GB1572532A

GB1572532A - Method for separation of a gaseous mixture

Info

Publication number: GB1572532A
Application number: GB40944/77A
Authority: GB
Original assignee: AGA AB
Current assignee: AGA AB
Priority date: 1976-10-04
Filing date: 1977-10-03
Publication date: 1980-07-30
Also published as: DE2743861C2; DK438977A; DE2743861A1; NL7710848A; FR2366050A1; SE409553B; NO773366L; NO145420B; FI62229B; BE859324A; FI62229C; SE7610947L; IT1085299B; FR2366050B1; DK148997C; DK148997B; NO145420C; JPS5345675A; FI772916A

Description

(54) METHOD FOR SEPARATION OF A GASEOUS MIXTURE (71) We, AGA AKTIEBOLAG, a Swedish Company, of S-181 81 Liying5, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention concerns a method for the separation of a gaseous mixture having at least two components, one of these components being periodically adsorbed at and subsequently desorbed from a material with the capacity of preferentially adsorbing this component at a relatively high pressure, whereas the desorption is achieved by lowering the pressure to a relatively low pressure. In this method at least two beds are employed, being alternately active, so that the adsorption takes place in one bed substantially simultaneously with an other bed being regenerated by desorption and vice versa.

A method according to this principle is previousyly known for instance from the Swedish patent 201 340 (or from the US patent 2 944 627). The present invention has for its purpose to achieve a more complete separation and a better recovery of the less readily adsorbable component of the gaseous mixture.

According to the invention, there is provided a method for fractionating a gaseous mixture using at least a first and a second bed of a material which at a relatively high pressure preferentially adsorbs one component of the gaseous mixture, in which each bed is alternately pressurised to and kept at the relatively high pressure and depressurised, and in which pressurisation of the first bed is made in three steps such that during the first step product gas substantially consisting of the less readily adsorbable component is supplied to the first bed through the outlet side thereof, while its inlet side is closed, during the second step the inlet side of the first bed is connected with the outlet side of the second bed with the second bed at adsorption pressure whilst both the outlet side of the first bed and the inlet side of the second bed are closed so as to cause a pressure equalisation between the beds, and during the third step the gaseous mixture is supplied to the inlet side of the first bed while its outlet side is closed, until the said relatively high pressure has been reached in the first bed.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings in which: - Figure 1 shows schematically a circuit diagram of a gas separation system; Figure 2 shows schematically a time sequence of operational phases of the system; Figure 3 shows gas flows occurring in the system during various operational phases; and Figure 4 shows concentration profiles in an adsorption bed at various times during an operational cycle.

The system which is schematically shown in fig. 1 may be used for the separation of an arbitrary gaseous mixture, but in the present case it is assumed that it is devised for fractionation of air so that the product gas will be oxygen with substantially no content of nitrogen. The system comprises two beds 1 and 2. Each one of these comprises a zeolitic molecular sieve with the capacity of preferentially adsorbing nitrogen rather than oxygen at a relatively high pressure. The raw gas which in this case is compressed air is supplied to a connection 3. The supplied air may be compressed to a pressure of about 4 bar (60psi). From the connection 3 the raw gas passes through a pressure regulator 4 and an adjustable choke 5 to a pair of controllable valves 6 and 7. Via these valves the raw gas can be directed either to the bed 1 or to the bed 2. The product gas which leaves the bed 1 at the outlet side thereof reaches, via a controllable valve 8 and an adjustable choke 9, the product tank 10 which is intended for storing the product gas. The product gas which is generated in the bed 2 reaches in a similar way the product tank 10 via a controllable valve 11 and the choke 9. The product gas is delivered from the product tank through an outlet conduit 12. The choke 9 is inserted in a product conduit 13.

In a manner known per se the outlet side of each bed can be connected to the inlet side of the other bed. This is achieved via controllable valves 14 and 15 at the outlet ends of the beds and controllable valves 16 and 17 at the inlet ends of the beds and an adjustable choke 18 inserted between these pairs of valves. Via controllable valves 19 and 20, the beds 1 and 2, respectively, can be connected with the surrounding atmosphere or with a vacuum pump via a purge conduit 21.

