CA2045905A1 - Process for recovering organic vapors from air - Google Patents
Process for recovering organic vapors from airInfo
- Publication number
- CA2045905A1 CA2045905A1 CA002045905A CA2045905A CA2045905A1 CA 2045905 A1 CA2045905 A1 CA 2045905A1 CA 002045905 A CA002045905 A CA 002045905A CA 2045905 A CA2045905 A CA 2045905A CA 2045905 A1 CA2045905 A1 CA 2045905A1
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- Prior art keywords
- organic vapor
- carbon adsorption
- semipermeable membrane
- air
- organic
- Prior art date
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Abstract
TITLE
IMPROVED PROCESS FOR RECOVERING
ORGANIC VAPORS FROM AIR
ABSTRACT OF THE DISCLOSURE
A process for separating and recovering organic vapors (e.g., chlorofluorocarbons) from a feed stream of organic vapors and air wherein organic vapors are present in the "window concentration" range being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery. By passing the organic vapor and air feed stream through the feed side of a semipermeable membrane unit, a vapor depleted stream exiting the membrane unit can then be processed by conventional carbon adsorption while a vapor enriched stream from the membrane unit can be processed by conventional compression and condensation.
IMPROVED PROCESS FOR RECOVERING
ORGANIC VAPORS FROM AIR
ABSTRACT OF THE DISCLOSURE
A process for separating and recovering organic vapors (e.g., chlorofluorocarbons) from a feed stream of organic vapors and air wherein organic vapors are present in the "window concentration" range being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery. By passing the organic vapor and air feed stream through the feed side of a semipermeable membrane unit, a vapor depleted stream exiting the membrane unit can then be processed by conventional carbon adsorption while a vapor enriched stream from the membrane unit can be processed by conventional compression and condensation.
Description
f TITLE
IMPROVED PROCESS FOR RECOVE~ING
ORGANIC VAPORS FROM AIR
BACKGROUND OF THE INVENTION
1~ Field of the Invention The present invention relates to an improved process for recovering organic vapors from an organic vapor/air mixture wherein organic vapors are present in the so-called ~window concentratiQn~ range, being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery. Msre specifically, the present invention relates to separating an organic vapor and air feed stream, by use of a semipermeable membrane unit, into a vapor depleted stream that is then process by carbon adsorption and a vapor ~nriched strQam that is then processed by compression and condensation.
IMPROVED PROCESS FOR RECOVE~ING
ORGANIC VAPORS FROM AIR
BACKGROUND OF THE INVENTION
1~ Field of the Invention The present invention relates to an improved process for recovering organic vapors from an organic vapor/air mixture wherein organic vapors are present in the so-called ~window concentratiQn~ range, being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery. Msre specifically, the present invention relates to separating an organic vapor and air feed stream, by use of a semipermeable membrane unit, into a vapor depleted stream that is then process by carbon adsorption and a vapor ~nriched strQam that is then processed by compression and condensation.
2. Description of Related Art Including Information Disclosed under 1.97-1.99 It is well known and a common commercial practice to employ organic solvents such as hydrocar- ::
bons, halogenated hydrocarbons, oxygenated hydrocarbons and the like in chemical processes, in the production of articles of manufacture, in the cleaning industry as well as in many other application~ wherein an organic vapor/air mixture is produced as a by-product or waste stream. Historically such spent organic vapor/air 30 streams would either be vented or flared to the atmosphere. However, contemporary emission standards, ~timulated by such considerations as the ngrePnhous~"
effect, potential depletion of ætratospheric ozone as well as general health considerations and conservation 35 of resource~ and basic economic considerations CH-1595-~ associated with wasting c05tly raw material, manda~e --1~
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that the organic solvent from the spent vapor/air mixtures be reclaimed.
Generally, the particular solvent and the concentration of its vapor in the air is dependent upon the particular process or application generating the spent stream. When the organic solYent i5 a high boiling liquid, its recovery can usually be carried out effectively by cooling to condense it to a liquid.
However, when the organic solvent is a low boiling liquid there are two traditional methods for its recovery. Thus it is generally known that at high loading of organic vapor in air the organic solvent can be economically reclaimed by compression and condensation and at low loading of organic vapor in the air the organic solvent can be economically reclaimed by carbon adsorption.
The difficulty with the traditional methods is that there is a so-called "window concentration"
range (typically from about 6 volume percent to 30 volume percent organic vapor) wherein neither method is practical. At too low of a concentration compression and condensation is inefficient in that greatly increased cost of compression together with r~duced amount o~ solvent recovery and increased emission o~
uncondensed vapor make the process prohibitively expensive. On the other hand, at too high of a concen-tration the exotherm associated with carbon adsorption raises the temperature of the adsorbent bed thus reducing the adsorption ~f~iciency and in some cases leading to the possibility of spontaneous combustion~
Furthermore, high vapor concentration requires more frequent regeneration of the carbQn adsorption tower thus again addiny to the cost. At present, if a low boiling organic vapor and air mixture in the ~window concentration" range is produced, one must further dilute the stream and then recover the organic phase by adsorption.
In U.S. Patent 4,553,983 a process for recovery of organic vapor from a feed stream of air having an organic vapor content of no more than 20,000 ppm by volume (2 volume percent) is disclosed wherein a thin sPmipermeable membrane with a permeation selectively of at least 50 favoring the organic vapor and a permeability of at least 3 x 10-7 cm3(STP) cm/cm2-sec-cmHg is used in combination with a partial vacuum on the permeate side to enrich the permeate with oxganic vapors. Similarly, in an article by D. L. Roberts et al. entitled nRecovery of Freon Gases with Silicone Rubber Membrane", Inq. Eng. Chem.
Process Desi~n. Dev., 1986, 25, pp 971-973, the recovery of fluorocarbons and chlorofluorocarbons from air mixtures using a semipermeable membrane is disclosed. In U.S. Patents 4,316,364 and 4,417,451 a refrigerant monitor system employing a membrane with a permeation selectivity favoring air relative to the refrigerant is disclosed.
SUMMARY OF T~E INVENTION
ThP present invention provides an improved process for recovering organic vapors from organic vapor/air mixtures, particularly mixtures wherein the concentration of the organic vapor to be recovered is in the so-called "window concentration~ range (i.e., too high for efficient recovery by the carbon adsorption method and too low for fficient recovery by the compression/condensation method). ThP improved method according to the present invention involves the imultaneous use of a membrane separation ~ystem in combination with a conventional carbon a~sorption system and a conventional compression/condensation system, thus producing a hybrid recovery unit.
bons, halogenated hydrocarbons, oxygenated hydrocarbons and the like in chemical processes, in the production of articles of manufacture, in the cleaning industry as well as in many other application~ wherein an organic vapor/air mixture is produced as a by-product or waste stream. Historically such spent organic vapor/air 30 streams would either be vented or flared to the atmosphere. However, contemporary emission standards, ~timulated by such considerations as the ngrePnhous~"
effect, potential depletion of ætratospheric ozone as well as general health considerations and conservation 35 of resource~ and basic economic considerations CH-1595-~ associated with wasting c05tly raw material, manda~e --1~
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that the organic solvent from the spent vapor/air mixtures be reclaimed.
Generally, the particular solvent and the concentration of its vapor in the air is dependent upon the particular process or application generating the spent stream. When the organic solYent i5 a high boiling liquid, its recovery can usually be carried out effectively by cooling to condense it to a liquid.
However, when the organic solvent is a low boiling liquid there are two traditional methods for its recovery. Thus it is generally known that at high loading of organic vapor in air the organic solvent can be economically reclaimed by compression and condensation and at low loading of organic vapor in the air the organic solvent can be economically reclaimed by carbon adsorption.
