TREATMENT OF ORGANIC-CONTAMINATED WASTEWATER BY PERVAPORATION

J.G. WIJMANS J. KASCHEMEKAT R.W. BAKER V.L. SIMMONS Research Director Design Engineer President Marketing Director

Membrane Technology and Research, Inc., Menlo Park, CA

ABSTRACT

The removal and recovery of organic contaminants from aqueous streams by pervaporation membrane systems is a viable and economical treatment for many waste streams. Specific opportunities for the technology are identified in this paper. La~oratory and pilot system data are used to develop system desIgns and to calculate the cost of treating specific streams.

BACKGROUND

Pervaporation is a membrane process in which a perm­selective membrane is used to separate mixtures of dissolved solvents. A liquid mixture contacts one side of a membrane and is removed as a vapor from the other side. Transport through the membrane is induced by maintaining the vapor pressure on the permeate side of the membrane lower than the vapor pressure of the feed liquid. In the :Jaboratory, the low-pressure permeate vapor pressure is most conveniently produced with a vacuum pump. On a commercial scale, however, the vacuum pumps required would be impossibly large. An attractive alternative to vacuum operations. illustrated in Figure I. is to cool the permeate vapor, thus condensing it to a liquid. In this process, the low pressure on the permeate side is maintained 'by spontaneous condensation of the permeate vapor, and the permeate vapor pressure is determined by the temperature of the condensed liquid. This system is preferred in commercial operations because the cost of providing cooling water for the condenser is much less than the cost of a vacuum pump. Also, a system using a condenser is inherently more reliable than one which requires a vacuum pump.

Purified feed

Feed liquid

The adoption of pervaporation as a viable, economical industrial separation process has occurred fairly recently ­within the last ten years. Today pervaporation is used for a number of liquid separations. The biggest current application is the dehydration of 90% ethanol/water solutions to yield 99.5% pure ethanol. 1,2 This paper describes an emerging application for pervaporation, the removal of organic solvents from dilute aqueous streams. In this application, membranes more

e!Jl>rmeable to organic compounds than to water are used.s-

Hydrophobic. sparingly water-soluble. volatile solvents are concentrated most efficiently by MTR's pervaporation membranes. For example. I, I ,2-trichloroethane can be concentrated 100- to 200-fold. Equally good separation is obtained with other dissolved halocarbons or hydrocarbons, such as benzene. toluene, etc. Sparingly soluble solvents such as butanol, methyl ethyl'ketone or ethyl acetate are also separated well, and can be concentrated 40- to 60-fold. Water-soluble solvents, such as alcohols, are concentrated less efficiently. However, enrichments of 5- to 20-fold can be obtained even with these solvents.

Membrane and module design

An economically viable pervaporation process requires relatively thin membranes packaged in high-membrane-area modules. When these requirements are met, high product flow rates are achieved at an economical cost. The types of membrane and modules used in this work are illustrated in Figu!e 2. Composite membranes are made by coating a relatively open microporous support membrane with a thin, d~nse film.

The flat-sheet composite membranes are formed into spiral-wound membranes modules similar to those used in membrane reverse osmosis and gas separation systems. However, in pervaporation modules more open permeate spacer materials are used to minimize parasitic pressure drops on the permeate (vapor) side of the membrane. Laboratory-scale modules have a membrane area of 0.2-0.3 m2 and industrial­

2,call> modules have a membrane area of 4-6 m .

__ Permselective layer

___ Microporous support layer

Condensed figure 2a. A cross-section of a multilayer composite permeate membrane. The permselective layer is

liquid 03B-l S approximately 0.5-2.0-J,lm-thick and performs the separation. The microporous support provides mechanical support for the selective layer.

Figure I. Schematic of the pervaporation process.

108

TREATMENT OF ORGANIC-CONTAMINATED WASTEWATER BY PERVAPORATION

J.G. WIJMANSResearch Director

J. KASCHEMEKAT R.W. BAKER V.L. SIMMONSDesign Engineer President Marketing Director

Membrane Technology and Research, Inc., Menlo Park, CA

ABSTRACT

The removal and recovery of organic contaminants fromaqueous streams by pervaporation membrane systems is a viableand economical treatment for many waste streams. Specificopportunities for the technology are identified in this paper.La~oratory and pilot system data are used! to develop systemdesIgns and to calculate the cost of treating specific streams.

