The Simultaneous Synthesis Of Multi-stream Heat Exchanger Design And Network Optimization

Nowadays, Multi-stream heat exchanger (MHEX) has been a hot research in process intensification field with its compact structure, high efficiency and low heat loss. Even though it is widely used in cryogenic process industrial, the current work is still incomplete and there are many vacancies on the equipment as well as the network researches. Under this situation, some work on MHEXs has been made, aimed at simultaneous synthesis of equipment design and their network optimization.Firstly, based on the special pattern of multi streams heat exchanging, the MHEXs thermal coupling model is founded. With the energy flow matching operator, the principle of multi stream heat exchanging is presented. Furthermore, it also reveals the topology relationship between MHEXs and conventional two-stream heat exchangers. Meanwhile, the state space (SS) superstructure is adopted in the process optimization. With the help of process operators, which are consist of MHEXs thermal coupling models, a synthesis method on MHEXs networks is proposed.Secondly, in consideration of the cryogenic industrial background, the environment and thermal insulation materials are introduced into MHEXs design. Base on the system preanalysis and plate geometry model assumption, a MHEX lumped model is constructed. Combining with the SS superstructure, a system analysis framework is created, which is able to quantitate the environment influence as well as the thermal insulation materials envelope effect.Then, some improvements at the stream level are taken, to ascertain the environment influence and thermal insulation materials envelope effect, exactly. Based on the plate assumption and stream arrangements strategy, a much specific and reasonable MHEXs model is proposed. On the other hand, a modified SS distribution network is proposed to overcome the redundancies. By eliminating feedback circulatory connections, a sequential superstructure more suitable for heat exchanger networks is achieved. Consequently, a methodology on simultaneous synthesis of MHEXs design under the environment influence and their network optimization is formulated.Next, with the analysis on isothermal phase changes, a new criterion rather than temperature is used to tell the stream state. By appropriate mathematical constraints, the processes for isothermal phase changes are expressed, effectively. And corresponding process synthesis on MHEXs design and network optimization is realized, with the help of the improved SS superstructure.Besides, there are several effective solution strategies used in the process, aimed at reducing the model and computational complexity. Through complementarity constraints, the Boolean variables (0,1) are replaced by continuous variables, and the MINLP is translated into NLP problem, which is much beneficial for solution procedure. Moreover, an iterative solution procedure with random variable initial value is used to enhance the global solution quality. With the screening of local optimums, the global optimum could be finally achieved.At last, several cases are illustrated to verify the feasibility and superiority of the methodology.