A comparison of control techniques for dairy falling film evaporators

Falling film evaporators (FFE) are widely used in the dairy industry to pre-concentrate milk for powder production. FFE control is, however, not performed well, with many plants still under operator or proportional and integral (PI) control. Several authors have created fundamental models to use for controller development, yet these models have various differences in structure and span feed flow rates ranging from laboratory scale (2 500kg/h) to industrial scale (27 000kg/h). This paper used a single semi-empirical model developed by Haasbroek (2013) to offer a sensible comparison of the most often seen dairy FFE controllers. Disturbance rejection was tested by introducing a feed dry mass fraction (WF) step and then comparing the product dry mass fraction (WP) increase as a percentage (ΔWP/ΔWFx100). It was found, as shown in figure 7, that linear quadratic (LQR) control (Haasbroek et al., 2013) and fuzzy predictive controllers showed the best performance (70% and 69% respectively), followed by cascade control (77%) and lastly PI control (123%). The fuzzy controller does, however, struggle with disturbances it has not been tuned for, while cascade and LQR controllers still perform well, as seen in figure 8. Taking into account the involved design required for LQR control, cascade control offers a well balanced approach to FFE disturbance rejection.