WO2001019728A1

WO2001019728A1 - Apparatus and method for drying hydrogen

Info

Publication number: WO2001019728A1
Application number: PCT/GB2000/003529
Authority: WO
Inventors: Edward Gobina
Original assignee: The Robert Gordon University
Priority date: 1999-09-16
Filing date: 2000-09-14
Publication date: 2001-03-22
Also published as: AU7431000A; GB9921785D0; EP1216201A1

Abstract

A gas dryer is disclosed that includes a membrane located between an inlet and an outlet in the dryer. A wet feed gas (e.g. hydrogen) is passed from the inlet to the outlet over the membrane. Water vapour from the gas passes through the membrane as it can permeate the membrane faster than the gas due to the relative permeability of the membrane. A sweep gas may be used to collect the water vapour, and further inlets and outlets may be provided for this purpose. A cellulose acelate membrane B used in the dryer.

Description

"Apparatus and Method for Drying Hydrogen"

The present invention relates to apparatus and method, particularly, but not exclusively, for drying hydrogen .

Hydrogen gas is typically used in a variety of different industries (including academic institutions) for a variety of different purposes. For example, it may be used as a carrier gas or as a fuel to supply chromatographic equipment during routine hydrocarbon analysis. Hydrogen gas is typically supplied in one of two ways; from cylinders that store compressed hydrogen gas, or by a hydrogen generator.

Hydrogen gas is usually stored within cylinders that are typically located within a building or other structure, close to where the gas is required. Hydrogen is very flammable and is generally stored under high pressure in the cylinders. The cylinders are typically rented from the supplier and the hydrogen itself can be relatively expensive. Even if the hydrogen gas within the cylinders is not being used, there is a cost involved for the rental of the cylinder (s) .

The storage of hydrogen gas within buildings and other structures is problematic and the cylinders are preferably stored externally, requiring pipework to be installed from the storage facility outwith the building to the place (s) where the hydrogen gas is to be used.

Furthermore, the hydrogen gas stored within the cylinders is relatively impure and requires further purification for many applications such as gas chromatography or other small-scale applications where hydrogen gas is required on-demand. The supply of hydrogen gas on-demand is useful, for example, on offshore oil and gas platforms or in remote locations for obtaining data on gas gravity and gas impurities; two key data required for estimating formation permeability. One of the primary sources of data requirements for estimating gas impurity and gas gravity involves the use of gas chromatography incorporating flame ionisation detectors. Hydrogen is used both as the fuel and the carrier gas for this equipment. Space is usually limited on offshore rigs and equipment deployment on rigs and in remote locations can be problematic. The use of pressurised hydrogen cylinders on an offshore rig can be a considerable risk to personnel.

Alternatively, hydrogen gas can be generated in hydrogen generators . These generators may be of a variety of different types, the most common incorporating a solid polymer electrolyte (SPE) for the electrolysis of de-ionised water. Such generators are primarily used in the lab-gas market where the hydrogen gas is used in gas chromatography either as fuel or as a carrier gas. To be useful in such applications, the hydrogen gas must be pure, delivered at high pressure and dry (ie moisture free) .

Hydrogen generators have many other applications where local generation of hydrogen gas is required. Such applications include the cooling of power station alternators, metallurgical furnace operations, catalytic process, in the electronics industry such as in semiconductor manufacturing, controlled atmospheres, and analytical applications. Each of these applications may require a different purity of hydrogen gas in terms of how dehydrated it is.

In SPE hydrogen generators, the hydrogen gas exits the electrolytic cell at pressure and is usually mixed with the de-ionised water used in the electrolytic process. Hydrogen gas is created in these generators by splitting up the de-ionised water into hydrogen and oxygen gases using the electrolytic cell. The cell typically incorporates a positive anode and a negative cathode, separated by an ion- exchange solid polymer membrane. The generators require only water and electricity to operate and are generally portable, thus producing hydrogen gas "on- demand" .

