GB2109266A

GB2109266A - Pressure swing process for the separation of gas mixtures by adsorption

Info

Publication number: GB2109266A
Application number: GB08231421A
Authority: GB
Inventors: Gerhard Reiss
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1981-11-05
Filing date: 1982-11-03
Publication date: 1983-06-02
Also published as: JPS5884020A; DE3144012C2; GB2109266B; JPH02962B2; DE3144012A1

Abstract

A continuous process of adsorption for purifying gases and separating gas mixtures by pressure swing adsorption using at least three beds (A, B, C) of adsorbent, in which desorption of the adsorbed components is carried out at a pressure below 1 bar (abs) in countercurrent to the direction of adsorption, the bed of adsorbent which is to be desorbed is rinsed by a stream of gas which is withdrawn, in the same direction of flow as that of adsorption, from that adsorber which has completed its step of adsorption or release of product, and said adsorber is either maintained at the adsorption pressure by the inlet for crude gas being kept open or it is lowered to a pressure below the adsorption pressure by the crude gas inlet being closed. <IMAGE>

Description

SPECIFICATION Pressure change process for the separation of gas mixtures by adsorption This invention relates to an improved pressure change process for the continuous separation of a gas mixture by adsorption, which process may advantageously be used for enriching air with oxygen.

Processes employing pressure change adsorption (PCA) are employed wherever the component to be removed from the crude product gas is present at a relatively high concentration, e.g. above 1 vol.-%, or if it is insufficiently adsorbed by the adsorbent so that large adsorption units and large regeneration quantities would be necessary for thermal regeneration. Separation by adsorption is generally carried out at a higher pressure than the desorption of adsorbed component following the adsorption step.

The process of desorption is in most cases assisted by rinsing the adsorbent with a portion of the product gas, e.g. when recovering nitrogen from combustion gases or when drying gases. When enriching air with oxygen by adsorption, this rinsing is carried out at 1 bar (abs) for adsorption pressures of from 2 to 4 bar (abs), using separated product gas (German Auslegeschrift No. 1,259,844) or part of the pressure releasing gas (German Auslegeschrift No.

2,338,964).

The enriching of airwith oxygen assumes a position of particular importance compared with other PCA processes since the molecular sieve zeolites used as adsorbent adsorb not only nitrogen from the air, but also oxygen and argon. It is therefore not possible to adsorb only nitrogen and obtain all the oxygen present in the crude product air. Since argon is adsorbed at much the same slow rate as oxygen, the processes for enriching air with oxygen result in oxygen products of only 95% with a 5% residual content of argon and nitrogen. In the processes hitherto known, even these high oxygen concentrations are only obtained if at the end of the adsorption stage the amount of nitrogen on the molecular sieve zeolite is kept as low as possible in the region of the adsorption outlet zone or if this zone is filled with oxygen.This means that during the ensuing desorption step the variable high percentage oxygen is lost to the adsorption outlet zone and consequently the recovery factor for oxygen (quantity of oxygen obtained in proportion to the quantity of oxygen in the air put into the process) and the yield of the PCA installation are considerably reduced.

This also applies to cases in which adsorption is carried out at a pressure of about 1 bar (abs) and desorption is carried out by pumping off the molecular sieve by means of a vacuum pump in countercurrent to the adsorption (German Auslegeschrift Nos.

1,265,724 and 1,817,004). When desorption is carried out at reduced pressure for enriching air with oxygen, the desorption of nitrogen is improved by the rinsing effect of the oxygen in the adsorption outlet zone. The process of enriching with oxygen is further improved to a considerable extent by filling the adsorber with oxygen product after evacuation to the adsorption pressure since this gas pushes quantities of nitrogen from the adsorption outlet part to the inlet zone (German Auslegeschrift No.