BACKGROUND

Pervaporation is a membrane process in which a perm­selective membrane is used to separate mixtures of dissolvedsolvents. A liquid mixture contacts one side of a membrane andis removed as a vapor from the other side. Transport throughthe membrane is induced by maintaining the vapor pressure onthe permeate side of the membrane lower than the vaporpressure of the feed liquid. In the laboratory, the low-pressurepermeate vapor pressure is most conveniently produced with avacuum pump. On a commercial scale, however, the vacuumpumps required would be impossibly large. An attractivealternative to vacuum operations. illustrated in Figure I, is tocool the permeate vapor, thus condensing it to a liquid. In thisprocess, the low pressure on the permeate side is maintained byspontaneous condensation of the permeate vapor, and thepermeate vapor pressure is determined by the temperature ofthe condensed liquid. This system is preferred in commercialoperations because the cost of providing cooling water for thecondenser is much less than the cost of a vacuum pump. Also, asystem using a condenser is inherently more reliable than onewhich requires a vacuum pump.

Purifiedfeed

Feedliquid

Condensedpermeate

liquid 038·1 S

Figure I. Schematic of the pervaporation process.

108

The adoption of pervaporation as a viable, economicalindustrial separation process has occurred fairly recently ­within the last ten years. Today pervaporation is used for anumber of liquid separations. The biggest current application isthe dehydration of 90% ethanol/water solutions to yield 99.5%pure ethanol. 1,3 This paper describes an emerging applicationfor pervaporation, the removal of organic solvents from diluteaqueous streams. In this application, membranes morepermeable to organic compounds than to water are used.s-s

Hydrophobic. sparingly water-soluble. volatile solventsare concentrated most efficiently by MTR's pervaporationmembranes. For example. I, I ,2-trichloroethane can beconcentrated 100- to 200-fold. Equal1y good separation isobtained with other dissolved halocarbons or hydrocarbons,such as benzene. toluene, etc. Sparingly soluble solvents such asbutanol, methyl ethyl"ketone or ethyl acetate are also separatedwel1, and can be concentrated 40- to 6O-fold. Water-solublesolvents, such as alcohols, are concentrated less efficiently.However, enrichments of 5- to 20-fold can be obtained evenwith these solvents.

Membrane and module design

An economical1y viable pervaporation process requiresrelatively thin membranes packaged in high-membrane-areamodules. When these requirements are met, high product flowrates are achieved at an economical cos\. The types ofmembrane and modules used in this work are illustrated inFigufe 2. Composite membranes are made by coating arelatively open microporous support membrane with a thin,dense film.

The flat-sheet composite membranes are formed intospiral-wound membranes modules similar to those used inmembrane reverse osmosis and gas separation systems.However, in pervaporation modules more open permeate spacermaterials are used to minimize parasitic pressure drops on thepermeate (vapor) side of the membrane. Laboratory-scalemodules have a membrane area of 0.2-0.3 m2 and industrial­,cal" modules have a membrane area of 4-6 m2.

__ Permselective layer

__ Microporoussupport layer

figure 2a. A cross-section of a multilayer compositemembrane. The permselective layer isapproximately 0.5-2.0-t'm-thick and performs theseparation. The microporous support providesmechanical support for the selective layer.

ESL-IE-91-06-25

Proceedings from the 13th National Industrial Energy Technology Conference, Houston, TX, June 12-13, 1991

Figure 2b. Composite membranes are formed into spiral-wound modules. A typical industrial-sized pervaporatioll module contains 405 m2 of membrane and produces 5-20 L of concentrated solvent permeate per hour

Applications

The opportunities for widespread use of pervaporati,," to remove organic solvents from aqueous streams can be divl.;~L1

into l"ree categories. listed helow Poilu/ion cOn/rol. This category is characterized by

streams containing low concentrations of hydrophobic solvents. The objective of the treatment is to lower the solvent concentration to 10 ppm or less, so that the water can be discharged or reused. A typical stream might come from a contaminated surface water or the evaporator condensate from a chemical plant.

