WO1994026394A1

WO1994026394A1 - Gas separation process and apparatus

Info

Publication number: WO1994026394A1
Application number: PCT/GB1994/001026
Authority: WO
Inventors: Mark Richard Hesketh; George Gaudern Le Hunte; John Kenneth Rurik Page
Original assignee: Calor Air Separation Limited
Priority date: 1993-05-17
Filing date: 1994-05-12
Publication date: 1994-11-24
Also published as: AU6661494A; GB9310136D0

Abstract

A gas separation process is described for producing two products which differ in composition. The process comprises passing a gaseous mixture through a membrane separator (4), the composition of the output of which depends upon the gas flow rate therethrough and on the composition of the gaseous mixture. The output of the sparator (4) is passed to a selector valve assembly (27, 28, 29, 30) whereby the output can be directed selectively to one or another of at least two product storage vessels (50, 60). The status of the selector valve assembly (27, 28, 29, 30) is adjusted in response to the gas pressure in at least one of the storage vessels.

Description

GAS SEPARATION PROCESS AND APPARATUS

Background to the Invention

This invention relates to a gas separation process and an apparatus suitable for use in such a process.

Processes for the separation of the components of a gas mixture are well known. In particular, membrane air separation processes are known whereby nitrogen rich gas is generated for use in contact with foods and beverages to reduce the oxidative degradation of these products. It is often found that once an apparatus for carrying out such a process has been installed, it is convenient to use the same gas product for other (secondary) applications which do not in fact require the same degree of nitrogen purity as the primary application. As an example, in the pressure dispensing of beverages from kegs or similar containers, the oxygen content of the gas is usually kept below 1%. As a further example, in the packaging of fresh or cut vegetables, the storage life of the product can be extended by enveloping the product in a dry inert gas atmosphere, typically primarily nitrogen, within a semi-permeable polymer film. However, for secondary applications such as purging tubing of beverage dispensing apparatus after cleaning or for operating pneumatically-driven pumps, this low level of oxygen is unnecessary. Furthermore, these secondary applications tend to place significant extra demands on the capacity of a given generator system, which, if these demands are to be met, require a larger and more complicated system to be installed.

Membrane gas generator systems normally incorporate an air compressor, which in theory could be used to generate compressed air for use in secondary applications, but ^'because the compressor capacitor may not be sufficient to cope with the flow rates demanded, it would thus be necessary to install storage vessels to contain a sufficient volume of compressed air. A disadvantage of this approach would be that the compressed air storage vessels would need to be inspected regularly for corrosion by moisture in the air, and periodically replaced.

It is thus an object of the present invention to provide a process and apparatus for generating two gaseous products of different composition, whereby one product can be used for a primary application while the other product can be used for secondary applications.

It is a further object of this invention to provide a process and apparatus for generating two gaseous products while minimising compressor demands.

Summary of the Invention

According to a first aspect of the invention, there is provided a gas separation process for producing two products which differ in composition, the process comprising the steps of:

(i) passing a gaseous mixture through gas separation means, the composition of the output of which depends upon the gas flow rate therethrough and on the composition of the gaseous mixture;

(ii) passing the output of the gas separation means to a selector valve assembly whereby the output can be directed selectively to one or another of at least two product storage vessels; and

(iii) adjusting the status of the selector valve assembly in response to the gas pressure in at least one of said storage vessels.

According to a first embodiment of the invention, step (iii) includes adjusting the gas flow rate through the gas separation means in response to the gas pressure in at least one of said storage vessels.

The invention thereby enables^~a-ga-s^~mlxLure"tσ be separated into at least two products of different composition. As one of these products becomes depleted by use, the pressure in the respective storage vessel falls, triggering an adjustment in the gas flow rate through the separator and a feeding of the output of the separator to the depleted storage vessel.

We prefer that the process effectively allocates priority to generation of the lower purity product, that is the product formed by separation at the higher flow rate. Thus, if the gas pressure in that vessel which stores the lower purity gas falls below pre-determined level, the process will commence generation of that gas irrespective of its prior activity.

