| In recent years,with the increase of fossil energy consumption,carbon dioxide emissions continue to rise,energy and environmental issues have increasingly attracted people’s extensive attention.The development of advanced technologies and methods is an effective way and an important means to improve energy utilization efficiency and achieve energy conservation and emission reduction goals.With the development of chemical systems engineering and thermal energy and power engineering,great progress has been made in the research of heat exchanger network synthesis,compression-absorption cascade refrigeration system(CACRS),organic Rankine cycle(ORC)and other advanced thermodynamic cycles that can realize waste heat recovery.If the heat exchanger network(HEN)can be used in cooperation with advanced thermal cycles such as CACRS and ORC,it will be of great significance to energy efficiency improvement,energy forms diversification,as well as consumption and emission reduction.Improving the waste heat recovery rate and thermal cycle performance are both effective methods to enhance the energy utilization efficiency.At present,most of the research on waste heat recovery only focuses on the optimization of a certain technology,ignoring the effect of operating conditions of the synergistic technology;the action mechanism and coupling relationship between different technologies have not been considered comprehensively and carefully;and systematic optimization from a global perspective also has not been implemented.Therefore,in this paper,the CACRS and HEN are simultaneously synthesized along with considering the optimization of operating condition,establishing the integrated mechanism between the HEN and thermal cycle.Subsequently,based on the multi-energy characteristics of refrigeration system,the ORC is introduced to cooperated with CACRS,and a multiobjective optimization design method which can balance thermodynamics and economy is proposed,clarifying the coupling relationship between ORC and CACRS.Then,in order to meet the diverse demands of energy and further improve the utilization efficiency of waste heat,a simultaneous integration method combining thermodynamic model and mathematical programming is proposed to realize the optimal design of HEN,CACRS and ORC from a global perspective.Finally,the method proposed in this paper is applied to industrial practice for verifying its effectiveness and feasibility.The main research contents of this paper are as follows:(1)Aiming at the problem that the effect of operating condition within refrigeration system on the structure of HEN is not deeply considered in the synthesis research of HEN integrating with refrigeration system,this paper proposes a simultaneous synthesis method for CACRS and HEN along with considering the optimization of operating condition.The coupling relationship of CACRS and HEN is quantitatively expressed based on simple discrete temperature model and strict thermodynamic model,respectively.A mixed integer non-linear programming(MINLP)model with the objective of minimizing total annualized cost is established,through which the integrated design for CACRS and HEN is realized.Compared with the scenario without CACRS,in the scenario where CACRS is integrated and the coupling relationship is established based on simple discrete temperature model,the total waste heat recovery rate of the integrated system is increased by 18.7%,and the cold utility consumption is reduced by29.3%.Compared with the simple discrete temperature model,the refrigeration performance corresponding to the optimal result obtained based on the strict thermodynamic model is improved by 0.17,and the total annualized cost is reduced by 36.4%.The result of the case shows that the integration of CACRS can effectively improve the energy utilization efficiency and reduce external utilities consumption,and the simultaneous optimization of operating condition can further improve the refrigeration performance and reduce the economic cost.The proposed method reveals the correlation between operating parameters of thermodynamic cycle and HEN structure and lays the integration mechanism for subsequent integrated study of HEN and multiple thermodynamic cycles.(2)Aiming at the single coupling structure of ORC and CACRS,and the weak correlation between heat source and ORC as well as CACRS,this paper comprehensively considers all coupling possibilities between different cycles and various waste heat utilization forms to simultaneously optimize the interaction between structure and parameters of ORC,CACRS and heat source.In addition,the trade-off between economic goal and thermodynamic goal is achieved through multi-objective optimization.The effectiveness of the method is verified through a case study.After optimization,a series of Pareto optimal solutions that compromise between economic cost and exergy destroy are obtained.Under the economic goal,the total annualiaed cost is-1650880 $/y,and the corresponding global exergy destroy is 11675 k W.Under the thermodynamic goal,the global exergy destroy is 7848 k W,and the corresponding total analized cost is 946017 $/y.Through the analysis,it can be found that with the enhancement of thermal coupling between ORC and CACRS,the global exergy destroy of the integrated system is decreased,while the total annualized cost is increased.In this study,a method framework for the optimal design of the CACRS-ORC integrated system is established,the influence rule of operating parameters and integrated structure on thermodynamic and economic performance is revealed,and the coupling mechanism between CACRS and ORC and their interactions with heat source under different optimization objectives are clarified.Based on the multi-objective optimization,sensitivity analysis of key parameters affecting the integrated structure and performance is further carried out,which will provide effective thermodynamic feasibility guidance for the efficient recovery of waste heat.(3)The global optimization and integration of multiple waste heat recovery technologies including cooling,heating and electricity applications is of great significance in terms of industrial demand and energy efficiency improvement.However,how to establish the relationship between driving and consumption of energy among various technologies,and consider the internal operation mechanism of all technologies in order to optimize the operating parameters simultaneously,is a challenge in the integration methodology.In this work,an extended superstructure incorporating CACRS,ORC and HEN with internal matching and operation as well as external interaction and coupling is proposed.In addition,a simultaneous integration method combining strict thermodynamic and mathematical programming model is established to realize the integrated optimization and comprehensive design of HEN,CACRS and ORC from a global perspective.Aiming at the large scale and computational difficulty of the simultaneous integration model,a series of summarized empirical rules related to structure and operation are adopted as assistant solving strategies to strengthen the solving process and shorten the solving time.Two cases under different application scenarios are studied.For Case1,the global exergy destroy under the economic goal is reduced by 31.5% compared with the previous method,verifying the superiority of the proposed method in improving energy efficiency and reducing energy loss.For Case 2,the produced cooling energy is stored to meet the refrigeration needs of other process,the total analized cost and global exergy destroy of the optimal economic structure are 1124778 $/y and 2531 k W,the global exergy destroy and total analized cost of the optimal thermodynamic structure are 900 k W and 1319326 $/y.(4)Finally,the proposed method is applied to the refrigerated methanol process,and the HEN corresponding to the process is integrated optimization and synthesized design,in order to realize the efficient and reasonable utilization of energy and reduce the economic cost of enterprises.Compared with the optimal solution of the original literature,the waste heat recovery rate corresponding to the economic optimal solution is improved by 13.4%,the use of cooling water is completely eliminated,and the consumption of ammonia refrigeration is reduced by 1.7%;meanwhile,the additional power generated by the integration system can be sold for profit,which reduces the economic cost to a certain extent.Through multi-objective optimization,decision makers can also choose the best solution for their needs according to their preferences.The research in this paper demonstrates the application potential and development value of the proposed method in improving energy efficiency and reducing economic cost,and can provide important theoretical guidance for the efficient and rational utilization of energy in the coal-to-methanol process. |
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