A simple valve mechanism then separates the bulk water from the hydrogen gas. However, the hydrogen typically remains fully saturated and is typically passed through a NAFION (RTM) drying tube and thereafter through a column of silica gel. The silica gel containment vessel is typically a long tube that is filled with the desiccant material. This three-stage process generally provides dry hydrogen to the detector.

However, this conventional method of generating and drying hydrogen has a number of associated limitations and disadvantages. For example, the silica gel column is consumable as it has a finite capacity for absorbing water vapour, and requires to be changed periodically as the gel becomes saturated. This requires the user to shut down the generator and any sampling equipment or the like that relies on the supply of hydrogen for a period of time. This is not convenient for applications where, for example, laboratories are required to operate twenty-four hours a day, seven days a week. Typical hydrogen generators can generate quantities of hydrogen ranging from 200 to 1200 cubic centimetres per minute (cc/min) . Although the volume of gas entering the hydrogen dryer may vary, the volume of silica gel required does not, and depending upon the size of the dryer and the usage thereof, the silica gel may require to be changed once a month or once every five days, for example.

Furthermore, the columns of silica gel are often comparatively large and cumbersome and thus result in a generator that is larger than it need be.

Conventional hydrogen generators have safety features, due to specific safety requirements for the usage of hydrogen that include various leak detection systems. These systems shut down the generator in the event of a hydrogen leak being detected. NAFION (RTM) drying tubes tend to leak with age and also with increasing hydrogen pressure, and the continual changing of the silica gel may be detrimental to the tube, thus introducing further leaks or reinforcing existing leaks. This often results in the leak detection systems shutting down the generators automatically. These shut downs require the generator to be repaired before it may be used again, this being detrimental to the operations requiring the use of dry hydrogen gas.

It is also known to use a palladium membrane that provides a very dry gas stream without the use of consumables. Palladium is a rare precious metal and thus palladium membranes tend to be very expensive. Additionally, palladium membranes require a high temperature to work effectively and a significant pressure drop to enable significant hydrogen permeatation rates. For the foregoing reasons, the use of palladium membranes in small-scale laboratories or other small-scale applications tends to be uneconomic.

Other methods and apparatus exist for generating hydrogen, such as filter presses and Milton-Roy membrane technology, but SPE is currently the most popular.

According to a first aspect of the present invention, there is provided a gas dryer comprising at least one inlet for gas, at least one outlet for gas, and at least one cellulose acetate membrane, the membrane being capable of allowing water vapour to pass therethrough whilst substantially restricting passage of gas.

According to a second aspect of the present invention, there is provided a method of dehydrating gas, the method comprising the steps of providing at least one cellulose acetate membrane, passing gas over the cellulose acetate membrane, wherein water vapour contained within the gas passes through the membrane.

The gas is preferably hydrogen. The gas dryer typically comprises a wet gas inlet and a dehydrated gas outlet. Typically, the membrane is located at least partially between the inlet and the outlet. Varying the area of the cellulose acetate membrane typically varies the extent of dehydration of the gas. Increasing the area of the cellulose acetate membrane typically increases the extent of dehydration of the gas. The wet gas may be passed over one or more cellulose acetate membranes. The cellulose acetate membranes are typically located in a stack or baffle. This allows the dryer to be made more compact .

The gas dryer preferably includes a second inlet and a second outlet to allow a sweep gas to flow through the dryer. The sweep gas typically removes water vapour from the dryer once the vapour has passed through the membrane. The sweep gas typically comprises nitrogen, although air or other gases may be used. The flow rate of the sweep gas can be varied. Increasing the flow rate of the sweep gas typically increases the extent of dehydration of the gas. Alternatively, a vacuum may be used to remove the water vapour, wherein a vacuum pump is coupled to the second outlet.

The dryer typically comprises a first and a second chamber, the first and second chambers being divided by the membrane. The first chamber typically includes the inlet and outlet for the wet gas. The second chamber may include the second inlet and second outlet for the sweep gas. Alternatively, the second chamber may include the second outlet for the vacuum.

