Synthesis Of Work-integrated Heat Exchange Networks Based On Extended Superstructure

With the global energy shortage and energy consumption increase,the importance of energy conservation has become increasingly prominent.In the whole social production process,energy consumption in process industry accounts for a large proportion.Two significant forms of energy widely used in chemical industries are heat and work.Since complex relationship exists in these strongly interacting properties(heat,work,temperature,pressure),where pressure manipulation leads to temperature changes while the amount of compression/expansion work is also affected by the temperature variations,thus the research on the simultaneous integration of work and heat has a significant impact on the overall energy-efficiency improvement of the process.In view of the single structure of heat exchange network(HEN)and the neglect of the influence of pressure operation on heat exchange matching in chemical process,this thesis carried out the study on synthesis of work-integrated heat exchange networks based on extended superstructure,focusing on the simultaneous synthesis of single pressure operation process with only compression or expansion and HEN design.The main contents of the thesis are as follows:(1)This thesis proposes an extended stage-wise superstructure that involves compression manipulation both in stages and between stages along with heat integration for each pressure-change cold sub-stream in stages.A mixed-integer nonlinear programming model is formulated to obtain the optimal network configuration.The sub-streams return to their parent state by adopting constant-pressure mixing and non-isothermal mixing.The superstructure enables an optimized selection of pre-coolers,end-heaters and end-coolers for the compressed streams for which the thermodynamic identity may change after compression to meet their desired target temperatures.The network structure is optimized with the objective of the minimum exergy consumption and the minimum annual total cost.The interactive mechanism between the compression placement and the HEN synthesis is revealed.Based on it,the multi-objective optimization is conducted.The example analysis proves the proposed method can further solve the synthesis problem of sub and above ambient heat exchange network including multi-stream compression(2)Inspired by coupling the compression process with the heat exchange network,an extended stage-wise superstructure that involves expansion manipulation and heat integration for each pressure-change hot sub-stream is proposed.Different from(1),the synthesis of HEN with expansion process considers the optimal selection of valves/expanders and makes the superstructure more comprehensive,because both valves and expanders have an effect on pressure reduction.A stream that expands through expanders can produce work and cold load,while expansion through valves barely affects heat integration and the exergy is completely lost.Through the example analysis,the trend of equipment selection under different conditions and objectives can be obtained.The optimal trade-off between thermodynamic and economic performances of the system can be achieved through multi-objective optimization.The Pareto curve is plotted for decision-makers to determine the optimal design scheme in terms of process requirement.(3)In order to expand the applicability of the method proposed in this thesis,the conditions of hot stream compression and cold stream expansion are further considered.Note that there is no need to consider the identification of the cold and hot properties of the stream.The influence of stream thermodynamic paths on the synthesis of heat exchange networks coupled with work integration is discussed from the aspects of mathematical model,network structure and multi-objective optimization characteristics.