GB2053021A - A gas-phase separation system - Google Patents

Abstract

A system for altering the relative concentration of the components of a mixture of gasses, includes a cell 1 having chambers 2, 3 separated by a semi-permeable membrane 4 and a means 5, 25 for creating a pressure differential across the membrane. The mixture flows through the inlet means 6 and around the cell 1 in countercurrent reflux flow. An altered mixture is recovered through outlet means 7, 8. The cell may be formed from modular units which can be arranged to achieve a tapered cell configuration so that the flow rate around the system is substantially constant. <IMAGE>

Description

SPECIFICATION A gas-phase separation system This invention relates generally to a gas-phase separation system and more specifically to concentrating selected gasses in a mixture of gasses by the passage of gasses across a membrane.

The separation of gasses across a membrane is known, for example, U.S. Patents No. 2,159,434 and No. 2,388,095 disclose gas separation devices which apply with pressure a mixture of gasses to one side of a semiperme#able membrane. The membrane allows one gas to pass in preference to another. It is also well known that the product gas from one such device can be processed through subsequent devices connected in series to further concentrate the gas.

Tapering of such separation devices is known. For example, the textbook Nuclear Chemical Engineering, by Benedict and Pigford, p. 391, (McGraw-Hill, New York, 1957) describes tapering a plurality of such gas separation devices which are connected in cascade fashion to distribute a gas flow rate evenly through a gas separation system. Tapering and cascade systems are described in Introduction to Nuclear Engineering, by Stephenson, pp. 362-368 (McGraw-Hill, New York, 1958). The article "Taperization of Step Cascade for Uranium Enrichment by Gaseous Diffusion Process", by Higashi and Myamoto (Journal of Nuclear Science and Technology, January 1976, pp. 30-34 also describes cascading and tapering. Cascading to achieve tapering is also described in Introduction to Nuclear Engineering, by Murray (George Allen, Unwin Ltd., London pp. 68-79).

Countercurrent reflux is well known in distillation, extraction and gas-absorption processes. Countercurrent reflux without back-mixing in gas diffusion devices is described in U.S. Patent No. 3,144,313 wherein the device separates a selected gas from a mixture of gasses across a membrane which has a high diffusion rate for the selected gas. The device described in U.S. Patent No.

3,144,313 which operates at relatively high temperatures (300 to 400 C) and pressures (2.75 x 106 to 3.45 x 106 Pa) and uses a metallic membrane.

Gas diffusion devices which employ hollow fibres or tubes as membranes to separate feed and recovery chambers are known from the U.S. Patent No. 3,144,313 and U.S. Patent No. 3,735,558.

Practical applications of gas-diffusion devices are many and varied. These include oxygen-enrichment for inhalation therapy, sweetening of natural gas, stack gas cleaning, nitrogen-enrichment to reduce fire hazard and the like. In most such applications, economy and efficiency of operation are important to the commercial advancement of such devices.

Devices which operate at relatively low temperatures and pressures and which require relatively little energy input are desirable.

Gas separation devices which avoid numerous cascading steps are desirable.

We have therefore sought to provide a system for concentrating a selected gas or gasses in a mixture of gasses, at relatively low temperatures and pressures.

We have further sought to provide a system to concentrate a gas or gasses without cascading and we have attempted to maintain a substantially homogeneous flow rate throughout a countercurrent reflux gas separation system.

We have also sought to provide a system which enables moisture to be removed from a gas.

Accordingly, we have found a system operable at room temperature for altering the relative concentrations of the components of a mixture of fluids moving in the system. The system includes at least one cell having chambers separated by a semi-permeable membrane. The chambers communicate in such a way as to produce countercurrent reflux flow of the mixture and to result in one chamber having a higher pressure than the other. The chambers have a geometry which avoids backmixing, and the membrane has a different permeability constant for at least two of the components of the mixture.

