of regeneration and/or purge gas can contribute to the final recovery and design of the overall process scheme in the system. Described herein are three example process schemes with different purge gases for regeneration of each bed. The process can be configured with different combinations of purge gas for different beds as well as operating one or more of the beds under vacuum for regeneration.
As used herein, the “product component” is the component in the gas stream that is considered to be the desired product, and is typically aimed to be recovered in as much high purity as possible. For example, in case of OCM process, ethylene can be the product component.
Usually PSAs are designed to handle streams that have few components in the feed (e.g. 2, 3, 4, or 5 components at greater than 3% concentration). In contrast, the systems and methods described herein can be capable of removing bulk of undesired components from a stream of multitude of components, like methane, hydrogen, ethane, carbon monoxide, and carbon dioxide, among others. The product stream can still be highly enriched in the product component (ethylene in this case) despite the large number of components.
Described herein is a method for separating a product from a gas mixture. The method may comprise, at a first total pressure, directing a gas mixture into a pressure swing adsorption (PSA) vessel. The gas mixture may comprise a product and at least one impurity (e.g., at least about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more impurities). The product may have a concentration of less than or equal to about 50, 45, 40, 35, 30, 25, 20, 18, 16, 14, 12, 10, 8, 6, 4, 2, 1 mol %, or less in the gas mixture. The at least one impurity may comprise carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethane (C2H6), hydrogen (H2), or any combinations thereof. The gas mixture may comprise an effluent(s) from one or more processing units including for example, an OCM unit, an ETL unit, a cracking unit, a separations unit, a methanation unit, or combinations thereof. The PSA vessel may comprise an adsorbent. The adsorbent may comprise a metal organic framework (MOF) such as M2(dobdc). The adsorbent may be configured to adsorb the product thereon. In some cases, the product has a first partial pressure.
Next, a sweep gas may be directed into the PSA vessel at a second total pressure. The sweep gas may be adsorbed on the adsorbent thereby displacing the product from the adsorbent to yield a displaced product. The sweep gas may have a second partial pressure. The second partial pressure may be greater than, less than or equal to the first partial pressure of the product in the gas mixture. Subsequently, the sweep has may be desorbed from the adsorbent at substantially (e.g., at least about 80%, 82%, 84%, 86%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or more) the first total pressure, such that additional product may be capable of adsorbing on the adsorbent. After the adsorption of the product from the gas mixture, the gas mixture may comprise a predetermined amount of a given impurity (e.g., CO2). In some cases, an amount of the product that adsorbs on the adsorbent at the first partial pressure may be substantially equivalent to the amount of the sweep gas that adsorbs on the adsorbent at the second partial pressure.
Heat may be released upon adsorption of the product and/or the sweep gas on the adsorbent. An amount of heat released by the adsorption of the product may be the same as or different from an amount of heat released by the adsorption of the sweep gas. In some cases, the amount of heat released by the adsorption of the product is greater than or equal to about 50%, 60%, 70%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or more of the amount of heat released by the adsorption of the sweep gas. In some cases, the amount of heat released by the adsorption of the product is less than or equal to about 100%, 98%, 96%, 94%, 92%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, or less of the amount of heat released by the adsorption of the sweep gas. In some cases, the amount of heat released by the adsorption of the product is substantially equivalent to (e.g., greater than or equal to about 90%, 95%, 99%, or more of) the amount of heat released by the adsorption of the sweep gas.
The displaced product may be enriched (e.g., concentration increased) relative to the product in the gas mixture by a factor of at least about 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, 45, 50, or more. In some cases, the displaced product may include some of the sweep gas, for example, at least about 0.001, 0.005, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20 mol %, or more of the sweep gas. The product and/or the sweep gas may comprise olefins (e.g., ethylene, propylene, and butadiene), paraffins (e.g., methane, ethane, propane), and/or other types of hydrocarbon or nonhydrocarbons compounds (e.g., CO2, CO, O2, N2, H2). In some examples, the product is ethylene. In some examples, the sweep gas is ethane.
In some cases, at the first pressure, a selectivity of the adsorbent for adsorbing the product as compared to the at least one impurity is at least about 2, 5, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 120, 140, 160, 180, 200 or more. As used herein, selectivity of component 1 over component 2 (e.g., the product over the at least one impurity), for an adsorbent at a specified total pressure, temperature, and gas phase composition, is defined as (x1*y2)/(x2*y1), where x1=mole fraction of component 1 in adsorbed phase, y1=mole fraction of component 1 in gas phase, x2=mole fraction of component 2 in adsorbed phase, y2=mole fraction of component 2 in gas phase. The gas mixture may be derived from an effluent from an OCM reactor. The at least one impurity may be recycled to the OCM reactor following the adsorption of the product from the gas mixture.
The PSA system described herein can recover the product component even when it is present in very low concentrations in the feed stream (e.g., less than or equal to about 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, or less). An example of such a case is an effluent of an OCM process. The bulk of OCM effluent may include methane and hydrogen. The product component (ethylene) can be in a very low concentration exiting the reactor. In an adsorbent based separation systems, a component with high partial pressure (i.e., having high concentration) may tend to adsorb more on the adsorbent, whereas a component with low partial pressure may tend to adsorb less strongly and/or in lesser amounts. In some cases, lighter components (e.g., methane and hydrogen in the case of OCM) tend to adsorb less, whereas heavier components tend to adsorb more (e.g., ethylene in the case of OCM). These competing tendencies of adsorption with respect to partial pressure and molecular weight can make it difficult to get a clear difference in adsorption capacities for the undesired and desired components. The systems and methods described herein resolve this conflict by using materials that preferentially adsorb the desired product (e.g., ethylene for OCM, which is the heavier component, but has a low partial pressure). Thus, the materials used herein can have the property of showing a strong adsorption preference for the product component over undesired components despite low partial pressure of the product component.
In PSA based separation systems, the product component may be the one that is not adsorbed and is recovered at a high purity in the light stream. The adsorbed undesired component may then be removed from the bed by desorbing it at a lower pressure (or even vacuum) using a portion of the light product as the sweep gas. In some cases, however, the desired product (e.g., ethylene in the case of OCM), may be the heavier component that gets adsorbed on the adsorbent. To desorb it and recover it as a product, the light product may not be used as a sweep gas, since this is the component that the product needs to be separated from in the first place.
In the systems and methods described herein, the sweep gas can be chosen on the basis of criteria including e.g., (a) it is separable from the product component (e.g., ethylene in the case of OCM) following the adsorptive separation; (b) it not just sweeps the desorbed product (e.g., as a convection effect), but also replaces it on the bed as a mass transfer effect, in order to enhance the rate of desorption; and/or (c) it is not detrimental to the reactor when a portion of it is carried over with the lighter stream during the adsorption step (which can be recycled back to the reactor). In some cases, the sweep gas achieves any two of (a)-(c). In some cases, the sweep gas achieves all of (a)-(c).
