Ind. Eng. Chem. Res. 1998, 37, 1821-1826
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Precipitation of Polystyrene from Toluene with HFC-134a by the GAS Process Chung-Sung Tan* and Wen-Wei Chang Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
Precipitation of polystyrene from toluene solution using HFC-134a as antisolvent was investigated. In this gas antisolvent (GAS) precipitation process, the pressurized HFC-134a was dissolved into a 20-g toluene solution containing 5 and 10 wt % polystyrene. The pressure required to expand toluene and cause precipitation of polystyrene by HFC-134a was observed to be much lower than that required by the frequently used antisolvent carbon dioxide. At a temperature of 293 K and a pressure of 5.17 bar, 67 and 74.5% yields were obtained for holding times of 50 and 120 min, respectively. The cloud-point pressure was found to be linearly dependent on temperature. Almost the same yield was obtained for any arbitrary combination of temperature and pressure having the same binary volume expansion. The volume expansion was found to be the major factor influencing yield. The operation with a higher pressure and a lower temperature would result in a higher yield. Introduction Polystyrene is a versatile thermoplastic material used in a wide range of applications. Because of the large quantity of polystyrene produced in the world, recycling of waste polystyrene is essential in order to make this industry sustainable. Although many attempts have been made to convert polystyrene into styrene monomer or fuel oils by thermal or catalytic degradation techniques (Cameron and MacCallum, 1967; Menzel et al., 1973; Audisio et al., 1990; Zhang et al., 1995), those techniques are very energy intensive since the reaction temperature is generally higher than 600 K. It is therefore worthwhile to pay attention to the dissolution/precipitation technique as concerned with energy consumed for reuse of waste polystyrene. The advantages of this technique include a massive reduction of the bulk volume, removal of some insoluble contaminants, and the quality of the precipitated polymer comparable with the virgin polymer (Kampouris et al., 1988). The GAS precipitation is a technique utilizing a highpressure gas as an antisolvent for recrystallization or precipitation of a solid solute dissolved in an organic solvent (Gallagher et al., 1989, 1992; Berends et al., 1996). In this process, the antisolvent gas causes a volume expansion of the liquid solvent and thus lowers the solvent strength. As a result, precipitation of the dissolved solutes occurs. For separating polymer solutions, 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 (McHugh and Guckes, 1985; Seckner et al., 1988; Dixon et al., 1993; Mawson et al., 1995). As indicated by McHugh and Guckes (1985), the lower critical solution temperature curve could be shifted by more than 100 °C to lower * To whom correspondence should be addressed. Telephone: 886-3-572-1189. Fax: 886-3-572-1684. E-mail: cstan@ che.nthu.edu.tw.
temperatures by introducing a high-pressure gas antisolvent into the polymer solution. Under this situation, the GAS technique offers an advantage over the conventional steam stripping technique due to the fact that the separation can be operated at lower temperatures where the thermal degradation of the polymer is less. Seckner et al. (1988) observed that more than 99 wt % polystyrene could be recovered from a 5 wt % polystyrene in toluene solution by adding ethane to the solution at a temperature of 343 K and pressures higher than 60 bar. Dixon et al. (1993) recovered polystyrene by spraying a 1 wt % polystyrene 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 of the precipitated polystyrene could be controlled by adjusting the carbon dioxide temperature and density. In addition to polystyrene, some other polymers could also be precipitated from organic solutions by the GAS technique using CO2 as antisolvent as well (Yeo et al., 1993a,b; Randolph et al., 1993). In all these studies, the operating pressures, however, are relatively high. Gallagher et al. (1989) employed CO2, chlorodifluoromethane (CFC-22), and dichlorodifluoromethane (CFC12) as antisolvents to recrystallize nitroquanidine from some organic solutions. They observed that the desired nitroquanidine particles could be obtained at much lower pressures when CFCs were used. This is because the critical pressures of CFCs are lower than those of CO2 and CFCs could expand organic solvent at lower pressures. However, this kind of CFC is prohibited from further use due to its destruction of ozone in the stratosphere. Currently, HFC-134a (CF3CH2F) is an environmentally acceptable chemical which has replaced CFCs in a wide range of application. The objective of this work is therefore to investigate the possibility of recovering polystyrene from a toluene solution utilizing HFC-134a as the antisolvent. To apply the GAS precipitation technique, the gas should have the capability of expanding organic solvent. The toluene expansion by HFC134a at various temperatures and pressures was thus
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1822 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998
Figure 1. Experimental apparatus for volume expansion and GAS precipitation measurement: 1, HFC-134a cylinder; 2, regulator; 3, heat exchanger; 4, filter; 5, pump; 6, check valve; 7, on-off valve; 8, surge tank; 9, metering valve; 10, three-way valve; 11, Jurguson gauge; 12, cold trap; 13, constant temperature oven; T, thermocouple; P, pressure indicator.