The system also includes a means for creating a pressure differential across the membrane and at least one inlet means for feeding the mixture into the system. At least one outlet means is also included in the system for recovering the mixture after the relative concentrations of its components have been altered. The means for creating a pressure differential may create any such suitable differential, although a pressure differential of less than about 3.45 x 105 Pa is preferred.

The means for creating a pressure differential between the chambers may be, for example, a pressure-reducer positioned in the communication between the chambers. The means may also be provided by a relatively high pressure feed mixture or a compressor which communicates between the chambers or both.

The outlet means is positioned in the system to recover an altered mixture having increased amounts of either the most permeable gas or the least permeable gas or both.

The inlet means can be positioned at any suitable location in the system, but it is preferably located at a point where the concentration of gasses in the system matches the concentration of gasses in the inlet mixture.

In the relatively low pressure, room temperature system according to the present invention, any suitable semi-permeable membrane material may be used. Representative useful materials include, for example, cellulose acetate, polytetrafluoroethylene, cellulose triacetate, cellulose acetate-styrene, cellulose acetate butyrate, polyethylmethacrylate, cellulose propionate, polypropylene, epoxy, ethyl cellulose, ethylene vinyl-acetate, methyl cellulose, nitrocellulose, polyvinylchlorde, polyvinyl acetate, nitroso rubber, polyamide, polybutadiene, polyvinylchloride, poly(butadiene methylmethacrylate), poly(butadiene styrene), polycarbonate, polydialkylsiloxane resins, silicone rubbers, polyethylene-acrylonitrile, polyethy lenimine-polyvinylbutyral, polyethylene, polyestermethane and polyethylene terephthalate.

Silicone materials are often preferred because of their permeability and relative chemical inertness. An especially preferred material is a poly(alphamethylstyene-co-dimethyl-siloxane) copolymer (described in U.S. Patent No.

4,107,227) which has many of the permeability characteristics of silicone rubber but which has shaping and handling characteristics typical of plastics. In the preferred embodiment, the membrane is a hollow fibre.

A cell may be formed from a plurality of interconnected modular units. Each such unit includes chambers separated by a membrane.

In the preferred embodiment, the modular units are interconnected in a tapered configuration so as to form a system in which the rate of movement of the mixture is substantially constant throughout the system.

Figure 1 shows schematically and in crosssection one embodiment of a system according to the present invention; Figure 2 shows schematically the relative membrane area required to obtain substantially even flow in the system of Fig. 1; Figure 3 shows schematically and in crosssection an arrangement of modular units which together form one embodiment of the system according to the invention having a membrane area approximating that shown in Fig. 2; and Figures 4 and 5 show alternative embodiments of the system according to the present invention.

Referring more specifically to Fig. 1, there is shown a system according to this invention for altering the relative concentrations of the components of a mixture of fluids moving in the system. The system includes a cell 1 which has a high pressure chamber 2 and a low pressure chamber 3 separated by a membrane 4. The chambers 2 and 3 communicate through a pressure reducer 25 at one end and a compressor 5 at the other.

Each chamber 2 and 3 has a geometry which avoids back-mixing of a gas flowing through the system. Such a geometric limitation usually requires that the chambers have a small cross-section. In a representative embodiment, for example, the cell 1 is a bundle of hollow microfibres packed in a tube, each hollow microfibre having an l.D. of 0.239 mm. and an O.D of 0.610 mm. The inside of the microfibres forms the chamber 2 and the area outside the fibres but inside the tube forms the chamber 3.

An inlet means 6 allows entry of a mixture of gasses into the system. Inlet means 6 may be a pressurized feed. The pressurized feed creates a pressure differential across the membrane with the aid of the pressure reducer 25.

The inlet means 6 can be located anywhere along either side of cell 1, but it is most preferably located at about the point where the concentration of gasses in the mixture moving through the system is about the same as the concentration of gasses entering the system.