1,544,152). It has already been attempted (German Auslegeschrift No. 2,707,745) to improve the rinsing effect by introducing part of the product stream into the molecular sieve bed which is required to be desorbed, but this has not led to a substantial improvement in the efficiency of the process since it is accompanied by loss of part of the valuable product gas. Experimental results obtained in Example 1 given below show that the production of 100 Nm3/h of oxygen (conc. 90%) requires a quantity of molecular sieve of about 5 m3 per adsorber, which means that if 10% of the product gas is branched off as scavenging gas, it provides a specific quantity of only 0.033 Nm3 of scavenging gas per m3 of zeolite, which is known from experience to be much too little to product a rinsing effect.

The present invention relates to a process for the separation of gas mixtures which obviates these disadvantages of vacuum rinsing with a proportion of the product gas, but substantially improves the efficiency of the separating process by pure vacuum desorption (without rinsing gas).

A process has been found by which the recovery factor of the proportion of product obtained is increased whereby the specific output of the vacuum pump (kwh/Nm3 of producted oxygen quantity) is improved and the quantity of adsorbent and quantity of crude product gas used may be reduced.

The present invention relates to a process for the separation of gas mixtures by adsorption with particular reference to the enriching of air with oxygen by pressure change adsorption, using at least three beds of adsorbent which may be charged with crude product, e.g. air, at one end, the adsorber inlet, while purified or separated product, e.g. air enriched with oxygen, may be withdrawn at the adsorber outlet, desorption of the adsorbed components being carried at a pressure below 1 bar (abs) and a rinsing gas obtained from the adsorber when the adsorber has completed its step of adsorption or release of product being used for improved desorption, this rinsing gas being withdrawn from the said charged adsorber in the same direction of flow as adsorption, the adsorber at the same time being maintained at its adsorption pressure by continued connection to the crude product or alternatively the pressure in the said adsorber being reduced during the release or rinsing gas to values below 1 bar by closing of the adsorption inlet end, this rinsing gas being withdrawn at pressures below 1 bar (abs) through the bed of adsorbent (which is to be desorbed) in countercurrent to the direction of adsorption.

The process according to the present invention is preferably employed for enriching air with oxygen. It is also suiable for the separation of gases and vapors having components which differ in ability to be adsorbed, e.g. the removal of CO2 or CO from N2 or CH4 or the removal of N2, CO or CH4 from H2.

A process has been found which is distinguished from processes hitherto employed by its improved efficiency and reduced operating costs. The constructional features of the process according to the present invention will now be described with reference to the accompanying drawing wherein: Figure 1 is a schematic representation of a conventional process as described in AICHE Symp. Ser. No.

134, Vol. 69 (1973) page 7; and Figure 2 is a flow chart of a process according to the present invention.

Referring now more particularly to the drawing, in Figure 1 valves (11) for the inlet of crude gas and valves (12) for the outlet of desorbed gas are situated at the bottom of the adsorber. The bed of adsorbent comprises at its lower end a protective layer, such as silica gel, for preliminary drying of the incoming crude gas and above this layer the main zone containing adsorbent for separation of the gas stream. Valves (14) for discharge of the gas which has been treated by adsorption and valves (13) to refill the adsorbers to the adsorption pressure are situated at the upper end of the adsorbers. Filling of the adsorbers may be controlled by the valve (15) to provide a constant increase in pressure or a constant supply of filling gas.

The apparatus of Figure 2 differs from that of Figure 1 in having additional valves (25) and a restrictor (26). The proportion of valve sizes is given approximately in the Figures. The additional valve (25) is relatively small owing to the low rate of flow of scavenging gas. The size of the stream of scavenging gas may be adjusted by means of the restrictor (26).

One essential feature of the process according to the present invention is that desorption of the adsorbent charged with adsorbate is carried out at a pressure below 1 bar (abs) and an additional stream of rinsing gas is used. This rinsing gas is not part of the product gas, but is obtained from the adsorber which has already completed its adsorption, i.e. its release of product. To obtain this gas, the adsorber which has been removed from the step of adsorption is kept in communication with the crude gas and a stream of gas is removed from the adsorption outlet at the adsorption pressure, the said gas stream being passed at a low pressure, in countercurrent to adsorption, through the adsorption bed which is to be desorbed.

