Study On Synthesis Of Mass Exchange Network Targeting Minimum Cost

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 utility and reducing the environmental pollution. Total annualized cost of a mass exchange network is mainly comprised of operating cost and capital cost. The minimum composition differences (e) reflect the impetus of mass transfer between rich and lean streams and affect the operating cost and capital cost significantly. However, so far they are either given by fixed values as in most literatures, or optimized using stepwise methods, so the minimum cost of mass exchange network can not achieved actually. This paper presents a simultaneous method to optimize the e and mass exchange networks for the single-contaminant or multi-contaminant system, which is based on comprehensive analyzing the similarity of heat exchange networks and mass exchange networks, the minimum composition differences are used to balance the operating cost and capital cost to achieve the objective of minimum total cost. The content of this paper is listed as follows:(1) As to the minimum cost of single-contaminant synthesis of MEN, this paper presents a combination method of composition interval diagram with mathematical programming, and the e between rich and lean streams are taken as variables for targeting the minimum total annualized cost, which is based on the pinch technology. The non-linear programming(NLP) model is formed consisting of the constraints of mass transfer equilibrium equations, outlet composition restricts, environmental standards and operating feasibility, then the optimal values ofεand the structure of MEN can be obtained simultaneously by solving NLP model in the feasible interval of e vaules. The proposed method is demonstrated with two examples.(2) Aiming at the shortages of pinch technology while optimizing theεvaules and network structure of multi-contaminant MEN, this paper presents a stage-wise superstructure method to consider all the feasible streams matching, which can be used to handle both the single-contaminant and multi-contaminant system, the superstructure model of MEN can be built by targeting the minimum total annualized cost and taking theεas variables to measure the operating cost and capital cost. The minimum total annualized cost and theεvalues can be optimized simultaneously by the adaptive simulated annealing genetic algorithm, then the global optimal MEN can be obtained.(3) In the condition of a little larger mass transfer of components between rich and lean streams, the flow rates of streams must be taken as a set of variables, which can be calculated by means of mass transfer equilibrium equations. Besides, in order to accord with the practical case of a multi-component system in industry, this paper takes the interactions of the components in the system into consideration, so the mass transfer equilibrium equations are correlated to the composition of components, the operating temperature and pressure of each unit. At feasible temperature and pressure, the optimal flow rates, along with the minimum composition differences and the mass exchange network, can be obtained simultaneously. Because the temperature and pressure of exchangers are taken as variables, the corresponding total annualized cost and minimum composition differences are globle optimum. At last, an practical application example of synthetic ammonia is used to validate the method's effectiveness.