GB2341561A

GB2341561A - Hydrogen purifier

Info

Publication number: GB2341561A
Application number: GB9820181A
Authority: GB
Inventors: Gregor Hoogers
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1998-09-17
Filing date: 1998-09-17
Publication date: 2000-03-22
Also published as: GB9820181D0

Abstract

Apparatus for purifying an impure hydrogen stream has: <SL> <LI>(a) a purification compartment for removing impurities from the impure hydrogen stream; <LI>(b) an inlet enabling the impure hydrogen stream to enter the purification compartment; and <LI>(c) an outlet enabling a purified hydrogen stream to leave the purification compartment, and is </SL> characterised in that the purification system is associated with <SL> <LI>(i) detection means for detecting that impurities are entering and/or are about to enter the outlet; <LI>(ii) cleaning means for removing impurities from the purification compartment when the detection means has so detected; and <LI>(iii) controlling means for preventing the impure hydrogen stream from entering and/or leaving the inlet and/or outlet when the cleaning means is in use. </SL> Typical impurities are CO as H<SB>2</SB>S. Utility is in fuel cells.

Description

1 2341561 APPA TUS The present invention relates to apparatus for use as a

hydrogen purifier, for example for use in the removal of carbon monoxide. Such a purifier may be used, for 5 example, in combination with a fuel cell.

In a fuel cell, a fuel, which is typically hydrogen, is oxidised at a fuel electrode (anode) and oxygen, typically from air, is reduced at a cathode, to produce an electric current and form product water.

In most practical fuel cell systems, the hydrogen fuel is produced by converting a hydrocarbon-based fuel, such as methane, or an oxygenated hydrocarbon fuel, such as methanol, to hydrogen in a process known as reforming. This reformate fuel contains, in addition to hydrogen, high levels of carbon dioxide (C02) of around 25%, and small amounts of impurities, such as carbon monoxide (C0) typically at levels of around 1%. In proton exchange membrane fuel cells (PEMFCs), the electrolyte is a solid proton-conducting polymer membrane, commonly based on perfluorosulphonic acid materials. The electrolyte must be maintained in a hydrated form during operation in order to prevent loss of ionic conduction through the electrolyte. This limits the operating temperature of the PEWC typically to between 7TC and 120T, depending on the operating pressure. For fuel cells operating at temperatures below 200T, it is well known that CO, even at levels of 1 - 1 Oppm, is a severe poison for the platinum electrocatalysts that are present in the electrodes to promote the rates of the electrochemical reactions. This leads to a significant reduction in fuel cell performance, ie the cell voltage at a given current density is reduced. This deleterious effect is especially pronounced in PEMFCs where the operating temperature is around 1OCC.

Various methods have been employed to alleviate anode CO poisoning. For example, reformer technology has been redesigned to incorporate an additional catalytic reactor, known as a preferential or selective oxidation reactor. This involves the in ection j of air or oxygen into the hydrogen-containing reactant gas stream prior to. passing over a selective oxidation catalyst to oxidise the CO to C02. This can reduce the levels of CO from 2 1-2% down to below 10Oppm. However, even at these levels the anode electrocatalyst in the PEWC is still poisoned.

A further technique for alleviating fuel cell performance reduction due to anode poisoning is to employ an anode electrocatalyst that is itself intrinsically more poison-tolerant, but that still functions as a hydrogen oxidation catalyst in the presence of CO. As described by, for example, L Niedrach et al in Electrochernical Technology, (1967), 318, the use of a bimetallic anode electrocatalyst comprising platinum/ruthenium, rather than the more conventionally used mono-metallic platinum only electrocatalyst, shows a reduction in the poisoning effect of the CO at typical PEWC operating temperatures. The bimetallic catalyst does not, however, reduce the levels of CO in the reactant fuel stream, but is slightly more tolerant towards the presence of CO than platinum electrocatailyst alone.

However, again, it is not generally possible to attain the performance observed on pure hydrogen, ie in the absence of CO in the fuel stream, by using this approach in isolation. In practice, this technique may be combined with the air bleed technique described below to obtain improved performance.

