| The multi-effect evaporation system is critical in diaphragm caustic soda industry. It utilizes heat energy to vaporize water of solution and product concentrated NaOH solution. The characteristic of the technical process is high energy consumption. So how to optimize the design of multi-effect evaporation system and improve its economic efficiency is the important task of many chemical engineering designers. With the improvement of computer science, the technology of chemical process simulation has become the important tool for the design and optimization of chemical process. Especially, advanced process simulation software which are developed by applying the method of process system engineering not only have accurate physical property models and general chemical engineering unit operation, but also have open modeling technique. So using advanced process simulation software to simulate and optimize multi-effect evaporation system can improve its design standards and process economic effectiveness and is of great practical purpose.In this paper, using the HeatX, Flash2, Fsplit, Mixer, Pipe, Pump, Sep model and Design modular of Aspen plus software, with C-PRES and C-CIRC calculator modular which are written by FORTRAN programming language, accurate mathematical models of four diaphragm caustic soda multi-effect evaporation technological processes are built. They include simple triple effect forward flow, triple effect forward flow with the flash vessel in which condensate is vaporized, triple effect forward flow with the preheater in which condensate is used to preheat the feed and triple effect forward flow with the preheater in which condensate and extra stream from first evaporating chamber are used to preheat the feed. The process mathematical models not only use accurate electrolyte thermodynamic physical property model and real composition of all electrolytes, but also contain the influence of the circulation and indirect steam pipeline system in every evaporator. According to equi-area method of every evaporator, primary steam economic efficiency, heat transfer area in every evaporator and important stream processing parameters of four technological processes are analyzed by using Aspen plus. The mass flow rate of extra stream from first evaporating chamber is analyzed by using sensitive analysis. The results show that the technological process with the preheater in which condensate is used to preheat feed is superior to the process flow with the flash vessel in which condensate is vaporized in terms of saving primary steam flow and heat transfer area in every evaporator and vacuum system cost. The process that extra steam from first evaporating chamber is used to preheat feed can further increase primary steam economic efficiency and decrease heat transfer area in every evaporator and vacuum system cost. But the mass flow rate of extra steam from first evaporating chamber is limited by heat transfer area of the third preheater.Based on accurate mathematical model of triple effect forward flow with the preheater in which condensate is used to preheat the feed, mathematical models of optimal design of minimal overall heat transfer area of every evaporator and minimal annual cost of multi-effect evaporation system are built. The power consumption of forced circulated pump is also included in the annual cost. The model of optimal design is solved using sequential quadratic programming method. The decision variables include primary steam flow and first evaporating chamber pressure and second evaporating chamber pressure. The optimization results is analyzed. The results show that compared with the equi-area design of every evaporator, the optimal design of minimal overall heat transfer area of every evaporator not only can decrease overall heat transfer area, but also can reduce primary steam flow. The optimal design of minima annual cost will further decrease heat transfer area of the second evaporator and increase heat transfer area of the third evaporator. |
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