GB2195097A

GB2195097A - Separation of gas mixtures by pressure swing adsorption

Info

Publication number: GB2195097A
Application number: GB08720717A
Authority: GB
Inventors: Michael Ernest Garrett
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1986-09-18
Filing date: 1987-09-03
Publication date: 1988-03-30
Anticipated expiration: 2007-09-03
Also published as: JPS63178821A; GB2195097B; GB8720717D0; GB8622509D0; JP2539846B2; ZA876844B

Abstract

A gas mixture, typically air, is separated by pressure swing adsorption. Compressed air is passed into a bed 12 of carbon molecular sieve. Nitrogen is the least readily adsorbed component of the mixture and a gas mixture enriched in nitrogen flows out of the bed 12 into a reservoir 28. At the end of the adsorption step communication between the bed 12 and air compressor 4 is ended and the bed 12 is placed in communication with bed 14 (at atmospheric pressure) to equalise the pressure therebetween. The bed 12 is then regenerated by placing it in communication with the atmosphere via conduit 34 while the bed 14 is placed in communication with the compressor 4 and nitrogen-enriched gas mixture continues to be produced. The pressure between the beds 12 and 14 is then equalised. During this cycle according to the invention there is a bleed of nitrogen-enriched gas mixture back into the beds 12 and 14 from reservoir 28 via conduit 42 and pipe 44 whenever the pressure in at least one of the beds 12 and 14 is less than the pressure in the reservoir 28. <IMAGE>

Description

SPECIFICATION Separation of gas mixtures by pressure swZ ing adsorption This invention relates to the separation of gas mixture by pressure swing adsorption.

Pressure swing adsorption is a well known process for separating gas mixtures. A bed of molecular sieve adsorbent is employed to separate or extract relatively more of at least one component of a gas mixture admitted to a bed of such adsorbent than of the remaining constitutent or constitutents of the gas mixture. Accordingly, a product gas enriched in the unadsorbed constitutents may be taken from the bed as a product. The bed is then regenerated by being subjected to a lower pressure than the one at which adsorption takes place.

In the first pressure swing adsorption processes that were developed, zeolite molcular sieves were employed as adsorbents. The adsorption rate of gaseous molecules into zeolite pores depends to some extent on the crystallite size, but in general adsorption is fast and equilibrium is reached typically in a few seconds. The sieving action is based on the difference in the equilibrium adsorption of a different components of the gas mixture. The amount adsorbed depends on the pressure of the gas, as well as on temperature. Because of this it is possible to adsorb a gas into the zeolite at high pressure and remove it again by reducing pressure.In the example of air separation, nitrogen is adsorbed preferentially to oxygen, and accordingly the use of pressure swing adsorption to produce oxygen-enriched air or substantially pure oxygen is well known and there are a number of different cycles available for effecting the separation on a commercial scale.

More recently, carbon molecular sieves have been developed as an alternative to zeolite.

Carbon molecular sieves do not produce their sieving action by differences in equilibrium adsorption, though such differences may be present, but by differences in the rate of adsorption of the different components of the gas mixture. Carbon molecular sieves can in particular be employed in air separation to adsorb oxygen from air thus produce nitrogen product.

In view of the different sieving action of carbon molecular sieves from zeolite molecular sieves, a new range of commercial pressure swing adsorptions cycles was developed for use with carbon molecular sieves. In general, these cycles are simplier than pressure swing adsorption cycles employing zeolite molecular sieves. One well know cycle which is described in UK patent Application No. 2073043 A employs two adsorbent beds that are operated on similiar cycles which are sequenced to be out of phase with one other by 1800 so that when one bed is on its adsorption step, the other bed is on its regeneration step and vice versa. Between the adsorption and regeneration steps, the pressures in the two beds are equalised by connecting the two bed inlets together and connecting the two bed outlets together.With these connections made, the gas within the void spaces of the bed which has just completed its adsorption step, that is the high pressure bed, flows into the bed which has just completed its regeneration step, that is the low pressure bed, by virtue of the pressure difference which exists between the beds at this stage. This equalisation step is found to be beneficial in improving the product purity because the gas in such void spaces will have already become somewhat enriched in nitrogen. During the equalisation step the purest gas, that is, that richest in product gas within the high pressure bed will tend reach the middle of the low pressure at the end of equalisation whereas a less pure gas from lower down the higher pressure bed will tend to reach the top of the low pressure bed by the end of the pressure equalisation.

