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Title:
RECOVERY OF OLEFINS FROM GASEOUS MIXTURES
Document Type and Number:
WIPO Patent Application WO/2001/017664
Kind Code:
A1
Abstract:
Process for recovering olefins from gas mixtures containing the paraffin having the same number of carbon atoms. A carbon membrane is provided in the form of hollow fibres and a membrane module is formed. The pores of the membrane are tailored to maximize the selectivity between the olefin (the permeating species) and the paraffin (the retained species). The membrane module is sealed into a pressure housing in such a way that the seal is stable to temperature changes and any individual defective CMSM hollow fibers can be plugged in situ. The gas mixture is fed to the membrane module, while heating the gas mixture and the membrane module and separately collecting the permeate and the retentate. The separation process is then completed by distillation.

Inventors:
AGAM GIORA (IL)
DAGAN GIL (IL)
GILRON JACK (IL)
KRAKOV VITALY (IL)
TSESIN NATALIA (IL)
Application Number:
PCT/IL2000/000536
Publication Date:
March 15, 2001
Filing Date:
September 07, 2000
Export Citation:
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Assignee:
CARBON MEMBRANES LTD (IL)
AGAM GIORA (IL)
DAGAN GIL (IL)
GILRON JACK (IL)
KRAKOV VITALY (IL)
TSESIN NATALIA (IL)
International Classes:
B01D53/22; B01D65/10; B01D69/08; B01D71/02; C07C7/144; C10G31/11; (IPC1-7): B01D53/22; B01D69/00; B01D69/04; B01D71/02; C07C7/144
Foreign References:
US5695818A1997-12-09
US5575963A1996-11-19
US4685940A1987-08-11
Attorney, Agent or Firm:
Luzzatto, Kfir (Luzzatto & Luzzatto P.O. Box 5352 Beer-sheva, IL)
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Claims:
CLAIMS
1. Process for recovering olefins from gas mixtures containing the paraffin having the same number of carbon atoms, which comprises: AProviding a carbon membrane in the form of hollow fibres and forming a membrane module. B. Tailoring the pores of the membrane to maximize the selectivity between the olefin (the permeating species) and the paraffin (the retained species). C. Sealing the membrane module into a pressure housing in such a way that the seal is stable to temperature changes and any individual defective CMSM hollow fibers can be plugged in situ.
2. Process according to claim 1, further comprising feeding the gas mixture to the membrane module, while heating said gas mixture and said membrane module and separately collecting the permeate and the retentate.
3. Process according to claim 1, further comprising completing the separation process by distillation.
4. Process according to claim 1, wherein the carbon membrane is provided by providing a fibre precursor and pyrolizing it.
5. Process according to claim 1, wherein the pore tailoring operation is carried out on a module formed by potting the hollow fibres together to form a module and mounted into a housing for pore tailoring, said housing being mounted into an apparatus for carrying out the pore tailoring operations.
6. Process according to claim 1, comprising, after completing the pore tailoring, removing the module from the pore tailoring housing and mounting and sealing the same in a pressure housing.
7. Process according to claim 1, wherein the distillation is carried out by passing the gas mixture fed to the membrane to a distillation column, and/or by feeding the retentate fraction to a different plate on the distillation column, and/or by feeding the membrane unit from the distillation column whether from a sidestream, overhead or other place on the column.
8. Process according to claim 1, wherein the gas mixture and the module are heated in the pressure housing to a temperature at which the nonpermeable component does not adsorb and block the pore mouth of the membrane.
9. Process according to claim 8, wherein the temperature between 70 and 120° C.
10. Process according to claim 1, wherein the olefin is propylene and the paraffin is propane.
11. Process according to claim 1, wherein the olefin is chosen from among ethylene, isobutane and 1butene propylene and the paraffin is chosen from among ethane and isobutane.
12. Process according to claim 1, wherein the membrane tailoring operations comprise preconditioning with air or oxygen, carrying out CVD and activating with air or oxygen, the activation being effected by feeding an activating gas to the shell side but not to the lumen side of the hollow fibers in the module, so that a pressure difference is maintained between the shell side and the lumen side, monitoring the flow of activating and terminating the activation when the flow thereof increases by a predetermined value.
13. Process according to claim 12, wherein the predetermined value of the flow increase is based on previous correlations between the flow rate of the activating gas and the measured, pure gas permeances of selected gases.
14. Membrane, having pores tailored to maximize the selectivity between an olefin and corresponding paraffin contained in gas mixtures, by preconditioning with air or oxygen, carrying out CVD and activating with air or oxygen, the activation being effected by feeding an activating gas to the shell side but not to the lumen side of the hollow fibers in the module, so that a pressure difference is maintained between the shell side and the lumen side, monitoring the flow of activating and terminating the activation when the flow thereof increases by a predetermined value.
15. Membrane according to claim 14, wherein the olefin is propylene and the paraffin is propane.
16. Use of a membrane according to claim 14 for the separation of olefins from gas mixtures containing the corresponding paraffins.
17. Process for separating olefins from paraffins contained in a gas mixture, substantially as described and illustrated.
18. Process according to claim 1, for separating propylene from propane.
Description:
RECOVERY OF OLEFINS FROM GASEOUS MIXTURES Field of the Invention This invention relates to a method for recovering olefins in gaseous mixtures from exhaust gases of petrochemical processes, and particularly for separating olefins from paraffins in such mixtures. The separation of propylene from propane is a specific aspect of this invention.

