Synthesis And Optimization Of Heat Exchanger Networks

It is well known that the process industry(petrochemical,power,metallurgy,papermaking,etc.)is an energy-intense and water-consuming industry.As the energy crisis and water shortage becoming more and more serious,the energy and water cost of process industry has also increased.Additionally,the environmental regulations and laws become stricter and stricter,with the strengthening of sustainable development concept.It is of significance to achieve energy conservation and emission reduction of process industry.Energy integration and mass integration is one of the important approaches to achieve energy saving and emission reduction in the process industry.Generally,the methods can be classified into two groups:conceptual design methods and mathematical programming methods.The conceptual design methods are mainly graphical methods based on pinch technology,which is easy to operate.Feasible solutions can be quickly obtained.However,conceptual design methods are sequential in nature,which include targeting and design steps.The mathematical programming methods are based on superstructure optimization.Models are developed and solved by the optimization algorithm.The trade-offs and connections among the entire network can be established and explored.Multiple factors can be considered and optimized simultaneously by mathematical programming methods.In this paper,aiming at the energy and mass integration of process industry,the synthesis of heat integrated water allocation networks,heat exchanger networks,inter-plant heat exchanger networks are addressed.Both conceptual design methods and mathematical programming methods are developed systematically.The main researches are as follows:(1)A new graphical tool has been developed for the heat exchanger network(HEN)design of heat integrated water allocation networks(HIWAN).An H-F diagram is proposed to deal with the heat exchange matches.Non-isothermal stream mixing and splitting can be achieved by the transformation of hot or cold stream composite curve in the H-F diagram.A matching composite curve is proposed for the design of HEN with parallel structure.The structure of HEN can be simplified by adjusting the shape of the matching composite curve.Both small scale and large scale examples are illustrated.Results show that the proposed method is as good as the previous methods for small scale problem,while it gets a better result for large scale problem.Besides,a graphical approach has been proposed to reveal the new insights of T-H/H-F diagram and fully exploit the HEN structure possibilities.A transformation method is proposed to generate HENs with different structures based on these two tools.Both simplex series or parallel structures and hybrid structures can be obtained via the transformation procedures.Two cases are illustrated to demonstrate the operability of the proposed procedure.(2)In order to overcome the deficiency of T-H diagram and H-F diagram in optimizing the capital cost,systematic procedures for the synthesis of HEN in HIWAN has been proposed.A graphical tool,namely heat transfer blocks diagram,is developed to improve the conceptual understanding for the implications of HEN structures in HIWAN.The minimum hot/cold utility consumption can be obtained by analyzing heat surpluses and heat deficits between cold stream and hot stream heat transfer blocks,while heat transfer matching blocks are introduced to deal with the heat exchange matches.Not only simplex series or parallel structures but also hybrid structures can be obtained via this graphical tool.Besides,a heuristic of step by step maximizing the heat load of heat exchangers is presented to guide the heat transfer matching blocks to the lower investment cost direction.Cost-effective HENs have been identified by the proposed systematic procedures.Two examples are illustrated to demonstrate the operability and availability of the systematic procedures.(3)It is known that models with plentiful HEN structures always result in complex mixed-integer nonlinear programming(MINLP)models with non-convexity of nonlinear constraints,which will cause heavy computational burdens.The computational time of MINLP models and even the feasibility of models without good initial points are still challenging researchers.This paper presents a novel transshipment type model for the synthesis of HEN,which is formulated as a MINLP problem with all linear constraints.Stream splitting,stream by-pass,isothermal and non-isothermal mixing,and recycling flows are all included,while multiple utilities are available in the model.Furthermore,new formulations are presented to calculate the temperature difference of heat exchangers,while a more precise heat exchanger area calculation method by piecewise calculation is developed in the new model.Six literature examples are presented to illustrate the effectiveness and applicability of the proposed model.It is shown that several better results are obtained by the proposed model.(4)Water and energy management issues in process industries are generally related to the synthesis of HIWAN,since water and energy are inextricably intertwined in process water networks.The aim of this study is to present a novel mathematical programming model for HIWAN synthesis problems and develop an efficient solution strategy.In this model,wastewater treatment units and multiple contaminants are considered,and the total annual cost(TAC)is optimized rather than targeting a simplified TAC.What's more,a three-step solution strategy is proposed to guarantee obtaining promising solutions.The freshwater consumption,the relaxed TAC(rTAC),and the TAC are optimized successively.Good initial points for the rigorous HIWAN model in the last step can be generated by the minimizing the rTAC in the second step.Four examples including two large-scale examples are illustrated to demonstrate the applicability of the proposed model and the efficiency of the solution strategy.The results indicate that the proposed model is suitable for the synthesis of HIWAN,and more significantly,better results for large-scale problems are achieved.(5)Inter-Plant Heat Integration(IPHI)using intermediate fluid circles provides an additional opportunity for energy saving and emission reduction,beyond traditional Intra-Plant Heat Integration.However,the number of intermediate fluid circles,as well as their heat capacity and temperatures are unknown a priori.It is difficult to find promising solutions,since such conditions generally lead to non-linearity.To deal with this problem,a transshipment type model,which holistically optimize the parameters and interconnectivity patterns of intermediate fluid circles and keep the constraints linear,is developed.New formulations are developed to identify exact distributions(namely heat transfer available intervals)among all temperature intervals,with unknown temperatures of intermediate fluid circles.Additionally,mixed-integer linear constraints are developed to describe interconnectivity patterns of intermediate fluid circles,so as to calculate piping and pumping cost.The performances of different interconnectivity patterns are explored,while not only the energy efficiency but also the total cost is optimized.Case studies show that better results with lower TAC and appropriate interconnectivity patterns of intermediate fluid circles can be obtained,compared to literature.(6)A new transshipment type model for IPHI directly using process streams is developed,while simultaneously optimizing the total annual cost of intra-plant and inter-plant HEN,including the piping and pumping cost.Thus,the interconnection between intra-plant and inter-plant HENs can be considered during optimization.New formulations are developed to keep track of the transportation of process streams across plants,so as to calculate the piping and pumping cost and compare different inter-plant heat integration opportunities.Case studies show that better results with lower TAC can be obtained,compared to literature.Besides,owing to the characteristic of more structures embedded in the model,results with unique structures can be obtained by the proposed model.