GB2042365A

GB2042365A - Gas separation

Info

Publication number: GB2042365A
Application number: GB7907721A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1979-03-05
Filing date: 1979-03-05
Publication date: 1980-09-24
Also published as: ZA801054B; JPS5645724A; GB2042365B; DE3007427A1

Abstract

A pressure swing adsorption (PSA) process for the separation of a component from a gas mixture also containing water vapour, eg the separation of nitrogen from air, employs adsorbers 1 and 2 each having a first stage 3, 3' containing a desiccant adsorbent and a second stage 4, 4' containing an adsorbent which preferentially adsorbs another component of the mixture eg a carbon molecular sieve which adsorbs oxygen more rapidly than nitrogen. Each adsorber is put through a cycle comprising an adsorption step during which the mixture is passed through the adsorber by a compressor 5 and unadsorbed product is withdrawn through a delivery line 11, and a desorption step during which the adsorber is vented to atmosphere through a waste gas line 9. The pressure reached in the adsorber during such venting is the lowest reached during the entire cycle, no vacuum pump being employed. A valved line 12 allows pressure equalisation between the adsorbers. <IMAGE>

Description

SPECIFICATION Gas separation The present invention relates to a method of gas separation by pressure swing adsorption (PSA) techniques, and is particularly, though not exclusively, concerned with a PSA process for the separation of nitrogen from air.

One known PSA process for the separation of nitrogen from air employs a molecular sieve carbon adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its more rapid adsorption of oxygen than of nitrogen. In operation a bed of this adsorbent is put through a cycle which includes an adsorption step during which time air is pumped through the bed, most of the oxygen and a proportion of the nitrogen plus substantially all of the carbon dioxide and water vapour in the feedstock are adsorbed, and a nitrogen rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and then evacuated through its inlet, so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step. In practice to adsorbent beds are employed and operated on similar cycles but sequenced to be out of phase with one another by 1 80', so that when one bed is on its adsorption step the other bed is on its desorption step, and vice versa.Furthermore it is usual to equalise the pressures in the two beds between each step by connecting the two bed inlets together and connecting the two bed outlets together; with these connections made the gas within the void spaces of the bed which has just completed its adsorption step is sucked into the bed which has just completed its desorption step by virtue of the pressure difference which exists between the beds at that stage, and this is found to be beneficial in maximising the product output because such void space gas will have already become somewhat enriched in nitrogen.

To effect the feed of air through the adsorbent beds a plant operating this process must include a compressor, and to evacuate the beds during desorption a vacuum pump is also included. Typically the bed pressure during adsorption rises sto a maximum value of 7 to 10 bar g (although lower adsorption pressures are sometimes employed), with the vacuum reached during desorption typically being in the region of 100 Torr. Plants operating the process as described above can readily achieve a nitrogen product purity in excess of 99% by volume.

Now, in order to reduce the capital and running costs of such a plant it has been proposed to eliminate the vacuum pump, so that during desorption the beds are vented to atmospheric pressure but are not reduced to a pressure below atmospheric. Clearly if costs are reduced in this manner a penalty will be paid in that, for the same product purity and all other factors being equal, the plant will not be able to process the same volume of feedstock as a plant operating with a vacuum pump and thereby providing a greater overall pressure swing. Nevertheless, if the after-mentioned problem can be overcome such operation may make economic sense in many instances, and the operator of a plant initially assembled without a vacuum pump is given the flexibility to up-rate the performance of his plant at a later date if circumstances should so require, simply by installing a vacuum pump.

The problem which has been found in operating the process without a vacuum pump is that, over a period, a gradual decrease in the product output at a given purity is observed, in one experiment the product flow rate falling by some 40% over a period of six hundred hours. From a consideration of the water adsorption isotherm of the molecular sieve carbon adsorbent we postulate that the reason for this decreasing performance is that, without vacuum, a proportion of the water vapour adsorbed during the adsorption step remains on the adsorbent after the desorption step, so that as the cycle is repeated moisture gradually builds up in the adsorbent bed thereby decreasing its capacity to effect the desired oxygen/nitrogen separation.

It is an aim of the invention to overcome this problem and, while the invention is principally concerned with the separation of nitrogen from air using a molecular sieve carbon adsorbent the invention may also be applicable to other PSA separations where similar problems arise. As an example there can be quoted the separation of hydrogen from a mixture with methane using the same carbon adsorbent and where moisture is also present in the feedstock.

In a broad aspect the invention accordingly resides in a process for the separation of a desired gaseous component from a gaseous mixture including the desired component, at least one other component and water vapour, which process comprises repeating a cycle of operation including successive adsorption and desorption steps, the adsorption step comprising passing the gaseous mixture at a superatmospheric pressure through an adsorber having a first stage containing a desiccant adsorbent and a second stage containing an adsorbent which adsorbs said other component more readily than said desired component, so as to obtain from the adsorber a product gas enriched in said desired component and depleted in said other component and water vapour; and the desorption step comprising venting the adsorber to a pressure not substantially below atmospheric pressure in a direction counter to the direction of passing the gaseous mixture, such pressure being the minimum pressure obtained in the adsorber during the cycle.