measured first. Since the morphology of the recycling polystyrene may not significantly affect subsequent reuse processing, though it may be crucial to some polymers for certain applications, more attention is thus paid to obtain a high recovery yield of polystyrene from a toluene solution in this study. The properties of the precipitated polystyrene, such as molecular weight and dispersity, are always of interest and were measured in this study. The operation variables including temperature, pressure, and injection and holding time of HFC-134a were systematically studied to examine their effects on yield. Experimental Section The GAS precipitation of polystyrene was conducted in a batchwise operation. The experimental apparatus is illustrated in Figure 1. HFC-134a (Daikin Industries Ltd., 99.95% purity) was first compressed by a minipump (Milton Roy, NSI-33R) and was then stored in a surge tank which was placed in a constant-temperature oven whose temperature could be maintained to within 0.2 K in a range of 253-333 K. A total of 20 g of a 5 or 10 wt % polystyrene in toluene solution was charged initially to a 70-mL sight glass (Jerguson Gauge, 13-R20). This sight glass served as a precipitator. The polystyrene-toluene solution was prepared by dissolving a known amount of polystyrene chips (Scientific Polymer Co., Mw ) 1.85 × 105, Mw/Mn ) 2.31) in a HPLC-grade toluene (Fisher Chemical, 99.9% purity). To trap the precipitated polystyrene, 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. HFC-134a flowed into the precipitator through the bottom supply tube. The injection rate was adjusted by a metering valve. When the pressure reached the desired value, no more HFC-134a was fed. To allow polystyrene to settle, the solution was held at the same pressure for a certain period of time. These injection and holding periods were recorded. During the injection, the pressure at which two liquid-phase splits occurred, denoted as the cloud point, was also recorded. After the holding period, the solution and HFC-134a were drained from the precipitator. The amount of HFC-134a in the precipitator was determined by meas-
uring the volume of water displaced in a column which was filled with water saturated with HFC-134a initially. HFC-134a was then fed again from the top of the precipitator to remove toluene adhering to the polystyrene trapped on the filter paper. After washing, 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 flushing toluene solution and in the drainage solution was precipitated by adding 3 times the volume of methanol to the solutions, and the resulting solution was heated in a vacuum at 323 K for 4 h before analysis. The molecular weights and the glass transition temperatures of the collected polystyrene from the precipitator and the flushing and drainage solutions were determined by a gel permeation chromatograph (Shimazu, LC-9A) using THF as the mobile phase and a differential scanning calorimeter (Seiko, SSC-5200), respectively. A scanning electron microscope (Hitachi, S-2300) was used to examine the structure of the collected polystyrene. The volume expansion curves for toluene were also measured using the same apparatuses. It was done by charging HFC-134a slowly into the sight glass containing a known volume of toluene to the desired pressure. To ensure reaching equilibrium, the system was allowed to sit for a certain period of time until no change in temperature and pressure was observed. The expanded volume of toluene was then recorded. Results and Discussion To ensure the validity of the present apparatus, the volume expansion of toluene (∆V/V) by carbon dioxide at different pressures and at 298 K was measured first. The volume expansion is defined by
∆V V(T,P) ) -1 V Voriginal The expansion values obtained were in excellent agreement with those reported by Chang and Randolph (1990). The expansion behavior of the binary system HFC-134a-toluene is depicted in Figure 2. It can be seen that toluene could be expanded by HFC-134a at much lower pressures compared with carbon dioxide. For example, at 298 K, toluene can be expanded to 3 times its original volume by HFC-134a at about 5.5 bar and by carbon dioxide at about 60 bar. This indicates that HFC-134a may be a potential antisolvent candidate. One of the possible reasons for the lower pressure required for HFC-134a to expand toluene than for carbon dioxide is that the critical pressure of HFC-134a (40.6 bar) is lower than that of carbon dioxide (73.8 bar). The compositions in the liquid and gas phases were also measured using a dual recirculation technique. The Peng-Robinson equation of state and the two-parameter mixing rules were used to correlate the binary VLE data. More details on measurement and modeling can be found elsewhere (Lee and Tan, 1998). The solid curves in Figure 2 represent the predicted volume expansion with the interaction parameters δ ) 0.0138 and η ) -0.118. The average absolute deviation in volume expansion between the predicted and experimental was about 8.0%. For the GAS precipitation experiments, the overall mass of polystyrene collected from the filter paper, the
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1823
Figure 2. Volume expansion behavior for the HFC-134a-toluene system.