In the embodiment of Fig. 1, the pressure differential across the membrane can be provided by the compressor 5 which increases the gas pressure as it moves from the chamber 3 to the chamber 2.

An outlet 7 for an altered mixture 11 enriched in the least permeable gas is located at one end of the system. Another outlet 8 is provided at the other end of the system to collect an altered mixture 12 enriched in the most permeable gas.

A plurality of outlets such as the outlets 7 and 8, or one such outlet, could be placed anywhere along either side of the cell to collect any desired mixture of gasses.

In operation of the embodiment of Fig. 1, a pressurized feed mixture of gasses 9 enters the system at inlet 6 and moves around the system in the direction shown by the arrows.

Portions of the most permeable component 10 pass through membrane 4 as the mixture 9 moves along the chamber 2. As mixture 9 approaches the end of chamber 2, its relative concentrations of least permeable and most permeable components had changed because of the loss of the most permeable component 10.

Altered mixture 11 which has a relatively low concentration of component 10 may be recovered at outlet 7.

As mixture 9 moves along chamber 3, countercurrent to its flow in chamber 2, it gathers additional amounts of most permeable component 10 so that an altered mixture 12 containing a relatively high ratio of the most permeable component 10 may be collected at outlet 8.

In most operations, the volume of the gas steam recirculated through the cell 1 may exceed considerably that of the feed stream 9; however, at steady state operation, the volume of altered mixtures 11 and 12 will total the volume of feed mixture 9.

It can readily be seen that such a flow pattern would normally result in a greater volume of the mixture 9 being present near the compressor end of the cell 1 where most permeable gas 10 tends to accumulate. For example, in a system for concentrating oxygen in air, consensed air (which contains 21.1% 2) is introduced at inlet 6 at a rate of about 0. 137 cc/sec under a pressure of 172.1 cm Hg (the system operates at a temperature of about 23.1 C). At inlet 6, mixture 9 already in cell 1 is flowing in the direction shown by the arrows at a rate of about 0.093 cc/sec.

By the time mixture 9 reaches the end of chamber 2, a distance of 2.11 m in a 35member bundle of silicone rubber hollow microfibres, it has a pressure of only 171.5 cm Hg because of passage of 02 through membrane 4. The gas stream has an oxygen concentration of only 1 5.1 % at the end of chamber 2. About 0.0972 cc/sec of the oxygen reduced (nitrogen-enriched) altered mixture 11 is drawn off at outlet 7 in this example.

None of the gas stream passes through the pressure reducer 25, but at a point in the chamber 3 opposite the inlet valve 6 the gas flow is 0.133 cc/sec and the 02 concentration is about 25.9%. The pressure in chamber 3 is 75.92 cm Hg.

By the time the gas stream reaches the end of chamber 3 (a distance of about 4.24 m), it has a flow rate of 0.284 cc/sec and an altered oxygen concentration of about 36.8%.

About 0.0401 cc/sec of the oxygen-enriched altered mixture 12 is recovered at outlet 8 and about 0.244 cc/sec is reintroduced into chamber 2 at a pressure of about 172.6 cm Hg. Because of passage of the most permeable gas 10 (oxygen through the membrane 4, the mixture 9 has a reduced pressure of about 172.1 cm Hg after it travels the 2.13 m back to inlet 6.

Fig. 2 shows schematically a sheet membrane 14 which is shaped to correspond with the changing volume of flow of the mixture 9 as it circulates around cell 1 of Fig. 1 in countercurrent fashion so that there is a homogeneous rate of flow. As mixture 9 enters chamber 2 at inlet 6, much of the most permeable component 10 passes through the membrane 4 and adds to the volume of the portion of the mixture 9 which is already in the chamber 3. The volume of the mixture 9 is continually reduced as it passes along the chamber 2 because of the continual passage of portions of the most permeable component 10 through the membrane 4.

Sheet membrane 14 shown in Fig. 2 is constructed to correspond with the changes in volume of mixture 9 so as to result in a substantially uniform flow of mixture 9.

