Method and apparatus for separating gases

Abstract

A method and apparatus for the separation of gases by preferential adsorption of at least one of the gases in which at least three adsorption zones are used, at least two of which are arranged in series to form the separation zone while at least one other is being regenerated for re-use in the separation zone, and in which periodically a fresh zone is added to the end of the separation section from which the product gas is recovered and the zone forming the first zone of the separation section is removed for regeneration whereby each zone is successively incorporated into the separation section and subsequently removed from it and regenerated ready for reincorporation into the separation section. Improved utilization of adsorbent and reduced loss of product gas for regeneration purposes is thereby achieved without loss of product purity.

Description

This is a continuation-in-part of U.S. Pat. application Ser. No. 418,027 filed Nov. 23, 1973, now abandoned and relates to a method and apparatus for separating gas mixtures by preferential adsorption of at least one of the gases.

The pressure swing adsorption method of separating a gaseous mixture involves passing the mixture at an elevated pressure through a bed of adsorbent, usually in the form of solid particles, which is designed to be inert towaste gas conduit means also arranged to receive the said gas, valve means adapted to direct said the gas from said compartment, to the one or the other of said compressor inlet or to waste and said waste gas conduit means, (i.e., to the waste gas conduit in the case where regeneration is by means of a purge gas or to the evacuation means in the case where regeneration is by means of evacuation), and valve control means adapted to control the valve means to direct said gas initially to said compressor inlet and subsequently to divert direct said gas to said waste gas conduit means at or before the occurrence of a rapid increase in the concentration of contaminant in said gas.

The composition of the exit gas can be readily monitored using known gas analysis techniques, e.g., katharometry, and therefore a rapid increase in the concentration of the adsorbable material can be readily determined.

For any given gas mixture and adsorption conditions such as of temperature and pressure, this rapid increase in concentration of the adsorbable material will generally occur at or near a given value for the concentration.

Thus, in accordance with one method of carrying out the recycle process of the invention, the gas exiting from the zone during the initial stages of depressurisation may be recovered and recycled until the concentration of adsorbable material in said gas reaches a predetermined level which is not greater than the level which corresponds to the occurrence of the abovementioned rapid increase. Preferably this predetermined level is substantially equal to the level corresponding to the rapid increase.

In apparatus for carrying out this embodiment, the valve control means suitably comprise sensor means for sensing the level of concentration of contaminant in said gas and means responsive to said sensor means to operate the valve to direct the gas to said compressor inlet when said sensed level is below a predetermined level and to direct the gas to said waste conduit means when said sensed level exceeds said predetermined level.

The pressure at which this occurs will vary depending on the nature of the material to be desorbed from the adsorber bed but experience shows that in general it will be found to be equivalent to about 15 to 30% of the total drop in pressure from the adsorption pressure to the pressure of the purge gas where a purge gas is used, or about 10 to 30% of the adsorption pressure expressed as absolute pressure where regeneration is effected by evacuation.

Thus in accordance with another method of carrying out the recycle process of the invention, the gas exiting from the zone during the initial stages of depressurisation may be recovered and recycled until its pressure falls to a predetermined pressure below the adsorption pressure.

This may be achieved for example by sensing the pressure of the gas exiting from the zone and stopping the recycling when this pressure reaches said predetermined pressure. Alternatively, the recycling may be stopped after the elapse of a period of time after commencement of depressurisation which is known, e.g., from trial and experiment or from calculation, to correspond to said pressure.

In apparatus for use with the first alternative, the valve control means may suitably comprise sensor means for sensing the pressure of said gas and means responsive to said sensor means to operate the valve to direct the gas to said compressor inlet when said sensed pressure exceeds a predetermined level and to direct the gas to said waste conduit means when said sensed level is below said predetermined level.

In apparatus for use with the second alternative, the valve control means may suitably comprise timing means adapted to operate said valve to divert the gas from said compressor inlet to said waste conduit means at a predetermined time after the commencement of evacuation of said third compartment.

