An investigation into sub-optimal control on the downstream processing of a large scale industrial process

Industrial processes can be divided into two main areas: upstream processing; involving the manufacture of product and downstream processing; product separation and purification. Although both of these operations are necessary to obtain the final product; optimisation and improvement efforts are generally biased towards upstream processing. The importance of downstream processing can often be neglected to concentrate on the “more important” aspect of upstream processing. However, if the purification and separation steps aren´t controlled effectively it can result in a significant reduction in the overall process yield. This investigation focuses on the control of a two stage counter current liquid-liquid extraction unit downstream of a batch process. A reduced separation capacity of this process is observed due to a flow oscillation in the solvent stream. The cause of this oscillation is investigated by analysing process variable behaviour, paying particular attention to the interplay between the control strategy and disturbances of the process. The effect on separation efficiency is shown through high frequency analysis of product concentrate in the output stream. The aim of this work is to highlight the importance of the process control strategy. Reviewing the linkages between the process controls, product yield and quality are essential as part of a continuous improvement strategy. Such reviews can highlight opportunities for significant increases in yield that are often masked out through infrequent sampling procedures.