Systems for the production of high purity gas

Abstract: Processes for the production of an inert gas-rich product gas from pressure swing adsorption are provided wherein the pressurization step is carried out by the use of feed gas only in a two step late feed profile, or by product gas and feed gas with the feed gas entering in either a typical one-step feed profile or in the two step late feed profile above.

Description: The present invention relates to the production of a highly purified product gas from a feed gas containing the product gas and impurities, utilizing pressure swing adsorption techniques. More particularly, this invention relates to a cyclic process wherein the treatment zone is repressurized to production pressure either by feed gas alone fed in an initial slow stage followed by a rapid second stage, or by production gas and feed gas with the feed gas being introduced in a two stage profile as described above. Adsorption techniques have been used in the separation and in the production of a variety of gases including hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen, and nitrogen. Feed gases useful in these adsorption separations include air; refinery off-gases; and land-fill, flue, and natural gases. Pressure swing adsorption (PSA) processes typically involve the cyclical pressurization, production, and regeneration of adsorbent beds or treatment zones to achieve a product of a specified purity. The pressurization step is usually carried out at a constant rate. This type of system is graphically illustrated in FIGS. 1-3. FIG. 1 represents a constant pressure pressurization step. FIG. 2 corresponds to the feed flow rate corresponding to the pressurization history in FIG. 1. FIG. 3 represents the hypothetical concentration profile of the impurities in the interstitial gas at the end of the pressurization step corresponding to FIG. 1, as a function of bed length. The feed rate for the constant pressure history is flat, and therefore, the final concentration profile should be higher in purity at the production end and lower at the injection end. However, such processes e.g. for the production of nitrogen having an oxygen content below 100 vpm (0.01%) from air are inefficient. PSA designs having multiple treatment zones are more effective for obtaining highly pure inert gases, but the cost for such systems are much higher than traditional two-zone PSA plants. The purity of a gas product of a PSA process is determined by the differences in the uptake rates of the feed gas components which in turn is dependent upon the length of time the feed gas is in contact with the adsorbent in the treatment zone. The initial product coming out of the treatment zone has the shortest residence time and is therefore of the lowest purity. The purity expectedly improves with time and then decreases as the adsorption wave approaches the bed exit. A catalytic combustion unit is occasionally combined with a PSA plant when very low oxygen concentration is required. The PSA plant is operated to produce nitrogen gas with oxygen concentrations between 10,000 and 1,000 vpm (1.0 and 0.1 percent, respectively) which is then combusted to produce a final nitrogen gas containing 100 vpm or less of oxygen. This operation is costly, however, as the catalytic units and the fuel used in the combustion are expensive. Furthermore, it is necessary to remove the combustion products from the effluent of the catalytic unit. Armond et al., U.S.-A-4,144,037, disclose a process for gas separation wherein the gaseous mixture is drawn through an adsorbent bed in a substantially unpressurized condition by applying reduced pressure to the outlet of the bed. Leitgeb, U.S.-A-4,256,469, offsets the initial low purity surge of product gas of a typical constant pressurization PSA system by slowing the PSA process concentration wave as it moves through the treatment zone. This was done by multi-stage pressurization where the final stage was conducted substantially more slowly than the initial stage. The Leitgeb process actually makes use of a lengthy adsorption time before product is taken though, and does not depend on the pressurization rates. This type of a system is graphically illustrated in FIGS. 4-6. FIG. 4 represents a rapid initial pressurization followed by a slow pressurization at the end of the pressurization step. FIG. 5 corresponds to the feed flow rate corresponding to the pressurization history in FIG. 4. FIG. 6 represents the hypothetical concentration profile of the impurities in the interstitial gas at the end of the pressurization step corresponding to FIG. 4, as a function of bed length. The feed rate for this "early" pressure history is initially so fast that the bed comes up to pressure before a lot of the feed gas is adsorbed. Consequently, the resultant final concentration history is flat, and, although said in US-A-4 256 469 to be better than other processes, the initial product purity remains somewhat lower than might be desired. Therefore, a PSA plant operating with an early feed history on a given cycle and at specified feed and product rates would produce the lowest purity. Ward, U.S.