Sol Yen/ recoYery. In this category, a typical stream contains relatively high concentrations of a single solvent in water. The objective is recovery of a concentrated solvent that can be reused, and 90% solvent removal from the water stream so that it can be discharged or further treated.

Volume reduction of mixed-so/Yent hazardous was/e s/reams. Because this type of stream contains several solvents, recycling a solvent concentrate back to the process is not usually viable. However, the stream contains too much solvent to be discharged. Currently, these waste streams would be trucked to an incinerator or perhaps to a solvent reclaimer, both of which are expensive alternatives. The objective of the pervaporation process is to achieve 95-98% removal of solvent from the feed stream so it can be discharged or reused, and to produce a concentrated small-volume solvent stream that could be sent to a reclaimer. Streams like this might be produced from a solvent air scrubber or the regeneration cycle of a carbon adsorber.

As an example, consider a pervaporation system designed for a pollution control application for the removal of benzene from a process wastewater. Figure 3 is a flow diagram of the system designed to remove 99% of the solvent from a 20,OOO-gpd stream containing 1,000 ppm benzene. The concentration of the benzene is reduced to less than 10 ppm in the final residue. This could be discharged, reused, or sent to a final carbon adsorption polishing step. The pervaporation system able to treat this benzene stream would have a membrane area of 200 m2 , producing a permeate with an average concentration of 26%. Because benzene is relatively insoluble in water, permeate vapor of this concentration would phase­separate on condensation to yield a pure benzene stream and a small benzene-saturated aqueous stream, which would be recycled to the feed. The operating costs of this system are $14/1,000 gal of feed treated. This compares favorably to other waste treatment methods, especially when the compact size, simple operation and completeness of the separation are considered.

MTR has constructed seven pervaporation pilot units and has demonstrated the process on a wide range of aqueo:us organic waste streams. Industrial demonstration of the proce" is \In,-!('rway.

< 10 ppm benzene

1000 ppm benzene

20,000 gallday

Membrane unit 200 n12

26% benzene 75 gal/day

_0.2% benzene 55 gal/day

:>99% benzene 20 gpl/day

Figure 3. Flow diagram of an MTR pervaporation systlm for 99% benzene removal.

REFERENCES

I. H.E.A. Brlischke, "State of the Art of Pervaporation," in Proceedings of Third International Conference on! Pervaporalion in the Chemical Industry, Nancy, France, R. Bakish (ed), Bakish Materials, Englewood, NJ (1988).

2. G.F. Tusel and H.E.A. Brlischke, "Use of Pervaporation Systems in the Chemical Industry," Desalination 52, 327 (1988).

3. I. Blume, J.G. Wijmans, and R.W. Baker, "The Separation of Dissolved Organics from Water by Pervaporation," J. Memb. Sci. 49, 253 (1990)

4. J. Kaschemekat, J.G. Wijmans, R.W. Baker and I., Blume, "Separation of Organics from Water Usin~ Pervaporation," in Proceedings of Third Internati~lPal Conference on Pervaporation, Nancy, France, R. Bakish (ed.), Bakish Materials, Englewood, NJ (1988).

5. C.-M. Bell, F.-J. Gerner and H. Strathmann, "Selbction of Polymers for Pervaporation Membranes," J, Memb, ~,315 (1988).

6. H.H. Nijhuis, M.H.V. Mulder and C.A, Smolders "Selection of Elastomeric Membranes for the Re~oval of Volatile Organic Components from Water," in Proceedings of the Third International Conference on Pervaporation in the Chemical Industry, Nancy, France, R. Bakish (ed.), Bakish Materials, Englewood, NJ (1988).

109

MTR has constructed seven pervaporation pilot unitsand has demonstrated the process on a wide range of aqueo;usorganic waste streams. Industrial demonstration of the proce"is un,-!('rway.

Membrane unit200 rrl2

< 10 ppm benzene

1000 ppmbenzene

20,000 gal/day

o

Figure 2b. Composite membranes are formed into spiral-woundmodules. A typical industrial-sized pervaporationmodule contains 405 m2 of membrane and produces5-20 L of concentrated solvent permeate per hour

.".~;:'::r~l:l:~:ugh /l........__o=>-=z=..,jm.mbt.l1.