In a more specific aspect the invention resides in a process for the separation of nitrogen from a gaseous mixture including nitrogen, oxygen and water vapour, which process comprises repeating a cycle of operation including successive adsorption and desorption steps, the adsorption step comprising passing the gaseous mixture at a superatmospheric pressure through an adsorber having a first stage containing a desiccant adsorbent and a second stage containing a molecular sieve carbon adsorbent which adsorbs oxygen more rapidly than nitrogen, so as to obtain from the adsorber a product gas enriched in nitrogen and depleted in oxygen and water vapour; and the desorption step comprising venting the adsorber to a pressure not substantially below atmospheric pressure in a direction counter to the direction of passing the gaseous mixture, such pressure being the minimum pressure obtained in the adsorber during the cycle.

Preferred desiccant adsorbents are alumina and silica gel.

The invention also resides in apparatus adapted to perform the above-defined processes.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1A is a schematic diagram of a plant for the separation of nitrogen from air in accordance with the invention; Figure 1B indicates the sequencing of the operating cycles for the two adsorbers of Fig.

1A; Figure 2A is a schematic diagram of a modification of the plant illustrated in Fig. 1A; and Figure 2B indicates the sequencing of the operating cycles for the two adsorbers of Fig.

2A.

Referring to Fig. 1A the illustrated plant comprises two adsorbers 1 and 2. Each adsorber comprises a single vertical column having a first (lower) stage 3, 3' packed with a layer of desiccant adsorbent such as alumina or silica gel, and a second (upper) stage 4, 4' packed with a layer of a molecular sieve carbon adsorbent which adsorbs oxygen more rapidly than nitrogen. Air is delivered by a compressor 5 into an inlet line 6 which branches in to two lines leading to the lower (inlet) ends of the adsorbers and controlled respectively by valves 7 and 7'. Two further lines controlled respectively by valves 8 and 8' are connected to the lower ends of the adsorbers and these join together in a waste gas outlet line 9. Leading from the upper (outlet) ends of the adsorbers are lines controlled respectively by vases 10 and 10' which join together in a product gas delivery line 11.A further line controlled by a valve 12 is provided whereby the outlet ends of the adsorbers can be connected together.

Considering an operating cycle for adsorber 1, valves 7 and 10 are opened and air is fed by compressor 5 through line 6, into the adsorber and through the two adsorbent beds 3 and 4. In passing through the desiccant bed 3 any water vapour in the incoming air is adsorbed and in passing through the carbon bed 4 a major proportion of the oxygen, a proportion of the nitrogen, and the carbon dioxide in the incoming air are all adsorbed, so that a product gas stream of typically 99% pure nitrogen exits through the outlet of the adsorber and into the delivery line 11.

After a predetermined interval valves 7 and 10 are closed and valve 12 is opened to equalize the pressures in the two adsorbers 1 and 2. At this stage the pressure in adsorber 1 will have reached its maximum value of say 7 bar g, and the pressure in adsorber 2 (which will have just completed its desorptionstep) will be at its lowest value of atmospheric pressure. Consequently the gas in the void spaces towards the outlet end of adsorber 1, which will be partially enriched in nitrogen and depleted in the other atmospheric gases, is sucked into the outlet end of adsorber 2 where it is further purified by the carbon bed 4' during the subsequent adsorption step of adsorber 2.

Valve 12 is then closed and valve 8 is opened to vent adsorber 1 to atmospheric pressure through outlet line 9. During this time the gases adsorbed on beds 3 and 4 are desorbed and flow out through the inlet end of the adsorber, i.e. in a direction counter to the direction of passing the air through the adsorber during the adsorption step. As the oxygen and other gas is desorbed from bed 4 it washes back through bed 3 to ensure that water vapour is removed from that bed in an amount equal to the amount deposited during the preceding adsorption step.

After a predetermined interval equal to the time for which valves 7 and 10 were open during the adsorption step, valve 8 is closed and valve 12 is reopened to once again equalize the pressures within the adsorbers 1 and 2. On this occasion, of course, adsorber 1 has just completed its desorption step and adsorber 2 has just completed its adsorption step and so the flow of void space gas will be from adsorber 2 to adsorber 1.

Valve 12 is then closed and valves 7 and 10 reopened and the whole cycle is repeated.

It will be appreciated that at the same time, by appropriate control of the valves designated by primed reference numerals, adsorber 2 will be put through a similar operating cycle to adsorber 1 but sequenced to be out of phase therewith by 1 80'.

It will be further appreciated that the carbon beds 4 and 4' of the illustrated plant operate to effect a nitrogen/oxygen separation in the same way as the beds of a plant operating the known process previously described, but that no vacuum pump is included in the plant thereby reducing both its capital and running costs. Furthermore, the incorporation of desiccant beds 3 and 3' which effectively function as heatless driers upstream of the main carbon beds ensures that there can be no undesirable build up of moisture in the adsorbers.

Turning now to Fig. 2A this shows a modification of the Fig. 1A plant to provide an additional purging operation during the desorption steps of each adsorber. To this end a purging line 13 including a throttle valve 1 3A branches from the product delivery line 11 and in turn branches into two lines leading to the outlet ends of the adsorbers 1 and 2 controlled respectively by valves 14 and 14'.

