Separation of multicomponent gas mixtures by selective adsorption

Abstract: Multicomponent gas mixtures containing: (1) hydrogen as the primary component, (2) a secondary key component that is more strongly sorbed by the adsorbent than hydrogen, and (3) a minor quantity of one or more dilute tertiary components less strongly sorbed than the secondary key component, are subject to selective adsorption in an adiabatic pressure swing adsorption (PSA) cycle for the separate recovery of high purity hydrogen and the secondary component. A given example is the treatment of the effluent gas from a shift converter in a hydrocarbon reformer plant, wherein hydrogen and carbon dioxide are separately recovered as key components substantially freed of minor dilute components such as methane, carbon monoxide and nitrogen.

Description: TECHNICAL FIELD The present invention relates generally to the separation and recovery of individual components of gas mixtures by pressure swing adsorption and is especially concerned with enhancing recovery of a primary component, such as hydrogen in particular, from such gas mixtures. BACKGROUND OF THE INVENTION Applicant's prior U.S. Patent 4,171,206 describes a pressure swing adsorption (PSA) process (hereinafter the "base cycle") which can be used for the fractionation of a multicomponent gas mixture composed of (1) a bulk primary component, (2) a bulk secondary component more strongly sorbed than the primary component and (3) a minor quantity of one or more dilute components less strongly sorbed than the secondary component. By way of example, the '206 patent applied the base cycle to the production and recovery of pure hydrogen (primary component) and CO₂ (secondary component) from the gaseous effluent of a shift converter in a hydrocarbon reforming plant where the minor quantity of dilute components consisted of CH₄ and CO. Applicants' EP-A-0383312 discloses a pressure swing adsorption process for the recovery of two individual components from a multicomponent gas mixture. The process utilizes a first group of adsorbent beds selective for the secondary component and a second group of beds selective for the tertiary impurities in the mixture. The gas mixture is passed through a first and a second bed in series, the very high purity primary component is discharged from the second bed, and the high purity secondary component is recovered by desorption from the first bed. A low pressure purge of the first bed with high-purity secondary component is utilized to minimize compression power and improve purity of the secondary component. Applicants' EP-A-0385174 discloses a pressure swing adsorption process for the recovery of two individual components from a multicomponent gas mixture. The process utilizes a first group of adsorbent beds selective for the secondary component and a second group of beds selective for the tertiary impurities in the mixture. The gas mixture is passed through the first and the second beds in series and a high purity component product is discharged from the second beds. The secondary component product is recovered by desorption from the first beds. Product recovery is supported by recycling depressurization gas from the first beds and by several pressure equalization steps. The present invention provides an improvement to the base cycle whereby the recovery of the primary component is significantly increased. SUMMARY OF THE INVENTION As indicated above the present invention provides an improvement to the base cycle disclosed in Applicant's prior U.S. Patent 4,171,206 whereby the recovery of the primary component is significantly increased. (The sepcification of U.S. Patent 4,171,206 is hereby incorporated by reference.) Specifically this improvement consists of redirecting the pressure equalization flow of the base cycle's second pressure equalization step (hereinafter "PE2 Redirection") while adding a third pressure equalization step (hereinafter "PE3"). As discussed infra, PE2 Redirection and PE3 complement each other and must be implemented together. The operation of the invention will be understood and the advantages afforded thereby more fully appreciated from the detailed description which follows read in connection with the accompanying drawing illustrating a practical embodiment in which the invention may be practiced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. I is a flow diagram of a preferred embodiment of the invention employing a group of six adsorbent vessels with connecting conduits designed for operation in parallel and in timed sequence, each of these vessels being connected to a second group of three adsorbent vessels through valved conduits designed to enable vessels of said first group to be placed in series flow communication with selected vessels of said second group. DETAILED DESCRIPTION As shown in Fig. I, the present invention employs two sets of adsorbent beds with the most strongly adsorbed secondary component recovered in the first set of beds (A beds) while the minor quantity of dilute components (also referred to as "impurities" or "tertiary components") are removed in