The desorption step in a PSA may be done at a lower pressure than the adsorption step. In some cases, the systems and methods of the present disclosure may be configured to perform the desorption step with a suitable sweep gas at a higher pressure than absorption. In some embodiments, the sweep gas shows a greater tendency to become adsorbed on adsorbent, and thus replace the product component (ethylene in case of OCM). This can be done to counter the adsorbent material's strong adsorption preference for the product component, and ensure recovery of the product component. The choice of the sweep gas is made such that it shows significantly higher tendency to replace the product component at a high pressure (in comparison to the partial pressure of the product component in the feed stream) during the desorption step, whereas the sweep gas itself can be easily desorbed from the bed by lowering the pressure to make way for the product component to take its place during the adsorption step in the next cycle. Thus, the adsorption isotherm of such a sweep gas can be very different from that of the product component such that product component shows high adsorption capacity even at very low partial pressure, whereas the proposed sweep gas shows high adsorption capacity only at high pressure.
PSA based separation systems can be designed to maximize recovery and purity of the product components. Even if one of these is compromised, it can be done as a trade-off to maximize the other objective. However, the intent remains to separate the components as much as possible. In the systems and methods described herein, however, a methodology is devised to get a specific split of a component (CO2 in the present example). This component is not necessarily being the product component. The split can be such that a specific desired proportion goes into the lighter stream, while the rest goes into the heavier stream. Such a split may be a requirement of the overall process scheme (for example, in the OCM process, a specific portion of CO2 is sent with the lighter stream to the methanation reactor, while the rest is separated out and purged). This can be done while still maximizing the capture of the product component (ethylene in the case of OCM). Therefore, the systems and methods described herein may use a combination of more than one type of materials in the adsorbent bed in such a way that one material is a weak adsorber of the component to be split, but a strong adsorber of the product component (e.g., Co2(m-dobdc)), while another material may be a strong adsorber of both the component to be split and the product component (e.g., CaX). By using a proportionate layered combination of the two materials in the same bed or in separate beds, the product component can be fully captured, while the component to be split can be split in the desired proportion.
Several examples of process configurations using adsorption purification to recover ethylene from OCM effluent are provided herein.
The regeneration of the bed can be performed using propane 1418, or propane diluted with a light gas that is weakly adsorbed on the bed. This step can be performed at a different pressure than adsorption to get a pressure swing effect and/or at a different temperature to get temperature swing effect. Propane being strongly adsorbed on the bed, is capable of replacing ethylene and CO2 from the adsorption sites, therefore desorbing the ethylene and CO2, including carryover of traces of light components. The effluent from the regeneration step can be the product of the first stage. Since the regeneration gas (propane) replaces ethylene during the desorption step, there can be a carryover of this propane with the lights stream during the adsorption step.
At this stage, propane can be separated 1420 (for example, using distillation) and resent into the purge gas system. This can result in a cleaner C2 stream that can be recovered more efficiently in a second bed.
The stream now containing ethylene, CO2, and a small portion of other components is sent into a second bed 1416 containing a layered combination of two materials such that one has strong adsorption for CO2 (e.g., CaX), while the other has weak affinity for CO2 (e.g., M2(dobdc)). Such a combination can aid in a desired split of CO2 in the adsorbed and non-adsorbed phases for further optimized processing of CO2, while achieving high ethylene recovery. Methane, ethane and a portion of CO2 can be released as lights during the adsorption step. This stream along with the stream from the adsorption step of the first bed can be combined to remove propane and then sent into the methanation unit 1422 for further conversion of CO and CO2 into methane. This step improves the overall carbon efficiency of the unit.
The second bed (i.e., CaX+M2(dobdc)) can be desorbed again using propane for efficient removal of the adsorbates. The desorbed gas now comprises the ethylene, the purge gas propane and minor amounts of ethane.
A CO2 removal unit 1424 (which can be much smaller than the designs not having the absorption systems described herein) can remove CO2 for venting. An acetylene hydrogenation 1426 step can be used for conversion of acetylene into ethylene.
A final purification column 1428 for ethylene can then separate the product ethylene 1430 from propane and ethane and other minor components, thus producing polymer grade ethylene as product. A propylene separation can be performed either as a small distillation column or in a combined ethylene/propylene separation unit in a membrane system that is capable of removing olefins over paraffins.
The regeneration of the bed can be performed using ethane 1500. Ethane at approximately the same pressure as adsorption or higher pressure can bind in the CaX sites and release the ethylene and CO2. This desorption gas can contain about 70% ethane (purge gas+adsorbed ethane), about 17% ethylene and about 10% CO2, which can be sent into a CO2 removal unit 1502.
At this stage, CO2 can be removed and separated into two streams, one going back into methanation reactor along with the methane rich stream containing hydrogen, and the rest being vented 1504. In the methanation reactor, hydrogen from the system may be utilized to convert CO and CO2 back into methane thus improving the overall carbon efficiency.
The stream now containing ethylene, ethane and a small portion of methane can be sent into a second PSA bed 1416 containing adsorbent (e.g., M2(dobdc)). The adsorption step can remove all of the methane from the ethylene product stream, with the unadsorbed gas containing ethane (about 97%) and the methane carried over from CaX bed (about 3%). The desorption can be performed at adsorption pressure or slightly higher pressure with ethane 1506, where ethane replaces the ethylene bound in the second adsorbent bed (e.g., containing M2(dobdc)). In some cases, the desorption gas can contain only two components: ethylene (about 16%) and ethane (about 84%). The high percentage of ethane in the desorption gas can be due to it being used as purge gas. However, this stream can be much easier to process and purify ethylene since all the other effluent components were thoroughly removed by the two PSA units. The purge gases can be resolved in a purge gas recovery module 1508. A C2 splitter 1510 can perform the final purification of ethylene and produce polymer grade ethylene product. An advantage can be realized compared to cryogenic separations in utilizing smaller process units for final purification instead of huge compressions systems, refrigeration systems, CO2 removal and distillation columns that process entire effluent streams.
The effluent from the desorption step of the first bed can be sent into a second bed 1416. The stream from the adsorption step can then be directly sent into the post bed cracking section 1406 of the reactor, for the conversion of ethane into ethylene, thus increasing the overall ethylene production rate. Alternatively, a similar scheme such as in
The second bed can be desorbed using propane 1602, for efficient removal of ethylene. The desorbed gas now contains mainly ethylene and propane, with minor quantity of ethane. This stream can then be sent to an acetylene hydrogenation unit 1426 for conversion of ethylene into ethane and finally into an ethylene purification column. The final column 1428 now containing much smaller volumes of gases can be cryogenically separated (or separated in an olefin selective membrane) for high purity ethylene 1430.
The desorbed stream can then be sent into a CO2 removal unit. A portion of the CO2 can be mixed along with the lights stream from adsorption step and sent into the methanation unit for further conversion into methane. The desorbed product can be sent into the second step of bed.
