Adsorption process with high and low pressure feed streams

Abstract: An adsorption process in which a low pressure feed gas stream and then a high pressure feed gas stream is introduced into an adsorbent bed to produce product streams by adsorbing one or more preferentially adsorbed components within the beds. Thereafter the beds are regenerated typically through depressurisation and purge stages. This cycle is conducted out of phase with corresponding cycles on at least one other bed so that high pressure and low pressure gas streams are both produced essentially continuously. The gas streams may be of air, and may be separated by rectification at cryogenic temperatures downstream of their purification.

Description: The present invention relates to an adsorption process in which a feed stream is introduced into a plurality of adsorbent beds, operated in accordance with an out of phase cycle, to adsorb one or more preferentially adsorbed components contained within a feed stream and thereby produce a product stream. More particularly, the present invention relates to such an adsorption process in which high and low pressure feed streams are introduced into each adsorbent bed to produce high and low pressure product streams. There are many industrial processes that require feed streams to be pre-treated by adsorption and thereafter to be delivered to the process at two or more pressures. As an example, air separation by rectification at cryogenic temperatures requires treatment of incoming feed air stream by adsorption beds to remove carbon dioxide and moisture from such feed stream. The treated feed stream is frequently required at different pressures according to the operating pressures of distillation columns employed in distilling the treated air, and the inlet pressure of at least one turbine used to generate refrigeration for the air separation process and, in some examples, for the production of the products of the air separation in liquid state. An example of a cryogenic air separation process can be found in US 4,964,901, in which air is separated by low temperature rectification in higher and lower pressure distillation columns operatively associated with one another by a condenser-reboiler. In this process, air is compressed and purified to remove carbon dioxide and water vapour. Thereafter, the air is cooled to a temperature suitable for its rectification and is introduced in two streams into the higher and lower pressure columns. The higher pressure column serves to separate the air into a nitrogen rich fraction and a crude liquid oxygen fraction ("crude liquid oxygen column bottoms"). The crude liquid oxygen is further refined in the lower pressure column to produce oxygen and nitrogen (liquid oxygen "column bottoms" and a nitrogen rich "tower overhead"). Separate purification units are used to purify the feed streams. In other processes requiring feed air streams at high and low pressures, all of the air is compressed to the pressure of the higher pressure column, pre-purified and then divided. One subsidiary stream is utilised at the higher pressure and the other subsidiary stream is expanded through a valve to the lower pressure of the lower pressure column. As can be appreciated, US 4, 964,901 is more energy efficient than the aforementioned process because not all of the air is compressed to the higher pressure but two separate pre-purification units are required. As will be discussed, the present invention provides an adsorption process which is capable of producing product streams at high and low pressures in which only a single apparatus or unit is required for such purpose. The present invention provides an adsorption process as set out in claim 1. In accordance with this process, at least a high and low pressure feed stream are introduced into a plurality of adsorption beds containing at least one adsorbent to adsorb at least one more preferentially adsorbed component contained_within the high and low pressure feed streams. This adsorption produces high and low pressure product streams from adsorption by the at least one adsorbent. The plurality of adsorbent beds are subjected to operational cycles, each having at least two adsorption stages comprising high and low pressure feed stages having the high and low pressure feed streams introduced into the adsorbent beds. Additionally, at least two repressurisation stages are provided in each cycle. These at least two repressurisation stages are situated just prior to the high and low pressure feed stages. The at least two repressurisation stages comprise a high pressure repressurisation stage prior to the high pressure feed stage and a low pressure repressurisation stage before the low pressure feed stage. A regeneration stage is provided for desorbing the at least one more preferentially adsorbed component. The operational cycles are conducted in an out of phase relationship with respect to one another so that the high and low pressure product streams can be essentially continuously. The present invention encompasses both pressure-swing and temperature-swing adsorption processes. In either type of adsorption process in accordance with the present invention, each bed is first subjected to a low pressure feed to produce a low pressure product stream and then a high pressure feed to produce a high pressure product