It has also been found that poisoning of the electrocatalyst by CO at levels of 1 - 1 00ppm, can be reduced by the use of an oxygen or air bleed directly into the anode gas stream just before it enters the anode chamber of the fuel cell itself. This is described by S Gottesfeld and J Pafford in J. Electrochem. Soc., 135, 2651 (1988). This technique is believed to have the effect of oxidising the residual CO in the fuel to C02, the reaction being catalysed by the electrocatalyst sites present in the anode:

1 CO + /202 - C02 An alternative, more recent approach to minimise the effect of CO poisoning by use of an air bleed is disclosed in US patent specification number 5 482 680. This specification discloses the use of a selective oxidation catalyst, present as a gas- porous bed or layer, placed between the fuel stream inlet of the fuel cell nd the anode catalyst layer. In particular, the catalyst bed or layer can be placed in a variety of positions within the fuel 3 stream manifold, including within the fuel stream inlet and fuel stream humidification apparatus.

These air bleed techniques provide fuel cell performance that is much closer to the performance observed when no CO is present in the fuel stream. Nevertheless, air bleed techniques generally result in decreased lifetimes of the anodes; oxidation of some hydrogen fuel reduces overall efficiency of such systems; and other disadvantages accrue, which include complexity (due to requiring addition of oxygen or air to the fuel stream).

The present invention seeks to overcome the problem of electrode performance decay due to poisoning of the anode electrocatalyst by CO, by providing a hydrogen purifier which will remove the CO present in the reformate before coming into contact with the anode electrocatalyst; but without the need continuously to inject air or oxygen into the fuel stream being fed to the fuel cell. The hydrogen purifier of the invention may also be used to purify a hydrogen stream in general and to remove impurities other than CO, such as hydrogen sulphide (H2S).

Accordingly, the present invention provides apparatus for purifying an impure hydrogen stream, which apparatus comprises a purification system comprising:

(a) a purification compartment for removing impurities from the impure hydrogen stream; (b) an inlet enabling the impure hydrogen stream to enter the purification compartment; and (c) an outlet enabling a purified hydrogen stream to leave the purification compartment characterised in that the purification system is associated with (i) detection means for detecting that impurities are entering and/or are about to enter the outlet; (ii) cleaning means for removing impurities from the purification compartment when the detection means has so detected; and (iii) controlling means for preventing the impure hydrogen stream from entering and/or leaving the inlet and/or outlet when the cleaning means is in use.

4 The apparatus of the present invention therefore provides a less complex purification technique than the prior art techniques, since the purification system is not itself part of the fuel cell; and particularly provides a more efficient technique since the impurities become more concentrated and their intermittent, rather than continuous, removal is more economic than with the prior art air bleed techniques.

Usually, in the apparatus of the present invention, the purification compartment comprises a reactor or reaction chamber wherein the impurities are absorbed and wherefrorn they are removed by the cleaning means, for example, by conversion eg by oxidation followed by exhaustion, as described further below.

Preferably, the apparatus comprises more than one purification system. More preferably, the controlling means is adapted to allow the impure hydrogen stream to enter and leave a second purification system when the cleaning means of the first purification system is in use.

The hydrogen purifier of the present invention if, therefore based on a system that allows removal of the impurities in the, or the first, purification compartment, thereby removing the impurities from the hydrogen strewn, the purified hydrogen continuing through the purifier and thence through the outlet. When the purification compartment is fully saturated with impurities, impurities can no longer be removed from the hydrogen stream and presence of impurities may be detected, for example, in the hydrogen stream flowing from the outlet. Once saturation of the purification compartment with impurities is detected, it is cleaned of the impurities. During the intermittent cleaning of the purification compartment, the flow of impure hydrogen into the, or the first, purifier can be temporarily stopped and, preferably diverted to a second or subsequent purification compartment.

Alternatively, the hydrogen stream could be diverted to a buffer, in the case of a single purification compartment, such as to a store of purified hydrogen that is replenished from the apparatus of the present invention once the purification compartment has been cleaned.

The invention therefore further provides a process for the removal of one or more impurities from an impure hydrogen stream, said process comprising the introduction of an impure hydrogen stream into a hydrogen purifier of the present invention, passing the impure hydrogen stream through a or a first purification compartment wherein one or more impurities are removed from the hydrogen stream and passing the purified hydrogen stream out of the purifier, characterised in that, when the or the first purification compartment is saturated with impurities, flow of the impure hydrogen stream to and/or from that purification compartment stops and is preferably re-directed to a second or subsequent purification compartment, and impurities are removed from the or the first purification compartment.

