Ind. Eng. Chem. Res. 2010, 49, 1997–2002
1997
Experimental Study of the Effect of Reflux Rate during Suspension Polymerization on Particle Properties of PVC Resin Nasrin Etesami, Mohsen Nasr Esfahany,* and Roohollah Bagheri Department of Chemical Engineering, Isfahan UniVersity of Technology, Isfahan 84156-83111, Iran
The particle morphological properties of poly(vinyl chloride) produced by suspension polymerization in a pilot-scale reactor under different reflux rates have been investigated. It was found that the monomer conversion decreased with increasing reflux rate. It was also observed that the cold plasticizer absorption (CPA) increased with reflux rate, whereas the bulk density (BD) and K value of the resin decreased. SEM micrographs showed that PVC resin with a rougher particle surface, more separate aggregates, and smaller primary particles was prepared at higher reflux rates. The particle size distribution determined by laser size analysis showed that the standard deviation of the particle size distribution and the mean particle size did not change significantly with reflux rate. Introduction Poly(vinyl chloride) (PVC) is one of the world’s major polymers, and suspension polymerization is used for the commercial production of approximately 80% of the world’s PVC. Suspension polymerization is usually carried out in a batch reactor at constant temperature. Because PVC formation is an exothermic polymerization reaction, it is necessary to appropriately remove the heat produced by the reaction. For very large commercial polymerization reactors, the heattransfer area of the cooling jacket is insufficient for removing heat of reaction. One of the most effective ways to appropriately remove part of the heat of the reaction is by using a reflux condenser. By condensing vapor to liquid and removing the latent heat of vaporization from the reactor contents, reflux condensers function as an effective heat-removal apparatus. The monomer reflux flow affects PVC particle properties such as the particle size, porosity, and molecular weight of the resin that subsequently affect plasticization and end-use properties. Regardless of the importance of the problem, few works are available in the literature on the effects of reflux condenser operation on the properties of PVC particles produced by the suspension polymerization process. Koyanagi et al.1 showed that commencement of refluxing before 5% conversion is reached increases the fraction of coarse particles in the final product. Kobayashi et al.2 reported that monomer refluxing increases the porosity and decreases the bulk density of the final PVC particles. Cheng and Langsam3 investigated the effects of monomer refluxing on the resin properties, in a 14 L reactor equipped with an internal cooling coil as a condenser. They observed that the fraction of coarse particles and the porosity of the final product increased with increasing reflux rate whereas the bulk density remained unaffected. Zerfa and Brooks4 studied the effect of condenser operation on product properties during the suspension polymerization of vinyl chloride monomer in a 1 L vessel. They observed that increasing the reflux rate decreased the conversion. They also found that the porosity of the final product obtained in the presence of refluxing was slightly greater than that of PVC produced in the absence of refluxing. Visentini5 reported that increasing the reflux rate increases porosity. * To whom correspondence should be addressed. E-mail: mnasr@ cc.iut.ac.ir. Tel.: (+98-311) 3915604. Fax: (+98-311) 3912677.
In this work, the influence of the percentage of heat removed by the condenser (i.e., the reflux flow rate) on the suspension poly(vinyl chloride) (S-PVC) resin produced in a 15 L reactor equipped with a vertical shell-and-tube condenser was investigated. An industrial recipe used at Bandar Imam Petrochemical Company (BIPC, Tehran, Iran) was followed. The vinyl chloride monomer (VCM)/water mass ratio was 0.6, which is quite a bit larger than the values used in previous studies. Experimental Procedures Polymerization Procedure. VCM polymerization was carried out in a 15 L stainless steel jacketed reactor with two baffles of circular cross section and an agitator consisting of two eight-
Figure 1. Schematic diagram of the polymerization reactor: (1) entry valve for VCM to the condenser, (2) output valve for condensate, (3) discharging valve, (4) baffles, (5) jacket, (6) condenser, (7) motor, (8) stirrer.