The pressure of the gas that is recycled to the separation section is restored to the adsorption pressure by means of the compressor. The same compressor may also be used to compress the feed gas mixture to the adsorption pressure where such compression is required.

The use of evacuation to regenerate the adsorbent and the recycling of the interstitial gas to the separation section are illustrated by means of the following Example and with the aid of the accompanying drawings in which:

FIG. 9 is a schematic flow sheet of a modification of the four zone pressure swing adsorption unit illustrated in FIG. 4 when in the mode identified as STAGE 1 in FIG. 4, and wherein regeneration of the adsorbent is effected by evacuation in place of purging;

FIG. 10 is a schematic flow sheet of another modification of the four zone pressure swing adsorption unit illustrated in FIG. 4, again when in the mode identified as STAGE 1 in FIG. 4, wherein provision is made for recycle of the interstitial gas recovered during depressurisation; and

FIG. 11 is a schematic flow sheet of a modification of the apparatus of FIG. 9 wherein provision is made for recycle of the interstitial gas recovered during depressurisation.

In this arrangement illustrated in FIG. 4 of the drawings, it will be recalled that in the period of the cycle illustrated in STAGE 1, feed gas for separation (FG) is fed at elevated pressure to the inlet of the separation section of the pressure swing absorber, consisting of adsorber zones A and B connected in series, where adsorbable contaminant is removed from the feed gas by adsorption. Product gas substantially free of the adsorbable material leaves the separation section from the outlet of zone B and is divided into three portions. A first portion is recovered as product gas (PE). A second portion (RG) is fed to adsorber zone C to re-pressurise this zone to adsorption pressure. A third portion (PG) is fed to expansion valve V where its pressure is reduced, and is then passed at reduced pressure to adsorber zone D as purge gas. The gas (DG) exiting from the adsorber zone D is waste gas and is vented to atmosphere or flared.

Referring now to the arrangement illustrated in FIG. 9, the adsorber illustrated differs from that of FIG. 4 in that the regeneration is effected by evacuation, no purge gas being used. Thus, in the arrangement illustrated in FIG. 9, the feed gas for separation is fed at elevated pressure in pipeline 302 (or is fed in pipeline 303 to be compressed to elevated pressure in compressor 304 and fed via pipeline 306) to pipeline 308 leading to the inlet of the separation section 310 of the adsorber, consisting, as before, of zones A and B connected in series. Product gas substantially free of the adsorbable material leaves the separation section in pipeline 312 but in this case it is divided into two portions only, one of which is recovered in pipeline 314 as product, and the other of which is fed via pipeline 316 to zone C to re-pressurise this zone to adsorption pressure. Zone D is undergoing regeneration by evacuation through pipelines 324 and 328 by means of vacuum pump 340. The evacuated material is passed for venting or flaring.

The cycle of operation of the apparatus corresponds to that of the apparatus shown in FIG. 4.

FIG. 10, in which the features which are common with FIG. 9 have the same reference numerals or letters, is again a flow sheet of a four zone adsorber which operates with the same cycles as that illustrated in FIG. 4 and is shown in the period of the cycle corresponding to STAGE 1 in FIG. 4. As in FIG. 4, the product gas leaving the separation zone is divided into three portions. The first portion is recovered through pipeline 314 as product gas. The second portion is fed via pipeline 316 to adsorber zone C to re-pressurise this zone to adsorption pressure. A third portion is fed via pipeline 318 to expansion valve 320 where its pressure is reduced, and is then passed to adsorber zone D as purge gas. In the adsorber of FIG. 10, however, provision is made for recycling the gas exiting from zone D, which is undergoing depressurisation and purging, to the separation section inlet. Thus, pipeline 324 is connected to two pipelines, 326 and 328, each of which is fitted with a valve, 330, 332 respectively. Pipeline 326 connects pipeline 324 to pipeline 303 and the inlet to compressor 304. Pipeline 328 connects pipeline 324 to the flare or vent to atmosphere. A pressure sensing device 334 is located in pipeline 324 to sense the pressure of the gas therein and is connected to a valve control means 336 in the form of a pressure controller which is adapted to control valves 330 and 332. The controller is adapted so that in operation it closes valve 330 and opens valve 332 when the sensed pressure of the gas in pipeline 324 falls below a predetermined level, the positions of these valves being reversed when the sensed pressure of the gas in pipeline 324 is above that level. In operation, the pressure controller is arranged so that it operates to close valve 330 and open valve 332 at a pressure which is at or slightly above the pressure at which the absorbed contaminant commences to be desorbed from the adsorbent in zone D, resulting in a rapid increase in the concentration of contaminant in the gas in pipeline 24.

