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WO1992019358A1 - Procede de separation utilisant une membrane - Google Patents

Procede de separation utilisant une membrane Download PDF

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Publication number
WO1992019358A1
WO1992019358A1 PCT/JP1991/000608 JP9100608W WO9219358A1 WO 1992019358 A1 WO1992019358 A1 WO 1992019358A1 JP 9100608 W JP9100608 W JP 9100608W WO 9219358 A1 WO9219358 A1 WO 9219358A1
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WO
WIPO (PCT)
Prior art keywords
membrane
permeate
organic component
feed
stream
Prior art date
Application number
PCT/JP1991/000608
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English (en)
Inventor
Richard W. Baker
Johannes G. Wijmans
Original Assignee
Membrane Technology And Research, Inc.
Nitto Denko Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Membrane Technology And Research, Inc., Nitto Denko Corporation filed Critical Membrane Technology And Research, Inc.
Priority to PCT/JP1991/000608 priority Critical patent/WO1992019358A1/fr
Priority to JP91508270A priority patent/JPH05508104A/ja
Publication of WO1992019358A1 publication Critical patent/WO1992019358A1/fr

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    • 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
    • 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
    • B01D53/225—Multiple stage diffusion
    • B01D53/226—Multiple stage diffusion in serial connexion

Definitions

  • Curing the emission of vent gases contaminated with organic vapors is an urgent environmental problem.
  • Treatment methods may be chemical, in which case the organic substance is destroyed completely or converted to another form, or physical treatment, in which case the waste is changed in volume or composition, and there is an opportunity for recovery of the organic.
  • separations or treatment technologies are described in the literature, yet few are in use, either because the technology has not been developed to the industrial level, or because the process is too costly for general application.
  • the cost of most treatment processes is concentration dependent.
  • the widely used processes are carbon adsorption, incineration and compression condensation.
  • Incineration because it involves burning the organic vapor with a supplementary fuel, such as natural gas, is best suited to relatively concentrated streams. Incineration is relatively costly when the concentration of organic in the stream is below about 5-10%. Neither carbon adsorption nor incineration is easily conductive to organic vapor recovery, and both can create secondary waste or pollution problems; in the case of carbon adsorption, spent contaminated carbon from the beds, and in the case of incineration, large volumes of carbon dioxide or other combustion products.
  • the invention is a membrane process that fractionates gas streams containing organic vapor at concentrations difficult to handle by other technologies, in other words, outside the optimum concentration ranges for carbon adsorption, incineration or condensation.
  • the result is two product streams, both of which are within concentration ranges suitable for treatment by these processes, and no other secondary streams.
  • the gas stream to be treated may be an effluent stream that would otherwise be discharged into the atmosphere untreated, or would be subject to some other treatment method or methods. Alternatively it may be an internal process stream from which it is desirable, for example, to recover one or more organic components for reuse.
  • Table 1 summarizes the representative features of carbon adsorption, incineration and condensation. The last column shows the corresponding features of membrane fractionation.
  • membrane fractionation offers a new technique for converting streams in the organic concentration range above about 0.1% and below about 20-50% saturation, where other processes are inefficient and costly, to streams that can be handled efficiently.
  • Membrane fractionation has various other advantages, including mobility; ease of operation; opportunities for organic recovery and reuse; versatility; absence of secondary streams; use at the source of the waste, without prior pooling, dilution or concentration of the stream; and favorable payback times.
  • the process of the invention involves running a feedstream containing organic vapor across a membrane that is selectively permeable to that vapor.
  • the vapor is therefore concentrated in the stream permeating the membrane; the residue, non-permeating, stream is correspondingly depleted in organic content.
  • the driving foce for permeation across the membrane is the pressure difference on the feed and permeate sides.
  • the fractionation produces permeate and residue streams containing organic vapors in concentrations that can be treated by existing separations technology. What is done with the two streams when they leave the membrane unit will depend on the nature of the organic, its economic value, its degree of harmfulness as a pollutant and other practical factors.
  • the efficiency of the process in terms of the relative proportions of organic vapor and other components in the feed, permeate and residue streams, will be determined by a number of factors, including the pressure difference, the selectivity of the membrane, the proportion of the feed that permeates the membrane, and the membrane thickness.
  • the membrane fractionation process may be configures in many possible way. Examples include a single membrane stage; a multistage system, where the membranes are connected in a cascade arrangement such that the permeate from one stage forms the feed for the next; a multistep system, where the membranes are connected in a series arrangement such that the residue from one step becomes the feed for the next; or mixed combinations of these, including those in which one or more of the permeate or residue streams is recycled within the membrane system.
  • Two-stage membrane systems are particularly advantageous, because they can achieve very high degrees of enrichment of the organic component in a highly cost-effective manner. Eighty per cent or above removal of the organic content of the feed can typically be achieved with an appropriately designed membrane fractionation process, leaving a residue stream containing only traces of organic. The permeate stram is typically concentrated 5- to 100-fold compared with the feedstream.
  • the membrane used in the fractionation process is selectively permeable to the organic component of the feedstream, so that the organic component is enriched in the permeate compared with the feed.
  • the permselective membrane materials used in the invention are preferably rubbery polymers at the operating conditions of the system, that is, they have glass transition temperatures below the temperature of the feed gas.
  • the permselective membrane should be made very thin.
  • a preferred embodiment of the invention involves the use of a composite membrane comprising a microporous support, onto which the rubbery permselective layer is deposited as an ultrathin coating. The preparation of such membranes is known in the art, and is discussed in detail below.
  • the membrane configuration is not critical to the invention.
  • the membranes are cast and coated as flat sheets, and then rolled into spiral-wound modules.
