GB2281229A

GB2281229A - An adsorber vessel

Info

Publication number: GB2281229A
Application number: GB9317869A
Authority: GB
Inventors: John Terence Lavin
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1993-08-27
Filing date: 1993-08-27
Publication date: 1995-03-01
Also published as: GB9317869D0

Abstract

An adsorber vessel (10) for separating a process gas such as air comprises a plurality of tubes (16) containing an adsorbent (19) capable of preferentially adsorbing one or more of the undesired constituents of the process gas. The tubes (16) are arranged for the passage of heating/cooling fluid over the outer surface thereof so as to enable rapid heating/cooling of the adsorbent to its upper desorption and lower adsorption temperatures. Due to the rapid heating/cooling capabilities of the arrangement, cycle times may be reduced and hence less adsorbent and hence less purge gas are needed. <IMAGE>

Description

AN ADSORBER VESSEL The present invention relates to an adsorber vessel and relates particularly, but not exclusively, to an adsorber vessel of the type used in the separation of air in a temperature swing adsorption (TSA) process.

Presently known TSA vessels generally comprise a large vertically or horizontally extending container having a quantity of adsorbent material, typically molecular sieve, provided therein. The adsorbent may comprise sect ions of 13X molecular sieve and activated Alumina, for example, for the adsorption of CO2 and B2O respectively. In operation, ambient air is passed through the sieve cooled, and then passed to a cryogenic distillation column where the dried ambient air is further separated into nitrogen and oxygen rich streams in a process which forms no part of the present invention and is therefore not described further herein. After a period of time the sieve material becomes saturated and must be regenerated. Regeneration takes place by reverse flowing heated product or by-product gas from the distillation process back through the sieve material which, at this elevated temperature, releases the adsorbed CO2 and H2O. Once regeneration is completed, the sieve is cooled down to its adsorption temperature by passing unheated product or by-product gas therethrough before the adsorption step can be restarted and treated air supplied to the distillation column once again. A TSA apparatus usually comprises two or more adsorption vessels operated out of phase to each other so as to ensure a constant supply of dried air.

One of the problems associated with presently known TSA adsorption processes is the amount of time it takes to heat and/or cool the sieve material between its respective adsorption/desorption cycles. As a consequence of the longer cycle times it is common to provide a larger quantity of sieve material than might be desired in order to ensure sufficient dry air can be provided to the distillation process before the TSA adsorber requires regenerating. The greater the quantity of sieve the greater the quantity of regeneration gas that will be required for a given cycle time and regeneration temperature. In addition to this, the regeneration gas is contaminated with H20, CO2 etc which reduces significantly its regeneration capacity.

It is an object of the present invention to provide an adsorber vessel suitable for use in a temperature swing adsorption process for the separation of air which reduces and possibly eliminates the disadvantages of presently known arrangements.

Accordingly, the present invention provides an adsorber vessel for receiving a process gas to be purified by the removal of impurities therefrom, the adsorber vessel comprising a plurality of tubes for containing one or more adsorbents capable of selectively adsorbing one or more of the impurities in the process gas, said tubes being connected for the flows of process gas therethrough and for the flow of heating/cooling fluid thereover.

It will be appreciated that by providing the adsorption material in a plurality of tubes rather than in a bulk bed arrangement and by passing the heating/cooling fluid over the outside of the tubes it will be possible to reduce the heating/cooling time periods and consequently reduce the quantity of sieve material. In addition to this, it will also be possible to reduce the quantity of regeneration gas in proportion (although not necessarily in direct proportion) to the reduction in the quantity of sieve. A further, and perhaps more important, advantage of employing a tube arrangement resides in the fact that any heating/cooling fluid may be used as it never comes into direct contact with the sieve material itself.

This could reduce the requirement to use heated product or by-product gas as heated air or water could be used as alternatives.

Advantageously, the tubes are right circular tubes thereby to allow for the smooth flow of heating/cooling fluid thereover whilst also providing an advantageous ratio of surface area of tube volume of sieve.

