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윤원중

경원대학교 화학공학과

A Two-Stage Continuous Polymerization Reactor Modeling

for Styrene Polymerization

Won Jung Yoon

Department of Chemical Engineering, Kyungwon University

Introduction

In our previous reports, we have been studied  filled tubular reactor with some axial dispersion effect, such as a near plug flow pattern, due to intensive radial mixing induced by static mixers [Yoon and Choi (1996)]. In-line mixing devices or static mixers are widely used in chemical industry. A static mixer is a specially designed geometric structure placed in a pipe. It has no moving parts or external power requirements. It mixes materials flowing through it solely by redirecting fluid flow using the geometry of the channels. Mixing elements are installed within the tube to promote radial flow and to alleviate the composition and temperature difference which would otherwise arise. Each static mixer element is a specially designed rigid structure that divides the incoming flow and recombines it in a geometric sequence so that intensive radial mixing is obtained. Mixing and contacting take place as the fluids are sheared and directed radially across the pipe or duck. The major objective of all element design is to eliminate radial temperature and/or composition gradients that would arise during the flow in an empty tube. In addition, the heat transfer through the tube walls is enhanced due to intensive radial mixing. A solution polymerization of styrene was investigated using a filled tubular reactor packed with static mixing elements. In order to characterize the liquid flow pattern in the filled tubular reactor, a residence time distribution (RTD) test was carried out [Yoon and Choi (1996)]. We reported Uz/Dax =70.4m-1 (or Peclet number=106), corresponding to approximately 35 ideal stirred tanks per meter. This RTD test indicated that the flow pattern in the filled tubular reactor was close to that of the plug flow but the axial mixing effect was not negligibly small.

In some industrial continuous styrene polymerization process, a train of reactors consisting of stirred tank reactors and linear flow reactors is often used (e.g. high impact polystyrene process). In this study, a two stage was proposed wherein a continuous stirred tank is used as a prepolymerization reactor for styrene polymerization in filled tubular reactor.

The purpose of this study is to investigate the feasibility of the two stage polymerization process. The polymerization reactor which used to only the filled tubular reactor must have a vessel to preheat and mix the reaction mixture. When the prepolymerizer can be replaced the vessel, the economical view of the industry have a good advantage. When the stirred tank reactor is used as a prepolymerizer, a feed stream to the filled tubular reactor is more viscous than the monomer/solvent mixture in our previous experiments. Therefore the main objective of this work is to investigate  how the performance of the filled tubular reactor will be affected by the feed of viscous prepolymer solution. Both start-up transient and steady state behaviors of the reactor was investigated through laboratory experimentation and model simulations.

Experiment

A bench scale continuous two stage polymerization reactor facility has been built for styrene polymerization experiments. A schematic diagram of the experimental reactor system is illustrated in Figure 1. The continuous reactor system is composed of two reactor units: a prepolymerization reactor and a filled tubular reactor. A jacketed glass reactor with a maximum working volume of 1 liters can be used either a continuous stirred tank prepolymerizer. The prepolymerizer is pumped to inlet of the main tubular reactor via a gear pump. The main reactor consists of two equal sized jacketed tubes made of stainless steel. The L/D ratio of each reactor segment is 28 and the tube diameter is 2.62cm(I.D). Each reactor segment is packed with 24 Sulzer SMX static mixers and the voidage of the packed tube is 0.91. Each static mixing element is turned around its main axis by 90 against its neighboring elements. The two reactor segments are connected by a short hollow connector unit(L=10.2) to which a ball valve has been attached for the sampling of small amounts of reactor fluids. There are three thermocouples(J-type) to measure the temperature at the reactor inlet, middle section and outlet. The 4 liter vented glass tanks are used as solvent and reaction mixture storage tanks. Predetermined amounts of bifunctional initiators, Lupersox 118 (initiator A) and Lupersol 256 (initiator B), are added to the measured quantity of purified styrene and solvent mixture to prepare the feed monomer solution. The feed monomer solution is always purged with nitrogen during the experiment. Ethylbenzene (solvent 1) is used as a solvent. Toluene (solvent 2) is used to flush the reactors feed lines and pumps after polymerization experiments are completed. The reactor feed is pumped to the filled tubular reactor via a gear pump. All the reactor feed lines are made of chemically inlet TEFLON, high density polyethylene, and stainless steel tubings. The reactor is first filled with ethylbenzene and heated to a desired reaction temperature. Then the feed monomer solution containing a mixture of initiators is pumped to the bottom of the reactor. The polymer samples is dissolved in toluene and precipitated by the adding excess methanol. This procedure is repeated several times to ensure that unreacted monomer is completely removed. The samples are dried overnight in vacuo at 40oC. Then, the monomer conversion is determined by gravimetric method. The polymer molecular weight has been determined by gel permeation chromatography using four Ultrastyragel columns (Waters; 104, 103, 500□, and linear)