-A-4,572,723, discloses a process for the production of low oxygen content nitrogen product gas wherein the cycle time is preferably 500 seconds and the pressure is allowed to rise at a constant slow rate. U.K.-A-2,195,097 describes a PSA gas separation wherein the product gas returns whenever the pressure in a product gas reservoir exceeds that of the treatment zone. Japanese-A-63-79714 discloses a three treatment zone merry-go-round PSA system wherein at any given time, two serially connected treatment zones are used for adsorption while a third treatment zone is regenerated. The PSA processes of the present invention alleviate the problems of the prior art by the cost-effective production of substantially pure product gas by a two step slow/fast pressurization with feed gas or by product gas pressurization in combination with two step slow/fast feed gas pressurization. The process of the present invention, in its broadest aspects is directed to the production of a highly purified gas through the use of a pressure swing adsorption system in which the treatment zone is repressurized to a production pressure by either a method employing a feed gas alone or in combination with the product gas. In particular, the present invention relates to a process for the production of highly purified product gas from a feed gas containing the product gas and impurities comprising (a) pressurizing to a production pressure a treatment zone comprising an inlet region, an outlet region, and at least one impurity adsorbing material by: (i) firstly introducing the feed gas for an initial period into the inlet region to raise the pressure of the treatment zone to an intermediate pressure less than the production pressure; subsequently (ii) secondly introducing the same or a different feed gas for a second period into the inlet region to further raise the pressure of the treatment zone to the production pressure; (b) introducing the same or different feed gas to the production pressurized treatment zone through the inlet region to remove the impurities from the feed gas and to produce the highly purified product gas; (c) removing the product gas from said outlet region, characterised in that the first introduction of gas in step (a)(i) is conducted at a slow rate and the introduction of gas in step (a)(ii) is conducted at a rapid rate for a period less than the initial period. The process of the present invention preferably includes the regeneration of the treatment zone to remove impurities therefrom which is particularly desired for the continuous production of product gas. Furthermore, any of these processes may be performed in a single treatment zone or in a plurality of treatment zones. When conducted in a plurality of treatment zones, at least one of the treatment zones is used to remove the impurities from the feed gas and at least one of the treatment zones is simultaneously regenerated to remove impurities therein to thereby allow the process to run continuously. Embodiments of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: FIG. 1 is a graphic illustration of the pressure history of a PSA system pressurized with a constant pressure pressurization step; FIG. 2 is a graphic illustration of the feed flow history of a PSA system pressurized with a constant pressure pressurization step as in FIG. 1; FIG. 3 is a graphic illustration of the concentration profile of impurities in interstitial gas at the end of the pressurization step of FIG. 1; FIG. 4 is a graphic illustration of the pressure history of a PSA system pressurized with feed gas with a rapid initial stage and a slow final stage; FIG. 5 is a graphic illustration of the feed flow history of a PSA system pressurized with a rapid initial stage and a slow final stage as in FIG. 4; FIG. 6 is a graphic illustration of the concentration profile of impurities in interstitial gas at the end of the pressurization step of FIG. 4; FIG. 7 is a graphic illustration of the pressure history of a PSA system pressurized with feed gas with a slow initial stage and a rapid final stage; FIG. 8 is a graphic illustration of the feed flow history of a PSA system pressurized with a slow initial stage and a rapid final stage; FIG. 9 is a graphic illustration of the concentration profile of impurities in interstitial gas at the end of the pressurization step of FIG. 7; FIG. 10 is a schematic diagram of a conventional two zone PSA process for the production of rich product gas; FIG. 11 is a schematic diagram of a PSA system comprising two treatment zones for the production of inert-gas rich product gas in accordance with the present invention; and The present invention is directed to a PSA system wherein a high purity gas is produced by passing a feed gas through a treatment zone, adsorbing a majority of the impurities from the gas, and withdrawing a purified product gas from the treatment zone. It has been found with the pressurization techniques of the present invention, that the efficiency of the PSA system in producing the high-purity