Once the pressure in that vessel is restored, the process will either turn off or revert to generation of the other, higher purity, gas if a demand for that other gas still exists.

According to a second aspect of the invention, there is provided a gas separation apparatus for producing and storing two products which differ in composition, the apparatus comprising:

(a) gas separation means, the composition of the output of which depends upon the gas flow rate therethrough;

(b) a selector valve assembly connected to the output of the gas separation means;

(c) at least two storage vessels connected to the selectable outputs of the selector valve assembly; and

(d) control means for adjusting the status of the selector valve assembly in response to the pressure in at least one of the storage vessels.

In one embodiment of the invention, the apparatus further comprises:

(e) means for adjusting the flow rate of a gaseous mixture through the gas separation means; and

(f) control means for adjusting the gas flow rate through the gas separation means in response to the pressure in at least one of the storage vessels.

In a preferred embodiment of the invention, the gaseous mixture is air, although the invention is equally applicable to other gaseous mixtures.

When the gaseous mixture is air, the separator will normally generate a useful product which is nitrogen containing a reduced level of oxygen and other constituents of air, such as water vapour, and a waste product which comprises oxygen-enriched air. Since the process makes use of a separator which produces a product whose composition is dependent on the gas flow rate therethrough, the level of oxygen and other contaminants in the nitrogen product depends on this flow rate. In this embodiment the process produces two products which comprise nitrogen containing different levels of oxygen therein, the level of oxygen being a function of the flow rate of the air through the separator. The invention has the advantage that the less pure nitrogen product, that is the product required for secondary applications, is generated at a higher rate than the more- pure nitrogen product. Hence, overall, the compressor will operate for shorter times than would be called for if all applications used the pure gas. A further advantage is that for a given size of installation, the apparatus is able to satisfy all demands for gas with greater reliability and shorter running times.

The gas separation means is preferably a permeable gas separator. Such separators are known in which the output gas concentration is a function of the flow-rate therethrough, and in particular where the relationship between composition and flow-rate are substantially linear. Thus, in the separation of air, an enriched nitrogen product may be obtained in which the level of oxygen is proportional to the flow rate of air through the membrane. We prefer to operate the process of the present invention under conditions in which the composition to flow-rate relationship is substantially linear.

In a preferred version of the invention, the gaseous mixture is air and the oxygen level in one of the two products is less than 1% by volume, while the oxygen level in the other of the two products is more than 1% by volume. The process described herein enables the moisture level in one of the two products, ie the purer nitrogen product, to be less than 100 vpm, although this level of dryness can be achieved in both products.

Nitrogen containing less than 1% oxygen and less than 100 vpm moisture has a number of uses. In particular, such a gas is useful in the dispensing of beverages under pressure. The process according to the invention may include the addition of further gaseous components to one or both products before use. Thus, dry oxygen-free carbon dioxide may be^~added^~to the niL-iu eii products. Such a nitrogen/carbon dioxide mixture is useful as a distribution gas for carbonated beverages.

The apparatus preferably includes means to cut off the flow of gaseous mixture through the separator when the pressure in each of the the storage vessels exceeds a predetermined threshold. Thus, when the storage vessels are in a "full" state, no further product is supplied. When one or both storage vessels become depleted, flow to the separator is restored thereby to generate further product to replace that which has been used.

The gaseous mixture may be fed to the separator by a compressor, which may be powered via at least one pressure switch.

In a first embodiment according to the invention, the selector valve assembly comprises an assembly of YES and NOT valves. In a particular example of this embodiment, the first flow control valve is connected to pass gas to the input port of a first YES valve, the second flow control valve is connected to pass gas to the input port of a first NOT valve, the pilot ports of the first YES and NOT valves are connected to a common pilot line, the output ports of the first YES and NOT valves are connected to a common output line leading to the input ports of a second YES valve and a second NOT valve, the pilot ports of the second NOT and YES valves are both connected to the common pilot line, the output of the second YES valve is connected to a first storage vessel, and the output of the second NOT valve is connected to the second storage vessel.

