Simultaneous Optimization Of Multi-component Mass Exchange Network Based On The Mass Transfer Composition Difference

As one of the branches of process integration, synthesis of mass exchange network (MEN) has been provided with dual benefits of improving the efficiency in resource utilization and reducing the environmental pollution. In this thesis, the strategies to solve synthesis problems for multi-component MEN are proposed based on the analysis of its characteristics and difficulties of the study from the views of thermodynamics. And the synthesis methods for multi-component MEN are studied in detail and the work focuses on the studies of simultaneous optimizing methods for the multi-component MEN based on the mass transfer composition differences (e) between the rich and lean streams with combined and superstructure methods respectively. The main contents and results of this paper are as follows:(1) Having studied the influences of e on the various costs of a MEN, it is found that the e affects the operating cost and capital cost reversely but in different degrees, so the optimal MEN targeting minimum total annual cost (TAC) can be obtained only by means of simultaneous methods. As a result the simultaneous methods have been studied thoroughly by taking the e as the variables to trade-off the operating cost and capital cost of the MEN.(2) A combined method of composition interval diagram (CID) with mathematical programming is presented in this thesis in order to synthesize compatible multi-component MEN. Having taken the e as optimal variable, the CID is established according to the corresponding composition scales. Then the nonlinear programming (NLP) model is formed based on the CID. So the optimal values of e and the structure of MEN with minimum TAC can be obtained simultaneously by solving NLP model. The proposed method based on the pinch technology is helpful to find the bottleneck of the process, eliminate unreasonable solutions and cut down the dimensions of the mathematical model. So the NLP model can be solved by the software of GAMS. The proposed method is demonstrated with examples and the TAC of the calculating results have dropped with maximum 20.1% comparing with that of literatures, which validated the feasibility and effectivity of the combined method for the synthesis of compatible multi-component MEN.(3) A superstructure simultaneous method to consider various feasible streams matching and constraints is presented in order to solve the synthesizing problems of complicated multi-stream and incompatible multi-component MEN. Different from literatures, a NLP model is formed rather than MINLP model, which can reduce dimensions of mathematical model and avoid the discrete of binary variables 0-1 in the MINLP, so that the solution difficulties are decreased. The adaptive simulated annealing genetic algorithm (ASA/GA) is used to solve this model. At last the minimum TAC and the e values of a MEN can be optimized simultaneously by ASA/GA. The proposed method is illustrated with two examples and the TAC of the calculating results have decreased 9% and 4% respectively comparing with the results of literatures, which demonstrated that the proposed method can be applied to multi-component MEN effectively and with the higher probability to find the global optimum solution.(4) The process condensate recovery system for an ammonia plant in our country is optimized by the superstructure method proposed in this thesis. The system has the characters of relatively larger mass transfer loads between the rich and lean streams, higher operating temperature and pressure, and stronger non-ideality, so it is belong to the problems of complicated multi-component MEN synthesis. Different from most ordinary MEN, the inlet and outlet flowrates of streams must be taken as variables. Besides the interactions of the components in the system and effects of operating temperature/pressure in each unit on the phase equilibrium equations are also considered. The superstructures in series-stage and in multi-stage are applied respectively. The results show that it is necessary to take flowrates, operating temperatures/pressures as optimizing variables, and the operating temperature/ pressure can affect the phase equilibriums obviously. At last, the minimum TAC close to global optimum can be obtained. The results illustrate that the proposed method can be successfully used in the guidance of optimizing the process of industry.(5) The ASA/GA is developed to solve the NLP model formed and evaluating function is modified according to the synthesis problems. The strategies of adjusting step size and temperature decreasing adaptively improve the operation speed and the ability to find solution with high quality of the hybrid algorithm. The results of case studies show that the ASA/GA has good capability in convergence speed and finding the global optimum with a high probability and can successfully solve the NLP model established from synthesis of multi-contaminant MEN.