Background of the Invention Separation of olefins from the paraffins having the same number of carbon atoms is a problem of the chemical industry that has not received a fully satisfactory solution in the art. An important case of such a separation is the recovery of propylene from propane in gaseous mixtures from exhaust gases of petrochemical processes, which is one of the most energy intensive separations practiced in the chemical industry. Because of the low boiling points of both of the components, the separation is carried out by cryogenic distillation, a very expensive process due to the compressors and heat exchangers needed and the large number of distillation plates required to effect the separation.

It has been proposed to render the separation of olefins from paraffins more economical by using membranes. Davis et al, in a communication to the 7th Symposium on Separation Science and Technology for Energy Applications, Knoxville, TN, USA, entitled"Facilitated transport membrane hybrid system for olefin purification", have proposed the use of liquid membranes trapped in a porous support. These membranes make use of a silver nitrate solution to complex the olefin on the upstream side of the membrane and carry it to the downstream side where it is stripped by vacuum or a carrier gas. They achieve high-80-300-olefin/paraffin selectivities, but have relatively low olefin fluxes of 5-30 SL/m2-hr-bar.

The disadvantage of these membranes are the difficulties encountered in stabilizing them to prevent the gradual loss of the silver nitrate extractant from the membrane pores, and the potential safety hazards posed by the formation of potentially explosive silver-organic complexes. A number of attempts have been made to overcome these disadvantages by using silver complexants that are bound to the membrane matrix. However, the resulting membranes have not been commercialized, probably due to the difficulty and expense involved in manufacturing them on a large scale.

USP 5,670,015 describes an olefin separation membrane comprising a solid solution of an ionic metal salt in a polymer, which is said to have selectivity for an unsaturated hydrocarbon over a saturated hydrocarbon having the same number of carbon atoms. This has an example of a solid ion solution in a membrane, which however does not have a high flux, not more than 15 L/m2-hr-bar for the unsaturated hydrocarbon.

Boom, J. P. et al. discuss zeolite-filled membranes in Stud. Surf. Sci. Catal., 84, Zeolites and Related Microporous Materials, Pt. B, 1167-74,1994.

They discuss the addition of the zeolites silicalite-1, NaX and AgX to rubbery polymeric membranes. It is stated that the addition of zeolites can lead to an increase in both permeability and selectivity of the membranes, the increase in selectivity resulting from a longer pathway for the slowest component around the zeolite particle, while the increase in permeability is explained by an increase in sorption of the components in the membrane.

It would be possible, in principle, to use molecular sieve membranes to separate paraffins from olefins and particularly propylene from propane.

Propylene and propane have different minimal molecular dimensions, and, in principle, could be separated by a membrane whose pore size is intermediate between the dimensions of the two molecules in question.

The molecular sieve membranes could be e. g. zeolite and silicalite based membranes or carbon molecular sieve membranes. The zeolite and silicalite membranes reported in the literature (see Nair et al,"Surface selective approach to separation of propylene from propane", in Separation Science and Technology, vol. 31, No. 14, August 1996, pp. 1907-1914) have not been commercialized to-date because of the great difficulty in synthesizing large areas of such membranes without grain boundary defects that would destroy the selectivity of the membrane. Also the problem of sealing large areas of such membranes, to separate the feed from the permeate side of a module composed of such membranes, is technologically formidable.

Attempts to make carbon molecular sieve membranes that could effect this separation are reported by Suda et al., in"Alkene/alkane permselectivities of a carbon molecular sieve membrane", Chem. Comm. 1997, pp. 93-94.