The second stage bed adsorbs ethylene from the stream. Ethane and methane can be released during the adsorption step, which can be recycled back into the OCM reactor. The MOF bed can be desorbed under vacuum for recovering high purity ethylene. This stream containing ethylene and minor portions of ethane and acetylene is then sent into an acetylene hydrogenation system for conversion of acetylene into ethylene. A final ethylene purification column can then separate ethylene as polymer grade from ethane.
A combination of different purge gases (including but not limited to ethane, propane, butane, CO2, hydrogen, etc.) and vacuum swing can also be utilized for each of the two beds, along with a combination of different layers of adsorbent materials in each bed in order to achieve different ethylene purity grades, recovery and overall carbon efficiency with the recycle.
For an OCM process, MOFs can be utilized for separation of various light hydrocarbons. In
In
In
In summary, different MOFs can be utilized for their specific selectivities and adsorption capabilities, for example in MMMs or adsorbent beds as PSA systems for hydrocarbon separation of the OCM effluent. MOFs can be very advantageous for their on-purpose synthesis and high surface areas (highest surface area/gram compared to any other material). MOFs in combination with other separation systems (such as polymeric membranes, zeolites, and cryogenic distillation) can be used in novel process schemes to produce OCM product (e.g., ethylene).
Adsorption may be an exothermic phenomenon that releases heat. In some cases, heat of adsorption is the amount of heat released when a unit amount of gas becomes adsorbed on the adsorbent. The heat of adsorption may be approximately equal to the amount of heat required to desorb the same amount of gas. Qualitatively, the greater the heat of adsorption, the more strongly the gas may be adsorbed on the adsorbent, and thus more heat may be required to desorb that gas.
In a PSA, after the adsorption step is complete, desorption of the adsorbed gas may be needed. Desorption can be facilitated by providing sufficient heat of desorption. This can be done by heating the adsorbent bed by various different methods, such as passing a hot purge gas through the bed. Such a mechanism may be referred to as Temperature Swing Adsorption (TSA). TSA may be a relatively slow process (e.g., because of limitation of the rate at which heat can be transported to each adsorption site). As described in the present disclosure, systems and methods are provided whereby the heat of desorption is provided directly at each site by desorbing using a purge gas that itself has high enough adsorption capacity on the adsorbent material such that the heat released by its adsorption can be used as desorption heat for the previously adsorbed feed gas component.
In some cases, the heat of adsorption of the purge gas on the adsorbent material is not equal to the heat of desorption of the feed gas component. Thus the adsorbent bed can either be under-heated (e.g., because of less heat of adsorption of the purge gas than the feed component), or over-heated (e.g., because of high heat of adsorption of the purge gas than the feed component)
The present disclosure provides methods and systems for overcoming such a problem by using a mixture of more than one adsorbent material, such that if one material has a high heat of adsorption for the purge gas, another material may have a low heat of adsorption for the purge gas. The mixture can be prepared homogeneously such that the extra heat released by the purge gas on one component may be readily used by the other material, which otherwise may not release sufficient energy on adsorption of the purge gas. The mixture can be prepared in such a proportion that overall, the heat required by the desorption of the feed gas may be sufficiently matched with adsorption of the purge gas on the mixture of the materials. The match may not need to be substantially precise, since there can be other methods employed at the same time to facilitate desorption, such as pressure swinging, which can have its own contribution on desorption. In some cases, the heat required by the desorption of the feed gas is greater than or equal to about 0.05%, 0.1%, 0.5%, 1%, 5%, 10%, 20%, 30%, 40%, or more of the heat released by the adsorption of the purge gas on the mixture of adsorbent materials. In some cases, the heat required by the desorption of the feed gas is less than or equal to about 40%, 30%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, or less of the heat released by the adsorption of the purge gas on the mixture of adsorbent materials. In some cases, the heat required by the desorption of the feed gas is from about 0.01% to about 5%, or from about 0.05% to about 10% of the heat released by the adsorption of the purge gas on the mixture of adsorbent materials. The heat required by the desorption of the feed gas may be within about 0.05%, within about 0.1%, within about 0.5%, within about 1%, within about 5%, or within about 10% of the heat released by the adsorption of the purge gas on the mixture of adsorbent materials.
In some examples, an adsorbent may comprise a first material and a second material that is different from the first material. The first material may adsorb a product gas and/or a sweep gas. The first material may adsorb the product gas at a first heat of adsorption. The first material may adsorb the sweep gas at a second heat of adsorption. The second material may adsorb the product gas and/or the sweep gas. In some cases, the second material adsorbs the product gas and/or the sweep gas at a third heat of adsorption. The first material and the second material may be mixed in a relative proportion, e.g., having a (first material/second material) molar ratio greater than or equal to about 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500 or more. The mixed material may have an average heat of adsorption of the sweep gas. The first heat of adsorption, the second heat of adsorption, the third heat of adsorption and the average heat of adsorption may be the same or different. In some cases, one or more of the first heat of adsorption, the second heat of adsorption and the third heat of adsorption have negative values (i.e., heat is released during the adsorption of the product gas and/or the sweep gas). In some cases, there is a difference between the first heat of adsorption and the average heat of adsorption. In some cases, there is a difference between the first heat of adsorption and the second heat of adsorption. In some cases, an absolute value of the difference between the first heat of adsorption and the average heat of adsorption is equal to, greater than or less than an absolute value of the difference between the first heat of adsorption and the second heat of adsorption. In some cases, the absolute value of the difference between the first heat of adsorption and the average heat of adsorption is less than or equal to about 50%, 40%, 30%, 20%, 10%, 5%, 4%, 3%, 2%, 1% or less of the absolute value of the difference between the first heat of adsorption and the second heat of adsorption.
In some cases, the first material adsorbs a product gas at a first heat of adsorption and adsorbs a sweep gas at a second heat of adsorption. In some cases, the second material adsorbs the product gas at a third heat of adsorption and the sweep gas at a fourth heat of adsorption. The first, second, third and fourth heat of adsorption may be the same or different. In some cases, one or more of the first, the second, the third and the fourth heat of adsorption have negative values (i.e., heat is released during the adsorption of the product gas and/or sweep gas). The first material and the second material may be mixed in a relative proportion, e.g., having a (first material/second material) molar ratio greater than or equal to about 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500 or more. Such mixed material may have a first average (or mean) heat of adsorption for the product gas. The mixed material may have a second average (or mean) heat of adsorption for the sweep gas. The first average heat of adsorption may be equal to, greater than or less than the second average heat of adsorption. In some cases, the second average heat of adsorption if less than or equal to about 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or less of the first average heat of adsorption. In some cases, the second average heat of adsorption is substantially equivalent to (e.g., within ±10%, ±5%, or less of) the first average heat of adsorption. In some cases, there is a difference between the first average heat of adsorption and the second average heat of adsorption. In some cases, there is a difference between the first heat of adsorption and the second heat of adsorption. In some cases, an absolute value of the difference between the first average heat of adsorption and the second average heat of adsorption is equal to, greater than or less than an absolute value of the difference between the first heat of adsorption and the second heat of adsorption.
