Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C17/00—Preparation of halogenated hydrocarbons
  • C07C17/38—Separation; Purification; Stabilisation; Use of additives
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
  • B01D53/228—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion characterised by specific membranes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2257/00—Components to be removed
  • B01D2257/20—Halogens or halogen compounds
  • B01D2257/204—Inorganic halogen compounds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2257/00—Components to be removed
  • B01D2257/20—Halogens or halogen compounds
  • B01D2257/206—Organic halogen compounds
  • B01D2257/2066—Fluorine
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
  • Y02C20/00—Capture or disposal of greenhouse gases
  • Y02C20/30—Capture or disposal of greenhouse gases of perfluorocarbons [PFC], hydrofluorocarbons [HFC] or sulfur hexafluoride [SF6]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/151—Reduction of greenhouse gas [GHG] emissions, e.g. CO2
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/151—Reduction of greenhouse gas [GHG] emissions, e.g. CO2
  • Y02P20/155—Perfluorocarbons [PFC]; Hydrofluorocarbons [HFC]; Hydrochlorofluorocarbons [HCFC]; Chlorofluorocarbons [CFC]

Definitions

• This invention relates to a method for recovering or concentrating perfluorinated compounds (PFC) and hydrofluorocarbon (HFC) gases from effluent gases in manufacturing processes.
• PFC perfluorinated compounds
• HFC hydrofluorocarbon
• PFCs polyfluorinated compounds
• the techniques presently used for PFC removal from waste streams include scrubbers (e.g. of Ecosys, ATMI) and scrubbers combined with cryogenic ⁇ distillation (Praxair US 5,502,969) and permeation and condensation by means of glassy membranes(EPA 0 754 487).
• scrubbers e.g. of Ecosys, ATMI
• CRIPair US 5,502,969 cryogenic ⁇ distillation
• permeation and condensation by means of glassy membranes EPA 0 754 487.
• the scrubber and distillation process has the advantage of recycling but the PFC must be separated from the scrubbing liquid and proper disposal of the scrubber liquid must still be dealt with.
• EPA 0 754 487 discloses a process for the separation and recovery of perfiuorocompound gases by means of glassy polymeric membranes. Such membranes, however, are not selective enough to completely retain all of the PFC and HFC compounds concerned. As a result more downstream processing of the PFC and HFC compounds that have escaped to the permeate is required either by scrubbing or by absorbers. This limitation of selectivity results in a limit to the volume concentration that the feed stream can be subjected to, which leads to a relatively high amount of non- condensable carrier gas (e.g. nitrogen) still being in the non-permeate (reject) stream. As a result higher pressures and temperatures would be required to reach the desirable extent of recovery of PFCs by condensation.
• non- condensable carrier gas e.g. nitrogen
• Example 1 of the cited EPA 0 754 487 An example of this limitation is shown in Example 1 of the cited EPA 0 754 487.
• Table 2 are shown the extent of PFC and HFC losses to permeate of an effluent stream which has been concentrated 15 fold. As can be seen in the table, while the concentration of the CF 4 in the permeate stream is about 1% of its concentration in the feed stream, , that of the C 2 F 6 is almost 2% and that of the CHF is over 40%.
• glassy polymeric membranes have a drastic flux drop as their operation temperature is reduced.
• SF ⁇ sulfur hexafluoride
• SF ⁇ is a very inert gas that is finding increasing use in the electrical power industry, magnesium and aluminum foundries,, and semiconductor manufacture (see E. Cook, "Lifetime Commitments: Why climate Policymakers Can't Afford to Overlook Fully-Fluorinated Compounds," World Resources Institute, Washington D.C, 1995 cited in Christophoru, L.G. and R.J. Van Brunt, IEEE Trans. Dielectrics and Electric Insulation, 2(5), pp. 952-991 (1995). Its dielectric properties make it three times as resistant as air to electric breakdown. As a result 80% of SFe is used in blanketing electrical installations, such as circuit breakers and switching stations.
• SFe has extremely strong IR adsorption bands at 522 and 345 cm- 1 and additional bands at wave numbers of 947, 770, 640, and 615 cm" 1 (Matheson, Handbook of Gases, "The Matheson Unabridged Gas Data Book - A compilation of physical and thermodynamic properties", 5th Ed., 4 Vols. (1974) published by Matheson Gas Company).
