GB2171927A - Method and apparatus for separating a gaseous mixture - Google Patents

Abstract

During a pressure swing adsorption (PSA) process, the pressure in the adsorption vessels 2 and 4 is reduced causing generally adiabatic expansion of the gas therein and a resultant temperature reduction. This temperature reduction is utilized by heat exchanging in a heat exchanger 20 the vent gas stream 34 from the PSA process with an incoming compressed gas stream 14 for separation so as to reduce the temperature of the incoming stream and thus facilitate the condensation of readily condensible vapour (e.g. water vapour) therefrom. The condensate is removed in filters 22 and 24. If desired, water can be introduced into the heat exchanger 20 through pipe 46 and caused to evaporate therein to give an enhanced cooling of the incoming gas stream. The invention reduces the likelihood of water condensing in the gas stream downstream of the filters 22 and 24 and thereby gives added protection to molecular sieve adsorbent in the beds 6 and 8. <IMAGE>

Description

SPECIFICATION Method and apparatus for separating a gaseous mixture This invention relates to a method and apparatus for separating a gaseous mixture. It is particularly concerned with gas separation by pressure swing adsorption techniques.

One known pressure swing adsorption process for the separation of nitrogen from air employs a molecular sieve in the form of a carbon adsorbent which has the ability to effect a separation of oxy gen from nitrogen by virtue of its more rapid ad sorption of oxygen than of nitrogen. In operation, a bed of this adsorbent is put through a cycle which icludes 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 prod uct gas is supplied through the outlet of the bed;; and a desorption step during which time the outlet of the bed is closed, and the bed is vented to at mospheric pressure through its inlet so that the adsorbed gas is substantially removed from the bed thereby preparing it for the next adsorption step. In practice, two adsorbent beds are employed and operated on similar cycles but sequenced to be be out of phase by one another by 1800 so that when one bed is on its adsorption step the other bed is on its desorption step, and vica versa. Fur thermore, 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 beds 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 re mains somewhat enriched in nitrogen.

To effect the feed of air through the adsorbent beds a plant operating this process will normally include a compressor. Typically the bed pressure during adsorption rises to a maximum value of 7 to 10 bar g (although sometimes lower adsorption pressures are employed). Before introducing the incoming air into the adsorbent beds, it is desira ble to remove from the air any liquid water, oil va pour and solid particulates that are entrained therein. For this purpose, two filters having means defining surfaces on which liquid water can be de posited are employed. Moreover, each bed prefera bly comprises a lower layer of a desciccant that removes water vapour from the air and an upper layer of adsorbent whose adsorption of oxygen is more rapid than that of nitrogen.Nonetheless, the air leaving the compressor is typically saturated with water vapour and is normally warmer than ambient. Consequently, as its temperature falls, water vapour is continually condensed from the air and under severe conditions some entrained droplets could reach the desiccant layers in the beds.

It is an aim of the present invention to provide a method and apparatus for the separation of gas mixtures including water vapour by pressure swing adsorption in which special means are provided to reduce the amount of water vapour or other readily condensible vapour in the gas entering the adsorbent bed.

According to the present invention, there is provided a gas separation process comprising repeating a cycle of operations including an adsorption step in which a gaseous mixture including water vapour, or other readily condensible vapour a product and at least one other gas is contacted with at least one adsorbent whereby the mixture is enriched in said product gas, and a desorption step in which the adsorbent is regenerated by being subjected to a reduction in pressure to produce a gas stream including desorbed gas, wherein upstream of the adsorbent said gaseous mixture is subjected to means for removing liquid droplets therefrom, the gaseous mixture being heat exchanged with said gas stream upstream of said means for removing liquid.

For performing this method, the invention also provides apparatus comprising at least one vessel for adsorbent; said vessel having a valved inlet and a valved outlet; a gas supply passage communicating with said inlet; a product withdrawal passage and a passage for a gas stream including desorbed gas, each able selectively to be placed in communication with the valve outlet, means for removing liquid droplets from the gaseous mixture to be separated, said means being located in the gas supply passage, and a heat exchanger effective to place said gas supply passage in heat exchange relationship with the said gas stream passage upstream of said liquid droplet removal means.

The pressure reduction step causes an expansion of the gas in the void spaces of the bed and in consequence there is a degree of substantially adiabatic cooling which effects a temperature reduction in the gas in the void spaces. Thus, the temperature of the vented gas stream is appreciably less than the temperature of the gaseous mixture for separation entering the adsorbent bed.

Thus, heat exchange of the vented gas stream with the incoming gas for separation will effect a reduction in the temperature of the latter. Where the incoming gas mixture for separation is saturated in water vapour, this temperature reduction will cause some of the water vapour to condense out.

The condensate may typically be removed in the filters which also remove solid particulate material from the incoming gas. Accordingly, the gas that enters the adsorbent beds has a reduced concentration of water vapour and, because the desorbed gas may be cooler than ambient, no further condensation will take place in the gaseous mixture for separation leaving it free of liquid droplets.

