Ind. Eng. Chem. Res. 1993,32, 2236-2241
2236
Membrane Vapor Separation: Recovery of Vinyl Chloride Monomer from PVC Reactor Vents Richard J. Lahiere' Vista Chemical Company, 900 Threadneedle, Houston, Texas 77079
Mark W. Hellums Vista Chemical Company, 12024 Vista Parke Drive, Austin, Texas 78726
Johannes G. Wijmans and Juergen Kaschemekat Membrane Technology and Research, Inc., 1360 Willow Road, Menlo Park, California 94025
Membrane vapor separation technology was employed in a PVC (poly(viny1 chloride)) plant to separate vinyl chloride monomer (VCM) from inert gases in a reactor vent stream. The purpose of the membrane separation process was to recover VCM that would normally be incinerated in the vent stream so that the VCM could be recycled and reused in the PVC reaction. In this paper we report on the operation of one of the first membrane vapor separation systems in the chemical industry. Pilot and laboratory studies demonstrated that the membrane system recovered 63-96 vol% of VCM from a vapor that is routinely incinerated. Computer simulations based on the experimental data showed that VCM recoveries greater than 99% can be achieved by increasing the membrane area. Benefits for using membrane technology include recovered VCM cost savings, lower acid neutralization expenditures, and reduced incinerator maintenance.
Introduction The United States PVC industry uses about 11billion pounds of VCM (vinyl chloride monomer) annually. Approximately 1%of the VCM is not reacted in the manufacturingprocess. This VCM is lost in a waste stream and typically incinerated. During the VCM incineration process, HCl acid is liberated and must be neutralized by caustic. The typical PVC manufacturer may incinerate an average of 500 000 lb of VCM annually. Tough, new environmental regulations and the need to increase production yields for economic purposes are driving PVC makers to consider technological alternativesfor recovering unreacted VCM. Membrane vapor separation has recently shown promise for recovery of VCM, as well as other types of organic vapors (Behling et al., 1989;Buys et al., 1990;Peinemann et al., 1986;Rose, 1991;Wijmans et al., 1989). Membrane vapor separation is a pressure-driven membrane process for separating organic vapors from air. Membrane vapor separation has already been effectively applied to the separation of other chlorinated hydrocarbons, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and gasoline vapors. The successfultesting and implementation of membrane vapor separation can be attributed to unique advantages the technologyoffers compared to conventional separation techniques: 1. High selectivities. Vapor separation membranes can separate mixtures that are not separable by other processes. Selectivity is a parameter for evaluating the efficiency of a membrane separation process. For example, a VCM/ nitrogen selectivity of 15 means that VCM permeates through a membrane 15 times faster than nitrogen, if the driving forces for permeation are equal. 2. Low maintenancelsimple operationlcompact design. The membrane unit consists primarily of membrane modules and valves. No moving parts are involved since the process can be integrated with the plant's existing VCM recovery compressor for most PVC operations.
Maintenance and operator attention is minimal. The membrane units are compact and have a small footprint. 3 . A b i l i t y t o recover, recycle, and reuse organic compounds. The membrane separation process does not chemically alter the recovered organic compounds so the membrane process is amenable to the recycle of recovered material and its reuse. 4. Handling of feeds with variable concentrations. Changes in the concentrations of organic vapors in the membrane feed do not affect the removal of the organic vapor, in most cases. The technology is particularly suited for streams subject to upsets and ones in which constant effluent quality is critical, such as those involving environmental compliance. 5 . Low energy costs. The operational cost of a membrane vapor separation system is low since no phase change is involved.
Test Objective The objective of the testing reported in this paper was to evaluate membrane vapor separation technology for separating vinyl chloride from nitrogen, carbon dioxide, and other inert gases in PVC reactor vent streams. In particular, the VCM permeate flux, the flux stability, the amount of VCM recovered, and the ability of the membrane to accommodate variable feed conditions were evaluated during the test program. Permeate flux, the flow of permeated VCM per unit of membrane area, determines the required membrane area and the membrane system cost. Testing included laboratory and onsite pilot studies. How Vapor Separation Membranes Work
Membranes used in the vapor separation process are selectively permeable to organic compounds such as vinyl chloride but relatively impermeable to air and inert gases like nitrogen and carbon dioxide. Separation occurs because different compounds transport across the membrane barrier at different rates. A pressure difference
Qass-5885/93/2632-2236$04.Q~lQ 0 1993 American Chemical Society
vocdepleted air
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2237 / Permselectlve layer Mlcroporous support layer
+ Condenser bleed air
Figure 2. Vapor separation membrane cross section.