Intermittent cleaning of the impurities from the purification compartment may proceed via a variety of methods and depends on the type of compartment present. A first method is based on a heterogeneous catalytic reaction. In this method, the apparatus is adapted to provide oxygen or air to the purification compartment only during the cleaning phase, ie intermittently, whereby the one or more impurities adsorbed onto the catalyst are oxidised. The oxidation products are then preferably removed from the purifier via an exhaust.

A second method for cleaning the purification compartment is an electrochemical method and is of use when the purification compartment comprises the anode of an electrochemical cell. In this method, completion of an electrochemical circuit and the provision or presence of oxygen or oxidising species during the cleaning phase allows the impurities to be oxidised and preferably removed from the cell via an exhaust. Completion of the circuit involves allowing a current to flow between the anode that is comprised in the purification compartment and the cathode of the cell, which is supplied with eg oxygen or air. During cleaning, electric power is therefore generated through the electrochemical oxidation of the impurities and any residual fuel.

Cleaning means may alternatively comprise physical means such as vacuum means for applying a vacuum to the impurity-saturated purification compartment, whereby the 6 impurities are intem-iittently removed via an exhaust. Other non- catalytic means for cleaning the purification compartment include the use of a co-adsorber such as charcoal or a zeolite for removing the impurities. Such co-adsorbers may themselves be incorporated into an electrode in an electrochemical cell and used in a similar manner to that described above.

Accordingly, the present invention ftirther provides apparatus finther comprising or in association with any of the cleaning means described hereinbefore. The hydrogen purifier of the invention has been found to be particularly successful in removing carbon monoxide from an impure hydrogen stream and consequently will be particularly useful in combination with a fuel cell that is designed to be operated using reformate fuel.

Detection of impurities can be carried out by any method known to a person skilled in the art. It is not necessary that the detection means be directly connected to the outlet, although such means may be convenient. For example, in the case where the apparatus of the present invention is combined with a fuel cell, the detection means may be associated with the fuel cell such that a deterioration in the output therefrom trigger(s) the cleaning and/or controlling means. Alternatively, the detection mqns may be directly associated with the purification compartment whereby the level of saturation thereof is monitored, such that once a pre-determined level is reached, which would result in impurities entering the outlet, then the cleaning and/or controlling means is/are triggered. Detection means therefore may comprise infra-red detection; semi-conducting, catalytic and electrochemical sensors; voltmeters (for detecting a change in fuel cell output voltage); and the like.

The controlling means for regulating the flow of impure hydrogen to and/or from the purification system may be any such means known to those skilled in the art. For example, two- or three-way valves may be suitably employed, such as solenoid valves.

Preferably, one or more valves is/are associated with the inlet and, more preferably, also with the outlet to stop or divert the hydrogen flow. Especially preferred is a system of co-operating valves to enable the use of a second or subsequent purification system whilst the first or other purification system is being cleaned. ' 7 In a first embodiment of the invention, the purification compartment is in the form of a single catalyst bed, the impurities being adsorbed onto the catalyst. When all catalytic sites are occupied, no more impurities are able to be adsorbed and impurities will pass through the purifier with the hydrogen into the outlet. The presence of impurities in the outlet will be detected by the detection means, which will trigger the controlling means so that the flow of impure hydrogen to the purification system is prevented. If only one purification compartment is present in the purifier, the inflow of impure hydrogen is stopped completely and the purification compartment is cleaned, after which the flow of hydrogen begins again. If two or more purification compartments are present in the purifier, the flow of hydrogen through the inlet is diverted to one of the other purification compartments, and the first purification compartment is cleaned. The hydrogen flow will continue to the second purification compartment until all its catalytic sites are saturated, at which point re-direction of the hydrogen flow as described above will occur and the catalyst on the second purification compartment cleaned.

In this embodiment, cleaning is carried out by a heterogeneous catalytic reaction.

Catalytic oxidation of the impurities occurs by the introduction of oxygen into the purification compartment. The catalyst is chosen such that the oxidative removal of adsorbed impurities is initiated at low light-off at temperatures, preferably below a few hundred degrees Celsius.

In a second embodiment of the invention, the purification compartment is incorporated in an electrochemical cell comprising an anode, a cathode and an electrolyte.