In another embodiment of the process according to the present invention, rinsing gas is obtained by closing the adsorber which has completed its adsorption step at the crude gas inlet end and passing gas from this adsorber in countercurrent to the direction of adsorption at a pressure below 1 bar through the adsorber which is to be desorbed.

The examples which follow illustrate the various steps of the process in detail and the values for the quantities of product gas obtained in the experiments demonstrate the important influence and advantage of the scavenging process according to the present invention.

Example 1 A pressure change adsorption installation as illustrated in Figure 1 was used. The total height of the bed in the adsorberwas 2500 mm. Each adsorber contained 3 kg of silica gel at the bottom covered by 25 kg of molecular sieve zeolite 5 A of 2 - 5 mm grain size. An oil-operated rotary vacuum pump (v) with a nominal capacity of 25 m3/h was used. The compressor (R) was provided to withdraw air enriched with oxygen from adsorbers A, B and C and compresss it to from 1.1 to 1.5 bar (abs).

A continuous process with continuous withdrawal of gas by the compressor (R) could be obtained with these three adsorbers. The following time program was selected: Step 1: 0 - 60 seconds Ambient airflowsthrough blower (G), pipe L 12 and valve 11 A into adsorber A at a constant pressure of about 1 bar (abs). Air enriched with oxygen is withdrawn as product at the blower R by way of the valve 14 A and pipe L 13. The valves 12 A, 13 A are closed. At the same time, part of the air enriched with oxygen flows from pipe L 13 through the flow control valve 15, pipe L 14 and valve 13 B into the adsorber B while valves 14 B, 11 B and 12 B are closed. Adsorber B, which was previously subjected to desorption, i.e. evacuated, is thus refilled to the adsorption pressure with air enriched with oxygen.To prevent a pressure drop in adsorber A, for example due to rapid removal of product (filling gas) through pipe L 13, the valve 15 is regulated to ensure a constant rate of product flow (expressed in Nm3/h) into the adsorber B by way of the pipe L 14 and valve 13 B.

During the step of adsorption in adsorber A and during the step of filling in adsorber B, adsorber C is evacuated by vacuum pump V by way of the valve 12 CandpipeLll,i.e.valvesllC,13Cand14Cof adsorber C are closed. After a desorption time or pumping off time of 60 seconds, a mercury manometer situated between valve 12 C and adsorber C indicates a final pressure of 200 mbar.

Step 2: 60 - 120 seconds Adsorber A is evacuated to a final pressure of 200 mbar by a vacuum pump (V) by way of valve 12 A and pipe L 11 while valves 11 A, A and 14 A are closed. Adsorber B has been charged with air by way of the blower (G), pipe L 12 and valve 11 B, and product gas is withdrawn from the adsorber B by the compressor (R) by way of the valve 14 B and pipe L 13. Valves 12 B and 13 B are closed. Adsorber C is charged with air enriched with oxygen by way of pipe L 13, flow control valve 15, pipe L 14 and valve 13 C so that the pressure in the adsorber C is raised from 200 mbar to the adsorption pressure of about 1 bar. At the same time valves 1 1 C, 12 C and 14 C of adsorber C are closed.

Step 3: 120 - 180 seconds Adsorber A is charged with air enriched with oxygen from pipe L 13 by way of valve 15, pipe L 14 and valve 13 A so that the pressure in adsorber A is raised from its minimum desorption pressure (200 mbar) to the adsorption pressure 1 bar (abs) while valves 11 A, 12 A and 14 A are closed.

Adsorber B is evacuated to a final pressure of 200 mbar by the vacuum pump (V) by way of pipe L 11 and valve 12 B, while valves 11 B, 13 B and 14 B are closed.

