SEPARATION OF GASES

Abstract: A method for the separation of at least one constituent gas from a gas mixture containing the at least one constituent gas and at least one other gas species is described, the method comprising the steps of: providing a stream of the gas mixture to a first membrane of a zeolite material under conditions of pressure and temperature such that the required gas is selectively adsorbed onto a first face of the zeolite material membrane and diffusion through the zeolite membrane towards a second face is promoted; retaining and containing the permeate gas emerging from said membrane second face and passing a retentate of said gas stream not passed through said zeolite membrane either back to the first face of the first zeolite membrane or to a first face of a second zeolite membrane; and, repeating said method steps until a content of the required gas in the gas mixture is reduced to a desired level.

Description: SEPARATION OF GASES The present invention relates to a method for the separation of constituent gases from mixtures of molecular gases or mixtures of monatomic and molecular gases. It is desirable to be able to separate the radioactive noble gases such as krypton for example from other gases such as oxygen and nitrogen prior to release of the latter to atmosphere. Such a need frequently arises, for example, to separate radioactive isotopes of the noble gases arising from nuclear reactions in nuclear fuel from air waste streams. Other needs may arise in separating and recycling of noble gases used as inert carrier gases in gas phase reactions such as the water gas shift reaction. It is also desirable to be able to separate the constituent gases of mixtures of molecular gases such as carbon dioxide from natural gas or carbon dioxide from air for example. It is known to selectively adsorb krypton and xenon by the use of beds of zeolite material. Once the noble gases have been adsorbed into the zeolite material, conditions of temperature and/or pressure are adjusted so as to liberate the gases therefrom so that they may be further processed, contained and stored in some manner. Such methods are essentially batch processes and employ multi- bed adsorption systems. Examples of such processes are described in US-A-4447353 and EP 0658364. Other methods based on continuous processes rather than batch processes have been suggested. Such processes have been proposed utilising polymeric membranes for gas separation systems. In this method, the gas molecules "dissolve" in the polymer matrix and diffuse through it to an area of lower pressure. This method has disadvantages in that the maximum temperature of the feed gas stream is limited to about 100°C. There are also concerns over radiolysis of the polymer membrane materials. However, the main disadvantage is that a very large surface area of the polymer membrane will be required due to very low flux levels of gases passing through the membranes, making the method expensive. Furthermore, since a pressure difference is required to drive the diffusion process, repressurisation between cascade stages will be required, casting further doubt on the economics of the method for separating gases. It is an object of the present invention to provide a continuous process for the separation of one or more desired gas species from a gas steam which process does not have the disadvantages of the proposed polymer membrane methods. According to the present invention there is provided a method for the separation of at least one constituent gas from a gas mixture containing the at least one constituent gas and at least one other gas species, the method comprising the steps of: providing a stream of said gas mixture to a first membrane of a zeolite material under conditions of pressure and temperature such that said at least one constituent gas is selectively adsorbed onto a first face of said zeolite material membrane and diffusion through said membrane towards a second face is promoted; containing and retaining a permeate gas containing said at least one constituent gas emerging from said membrane second face; passing a retentate gas of said gas stream not passed through said zeolite membrane either to said first face of said first zeolite membrane or to a first face of a second zeolite membrane; and, repeating said method steps until a content of said at least one constituent gas in said gas mixture is reduced to a desired level . Zeolite materials are aluminosilicate molecular sieve materials but the term "zeolite" is also sometimes used to refer to crystalline molecular sieves as a generic description. The use of the word "zeolite" in this specification is intended to refer to molecular sieves as a generic term, including for example, silicates, aluminophosphates, gallium phosphates and metal substituted variants of these materials. The term "containing and retaining" as used herein is used to indicate that the permeate gas is retained or contained for perhaps storage or bottling or merely vented to atmosphere if appropriate. The permeate gas may be further treated by any method or means known in the art and the term "containing and retaining" is not to be construed in a limiting manner. The at least one constituent gas may be a monatomic gas such as a noble gas such as krypton for example or may be a molecular