Synthesis Of Heat Integrated Mass Exchanger Networks Using Stage-wise Superstructure Approach

Energy shortage and environment pollution are serious problems for human society. Recently, many countries set down stricter regulations for industrial enterprise to reduce energy consumption and discharge of pollutants into the environment. Synthesis of mass exchange network (MEN) and heat exchange net work (HEN), which are two branches of process system engineering, can provide systematic methodologies to design cost effective process frameworks that meet the requirements of environment and achieve wastes and energy recovery respectively. However, strong interaction exists between those two networks because equilibrium relations are mainly dependent on the temperatures of streams when the total pressure is not extremely high. Mass exchange processes can be improved by heating or cooling streams to reduce the capital and operating costs, so it is necessary to design a optimal heat-integrated mass exchange network in a cost effective manner. Recently, several researchers have proposed new methods to synthesize water allocation and heat exchange networks, but equipment costs as well as the relationship between temperature and equilibrium coefficient are generally neglecteded in synthesis of water allocation heat exchanger network (WAHEN) and much less attention was paid to heat-integrated mass exchange network synthesis.(1) This paper deals with the synthesis problem of heat integrated mass exchanger networks by mathematical programming approach based on the stage-wise superstructure representation. A design strategy is proposed after analyzing all the parameters connecting mass exchanger networks with heat exchanger networks. This study discretizes those parameters first and then combines them to obtain multiple options. In each option, a step-wise method is used:firstly, MEN is synthesized targeting minimum total annual cost of MEN by superstructure-based approach; secondly, the HEN whose data is got from the MEN in first step is synthesized by solving non-linear programming of modified HEN based on HEN stage-wise superstructure. Then sum the TAC of two networks to obtain minimum TAC of whole networks in each option. Finally the TAC of all options are compared and the option with the lowest TAC of heat integrated mass exchanger network is chosen as the best option. Two examples are examined to demonstrate the feasibility and effectiveness of the method.(2) This paper develops a new simultaneously design methodology for the synthesis of CHAMEN based on stage-wise superstructure for MEN and HEN. All the variables of combined network are optimized simultaneously. The temperatures of the lean streams are optimized as continuous variables. Adaptive simulated annealing genetic algorithm is used to solve the NLP model of proposed to obtain global optimum or approximate optimum. Examples with regeneration and no regeneration are presented to show the effectiveness of this approach.(3) The approach used in simultaneous synthesis of mass exchange network is applied to the recovery system of condensed water from the process of synthetic ammonia. This system is different from traditional mass exchange network because it is a multi-component system in which several possible alternative processes could be selected and the flowrates of streams shouldn't be seen as constant due to huge load of mass exchanging. Since units of different types have various ranges of operating temperatures, medium heating or cooling which can not be identified clearly is needed to set between two equipments. So discussion of two situations is implemented and the global optimal solution is obtained by comparing the best results in two situations respectively. At last, a practical application example is used to show the method's effectiveness.