• a gas that absorbs IR radiation emitted by the earth's surface especially in wavelengths between 7-13 urn.
• SF ⁇ is a condensable gas and liquefies at room temperature at 23 atmospheres. Its vapor pressure diagram is provided in Fig. 7 (based on data from the above cited Matheson, Gas Handbook). As such, it could ' be conceivably condensed from a mixture of non-condensing gases (e.g. air, argon, etc.). However, if mixed with CO2 it would be hard differentially to condense it and a cryogenic distillation system would be required for this purpose.
• non-condensing gases e.g. air, argon, etc.
• a microporous membrane will act as a molecular sieve when the effective smallest dimension of its pores lies between that of the molecules to be permeated and that of the larger molecules to be retained.
• a molecule retained by a molecular sieving membrane still have a positive energy of activation for diffusion across the membrane which is significantly higher than that of the molecules which are passed by the membrane.
• the nature of the molecular sieving membranes is such that the selectivities tend to be very high relative to polymer membranes as can be seen from the selectivities found in the cited references.
• PFCs perfluorinated compounds
• HFC's hydrofluorocarbons
• the membrane is a molecular sieve membrane which passes nitrogen but not PFC and HFC molecules, and has a retentivity ratio of the PFC or HFC over nitrogen that is at least 50, and preferably 300, and a permeance for nitrogen that is at least 80 SL/m 2 -hr-bar (standard liters per square meter per hour per bar), and preferably at least 200 SL/m 2 -hr-bar, provided that if the PFC or HFC has a molecular diameter equal to or smaller than that of CHF3 , the permeance for N2 should be at least 50 SL/m 2 -hr-bar.
• the molecular sieve membranes which have the aforesaid, required retentivity ratio and permeance are chosen from among inorganic, microporous, molecular sieve membranes.
• the retentivity of the membrane Since the gases of interest in this invention are required to be retained instead of being passed by the membrane, it is found convenient to define the retentivity of the membrane as the reciprocal of the permeance of the gas, and the retentivity ratio is the retentivity of the PFC or HFC gas divided by the retentivity of the non-condensable gas with which it is mixed. Therefore, the lower the permeance of a gas, the higher its retentivity.
• the retention ratio of a membrane will be the ratio of the retentivity of the slowest gas to the other gases in the processed mixture.
• Said retentivity ratio is equal to the permselectivity, which is the ratio of the permeance of the more permeable non-condensing gas to that of the less permeable (PFC or HFC) gases.
• non-condensable gases are meant gases whose critical temperature is lower than 0°C.
• the molecular sieve membranes preferably have a pore size larger than the effective molecular diameter of the non-condensable gases mixed with the PFC or HFC's in the industrial exhaust effluent gas to be treated, but smaller than the effective molecular diameters of the PFC and HFC's. Since the most common non-condensable gas nitrogen, the membrane should have a pore size of at least 3.7 Angstrom. Should tho gas stream to be treated contain, in place of nitrogen, another non-condensable gas having a larger effective molecular diameter, the minimum pore size of the membrane should be proportionally higher to permit said gas to pass through the membrane.
• the maximum pore size must be such as to retain the PFC or HFC's which it is wished to recover, it must not exceed the molecular diameter of the PFC or HFC and may vary depending on said diameter. In most cases, it would not be higher than 5.5 Angstrom.
• the gases with which the PFC or HFC's are usually diluted are permanent gases fo -.d in air (O2, N2, Ar) and gases used in certain industrial processes (O2, Ar, CO2).
• gases used in certain industrial processes O2, Ar, CO2.
• Table 3 Kinetic diameters of molecules after Breck. in Angstrom C0 2 0 2 Ar N 2 SF ⁇ CF2CI2 CF 4
• Preferred inorganic, molecular sieve membranes are carbon membranes.
• a suitable carbon molecular sieve membrane can be prepared for example by a method described in PCT application WO 96/22260. Examples of such membranes, having the desired retentivity and permeance parameters, are e.g. those prepared according to steps 1-14 in Table V of Example 1 of said PCT appHcation and those prepared according to steps 1-2 in Table VI of Example 2 of said PCT appHcation, but with a time of 20-75 minutes in step 1.