stream. Generally, after subjecting an adsorbent bed to adsorption at low pressure, there will still be added capacity in the adsorbent bed to adsorb at high pressure. Hence, the use of a single adsorbent bed to adsorb at both low and high pressures requires less adsorbent than if two separate beds were provided that were respectively dedicated to adsorption at low and high pressures. Since only a single PSA or TSA unit is required in accordance with the present invention and utilisation of the adsorbent therein can be optimised, the apparatus can be considerably simplified. Additionally, since only a portion of the feed need be compressed to high pressure (as compared with the prior art in which all of the feed is compressed to high pressure) power savings can also be realised in conducing a process in accordance with the present invention. The process according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a pre-purification unit integrated with an air separation plant in accordance with the present invention; Figure 2 is a schematic illustration of the pre-purification unit illustrated in Figure 1; Figure 3 is a cycle chart of an adsorption process in accordance with the present invention that involves temperature swing adsorption; and Figure 4 is a cycle chart of an adsorption process in accordance with the present invention that involves pressure swing adsorption. With reference to Figure 1, an integrated air separation plant 1 is disclosed in which filtered air is compressed in a main compressor 10. The air is then divided into a main air stream 12, and a subsidiary air stream 14 which is further compressed within a booster compressor 16. Streams 12 and 14 are thereafter introduced into a pre-purification unit 18 which is operated in accordance with the present invention. Pre-purification unit 18 purifies main and subsidiary air streams 12 and 14 and produces low and high pressure air streams 20 and 22 (as product) that have substantially the pressures that were imparted to them by main air compressor 10 and booster compressor 16, respectively. Low and high pressure air streams 20 and 22 are cooled in a main heat exchanger 24 to a temperature suitable for their rectification and are thereafter introduced into an air separation unit 26 which separates the air by cryogenic rectification to produce nitrogen, oxygen and optionally argon rich products. Air separation unit 26 can be a single column oxygen generator, a single column nitrogen generator (in which case no argon rich product is produced). Alternatively, air separation unit 26 can be a double column air separation plant having columns operating at higher and lower pressures, operatively associated with one another by a condenser-reboiler arrangement. Such a double column can have associated therewith an argon column to produce an argon rich product. In the example of the double column, the lower pressure air stream 20 might be fed directly into the higher pressure column while the higher pressure air stream 22 might be expanded, through engine expansion (e.g. an expansion turbine), to produce refrigeration and thereafter, be introduced into the lower pressure or higher pressure column. As is known by those skilled in the art, even in single column plants (oxygen and nitrogen generators) boosted pressure streams are frequently used. The present invention is not, though, limited to air separation plants and has application to any process requiring both high and low pressure streams either as reactants or as streams to be purified or otherwise treated. With reference to Figure 2, pre-purification unit 18 has first, second and third adsorbent beds 30, 32 and 34, respectively. First, second and third adsorbent beds 30-34 are for example formed of silica gel, activated carbon, activated alumina, or a zeolite, or combinations thereof, in a manner well known in the art, to adsorb moisture and carbon dioxide present within the air. Each bed is typically provided in a suitable vessel. Main and subsidiary air streams 12 and 14 are routed to first, second and third adsorbent beds 30-34 by on-off valves 36-70. During the operation of pre-purification unit 18, main and subsidiary air streams 12 and 14 are introduced by means of low and high pressure inlet manifolds 74 and 76 respectively, into first, second and third adsorbent beds 30-34. Low and high pressure purified air streams 20 and 22 are discharged through the use of low and high pressure discharge manifolds 78 and 80, respectively. Purge gas, which can be waste nitrogen, oxygen or oxygen-enriched fluid (depending upon the type of air separation unit 26) is introduced into first, second and third beds 30-34 through purge manifold 82. Waste is discharged from first, second and third beds 30-34 through waste manifold 84. In a TSA process, a relatively dry and CO 2 free purge gas at elevated temperature is employed to cause desorption of the adsorbed components. This purge gas can be raised in temperature in a heat exchanger or by electrical heating. When the bed is at or approaching a point at which all the impurities have been desorbed, the heating can be discontinued to form a relatively cold purge gas to bring the bed back to an adsorption temperature. The following chart is a valve sequence for an operation of valves 36-70 in either a TSA cycle or a PSA cycle "0" indicates the