  • other types of configuration such as hollow fibers, plate-and-frame, or flat sheet membranes are also possible, and are intended to be within the scope of the invention.
  • the flux of a gas or vapor through a polymer membrane is proportional to the pressure difference of that gas or vapor across the membrane.
  • the pressure drop is preferably achieved by lowering the pressure on the permeate side of the membrane. This may be done by cooling and compressing the permeate, for example, or by means of a vacuum pump.
  • the ratio of the pressures on the permeate and feed sides of the membrane, and the ratio of the permeate flow and feed flow volumes also influence the performance of the process. It is an object of the invention to provide a fractionation process for dividing organic-vapor laden streams into two streams, both within concentration ranges suitable for treatment by other separations or waste treatment technology.
  • Figure 1 is a graph showing the relationship between permeate vapor concentration and pressure ratio for membranes of varying selectivities.
  • Figure 2 is a schematic diagram of a single stage membrane system for treating an organaic-containing stream.
  • Figure 3 is a schematic diagram of a two-stage membrane system for treating an organic-containing stream.
  • Figure 4 is a schematic diagram of a two-stage membrane system for treating an organic-containing stream, with condensation of the permeate and recycle of the non-condensed fraction of the permeate.
  • Figure 5 is a schematic diagram of a two-step membrane system for treating an organic-containing stream.
  • Figure 6 is a graph showing the relationship between feed and permeate concentrations of acetone, 1,1,1-trichloroethane, toluene and octane.
  • Figure 7 is a graph showing the relationship between feed and permeate concentrations of perchloroethylene.
  • Figure 8 is a graph showing the relationship between feed and permeate concentrations of CFC-11 at low CFC feed concentrations.
  • Figure 9 is a graph showing the relationship between feed and permeate concentrations of CFC-11 at CFC feed concentrations up to about 35 vol%.
  • Figure 10 is a graph showing the relationship between feed and permeate concentrations of CFC-113 at CFC feed concentrations up to about 6 vol%.
  • Figure 11 is a graph showing the relationship between feed and permeate concentrations of HCFC-123 at CFC feed concentrations up to about 8 vol%.
  • Figure 12 is a graph showing the relationship between feed and permeate concentrations of Halon-1301 at concentrations from about 0.1% to 5%.
  • Figure 13 is a graph showing the relationship between feed and permeate concentrations of methylene chloride at feed concentrations up to about 8%.
  • Figure 14 is a graph showing the relationship between operating cost and vapor concentration for carbon adsorption and for membrane fractionation, for a 1,000 scfm halocarbon-containing stream.
  • Figure 15 is a graph showing the relationship between operating cost and vapor concentration for carbon adsorption and for membrane fractionation, for a 200 scfm halocarbon-containing stream.
  • vapor refers to organic compounds in the gaseous phase below their critical temperatures.
  • CFC refers to fluorinated hydrocarbons containing at least one fluorine atom and one chlorine atom.
  • permselective refers to polymers, or membranes made from those polymers, that exhibit selective permeation for at least one gas or vapor in a mixture over the other components of the mixture, enabling a measure of separation between the components to be achieved.
  • multilayer means comprising a support membrane and one or more coating layers.
  • selectivity means the ratio of the permeabilities of gases or vapors as measured with gas or vapor samaples under the normal operating conditions of the membrane.
  • residue stream means that portion of the feedstream that does not pass through the membrane.
  • permeate stream means that portion of the feedstream that passes through the membrane.
  • series arrangement means an arrangement of membranes connected together such that the residue stream from one becomes the feedstream for the next.
  • cascade arrangement means an arrangement of membranes connected together such that the permeate stream from one membrane becomes the feedstream for the next.
  • membrane array means a set of individual membranes connected in a series arrangement, a cascade arrangement, or mixtures or combinations of these.
  • product residue stream means the residue stream exiting a membrane array when the membrane fractionation process is complete. This stream may be derived from one membrane, or may be the pooled residue streams from several membranes.
  • product permeate stream means the permeate stream exiting a membrane array when the membrane fractionation process is complete. This stream may be derived from one membrane, or may be the pooled permeate streams from several membranes.
  • a feedstream containing an organic vapor is passed across a thin, permselective membrane.
  • the sources of the feedstreams to be fractionated are diverse. Many industrial processes produce waste gas streams containing low concentrations of organic vapors.
  • solvent-containing airstreams are produced as a result of solvent vaporization in the drying of synthetic fibers and films, plastics, printing inks, paints and lacquers, enamels and other organic coatings. Solvents are also used in the preparation of adhesive coatings and tapes. Waste gases containing organic vapors are generated by solvent degreasing operations in the metal and semiconductor industries. Hydrocarbon vapors are released from petroleum storage tanks during transfer operations.
  • Chlorinated fluorocarbons are emitted to the atmosphere in huge quantities from plants manufacturing polyurethane and other plastic foams.
  • Other sources of extensive CFC pollution are refrigeration operations, air conditioning and fire extinguisher filling and use.
  • the concentration of these streams varies widely, from a few ppm to as high as 40-50% organic.
  • Organic vapors that can be handled by the process include, but are not limited to, chlorofluorocarbons such as CFC-11 (CCl 3 F), CFC-12(CCl 2 F 2 ), CFC-113(C 2 Cl 3 F 3 ), CFC-114(C 2 Cl 2 F 4 ), CFC-115(C 2 ClF 5 ), HCFC-21 (CHCl 2 F), HCFC-22 (CHClF 2 ), HCFC-23(CHF 3 ), HCFC-123 (C 2 HCl 2 F 3 ), HCFC-142b(C 2 H 3 ClF 2 ), Halon 1211 (CF 2 ClBr), Halon-1301 (CF 3 Br ) and Halon-2402 (C 2 F 4 .Br 2 ); chlorin
  • organic-componentcontaining streams will comprise the organic material in air.