• the retentate can be further purified by feeding it to a condenser as taught in EPA 0 754 487. Since higher volume concentration ratios can be obtained by the use of molecular sieve membrane than in other ways, higher temperatures can be used in the condenser, or the feed stream can be less compressed. Since the presence of PFCs and HFC's in the permeate will be extremely low, the permeate can either be directly discharged to the atmosphere or purified by passing it through an adsorber bed, such as sold by supphers such as Air Products, or through scrubbers such as those suppHed by Ecosys.
• an adsorber bed such as sold by supphers such as Air Products, or through scrubbers such as those suppHed by Ecosys.
• these permeate purifying devices can be smaHer, or have a longer cycle time between regeneration steps, than similar devices when used in prior art processes.
• the permeate car. be purified, and the residual PFC or HFC contained therein be recovered, by passing said permeate through a second type of membrane, that is permselective to the PFC or HFC over the non-condensable gas, as is taught in PCT WO 92/19359.
• the resulting concentrated permeate can then be mixed with the feed to the molecular sieve membrane.
• the pressure difference on the molecular sieve membrane module can be produced either by compressing the feed gas stream or/and by applying a vacuum on the permeate gas stream.
• the retentate can be recycled or otherwise reused as such, or further treated to further increase the content of the PFC or HFC's.
• the permeate can be discharged to the atmosphere or further purified to recover more PFC or HFC's and make the final waste still more environment-friendly.
• the retentate which has a high concentration of the PFC or HFC's , is available on the feed side of the membrane module, at the high pressure at which it has been fed to said module. It is preferable to maintain said high pressure, either for recycling the retentate to the process which has produced the outlet gas mixture, thus saving compression capital costs and energy needed to store the gas in cylinders, or for sending said retentate to subsequent condensation-purification stages.
• the invention further provides an apparatus for the separation and recovery ⁇ or concentration of perfluorinated compounds, which comprises at least one membrane module instaHed on the outlet of the duct system of an apparatus in which an industrial process is carried out, and means for recovering the retentate of said module, wherein the permeable membrane of the module has the foUowing characteristics: a retentivity ratio of the PFC or HFC's over nitrogen that is at least 50, and preferably 300, and a permeance for nitrogen that is at least 80 SL/m 2 -hr-bar (standard Hters per square meter per hour per bar), and preferably at least 200 SL/m 2 -hr-bar, provided that if the apparatus is intended for the separation of PFC or HFC's which have a molecular diameter equal to or smaHer than that of CHF 3 , the membrane should have a permeance for N2 which is at least 50 SL/m 2 -hr-bar.
• the driving pressure for forcing the permeating outlet gases through the membrane can be suppHed by the blower gathering the duct gas, by an auxiliary compressor taking the gas from the duct, or by any device for creating a vacuum on the permeate side of the membrane, such as a pump or an ejector.
• the apparatus further comprises means for returning the retentate gas, leaving the membrane enriched in PFC or HFC, to the process or to further purification apparatus, such as compressor and condenser apparatus.
• the apparatus further comprises a second membrane module or modules that are permselective for the PFC or HFC's.
• permselective is used in the conventional sense used by membranologists, viz. to mean that the gas in question permeates the membrane in preference to other gases in the mixture.
• Fig. 1 shows a typical flow sheet or schematic apparatus illustration using a molecular sieve membrane to recover the PFC or HFC from the effluent, according to an embodiment of the invention
• Fig. 2 shows an embodiment of a two stage membrane process using one compressor and one vacuum pump to provide the transmembrane pressure driving force
• Fig. 3 is a process flowsheet for fabricating a carbon molecular sieve membrane by a combination of pyrolysis foUowed by one or more CVD and activation steps;
• Figs. 4 and 5 are schematic illustrations of apparatus for the recovery of PFC and HFC's from waste gas mixtures, according to two further embodiments of the invention.
• Fig. 6 illustrates a typical test system for measuring separating power of the molecular sieve membrane base
• Fig. 7 is a SF ⁇ vapor pressure diagram
• Fig. 8 is a diagram illustrating SF ⁇ losses in cryogenic recovery from waste gases.