A system according to the present invention can be constructed to correspond to the shape of sheet membrane 14, with an outlet for the altered mixture 11 enriched with the least permeable product and outlet 8 for the altered mixture 12 enriched with the most permeable product.

However, the construction of such a system presents practical construction difficulties which do not make it a preferred embodiment. Such difficulties are largely overcome by the representative embodiment shown schematically in Fig. 3.

With reference to Fig. 3, a cell 15 is constructed of modular units 16. Each unit 16 comprises a high pressure chamber 17 and a low pressure chamber 18 separated by a membrane 19. The units 16 are interconnected in such a way as to result in a membrane surface area and configuration approximating that shown in Fig. 2 so that the rate of flow is substantially constant at all parts of the cell 15.

In the cell 15, a mixture of gasses is introduced at an inlet 20. It moves through chambers 17 and 18 and through a membrane 19 as in Fig. 1. In the embodiment of Fig. 3 the gas moves at a substantially uniform rate throughout the system because of the tapering effect, provided by the arrangement of modular units 16. A gas mixture having an altered concentration of components can be removed at an outlet 21 (for mixtures having a reduced concentration of the most permeable component) or at an outlet 22 (for mixtures having an increased concentration of the most permeable component).

Pressure reducer 23 and compressor 24 function in the same manner as pressure reducer 25 and compressor 5 of Fig. 1.

Fig. 4 shows schematically and in crosssection an embodiment of the present invention where the pressure differential is accomplished by a pressurized feed of a mixture 27 through an inlet 26, as the mixture 27 moves through a high pressure chamber 28 to a pressure reducer 29, the most permeable component 30 moves through a semi-permeable membrane 31. In a low pressure chamber 32 the most permeable component 30 joins the mixture 27 in countercurrent reflux flow.

An altered mixture 32 having a high concentration of the most permeable gas can be collected or discharged at an outlet 33. Similarly, in this embodiment an altered mixture 34 is collected at an outlet 35. The altered mixture 34 has been stripped of some of its most permeable gas 30, leaving it with an enriched concentration of the least permeable gas.

Referring more specifically to Fig. 5, there is shown a further embodiment of the present invention. The pressure differential across membrane 36 is accomplished by a compressor 37 which draws a feed mixture 38 (such as air) into a low pressure chamber 39 at an inlet 41, increases its pressure and moves it into a high pressure chamber 40 in countercurrent flow. The most permeable component 42 of mixture 38 moves through the membrane 36 in the direction shown by the arrows and countercurrent flow of the mixture 38 results in an altered mixture 43 which is enriched in the most permeable component.

The altered mixture 43 may be collected at outlet 44. An altered mixture 45, which is usually considered as residue in this embodiment may be collected at outlet 46.

Claims (5)

1. A system operable at room temperature for altering the relative concentration of the components of a mixture of fluids moving in the system, the system comprising: (a) at least one cell having chambers separated by a semi-permeable membrane, the chambers communicating in such a way as to produce countercurrent reflux flow of the mixture and to result in one chamber having a higher pressure than the other, the chambers having a geometry which avoids back-mixing and the membrane having a different permeability constant for at least.two of the components of the mixture; (b) means for creating a pressure differential across the membrane; (c) at least one inlet means for feeding the mixture into the system; and (d) at least one outlet means for recovering the mixture from the system after the relative concentrations of its components have been altered.

2. A system according to claim 1, wherein the communication between the chambers includes a pressure reducing means.

3. A system according to claim 1 or 2, wherein the cell is formed from a plurality of interconnected modular units, each unit comprising chambers separated by a membrane.

4. A system according to claim 3, wherein the units are interconnected to form in effect a tapered configuration in such a way that the rate of movement is substantially constant throughout the system.

5. A system operable at room temperature for altering the relative concentration of the components of a mixture of fluids in the system substantially as herein described with reference to any of the accompanying drawings.