However, all of the data reported by Suda is for pure gases. It is often found that the behavior of a membrane with pure gases is not the same as its behavior with mixtures. In the latter case, the less permeable gas may adsorb at the entrance to the membrane pore and block the transport of the more permeable gas, causing a disastrous loss in flux. This is illustrated in the article by Kaptejin et al.,"Permeation and separation of light hydrocarbons through a silicalite-1 membrane application of the generalized Maxwell-Stefan equations", Chemical-Engineering-Journal-and-Biochemical-Engineering Journal, vol. 57. n. 2, April 1995, pp. 145-153. This article describes results of the use of silicalite membranes (membranes formed from a molecular sieve material) and compares the permeances of mixtures ethane/ethylene and propane/propylene with those of the pure alkane and alkene gases. It is reported that, while the permeabilities of the pure alkenes were significantly higher than those of the pure alkanes with the same carbon number, in the mixtures the selectivities were reversed and the alkanes were more permeant than the alkenes. Since the less permeable gas is a significant component of the feed mixture, it cannot be removed in a pretreatment step. Therefore, any attempt to prepare and operate carbon molecular sieve membranes on a practical level must demonstrate their effectiveness on mixtures and specify operating conditions in which the adsorption of the less permeable component is prevented. In addition, the practical operation of CMSM requires that a significant area of carbon membrane be mounted in the membrane module and an effective sealing method be found to separate the feed from the permeate side of the module. Neither Suda et al. nor any other publication of the prior art gives adequate teachings to this effect.

It is therefore a purpose of this invention to provide a process for efficiently separating propylene from propane contained in gaseous mixtures, particularly mixtures that are effluents from industrial chemical processes.

It is another purpose to provide a process by which the problems of molecular sieve membrane separation, set forth hereinbefore, are solved.

It is a further purpose to provide such a process in which a carbon molecular sieve membrane (CMSM) is used and its pore size is tailored to allow the passage of propylene molecules and the retention of paraffins with which the propylene is mixed in a feed stream, so that a product stream enriched in propylene and a second product stream reduced in propylene content are produced.

It is a still further purpose to provide such a process that reduces the energy costs and increases the per unit capacities of any standard separation operations, by which the product stream can be further purified, such as distillation and adsorption.

Other purposes and advantages of the invention will appear as the description proceeds.

Summarv of the invention The process of the invention for separating olefins from paraffins having the same number of carbon atoms in gas mixtures comprises carrying out a partial separation by using a carbon molecular sieve membrane in the form of hollow fibres, which process comprises the following steps: 1. Providing a carbon membrane in the form of hollow fibres and forming a membrane module; 2. Tailoring the pores of the membrane to maximize the selectivity between the olefin (the permeating species) and the paraffin (the retained species); and 3. Sealing the membrane module into a pressure housing in such a way that the seal is stable to temperature changes and any individual defective CMSM hollow fibers can be plugged in situ.

Optionally, the gas mixture and the membrane module may be heated to a temperature at which the non-permeable component does not adsorb and block the pore mouth of the membrane, said temperature being between 50 and 200° C and preferably between 70 and 120° C.

Further, the separation process may be optionally completed by distillation.

Step 1 is carried out by providing a fibre precursor and pyrolizing it, particularly as described in patents mentioned hereinafter.

In order to carry out step 2, the hollow fibres are potted together to form a module, which is mounted into a housing for pore tailoring and then mounted into an apparatus for carrying out the pore tailoring operations, preferably all as described hereinafter.

In order to carry out steps 3, the module is removed from the pore tailoring housing and is mounted and sealed in a pressure housing.

The distillation step is carried out by passing the gas mixture fed to the membrane to a distillation column, and/or by feeding the retentate fraction to a different plate on the distillation column, and/or by feeding the membrane unit from the distillation column whether from a sidestream, overhead or other place on the column.

In a preferred aspect of the invention, the olefin is propylene and the paraffin is propane.

Brief Description of the Drawings In the drawings: Figs. 1 and 2 are diagrams showing the effect of temperature on ethylene permeability and selectivity for CMSM ethylene/ethane separation; and Fig. 3 is a diagram illustrating the relationship between ethylene permeance and selectivity in the same separation.

Detailed Description of the Invention Preparation of the membrane The membrane is prepared in a variation on the method taught in US Patent 5,695,818 (Sofer et al.). A polymeric precursor in hollow fiber form is pyrolized under standard conditions, e. g. as described in EPA 95103272.

