WO1993013854A1

WO1993013854A1 - Mixed gas supply system

Info

Publication number: WO1993013854A1
Application number: PCT/GB1993/000074
Authority: WO
Inventors: John Kenneth Rurik Page
Original assignee: Calor Air Separation Limited
Priority date: 1992-01-14
Filing date: 1993-01-14
Publication date: 1993-07-22
Also published as: GB9200648D0; AU3264893A

Abstract

A system is described for the supply of a gas mixture having a selected ratio of constituent gases. The system comprises a supply line (14, 30) for a first constituent gas under pressure, a supply line (41, 50) for a second constituent gas under pressure, a mixing zone (51) for the said constituent gases and, in each of the constituent gas supply lines, a pilot-controlled biased-diaphragm control valve (26, 46) having a main chamber (6) and a pilot chamber (7). The supply lines include an adjustable restrictor valve (25, 45) upstream of the main chamber (6) of its diaphragm valve (26, 46). The pilot chambers (7) of both of the said diaphragm valves (26, 46) are connected to the first supply line (14, 30) at a point upstream of its restrictor valve (25, 45). The invention offers the advantage that the volumes and relative proportions of the first and second constituent gases are simply controlled by the relative settings of the restrictor valves in the respective supply lines.

Description

MIXED GAS SUPPLY SYSTEM

This invention relates to the supply of a mixture of gases. In particular it relates to a system which facilitates selection of the proportions of the constituent gases in the mixture. It is primarily concerned with the provision of gas mixtures in which nitrogen and carbon dioxide are the main constituents.

The use of nitrogen/carbon dioxide gas mixtures is known in the processing and distribution of food and drink products. For example, in the pressure-dispensing from kegs or barrels of carbonated beverages such as ales and lagers suitable nitrogen/carbon dioxide gas mixtures permit acceptably short serving times without leading to over-carbonation and consequent wastage of the beverages. The specific gas mixture for a given beverage is determined by the level of carbonation in the beverage, its temperature and the required tap-pressure. If the correct gas mixture is used, this will provide the partial pressure of carbon dioxide which results in no change in the beverage's carbonation, yet the overall tap gas pressure will permit its rapid displacement.

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 an mixed gas atmosphere, comprising typically 85% nitrogen, within a semi-permeable polymer film.

It is often advantageous to generate the mixed gas or gases on site instead of purchasing high-pressure cylinders containing pre-mixed gas. The normal method of on-site generation is to mix carbon dioxide, derived conveniently from such a source as a cylinder of the liquefied gas, with nitrogen gas supplied from receivers which are charged from an air separation system. The mixing of the two gases is conventionally carried out with special valves but these are generally complex, bulky and expensive.

According to the invention there is provided a system for the supply of a gas mixture having a selected ratio of constituent gases which system comprises a plurality of supply lines for respective constituent gases under pressure, a mixing zone for the said constituent gases and, in each of the constituent gas supply lines, a pilot-controlled biased-diaphragm control valve having a main chamber and a pilot chamber, characterised in that each supply line includes an adjustable restrictor valve upstream of the main chamber of its diaphragm valve, and in that the pilot chambers of the said diaphragm valves are all connected to the first supply line at a point upstream of its restrictor valve.

Although the invention can be applied to the mixing of three or more constituent gases, its major application is to the mixing of two constituent gases, and only the two-gas aspect will be considered from here on.

The invention offers the advantage that the volumes and relative proportions of the first and second constituent gases are simply controlled by the relative settings of the restrictor valves in the respective supply lines.

It offers the further advantage that any fluctuations in the supply pressure of the first constituent gas do not affect the concentration of the second constituent gas in the mixed gas.

In one important embodiment the present invention represents a simpler and lower cost method of generating mixed gas directly from a membrane air separation unit which is provided with compressed air and carbon dioxide as input gases. The mixed gas composition is fixed by adjustment of the carbon dioxide flow-rate in relation to the nitrogen flow-rate from the membrane, and the whole system is controlled in a manner similar to that used in a conventional nitrogen-generating air separator system.