The difficulty with the traditional methods is that there is a so-called "window concentration"
range (typically from about 6 volume percent to 30 volume percent organic vapor) wherein neither method is practical. At too low of a concentration compression and condensation is inefficient in that greatly increased cost of compression together with r~duced amount o~ solvent recovery and increased emission o~
uncondensed vapor make the process prohibitively expensive. On the other hand, at too high of a concen-tration the exotherm associated with carbon adsorption raises the temperature of the adsorbent bed thus reducing the adsorption ~f~iciency and in some cases leading to the possibility of spontaneous combustion~
Furthermore, high vapor concentration requires more frequent regeneration of the carbQn adsorption tower thus again addiny to the cost. At present, if a low boiling organic vapor and air mixture in the ~window concentration" range is produced, one must further dilute the stream and then recover the organic phase by adsorption.
In U.S. Patent 4,553,983 a process for recovery of organic vapor from a feed stream of air having an organic vapor content of no more than 20,000 ppm by volume (2 volume percent) is disclosed wherein a thin sPmipermeable membrane with a permeation selectively of at least 50 favoring the organic vapor and a permeability of at least 3 x 10-7 cm3(STP) cm/cm2-sec-cmHg is used in combination with a partial vacuum on the permeate side to enrich the permeate with oxganic vapors. Similarly, in an article by D. L. Roberts et al. entitled nRecovery of Freon Gases with Silicone Rubber Membrane", Inq. Eng. Chem.
Process Desi~n. Dev., 1986, 25, pp 971-973, the recovery of fluorocarbons and chlorofluorocarbons from air mixtures using a semipermeable membrane is disclosed. In U.S. Patents 4,316,364 and 4,417,451 a refrigerant monitor system employing a membrane with a permeation selectivity favoring air relative to the refrigerant is disclosed.
SUMMARY OF T~E INVENTION
ThP present invention provides an improved process for recovering organic vapors from organic vapor/air mixtures, particularly mixtures wherein the concentration of the organic vapor to be recovered is in the so-called "window concentration~ range (i.e., too high for efficient recovery by the carbon adsorption method and too low for fficient recovery by the compression/condensation method). ThP improved method according to the present invention involves the imultaneous use of a membrane separation ~ystem in combination with a conventional carbon a~sorption system and a conventional compression/condensation system, thus producing a hybrid recovery unit.
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According to one embodiment of th~ present invention, a two-stage membrane separation system is used to produce a feed side effluent at the first stage that is sufficiently depleted of organic vapors that it can be treated by conventional carbon adsorption~ The permeate from the first stage is then introduced to the feed side of the second stage of the membrane separation system to produce further organic vapor enriching of the second stage permeate that is then treated by conventional compression and condensation.
The feed side effluent of the second stage is recycled to the feed side of the first stage. Such a process is particularly useful in recovering chlorofluorocarbon, hydrochlorofluorocarbon, and hydrofluorocarbon vapors emitted to the air during the manufacture of various foamed plastic products and the like. In another embodiment of the present invention, a feed side effluent at the first stage of a two~stage membrane separation system is organic vapor enriched and sent to the compression/condensation separator while the permeate from the second stage is organic vapor depleted and, therefore, processed by carbon adsorption. FurthPr according to the present invention, a regeneration/recycle procedure employing multiple carbon adsorption bed and the return of effluent from an adsorption bed being regenerated with an in situ stream is provided to further reduce emission of the halocarbon.
Thus, the present invention provides a process for separating and recovering organic vapors from a ~eed str am of organic vapor and air wherein or~anic vapors are present in the ~window concentration~ range, being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery, comprising the steps of:
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(a) providing a semipermeable membrane means for separating organic vapors from air having a feed side and a permeate side wherein said semipermeable membrane means is characterized as having a selectivity for allowing the passage of organic vapor relativs to air or for allowing the passage of air relative to organic vapor of at least 10 and permeability for the permeate gas of greater than 1 x 10-7 cm3(STP) cm cm~2 cmHg~1 sec~1;
(b) passing a feed stream of oryanic vapor and air, wherein said organic vapor is present in the ~window concentrationn range, across the feed side of the semipermeable membrane such that organic vapor or air, but not both, : passes preferentially through the membrane to form an organic vapor depleted ~ir stream characterized by an organic vapor concentration below the nwindow concentration'J range and an organic vapor enriched stream characterized by an organic vapor concentration above the ~window concentrationN range;
(c) subj~cting said or~anic vapor depleted air stream produced in step (b) to carbon adsorption, thus separating and recovering organic vapor therefrom; and (d~ subjecting ~aid enrichPd stream produced in step (b) to compression and condensation, thus separating and recovering organic vapor therefrom.
It is an object of the preæent invention to provide an improved proc~s for the efficient and economic recovery of low boiling or~anic solvent vapors - 35 from an organic vapor and air mixturs, wherein the organic vapor to air mixture is at a concentration such , .. . . .
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that the organic vapor concentration i6 too low for efficient recovery by compression/condensation methods and too high for efficient recovery by carbon adsorption methods. It is an additional object of the present invention to provide an efficient method of recovering low boiling organic solvent vapors from the above-described vapor/air mixtures by use of a semiper-meable membrane separation stage that works in combina-tion, simultaneously, with a conventional carbon adsorption system and a conventional compression/con-densation system. It is still a further ob~ect of the present invention to provide a hybrid membrane-carbon-adsorption-compreRsion/condensation vapor recovery system that will recover chlorofluorocarbons from a chlorofluorocarbon and air mixture particularly those associated with the commercial use of blowing and foaming agents in the manufacture of foamed plastics, fibers, films and the like. Fulfillment of these objects and the presence and fulfillment of other objects will be apparent upon complete reading of th~
specification and claims taken in conjunction with the attached drawing.
BRIEF DESCRIPTION OF THE DRAWING
'rhe Figure represents a schematic illustra-tion of a typical improved process ~or recovery of organic vapors according to one embodiment o~ the present invention.
~ESCRIPTION OF THE PREFERRED EMBODIMENTS
The improved process for recovering organic vapors from an organic vapor/air mixture according to the present invention, how it differs from yet is related to previously known methods of recov~ring organic vapors, what advantages there are associated with the use of the improved method relative to these ,, . . : , -, , ' prior art methods, and under what circumstances the present invention provides these advantages can be best explained and understood by reference to the drawing.
The schematic flow diagram of the Figure illustrates the basic components and stages of one particular embodiment of the present invention. As illustrated in the Figure, a feed stream (i.e., a halocarbon/air mixture in the ~window concentrationn range) is continuously introduced into the first stage 10 of the two-stage semipermeable membrane units via line 12. This gaseous feed stream entering the feed side of first stage membrane separator 10 is continuously commingled with the recycle gas~ous effluent from the feed side of the second membrane separator 14, via line 16, prior to the combined mixture being sent to membrane separator 10. Within separator 10 (in this particular illustrated embodiment) an organic vapor enriched permeate is continuously withdrawn via line 18 by virtue of vacuum blower 20 while an organic vapor depleted stream is withdrawn from the feed side via line 22 and sent to (again in this particular illustrated embodiment) one of two essentially equivalent carbon adsorber units, 24, for recovery of the organic vapors. The permeate from the first stage membrane separator 10 i5 then delivered to the feed side of the second stage separator 14 via line 26. Within separator 14 further organic vapor enriching of the permeate occurs with the permeate being withdrawn via line 28 by virtue of vacuum blower/compressor 30 and sent, via line 32, ~o condenser/separator 34. The effluent from the feed side of membrane separator 14, having an organic vapor concentration substantially the same as the original feed, is continuously returned and commingled with the feed as previously mentioned.