The membrane pore size is tailored to the required pore size by the following generic steps: 1. Preconditioning with air or oxygen.

2. CVD (chemical vapor deposition) 3. Activation with air or oxygen.

The preconditioning is preferably carried out at a temperature of about 200°C.

The activation step in the present invention involves feeding the activating gas to the shell side, but not to the lumen or bore side of the hollow fibers in the module, so that a pressure difference is maintained between the shell side and the lumen side. This pressure difference generates a flux of gas through the membrane. This pressure difference can be between 50 and 1000 mbar and preferably around 500 mbar. As the activating gas oxidizes carbon atoms off the pore surface, the pore diameter is expanded and there is an increasing rate of permeation of the activating gas through the membrane from the shell to the lumen side of the fiber. The flow of activating gas is monitored and the activation step is terminated when the flow of activating gas has increased by a predetermined value. This value is based on previous correlations between the flow rate of the permeating activating gas and the pure gas permeances of selected gases that were measured after the activation: there exists an approximately monotonic correspondence between the increase of the oxygen or air permeance due to the activation step and that of the permeances of other individual gases. This activation step is denoted asymmetric activation, since there is a higher partial pressure of the activating species on one side of the membrane relative to the other. The use of both the asymmetric activation and the monitoring of the increase in the permeation of standard gases are both innovations over the existing art taught in making CMSM membranes for gas separation.

The resulting membranes and their use are also parts of this invention.

They preferably have pore sizes from 3 to 5 Å and more preferably from 3.2 o to 4.2 A., and preferably have a capacity of at least 0.2 m2 The membrane module is sealed into a pressure housing, to obtain a seal stable to temperature changes and permitting the plugging in situ of any defective hollow fibers, preferably as described in copending Israeli patent application 123462, the contents of which are incorporated herein by reference, and in US Patent 5,575,963.

As an example, one module containing 1000 carbon hollow fibers (-2 ft2) was mounted in an apparatus such as shown in Figures 5a to 5d of USP 5,695,818, with separate feed and exit ports for the shell side and the lumen side of the fibers of the module, so that gas could not be transported from one side to the other without traversing across the membrane wall.

The module was then subjected to the following operations: 1. Pre-conditioning; oxygen was fed at 50 mbarg to both the shell and lumen side of the fibers which were maintained at 200°C for 20 minutes.

The gas fed to both sides was carried out through the exit pores.

2. CVD: propylene was fed at 50 mbarg to the bore side of the membrane for 2 minutes while the module was kept at 700°C.

3. Asymmetric activation: oxygen was fed to the shell side of the module kept at 295°C until the permeate flow rate increased by 125 scc/min from its initial value. The pressure difference was about 500 mbar. The activation step was quenched by removing the activating gas, rinsing the module with an inert gas and then feeding hydrogen to both sides of the module held at 620°C.

4. The module was heated with hydrogen at up to 70 mbarg on both sides for 30 minutes at 620°C.

The result of this treatment gave a membrane with the pure gas separation characteristics set forth in Table 1.

Table 1: Pure gas permeances of CMSM membrane whose pores were adjusted for olefin/paraiBln separation Test Lp Gas (SL/M2-Hr-Bar) SF6 1 H2 3100 N2 660 02 1360 About 0.2m2 of this same membrane module was then mounted in a pressure housing (Module #638) and sealed at both ends according to the technique taught in copending Israeli patent application, No. 123462, the contents of which are incorporated herein by reference, and US Patent 5,575,963. This provided a membrane module to which could be fed a gas mixture containing an olefin and the corresponding paraffin. The term "corresponding", as herein used, means"having the same number of carbon atoms".

The use of the aforesaid membrane for the separation of propylene from propane and propane at above ambient temperatures will now be described.

Example 1 The membrane module was fed a mixture of propylene and propane at both room temperature and at elevated temperatures (70 and 120°C). The composition of the feed, retentate and permeate streams were determined by gas chromatography. The flow rates were measured by both bubble flowmeters and mass flowmeters which were calibrated for the different gas mixtures. The results of these measurements are given in tables 2-3. Table 2: Separation of propylene/propane with CMSM 1. Separation at room temperature Pf, Qp Qr Fee Permea Retent Alpha Lp d te ate bar Cc/mi Cc/mi mol fraction of C3H6/C3 (SL/M2-H n n e olefin H8 r-Bar 2 763 1700 74.4 93.3% 66.0% 15 497 2.4 1004 1500 % 93.4% 62.1% 13.5 443 4 1400 700 74.2 89.8% 45.1% 8.4 285 % 74.2 % Table 3: Separation at 70°C.

