Predicting batch pan operation when using low pressure vapour

IF THE AUSTRALIAN sugar industry is to substantially increase the revenue from bagasse (e.g. from cogeneration, biofuels, animal feeds etc.) the pan stages must operate effectively on low pressure vapour e.g. vapour from the third evaporation stage, in order to reduce the steam consumption to below 35% on cane. Currently the lowest steam consumption in Australian factories is approximately 40% on cane. The designs of the majority of the strike pans currently in the industry (viz. large, unstirred batch pans) are appropriate for operation on steam/vapour of pressure 180 kPa abs and higher but are unsuited to operation on vapour of 100 to 95 kPa abs, as would be required. A dynamic model using the SysCAD software has been developed to simulate batch pan operation and used to investigate the changes in pan cycle times and exhaustion of massecuites at pan drop as a function of vapour supply pressure. The control strategy in the model was to regulate the feed rate to suit a predefined crystal content profile and to regulate the supersaturation according to the available evaporation rate. Supersaturation was kept below the nucleation limit. The available evaporation rate is the lower of (1) evaporation using a pre-set steam rate set point and (2) a modified form of the Rouillard (1985) equation. The modelling is considered to be a preliminary study as little data are available on the changes in evaporation rates (and hence crystallisation rates) that result when a low pressure vapour is used. The model was used to investigate typical duties for the A, B and C massecuite strikes. The investigations have shown, as expected, that when low pressure vapour is used the cycle times are increased (due to both run up times and heavy up times being increased) and the massecuites at pan drop may be at reduced exhaustion. Reduced exhaustion has implications for the massecuite production loadings on the stage and on sugar recovery. This paper describes the model, the preliminary results, and the future plans for the investigations. The future plans include obtaining evaporation rate data from factory pans when the calandria pressures are restricted to lower values, incorporating these evaporation rates into the dynamic model and determining control profiles and operating procedures to minimise the impacts of using low pressure vapour.