Optimal setpoints for a pH neutralization process

Abstract

The purpose of this thesis was to determine the optimal setpoints for a pH neutralization process in three stages. A strong acid is to be neutralized by adding a strong base (reagent) in a series of three mixing tanks. The setpoints for the outlet concentration of each tank have to be selected such that the outlet concentration of the last times always satisfies the specification 0 plus/minus 10E-06 mol/l excess H+. The critical issue is that we can never have a negative reagent flow rate.We considered a worst-case scenario where all disturbances occur at the same time. The disturbances can be positive or negative, so we first had to determine the worst-case combination of signs. Then several cases with different setpoints were tested by invoking the disturbance combinations in a nonlinear Simulink model to obtain the maximum deviation from the setpoint in each tank. We also checked whether the reagent flow saturated or not.This "wild guess" approach turned out to be very time consuming and not very iterative. A set of setpoints that would satisfy the specification for the last tank was not found. The best result was a deviation of 2.19E-06. However, it became clear that the setpoint for Tank 2 must not be too low, as this leads to saturation of the reagent flow rate q_r to Tank 3. A better approach would be to develop a model that could simulate a number of cases without changing the parameters for each case manually, and at the same time check whether q_r saturates or not.