At a predetermined interval after the opening of the valve 8 or 8' during the desorption step of the appropriate adsorber the valve 14 or 14' is also opened and remains open with valve 8 or 8' until the end of the desorption step. During this time a proportion of the product gas from line 11 is diverted through line 13 (the proportion being determined by the setting of throttle valve 1 3A) and flows downwardly through the appropriate adsorber thereby purging out the void space gas from each of the adsorber stages and replacing such gas with product-quality gas in preparation for the following adsorption step. Such a purging step may be especially valuable in an embodiment having the same adsorbents as described above but used for separating hydrogen from a mixture with methane and water vapour.

As a further variation of either plant indicated in broken line in Fig. 2A an additional line controlled by a valve 15 may be provided whereby the inlet ends of the adsorbers can be connected together during the pressure equalization steps.

Claims

1. A process for the separation of a desired gaseous component from a gaseous mixture including the desired component, at least one other component and water vapour, which process comprises repeating a cycle of operation including successive adsorption and desorption steps, the adsorption step comprising passing the gaseous mixture at a superatmospheric pressure through an adsorber having a first stage containing a desiccant adsorbent and a second stage containing an adsorbent which adsorbs said other component more readily than said desired component, so as to obtain from the adsorber a product gas enriched in said desired component and depleted in said other component and water vapour; and the desorption step comprising venting the adsorber to a pressure not substantially below atmospheric pressure in a direction counter to the direction of passing the gaseous mixture, such pressure being the minimum pressure obtained in the adsorber during the cycle.

2. A process for the separation of nitrogen from a gaseous mixture including nitrogen, oxygen and water vapour, which process comprises repeating a cycle of operation including successive adsorption and desorption steps, the adsorption step comprising passing the gaseous mixture at a superatmospheric pressure through an adsorber having a first stage containing a desiccant adsorbent and a second stage containing a molecular sieve carbon adsorbent which adsorbs oxygen more rapidly than nitrogen, so as to obtain from the adsorber a product gas enriched in nitrogen and depleted in oxygen and water vapour; and the desorption step comprising venting the adsorber to a pressure not substantially below atmospheric pressure in a direction counter to the direction of passing the gaseous mixture, such pressure being the minimum pressure obtained in the adsorber during the cycle.

3. A process according to claim 1 or claim 2 wherein said desiccant adsorbent is alumina or silica gel.

4. A process according to any preceding claim wherein the desorption step includes, subsequent to venting the adsorber, purging the adsorber with a stream of said product gas in a direction counter to the directiori of passing the gaseous mixture.

5. A process according to any preceding claim wherein the maximum pressure reached within the adsorber as a result of the adsorption step is not above 10 bar g

6. A process according to any preceding claim employing two such adsorbers and wherein each undergoes a similar such cycle of operation but sequenced to be out of phase with one another by 1 80' so that when one adsorber is on its adsorption step the other adsorber is on its desorption step, and vice versa.

7. A process according to claim 6 which includes the step of equalizing the pressures within the two adsorbers between each adsorption and desorption step.

8. A process according to claim 7 wherein said pressure equalization is achieved by connecting together the outlets of the adsorbers.

9. A process according to claim 7 wherein said pressure equalization is achieved by connecting together the outlets of the adsorbers and connecting together the inlets of the adsorbers.

10. A process for the separation of a desired gaseous component from a gaseous mixture including the desired component, at least one other component and water vapour, substantially as hereinbefore described with reference to Figs. 1 A and 1 B or Figs. 2A and 2B of the accompanying drawings.

11. Apparatus for use in the separation of a desired gaseous component from a gaseous mixture including the desired component, at least one other component and water vapour, comprising an adsorber having a first stage containing a desiccant adsorbent and a second stage containing an adsorbent which adsorbs said other component more readily than said desired component; a compressor adapted to pass the gaseous mixture through the adsorber in a given direction; a line for venting the adsorber in a direction counter to the first-mentioned direction; and control means whereby the apparatus is adapted to perform the process defined in claim 1.

12. Apparatus for use in the separation of nitrogen from a gaseous mixture including nitrogen, oxygen and water vapour, comprising an adsorber having a first stage containing a desiccant adsorbent and a second stage containing a molecular sieve carbon adsorbent which adsorbs oxygen more rapidly than nitrogen; a compressor adapted to pass the gaseous mixture through the adsorber in a given direction; a line for venting the adsorber in a direction counter to the first-mentioned direction; and control means whereby the apparatus is adapted to perform the process defined in claim 2.

13. Apparatus according to claim 11 adapted to perform any of the processes defined in any one of claims 3 to 9 when appended to claim 1.

14. Apparatus according to claim 12 adapted to perform any of the processes defined in any one of claims 3 to 9 when appended to claim 2.

15. Apparatus for use in the separation of a desired gaseous component from a gaseous mixture including the desired component, at least one other component and water vapour, substantially as hereinbefore described with reference to Figs. 1 A and 1 B or Figs. 2A and 2B of the accompanying drawings.