  • Mixtures of organic components in nitrogen are also commonly encountered, because nitrogen is frequently used as a blanketing gas.
  • Streams of organic compounds in other gases, orstreams comprising mixtures of organics are also found.
  • hydrogenation reactions in the chemical industry yield off-gas streams containing hydrogen and various hydrocarbons. Treatment ofsuch streams could be carried out using a membrane type preferentially permeable to the hydrocarbon component.
  • Mixed organic component streams might arise, for example, from natural gas processing or petrochemical refining, where the stream could contain a mixture of methane, ethane, propane, butane and so on.
  • the permselective membrane forms a barrier that is relatively permeable to an organic vapor component of the stream, but relatively impermeableto other gases in the stream.
  • the membrane may take the from of a homogeneous membrane, a membrane incorporating a gel or liquid layer, or dispersed particulates, or any other form known in the art.
  • Preferred embodiments of the invention employ a multilayer membrane comprising a microporous support onto which is coated an ultrathin permselective layer of a rubbery polymer.
  • the microporous support membrane should have a flow resistance that is very small compared to the permselective layer.
  • a preferred support membrane is an asymmetric Loeb-Sourirajan type membrane, which consists of a relatively open, porous substrate with a thin, dense, finely porous skin layer. Preferably the pores in the skin layer should be less than 1 micron in diameter, to enable it to be coated with a defect-free permselective layer.
  • the support membrane should resist the solvents used in applying the permselective layer.
  • Polyimide or polysulfone supports are preferred for solvent resistance.
  • Commercial ultrafiltration membranes for example, NTU 4220 (Crosslinked polyimide), or NTU 3050 (polysulfone) from Nitto Electric Industrial Company, Osaka, Japan, are suitable as supports.
  • Suitable support membranes may be made by the processes for making finely microporous or asymmetric membranes known in the art.
  • Polymers which may be used in addition to polysulfone or polyimide include polyvinylidene fluoride (for example, Kynar 461, Pennwalt Corp., Philadelphia, Pennsylvania), or aromatic polyamides (for example, Nomex 450, DuPont, Wilmington, Delaware).
  • Simple isotropic supports such as microporous polypropylene or polytetrafluoroethylene can also be used.
  • the thickness of the support membrane is not critical, since its permeability is high compared to that of the permselective layer. However, the thickness would normally be in therange 100 to 300 microns, with about 150 microns being the preferred value.
  • the support membrane may be reinforced by casting it on a fabric or paper web.
  • the multilayer membrane then comprises the web, the microporous membrane, and the ultrathin permselective membrane.
  • the web material may be made from polyester or the like. The permselective layer could not be cast directly on the fabric web, because it would penetrate the web material, rather than forming an unbroken surface coating.
  • J Dk ⁇ p , (1) l
  • J is the membrane flux (cm 3 (STP)/cm 2 ⁇ s ⁇ cmHg)
  • D is the diffusion coefficient of the gas or vapor in the membrane (cm 2 /sec) and is a measure of the gas mobility
  • l is the membrane thickness
  • k is the Henry's law sorption coefficient linking the concentration of the gas or vapor in the membrane material to the pressure in the adjacent gas (cm 3 (STP)/cm 3 ⁇ cmHg)
  • ⁇ p is the pressure difference across the membrane.
  • the product Dk can also be expressed as the permeability, P, a measure of the rate at which a particular gas or vapor moves through a membrane of standard thickness (1 cm) under a standard pressure difference (1 cmHg).
  • a measure of the ability of a membrane to separate two components, (1) and (2), or a feedstream is the ratio of their permeabilities, , called the membrane selectivity
  • membrane materials possess an intrinsically high selectivity for organic solvents over air and can therefore be used in a membrane separation process.
  • Preferred permselective membranes used in the invention therefore are rubbery non-crystalline polymers, that is, they have a glass transition temperaturebelow the normal operating temperature of the system.
  • Thermoplastic elastomers are also useful. These polymers combine hard and soft segments or domains in the polymer structure. Provided the soft segments are rubbery at the temperature and operating conditions of the invention, polymers of this type could make suitable membranes for use in the invention.
  • Polymers that may be used include, but are not limited to, nitrile rubber, neoprene, silicones rubbers, including polydimethylsiloxane, chlorosulfonated polyethylene, polysilicone-carbonate copolymers, fluoroelastomers, plasticized polyvinylchloride, polyurethane, cis-polybutadiene, cis-polyisoprene, poly(butene-1), polystyrene-butadiene copolymers, styrene/butadiene/styrene block copolymers, styrene/ethylene/butylene block copolymers, thermoplastic polyolefin elastomers, and block copolymers of polyethers and polyesters.
  • nitrile rubber neoprene
  • silicones rubbers including polydimethylsiloxane, chlorosulfonated polyethylene, polysilicone-carbonate copoly
  • the permselective layer should be made very thin.
  • the permselective layer must also be free of pinholes or other defects that could destroy the selectivity of the membrane by permitting bulk flow-through of gases.
  • Particularly preferred rubbers are styrene-butadiene copolymers or silicone rubbers.
  • the preferred membrane is one in which the permselective coating is deposited directly on the microporous support.
  • optional embodiments that include additional sealing or protective layers above or below the permselective layer are also intended to be encompassed by the invention.
  • the preferred method of depositing the permselective layer is by dip coating.
