PDF simulations of ethylene decomposition in tubular LDPE reactors

The present study deals with turbulent reacting flow simulation inside low-density polyethylene (LDPE) tubular reactors, based on a detailed computational fluid dynamics (CFD) technique-transported probability density function (PDF) methods. The ability of the PDF methods to provide an exact representation of chemical source terms is ideally suited for coupling complex LDPE chemistry with small-scale fluid dynamic fluctuations in turbulent flow. LDPE chemistry with a total of 16 scalars provides an ideal test case for illustrating the applicability of an efficient chemistry algorithm based on in-situ adaptive tabulation. A particle-based Monte Carlo algorithm is used to solve the joint-composition PDF equation, whereas a finite-volume code is used to obtain hydrodynamic fields from the standard k-epsilon turbulence model. The influence of feed temperature, initiator concentration, and degree of premixing is investigated to gain detailed knowledge of micromixing effects on steady-state reactor performance. The computational approach provides a low-cost alternative to experimental and pilot-plant tests for exploring a variety of design options when making important design and operational decisions, or for investigating unstable reactor operating conditions. The ability of a simplified, but otherwise equivalent multi-environment presumed PDF model to predict turbulence-chemistry interactions close to physical reality is validated using the detailed transported PDF simulations. The transported PDF method is shown to be an excellent tool for obtaining fundamental information on turbulent reacting flows, as well as for deriving simplified models for faster and easier interpretation of these flows when developing safe and efficient chemical processes. (C) 2005 American Institute of Chemical Engineers.