product gas is substantially increased. The PSA processes of the present invention, in general, can be carried out using any adsorbent material having selectivity for one or more of the impurities of the gaseous feed mixture. Suitable adsorbents include zeolite molecular sieves, zeolites, activated carbon, carbon molecular sieves, silica compounds, aluminium compounds and the like. The difference between the amounts of the impurities adsorbed at the lowest and highest pressures during the PSA cycle determine the efficiency of the separation. In accordance with the invention, the treatment zone may contain one or more different adsorbents. The present processes may be conducted either batchwise or continuously. In either case, the treatment zone must be periodically regenerated by purging the accumulated adsorbed impurities if the treatment zone is to be reused. In a batchwise system, purification of the feed gas must be stopped during regeneration of the treatment zone. In a continuous system, a plurality of treatment zones are used with at least one treatment zone producing purified gas while at least one other treatment zone is undergoing regeneration. The feed gas pressurization is carried out in two stages, the first stage comprising a slow introduction of feed gas for an initial period into the inlet region of the treatment zone in order to raise the pressure of the treatment zone to an intermediate pressure. The intermediate pressure is less than the production pressure of the system yet greater than the pressure of the treatment zone after regeneration. In a subsequent stage, the same or different feed gas is rapidly introduced for a second period, which is less than the initial period, into the inlet region of the treatment zone in order to raise the pressure of the treatment zone to the production pressure. This type of "late" pressurization is illustrated by FIG. 7. FIG. 8 corresponds to the feed flow rate corresponding to the pressurization history in FIG. 7. FIG. 9 represents the hypothetical concentration profile of the impurities in the interstitial gas at the end of the pressurization step corresponding to FIG. 7, as a function of bed length. Therefore, when compared with the pressurization profiles, feed histories, and impurity concentrations of prior art FIGS. 1-3 and FIGS. 4-6, the "late" pressurization and feed history of the process of the present invention yields the highest purity of product gas. A typical full cycle of the PSA unit in the present process with two step late profile feed gas pressurization is as follows: FLOWCHART I Step 1 Slow Pressurization by Feed Gas to Inter-mediate Pressure Step 2 Rapid Pressurization by Feed Gas to Production Pressure Step 3 Continued Feed and Product Release Step 4 Regeneration A conventional two zone process with constant pressure feed pressurization is shown schematically in FIG. 10. Here, the valves controlling flow of the feed gas into the system, high-pressure product withdrawal and waste gas venting from treatment zones A and B are numbered 1 through 10. A full cycle on conventional PSA units as shown in FIG. 10 is as follows: FLOW CHART II STEP TREATMENT ZONE A TREATMENT ZONE B 1 Pressurization Regeneration 2 Continued Feed and Product Release Regeneration 3 Regeneration Pressurization 4 Regeneration Continued Feed and Product Release An embodiment of the present invention as a two zone PSA process with the equipment necessary for the pressurization techniques of the present invention is shown schematically in FIG. 11. In FIG. 11, the valves controlling flow of feed into the PSA system, product withdrawal, and waste gas venting from treatment zones A and B are numbered 1 through 12. A typical full cycle on the PSA unit incorporating two step "late" profile pressurization with feed gas in accordance with the present invention is as follows: FLOW CHART III STEP TREATMENT ZONE A TREATMENT ZONE B 1 Slow Pressurization by Feed Gas to Intermediate Pressure Regeneration 2 Rapid Pressurization by Feed Gas to Production Pressure Regeneration 3 Continued Feed and Product Release Regeneration 4 Regeneration Slow Pressurization by Feed Gas to Intermediate Pressure 5 Regeneration Rapid Pressurization by Feed Gas to Production Pressure 6 Regeneration Continued Feed and Product Release Typical full cycles of two treatment zone processes having feed and product pressurization as embodied in the present invention are as follows: FLOW CHART V STEP TREATMENT ZONE A TREATMENT ZONE B 1 Pressurization by Product Backfill to First Intermediate Pressure Regeneration 2 Slow Pressurization by Feed Gas to Second Intermediate Pressure Regeneration 3 Rapid Pressurization by Feed Gas to Production Pressure Regeneration 4 Continued Feed and Product Release Regeneration 5 Regeneration Pressurization by Product Backfill 6 Regeneration Slow Pressurization by Feed Gas to Intermediate Pressure 7 Regeneration Rapid Pressurization by Feed Gas to Production Pressure 8 Regeneration Continued Feed and Product Release Pressurization in multiple treatment zone systems may also optionally