GB2161717A - Improved apparatus for the separation of a gaseous mixture - Google Patents

Abstract

In an apparatus for the separation of a gaseous mixture using PSA techniques two vessels 8, 10 each containing a bed of adsorbent material 12, 14 are interconnected during a pressure equalisation step by pipelines extending between the top of the one vessel and the bottom of the other vessel. <IMAGE>

Description

SPECIFICATION Improved apparatus for the separation of a gaseous mixture The present invention relates to apparatus for the separation of a gaseous mixture using pressure swing adsorption techniques (PSA).

It is known from United Kingdom published patent application No. 2073043A to separate nitrogen from air using an 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. The adsorbent is usually molecular sieve carbon.In operation, a bed of the 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 and substantially all of the carbon dioxide and water vapour in the feed are adsorbed and a nitrogen-rich product gas is supplied from the outlet of the bed; and a desorption or regeneration step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and/or 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, two adsorbent beds are employed and operated on similar cycles which are sequenced to be out of phase with one another by 180 so that when one bed is on its adsorption step, the other bed is on its regeneration step and vice-versa.

Between the adsorption and the regeneration steps, the pressures in the two beds are equalised 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, the high pressure bed, flows into the bed which has just completed its regeneration step, the low pressure bed, by virtue of the pressure difference which exists between the beds at this stage. This equalisation step is found to be beneficial in maximising the product output because the gas in such void spaces will have already become somewhat enriched in nitrogen.

However, it will be apparent that during the equalisation step the purest gas, that is, that richest in product gas, within the high pressure bed will flow into the middle of the low pressure bed whereas a less pure gas from lower down the high pressure bed will end up at the end of the equalisation step, at the top of the low pressure bed.

This less pure gas is then delivered first during the next adsorption step of the low pressure bed rather than the purer gas.

Further, gas passing during the equalisation step from the bottom/inlet of the high pressure bed into the bottom/inlet of the low pressure bed is the least pure of all the gas in the high pressure bed. This least pure gas ends up in the middle of the low pressure bed, interfacing with the purest gas from the top of the high bed.

It is an aim of the present invention, by improving the apparatus described in United Kingdom published patent application No. 2073043A to increase the efficiency and the product purity when operating a pressure swing adsorption process with said apparatus.

According to one aspect of the present invention, an apparatus for the separation of a gaseous mixture by pressure swing adsorption comprises first and second vessels each containing a bed of adsorbent material which adsorbs preferentially at least one constituent of a feed gas mixture, each vessel at one end having an inlet for the feed gas mixture and at an opposite end an outlet for product gas; a first pipeline incorporating a first valve for permitting communication between the vessels when said first valve is open and a second pipeline incorporating a second valve for permitting communication between the vessels when said second valve is open, in which the first and second pipelines each extend from or adjacent to the top of one vessel to or adjacent to the bottom of the other vessel, and in which a purge pipeline extends from the outlet of a vessel to communicate with a product outlet pipeline, which purge pipeline by-passes a stop valve normally controlling the flow of product gas from the top of the vessel to the product outlet pipeline.

According to a further aspect of the present invention, a process for the separation of a gas mixture by pressure swing adsorption comprises the steps of: (a) repeatedly performing a cycle of operations employing a first vessel containing a bed of adsorbent which adsorbs one component of the gas mixture more readily than another or the other component of the gas mixture, the cycle of operations comprising; (i) passing the gaseous mixture at superatmospheric pressure through the vessel from a gaseous mixture inlet end to a product gas outlet end whereby said one component is adsorbed and a product gas stream relatively lean in the adsorbed component flows out of the bed;; (ii) regenerating the adsorbent by desorbing gas therefrom, causing a bleed of product gas to flow into the bed at or near the said outlet end causing a gas stream relatively rich in the desorbed gas to flow out of the said inlet end of the vessel; (b) repeatedly perforr ling sue'1 a cycle of operations employing a second iessel containing a bed of said adsorbent, the cycles being phased relative to one another such that at no time is step (i) or step (ii) in one cycle performed simultaneously with the corresponding step in the other cycle; (c) at intervals between successive adsorption steps placing the two beds in flow communication with each other such that gas at or adjacent the top of the bed which has just completed its adsorption step flows towards the bottom or adjacent the bottom of the bed which has just completed its regeneration step.