• Fig. 9 shows diagrams illustrating the effect of membrane selectivity on SF ⁇ losses.
• block 10 indicates the apparatus of a manufacturing process that produces a PFC or HFC containing effluent.
• the gaseous feed to the process is indicated as stream 11. It is mixed with recycled gas (stream 12) originating from the recovery apparatus to form the gas feed 13 to the process 10, unless said stream 12 is utilized in any other desired way.
• the waste gas from the process 14 is collected in a duct system 15.
• the driving force for coUecting the waste gas is provided by the underpressure generated by the inlet to a blower or -compressor 16.
• the waste gas may be diluted by the surrounding air 17 which is also swept into the duct system.
• the effluent 18 flows optionaHy through a guard bed 19 that removes potentiaHy fouling contaminants of the molecular sieve membrane, e.g. moisture.
• the gas stream then flows into one or more highly productive molecular sieve membrane modules 20, which are highly retentive for the PFC or HFC's.
• a vacuum pump 22 can be placed on the permeate (low pressure side or the module) to increase the permeate to feed pressure ratio.
• the membrane module 20 can actuaHy represent a module array with successively staged concentration steps with gas recycle within each concentration step to provide good flow thrcugn the modules.
• the module can represent an array with a tapered flow structure in which there are successively fewer modules in paraUel in each successive concentration stage. All of these arrangements are well known to the practitioner of membrane based separations seeking high volume concentration ratios.
• the retentate stream 23 can be sent optionally to a further concentration or purification process such as condensation and/or distillation and/or selective adsorption before being returned to the process. This optional further step is represented by block 26.
• the permeate 21 can be vented or sent to a further cleaning step such as a scrubber or adsorber (block 30) to remove any vestiges of the PFC or HFC gas.
• Fig. 2 represents another preferred embodiment.
• An effluent stream 31 is fed to a compressor A and the pressurized feed 32 is sent to a first stage membrane module array B operated with the permeate 35 at ambient pressure.
• the concentrated retentate 33 from the first stage is fed to a final membrane concentration stage equipped with molecular sieve membranes C, in which the permeate is maintained at a vacuum to increase the fraction of the more non-condensable gas which is passed to the permeate without a corresponding loss of the PFC or HFC to the permeate.
• the retentate 34 is used as desired, either by recycling it or in any other way, while the permeate 36 may be sent, if desired, to a further cleaning apparatus D and then vented, as indicated at 37.
• a molecular sieve membrane can be prepared with appropriate pore diameter using the techniques reported in the Hterature for inorganic membranes for gas separation, such as zeohte, amorphous carbon or siHcaHte materials.
• a carbon molecular sieve membrane that can be used according to the invention can be prepared by the methods describe in said WO 96/22260 and if necessary with the additional steps described in EPA 94200680.0. The method of preparation is illustrated in the flowsheet of Fig. 3.
• Said membrane is prepared by first pyrolysing a polymer precursor, such as polyimide or cellulose, which is already in the geometry of a membrane form 51 either in vacuum or in an inert gas stream.
• the permeabiHty of the resultant carbon membrane can be increased by a series of one or more activation steps 52.
• the carbon membrane should have a defined asymmetric structure. If it does not, the thermochemical treatments can include transforming the carbon hoUow fiber membrane into an asymmetric membrane by chemical vapor deposition on one of the membrane surfaces 53. Further opening of the carbon matrix to increase the permeability to non-condensing smaHer gases can be achieved with further activations 54.
• Pure gas permeances were measured by the pressure change method, applying a vacuum to the permeate side of the membrane and measuring the rate of pressure change on the feed side of the membrane module and test apparatus, the volumes of which were previously caHbrated.
• the units "mol/m 2 -min” can be converted to SL/m 2 -hr, taking inLo account that 1 mol is equivalent to 22.4 SL.
• the permeance is then obtained by dividing the molar flux by the average pressure (in bar) found on the feed side of the membrane.
• the volume on the feed side of the measurement apparatus is arranged to be of such a magnitude that the change of pressure during the measurement period is a smaH fraction of the total apphed pressure so that the transmembrane pressure difference is relatively constant, yet the change is large enough to be accurately measured with the pressure sensors used in the measurement.