Alternatively, the tubes may comprise substantially flat tubes thereby to improve the flow of heat transfer fluid and hence increase the heat transfer rate.

The vessel may further include a first plenum chamber within the vessel for receiving and distributing process gas within the vessel prior to the passage thereof through the tubes and may additionally be provided with a second plenum chamber for receiving and collecting process gas after it has passed through the tubes.

For purposes of strength and thermal conductivity the tubes may comprise metal tubes.

In one arrangement the tubes may extend substantially vertically, thereby to reduce the footprint of the vessel.

In an alternative arrangement the tubes may extend substantially horizontally thereby to enable a reduction in the height of the vessel to be achieved.

In a particularly advantageous arrangement the vessel further comprises a first and a second heating/cooling fluid inlet/outlet each positioned for directing heating/cooling fluid between and along a substantial portion of said tubes, thereby to facilitate rapid heat transfer.

The present invention will now be more particularly described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic cross sectional view of an adsorber vessel according to the present invention; Figure 2 is a partial cross sectional view in the direction of arrows A-A in Figure 1; and Figure 3 is a blow up cross sectional view taken in the direction of arrows B-B in Figure 2.

Figure 4 is a blown up cross sectional view of an alternative flatter tube arrangement.

Referring now to the drawings in general but particularly to Figure 1, an adsorber vessel 10 is provided with an inlet 12, an outlet 14 and a plurality of spaced tubes 16 extending between first and second plenum chambers 18, 20. Preferably, each tube 16 is made from a material having a high thermal conductivity, such as for example a metal. An adsorbent 19 capable of selectively adsorbing one or possibly more of the components of a process gas to be passed through the vessel is provided within the tubes 16 such that, in operation, process gas passes through the adsorbent 19 as it is passed from the first to the second plenum chamber.

The outer surface 16a of the tubes 16 and the inner surface 10a of the vessel 10 act to define a flow passage 21 through which, in operation, cooling/heating fluid is passed. First and second inlets/outlets 22, 24 are provided for allowing the passage of fluid into and out of the passage 21. In one particularly advantageous arrangement the inlets/outlets 22, 24 are positioned at opposite ends and opposite sides of the vessel to each other, thereby to facilitate the flow of cooling/heating fluid between and along a substantial portion of said tubes 16.

Figure 1 illustrates a vessel with vertically extending tubes 16 and one upper 22 and one lower 24 inlet/outlet. It will however be appreciated that other arrangements, such as for example horizontally extending tubes 16 may be employed without departing from the spirit of the present invention.

The tubes 16, best seen in Figures 2 and 3, comprise a thin metal wall 16a in which is provided the adsorbent material 19 such as for example molecular sieve, alumina 13X, etc. The adsorbent or sieve is generally granular in form and therefore allows for the passage of process gas through the gaps 30 (Figure 3) formed between the granules when placed in the tubes 16. The adsorbent material 19 may be provided in layers, each layer comprising a different adsorbent or may comprise a mix of various adsorbents randomly dispersed therein.

It is important to note that the tubes may be made of any material having a good thermal conductivity and metal has been selected herein for illustration purposes only.

Referring now briefly to Figure 2, the tubes 16 may be of any cross sectional form but advantageously have a right circular cross section as illustrated. such an arrangement allows for the smooth passage of heating/cooling fluid between the tubes 16. The right circular cross section also provides a good ratio of surface area/unit volume of adsorbent placed therein thereby providing an advantageous heat transfer capability.

Alternatively, the tubes 16 may take a flatter form as shown in figure 4.

Such an arrangement allows for a more efficient passage of cooling/heating fluid between the tubes thereby increasing the heating/cooling rate.

One of the advantages of the above described tube arrangement resides in the fact that, because the heating/cooling fluid is actually separated from the adsorbent 19 by the tubes 16, it will be possible to use any form of fluid and not just dried product or by-product gas as is required by the prior art arrangements.

