반회분식 폴리카보네이트 용융중합 공정에 대한

공정모델의 비교

김양수

인제대학교 화학공학과

Comparison of Process Models for the Semibatch

Polycarbonate Melt Polymerization Process

Yangsoo Kim

Dept. of Chemical Engineering, Inje University

INTRODUCTION

In reversible polycondensation process, volatile condensation by-products are removed from the reaction medium by means of applying vacuum to facilitate forward chain growth reaction and thus to obtain high molecular weight of polymer. As the reaction proceeds the viscosity of reaction medium increases significantly, making the removal of by-product difficult.

In the melt polycondensation of diphenyl carbonate (DPC) with bisphenol-A (BPA) to produce polycarbonate, volatile reaction by-products (phenol) form bubbles in the melt phase and pass through the melt phase to a reactor head space. As phenol is produced by the reaction, it diffuses to liquid-gas interface. The gas phase may consist of bubbles of phenol vapor and reactor head space. In the earlier stage of melt polycondensation of BPA and DPC, reaction mass resembles a boiling liquid and the reaction mixture can be well mixed. Then phenol produced by reaction is fully exposed to vapor phase in this stage of reaction. Therefore, it can be assumed that the whole liquid phase is at equilibrium with a vapor phase in the beginning of melt polycondensation reaction, in which the reaction process is kinetically controlled. Kim and Choi[1] presented a vapor-liquid equilibrium model to explain the experimental data for the multistage polycondensation of BPA and DPC to polycarbonate. However, if phenol gas bubbles are formed in the later stage of polycondensation, there will be finite interfacial mass transfer resistance, and the overall process becomes mass transfer controlled.

The mass transfer process is represented by the model using a volumetric mass transfer coefficient, which is the product (i.e., [eqn: k_L ``a`]) of the mass transfer coefficient, [eqn: k_L], and the interfacial area per unit volume, [eqn: a`]. In this study, a two phase mass transfer model using the volumetric mass transfer coefficient ([eqn: k_L ``a`]) has been applied to the multistage of polycondensation of BPA and DPC to produce polycarbonate. The value of [eqn: k_L ``a`] for each semibatch reaction stage is estimated based on the experimental data. Also model predictions for the multistage melt polycarbonate polymerization process predicted by both the vapor-liquid equilibrium model and the two phase mass transfer model are compared each other.

MODEL DEVELOPMENT

A mass transfer rate of the volatiles from the liquid phase to the vapor phase is described via an effective mass transfer coefficient ([eqn: k_L `]; cm/min) and the specific interfacial contact area ([eqn: a`]; cm2/cm3). The parameter [eqn: a`] can be interpreted as a characteristic of a given reactor geometry and the melt flow distribution pattern in the reactor. As the number of bubbles in the liquid phase increases, it will contribute to the increase of the specific interfacial contact area.

Several assumptions has been made in the development of a two phase mass transfer model: (1). Reaction (i.e., polymerization) occurs only in the liquid phase. (2). Mass transfer of the volatiles at the gas-liquid interface is based on the two film model[2]. (3). Only phenol and DPC vaporize during the reaction, however, BPA and polymer do not vaporize. (4). Phenol is completely distilled off and the partial amount of DPC evaporated is refluxed back to the liquid phase. The rectification efficiency factor ([eqn: phi`]) is introduced to quantify the partial loss of DPC. (5). The interfacial concentrations of the volatiles are determined by the Flory-Huggins equation[3]. (6). The ratio of the mass transfer coefficient of DPC to that of phenol is proportional to the ratio of the saturated vapor pressure of DPC ([eqn: P_{B_0}^o `]) to that of phenol ([eqn: P_P^o `]) at given temperature. Therefore, the volumetric mass transfer coefficient of DPC [[eqn: (k_L ``a)_{B_0}`]] is determined by that of phenol [[eqn: (k_L ``a)_P `]] given, i.e.,

[eqn: (k_L ``a)_{B_0} ``=`` (k_L ``a)_P cdot {P_{B_0}^o} over {P_{P}^o}`]

The polymer molecular weight averages are computed by using the molecular weight moment equations with the contribution of two functional end groups to the molecular weight determination of the three polymeric species ([eqn: A_n `], [eqn: B_n `] & [eqn: C_n `]).

RESULTS AND DISCUSSION

The two phase mass transfer model requires two unknown parameters [i.e., [eqn: phi `] and [eqn: (k_L a)_P `]] to be estimated. Reflux efficiency factor ([eqn: phi `]) for DPC was introduced to account for the imperfect distillation efficiency. The reflux efficiency factor ([eqn: phi `]) estimated by Kim and Choi[1] has been used in the mass transfer model simulations.

In this study, it was also assumed that the mass transfer coefficients for phenol in the stages 4 and 5 were same, that is, [eqn: (k_L a)_{P,4} = (k_L a)_{P,5}]. The estimated mass transfer coefficients for phenol in stage-3 and stage-4 are 10.0 min-1 and 2.5 min-1 for Run-I and 13.7 min-1 and 3.1 min-1 for Run-II. The reaction rate constant value based on the reaction condition for stage-3 of Run-I is 9.0 □/mol□min, and hence it shows that the polymerization is kinetically controlled up to stage-3 and after then it is mass transfer controlled.

Figure 1 shows that the two phase mass transfer model gives reasonable predictions of molecular weights over the entire stage and it is comparable to the vapor-liquid equilibrium model in the prediction of the semibatch melt polymerization process. Note that polymer molecular weight profiles predicted by two process models are almost superimposed up to stage-4, however, two profiles are deviated each other after then. In Figure 2, the agreement between the experimental data and the two phase model predictions for concentrations (i.e., wt%) of oligomer, DPC, BPA, and phenol consisted in the reaction product is very excellent over the whole multistage melt polymerization process.

The two phase mass transfer model has been presented in order to theoretically predict the semibatch polycarbonate melt polymerization process, which is operated in the mass transfer controlling state. There has been some discrepancies between two process models in polymer molecular weight predictions for the later stage of multistage melt polymerization process of polycarbonate (especially stage-5), in which it is believed that a mass transfer of phenol from the liquid phase to the vapor phase is controlling the whole polymerization process. It should be noted that the vapor-liquid equilibrium model does not explain the mass transfer resistance process occurring in highly viscous reaction medium. The main theoretical background of the two phase mass transfer model is that there is a mass transfer resistance at the gas-liquid interface. In the later stage of semibatch melt polymerization process producing high molecular weight of polycarbonate, a reaction medium is very viscous and the mass transfer resistance increases. Such a presence of mass transfer resistance is represented by the two phase mass transfer model.

REFERENCES

[1] Kim, Y. and Choi, K. Y., □Multistage Melt Polymerization of Bisphenol-A     and Diphenyl Carbonate to Polycarbonate.□, J. Appl. Polym. Sci., 49, 747      (1993).

[2] Sherwood, T. K., Pigford, R. L., and Wilke, C. R., □Mass Transfer□,           McGraw Hill. N.Y., 1975.

[3] Prausnitz, J. M., □Molecular Thermodynamics of Fluid-Phase Equilibria□,       Prentice Hall, Englewood Cliffs, N. J., 1969.

Figure 1. Comparison of the number average polymer molecular weight              profiles predicted by the two phase mass transfer model and the              vapor-liquid equilibrium model along with the experimental data.

Figure 2. Comparison of the reaction product composition profiles                     predicted by the two phase mass transfer model and the                     experimental data