Study Of Refrigerant Composition Shift And Performance Optimization For Zeotropic Refrigeration System

Non-azeotropic refrigeration is widely used in the cryogenic field, of which study is ofinterest from an academic as well as a practical point of view.In the refrigeration cycle, gas-liquid flow with phase transition is inevitable. Intwo-phase flow of mixed working fluid, there will be a difference between local compositionand circulating composition, i.e. composition shift. Based on conservation equations, themathematical model of the composition shift was set up and solved. The result calculated withpropane/I-butane binary mixture was verified by the experiment in the evaporator of arefrigerator. Plus, the composition shift principle was gotten by the analysis from themathematical model. The composition shift makes the difference between the circulatingcomposition and charged composition in a refrigerator. The flowing refrigerant contains somemore low-boiling-point components and some less high-boiling-point components.Additionally, in the auto cascade refrigeration (ACR) process, the variation of J-T valvesopening also causes the change of the circulating composition of refrigerant. This paperapproximately simulated the ACR system, then analyzed and experimentally verified thecirculated composition variation responding to the alteration of the opening of valves.In order to improve the efficiency of ACR system, the causes of low efficiency in ACRexperiment were analyzed, and the ACR process running with the ideal heat transfer processeswas obtained. Then, based on the characters of temperature variation of heat source, thedemands for the properties of mixed refrigerant were analyzed and listed. At last, thecomposition and operating pressures were optimized for the one-stage ACR heat pumpsoperated with large temperature difference using R41/R125/R124ternary-componentsmixture as refrigerant.The refrigerating processes a none-azeotropic as working fluid shares a significantpart in liquefaction field. To optimize the refrigeration process using multi-componentsnon-azeotropic mixture, this paper proposed a new optimization process which applies thecommercial simulation software Aspen plus to simulate the process and searches theoptimal solution using Genetic Algorithm (GA). It was applied to optimize the SMR LNGprocess's refrigerant composition and running pressures in this paper. In addition, theperformances' sensitivity to each optimized variable and the solution adaption to theenvironment change were studied. When ambient condition alters, the operating pressureswere fixed, the mixed-refrigerant compositions were optimized under different inlet temperatures of the cold box (T2) and performed linear regression. When the liquefaction loadalters, the refrigerant composition was fixed, the operating pressures were optimized. Thenthe corresponding rotate speeds of centrifugal compressor were calculated and quadraticallyregressed with the liquefaction load.Of course, it is impossible to make the refrigerant composition timely alters as theambient condition changes. Therefore, at last of this paper, the LNG process containingprecooling pure refrigerant cycle and deep-cooling mixed refrigerant cycle was proposed. Itcan adjust the precooling load to adapt the change of ambient condition and sequentially keepthe stability of mixed refrigerant cycle. The process synthesizes the stability of purerefrigerant cycle in load adjustment and high efficiency of mixed refrigerant cycle. Thetheoretical basis of energy saving was proposed and the payback time of the innovation investwas analyzed.