gaε such as carbon dioxide for example. Thus, the gas mixture may be a mixture of monatomic gases such as zenon and krypton, where xenon may be separated from krypton; monatomic and molecular gases,- or, may be a mixture of only molecular gases. However, there may be two or more gas species within the mixture. In the following general description of the method, krypton is specifically referred to, however, this is merely exemplary and reference to krypton should be read as also including other noble gases such as xenon and radon for example or to molecular gases such as carbon dioxide for example. In the above invention, the permeate is that portion of the gas stream which passes through the zeolite membrane and contains a high proportion of noble gas relative to the feed stream and the retentate is that portion of the gas feed stream which does not pass through the zeolite membrane. The relative proportions of constituents of a gas mixture which pass through the zeolite membrane is known as the "separation factor" and is defined as the ratio of the permeations of two gases through the membrane. The retentate is either recycled into the original feed gas stream and passed across the first face of the first zeolite membrane or is conducted to a second zeolite membrane in series with the first membrane. At a suitable stage, when the radioactive gas content of the retentate is reduced to an acceptable level, the retentate is discharged to atmosphere Depending upon the degree of separation of krypton from the feed gas stream, the permeate may either be retained and/or contained or itself fed to a further zeolite membrane so as to further increase the degree of separation. The zeolite membrane may comprise a porous substrate such as a sintered metal or ceramic having a zeolite membrane layer formed thereon. It is important that the zeolite membrane is substantially defect free so that there are no "pm holes" or voids extending the full thickness of the membrane which are of similar or greater dimensions than the pores of the zeolite material itself. A membrane of the type described m WO94/01209 may be suitable for the purpose The form of the membrane may be planar or cylindrical and suitably housed with process equipment to contain and direct the gas stream. As is well known, zeolite materials are frequently referred to as being molecular sieve materials, their structures being such that by control of their composition and manufacture, channels and cavities of specific dimensions may be incorporated therein such that atoms and molecules of desired maximum sizes may effectively be filtered by and/or adsorbed to them Furthermore, zeolite materials may also be produced having desired electrical polarisation properties such that polar molecules or easily polarised atoms or molecules may be selectively attracted to them. Therefore, by a combination of size selectivity due to the molecular dimensions of the pores and channels of the zeolite material and by control of the electrical properties of the zeolite material, control over the gas species which are attracted to and adsorbed onto the membrane may be exercised. The zeolite membrane material may be chosen so that its crystal structure is such that the atoms or molecules of the gas species which it is desired to separate are able to be adsorbed therein and able to diffuse therethrough. In some circumstances it may be preferable for the first and subsequent zeolite material membranes to comprise a zeolite material which is electrically polarised, an example of such a zeolite material being that known as chabazite. This may be desirable because the noble gases, in particular krypton, are relatively easily polarised and are therefore attracted to the polarised zeolite material thus, increasing the rate at which the krypton is initially adsorbed onto the zeolite membrane material. Once a krypton atom has been adsorbed into the membrane, the channel size of the zeolite material may be such it is not possible for nitrogen molecules for example co diffuse past thus, the efficiency of the process may be enhanced by controlling conditions such that the rate of adsorption of krypton onto the membrane is greater than the rate of adsorption of other gas species in the mixture. In the case of a gas stream containing radioactive noble gases which it is desired to separate, the feed gas mixture frequently contains many species of gas molecules including for example water (H 2 0) , carbon dioxide (C0 2 ) , oxides of nitrogen (No x ) , organic hydrocarbons (HC) , oxygen (0 2 ) and nitrogen (N 2 ) in addition to noble gases such as krypton (Kr) and xenon (Xe) . Prior to passing the feed gas mixture stream into the first zeolite membrane it is desirable to initially separate as many other species as possible from the gas stream. It is desirable to remove H 2 0, C0 2 , NO x , and HC from the feed gas stream prior to removing the krypton by the first zeolite membrane. Such polar molecules will be particularly prone to adsorption in the membrane and may block the pores and cause a reduction in the diffusion rate and thus the separation factor, and therefore in the efficiency of the process, of krypton through the membrane. In this regard, it may be preferable to initially pass the gas mixture stream through separation means to first remove these large