Patent Application
20090320519
None.
Hydrofluoroalkanes (HFAs), alternatively named hydrofluorocarbons, are saturated alkanes wherein one or more of the hydrogens are substituted with a fluorine atom. Chlorofluorocarbons (CFCs) are saturated alkanes wherein at least one or more of the hydrogens are substituted with a fluorine atom and at least one or more of the other hydrogens are substituted with a chlorine atom. Several types of HFAs and CFCs are used in many medical products to propel an active ingredient dispersed or solubilized therein, such as metered dose inhalers, nasal sprays, foam sprays, and other oral sprays. The combined types of HFAs and CFCs may be describe by the compound of formula I:
(CX3(CX2)nCX3 (I)
wherein each X is individually H, F, or Cl and at least one X is F and n is an integer in the range of 0-3.
Various types of HFAs include heptafluoropropane (CF3CFHCF3), tetrafluoroethane (CF3—CFH2), 1,1,1,3,3-pentafluoropropane, 1,1,1,3,3-pentafluorobutane, difluoroethane (CH3CHF2) and 1,1,1,2,3,4,4,5,5,5-decafluoropentane. There are numerous CFCs known in the field of pharmaceutical propellants and an exhaustive list need not be recited herein. HFAs and CFCs typically used as propellants in medical products include heptafluoropropane, tetrafluoroethane, trichloromonofluoromethane, dichlorodifluoromethane, and dichlorotetrafluoroethane.
In the final step before packaging, these products are tested at the production facility to make sure proper dose of active ingredients are delivered (Assay test). The resultant sprayed doses containing HFAs are typically driven out by vents using air. Testing large quantities of medical products simultaneously can increase the effluent concentration of HFAs beyond acceptable limits. Since these organic compounds are highly volatile and very potent green house gases, their discharge to the atmosphere needs to be controlled.
There is disclosed a method for recovering a halogenated alkane of the formula CX3(CX2)nCX3 wherein each X is individually H, F, or Cl and at least one X is F and n is an integer in the range of 0-3. The method comprises the following steps. The gas mixture is directed to a gas separation membrane unit, the gas mixture comprising air and the halogenated alkane. The gas mixture is separated with the gas separation membrane unit into a permeate enriched in oxygen and nitrogen, and depleted in the halogenated alkane and a retentate enriched in the halogenated alkane and depleted in oxygen and nitrogen. The retentate is directed to a cryogenic condenser. An amount of halogenated alkane is condensed from the retentate in the cryogenic condenser. A non-condensed portion of the retentate is condensed from the cryogenic condenser.
There is also disclosed a system for recovering hydrofluoroalkanes from a gas mixture that comprises: a gas separation membrane unit, a cryogenic condenser, a source of liquid nitrogen, and a heat exchanger. The gas separation membrane unit is adapted and configured to separate a gas mixture containing a hydrofluoroalkane and air into a permeate enriched in oxygen and nitrogen and depleted in the hydrofluoroalkane and a retentate enriched in the hydrofluoroalkane and depleted in oxygen and nitrogen. The cryogenic condenser comprises a housing enclosing an inner space, a retentate inlet adapted and configured to receive the permeate from the permeate outlet, a non-condensate outlet adapted and configured to vent a portion of the retentate not condensed by the cryogenic condenser, and a condensate outlet adapted and configured to discharge hydrofluoroalkane condensed from the retentate by the cryogenic condenser. The heat exchanger is disposed within the inner space and including a liquid nitrogen inlet in fluid communication with the source of liquid nitrogen, a gaseous nitrogen outlet, and a metallic heat exchange element having an inner and an outer surface, the inner surface of the heat exchange element defining a flow path in fluid communication between the liquid nitrogen inlet and the gaseous nitrogen outlet, the metallic heat exchange being adapted and configured to condense hydrofluoroalkane from the retentate on the outer surface of the heat exchange element through exchange of heat between the retentate and liquid nitrogen flowing through the flow path.