  • the polymer material that forms the permselective layer should be a film-forming material that is soluble in an organic solvent.
  • the dip coating method is described, for example, in U.S. Patent 4,243,701 to Riley et al., incorporated herein by reference.
  • a support membrane from a feed roll is passed through acoating station, then to a drying oven, and is then wound onto a product roll.
  • the coating station may be a tank containing a dilute polymer or prepolymer solution, in which a coating, typically 50 to 100 microns thick, is deposited onthe support. Assuming a 1% concentration of polymer in the solution, then after evaporation a film 0.5 to 1 micron thick is left on the support.
  • the permselective membrane may be cast by spreading a thin film of the polymer solution on the surface of a water bath. After evaporation of the solvent, the permselective layer may be picked up onto the microporous support. This method is more difficult in practice, but may be useful if the desired support is attacked by the solvent used to disolve the permselective material.
  • the thickness of the permselective layer should normally be in the range 0.1 to 20 microns, more preferably 0.1-5 microns.
  • the form in which the membranes are used in the invention is not critical. They may be used, for example, as flat sheets or discs, coated hollow fibers, or spiral-wound modules, all forms that are known in the art. Spiral-wound modules are a preferred choice. References that teach the preparation of spiral-wound modules are S.S. Kremen, "Technology and Engineering of ROGA Spiral Wound Reverse Osmosis Membrane Modules", in Reverse Osmosis and Synthetic Membranes, S.Sourirajan (Ed.), National Research Council of Canada, Ottawa, 1977; and U.S. Patent 4,553,983, column 10, lines 40-60. Alternatively the membranes may be configured as microporous hollow fibers coated with the permselective polymer material and then potted into a module .
  • the permselective membranes used in the present invention should preferably have a selectivity for the organic vapor of at least 5, more preferably at least 10, and most preferably at least 20.
  • a selectivity for the organic vapor of at least 5, more preferably at least 10, and most preferably at least 20.
  • extremely high selectivities are not necessary desirable or advantageous, as the examples and accompanying discussion show.
  • other factors determine the degree of enrichment of organic vapor obtianed in a membrane process. The first is the extent of removal of organic vapor from the feed. When a given volume of the feedstream enters the membrane, it immediately begins to lose organic vapor, as the organic vapor preferentially permeates the membrane. Thus, the concentration of organic vapor in the feedstream decreases as it passes through the membrane module.
  • the average concentration of the organic vapor on the feed side of the membrane will determine the average concentration of vapor on the permeate side of the membrane. If concentration of organic in the feed is reduced to a small value before it leaves the module, the average feedstream concentration will be low. as a result, the vapor enrichment in the permeate stream is low also. Thus, as organic removal from the feedstream is increased, the average concentration of organic vapor in the permeate decreases.
  • a second factor affecting the performance of a membrane system is the pressure of feed and permeate gas streams.
  • the driving force for permeation is the difference between the partial pressures of the components on the feed and permeate sides.
  • the ratio of the feed to the permeate pressures defined as total permeate pressure (p)
  • P 1 and P 2 are the permeabilities of components 1 and 2
  • l is the membrane thickness
  • p' 1 , p' 2 and p" 1 , P" 2 are the partial pressures of the two gases or vapors in the feed and permeate streams, respectively.
  • the total gas pressure is equal tothe sum of the partial pressures, i.e.,
  • volume fractions, C' 1 and C' 2 of the two components in the feed, and in the permeate, C" 1 and C" 2 are given by:
  • the pressure drop across the membrane can be achieved by pressurizing the feed, by evacuating the permeate or by both. Because the volume of the permeate stream is much less than the volume of the feed, it is energy and cost effective, and therefore preferable in the context of the invention, to operate in the vacuum mode, i.e., drawing a partial vacuum on the permeate side. At pressure ratios between 0.01 and 0.001, very large differences in performance with differing selectivity can be achieved. However, to achieve such pressure ratios involves drawing a hard vacuum on the permeate side.
  • the feedstream is only slightly pressurized to force the feed gas through the membrane module, preferably not more than to about 5 atm pressure, more preferably to not more than 2atm pressure, and most preferably to only a few psig over atmospheric pressure. Therefore, a pressure ratio of 0.1 is roughly equivalent to a permeate pressure of 8 cmHg, a pressure ratio of 0.01 to a permeate pressure of 0.8 cmHg, and a pressure ratio of 0.001 to a permeate pressure of 0.08 cmHg. Pressures less than 1 cmHg can be achieved in a laboratory-scale experiment, but will be difficult and very expensive to realize in a full-scale industrial process. Therefore, a value of 0.005 is probably the preferable lower limit for practical pressure ratios in an industrial setting.
  • Figure 1 shows that for pressure ratios ranging from 0.1 to 1 , the separation achieved is modest and is largely independent of the membrane selectivity, i.e., the separation is pressure ratio controlled.
  • the preferred operating zone for the processes of the invention therefore, is generally in the middle region of Figure 1, where a good separation can be achieved by combining a membrane with a good, but not excessively high, selectivity, typically in the range 5-200, with a pressure ratio in an economically sustainable range, such as 0.005 - 0.5. This limits the maximum enrichment of organic vapor obtained in a single-stage industrial system to this range.
  • the ratio of the permeate flow to the feed flow is called the stage cut.
  • the degree of depletion of the more permeable component from the feed depends on the stage-cut.
  • the feed gas becomes substantially depleted in the more permeable components.
  • the average concentration of the more permeable component seen by the membrane is substantially lower than the initial feed gas concentration.
  • the result is a fall in the concentration of the more permeable component in the permeate stream.