Condenser
Liquid
voc
Figure 1. Vapor separation membrane system schematic.
between the feed side of the membrane and the permeate side (downstream) is the driving force for the separation. A vacuum pump can be used to maintain pressure on the permeate side of the membrane lower than the pressure of the feed (see Figure 1). The flow of permeate is directly proportional to the magnitude of the pressure difference. The permeate flux in a membrane vapor separation process can be described by the following relationship: (1) Q J A = (Pi/O@$i - PPJ where xi is the mole fraction of component i in the feed, yi is the mole fraction of component i in the permeate, Qi/A is the membrane flux (volumetric flow rate per unit of membrane area), I is the membrane thickness, p j is the feed-side pressure, ppis the permeate-side pressure, and Pi is the membrane permeability coefficient. The quantity Pill will be referred to as the pressure-normalized flux of compound i (volumetric flow rate of i per unit membrane area and per unit partial pressure driving force). The permeability coefficient, Pi, can be expressed as product of a solubility coefficient, Si, and a diffusion coefficient, Di: Pi = SiDi (2) The permeate is 10-50 times more concentrated with organic vapor than the feed, depending on the selectivity. The organic in the permeate is readily recovered by condensation. Purified air is rejected by the membrane as residue. Membranes used for vapor separation have a composite structure, consisting of a thin, dense film of a rubbery polymer cast on a tough, microporous support layer made of an engineering resin. The support layer material for the membrane used in this study, manufactured by MTR (MembraneTechnology and Research, Inc., of MenloPark, CA), is resistant to chlorinated solvents in either the vapor or liquid phase. The polymer layer performs the separation and the support provides mechanical strength. Permeation rates through the membrane are high since the polymer layer is very thin, only 0.2-5 pm in thickness. A cross-sectional diagram of a membrane is given in Figure 2.
Membrane polymers are selected to give the optimal selectivity for the highest flux rate. The membrane materials can generally withstand temperatures up to 60 "C and have lifetimes of 3 years. The materials most commonly used for vapor separation membranes are rubbery polymers, such as polysiloxane and polyolefins. The solubility coefficient of organic vapors in rubbery materials typically is 100-1000 times larger than that of
permanent gases such as nitrogen and oxygen. Due to their larger molecular size, the diffusion coefficient of organicsin rubbery materials is typically 2-10 times smaller than the diffusion coefficient of permanent gases. The net result is that rubbery materials have higher permeability coefficients for organic vapors than for permanent gases (see eq 2). The membranes used in the study are configured in spiral-wound modules. The module consists of layers of membranes that are wrapped around a porous collection pipe. The individual membrane layers are separated by spacers (see Figure 3). The flow pattern in the membrane module is crossflow. Feed passes parallel to the membrane surface, and organic vapors permeate through the membrane perpendicular to the flow.
Laboratory Permeation Tests
A limited number of laboratory experiments were performed to determine the permeation behavior of VCM using the MTR-135 vapor separation membrane. The membrane is more permeable to VCM than to nitrogen. Experimental System. A nitrogen stream containing 2.4-6.2 vol 7% VCM at 136 kPa (5 psig) pressure was fed to a 0.064-m- (2.5-inJ-diameter spiral-wound module containing 1800 om2 (0.18 m2) of the vapor separation membrane. A permeate stream was generated by pulling a vacuum on the permeate side of the membrane. A typical permeate pressure was 7 kPa (1 psia). The VCM concentrations in the feed, permeate, and residue streams were determined by gas chromatography. The module was operated at small stage cuts (stage cut is defined as permeate flow rate divided by feed flow rate), meaning that the feed and residue streams were essentially identical in composition. Composition, pressure, and flow rate data were used to calculate the pressure-normalized flux of VCM by means of eq 1. Discussion of Results. The permeate typically contains 10times more VCM than the feed gas, demonstrating that the membrane permeated VCM preferentially over nitrogen. Figure 4 is a plot of the VCM concentration in the permeate stream as a function of VCM concentration in the feed stream. The preasure-normalized VCM flux as a function of the partial pressure of VCM in the feed gas is shown in Figure 5. The pressure-normalized flux is given in arbitrary units. In general, the nitrogen pressure-normalized flux of commercially available vapor separation membranes ranges from 5 X 106 to 5 X 10-4 cm3(STP)/(cm2-s.cmHg) (or 3.1-31 scfh/(ft2.100 psi); scfh, standard cubic feet per hour), whereas the pressure-normalized fluxes of organic vapors are typically 15-30 times higher. As demonstrated in Figure 5, the pressure-normalized flux of VCM increases with the VCM partial pressure. This behavior has been observed for other organic vapors as well (Behling et al., 1989). The most likely explanation is that the sorption
2238 Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 Module housing
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.’