In one embodiment, a plurality of dryers are provided, wherein the inlets and outlets for the gas and the second inlets and second outlets for the sweep gas are coupled so that the gas and/or the sweep gas flow in a criss-cross pattern through the dryers. In one embodiment, the gas flows in a first direction and the sweep gas flows in a second direction, wherein the second direction is typically opposite to the first direction. This is termed a counter-current flow configuration and is more efficient than a co-current flow configuration where the wet gas and the sweep gas flow in the same direction. Alternatively, the membranes may be located in a series. The path that the gas and/or the sweep gas follows can be of any suitable configuration, and may be convoluted for example.

The dryer is preferably manufactured from a stainless material that may be stainless steel.

The cellulose acetate membrane preferably comprises a regenerated cellulose acetate. The cellulose acetate membrane is typically regenerated by a xanthate process. The method preferably includes the additional steps of providing a second inlet and a second outlet for passing a sweep gas therethrough, and passing the sweep gas from the second inlet to the second outlet to remove the water vapour. Alternatively, the method may comprise the additional steps of coupling a vacuum pump between the second inlet and the second outlet and activating the pump to remove the water vapour .

Embodiments of the present invention shall now be described, by way of example only, with reference to the accompanying drawings, in which: -

Fig. 1 is a schematic diagram illustrating the use of a cellulose acetate membrane to dehydrate hydrogen gas; Fig. 2 is a schematic diagram of an exemplary hydrogen dryer; Fig. 3 is a graph of dew point against time illustrating the differences between various methods and apparatus for dehydrating hydrogen gas; Fig. 4 is a graph of flow rate and dew point against time for a regenerated cellulose acetate membrane used in the dryer of Fig. 2; and Fig. 5 is a schematic diagram of an alternative embodiment of a hydrogen dryer, where a plurality of the dryers of Fig. 2 are coupled in series. Referring to the drawings, Fig. 1 is a schematic diagram illustrating how a cellulose acetate membrane can be used to dehydrate hydrogen gas. It should be noted that the following description relates to dehydrating hydrogen gas, but it will be appreciated that the method and apparatus described herein can be used to dehydrate other gases .

Referring to Fig. 1, "wet" hydrogen gas (comprising hydrogen H₂ and water vapour H₂0) , indicated generally at 10, is passed over a cellulose acetate membrane 12.

The thin sheet of cellulose acetate membrane 12 separates the components of a gas mixture as the components within the gaseous mixture will pass through (or permeate) the structure of the membrane 12 at different rates due to the different molecular characteristics of the components.

The membrane 12 preferably comprises a regenerated cellulose acetate. Goodfellow supply cellulose acetate that have brand names of Clarifoil, Dexel and Tenite Acetate. However, cellulose acetate can be regenerated, for example by heating to specific temperatures, to give the acetate slightly different properties. Examples of brand names for regenerated cellulose acetates are CELLOPHANE (RTM) and Rayophane . Cellulose acetate is regenerated using a xanthate process to manufacture fibres, commonly called rayon or viscose, and film such as CELLOPHANE (RTM) . The films are often plasticised using glycols and water to reduce the brittleness thereof.

Referring again to Fig. 1, when a wet stream of hydrogen gas (H₂ + H₂0) is passed over the membrane 12, the water vapour H₂0 permeates through the membrane 12 at a faster rate than the hydrogen H₂. Referring to Fig. 1, this is shown schematically as water vapour H₂0 14 passing through the membrane 12, whereas hydrogen gas H₂ 16 does not permeate through the membrane 12. The hydrogen gas H₂ 16 can therefore be channelled between the membrane 12 and a surface (not shown) to produce a stream of dehydrated hydrogen gas H₂ 18 (or "dry" hydrogen) .

In summary, a wet hydrogen gas 10 is passed over the membrane 12 whereby water vapour 14, having a different molecular structure than hydrogen gas 16, permeates through the membrane 12 faster than the hydrogen 16, producing a dry hydrogen gas 18.