2.70 Pf, Qp Qr Fee Perme Retenta Alpha Lp d ate te ba Cc/mi Cc/mi mol fractio of olefin C3H6/C3 (SL/ rg n n e n H8 M2-H r-Bar 2.1130 1400 75.0 92.5% 60.6% 13 606 2 1060 1500 % 93.0% 61.0% 14.5 592 2.1670 500 74.3 86.7% 30.5% 8.9 451 2 % 3.74.2 8 % 5.2190 2900 71.5 91.7% 56.4% 8.1 294 0 % In the Tables: Lp = membrane gas permeance in units of L/m2-hr-bar barg = bar gauge (pressure above ambient pressure) Pp = pressure on permeate side of membrane module Pf = pressure on feed side of membrane module sccm = cm3/min at standard temperature and pressure Xf = mole fraction of olefin in the feed stream Xret = mole fraction of olefin in the retentate stream Xp = mole fraction of olefin in the permeate stream Stage-cut = ratio of permeate flow rate to feed flow rate (represents the fraction of the feed stream which ends up as permeate ALPHA = permeance selectivity ratio, in this case the ratio of the olefin permeance to the paraffin permeance It can be seen that the increase in temperature increases the olefin permeance (Lp) without significantly harming the selectivity. Also note that the olefin permeances achieved are an order of magnitude higher than those obtained by Davis et al., in the aforecited paper, using silver nitrate impregnated supported liquid membranes. In addition, the membrane tends to lose specific flux as the pressure is increased, leading to the speculation that partial condensation in membrane pores may be occurring as the pressure increases. By raising the temperature, this is delayed to higher pressures and there is a higher specific flux at higher pressures allowing higher membrane fluxes.

The membrane described achieves a partial separation of propylene from propane. To complete said separation, the permeate fraction obtained from the membrane module can be passed through a distillation column. A way of doing this is described in the article of Stephan et al.,"Design Methodology for a membrane/distillation hybrid process", J. Memb. Sci., 99, pp. 259-272 (1995), the contents of which are incorporated herein by reference.

In said article, four different configurations are presented for combining a membrane with a distillation column (Figure 1). All of these configurations can be carried into practice with the membrane module described herein. It should be noted that the membrane of this invention has a separation factor that is close to that of the silver nitrate membrane that formed the basis of Stephan et al.'s calculations, which is 18.

Therefore, the results presented in their article are applicable to the membrane of this invention. For example, if the separation factor of said membrane is calculated from the equation 23 of Stephan et al., it is found to range from 8 to 15, depending on the pressure and stage cut. Examining Figure 13 of Stephan et al., it is seen that such a membrane can save between 25-35 trays from a 135 tray distillation column.

Completion of the separation process by distillation can also be carried out as described in the cited paper of Davis et al.

The following examples illustrate that this invention is effective for separating a variety of mixtures in which the mixture element to be passed into the permeate is an olefin and the mixture element to be retained is a paraffin. In these examples, the test apparatus is the same as described hereinbefore and as illustrated in Figs. 5a-5d of aforecited U. S. Patent 5,695,818. The modules are prepared as described hereinbefore, with particular details in the activation-CVD procedure described in each example.

Example 2 A membrane module with-0.2 m2 of membrane surface was prepared as described hereinbefore, with the following particular steps in the preparation: a) Conditioning with oxygen for 20 minutes at 200°C. b) CVD with propylene for 2 minutes at 700°C. c) Asymmetric activation with oxygen for 17 minutes at 290°C followed by annealing with hydrogen for 10 minutes at 620°C. d) Symmetric activation with oxygen for 3 hours at 280°C followed by annealing with hydrogen for 10 minutes at 620°C.

The pure gas permeabilities for this membrane were measured for several gases, and the results obtained are given in Table 4: Table 4: Pure Gas Permeabilities (SL/M2-hr-bar) Lp of pure gases at 1 bar g T = 45°C 1 bar isobutene 347 isobutane 15 Ideal Selectivity 23. 1 This module was then mounted in the test apparatus and fed a mixture of isobutane and isobutene with 76.2 mol% isobutene at 45°C. The composition of the feed, permeate and retentate streams and the permeances and selectivities are given in Table 5.