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Figure 9. Spud-woundmembrane module.
0
2
0
4
6 Feed VCM Cmcemtlon (Val %I
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Figure 4. Laboratorydata: VCM concentrationin the permeate aa a function of VCM concentrationin the feed. Data obtained at room temperature with MTR-135 membrane.
P-um - NwmalW Flux of VCM (8lb“rary “““SI
1m
Figure 5. Laboratory data: preasurpnormalizedVCM flux aa a function of VCM partial pressure in the feed gaa. Data obtained at room temperature with MTR135 membrane.
of VCM into the membrane material increases rapidly with increasing VCM partial pressure. Pilot Plant Tests
A pilot vapor test system containing one membrane modulesuppliedby MTRwasinstalledataVistaChemical Company PVC manufacturing site. Feed Stream. The membrane module was installed to recover VCM from gas periodically bled from the plant’s VCM recovery system to purge inerts (see Figure 6). Gas was fed to the membrane intermittently as inerts build up in the VCM recovery condenser and were vented to the incinerator. Incinerator venting occurs several times per day and lasts approximately 30 min.
The feed to the pilot plant membrane unit was the overhead from the VCM recovery condenser. The stream contained noncondensable components, including 20-60 wt 9% nitrogen, 10-30 wt % carbon dioxide, and 15-70 wt 9% VCM. The temperature of the stream varied from 0 to 27 ‘C and the pressure ranged from 377 to 550 kPa (40-65 psig). Pilot Plant System. The pilot plant vapor separation test unit consisted of the membrane module, a liquid knock-out filter, and various valves and gauges. The diameter of the membrane module was 0.10 m (4 in.). The pressure of the feed to the membrane was sufficient so that an auxiliary compressor was not necessary. Check and relief valves were installed on the permeate line to prevent the permeate pressure from exceeding the feed pressure. The entire unit was mounted on a 0.7-m-wide, l.4m-long, and 0.9-m-high skid. A flow diagram of the test system, Figure 7, shows that the feed to the membrane, containing VCM, nitrogen, carbon dioxide, and small amounts of other inerts, flowed into a liquid knock-out filter. The purpose of the filter was to remove liquid VCM entrained in the feed stream. Downstream of the filter, vapor from the condenser flowed into the membrane module. A VCM-rich stream permeated through the membrane into a collection pipe (Figure 3) and exited the module. The pressure of the permeate varied from 108 to 150 kPa (1-7 psig). On a permanent installation, the permeateshould be connected to a vacuum pump to maximize the pressure driving force. The VCM-rich permeate flowed to the VCM condenser and was recycled to the PVC reactors. Carbon dioxide and nitrogen (residue) were rejected hy the membrane and exited the module on the outside of the collection pipe. The residue pressure was 184 kPa (12 psig). The residue, depleted in VCM, flowed to the incinerator. During the test, the temperature, pressure, and flow rates of the feed, permeate, and residue streams were recorded. The three streams were sampled 2-3 times per day with stainless steel sample bombs and analyzed for VCM, nitrogen, and CO2 with a gas chromatograph. Discussion of Results. The membrane module separation performance was evaluated at several feed conditions. The feed gas composition varied from 15.0 to 68.9 vol 9% VCM during the ‘burn” as inerts were purged from the system. Samples were collected a t various times during the “burn” cycle. The flow rate of feed gas fluctuated from 20 to 50 mS(STP)/h. Typical gas sample
Ind. Eng. Chem. Res., VoL 32,No. 10,1993 2239
Recovery of VCM from PVC Reactor Vents wlth Vapor Separation Membranes
Incinerator
Recycled VCM
Figure 6. Vapor mpmation membrane pilot procesa for VCM recovery.
to Incinerator
Figure 1. Detailed achematic of vapor separation membrane Bystem.