the second set of beds (B beds). In addition to these adsorption columns, the illustrated PSA system further comprises a compressor identified by "C", a vacuum pump identified by "V", and switching valves identified by number designation. Each set of adsorption vessels undergoes a certain sequence of cyclic steps as described below. These steps are similar to those disclosed in the base cycle except for steps (B)(3)(b) and (B)(7)(a) which incorporate PE2 Redirection and steps (A)(5) and (B)(4) which incorporate PE3. A) Cycle Steps for A Columns 1) Adsorption Step , which consists of: (a) Flowing the multicomponent gas mixture at a pressure of approximately 2,07-34,5 bar (30-500 psia) and a temperature of approximately 4,44-65,56°C (40-150°F) through the A column packed with an adsorbent capable of selectively adsorbing the desired secondary component from the gas mixture. (b) Withdrawing an effluent stream which now is essentially free of the secondary component and feeding the effluent stream to a B column which is on step (B)(1). (c) Continuing steps (1)(a) and (1)(b) above for a predetermined cycle time or until the concentration of the secondary component in the effluent stream reaches a preset limit. The A column is now called "spent" because it has exhausted its capacity for adsorbing the secondary component from the gas mixture. 2) Rinse Step , which consists of: (a) Raising the pressure of the high purity secondary component desorbed in Step (A)(3)(b) to the adsorption pressure by a compressor. (b) Passing this gas through the "spent" A column cocurrently to the feed flow to displace any void gas or adsorbed impurities. (c) Mixing the effluent gas from step (A)(2)(b) with fresh feed gas in step (A)(1)(a). 3) Depressurization Step , which consists of: (a) Discontinuing the rinse flow through the A column and transferring the feed to another A column. (b) Reducing the pressure in the "rinsed" A column to approximately 1,03 bar (15 psia) by venting the gas in a direction opposite to the feed flow. 4) Evacuation Step , which consists of: (a) Further lowering the pressure of the depressurized A column from a pressure of approximately 1,03 bar (15 psia) to a pressure of approximately 0,053-0,267 bar (40-200 torr) by evacuating the gas from the feed end of the A column. (b) Storing the evacuated gas, which is essentially the secondary component, at ambient pressure for further use. 5) Pressure Equalization Step III (PE3) , which consists of: (a) Discontinuing the pressure reduction of the A column. The A column is now called "regenerated" because its capacity for removing the secondary component has been restored. (b) Raising the pressure in the regenerated A column from evacuation level by connecting the feed end of the regenerated A column with the feed end of a B column which is on step (B)(4). (c) Discontinuing the above step when the pressures in the connected A and B columns are about equal. (d) Note: As noted supra, step (A)(5) is an improvement to the base cycle. In the base cycle this step does not exist. 6) Pressure Equalization Step I (PE1) , which consists of: (a) Disconnecting the A column from the B column at the end of the above step. (b) Further raising the pressure in the A column by connecting the feed end of the A column with the feed end of a B column, which is on step (B)(2). (c) Discontinuing the above step when the pressures in the connected A and B columns are about equal. 7) Repressurization Step , which consists of: (a) Connecting the product end of the A column with the feed end of a B column which has just finished step (B)(7) and is on step (B)(8). (b) Raising the pressure in the A column to the adsorption pressure level (approximately 2,07-34,5 bar (30-500 psia)) via the effluent gas from a B column on step (B)(8). (c) Discontinuing the above step when the pressure in the regenerated A column has reached the desired level. The A column is now ready to undergo a new cycle starting at step (A)(1). B) Cycle Steps for B Columns 1) Adsorption Step , which consists of: (a) Flowing the effluent from an A column on step (A)(1), which effluent has been depleted of the secondary component, at a pressure of approximately 2,07-34,5 bar (30-500 psia) and a temperature of approximately 4,44-65,56°C (40-150°F) through the B column packed with adsorbents capable of selectively adsorbing all the components of the mixture except the desired primary product. (b) Withdrawing an effluent stream, which is the very high purity primary component. (c) Continuing steps (1)(a) and (1)(b) above for a predetermined cycle time or until the concentration of impurities in the effluent stream reaches a preset limit. This column is now called "spent" because it has exhausted its capacity for removing the impurities from the primary component. 2) Pressure Equalization Step I (PE1) , which consists of: (a) Discontinuing the feed flow through the "spent" A and B columns and transferring the feed to another pair of A and B columns which are in direct communication. (b) Reducing the pressure in the B column by connecting the feed end of this B column with the feed end of an A column on step (A)(6). (c) Discontinuing the above step when the pressures in the connected A and B columns are about equal. 