In order to enable purging and repressurizing of one bed with product gas independently of the operational phase of the other bed the system is so designed that product gas from the tank 10 can be supplied via a conduit 22 and one of the controllable valves 14 and 15 to the outlet side of the beds. Due to reasons explained below, the conduit 22 consists of two parallel branches, each branch including a controllable valve (23 and 24, respectively) and an adjustable choke (25 and 26, respectively).

The different controllable valves are actuated, preferably automatically, so that during the operational phases designated I X, which together form a complete operational cycle, those gas flows are created which are shown in fig. 3. The corresponding operational phases in relation to the time are shown in fig. 2, in which the upper part shows the operational phases of the bed 1 and the lower part shows the operational phases of the bed 2. In the bed 1 an adsorption of the more readily adsorbable component and production of product gas takes place during the phases I and II, during the phase III a pressure decrease by pressure equalizing between the bed 1 and 2 (Ts 1) and during the phase IV a pressure decrease to the relatively low desorption pressure (Ts 2). During the phases V and VI the bed 1 is purged by the product gas whereupon during the phases VII-X a repressurizing of the bed 1 is made, i.e. during the phase VII with the product gas (To 2) during the phase VIII by pressure equalizing between the beds (To 1) and finally during the phases IX and X with the raw gas (To 3). Thereafter this bed is again ready for the production of product gas.

In the present example a complete cycle time = 208 seconds, and fig. 2 indicates the times when the system shifts from one operational phase to the next one. From figures 2 and 3 it is seen that the operational cycle for the bed 2 is phase displaced in relation to the operational cycle for the bed 1 in such a way that a depressurizing of one bed and a repressurizing of the other bed takes place during the pressure equalization between the beds.

Fig. 4 shows as an example the form of the profile of concentration, i.e. the content of oxygen, within different parts of the bed 1 from the upper inlet side to the lower outlet side. Each sub-figure shows the profile of concentration at a time indicated in the corresponding sub-figure at the transistion from one operational phase to the succeeding phase.

With reference to the figures, a more detailed description is given below of the processes which take place in the bed 1.

At the time 0 the adsorption pressure has been reached in the bed 1 by the supply of air during the operational phases IX and X. At the inlet side of the bed the molecular sieve is saturated with air having the existing pressure. The outlet end contains "pure" product gas, i.e. a gas which approximately consists of 95 % oxygen and 5 % argon. During the operational phase I the product gas is leaving the bed at a rate which is exactly controlled by the choke 9 while the pressure regulator 4 at the inlet end ensures that air is supplied in such a way that the pressure in the bed is kept constant. When the gas passes the bed a fractioning takes place since nitrogen is adsorbed to a higher extent than oxygen. As the adsorption proceeds, the bed will be more and more saturated with air.

By keeping the gas flow rate below a maximum critical value, a nitrogen-free zone is maintained nearest the outlet end during all the time when product gas is delivered from the bed.

During the operational phase II the production of a nitrogen-free product gas from the bed 1 continues. This production is interrupted at the time 66 (seconds) just before the content of nitrogen increases in the delivered product gas.

During the operational phase III, the outlet end of bed 1 has been connected with the inlet end of bed 2 simultaneously with the interruption of air to the bed 1 and as a consequence thereof a pressure decrease occurs in the bed 1. The resulting decrease in the partial pressure for the components of the gas causes a desorption of them in the bed. The first amount of the gas which leaves the bed 1 has a oxygen content of about 95 %, while the latest amount has a oxygen content of about 40-60 %. The pro portion of oxygen in the gas which leaves the bed 1 is accordingly much higher than the oxygen content in normal air.

During the operational phase IV a pressure decrease takes place in the bed 1 in that the inlet end thereof is connected with the surrounding atmosphere or with a vacuum pump. The decrease in the partial pressures causes a further desorption and a certain part of the desorbed material is purged.

During the operational phase V the bed 1 is purged from the outlet end to the inlet end in that nitrogen-free product gas is supplied from the product tank 10 over the controllable valve 23 and the adjustable choke 25 in the conduit 22. The adjustable choke is then so designed that the supply of product gas takes place comparatively slowly.

During the operational phase VI the purging of the bed 1 with product gas via the valve 23 and the choke 25 continues. The low partial pressure of nitrogen in the purging gas results in a desorption of nitrogen from the molecular sieve which is purged out of the bed.

