IMPROVED CFD MODELLING OF NATURAL CIRCULATION VACUUM PANS

NUMERICAL and especially CFD modelling are becoming cost-effective and reliable ways to develop improvements in vessel designs. Simulating boiling in crystallisation vacuum pans is a very complex process that needs to consider non-isothermal conditions, multi-component, multiphase boiling and condensation. This level of complexity which has been captured in the SRI CFD model was previously too complicated for most CFD software. With improvements to the software, these complex physical processes can now be modelled, albeit a little cumbersomely. This paper details the development of CFD models to predict the circulation patterns and heat transfer occurring in natural circulation crystallisation vacuum pans. Model validation involved checking the circulation velocities predicted by the CFD model with circulation velocity data measured on factory pans. The predictions were in reasonable agreement with factory measurements. The validated CFD model was used to investigate the effect of altering key dimensions on batch pans and on two different continuous pan designs. The batch pan investigations included: increasing the volumetric region above the calandria by flaring the pan body; reducing the clearance underneath the calandria; and changing the dimensions of the tubes (tube diameter and length) while maintaining the same heat transfer area and keeping the evaporation rate constant. The effect of variations in pan geometry, massecuite viscosity and operating level above the calandria were considered for the batch pan simulations. These results provide interesting insight into the complicated processes involved in the operation of natural circulation vacuum pans.