Study on Simultaneous Synthesis of Work and Heat Exchange Networks Based on Extended Superstructure
Due to the rising demand of energy,the rapid depletion of oil reserves,technologies barriers in renewable energy and concerns over energy security,energy usage has attracted increasing attention in recent decades.With the development of chemical system engineering theory and process integration technology,the current research on heat exchange network(HEN)has made great progress for enhancing heat recovery and reducing energy consumption.However,for hydrotreating process,ammonia and methanol synthesis process,liquefied natural gas(LNG)purification and low temperature process of liquefaction process,the pressure change of streams will lead to energy consumption.The individual utilization of expanders and compressors results in little work recovery.If work integration and even simultaneous integration of work and heat can be considered in the process industry,it will help improve energy efficiency and further achieve the goal of energy saving and emission reduction.Although work and heat recovery is an effective method to improve energy utilization efficiency,the current research is limited to respectively synthesize work exchange network(WEN)and HEN.In addition,there exists a coupling relationship between WEN and HEN,in which heat integration affects compression and expansion while work exchange changes the heat transfer matches.Therefore,this thesis investigates the method for simultaneous synthesis of work and heat exchange networks(WHEN)based on extended superstructure.First,an efficient approach to WEN synthesis based on thermodynamic analysis and mathematical model is developed.Second,the sequential synthesis of WEN and HEN with stream matching rules is proposed,further to reveal the interaction mechanism of work and heat.Finally,the strategies for coupling integration of work and heat are explored.The main contents and results of this thesis are as follows:(1)Since the operating mode of work exchangers is dramatically different from that of heat exchangers,pinch analysis for HEN synthesis cannot be directly applied to work integration.To facilitate WEN synthesis,an upgraded thermodynamic graphical methodology is proposed under isothermal and adiabatic condition.In this method,the pressure intervals are divided according to the pressure of work sinks and then the improved composite curves of work sources and work sinks are plotted in the pressure index(μ)versus work(W)diagram.Furthermore,the improved linear-approximation auxiliary lines of work sinks and three matching rules are proposed to assist identifying the feasible matches,aiming at the optimal work exchange network with maximum mechanical energy recovery and minimum external utility consumption.Two examples from literatures are investigated to justify the efficacy of the proposed method.Our presented method intuitively clarifies the bottleneck of mechanical energy recovery through work exchange and achieves one-to-one match as well as configuration design between work sources and work sinks.(2)Due to the limitations of the graphical method with a tediously iterative procedure for searching feasible matches,which cannot ensure the optimal solutions due to the overlarge minimum pressure difference,to effectively adopt mathematical programming method for WEN synthesis,a two-stage transshipment model is established to sequentially synthesize WEN under adiabatic condition with the objective of minimum work utility and the minimal number of units.The strategies of constructing an intermediate pressure of low-pressure streams and of merging the adjacent pressure interval are employed respectively to automatically obtain the initial WEN and further optimize the configurations on the basis of the presented matching rules and structure optimization strategies.Hence,the utility consumption and the number of units are reduced.Based on this,the notion of coupling optimal placement of heat-exchange units into WEN is proposed by aiming at the minimal total annual cost,which is achieved by the following strategies in sequence:introducing heat-exchange units directly,adjusting the work quantity of the adjacent utility compressors or expanders,and approximating upper/lower pressure limits,consequently to obtain considerable cost savings of expanders or compressors and work utility.Moreover,the in-depth discussion of the influence of isentropic efficiency on the coupling of WEN and heat-exchange units placement is performed,which provides the basis for synthesis of WHEN.(3)To efficiently balance the capital investment and operational expense,a stepwise synthesis method for WHEN synthesis is proposed,sequentially implemented by WEN synthesis,determination of hot or cold identities and HEN synthesis.An extended multi-stage superstructure with stream splits and work utility in stages is constructed for synthesizing WEN under adiabatic processes,and then a mixed-integer nonlinear programming(MINLP)model is formulated with the goal of minimizing total annual cost,to obtain the optimal WEN configurations.The approach for identifying hot and cold identities and the strategies for coupling WEN and HEN superstructures are proposed for stepwise synthesis of WHEN based on the determined hot and cold streams.Regarding different cases with consistent and/or inconsistent temperature and pressure changes,the impact of work and heat integration sequence and the variable thermal identities on the economy and configurations of the overall system is investigated.Based on the theory of exergy destruction,thermodynamic analysis for derived WHEN configuration is conducted to explore the rational energy utilization among work and heat sub-networks,so as to clarify the relationship between energy loss and economy,which will lay the theoretical foundation for the simultaneous synthesis of WHEN.(4)On the basis of sequential integration,a simultaneous synthesis method for WHEN is proposed based on WEN and indistinct HEN coupled topological superstructure,to further explore the overall optimum of WEN and HEN sub-networks.A simultaneous synthesis strategy is proposed based on the combination of identification for thermal identities and work-exchange as well as heat-exchange matches.The MINLP formulations targeting the minimum total annual cost are established for WHEN synthesis that concurrently considers heat integration between unclassified hot/cold streams and work integration among pre-classified high-/low-pressure streams.The upgraded WHEN superstructure considers work exchange concurrently with heat integration between stages of and at the entry and exit of WEN combined with end-coolers and end-heaters optimization.Regarding the problem that simultaneous model features greater complexity and heavier computation load,the combination of thermodynamic analysis and deterministic algorithms are taken to simplify the model and accelerate the solution process.The presented method is applied to two examples from literatures for performing the simultaneous integration of work and heat,which proves that the simultaneous synthesis strategy can efficiently improve the energy efficiency and economic benefits.