SEPARATION OF GAS MIXTURES

Abstract: A compressed gas mixture, particularly air, is separated in adsorbent beds 6 and 8 in turn. The adsorbent adsorbs nitrogen in preference to oxygen and argon. Accordingly, product gas rich in oxygen is delivered to pipeline 44. While one bed adsorbs nitrogen from compressed air, the other is regenerated by being placed in communication with the atmosphere through vent pipe 30. At the end of each adsorption step the respective bed 6 or 8 is placed in communication with a bed 12 of adsorbent that adsorbs oxygen more rapidly than nitrogen and argon, and the pressure between the two beds is equalised. The pressure between the beds 6 and 8 is then equalised. Unadsorbed gas comprising argon and nitrogen is vented from bed 12 to the atmosphere and then the bed 12 is placed in communication with either bed 6 or bed 8 to cause oxygen to be desorbed from bed 12 and to enter the bed 6 or 8. The venting of argon to the atmosphere reduces the argon content of the product.

Description: This invention relates to the separation of gas mixtures and particularly the separation of air by pressure swing adsorption. One known PSA process for separation of oxygen from air employs a zeolite molecular sieve adsorbent which has the ability to adsorb nitrogen preferentially to oxygen and argon. In operation, a bed of this adsorbent is put through a cycle which includes an adsorption step during which time air is passed through the bed, most of the nitrogen and proportions of the argon and oxygen together with substantially all the carbon dioxide and water vapour in the incoming air are adsorbed, and an oxygen-rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and is then evacuated through its inlet, so that the adsorbed gas are substantially removed from the bed thereby preparing it for the next adsorption steps. In practice two adsorbent beds are employed and operated on similar cycles but sequenced to be outer step with one another by 180°, so that when one bed is on its adsorption step the other bed is on its desorption step, and vice versa. One disadvantage of the known process is that there is a waste of the energy employed in compressing the incoming air by virtue of venting the bed to atmospheric pressure during its regeneration. Another feature of the known process, which may be disadvantageous when the oxygen-rich gas is administered to a patient with an anaesthetic, is that the oxygen-rich product gas is also enriched in argon. It is also known to produce nitrogen-rich product by employing in the pressure swing adsorption process a carbon molecular sieve that adsorbs oxygen more rapidly than either nitrogen or argon. Accordingly, the product withdrawn from the outlet of the bed is enriched in nitrogen. An example of a process, used commercially, which separates nitrogen by pressure swing adsorption is described in GB-B- 2 042 365. In this process, at the end of each adsorption and each regeneration step one bed is placed in communication with the other bed so as to equalise the pressure therebetween. It is thus possible to conserve a proportion of the energy that is used to compress the incoming air. The present invention relates to a method and apparatus for separating a gas mixture comprising at least three components which employs a plurality of discrete bed pressure reduction steps prior to placing the bed in communication with the atmosphere so as to regenerate the bed while also helping to reduce the impurity level of single component product gas. According to the present invention there is provided a method of separating a gas mixture comprising at least three components by pressure swing adsorption employing first and second beds of adsorbent that adsorbs a first component preferentially to or more rapidly than second and third components of the gas mixture and a third bed that adsorbs the second component more rapidly than or preferentially to the first and third components, the method comprising repeatedly performing a cycle of steps in which: a) the gas mixture is admitted under pressure to the first bed, and unadsorbed gas comprising the second component is withdrawn from the first bed as product; b) admission of the gas mixture to the first bed is ceased and the pressure in the first bed is reduced by placing it in communication first with the third bed and then the second bed so as to allow gas to flow out of the first bed; c) the first bed is regenerated by placing it in communication with the atmosphere so as to desorb previously adsorbed gas and venting desorbed gas to the atmosphere; d) unadsorbed gas in the third bed is vented to the atmosphere which venting is stopped whilst the pressure in the third bed is higher than that exiting in the first and second beds, and then the third bed is placed in communication with one or both of the first and second beds so as to cause