Thus, when the next adsorption step is performed, the product gas initially taken tends to have a greater than average contamination with impurities.

It has been proposed to place the top of the bed undergoing adsorption in communication with the top of the bed undergoing desorption for at least a latter part of each adsorption step. It is found that by this means either the yield or the purity of the product, or both, can be increased. We have now found that in pressure swing adsorption processes utilising carbon molecular sieve, a surprisingly by large increase in product purity or product yield, or both, may be achieved, if, instead of placing the bed undergoing desorption in communication with the bed undergoing regeneration, it is placed in communication with a reservoir in which the product gas is collected.

Accordingly, the invention provides a process for the separation of a desired gas component from a gas mixture including said desired component, which process comprises repeatedly performing the following sequence of steps: a) passing the gas mixture into one end region of a bed of carbon molecular sieve that adsorbs at least one other component of the gas mixture than the desired component more readily than the desired component, and thereby pressurising the bed; b) withdrawing from the other end of the bed a gaseous product enriched in said desired component while continuing to admit the gas mixture into the bed, and passing said gaseous product into a reservoir; and c) desorbing said at least one other component of the gas mixture from said carbon molecular sieve by placing said one end region of the bed in communication with atmosphere or with vacuum; wherein there is a flow of gas from the reservoir to the said bed whenever the pressure in the reservoir exceeds that of the bed.

The invention also provides apparatus for use in the separation of a desired gaseous component from a gas mixture including said desired component, comprising an adsorbent vessel having an inlet conduit at one of its end regions in communication with a source of the gas mixture to be separated, an outlet conduit for venting desorbed gas from said one end region of the bed, and at its other end region another outlet conduit able to place the bed in communication with a reservoir for product gas, each of said conduits having an automatically-operable stop valve disposed therein; a passage extending back from the reservoir to the other end of the bed; and valve control means for controlling opening and closing of the said valves, wherein there is no stop valve disposed in the passage leading back from the reservoir to the said other end of the bed, whereby there is a flow of gas from the reservoir to the bed whenever the pressure in the reservoir exceeds the bed pressure.

Although there may be just one bed of carbon molecular sieve, the apparatus and process according to the invention preferably employ at least two beds of carbon molecular sieve. When there are two beds of carbon molecular sieve, each undergoes said sequence of steps 1800 out of phase with the other so that when one bed is on its adsorption step, the other bed is on its desorption step, and vice versa.Preferably, in such an example of a process according to the invention, upon one bed completing said step (b) it is placed in flow commuication with the other bed whereby there is gas flow from said one bed to the other bed, and said gas flow is ended prior to commencement by said one bed of the next step (c), and upon said one bed completing a deporption step it is placed in flow communication with the other bed whereby there is gas flow from the other bed to said one bed, this gas flow being ended prior to the commencement by said one bed of the next step (a). These gas flows preferably serve to equalise the pressures between the two beds.The gas flows may take place through a conduit placing said other end region of one bed in communication with said other end region of the other bed, or, preferably, by means of such a conduit and another conduit which places the said one end region of one bed in communication with the said one end region of the other bed. Typically, both conduits are provided with automaticallyoperable stop valves operatively associated with said control means.

When two beds of carbon molecular sieve are employed the total flow of gas from the reservoir back to the beds in a period defined by the start of one step (a) on one bed and the start of the next step (a) to be performed by said one bed is preferably in the range of from half a bed volume to two bed volumes (at atmospheric pressure). Typically, therefore, the back flow of gas from the product reservoir will amount to a small proportion of the gas that is withdrawn from the reservoir as product.

The apparatus and process according to the invention are particularly useful in separating nitrogen product from air. Typically, in air separation, the said one end of the or each carbon molecular sieve is protected from water vapour by a layer of desiccant such as alumina or silica gel. Normally, the beds are columnar and said one end of each bed constitutes the bottom of such bed and said other end constitutes the top of such bed. Thus, during adsorption and product withdrawal there is an upward gas flow, and during desorption there is a downward gas flow.

The process and apparatus according to the invention make possible an improvement in the yield or purity of product gas over comparable processes and apparatus in which there is no passage of gas back from the reservoir to the bed or beds.

A process and apparatus according to the invention will now be described by way of example with reference to the accompanying drawing, which is a schematic diagram of a plant for the separation of nitrogen from air in accordance with the invention.