In some cases, the mixture of adsorbent materials comprises two different zeolites and/or MOFs. One of the adsorbent materials can be M2(dobdc), including para or meta versions of the dobdc and any suitable metal. The mixed adsorbent can have at least about 2, 3, 4, 5, 6, 7, 8, 9, 10, or more different materials. The mixed adsorbent can be intimately mixed, such as having particles of a single material having an average diameter of no more than about 10 micrometers (um), no more than about 50 um, no more than about 100 um, no more than about 500 um, no more than about 1 millimeter (mm), no more than about 5 mm, no more than about 10 mm, no more than about 100 um, or no more than about 500 mm. Adsorbents can be formed materials, which can include mixing powders of the materials (e.g., intimately) adding suitable binders and either tableting or extruding to form the mixed material. Tableted or extruded material can be calcined (e.g., heated) or have other treatments applied to form a robust adsorbent having a suitable crush strength and suitable for being used in a PSA or TSA separation unit.
CO2 SeparationCarbon dioxide (CO2) may be captured from various sources, such as from flue gases, natural gas, or from any process gas rich in CO2. Various processes for post-combustion or pre-combustion capture can be used reduce CO2 emissions.
In some examples, OCM processes can utilize an amine based absorption system for CO2 removal, which can be followed by use of a caustic scrubber to obtain high degree of separation. The amine system may be prone to corrosion, solvent degradation, and above all, may have high energy requirements. Separations with sorbents and/or solvents can involve placing the CO2 containing gas in intimate contact with a liquid absorbent or a solid sorbent that is capable of capturing the CO2. As shown in
Amine scrubbing technology can be used to remove acid gases from process gases. Primary amines (e.g., monoethanolamine (MEA), aminoethoxyethanol (DGA)), secondary amines (e.g., diethanolamine (DEA), diisopropanolamine (DIPA)), tertiary (e.g., methyldiethanolamine (MDEA), triethanolamine (TEA)), sterically hindered amines, chilled ammonia, potassium carbonate, and other compounds can be used to remove CO2 from process gases. Traditional amine based systems can be characterized by high energy requirements and solvent degradation. Improved solvents, which can require less energy for regeneration of the solution, include the Benfield process and two stage diethanolamine. Combination with an OCM process can reduce the energy consumption of amine scrubbing processes. Improved solvents can reduce the energy requirements by at least about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or more, compared to the traditional MEA solvents. This has the potential of reducing the energy, and hence steam, consumption of the OCM process, thereby increasing the amount of steam available for export from the OCM, or making alternative waste heat recovery methods feasible.
Physical absorption solvents used can include but are not limited to glycol dimethylethers (e.g., Selexol) and propylene carbonate (e.g., IPTS/EC). Regeneration of the solution can be performed by vacuum flashing and air stripping. This approach can consume significantly less energy than in chemical absorption. In using physical solvents CO2 can be released mainly by depressurization, thereby avoiding the high heat of consumption of amine scrubbing processes.
Mixed or hybrid solvents can include but are not limited to sulfinol (sulfolane, water, and amine), such as sulfinol-M and sulfinol-X.
Solid adsorbents, such as zeolites and activated carbon, can be used to separate CO2 from gas mixtures. In pressure swing adsorption (PSA), a gas mixture can flow through a packed bed of adsorbent at elevated pressure until the concentration of the desired gas approaches equilibrium. The bed can be regenerated by reducing the pressure. In temperature swing adsorption (TSA), the adsorbent can be regenerated by raising its temperature. In general usage, adsorption is not yet considered attractive for large scale separation of CO2 because the capacity and CO2 selectivity of available adsorbents are low. However, when the OCM process is a recycle process, an adsorbent based separation method can be used to separate bulk CO2 followed by consuming the remaining CO2 in a methanation reactor system, or by using a caustic scrubber to treat the remaining CO2.
Many different types of membrane materials (e.g., polymeric, metallic, ceramic) can be used for CO2 capture to preferentially separate CO2 from a range of process streams.
Membrane and amine technologies can be combined to form a hybrid process to capture CO2. Micro-porous hollow fiber membranes can be used for CO2 separation using amine-based chemical absorption processes. Micro-porous membranes can be used in a gas-liquid unit where the amine solution is contacted with CO2 containing gas. Use of membrane can lead to a reduction in the physical size and weight of the gas-liquid contacting unit. The separation is based on reversible chemical reaction, and mass transfer occurs by diffusion of the gas through the gas/liquid interface as in traditional contacting columns. Such a hybrid membrane contactor can provide a high contact area between gas and liquid, reduce or essentially eliminate foaming and flooding problems, and give better operational flexibility while reducing solvent degradation problems.
A membrane contactor can combine the advantages of membrane technology and solvent absorption for CO2 separation. A membrane contactor is a combination of advanced membrane techniques with an effective absorption process. A membrane contactor is a hybrid mass exchanger where a porous membrane separates two phases. The selective sorbent performs the separation while the membrane facilitates the mass exchange process by expanding the phase contact surface area. The modified surface properties can improve the selectivity of the process by selectively inhibiting the transport of one of the mixture constituents. Compared to a conventional column device, membranes can allow for up to five times increase in yield per unit volume. Since the sorptive liquid flows within capillaries and both phases are not directly contacting each other, membrane absorbers can operate in any spatial configuration (horizontal or vertical) and at any flux rations between both phases. Also, there is no flooding or uneven packing moisturization. Since the system operates with unchanging yields, independent of the diameter and height; scaling up is fairly simple. Membranes used can be micromembranes or ultrafiltration membranes made up of a variety of different polymer and ceramic materials. Polypropylene fiber membranes can be used to separate CO2 from CH4, for example by using amines like MEA as absorption liquid. Hollow fiber membranes, such as porous polypropylene, perfluoroalkoxy (PFS), and asymmetric poly(phenylene oxide) hollow fiber membranes with a dense ultrathin skin at the outside of the membrane can also be used. In some cases, absorption liquids such as aqueous sarcosine salt solutions, for example in a gas-liquid membrane contactor system, may be used. A membrane contactor can be used to separate the CO2 from the OCM effluent in which CH4 is the major component. Membrane contactors can also be used for separation of olefins and paraffins, and the separation of CO2 from light gases.
An activator, such as piperazine, diethanolamine, and arsenic trioxide, can be used to further enhance the effectiveness of CO2 capture. DGA and tertiary amines may provide more improvement than primary or secondary amines.