The anode comprises a catalyst, suitably one which pref;rentially adsorbs the one or more impurities in preference to hydrogen. Examples of such catalysts include copper and precious metals, for example ruthenium and platinum, or oxides thereof, and metal alloys.

The impure hydrogen is passed over the anode wherein the impurities are adsorbed onto the catalyst. The purified hydrogen then continues through the outlet, free from impurities.

When the anode catalyst becomes saturated with impurities, the hydrogen stream is not able to be purified completely and the presence of impurities becomes detectable in the outlet.

Once impurities are detected in the outlet, the flow of hydrogen to the purification 8 compartment ceases due to the action of the controlling means, and the anode catalyst is cleaned ready for re-use. If more than one purification compartments are present, the flow of impure hydrogen is re-directed to a second or subsequent purification compartment, and the purification process continues as described above. The hydrogen flow continues to the second purification compartment until all the catalytic sites on that anode are saturated, at which point the flow is re-directed as described above and the catalyst on the second absorption unit is cleaned.

In this embodiment, cleaning of the purification compartment occurs electrochemically, by completing an external electrical circuit between the anode and cathode, which causes the adsorbed impurities to be oxidised at the anode and supplied oxygen (or an oxygen-containing species) to be reduced at the cathode. The oxidised impurities are removed from the absorption unit via an exhaust.

The invention will now be further described with reference to the accompanying Figures, in which:

Figure I A is a schematic representation of a simple hydrogen purifier of the present invention that uses a single absorption unit requiring cleaning by a heterogeneous catalytic reaction; Figure IB is a schematic representation of a hydrogen purifier of the present invention that uses two absorption units requiring cleaning by a heterogeneous catalytic reaction; Figure 2A is a schematic representation of a hydrogen purifier of the present invention using a single absorption unit comprising an electrochemical cell; and Figure 2B is a schematic representation of a hydrogen purifier of the present invention which uses two absorption units, each of which comprises an electrochemical cell.

Referring to Figure I A: During operation, impure hydrogen flows along inlet (1) through open valve (2) and into the absorption unit (3) comprising catalyst (4). Impurities in the hydrogen stream are adsorbed onto the catalyst sites, allowing purified hydrogen to 9 flow out of the absorption unit along outlet (5) through valve (6). When all catalytic sites are saturated, impurities also begin to flow along outlet (5) and detection by a sensor (not shown) will cause valves (2) and (6) to close. Cleaning of the absorption unit will then occur by opening valve (7) temporarily to allow oxygen or an oxygen- containing species to enter the absorption unit through conduit (8). A catalytic reaction occurs in the absorption unit, oxidising the impurities which are purged from the absorption unit through exhaust (9) and open valve (10). Once all the impurities have been removed from the absorption unit, valves (7) and (10) are closed to prevent ingress of oxygen or the oxidising species, and valves (2) and (6) opened enabling the purification cycle to begin again.

Referring to Figure I B: During operation, impire hydrogen flows along inlet (1) and through open valve (2) into the first absorption unit (3) comprising catalyst (4). Valve (2A) is closed to prevent flow of impure hydrogen into the second absorption unit (3A).

Impurities in the hydrogen stream are adsorbed onto the catalyst sites (4), allowing purified hydrogen to flow out of the absorption unit (3) along outlet (5) through valve (6). When all catalytic sites are saturated, impurities also begin to flow along outlet (5) and detection by a detector (not shown) will cause valves (2) and (6) to close. Simultaneously, valve (2A) will open, allowing impure hydrogen to flow into the second absorption unit (3A) comprising catalyst (4A). Impurities in the hydrogen stream are adsorbed onto the catalyst sites (4A), allowing purified hydrogen to flow out of the absorption unit (3 A) through open valve (6A) and along outlet (5). During operation of absorption unit (3A), absorption unit (3) is cleaned of impurities. Valve (7) allowing oxygen or an oxygen-containing species to enter absorption unit (3) through conduit (8) is opened, keeping valve (7A) closed. A catalytic reaction occurs in absorption unit (3) oxidising the impurities, which are purged from the absorption unit through exhaust (9) and open valve (10). Once all the impurities have been removed from the absorption unit, valves (7) and (10) are closed. When all catalyst sites (4A) are saturated, impurities also begin to flow along outlet (5) and detection by a detector (not shown) will cause valves (2A) and (6A) to close. Simultaneously, valves (2) and (6) will open and the purification cycle will continue using absorption unit 30 (3). Absorption unit (3A) will be cleaned of all impurities by opening of valves (7A) and (I OA), introducing oxygen or an oxygen-containing spe;ies into absorption unit (3 A) and removing oxidised impurities through exhaust (9).