A sweep gas 20, such as nitrogen or air, can be used to remove the water vapour H₂0 14 that has permeated through the membrane 12 from the other side of membrane 12. Alternatively, a vacuum pump (not shown) may be used to remove the water vapour from the permeate side of the membrane 12.

Referring to Fig. 2, there is shown a schematic drawing of a hydrogen dryer 50. The dryer 50 includes a body 52 that is typically of stainless steel. The dryer 50 is divided into first and second chambers 54, 56 by a cellulose acetate membrane 58. Membrane 58 is similar to membrane 12 in Fig. 1 and is used to perform the same function.

The dryer 50 is preferably leak tight to hydrogen whereby the two chambers 54, 56 are isolated from one another.

The dryer 50 includes a first inlet 60 through which the wet hydrogen gas (H₂ + H₂0) is passed into the dryer 50. The first inlet 52 typically comprises an aperture that is drilled in the side of the body 52. However, it should be noted that the body 52 and the first inlet 60 may be of any suitable configuration.

The dryer 50 also includes a first outlet 62 that is typically provided on the opposite side of body 52 from the first inlet 60. The first outlet 62 typically comprises an aperture that is drilled in the side of the body 52, and provides an outlet from the body 52 for the dry hydrogen gas.

A second inlet 64 and second outlet 66 are optionally provided in body 52. Second inlet 64 and second outlet 66 typically comprise apertures drilled in the side of body 52, and provide an inlet and an outlet, respectively, through which a sweep gas, such as nitrogen or air, can be passed to collect the water vapour in second chamber 56. Use of a sweep gas is optional, but as it removes the water vapour from the dryer 50, it provides a more efficient dryer 50. This is because the water vapour that collects in the second chamber 56 is removed continuously, thus allowing more vapour to permeate the membrane 58.

In use, wet hydrogen gas is supplied to the inlet 60. A sweep gas such as nitrogen or air is supplied between the second inlet 64 and the second outlet 66 to remove water vapour that collects on a permeate side of the membrane 58 (i.e. in second chamber 56) . The water vapour in the wet hydrogen gas permeates through the cellulose acetate membrane 58 leaving dry hydrogen to be collected at outlet 62.

Referring to Fig. 5, there is shown an alternative embodiment of a hydrogen dryer, generally designated 150. Dryer 100 is substantially a plurality of dryers 50a to 50f stacked one above the other to form a second embodiment of a hydrogen dryer 150. A plurality of the dryers 50a to 50f are provided (six shown in Fig. 5) in series, with the output 62a of the first dryer 50a being coupled to the outlet 62b of the next dryer 50b. The inlet 60b of the second dryer 50b is coupled to the inlet 60c of the third dryer 60c and so on. The couplings between the inlets 60 and outlets 62 may be made using suitable conduits, or may be integral within a frame or body 102. In use, the wet hydrogen gas enters dryer 150 at inlet 60a of dryer 50a and flows through the dryer 150 in a criss-cross pattern, as shown by arrows 112. The wet hydrogen gas is thus passed over the six membranes 58a to 58f in sequence, thus resulting in a dryer hydrogen gas exiting the dryer 150 at inlet 60f of the uppermost dryer 50f.

The second inlets 64 and second outlets 66 for the sweep gas are also coupled in a similar manner to the inlets 60 and outlets 62. It should be noted that the sweep gas flows through the dryer 150 in the opposite direction to the wet hydrogen gas. This is termed counter-current flow and results in a more efficient dryer. The sweep gas may flow through the dryer 150 in the same direction as the wet hydrogen gas (termed co-current flow) , but this has been found to be less efficient in practice. With reference to Fig. 5, the sweep gas enters dryer 150 at second inlet 64f of dryer 50f and exits at second outlet 66f. Second outlet 66f is coupled to second outlet 66e of dryer 50e, and second inlet 66e is coupled to second inlet 66d of dryer 50d and so on. The sweep gas with the collected water vapour exits the dryer 150 at second inlet 64a of dryer 50a. The sweep gas thus flows in a criss-cross pattern in the direction of arrows 114.

