Research On New Control Method Of Cyclic Total Reflux Batch Distillation

The operation mode of cyclic total reflux batch distillation in the way of total reflux-total withdrawal without reflux drum was proposed in this paper. The existence of the concentration platform of the top of the column during the total withdrawal period was proven by simulation and experiment. A middle tank which improves the slop cut operation was set up. The new operation mode was applied in separation of the ternary mixture: methanol-ethanol-propanol system. The mathematical model for the slop cut operation process with a middle tank was set up. A control policy based on the multi-temperature measured and tri-temperature controlled was proposed according to the new operation mode. Combined with the system identification and predictive control theory, the automatic control and on-line optimization platform was developed for the cyclic total reflux batch distillation. The operational parameters were set with the assistance of the theoretical plate method, and furthermore, the feasibility of the tri-temperature control policy was validated by the mathematical simulation and experiments in the paper.First, total withdrawal process which is the principle basis of the no-reflux-drum cyclic total reflux batch distillation process was studied computationally and experimentally. The effect of different column parameters on total withdrawal process was discussed respectively. The existence of the concentration platform at the top of the column, which acts as the theoretical basis of total reflux-total withdrawal cyclic batch distillation, was proven. Then, the withdrawal of slop cut of the cycle total reflux batch distillation with an additional middle tank was studied. The effect of filling and dumping of the liquid in middle tank on the change of the top concentration of light component and the rising of the heavy component in still was discussed. Furthermore, the mathematic model of the process of the new operation mode was set up.Based on the above research, tri-temperature control strategy assisted by the theoretical plate method was proposed. RBF (Radial Basis Function) neutral network was adopted to carry out the temperature system identification, and EKF (Extended Kalman Filter) was adopted to estimate the temperature in the time-varying system and predict the concentration variation in the column. In order to realize the on-line control of the switch between total-reflux and total-withdrawal, a closed loop control system based on FNNC (Fuzzy Neural Network Control) method was formed by the control of the withdrawal and reflux process.Finally, the experimental apparatus with tri-temperature controlled cyclic total reflux batch distillation with middle tank was set up. The separation of the ternary mixture methanol-ethanol-propanol was carried out in the new operation mode and control method proposed by the paper. During the process the control performance showed good robustness. Compared with the constant reflux operation, the product amount was increased by 26.2%, and the operation time for slop cut withdrawal was decreased by 53.4%, the efficiency of the distillation column was increased by 66.2%. Therefore in the separation of ternary mixture, the tri-temperature controlled cyclic total reflux batch distillation with middle tank is obviously advantageous over the conventional constant reflux control policy. And the feasibility of this control policy was verified.