valves are open. Otherwise the valves are closed. VALVE SEQUENCE FOR TYPICAL TSA AND PSA PPUs, FIGURE 2 VALVE #'S TSA TIME (MIN) 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 PSA TIME (MIN) 0-15 0 0 0 0 0 0-3 15-25 0 0 0 0 0 3-4 25-125 0 0 0 0 0 0 4-7 125-225 0 0 0 0 0 0 * 225-240 0 0 0 0 0 7-10 240-255 0 0 0 0 0 10-13 255-265 0 0 0 0 0 13-14 265-365 0 0 0 0 0 0 14-17 365-465 0 0 0 0 0 0 * 465-480 0 0 0 0 0 17-20 480-495 0 0 0 0 0 20-23 495-505 0 0 0 0 0 23-24 505-605 0 0 0 0 0 0 24-27 605-705 0 0 0 0 0 0 * 705-720 0 0 0 0 0 27-30 0 = VALVE IS OPEN With additional reference to Figure 3, the temperature swing adsorption process that is performed when the valves are operated in accordance with Table 1 is illustrated. In this process, each of the first, second and third beds 30-34 is subjected in our-of-phase operating cycles to a low pressure feed, a (re)pressurisation up to the high feed pressure (RPH) followed by high pressure feed stage or step. In order to regenerate it each bed is subjected to a depressurisation stage or step to initiate desorption of the more preferentially adsorbable components that were adsorbed during the feed steps. The depressurisation stage is followed by a hot purge stage and then a cold purge stage. These latter two purge stages further desorb the more preferentially adsorbed components. Thereafter, each bed 30 subjected to a repressurisation (RPL) back to that the low pressure feed. As is evident, while one of the first, second and third beds 30-34 is being subjected to a low pressure feed stage and thereby produces the low pressure air stream 20, another bed is subjected to high pressure feed stage and is thereby producing high pressure air stream 22. Further, purified low pressure and high pressure air stream are both produced continuously. In the feed steps, water vapour and carbon dioxide impurities are preferentially adsorbed and an air stream essentially free of these impurities passes out of the opposite end of the bed to that at which the feed air is introduced. With specific reference to Table 1, from 0-15 minutes, valves 36, 44, 46, 62 and 66 are set in the open position. This allows first bed 30 to be subjected to the low pressure feed step (stage) by virtue of valve 36 being opened. The low pressure product stream is discharged from first bed 30 to low pressure product header 78 by virtue of valve 66 being open. Valves 44 and 62 are both set in the open position to allow second bed 32 to be subjected to the high pressure feed step and to discharge high pressure product stream 22 from high pressure product header 80. Third adsorbent bed 34 is being repressurised to high pressure and, as a result, valve 46 is open so that part of the high pressure air can be introduced into third adsorbent bed 34. During the next time interval of 15-25 minutes, second adsorbent bed 32 is subjected to the depressurisation stage while third adsorbent bed 34 is to be subjected to the high pressure feed stage. In order to accomplish this, valve 50 is opened to allow second adsorbent bed to countercurrently depressurise through waste header 84. Valve 64 opens to allow high pressure product to be introduced into high pressure product manifold 80. During the 25-125 minute time interval, second adsorbent bed is subjected to a hot purge stage by opening valves 50 and 56. During the cold purge stage (the 125-225 time interval), valves 50 and 56 remain open. The hot purge stage is brought about by either heating the purge stream or heating the relevant adsorbent bed itself. In the 225-240 minute time interval, in order to prepare second adsorbent bed 32 to be brought on as a producing bed acting in a low pressure feed stage, valve 38 is opened in order to pressurise second adsorbent bed 32 with main air stream 12. Valves 50 and 56 have to be re-set in the closed position during this time interval to end the purging of second adsorbent bed 32. During the next 15 minutes, (the 240-255 minute time interval) first adsorbent bed 30 is subjected to high pressure repressurisation stage by opening valve 42. Since first adsorbent bed is not producing at this point, valves 36 and 66 are closed. Second adsorbent bed 32 is now acting in place of first adsorbent bed 30 as a producing bed. As such, valve 68 is opened to produce low pressure air stream 20 from second adsorbent bed 32. The cycle continues at 255 minutes with first adsorbent bed 30 being subjected to the high pressure feed stage by opening valve 60. At this point, third adsorbent bed 34 is subjected to a depressurisation stage by opening valve 52. Since third adsorbent bed is therefore no longer serving as a producing bed, valves 46 and 64 are closed. At 265 minutes and then at 365 minutes, third adsorbent bed is subject to hot and cold purge stages by opening valves 52 and 58. First and second adsorbent beds are still being subjected to high and low pressure feed stages. During the succeeding 465 to 480 minute time interval, third adsorbent bed 34 is subject to a low pressure repressurisation stage by opening valve 40 and closing valves 52 and 58. At 480 minutes into the cycle, third adsorbent bed 34 is subject to the low pressure feed stage by opening valve 70. Valve 44 is opened and valves 38 and 68 are closed to permit second adsorbent bed 32 to be subject to a high pressure repressurisation stage. During the 