and/or polar molecule species. This may be achieved by first passing the gas stream through a membrane of a non-polar zeolite material such as for example silicalite. Due to the zeolite being non-polar, the unwanted polar molecules are rejected and allows the krypton and nitrogen at least to pass therethrough; also, other large molecules such as HCs are prevented from passing through. Alternatively, a polymeric gas separation membrane may be used as they are effective for the exclusion of polar molecules. Thus, a process employing a non-polar zeolitic membrane, or alternatively, a polymeric membrane, to exclude water and N0 X , for example, and one or more polar zeolitic membranes for the separation of krypton downstream is envisaged in the present invention. Although the above generalised description of the method of the present invention has been exemplified by the separation of the noble gas krypton from a gas stream, the method may be used to separate many other and different gas species from a gas stream. In particular, the removal of carbon dioxide from natural gas, which reduces the calorific value of natural gas as a fuel and the inclusion of which results in the transportation of unwanted material, is particularly envisaged. It is also envisaged that the method of the present invention may be used for the manufacture of carbon dioxide from air. It is also envisaged to separate carbon dioxide from power station flue gases so as ultimately to be able reduce the volume of such "greenhouse" gas from entering the atmosphere. A particular advantage of the method of the present invention is the greatly increased gas fluxes which may be employed with a zeolite material membrane compared to those attainable with polymeric membrane methods of gas separation where the gas fluxes are relatively extremely low although comparable separation factors may be achieved. In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure l shows a graph of separation of krypton from a mixture of nitrogen and krypton, the graph showing gas flux through a zeolitic membrane vs temperature,- Figure 2 shows a cross section through a schematic zeolite membrane,- Figure 3 shows an explanatory diagram of a cross-flow type filter arrangement corresponding to a membrane of the type envisaged in the present invention,- Figure 4 shows a flow diagram representing one embodiment of a method according to the present invention; Figure 5 shows a flow diagram representing a second embodiment of a method according to the present invention; and Figureε 6 to 11 which show graphs relating to the separation of various gases under stated conditions relating to Examples 1 to 6 as set out hereinbelow. Referring now to the drawings and where the same features are denoted by common reference numerals. Figure 1 shows a graph of gas species flux through a membrane of silicalite zeolite material against temperature It may be seen that at about ambient temperature, i.e about 300K, that there is a separation factor between krypton and nitrogen of about 1.5, I e approximately 1.5x more krypton diffuses through the membrane than nitrogen. The zeolite material this case is silicalite which is non-polar. The zeolite membrane was 40-50μm in thickness and grown on a porous sintered stainless steel substrate of 3mm thickness Figure 2 shows a cross section through a schematic structure 10 housmg a cylindrical zeolite membrane The structure includes a cylindrical separation member 12 comprising a zeolite membrane 14 which is grown on a porous sintered stainless steel or alumina substrate 16 The member 12 is contained withm a cylindrical housmg 18 and has annular seals 20 to prevent leakage of gases The housing 18 has an inlet conduit 22 for the admission of feed gas, indicated by the arrow 24, and two outlet conduits 26, 28 to take off the permeate gas, indicated by the arrow 30, enriched with krypton and the retentate gas, indicated by the arrow 32, depleted m krypton, respectively The conduits 22, 26, 28 are also provided with suitable presεure ad]ustmg means such as flow restricting valves and/or presεure or vacuum pumps for example and indicated only schematically at 34, 36, 38 A suitable apparatus may comprise a multiplicity of such units as are indicated at Figure 2 connected m parallel, the permeate flux being driven through the membrane by pressure difference between feed and permeate sides. The retentate is rejected by the zeolite membrane as the molecules are either too large to enter the pores of the structure or are rejected due to electrical polarisation effects or otherwise. The type of cross-flow membrane structure 10 described with reference to Figure 2 will be indicated in subsequent figures by the schematic model shown in Figure 3 where the relevant reference numerals are similarly employed. It will be appreciated by those skilled in the art, however, that the simplified schematicised units 10 shown in subsequent figures will also include all the necessary process control features deεcribed with reference to Figure 2. Figure 4 shows the manner of connection of units 10 into a cascade to purify the permeate to a desired level, wherein the retentate 32 is recycled into the feed 24 of each previous stage. Figure 5 shows a process flow diagram for an apparatus comprising a preliminary separation stage for removal