include a zone pressure equalization step before the first stage or step of two stage slow/fast or two step backfill/slow/fast feed pressurization processes. Equalization pressure is generally higher than regeneration pressure but less than production pressure. Regeneration is accomplished by subjecting nearly saturated or saturated treatment zones to a reduced pressure (atmosphere and/or sub-atmospheric pressure) provided by a mechanical vacuum pump, by venting to the atmosphere, by flow of a purge gas containing little or none of the more-readily adsorbed components or by a combination of any of the above. Referring to FIG. 11 in a two treatment zone system operated in accordance with Flow Chart V with optional pressure equalization steps, zones A and B contain an adsorbent or combination of adsorbents that selectively remove one or more components from a feed gas stream. The valves 1 and 2 control the flow of feed gas to zones A and B, respectively, while the valves 9 and 10 control the flow of product from the two zones. Flow of the desorbed gases during regeneration is allowed by the operation of the valves 5 and 6. The valves 3, 4, 7, and 8, which are closed during all adsorption and desorption steps, open during the equalization step to allow the flow of gas from the inlet region and/or the outlet region of the zone at high pressure to the inlet region and/or outlet region of the zone at low pressure. Following the equalization step, the valves 3, 4, 7, and 8 close, and repressurization is continued. If, for example zone A is to be repressurized while zone B is to be regenerated, valve 6 is opened for venting zone B while product gas is introduced to Bed A through the opening of valves 9, 11, and 12. The valve 12 in this schematic is not an actuatable valve like valves 1 through 11, but merely serves to throttle the backflow of product gas so as to control pressurization. In a like manner, the flow of feed gas to the conduit connecting the valves 1 and 2 is controlled by a similar throttling valve (not shown in this schematic) to allow for two step slow initial pressurization followed by rapid feed pressurization. When the valves 9, 11, and 12 are open, a steady flow of product gas enters the zone for a period of time of between 5 and 10 seconds. The zone, which at the end of the equalization step was at a pressure between 40 and 50 percent of the production pressure, should finish backfilling at a pressure between 60 and 70 percent of the production pressure. At the end of the backfill step, valve 11 closes and valve 1 opens to allow the flow of the feed gas to proceed slowly into zone A. The flowrate, controlled by a throttling valve upstream of the valves 1 and 2, is set to bring the pressure from 40 to 50 percent of the production pressure up to 90 to 100 percent of the production pressure within 70 percent of the remaining production time, typically 60 to 120 seconds for a 240 to 360 second full cycle time. The combined feed and product pressurization schemes establish multiple regions of varying product gas purity in the regenerated treatment zones upon repressurization. The region nearest the inlet of the treatment zone (the "first region") has a composition like that of the feed gas. The region just above the "first region" (the "second region") has a purity intermediate that of the feed gas and the product gas. The purity of the region above the "second region" (the "third region") is intermediate that of the "second region" and the product gas, and the region nearest the outlet (the "fourth region") has a purity like that of the product gas. Any additional regions of gas purity are introduced into the repressurizing PSA vessel in such a way that the purity of the additional region lies between the purities of its neighbours. In combined product/feed gas pressurizations, product pressurization brings a treatment zone to from about 60 to about 70 percent of the production pressure. All processes of the present invention typically are completed in from about 240 to about 480 seconds. The initial periods of late profile feed gas pressurization typically is from about 75 to about 160 seconds, and feed gas flow rate during the initial period ranges from about 50 to about 120 m³ per hour at standard atmospheric pressure and temperature/m³ adsorbent (about 50 to about 120 scfh/cf adsorbent). The second period typically is from about 10 to about 70 seconds, and the feed gas flow rate during the secondary period ranges from about 100 to about 350 m³h/m³ adsorbent (about 100 to about 350 scfh/cf adsorbent). Product pressurization typically lasts from about 5 to about 10 seconds, and flow rate is from about 57x10⁻³ to about 141x10⁻³ cm per hour at standard atmospheric pressure and temperature (about 2.0 to about 5.0 scfh). Production pressure ranges from 518x10³Pa to about 1035x10³Pa (about 75 to about 150 psig) and preferably about 590x10³Pa (about 85 psig). Regeneration pressure is typically ambient pressure (0 psig), initializing pressure typically ranges from about 450x10³Pa to about 932x10³Pa (65 to about 135 