Instrumentation, modelling and control for reactive extrusion

The topic of this thesis is the development of process control schemes suitable for reactive extrusion processes. The process that serves as a basis for this investigation is the free radical initiated degradation of polypropylene. The principal objectives were: (i) the development of an improved rheometer designed to measure the rheological properties of reactive extrusion products on-line and to provide feedback signals for process control schemes; (ii) the development of a process control scheme for the reactive extrusion polypropylene degradation process which is designed to continuously produce polypropylene with desired properties despite variations in the properties of the feed material; (iii) to investigate the kinetic modelling of the free radical initiated degradation reaction and to determine the possible uses of kinetic models in the development of model-based process control schemes for reactive extrusion processes.; In accomplishing the above objectives the following contributions have been made. A novel rheometer which is based on the flow of a power-law fluid through a wedge (vertically tapered slit) was developed to measure the viscosity of reactive extrusion products on-line. A combination analog-digital anti-aliasing filter was developed to improve the reliability of extrusion pressure measurements when digital data acquisition equipment is used and to reduce the effect of various pressure fluctuations on the resulting viscosity measurements. An empirical model of the reactive extrusion polypropylene degradation process was developed which describes the process dynamics and characteristic process disturbances. A digital model-based process control scheme based on a pole placement controller was designed and used to control the viscosity of polypropylene degraded during reactive extrusion despite variations in the viscosity of the feed material. Finally, an improved kinetic model of the polypropylene degradation reaction, which incorporates a simplified model of the melting mechanism in the extruder, was developed and was used to determine the gain of the polypropylene degradation process as a function of the initiator concentration. Such a relationship can be used to significantly improve the performance of process control schemes for the reactive extrusion polypropylene degradation process.