of polar/large molecules. The apparatus comprises a preliminary separation unit 40 having a zeolite membrane 42 as described with reference to Figure 2, the zeolite comprising silicalite which is non-polar. This membrane 42 has the effect of rejecting large/polar molecules and some of the oxygen and nitrogen to the retentate 44 and allowing the krypton, most of the nitrogen and oxygen to pass through to the permeate 46. The permeate purified of water, oxides of nitrogen and HCs for example then passes to the feed 24 of a first separation unit 10 for separation of the krypton from the nitrogen and oxygen. The retentate and permeate of the unit 10 may then be treated as described above with reference to Figure 4. The zeolite membrane of the unit 10 of Figure 5 may comprise a polar zeolite such as chabazite to enhance separation of the krypton from the feed gas. The following examples are given of particular gas mixtures containing the gas species stated in the proportions stated and under the conditions of pressure and temperature as stated in each example. Example 1 A feed gas at a total pressure of lOOkPa containing krypton and nitrogen was passed across a silicalite membrane at a temperature of 303K. The mole fraction of krypton in nitrogen was varied between 0 and 1. As may be seen from Figure 6, the separation factor was found to be 1.6 in favour of krypton and was substantially constant throughout the whole range of krypton concentrations in nitrogen. Thuε, it may be seen that the zeolite membrane is gaε εpecific rather than the separation factor being partially dependant upon the relative proportions of the constituent gaseε. Example 2 Figure 7 shows a graph of the separation factor vs temperature of binary feed gas systems containing nitrogen and krypton in the ratios 50-50, 75-25 and 95- 05. The total presεure of the gaε mixture εystems was lOOkPa and they were pasεed across a silicalite membrane at temperatures ranging from 175K to 675K. It may be seen that the temperature at which the optimum separation factor was achieved was in the range from about 325K to about 350K, the most effective temperature substantially coinciding for all three gas mixture systems. Generally, it was found that a temperature range from about 275K to about 400K was the optimum range with a preferred temperature range lying from about 325K to about 375K. The highest separation factor achieved was 1.8 in favour of krypton. Again, it may be seen that the optimum temperatures for achieving the highest separation factors were largely independent of gas mixture composition. Example 3 Figure 8 shows a graph of separation factors of krypton and nitrogen vs total pressure of the feed gas mixture system. Gas temperature was kept constant at 303K and passed acrosε a silicalite membrane at pressure from 100 to 600kPa. Three feed gas mixtures of nitrogen and krypton were employed, the mixtures containing 50-50, 75- 25 and 95-05 proportions of the gases nitrogen and krypton, respectively. As may be seen from the graph, the maximum separation factor waε 1.8 achieved at the loweεt pressure of lOOkPa used in the examples. Although the 50-50 and 75-25 gas mixtures were εubεtantially the same with respect to their εeparation factors at each pressure, the relatively dilute 95-05 gas mixture showed the highest separation factor at each pressure. Example 4 A feed gas mixture with a total presεure of lOOkPa containing krypton and carbon dioxide was passed across a silicalite membrane at a temperature of 303K. As may be seen from the horizontal axis of Figure 9, the mole fraction of krypton in carbon dioxide waε varied between 0 and 1. The left hand vertical axis of Figure 9 εhows the flux through the silicalite membrane of each gas whilst the right hand vertical axiε shows the separation factor achieved at each concentration. The maximum separation factor was 2.7 in favour of carbon dioxide. Example 5 A 50-50 mixture of carbon dioxide and krypton waε paεεed acroεε a silicalite membrane at 303K and the total presεure of the feed gas was increased from lOOkPa to 450kPa. Figure 10 showε a graph of gas flux and separation factor for a carbon dioxide and krypton gaε mixture vs total pressure of the gas mixture on the left hand side of the graph and separation factor vs total pressure of gas mixture system on the right hand side of the graph. As may be seen from Figure 10, a best separation factor of 2.4 in favour of carbon dioxide at the lowest gas presεure of lOOkPa. Although total flux improved conεiderably with increaεe in total gaε preεεure, the εeparation factor tended to decreaεe with increaεing gas presεure. Example 6 A gas mixture containing 50kPa of krypton and 50kPa of carbon dioxide was passed across a silicalite membrane, the temperature being increased within a range from 200K to 675K. The maximum separation factor was 20 in favour of C0 2 at a temperature of 200K as may be seen from Figure ll. The separation factor decreased rapidly with increasing temperature. The optimum preferred range for carrying out the method in respect of krypton and C0 2 would lie in the range from 200 to about 300K. Example 7 A gas mixture containing 50kPa carbon dioxide and 50kPa nitrogen was passed across a silicalite membrane at a temperature of 303K. A separation factor of 3.7 in favour of carbon dioxide was observed.