• the next table gives typical results of several carbon hoUow fiber molecular sieve membrane modules subjected to this treatment.
• the results are given as pure gas permeances in SL/m 2 -hr-bar, and as selectivities (ratio of N 2 permeance to CF 4 permeance) and were measured as described above.
• the process of the invention does not require that the permeable membrane have a permeance for N 2 which is at least 80 SL m 2 -hr-bar, but said permeance may be as low as 50 SL/m 2 -hr-bar.
• N 2 which is at least 80 SL m 2 -hr-bar
• said permeance may be as low as 50 SL/m 2 -hr-bar.
• Steps 5- 6 are repeated two more times.
• a carbon membrane module prepared with this protocol gave CF 4 permeance at 25 °C of 0.39 SL/m 2 -Hr-bar and an N 2 permeance of 310 SL/m 2 -Hr-Bar, which gives a selectivity of 795.
• FIGs. 4 and 5 Other embodiments of apparatus according to the invention are illustrated in Figs. 4 and 5.
• Fig. 4 schematically illustrates an embodiment, in which 10 indicates the apparatus which produces the effluent stream, 13 is the feed to said apparatus, 14 the effluent stream or waste gas produced by apparatus 10, , which may be diluted by surrounding air 17 and is coHected in duct system 15, 16 is a blower or vacuum ejector pump providing the driving force for coUecting said gas, 18 is the stream outlet by it, 20 is a membrane module or modules for retaining the PCF or HCF, and 21 is the permeate from module 20.
• the PCF or HCF retentate or concentrate 55 is fed to a compressor 56, then to a condenser composed of a heat exchanger 57 and a Hquid-vapor phase separator 58, where the PCF or HCF, which " is now in concentrated form, can be more easily compressed and condensed, thereby further purifying it from its non-condensable contaminants.
• the purified retentate can then be recycled to the process or otherwise used, as indicated at 59, while the waste gases are vented to the atmosphere, as indicated at 60.
• Fig. 5 schematically illustrates still another embodiment, wherein the parts that are essentiaHy the same as in Fig. 4 are indicated by the same numerals.
• the retentate or c centrate stream from membrane module 20, which stream is here indicated at 61 is fed to a second membrane module 62, which is different from module 20 and is permselective for the PFC or HFC's to be separated, viz. selectively permeates them over non-condensable gases.
• the enriched retentate presents a higher driving force (the driving force for a given gas being the partial pressure difference across the membrane for that gas) to the membrane 62, which is permselective to the PFC or HFC, and produces a permeate 63 of even higher concentration of these latter, which is recycled or otherwise used.
• the stream 64 from membrane 62, depleted of PFC or HFC is recycled to the feed 18 of membrane 20.
• the permeate 63 of membrane 62 can be used as is, or alternatively fed to a compressor-condenser combination, not illustrated, for further purification of the PFC or HFC from non-condensable gases by conventional means.
• Fig. 6 shows a flow system with a gas manifold generaHy indicated at 41 to provide high pressure gas to the feed side of the membrane module and draw away permeate from the low pressure side of the membrane.
• the module pressure housing 42 containing the membrane is of stainless steel with bore and shell gas inlet/outlets at each end, so that the module can be operated in bore feed or shell feed mode, as needed.
• the bore side is kept separate from the shell side by an epoxy potting materi-- 1 in the tube ends, which seals the fibers to each other and to the inner surface of the stainless steel tube.
• the feed manifold generally indicated at 41, comprises three conduits (41-1 through 41-3) connected to pressure gauges 47 and mass flow controUers 44.
• the permeate rate is measured by mass flow meters or bubble flow meters indicated at 45.
• the oxygen content of the feed, retentate and permeate streams can be monitored with a Model 570 A ServomexTM oxygen analyzer made by Sybron Corp., indicated by numeral 46.
• Numeral 47 indicates all pressure gages;
• numeral 48 indicates pressure transducers and numeral 49 differential pressure transducers.
• This example uses the apparatus shown in Fig. 6.
• the gases N 2 and CF were fed in through the gas manifold 41 using mass flow controUers 41-1 and 41-2.
• the mixture was fed to the bore side of a hoUow fiber module containing 0.16 m 2 of carbon molecular sieve membrane as hereinbefore described.
