Study On Synthesis Of Flexible Heat Exchanger Networks Embedded With System Reliability Analysis

Rapid progress of process industry urges heat exchanger network (HEN), which isessentially supportive to process industry, to perform reliably during operation period. Heatexchanger network is embedded with intense connections. The heat exchangers those arelinked with the same hot/cold stream produce complicated interactions. If failure of one orseveral heat exchangers within the HEN occurs, variations of inlet/outlet temperature for therelated heat exchanger appear, then the target temperature of process streams can not bereached and heat exchange task that require HEN to provide also can not be finished. So theanalysis of system reliability for HEN is important in HEN design. The approach with systemreliability in flexible HEN synthesis is presented by the paper. Main research processes arerecorded as the following contents:（1） In order to soften computational difficulty, two-step procedure is taken for synthesisof flexible heat exchanger networks with system reliability. Initial flexible HEN designproceeds on the basis of pseudo-T-H diagram approach, then during the next optimizationprocess, as for the HEN arrangement that is unable to fit the system reliability, networkdecoupling for the HEN system is used. The connections among the heat exchangers arechanged so as to raise the system reliability of the HEN. The heat exchanger areas and utilityconsumption are further optimized by genetic/simulated annealing algorithms (GA/SA). Themost economical flexible network arrangement within the tolerance of the system reliabilityrequirement can be received.（2） Based on the system analysis of HEN, the system reliability of HEN is calculated,moreover network decoupling is applied to improve the system reliability of HEN. Networkstructure matrix is introduced to recognize independent subsystems of the HEN. Thennecessary data for evaluating system reliability of HEN, that means the quantity andcomposition of subsystems are obtained. It is found that system reliability of HEN is decided bythe independent subsystem, which includes maximum quantity of heat exchanger units (notedasN sub sysmax). Then the system reliability of HEN is calculated based on the composition ofN sub sysmax.As for the HEN structure in flexible HEN synthesis results that below the systemreliability requirement, network decoupling adjustments are applied. The distributions of subsystems in the HEN are reshaped, and system reliability of HEN is promoted to thepermitted degree.（3） Some heuristic parameters are introduced by mathematical transformation forstructure matrix of HEN, the selection methods of suitable decoupling heat exchangers, whichcan most effectively improve system reliability of HEN, are founded by the heuristicparameters. Features of suitable decoupling heat exchangers based on the heuristic parametersare analyzed. Computational cases show that those heuristic parameters can effectivelydescribe distributions of subsystems in the HEN after network decoupling, therefore canprovide reference for searching suitable decoupling heat exchanger.（4） Different types of calculation cases are analyzed to further verify the selectionmethod of suitable decoupling heat exchangers. For the1stcase, the quantity of subsystems inthe HEN is unchanged after network decoupling. It is also the case that new subsystem can notbe formed by deleting one heat exchanger in the HEN. The values of heuristic parameters foreach heat exchanger are the same. For the2ndcase, the quantity of subsystems in the HEN ischanged after network decoupling. Based on the selection method of suitable decoupling heatexchangers by heuristic parameters, network decoupling is applied to facilitate the HEN, whosesystem reliability is below expectation, to meet requirement. The3rdcase is the engineeringapplication, in which the quantity of subsystems in the HEN is changed after networkdecoupling. For the3rdcase, network decoupling on the basis of the selection method ofsuitable decoupling heat exchangers by heuristic parameters is also applied, flexible HEN thatmeets system reliability requirement is found. These calculation cases not only further verifythat the selection method of suitable decoupling heat exchangers by heuristic parameters fornetwork decoupling is effective, but also remind the method in this work is applicable to solvelarge scale flexible HEN with system reliability measurement.