Gas selective poly ionic liquid membranes, which are polymerized room temperature ionic liquids (RTIL), can be used to selectively separate CO2. RTILs can be synthesized as a monomer and subsequently polymerized to obtain gas selective membranes. The ionic nature of the polymers can result in tight arrangements between the oppositely charged ionic domains in the poly RTIL, which can prevent the membrane from excessive swelling and deterioration of its performance at elevated pressure and/or temperature. This intrinsic property of poly RTIL can be used to increase the resistance against plasticization and/or to restrict strong swelling of the polymer membrane so as to maintain its permeation properties in the presence of a strong plasticizing agent such as CO2 at higher pressures. For example, an imidazolium-based poly RTIL can be used as base material and the length of the alkyl chain can serve to strengthen or weaken the ionic interactions within the poly RTIL. High pressure mixed CO2/CH4 gas separation measurements at different temperatures.
Gas components like CO2, from N2 or CH4 can be separated with supported ionic liquid membranes. Ionic liquids are molten salts with a very low melting point (many are liquids at room temperature). Many ionic liquids show a high solubility for carbon dioxide and hence can be highly suitable for use with an OCM process. For example, ionic liquids can include but are not limited to imidazolium, pyrollidinium, pyridinium, cuanidinium, phosphonium, morpholinium, piperidinium, sulfonium, ammonium, hexafluorophosphate, tetraflouroborate, alkylsulphate, triflate, dicyanamide, bis(trifluoromethylsulfonyl)imide, and combinations thereof. Specific advantages of ionic liquids include very low to negligible vapor pressure, good dissolution characteristics for many substances, and lack of flammability or toxicity. Ionic liquids can have good thermal, mechanical and chemical stability as well as favorable densities and viscosities. The required specifications can be adjusted easily by the large number of possible combinations of anions and cations when formulating an ionic liquid. Ionic liquids can be used as chemical solvents, catalysts, electrolytes in fuel cells as well as for gas-separation and storage by absorption. Ionic liquid membrane systems can comprise an adequate porous support material, e.g. a polymer film, coated by ionic liquids. The system can separate CO2 and sulfur compounds from different gas mixtures. Competitive selectivity and permeability are obtained for the separations.
Novel membrane materials, such as polyetherimides, can be used as membrane material with improved plasticization resistance for CO2 removal, for example with an OCM process. Other membrane materials that can be used include, but are not limited to, polymeric membranes based on or comprising polyamides, polysemicarbazides, polycarbonates, polyarylates, polyaniline, poly(phenylen oxide), polysulfones, polypyrrolones, or combinations thereof. In some cases, the polymeric membrane is solvent resistant and can reduce the plasticization effects of hydrocarbons in the feed stream, e.g., polyketone, polyether ketone, polyarylene ether ketone, polyimide, polyetherimide, and/or polyphenylene sulphide, which have intrinsic solvent inertness and can therefore withstand organic rich operation conditions.
An adequate porous support material, e.g. a polymer film, coated by ionic liquids can be used in continuous separation of CO2 and sulfur compounds from different gas mixtures, including a methane rich stream. This separation can improve the efficiency of OCM processes. The OCM reactor effluent can enter the supported ionic liquid separation subsystem, and CO2 and other contaminants can be removed from the process gas. Other contaminants can include but are not limited to traces of sulfur compounds, inerts, CO, SO2, H2S, and tetrahydrothiophene (THT).
CO2 can be separated from other gases by cooling and condensation, for example as shown in
In some cases, low temperature distillation is used to separate CO2. Low temperature distillation can give better results when there is a high concentration of CO2 in the feed gas. For the OCM process gas, the CO2 concentration can be increased by, for example, having a recycle stream, or by using a modified OCM reactor where excess CO2 is used as a quench medium for the reaction heat. Low temperature separation can refer to separations using temperature levels above −90° C.
As shown in
As shown in
The first (bulk) CO2 separations system can be a membrane or a PSA system, an amine removal system, or any other solvent based CO2 removal system as described herein. The final CO2 removal step can be a caustic tower, a membrane based system, a PSA based system, or any other CO2 removal system as described herein. The ethylene product CO2 removal system 2480 can be followed by further drying and/or purification steps.
One advantage of the two step process described herein can be energy saving that arise from decreasing the gas volumes being processed for the final CO2 removal step. If CO2 removal is done entirely upstream of the demethanizer, the energy consumption may be much greater than described in
The concentration of CO2 going into the de-methanizer (following the first CO2 removal unit) can be any suitable amount (i.e., such that CO2 doesn't freeze in the de-methanizer). In some cases, the concentration of CO2 going into the de-methanizer is greater than or equal to about 0.1 mol %, 0.2 mol %, 0.3 mol %, 0.4 mol %, 0.5 mol %, 0.6 mol %, 0.7 mol %, 0.8 mol %, 0.9 mol %, 1.0 mol %, 1.2 mol %, 1.4 mol %, 1.6 mol %, 1.8 mol %, 2.0 mol %, 2.2 mol %, 2.4 mol %, 2.6 mol %, 2.8 mol %, 3.0 mol %, 3.5 mol %, 4.0 mol %, 5.0 mol %, or more. In some cases, the concentration of CO2 going into the de-methanizer is less than or equal to about 5.0 mol %, 4.0 mol %, 3.5 mol %, 3.0 mol %, 2.8 mol %, 2.6 mol %, 2.4 mol %, at 2.2 mol %, 2.0 mol %, 1.8 mol %, 1.6 mol %, 1.4 mol %, 1.2 mol %, 1.0 mol %, 0.9 mol %, 0.8 mol %, 0.7 mol %, 0.6 mol %, 0.5 mol %, 0.4 mol %, 0.3 mol %, 0.2 mol %, 0.1 mol %, or less. In some cases, the concentration of CO2 going into the de-methanizer is between any of the two values described above, for example, between about 0.5 mol % and about 2.0 mol %.
Alkaline salt-based processes can be used for CO2 removal. These processes can utilize the alkali salts of various weak acids, such as sodium carbonate and potassium carbonate. These processes can provide advantages such as low cost and minimal solvent degradation. Processes that can be used for H2S and CO2 absorption include those using aqueous solutions of sodium or potassium compounds. For example, potassium carbonate can absorb CO2 at high temperatures, an advantage over amine-based solvents.
Hot potassium carbonate (K2CO3) solutions can be used for the removal of CO2 from high-pressure gas streams, among other applications. Potassium carbonate has a low rate of reaction. To improve CO2 absorption, mass transfer promoters such as piperazine, diethanolamine, and arsenic trioxide can be used. Less toxic promoters such as borate can also be used, for example with flue gas streams (see, e.g., Ghosh et al., “Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid,” Energy Procedia, pages 1075-1081, February 2009, which is hereby incorporated by reference in its entirety). To limit corrosion, inhibitors can be added. These systems can be known as activated hot potassium carbonate systems. Licensed hot activated potassium carbonate systems include the Benfield and the Catacarb process. The processes can be used for bulk CO2 removal from high-pressure streams, but can also produce high-purity CO2.
