Optimising imbibition in a sugar mill with cogeneration.

Design of raw sugar factories has traditionally focused on achieving a balance between the energy demand of the factory and the energy contained in the available fuel supply (ie. bagasse). This has been particularly true in South Africa, where historically low prices have discouraged any investment for the purposes of exporting electrical energy to the national grid. This approach to factory design yields factories with a relatively low capital cost, but often with reasonably poor levels of energy efficiency. However, the current drive towards cogeneration has changed industry thinking in this regard. Standard sugar industry practice aims at maximising the recovery of sucrose from cane by maximising the amount of imbibition water that is applied to the extraction plant, while remaining within the constraints of the bagasse supply. This is a sensible approach for a factory producing only one valuable product. However, this optimisation method is no longer valid for a cogeneration factory producing a product mix which includes both raw sugar and electrical power. While sugar recovery is maximised by increasing the rate of imbibition, this reduces the potential for electrical power export. The current study demonstrates techniques that can be used to determine the economic optimum for a typical South African raw sugar mill, based on detailed modelling of the extraction plant (eg. a diffuser) and the energy balance of the factory. The best financial outcome for the mill occurs at an imbibition rate lower than that conventionally applied in the local industry.