gas to be desorbed from the third bed and to flow to one or both of the first and second beds; e) said steps a) to d) are performed again but with the second bed taking the place of the first bed, and vice versa and wherein the step (e) of the cycle is phased with the steps a) to d) such that the first and second beds neither receive the gas mixture simultaneously nor simultaneously vent gas to the atmosphere. The invention also provides apparatus for performing the above method. The method according to the invention may typically be used to separate oxygen from air. In this example, the first component is nitrogen, the second component is oxygen and the third component is argon. In this example, the third bed keeps down the argon-enrichment of the product gas. Preferably, the step of placing the first bed at the end of its adsorption step in communication with the third bed has a sufficient duration to equalise the pressures therebetween. Similarly, it is preferred that each step of placing of the first and second beds in communication with one another is of sufficient duration to equalise the pressures therebetween. By equalising the pressure of the first bed at the end of its adsorption step with the third bed and then the second bed it becomes possible to keep down the effective loss of work of compression of the incoming air. Each venting of the third bed to atmosphere preferably has a duration of less than a second. This time is sufficent for a significant amount of unsorbed first and third components to be vented from the third bed. Accordingly, when gas is returned from the third bed to the first or second bed (or both) it is relatively rich in the second component (the product) in comparison to the incoming gas mixture but is typically not significantly enriched in the third component. Accordingly, the gas returned to the first and second beds from the third bed helps to reduce the amount of impurity in the product. Steps a), b) and c) are performed in sequence. Typically, the venting part of step d) is performed in a period intermediate ending communication between the first and third beds in step b) and starting step c). While step c) is performed, typically first the gas mixture is admitted to the second bed (step(e)) and then the pressure in the second bed is reduced by placing it in communication with the third bed (as part of step e)). In the separation of air, the adsorbent in the first and second beds is preferably a zeolite molecular sieve that adsorbs nitrogen in preference to argon and oxygen and the adsorbent in the third bed is preferably a carbon molecular sieve that adsorbs oxygen more rapidly than either nitrogen or argon. Preferably in step d), the third bed communicates with the second bed immediately prior to admission of the gas mixture to the second bed, and is subsequently placed in communication with the first bed undergoing regeneration. Both these measures enhance the purity of the product gas. The third bed typically has a smaller volume than either of the first and second beds which are typically of equal volume. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawing, which is a schematic flow diagram of one apparatus for performing the method according to the invention. Referring to the drawing, the apparatus includes first and second cylindrical vessels 2 and 4 that are substantially identical to one another and which contain respectively first and second beds 6 and 8 of zeolite molecular sieve adsorbent. There is a third cylindrical vessel 10, smaller than the vessels 2 and 4 which contains a bed of carbon molecular sieve 12. The vessel 2 has a gas inlet 14 at its bottom and a gas pipe 18 extending out of its top. Similarly, the bed 4 has a gas inlet 16 at its bottom and a gas pipe 20 extending out of its top. The inlet 14 terminates in the union of an air inlet line 22 (having an automatically operable on-off (ie stop) valve 24 disposed therein) with a gas vent conduit 26 (having an automatically operable on-off valve 28 disposed therein) which terminates in an outlet 30 communicating with the atmosphere. The inlet 16 to the vessel 4 terminates in the union of an air inlet pipe 32 with a vent pipe 34. The air inlet pipe 32 has an automatically operable on-off valve 36 disposed therein and the vent conduit 34 has an automatically operable on-off valve 38 disposed therein. The conduit 34 terminates in the vent pipe 30. Although not shown, both the air inlets 22 and 32 communicate at their upstream ends with a source of compressed air. In a hospital, therefore, the pipes 22 and 32 may be placed in communication with the normal compressed, dried, air supply. In such situations where there is a supply of compressed air on tap, the apparatus shown in the drawing need not be provided with its own dedicated compressor, and since regeneration of the beds may be effected merely placing