Mathematical modelling of climbing film evaporators.

Abstract

Climbing film evaporators are in widespread use in the South African sugar industry, with the vast majority of the local sugar mills currently utilising these evaporators as first effect vessels in multiple effect evaporator sets. However, it is generally considered that the performance of these evaporators has not been maximised, and that improvements could be achieved by proper optimisation of the operating parameters. Unfortunately, very little comprehensive design information has been published in the literature. owing to the complexity of the heat transfer and hydrodynamic interactions in the evaporator tube. Attempts at performance improvement have been hampered by the lack of any theory to explain fully the effects of the operating parameters and physical properties of the feed liquor on the performance of the evaporator. In this study. a mathematical model of the climbing film evaporator system was developed in order to assess the effects of changing operating conditions on evaporator performance, based on as solid a theoretical foundation as cunendy possible. The model was tested against experimental data from a pilot plant climbing film evaporator and this experimental data was used to enhance the accuracy of the model by means of process identification. Because of the complexity of the model and the extensive computational time required for its solution, a simplified evaporator model was also developed, based on Iinearisation of the system of ordinary differential equations describing the climbing film evaporator system. This simplified model was used to predict trends in evaporator behaviour under various operating conditions.