At the beginning of the operational phase VII the outlet to the atmosphere is closed and the supply of product gas to the bed 1 continues, but now through the controllable valve 24 and the adjustable choke 26. The nitrogen-free product gas which now flows into the bed causes a pressure increase and also a desorption of nitrogen in the lower outlet end. The desorbed nitrogen is carried by the gasflow towards the upper inlet end of the bed and it is again adsorbed by the molecular sieve when the partial pressure of nitrogen exceeds the value which corresponds to an equilibrium status of the molecular sieve.

Accordingly, a transfer of nitrogen from the outlet end of the bed to the inlet end thereof takes place during the operational phase VII and simultaneously therewith a nitrogenfree zone is created at the outlet end of the bed.

At the beginning of the operational phase VIII the supply of product gas to the bed 1 is interrupted and the inlet end of the bed 1 is connected with the outlet end of the bed 2. Thus, gas will flow from the bed 2 to the bed 1 resulting in a pressure increase in the bed 1. This operational phase continues until a pressure equilibrium has been reached between the beds. To achieve this pressure increase in the bed 1 a gas having a relatively high content of oxygen has been used, as explained above in connection with the description of the operational phase III, however, this gas has a much lower content of oxygen than the gas which is at the outlet end of the bed 1 and the pressure equilization may not be performed so fast that the gas with a high content of nitrogen can flow through the bed 1 at a rate which is so high that the nitrogen will not have time to be adsorbed.

The necessary low flow rate is in this case achieved by means of the adjustable choke 18.

During the operational phase IX compressed air is supplied to the bed 1 through the inlet end. This results in a continued slow pressure increase and during this operational phase the bed becomes saturated with air at the inlet end at the same time as a nitrogen-free zone is maintained at the outlet end of the bed.

During the operational phase X the increase of pressure in the bed 1 continues by the supply of air until the adsorption pressure has been reached, whereupon the bed is ready for production which means that the operational phases described above will be repeated periodically.

It is clear from the above given description that the pressure increase in the bed 1 to the final pressure takes place in three steps. During the first step, the operational phase VII, the pressure increase is made with product gas from the product tank 10. Immediately before this operational phase a purging with product gas has taken place and the profile of concentration in the bed is then approximately linear, which is shown in the sub-figure of fig. 4 which is designated t = 152. When nitrogen-free product gas is supplied through the outlet end of the bed 1 during the operational phase VII and the pressure is increased, the relation between the partial pressure of oxygen and nitrogen is increased. The nitrogen is desorbed and carried further into the bed towards the inlet end thereof.

When this pressure increase is finished the bed has a profile of concentration which in fig. 4 is shown in the sub-figure designated t = 170. From this sub-figure it is clear that a zone with a nitrogen-free gas has been created nearest the outlet end. In this zone the final separation will later on take place during the adsorption. Gas containing nitrogen can now be admitted to the bed 1, but this has to be done through the inlet end of the bed since otherwise said nitrogen-free zone would be spoiled. In this connection it can be pointed out that if gas containing nitrogen had been admitted to the inlet end of the bed immediately after the purging during the operational phase VI then a certain amount of this nitrogen could have passed through the bed without being adsorbed. In such case gas containing nitrogen would be present at the outlet end and as a consequence thereof the product gas would contain nitrogen.

The second step of the pressure increase is achieved by a pressure equalization between the beds. For the bed 1 this takes place during the operational phase VIII.

During this pressure equalization a pressure decrease takes place in the bed 2 and a certain amount of nitrogen is then desorbed and is carried by the gas flow into the bed 1 which is to be repressurized. For that reason the gas is supplied to the inlet end of the bed 1 so that a nitrogen-free zone can be maintained at the outlet end.

The supplied gas containing nitrogen is fractionated in the bed 1 during this operational phase and at the end of the pressure equalization the profile of concentration has been reached which in fig. 4 is shown in the sub-figure designated t = 183.

The pressure increase to the final adsorption pressure takes place by the supply of air from the compressor through the inlet end of the bed 1. During this pressure increase a fractioning of the inflowing air takes place and when the final adsorption pressure has been reached after the operational phases IX and X the bed 1 has a profile of concentration according to the sub-figure of fig. 4 which is designated t = 208. The bed 1 is then again ready for production.

The repressurizing sequence described above differs from and presents improvements over previously known sequences in the following respects. A pressure increase with a product gas which is free from the more readily adsorbable component takes place as the first step and the product gas is supplied over the outlet end of the bed.

