Continuous blending models for free radical polymerization systems

Abstract

The dynamics of change of molecular weight distribution in free radical polymerization systems was modelled using a continums blmding concept which recognizes that the changes of molecular weight distribution are caused by continuous blending of increments af new polymer formed by the reaction into the already existing polymer, continuous blending of polymer at various parts of reactor by diffusion and convection, and continuous blending of polymer from various parts d reactor into the final product, The models have been applied to the analysis of both lumped and distributed-parameter systems using cither empirical or mechanistic relations for molecular weight of new polymer.he designer of a chemical reactor for polymeriz-T ation must be able to predict the molecular weight distribution ( M W D ) of the polymer product in addition to solving the problems associated with heat and mass transfer, reaction kinetics, fluid flow, etc. Numerous papers have been published in literature on the methods of calculating the M W D of polymer produced in polymerization systems. Amundson and LUUS"' presented an extensive review of these methods.Tn attempting to apply the methods to the design of polymerization reactor systems, one discovers two important limitations. The first is that most of the methods are based on the polymerization mechanisms which are taken to be known. Frequently, however, the polymerization mechanisms are not known clearly even after the reaction has been used for a long time in commercial production a s in the case of styrene polymerization"', In the initial engineering stages of reactor design under such situations, i t is often necessary to use empirical correlations of experimental measurements in the design calculations. The second limitation to existing methods for calculating M W D is that they are restricted to homogeneous (wellmixed) lumped-parameter systems without the distributed gradients of temperature and composition of the reactant. In the actual operation of polymerization systems, however, such homogeneity rarely exists. Whenever the homogeneity can not be assumed, the effect of inhomogeneity needs to be accounted for in the prediction of MWI) of the polymer product. Thus, the applicabilities of the published methods of calculating the MWD are limited to homogeneous polymerization systems for which the polymerization mechanisms are known.In the present paper, we discuss the continuous blending model for describing the dynamics of change of M W D in free radical polymerization systems. 1Prfaent address : Allied Chemlcal Corporation. Morriatown. N.J. 07960. U.S.A. 011 a model6 la dynariiique du changeiiient de distribution du poids molhdaire daris des svsttmes de polymerisation de radicaux libres, en eniployaiit un concept dc d h g e c m l h qui reconnaft que les changements de distribution du poi& molCculaire sont causes par: (a) lc mbltlnge continu, dam le polyndre dkjh existant, des particulcs d'un nouvedu polymhe form6 par la reaction; (b) melange continu du plymPre, par diffusion et co...

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