Adsorber C supplies air enriched with oxygen, i.e.

ambient air enters adsorber C by way of the blower (G), pipe L 12 and valve 11 C and product gas is withdrawn through the compressor (R) by way of valve 14 C and pipe L 13 while valves 12 C and 13 C are closed.

After a cycle of 180 seconds, the process repeats itself, i.e. adsorber A is at adsorption, adsorber B is being filled and adsorber C evacuated.

A stream of product having a constant oxygen concentration could be obtained from the compressor (R) within from 0.5 to 1 hour after the beginning of the experiment. To determine the product rates at oxygen contents of 90% and 80%, the process is adjusted to various product rates by adjusting the blower (G) to a bypass setting (not shown in Figure 1). At an oxygen concentration of 90%, an oxygen product rate of 0.675 Nm3/h, based on 100% oxygen, could be obtained. At an oxygen concentration of 80%. the oxygen product rate based on 100% oxygen was 0.90 Nm3/h.

Example Figure 2 is the flow chart of a process according to the present invention for enriching air with oxygen by the PCA tech nique, using three adsorbers. Desorption is carried out by evacuation of the bed of adsorbent in countercurrent to the adsorption, and, at the end of desorption, a second part of a product is removed as rinsing gas from an adsorber which has completed its release of product. This rinsing gas is passed in countercurrent to adsorption through the bed which is to be desorbed. During this step of desorption, the adsorber from which the second part of the product is taken is maintained at its previous adsorption pressure by the valve at the air inlet end being kept open. In the experiment of Example 2, the quantity of the afore-said scavenging gas was 1.98 Nm3/h, i.e. 11 N1 per adsorption cycle.

The size of the adsorber, the temperatures, adsorption pressure, quantities and types of adsorbent and size of the vacuum pump were the same as in Example 1. The following program was employed in one experiment: Step 1:0-20 seconds Ambient air enters adsorber A at an adsorption pressure of 1 bar (abs) by way of the blower (G), pipe L 22 and valve 21 A. Air enriched with oxygen leaves adsorber A by way of valve 24 A and pipe L 23 and is withdrawn as product through compressor (R).

Adsorber B reaches the last phase of its regeneration step, i.e. adsorber B is evacuated by the vacuum pump (V) by way of valve 22 B and pipe L 21, and air enriched with oxygen is transferred from adsorber C to adsorber B by way of the valve 25 C, restrictor 26, pipe L 24 and valve 23 B. During this time, valve 21 C remains open, i.e. adsorber C remains at the adsorption pressure and is charged with air from blower (G). The valve 26 used in the experiment was a simple restrictor valve, but valve 26 could also be designed to be controlled to ensure a constant rate of flow. Valves 22 A, 23 A, 25 A, 21 B, 24B, 25 B, 22 C, 23 C, 24 C and 25 are closed.

Step 2: 20 to 60 seconds Ambient air continues to flow through the blower (G), pipe L 22 and valve 21 A into adsorber A and air enriched with oxygen leaves adsorber A through valve 24A and pipe L 23 and is withdrawn as product by the compressor (R).

Adsorber B is charged to the adsorption pressure with air enriched with oxygen from pipe L 23 by way of the valve 25, pipe L 24 and valve 23 B.

Adsorber C is evacuated by the vacuum pump (V) by way of the valve 22 C and pipe L21.Valve22A,23 A, 25A, 21B, 22 B, 24 B, 25 B, 21 C,23C,24Cand25 C are closed.

Step 3: 60 to 80 seconds Process step analogous to step 0 to 20 seconds, i.e. adsorber C is evacuated. Air enriched with oxygen is withdrawn from adsorber A which has completed its release of oxygen product and this air enters adsorber C as scavenging gas, while adsorber C is evacuated by the vacuum pump (V). Ambient air is introduced into adsorber B by the blower (G) and yuields air enriched with oxygen as product, which is withdrawn by the blower (R).