Figure 3. Cloud points observed visually for 5 and 10 wt % polystyrene solutions.
flush solution, and the drainage solution was observed to be sufficiently close to the amount initially charged in the precipitator. The difference was no more than 4.0%. The reproducibility test was also performed at various temperatures and pressures. The difference in the amount of polystyrene collected from the precipitator was found to be always less than 5.0%. Figure 3 depicts the cloud points observed visually for 5 and 10 wt % polystyrene in toluene solutions. While hystereses in the cloud-point pressures were observed for the polystyrene-toluene solution (Seckner et al., 1988), the cloud-point pressures shown in Figure 3 just represent the pressure at which two phase splits occurred determined in a way by a gradual increase in pressure at constant temperature and polystyrene composition. Figure 3 shows that the cloud-point pressure exhibited a linear dependence on temperature for a fixed polystyrene composition in toluene. It was also found that the volume expansions of toluene in the binary HFC-134a-toluene system at temperatures and pressures where two phase splits occur in the ternary system were almost identical and were about 1.1 and 0.7 for 5 and 10 wt % polystyrene solutions, respectively, as can be seen from Figures 2 and 3. Since precipitation of polystyrene can only occur at pressures higher than the cloud-point pressure, this observation means that, for a constant-temperature operation, the pressure could be chosen from the binary expansion behavior so as to ensure the volume expansion will be higher than the corresponding expansion observed for a fixed polystyrene composition. Figure 3 also indicates that a lower operating pressure is required to precipitate polystyrene from the polymer solution containing more polystyrene. In our extensive runs for a 10 wt % polystyrene in toluene solution, no polystyrene microspheres were observed at various combinations of temperatures, pressures, and injection time of HFC-134a presently
studied. Some typical resulting polystyrene morphologies are illustrated in Figure 4. When the polystyrene concentration was reduced to 5 wt %, polystyrene microspheres though agglomerated were observed at certain operating conditions, shown in Figure 5. The injection time seems to play an important role for the precipitated polystyrene morphology. Since the morphology of the recycling polystyrene may not significantly affect subsequent reuse processing, attention was thus paid to obtain a high recovery yield of polystyrene from toluene solution in the following. Table 1 shows the effect of pressure on yield when the temperature, injection time, and holding time were fixed at 293 K, 50 min, and 50 min, respectively, for a 10 wt % polystyrene in toluene solution. Though a 50min holding time is not long enough to reach equilibrium, it was used to observe the pressure effect based on the fact that the same conclusion could be drawn when the holding time was extended to 120 min from some additional runs and the time required to reach equilibrium for different sets of temperature and pressure was not the same. When the pressure was below the cloud-point pressure, no phase split occurred. As a result, polystyrene could not be recovered from toluene solution. The same phenomenon also happened to a 5 wt % polystyrene solution. When the pressure was beyond the cloud-point pressure, the yield was observed to increase with increasing pressure. This is due to a higher volume expansion at higher pressure, as shown in Figure 2. The molecular weights of the precipitated polystyrene at different pressures were all less than that of the original polystyrene chips; however, a narrower distribution of molecular weight of the precipitated polystyrene over the original could be obtained, indicated by smaller dispersity (Mw/Mn) in Table 1. Due to the fact that a higher pressure is more favorable to yield, the effect of temperature on yield was
1824 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998
Figure 4. SEM of the precipitated polystyrene from a 10 wt % polystyrene solution at 293 K, 5.17 bar, and holding time of 50 min for injection times of (a) 30, (b) 50, and (c) 90 min.