the beds in communication with the ambient atmosphere, no moving parts are required other than the on-off valves. A pressure equalisation conduit 40 having an automatically operable valve 42 disposed therein extends from the gas pipe 18 to the gas pipe 20 and thus when open permits gas to flow from the bed 6 to the bed 8 or vice versa according to which of the bed 6 and 8 is for the time being at the higher pressure. The gas pipes 18 and 20 lead to a product outlet line 44 and have automatically operable on-off valves 46 and 48 respectively disposed therein. The vessel 10 has a first conduit 50 communicating with a region of the pipe 18 intermediate the valve 46 and the union of the pipe 18 and the conduit 40. The vessel 10 has a second conduit 52 near its bottom communicating with the pipe 20 at a region intermediate the valve 48 and the Union of the pipe 20 with the conduit 40. The conduits 50 and 52 have automatically operable on-off valves 54 and 56 respectively disposed therein. The vessel 10 has at its top a vent pipe 58 communicating with the atmosphere and having an automatic operable on-off 60 valve disposed therein. In operation the apparatus shown in the drawing is operated to perform the cycle set out in Table 1. During the twelve steps of the cycle, the on-off valves of the apparatus shown in the drawing are in the positions indicated in Table 2. In the compressed air feed steps of the cycle, dried air is fed to the apparatus shown in the drawing at a pressure in the range of 2 to 10.1 bar (2 to 10 atmospheres) (typically in the order of 6.1 bar (6 atmospheres)). The zeolite molecular sieve is effective to adsorb carbon dioxide and nitrogen preferentially to argon and oxygen. The carbon dioxide is more readily adsorbed and therefore tends to be adsorbed predominantly in the bottom half of the bed. During each compressed air feed step (Bed 6 - 1 and 12; bed 8 - 6 and 7) unadsorbed gas comprising oxygen-enriched air passes out of the zeolite bed into the product pipeline 44. At steps 2 and 8 respectively the gas from the zeolite bed 6 or 8, as the case may be, flows into the carbon molecular sieve bed 12. This step is intended to have a very short duration (typically less than one second) so that the gas flow into the bed of carbon molecular sieve is predominantly of unadsorbed gas in the interspaces of the zeolite bed at the start of the pressure equalisation step. The gas entering the bed 12 thus comprises a mixture of oxygen and argon with some nitrogen but substantially no carbon dioxide, the carbon dioxide being retained on the zeolite adsorbent during the pressure equalisation step. The oxygen is largely adsorbed in the bed 12 leaving nitrogen and argon as the void or unadsorbed gas. Subsequent venting of the unadsorbed gas to the atmosphere, a step which takes less than one second (steps 4 and 10) leaves the contents of the bed 12 rich in oxygen but depleted in argon and nitrogen. After flow of gas from the bed 6 or 8, as the case may be, to the bed 12, (steps 2 and 8) the pressure between the bed 6 and 8 is equalised by allowing gas to flow from a higher pressure bed to the lower pressure bed (steps 3 and 9). Steps 3 and 9 may start at the same time as steps 4 and 10 respectively or may be completed before steps 4 and 10 respectively are performed. Each step of equalising the pressure of the beds 6 and 8 with one another (steps 3 and 9) creates in those beds a pressure lower than that which obtains in the bed 12 after void or unadsorbed gas in bed 12 has been vented to the atmosphere (steps 4 and 10). Accordingly, there is then performed (steps 5 and 11) a third pressure equalisation step in which gas, comprising relatively pure oxygen (having a reduced argon content), flows from the bed 12 into the one of the beds 6 and 8 whose pressure was increased in the preceding equalisation step between the beds 6 and 8. Such third pressure equalisation provides this bed at its upper end with a charge of oxygen-enriched gas that is depleted in argon and thus enhances the purity of the oxygen product. The bed that receives this charge of oxygen from the bed 12 then receives the compressed air feed while the other bed is regenerated by being vented to atmosphere. Since in steps 5 and 11 the flow of gas from the bed 12 into the beds 6 and 8 does not reduce the pressure of bed 12 to atmospheric pressure the bed 12 is used in steps 1 and 7 to provide an oxygen purge which again helps to reduce the level of impurities left in this bed at the end of venting it to the atmosphere. If desired, the product pipeline 44 be provided with a reservoir to enable product oxygen to be supplied to a user continuously. In addition, the product pipeline 44 may include a pressure regulator so as to prevent gas being withdrawn from the bed 6 and 8 at too low a pressure during the compressed air feed steps (steps 1, 6, 7 and 12).