Step 4: 80 to 120 seconds Process step analogous to step 20 to 60 seconds, i.e. adsorber A is evacuated by the vacuum pump (V). Adsorber B yields product air enriched with oxygen by way of the blower (G) and compressor (R). Adsorber C is filled with air enriched with oxygen from adsorber B until it reaches the adsorption pressure.

Step 5: 120 to 140 seconds Process step analogous to step 0 to 20 seconds, i.e. adsorber A is rinsed with air enriched with oxygen from adsorber B which has completed its release of product, and adsorber A is evacuated by the vacuum pump (V). Adsorber C is charged with ambient air at the adsorption pressure of 1 bar (abs) by means of the blower (G) and air enriched with oxygen leaves adsorber B and is withdawn as product by the compressor (R).

Step 6: 140 to 180 seconds Process step analogous to the time cycle 20 to 60 seconds, i.e. adsorber A is charged to the adsorption pressure with air enriched with oxygen from adsorber C. Ambient air enters adsorber C by way of the blower (G) while air enriched with oxygen leaves adsorber C and is withdrawn as product by the compressor (R). Adsorber B is evacuated by the vacuum pump (V).

When the oxygen concentration was 90%, an oxygen product rate of 0.76 Nm3/h, based on 100% oxygen, could be obtained. When the oxygen concentration was 80%, the oxygen product rate, based on 100% oxygen, was 1.02 Nm3/h.

Compared with the known processes of Example 1, the oxygen product rate could thus be increased by 12.6% when the oxygen concentration was 90% and by 7% when the oxygen concentration was 80%.

Example 3 Figure 2 is the flow chart of a process according to the present invention for enriching air with oxygen by the pressure change technique using three adsorbers. Desorption is carried out by evacuating the bed of adsorbent in countercurrent to the adsorption. At the end of desorption, air or rinsing gas enriched with oxygen is withdrawn, in the same direction of flow as that of adsorption, from the adsorber which has previously completed its step of adsorption or release of product. This rinsing gas is passed through the adsorber which is to be desorbed by means of a vacuum pump in counter-current to the adsorption, but during this step the adsorber from which the rinsing gas is removed undergoes a pressure drop due to the adsorption inlet end being closed.

In Example 3, this pressure fell from 1 bar (abs) to 770 mbar (abs), The optimum rate of removal of rinsing gas, i.e. the optimum lowering of pressure, needs to be determined experimentally since it depends, interalia, upon the quality of the adsorbent used and the nature of the vacuum pump.

A process progam analogous to that of Example 2 was used in the experiment of Example 3. The size of the adsorbers, the temperatures, adsorption pressure, quantities and types of adsorbent and size of the vacuum pump were the same as in the experiment of Example 1.

The various steps of the process of Example 3 will now be explained in detail with respect to the first two cycles.

Step 1: Oto 20 seconds Ambient air enters adsorber A at the adsorption pressure of 1 bar (abs) by way of the blower (G), pipe L 22 and valve 21 A. Air enriched with oxygen leaves the adsorber A by way of the valve 24 A and pipe L 23 and is removed as product by the compressor (R).

Adsorber B is at its last phase of regeneration, i.e.

adsorber B is evacuated by the vacuum pump (V) by way of the valve 22 B and pipe L 21 and air enriched with oxygen is transferred from adsorber C to adsorber B by way of the valve 25 C, valve 26, pipe L 24 and valve 23 B. During this time, valve 21 Cis closed so that the pressure in adsorber C falls from 1 bar (abs), for example, to 770 mbar (abs). The rinsing gas introduced into adsorber B does not, as in Example 1, result in a final pressure of 200 mbar at the end of the evacuation cycle, but in a final pressure of from 220 to 300 mbar, depending on the quantity of rinsing gas. Valves 22 A, 23A, 25 A, 21 B, 24 B, 25 B, 21 C, 22 C, 23 C, 24 C, and 25 are closed.