Figure 5. SEM of the precipitated polystyrene from a 5 wt % polystyrene solution at 293 K, 5.17 bar, and holding time of 50 min for injection times of (a) 30, (b) 50, and (c) 70 min.
therefore examined at the highest pressure in Table 1, i.e., at 5.17 bar. Table 2 shows that a lower temperature operation resulted in a higher yield for a 10 wt % polystyrene solution. Again, volume expansion seems to reasonably explain the results, since more volume can be expanded at lower temperatures for a fixed pressure operation, as shown in Figure 2. A lower molecular weight but with a narrower distribution of the precipitated polystyrene over the original was also observed for all different temperature operations. It is noted that the same effect of temperature on yield and molecular weight happened to a 5 wt % polystyrene solution.
Since volume expansion seems to be the dominant factor influencing yield, it is of interest to compare the yields obtained at different combinations of temperature and pressure which have the same binary volume expansion. Table 3 shows that almost the same yield could be obtained at different combinations of temperature and pressure having similar volume expansions for both 10 and 5 wt % polystyrene in toluene solutions. This observation means that the temperature and pressure in recycling of polystyrene from toluene solution could be chosen a priori from the binary volume expansion data for a desired yield. This will especially
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1825 Table 1. Effect of Pressure on Yield at a Temperature of 293 K, an Injection Time of 50 min, and a Holding Time of 50 mina P, bar
yield, %
∆V/V
37.5 51.5 67.0
0.36 0.42 0.73 1.05 2.95
3.66 3.79 4.00 4.41 5.17
Mw × 10-5
Mw/Mn
Conclusion 1.69 1.78 1.80
2.15 2.19 2.19
a The original polystyrene chips possess a M of 1.85 × 105 and w a Mw/Mn of 2.31.
Table 2. Effect of Temperature on Yield at a Pressure of 5.17 bar, an Injection Time of 50 min, and a Holding Time of 50 mina T, K
yield, %
∆V/V
Mw × 10-5
Mw/Mn
283 288 293 298
72.5 71.0 67.0 53.5
8.34 4.63 2.95 1.40
1.75 1.72 1.80 1.77
2.19 2.22 2.19 2.11
a The original polystyrene chips possess a M of 1.85 × 105 and w a Mw/Mn of 2.31.
Table 3. Yield Obtained at Similar Binary Volume Expansions for the Injection Time of 50 min and Holding Time of 50 mina ∆V/V
differential scanning calorimeter in this study. It was observed that Tg of all the precipitated polystyrenes was not changed significantly compared to the original.
yield, %
Mw × 10-5
T, K
P, bar
Mw/Mn
283 288 298
3.10 3.58 4.55
10 wt % Polystyrene Solution 0.70 31.5 1.75 0.71 31.5 1.82. 0.70 31.5 1.82
2.16 2.22 2.15
288 293 298
4.69 5.10 5.79
5 wt % Polystyrene Solution 2.69 67.0 1.78 2.70 65.0 1.78 2.78 66.0 1.85
1.83 1.97 2.31
a The original polystyrene chips possess a M of 1.85 × 105 and w a Mw/Mn of 2.31.
Table 4. Yields at Various Holding Times for 298 K, 5.17 bar, and Injection Time of 50 mina tholding, min
yield, %
Mw × 10-5
Mw/Mn
20 35 50 70 90 120 180 240
60.0 64.5 67.0 68.3 72.0 74.5 74.9 74.9
1.76 1.72 1.80 1.79 1.80 1.83 1.84 1.84
2.11 2.06 2.19 1.85 1.86 2.02 2.25 2.25
a The original polystyrene chips possess a M of 1.85 × 105 and w a Mw/Mn of 2.31.
