The conceptual design of polymerisation processes

• Main Author: McKenna, Timothy Frederick L
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1990
• Subjects: Chemical engineering|Plastics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9022716

A hierarchical procedure for the conceptual design of polymer production processes has been developed. The procedure is based on a set of heuristics and simple models to help choose among alternatives and to identify the most promising candidate designs. Economic estimates of raw material and equipment capital and operating costs are used to identify the most profitable designs and to discriminate between design alternatives when the heuristics are ambiguous. A sensitivity analysis is used to identify critical design parameters and to generate process alternatives. The procedure has been used as the basis for software for the design of processes for the production of chain growth homopolymers and copolymers in solution. Kinetic models for homopolymerisation and copolymerisation are used to estimate reactor volumes and the leading moments of the molecular weight distribution for free radical systems. A simplified rate expression for Ziegler-type catalysts was used to find reactor volumes otherwise. The procedure was found to be useful in the design of processes for several commercially important polymers; the results are similar to existing industrial designs for polystyrene, poly(methyl methacrylate), polyethylene and poly(styrene co-acrylonitrile). In most cases, it is found that the optimum process flow represents a trade-off between expensive separations at low conversions and expensive reactors at high conversions. In the particular case of polyethylene, where there is a vapour recycle and purge stream, the cost of raw materials and vapour recycle were also important in determining the process optimum. Regions of steady state reactor multiplicity are identified and the economic implications discussed. For the cases of poly(methyl methacrylate) and poly(vinyl acetate) it is found that using a plug flow reactor in the finishing stages of polymerisation was the most economical means to circumvent regions where multiple steady states are predicted. However, for copolymers, the restrictions placed on reactor separation prevented this solution. Finally, a model for the design of a wiped film evaporator for the devolatilisation of polymer melts is presented.