Design And Control Of Heat Pump Assisted Dividing Wall Column

Heat pump assisted dividing wall column integrates heat pump and dividing wall column,which greatly improves the energy efficiency and economic benefit of dividing wall column.In heat pump assisted dividing wall column,the overhead vapor of dividing wall column is compressed by a heat pump and used as heating medium in reboiler or intermediate reboiler.However,due to the complicated structure and strong interactions between the variables in heat pump assisted dividing wall column,it is difficult to develop an effective control structure for this highly integrated system.The objectives of this paper are to study the controllability of heat pump assisted dividing wall column,and to establish PID control structures and model predictive control scheme which can resist the large disturbances in feed flowrate and feed composition.The rigorous models of conventional distillation column,dividing wall column and heat pump assisted dividing wall column are performed using Aspen Plus based on the separation of n-pentane/n-hexane/n-heptane mixture.In the configuration of heat pump assisted dividing wall column,only a part of overhead vapor is compressed and then provide heat to the intermediate reboiler.The three processes are optimized using total annual cost（TAC）as objective function.Compared with the conventional distillation column,the dividing wall column saves 33.27%of the TAC,and the heat pump assisted dividing wall column saves 37.63%of the TAC.Then,Aspen Dynamics is used to study the control performance of the heat pump assisted dividing wall column,and four PID temperature control structures are established.The dynamic responses show that the conventional three-point temperature control structure is difficult to control+20%disturbance in feed flowrate.The control structure with fixed V_R/F ratio and the control structure with the fixed V_R/Q_R ratio can handle±20%disturbances in feed flowrate and feed composition well.Moreover,the control performance of the control structure with the fixed V_R/Q_R ratio is better than that of the control structure with fixed V_R/F ratio in reducing product purity overshoot.In this paper,the dynamic behaviors of advanced control strategy based on model predictive control is also explored using Aspen Dynamics and Matlab/Simulink.According to the simulation results,the MPC scheme shows enhanced control performances over the PID control scheme in terms of reducing overshoot and oscillations.The rigorous models of conventional reactive distillation column,reactive dividing wall column and heat pump assisted reactive dividing wall column are performed for methyl acetate hydrolysis process.The three processes are optimized using total annual cost（TAC）as objective function.Compared with the conventional reactive distillation column,the reactive dividing wall column saves 16.6%of the TAC,and the heat pump assisted reactive dividing wall column saves up to 38.59%of the TAC.Different PID control strategies are developed to handle the large disturbances in feed flowrate and feed composition.The results show that the temperature control structure with fixed F_IR/V_R ratio can resist the large disturbances in the feed flowrate,and keep the products at their purities.Then,the model predictive control of the heat pump assisted reactive dividing wall column is studied.The results show that the overall performance of the model predictive control scheme is better than PID control in resisting feed disturbance,reducing overshoot and oscillation.