3) Pressure Equalization Step II (PE2) , which consists of: (a) Disconnecting the A and B columns at the end of the above step. (b) Further reducing the pressure in the B column by connecting the effluent end of the B column with the effluent end of another B column on step (B)(7). Note: As noted supra, this step (B)(3)(b) is an improvement to the base cycle specifically with respect to how the connection is made. In the base cycle, the connection was made between the feed ends (as opposed to the effluent ends) of the B columns. (c) Discontinuing the above step when the pressures in the connected B columns are about equal. 4) Pressure Equalization Step III (PE3) which consists of: (a) Disconnecting the B column from the other B column at the end of the above step. (b) Further reducing the pressure in the B column by connecting the feed end of the B column with the feed end of an A column on step (A)(5). (c) Discontinuing the above step when the pressures in the connected A and B columns are about equal. (d) Note: As noted supra, step (b)(4) is an improvement to the base cycle. In the base cycle, this step did not exist. 5) Depressurization Step , which consists of: (a) Disconnecting the A and B columns at the end of the above step. (b) Further reducing the pressure in the B column to approximately 1,03-2,07 bar (15-30 psia) by venting the gas in direction opposite to the feed flow. The desorbed gas may be vented as waste, used as fuel, or used as feed to another process. (c) Discontinuing the above step when the pressure in the B column has reached approximately 1,03-2,07 bar (15-30 psia). 6) Purge Step , which consists of: (a) Flowing the very high purity primary component in a direction opposite to the feed flow through the B column at a pressure of approximately 1,03-2,07 bar (15-30 psia). (b) Continuing this purge gas flow for a predetermined time such that the B column is sufficiently clean of the impurities. The column is now called "regenerated" since its capacity for removing the impurities from the primary component has been restored. (c) Discontinuing the purge gas flow by transferring it to another B column which has just finished step (B)(5). The purge gas effluent may be vented as waste, used as fuel, or used as feed to another process. 7) Pressure Equalization Step II (PE2) , which consists of: (a) Raising the pressure in the B column from a pressure of approximately 1,03-2,07 bar (15-30 psia) by connecting the effluent end of this B column with the effluent end of another B column on step (B)(3). Note: As noted supra, this step (B)(7)(a) is an improvement to the base cycle, specifically with respect to how the connection is made. In the base cycle, this connection was between the feed ends (as opposed to the effluent ends) of the B columns. (b) Discontinuing the above step when the pressures in the connected B columns are about equal. 8) Repressurization Step , which consists of: (a) Raising the pressure in the B column to adsorption pressure level (approximately 2,07-34,5 bar (30-500 psia)) by flowing the very high purity primary component through the B column. (b) Effluent from the feed end of this column is used to repressurize the A column on step (A)(7). (c) Discontinuing the above step when the pressure in the regenerated B column has reached the desired level. The B column is now ready to undergo a new cycle starting at step (B)(1). Operation of an embodiment of the invention will now be explained based on the flow diagram of Figure I and on the arbitrarily chosen 24 minute cycle as set out in Figure II. It may be seen from Figure II that while the A columns go thru one complete cycle, the B columns go thru two complete cycles. The valve sequence for Figure II's cycle is given in Figure III. Together, Figures I, II and III represent an integration of the invention into the base cycle. Feed at a pressure of approximately 20,7 bar (300 psia) and a temperature of approximately 4,44-37,8°C (40-100°F) is fed to the first integrated train, columns 1A and 1B, via open valves 1, 43, and 49. These columns have previously been pressurized to the adsorption pressure of approximately 20,7 bar (300 psia). The primary component is withdrawn from column 1B via open valve 64 at a pressure of approximately 20,7 bar (300 psia). The adsorption step is continued for a fixed time or until the effluent concentrations from both columns have reached a predetermined level. At the end of the adsorption step, the feed is directed to the second integrated trains, columns 2A and 2B. Column 1A is now rinsed with the high purity secondary component through open valve 7. The effluent which is feed like in composition passes through open valve 31 to be mixed with the feed. Column 1A is then depressurized to atmospheric pressure through open valve 13. This gas is compressed to a pressure of approximately 20,7 bar (300 psia), used as a rinse gas for Column 2A, and then recycled to the feed stream where it is mixed with fresh feed. During