| The distillation is one of the most energy-consuming unit operations in the chemical engineering and industry. Nowadays, since the price of energy source keeps increasing, it is of great significance to reduce the energy consumption of distillation operation. As the main approach to energy-saving, the distillation with the heat integration has gained intensive concern and further study in the aspect of development and application.In this dissertation, a novel DVH process is proposed for the purification and separation process of plant polysaccharide extract. By means of combination of side draw of distillation column and vapor jet pump, the evaporation process is accomplished, at the same time, the latent heat of vapor is recycled. The integration of the three unit operation unit-distillation, evaporation and heat pump, not only saves one evaporator and then decreases the equipment cost, but also makes the DVH process reduces the heat consumption by at least 34% compared with the traditional process, by the recycle of the latent heat of product vapor. However, at the same time, this novel process increases the coupling of the system, and leads to significant difficulties in the control during operation of production and start-up. It is necessary to study the dynamic behavior and controllability analysis of this novel system on purpose of industrial application.In this dissertation, a high-fidelity model is developed, dynamic behavior is studied, control structure is selected, the closed-loop dynamic behavior of different control schemes are compared, the start-up scheme is designed, and the start-up process is simulated and optimized. The detailed research work and contributions of this dissertation are as follows:1. The steady-state simulation of DVH process is completed using rigid non-linear model in the commercial simulation tools Aspen Plus. In the simulation, distillation column uses RADFRAC models with rigorous hydraulics method. The heat-exchanger use independent Heatx models. All the parameters needed in the calculation of unit operation are derived from the practice factory data. The parameters in the steady-state run are optimized by the method of Aspen Plus coupled with a distributed parallel genetic algorithm.2. The dynamic process of DVH is simulated using the software Aspen Dynamics and Aspen Custom Modeler. The detailed open-loop dynamic behavior study about the feed stream and manipulating variables disturbances is proposed to analyze the interaction between system parameters, and summarize the open-loop dynamic behavior characteristic of the system to establish suitable control structure.3. The transfer matrices of different control structures are compared and analyzed using the relative gain matrix and dimensionless number NI and MRI, the controllability of the system is judged, and then the better control structure is picked out for the further study.4. Specific to the above-mentioned control structure, in this dissertation, the closed-loop dynamic behavior study about the operation parameters and feed stream or vapor pressure disturbances are analyzed with different mixed vapor pressure separately using the control strategy-direct composition control and predictive control. Under different disturbances, the best control strategy is chosen, the advantages and disadvantages of each control strategy are summarized.5. Finally, in this dissertation, DVH process is decoupled, a special start-up strategy is proposed, soft water from the process is set as initial feed, when the steam jet pump can supply sufficient heat, extract is then set as feed. The trend of the system changing with time and better operation parameters are achieved by the dynamic simulation of the start-up. At the same time, the out-put timing and composition of both column top and bottom are optimized, avoiding the recycle of the disqualified product and make sure the stabilization of the system. At the same time, the extract gets zero loss and no waste discharged. |
PDF Full Text Request