Need for Control: Instability of Continuous Stirred Tank Polymerization Reactor Simulations

High Resolution Image Download MS PowerPoint Slide Free radical polymerization of low-density polyethylene (LDPE) is highly exothermic. If left uncontrolled, pressures in excess of the reactor’s design can occur, potentially leading to catastrophic reactor explosions. Because of this, research and development studies are often performed using computer simulations. However, the instability of this system makes accurate results difficult to obtain because of steady-state multiplicity and oscillatory behavior. This work utilizes computational fluid dynamics (CFD) to develop a simplified continuous stirred tank reactor (CSTR) model for LDPE. After a host of tests covering numerical settings, startup processes, kinetic simplifications, and reactor design, it was determined that a LDPE CSTR steady-state result cannot be achieved in CFD. This was exemplified in two identical reactor simulations that differed in initial temperature by only 0.01 K that exhibited results divergent from one another. Because of this extreme sensitivity, active PID control was necessary for a stable CFD solution. We demonstrate that active control is the sole enabler for stable adiabatic CSTR CFD solutions involving any exothermic reaction, allowing engineers and scientists to efficiently simulate reactor improvements without danger or loss of plant reactor productivity.