CFD modeling of LDPE autoclave reactor to reduce ethylene decomposition: Part 1 validating computational methods

Abstract

• Implemented five simultaneously independent proportional integral derivative controllers to maintain thermal stability in CFD using a similar methodology as employed in the plant. • Developed a comprehensive model of an industrial scale LDPE autoclave reactor consisting of rotating mesh elements, reaction kinetics, and complex geometry. • Validated numerical methods of time step size, grid resolution, and turbulence model sensitivity. • Replicated proposed mixing patterns in CFD. CFD was employed to develop a rigorous model of an LDPE autoclave reactor. Different numerical settings within the solver are evaluated to eliminate false diffusion and to reflect the sensitive heat generation taking place during free radical polymerization. An accurate model can allow geometry and process adaptations to be evaluated for much lower costs than physical experiments. Improving the reactor design allows for longer run times and a higher degree of catalyst conversion. The rigorous CFD model employed reaction kinetics, PID-automated thermal management, and a rotating stirrer shaft. Validation was carried out to determine the sensitivity to time-step size, turbulence model, and grid resolution. Data were compared to an industrial scale plant autoclave to guide the development of CFD. In a comparison of turbulence models, the shear stress transport (SST) model was found to predict higher concentrations of turbulent kinetic energy (TKE) resulting in a lower temperature distribution throughout the reactor than the differential Reynolds stress model (DRSM). The less diffusive DRSM was recommended for future studies. A mesh refinement study revealed slight variation in the results between the base mesh of 6 million computational elements and the refined mesh consisting of 40 million. Ultimately, the variation between different grid resolutions was not significant enough to justify slowing down the solver speed by 14X by using the refined mesh. Increased rigor improved the model’s ability to match plant data, and CFD thermocouples were within 2.5% of temperatures from plant data.