Application of Adaptive Control to a Reciprocating Plate Liquid-Liquid Solvent Extraction Column

Abstract

<p>An experimental investigation of the application of direct digital control (DDC) has been carried out with continuous chemical processing equipment, namely a liquid-liquid solvent extraction column. The type of extractor used was the Karr reciprocating plate column.</p> <p>Steady state simulations of the column were carried out using both a semi-empirical model for studying the hydrodynamics of the column, and a mechanistic model for studying the mass transfer characteristics of the column. The results of these simulations were used to interpret the closed loop response characteristics of the column. Two types of control schemes were studied; (i) an adaptive self tuning regulator (STR), (ii) the Dahlin algorithm.</p> <p>Though the dispersed phase holdup is not the primary control variable, its control is essential to prevent the hydrodynamic instability. The frequency of reciprocation was used to control the holdup. The relationship between the frequency of reciprocation and the dispersed phase holdup is highly nonlinear therefore an adaptive controller (in this work, a self tuning regulator) has to be used to control this process. Previous work on holdup control has been extended to include a wider, range of operating conditions and prevention of covariance windup by using an empirical discounting factor.</p> <p>The primary control objective was the control of the extract concentration. This was done in two ways. The first scheme was a cascaded control to manipulate the frequency of reciprocation whereby the inner loop was the holdup control. An STR-STR cascade arrangement was compared with cascade a PI-STR arrangement. It was found that the restrictions imposed on the outer loop meant that the PI-STR arrangement, was as effective as the STR-STR arrangement. The second control scheme manipulated the continuous phase flow rate with (MIMO) and without (SISO) the simultaneous control of the holdup via manipulation of the frequency of reciprocation. A variable dead time Dahlin controller was used to control the extract concentration in this second control scheme. In the MlMO case, decoupling was not necessary since one loop had a positive gain while the other had a negative gain and so the interactions were constructive.</p>