Step 2: 20 to 60 seconds Ambient air continues to enter adsorber A by way of blower (G), pipe L 22 and valve 21 A. Air enriched with oxygen leaves adsorber A through valve 24 A and pipe L 23 and is removed as product by the compressor (R).

Adsorber B is charged to the adsorption pressure with air enriched with oxygen, gas from pipe L 23 entering the adsorber B by way of the valve 25, pipe L 24 and valve 23 B.

Adsorber C is evacuated by the vacuum pump (V) by way of valve 22 C and pipe L 21. Valves 22 A, 23 A, 25A,21 B, 22 B, 24 B, 25 B, 21 C, 23 C, 24 C and 25 C are closed.

Step 3: 60 to 80 seconds Process step analogous to the time cycle of 0 to 20 seconds, i.e. adsorber C continues to be evacuated, the adsorption inlet end of adsorber A is closed (valves 21 A/22 A) and gas enriched with oxygen is evacuated from adsorber A in the same direction of flow as that of adsorption, and flows as rinsing gas through adsorber C. Adsorber B yields air enriched with oxygen as product.

Step 4: 80 to 120 seconds Process step analogous to the time cycle 20 to 60 seconds, i.e. adsorber B delivers air enriched with oxygen, which is recieved as product by the compressor (R). The adsorber C is charged to the adsorption pressure with air enriched with oxygen from adsorber B. Adsorber A is evacuated by the vacuum pump (V).

Step 5: 120 to 140 seconds Analgous to the time cycle of 0 to 20 seconds, i.e.

adsorber A continues to be evacuated while the adsorption inlet end of adsorber B is closed (valve 21 B/22 B) and gas enriched with oxygen is withdrawn from adsorber B in the same direction of flow as that of adsorption and flows through adsorber A as rinsing gas. Adsorber C yields air enriched with oxygen as product.

Step 6: 140 to 180 seconds Process step analogous to the time cycle of 20 to 60 seconds, i.e. adsorber C yields air enriched with oxygem which is delivered as product to the compressor (R). Adsorber A is charged to the adsorption pressure with air enriched with oxygen from adsorber C. Adsorber B is evacuated by the vacuum pump (V).

In the experiment of Example 3, an oxygen product rate of 0.87 Nm3/h, based on 100% oxygen, could be obtained from the compressor (R) at an oxygen concentration of 90%. When the oxygen concentration was 80%, the oxygen product rate, based on 100% oxygen, was 1.02 Nm3/h. Compared with the experiment from Example 2, the oxygen product rate could thus be increased by 29% when the oxygen concentration was 90% and by 13.5% when the oxygen concentration was 80%.

Further improvement in the process according to the present invention is obtained by using four adsorbers. In the system using three adsorbers, the time required for filling the adsorbers to the adsorption pressure is shortened by the final rinsing step, i.e. it is not equal to the adsorption time. During the time of the final rinsing the blower (G) supplies air at a rate below the average while during the filling process it supplies air at an above average rate.

By providing four adsorbers, a constant delivery rate is obtained from the blower (G) and, as Example 4 indicates, a higher oxygen product rate is obtained, e.g. compared with the process of Example 3. The investment costs for a four-adsorber system are, however, higher than for a three-adsorber system.

Since in the four adsorber system a lower energy consumption of vacuum pump is possible for the same oxygen product rates (Nm3/h) owing to the higher specific oxygen product rates (Nm3 oxygen/h times kg of zeolite of an adsorber), the overall investment and operating costs for the four adsorber system may prove to be advantageous.

A program analogous to that of Example 1 was used for the experiments in Example 4. The size of the adsorbers, the temperatures, adsorption pressure, quantities and types of adsorbents and size of vacuum pump were the same as in the experiment of Example 1.