In a second embodiment of the apparatus according to the invention, a first leasure switch: is associated with the first storage vessel and a second pressure switch is associated with the second storage vessel, the arrangement being such that when the pressure in either of the storage vessels falls below a predetermined threshold, the associated pressure switch is closed to complete the connection of power to the compressor.

In alternative embodiments of the invention, the selector valve assembly is electrically operated. At least one of the storage vessels may have a pressure sensor associated therewith, this pressure sensor generating an electrical signal indicative of the pressure in the associated storage vessel. The control means may enable the generation of an electric signal to operate the selector valve assembly in response to electrical signals received from the or each such pressure sensor.

It is to be understood that while the description herein describes the production of two useful products, the invention is equally applicable to processes and apparatus for the production of three or more useful products. The accompanying drawings

The invention will now be further described with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of one embodiment of an apparatus according to the invention;

Figure 2 is a schematic representation of an alternative embodiment of an apparatus according to the invention; and

Figure 2A is a cross-section through- the—control- assembly of the embodiment shown in Figure 2.

In the apparatus shown in Figure 1, an air compressor 1 supplies compressed air via an inlet filter 2 to a permeable membrane separator 4. The inlet pressure to the separator is controlled by a biased relief valve 3. The permeable membrane separator, such as a Permea Inc. PPA-21A (2.5cm x 30cm) separates the compressed air into an oxygen-rich permeate gas which is vented via port 41, and a dry nitrogen-rich retentate gas which leaves the separator at exit 42.

Two flow rate control valves 5, 6 are connected in parallel to the exit 42 of the separator 4. The control valves each have a manually adjustable flow adjuster 5A, 6A by means of which the flow rate through the respective flow control valve can be set. As shown, the flow control valve 5 is set to allow a higher flow rate than flow control valve 6. Since the oxygen level in the nitrogen output of the separator depends on the flow rate therethrough, this in turn depends on through which of the flow control valves 5, 6 the gas is passing. Thus, relative pure nitrogen is generated when the gas passes through flow control valve 6 while less pure nitrogen is generated when the gas passes through control valve 5. The outputs 57, 67 from the flow control valves 5, 6 are fed to a selector valve assembly which consists of an assembly of YES and NOT logic valves 27, 28, 29 and 30.

Relatively impure gas is passed from the flow control valve 5 to the input port of a first YES valve 27, while relatively pure gas from flow control valve 6 is passed to the input port of a first NOT valve 28. The pilot ports of valves^~27 and 28 are connected to a common pilot line 31. The output ports of the valves 27 and 28 are connected via non-return valves 32, 33 to a common output line 34 leading to the input ports of a second YES valve 29 and a second NOT valve 30.

The electrical power to the compressor 1 is fed via a pressure switch 11, to which the common output line 34 is connected. The pilot ports of the second YES and NOT valves 29 and 30 are both connected to the common pilot line 31. The output of the second YES valve 29 is connected via a non-return valve 36 via line 51 to an impure gas storage vessel 50 while the output of the second NOT valve 30 is connected via a non-return valve 36 and line 61 to a pure gas storage vessel 60.

The common pilot line 31 is connected to the output of the compressor l via a YES valve 40, the pilot port of which is connected to the pure gas storage vessel 60 via a line 43.

The arrangement shown in Figure 1 operates as follows. When the pressure in the storage vessel 60 becomes depleted, this is sensed via the common output line 34 and causes the pressure switch 11 to turn on the compressor 1. The reduced pressure in the storage vessel 60 causes the valve 40 to close, preventing the output of the compressor 1 being fed to the common pilot line 31. In the absence of pressure in the pilot line 31, the NOT valves 28 and 30 are in an open status. The compressor feeds compressed air through the separator 4, the flow control valve 6, and the NOT valves 28 and 30 to the storage vessel 60. Once pressure in the storage vessel 60 is restored, the YES valve 40 opens in response to pressure in the line 43 and the output of the compressor 1 is thereby applied to the pilot line 31. This causes tiie- NOTc lve 28 and 30 to close and simultaneously the YES valves 27 and 29 to open. The compressor now feeds compressed air through the separator 4, the flow control valve 5, and the YES valves 27 and 29 to the storage vessel 50, replacing any depletion of gas pressure therein. Once pressure in the storage vessel 50 is restored, this is sensed by a build up of pressure in the line 34, whereupon the pressure valve 11 is triggered to turn the compressor 1 off.