As there is no longer a requirement for the heating/cooling gas to be dry, it will be possible to use, for example, heated or cold water as an alternative to dry heated/unheated product or by-product gas.

In operation, process gas to be separated (such as for example air to be dried and to have C02 removed therefrom) is passed into the first plenum chamber 18 via inlet 12. The plenum chamber 18 acts to distribute the process gas fairly evenly between the tubes 16 through which it subsequently passes before entering the second plenum chamber 20 which acts to collect the now processed process gas and direct it to outlet 14.

The adsorbent, and the operation thereof, forms no part of the present invention and is therefore not described in detail herein. However, such adsorbents are generally capable of adsorbing one particular constituent of a process gas at around ambient temperature and desorbing it at a higher temperature. This capability is exploited in the present application by passing heating/cooling fluid through flow passage 21 where it acts to heat or cool the adsorbent contained in the tubes 16. An example of an adsorbent capable of adsorbing H20 is Alumina, whilst molecular sieve materials may be employed to adsorb a combination of impurities such as, for example, H20 and C02 etc.

A typical cycle would involve the following steps: (a) Firstly, the adsorbent must be cooled to its 'adsorption' temperature.

This is done by passing cooled fluid through passageway 21 and allowing the chill associated therewith to pass through the tubes and thereby cool the adsorbent.

(b) Secondly, process gas is passed through the adsorbent 19 which preferentially adsorbs the undesirable components thereof. In a typical TSA air separation step C02 and H20 are adsorbed and the dried gas mixture passed to a cryogenic distillation apparatus for separation into oxygen and nitrogen enriched streams.

(c) As the adsorbent approaches its saturation point, that is to say the point at which it can no longer adsorb the undesirable components of the process gas, the flow of process gas is discontinued and a desorption step initiated.

(d) The desorption step involves raising the temperature of the adsorbent to its desorption temperature and reverse flowing a quantity of dry air, product gas or by-product gas through the adsorbent until all or substantially all the adsorbed elements are desorbed and passed to atmosphere. The temperature increase is achieved by passing heating fluid such as, for example, low pressure steam through flow passage 21 and allowing the tubes and hence the adsorbent to be heated thereby to facilitate desorption. Once desorption is completed the vessel 10 may once again be used for adsorption as outlined in (a) above.

It has been found that the passage of heating/cooling fluid through passageway 21 allows more rapid heating or cooling to be achieved than was previously considered possible. As a consequence of this, shorter cycle times may be employed and hence much less adsorbent is required. As a direct consequence of being able to reduce the quantity of adsorbent and provide heating and cooling indirectly it will be possible to reduce the quantity of purge gas (usually nitrogen) passing through the adsorbent and consequently becoming contaminated, leaving a greater yield of pure product gas.

Claims

1. An adsorber vessel for receiving a process gas to be purified by the removal of impurities therefrom, the adsorber vessel comprising a plurality of tubes for containing one or more adsorbents capable of selectively adsorbing one or more of the impurities of the process gas, said tubes being connected for the flow of process gas therethrough and for the flow of heating/cooling fluid thereover.

2. An adsorber vessel as claimed in claim 1, in which said tubes are right circular tubes.

3. An adsorber vessel as claimed in claim 1, in which said tubes comprise substantially flat tubes.

4. An adsorber vessel as claimed in any one of claims 1 to 3 further including a first plenum chamber within the vessel for receiving process gas within the vessel prior to the passage thereof through the tubes.

5. An adsorber vessel as claimed in any one of claims 1 to 4 further including a second plenum chamber within the vessel for receiving process gas after it has passed through the tubes and before it is passed from the vessel itself.

6. An adsorber vessel as claimed in any one of claims 1 to 5 in which the tubes comprise metal tubes.

7. An adsorber vessel as claimed in any one of the preceding claims further including a first upper and a second lower heating/cooling inlet/outlet positioned so as to direct heating/cooling fluid between and along a substantial portion of said tubes.

8. An adsorber vessel substantially as herein described with reference to and as illustrated in Figures 1 to 4 of the accompanying drawings.