this time column 1B has undergone four pressure reduction steps. In the first, the pressure in column 1B is reduced by connecting the feed end of column 1B with the feed end of column 3A via open valves 58 and 21. Column 3A has Just completed a previous pressure equalization with column 3B. When the pressures in the columns are about equal, the second pressure reduction begins by connecting the effluent end of column 1B to the effluent end of column 3B through open valves 61 and 63. Column 3B has just finished purging and is at a pressure of approximately 1,38 bar (20 psia.). Again, when the pressures are approximately equal, the third pressure reduction begins by connecting the feed end of column 1B to the feed end of column 4A through valves 58 and 22. Column 4A has just completed the evacuation step and is at a pressure of approximately 0,133 bar (100 mmHg). After this third pressure equalization, the fourth pressure reduction begins by venting column 1B to approximately 1,38 bar (20 psia) through open valve 55. The gas from this step is either vented, used as fuel, or as feed gas to another process. Following column 1A's depressurization, column 1A undergoes further pressure reduction by evacuation via valve 25. The pressure in column 1A is reduced to approximately 0,133 bar (100 mmHg) and the evacuated gas is the most strongly adsorbed component of the feed mixture which is the secondary component. The feed end of column 1A is now connected to the feed end of column 1B via open valves 19 and 58. Column 1B has just finished two consecutive pressure equalization steps with columns 6A and 3B respectively. After equalization, the feed end of column 1A is connected with the feed end of column 2B through open valves 19 and 59. During the evacuation and pressure equalization steps for column 1A, column 1B is purged with the primary component countercurrently to the feed flow. The primary product gas enters column 1B via open valve 67 and exits via open valve 55 at a pressure of approximately 1,38 bar (20 psia). The purge effluent is either vented, used as fuel, or as a feed to another process. Following the purge step, the effluent end of column 1B is connected to the effluent end of column 2B which has just finished a pressure equalization with column 1A. The two columns are equalized in this step. Column 1A and 1B are now repressurized via open valves 67, 52, and 37. The primary component is used for this repressurization step. Columns 1A and 1B are now ready to undergo another cycle. As noted previously, there are two aspects of this invention that act in concert to improve the recovery of the primary component relative to the base cycle: redirecting the pressure equalization flow of the base cycle's second pressure equalization step (PE2 Redirection) and adding a third pressure equalization step (PE3). As will be shown in the Example, infra, PE2 Redirection and PE3 improved the base cycle's recovery of the primary component by two percentage points. The PE2 Redirection changes the connection between the B columns during the second pressure equalization step. In the base cycle, this connection is from the feed end of the "transferring" B column (i.e. the column which is initially at the higher pressure prior to PE2's pressure equalization) to the feed end at the "receiving" B column (i.e. the B column which is initially at the lower pressure prior to PE2's pressure equalization). PE2 Redirection changes this connection to be from the effluent end of the transferring B column to the effluent end of the receiving B column. The result is less impurity transfer to the receiving B column during PE2 since the impurities are concentrated at the feed end of the transferring B column prior to PE2. This in turn increases the adsorption capacity of the receiving B column's next adsorption step thereby improving the cycle's efficiency and primary component recovery. Although PE2 redirection does tend to stretch the transferring B column's mass transfer zone thereby requiring more primary component to eventually purge this B column (thereby reducing the cycle's efficiency), this negative effect of PE2 Redirection is compensated for by the immediately following PE3 step. PE3, by subsequently transferring from the feed end of this B column, recompacts the mass transfer zone. This is why PE2 redirection and PE3 complement each other and must be implemented together. In addition to recompacting the mass transfer zone, PE3 also lowers the pressure of the transferring B column prior to said column's subsequent depressurization step. The result is that less primary component is vented as waste which further improves recovery of the primary component. EXAMPLE A laboratory test trial for the improved cycle was made to confirm its performance. The feed gas was 75% hydrogen, 18.1% CO₂, 0.5% CO, 6.2% CH₄, and 0.2% N₂. The feed pressure was 20,7 bar (300 psia) and the temperature was 21,1°C (70°F). The A columns were 390,1 cm (12.8 feet) long and 5,08 cm (2 inches) in diameter and were packed with NaX zeolite for selective adsorption of CO₂ from the multicomponent gas mixture. The