Static and dynamic bifurcation characteristics of industrial fluidized bed polyethylene reactors (UNIPOL process) and their implications on polyethylene production rate

Static and dynamic bifurcation behaviour dominates the operation of fluidized bed catalytic reactors for the production of polyethylene (UNIPOL process) and have important implications on safe operating temperature and polyethylene production rate. The multiplicity of the steady states phenomenon covers a wide range of parameters together with the phenomenon of periodic oscillations with sharply changing amplitudes with change of the chosen bifurcation parameter. In some cases, the periodic branches terminate through periodic limit point (PLP), while in other cases, it terminates homoclinically. A detailed parametric investigation using two-parameter continuation diagrams for the loci of static and Hopf bifurcation points as well as one parameter bifurcation diagrams showed that it is possible to increase the productivity of the unit significantly without exceeding the constraint of the polymer melting point. By careful choice of the operating parameters to operate the reactor on a periodic branch basis with small oscillations led to higher polyethylene yield than the corresponding steady state operation. Gas velocity, catalyst feed rate and polymer particle size distribution are important operating parameters in polyethylene fluidized bed reactors. Different control models (P, PI, PID) are considered for the stabilization of desirable unstable steady states and the characteristics of the closed loop control system are discussed.