Referring to the drawing, the illustrated plant includes an air feed pipeline 2 leading to a compressor 4. The outlet of the compressor 4 communicates with an air inlet conduit 6. The air inlet conduit 6 is able to be placed in communication with either one of columns 8 and 10 containing beds 12 and 14 respectively of carbon molecular sieve adsorbent.

The columns 8 and 10 are substantially identical to one another, as are the beds 12 and 14. A stop valve 16 (also referred to herein as an on-off valve) is operable to place the bottom of the bed 12 in communication with the inlet conduit 6 or to deny communication between the bed 12 and the inlet conduit 6.

Analogously, a stop valve 18 is operable to place the bottom of the bed 14 in communication with the inlet conduit 6 or to deny such communication.

The plant shown in Figure 1 is provided with an outlet conduit 20 terminating in a reservoir 28. A stop valve 22 is operable to place the top of the bed 12 in communication with the reservoir 28 via the conduit 20 or to prevent such communication. Analogously, a stop valve 24 is operable to place the top of the bed 14 in communication with the reservoir 28 via the conduit 20 or to deny such communication. The reservoir 28 typically has a larger volume than either the bed 12 or the bed 14. In the conduit 20 there is disposed a non-return valve 26 which is adapted to be open only when the pressure upstream of it exceeds the downstream pressure (i.e. the pressure in reservoir 28). The reservoir 28 has an outlet communicating with a pipeline 30 for product gas.In the pipeline 30 is disposed a flow control valve 32 which can be operated to withdraw nitrogen product from the reservoir 28 at a chosen rate.

There is also an outlet conduit 34 for waste or desorbed gas. A stop valve 36 is operable to place the bottom of the bed 12 in communication with the atmosphere via the conduit 34 or to deny such communication. A similar stop valve 38 is operable to place the bottom of the bed 14 in communication with the atmosphere via the conduit 34 or to deny such communication. There is also a conduit 50 which when open places the bottom of the bed 12 in communication with the bottom of the bed 14. A similar conduit 52 places the top of the bed 12 in communication with the top of the bed 14. A stop valve 54 is disposed in the conduit 50 and when closed denies communication between the two beds, a similar stop valve 56 being located in the conduit 52.

In accordance with the invention, there is a narrow pipe or passage 44 which terminates at one end in the top of the column 8 and at its other end in the top of column 10. The passage or pipe 44 makes a union with a conduit 42 that terminates in the reservoir 28.

Accordingly, in operation there is a bed or flow of product gas from the reservoir 28 back into one or both of the beds 12 and 14 according to the respective pressures in the reservoir 28 and the beds 12 and 14. This flow is typically relatively small compared with the flow of product gas taken from the reservoir through the outlet pipeline 30. Non-return valves 46 and 48 are disposed on the respective sides of the union of conduit 42 with pipe 44 and function to prevent flow of gas from beds 12 and 1.4 via pipe 44 and conduit into the reservoir 22.

The valves 16, 18, 22, 24, 36, 38, 54 and 56 are all automatically operated in a manner well known in the art and the timing of the opening and closing of these valves may be dictated by a programmable valve controller 60.

In operation, at the start of a cycle, the bed 12 is at atmospherio pressure and the bed 14 is at its maximum pressure being in communication with the compressor 4. The cycle starts with the beds 12 and 14 being placed in communication with one another through the conduits 50 and 52. In this phase of the cycle, valves 16, 18, 22, 24, 36 and 38 are all in their closed positions and valves 54 and 56 are open. Since the pressure in bed 14 is greater than that in the bed 12, unadsorbed nitrogen-enriched gas in the spaces between individual particles of adsorbent in the bed 14 flows through conduits 50 and 52 into the bed 12. During this first phase of the cycle, and all others nitrogen product is continuously withdrawn from the reservoir 28 through the outlet pipeline 30.There will typically be fluctuations in the pressure in the reservoir 28, but the reservoir 28 is preferably sized such that such fluctuations in delivery pressure are relatively small and such that the average delivery pressure is in the order of or a little below the maximum pressure that obtains during the cycle in the vessels 8 and 10. For example, the pressure in the reservoir 28 may range between 7 and 8 atmospheres absolute and the compressor 4 may typically have an outlet of 8 atmospheres absolute. At the start of the step in which the beds 12 and 14 are placed in communication with one another there is a small flow of product purity gas back into the bed 12 from the reservoir 28 via the conduits 42 and 44.By the time, typically a few seconds, when the pressure in the beds 12 and 14 is equalised, there will also be a bleed of gas from the reservoir 28 into the bed 14, since the pressure in this bed is at this stage of the cycle below that in the reservoir 28.