495 to 505 minute time interval, first adsorbent bed 30 is subject to a depressurisation stage by opening 48 and closing valves 36 and 60. Thereafter, first adsorbent bed 30 is subject to high and cold purge stages at 505 and 605 minutes by opening valve 54. During the last time interval of the cycle, from 705 to 720 minutes, first adsorbent bed 30 is subject to a low pressure repressurisation stage by opening valve 36 and closing valves 48 and 54. With additional reference to Figure 4, pre-purification unit 18 can be operated in accordance with a pressure swing adsorption cycle. In such cycle, each bed is subjected to a low pressure feed stage, a high pressure repressurisation stage (RPH), a high pressure feed stage, followed by depressurisation, purge and low pressure repressurisation stages. As is evident, one bed is undergoing a low pressure feed stage and is therefore producing a low pressure air stream 20 while another bed undergoes a high pressure feed stage and thus produces high pressure air stream 22. For instance, when first adsorbent bed 30 is undergoing a low pressure feed stage, second adsorption bed 32 is undergoing a high pressure feed stage and third adsorbent bed is undergoing a high pressure (re)pressurisation stage. Thereafter, second adsorbent bed 32 is depressurised while third adsorbent bed 34 undergoes the high pressure feed stage. Following a depressurisation stage to initiate desorption and a purge stage with a purge stream to complete desorption, second adsorbent bed 32 is subjected to a low pressure repressurisation stage so that it can next serve as a producing bed, producing low pressure product stream. With reference to the valve sequence chart, the valve sequences are the same as for the temperature swing adsorption process. For the sake of brevity, the above discussion relating to each valve opening and closing will not be repeated. However, the pressure swing adsorption process has a total cycle time that is far less than the temperature swing adsorption process. Moreover, there are no separate hot and cold purge stages. As such, an asterisk is placed after 4-7 minutes, 14-17 minutes, and 24-27 minutes to indicate that such rows are not part of the PSA cycle. It should be mentioned that the present invention is not limited to the use of high and low pressure feed streams such as main and subsidiary air streams 12 and 14. An intermediate pressure stream could for example be additionally purified to produce an intermediate pressure product stream. The modification required for a three pressure system, to the process outlined above, involves adding an adsorbent bed so that the cycle remains essentially continuous (product streams are continuously being produced) and adding intermediate pressure repressurisation and feed stages between the low pressure feed stage and the high pressure repressurisation stage.

Claims: An adsorption process including subjecting to repeating operational cycles a plurality of adsorption beds each containing at least one adsorbent to adsorb at least one more preferentially adsorbed component from a feed gas, each operational cycle comprising, in sequence; a) a first adsorption step in which a low pressure stream of the feed gas is introduced into a chosen one of the adsorbent beds and said more preferentially adsorbed component is adsorbed therefrom; b) a first pressurisation step to raise the pressure of the chosen bed to a chosen high pressure; c) a second adsorption step in which a high pressure stream of the feed gas is introduced into the pressurised adsorbent bed and said more preferentially adsorbed component is adsorbed therefrom; d) a regeneration step for desorbing said more preferentially adsorbed component from the chosen Adsorbent bed; and e) a second pressurisation step to raise the pressure of the regenerated bed to said low pressure; wherein each adsorbent bed performs the repeating operational cycles out of phase with each other adsorbed bed so as to produce essentially continuously both high and low pressure streams of the feed gas depleted of the more preferentially adsorbed component. An adsorption process according to claim 1, in which said regeneration step comprises depressurising said Adsorbent bed and purging the depressurised bed with a purge gas. An adsorption process according to claim 1 or claim 2, in which the first and second pressurisation steps are performed with said high pressure feed gas and said low pressure feed gas respectively. An adsorption process according to any one of the preceding claims, wherein said high and low pressure feed streams are made-up of air and said at least one more preferentially adsorbed component includes carbondioxide and water vapour. An adsorption process according to any one of the preceding claims, in which there are just two feed streams and three adsorbent beds. A cryogenic air separation method comprising forming a high pressure and a low pressure feed air stream, purifying the high and low pressure feed air streams by removing water vapour and carbon dioxide impurities therefrom by a process as claimed in any one of the preceding claims, and separating the purified air streams by rectification at cryogenic temperatures.

Inventors: KUMAR RAVI, KUMAR, RAVI
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1996
Main classification: F25J3
IPC codes: B01D53