• the pressure drop across the membrane is monitored and maintained with the differential pressure gauge 49.
• the permeate is removed from the sheH side outlet of the module that is closest to the feed entrance 50.
• the pressure in the module feed side is maintained using a backpressure regulator 43.
• the permeate and retentate feed rates are sampled by the mass flow meters 45b and 45a respectively and fed alternatively to a gas chromatograph 46 to analyze the gas composition.
• 1 Hter of gas mixture was fed to the module and 0.95 Hter was taken out as permeate 50 and 0.05 Hter of gas mixture was taken out as retentate 51.
• Table 8 The results of this experiment; are shown in Table 8.
• the foUowing simulation shows the recovery of PFCs from a process using a system such as shown in Figure 2.
• the simulation was calculated for a carbon molecular sieve membrane with selectivities as indicated in the tables. The calculation was done assuming crossflow flow pattern and calculating driving forces by a log mean average of inlet and exit conditions, where the local composition of gas on the permeate side is given by the WeHer-Steiner equation.
• SFe is of particular interest.
• it is preferable to retain it and pass the permanent gases through since the permanent gases are to be vented to the atmosphere in most cases, while the SFe is to be recovered and it is desirable to recover it at pressure.
• the most common contaminant to be found with SFe is air, which is made up largely of nitrogen, it is convenient to define the selectivity of the membrane as the selectivity between SFe and nitrogen. The required selectivity will depend on the concentration of SFe and the concentration , factor required to return it to the process. The higher the concentration factor, the higher the selectivity required to prevent significant SFe losses.
• One ot the advantages of the molecular sieving membranes from inorganic materials is that their matrices are fixed and defined at a wide range of temperatures and do not undergo swelling when permeated by molecules that interact with the matrix.
• the required selectivity is a function of the concentration of the SF ⁇ in the waste gas and the concentration factor by which it must be raised in the recovered product gas.
• a carbon sieve membrane that can be used according to the invention for separating SFe from other, particularly non-condensable gases can be prepared, as has been said, by the method described in said WO 96/22260, and if necessary with the . additional steps described in EPA 94200680.0, e.g. as follows.
• Said membrane is prepared by a series of thermochemical treatments of a carbon hoUow fiber membrane which has previously been formed by pyrolyzing a non-melting polymer such as ceUulose in an inert gas stream.
• the carbon hoHow fiber membrane should have a defined asymmetric structure.
• thermochemical treatments include firstly transforming the carbon hoUow fiber membrane into an asymmetric membrane by chemical vapor deposition on one of the membrane surfaces.
• FoUowing the transformation of the membrane into an asymmetric membrane its permeance is then increased by a series of activations by exposing it to oxygen at temperatures ranging from 200 to 300°C and times ranging from 10 to 90 minutes. .
• residual oxygen is removed by treating the membrane with hydrogen or other reducing gases at elevated temperatures.
• These activation steps are continued untU the required permeance to the slowest non- condensable gas (e.g. N2) is achieved, as indicated by the point at which the selectivity between oxygen and nitrogen drops to less than 2 and preferably to less than 1.5.
• the slowest non- condensable gas e.g. N2
• the pure gas permeances were measured by the pressure change method, applying a vacuum to the permeate side of the membrane and measuring 'the rate of pressure change on the feed side of the membrane module and test apparatus, the volumes of which were previously cahbrated.
• the units "mol/m 2 • min” can be converted to SL/m 2 -hr, taking into account that 1 mol is equivalent to 22.4 SL.
• the permeance is then obtained by dividing the molar flux by the average pressure (in bar) found on the feed side of the membrane.
• the volume on the feed side of the measurement apparatus is arranged to be of . c uch a magnitude that the change of pressure during the measurement period is a smaU fraction of the total apphed pressure so that the transmembrane pressure difference is relatively constant, yet large enough to be accurately measured with the pressure sensors used in the measurement.
• a typical treatment which was successful in developing such a membrane is defined in the foUowing table:
• the said membrane has the pure gas permeances set forth in Table 12.
• CMSM carbon molecular sieve membrane
• the membrane can be opened in one additional activation step " to increase the permeabUity to the slowest of the non-condensable gases, which is N2 in this case, but at the expense of selectivity.