After a period of time of a few seconds selected to be sufficient for the pressure in the bed 12 have equalised or nearly equalised with the pressure in the bed 14, the next phase of the operating cycle starts. Thus, the controller 60 generates signals to close valves 54 and 56 and to open valves 16, 22 and 38. Accordingly, compressed air is fed into the bed 12 from the compressor 4 and the pressure in the bed 12 is gradually increased and reaches the value of the pressure in the reservoir 28. During this period oxygen is adsorbed by the carbon molecular sieve to leave in the vessel 8 an unadsorbed gas mixture enriched in nitorgen. It will be appreciated that during the previous pressure equalisation step unadsorbed nitrogen-enriched gas has been supplied from the vessel 14 to the vessel 12.

In addition, the flow of product quality gas from the reservoir 28 to the top of the bed 12 which takes place whenever there is a pressure difference therebetween helps to ensure that the first gas delivered to the reservoir 28 during the adsorption phase of the bed 12 is of satisfactory purity. This first delivery takes place once the pressure in the bed 12 exceeds that in the reservoir 28. The non-return valve 26 then permits gas to flow into the reservoir 28. This flow of gas from the bed 12 into the reservoir 28 continues for a chosen period of time. Typically, the bed 12 receives compressed air from the compressor 4 for a period in the order of a minute or so during any one adsorption step. The adsorption step is typically allowed to continue for just so long as the purity of the gas delivered to the reservoir 28 remains acceptable.

During all the period in which the bed 12 is in communication with the compressor 4 the bottom of the bed 14 communicates with the atmosphere through the valve 38. Accord ingly, residual unadsorbed gas in the bed 14 is vented and, as the pressure in the vessel 10 drops, so previously adsorbed oxygen is desorbed and this gas is vented as well. By this means, the adsorbent in the bed 14 is regenerated ready for the next adsorption step that it has to perform. During the desorption step, the pressure of the bed 14 is less than the pressure in the reservoir 28 so-there is a flow of product purity gas from the reservoir 28 into the top of the bed 14. Typically this flow is arranged at such a rate that little if any of the product purity nitrogen so supplied to the bed 14 reaches the bottom thereof to be vented through the valve 28 to the atmos phere.However, it can be seen at the end of the desorption step (which has the same duration as the adsorption step being performed on the bed 12 at the same time) the gas at the top of the column 10 approximates to product purity.

The next step in the operating cycle is to perform another equalisation of the pressure between the beds 12 and 14. The previous phase ends and the pressure phase equalisa tion is started by the controller 60 generating signals to close the valves 16, 22 and 38 and to open the valves 54 and 56. This pressure equalisation phase of the operating cycle is analogous to the first phase, except that in this instance the flow of nitrogen-rich gas is from the bed 12 to the bed 14. Moreover, the bed 14 will continue to receive nitrogen of product purity from the reservoir 28 via the conduit 42 and the pipe 44 and there is also such flow of such gas into the top of the bed 12.The duration of this pressure equalisation phase is equal to the duration of the first pressure equalisation phase and its end is marked by the valves 54 and 56 being closed by virtue of signals from the controller 60 and the valves 18, 24 and 36 being opened by such signals. The final phase of the operating cycle the commences. This phase is analo gous to the second phase (in which the bed 12 is adsorbing and the 14 is being regene rated) except that in this instance it is the bed 14 that adsorbs oxygen from incoming compressed air while the bed 12 is regenerated by being subjected to atmospheric pressures.

During the desorption step there will be pas sage gas from the reservoir 28 into the top of -the bed 12 via the conduit 42 and the pipe 44. At the end of this fourth and final phase of the operating cycle the valves 18 and 24 and 36 are closed by virtue of signals from the controller 60 and the valves 54- and 56 are opened again by such signals so that the cycle can be repeated.