10.1021/ie9008259 2010 American Chemical Society Published on Web 01/26/2010
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Table 1. Materials Used in the Suspension Polymerization of Vinyl Chloride name
role
vinyl chloride monomer (VCM)
trade name
manufacturer
monomer
Bandar Imam Petrochemical Co., Mahshahr, Iran hydroxypropyl primary Methocel Shin-Etsu methyl cellulose suspending 65SH-50 Chemical Co., (HPMC) agent Tokyo, Japan hydroxypropyl primary Klucle J Hercules cellulose (HPC) suspending International Ltd., agent Huntington, WV sorbitan monolaurate secondary Span 20 Beckmann Chemikalien suspending KG (Becksurf 7125), agent Bassum, Germany dimyristyl initiator Perkadox 26 Akzo Nobel Co. peroxydicarbonate (Amersfoort, The (MYPC) Netherlands) Table 2. Suspension Polymerization Recipea ingredient
units
value
VCM/water (phase ratio, φ) (3.5 wt % HPMC in water)/water (3.5 wt % HPC in water)/water Span20/distilled water MYPC/VCM
(g/g) (g/g) (g/g) (g/g) (g/g)
0.6 0.015 0.0143 0.00073 0.00106
a Total initial volume of liquid in the reactor was 50% of the total available volume of the reactor.
flat-bladed turbines, equipped with a vertical shell-and-tube heat exchanger as a condenser. The condenser contained 19 tubes with 12-mm inner diameter and 40 cm length. VCM flowed in tubes, and water passed countercurrently from the shell side. The reflux rate was adjusted by controlling the temperature and flow rate of water flowing in the shell. Figure 1 shows a schematic of the polymerization reactor. The reflux rate was calculated by an energy balance on the isolated condenser m ˙ VCMλ ) m ˙ waterCp∆T
(1)
where m ˙ VCM and m ˙ water are the flow rates of condensed VCM and cold water, respectively. λ is the latent heat of vaporization of VCM at the reaction temperature (52 °C), which was calculated from the Watson equation6 to be 292.59 kJ/kg. Cp is the heat capacity of water, and ∆T is the difference between the inlet and outlet water temperatures. Materials (Table 1) were charged to the reactor according to Table 2, the contents of the reactor were heated to 52 ( 1 °C by flowing hot water in the jacket, and polymerization was started. The instant when the temperature of the reactor contents reached 52 °C was defined as the reaction start time. At the end of the polymeri-
Figure 2. Variation of monomer conversion in VCM polymerization with reflux rate for two series of experiments: (() series A, delayed reflux of 150 min; (∆) series B, reflux without delay.
zation, the reactor was cooled to about 20 °C, and the excess vinyl chloride was slowly vented through the extractor via a venting valve above the condenser. When the venting was completed, the reactor contents were heated to 60 °C to remove the residual VCM. The produced PVC resin was filtered, and the resulting wet cake was dried in a vacuum oven at 40 °C for 24 h. The dried PVC was weighed, and the final conversion was calculated by gravimetry. Characterization of the S-PVC Particles. The mean particle sizes and the particle size distribution (PSD) of the dried PVC samples were measured with Malvern particle size analyzer, model 2603LC. The degree of porosity of the samples was characterized by cold plasticizer absorption (CPA) according to the standard test method for plasticizer sorption of poly(vinyl chloride) resins under an applied centrifugal force (ASTM D3367-95). The bulk density (BD) was measured by the DIN 53466 standard method. Scanning electron microscopy (SEM; Philips Company, model XL30) was used to study the quality of the produced particles. The K value is a number calculated from dilute-solution viscosity measurements of a polymer, used to denote the degree of polymerization or molecular size. This parameter was calculated from relative viscosity measurements made with a Ubbelohde viscometer (ASTM D1243-95). Results and Discussion To investigate the effect of reflux rate on particle properties, two series of experiments were performed. The first set of experiments was carried out using a delayed reflux (Table 3, series A) in which reflux was started 150 min after the beginning of the polymerization reaction, when the conversion was about
Table 3. Two Series of Experiments Designed with Different Reflux Rates run
reflux rate ((6%, mL/min)
time delay in refluxing (min)
conversion after 200 min (Xfinal)
1 2 3 4 5 6
40 96 153 198 232 303
150 150 150 150 150 150
38 34.1 35.3 31.7 33.2 36.4
7 8 9 10
42 155 193 301
0 0 0 0
47.3 39 45.2 41.4
CPA (gDOP/100 gPVC)
BD (g/L)
K value
mean particle diameter (µm)
74.52 77.31 74.46 78.44 76.91 81.71
291.9 264.6 276.8 286.1 254.4
66.4 65.7 59.1 58.2 60.7
129.0 147.4 120.2 110.8 134.9
52.54 64.32 58.23 68.56
420.8 338.8 382.2 359.1
69.8 64.4 65.8 65.7
Series A
Series B
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010
1999
Figure 3. Particle size distribution of PVC produced at different reflux rates (series A): (a) run 1, (b) run 2, (c) run 3, (d) run 4, (e) run 6.
