DEVELOPMENTS IN SOLVENT RECOVERY BY MEMBRANES J.G. Wijmans, J. Kaschemekat and R.W. Baker Membrane Technology and Research, Inc. 1360 Willow Road Menlo Park, California 94025 SUMMARY For the past eight years, Membrane Technology and Research, Inc. (MTR) has been developing a membrane process to separate organic vapors from effluent air streams. This process is now at the demonstration stage, and a number of pilot plants and small commercial systems are in operation. The process appears to be particularly suited to the recovery of organics from relatively concentrated streams in the range of 0.5-10% organic solvent. The process works well with a large variety of solvents including hydrocarbons and halogenated hydrocarbons. In this concentration range, the principal competitive technologies are condensation, carbon adsorption and incineration. However, these well-developed processes have a number of drawbacks. In this paper, we describe the principles of the membrane process. The factors determining the design of systems are then discussed. Finally, some commercial applications and installations of the process are described. BACKGROUND The vapor separation process is shown in its simplest form in Figure 1. A contaminated air stream is introduced into an array of membrane modules. Organic vapors are preferentially drawn through the membrane by a vacuum pump. The organic vapor is condensed and removed as a liquid. The purified airstream is removed as the residue. Transport through the membranes is induced by maintaining the vapor partial pressure on the permeate side of the membrane, lower than the vapor partial pressure of the feed air stream. This pressure difference is most commonly achieved by means of a vacuum pump. However, the feed stream can also be compressed. Air and organic vapor permeate the membrane at a rate determined by their relative permeabilities and the pressure difference across the membrane. Because the membrane is 10-100 times more permeable to organic vapor than air, a significant enrichment of organic vapor on the permeate side of the membrane is achieved. Depending on the system design, between 90-9996 of the organic vapor is removed from the feed air stream and a permeate stream, enriched 5- to 50-fold in organic vapor, is produced. (1-6) 199
Figure 1. Flow diagram of a membrane vapor separation process. To achieve an economical separation, the membrane system shown in Figure 1 must meet three requirements. First, the membrane materials must have adequate selectivity for organic vapors from air. Second, these materials must be formed into high-flux, defect-free membranes. Third, these membranes must be formed into economical membrane modules. MTR has developed composite membranes and spiral-wound modules that meet these requirements. SYSTEM DESIGN CONSIDERATIONS During the last two years, MTR has constructed six pilot vapor separation units of various designs and installed three small commercial plants. A schematic of one of our larger pilot units is shown in Figure 2. This unit, containing 40 m 2 of membrane, is able to remove 90% of the organic vapor from a 100-scfm air stream, producing a concentrated permeate enriched 20- to SO-fold in organic vapor. Alternatively, by altering the flow of gas through the membrane system, the unit could remove 99% of the organic vapor from a 50-scfm air stream, again producing a permeate enriched 20- to 50-fold in organic vapor. 200
Figure 2. Flow diagram of a 100-scfm pilot unit. The membrane system design used in a particular application depends on the vapor to be removed and the separation required. Figure 1 shows a simple singlestage system, whereas Figure 2 shows a two-stage system. In a two-stage system, the permeate produced by the first stage is subsequently used as the feed for the second stage. The advantage of a two-stage system is additional enrichment of the vapor, which improves the efficiency of the condensation step. Adding a second stage only marginally increases the capital and operating cost of the membrane system since the second stage operates on a feed stream much smaller than the first-stage feed stream. Other system configurations are also possible. Some examples of these are presented in the following section. APPLICATIONS VaporSep systems can be used to treat a great many organic vapor streams. One of the most promising application areas is the recovery of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). These solvents are expensive (typically $1-5/lb) and difficult to treat using conventional processes, like incineration, carbon adsorption and compression/condensation. VaporSep systems for the recovery of CFCs and HCFCs represent a major advance in separation technology for these streams. Two representative applications for the technology are described below. 201