Claims: 1. A method of separating a gas mixture comprising at least three components by pressure swing adsorption employing first and second beds of adsorbent that adsorbs a first component preferentially to or more rapidly than second and third components of the gas mixture and a third bed that adsorbs the second component more rapidly than or preferentially to the first and third components, the method comprising repeatedly performing a cycle of steps in which: a) the gas mixture is admitted under pressure to the first bed, and unadsorbed gas comprising the second component is withdrawn from the first bed as product; b) admission of the gas mixture to the first bed is ceased and the pressure in the first bed is reduced by placing it in communication first with the third bed and then the second bed so as to allow gas to flow out of the first bed; c) the first bed is regenerated by placing it in communication with the atmosphere so as to desorb previously adsorbed gas and venting desorbed gas to the atmosphere; d) unadsorbed gas in the third bed is vented to the atmosphere which venting is stopped whilst the pressure in the third bed is higher than that existing in the first and second beds, and then the third bed is placed in communication with one or both of the first and second beds so as to cause gas to be desorbed from the third bed and to flow to one or both of the first and second beds; e) said steps a) to d) are performed again but with the second bed taking the place of the first bed, and vice versa and wherein the step (e) of the cycle is phased with the steps a) to d) such that the first and second beds neither receive the gas mixture simultaneously nor simultaneously vent gas to the atmosphere. 2. A method as claimed in claim 1, in which that part of step (d) in which unadsorbed gas in the third bed is vented to the atmosphere is performed in a period intermediate the ending of communication between the first and third beds in step (b) and starting step (c). 3. A method as claimed in claim 1 or claim 2, in which while step (c) is performed, the gas mixture is admitted to the second bed (step (e)) and the pressure in the second bed is reduced by placing it in communication with the third bed (as part of step (e)). 4. A method as claimed in claim 3, in which in step (d) the third bed communicates with the second bed immediately prior to the admssion of the gas mixture to the second bed, and is subsequently placed in communication with the first bed undergoing regeneration. 5. A method as claimed in any one of the preceding claims in which the third bed has a smaller volume than either of the first and second beds. 6. A method as claimed in any one of the preceding claims in which, the gas mixture is air, the first component is nitrogen, the second component is oxygen and the third component is argon. 7. A method as claimed in claim 6, in which the adsorbent in the first and second beds is zeolite molecular sieve that adsorbs nitrogen in preference to oxygen and argon and the third bed is of a carbon molecular sieve that adsorbs oxygen more readily than nitrogen and argon. 8. Apparatus for separating a gas mixture comprising at least three components, including first and second vessels (2, 4) containing respectively first and second beds (6, 8) of adsorbent that adsorbs a first component preferentially to or more rapidly than second and third components of the mixture; a third vessel (10) containing a third bed (12) of adsorbent that adsorbs the second component more rapidly than or preferentially to the first and third components; a product delivery pipeline (44) for delivering product gas comprising said second component; a conduit (40) affording communication when open between upper regions of the first and second beds (6, 8); a conduit (50) affording communication when open between upper region of the first bed (6) and a lower region of the third bed (12); a conduit (52) affording communication when open between an upper region of the second bed (8) and a lower region of the third bed (12); and a vent pipe (58) affording communication when open between the top of the third bed (12) and the atmosphere, and wherein the first and second beds (6, 8) each have an inlet 14, 22, 16, 32 affording communication when open between the bottom of the respective bed and a source of said gas mixture, an outlet 14, 26, 30, 16, 34, 30 affording communication when open between the bottom of the respective bed and the atmosphere and an outlet (18, 20) affording communication when open between the top of the respective bed and the product delivery pipeline (44), and wherein the said inlets, outlets, conduits and the said pipe each have a stop valve disposed therein.

Inventors: SADKOWSKI, PIOTR JAN
Applicants: THE BOC GROUP PLC
Application year: 1988
Main classification: B01D53
IPC codes: B01D53, C01B13, C01B21