Preferably, a liquid is caused to evaporate in the vented gas stream so as to effect a further reduction in the temperature of such stream. Typically, therefore, the heat exchanger is of the kind in which the liquid can be evaporated. Typically, the liquid is water and it is introduced at a low flow rate or intermittently into the heat exchanger. By this means, it is possible to reduce the temperature of the incoming gas stream to a value approaching from above that of the freezing point of water (0 C). Not only does this offer the advantage of a greater removal of water vapour upstream of the adsorbent beds but also it will reduce the effective operating temperature of the adsorbent beds and thereby increase the adsorptive. capacity of these beds.

The method and apparatus according to the invention are particularly suited for use in the separation of air, and more particularly if the bed includes carbon molecular sieve. The method and apparatus according to the present invention may however be used to separate other gases than air.

Moreover, molecular sieves other than of the carbon kind may be employed in the method and apparatus and according to the present invention.

The method and apparatus according to the present invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram of the separation of nitrogen from air in accordance with the invention.

Referring to the drawing, the illustrated plant includes two adsorbers 2 and 4, each adsorber comprises a single vertical column having a lower stage 6 packed with a layer of a desiccant such as alumina or silica gel, and an upper stage 8 packed with a layer of a molecular sieve carbon adsorbent which adsorbs oxygen more rapidly than nitrogen.

Air is delivered by a compressor 10 (having an aftercooler 12) into a gas supply passage 14. Located in the gas supply passage 14 is a buffer tank 16 whose purpose will be described below. The outlet from the buffer tank 16 is controlled by pressure regulator 18 disposed in the passage 14. Downstream of the regulator 18 is a heat exchanger 20 through which the passage 14 extends. Two filters 22 and 24 of a conventional kind and effective to remove most of the particulate solids, oil vapour and liquid water carried in the incoming air are located in the passage 14 downstream of the heat exchanger 20. The gas supply passage 14 branches into two pipes leading through the lower, that is inlet, ends of the adsorbers2 and 4 and controlled respectively by valves 26 and 28.Two further pipes controlled respectively by valves 30 and 32 are connected to the lower ends of the adsorbers and these join together in a waste or vent gas outlet pipe 34. Leading from the upper, that is outlet, ends of the adsorbers 2 and 4 are pipes controlled respectively by valves 36 and 38 which join together in a product gas delivery pipe 40. A further pipe controlled by a valve 42 is provided whereby the outlet ends of the adsorbers can be connected together and a similar pipe controlled by a valve 44 is also provided whereby the inlet ends of the adsorbers can be connected together.

In operation, air is compressed to a suitable pressure of, say, 8 bar g in the compressor 10 and is then cooled to a temperature near to but typically above ambient in the aftercooler 12. The compressed air flows from the buffer tank 16 through the heat exchanger 20 where it is cooled typically to a temperature below 10 C. Depending on the prevailing climatic conditions, the incoming air will contain a greater or a lesser quantity of water vapour. Typically, however, the reduction in temperature from above ambient to the temperature in the order of 10 C will cause some of the water vapour in the air to condense. The resultant condensed water vapour together with any other liquid water that was present in the air is removed from the incoming air with particulates by means of the filters 22 and 24.The filtered air is then subjected.to separation in the adsorbers.

Considering an operating cycle for the adsorber 1, valves 26 and 36 are opened and air passes through the passage 14 into the adsorber and through the two beds 6 and 8. (Valve 32 is also open during this part of the cycle). In passing through the desiccant bed 6 any remaining water vapour in the incoming air is adsorbed andin passing through the carbon bed 8 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 98 to 99% pure nitrogen exits through the outlet of the adsorber and into the product delivery pipe 40.

After a predetermined interval, valves 26, 32 and.

36 are closed and valves 42 and 44 are opened to equalise the pressures in the two adsorbers 2 and 4. At this stage the pressure in adsorber 2 will have reached its maximum value of say 8 bar g and the pressure in adsorber 2 (which will have just completed its desorption step) will be at its lowest value of atmospheric pressure. Consequently the gas in the void spaces towards the outlet end of adsorber 2 which will be partially enriched in nitrogen and depleted in the other atmospheric gases is sucked into the outlet end of adsorber 4 where it is further purified by the carbon bed 8 during the subsequent adsorption step of the adsorber 4. The reldtivelylmpure gas at the inlet end of the adsorber 2 will similarly be sucked in to the inlet end of the adsorber 4 and will be purified during the subsequent adsorption step of adsorber 4.

The valves 42 and 44 will then close and the valve 30 is opened to vent adsorber 2 to atmospheric pressure through the vent gas pipe 34. (At the same time the valves 28 and 38 are opened).

During this time the gases adsorbed on beds 6 and 8 of the adsorber 2 are desorbed and flow out through the inlet end of the adsorber 2, that is in a direction counter to the direction of passage of air through the adsorber during the adsorption step.

As the oxygen and other gas is desorbed from the bed 8 it washes back through the bed 6 to ensure that water vapour is removed from that bed in an amount substantially equal to the amount deposited during the previous adsorption step.