Claims: CLAIMS 1. A method for the separation of at least one constituent gas from a gas mixture containing the at least one constituent gas and at least one other gas species, the method comprising the steps of: providing a stream of said gas mixture to a first membrane of a zeolite material under conditions of pressure and temperature such that said at leaεt one constituent gaε iε εelectively adsorbed onto a first face of said zeolite material membrane and diffusion through said membrane towards a second face is promoted; containing and retaining a permeate gas enriched with said at least one constituent gas emerging from said membrane second face or passing said permeate gas to the first face of another zeolite material membrane,- pasεing a retentate of εaid gaε εtream not paεεed through εaid zeolite membrane either to said first face of said first zeolite membrane or to a first face of a second zeolite membrane; and, repeating said method steps until a content of said at least one constituent gas in εaid gas mixture is reduced to a desired level. 2. A method according to claim l wherein the at least one constituent gaε is a noble gas. 3. A method according to claim 2 wherein the noble gas compriseε a radioactive species . 4. A method according to claim 1 wherein said at least one constituent gas iε a molecular gaε. 5. A method according to claim 4 wherein said molecular gas is carbon dioxide. 6. A method according to any one of preceding claims 1 to 3 wherein the at least one other gas species is a molecular gas. 7. A method according to any one of preceding claims 1, 4 or 5 wherein said at least one other gas species is a monatomic gas. 8. A method according to any one of preceding claims l to 3 or 6 wherein said at leaεt one conεtituent gaε is krypton. 9. A method according to claim 8 wherein said at least one other gas species is nitrogen. 10. A method according to claim 9 wherein separation is effected in a temperature range of from 200 to 450K. 11. A method according to claim 8 wherein said at least one other gas species is carbon dioxide. 12. A method according to claim 11 wherein separation is carried out in the temperature range from 200 to 375K. 13. A method according to claim 1 wherein said at least one other gas species are component gas εpecieε of air and separation iε carried out in the range from 200 to 450K. 14. A method according to claim 1 wherein εaid at leaεt one constituent gas is carbon dioxide and said at least one other gas species is nitrogen. 15 A method accordmg to any one preceding claim where the zeolite material is silicalite. 16. A method accordmg to any one preceding claim from 1 to 14 wherein the zeolite material is chabazite. 17 A method accordmg to any one of preceding claims 1 to 3 further mcludmg the step of first passing said gas stream across a preliminary separation means so as separate molecules of at least one of water, carbon dioxide, No x , and hydrocarbons from said gas stream. 18 A method accordmg to claim 17 wherein sa d preliminary separation means is a non-polar zeolite material membrane 19 A method according to claim 17 wherein said preliminary separation means is a polymeric membrane 20 A method accordmg to any one preceding claim wherem said firεt zeolite material membrane compriεes a polar zeolite material. 21 A method accordmg to claim 1 wherein εaid at least one constituent gas is xenon and said at least one other gas s krypton.

Inventors: BROAN CHRISTOPHER JOHN, HUDSON IAN DAVID, BROAN, CHRISTOPHER, JOHN, HUDSON, IAN, DAVID
Applicants: BRITISH NUCLEAR FUELS PLC, BROAN CHRISTOPHER JOHN, HUDSON IAN DAVID, BROAN, CHRISTOPHER, JOHN, HUDSON, IAN, DAVID
Application year: 1996
Main classification: G21F9
IPC codes: B01D53, B01D71, C01B21, C01B23, C01B31, G21F9