As a further alternative, the membranes 58 within the dryer 50 (Fig. 2) may be stacked vertically and the wet hydrogen gas fed in at the inlet 60, the gas following a criss-cross path through the stacked membranes 58. In addition, the arrangement/configuration of the dryer 50, 150 can be varied, e.g. the path that the hydrogen follows through the dryer 50, 150 can be varied, e.g. by convoluting the pathway.

It should be noted that conventional hydrogen generators, such as SPE generators, lose pressure during the dehydrating process. That is, if the wet hydrogen gas is supplied at a first pressure, the dry hydrogen gas is dispensed at a second lower pressure. However, with the dryer 50, 150, there is no noticeable reduction in pressure during the dehydrating process. This is advantageous, particularly as the supply of hydrogen gas is often required at high pressures.

The conventional method for detecting how dry a gas has become is by measuring the temperature at which the water vapour in the gas condenses when it is cooled. This temperature is generally referred to as the dew point of the gas. A relationship exists between the dew point and the dryness of the gas wherein the lower the temperature to which the gas has to be cooled to condense the water vapour (ie the lower the dew point) then the dryer the gas is. A dew point meter can be used to measure this temperature, usually in units of degrees centigrade (°C) . Conversion tables are typically used to convert the dew point of gas in degrees centigrade to the concentration of water vapour in the gas in parts per million (ppm) .

Fig. 3 is a graph of dew point against time illustrating the differences between various methods and apparatus for dehydrating hydrogen gas.

Six membrane dryers 50 (Fig. 2) were used, the dryers 50a to 50f being coupled in series, similar to dryer 150 shown in Fig. 5. A varying flow rate of the (nitrogen) sweep gas was used to determine the performance of the dryer 50, 150 and to obtain the optimum value for the sweep gas flow rate. To aid the comparison, a conventional silica gel and NAFION (RTM) tube (apparatus not shown) were also used.

As can be seen from Fig. 3, the best result was obtained using the membrane dryer 150 with a nitrogen sweep gas flowing at 400 cubic centimetres per minute (cc/min) , the sweep gas being unsplit, that is that the sweep gas was fed into the series of dryers 50a to 50f (e.g. dryer 150) and allowed to flow therethrough. It should be noted that only a single point 100 for this test is shown in Fig. 3. After 15 hours, the dew point fell to -45.6 °C indicating that this gave the driest hydrogen output. A dew point of -45.6 °C corresponds to approximately 105 parts per million (ppm) water vapour in hydrogen by volume, and approximately 945 ppm water vapour in hydrogen by weight. The six membrane dryers 50a to 50f (e.g. dryer 150) were also used with a nitrogen sweep gas flowing at 400 cc/min, which is represented in Fig. 3 as trace 102. The flow of nitrogen sweep gas was spilt three ways, that is three different streams of nitrogen gas were used; one stream was passed through the first two dryers 50e, 50f, the second was passed through the second two dryers 50c, 50d; and the third stream was passed through the last two dryers 50a, 50b in series. As shown in Fig. 3, the dew point for trace 102 fell to -40.82 °C after 13 hours. A dew point of -40.82 °C corresponds to approximately 188 ppm water vapour in hydrogen by volume, and approximately 1691 ppm water vapour in hydrogen by weight .

Comparable with this was the use of the six dryers 50a to 50f (e.g. dryer 150) with an unsplit nitrogen sweep gas flowing at 600 cc/min, represented in Fig. 3 as trace 104. The dew point for trace 104 fell to -40.87 °C after 13 hours. A dew point of -40.87 °C corresponds to approximately 171 ppm water vapour in hydrogen by volume, and approximately 1550 ppm water vapour in hydrogen by weight.