FIG. 11, in which the features common with FIGS. 9 and 10 have the same reference numerals or letters, is a flow sheet of the four zone adsorber of FIG. 9 but modified, as in the adsorber of FIG. 10, to provide for recycling of gas exiting from the zone D to the separation section inlet.

EXAMPLE

A. In a first experiment, a feed gas mixture consisting of 94 mol % He and 6 mol % of adsorbable impurities was separated using the adsorber arrangement of FIG. 4 using coconut shell activated carbon as the adsorbent.

The pressure in the separation section was 350 psig, the length of each period of the cycle was 10 minutes, and the purge gas pressure was 5 psig.

With a feed gas rate of 149.5 SCFH the total product recovered from the separation section was 140.5 SCFH of 99.995% pure helium of which 15 SCFH was used as purge gas and 40.5 SCFH was required for re-pressurisation since each zone required 6.75 SCF of gas for re-pressurisation to 350 psig and six zones required re-pressurisation each hour.

Thus, the hourly recovery of helium was

[140.5- (15+ 40.5)] × 0.99995

and the yield of helium, expressed as percentage of theoretical was ##EQU1##

B. In a second experiment, the same feed gas mixture was separated in the adsorber of FIG. 9 using the same volume of the same absorbent as in the first experiment, to produce a product gas of the same 99.995% purity.

It was found that using the same adsorption pressure of 350 psig and the same rate of feed of 149.5 SCFH, as a result of the increased efficiency of regeneration by use of vacuum rather than purging, the length of each period of the cycle could be increased safely to about 15 minutes duration so that only 4 zones required regeneration in each hour instead of 6. However, as it was now necessary to re-pressurise each zone from vacuum to 350 psig, the quantity of product gas required to re-pressurise each zone was found to have increased to 7.05 SCF. On the other hand, no product gas was lost as purge gas.

Thus, the yield of He was ##EQU2##

C. In a third experiment, the same feed gas mixture was separated in the adsorber of FIG. 10 using the same volume of the same adsorbent as in the first experiment, to produce a product gas of the same 99.995% purity.

The pressure controller 336 of FIG. 10 was set to close valve 330 in each period after the pressure of the gas exiting from the zone undergoing regeneration had dropped to 50 psig.

The pressure and total rate of feed of the gas mixture to the separation section was maintained the same as the first experiment at 350 psig and 149.5 SCFH, as were the purge gas pressure (5 psig) and the length of each period of the cycle (10 minutes).

Since the adsorption and purge gas pressures were the same as for Experiment A, the volume of gas required to re-pressurise each zone remained the same at 6.75 SCF.

It was found that under these conditions 5.55 SCF of interstitial gas was recovered and recycled from each zone during depressurisation.

Thus, the net flow of fresh feed to the absorber was 149.5- (6× 5.55)= 116.2 SCFH.

The yield of He was thus ##EQU3##

D. In the fourth experiment, the same feed gas mixture was separated in the adsorber of FIG. 11 using the same volume of the same adsorbent as in all the other experiments, again to produce a product gas of 99.995% purity. Total feed rate to the separator section was 149.5 SCFH at 350 psig.