The method and/or system may include one or more of the following aspects:
the method further comprises the step of compressing the gas mixture before the step of separating is performed.
the cryogenic condenser comprises:
the method further comprises the step of combining a portion of non-condensed retentate from the non-condensate outlet with the gas mixture upstream of the gas separation membrane unit.
the gas separation membrane unit comprises at least one gas separation membrane.
the method further comprises the step of directing at least a portion of non-condensed retentate from the non-condensate outlet to a permeate side of the gas separation membrane to enhance permeance of oxygen and nitrogen through the membrane.
the membrane is configured as a plurality of hollow fibers each comprising a core surrounded by a sheath comprised of a primary gas separation medium.
the primary gas separation medium comprises a polymeric condensation product of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 5(6)-amino-1-(4′-aminophenyl)-1,3,3′-trimethylindane.
the primary gas separation medium comprises a 60%:40% blend of a polymer A and polymer B, wherein:
the primary gas separation medium comprises a copolyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 80%/20% mixture of toluenediisocyanate and 4,4′-methylene-bis(phenylisocyanate).
the permeate is enriched in water and the retentate is depleted in water.
the halogenated alkane is a hydrofluoroalkane of the formula CX3(CX2)nCX3 wherein each X is individually H or F and at least one X is F and n is an integer in the range of 0-3.
the system further comprises a compressor adapted and configured to compress and direct the gas mixture to said gas separation membrane unit.
the system further comprises a recycle conduit fluidly communicating between said non-condensate outlet and said feed inlet.
the system further comprising a sweep gas conduit fluidly communicating between said non-condensate outlet and a permeate side of said gas separation membrane such that flow of the non-condensate therethrough drives permeation of the oxygen and nitrogen through the membrane.
the method further comprises the step of directing at least a portion of gaseous nitrogen from the gaseous nitrogen outlet to a permeate side of the gas separation membrane.
the system further comprises a sweep gas conduit fluidly communicating between the gaseous nitrogen outlet to a permeate side of the gas separation membrane.
the primary gas separation medium comprises a polymeric material having a selectivity of nitrogen to the halogenated alkane present in the gas mixture of at least 45.
For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:
The concentration of halogenated alkane in an effluent gas mixture of air and sprayed doses of pharmaceutical (containing one or more halogenated alkanes as a propellant) may be significant from an environmental perspective, but is rather low for condensing it out with a cryogenic condenser. Significant energy is required to recover the halogenated alkanes by the cryogenic condensation method. Presence of water in the effluent stream also results in the need for mechanical refrigeration unit and a precondenser for removing water before treating it with the cryogenic condenser.
By first removing substantial amounts of oxygen, nitrogen, and moisture from a gas mixture of one or more halogenated alkanes and air, subsequent cryogenic condensation of the retentate gas can be performed using significantly less liquid cryogen and energy. The water, oxygen and nitrogen preferentially permeate through the membrane over halogenated alkanes. Since water is the fastest molecule with permeability significantly higher than other components in the stream, almost all of water could be removed. At the same time, large portions of oxygen and nitrogen permeate thereby concentrating the concentrations of the halogenated alkanes in the retentate. Removal of water also eliminates the need for mechanical refrigeration and precondenser.
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The membrane includes a primary gas separation medium. The membrane may be configured in a variety of ways: sheet, tube, hollow fiber, etc. In the case of a hollow fiber membrane, either a monolithic or conjugate configuration (a sheath surrounding a core) may be selected. If the monolithic configuration is selected, the primary gas separation medium is uniformly distributed throughout the fiber.
If the conjugate configuration is selected, while the primary gas separation medium present may be present in the core, preferably it is present in the sheath (in such a case the sheath is also called the selective layer) around a core. In this latter configuration, the core has an OD in the range of from about 100 and 2,000 μm, preferably from about 300 μm and 1,500 μm. The core wall thickness is in a range of from about 30 μm to 300 μm, preferably no greater than about 200 μm. The core inner diameter is from about 50 to 90% of its outer diameter. The selective layer is less than about 1 μm thick, preferably less than about 0.5 μm thick. Preferably, the thickness is in a range of from about 150 to 1,000 angstroms. More preferably, the thickness is in a range of from about 300 to 500 angstroms.
When the primary gas separation medium is present in the sheath, the core may be made of several different types of polymeric materials. A non-limiting list of materials suitable for use as the core include polysulfones, ULTEM 1000, or a blend of ULTEM and a polymeric material available under the trade name MATRIMIDE 5218. Ultem 1000 is a polymer represented by Formula II below and is available from a variety of commercial sources, including Polymer Plastics Corp., Reno, Nev. or Modern Plastics, Bridgeport, Conn.).