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During operation of the embodiment of the Figure, liquid halocarbon is continuously removed from the bottom of the condenser/separator 34 while halocarbon is simultaneously being deposited on the carbon bed within adsorber 24. Halocarbon depleted air may be vented from both the adsorber 24 and condenser/separator 34. Because of the finite capacity of the carbon adsorption bed, an additional adsorption unit 36 is provided for continuous operation. While bed 24 is actively adsorbing halocarbon the other adsorption unit 36 can be regenerating~ The Figure illustrates regeneration in that a portion of the air effluent from adsorber 24 is passed through heater 38 and introduced into adsorber 36 via line 40. The halocarbon thus removed from the regenerating adsor~er 36 is then sent back to an appropriate location in the overall process for ~urther isolation and recovery of halocarbon. As illustrated in the Figure the recycle of halocarbon enriched regeneration gas is being directed via line 42to the feed stream entering the first membrane separation unit lO. It should be appreciated that depending on the level or concentration of halocarbon in this recycle loop, the regeneration stream can be reintroduced advantageously to other locations in the overall process, including the permeate side of the first membrane unit 10 or even directly to the compression/conden~ation unit if the halocarbon concentration is sufficiently high. It should also be appreciated that various additional valves, conduits, and heat exchangers and the like (not shown in the Figure) can be incorporated into the process, as generally known in the art, to accomplish the switching of adsorber units and regeneration with recycle features of the present invention.
3~ It should be appreciated that the particular two stage embodiment illustrated in the Figure is most --8--.
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' , ' useful when the feed stream is relatively dilute with respect to the so-called ~window concentration" range or, stated in other terms, when the feed stream is su~ficiently dilute to be further depleted of organic vapors in a single membrane stage such as to be amenable to carbon adsorption. Thus, in principle, the process according to the present invention is contemplated as being use~ul in embodiments that involve a single staye membrane unit, a series or lo sequence of membrane units wherein the final pair of stags invslve a recycle loop. The basic point of interest in each of these embodiments is that the use of the semipermeable membrane unit, although not achieving isolation and recovery of the organic vapor per se, does produce simultaneously an organic vapor enriched and an an organic vapor depleted gaseous stream without external dilution which can then be further treated by conventional carbon adsorption and compression/condensation to recover the organic phase.
It should be further ~ppreciated that the specific embodim2nt illustrated in the Figure is to be employed with a semipermeable membrane that is selective with respect to the preferential passage of organic vapor relative to air. When a æemipermeable membrane selective to the passage of air relative to organic vapor is employed, the role of the carbon adsorption unit 24 and the compression/condensation unit, 30 and 34, are interchanged.
The advantages of the improved process for recovering organic solvent vapors according to the present invention are significant and numerous. First and foremost, the improved method affords an efficient and technically convenient method of recovering solvent vapors from feed streams that are in the J'window concentration~' range, without resorting to excessive dilution, oversized adsorption towers, frequent _g_ -:, . ,, - : "
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regeneration of columns or taxing the capacity of existing adsorption tower to its limit or, alternatively, forcing the compression/condensation equipment to operate under impractical and inefficient conditions including possible excessive emission losses. By eliminating the need for further dilution yet producing a vapor depleted effluent stream for conventional carbon adsorption, the overall capacity of a given adsorption tower is optimized and the risks associated with relatively high conc~ntration of vapors (excessive exotherms, hot spots and/or spontaneous combustion) is significantly reduced. Simultaneously, the production of an effluent stream enriched in organic vapors further optimiz~s the compressionlcon-densation step used to isolate and recover the organicsolvent.
The particular feed streams that are amenable to the aforementioned advantages and benefits of the present invention are generally any low-boiling organic solvent vapor mixed with air or similar carrier gas wherein the concentration of the organic vapor phase is within the "window conc~ntration~ range. For purposes of this invention, the organic vapor would include, by way of example, but not limited thereto, low boiling hydrocarbons, halogenated hydrocarbons, oxygenated hydrocarbons and the like. The present invention is viewed as bein~ particularly useful in the recovery of halogenated hydrocarbons fr~quently employed in commercial cleaning processes and as solvent or blowing/foaming agents in many plastic ~abrication and article manufacturing processes. In particular, thi~
would include the chloro- and/or ~luorocarbons, the chlorofluorocarbons and the hydrochloro~luorocarbons as commonly known and used in industry.
Generally when the organic solvent is a liquid with relatively high boiling point, its recovery ..
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is relatively simple (i.e., cooling to bring about condensation provides efficient recovery). In contrast, when the organic solvent is a liquid with relatively low boiling point, such as a boiling point below about 40C wherein a large portion of the solvent is in the vapor state at ordlnary temperatures, simple condensation is not sufficient. Currently, there are two traditional methods available ~or vapor recovery for low boiling organic solvents, i.e., eithPr by compression/condensation or by use of an adsorbent.
Unfortunately, for many organic vapor/air mixtures th~re is an upper limit to that concentration of organic vapors that can be processed by say caxbon adsorption and a minimum concentration required to efficiently recover vapor by compression/condensation.
In such ca es a "window concentration~ range may exist wherein the concentration of organic vapor present in the air or carrier gas is too high for conventional carbon adsorption and too low ~or efficient compression/condensation recovery. It should be appreciated that the specific concentration associated with this ~o-called ~window concentration~ range may vary according to the ~pecific organic vapor and the carrier gas as well as the operating conditions being employed. Thus, generally and for purposes of this invention, the "window concentrationn range can be defined by viewing or considering what concentration cannot be e~ficiently processed by conventional means.
However, as an al~ernative to this defini-tion, the "window concentration~ range can be inter-preted from an affirmative viewpoint by considering the concentration ranges that can be advantageously processed by the improved process according to the present invention. To a great extent these viewpoints arè synonymous but the latter de~inition may tend to broaden the specific quantitative concGntration range.
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; ~ ' ' ', Thus, for example, when the concentrations of solvent vapor are sufficiently high in solventlair mixture, say about 30 volume percent, efficient recovery is possible by the compression/condensation method wherein the solvent/air mixture is compressed and cooled to condense the solvent vapor into liquid solvent. The liquefied solvent can then b~ separat~d and stored for further use. When the concentration of solvent vapor in solvent/air mixture is low, for lo example about 6 volume percent or lower, efficient recovery is possible by the carbon adsorption method wherein such solvent air mixture is contacted with activated carbon. Typically this is performed in an adsorption tower, wherein the solvent is preferentially adsorbed on the carbon and air devoid of solvent can be relea~d into the atmosphere. When the carbon in the adsorption tower is saturated with the solvent, the solvent/air stream is directed to another adsorption tower. Solvent adsorbed on carbon can then be o recovered by a number of ways such by raising the temperature or passing steam through the tower.
However, when the concentration of low boiling solvent in a solvent/air mixture is between from about 6 volume percent and about 30 volume percent, the above-described solvent recovery processes - become inefficient and uneconomical. The present invention provides an improvement in solvent recovery process wherein low boiling solvent in a solvent/air mixture is present in a concentration range which makes recovery by either compr~ssion/condensation or carbon adsorption method impractical and uneconomical. In principle, the present invention provides a process for treating a solvent/air mixture in the above-described nwindow concentration~ range to obtain two different solvent/air mixtures, i.e., one in which the solvent concentration is sufficiently high for efficient ,, . ,~,.. . .. . ..
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recovery by the compression/condensation method and the other in which the concentration of the solvent is sufficiently low ~or efficient recovery by the carbon adsorption method.