In this way a distribution of the concentration in the bed is created which is suitable for the fractioning of the gas which is supplied later on during the operational cycle.

Further on, all gas which contains the more readily adsorbable component is always supplied through the inlet end of the bed and only after the product gas which is free from the more readily adsorbable component has been supplied through the outlet end. Fractionation takes place for the gas which is supplied through the inlet end of the bed and the zone at the outlet end of the bed which is free from the more readily adsorbable component is maintained substantially intact. This is a condition suitable for the generation of a product gas which is free from the more readily adsorbs able component.

WHAT WE CLAIM IS: - 1. A method for fractionating a gaseous mixture using at least a first and a second bed of a material which at a relatively high pressure preferentially adsorbs one component of the gaseous mixture, in which each bed is alternately pressurized to and kept at the relatively high pressure and depressurized, and in which pressurization of the first bed is made in three steps such that during the first step product gas substantially consisting of the less readily adsorbable component is supplied to the first bed through the outlet side thereof, while its inlet side is closed, during the second step the inlet side of the first bed is connected with the outlet side of the second bed with the second bed at adsorption pressure whilst both the outlet side of the first bed and the inlet side of the second bed are closed so as to cause a pressure equalization between the beds, and during the third step the gaseous mixture is supplied to the inlet side of the first bed while its outlet side is closed, until the said relatively high pressure has been reached in the first bed.

2. A method as claimed in claim 1 in which the first step of the pressurization of the first bed is made simultaneously with the production from the second bed of a product gas consisting of the less readily adsorbable component of the gaseous mixture.

3. A method as claimed in claim 2, in which the product gas is supplied to a product tank through a product conduit and product gas for the first step of the pressurization of a bed is taken from the product tank through a conduit which is separate from the product conduit.

4. A method for fractionating a gaseous mixture substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. place during the operational phase VIII. During this pressure equalization a pressure decrease takes place in the bed 2 and a certain amount of nitrogen is then desorbed and is carried by the gas flow into the bed 1 which is to be repressurized. For that reason the gas is supplied to the inlet end of the bed 1 so that a nitrogen-free zone can be maintained at the outlet end. The supplied gas containing nitrogen is fractionated in the bed 1 during this operational phase and at the end of the pressure equalization the profile of concentration has been reached which in fig. 4 is shown in the sub-figure designated t = 183. The pressure increase to the final adsorption pressure takes place by the supply of air from the compressor through the inlet end of the bed 1. During this pressure increase a fractioning of the inflowing air takes place and when the final adsorption pressure has been reached after the operational phases IX and X the bed 1 has a profile of concentration according to the sub-figure of fig. 4 which is designated t = 208. The bed 1 is then again ready for production. The repressurizing sequence described above differs from and presents improvements over previously known sequences in the following respects. A pressure increase with a product gas which is free from the more readily adsorbable component takes place as the first step and the product gas is supplied over the outlet end of the bed. In this way a distribution of the concentration in the bed is created which is suitable for the fractioning of the gas which is supplied later on during the operational cycle. Further on, all gas which contains the more readily adsorbable component is always supplied through the inlet end of the bed and only after the product gas which is free from the more readily adsorbable component has been supplied through the outlet end. Fractionation takes place for the gas which is supplied through the inlet end of the bed and the zone at the outlet end of the bed which is free from the more readily adsorbable component is maintained substantially intact. This is a condition suitable for the generation of a product gas which is free from the more readily adsorbs able component. WHAT WE CLAIM IS: -

1. A method for fractionating a gaseous mixture using at least a first and a second bed of a material which at a relatively high pressure preferentially adsorbs one component of the gaseous mixture, in which each bed is alternately pressurized to and kept at the relatively high pressure and depressurized, and in which pressurization of the first bed is made in three steps such that during the first step product gas substantially consisting of the less readily adsorbable component is supplied to the first bed through the outlet side thereof, while its inlet side is closed, during the second step the inlet side of the first bed is connected with the outlet side of the second bed with the second bed at adsorption pressure whilst both the outlet side of the first bed and the inlet side of the second bed are closed so as to cause a pressure equalization between the beds, and during the third step the gaseous mixture is supplied to the inlet side of the first bed while its outlet side is closed, until the said relatively high pressure has been reached in the first bed.