The arrangement shown in Figure 1 is triggered into operation by a depletion of the pure gas in storage vessel 60, and then acts to restore the supply of pure gas to storage vessel 60 and to restore the supply of less pure gas in storage vessel 50, should that have also become depleted.

It may be desirable that the system should give priority, not to the depletion of the pure gas in vessel 60, but to the depletion of the less pure gas in vessel 50. This can be achieved by a modification of the arrangement shown in Figure 1, whereby the YES valve 40, instead of being connected to the storage vessel 60, is connected to the storage vessel 50, while it would also be necessary to exchange the second YES and NOT valves 29 and 30. Loss of pressure in the storage vessel 50 will then trigger the supply of less pure gas thereto until pressure is restored, and thereafter any loss of pressure in the pure gas storage vessel 60 is restored.

In a modification of the arrangement shown in Figure 1, a leak valve is connected to the common output line 34, so that in the at-rest position, pressure in the common output line 34 slowly falls, eventually reaching the point where the pressure switch 11 turns the compressor 1 on, causing depletion, II any, in- ither of the storage vessels to be restored. This arrangement would have the advantage that a significant depletion in the storage vessel 50 would not go unrestored, despite a maintenance of pressure in the storage vessel 60.

In the embodiment shown in Figures 2 and 2A, a number of elements of the embodiment of Figure 1 are retained, and these are given reference numerals similar to those used in Figure 1, but with a 300 pre-fix. Thus, an air compressor 301 supplies compressed air via an inlet filter 302 to a permeable membrane separator 304. Power to the compressor 301 is fed via two pressure switches 312, 313 connected in parallel, such that electrical power is fed to the compressor when either of these pressure switches is activated. The pressure switches 313, 312 are connected to the nitrogen reservoirs 350 and 360 via lines 316, 317 respectively.

The inlet pressure to the separator 304 is controlled by a biased relief valve 303. The permeable membrane separator separates the compressed air into an oxygen-rich permeate gas which is vented via port 341, and a dry nitrogen-rich retentate gas which leaves the separator at exit 342.

In this embodiment, the control system is mainly located in a common unit, indicated by the broken lines 300 in Figure 2 and shown in detail in Figure 2A. Output chambers 380 and 381 are provided with carbon filters 382, 383 to filter output gases passing respectively to reservoirs 350 and 360. Flow rates to the high purity nitrogen reservoir 360 and the low purity reservoir 350 are maintained at a constant rate by flow control valves 305 and 306 respectively. This arrangement has the advantage that the switching functions are now ca -tied out upstream of the flow control valves. Since the latter are here designed with integral non-return valves 335, 336, it is possible to omit the four external non-return valves present in the arrangement shown in Figure 1, namely 32, 33, 35 and 36.

Selection of the appropriate flow control valve is made by the combination of a YES valve 372 and a NOT valve 374 whose pilot ports are connected via line 331 to the output from an adjustable YES valve 370. As will be apparent from Figure 2A, the input port of adjustable YES valve 370 is connected to the compressor output via line 315 and the pilot port of valve 370 is connected to the storage vessel 350 via line 314.

The pressure in the high purity reservoir 360 is set by its pressure switch 312 turning off the compressor 301. The pressure in the low purity reservoir 350 is set by the adjustable YES valve 370 and must be above the setting of the pressure switch 313 for this reservoir.

On initial start-up, both pressure switches 312 and 313 will be on and the system will charge the low purity reservoir 350, until the pressure set by the adjustable YES valve 370 is reached (for example more than 10.5 bar) . The adjustable YES valve 370 will then switch the YES and NOT valves 372, 374 and allow the high purity reservoir 360 to charge. When a pressure of, for example, 11 bar is reached, the pressure switch 312 will open and shut down the system.

