Improvements in pressure swing adsorption plants

Abstract: A pressure vessel 1 forming part of a PSA plant contains a molecular sieve adsorbent. A plurality of thermocouples are located within the molecular sieve adsorbent such that one junction 8 of each thermocouple is positioned adjacent the feed gas inlet 2 of the vessel 1 whilst the second junction 10 of each thermocouple is positioned adjacent the product gas outlet 4 of the vessel.

Description: The present invention relates to the separation of a gas mixture by pressure swing adsorption (PSA) techniques. United States Patent 4026680 addresses the problem of cold and hot spots or zones which are formed in the pressure vessels of PSA plants, particularly large scale PSA plants for the production of oxygen from feed air where the adsorbent material used is a zeolite molecular sieve. United States Patent 4026680 discloses that temperature drops in the order of 38°C (100°F) below feed air temperature have been observed at the inlet end of pressure vessels forming part of a PSA plant for the production of oxygen from feed air. Since the molecular sieve material is usually selected to operate best, as regards oxygen recovery efficiency, at ambient conditions, the effect of hot and cold spots is to reduce the performance of the molecular sieve material. In order to meet this problem United States Patent 4026680 teaches the transference of heat within the pressure vessel from the hot spot or zone to the cold spot or zone by metal solid conduction. The metal solid conduction can take the form of a plurality of metal plates positioned parallel and equi-distant from each other and extending through the adsorbent bed. An alternative embodiment teaches the use of metal rods. Such methods form a passive control system in as much as a temperature difference has to exist within the pressure vessel before they can operate. That is to say, they reduce rather than eliminate the hot and cold spot effect. Although the temperature differences are very pronounced, in the order of minus 70°C in the cold part of the pressure vessel and plus 40°C in the warm part of the pressure vessel, the net heat flow is relatively small and the effect of metal solid conduction can take several days to become established. It is an aim of the present invention to provide either passive or active means for inhibiting the effect of hot and cold spots in a pressure vessel of a PSA plant in order to maintain optimum performance of the adsorbent material used in the pressure vessel. According to the present invention an apparatus for the separation of at least one constituent of a feed gas mixture by pressure swing adsorption comprises a pressure vessel containing a molecular sieve adsorbent, an inlet in said pressure vessel for said feed air, an outlet from said pressure vessel for product gas and is characterised by a plurality of thermocouples located within the molecular sieve adsorbent such that one junction of each thermocouple is positioned at or adjacent the inlet whilst the second junction of each thermocouple is positioned at or adjacent the outlet. Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:- Figure 1 is a schematic view of a pressure vessel forming part of a pressure swing adsorption plant for the production of oxygen enriched gas from air; Figure 2 is a diagrammatic sketch of thermocouples located within the pressure vessel of Figure 1; and Figure 3 is a diagrammatic sketch of a modification to the thermocouple circuit illustrated in Figure 1. As shown in Figure 1, a pressure vessel 1 forming part of a pressure swing adsorption plant for the production of an oxygen enriched gas from a feed air stream contains a bed 6 of adsorbent material in the form of a zeolite molecular sieve. Adjacent an inlet 2 of the pressure vessel 1 is an alumina layer 16. As is known in the art, when the pressure vessel is in the adsorbing mode air under pressure passes from the inlet 2 through the alumina layer 16 and hence through the adsorbent bed 6 towards an outlet 4. The alumina layer 16 removes water vapour and possibly some carbon dioxide; and the adsorbent bed 6 preferentially adsorbs nitrogen and carbon dioxide such that an oxygen rich product gas exits from the pressure vessel 1 via the outlet 4. During the desorption mode of the pressure vessel 1, the interior of the vessel 1 is exhausted, usually to a vacuum of between 26.6-62.5 kN (200 and 500 torr), such that nitrogen rich gas is desorbed from the adsorbent bed 6 thus preparing the pressure vessel 1 for the next adsorption step. As explained earlier, this continuous cycling of the adsorption bed 6, particularly in larger pressure vessels, in the order of 65 cm (26 inches) diameter, causes a cold spot or zone to be formed at or adjacent the inlet 2 whilst a relatively hot spot or zone is formed at or adjacent the outlet 4. Referring now to Figure 2, a plurality of thermocouples are located in the bed of adsorbent material 6 and arranged in series such that one set of junctions 8 is located in the cold zone whilst the other set of junctions 10 is located in the warm zone. A resistor 12 is positioned within the thermocouple circuit at or adjacent the inlet 2 of the pressure vessel 1. The electrical current generated by the differences in temperature between the hot zone and the cold zone and which flows through the resistor causes said resistor to heat up thereby heating up the cold zone and reducing the temperature difference between the hot zone and the cold zone. Referring now to Figure 3 where the thermocouple circuit is substantially the same as with Figure 2 except that no resistor 12 is provided. A current is actively passed through the circuit from an external electrical power supply 14. This has the effect of causing one set of junctions to cool and the other set of junctions to warm and by arranging the junctions in the hot and cold zones external current can be adjusted to keep the temperature within the adsorbent bed 6 fully stabilised.

Claims: An apparatus for the separation of at least one constituent of a feed gas mixture by pressure swing adsorption comprising a pressure vessel (1) containing a molecular sieve adsorbent, an inlet (2) for said feed gas, an outlet (4) for product gas and characterised by a plurality of thermocouples located within the molecular sieve adsorbent such that one junction (8) of each thermocouple is positioned at or adjacent the inlet (2) whilst the second junction (10) of each thermocouple is positioned at or adjacent the outlet (4) of the pressure vessel (1). An apparatus as claimed in claim 1, characterised in that the thermocouples are arranged in series and in which a resistor (12) is positioned in the thermocouple circuit at or adjacent the inlet (2) of the pressure vessel (1). An apparatus as claimed in claim 1, characterised in that the thermocouples are arranged in series and a source (14) of electrical energy is provided for passing a current of electricity through the thermocouple circuit.

Inventors: GARRETT MICHAEL ERNEST, GARRETT, MICHAEL ERNEST
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1991
Main classification: B01D53
IPC codes: B01D53