  • a stream contains 4% organic vapor and it is desired to reduce the concentration to 0.5%. If only the organic vapor permeated the membrane, then the permeate flow would be pure organic, and would be 3.5% of the total feed flow.
  • the minimum stage cut to achieve this degree of separation would be 3.5%.
  • the stage-cut will always be higher than this, because the other gases in the feed will also permeate the membrane to some extent.
  • the stage cut should be kept low, preferably below 40% and most preferably below 30%.
  • the process of the invention is carried out using system designs tailored to particular requirements in terms of percentage of organic vapor removed from the feed, or the degree of concentration of the permeate.
  • the membrane fractionation process should preferably remove 80% or more of the organic content of the feed. This level of removal may frequently be obtained with a single membrane step. Removals up to 95%, or even 99%, are also possible with a membrane fractionation process. To achieve a very high degree of organic removal may require a two-step or multistep design.
  • the residue stream from the fractionation process should be reduced in organic vapor content to the point where treatment by carbon adsorption or another treatment appropriate for low concentrations of organic contamination can be used in a cost-effective manner.
  • the organic concentration in the residue will generally be below about 0.5%, and most preferably below about 0.5%.
  • a particular advantage of membrane fractionation, compared with straight dilution of the feedstream with clean air or gas, is that the total volume of waste gas to be treated is not increased.
  • the permeate stream from the fractionation process should be increased in organic vapor content to the point where treatment by incineration or compression condensation becomes cost-effective.
  • the degree of concentration required to reach this point will vary from organic to organic. If incineration is to be used, then the most preferred concentration of organic in the permeate will be such that the stream can be burnt with a minimal addition of supplementary fuel. Condensation or compression condensation is preferable to incineration in the case of most organics, because air pollution is minimized and the organic material can be recovered.
  • the degree of concentration of the organic in the permeate required to make compression condensation efficient will depend in part on the boiling point of the organic. The concentration should be such that the permeate stream can be brought to the dewpoint of the organic without excessive compression or cooling.
  • the dewpoint can be reached at a pressure less than 5-10 atmospheres, more preferably less than 2 atmospheres, and a temperature above 0°C.
  • the dewpoint will be reached by compressing to 2 atmospheres. Therefore, compressing the permeate to 10 atmospheres and chilling will remove more than 80% of the organic vapor.
  • the permeate stream is 30% saturated at room temperature, the dewpoint will be reached by compressing to 3.3 atmospheres. Therefore, compressing the permeate to about 15 atmospheres and chilling will achieve 80+% removal. If the permeate stream is 20% saturated at room temperature, the dewpoint will be reached by compressing to 5 atmospheres.
  • the fractionation process should be designed to produce a permeate organic vapor concentration that is greater than the 20% saturation concentration at 1 atmosphere pressure and 20oC. More preferably, the permeate organic vapor concentration should be greater than 30% saturation concentration at 1 atmosphere pressure and 20oC, and most preferably, greater than 50% saturation concentration.
  • fractionation process tailored to meet the above requirements will depend on the selectivity, operating pressure and stage cut, as discussed above.
  • a basic fractionation process is shown schematically in
  • the system comprises a feed air compressor, 1, membrane unit containing one or more membrane modules, 2, and permeate vacuum pump, 3.
  • the feedstream, 4 is compressed and passed through the membrane module.
  • the treated residue stream, 5, contains a small percentage of vapor, in a concentration appropriate for treatment by carbon adsorption, for recycling to the process that generated it, or for discharge to the atmosphere.
  • the permeate stream, 6, is enriched in the organic vapor so that it contains the vapor in a concentration appropriate for incineration or compression condensation.
  • a single-stage vapor separation system such as this is generally able to remove 80-90% of the organic vapor from the feed gas to produce a permeate that has five to en times the concentration of the feed gas.
  • the permeate stream from the first membrane unit, 16, is compressed in compressor, 17, and forms the feed to the second stage membrane unit, 18.
  • the pressure drop across this unit is provided by vacuum pump, 19, and the permeate stream, 20, can be treated by compression condensation or incineration.
  • the residue stream, 21, from the second membrane unit may optionally be combined with the feedstream, 11. This configuration allows organic enrichments typically up 100-fold to be achieved. Because the feedstream to the second stage is very much smaller than the feed to the first, the second stage is normally only 10-20% as large as the first stage.
  • the feedstream, 26, passes through a compressor, 22, and thence to a first stage membrane unit, 23.
  • the treated residue stream, 27, is sent to carbon adsorption unit, 38.
  • a vacuum pump, 24, is used on the permeate side of the membrane unit, and the permeating vapor stream, 28, is condensed to a liquid organic stream, 29, by condenser, 25.
  • the non-condensed vapor and gas stream, 30, becomes the feed for a second membrane unit, 31.
  • the permeate from this unit, 33 passes to vacuum pump, 32, and condenser, 34, to form a liquid organic stream, 35, and a non-condensed stream, 36, that can be recycled to the feed side of the membrane.
  • the residue stream, 37, from the second membrane unit is combined with the feedstream, 22.
  • Such a two-stage process could also be run without the condensation step between the two membrane stages, if the organic concentration in the permeate after the first stage was still relatively low. Recycling both the non-condensed fraction of the second permeate, and the residue from the second membrane stage, within the membrane system is also optional. However, recycle is preferred where the composition of these streams is still within the range that is best treatable by membrane fractionation.
  • a third system design may be preferred when a high degree of organic vapor removal from the feed is necessary.
  • a two-step process in which the residue from the first step is subjected to further treatment, can then be used. Organic removals of 95-99% can be achieved.
  • the second step required to reduce the feed concentration from 10% to 1% of the initial value is approximately as large as the first step required to reduce the feed concentration from 100% to 10% of the initial value.