In a particular embodiment of the invention described in detail herein (the "nitrogen/carbon dioxide'' embodiment) the first constituent gas is a nitrogen-enriched air fraction obtained from compressed air fed through a membrane separator unit and the second constituent gas is carbon dioxide. A further advantage of the invention when employed with these constituent gases is that the purity of the nitrogen in the mixed gas is relatively unaffected by fluctuations in the pressure of air supplied from the air compressor.

It will be realised that in this embodiment, the valve fed from the membrane separator is controlled substantially in accordance with the principles described in patent specification WO 92/20956 (PCT/GB92/00837) (Calor Air Separation Limited) entitled "Gas supply system", published 26 November 1992.

A pressure equalisation valve is preferably included in the second supply line upstream of the biased-diaphragm control valve. This pressure equalisation valve is also conveniently a pilot-operated diaphragm valve with its pilot chamber also connected to the first supply line at a point upstream of its restrictor valve. This arrangement ensures close tracking between the respective pressures of the first and second constituent gas streams applied to their respective restrictor valves. In the nitrogen/carbon dioxide embodiment, a first pressure switch is preferably connected via a suitable detector line to the mixing zone and a second pressure switch is preferably connected to the carbon dioxide supply line. These pressure switches are employed with a controller for the air compressor such that the compressor operates only when the pressure in the mixing zone is low and the carbon dioxide pressure is above a pre-determined level.

It is generally preferred to include one pressure regulator in the second constituent gas supply line upstream of the pressure equalising valve. Non-return valves can usefully be provided at various points in the system, for example in one or more of the the lines downstream of the adjustable flow control valves so as to prevent back-flow of one source gas into the feed line of another gas, or in the final supply line to the gas receiver so as to prevent back-flow of stored gas when the supply is stopped.

Filters can also be provided if desired at various points in the system, for example an active carbon filter in a nitrogen supply line prior to the point of mixing with a carbon dioxide stream.

The mixing zone is conveniently provided by a tubular pipeline extending from the point at which the constituent gas supply lines first meet and towards the product receiver(s), although an enlarged chamber and specific mixing aids such as baffles can be provided if desired.

The system preferably includes at least one storage receiver for the mixed gas. Provision of a receiver allows mixed gas to be withdrawn on demand, with the production of the mixed gas being resumed when the reducing pressure in the mixing zone activates the first pressure switch and restarts the compressor. In one convenient embodiment there are provided interconnected receivers, one or more normally operational and one or more normally in reserve. Provision of both a . change-over valve and a non-return valve between the two receivers permits automatic switching to the reserve receiver(s) if the demand cannot be met by the operational receiver(s) .

The system can be elaborated to produce two different output mixtures at two different outputs, by providing two restrictors in parallel in the second supply line, two switching valve assemblies coupled together, the first selecting one or other of those two restrictors and the second coupling the mixing zone to the appropriate output. The two outputs may have respective receivers, and control circuitry may be provided for controlling the switching valve assemblies to select the mixture according to whichever of the two receivers is at lower pressure.

Biased-diaphragm flow control valves, as used in the present invention, typically comprise an enclosed zone with a gas inlet and gas outlet. The outlet is closed and opened by a closure plug which in the closed position fits into a seat in the outlet. The plug is in contact with a flexible diaphragm which divides the enclosed zone into a main chamber and a pilot chamber. Means are provided to bias the diaphragm and plug either towards or away from closing the valve. In the bias-closing version, gas introduced into the main chamber applies a force to the diaphragm and when this force exceeds the bias it displaces the diaphragm, thereby lifting the plug from its seat and permitting gas flow through the valve. If the inlet pressure falls to a level at which the force it applies to the diaphragm is less than the bias, the plug returns to its seat and stops the gas flow. The bias-open version operates in a similar manner but so as to keep the valve open until the pressure in the pilot chamber is sufficient to close the valve.

In the present invention the pilot chambers of the diaphragm valves used as flow controllers in the supply lines of the respective gases to be mixed are supplied with pressurising gas from a common source, which can be one of the gases to be mixed. Both valves are of the bias-open type. This common pilot pressurisation has the advantage that it permits closer "tracking" of the flow controllers for the respective gases when their gas throughput reduces just before the stopping of mixed gas flow by closure of a valve in the mixed gas line.

