Prediction of the spatial distribution of the average molecular weights inliving polymerisation reactors using CFD methods

Full Paper: Polymerisation reactor design and mode of operation are particu larly important for polymerisation processes where the lifetime of the grow ing species is long compared to the mean residence time of reactants in the reactor. For these reasons, reactor control strategies can be devised for living polymerisation processes in order to tailor the nature of the polyme r produced, for example, molecular weight distribution. The strategies can only be devised for ideal reaction and reactor behavior. Since neither are ideal, it is necessary to understand those factors which cause deviation fr om ideality. In this work the influence of the spatial distribution of temp erature and flow velocity is examined for an ideal classical living polymer isation reaction carried out in a tubular reactor under steady-state reacto r conditions using a computational fluid dynamics (CFD) code (PHOENICS) to explore the influence of the spatial distribution of species and temperatur e on the product with all other reactor features being ideal. Particular at tention is given to the problem of describing the spatial distribution of t he dispersity index (moments of the molecular weight distribution) of the p olymer in addition to flow velocity, temperature, component concentrations and kinematic viscosity. Comparisons ate made between he CFD predictions, s imulations based on the ideal behaviour of the process, and the experimenta l results from a laboratory-scale reactor. It is shown that the CFD predict ions give a more realistic description of the tubular reactor behaviour for the conditions employed. The limitations of the CFD approach and the addit ional problems still needing to be addressed, such as mixing quality, are d iscussed.