The semipermeable membrane unit useful in the present invention can be generally any such device as well known in the art, including by way of example, but not limited thereto, semipermeable membrane thin layer of film, spiral wound membrane, hollow fiber semipormeable membrane or the like. For the semipermeable membrane to eeparate the organic vapor/air mixture into a vapor enriched component and a vapor deplete component, there must be a difference in the permeation rates for the organic vapor and air (i.e., 2 and N2). For purpos~s of this invention the ratio of permeation rate for the organic vapor through the barrier membrane to the permeation rate ~or air (usually measured with respect to nitrogen) should be at least 10. Preferably this selectivity (or separation factor) should be lQ0 or even up to 10,000 with the actual absolute permeation rate of organic vapor being typically at least 1 x 10-7 cm3(STP)-cm-cm~2-cmHg~1-sec~1 or greater. Typically, the barrier membrane is an elastomeric polymer film made from natural rubbers, polyiosprenes, polybutenes, polybutadienes, silicone rubbers, neoprene or the like as generally known in the art. Preferably, for the separation of ¢hlorofluorocarbons and hydrochloro-fluorocarbons ~rom air, wherein the permeation selectivity is to favor passage oP the halocarbon, a barrier membrane of dimethyl sllicone rubber is employed. When the permeation selectivity i~ to favor the passage of air/ potential harrier membrane materials include glasses, ceramics, polymeric plastics, films and elastomers, natural products such as ~ellulose and rubber as w211 as porous metals or .. ..
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metal films, such as stainless steel, palladium, platlnum and cold rolled steel, as generally known in the art.
The organic vapor depleted effluent stream 5 from the feed side of the membrane separation unit can be processed for recovery of organic solvent by any of the adsorption tower techniques as generally known in the art. Preferably a carbon adsorption is used, but ~or purposes of this invention other convent1onal adsorption units based on molecular sieve or adsorption media should be considered equivalent to the pref~rred carbon adsorption. Typically such systems involve a plurality of individual adsorption towers wherein one unit is selectively adsorbing organic vapor as the effluent stream from the membrane section passes through while the other units are being stripped of the organic solvent and regenerated for further vapor adsorption, again as generally known in the art.
The organic vapor enriched permeate stream from the membrane separation unit can ~e processed by any of the conventional compression and conden~ation techniques as generally known in the art. Typically the process involves a compression followed by a condenser/heat exchange which is adapted to recover liquid condensate. The liquid phase organic solvent recovered as well as that recovered ~rom the carbon adsorption process can either be sent to stor3ge or recycled to the particular manu~acturing or process system that generated the organic vaporJair mixture being processed.
An example of a process which produces organic vapor/air ~ixtures wherein the concentration of the organic solvent in the vapor/air mixture is from about 10 to 25 volume percent, i.e., in the ~window concentrationn range, is in the manu~acturing of continuous-fiber, spunbonded polyethylen2 fabric sheets . " . . , ~ , , , , ~J J, ~
which ars used, for example, in the housing industry.
In this flash-extrusion process a chloroforocarbon, namely, monofluorotrichloromethane (CFC13, CFC-11; b.p.
23.8C) is used as the processing solvent. This solvent is particularly useful because it provides certain processing advantages which lead to superior properties in the finishPd product. However, this solvent is comparatively expensive and is considered to be a contributor to the ~tratospheric ozone layer depletion process. Thus, there is a two fold incentive ~environmental and economic) to recover as much of this solvent as possible and in a cost effective way.
In this process, polyethylene is intimately mixed and heated with mono~luorotrichloromethane in a mixing tank to form a solution of polyethylene in CFC-11. This solution is flash-extruded onto a moving continuous belt in a chamber whereby the CFC-11 is rapidly vaporized. The continuos-fiber polyethylene mat is then bonded by heat and calender pressure to provide the desired spunbonded sheet The vaporized CFC-ll, now admixed with air present in the chamber, has a concentration in the range of from about 10 volume percent to about 25 volume percent. Since this concentration of CFC-ll in the CFC-ll/air mixture is too low for efficient recovery by the compression/con-densation method and too high for efficient recovery by the car~on adsorption method, it was necessary to add additional air to this mixture ~uch that the concentration of CFC-ll was raduced to below 7 volume percent 60 that recovery can be made by the carbon ad~orption method. The step o~ adding additional air so the solvent recovery by carbon adsorption method can be used provided problems such as (13 considerably greater volume of solvent/air mixture must now be treated, (2~ if lesser amount of air is used to reduce problem (1) above, the efficiency of the adsorption C? r~
tower is reduced due to temperature rise in the adsorption tower from the heat of adsorption, and, in ~act, in some situations, temperature may rise so high that fire may cccur in the tower; and (3) since all of the solvent is recovered via the carbon adsorption tower, there is a constant need to switch adsorption towers when it is saturated with CFC-11, and with the attendant need to desorb CFC-11 from carbon and reqeneration of the carbon adsorbent, certain loss of CFC-11 into the atmosphere was unavoidable. By the use of the present invention process, there is almost complete recovery of CFC-11 for reuse and the atmospheric release of CFC-ll has been greatly reduced.
The following examples are presented to further illustrate specific embodimen~s of the invention. In presenting these examples all ref~rences to percentages of components in the gaseous phase are by volume percent unless otherwise indicated.
The improved method according to the present invention was carried out during a commercial process for making polyethylene sheet~ wherein the process comprised the steps of intimately mi~ing poly~thylene beads with a chlorofluorocarbon (CFC-11), hea~ing and mixing the mixture under pressure and then flash-extruding the solution o~ polyethylene in CFC-11 onto a continuous moving belt in a chamber at atmospheric pressure containing air. Under these conditions CFC-11 which boils at 23.8C vaporizes very rapidly leaving the desired polyethylene sheet on the moving belt. The CFC-ll/air mixture exiting the chamber measured during the 51 runs carried out averaged 17.9 volume percent (range of from 10 volume percent to 29 volume percent). Previously this CFC-11/air mixture was diluted with additional air so , .-: . , : -, : . , . , ,.. :
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that CFC concentration was rsducPd to a few volume percent so that CFC-11 could be recovered by the carbon adsorption method. In carrying out the process o~ the invention, the CFC-11/air mixture was contacted with semipermeable membrane unit. The semipermeable membrane utilized was an elastomeric dimethyl silicone wherein the membrane is incorporated into spiral wound membrane module. The unit is operated by maintaining the vapor pressure on the permeate side lower than the vapor pressure on the feed side. This is done by operating a vacuum pump on the permeate side. In the 51 runs, the feed to the permeation unit which averaged 17.9 volume percent o~ CFC-11 was separated into CFC-11 enriched component which averaged 73.82 volume percent CFC-11 (range of 50 volume percent to approaching 100 volume percent) and CFC-ll depleted component which averaged a 6.8 volume percent CFC-ll (range of 3.3 volume percent to 12.0 volume percent). With the average concentration of CFC-ll of 73.8 volume percent which on the average represents enrichment o~ 4.1 fold, CFC-11 from such a mixture was recovered efficiently by the usual compression/condensation method. The CFC-11 depleted component with an average concentration of CFC-11 of 6.8 volume percent was readily treated in the carbon adsorption tower.
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A continuously feed str~am o~ CFC-11 and air withdrawn from a chamber containing a moving belt upon which a spunbonded, random weave, polyethylene fabric, is being cbntinuously manufactured is fed to a hybrid membrane-carbon-adsorption-c:ompression/condensation ~ystem as illustrated in the Figure~ This feed stream is delivered at a rate of 1000 scfm and contains an average of 12 volume percent CFC-ll. Prior to introduction of the feed stream to the first stage " , , ' ~ ~ . . : ' .
.~ f . . .
: ' ~ -~s ~piral wound membrane unit hav~ng a silicon polymer semipermeable membrane, it is combined with a 12 volume percent CFC-ll recycle effluent at a rate o~ 71.3 scfm from the feed side of the second ~tage membrane unit.