MATRIMID 5218 is the polymeric condensation product of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 5(6)-amino-1-(4′-aminophenyl)-1,3,3′-trimethylindane, commercially available from Ciba Specialty Chemicals Corp.
Preferably, the material comprising the primary gas separation medium has a nitrogen to halogenated alkane selectivity of at least 45. A non-limiting list of particular materials for use as the primary gas separation medium includes but is not limited to
The stream of retentate 9 is directed from the gas separation membrane unit 5 and through a retentate inlet formed in a housing 12 of a cryogenic condenser 11. From there, it proceeds into to an inner space 13 enclosed by the housing 12. A stream of a liquid cryogen 15 (preferably nitrogen) is directed through a heat exchange element 17 contained within inner space 13. The retentate is allowed to flow past the element 17. The relatively low temperature of the liquid cryogen flowing through the element 17 causes the halogenated alkane to condense upon it and collect at a bottom of the condenser 11. The non-condensed portion of the retentate exits the condenser 11 as non-condensate stream 23. The condensed retentate may be allowed to exit the condenser 11 continuously or in batch-wise fashion via condensate stream 21. The condensate may be then be purified and reused or contained or destroyed. Through heat exchange with the retentate, the liquid cryogen vaporizes and exits the condenser 11 via gaseous nitrogen stream 19.
There are many commercially available cryogenic condensers, including the VOXAL cryogenic condenser available from DTA (Grenoble, France) and Air Liquide Advanced Technologies U.S., subsidiaries of Air Liquide. The VOXAL CRYO model can handle a flow of gas to be treated up to 2000 Nm3/hr (1200 scfm) having an inlet VOC (volatile organic compound) concentration greater than 5 g/Nm3 (5000 ppmv). Depending upon the amount of retentate flow into such a unit, the operating temperature within will range from −30° C. (−22° F.) to as low as −170° C. (−274° F.).
While a shell and tube type configuration is shown for condenser 11, many other suitable configurations are possible and are considered within the scope of the invention. One example includes a heat pipe where cold is transferred between the liquid cryogen and the gas mixture by means of an intermediate fluid boiling in tubes maintained at constant temperature. The condenser 11 may also be configured as an exchanger block where heat transfer between the liquid cryogen and gas mixture takes place within two separate flow circuits within a cast aluminum block. Another example is the direct contact exchanger configuration. In this configuration, liquid cryogen is sprayed from a top of a column and onto the gas mixture. The cooled condensate also further cools the gas mixture.
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Depending upon the pressure of the effluent stream, it may need to be compressed for carrying out the desired separation.
The membrane's performance may be optimized by adjusting the operating temperature and pressure of the gas separation membrane in an empirical manner. For example, higher temperatures will increase the permeance and reduce the membrane area required for the separation whereas lower temperature will increase the selectivity and reduce the amount of halogenated alkane permeating to the low pressure side.
Likewise, the performance of the cryogenic condenser may be optimized by adjusting its operating temperature and pressure in an empirical manner.
A simulation tool was used to estimate outlet compositions based upon several operating parameters. A gas mixture feed with a temperature of 22 C, a pressure of ambient, and a flow rate of 18.294 kmol/hr was assumed. An HFA concentration of the gas mixture feed was set to 5 g/Nm3. Overall, the molar ratio of the gas mixture feed was set to 0.0135 mols H2O, 0.2070 mols O2, 0.7788 mols N2, and 6.59×10−4 mols HFA. The membrane simulated was a hollow fiber membrane having a core of a 95%/5% blend of Ultem/Matrimide and a sheath of Matrimide.
As seen in Table I, use of the membrane at the given parameters decreased the water, O2, and N2 concentrations and increased the HFA concentration such that no precondenser is necessary upstream of the cryogenic condenser and 17.25-127.0 kg/hr of liquid N2 was consumed. When a more moderate operating temperature of pressure is used or greater than 2 membrane bundles are used, the liquid N2 consumption drops to only 17.25-36.04 kg/hr. As a result of these simulations, it is seen that the use of the Case 5 conditions is optimal in comparison to Cases 1-4 and 6.
Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims.