B columns were 390,1 cm (12.8 feet) and 5,08 cm (2 inches) in diameter and were packed wlth 85,34 cm (2.8 feet) of NaX zeolite at the feed end and 304,8 cm (10 feet) of BaX zeolite at the product end. NaX zeolite in the B columns removed CO₂ which broke through the A beds during the adsorption step and BaX zeolite in the B beds removed the CO, CH₄, and N₂ from the hydrogen. The hydrogen recovery was 88.1% with a purity of 99.9987. A comparison of experimental results for the base cycle in U.S. Patent 4,171,206 and the improved cycle in this invention is given in the following table. The feed represents a typical case for feed gas from a steam methane reformer. As can be seen, the improved cycle increased the hydrogen recovery by two percentage points over the base cycle. Feed Comp. Base Cycle Improved Cycle H₂ 74.8 75.0 CO₂ 18.3 18.1 CO 0.5 0.5 CH₄ 6.2 6.2 N₂ 0.2 0.2 H₂ Recovery 86.0 88.1 H₂ Purity 99.9988 99.9987 While the invention has been particularly described in connection with the separation and recovery of hydrogen as the primary key component and CO₂ as the secondary key component from a gas mixture containing these components accompanied by a minor quantity at dilute components, it will be understood that the described system and operation can be advantageously employed in the separation of other multicomponent gas mixtures having present therein a major portion of (1) primary key component which is not substantially adsorbed in either of the adsorbent columns of a train, (2) a secondary key component desired to be recovered and which is preferentially adsorbed and (3) one of more dilute components which are less strongly adsorbed than the secondary key component. Any sorbent which is selective towards the secondary key component of the feed mixture can be used in the A columns. The sorbent in the B columns should be selective for the minor tertiary components of the feed mixture or there may be employed in the B columns a combination of sorbents for that purpose. Having thus described the invention, what is desired to be secured by letters patents of the United States is set forth in the following claims.

Claims: A method of separation of a multicomponent feed gas mixture with the individual recovery of a primary key component and a secondary key component present in said mixture, by selective sorption, wherein said secondary key component is more strongly sorbed than said primary key component and there is present in said mixture at least one minor dilute tertiary gas component less strongly sorbed than said secondary key component and wherein the separation of said multicomponent feed gas mixture is effected in a defined system comprising a plurality of first sorbent beds (A) operating sequentially in parallel during a fixed cycle and a plurality of second sorbent beds (B), also operating sequentially in parallel during the fixed cycle, at least one of which second beds (B) is selectively coupled in series with one or another first bed (A) during the passage of the feed gas mixture into said first bed (A); the method which comprises, in an adiabatic adsorption pressure swing cycle, the steps of: (a) passing said multicomponent feed gas mixture at initial superatmospheric pressure and in selected flow direction through a first sorbent bed (A) selective for preferential retention of said secondary key component and then passing the effluent from said first bed through a second sorbent bed (B) selective for retention of said tertiary components, and discharging from said second bed unadsorbed primary key component, said passing of the multicomponent gas mixture being continued for a controlled time period until or short of breakthrough of said secondary key component from said first bed, while retaining all of the said tertiary components in said second bed; (b) thereafter discontinuing gas flow communication between said first and second beds, and (i) rinsing said first bed by flowing a stream of secondary key component therethrough at substantially the initial superatmospheric feed pressure level for a controlled time period effective to purge most of the void and desorbed gases from said first bed, and during this time period (b), (ii) lowering the pressure in said second bed to an intermediate level by withdrawing a gas stream therefrom including void and desorbed gases, wherein said pressure lowering is effected in three sequential pressure equalization transfers, (a) during the first of which pressure equalization transfers withdrawn gas from the feed end of said second bed is transferred to the feed end of a first bed of the defined system undergoing step (e)(ii) below thereby equalizing the pressures between the two beds involved in said first pressure equalization transfer, and (b) during the second pressure equalization transfer withdrawn gas from the effluent end of said second bed is transferred to the effluent end of a second bed of the defined system undergoing step (c)(ii) below thereby equalizing the pressures between the two beds involved in said second pressure equalization transfer, and (c) during the third pressure