Either pressure switch 312 or 313 will turn on the compressor if the the pressures in reservoirs fall below 9.0 bar for low purity nitrogen and 9.5 bar for the high purity nitrogen.

Claims

1. A gas separation process for producing two products which differ in composition, the process comprising the steps of:

2. A gas separation process according to claim 1, wherein the gas separation means generates a relatively pure product when operated at a low gas flow rate therethrough and a relatively impure product when operated at a high gas flow rate therethrough.

3. A gas separation process according to claim 2, wherein step (iii) includes adjusting the gas flow rate through the gas separation means in response to the gas pressure in at least one of said storage vessels.

4. A gas separation process according to claim 3, wherein ^"the adjustment of gas flow rate and the status of the selector valve assembly in step (iii) is made in response to the gas pressure in the storage vessel to which the relatively pure product is directed.

5. A gas separation process according to any preceding claim, wherein means are provided to cut off the flow of gaseous mixture through the gas separation means when the pressure in each of the storage vessels exceeds a pre-determined threshold.

6. A gas separation process according to any preceding claim, wherein the gaseous mixture is air.

7. A gas separation process according to claim 6, wherein the two products comprise nitrogen containing different levels of oxygen therein.

8. A gas separation process according to claim 7, wherein the oxygen level in one of the two products is less than 1% by volume.

9. A gas separation process according to claim 8, wherein the moisture level in one of the two products is less than 100 vpm.

10. A gas separation process according to claims 8 or 9, wherein the oxygen level in the other of the two products is more than 1% by volume.

11. A gas separation apparatus for producing and storing two products which differ in composition, the apparatus comprising:

(a) gas separation means, the composition of the output of which depends upon the gas flow rate therethrough and on the composition of the gaseous mixture;

12. A gas separation apparatus according to claim 11, the apparatus further comprising

13. A gas separation apparatus according to claim 11 or 12, wherein the gas separation means is a permeable gas separator.

14. A gas separation apparatus according to any one of claims 11 to 13, wherein the gaseous mixture is fed to the gas separation means by a compressor.

15. A gas separation apparatus according to claim 14, wherein the compressor is powered via at least one pressure switch.

16. A gas separation apparatus according to claim 15, wherein the pressure switch is connected to the inlet of the selector valve assembly, the arrangement being such that the pressure switch is triggered to operate the compressor in response to the pressure in the storage vessels falling below a predetermined threshold.

17. A gas separation apparatus according to any one of claims 11 to 16, wherein the selector valve assembly comprises an assembly of YES and NOT valves.

18. A gas separation apparatus according to claim 17, wherein the first flow control valve is connected to pass gas to the input port of a first YES valve, the second flow control valve is connected to pass gas to the input port of a first NOT valve, the pilot ports of the first YES and NOT valves are connected to a common pilot line, the output ports of the first YES and NOT valves are connected to a common output line leading to the input ports of a second YES valve and a second NOT valve, the pilot ports of the second NOT and YES valves are both connected to the common pilot line, the output of the second YES valve is connected to a first storage vessel, and the output of the second NOT valve is connected to the second storage vessel.

19. A gas separation apparatus according to claim 15, wherein a first pressure switch is associated with the first storage vessel and a second pressure switch is associated with the second storage vessel, the arrangement being such that when the pressure in either of the storage vessels falls below a predetermined threshold, the associated pressure switch is closed to complete the connection of power to the compressor.

20. A gas separation apparatus according to any one of claims 11 to 16, wherein said selector valve assembly is electrically operated.

21. A gas separation apparatus according to claim 20, wherein at least one of said storage vessels has a pressure sensor associated therewith, said pressure sensor generating an electrical signal indicative of the pressure in the associated storage vessel.

22. A gas separator apparatus according to claim 21, wherein said control means enables the generation of an electric signal to operate said selector valve assembly in response to electrical signals received from the or each said pressure sensor.