A vacuum blower delivers a 60 volume percent CFC-11 permeate stream from the first ~tage membrane unit to the ~econd stage membrane unit at a flow rate of 169.1 scfm. The second stage silicone polymer spiral wound membrane unit is used in combination with a ~econd lo vacuum blower to withdraw a 95 volume percent CFC-11 permeate stream at 97.8 scfm, while the previously mentioned 71.3 scfm, 12 volume percent CFC-ll feed side effluent is recycled to the inlet of the first membrane separator. The 9S volume percent CFC-11 permeate stream at 97.8 scfm is directed to a conventional compressor/condenser stage for final removal and recovery of CFC-11 while an 3 volume percent CFC-11 effluent stream from the feed side of the first membrane separator at a rate of 9C2.2 scfm is directed to a conventional carbon adsorption unit for separation and recovery of CFC-11.
Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following claims are not to be so limited but are to be aforded a scope commensurate with the wording of each element of the claim and equivalents thereof.
: . , , . . .
~J S , ~
According to one embodiment of th~ present invention, a two-stage membrane separation system is used to produce a feed side effluent at the first stage that is sufficiently depleted of organic vapors that it can be treated by conventional carbon adsorption~ The permeate from the first stage is then introduced to the feed side of the second stage of the membrane separation system to produce further organic vapor enriching of the second stage permeate that is then treated by conventional compression and condensation.
The feed side effluent of the second stage is recycled to the feed side of the first stage. Such a process is particularly useful in recovering chlorofluorocarbon, hydrochlorofluorocarbon, and hydrofluorocarbon vapors emitted to the air during the manufacture of various foamed plastic products and the like. In another embodiment of the present invention, a feed side effluent at the first stage of a two~stage membrane separation system is organic vapor enriched and sent to the compression/condensation separator while the permeate from the second stage is organic vapor depleted and, therefore, processed by carbon adsorption. FurthPr according to the present invention, a regeneration/recycle procedure employing multiple carbon adsorption bed and the return of effluent from an adsorption bed being regenerated with an in situ stream is provided to further reduce emission of the halocarbon.
Thus, the present invention provides a process for separating and recovering organic vapors from a ~eed str am of organic vapor and air wherein or~anic vapors are present in the ~window concentration~ range, being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery, comprising the steps of:
~, Ø~ . . J
(a) providing a semipermeable membrane means for separating organic vapors from air having a feed side and a permeate side wherein said semipermeable membrane means is characterized as having a selectivity for allowing the passage of organic vapor relativs to air or for allowing the passage of air relative to organic vapor of at least 10 and permeability for the permeate gas of greater than 1 x 10-7 cm3(STP) cm cm~2 cmHg~1 sec~1;
(b) passing a feed stream of oryanic vapor and air, wherein said organic vapor is present in the ~window concentrationn range, across the feed side of the semipermeable membrane such that organic vapor or air, but not both, : passes preferentially through the membrane to form an organic vapor depleted ~ir stream characterized by an organic vapor concentration below the nwindow concentration'J range and an organic vapor enriched stream characterized by an organic vapor concentration above the ~window concentrationN range;
(c) subj~cting said or~anic vapor depleted air stream produced in step (b) to carbon adsorption, thus separating and recovering organic vapor therefrom; and (d~ subjecting ~aid enrichPd stream produced in step (b) to compression and condensation, thus separating and recovering organic vapor therefrom.
It is an object of the preæent invention to provide an improved proc~s for the efficient and economic recovery of low boiling or~anic solvent vapors - 35 from an organic vapor and air mixturs, wherein the organic vapor to air mixture is at a concentration such , .. . . .
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that the organic vapor concentration i6 too low for efficient recovery by compression/condensation methods and too high for efficient recovery by carbon adsorption methods. It is an additional object of the present invention to provide an efficient method of recovering low boiling organic solvent vapors from the above-described vapor/air mixtures by use of a semiper-meable membrane separation stage that works in combina-tion, simultaneously, with a conventional carbon adsorption system and a conventional compression/con-densation system. It is still a further ob~ect of the present invention to provide a hybrid membrane-carbon-adsorption-compreRsion/condensation vapor recovery system that will recover chlorofluorocarbons from a chlorofluorocarbon and air mixture particularly those associated with the commercial use of blowing and foaming agents in the manufacture of foamed plastics, fibers, films and the like. Fulfillment of these objects and the presence and fulfillment of other objects will be apparent upon complete reading of th~
specification and claims taken in conjunction with the attached drawing.
BRIEF DESCRIPTION OF THE DRAWING
'rhe Figure represents a schematic illustra-tion of a typical improved process ~or recovery of organic vapors according to one embodiment o~ the present invention.
~ESCRIPTION OF THE PREFERRED EMBODIMENTS
The improved process for recovering organic vapors from an organic vapor/air mixture according to the present invention, how it differs from yet is related to previously known methods of recov~ring organic vapors, what advantages there are associated with the use of the improved method relative to these ,, . . : , -, , ' prior art methods, and under what circumstances the present invention provides these advantages can be best explained and understood by reference to the drawing.
The schematic flow diagram of the Figure illustrates the basic components and stages of one particular embodiment of the present invention. As illustrated in the Figure, a feed stream (i.e., a halocarbon/air mixture in the ~window concentrationn range) is continuously introduced into the first stage 10 of the two-stage semipermeable membrane units via line 12. This gaseous feed stream entering the feed side of first stage membrane separator 10 is continuously commingled with the recycle gas~ous effluent from the feed side of the second membrane separator 14, via line 16, prior to the combined mixture being sent to membrane separator 10. Within separator 10 (in this particular illustrated embodiment) an organic vapor enriched permeate is continuously withdrawn via line 18 by virtue of vacuum blower 20 while an organic vapor depleted stream is withdrawn from the feed side via line 22 and sent to (again in this particular illustrated embodiment) one of two essentially equivalent carbon adsorber units, 24, for recovery of the organic vapors. The permeate from the first stage membrane separator 10 i5 then delivered to the feed side of the second stage separator 14 via line 26. Within separator 14 further organic vapor enriching of the permeate occurs with the permeate being withdrawn via line 28 by virtue of vacuum blower/compressor 30 and sent, via line 32, ~o condenser/separator 34. The effluent from the feed side of membrane separator 14, having an organic vapor concentration substantially the same as the original feed, is continuously returned and commingled with the feed as previously mentioned.
, .
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During operation of the embodiment of the Figure, liquid halocarbon is continuously removed from the bottom of the condenser/separator 34 while halocarbon is simultaneously being deposited on the carbon bed within adsorber 24. Halocarbon depleted air may be vented from both the adsorber 24 and condenser/separator 34. Because of the finite capacity of the carbon adsorption bed, an additional adsorption unit 36 is provided for continuous operation. While bed 24 is actively adsorbing halocarbon the other adsorption unit 36 can be regenerating~ The Figure illustrates regeneration in that a portion of the air effluent from adsorber 24 is passed through heater 38 and introduced into adsorber 36 via line 40. The halocarbon thus removed from the regenerating adsor~er 36 is then sent back to an appropriate location in the overall process for ~urther isolation and recovery of halocarbon. As illustrated in the Figure the recycle of halocarbon enriched regeneration gas is being directed via line 42to the feed stream entering the first membrane separation unit lO. It should be appreciated that depending on the level or concentration of halocarbon in this recycle loop, the regeneration stream can be reintroduced advantageously to other locations in the overall process, including the permeate side of the first membrane unit 10 or even directly to the compression/conden~ation unit if the halocarbon concentration is sufficiently high. It should also be appreciated that various additional valves, conduits, and heat exchangers and the like (not shown in the Figure) can be incorporated into the process, as generally known in the art, to accomplish the switching of adsorber units and regeneration with recycle features of the present invention.
3~ It should be appreciated that the particular two stage embodiment illustrated in the Figure is most --8--.
.