During the entirity of the cycle, nitrogen product is withdrawn at a chosen rate from the reservoir 28 through the outlet 30. Moreover, there is always a bleed of product purity nitrogen from the reservoir 28 back into one or both of the beds 12 and 14. It is found that the performance of this bleed (typically at a rate such that the total back flow of gas into the vessels 8 and 10 during operating cycle is in the range from a half to two bed volumes (at atmospheric pressure)) enables a significant gain in product purity or in yield (or both) to be made in comparison with a comparable process in which the conduit 42, the pipe 44 and the non-return valves 46 and 48 are omitted, or indeed, in comparison with one in which merely the conduit 42 and the valves 46 and 48 are omitted such that there is an equivalent bleed of gas directly from the higher pressure bed to the lower pressure bed.

Claims

1. A process for the separation of a desired gas component from a gas mixture including said desired component, which process comprises repeatedly performing the following sequence of steps; a) passing the gas mixture into one end region of a bed of carbon molecular sieve that adsorbs at least one other component of the gas mixture than the desired component more readily than the desired component, and thereby pressurising the bed; b) withdrawing from the other end of the bed a gaseous product enriched in said desired component while continuing to admit the gas mixture into the bed, and passing said gaseous prcduct into a reservoir; and c) desorbing said at least one other component of the gas mixture from said carbon molecular sieve by placing said one end region of the bed in communication with atmosphere or with vacuum; wherein there is a flow of gas from the reservoir to the said bed whenever the pressure in the reservoir exceeds that of the bed.

2. A process as claimed in claim 1 employing at least two beds of carbon molecular sieve.

3. A process as claimed in claim 2 employing two beds of carbon molecular sieve, in which each bed undergoes said sequence of steps 1800 out of phase with the other so that when one bed is on its adsorption step, the other bed is on its desorption step, and vice versa.

4. A process as claimed in claim 3, wherein upon one bed completing said step (b) it is placed on flow communication with the other bed whereby there is gas flow frcm said one bed to the other bed, and said gas flow is ended prior to commencement by said one bed of the next step (c), and upon said one bed completing a desorption step it is placed in flow communication with the other bed whereby there is gas flow from the other bed to said one bed, this gas flow being ended prior to the commencement by said one bed of the next step (a).

5. A process as claimed in claim 4, in which the gas flows between the vessels serve to equalise the pressure therebetween.

6. A process as claimed in any one of claims 3 to 5, wherein the total flow of gas from the reservoir back to the beds in a period defined by the start of one step (a) on one bed and the start of the next step (a) to be performed by said one bed is in the range of from half a bed volume to two bed volumes (at atmospheric pressure).

7. A process as claimed in any one of the preceding claims, in which the product comprises nitrogen and the said gas mixture is air.

8. A process for separating air, substantially as herein described with reference to the accompanying drawing.

9. Apparatus for use in the separation of a desired gaseous component from a gas mixture including said desired component, comprising an adsorbent vessel having an inlet conduit at one of its end regions in communication with à source of the gas mixture to be separated an outlet conduit for venting desorbed gas from said one end region of the bed, and at its other end region another outlet conduit able to place the bed in communication with a reservoir for product gas, each of said conduits having an automatically-operable stop valve disposed therein; a passage extending back from the reservoir to the other end of the bed; and valve control means for controlling opening and closing of the said valves.

10. Apparatus as claimed in claim 8 or claim 9, employing at least two beds of carbon molecular sieve.

11. Apparatus as claimed in claim 10, employing two beds of carbon molecular sieve, wherein said valve controller is adapted to control operation of said valves such that each bed undergoes said sequence of steps 1800 out of phase with the other so that when one bed is on its adsorption step, the other bed is on its desorption step, and vice versa.

12. Apparatus as claimed in claim 11, additionally including a conduit (when open) placing said other end region of one bed in communication with said other end region of the other bed.

13. Apparatus as claimed in claim 12, additionally including an automatically-operable stop valve in the conduit that (when open) places said other end region of one bed in communication with said other end region of the other bed, the stop valve being operatively associated with said valve control means.

14. Apparatus as claimed in claim 12 or claim 13, additionally including a conduit (when open) placing said one end region of one bed in communication with said one end region of the other bed.

15. Apparatus as claimed in claim 14, additionally including an automatically-operable stop valve in the conduit that (when open) places said one end region of one bed in communication with said one end region of the other bed, the stop valve being operatively associated with said valve control means.

16. Apparatus as claimed in any one of claims 11 to 15, wherein each said bed has a non-return valve in its said passage extending back from the reservoir to the said other end thereof, whereby flow of gas through the passage to the reservoir is prevented.

17. Gas separation apparatus substantially as herein described with reference to the accompanying drawing.