Modeling and simulation of bubble column reactors

Abstract

The mathematical equations and a comprehensive computer program for modeling and simulating bubble column reactors with internal heat exchange installations are presented. Detailed reactor model equation systems on the basis of the axial dispersion model and the cell model with backflow are developed by linking together the mathematical description o the multiphase flow properties with the momentum, mass and energy balances of the reactor. A highly modular constructed computer program is presented summarizing a lot of theoretical and empirical knowledge about multiphase flow and heat and mass transfer processes in bubble columns. The enhanced possibilities of this new software system are demonstrated by simulating two industrial processes. The methanol synthesis in the slurry phase is chosen in oder to test the model accuracy for a multicomponent reaction system including a hermodynamically balanced chemical reaction. Secondly, the wet air oxidation of municipal sewage sludge in a large-scale slurry bubble column reactor is simulated as an example of a highly exothermic oxidation carried out at extreme pressure and temperature conditions. This chemical process requires special attention for reaction heat removal. The numerical treatment of the algebraic equation system based on the cell model with backflow is easier and leads to average computing times up to 100 times lower than for the reactor model based on the axial dispersion model. The iteration process for solving the boundary value problem runs into numerical convergence difficulties for the case of highly exothermic process conditions with considerable temperature gradients in the reactor. So the finite-difference technique cannot be recommended from a numerical point of view, whereas the cell model formulation allows computations on an IBM-compatible PC (386, 25 MHz) with tolerable computing times of the order of minutes.