Assessment and Future Directions of Nonlinear Model Predictive ...

Experiences with Nonlinear MPC in Polymer Manufacturing 385variables refer to the feed components in the reactor and to general reactorcondition variables like temperature and pressure.Perhaps the most critical controlled variable is the reactor temperature. Thisis too fast for conventional MPC and usually has to be a non-linear PID loop.If the temperature is unstable or poorly controlled, all other variables on thereactor, both quality and compositions will be difficult, if not impossible tomaintain.For MPC purposes, the controller design has to focus on those compositionsand reactor conditions that have an impact on the various product quality attributes.To do this, the MPC will manipulate the feed and additive flow rates.To control the product quality, the controller has to determine the correct compositionsand reactor conditions and control to these set-points.Typically these processes are constrained by cooling capacity, powder dischargesystem limits or downstream extrusion constraints. The physical constraintset will differ from plant to plant. It is up to the MPC system to ensurethat the process is always riding these constraints in order to maximize productionor maintain powder qualities.2.2 Polyethylene High Pressure Tube ReactorOne method of manufacturing low-density polyethylene is via a high-pressuretube reaction system. Figure 2 presents a schematic of a typical LDPE highpressuretube process.Ethylene is pressurised to over 2000 bar in two stages. A chain transfer agentis then injected which promotes longer chain molecules. A cocktail of initiatoris injected at various points along the tube producing an exothermic reactionthat peaks in temperature and then decays again until the polymer reaches thenext initiator injection point. Increasing the injection of initiator increases thereaction resulting in a higher peak temperature and increased conversion. ThereFig. 2. High Pressure Tube LDPE Process