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' , ' useful when the feed stream is relatively dilute with respect to the so-called ~window concentration" range or, stated in other terms, when the feed stream is su~ficiently dilute to be further depleted of organic vapors in a single membrane stage such as to be amenable to carbon adsorption. Thus, in principle, the process according to the present invention is contemplated as being use~ul in embodiments that involve a single staye membrane unit, a series or lo sequence of membrane units wherein the final pair of stags invslve a recycle loop. The basic point of interest in each of these embodiments is that the use of the semipermeable membrane unit, although not achieving isolation and recovery of the organic vapor per se, does produce simultaneously an organic vapor enriched and an an organic vapor depleted gaseous stream without external dilution which can then be further treated by conventional carbon adsorption and compression/condensation to recover the organic phase.
It should be further ~ppreciated that the specific embodim2nt illustrated in the Figure is to be employed with a semipermeable membrane that is selective with respect to the preferential passage of organic vapor relative to air. When a æemipermeable membrane selective to the passage of air relative to organic vapor is employed, the role of the carbon adsorption unit 24 and the compression/condensation unit, 30 and 34, are interchanged.
The advantages of the improved process for recovering organic solvent vapors according to the present invention are significant and numerous. First and foremost, the improved method affords an efficient and technically convenient method of recovering solvent vapors from feed streams that are in the J'window concentration~' range, without resorting to excessive dilution, oversized adsorption towers, frequent _g_ -:, . ,, - : "
.
, _ ~ - r~
regeneration of columns or taxing the capacity of existing adsorption tower to its limit or, alternatively, forcing the compression/condensation equipment to operate under impractical and inefficient conditions including possible excessive emission losses. By eliminating the need for further dilution yet producing a vapor depleted effluent stream for conventional carbon adsorption, the overall capacity of a given adsorption tower is optimized and the risks associated with relatively high conc~ntration of vapors (excessive exotherms, hot spots and/or spontaneous combustion) is significantly reduced. Simultaneously, the production of an effluent stream enriched in organic vapors further optimiz~s the compressionlcon-densation step used to isolate and recover the organicsolvent.
The particular feed streams that are amenable to the aforementioned advantages and benefits of the present invention are generally any low-boiling organic solvent vapor mixed with air or similar carrier gas wherein the concentration of the organic vapor phase is within the "window conc~ntration~ range. For purposes of this invention, the organic vapor would include, by way of example, but not limited thereto, low boiling hydrocarbons, halogenated hydrocarbons, oxygenated hydrocarbons and the like. The present invention is viewed as bein~ particularly useful in the recovery of halogenated hydrocarbons fr~quently employed in commercial cleaning processes and as solvent or blowing/foaming agents in many plastic ~abrication and article manufacturing processes. In particular, thi~
would include the chloro- and/or ~luorocarbons, the chlorofluorocarbons and the hydrochloro~luorocarbons as commonly known and used in industry.
Generally when the organic solvent is a liquid with relatively high boiling point, its recovery ..
-~
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is relatively simple (i.e., cooling to bring about condensation provides efficient recovery). In contrast, when the organic solvent is a liquid with relatively low boiling point, such as a boiling point below about 40C wherein a large portion of the solvent is in the vapor state at ordlnary temperatures, simple condensation is not sufficient. Currently, there are two traditional methods available ~or vapor recovery for low boiling organic solvents, i.e., eithPr by compression/condensation or by use of an adsorbent.
Unfortunately, for many organic vapor/air mixtures th~re is an upper limit to that concentration of organic vapors that can be processed by say caxbon adsorption and a minimum concentration required to efficiently recover vapor by compression/condensation.
In such ca es a "window concentration~ range may exist wherein the concentration of organic vapor present in the air or carrier gas is too high for conventional carbon adsorption and too low ~or efficient compression/condensation recovery. It should be appreciated that the specific concentration associated with this ~o-called ~window concentration~ range may vary according to the ~pecific organic vapor and the carrier gas as well as the operating conditions being employed. Thus, generally and for purposes of this invention, the "window concentrationn range can be defined by viewing or considering what concentration cannot be e~ficiently processed by conventional means.
However, as an al~ernative to this defini-tion, the "window concentration~ range can be inter-preted from an affirmative viewpoint by considering the concentration ranges that can be advantageously processed by the improved process according to the present invention. To a great extent these viewpoints arè synonymous but the latter de~inition may tend to broaden the specific quantitative concGntration range.
: , - : ~
; ~ ' ' ', Thus, for example, when the concentrations of solvent vapor are sufficiently high in solventlair mixture, say about 30 volume percent, efficient recovery is possible by the compression/condensation method wherein the solvent/air mixture is compressed and cooled to condense the solvent vapor into liquid solvent. The liquefied solvent can then b~ separat~d and stored for further use. When the concentration of solvent vapor in solvent/air mixture is low, for lo example about 6 volume percent or lower, efficient recovery is possible by the carbon adsorption method wherein such solvent air mixture is contacted with activated carbon. Typically this is performed in an adsorption tower, wherein the solvent is preferentially adsorbed on the carbon and air devoid of solvent can be relea~d into the atmosphere. When the carbon in the adsorption tower is saturated with the solvent, the solvent/air stream is directed to another adsorption tower. Solvent adsorbed on carbon can then be o recovered by a number of ways such by raising the temperature or passing steam through the tower.
However, when the concentration of low boiling solvent in a solvent/air mixture is between from about 6 volume percent and about 30 volume percent, the above-described solvent recovery processes - become inefficient and uneconomical. The present invention provides an improvement in solvent recovery process wherein low boiling solvent in a solvent/air mixture is present in a concentration range which makes recovery by either compr~ssion/condensation or carbon adsorption method impractical and uneconomical. In principle, the present invention provides a process for treating a solvent/air mixture in the above-described nwindow concentration~ range to obtain two different solvent/air mixtures, i.e., one in which the solvent concentration is sufficiently high for efficient ,, . ,~,.. . .. . ..
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recovery by the compression/condensation method and the other in which the concentration of the solvent is sufficiently low ~or efficient recovery by the carbon adsorption method.
The semipermeable membrane unit useful in the present invention can be generally any such device as well known in the art, including by way of example, but not limited thereto, semipermeable membrane thin layer of film, spiral wound membrane, hollow fiber semipormeable membrane or the like. For the semipermeable membrane to eeparate the organic vapor/air mixture into a vapor enriched component and a vapor deplete component, there must be a difference in the permeation rates for the organic vapor and air (i.e., 2 and N2). For purpos~s of this invention the ratio of permeation rate for the organic vapor through the barrier membrane to the permeation rate ~or air (usually measured with respect to nitrogen) should be at least 10. Preferably this selectivity (or separation factor) should be lQ0 or even up to 10,000 with the actual absolute permeation rate of organic vapor being typically at least 1 x 10-7 cm3(STP)-cm-cm~2-cmHg~1-sec~1 or greater. Typically, the barrier membrane is an elastomeric polymer film made from natural rubbers, polyiosprenes, polybutenes, polybutadienes, silicone rubbers, neoprene or the like as generally known in the art. Preferably, for the separation of ¢hlorofluorocarbons and hydrochloro-fluorocarbons ~rom air, wherein the permeation selectivity is to favor passage oP the halocarbon, a barrier membrane of dimethyl sllicone rubber is employed. When the permeation selectivity i~ to favor the passage of air/ potential harrier membrane materials include glasses, ceramics, polymeric plastics, films and elastomers, natural products such as ~ellulose and rubber as w211 as porous metals or .. ..
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metal films, such as stainless steel, palladium, platlnum and cold rolled steel, as generally known in the art.
The organic vapor depleted effluent stream 5 from the feed side of the membrane separation unit can be processed for recovery of organic solvent by any of the adsorption tower techniques as generally known in the art. Preferably a carbon adsorption is used, but ~or purposes of this invention other convent1onal adsorption units based on molecular sieve or adsorption media should be considered equivalent to the pref~rred carbon adsorption. Typically such systems involve a plurality of individual adsorption towers wherein one unit is selectively adsorbing organic vapor as the effluent stream from the membrane section passes through while the other units are being stripped of the organic solvent and regenerated for further vapor adsorption, again as generally known in the art.