20%. The second set was performed with refluxing from the beginning of polymerization (Table 3, series B). A separate study of the effect of a time delay in refluxing on PVC particle properties showed that, for a reflux delay of 150 min (∼20% conversion), the PSD, CPA, and BD were significantly changed.7 In this work, the reaction duration and other parameters were kept unchanged while different reflux rates were employed to produce PVC particles. Effect of Reflux Rate on Conversion. Figure 2 shows the change in monomer conversion as a function of reflux rate. It can be seen that conversion decreased with increasing reflux
rate. Zerfa and Brooks4 showed that the refluxed monomers coming from the condenser (“new” drops) get their initiator through the continuous phase (by diffusion from the old drops or continuous phase into the new drops or via latex particles) and start polymerization. Diffusion presents a strong barrier for reaction, and therefore, the conversion rate decreases. Figure 2 shows that the decreasing trend in final conversion after 200 min is more significant for series B than for series A. When refluxing is delayed to later stages of polymerization (series A), the monomer conversion increases, and the fraction of polymer phase in droplets/particles increases. In this time, monomer
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Figure 4. Particle size distribution of PVC produced at different reflux rates when refluxing starts at the beginning of the reaction (series B): (a) run 7, (b) run 8, (c) run 9, (d) run 10.
refluxing causes a decrease in the amount of monomer in droplets. The reaction proceeds mostly in the polymer-rich phase, where it is diffusion controlled, and thus, the reaction rate decreases, resulting in lower conversions at equal times compared to series B in which refluxing begins at the start of the polymerization reaction. Effect of Reflux Rate on Particle Size and Size Distribution. Figures 3 and 4 show particle size distributions for PVC grains produced at different reflux rates in series A and B, respectively. As can be seen from Figures 3 and 4, monomodal
distributions were obtained for series A, whereas bimodal distributions were produced for series B except for run 7. Because of the very low reflux rate used in run 7, the effect of refluxing is not pronounced, which can explain the different PSD obtained for run 7 compared to runs 8-10. It was found in another work that monomodal PSDs are obtained for time delays in refluxing of longer than 120 min from the start of the reaction, whereas bimodal distributions are obtained for shorter time delays in refluxing.7 Rate data from BIPC show that conversion reaches 0.15 after 120 min from the start of the
Figure 5. Sauter mean diameter (D32) and coefficient of variation of PVC particles produced at different reflux rates (series A).
Figure 6. CPAs of PVC resins produced at different reflux rates in series A and B.
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010 5
Figure 7. BDs of PVC resins produced at different reflux rates in series A and B.