Since, as in the second experiment, regeneration was effected by evacuation, there was no purge gas stream and, as in said second experiment, each period could be safely extended to 15 minutes duration. Again, as in the second experiment, since re-pressurisation had to be effected from vacuum, 7.05 SCF of gas was required to re-pressurise each zone.

The pressure controller 336 was set so that in each period the valve 330 closed and valve 332 opened after 5.2875 SCF of interstitial gas had been recovered and recycled from each zone during evacuation.

Thus the net flow of fresh feed to the adsorber is

[149.5- (4× 5.2875)] = 128.35 SCFH.

The net product flow is 140.5- (4× 7.05× 0.99995)= 112.3 SCFH.

Thus, the yield of He is 112.3/128.35= 87.5%

Claims

1. A method of separating a purified gas from a mixture thereof with at least one gaseous contaminant by pressure swing adsorption, said method comprising the steps of

a. providing an adsorption unit comprising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole number of at least three;

b. passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x-1 and said y zones constituting a separation section of said adsorption unit;

c. recovering from the last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the gas mixture;

d. before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section;

e. removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone;

f. purging the adsorbent in said removed zone of adsorbed material by passing a purge gas through it at a second pressure which is lower than the first pressure whereby to prepare said zone for re-use in the adsorption section commencing as the last zone of said section; and

g. periodically repeating steps (d), (e) and (f) whereby each zone of said unit is in successive periods incorporated into the separation section as the last zone thereof, subsequently periodically progressed along said section until it becomes the first zone thereof, and then purged ready for re-incorporation into the separation section in a subsequent period.

2. A method as claimed in claim 1 in which x is at least 4 and y equals x-2, and in which each zone is subjected to a separate step of re-pressurisation subsequent to the purging step but before being incorporated into the separation section.

3. A method as claimed in claim 2 in which steps (d) and (e) are operated simultaneously.

4. A method as claimed in claim 2 in which substantially equal periods of time are alowed for the step of purging and the step of re-pressurisation.

5. A method as claimed in claim 2 in which x equals 4.

6. A method as claimed in claim 1 in which x is at least 5 and y equals x-3, and in which each zone is subjected to separate steps of depressurisation after being removed from the separation section and prior to the purging step, and of re-pressurisation subsequent to the purging step and before being incorporated into the separation section.

7. A method as claimed in claim 1 in which the purge gas is provided by a portion of the purified gas at reduced pressure.

8. A method as claimed in claim 1 which further includes causing a part of the gas mixture to by-pass the separation section and thereafter combining said part with the purified gas recovered from said separation section to form a combined gas stream, and controlling the flow rate of said part relative to the flow of the purified gas from said separation section so as to maintain the composition of the combined stream constant.

9. A method as claimed in claim 1 for the recovery of helium from helium-containing gas exhaled by divers inhaling a helium-diluted air composition under pressure.

10. A method as claimed in claim 9 in which the adsorbent is coconut shell-activated carbon.

11. A method as claimed in claim 9 in which the exhaled helium-containing gas is passed to a buffer vessel and thereafter compressed to a first elevated pressure and passed at said elevated pressure to the adsorption unit, and the purified helium recovered from the adsorption unit is further compressed to a second elevated pressure for subsequent recirculation for inhalation.

12. Apparatus for separating a purified gas from a mixture thereof with at least one gaseous contaminant, said apparatus comprising a pressure swing adsorption unit comprising a plurality of adsorption zones for adsorbing said contaminant from said mixture and means for purging adsobent of adsorbed contaminant, said adsorption unit including

at least four compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorbent in the compartment, each compartment forming a separately isolatable adsorption zone,

an inlet for the gas mixtures to be separated in said unit,

an outlet for purified gas,

an inlet manifold connected to the inlet for the gas mixture,

a purified gas outlet manifold connected to the outlet for the purified product,

said means for purging adsorbent comprising a purge gas manifold,

a waste gas manifold,

first conduit means for connecting the inlet of each compartment to the inlet manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold,