Vent Gas from Transfer Operations An example of an application where membrane vapor separation has advantages over alternative technologies is retrofitting an existing vent condenser with a membrane unit to improve the solvent collection efficiency of the system. In this example, we consider a stream produced during CFC-11 tank filling operations. The vent gas saturated with the CFC vapor is normally passed through a molecular sieve dehydrator and then to a -15 C condenser. The condenser reduces the CFC concentration substantially to approximately 21% CFC- 11. In the past, this 21% stream would have been vented without further treatment. Now, however, recovery of the CFC in the vent is both economically and environmentally driven. A single-stage membrane unit easily reduces the vented gas CFC concentration by a further 90%, producing a CFC-enriched permeate that is recycled to the front of the condenser. The final CFC-11 concentration in the vent gas is reduced from 21% to 1.2%. The flow chart and design calculations for this system are shown in Figure 3. The cost of operating this unit is a tiny fraction of the value of the recovered CFC. The system pays for itself after only a few hundred hours operation. An alternative to the membrane process would be lowering the temperature of the condenser. However, to achieve 1.2% CFC- 11 in the final vent gas would require a condenser operating at -50 C. at a cost substantially more expensive than the membrane unit. Refrigerant Vent Streams Industrial refrigerators use large quantities of CFCs, particularly CFC- 1 1, CFC-12, and HFC-22. It is estimated that about one-third of all CFCs produced in the United States are used for refrigeration of air conditioning applications. 8 Because a large portion of the refrigeration system is at sub-atmospheric pressure, air leaks into the system on the low-pressure side. Air leaks are almost unavoidable in large industrial refrigerators; thus, air contaminated with refrigerant vapor must be periodically purged from the system. The design of a purge system for a refrigerator using R-12 (CFC-12) as the refrigerant is shown in Figure 4. An air leak into the systern of 10 scfm, typical of a large industrial unit, is shown. The air leak is removed through a pressure-actuated purge valve. The purge stream consists of 10 scfm of air mixed with 6-7 scfm of CFC. To reduce the CFC loss, the stream is maintained at the purge pressure of 90 psia and cooled to -60 F. Under these conditions, the bulk of the CFC contained in the stream is condensed and passed back to the refrigerator. However, the vent gas still contains approximately 5% CFC, which is discharged to the atmosphere. These refrigerator discharges represent a serious environmental problem. A 1 - scfm stream containing 5% CFC corresponds to a CFC loss of 0.016 lb/min or approximately 8,000 lb/yr. The refrigerant recovery plant we have constructed is designed to treat the vent stream produced from a very large low-temperature chiller typical of some used in food processing plants. The membrane system, shown schematically in Figure 5, is designed to recover 95 to 99% of the CFC-12 emissions from this refrigeration plant. 202
Capital Cost $45,000 Membrane selectivity 45 Annual Operating Cost Membrane area 10 m 2 Labor and maintenance $4,500 Feed pressure 80 cmhg abs Module replacement 2,000 Permeate pressure 5 cmhg abs Energy 2,000 Total Operating Cost $ 8,500/yr $0.01 /Ib of CFC- 11 recovered Figure 3. A membrane system for solvent recovery from tank filling operations. The system recovers 98% of the CFC-11 contained in the original feed stream. 203
Figure 4. Operation of a compression refrigerator, shown here with a purge system for evacuating air leaks. To low pressure system Figure 5. Flow schematic of a membrane VaporSep system designed to remove 95-99% CFC-I2 from an industrial refrigeration purge gas stream. 204
The total number of very large refrigeration systems of the type shown in Figure 4 is probably not more than 100 worldwide. Much more common are CFC- 11 centrifugal chiller units used to provide central air conditioning to large buildings. There are approximately 40,000 of these units in the U.S. Although each unit individually only releases 100 to 200 lb of CFC-11 per year, these combined sources are a major contribution to CFC- 11 pollution. Retrofitting these systems with small membrane recovery systems could extend the lifetime of this installed equipment by delaying the purchase of new equipment designed for alternative refrigerant compounds. CONCLUSIONS Membrane vapor recovery is a new technology that is able to separate and recover organic solvent vapors from air. The process is particularly suited to the treatment of streams containing more than 0.5% solvent. With these streams, the value of the recovered solvent frequently exceeds the cost of the processes. A number of small plants and pilot units have been installed (August 1990). No major problems have emerged and the process appears to be reliable and efficient. REFERENCES 1. R.W. Baker, N. Yoshioka, J.M. Mohr and A.J. Khan, "Separation of Organic Vapors from Air," J. Meb. Sci. 31, 259 (1987). 2. K-V. Peinemann, J.M. Mohr and R.W. Baker, "The Separation of Organic Liquids through Highly Swollen Polymer Membranes," J. Appl. Polym. Sci. 14, 2201 ( 1970). 3. J.G. Wijmans and V.D. Helm, "A Membrane System for the Separation and Recovery of Organic Vapors from Gas Streams," AIChE Symposium Series No. 272, Vol. 85, 74 (1989). 4. R.W. Baker, "Process for Recovering Organic Vapors from Air," U.S. Patent 4,553,983 (November 19, 1985). 5. R.W. Baker, C.-M. Bell, J.G. Wijmans and B. Ahlers, "Membrane Process for Treatment of Fluorinated Hydrocarbon-Laden Streams," U.S. Patent 4,906,256 (March, 1990). 6. R.D. Behling, L. Ohlorogge, K-V. Peinemann and E. Kybun, "The Separation of Hydrocarbons from Waste Vapor Streams," AIChE Symposium No. 272, Vol. 88, 68 (1989). 7. Y. Shindo, T. Habuta, H: Yoshitome and H. Inous, "Calculation Methods for Multicomponent Gas Separation by Permeation," Sep. Sci. Technol. 20, 445-459 ( 1985). 8. Federal Register 52, 239 (1987). 205