Applications

The opportunities for widespread use of pervaporatit·"to remove organic solvents from aqueous streams can be divl,;~L1

into Three categories. listed helowPoilu/ion con/rol. This category is characterized by

streams containing low concentrations of hydrophobic solvents.The objective of the treatment is to lower the solventconcentration to 10 ppm or less, so that the water can bedischarged or reused. A typical stream might come from acontaminated surface water or the evaporator condensate from achemical plant. Figure 3.

_0.2% benzene55 gal/day

:>99% benzene20 gpl/day

Flow diagram of an MTR pervaporation system for99% benzene removal. I

Sol Yen/ recoyery. In this category, a typical streamcontains relatively high concentrations of a single solvent inwater. The objective is recovery of a concentrated solvent thatcan be reused, and 90% solvent removal from the water streamso that it can be discharged or further treated.

Volume reduction of mixed-solYen/ hazardous was/estreams. Because this type of stream contains several solvents,recycling a solvent concentrate back to the process is not usuallyviable. However, the stream contains too much solvent to bedischarged. Currently, these waste streams would be trucked toan incinerator or perhaps to a solvent reclaimer, both of whichare expensive alternatives. The objective of the pervaporationprocess is to achieve 95-98% removal of solvent from the feedstream so it can be discharged or reused, and to produce aconcentrated small-volume solvent stream that could be sent toa reclaimer. Streams like this might be produced from a solventair scrubber or the regeneration cycle of a carbon adsorber.

As an example, consider a pervaporation systemdesigned for a pollution control application for the removal ofbenzene from a process wastewater. Figure 3 is a flow diagramof the system designed to remove 99% of the solvent from a20,OOO-gpd stream containing 1,000 ppm benzene. Theconcentration of the benzene is reduced to less than 10 ppm inthe final residue. This could be discharged, reused, or sent to afinal carbon adsorption polishing step. The pervaporationsystem able to treat this benzene stream would have a membranearea of 200 m2 , producing a permeate with an averageconcentration of 26%. Because benzene is relatively insoluble inwater, permeate vapor of this concentration would phase­separate on condensation to yield a pure benzene stream and asmall benzene-saturated aqueous stream, which would berecycled to the feed. The operating costs of this system are$14/1,000 gal of feed treated. This compares favorably to otherwaste treatment methods, especially when the compact size,simple operation and completeness of the separation areconsidered.

REFERENCES

I. H.E.A. Brlischke, "State of the Art of Pervaporation," inProceedings of Third International Conference on:Pervaporalion in the Chemical Industry, Nancy, France,R. Bakish (ed), Bakish Materials, Englewood, NJ (1988).

2. G.F. Tusel and H.E.A. Brlischke, "Use of PervaporationSystems in the Chemical Industry," Desalination 5~, 327(1988).

3. I. Blume, J.G. Wijmans, and R.W. Baker, "TheSeparation of Dissolved Organics from Water byPervaporation," J. Memb. Sci. 49, 253 (1990)

4. J. Kaschemekat, J.G. Wijmans, R.W. Baker and LBlume, "Separation of Organics from Water Usin~Pervaporation," in Proceedings of Third InternatiOnalConference on Pervaporation, Nancy, France, R. 'Bakish(ed.), Bakish Materials, Englewood, NJ (1988).

5. C.-M. Bell, F.-J. Gerner and H. Strathmann, "Selectionof Polymers for Pervaporation Membranes," J. Memb.~,315 (1988).

6. H.H. Nijhuis, M.H.V. Mulder and C.A. Smolders,"Selection of Elastomeric Membranes for the Removal ofVolatile Organic Components from Water," inProceedings of the Third International Conference onPervaporation in the Chemical Industry, Nancy, France,R. Bakish (ed.), Bakish Materials, Englewood, NJ(1988).

109

ESL-IE-91-06-25

Proceedings from the 13th National Industrial Energy Technology Conference, Houston, TX, June 12-13, 1991