Referring to Figure 2A: During operation, impure hydrogen flows along inlet (1) through valve (2), which is open, and into the absorption unit (3) comprising anode (11) with catalyst sites (12), cathode (13) with catalyst sites (14) and electrolyte (15). Impurities in the hydrogen stream are adsorbed onto the anode catalyst sites (12) and purified hydrogen flows out of the absorption unit (3) through valve (6) and along outlet (5). When all anode catalytic sites (12) are saturated, impurities also begin to flow along outlet (5) and detection by a sensor (not shown) will cause valves (2) and (6) to close. Cleaning of the absorption unit will then occur by opening of valve (7) allowing oxygen or an oxygen-containing species to enter the absorption unit through conduit (8) and flow to the cathode (13). When an electrical circuit is completed between the anQde (11) and cathode (13), an electrochemical reaction occurs wherein the impurities on anode catalyst sites (12) are oxidised, and are removed from the absorption unit through exhaust (9) and open valve (10).

Reduction of the oxygen or oxygen-containing species occurs at the cathode, forming product water which is removed from the absorption unit via outlet (16). Once all the impurities have been removed from the anode, valves (7) and (10) are closed, and valves (2) and (6) opened enabling the purification cycle to begin again.

Referring to Figure 2B: During operation, impure hydrogen flows along inlet (1) through open valve (2), and into absorption unit (3) comprising anode (11) with catalyst sites (12), cathode (13) with catalyst sites (14) and electrolyte (15). Impurities in the hydrogen stream are adsorbed onto the anode catalyst sites (12), and purified hydrogen flows out of the absorption unit (3) through valve (6) and along outlet (5). When all the catalytic sites (12) are saturated with impurities, impurities also beiin to flow along outlet (5) and detection by a sensor (not shown) will cause valves (2) and (6) to close. Simultaneously, valve (2A) will open, allowing impure hydrogen to flow into the second absorption unit (3A) comprising anode (I I A) with catalyst sites (1 2A), cathode (13 A) with catalyst sites (I 4A) and electrolyte (I 5A). Impurities in the hydrogen stream are adsorbed onto the anode catalyst sites (12A) and purified hydrogen flows out of the absorption unit (3A) through 11 valve (6A) and along outlet (5). During operation of absorption unit (3A), absorption unit (3) is cleaned of impurities. Valve (7) allowing oxygen or an oxygencontaining species to enter absorption unit (3) through conduit (8) is opened, keeping valve (7A) closed. When switch (S 1) is closed, an electrochemical reaction occurs, wherein the impurities on anode catalyst sites (12) are oxidised and removed from the absorption unit (3) through exhaust (9) and open valve (10). Reduction of the oxygen or oxygen-containing species occurs at the cathode, forming product water which is removed flom, absorption unit (3) via outlet (16). Once all the impurities have been removed from the absorption unit, valves (7) and (10) are closed. When all catalyst sites (12A) are saturated, impurities also begin to flow along outlet (5) and detection by a detector (not shown) will cause valves (2A) and (6A) to close. Simultaneously, valves (2) and (6) will open and the purification cycle will continue, using absorption unit (3). Absorption unit (3A) will be cleaned of all impurities by opening of valves (7A) and (lOA), introducing oxygen or an oxygen-containing species to the cathode (13A), closing switch (S2) and removing oxidised impurities through exhaust (9).

3 1-2.

Claims

CIAIMS

1. Apparatus for purifying an impure hydrogen stream, which apparatus comprises a purification system comprising:

(a) a purification compartment for removing impurities from the impure hydrogen stream; (b) an inlet enabling the impure hydrogen stream to enter the purification compartment; and (c) an outlet enabling a purified hydrogen stream to leave the purification compartment, characterised in that the purification system is associated with (i) detection means for detecting that impurities are entering and/or are about to enter the outlet; (ii) cleaning means for removing impurities from the purification compartment is when the detection means has so detected; and (iii) controlling means for preventing the impure hydrogen stream from entering and/or leaving the inlet and/or outlet when the cleaning means is in use.

