Study On Pollution Minimization Oriented Mass Exchange Network In Chemical Process

As environmental pollution becomes more and more serious,searching for solution to minimize waste in the chemical process has been paid more attention in China.The chemical industry urgently needs a systematic design scheme upgrade to achieve the goal of ultimately minimizing process waste within the battery limit of a unit or a plant.Development of mass integration(MI)and mass exchange network(MEN)as the effective tools for directly dealing with polluting stream in the chemical industry have been attracted widespread attention.This paper reviewed the research progress of MEN design both abroad and at home in recent years,and the shortcomings of some of the existing mass integration strategies and MEN design have been analyzed.Based on these studies,this work was mainly focused the following areas:(1)In the existing mass integration strategies,selection of the interception position was always targeted at single point.In this paper,a new mixed integer nonlinear program(MINLP)model was established to optimize the solution,and the LINGO software was used to obtain the model solution.The new MINLP model included both single-position interception and the combination of multiple positions intercepting simultaneously in order to obtain the optimal interception scheme in a more complete solution space,and thus the best mass integration strategy.This paper validated the validity of this model using a case study on the removal of a chlorinated hydrocarbon as demonstration.(2)MEN is a useful process synthesis tool for pollution prevention in process industries.When a staged distillation column is employed as a mass exchanger to synthesize a MEN focused process,the number of trays must be rounded up to the next largest integer after obtaining an optimal design,which increases the capital cost.While designing a chemical process plant,increase in capital cost normally leads to the decrease in operating cost.However,the operating cost is considered to be a fixed value in previous research even if the capital cost increased after rounding up the number of trays.Therefore,the total annual cost(TAC,including capital and operating cost)remained higher than the real optimal objective.To solve this problem,an integral function was added to the mathematical model to obtain the optimal MEN structure in this study.The application on two universal MEN synthesis mathematical models based on the Composition-Interval Diagram(CID)and stage-wise superstructure illustrate the applicability of the integral function.These modified models were applied in two demonstration examples of coke-oven gas sweetening and dephenolization of wastewater processes.The optimal approach employed in this study yielded better results than other works presented in the literature.This study demonstrated that the integral function can solve a series of MEN synthesis problems involving distillation columns.Furthermore,a more accurate design scheme for MEN synthesis and a structure with lower TAC can be obtained by this function.