A DISTRIBUTED PARAMETER APPROACH TO THE DYNAMICS OF ROTARY DRYING PROCESSES

Abstract

A nonequilibrium distributed parameter model for rotary drying and cooling processes described by a set of partial differential equations coupled with nonlinear algebraic constraints is developed in this work. These equations arise from the multi-phase heat and mass balances on a typical rotary dryer, A computational algorithm is developed by employing a polynomial approximation (orthogonal collocation) with a global spline technique leading to a differential-algebraic equation (DAE) system. The numerical solution is carried out by using a standard DAE solver. The two-phase-flow heat transfer coefficient is computed by introducing a correction factor to the commonly accepted correlations. Since interactions between the falling particles are considered in the correction factor, the results are more reliable than those computed by assuming that the heat transfer between a single falling particle and the drying air is unaffected by other particles. The heat transfer computations can be further justified via a study on the analogies between heat and mass transfer. The general model developed in this work is mathematically more rigorous yet more flexible than the lumped parameter models established by one of the authors (Douglas et al., (1993)). The three major assumptions of an equilibrium operation, perfect mixing and constant drying rate, are removed in the distributed parameter model. The simulation results are compared with the operational data from an industrial sugar dryer and predictions from earlier models. The model and algorithm successfully predict the steady state behaviour of rotary dryers and coolers. The generalized model can be applied to fertilizer drying processes in which the assumption of constant drying rate is no longer valid and the existing dynamic models are not applicable.