Flue gas impurities such as SOx and NOx can reduce the operational efficiency of the potassium carbonate as a solvent. SO2 and NO2 may not able to be released from the solvent under industrial conditions. Selective precipitation of the impurity salts formed by SOx and NOx can be used to remove such compounds (see, e.g., Smith et al., “Recent developments in solvent absorption technologies at the CO2CRC in Australia” Energy Procedia, pages 1549-1555, February 2009, which is hereby incorporated by reference in its entirety).
A variety of materials can be used as CO2 sorbents through chemical reactions and physical absorptions, including but not limited to soda-lime, active carbon, zeolites, molecular sieves, alkali metal oxides, silver oxide, lithium oxide, lithium silicate, carbonates, silica gel, alumina, amine solid sorbents, metal organic frameworks and others.
Physical impregnation of CO2-reactive polymers, such as tetraethylene pentamine or polyethyleneimine, inside a porous support, such as alumina, pumice, clay or activated carbon, can be used for CO2 removal. Amine based sorbents can be easily regenerated. Alternatively, a mixture of an amine compound with a polyol compound can be impregnated in a porous support. The polyol compound can be used to increase the CO2 desorption rate of the amine. The supported amine-polyol sorbent can comprise at least about 1 wt %, 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, or more amine and/or polyol. In some cases, the supported amine-polyol sorbent can comprise from about 1 wt % to about 25 wt % amine and from about 1 wt % to about 25 wt % polyol, with the balance being the support. Solid sorbent can adsorb and desorb CO2 a relatively high rates at ambient temperatures. Enhanced CO2 cyclic removal capacities in either dry or humid air flows can further be achieved by using a solid sorbent at an increased amine concentration of amines from about 35 wt % to about 75 wt %.
Solid sorbents that can selectively remove multiple gases can be used to remove CO2, H2O, nitrogen oxides, and hydrocarbons. This can be achieved by using composite adsorbents, for example by using a mixed adsorbent of alumina and zeolite to remove CO2 and H2O simultaneously.
CO2 can be separated from flue gas using an ion pump method instead of relying on large temperature and pressure changes to remove CO2 from a solvent. Ion pump methods can substantially increase the overlying vapor pressure of CO2. As a result, the CO2 can be removed from the downstream side of the ion pump as a pure gas. The ion pumping can be obtained from techniques including but not limited to reverse osmosis, electro dialysis, thermal desalination methods, or an ion pump system having an oscillation flow in synchronization with an induced electric field.
By making use of energy such as renewable or nuclear energy, CO2 and water can be recycled into sustainable hydrocarbon fuels in a non-biological process. Various pathways can enable such a conversion, for example by H2O and CO2 dissociation followed by fuel synthesis. The methods of dissociation can include heat, electricity, and solar driven methods such as thermolysis, thermochemical loops, electrolysis, and photoelectrolysis. High temperature electrolysis can make efficient use of electricity and heat, provide high reaction rates, and integrate well with fuel synthesis.
Synthetic analogues of enzymes as a polymer thin film supported on micro-porous substrates can be used to separate CO2 from gas mixtures. For example, a polymer thin film containing carbonic anhydrase mimicking sites can supported on a porous substrate and can separate CO2 from a stream containing O2 and N2. The system can be, for example, about 30% lower in cost compared to amine-based systems.
CO2 anti-sublimation can be used to remove CO2. CO2 anti-sublimation can use an SO2 removal unit followed by a water cooling step. The water can be eventually removed, for example first as liquid then below the triple point as ice. Dry flue gas can be further cooled until CO2 precipitates. The process can employ anti-sublimating CO2 on a low temperature surface, thus transforming the carbon dioxide from its gaseous phase to a solid phase frosted on a cold surface. Anti-sublimation can allow CO2 capture at a pressure slightly higher than atmospheric. CO2 anti-sublimation can be used with a flue gas system (flue gas composition, e.g., in mol %: CO2 15%, H2O 13%, N2 70% and O2 3%) at various temperatures (e.g., about 51° C.).
The triple point of CO2 is −56.4° C. and 5.11 atm. For 100% pure CO2 at a pressure P′ (where P′ is less than 5.11 atm) the frosting temperature can be given by T′=(P′−15.6)*(22.1/4.11). Accordingly, for a pressure of 4.5 atm, T=−59.6° C.
The sublimation temperature of a substance within a gas mixture can depend on its partial pressure (its corresponding concentration within the mixture). Table 1 shows frosting temperatures at different exemplary CO2 concentrations.
For use in an OCM process, a CO2 anti-sublimation unit may encounter higher pressure of OCM effluent (e.g., feed to CO2 capture system), lower CO2 concentration, and higher hydrocarbon content (e.g., methane, ethane, ethylene). Lower CO2 concentration can be addressed by a recycle.
The OCM reaction is highly exothermic. Various quenching media can be used to extract the OCM reaction heat. For example, CO2 can be injected to extract the heat, which results in the OCM effluent containing excess CO2. Such effluent can be suitable for the advanced CO2 recovery methods described herein.
Such an approach can provide advantages including a smaller recycle loop and more efficient CO2 removal methods, resulting in lower capital expenditure (CAPEX). This can also result in the feasibility of small distributed scale OCM units, since after the removal of excess CO2, the relatively richer ethylene stream needs fewer treatment and recovery steps.
Separation of AirProduction of O2 or N2 may be performed using cryogenic distillation of air which requires liquefaction. In some cases, oxygen condenses at about −183° C. at atmospheric pressure, which makes the cryogenic process very energy intensive. Pressure Swing Adsorption (PSA) systems based on zeolites may be used for air separation to reduce energy consumption in cryogenic processes. However, there is a need for new systems and methods for separating air into O2 and/or N2 fractions at reduced operational (OPEX) and capital (CAPEX) expense, particularly in cases where pure O2 and/or N2 is not required.
In some cases, the systems and methods described herein use materials having much higher surface areas when compared to zeolites (e.g., at least about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, about 10, or more fold increase in surface area). Examples of suitable materials include metal organic frameworks (MOFs) such as those described herein. In some cases, these materials exhibit very high selectivity (as compared to zeolites) towards one component (e.g., O2 and/or N2) in a mixture of gases (e.g., air). For example, some M2(dobdc) MOFs have excellent adsorption capacity and selectivity for O2 (e.g., Fe2(dobdc) MOF).
However, it can be very difficult to make a viable process that extracts the desired product at high recovery and purity, especially if the desired component is the one that gets strongly adsorbed on the material (such as, O2 on Fe2(dobdc)). Therefore, provided herein are methods and system configurations that make MOF materials economically viable for air separation by careful choice of purge gas and cycle conditions.
