Heat Transfer in Gas‐Phase Olefin Polymerisation: A Study of Particle/Surface Interactions

Abstract

This work focuses on the heat exchange between a polymer particle and the reactor wall for low gas velocities. The role of different wall materials (or heat transfer conditions) is investigated with an eye to understanding how this influences the likelihood of build up of wall sheeting via melting of particles. The temperature profiles inside growing polymer particles in the vicinity of the reactor wall when it is clean (steel) or covered with a layer of non‐reactive polymer are simulated. As a comparison, we have also simulated glass reactor walls to represent bench‐scale polymerisation equipment. The results can help to understand how how we can compare results from bench‐scale experiments to industrial scale units.

Particle Responses to Flow Field Oscillations in Heterogeneous Polymerizations Performed in Tank Reactors

“…Based on the previous remarks, it can be said that the coupling between the polymerization kinetics and the flow field simulations is very weak in most heterogeneous polymerization problems. As a consequence, it seems that detailed CFD simulations can only be justified in these problems when: (i) the rates of heat and mass transfer between the suspended particles and the suspending fluid is sufficiently low, as in some gas-phase olefin polymerizations (although the particle trajectories can depend weakly on the suspending fluid properties in these cases, as discussed previously and studied by Eriksson and McKenna, 2004, and Eriksson et al, 2010); [13,17] (ii) the characteristic particle circulation time is sufficiently large, as when stagnant zones are formed inside the reaction vessel (although this can be regarded as a serious reactor design problem and should not constitute a serious problem in real commercial vessels); (iii) the reaction rates are sufficiently high and the characteristic reaction times is sufficiently small, as in reactive injection molding processes (although trajectories approach the adiabatic trajectories in this case and the coupling between the kinetics and the flow field simulations may remain weak); (iv) temperature and concentration gradients are sufficiently high inside the reaction vessel (although this can also be regarded as a serious reactor design problem and should not constitute a serious problem in real commercial vessels); (v) simulations are intended to characterize other important particle properties, such as the particle size distributions calculated with help of population balances. In short, it seems that the uniform flow field assumption (using averages to represent the reactor states) and the uniform particle states (assuming that temperatures and concentrations are uniform inside each particle) are appropriate to represent the kinetic behavior of the vast majority of the real heterogeneous polymerization processes.…”

“…Second, the resistance to heat and mass transfer between the medium and the particle may not be negligible. [13,17] In this case, the fluctuations in temperature and concentration of the medium can be mitigated by the limited exchange capacity of the system, causing the particle to experience fluctuations in temperature and concentration that are much lower than those observed in the suspending fluid. Besides, in these cases, the evolution of temperatures and concentrations inside the particles can be much less sensitive to the suspending fluid conditions.…”

“…(2010) [17] studied the development of temperature and concentration profiles in polymer particles in gas-phase olefin polymerizations, concluding that the formation of particle clusters can severely affect the performances of these polymerization systems and that the interaction with the reactor walls can affect the reactor performance. Pohn et al (2010Pohn et al ( , 2011Pohn et al ( , 2013aPohn et al ( , 2013b) studied the evolution of particle size distributions in emulsion polymerization reactors with help of CFD tools.…”

Uma revisão sobre polimerização de olefinas usando catalisadores Ziegler-Natta heterogêneos

“…Em trabalhos recentes, McKenna et al [31,48,49] e Eriksson et al [50,51] utilizaram fluido dinâmica computacional para avaliar o efeito dos mecanismos de transferência de massa e calor sobre o processo de polimerização, levando em consideração a influência do tamanho e conformação (relativo a posição e ao contato mútuo) das partículas de catalisador no meio reacional. De forma geral, os gradientes de calor e massa devem ser minimizados durante a operação normal do processo, o que resulta na necessidade de realizar um estágio de pré-polimerização no processo, para diluir a atividade do catalisador por volume de partícula.…”