reaction for the recipe used in this work. This conversion is called the critical conversion at which a continuous threedimensional primary particle network within the droplets has formed. Before this conversion, the droplets are sticky and very susceptible to coalescence, whereas after the primary particle network has formed, the droplets are very unlikely to coalesce. When refluxing starts at the beginning of the reaction (series B), the droplets returning from the condenser coalesce with the sticky old particles present in the reaction medium. Polymerization proceeds in both larger droplets produced by coalescence and smaller droplets present in the reaction medium without coalescence, resulting in a final product with a bimodal PSD. On the other hand, refluxing started 150 min after the start of the reaction, when the old droplets are no longer sticky, does not produce large droplets. Polymerization proceeds in the old droplets and in new droplets returning from the condenser and acquiring initiator by diffusion through the continuous phase, and the final product shows a monomodal PSD. More details can be found in ref 7. Figure 5 shows the particle size and coefficient of variation for samples prepared at different reflux rates in series A. It can be seen that the coefficient of variation (standard deviation/ mean particle size) does not change with reflux rate. Also, the Sauter mean diameter of particles shows no appreciable variations with reflux rate. When reflux starts 150 min after the start of the reaction (series A), the skeleton of the particles has formed, and coalescence is unlikely. The number of new droplets returning from the condenser and the collision frequency increase with the rate of refluxing, but coalescence does not occur. Therefore, larger droplets do not form, and the mean particle size remains unchanged. Effect of Reflux Rate on Porosity and Bulk Density. As shown in Figure 6, with increasing reflux rate in both series, the particle porosity (or CPA) increases. Similar results were
3
2001
obtained by Visentini and by Cheng and Langsam. Monomer refluxing is, in fact, another way of increasing the agitation that is felt inside the forming grains.5,7 In the presence of shear, the primary particles in droplets will not pack closely together and form a continuous open network, and the product has a high porosity. With an increase in the reflux rate, the applied shear to the primary particles increases, and thus, the porosity increases. Many researchers have indicated that the porosity of PVC grains decreases with conversion.8-10 With increasing reflux rate, conversion decreases (Figure 2), and thus, porosity increases. It is also seen from Figure 6 that the CPA for PVC grains produced with a time delay in reflux is greater than that produced without a delay in reflux. Commencement of refluxing at the later stages of polymerization (∼20% conversion), when the primary particle network is formed, extracts monomer from the network while leaving the pores unaffected. If reflux begins at the early stages of the reaction, when the primary particle network is not yet rigid, evaporation of monomer collapses the network and results in smaller porosity.4,7 As shown in Figure 7, the bulk density of resin decreases with increasing reflux rate because the porosity increases. Cheng and Langsam3 claimed that bulk density is not affected by monomer refluxing. It is expected that BD decreases with increasing CPA,11 as shown in Figure 7. SEM micrographs of sections of typical PVC grains prepared at two different reflux rates are shown in Figure 8. As seen in this figure, in run 4, the primary particles were smaller, and the space between them was higher, whereas in run 1, the primary particles were larger and more fused. With increasing reflux rate, the conversion decreases (Figure 2), and the primary particles in the PVC grains cannot grow enough. Therefore, smaller primary particles in separate aggregates are produced. Figure 8b shows a micrograph of particles produced at a reflux rate of 40 mL/min. It can be seen that the primary particles are larger and more fused, because of the higher conversion resulting in a lower porosity. Effect of Reflux Rate on K Value. The K value was found to decrease with increasing reflux rate, as shown in Figure 9. Several investigations have shown that molecular weight decreases slightly with initiator concentration.12,13 In the presence of refluxing, some monomer evaporates from the droplets, causing an increase in the initiator concentration and, therefore, a decrease in the molecular weight. In the presence of a high reflux rate, especially when reflux is delayed to later stages of polymerization when the fraction of polymer in the droplets becomes higher, the probability of chain-transfer reactions increases, and as a result, the K value decreases.
Figure 8. SEM micrographs of sectioned PVC grains produced at different reflux rates: (a) 198 mL/min (run 4), (b) 40 mL/min (run 1).
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Owing to the evaporation of VCM from the droplets/particles, the particles shrink and produce rough a surface, and this effect is more significant with increasing reflux rate (Figure 10c). Conclusions
Figure 9. K value of PVC resins produced at different reflux rates in series A and B.