Embodiments of the invention will now be described by way of example, reference being made to the Figures of the accompanying diagrammatic drawings: Figure 1 is a schematic diagram of an apparatus for the separation of nitrogen from air according to the present invention; and Figure 2 is a schematic diagram of a further apparatus for the separation of nitrogen from air in accordance with the present invention.

Referring first to Figure 1,this illustrates an apparatus for the separation of nitrogen from air.

The apparatus includes an air feed line 2 leading from a compressor (not shown). The air feed line 2 from the compressor communicates with an air inlet pipeline 6. The air inlet pipeline 6 is able to be placed in communication with either one of vessels 8 and 10 containing beds 12 and 14 of carbon molecular sieve adsorbent. Valve 16 is operable to place the bottom of bed 12 in communication with the inlet pipe 6 orto deny communication between the bed 12 and the inlet pipeline 6. Analogously, valve 18 is operable to place the bottom of the bed 14 in communication with the inlet pipeline 6 orto deny such communication.

The apparatus includes an outlet pipeline 15 extending from the top of the vessel 8 to an outlet product gas pipeline 20. A stop valve 22 in pipeline 15 is operable to place the top of the bed 12 in communication with the pipeline 20 or to prevent communication between the bed 12 and the pipeline 20. Analogously, a stop valve 24 in an outlet pipeline 17 is operable to place the top of the bed 14 in communication with the pipeline 20 orto deny such communication.

The apparatus includes an outlet pipeline 34for waste gas. A valve 36 is operable to place the bottom of the bed 12 in communication with the pipeline 34 or may be closed to deny such communication. A valve 38 is operable to place the bottom of the bed 14 in communication with the pipeline 34 or, when closed, deny such communication. The pipeline 34 normally communicates with the atmosphere but may be connected to a vacuum system to achieve a sub atmospheric pressure.

A pipeline 50 allows communication between the bottom of vessel 8 and the top of vessel 10. A similar pipeline 52 allows communication between the top of vessel 8 and the bottom of vessel 10. In the pipeline 50 is disposed a valve 54 and in the pipeline 52 is disposed a valve 56. When either valve 54 or 56 is open, there is communication between the beds 12 and 14.

Product purge pipelines 60, 62 containing restrictors connect respectively the tops of vessels 8, 10 with outlet product gas pipe 20 by-passing their respective stop valves 22, 24.

Describing very briefly a process using the apparatus in Figure 1 fortheseparation of nitrogen from air and assuming at the start of a cycle of operations that the bed 12 is at atmospheric pressure and the bed 14 is at its maximum pressure being in communication with the compressor. The cycle starts with the beds 12, 14 being placed in communication with one anotherthrough the pipeline 50. In this phase of the cycle valves 16, 18, 22, 24,36,38 and 56 are all in their closed position and valve 54 is open. Unadsorbed nitrogen-rich gas in the spaces between individual particles of adsorbent in the bed 14 flows through pipeline 50 into the bed 12. The unadsorbed gas in the bed 14 which passes to the bed 12 is rich in nitrogen.As it passes to the bed 12, so the pressure in the bed 12 increased from below atmospheric pressure and the pressure in the bed 14 undergoes a corresponding reduction.

Following equalization the cycle will then continue with the bed 12 performing an adsorption step whilst the bed 14 is regenerated in a manner well known in the art.

It will be apparent that equalisation of the two beds 12, 14 is effected by caausing gas to pass from the top only of the high pressure bed 14 directly into the bottom of the low pressure bed 12, or at least a region near to the bottom of this bed (see Figure 2).

Gas will then flow upwardly through the bed 12. It may therefore be "stacked" in the same way as it was in the high pressure bed 14. Thus, the cleanest gas at the top of the high pressure bed 14 at the start of equalisation passed to the top of the low pressure bed 12, and the gas that finished at the bottom of the low pressure bed 12 at the end of equalisation is the least "clean" of the gas passing from the high pressure to the low pressure bed although it has been partially purified. Moreover, the residual gas in the high pressure bed 14 at the end of equalisation which is subsequently vented is the least pure of the gas in that bed, at the start of equalisation.