  • Figure 5 shows such a system. Referring now to this figure, the feedstream, 41, passes through a compressor, 39, and thence to a first step membrane unit, 40.
  • the residue stream, 42 is fed to the second step membrane unit 43.
  • the treated residue stream, 44 is passed to further treatment, vented or recycled as above.
  • Vacuum pump, 45 is used on the permeate side of the first step membrane unit, and the permeating vapor stream 46, is sufficiently enriched in organic vapor content for treatment by compression condensation or incineration.
  • the permeate stream, 47, from the second step membrane unit may optionally be recycled via vacuum pump, 48, to be combined with the incoming feedstream.
  • the membrane system for carrying out the process should be configured based on the composition of the feedstream and the desired compositions of the product residue and product permeate streams.
  • the performance that can be obtained from the system will depend on the membrane selectivity and thickness, and the operating parameters, such as feed and permeate pressures and stage cut.
  • To achieve the desired performance may require an array of membrane units in a cascade arrangement such that the permeate from one unit becomes the feed for the next.
  • a cascade array may contain two, three or more sets of membrane units or stages.
  • the residue stream from the individual stages may conveniently, although not necessarily, be recycled within the array and mixed with a feed of similar organic concentration.
  • Such a cascade arrangement may be appropriate, for example, where a relatively dilute feedstream is to be treated to yield a product permeate stream with a high degree of enrichment of the organic component.
  • the array may be a series arrangement such that the residue from one unit becomes the feed for the next.
  • a series array may contain two, three or more sets of membrane units or steps. The permeate streams from the individual steps may conveniently, although not necessarily, be recycled within the array and mixed with a feed of similar organic concentration.
  • the membrane system for carrying out the process may contain both multiple steps and/or multiple stages, arranged in any combination.
  • the factors influencing the system design will be the capital and operting costs of the system, the energy requirements, the feed composition and the desired product compositions.
  • a particularly preferred membrane configuration is a two-stage unit, a representative example of which is shown in Figure 4.
  • two-stage systems are viewed as disadvantageous, and in fact tend to be disadvantageous, because costs are in direct proportion to the number of srtages required.
  • costs scale in proportion to the volume of feed to be handled.
  • a single stage membrane system selectively permeable to the organic component, can produce a permeate five-fold enriched in the organic component compared with the feed. The bulk of the feed will pass through the membrane module and membrane module and exit on the residue.
  • the volume of the permeate will be significantly less than that of the feed, and may be as low as 20% or less of the feed volume. It is this permeate stream that forms the feed to the second membrane stage. Therefore the membrane area required to process this stream may be 20% or less of that required to handle the original feed. If the membrane type is the same as that in the first stage, a similar five-fold enrichment could be obtained. The membrane system as a whole, then could produce an overall 25-fold enrichment of the feed, using a system only 20% larger than that needed to obtain a five-fold enrichment. In terms of capital costs and performance, therefore, two-stage membrane systems are highly efficient and economical.
  • a stream that was 20% saturated would be brought to saturation if the organic concentration were increased five-fold; a stream that was originally 10% saturated would be brought to saturation by 10-fold enrichment, and so on.
  • a stream that is saturated at ambient conditions can usually be condensed to recover the bulk of the organic without having to resort to excessive pressure/temperature conditions. Therefore membrane fractionation processes that can achieve five-fold or better organic enrichment and most preferably 10-fold or better organic enrichment, are preferred.
  • the examples are in two groups.
  • the first group covers the results obtained in a series of experiments carried out according to the general procedure described below. These experiments were performed to determine that separation of organic vapors from gas streams, with adequate selectivity, can be achieved.
  • the experiments were performed with a single membrane module, usually operated at low stage cut, to optimize the concentration of organic vapor in the permeate stream. There was no attempt made in these simple experiments to control the concentration of organic in the residue stream.
  • the second set of examples takes representative separations and illustrates how membrane fractionation systems can be designed to achieve both permeate and residue streams with organic vapor concentrations in the desired ranges.
  • a small bypass stream was used to take the samples at atmospheric pressure instead of the elevated pressure in the lines.
  • Two liquid nitrogen traps were used to condense the organic contained in the permeate stream.
  • a non-lubricated rotary-vane vacuum pump was used on the permeate side of the module.
  • the permeate pressure used in the experiments was about 1-5 cmHg.
  • the samples from the permeate stream were taken using a detachable glass vessel constantly purged with a bypass stream of the permeate. Upon sampling, the vessel was determined by gas chromatography. The permeate concentration was then calculated from the relationship:
  • the nitrogen permeate flow rate was determined by measuring the vacuum pump exhaust flow rate. This provided a quality check on the module.
  • Step 6 was repeated after 10-20 minutes. The feed concentration was monitored after each parameter change to ensure steady state had been reached.
  • the experimental procedures described above were carried out using a membrane module containing a composite membrane with an area of 1,100 cm 2 .
  • the feedstream comprised nitrogen and acetone, the acetone concentration in the feed varying from about 0.4% to 2%.
  • a plot of acetone concentration in the feed agaisnt acetone concentration in the permeate is given by the lowest curve in Figure 6.
  • the permeate was enriched about 18-fold compared with the feed.
  • a feedstream containing 0.45% acetone yielded a permeate containing 8% acetone.
  • the selectivity for acetone over nitrogen was found to be in the range 15-25, depending on the feed concentration of acetone and other operating parameters.
  • the experimental procedures described above were carried out using a membrane module containing a composite membrane with an area of 1,100 cm 2 .