Table 5: Results for separating isobutane/isobutene mixture Feed: isobutene/isobutane mixture 45Tmodule,°C Pp, bara 1.0 Pf, bara 2 Permeation Rate, 33 Retention Rate, scem 155 Xf (isobutene) = 76.2% Xret (iso-butene =) 71.4% Xp (iso-butene =) 96.7% Stage-Cut 19.0% ALPHA (olefin/paraffin) 28 Lp of iso-butene, L/m2-bar-h 226 Example 3 A membrane module with-0.2 m2 of membrane surface was prepared as described hereinbefore with the following particular steps in the preparation: e) Conditioning with oxygen for 20 minutes at 200°C. f) CVD with propylene for 2 minutes at 700°C. g) Asymmetric activation with oxygen fed at 4.3 cm3/min. for 10 minutes at 290°C followed by annealing with hydrogen for 10 minutes at 620°C. h) Symmetric activation with oxygen for 0.5 hours at 280°C followed by annealing with hydrogen for 10 minutes at 620°C. i) Symmetric activation with oxygen for 2 hours at 290°C followed by annealing with hydrogen for 2 hours at 620°C.

The pure gas permeabilities for this membrane were measured for several gases, and the results obtained are given in Table 6.

Table 6: Pure Gas Permeances (SL/m2-h-bar) at 45°C Gas 2 bar 3 bar 1-butene 107 90 Isobutane- « 1 Ideal Selectivity infinite The module was then mounted in the test apparatus and fed a mixture of isobutane and 1-butene with 83.4 mol% 1-butene at 45°C. The composition of the feed, permeate and retentate streams and the permeances and selectivities are given in Table 7.

Table 7: Results of separating a mixture of 1-butene and isobutane 45Tmodule,°C Pp, bara 1 Pf, bara 2 Perm. Rate, sccm 19 Reten. Rate, sccm 331 Xf (1-butene) = 83.4% Xret 1-butent) = 82. 6% Xp 99.7% Stage-Cut (Qp/Qf) 5.4% ALPHA (olefinlparaffin) 169 Lp of 1-butene, L/m2-bar-h 103 These examples illustrate that the proposed invention is effective for separating a variety of mixtures in which the mixture element to be passed into the permeate is an olefin and the mixture element to be retained is a paraffin. In these examples, the test apparatus is the same as described hereinbefore and illustrated in patent Figures 5a-5d of the aforecited U. S. Patent 5,695, 818. The modules are prepared as described in the body of the patent with particular details in the activation-CVD procedure described in each example.

Example 4: Separation of ethylene from ethane Two different membrane modules with #0. 2 m2 of membrane surface were prepared as described in the body of the patent. The particular pore-tailoring conditions for each module are given as follows: Module 644-4 a) Conditioning with oxygen for 20 minutes at 200°C. b) CVD with propylene for 2 minutes at 700°C. c) Asymmetric activation with oxygen fed for 94 minutes at 290°C until oxygen permeation rate increased by 6 scm3/min. This was followed by annealing with hydrogen for 10 minutes at 620°C.

Module 636-6 a) Conditioning with oxygen for 20 minutes at 200°C. b) CVD with propylene for 2 minutes at 700°C. c) Asymmetric activation with oxygen fed for 120 minutes at 290°C until oxygen permeation rate increased by 4.8 scm3/min. This was followed by annealing with hydrogen for 10 minutes at 620°C.

They were each mounted on a test stand as described in the previous examples and the permeances of pure gases were measured. The results are given in Table 8.

Table 8: Pure gas permeances for modules in Example 3. Gas L (SL/M2-Hr-Bar) 644-4 636-6 SF6 0 0 H2 1376 2614 N2 0 212 02 433 825 As can be seen, the first module, 644-4, has lower fluxes, but higher ideal selectivities between the various gas pairs.

Each module was then subsequently mounted on the test apparatus and fed a gas mixture which was 70 mol% ethylene/30 mol% ethane. Each module was tested on this mixture for pressures ranging from 2 bara to 7 bara and for temperatures ranging from room temperature (RT) to 120°C.

The results can be seen in Figures 1 and 2. Consistent with the results for pure gas permeabilities, module 644-4 has lower ethylene permeances but higher selectivities than module 636-6 when operated on the same gas mixture. However, when the performance of both modules are plotted together on a graph of mixture selectivity versus ethylene permeance based on mixture data, it is seen that the data fall around a common curve.

This is displayed in Figure 3.

While embodiments of the invention have been described by way of illustration, it will be apparent that the invention may be carried out with many variations, modifications and adaptations, without departing from its spirit or exceeding the scope of the claims.