be useful once economic analysis for different combinations of temperature and pressure is carried out. Table 4 indicates that at least 120 min was required to reach equilibrium at a temperature of 293 K, a pressure of 5.17 bar, and an injection time of about 50 min for a 10 wt % polystyrene solution. It is not surprising that the yield varied with time before equilibrium because it took time to precipitate polystyrene from the polymer-rich phase. The molecular weight of the precipitated polystyrene, however, was found to increase with holding time and approached the original when the holding time was sufficiently long. This observation provides a means to collect recycled polystyrene with a desired molecular weight. On the other hand, the reduction in molecular weight of the recycled polystyrene may hinder subsequent application after several recycles. The glass transition temperature (Tg) of the precipitated polystyrene was also measured by a
The environmentally acceptable HFC-134a has been shown to be an effective antisolvent to precipitate polystyrene from toluene solution. The pressure required to expand toluene by HFC-134a is much lower than that required by carbon dioxide, and the temperature required for precipitation of polystyrene can be at room temperature. These make the recycling process using HFC-134a as the antisolvent more attractive than that using carbon dioxide. With a proper combination of temperature, pressure, and holding time the recovery yield of polystyrene could be higher than 70%. Though the molecular weight of the precipitated polystyrene was always smaller than that of the original polystyrene chips, a narrower molecular weight distribution could be obtained. The volume expansion was observed to be the major factor affecting recovery yield. The operations at a lower temperature and a higher pressure would be favorable to obtain a higher yield. At different combinations of the antisolvent temperature and pressure which exhibit the same volume expansion in the binary system of HFC-134a and toluene, almost the same yield could be obtained. This observation provides a means to select the operating conditions from a point of view of operation cost. Though morphology is not the major concern in this study, relatively uniform microspheres of the precipitated polystyrene with agglomeration were observed under a certain injection time of HFC-134a for a 5 wt % polystyrene solution but not for a 10 wt % polystyrene solution under the operating conditions presently studied. This observation indicates that the morphology of the precipitated polystyrene may be in a controllable manner with temperature, pressure, injection time, and polystyrene concentration. A continuous operation by spraying 5 and 10 wt % polystyrene in toluene into HFC-134a through a nozzle is now under way in our laboratory to see the effects of the operating parameters on morphology of the precipitated polystyrene. Acknowledgment The financial support from the National Science Council of ROC, Grant No. NSC 86-2214-E-007-003, is gratefully acknowledged. Literature Cited Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Catalytic Degradation of Polymers: Part III-Degradation of Polystyrene. Polym. Degrad. Stab. 1990, 29, 191. 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. Cameron, G. G.; MacCallum, J. R. The Thermal Degradation of Polystyrene. J. Macromol. Sci., Rev. Macromol. Chem. 1967, C1 (2), 327. Chang, C. J.; Randolph, A. D. Solvent Expansion and Solute Solubility Predictions in Gas-Expanded Liquid. AIChE J. 1990, 36, 339. Dixon, D. J.; Johnston, K. P.; Bodmeier, R. A. Polymeric Materials Formed by Precipitation with a Compressed Fluid Antisolvent. AIChE J. 1993, 39, 127. Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Klasutis, N. Gas Antisolvent Recrystallization: New Process to Recrystallize
1826 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 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. Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Hillstrom, W. W. Gas Antisolvent Recrystallization of RDX: Formation of Ultrafine Particles of a Difficult-to-Comminute Explosive. J. Supercrit. Fluids 1992, 5, 130. Kampouris, E. M.; Papaspyrides, C. D.; Lekakou, C. N. A Model Process of the Solvent Recycling of Polystyrene. Polym. Eng. Sci. 1988, 28, 534. Lee, K. R.; Tan, C. S. Vapor-Liquid Equilibria for the 1,1,1,2Tetrafluoroethane + m-Cresol, and + p-Cresol and 1,1,1,2Tetrafluoroethane + m-Cresol + p-Cresol Systems. J. Chem. Eng. Data 1998 (to appear). Mawson, S.; Johnston, K. P.; Combes, J. R.; DeSimone, J. M. Formation of Poly(1,1,2,2-tetrahydroperfluoradecyl acrylate) Submicron Fibers and Particles from Supercritical Carbon Dioxide Solutions. Macromolecules 1995, 28, 3182. McHugh, M. A.; Guckes, T. L. Separating Polymer Solutions with Supercritical Fluids. Macromolecules 1985, 18, 674. Menzel, J.; Perkow, H.; Sinn, H. Recycling Plastics. Chem. Ind. 1973, 16 June, 570. 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. Seckner, A. J.; McClellan, A. K.; McHugh, M. A. High-Pressure Solution Behavior of the Polystyrene-Toluene-Ethane System. AIChE J. 1988, 34, 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 1993a, 26, 6207. Yeo, S. D.; Lim, G. B.; Debenedetti, P. G.; Bernstein, H. Formation of Microparticulate Protein Powders Using a Supercritical Fluid Antisolvent. Biotechnol. Bioeng. 1993b, 41, 341. Zhang, Z.; Hirose, T.; Nishio, S.; Morioka, Y.; Azuma, N.; Ueno, A.; Ohkita, H.; Okada, M. Chemical Recycling of Waste Polystyrene into Styrene over Solid Acids and Bases. Ind. Eng. Chem. Res. 1995, 34, 4514.
Received for review September 10, 1997 Revised manuscript received January 27, 1998 Accepted February 6, 1998 IE970630I