psig), secondary period pressure typically ranges from about 518x10³Pa to about 1035x10³Pa (75 to about 150 psig), and backfill pressure typically ranges from about 311x10³Pa to about 725x10³Pa (45 to about 105 psig). The following examples illustrate the invention without limitation. All parts are given by weight unless otherwise indicated. EXAMPLES 16-18 A series of experimental runs was conducted utilizing a PSA system with a two step feed pressurization having a profile as illustrated in FIG. 8, a cycle time of 240 seconds, a production pressure of 590x10³Pa (85 psig), a regeneration pressure of ambient pressure (0 psig) and a specific range of 0.75. The adsorbent was carbon molecular sieve. Results are summarized in Table 5. COMPARATIVE EXAMPLES 19*-25* The procedure of Examples 16-18 was followed substituting a feed pressurization profile as illustrated in FIG. 1. The results are summarized in Table 5. COMPARATIVE EXAMPLES 26*-36* The procedure of Examples 16-18 was followed substituting a feed pressurization profile as illustrated in FIG. 4. The results are summarized in Table 5. Examples 16-18 when compared with Comparative Examples 19*-36* demonstrate the improved productivity and/or yield that two step late feed pressurization in accordance with the present invention imparts over constant or two step early feed pressurization. Many variations of the present invention will suggest themselves to those skilled in the art in light of the above, detailed description. All such modifications and variations are within the full intended scope of the appended claims.

Claims: A process for the production of a highly purified product gas from a feed gas containing the product gas and impurities including the steps of (a) pressurizing to a production pressure a treatment zone comprising an inlet region, an outlet region, and at least one impurity adsorbing material by: (i) firstly introducing the feed gas for an initial period into the inlet region to raise the pressure of the treatment zone to an intermediate pressure less than the production pressure; subsequently (ii) secondly introducing the same or a different feed gas for a second period less than the initial period into the inlet region to further raise the pressure of the treatment zone to the production pressure; (b) introducing the same or different feed gas to the production pressurized treatment zone through the inlet region to remove the impurities from the feed gas and to produce the highly purified product gas; (c) removing the product gas from said outlet region characterised in that the first introduction of gas in step (i) is conducted at a slow rate and the introduction of gas in step (ii) is conducted at a rapid rate for a period less than the initial period. A process according to Claim 1 characterised by the step of introducing said product gas into said outlet region to raise the pressure of said treatment zone to a first intermediate pressure less than said product pressure prior to step (a)(i). A process according to Claim 1 or Claim 2, characterised in that the feed gas comprises air and the product gas comprises nitrogen. A process according to Claim 3, characterised in that the product gas contains no more than 10,000 ppm of oxygen. A process according to any one of Claims 1 to 4, characterised by the further step (d) of regenerating said treatment zone to thereby remove said impurities from said impurity adsorbing material. A process according to any preceding claim characterised in that the intermediate pressure obtained in step (a)(i) is in the range of from about 450 x 10³Pa to about 932x10³Pa (65 psig to about 135 psig). A process according to any preceding claim characterised in that said production pressure is in the range of from about 518x10³Pa to about 1035x10³Pa (75 to about 150 psig). A process according to any preceding claim characterised in that said initial period in step (a)(i) is in the range of from about 75 to about 160 seconds. A process according to Claim 8, characterised in that the flow rate of said feed gas during said initial period in step (a)(i) is in the range of from about 50 to about 120 standard m³ per hour/m³ adsorbent (50 to about 120 scfh/cf adsorbent). A process according to any preceding claim characterised in that said second period in step (a)(ii) is in the range of from about 10 to about 70 seconds. A process according to Claim 10, characterised in that the flow rate of said feed gas during said second period in step (a)(ii) is in the range of from about 100 to about 350 standard m³ per hour/m³ adsorbent (100 to about 350 scfh/cf adsorbent). A process according to Claim 5, characterised in that steps (a) - (d) are conducted in a plurality of treatment zones, at least one of said treatment zones being used to remove said impurities from said feed gas and at least one of said treatment zones being simultaneously regenerated to remove impurities contained therein.

Inventors: LACAVA ALBERTO I, SHIRLEY ARTHUR I, LACAVA, ALBERTO I., SHIRLEY, ARTHUR I.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, C01B21