• An additional step will result in the increase of nitrogen permeance to 1300 SL/m 2 -hr-bar, but at the cost of the N 2 /SF6 permeance ratio dropping to 810.
• the O2 partial pressure driving force is calculated from the measured transmembrane pressure and the measured O2 content in feed, retentate and permeate streams.
• the SFe content of each stream is calculated by the difference, I-X02 ' .. Tni expression
• I-X02 means the mol fraction of the gas mixture which remains after subtracting the mol fraction of oxygen.
• the mol fraction of SFe is given by I-X02 .
• a carbon molecular sieve membrane was subjected to thermochemical treatments according to the general procedure described in said WO 96/22260.
• the membrane thus prepared composed of a bundle of 100 fibers having an area of 0.18 ft 2 , was loaded in a high pressure housing formed of a V inch stainless steel tubing with end tube fittings to aUow separate access to the bore and sheU side of the membrane bundle. This membrane module was then mounted on the mixed gas test system described in Fig. 6.
• a carbon molecular sieve membrane with appropriate selectivity for SF ⁇ recovery was prepared according to the preparation conditions summarized in the foUowing Table 13.
• a carbon molecular sieve membrane with appropriate selectivity for SF ⁇ recovery was prepared according to detailed preparation conditions, as summarized in the foUowing Table 15.
• This membrane was mounted in a stainless steel pressure housing and mounted on the gas test system hereinbefore described.
• the pure oxygen permeance was measured and found to be 1118 SL/m 2 -hr-bar.
• no permeation could be detected, as seen by the absence of bubbles in a bubbler attached to the permeate port of the module.
• the module was fed a mixture (v/v%) of 98% O2 and 2% SFe to the bore side at a pressure of approximately 3 bar transmembrane pressure. Because the oxygen monitor was not sensitive enough to reHably measure with +/- 0.1% at values above 98%, the O2 content of the permeate was calculated from the O2 mass balance, and the SFe content was calculated by the difference. The results are summarized in the foUowing Table 16. Table 16 Results of mixed gas experiment on CMSM module
• Example 5 Recovery of SF ⁇ from a Hght metals foundry
• SFe is used to protect the surface of the cast metal.
• the SFe content of the protective gas mixture (stream 13) fed at 10 m 3 /hr to the casting ceU is 1% (v/v) and the exit gas (stream 14) is dfluted by the blowers coUecting the exit gas to 0.05% (v/v) with a flow of 200 m 3 /hr.
• the SFe gas is recovered by compressing the waste gas stream to 3 bara (stream 18) and passing the fast gases through the membrane module (stream 21) and returning -10 m 3 /hr of SFe enriched retentate (stream 23) baci-: to the casting ceUs as stream 12 where makeup SF ⁇ can be added (as stream 11).
• the membrane has the permeation characteristics Hsted in Table 12. The flow and composition balance of the stream of this example are given in the foUowing Table 17.
• This table can be read ns the maximal temperatures aUowed for a particular pressure to which the effluent stream has been compressed, or as the minimum pressure to which the effluent stream has been compressed, or as the minimum pressure to which the effluent stream must be compressed if the minimum temperature in the condenser is as Hsted in the foUowing table.
• Table 18
• the concentrated waste stream need only be condensed to 5 bar instead of 40 bar for a final condensation temperature of -77, and only 450 mol hr instead of 450C mol h; need be compressed, a tremendous savings in compression energy. Also, the compression ratio is half what it would be if ambient pressure gas were compressed, since the retentate is already at 2 bara.
• a membrane is prepared in a simUar manner as in example 4. This membrane was then mounted in a stainless steel housing and tested in a gas mixture test system, hereinbefore described and iUustrated in Example 3. However, in the present example the feed gas contained 91% O2 and 9% CHF 3 . The feed was fed to the sheU side of the membrane at 7 bar absolute pressure and the bore side was maintained at 1 bar absolute pressure, giving a pressure ratio of 7. Measurements and calculations where made in a manner similar to example 4. The results are given in Table 20. Table 20: Results of test separating mixture of CHFa and 0?

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• 1998
  • 1998-01-22 CA CA002275960A patent/CA2275960A1/en not_active Abandoned
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