equalization transfer withdrawn gas from the feed end of said second bed is transferred to the feed end of a first bed of the defined system undergoing step (e)(i) below thereby equalizing the pressures between the two beds involved in said third pressure equalization transfer; and thereafter (iii) further depressuring said second bed to near ambient pressure; (c) after said rinsing step in (b)(i) above reducing the pressure in said first bed to substantially ambient level by desorption of gas therefrom including previously sorbed secondary key component and during this step (c), (i) purging said second bed at near ambient pressure with a stream of primary key component, and (ii) partially repressurizing said second bed to an intermediate pressure level by a pressure equalization transfer into the effluent end of said second bed of withdrawn gas from the effluent end of a second bed of the defined system undergoing step (b)(ii)(b) above thereby equalizing the pressures between the two beds involved in said pressure equalization transfer; (d) following step (c) above evacuating said first bed to subatmospheric level; (e) after attaining the subatmospheric level in said first bed, repressurizing said first bed to an intermediate pressure level by flow thereinto of gas substantially free of the secondary key component wherein said partial repressuring is effected in two sequential pressure equalization transfers, (i) during the initial of which pressure equalization transfers withdrawn gas from the feed end of a second bed of the defined system undergoing step (b)(ii)(c) above is transferred into the feed end of said first bed thereby equalizing the pressure between the two beds involved in said initial pressure equalization transfer, and (ii) during the subsequent pressure equalization transfer withdrawn gas from the feed end of a second bed of the defined system undergoing step (b)(ii)(a) above is transferred into the feed end of said first bed thereby equalizing the pressures between the two beds involved in said subsequent pressure equalization transfer; and thereafter (f) further repressurizing said first bed to the initial superatmospheric feed pressure level by flowing thereinto primary key component via second bed already pressurized to the intermediate pressure level per step (c)(ii) above, thereby bringing both beds to the feed pressure level and making them ready to repeat the defined sequence of steps beginning with the reintroduction of the multi component feed gas mixture into said first sorbent bed. The method as defined in Claim 1 wherein said multicomponent feed gas mixture comprises hydrogen as the primary key component, carbon dioxide as the secondary key component and as tertiary gas component at least one gas from the group consisting of methane, carbon monoxide and nitrogen. The method as defined in Claim 2 wherein a carbonaceous solid adsorbent is employed in at least said first bed. The method as defined in Claim 2 wherein said rinsing of the first bed at substantially feed pressure level during step (b)(i) above is effected in the same flow direction as that employed during initial introduction of the multicomponent feed gas mixture. The method as defined in Claim 4 wherein the effluent gas discharged during said rinsing step is added to the multicomponent feed gas mixture. The method as defined in Claim 2 wherein the defined purging of the said second bed at near ambient pressure during step (c)(i) above using part of the primary key component is carried out in a direction opposite to that of initial passage of the multicomponent feed gas mixture into the said second bed. The method as described in Claim 2 wherein the desorbed gas during pressure reduction of said first bed as defined in step (c) above is recompressed to feed pressure level and recirculated as the rinse gas into another first bed of the system then undergoing rinsing as defined in step (b)(i) above. The method as defined in Claim 7 wherein the said desorption step is carried out by flowing the gas in a direction opposite to that of the initial feed flow through the bed. The method as defined in Claim 2 wherein said evacuation of the first bed as recited in step (d) above is effected in a flow direction therein opposite to that of the initial feed gas flow therein. The method as defined in Claim 2 wherein said evacuation of the first bed defined in step (d) is effected by pulling vacuum at an intermediate level of said first bed. The method as defined in Claim 2 wherein final pressurization of a first and a second adsorbent bed to the initial superatmospheric feed pressure during step (f) above is effected by bringing a second bed in flow communication with a first bed, both of which are previously pressurized to the same intermediate pressure, and then flowing the primary key component into the said second bed in a flow direction opposite to that of the feed gas flow through those beds.

Inventors: KRATZ WILBUR CLYMER, KUMAR RAVI, KRATZ, WILBUR CLYMER, KUMAR, RAVI
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1992
Main classification: B01D53
IPC codes: C01B3, B01D53