Embodiments of the invention are illustrated below with reference to the accompanying drawings (which are not to scale) in which:

Figure 1 is a block diagram of a basic mixed gas supply system for supplying a nitrogen/carbon dioxide mixed gas;

Figure 2 is a more detailed diagram of a system using a membrane unit nitrogen generator and carbon dioxide source for supplying a nitrogen/carbon dioxide mixed gas;

Figure 3 is a block diagram of a more elaborate mixed gas supply system for supplying two different mixtures;

Figure 3A is a more detailed diagram of a component of the Figure 3 system, and;

Figure 4 is a block diagram of an electrical control circuit for the Figure 3 system.

Figure 1 shows the basic principles of the present mixed gas supply system for supplying a nitrogen/carbon dioxide mixture (using that term for convenience, although, as will be seen, there will in practice be a small proportion of oxygen included as well) . It will be realised that in practice, there will be a number of elaborations, eg as discussed with reference to Figures 2 and/or 3 below.

The nitrogen supply means comprises a compressor 10 which feeds compressed air to an air separation membrane unit 13. The unit 13 separates the feed air into a nitrogen-rich non-permeate gas stream gas which emerges via outlet line 14 and an oxygen-rich permeate gas which is vented away. The nitrogen line 14 leads to an adjustable flow-controlling restrictor valve 25, and a pilot-controlled biased-diaphragm valve 26.

Carbon dioxide gas from a compressed source (shown symbolically as a cylinder) is fed to a pilot-controlled diaphragm valve 42, a flow-controlling restrictor valve 45 and a pilot-controlled biased-diaphragm valve 46.

Each of the diaphragm valves 26, 42 and 46 has a main chamber 6 and a pilot chamber 7. In each of these valves the pilot chamber is connected to the nitrogen outlet line from the membrane unit 13. The pilot chambers of all three valves are thus maintained at substantially the same pressure as the nitrogen generated by the membrane unit 13.

A nitrogen outlet line from the valve 26 and a carbon dioxide outlet line from the valve 46 combine to form a mixing line 51 which leads to a gas receiver 58. The flow-rate of nitrogen gas into the mixed gas line 51 is determined by the adjustable restrictor valve 25 and diaphragm valve 26. The pressure of carbon dioxide gas is regulated to a pressure above that of the nitrogen gas leaving the membrane unit 13 when the air compressor 10 is operating. The flow-rate of carbon dioxide is controlled by the restrictor valve 45 and biased-diaphragm valve 46. Because of the nature of membrane separators, in which product purity is approximately inversely proportional to the product flow-rate, the setting of the restrictor valve 25 also determines the purity of the nitrogen gas stream.

The valve 42 functions as a pressure equalisation valve, closing under pressure on the diaphragm if the carbon dioxide feed pressure rises above the nitrogen pressure in its pilot chamber 7. Carbon dioxide is thus supplied to the adjustable restrictor valve 45 at a pressure substantially equal to the pilot nitrogen pressure, the valve 45 being set to give the required carbon dioxide flow-rate into the mixing line 51. The volumes and relative proportions of nitrogen and carbon dioxide, in the mixed gas are thus controlled by the relative settings of the restrictor valves 25 and 45.

The oxygen content in the mixed gas may be set to any desired level by adjustment of the restrictor 25, and the carbon dioxide content is similarly adjusted to the desired level by means of the restrictor 45. The total flow-rate of mixed gas is the sum of the flow-rates set by restrictors 25 and 45.

This arrangement offers the advantage that any fluctuations in the pressure of nitrogen leaving the membrane unit 13 do not affect the carbon dioxide concentration in the mixed gas.

A further advantage arising from the use of flow control valves 25 is that the purity of the nitrogen gas constituent in the mixed gas is relatively unaffected by fluctuations in the pressure of air supplied from the compressor 10.