Example 4 Step 1: 0 to 60 seconds Adsorber A supplies air enriched with oxygen, i.e.

the blower (G) supplies air into adsorber A by way of the pipe L 32 and valve 31 A and air enriched with oxygen leaves the adsorber A by way of the valve 34 A and pipe L 33 and is withdrawn as product by the compressor (R). Adsorber B is charged with air enriched with oxygen from adsorber A by way of a flow control valve 36, pipe L 34 and valve 33 B so that the pressure in adsorber B is raised from its lowest desorption pressure to the adsorption pressure of 1 bar (abs). Adsorber B is desorbed or evacuated by the vacuum pump (V) by way of the pipe L 31 and valve 32 B and rinsing gas is withdawn from adsorber Din countercurrent to adsorption. Since the valves 31 D and 32 D are closed, the pressure in adsorber D falls, e.g. from 1 bar (abs) to 770 mbar (abs) and the rinsing gas from adsorber D enters adsorber C by way of the valve 35 D, pipe L 35, restrictor 39 and valve 35 C.Valves 32 A, 33 A, 35 A, 31 B, 32 B, 34 B, 35 B, 31 C,33 C,34 C,31 D, 32 D, 33 D, 34 D are closed.

Step 2: 60 to 120 seconds Analogously to the time cycle to 0 to 60 seconds, adsorber B delivers air enriched with oxygen, and part of this enriched air is used to charge adsorber C to an adsorption pressure of 1 bar (abs). Adsorber D is evacuated and rinsing gas is withdrawn from adsorber A and introduced into adsorber D.

Step 3: 120 to 180 seconds Analogously to the time cycle of 0 to 60 seconds, adsorber C yields air enriched with oxygen, part of which is used to charge adsorber D to the adsorption pressure. Adsorber A is evacuated and rinsing gas is withdrawn from adsorber B and introduced into adsorber A.

Step 4: 180 to 240 seconds Analogously to the time cycle of 0 to 60 seconds, adsorber D delivers air enriched with oxygen, and part of this enriched air is used to charge adsorber A to the adsorption pressure. Adsorber B is evacuated and rinsing gas is withdrawn from adsorber C and introduced into adsorber B.

In the experiment of Example 4, an oxygen product rate of 0.93 Nm3/h, based on 100% oxygen, could be obtained from compressor (R) when the oxygen concentration was 90%. At an oxygen con centration of 80 %, the oxygen product rate, based on 100% oxygen, was 1.05 Nm3/h. The process of Example 4 thus provided a product increase of 38% compared with Example 1 at an oxygen concentra tion of 90% and an increase of 17% at an oxygen concentration of 80%.

It will be appreciated that the instant specification and examples are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.

Claims

1. A pressure change adsorption process for the purification and separation of a gaseous mixture which comprises, using at least three adsorbent zones, desorbing the adsorbed component(s) at a pressure below 1 bar (abs) in counter-current to the direction of adsorption, the adsorbent to be desorbed being scavenged with a stream of gas withdrawn in the direction of adsorption from the adsorber which has completed its step of adsorption or release of product, the adsorber simultaneously being maintained at the adsorption pressure by the crude gas inlet being open, or the pressure in the adsorber being reduced to below the adsorption pressure by the crude gas inlet being closed.

2. A process as claimed in claim 1 in which a molecular sieve zeolite of type A and/or X is used in at least one of the adsorbent zones.

3. A process as claimed in claim 1 or claim 2 in which an adsorption pressure of from 1 to 4 bar (abs) is employed at a constant scavenging gas delivery pressure of from 1 to 4 bar (abs) and a desorption pressure below 1 bar (abs).

4. A process as claimed in any of claims 1 to 3 in which an adsorption pressure of from 1 to 4 bar (abs), an initial scavenging gas delivery pressure of from 1 to 4 bar (abs) and a final scavenging gas delivery pressure below 1 bar (abs) are employed.

5. A process as claimed in any of claims 1 to 4 in which the gaseous mixture is air.

6. A process as claimed in claim 1 substantially as herein described with particular reference to the Examples and/or the accompanying drawings.