second conduit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold,

third conduit means for connecting the purge gas manifold to the outlet from each compartment, with a purge gas inlet valve in the conduit between each compartment outlet and the purge gas manifold,

fourth conduit means for connecting the inlet to each compartment to the waste gas manifold, there being a waste gas outlet valve between each compartment inlet and the waste gas manifold, and

fifth conduit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each compartment connected to the inlet of the next compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet manifold for flow of gas therethrough and in that order with respect to the gas flow, at least a third compartment is connected to the outlet manifold for re-pressurisation and at least a fourth compartment is connected for gas flow therethrough from the purge gas manifold to the waste gas manifold for purging, and automatic valve control means adapted periodically to switch the valves to connect said third compartment in series with said second compartment in said separation section to form the last compartment in said section, expressed in terms of flow of gas therethrough, disconnect said fourth compartment from said purge gas manifold and said waste gas manifold and connect it to said outlet manifold for re-pressurisation, and disconnect said first compartment from said separation section and connect it between the purge gas manifold and the waste gas manifold for purging.

13. Apparatus as claimed in claim 12 in which the valves are electrically operated solenoid valves.

14. Apparatus as claimed in claim 12 including electronic timing means for controlling the operation of the valves.

15. Apparatus as claimed in claim 12 further including means for compressing feed gas to be supplied to the unit, means for compressing product gas recovered from the unit, and prime mover means connected to each of said compressing means for driving same, and means drivably connected to said prime mover means for powering the operation of the pressure swing adsorption unit.

16. Apparatus as claimed in claim 15 further including means for cooling each of the compressing means, said cooling means including means drivably connected to said prime mover means for circulating a cooling medium through said compressing means.

17. Apparatus as claimed in claim 15 further including a buffer bessel adapted to store gas mixture for feeding to the first-mentioned compressor means.

18. Apparatus as claimed in claim 15 arranged in two packs, the first comprising the pressure swing adsorption unit and the second comprising the two compressing means, the prime mover means and the means for powering the operation of the pressure swing adsorption unit.

19. Apparatus as claimed in claim 15 applied to the recovery of helium from helium-containing gas exhaled by divers inhaling a helium-diluted air composition under pressure.

20. Apparatus as claimed in claim 12 further including conduit means connecting said outlet manifold with said purge gas manifold, said conduit means including a pressure reducing valve whereby a portion of said purified gas in said outlet manifold is passed at reduced pressure as purge gas to said purge gas manifold.

21. Apparartus as claimed in claim 20 in which said adsorption unit contains four compartments.

22. Apparatus for separating a purified gas from a mixture thereof with at least one gaseous contaminant, said apparatus comprising a pressure swing adsorption unit comprising a plurality of adsorption zones for adsorbing said contaminant from said mixture and means for purging adsorbent of adsorbed contaminant, said adsorption unit including

at least three compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorbent in the compartment, each compartment forming a separately isolatable adsorption zone,

an inlet for the gas mixture to be separated in said unit,

an outlet for purified gas,

an inlet manifold connected to the inlet for the gas mixture,

a purified gas outlet manifold connected to the outlet for the purified product,

a purge gas manifold,

a waste gas manifold,

first conduit means for connecting the inlet of each compartment to the inlet manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold,

second conduit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold,

third conduit means for connecting the purge gas manifold to the outlet from each compartment, with a purge gas inlet valve in the conduit between each compartment outlet and the purge gas manifold,

fourth conduit means for connecting the inlet to each compartment to the waste gas manifold, there being a waste gas outlet valve between each compartment inlet and the waste gas manifold, and

fifth conduit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each compartment connected to the inlet compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet manifold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected for gas flow therethrough from the purge gas manifold to the waste gas manifold for purging, and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said purge gas manifold and said waste gas manifold and connect it in series with said second compartment in said separation section to form the last compartment in said section, expressed in terms of flow of gas therethrough and disconnect said first compartment from said separation section and connect it between the purge gas manifold and the waste gas manifold for purging.

23. Apparatus as claimed in claim 22 and further including gas compressor means having a gas inlet communicating with the outlet of said waste gas manifold and a compressed gas outlet communicating with the inlet to said separation section, waste gas conduit means also communicating with said second waste gas manifold, valve means adapted to direct gas recovered from said third compartment to the one or the other of said waste gas conduit means and said compressor inlet and valve control means adapted to control the valve means to direct the gas recovered from said third compartment initially to said compressor inlet and subsequently to divert gas to said waste gas conduit means at or before the occurrence of a rapid increase in the concentration of contaminant in said gas.

24. Apparatus as claimed in claim 23 wherein said valve control means comprise sensor means for sensing the level of concentration of contaminant in said gas recovered from said third compartment and means responsive to said sensor means to operate the valve to direct said gas to said compressor inlet when said sensed level is below a predetermined level and to direct said gas to said waste conduit means when said sensed level exceeds said predetermined level.

25. Apparatus as claimed in claim 23 wherein said valve control means comprise sensor means for sensing the pressure of said gas recovered from said third compartment and means responsive to said sensor means to operate the valve to direct said gas to said compressor inlet when said sensed pressure exceeds a predetermined level and to direct said gas to said waste conduit means when said sensed level is below said predetermined level.

26. Apparatus as claimed in claim 23 wherein said valve control means comprises timing means adapted to operate said valve to divert said gas recovered from said third compartment from said compressor inlet to said waste conduit means at a predetermined time after the commencement of evacuation of said third compartment.

27. A method of separating a purified gas from a mixture thereof with at least one gaseous contaminant, said method comprising the steps of

a. providing an adsorption unit comprising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole number of at least three;

b. passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x-1 and said y zones constituting a separation section of said adsorption unit;

c. recovering from the last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the gas mixture;

d. before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section;

e. removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone;

f. regenerating the adsorbent in said removed zone by evacuation of said zone;

g. periodically repeating steps (d), (e) and (f) whereby each zone of said unit is in successive periods incorporated into the separation section as the last zone thereof, subsequently periodically progressed along said section until it becomes the first zone thereof, and then treated to regenerate the adsorbent therein ready for re-incorporation into the separation section in a subsequent period.

28. A method as claimed in claim 27 further including the step of recycling the gas initially exiting from said removed zone during the evacuation procedure of step (f) to the inlet of the separation section to be passed therethrough at substantially said first pressure, said recycle being continued until not later than the occurrence of a rapid increase in concentration of said contaminant in said exit gas.

29. A method as claimed in claim 28 which includes sensing the level of concentration of said contaminant in said exit gas and terminating the recycle of said exit gas when the level of contaminant rises to a predetermined level which is not greater than that which corresponds to the occurrence of said rapid increase.

30. A method as claimed in claim 27 which includes sensing the pressure of said exit gas and terminating the recycle of said exit gas when said pressure drops to a predetermined pressure which is not lower than the pressure at which said rapid increase in concentration of contaminant occurs.

31. A method as claimed in claim 27 in which said recycle is terminated at the end of a predetermined time interval after the commencement of the evacuation procedure of step (f), the length of said time interval being at most the length of the time interval at which said rapid increase of concentration of contaminant occurs.

32. A method of separating a purified gas from a mixture thereof with at least one gaseous contaminant by pressure swing adsorption, said method comprising the steps of

a. providing an adsorption unit comprising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole number of at least three;

b. passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x-1 and said y zones constituting a separation section of said adsorption unit;

c. recovering from the last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the gas mixture;

d. before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section;

e. removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone;

f. reducing the pressure in said removed zone and purging the adsorbent in said removed zone of adsorbed material by passing a purge gas through it at a second pressure which is lower than the first pressure whereby to prepare said zone for re-use in the adsorption section commencing as the last zone of said section;

g. periodically repeating steps (d), (e) and (f) whereby each zone of said unit is in successive periods incorporated into the separation section as the last zone thereof, subsequently periodicaly progressed along said section until it becomes the first zone thereof, and then purged ready for re-incorporation into the separation section in a subsequent period; and

h. recycling the gas initially exiting from said removed zone during the step of reducing the pressure in said zone to the inlet of the separation section to be passed therethrough at substantially said first pressure, said recycle being continued until not later than the occurrence of a rapid increase in concentration of said contaminant in said exit gas.

33. A method as claimed in claim 32 which includes sensing the level of concentration of said contaminant in said exit gas and terminating the recycle of said exit gas when the level of contaminant rises to a predetermined level which is not greater than that which corresponds to the occurrence of said rapid increase.

34. A method as claimed in claim 32 which includes sensing the pressure of said exit gas and terminating the recycle of said exit gas when said pressure drops to a predetermined pressure which is not lower than the pressure at which said rapid increase in concentration of contaminant occurs.

35. A method as claimed in claim 32 in which said recycle is terminated at the end of a predetermined time interval after the commencement of the evacuation procedure of step (f), the length of said time interval being at most the length of the time interval at which said rapid increase of concentration of contaminant occurs.

36. Apparatus for separating a purified gas from a mixture thereof with at least one gaseous contaminant, said apparatus comprising an adsorption unit comprising a plurality of adsorption zones for adsorbing said contaminant from said mixture and means for regenerating adsorbent, said adsorption unit including

at least three compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorbent in the compartment, each compartment forming a separately isolatable adsorption zone,

an inlet for the gas mixture to be separated in said unit,

an outlet for purified gas,

an inlet manifold connected to the inlet for the gas mixture,

a purified gas outlet manifold connected to the outlet for the purified product,

said regenerating means comprising means for evacuating the adsorption zones,

first conduit means for connecting the inlet of each compartment to the inlet manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold,

second conduit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the interior of each compartment to said evacuating means, there being a valve between each compartment interior and the evacuating means, and

fourth conduit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each compartment connected to the inlet of the next compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet manifold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected to said evacuating means for regeneration of the adsorbent therein, and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said evacuating means and connect it in series with said second compartment in said separation section to form the last compartment in said section, expressed in terms of flow of gas therethrough and disconnect said first compartment from said separation section and connect it to said evacuating means for regeneration of the adsorbent therein.

37. Apparatus as claimed in claim 36 further including gas compressor means having a gas inlet communicating with the gas exhaust of said evacuating means said third compartment and a compressed gas outlet communicating with the inlet to said separation section, waste gas conduit means also communicating with said gas exhaust, valve means adapted to direct gas evacuated from said third compartment by said evacuating means to the one or the other of said compressor inlet and said waste gas conduit evacuating means and valve control means adapted to control the valve means to direct the gas initially to said compressor inlet and subsequently to divert the gas to said waste gas conduit evacuating means at or before the occurrence of a rapid increase in the concentration of contaminant in said gas.

38. Apparatus as claimed in claim 37 wherein said valve control means comprise sensor means for sensing the level of concentration of contaminant in said gas and means responsive to said sensor means to operate the valve to direct the gas to said compressor inlet when said sensed level is below a predetermined level and to direct the gas to said waste conduit evacuating means when said sensed level exceeds said predetermined level.

39. Apparatus as claimed in claim 37 wherein said valve control means comprise sensor means for sensing the pressure of said gas and means responsive to said sensor means to operate the valve to direct the gas to said compressor inlet when said sensed pressure exceeds a predetermined level and to direct the gas to said waste conduit evacuating means when said sensed level is below said predetermined level.

40. Apparatus as claimed in claim 37 wherein said valve control means comprises timing means adapted to operate said valve to divert the gas from said compressor inlet to said waste conduit evacuating means at a predetermined time after the commencement of evacuation of said third compartment.