analyses and percent removal of VCM are shown in Table I. The gas streams were composed of VCM, nitrogen, and COa. Table I shows that the module was operated at relatively large stage cuts. It should be noted that large stage cuts are required if a major component of the feed gas is to be removed. For example, at a VCM concentration of 70%, a stage cut equal to 63% is required to achieve 90% VCM removal, even if the permeate consists of pure VCM. The membrane pressure-normalized flux (flow per unit membrane area) and selectivity were satisfactory throughout the test. The pressure-normalized fluxes measured in the plant test were slightly higher and the selectivities
wereslightlylowerthanthosadete~nedinthelaborato~ studies. Pilot test pressurenormalized flux data are plotted in Figures 8-10 as a function of the days of operation, the partial preeaure of VCM in the feed, and the feed gas temperature. The test system operated successfullyfor over 1 month until sufficient design data were collected and the test was terminated. VCM and inert gaa pressure-normalized fluxes, plotted in Figure 8, decreased gradually over time. The downward trend in pressure-normalized flux waa a result of increasing ambient temperature (from midMay to mid-June), producing arise in the gas temperature feeding the membrane module. The trend is shown in
2240 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 Table I. Typical Gas Compositions and VCM Recoveries residue permeate stage VCM % cut feed concn concn concn VCM) removal (%) run ( ~ 0 1 % VCM) (~017% VCM) ( ~ 0 1 % 55.1 92 77 12.9 1 45.5 52.3 58 9.8 86 2 34.3 53 22.2 84 73.3 3 49.0 72.1 88 45.6 92 4 68.9 48 4.4 26.8 85 5 15.0 57 83.8 73 41.5 6 65.5 56.7 62 6.7 93 7 37.7 44 47.5 7.9 82 8 25.1 78.8 52 50.8 63 9 65.4 89 26.4 68.2 96 10 63.8 loo0
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I
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=.
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Table 111. Simulated Performance of MTR Membranes for VCM Recovery in Winter Conditions.
.=
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15
Figure 8. VCM and inert gas pressure-normalized flux vs days of operation of the pilot test.
Flux
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14
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, 5 10 15 20 Operating Temperature (C)
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25
30
Figure 9. VCM and inert gas pressure-normalized flux va operating temperature of the pilot test.
1
- -1
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. =
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' 0
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100 150 200 250 300 350 Parllal Pressure of VCM in Feed (kPa)
Table 11. Simulated Performance of MTR Membranes for VCM Recovery in Summer Conditions. 1module 2modules 3modules feed VCM (vol %) 60.0 60.0 60.0 C02 and N2 ( V O ~%) 40.0 40.0 40.0 flow (scfm) 17.0 17.0 17.0 residue VCM (vol % ) 35.6 5.2 0.1 64.4 C02 and Nz ( V O ~%) 94.8 99.9 flow (scfm) 7.8 2.4 0.1 permeate 80.5 68.9 60.3 VCM (vol %) C02 and Nz ( V O ~% ) 19.5 31.1 39.7 flow (scfm) 9.2 14.6 16.9 VCM recovery (% ) 73 98 >99 a Summer conditione w u m e stream temperatures of 21 OC, feed pressures of 550 kPa (80psia), and permeate pressures of 69 kPa (10 psia).
I
400
Figure 10. VCM and inert gas pressure-normalized flux vs partial pressure of VCM in the feed gas in the pilot test.
Figure 9. VCM permeability is a function of the VCM sorption (solubility coefficient) and diffusivity in the membrane polymer (eq 2). Higher feed temperature reduces VCM sorption into the membrane polymer but increasesthe diffusion coefficientof VCM. Since the VCM pressure-normalized flux decreased a t high feed temperatures, it may be concluded that the effect of VCM sorption on pressure-normalized flux exceeded the diffusion effect, resulting in lower VCM permeation rates.
lmodule 2modules 3modules feed VCM (vol % ) 35.0 35.0 35.0 C02 and N2 ( V O ~%) 65.0 65.0 65.0 flow (scfm) 17.0 17.0 17.0 residue VCM (vol %) 8.3 1.9 0.3 COz and Nz ( V O ~% ) 91.7 99.7 98.1 flow (scfm) 9.7 6.7 4.1 permeate VCM (vol %) 70.5 56.3 46.1 COSand N2 ( V O ~%) 29.5 43.7 53.9 flow (scfm) 7.3 10.4 12.9 VCM recovery ( % ) 86 98 >99 0 Winter conditions assume stream temperatures of -1 OC, feed pressures of 450 kPa (65 psia), and permeate pressures of 69 kPa (10 psia).