During regeneration, a bleed of product gas is passed downwardly through the bed 12 being renegerated from its top via pipeline 60 so as to reduce the amount of oxygen (impurity) that remains in the bed 12 at the end of its regeneration.

Preferably, the volume of such purge gas entering each bed during its regeneration is similar to the free gas volume of that bed at one atmosphere. In general, this volume is matched to the position at which the equalisation gas is introduced into the low pressure bed 12 during the equalisation step.

Thus, if the equalisation line enters the low pressure bed 12 1/5 its way up (see Figure 2), the purge volume can be reduced. This way, at the end of the regeneration step the "equalisation" gas from the high pressure bed 14 encounters relatively pure gas rather than desorbed impurity gas.

As shown, it is not necessary to employ one-off valves in the purge lines 60,62. These lines can be made valveless but relatively restricted so as to ensure that precisely the desired volume of purge gas is delivered during regeneration. At least some of this nitrogen will be collected as product during the next product delivery step. No gas will be delivered via the purge line to the bed while it is at product delivery pressure as there is no pressure differential between the product gas and the pressurised bed. If desired, however, on-off valves can be incorporated into the purge line, and may be of particular use in plants with a very intermittent duty.

The apparatus of Figure 1 can be modified as will be explained by reference to Figure 2. In Figure 2 it will be seen that the pipelines 50, 52 and enter their respective vessels 8, 10 to communicate with beds 12,14 one-fifth of the total length of the vessel 8,10 from the bottom of said vessel.

Although reference haas been made in the above described embodiments to the production of nitrogen from air, the apparatus could be used for the separation of any one constituent of a gaseous mixture using PSAtechniques.

Claims (6)

1. An apparatus for the separation of a gaseous mixture by pressure swing adsorption comprising first and second vessels each containing a bed of adsorbent material which adsorbs preferentially at least one constituent of a feed gas mixture, each vessel at one end having an inlet for the feed gas mixture and at an opposite end an outlet for product gas; a first pipeline incorporating a first valve for permitting communication between the vessels when said first valve is open and a second pipeline incorporating a valve for permitting communication between the vessels when said second valve is open, in which the first and second pipelines each extend from or adjacent to the top of one vessel to or adjacent to the bottom of the other vessel, and in which a purge pipeline extends from the outlet of a vessel to communicate with a product outlet pipeline, which purge pipeline by-passes a stop valve normally controlling the flow of product gas from the top of the vessel to the product outlet pipeline.

2. An apparatus as claimed in claim 1, in which the first and second pipelines at their lower ends extend into the interior of an individual vessel up to one fifth of the total length of that vessel from the bottom of that vessel.

3. A process for the separation of a gas mixture by pressure swing adsorption comprising the steps of: (a) repeatedly performing a cycle of operations employing a first vessel containing a bed of adsorbent which adsorbs one component of the gas mixture more readily than another or the other component of the gas mixture, the cycle of operations comprising:: (i) passing the gaseous mixture at superatmospheric pressure through the vessel from a gaseous mixture inlet end to a product gas outlet end whereby said one component is adsorbed and a gas stream relatively lean in the adsorbed component flows out of the bed; (ii) regenerating the adsorbent by desorbing gas therefrom, causing a bleed of product gas to flow into the bed at or near the said outlet end and causing a gas stream relatively rich in the desorbed gas to flow out of the said inlet end of the vessel; (b) repeatedly performing such a cycle of operations employing a second vessel containing a bed of said adsorbent, the cycles being phased relative to one another such that at no time is step (i) or step (ii) in one cycle performed simultaneously with the corresponding step in the other cycle; ; (c) at intervals between successive adsorption steps placing the two beds in flow communication with each other such that gas at or adjacent the top of the bed which has just completed its adsorption step flows towards the bottom or adjacent the bottom of the bed which has just completed its regeneration step.

4. A process as claimed in claim 3, in which the volume of product gas passed downwardly through the bed is generally equal to the free gas volume of the bed being regenerated at one atmosphere.

5. A process for separating a gas mixture, substantially as hereinbefore described with reference to the accompanying drawings.

6. An apparatus for the separation of a gaseous mixture by pressure swing adsorption constructed and arranged substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.