Optimising milling-train extraction through controlling added water

There is a limit to the amount of water that can be added to sugarcane to aid in the extraction of sucrose, due to the capacity of the evaporator station that needs to evaporate the water before crystal sugar production can occur. This paper reports on attempting to optimise that water addition by increasing the water addition rate when there was a greater benefit to extraction and reducing it when the benefit expected was less. Experiments conducted at Tully Mill concluded that higher extraction was achieved when the ratio of sucrose to fibre in cane was higher. Subsequent modelling predicted the benefit of adding more water was greater when extraction was lower. Using these results, a strategy to optimise extraction by varying the rate of water addition based on the ratio of sucrose to fibre in the cane was developed. However, the results of modelling undertaken to test the strategy found that there was very little benefit in terms of extraction to be gained by the implementation of this strategy. The project highlighted the difficulty of maintaining mill torque when varying the rate of water addition, due to limitations in torque control. It did support a strategy of reducing mill settings during maintenance days when evaporators are cleaned so that higher water addition rates can be achieved while maintaining mill torque. Following a stop, evaporator capacity reduces as scale forms and mill settings increase following roll wear. These two processes enable the water addition rate to reduce with minimal impact on mill torque.