The key process parameters which affect membrane performance are the feed gas flow, feed temperature, and feed and permeate pressure. Each of these parameters was highly variable during the test period due to the intermittent nature of the inert gas purging. The effect of the variation of VCM partial pressure in the feed is shown in Figure 10. The VCM pressurenormalized flux appears to decrease as the partial pressure of VCM in the feed gas increases. However, the increase in VCM partial pressure is caused by an increase in the temperature of the upstream VCM recovery condenser (see Figure 7). The increase in VCM partial pressure is thus accompanied by a simultaneous increase in feed gas temperature, which decreases the VCM pressure-normalized flux (see Figure 9). Laboratory experiments at constant feed temperature have shown that the VCM pressure-normalized flux increases with increasing VCM partial pressure (see Figure 5 ) . Computer Model Predictions. A computer model based on the gas permeation transport equation (eq 1) was used to simulate the vapor separation system performance. Results of the simulations, shown in Tables I1 and 111, were used to predict the performance of a commercial installation under typical summer and winter conditions. The results are shown for one-,two-, and threemodule systems in which the modules are connected in series. For example, in a two-module system, the residue gas from the first module is the feed gas for the second module. The results indicate that a two-module system is the most favorable option in terms of VCM recovery per unit
Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2241 of membrane area. VCM recoveries of 98% are attainable with a two-module design.
Conclusions The MTR membrane system performed well under highly variable feed conditions. No special equipment or adjustments were required to respond to the varying feed conditions. The single 0.10-m-diameter module used in the pilot test processed the entire vent stream that is sent to the incinerator. The recovery of VCM generally ranged from 63 to 96 vol % during the pilot test. The VCM pressurenormalized flux decreased slightly during the course of the test due to an increase in ambient temperature, which consequently increased the membrane feed temperature and reduced VCM sorption in the membrane polymer. The most optimum design for the permanent system will employ two 0.10-m- (4-in.) diameter modules in series. On the basis of computer simulation results, over 98% of the VCM currently incinerated will be recovered with a two-module system.
Acknowledgment The work of Dr. James R. Fair of The University of Texas a t Austin has been a tremendous inspiration to many in the separations community, including the authors of this paper. Dr. Fair’s contribution to his profession, in particular his energy and dedication in founding the Separations Research Program at The University of Texas, will be a legacy to chemical engineers in academia and industry for many years after he has retired. H.D. Kamaruddin and S. V. Segelke of Membrane Technology and Research, Inc., performed the laboratory experimental work and assisted with interpretation of the pilot plant data. The pilot plant system was operated by Mary O’Connell and Clint Osborne of Vista Chemical Company. Wayne Wilson of Vista Chemicals organized the pilot plant test and reviewed the pilot test system design. The cooperation and support of the personnel at the Vista Chemicals Oklahoma City PVC plant are gratefully acknowledged.
Nomenclature A: surface area of membrane, m2 Di: diffusion coefficient of component i, m2/s I: membrane separation layer, m Pi: membrane permeability coefficient, mS(STP)(m/ (s.m2.kPa)) p f : feed pressure, kPa pp: permeate pressure, kPa Qi: volumetric flow of component i, ma/s Si: solubility coefficient of component i, mS(STP)/(mS of polymer/kPa) xi: mole fraction of component i in feed, dimensionless yi: mole fraction of component i in permeate, dimensionless
Literature Cited Behling, R.-D.; Ohlrogge, K.; Peinemann, K.-V.; Kyburz, E. The Separation of Hydrocarbons from Waste Vapor Streams. Membrane Separations in Chemical Engineering;AIChE Symposium Series; AIChE: New York, 1989;Vol. 85 (No. 272), pp 68-73. Buys, H. C. W. M.; Martens, H. F.; Troost, L. M.; Van Heuven, J. W.; Tinnemans, A. H. A. New Intrinsic Separation Characteristics of Poly(dimethylai1oxane)Membranes of Organic Vapor/Ns Gas Mixtures. Proceedings of the Zntemutionul Conferenceon Membranes,Chicago; 1990;North American Membrane Society: p 833. Peinemann, K.-V.; Mohr, J. M.; Baker, R. W. The Separation of Organic Vapors from Air. Recent Advances in Separation Techniques-ZZI, AIChE Symposium Series; AIChE: New York, 1986; Vol. 82 (No. 250), pp 19-26. Rose, B. Recovery of Vinyl Chloride Monomer by Membranes.
Proceedings of the Annual Meeting of the Vinyl Chloride Safety Association, Denver, 1991. Wijmans, J. G.;Helm, V. D. A Membrane System for the Separation and Recovery of Organic Vapors from Gas Streams. Membrane Separations in Chemical Engineering; AIChE Symposium Series; AIChE: New York, 1989; Vol. 85 (No. 272), pp 74-79.
Received for review January 26, 1993 Revised manuscript received July 8, 1993 Accepted July 13, 1993. Abstract published in Advance ACS Abstracts, September 15,1993.