Results & Discussion

The validity of the proposed reactor model is evaluated by comparing the simulation results with laboratory experimental data. In conducting the reaction experiments, the feed temperature was maintained at 24oC at which the polymerization of styrene was negligible. In theses experiments, the solvent (ethylbenzene) volume fraction (fs) was held constant at 0.3. Two experimental runs  have been carried out using a CSTR-Filled Tubular Reactor train.  The experimental data show the polymer weight fraction (Wp) and molecular weight averages (Mn and Mw) of the samples taken from  the exit of the reactor at transient and steady state.

The first experiment was carried out at different reactor jacket temperature where the CSTR temperature is 100oC, and the temperature of the first and second tubular reactor segments are both of 100oC with yAf =0.5 and □ =80 min. Figure 2 shows the polymer weight fractions and molecular weight profiles in both the CSTR and the filled tubular reactor. Also shown in Figure 2 (c),(d) are the experimental date  in which the prepolymerizer was not used. Here, ◆ and □ represent the data for the samples with CSTR and □ represents the data for the samples  without CSTR. The solid lines are model simulation results. It is first observed that although the feed polymer solution to the tubular reactor contains 20% polymer, the final polymer weight fraction from the tubular reactor is only about 38% higher than the feed     value (NOTE: The polymer weight fraction of 0.38 corresponds to 54.2% conversion of monomer.). Figure 2 (b), (d) shows the polymer molecular weight averages for the first experiment. It is also seen that the polydispersity is about 2.0. The polymer weight fraction and molecular weight profiles of the first experiment is also very similar to those of without CSTR in Figure 2 (b), (d). This indicates that the addition of a prepolymer has essentially no effect on the performance of the filled tubular reactor.

Figure 3 shows the experimental results of the second experiment where the CSTR temperature is 90oC, and the temperature of the first and second tubular reactor segments are 100oC and 110oC, respectively. The quantitative behavior of the reactor is similar to that of the first run. Both Figure 2 and Figure 3  indicate that proposed axial dispersion model prediction of monomer conversion and polymer molecular weight averages without any model parameters adjusted to fit the data.  As a  result, a filled tubular reactor as a polymerization reactor has no effect on the viscosity as a  reaction go on and for the two stage polymerization process, good agreements between the model predictions and experimental data is obtained. When the vessel to mix or preheat for tubular reactor system  is replaced by the prepolymerizer, it can bring some advantages in the economical point of the industry. For  example, the industrial product per hour can be increased by the same amount of investments.

Conclusions

A mathematical model of a two stage polymerization process has been developed and validated through laboratory experimentation. The kinetic parameters and transport parameters used in the model simulations were taken from the literature and no adjustment parameters were used to fit the model with experimental data. The overall quality of the model predictions were quite reasonable, indicating that the proposed model is adequate in describing the reactor behavior. It was found that the addition of a prepolymerization reactor has almost no effect on the performance of the filled tubular reactor. The experimental results suggest that the filled tubular reactor can be potentially a good continuous reactor for the polymerization of vinyl monomers. Therefore, the two stage polymerization using filled tubular reactor is potentially a very important new industrial polymerization technology.

Reference

1. Yoon, W. J. and Choi, K. Y., "Polymerization of Styrene in a Continuous Filled Tubular Reactor", Polymer, 36(1), 66, (1996)

2. Yoon, W. J. and Choi, K. Y., "Free Radical Polymerization of Styrene with a Binary Mixture of Symmetrical Bifunctinal Initiators", J. Appl. Polym. Sci., 46, 1353(1992).

3. Yoon, W. J. and Choi, K. Y., "Kinetics of Free Radical Styrene Polymerization with the Symmetrical Bifunctinal Initiator 2,5-dimethyl-2,5-bis (2-ethyl hexanoyl peroxy) hexane", Polymer, 33(21), 4582(1992).

4. Yoon, W. J. "Experimental Study and Model Simulation of a Two-Stage Continuous Polymerization Process for Polystyrene", Korean J. of Chem. Eng., 13(1), 88 (1996).