Figure 2 shows a nitrogen/carbon dioxide mixed gas supply system based on the Figure 1 system but incorporating a variety of additional features. The compressor 10 feeds compressed air via a filter 12 and a pressure control means (not shown) to the air separation membrane unit 13. The unit 13 is preferably maintained at a constant temperature, and the inlet air pressure from 12 is also preferably maintained at a constant level. The membrane unit 13 has an outlet 11 at which the oxygen-rich permeate gas which is vented away. The carbon dioxide gas from a compressed source (not shown) is fed to the valve 42, via an inlet regulator valve 40 and inlet line 41.

The pilot chambers of the diaphragm valves 26, 42 and 46 are fed from the nitrogen outlet line 14 from the membrane unit 13 via lines 15; 16 and 17; and 16 and 18 respectively. Valve 26 feeds the mixing line 51 via a non-return valve 28, a filter 29, and a nitrogen outlet line 30. Valve 46 feeds the mixing line 51 via a non-return valve 48.

The mixing line 51 leads to twin gas receivers 59 and 60, interconnected by a change-over valve 54 and non-return valve 55. The receivers 59 and 60 also have individual on/off valves, respectively 52 and 53.

A pressure switch 76 detects via line 72 the pressure in the carbon dioxide feed line 41 between the valves 40 and 42. Similarly, a pressure switch 77 detects via line 70 the pressure in the line 50 and thus of the mixing line 51.

Downstream of the change-over valve 54 a mixed gas supply line 62, incorporating a manual check valve 63, a pressure regulator valve 64 and pressure release valves 66 and 67, leads to a draw-off point 68 from which mixed gas can be taken on demand. The change-over valve 54 allows the receivers 59 and 60 to be changed manually in case of a power failure or other fault. Normal on/off control of the system is effected by the pressure switch 77 which senses the mixed gas pressure in the mixing line 50 via a line 70. A pressure switch 76 senses the carbon dioxide pressure supplied to the pressure equalisation valve 42 via a line 72. The contacts of the pressure switches are connected in series to the compressor 10 such that it operates only when the pressure in the operational gas receiver is low and the carbon dioxide pressure is above a pre-determined level.

The particular arrangement illustrated in Figure 2 is suitable for the dispensing of beverages and allows the receiver 59 to be the main mixed gas storage vessel and receiver 60 to be a reserve. The non-return valve 55 and change-over valve 54 ensure that receiver 60 is automatically filled to the pressure permitted by the compressor controller. The receiver 59 may also be charged at the same time as mixed gas is being withdrawn at point 68. In the event that the rate of withdrawal of mixed gas exceeds the rate of mixed gas feed from line 51 for sufficiently long to empty the receiver 59 then the valve 54 can be switched to the reserve in receiver 60.

Figure 3 shows a mixed gas supply system for supplying two different nitrogen/carbon dioxide mixtures. In comparison with the Figure 2 system, this system has a pair of carbon dioxide flow restrictors 45A and 45B and a switching valve assembly 80 which selects one or other of them, a pair of gas receivers 58A and 58B and a switching valve assembly 81 which selects one or other of them, and pneumatic and electrical control circuitry which controls the switching valve assemblies 80 and 81.

The two flow restrictors 45A and 45B are set to different flow rates, and so determine different constitutions for the two gas mixtures. The two switching valve assemblies 80 and 81 are controlled together; when valve assembly 80 allows the carbon dioxide stream to pass through the restrictor 45A, valve assembly 81 directs the resulting mixture into- receiver 58A, and when valve assembly 80 allows the carbon dioxide stream to pass through the restrictor 45B, valve assembly 81 directs the resulting mixture into receiver 58B. One of the restrictors 45A and 45B may be closed if desired, so that the corresponding gas mixture is pure nitrogen.

The control circuit of Figure 4 operates to control the compressor 10. As in the Figure 2 system, the pressure switch 76 senses the pressure in the carbon dioxide supply line 41 and pressure switch 77 senses the pressure in the mixing line 51 to the switching valve assembly 81. When the carbon dioxide pressure is high and the mixed gas pressure is low, the electrical circuit closes to operate the compressor 10.