Trace 106 in Fig. 3 represents a similar test using the conventional silica gel and NAFION (RTM) tube. It should be noted that a nitrogen sweep gas is not used in the conventional method using silica gel and the NAFION (RTM) tube. As shown in Fig. 3, the dew point of the hydrogen gas after being processed using silica gel and the NAFION (RTM) tube was -36.53 °C after 13 hours. A dew point of -36.53 °C corresponds to approximately 270 ppm water vapour in hydrogen by volume, and approximately 2410 ppm water vapour in hydrogen by weight. Thus, use of the six hydrogen dryers 50a to 50f (e.g. dryer 150) with sweep gas flow rates in excess of 400 cc/min result in a dryer hydrogen gas output than the conventional method.

Trace 108 in Fig. 3 represents the use of the six hydrogen dryers 50a to 50f (e.g. dryer 150) with a nitrogen sweep gas flowing at 300 cc/min, split three ways. After 13 hours, the dew point of the output hydrogen gas fell to -34.55 °C, which is comparative with conventional silica gel method. A dew point of -34.55 °C corresponds to approximately 320 ppm water vapour in hydrogen by volume, and approximately 2900 ppm water vapour in hydrogen by weight. This indicates that reducing the flow of the nitrogen sweep gas has an adverse effect on the dew point. That is, the dew point increases as the flow rate of the sweep gas decreases (ie they are inversely proportional) .

Referring to Fig. 4, there is shown a graph of flow rate in cc/min (left hand y-axis) and dew point in °C (right hand y-axis) against time in minutes (x-axis) for a regenerated cellulose acetate membrane used in a hydrogen dryer, similar to that shown in Fig. 2. It should be noted that the x-axis is not to scale. Experiments were carried out on the membrane to determine what effect varying the wet hydrogen flow rate and/or the flow rate of the nitrogen sweep gas had on the dew point .

Referring to Fig. 4, trace 120 depicts the change in dew point over a nine hour period in °C; trace 122 depicts the change in wet hydrogen flow rate over the nine hour period in cc/min; and trace 124 depicts the change in nitrogen sweep gas flow rate over the nine hour period in cc/min.

In the initial three hour period, no nitrogen sweep gas was used to remove the water vapour, and the hydrogen flow rate was reduced from approximately 145 cc/min to approximately 35 cc/min. In this period, the dew point fell from 0°C to approximately -4.5°C after half an hour, at which point the hydrogen flow rate was decreased. The dew point then remained substantially constant at approximately -3.5°C over a period of two hours despite the reduction in hydrogen flow rate. This suggests that the rate of flow of wet hydrogen has very little effect on the dew point of the dry hydrogen.

After three hours, a flow rate of nitrogen sweep gas was introduced which was gradually increased to approximately 50 cc/min over a period of one hour. As can be seen from trace 120, the dew point in this period fell to less than -6°C. This suggests that a flow of nitrogen sweep gas aids in dehydrating hydrogen, as the nitrogen dilutes the water vapour, increasing the driving force of its permeation and thus reduces the dew point .

From this, it is preferable to use a sweep gas to remove the water vapour from the permeate side (ie in second chamber 56) of the membrane 58 (or some other method of removing the water vapour, eg a vacuum pump) , as this prevents the second chamber 56 from becoming saturated during use .

It should be noted that the membrane 58 is not consumable and does not require to be replaced periodically, unlike the silica gel in conventional SPE hydrogen generators.

A number of experiments were carried out using the dryer 50 (Fig. 2) , the experiments involving variations in the hydrogen input pressure, flow rate of the hydrogen and sweep gases and the membrane area. These experiments were carried out to determine what affect varying these parameters would have on the performance of the dryer 50.

The results of the experiments showed that under similar conditions of hydrogen input pressure and flow rate, the hydrogen dryer 50 significantly out- performed conventional dryer systems wherein hydrogen was produced that was upto three times dryer using the dryer 50 with only 39 ppm water vapour in the gas stream compared with 121 ppm using conventional systems. With gas chromatography being extensively used worldwide, this is advantageous.