The effects of reflux rate on the particle properties of S-PVC resin were investigated. The following results were observed: The conversion and bulk density decrease with increasing reflux rate, whereas the porosity increases. The effects become more significant with delayed reflux. The Sauter mean diameter and coefficient of variation in the particle size distribution do not change significantly with reflux rate, for a delayed reflux of 150 min. The K value of the resin decreases with increasing reflux rate. This is due to the increase in initiator concentration and the dominance of chain transfer to monomer. With increasing reflux rate, the surface of the particles becomes rougher. Acknowledgment The financial support of National Petrochemical Company, Research and Technology, through Contract 84269 is appreciated. Literature Cited
Figure 10. SEM micrographs of grains prepared by (a) no reflux, (b) refluxing at 140 mL/min (run 1), and (c) refluxing at 198 mL/min (run 2).
Comparing the K values of PVC resin samples obtained with and without delayed reflux in Figure 9, it can be seen that the K values of PVC samples produced with a time delay in reflux were lower because of the higher conversion and fraction of the polymer-rich phase in the droplets. Effect of Reflux Rate on Particle Morphology. Typical SEM micrographs for the whole grain at different reflux rates are shown in Figure 10. It can be seen that the reflux process causes the particle surface to become rough (Figure 10b,c).
(1) Koyanagi, Sh.; Tajima, Sh.; Kurimoto, K.; Kiri, Y. Method for suspension polymerizing vinyl chloride. U.S. Patent 4,136,242, Jan 23, 1979. (2) Kobayashi, T.; Tomishima, Y.; Najia, Y. Process for suspension polymerization of vinyl chloride in a reactor equipped with a efflux condenser and modified brumaging impeller. U.S. Patent 4,849,482, Jul 18, 1959. (3) Cheng, J. T.; Langsam, M. Particle structure of PVC Based on Cellulosic Suspension System III. Effect of Monomer Refluxing. J. Appl. Polym. Sci. 1985, 30, 1365–1378. (4) Zerfa, M.; Brooks, B. W. Influence of condenser operation in vinyl chloride suspension polymerization reaction, an exprimental study. Chem. Eng. Sci. 1997, 52 (14), 2421–2429. (5) Visentini, A. Development of morphology in suspension polymerization of vinyl chloride monomer. Plast. Rubber Compos. 1999, 28 (4), 142–144. (6) Yaws, C. L. Thermodynamic and Physical Property Data; Gulf Publishing Co.: Houston, TX, 1992. (7) Etesami, N.; Nasr Esfahany, M. Investigation of the Effect of Delayed Reflux on PVC Grain Properties Produced by Suspension Polymerization. J. Appl. Polym. Sci., accepted for publication. (8) Cebollada, A. F.; Schmidt, M. J.; Farber, J. N.; Capiti, N. J.; Valles, E. M. Suspension polymerization of vinyl chloride. I. Influence of viscosity of suspension Medium on Resin properties. J. Appl. Polym. Sci. 1989, 37, 145–166. (9) Alexopoulos, A. H.; Kiparissides, C. On the prediction of internal particle morphology in suspension polymerization of vinyl chloride. Part I: The effect of primary particle size distribution. Chem. Eng. Sci. 2007, 62 (15), 3970–3983. (10) Bao, Y. Z.; Brooks, B. W. Influences of some polymerization condition particle properties of suspension poly(vinyl chloride) resin. J. Appl. Polym. Sci. 2002, 85, 1544. (11) Bao, Y. Z.; Liao, J. G.; Huang, Z. M.; Wenr, Z. X. Influence s of Individual and Composed Poly(vinyl alcohol) Suspending Agents on Particle Morphology of Suspension Poly(vinyl chloride) Resin. J. Appl. Polym. Sci. 2004, 90, 3848–3855. (12) Xie, T. Y.; Hamielec, A. E.; Wood, P. E.; Woods, D. R. Experimental investigation of vinyl chloride polymerization at high conversion: Molecular-weight development. Polymer 1991, 32 (6), 1098–1111. (13) Kiparissides, C.; Daskalakis, G.; Achilias, D. S.; Sidiropulou, E. Dynamic Simulation of Industrial Poly(vinyl chloride) Batch Suspension Polymerization Reactors. Ind. Eng. Chem. Res. 1997, 36, 1253–1267.
ReceiVed for reView May 20, 2009 ReVised manuscript receiVed January 9, 2010 Accepted January 14, 2010 IE9008259