Presented herein are four examples (Cases I-IV) based upon which component is adsorbed and which component is the desired product. In Case I, O2 is preferentially adsorbed on the adsorbent material and O2 is the desired product. In an adsorption phase of a PSA cycle, air can be fed to the adsorbent bed at a pressure such that the partial pressure of O2 falls in a range at which it substantially saturates the bed, thus utilizing the bed's capacity. For example, the O2 partial pressure can be about 0.2 bar in case of Fe2(dobdc) at a temperature of 226K. Note that that the total pressure of the feed air would be greater, since air has approximately 21% O2. Due to the material's high selectivity for O2, O2 gets adsorbed whereas N2 adsorption is much lower, and negligible in some cases. Adsorption can be performed at high feed flow rate, so that the PSA cycle time is very short (for example, less than a minute). Short cycle times can require less of the adsorbent material per ton of O2 produced in a day, thus taking advantage of the high capacity of the adsorbent material. The lighter component stream from the PSA can contain N2 and a portion of the purge gas that replaces O2 (as described below).
Following adsorption for Case I, the system can be depressurized (e.g., for a few seconds) to remove gas from the void spaces of the system. This gas can have a composition similar to air. Desorption can follow depressurization. Since the adsorbed component (O2) is the desired product, the choice of purge gas for desorbing it from the bed can be carefully chosen as described herein. A suitable purge gas may have one or more of the following characteristics: (a) it can readily replace the adsorbed O2, thus resulting in quick desorption of the O2 (i.e., short desorption times can result in a smaller adsorbent bed, thus reducing the amount of adsorbent required); (b) it can be compatible with the downstream process where the O2 is consumed (e.g., if the O2 is to be consumed in an OCM process, CO2, CH4, steam, or any other downstream compatible gas can be used as purge gas); and (c) the lighter gas stream from the adsorption step (e.g., nitrogen) can be used as the purge gas (although the product then would not be ultra-high purity O2, yet the process can be economical in case lower levels of O2 purity is tolerable by the downstream process). In some cases, the purge gas satisfies and two of (a)-(c). In some embodiments, the purge gas satisfies all of (a)-(c).
In some embodiments, the desorption step can be performed at a pressure lower than the adsorption pressure. The bed can then be repressurized with air before the next adsorption step of the cycle.
In Case II, O2 preferentially adsorbs on the adsorbent material and N2 is the desired product. In the adsorption step for Case II, air can be fed to the adsorbent bed at a pressure such that the partial pressure of O2 falls in a range at which it substantially saturates the bed, thus utilizing its capacity. For example, the O2 partial pressure can be around 0.2 bar in case of Fe2(dobdc) at a temperature of 226 K. Note that the total pressure of the feed air would be greater, since air has approximately 21% O2. Due to the high selectivity of the adsorbent, O2 gets adsorbed whereas N2 adsorption in small or insignificant amounts. The lighter component stream from the PSA contains N2 (which is considered to be the product in this Case II) and a portion of the purge gas that replaces O2 (as described below).
Following adsorption for Case II, the system can be depressurized to remove gas from the void spaces of the system. This gas can have a composition similar to air. Desorption can follow depressurization. Since the purge gas replaces O2 on the adsorbent and sweeps the O2 out and the purge gas later gets replaced by O2 during the adsorption step, choice of the purge gas can be crucial as described herein. A suitable purge gas can have one or more (i.e., 1, 2 or 3) of the following characteristics: (a) the purge gas can replace the adsorbed O2 (e.g., CO2), thus resulting in quick desorption of the O2 (i.e., short desorption steps can result in smaller bed having less adsorbent material); (b) the purge gas can be compatible with the downstream process where N2 is being used (i.e., even though the N2 may not be 100% pure due to presence of a portion of purge gas, it is still almost nearly separated from O2); and (c) the lighter gas stream from the adsorption step (e.g., N2 in this case) can be used as the purge gas. The desorption step can be performed at a pressure lower than the adsorption pressure. The bed can be repressurized with air before the adsorption step restarts.
In Case III, N2 preferentially adsorbs on the adsorbent material and O2 is the desired product. In the adsorption step for Case III, air can be fed to the adsorbent bed at a pressure such that the partial pressure of N2 falls in a range at which it substantially saturates the bed, thus utilizing the capacity of the adsorbent material. Note that the total pressure of the feed air would be slightly more because air has approximately 79% N2. Due to the high selectivity of the adsorbent, N2 gets adsorbed whereas O2 adsorption is low or even insignificant. This adsorption step can be performed at high feed flow rate, such that the cycle time is very short (e.g., less than about a minute). The lighter component stream from the PSA contains O2 and a portion of the purge gas that replaces N2 (as described below). The lighter component is considered to be the product in this case.
Following adsorption for Case II, the system can be depressurized (e.g., for few seconds) to remove gas from the void spaces of the whole system. This gas can have a composition similar to air. For desorption, since the purge gas replaces N2 while sweeping it out and itself gets replaced by N2 during the adsorption step, the choice of the purge gas can be crucial as described herein. A suitable purge gas can have one or more (e.g., 1, 2 or 3) of the following characteristics: (a) the purge gas may readily replace the adsorbed N2, thus resulting in quick desorption of the N2; (b) the purge gas can be compatible with the downstream process where O2 is consumed (e.g., if the O2 is to be consumed in an OCM process, CO2, CH4, or steam can be used as purge gas); and (c) the lighter gas stream from the adsorption step (i.e., O2 in this case) can be used as the purge gas. The desorption step can be performed at a pressure lower than the adsorption pressure. The bed can be repressurized with air before the adsorption step restarts.
In Case IV, N2 preferentially adsorbs on the adsorbent material and N2 is the desired product. In adsorption, air can be fed to the adsorbent bed at a pressure such that the partial pressure of N2 falls in a range at which it substantially saturates the bed, thus utilizing its capacity. Note that the total pressure of the feed air would be slightly more, since air has approximately 79% N2. Due to its high selectivity, N2 gets adsorbed whereas O2 adsorption is less, or even insignificant in some cases. This step can be performed at high feed flow rate, such that the cycle time is very short (for example, less than a minute). The lighter component contains O2 and a portion of the purge gas that replaces N2 (as described below). The system can be depressurized for few seconds to remove gas from the void spaces of the system. This gas can have a composition similar to air.
For desorption for Case II, the purge gas replaces N2 while sweeping it out and itself being replaced by N2 during the adsorption step, so choice of the purge gas can be crucial as described herein. A suitable purge gas can have one or more (e.g., 1, 2) of the following characteristics: (a) the purge gas can replace the adsorbed N2 (for example, CO2), thus resulting in quick desorption of the N2; and/or (b) the purge gas can be compatible with the downstream process where N2 is consumed (even though the N2 may not be 100% pure due to presence of a portion of purge gas, it is still almost entirely separated from O2). The desorption step can be performed at a pressure lower than the adsorption pressure. The bed can be repressurized with air before the adsorption step restarts.