Thus, there is provided a method and apparatus for dehydrating wet hydrogen. Certain embodiments of the apparatus incorporate a cellulose acetate membrane, that is preferably a regenerated cellulose acetate, that allows water vapour in the wet hydrogen to permeate, whilst hydrogen gas will not. Certain embodiments of the apparatus do not affect the pressure of the hydrogen gas supply and thus if wet hydrogen is supplied to the apparatus at 7 bar, then dry hydrogen can be supplied also at 7 bar. Furthermore, the cellulose acetate is not consumable, unlike conventional hydrogen generators that have silica gel. The dryer, in certain embodiments, is relatively inexpensive to manufacture and has no components that are consumable (i.e. no components require to be replaced) . The dryer, in certain embodiments, is thus ideal for use in applications where dry hydrogen is continuously required on-demand

Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.

Claims

CLAIMS 1. A gas dryer comprising at least one inlet for gas, at least one outlet for gas, and at least one cellulose acetate membrane, the membrane being capable of allowing water vapour to pass therethrough whilst restricting passage of gas.

2. A gas dryer according to claim 1, wherein the at least one inlet comprises a wet gas inlet, and the at least one outlet comprises a dehydrated gas outlet.

3. A gas dryer according to either preceding claim, wherein the membrane is located at least partially between the inlet and the outlet .

4. A gas dryer according to any preceding claim, wherein the dryer includes a second inlet and a second outlet.

5. A gas dryer according to claim 4, wherein the second inlet and second outlet allow a sweep gas to flow through the dryer.

6. A gas dryer according to claim 5, wherein the sweep gas removes water vapour from the dryer once the vapour has passed through the membrane.

7. A gas dryer according to claim 5 or claim 6, wherein the flow rate of the sweep gas can be varied.

8. A gas dryer according to claim 7, wherein increasing the flow rate of the sweep gas increases the extent of dehydration of the gas.

9. A gas dryer according to claim 4, wherein a vacuum is used to remove the water vapour.

10. A gas dryer according to claim 9, wherein a vacuum pump is coupled to the second outlet.

11. A gas dryer according to any one of claims 4 to 10, wherein the dryer includes a first chamber and a second chamber, the first and second chambers being divided by the membrane .

12. A gas dryer according to claim 11, wherein the first chamber includes the inlet and outlet.

13. A gas dryer according to claim 11 or claim 12, wherein the second chamber includes the second inlet and second outlet.

14. A gas dryer according to any preceding claim, wherein the gas is passed over one or more membranes.

15. A gas dryer according to any preceding claim, wherein the membranes are located in a stack or baffle.

16. A gas dryer according to any one of claims 4 to 15, wherein a plurality of gas dryers are provided, wherein the inlets and outlets for the gas, and the second inlets and second outlets for the sweep gas are coupled so that the gas and/or the sweep gas flow in a criss-cross pattern through the dryers.

17. A gas dryer according to claim 16, wherein the gas flows in a first direction, and the sweep gas flows in a second direction, wherein the second direction is opposite to the first direction.

18. A gas dryer according to any preceding claim, wherein the dryer is manufactured from a stainless material .

19. A gas dryer according to any preceding claim, wherein the membrane comprises a regenerated cellulose acetate membrane.

20. A gas dryer according to claim 19, wherein the membrane is regenerated by a xanthate process.

21. A gas dryer according to any preceding claim, wherein the gas is hydrogen.

22. A gas dryer according to any preceding claim, wherein the sweep gas is nitrogen.

23. A gas dryer according to any preceding claim, wherein the sweep gas is air.

24. A method of dehydrating gas, the method comprising the steps of providing at least one cellulose acetate membrane, passing gas over the cellulose acetate membrane, wherein water vapour contained within the gas passes through the membrane.

25. A method according to claim 24, wherein the least one membrane is provided in a gas dryer, the dryer comprising at least one inlet for the gas and at least one outlet for the gas, the membrane being located at least partially between the inlet and the outlet.

26. A method according to claim 25, wherein the dryer further includes a second inlet and a second outlet.

27. A method according to claim 26, wherein the method includes the additional step of passing a sweep gas from the second inlet to the second outlet to remove water vapour.