Dynamic Behavior Of A Novel Double-effect Distillation Process For Methanol Purification

Methanol is not only the third massive organic product in chemical industry but also is a promising fuel of vehicles. It is important for promoting the development of methanol production to study the new techniques for distillation of methanol with saving the energy and water consumptions. Industrial practice concluded that the 2-column single-effect methanol distillation process, which has been widely used in China, is simple and cheap, easy to control, but consumes too much energy. Therefore, the 3-column double-effect distillation process has been widely adopted by developed countries because of its ability of reducing energy consumption and annual cost.We have optimized the 3-column double-effect distillation process widely used, and proposed a novel water-integrated and heat-integrated distillation system for purification of synthesized methanol. The new process gets the feed to low-pressure column from a side stream of high-pressure column, which balanced better the separation duties of the high- and low-pressure columns and implemented the integration of extracting soft-water. It was found by steady simulation that 25% of annual cost could be saved compared to the widely used heat-integrated process and so did 34% of soft water because of the water integration.In this dissertation, a high-fidelity dynamic model is developed and a control scheme is designed for the new process. By introducing various controller units into the model, the dynamic behavior of the system was explored and the feasibility of the control scheme was examined. In this system, it is impossible to use V to control the quality of product drawn from the bottom of LP column because of freedom reduce due to heat-integration between HP and LP columns, so the strategy of SV is not complete. The top pressure of HP column was controlled by adjusting the feed flow rate to LP, FLP. It means that FLP is used to balance the heat duties of HP condenser and LP reboiler, and the top pressure of HP column acted as a indicator whether the heat duties are in balance.The dynamic response behavior of this system to step changes in feed flow and feed composition are carefully studied. The response curves of process variables suchas the product rates, compositions and temperatures of HP and LP were obtained. The variation of heat-duty distribution of HP and LP and the fluctuation of purity of mixed product were analyzed. The performance parameters in time-domain showed that this control scheme is practical and effective for the new process.Based on the dynamic parameters, such as the hydraulic lags, obtained from the above simulation, a predictive control strategy was proposed to improve the controllability performance of the process by using the concept of advanced control. Comparing with the conventional PID control scheme, the predictive control scheme could dramatically improve the controllability so that the fluctuation of process variables was much smaller and the transitional time to approach new steady state reduced about 50%.Further work is being carried out on the interaction of system variables and optimization of control strategy and scheme.