The pressure equalisation step and the subsequent desorbtion step to atmospheric pressure cause a substantially adiabatic expansion of gas in the adsorber 2. This expansion is accompanied by reduction in the temperature of the gas. Accord-# ingly, this temperature reduction is employed in the heat exchanger 20 by using the vented gas in the stream flowing through the gas pipe 34 to cool the incoming gas passing through the passage 14 of the heat exchanger 20. In order to enhance the cooling provided, water is trickled into the vent gas passage in the heat exchanger 20 from a source (not shown) via a pipe 46. Evaporation of the water in the waste gas will cause the temperature to fall to near to the freezing point of water. Typically, the heat exchanger 20 may thus be provided with a fan or other means to assist in the evaporation of water.It is to be appreciated that during the period of time in which the adsorber 2 is receiving air for separation, the adsorber 4 is being regenerated after a previous adsorption step. During regeneration of the adsorber 4 the valves 28, 38, 42 and 44 associated therewith are in their closed positions while the valve 32 is in its open position. For each equalisation step all the valves associated with the beds are closed save for the valves 42 and 44. At the same time as the adsorber 2 is being regenerated, the adsorber 4 is employed to separate air. In this mode of operation, the valves 28 and 38 associated with the adsorber 4 are in their open positions and the valve 32 is in its closed position, and the valves 42 and 44 are also in their closed position.

After a predetermined interval of time equal to the time for which the valves 26 and 36 are open during the adsorption step of the adsorber 2 the valves 28 and 38 are closed and the valves 42 and 44 are reopened to equalise once again the pressures within the adsorbers 2 and 4. On this occasion, the flow of void gas is from adsorber 4 to adsorber 2. The valves 42 and 44 are then closed and the valves 26, 36 and 32 are reopened and the whole cycle is repeated.

It is to be appreciated that the buffer tank 16 is not an essential part of the apparatus illustrated in the accompanying drawings. Its purpose is to help in the rgulation of the supply of air to the adsorbers 2 and 4 in such a manner that the power consumption of the compressor 10 is minimised.

Although the apparatus and process according to the present invention having been described by way of example with reference to a plant in which the adsorption step is performed at a pressure well above atmospheric, and the regeneration step is performed by venting the respective adsorbent beds to atmosphere, it is possible to employ alternative cycles in which regeneration is effected in part or entirely by connecting the beds to a source of sub-atmospheric pressure, for example, a vacuum pump. In such cycles it is possible to dispense with the compressor 10 such that the adsorption step takes place at substantially atmospheric pressure. Nor is the invention limited to the separation of nitrogen from air. It is to be appreciated that coke molecular sieves are in particular damaged by liquid water. However, the presence of liquid water in any sieve material is undesirable and thus the invention may be employed in any process in which it is desired to use a PSA technique to separate a gas mixture including water vapour or other in the event that there are two or more readily condensible vapours volatile liquid.

The method according to the invention may be used in the event that there are two or more readily condensible vapours in the incoming gas mixture. Condensation of a main such readily condensible vapour (say, water) will facilitate cocondensation of traces of other condensible vapours (such as acetone).

Claims (11)

1. A gas separation process comprising repeating a cycle of operations including an adsorption step in which a gaseous mixture including water vapour or other readily condensible vapour, a product and at least one other gas is contacted with at least one adsorbent whereby the mixture is enriched in said product gas, and a desorption step in which the adsorbent is regenerated by being subjected to a reduction in pressure to produce a gas stream including desorbed gas, wherein upstream of the adsorbent said gaseous mixture is subjected to means for removing liquid droplets therefrom, the gaseous mixture being heat exchanged with said gas stream upstream of said means for removing liquid.

2. A process as claimed in claim 1, in which a liquid is caused to evaporate in the said gas stream.

3. A process as claimed in claim 2, in which the liquid is water.

4. A process as claimed in claim 3, in which the temperature of the gaseous mixture is reduced to a temperature approaching 0 C from above.

5. A process as claimed in any one of the preceding claims, in which the adsorbent is carbon molecular sieve.

6. A process as claimed in any one of the preceding claims, in which the gaseous mixture encounters a layer of desiccant immediately upstream of the adsorbent.

7. A process as claimed in any one of the preceding claims, in which the gaseous mixture is air.

8. A gas separation process substantially as hereinbefore described with reference to the accompanying drawings.

9. Apparatus comprising at least one vessel for adsorbent; said vessel having a valved inlet and a valved outlet; a gas supply passage communicating with said inlet; a product withdrawal passage and a passage for a gas stream including desorbed gas, each able selectively to be placed in communication with the valve outlet, means for removing liquid droplets from the gaseous mixture to be separated, said means being Icoated in the gas supply passage, an a heat exchanger effective to place said gas supply passage in heat exchange relationship with the said gas stream passage upstream of said liquid droplet removal means.

10. Apparatus as claimed in claim 9, in which the heat exchanger is of the kind in which a liquid can be evaporated.

11. Apparatus for the separation of a gaseous mixture substantially as hereinbefore described with reference to the accompanying drawing.