In some cases, the feed to the separation system can be the product from either an OCM reactor as discussed above, an OCM process integrated with a Methanol to Olefins (MTO) unit, an OCM process integrated with a steam cracker, or an OCM process integrated with a dimerization and metathesis unit for example.
Methods and systems of the present disclosure can be combined with or modified by other methods and systems, such as those described in U.S. Patent Publication No. 2015/0232395; U.S. Patent Publication No. 2014/0012053; U.S. Patent Publication No. 2014/0018589; U.S. Pat. No. 9,469,577; U.S. Patent Publication No. 2015/0152025; U.S. Patent Publication No. 2015/0210610; and U.S. Pat. No. 9,334,204, each of which is entirely incorporated herein by reference.
While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. It is not intended that the invention be limited by the specific examples provided within the specification. While the invention has been described with reference to the aforementioned specification, the descriptions and illustrations of the embodiments herein are not meant to be construed in a limiting sense. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. Furthermore, it shall be understood that all aspects of the invention are not limited to the specific depictions, configurations or relative proportions set forth herein which depend upon a variety of conditions and variables. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is therefore contemplated that the invention shall also cover any such alternatives, modifications, variations or equivalents. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.
Claims
1. A method for separating a product from a gas mixture, the method comprising:
- (a) at a first total pressure, directing a gas mixture comprising at least one impurity and a product gas into a pressure swing adsorption (PSA) vessel containing an adsorbent to adsorb the product gas on the adsorbent, wherein the product gas has a first partial pressure;
- (b) at a second total pressure, directing a sweep gas into the PSA vessel to adsorb the sweep gas on the adsorbent and displace the product from the adsorbent to yield a displaced product, wherein the sweep gas has a second partial pressure that is greater than the first partial pressure of the product in the gas mixture; and
- (c) desorbing the sweep gas from the adsorbent such that additional product is capable of adsorbing on the adsorbent.
2. The method of claim 1, wherein (c) is performed at substantially the first total pressure.
3. The method of claim 1, wherein an amount of the product that adsorbs on the adsorbent at the first partial pressure in (a) is substantially equivalent to the amount of sweep gas that adsorbs on the adsorbent at the second partial pressure in (b).
4. The method of claim 1, wherein an amount of heat released by the adsorption of the product in (a) is substantially equivalent to an amount of heat released by the adsorption of the sweep gas in (b).
5. The method of claim 1, wherein the displaced product is enriched relative to the concentration of the product in the gas mixture by a factor of at least about 2.
6. The method of claim 1, wherein the displaced product includes at least some of the sweep gas.
7. The method of claim 6, further comprising, following displacing the product from the adsorbent, separating the product from the sweep gas (e.g., by distillation).
8. The method of claim 1, wherein the product is ethylene.
9. The method of claim 1, wherein the sweep gas is ethane.
10. The method of claim 1, wherein the gas mixture comprises at least 3 impurities.
11. The method of claim 1, wherein the at least one impurity comprise carbon monoxide (CO), carbon dioxide (CO2), methane, ethane, hydrogen (H2), or any combination thereof.
12. The method of claim 1, wherein the product has a concentration of less than about 30% in the gas mixture.
13. The method of claim 1, wherein the adsorbent is a metal organic framework (MOF).
14. The method of claim 13, wherein the MOF is M2(dobdc).
15. The method of claim 1, wherein at the first pressure, a selectivity of the adsorbent for adsorbing the product as compared to the at least one impurity is at least about 5.
16. The method of claim 15, wherein the at least one impurity is methane.
17. The method of claim 1, wherein the gas mixture is derived from an effluent from an oxidative coupling of methane (OCM) reactor.
18. The method of claim 17, further comprising recycling the at least one impurity to the OCM reactor following the adsorption of the product from the gas mixture.
19. The method of claim 18, further comprising, prior to recycling the at least one impurity to the OCM reactor, converting carbon dioxide (CO2), carbon monoxide (CO), and/or hydrogen (H2) components of the at least one impurity to methane (CH4).
20. The method of claim 1, wherein following the adsorption of the product from the gas mixture, the gas mixture comprises a predetermined amount of a given impurity.
21. The method of claim 20, wherein the given impurity is CO2.
22. The method of claim 20, wherein the adsorbent comprises (i) a first material that adsorbs the product and the given impurity, and (ii) a second material that adsorbs the product but does not substantially adsorb the given impurity.
23. The method of claim 22, wherein an amount of the first material relative to an amount of the second material is selected to achieve the desired amount of the desired impurity.
24. The method of claim 22, wherein the first material is a CaX zeolite and the second material is a metal organic framework.
25.-40. (canceled)
41. A method for separating oxygen (O2) from nitrogen (N2), the method comprising:
- (a) at a first pressure, directing a mixture of O2 and N2 into a pressure swing adsorption (PSA) vessel containing an adsorbent to adsorb the O2 on the adsorbent, wherein the adsorbent is a metal organic framework (MOF) that is selective for O2; and
- (b) at a second pressure that is less than the first pressure, desorbing the O2 from the adsorbent with a purge gas.
42.-81. (canceled)
82. A method for generating compounds with two or more carbon atoms (C2+ compounds), comprising:
- (a) directing oxygen (O2) and methane (CH4) into an oxidative coupling of methane (OCM) reactor having a catalytic section and a cracking section to produce an OCM product stream, which catalytic section reacts the O2 and CH4 to yield ethylene (C2H4), ethane (C2H6) and heat, which cracking section uses the heat to convert C2H6 into C2H4, and which product stream comprises (i) C2+ compounds including ethylene (C2H4) and ethane (C2H6) and (ii) C1 compounds including un-reacted CH4;
- (b) directing the product stream into a separations unit containing a metal organic framework (MOF) that produces (i) a first stream comprising the C2H4, (ii) a second stream comprising the C2H6 and (iii) a third stream comprising the C1 compounds;
- (c) directing the second stream into the cracking section; and
- (d) directing the third stream into the catalytic section.
83.-111. (canceled)
112. An adsorbent, comprising:
- a. a first material that adsorbs (i) a product gas at a first heat of adsorption and (ii) a sweep gas at a second heat of adsorption; and
- b. a second material that adsorbs the sweep gas at a third heat of adsorption,
- wherein the first material and the second material are included in a mixed material in a relative proportion such that the mixed material has an average heat of adsorption for the sweep gas, and
- wherein an absolute value of a difference between the first heat of adsorption and the average heat of adsorption is less than an absolute value of a difference between the first heat of adsorption and the second heat of adsorption.
113.-135. (canceled)
Type: Application
Filed: Mar 31, 2017
Publication Date: Jun 21, 2018
Inventors: Divya Jonnavittula (San Ramon, CA), Gaurav Chachra (Berkeley, CA), Guido Radaelli (Pleasant Hill, CA)
Application Number: 15/476,889