Figure 3A shows a convenient form of construction for the switching valve assembly 80; switching valve 81 is identical. The valve assembly comprises a cylinder 90 containing a sliding piston 91 with two transverse passages 92 and 93. In its upper position, passage 92 connects the diaphragm valve 42 to the restrictor 45A; in its lower position, to the restrictor 45B. The position of the piston is controlled by the pressures in two control lines 83A and 83B which are connected to the two ends of the cylinder as shown.

The two valve assemblies 80 and 81 are controlled by the pair of lines 83A and 83B which connect the two receivers 58A and 58B and the control chambers of the valve assemblies 80 and 81 as shown. If the pressure in the receiver 58A is higher than that in receiver 58B, then valve assembly 80 will be driven to connect the carbon dioxide supply to the restrictor 45B and valve assembly 81 will be driven to connect the mixed gas output in line 50 to receiver 58B, and mutatis mutandis if the pressure in receiver 58A is the higher. If the pressures in the two receivers are approximately equal, then the system will alternate at intervals between them until they are both filled to the pressure set by the pressure sensing switch 77.

A pair of motorised valves 82A and 82B are preferably included in the lines 83A and 83B as shown in Figure 3, and are connected in the control circuit of Figure 4 as shown. These two valves are open in the un-energised state. When the compressor is energised, these two valves are also energised to closed them. So just before the compressor is energised, these two valves set valve assembly 81 to feed whichever of the two receivers 58A and 58B is at the lower pressure, and valve assembly 80 to select the corresponding one of restrictors 45A and 45B. When the mixed gas pressure in the mixing line 51 falls below the level set by the pressure switch 77, the valves 82A and 82B are energised to isolate the control pressures to the valve assemblies 80 and 81. This prevents possible switching between the two receivers in a single charging cycle.

In addition to the features already described, the system of Figure 3 includes various further features which produce improvements in its general performance. The compressor 10 has an inlet filter 85 for filtering the air entering it, and the diaphragm valve 26 has a filter 87 at its output. The carbon dioxide supply includes a manual valve, an automatic pressure reducing valve and a pressure relief valve shown together as 86. Each of the receiver 58A and 58B has an output assembly comprising a manual valve and pressure relief means, shown together as 88A and 88B.

Claims

1. A system for the supply of a gas mixture having a selected ratio of constituent gases which system comprises a a plurality of supply lines (14, 30, 41, 50) for respective constituent gases under pressure, a mixing zone (51) for the said constituent gases and, in each of the constituent gas supply lines, a pilot-controlled biased-diaphragm control valve (26, 46) having a main chamber (6) and a pilot chamber (7), characterised in that each supply line includes an adjustable restrictor valve (25, 45) upstream of the main chamber (6) of its diaphragm valve, and in that the pilot chambers (7) of said diaphragm valves (26, 46) are all connected to the first supply line (14, 30) at a point upstream of its restrictor valve (25).

2. A system according to claim 1, wherein the first constituent gas is a nitrogen-enriched air fraction obtained from compressed air fed through a membrane separator unit (13) and the second constituent gas is carbon dioxide.

3. A system according to claim 2, wherein a first pressure switch (77) is connected via a detector line (70) to the mixing zone (51) and a second pressure switch (76) is connected to the supply line (41,50) for a second constituent gas, these pressure switches being employed with a controller for an air compressor (10) such that the compressor operates only when the pressure in the mixing zone (51) is low and the pressure in the supply line (41,50) for a second constituent gas. is above a pre-determined level.

4. A system according to any preceding claim, wherein a pressure equalisation valve (42) is included in the second supply line (41,50) upstream of the biased-diaphragm control valve (46) .

5. A system according to any claim 4, wherein the pressure equalisation valve (42) is a pilot-operated diaphragm valve with its pilot chamber (7) connected to the first supply line (14) at a point upstream of its restrictor valve (25) .

6. A system according to any preceding claim, including at least one storage receiver (59, 60) for the mixed gas.

7. A system according to any claim 6, wherein the mixing zone (51) is provided by a tubular pipeline extending from the point at which the constituent gas supply lines (30, 50) first meet and towards the product receiver(s) .