The organic vapor enriched permeate stream from the membrane separation unit can ~e processed by any of the conventional compression and conden~ation techniques as generally known in the art. Typically the process involves a compression followed by a condenser/heat exchange which is adapted to recover liquid condensate. The liquid phase organic solvent recovered as well as that recovered ~rom the carbon adsorption process can either be sent to stor3ge or recycled to the particular manu~acturing or process system that generated the organic vaporJair mixture being processed.
An example of a process which produces organic vapor/air ~ixtures wherein the concentration of the organic solvent in the vapor/air mixture is from about 10 to 25 volume percent, i.e., in the ~window concentrationn range, is in the manu~acturing of continuous-fiber, spunbonded polyethylen2 fabric sheets . " . . , ~ , , , , ~J J, ~
which ars used, for example, in the housing industry.
In this flash-extrusion process a chloroforocarbon, namely, monofluorotrichloromethane (CFC13, CFC-11; b.p.
23.8C) is used as the processing solvent. This solvent is particularly useful because it provides certain processing advantages which lead to superior properties in the finishPd product. However, this solvent is comparatively expensive and is considered to be a contributor to the ~tratospheric ozone layer depletion process. Thus, there is a two fold incentive ~environmental and economic) to recover as much of this solvent as possible and in a cost effective way.
In this process, polyethylene is intimately mixed and heated with mono~luorotrichloromethane in a mixing tank to form a solution of polyethylene in CFC-11. This solution is flash-extruded onto a moving continuous belt in a chamber whereby the CFC-11 is rapidly vaporized. The continuos-fiber polyethylene mat is then bonded by heat and calender pressure to provide the desired spunbonded sheet The vaporized CFC-ll, now admixed with air present in the chamber, has a concentration in the range of from about 10 volume percent to about 25 volume percent. Since this concentration of CFC-ll in the CFC-ll/air mixture is too low for efficient recovery by the compression/con-densation method and too high for efficient recovery by the car~on adsorption method, it was necessary to add additional air to this mixture ~uch that the concentration of CFC-ll was raduced to below 7 volume percent 60 that recovery can be made by the carbon ad~orption method. The step o~ adding additional air so the solvent recovery by carbon adsorption method can be used provided problems such as (13 considerably greater volume of solvent/air mixture must now be treated, (2~ if lesser amount of air is used to reduce problem (1) above, the efficiency of the adsorption C? r~
tower is reduced due to temperature rise in the adsorption tower from the heat of adsorption, and, in ~act, in some situations, temperature may rise so high that fire may cccur in the tower; and (3) since all of the solvent is recovered via the carbon adsorption tower, there is a constant need to switch adsorption towers when it is saturated with CFC-11, and with the attendant need to desorb CFC-11 from carbon and reqeneration of the carbon adsorbent, certain loss of CFC-11 into the atmosphere was unavoidable. By the use of the present invention process, there is almost complete recovery of CFC-11 for reuse and the atmospheric release of CFC-ll has been greatly reduced.
The following examples are presented to further illustrate specific embodimen~s of the invention. In presenting these examples all ref~rences to percentages of components in the gaseous phase are by volume percent unless otherwise indicated.
The improved method according to the present invention was carried out during a commercial process for making polyethylene sheet~ wherein the process comprised the steps of intimately mi~ing poly~thylene beads with a chlorofluorocarbon (CFC-11), hea~ing and mixing the mixture under pressure and then flash-extruding the solution o~ polyethylene in CFC-11 onto a continuous moving belt in a chamber at atmospheric pressure containing air. Under these conditions CFC-11 which boils at 23.8C vaporizes very rapidly leaving the desired polyethylene sheet on the moving belt. The CFC-ll/air mixture exiting the chamber measured during the 51 runs carried out averaged 17.9 volume percent (range of from 10 volume percent to 29 volume percent). Previously this CFC-11/air mixture was diluted with additional air so , .-: . , : -, : . , . , ,.. :
., :, .:-, . , --17 ~
that CFC concentration was rsducPd to a few volume percent so that CFC-11 could be recovered by the carbon adsorption method. In carrying out the process o~ the invention, the CFC-11/air mixture was contacted with semipermeable membrane unit. The semipermeable membrane utilized was an elastomeric dimethyl silicone wherein the membrane is incorporated into spiral wound membrane module. The unit is operated by maintaining the vapor pressure on the permeate side lower than the vapor pressure on the feed side. This is done by operating a vacuum pump on the permeate side. In the 51 runs, the feed to the permeation unit which averaged 17.9 volume percent o~ CFC-11 was separated into CFC-11 enriched component which averaged 73.82 volume percent CFC-11 (range of 50 volume percent to approaching 100 volume percent) and CFC-ll depleted component which averaged a 6.8 volume percent CFC-ll (range of 3.3 volume percent to 12.0 volume percent). With the average concentration of CFC-ll of 73.8 volume percent which on the average represents enrichment o~ 4.1 fold, CFC-11 from such a mixture was recovered efficiently by the usual compression/condensation method. The CFC-11 depleted component with an average concentration of CFC-11 of 6.8 volume percent was readily treated in the carbon adsorption tower.
E~A~
A continuously feed str~am o~ CFC-11 and air withdrawn from a chamber containing a moving belt upon which a spunbonded, random weave, polyethylene fabric, is being cbntinuously manufactured is fed to a hybrid membrane-carbon-adsorption-c:ompression/condensation ~ystem as illustrated in the Figure~ This feed stream is delivered at a rate of 1000 scfm and contains an average of 12 volume percent CFC-ll. Prior to introduction of the feed stream to the first stage " , , ' ~ ~ . . : ' .
.~ f . . .
: ' ~ -~s ~piral wound membrane unit hav~ng a silicon polymer semipermeable membrane, it is combined with a 12 volume percent CFC-ll recycle effluent at a rate o~ 71.3 scfm from the feed side of the second ~tage membrane unit.
A vacuum blower delivers a 60 volume percent CFC-11 permeate stream from the first ~tage membrane unit to the ~econd stage membrane unit at a flow rate of 169.1 scfm. The second stage silicone polymer spiral wound membrane unit is used in combination with a ~econd lo vacuum blower to withdraw a 95 volume percent CFC-11 permeate stream at 97.8 scfm, while the previously mentioned 71.3 scfm, 12 volume percent CFC-ll feed side effluent is recycled to the inlet of the first membrane separator. The 9S volume percent CFC-11 permeate stream at 97.8 scfm is directed to a conventional compressor/condenser stage for final removal and recovery of CFC-11 while an 3 volume percent CFC-11 effluent stream from the feed side of the first membrane separator at a rate of 9C2.2 scfm is directed to a conventional carbon adsorption unit for separation and recovery of CFC-11.
Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following claims are not to be so limited but are to be aforded a scope commensurate with the wording of each element of the claim and equivalents thereof.
Claims (11)
1. A process for separating and recovering organic vapors from a feed stream of organic vapor and air wherein organic vapors are present in the "window concentration" range, being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery, comprising the steps of:
(a) providing a semipermeable membrane means for separating organic vapors from air having a feed side and a permeate side wherein said semipermeable membrane means is characterized as having a selectivity for allowing the passage of organic vapor relative to air or for allowing the passage of air relative to organic vapor of at least 10 and permeability for the permeate gas of greater than 1 x 10-7 cm3(STP)?cm?cm-2?cmHg-1?sec-1;
(b) passing a feed stream of organic vapor and air, wherein said organic vapor is present in the "window concentration" range of about 6 volume percent to about 30 volume percent organic vapor, across the feed side of the semipermeable membrane such that organic vapor or air, but not both, passes preferentially through the membrane to form an organic vapor depleted air stream characterized by an organic vapor concentration below the "window concentration" range and an organic vapor enriched stream characterized by an organic vapor concentration above the "window concentration" range;
(c) subjecting said organic vapor depleted air stream produced in step (b) to carbon adsorption, thus separating and recovering organic vapor therefrom; and (d) subjecting said enriched stream produced in step (b) to compression and condensation, thus separating and recovering organic vapor therefrom.