2. Apparatus according to claim 1 wherein the purification compartment comprises a reactor or reaction chamber wherein the impurities are absorbed and wherefrom they are removed by the cleaning means.

3. Apparatus according to any one of claims 1 or 2 comprising more than one purification system.

4. Apparatus according to claim 3 wherein the controlling means is adapted to allow the impure hydrogen stream to enter and leave a second purification system when the cleaning means of the first purification system is in use.

5. Apparatus according to claim 1 or 2 wherein when the cleaning means of the first purification system is in use, the impure hydrogen stream is diverted to a buffer.

6. Apparatus according to any one of claims 1 to 5 wherein the detection means is selected from the group consisting of infra-red detection; semi conducting, catalytic and electrochemical sensors; and voltmeters.

7. Apparatus according to any one of claims 1 to 6 wherein the cleaning means is based on a heterogeneous catalytic reaction.

8. Apparatus according to any one of claims 1 to 6 wherein the cleaning means is based on an electrochemical reaction.

9. Apparatus according to any one of claims 1 to 6 wherein the cleaning means is a physical means.

10. Apparatus according to claim 9 wherein the physical means is a vacuum means for applying a vacuum to the impurity-saturated compartment, whereby the impurities are removed via an exhaust.

11. Apparatus according to claim 9 wherein the cleaning means includes the use of a co adsorber such as charcoal or a zeolite.

12. Apparatus according to any one of claims I to 11 wherein the controlling means is one or more valves, such as solenoid valves.

13. Apparatus according to claim 12 wherein the one or more valves is/are associated with the inlet.

14. Apparatus according to claim 13 wherein the one or more valves is/are also associated with the outlet.

15. Apparatus according to any one of claims 1 to 11 wherein the controlling means comprises a system of co-operating valves to enable the use of a second or subsequent purification system whilst the first or other purification system is being cleaned.

16. A process for the removal of carbon monoxide from an impure hydrogen stream, said process comprising the introduction of an impure hydrogen stream into an apparatus according to any one of claims 1 to 17, passing the impure hydrogen stream through a or a first purification system wherein the carbon monoxide is removed from the hydrogen stream and passing the purified hydrogen stream out of the purifier, characterised in that, when the or the first purification compartment is saturated with carbon monoxide, flow of the impure hydrogen stream to and/or from that purification compartment stops and is optionally re-directed to a second or subsequent purification compartment, and carbon monoxide is removed from the or the first purification compartment.

16. Apparatus according to any one of claims 1 to 7 and 12 to 15 wherein the purification compartment is in the form of a single catalyst bed, the impurities being adsorbed onto the catalyst.

17. Apparatus according to any one of claims 1 to 6, 8 and 12 to 15 wherein the purification compartment is incorporated in an electrochemical cell comprising an anode, a cathode and an electrolyte.

18. A process for the removal of one or more impurities from an impure hydrogen stream, said process comprising the introduction of an impure hydrogen stream into an apparatus according to any one of claims 1 to 17, passing the impure hydrogen stream through a or a first purification system wherein one or more of the impurities are removed from the hydrogen stream and passing the purified hydrogen stream out of the purifier, characterised in that, when the or the first purification compartment is saturated with impurities, flow of the impure hydrogen stream to and/or from that purification compartment stops and is optionally re-directed to a second or subsequent purification compartment, and impurities are removed from the or the first purification compartment.

19. A process according to claim 18 wherein impurities are removed from the or the first purification compartment by means of a heterogeneous catalytic reaction.

20. A process according to claim 19 wherein oxygen or air is provided to the or the first purification compartment only during the cleaning phase and wherein the one or more impurities are oxidised by means of the heterogeneous catalytic reaction.

21. A process according to claim 20 wherein the one or more oxidised impurities are removed from the purifier via an exhaust.

22. A process according to claim 18 wherein the impurities are removed from the or the first purification compartment by means of an electrochemical reaction.

23. A process according to claim 18 wherein the impurities are removed from the or the first purification compartment by physical means.

24. A process according to claim 23 wherein the physical means is a vacuum means for applying a vacuum to the impurity-saturated purification compartment, whereby the impurities are intermittently removed via an exhaust.

25. A process according to claim 23 wherein the physical means includes the use of a co adsorber such as charcoal or a zeolite for removing the impurities.