  • the feedstream comprises nitrogen and 1,1,1-trichloroethane, the trichloroethane concentration in the feed varying from about 0.5% to 1.5%.
  • a plot of trichloroethane concentration in the feed against trichloroethane concentration in the permeate is given by the second lowest curve in Figure 6.
  • the permeate was enriched about 24-fold compared with the feed.
  • a feedstream containing 0.5% trichloroethane yielded a permeate containing 13% trichloroethane.
  • EXAMPLE 3 EXAMPLE 3 .
  • the feedstream comprised nitrogen and toluene, the toluene concentration in the feed varying from about 0.2% to 1%.
  • a plot of toluene concentration in the feed against toluene concentration in the permeate is given by the third curve in Figure 6. Typically the permeate was enriched about 48-fold compared with the feed.
  • the experimental procedures described above were carried out using a membrane module containing a composite membrane with an area of 1,100 cm 2 .
  • the feedstream comprised nitrogen and octane, the octane concentration in the feed varying from about 0.1% to 0.6%.
  • a plot of octane concentration in the feed against octane concentration in the permeate is given by the uppermost curve in Figure 6.
  • the permeate was enriched at least 50-60 fold compared with the feed.
  • a feedstream containing 0.3% octane yielded a permeate containing 14% octane.
  • the experimental procedures described above were carried out using two different membrane modules containing composite membranes with different rubbers as the permselective layer, but both with membrane areas of 3,200 cm 2 .
  • the feedstream comprised nitrogen and perchloroethylene, the perchloroethylene concentration in the feed varying from about 0.2% to 0.8%.
  • a plot of perchloroethylene concentration in the feed against perchloroethylene concentration in the permeate is given in
  • FIG. 7 The open circles are for one module; the triangles for the other.
  • the permeate was enriched at least 10-12 fold compared with the feed.
  • a feedstream containing 0.2% perchloroethylene yielded a permeate containing 2.2% perchloroethylene.
  • a feedstream containing 0.5% perchloroethylene yielded a permeate containing 8.3% perchloroethylene.
  • the experimental procedures described above were carried out using a feedstream containing CFC-11 (CCl 3 F) in nitrogen in concentrations from 100-2,000 ppm.
  • the module contained a composite;
  • EXAMPLE 7 The experimental procedures described were carried out using a feedstream containing CFC-11 (CCl 3 F) in nitrogen in concentrations from 1-35%.
  • the module contained a composite membrane with an area of approximately 2,000 cm 2 . The results are summarized in Figure 9. The calculated
  • CFC/N selectivity of the module increased from 30 at 1 vol% to 50 at 35 vol%. This effect may be attributable to plasticization of the membrane material by sorbed hydrocarbon. Both hydrocarbon and nitrogen fluxes increased with increasing hydrocarbon feed concentration.
  • the selectivity for CFC-11 over nitrogen was found to be in the range 30-50.
  • the experimental procedures described were carried out using a feedstream containing CFC-113 (C 2 Cl 3 F 3 ) in nitrogen in concentrations from 0.5-6%.
  • the module contained a composite membrane with an area of approximately 2,000 cm 2 .
  • the experimental procedures described above were carried out using a membrane module containing a composite membrane with an area of 2,0,0 cm 2 .
  • the feedstream comprised nitrogen and methylene chloride, the methylene chloride concentration in the feed varying from about 0.1% to 8%.
  • a plot of methylene chloride concentration in the feed against methylene chloride concentration in the permeate is given in Figure 13.
  • the permeate was enriched about 30-fold compared with the feed at low feed concentrations. At high concentrations the degree of enrichment dropped to about 10-20 fold.
  • a feedstream containing 2% methylene chloride yielded a permeate containing 44% methylene chloride.
  • a feedstream containing 8% methylene chloride yielded a permeate containing 84% methylene chloride.
  • the residue stream contains 250 ppm CFC-113 at 995 scfm and the permeate stream produced by this first stage contains 2.3%
  • This permeate stream is recompressed and passed to the second membrane stage, having an area of 200 m 2 , where the CFC content is reduced to 0.5%.
  • the residue stream from the second stage is recirculated to the inlet of the first membrane stage.
  • the permeate stream produced by the second stage contains 11.2% CFC-113 and is suitable for treatment by condensation.
  • the stream could be compressed to 100 psig and chilled to 5°c to recover the bulk of the CFC-113.
  • the condenser bleed stream could be returned to the inlet of the second membrane stage.
  • the function of the second stage is thus to further concentrate the CFC-113 to make condensation feasible.
  • the second stage is one-fifth of the size of the first stage. Table 2 summarizes the performance of such a system, including the compression condensation operation.
  • Compressors 155 hp The process as configured yields only two streams: a residue stream containing 250 ppm CFC and a clean liquid CFC permeate stream.
  • CFC-113 currently costs about $1-1 .50/lb , and other CFCs can cost up to $7/lb, so membrane fractionation leading to recovery of the CFC is extremely attractive.
  • a second system designed to treat the same 1,000 scfm stream of 0.5% CFC-113, was evaluated.
  • the system design used for the calculations was a two-step, two-stage system, combining system designs of the type shown in Figures 3 and 5, so that both the permeate and residue streams from the first membrane unit are passed to second membrane units.
  • the two-step first stage similar to that in Figure 5, uses two membrane units with membrane areas of 850 m 2 and 820 m2 respectively, producing two permeate streams. Pressure drops across the membranes are provided by vacuum pumps on the permeate side.
  • the permeate stream from the first step contains 3.1% CFC-113 and becomes the feedstream for the second stage.
  • the permeate stream from the second step contains 0.4% CFC-111 and is recompressed and recirculated to the feed of the first stage.