(a) providing a semipermeable membrane means for separating organic vapors from air having a feed side and a permeate side wherein said semipermeable membrane means is characterized as having a selectivity for allowing the passage of organic vapor relative to air or for allowing the passage of air relative to organic vapor of at least 10 and permeability for the permeate gas of greater than 1 x 10-7 cm3(STP)?cm?cm-2?cmHg-1?sec-1;
(b) passing a feed stream of organic vapor and air, wherein said organic vapor is present in the "window concentration" range of about 6 volume percent to about 30 volume percent organic vapor, across the feed side of the semipermeable membrane such that organic vapor or air, but not both, passes preferentially through the membrane to form an organic vapor depleted air stream characterized by an organic vapor concentration below the "window concentration" range and an organic vapor enriched stream characterized by an organic vapor concentration above the "window concentration" range;
(c) subjecting said organic vapor depleted air stream produced in step (b) to carbon adsorption, thus separating and recovering organic vapor therefrom; and (d) subjecting said enriched stream produced in step (b) to compression and condensation, thus separating and recovering organic vapor therefrom.
2. A process of Claim 1 wherein said semiperme-able membrane means further comprises a first and second separate semipermeable membrane stage wherein each stage has a feed side and a permeate side and a separate vacuum is provided on the permeate side of each of said stages and wherein said feed stream of organic vapor and air enters the feed side of said first semipermeable membrane stage, the stream exiting the feed side of said first semipermeable membrane stage is subjected to the carbon adsorption or to the compression and condensation, the permeate from said first semipermeable membrane stage is direct to and enters the feed side of said second semipermeable membrane stage, the organic vapor and air stream exiting the feed side of said second semipermeable membrane stage is recycled to said feed stream entering the feed side of said first semipermeable membrane stage, and the permeate from said second semipermeable membrane stage is subjected to the compression and condensation or to the carbon adsorption.
3. A process of Claim 1 wherein the semipermeable membrane means allows for the selective passage of organic vapor relative to air and wherein the organic vapor depleted air stream exiting the feed side of said semipermeable membrane means is characterized by an organic vapor concentration below the "window concentration" range and is subjected to the carbon adsorption and the organic vapor enriched permeate from said semipermeable membrane means is characterized by an organic vapor concentration above the "window concentration"
range and is subjected to compression and condensation.
range and is subjected to compression and condensation.
4. A process of Claim 1 wherein the semipermeable membrane means allows for the selective passage of air relative to organic vapor and wherein the organic vapor enriched stream exiting the feed side of said semipermeable membrane means is characterized by an organic vapor concentration above the "window concentration" range and is subjected to compression and condensation and the organic vapor depleted permeate from said semipermeable membrane means is characterized by an organic vapor concentration below the "window concentration" range and is subjected to the carbon adsorption.
5. A process of Claim 2 wherein the semipermeable membrane means allow for the selective passage of organic vapor relative to air and wherein the organic vapor depleted air stream exiting the feed side of said first semipermeable membrane stage is characterized by an organic vapor concentration below the "window concentration" range and is subjected to the carbon adsorption and the organic vapor enriched permeate from said second semipermeable membrane stage is characterized by an organic vapor concentration above the "window concentration" range and is subjected to compression and condensation.
6. A process of Claim 2 wherein the semipermeable membrane means allow for the selective passage of air relative to organic vapor and wherein the organic vapor enriched stream exiting the feed side of said first semipermeable membrane stage is characterized by an organic vapor concentration above the "window concentration" range and is subjected to compression and condensation and the organic vapor depleted permeate from said second semipermeable membrane stage is characterized by an organic vapor concentration below the "window concentration" range and is subjected to the carbon adsorption.
7. A process of Claim 1 wherein said organic vapor and air feed stream comprise a halocarbon.
8. A process of Claim 2 wherein said organic vapor and air feed stream comprises a halocarbont.
9. A process of Claim 5 wherein said organic vapor and air feed stream comprises a halocarbon.
10. A process of Claim 1 wherein the subjecting of said organic vapor depleted air stream to carbon adsorption is accomplished by use of at least two carbon adsorption units wherein while one of said carbon adsorption units is being employed to adsorb organic vapors at least one other carbon adsorption unit is being regenerated by the steps comprising:
(a) directing at least a portion of the effluent from the carbon adsorption unit adsorbing organic vapors through said other carbon adsorption unit being regenerated at a temperature sufficiently high to remove adsorbed organic vapors from said other carbon adsorption unit; and (b) recycling the effluent from said other carbon adsorption unit during regeneration to either the feed or permeate side of the semipermeable membrane means for further separation and recovery of organic vapor.
(a) directing at least a portion of the effluent from the carbon adsorption unit adsorbing organic vapors through said other carbon adsorption unit being regenerated at a temperature sufficiently high to remove adsorbed organic vapors from said other carbon adsorption unit; and (b) recycling the effluent from said other carbon adsorption unit during regeneration to either the feed or permeate side of the semipermeable membrane means for further separation and recovery of organic vapor.
11. A process of Claim 2 wherein the subjecting of said organic vapor depleted air stream to carbon adsorption is accomplished by use of at least two carbon adsorption units wherein while one of said carbon adsorption units is being employed to adsorb organic vapors at least one other carbon adsorption unit is being regenerated by the steps comprising:
(a) directing at least a portion of the effluent from the carbon adsorption unit adsorbing organic vapors through said other carbon adsorption unit being regenerated at a temperature sufficiently high to remove adsorbed organic vapors from said other carbon adsorption unit; and (b) recycling the effluent from said other carbon adsorption unit during regeneration to either the feed or permeate side of a semipermeable membrane stage for further separation and recovery of organic vapor.
(a) directing at least a portion of the effluent from the carbon adsorption unit adsorbing organic vapors through said other carbon adsorption unit being regenerated at a temperature sufficiently high to remove adsorbed organic vapors from said other carbon adsorption unit; and (b) recycling the effluent from said other carbon adsorption unit during regeneration to either the feed or permeate side of a semipermeable membrane stage for further separation and recovery of organic vapor.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| ZA915021A ZA915021B (en) | 1991-06-28 | 1991-06-28 | Process for recovering organic vapors from air |
| CA002045905A CA2045905A1 (en) | 1976-11-19 | 1991-06-28 | Process for recovering organic vapors from air |
| CN91104898A CN1068752A (en) | 1991-06-28 | 1991-07-13 | From air, reclaim improving one's methods of organic steam |
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US74336376A | 1976-11-19 | 1976-11-19 | |
| US74337076A | 1976-11-19 | 1976-11-19 | |
| US84317177A | 1977-10-19 | 1977-10-19 | |
| CA002045905A CA2045905A1 (en) | 1976-11-19 | 1991-06-28 | Process for recovering organic vapors from air |
| AU79443/91A AU645095B2 (en) | 1976-11-19 | 1991-07-01 | Improved process for recovering organic vapors from air |
| CN91104898A CN1068752A (en) | 1991-06-28 | 1991-07-13 | From air, reclaim improving one's methods of organic steam |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2045905A1 true CA2045905A1 (en) | 1992-12-29 |
Family
ID=27542851
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002045905A Abandoned CA2045905A1 (en) | 1976-11-19 | 1991-06-28 | Process for recovering organic vapors from air |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2045905A1 (en) |
-
1991
- 1991-06-28 CA CA002045905A patent/CA2045905A1/en not_active Abandoned
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |