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Ind. Eng. Chem. Res. 1999, 38, 3898-3902
Precipitation of Polystyrene by Spraying Polystyrene-Toluene Solution into Compressed HFC-134a Chung-Sung Tan* and Hung-Yuan Lin Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, ROC
A precipitation process employing compressed 1,1,1,2- tetrafluoroethane (HFC-134a) as antisolvent was used to recover polystyrene from toluene solution. In a continuous mode of operation, almost all the dissolved polystyrene could be precipitated under the condition that liquid HFC134a was present in the precipitator. When the precipitator was full of gaseous HFC-134a only, a significant temperature rise was observed and the amount of the precipitated polystyrene was small. The effects of temperature, pressure, toluene solution flow rate, HFC-134a flow rate, and polystyrene concentration on the yield and morphology for the precipitated polystyrene were examined in this study. Microparticles of the precipitated polystyrene were obtained only when the solution jet traveled through gaseous HFC-134a first and then contacted with liquid HFC134a in the precipitator. Introduction Because polystyrene (PS) has a wide range of application, a large amount of polystyrene is produced each year. From an environmental concern, recycling of waste polystyrene is essential. One method of recycling is to precipitate polystyrene from solution with a compressed fluid antisolvent. In this process a compressed gas, liquid, or supercritical fluid is used as an antisolvent. It causes a volume expansion of the liquid solution and thus lowers the solvent strength. As a result the precipitation of the dissolved solute occurs.1-3 For separation of a polymer solution, the addition of an antisolvent would lead to a liquid-liquid phase split into a polymer-rich phase and a solvent-rich phase. The polymer would eventually precipitate from the polymerrich phase by addition of more antisolvent.4-11 As indicated by McHugh and Guckes,4 the lower critical solution temperature curve could be shifted by more than 100 K after introduction of a high-pressure gas antisolvent into the polymer solution. With this situation, the precipitation with a compressed fluid antisolvent technique can be operated at much lower temperatures where there is less thermal degradation of polymer. Regarding precipitation of PS from solution, Seckner et al.5 observed that more than 99 wt % of PS could be recovered from a 5 wt % PS in toluene solution by adding ethane to the solution at a temperature of 343 K and pressures higher than 60 bar. Dixon et al.6 recovered PS by spraying 1 wt % PS in toluene solution into carbon dioxide through a 100 µm nozzle at temperatures of 273-313 K and pressures of 45-225 bar. The size and morphology for the polystyrene precipitated could be controlled by adjusting the carbon dioxide temperature and density. Dixon and Johnston7 extensively studied the morphology for the precipitated PS obtained by spraying solution containing 3.5-25.8 wt % of PS into compressed CO2 at the pressures higher than 45.6 bar. All these studies indicated that the operating pressure should be high when ethane and * To whom correspondence should be addressed Tel: 886-3-572-1189. Fax: 886-3-572-1684. E-mail: cstan@ che.nthu.edu.tw.
carbon dioxide are used as antisolvent. In a recent publication, Tan and Chang12 observed that HFC-134a could expand toluene at much lower pressures than CO2 and ethane. Under this situation, PS could be recovered from toluene solution at relatively lower pressures using HFC-134a as antisolvent. In a semibatch mode of operation, more than 65% of PS could be recovered from toluene solutions containing 5 and 10 wt % PS at a temperature of 293 K and a pressure of 5.17 bar. In their work, it was also found that the cloud-point pressure exhibited a linear dependence on temperature for a fixed PS concentration in toluene. The objective of this study is to evaluate the continuous precipitation process for the formation of polystyrene from toluene solutions containing 1.0-10 wt % of PS using HFC-134a as antisolvent. In this mode of operation, toluene solution and HFC-134a were fed continuously into a precipitator. The morphology for the precipitated PS was examined, and the properties, such as molecular weight and molecular weight distribution, were measured. The operation variables including temperature, pressure, and flow rates of toluene solution and HFC-134a were varied to examine their effects on the yield and morphology for the precipitated PS. Experimental Section The experimental apparatus used for the continuous GAS precipitation of PS from toluene solution is shown in Figure. 1. The precipitator was made of 316 stainless steel and had an inside diameter of 6.3 cm and a total volume of approximately 900 mL. It was equipped with two sapphire windows through which the precipitation of PS inside the precipitator could be observed. To trap the precipitated PS, a 0.2-µm filter paper (Gelman Sciences, FP200) sitting on a 100 mesh stainless steel wire was placed in the bottom of the precipitator. The precipitator was placed in a constant temperature oven where the temperature could be maintained to within 0.2 K in the range of 253-333 K. During the operation, HFC-134a (Daikin Industries Ltd., 99.95% purity) was compressed by a minipump (Milton Roy, NSI-33R) and flowed to the top of the precipitator. The pressure of the HFC-134a stream was
10.1021/ie9902277 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/03/1999
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3899
controlled by a back pressure regulator (Tescom, 261725-26-043). The flow rate of HFC-134a was determined by a mass flow meter (Teledyne HastingsRaydist, HFM-201) located downstream of the precipitator. A total of 50 g of toluene solution containing 1.010 wt % PS was charged initially to a 130-mL sight glass (Jerguson Gauge, 19-R-32) from which the amount of solution that flowed into the precipitator could be read. The PS-toluene solution was prepared by dissolving a known amount of PS chips (Scientific Polymer Co., Mw ) 2.05 × 105, Mw/Mn ) 3.48) in HPLC-grade toluene (Fisher Chemical, 99.9% purity). The compressed nitrogen was used to pressurize the toluene solution. When the temperature and pressure inside the precipitator reached the desired values, the PS-toluene solution was allowed to enter the precipitator through a nozzle (Spraying Systems, TN-SS40) with a diameter of 0.4 mm. The flow rate of the toluene solution was adjusted with a metering valve (Whitey, SS-21RS2). The solution drained from the precipitator was collected in a cold trap, where the temperature was maintained at about 263 K. During the spray of the toluene solution, the pressure was observed to vary within (0.2 bar. After stopping the spray of the toluene solution, HFC134a continued to flow through the precipitator for 2 h to remove toluene adhering to the polystyrene trapped on the filter paper. After this drying process, the polystyrene on the filter paper was collected and a known amount of toluene was used to flush the precipitator in order to collect the residual polystyrene deposited on the wall of the precipitator. Polystyrene in the drainage solution and in the flushing toluene solution was precipitated by adding three times the volume of methanol to the solutions and was heated in a vacuum at 323 K for 4 h before analysis. The collected PS from the filter paper and from the drainage and flushing solutions was weighed to check the overall mass balance. The molecular weights and the glass transition temperatures of the polystyrene collected were determined by gel permeation chromatography (Shimazu, LC-9A) using THF as mobile phase and a differential scanning calorimeter (Seiko, SSC-5200), respectively. Scanning electron microscopy (Hitachi, S-2300) was used to examine the morphology for the collected polystyrene. Results and Discussion For each continuous precipitation experiment, the overall mass of polystyrene collected from the filter paper, the drainage solution, and the flushing solution was observed to be sufficiently close to the amount charged to the precipitator. The difference was always less than 5.0%. The reproducibility test was also performed at various temperatures, pressures, and flow rates of HFC-134a and toluene solution. The difference in the amount of polystyrene collected from the precipitator was found to be always less than 4.0%. Effects of Pressure and Temperature. As pointed out by Tan and Chang,12 the precipitation of PS from toluene solution using HFC-134a as antisolvent could only occur at pressures higher than the cloud-point pressure in a semibatch mode of operation. The pressures higher than the cloud-point pressures were therefore chosen to study the effect of pressure on the amount of PS precipitated in this continuous mode of operation. The cloud-point pressure, defined as the pressure at which two phase splits occur, is 5.52 bar for the toluene
Table 1. Yields Obtained by Spraying a 5.0 wt % PS Solution at Different Temperatures and Pressures for a HFC-134A Flow Rate of 2000 mL/min and a Solution Flow Rate of 60 mL/min P (bar) 5.79 6.00 6.48 7.10 7.59 7.79 8.97 3.79 4.41 4.83 5.79 7.79 8.97 3.45 3.79 4.41 5.79 7.79 8.97 a
density of HFC-134a (g/cm3) T ) 303 K, Pcpa ) 5.52 bar 0.026 0.028 0.031 0.033 0.036 0.037 (vapor) 1.188 (liquid) 1.189 T ) 293 K, Pcp ) 4.41 bar 0.018 0.021 0.023 0.028 (vapor) 1.226 (liquid) 1.231 1.231 T ) 283 K, Pcp ) 3.38 bar 0.017 0.018 0.020 (vapor) 1.262 (liquid) 1.262 1.263 1.264
yield (%)
15.9 18.4 61.0 92.4 96.0
38.8 94.4 96.8 97.6
36.7 98.0 99.2 99.2 99.4
Pcp is the cloud-point pressure.
solution of 5.0 wt % PS and the temperature of 303 K.12 But no precipitation of PS was observed at the pressures of 5.79 and 6.0 bar. When the pressure was raised to 6.48 bar, precipitation occurred, but the yield was only 15.9%, indicated in Table 1. The yield is defined as the fraction of the dissolved PS in toluene solution that precipitated. The yield increased with further increase in pressure; however, the yields at the pressures below 7.6 bar were lower than 74.5% obtained in a semibatch operation.12 In these runs, only the vapor phase of HFC134a was present in the precipitator (the density of HFC-134a is shown in Table 1) and a significant rise of temperature as high as 7 K was observed. Because of the temperature rise, a higher pressure is required to expand the polymer solution due to the fact that the cloud-point pressure generally increases with temperature. When the pressures were raised to 7.79 bar, at which the vapor and liquid phases of HFC-134a coexisted, and to 8.97 bar, where only liquid phase of HFC-134a existed, Table 1 shows that more than 90% of the dissolved PS could be precipitated. In these two runs, no temperature rise was observed during the spray of the toluene solution. These results indicated that the presence of a liquid phase of HFC-134a is necessary in order to precipitate more PS from toluene solution. Though the yields for these two runs were almost the same, the morphology for the precipitated PS was quite different, as illustrated in Figure 2. For the run with the pressure at 7.79 bar, the toluene solution jet could breakup into droplets when it traveled through gaseous HFC-134a. These droplets then fell into liquid HFC134a where they were expanded to cause precipitation of PS. For this situation PS microparticles with the size ranging from 1 to 6 µm were formed. This result is similar to the cases for the formation of polystyrene and poly(L-lactide) microparticles by spraying dilute polymer solutions into compressed carbon dioxide6,7,11 and is
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 Table 2. Yields Obtained by Spraying a 5.0 wt % PS Toluene Solution at Different HFC-134A and Solution Flow Rates for a Temperature of 293 K and a Pressure of 5.79 Bar
Figure 1. Schematic diagram of the experimental apparatus for the continuous compressed fluid antisolvent precipitation process.
Figure 2. SEM of the precipitated polystyrene by spraying a 5 wt % PS solution at 303 K, with a solution flow rate of 60 mL/min and an HFC-134a flow rate of 2000 mL/min for pressures of (a) 7.79 bar and (b) 8.97 bar.
consistent with the postulation proposed by Bodmeier at al.11 that the formation of polymeric microparticles requires the atomization of the polymer solution into droplets and the hardening of the droplets into microparticles. As for the run operated at the pressure of 8.97
solution flow rate (mL/min)
HFC-134a flow rate (mL/min)
yield (%)
60 60 60 30
2000 3500 5000 5000
94.4 99.6 99.6 98.8
90
5000
98.0
morphology microparticles, 3-6 µm microparticles, 1-6 µm microparticles 0.5-3 µm fused and coalesced microparticles, fibrils microparticles, 0.5-3 µm
bar, the toluene solution jet mixed with liquid HFC134a right after it left the tip of the nozzle, since the precipitator was full of liquid HFC-134a. For this situation the precipitation of PS occurred immediately before the individual droplets were formed. As a result, irregularly shaped chunks were obtained. The same phenomenon was observed when the temperatures were initially fixed at 283 and 293 K. A temperature rise was observed when only gaseous HFC134a was present in the precipitator. Under this condition the yield was small or even not observed, as shown in Table 1. It was also found that more than 90% of PS could be precipitated when liquid HFC-134a was present in the precipitator. In addition, microparticles of PS were formed only at the pressures where both gas and liquid HFC-134a were present. Table 1 shows that the pressures required to precipitate more PS from toluene solution were lower at lower temperature operations. This is not a surprising result, since the cloud-point pressure increases with an increase in temperature. Effect of HFC-134a Flow Rate. When the temperature, pressure, PS concentration, and toluene solution flow rate were at 293 K, 5.79 bar, 5 wt %, and 60 mL/ min, respectively, Table 2 indicates that almost all the dissolved PS in toluene solution could be precipitated for the flow rates of HFC-134a higher than 2000 mL/ min. At this operating temperature and pressure, both liquid and gas phases of HFC-134a were present. In these runs the liquid level was maintained at about halfheight in the precipitator. Figure 3 shows that microparticles of PS were formed, regardless of the HFC-134a flow rates, but the size of the microparticles became smaller when the HFC-134a flow rate increased. This is probably due to the breakup of the solution jet being more intensive and the mass transfer resistance between finer droplets and liquid HFC-134a being reduced when the HFC-134a flow rate was increased. Effect of Toluene Solution Flow Rate. Table 2 shows that when the temperature was 293 K, the pressure 5.79 bar, and the HFC-134a flow rate 5000 mL/ min, the yield did not vary significantly at different toluene solution flow rates. When the flow rate was 30 mL/min, it was visualized that the solution jet could not have a complete breakup. Under this condition, fused and coalesced microparticles and small fibrils for the precipitated PS were formed. When the flow rates were increased to 60 mL/min, the toluene solution leaving the tip of the nozzle was completely atomized. As a result, microparticles of PS were obtained. When the flow rate was increased to 90 mL/min, the amount and morphology for the precipitated PS were not distinguished as compared to the run at 60 mL/min. The sizes of the microparticles were both in the range of 0.5-3.0 µm. Effect of PS Concentration. At a temperature of 293 K and a pressure of 5.79 bar, the yield was found
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3901 Table 3. Effect of PS Concentration on Yield and Morphology at a Temperature of 293 K, a Solution Flow Rate of 60 mL/min, and a HFC-134A Flow Rate of 5000 mL/min PS concentration (%)
Figure 3. SEM of the precipitated polystyrene by spraying a 5 wt % PS solution at 293 K and 5.79 bar, with a solution flow rate of 60 mL/min for HFC-134a flow rates of (a) 2000, (b) 3500, and (c) 5000 mL/min.
to increase with increasing PS concentration in the range of 1.0-5.0 wt %, indicated in Table 3. This is because a higher PS concentration resulted in a higher supersaturation. When the PS concentration was larger than 5.0 wt %, almost all the dissolved PS was precipitated. Figure 4 illustrates the morphologies for the precipitated PS for different PS concentrations. It can be seen that microparticles with similar size were
yield (%)
morphology
1.0 3.0 5.0 8.0 10.0
P ) 5.79 bar 88.0 microparticles, 0.5-3 µm 94.7 microparticles, 0.5-2 µm 99.6 microparticles, 1-6 µm 99.2 fibrils 99.7 fibrils
1.0 3.0 5.0 8.0 10.0
P ) 7.79 bar 96.1 irregular agglomerates 97.4 irregular agglomerates 96.8 chunks 95.0 chunks 98.3 chunks
formed when the PS concentrations were less than 5.0 wt %. But when the PS concentration was raised to 8.0 wt %, microparticles were no longer obtained. The same results were also observed for spraying solution containing more than 8.0 wt % of PS in compressed CO2.7 This may be due to the stabilization of the liquid jet by viscous forces and a rapid loss of toluene from the surface of the jet.7 When the temperature was still fixed at 293 K but the pressure was raised to 7.79 bar, conditions under which the precipitator was full of liquid HFC-134a, Table 3 shows that the yield at any PS concentration could still exceed 95%, but microparticles were not obtained. This might be due to failure to generate of fine droplets of the polymer solution in the precipitator. The molecular weight and the molecular weight distribution (Mw/Mn) of the precipitated PS measured by a gel permeation chromatography at different pressures for a temperature of 303 K, a PS concentration of 5 wt %, a solution flow rate of 60 mL/min, and a HFC134a flow rate of 5000 mL/min were found to be close to those of the original PS chips. The differences in molecular weight were within 3-9%. In addition to molecular weight, the glass transition temperature (Tg) of the precipitated PS was also measured by a differential scanning calorimeter in this study. It was observed that the Tg of the precipitated PS for the runs with more than 90% yield were sufficiently close to that of the original PS chips. Separate experiments in which the PS chips placed in a cell filled with HFC-134a at different temperatures and pressures were also performed. After depressurization, the morphology of the PS chips was found unchanged and the Tg was observed to be sufficiently close to that of the starting chips. These results showed a difference from that when compressed CO2 was used as antisolvent. A significant Tg depression of the PS caused by sorption of CO2 was reported.13,14 The present results thus indicated that the sorption of HFC-134a in PS did not cause a significant effect on the Tg of PS. One of the reasons may be that HFC-134a is not quite soluble in PS at the current operating temperature and pressure conditions. To ascertain this, more information on sorption of HFC134a in PS and Tg at elevated pressures is required. Conclusion From the experiments carried out by continuously spraying the PS-toluene solution into compressed HFC-
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
addition, the molecular weight and the molecular weight distribution of the precipitated PS were sufficiently close to those of the original PS chips. Because the pressures required to cause precipitation are less than 10 bar at room temperatures, the precipitation process using HFC-134a as antisolvent is more feasible than that using carbon dioxide or ethane, in which a higher pressure is needed. The morphology for the precipitated PS was found to depend on the operating conditions. At pressures where both gas and liquid HFC-134a were present in the precipitator, microparticles were formed for the PS concentration of less than 5.0 wt %. But when the precipitator contained liquid HFC-134a only, microparticles could not be obtained. Literature Cited
Figure 4. SEM of the precipitated polystyrene at 293 K and 5.79 bar, with a solution flow rate of 60 mL/min and an HFC-134a flow rate of 5000 mL/min for PS concentrations of (a) 3.0, (b) 5.0, and (c) 8.0 wt %.
134a, it was found that almost all the dissolved PS could be precipitated from the solution when the precipitator was filled with only liquid phase or both gas and liquid phases of HFC-134a. Compared to the results in a semibatch mode of operation,12 this continuous operation was proven to be more effective from the fact that the time required for precipitation was reduced significantly and a higher yield of PS could be obtained. In
(1) Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Klasutis, N. Gas Antisolvent Recrystallization: New Process to Recrystallize Compounds Insoluble in Supercritical Fluids. In Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; American Chemical Society: Washington, DC, 1989; Vol. 406, p 334. (2) Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Hillstrom, W. W. Gas Antisolvent Recrystallization of RDX: Formation of Ultra-fine Particles of a Difficult-to-Comminute Explosive. J. Supercritical Fluids 1992, 5, 130. (3) Berends, E. W.; Bruinsma, O. S. L.; de Graauw, J.; van Rosmalen, G. M. Crystallization of Phenanthrene from Toluene with Carbon Dioxide by the GAS Process. AIChE J. 1996, 42, 431. (4) McHugh, M. A.; Guckes, T. L. Separating Polymer Solutions with Supercritical Fluids. Macromolecules 1985, 18, 674. (5) Seckner A. J.; McClellan, A. K.; McHugh, M. A. HighPressure Solution Behavior of the Polystyrene-Toluene-Ethane System. AIChE J. 1988, 34, 9. (6) Dixon, D. J.; Johnston, K. P.; Bodmeier, R. A. Polymeric Materials Formed by Precipitation with a Compressed Fluid Antisolvent. AIChE J. 1993, 39, 127. (7) Dixon, D. J.; Johnston, K. P. Formation of Microporous Polymer Fibers and Oriented Fabrils by Precipitation with a Compressed Fluid Antisolvent. J. Applied Polym. Sci. 1993, 50, 1929. (8) Randolph, T. W.; Randolph, A. D.; Mebes, M.; Yeung, S. SubMicrometer-Sized Biodegradable Particles of Poly(L-Lactic Acid) via the Gas Antisolvent Spray Precipitation Process. Biotechnol. Prog. 1993, 9, 429. (9) Yeo, S. D.; Lim, G. B.; Debenedetti, P. G.; Bernstein, H. Supercritical Antisolvent Process for Substituted Para-Linked Aromatic Polyamides: Phase Equilibrium and Morphology Study. Macromolecules. 1993, 26, 6207. (10) Yeo, S. D.; Lim, G. B.; Debenedetti, P. G.; Bernstein, H. Formation of Microparticulate Protein Powders Using a Supercritical Fluid Antisolvent. Biotechnol. Bioeng. 1993, 41, 341. (11) Bodmeier, R.; Wang, H.; Dixon, D. J.; Mawson, S.; Johnston, K. P. Polymeric Microspheres Prepared by Spraying into Compressed Carbon Dioxide. Pharm. Res. 1995, 12, 1211. (12) Tan, C. S.; Chang, W. W. Precipitation of Polystyrene from Toluene with HFC-134a by the GAS Process. Ind. Eng. Chem. Res. 1998, 37, 1821. (13) Wissinger, R. G.; Paulaitis, M. E. Glass Transitions in Polymer/CO2 Mixtures at Elevated Pressures. J. Polym. Sci., Part B: Polym. Phys. 1991, 29, 631. (14) Condo, P. D.; Sanchez, I. C.; Panayiotou, C. G.; Johnston, K. P. Glass Transition Behavior Including Retrograde Vitrification of Polymers with Compressed Fluid Diluents. Macromolecues 1992, 25, 6119.
Received for review March 30, 1999 Revised manuscript received July 7, 1999 Accepted July 9, 1999 IE9902277