  • the residue from the second step contains 50 ppm CFC-113 at 995 scfm.
  • the second stage has a membrane area of 200 m 2 , and produces a residue stream containing 0.5% CFC-113, which can be recycled to the feed of the first stage.
  • the permeate from the second stage contains 11.2% CFC-113, and is suitable for treatment by compression condensation. For example, the stream can be compressed and condensed to yield liquid CFC-113 at a rate of 140 lb/h.
  • a two-step, two-stage configuration is more effective than a simple two-stage configuration in applications where high solvent recoveries are required. Table 3 summarizes the system performance. Table 3 .
  • the process as configures yields only two streams: a residue stream containing 50 ppm CFC and a clean liquid CFC permeate stream.
  • the estimated capital cost of the system was $956,000 or $956/scfm feed. Operating cost was estimated to be $446,000 per year or 40 cents/lb CFC-113 recovered.
  • the residue stream from the second stage having an organic concentration of 2% and a flow of 159 scfm, is recirculated to the inlet of the first membrane stage.
  • the permeate stream produced by the second stage contains 46.6% 1 ,1,1-trichloroethane, and has a voluem of 38 scfm.
  • Such a stream could be treated by compressing and cooling to about 5°c to recover pure 1,1,1-trichloroethane in liquid form. Any non-condensed fraction could be returned to the membrane unit.
  • Table 4 summarizes the performance of the system.
  • Compressors 85hp Capital cost was estimated to be roughly $900,000, and operating costs would be roughly 381,000/year.
  • the permeate from the first step contains 50.6% methylene chloride, has a flow rate of 194 scfm, and could be treated by condensation.
  • the permeate from the second step contains 9.8% methylene chloride, has a flow rate of 93 scmf, and could be returned to the inlet to the first membrane step.
  • Table 5 summarizes the performance of the system. Table 5
  • feed flow is 1,000 scfm input
  • Table 6 summarizes the operating conditions assumed for the carbon adsorption process in this comparison. Table 6. Operating Conditions for Carbon Adsorption
  • FIG. 14 shows the operating costs a function of the inlet concentration at a feed flow rate of 1,000 scfm.
  • Membrane separation systems have lower operating costs than carbon adsorption systems if the inlet concentration is higher than 1.2%.
  • Figure 15 shows the operating costs for a system treating 200 scfm of air.
  • the membrane system has lower operating costs than carbon adsorption if the halocarbon inlet concentration is more than 0.7%.
  • the operating costs used in producing Figures 14 and 15 include depreciation and interest on the invested capital. Thus, the relatively high capital cost of membrane sysems has been taken into account. Any decrease in carbon adsorption system life when operated with halocarbons is not included in the operating cost data.

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Abstract

Procédé de séparation destiné au traitement d'un flux gazeux contenant une vapeur organique en une concentration telle qu'il est difficile pour des raisons techniques et financières de traiter ledit flux de gaz selon des procédés normalisés d'élimination des déchets. Cette concentration en vapeur organique est, de façon typique, d'environ 0,1 à 10 %. Le procédé selon l'invention consiste à faire passer le flux gazeux à travers un système à membrane comprenant une ou plusieurs membranes, laissant passer de façon sélective la vapeur organique contenue dans le flux gazeux. Deux flux résultent de la séparation effectuée: un flux résiduel contenant la vapeur organique en une concentration inférieure à environ 0,5 % et un flux de perméat hautement enrichi en vapeur organique. Le flux résiduel et le flux de perméat conviennent ensuite au traitement de séparation ou aux techniques d'élimination des déchets classiques. On peut faire passer le flux résiduel à faible concentration en vapeur organique dans des lits d'adsorption carbonés, par exemple, et le flux de perméat à haute concentration en vapeur organique peut être soumis à la condensation ou l'incinération.
PCT/JP1991/000608 1991-05-07 1991-05-07 Procede de separation utilisant une membrane WO1992019358A1 (fr)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0785818A4 (fr) * 1994-10-11 1998-01-07 Enerfex Inc Procede a plusieurs etapes de separation de gaz par membranes semi-permeables
US9358498B2 (en) 2011-10-19 2016-06-07 Fuji Electric Co., Ltd. Mixed air removal device and power generator including the same

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN120731122A (zh) * 2023-03-16 2025-09-30 日东电工株式会社 气体分离系统及混合气体的分离方法

Citations (3)

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Publication number Priority date Publication date Assignee Title
US4553983A (en) * 1984-07-31 1985-11-19 Membrane Technology And Research, Inc. Process for recovering organic vapors from air
EP0389126A2 (fr) * 1989-03-23 1990-09-26 Membrane Technology And Research, Inc. Procédé à membrane pour le traitement de courants de gaz chargés d'hydrocarbures fluorées
US5032148A (en) * 1989-11-07 1991-07-16 Membrane Technology & Research, Inc. Membrane fractionation process

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4553983A (en) * 1984-07-31 1985-11-19 Membrane Technology And Research, Inc. Process for recovering organic vapors from air
EP0389126A2 (fr) * 1989-03-23 1990-09-26 Membrane Technology And Research, Inc. Procédé à membrane pour le traitement de courants de gaz chargés d'hydrocarbures fluorées
US5032148A (en) * 1989-11-07 1991-07-16 Membrane Technology & Research, Inc. Membrane fractionation process

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0785818A4 (fr) * 1994-10-11 1998-01-07 Enerfex Inc Procede a plusieurs etapes de separation de gaz par membranes semi-permeables
US9358498B2 (en) 2011-10-19 2016-06-07 Fuji Electric Co., Ltd. Mixed air removal device and power generator including the same

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