Improvement For The Pinch-Based Method And Optimization Of Heat Exchanger Networks With Cost Savings

With global energy demand increasing and warming intensifying,efficient energy utilization continues to be a concern.Process industries are strongly encouraged to make their production more energy-efficient.Heat recovery through the heat exchanger network(HEN)can realize efficient energy use and improve economic benefits.Pinch analysis is widely used in HEN optimization,which makes contributions to green and sustainable development.Strengthening the optimization guiding ability of pinch analysis has important engineering application value.When applying pinch analysis,the reasonable pinch position should be determined first,and then the specific HEN design can be performed with the help of pinch analysis principles.Determining the pinch position with the most cost-saving potential for the HEN depends very much on the prediction accuracy of the capital cost targeting method for the optimal capital cost at the targeting stage of pinch analysis.For the specific HEN design,graphical tools provide convenience for the decision-making and implementation of optimization schemes by giving clear insights.In order to overcome the deficiency of current capital cost targeting methods and graphical tools,this study proposes improvement approaches to make pinch analysis more effective in guiding the energy efficiency increase and overall cost reduction of the HEN.The main work contents are as follows:(1)Since the accuracy of conventional capital cost targeting methods is insufficient,a new approach for method establishment is provided.Conventional methods based on the spaghetti(SPA)structure ignore matching optimization and might result in cost targets with large deviations.Therefore,the SPA structure is improved by loop elimination to shift energy toward low-cost matching.The fourth structure evolved from the SPA structure(ESPA-Ⅳ structure)has the optimal matching distribution and forms the basis for establishing the ESPA method.It is validated by numerical experiments and applied to a case reported in the literature;meanwhile,comparisons are always made to the SPA method.The numerical experiments prove that the ESPA method can obtain capital cost targets more accurately than the SPA method.The absolute target deviations(often within ±5%)given by the ESPA method are much lower than those(always above 10%)derived by the SPA method.In the case study,the given HEN is further optimized,as hinted by the ESPA method results.Of the two target methods,the cost target indicated by the ESPA method is closer to the optimal capital cost of the case.The high accuracy of the ESPA method is further verified.(2)Since existing capital cost targeting methods for the capital cost of HEN are not suitable for wide application scenarios,a general capital cost targeting method is further developed.For achieving high adaptability,the establishment of this method considers the different heat exchanger cost categories,different cost laws for one stream pair,and area limitations of heat exchangers,which may be encountered in practice.In addition,allowing streams to use individual temperature difference contributions makes the method can be used to generate a more cost-effective diverse pinch analysis.The targeting method is still built on a final ESPA structure that evolved from the SPA structure of HEN through four loop elimination stages.This structure helps reduce the prediction deviation of the method.By taking improvement measures to overcome the deficiencies in establishing the method,the accuracy and stability of the targeting method are further enhanced,with absolute target deviations generally within 10%and often within 5%.The developed general capital cost targeting method provides a benchmark for the optimal capital cost of HEN and can be used to determine a pinch position with high cost-saving potential for pinch analysis at the targeting stage.(3)In order to improve the efficiency of the graphical tool in retrofitting the HEN with multiple minimum temperature difference constraints,a new graphical tool is developed.The retrofit of HEN often needs topology changes of the network structure to overcome the network pinch,thereby increasing heat recovery.It is difficult to maximize energy recovery while minimizing topology changes.Graphical tools are widely applied in the retrofit of HEN due to their good interactivity and visualized effects.However,for a HEN with different minimum temperature differences,it becomes difficult to screen retrofit options to avoid crossing the pinch.Therefore,the shifted temperature driving force plot is proposed to solve this problem.The retrofit for the HEN with multiple minimum temperature difference constraints belongs to the problem of diverse pinch analysis.With the aid of pinch analysis,the ideal regions are divided to indicate where heat exchange matches should be located.By shifting the stream temperature,the stagger of ideal regions that emerges before shifting the stream temperature can be avoided.This makes the screening of retrofit strategies easy and convenient.The retrofit of an industrial case is conducted with this graphical tool.The key changes are for the heat exchange matches that cross the pinch.The application results show that with fewer topology changes(five topology changes),more energy is recovered(the annual energy cost saving of 1.600 M$/y),and a better economic benefit(the annual profit of 1.125 M$/y)is generated.The application effectiveness of the shifted temperature driving force plot is verified.(4)In order to solve the problem of insufficient display of stream data in the graphical tool,a new comprehensive graphical tool is further developed,and then it is combined with the optimal pinch analysis to maximize the cost-effectiveness of the HEN design.Physical insights for optimizing HENs rely heavily on the reading of stream data in graphical tools.Enriching stream data is an effective way to enhance insights.Therefore,this study develops a shifted TQCH diagram for HEN optimization.This graphical tool organically combines stream data such as temperature,energy,heat capacity flow rate,and heat transfer coefficient for the first time,which is specifically realized by five independent and interrelated subgraphs.With this graphical tool,the stream data for the individual streams or heat exchange matches can be easily referred to during HEN optimization.The use of shifted stream temperatures ensures a clear indication for the pinch analysis applied in this graphical tool,even for the problem of the diverse pinch.In order to maximize the cost-effectiveness of HEN design,the graphical tool uses the optimum pinch analysis with the maximum cost-saving potential.In the studied synthesis and retrofit cases,in conjunction with the optimization strategies provided,an optimal synthesis solution(the optimal capital cost of 323800 $ under fixed heating demand)is readily obtained,and a more cost-effective retrofit solution(the lowest annual total cost of 4.37 M$/y)is found.