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The existence of huge gas resource in Iran and the global demand for the replacement of fossil fuels with this cleaner energy resource has caused that the large-scale gas export becomes an interesting topic. One of the methods for large-scale gas exports is liquefaction which is done by refrigeration cycle. Considering the importance of the efficient use and the reduction in energy consumption, particularly in large energy consumers like liquefaction plants, it is imperative to optimize the refrigeration cycles used in these plants. While there have been many studies focusing on the power consumption minimization of refrigeration cycles, however, in most of these studies the performance limitations of the refrigeration cycle components have not been considered. Therefore, the results of such studies are not practical for in-use refrigeration cycles in gas refineries. The main goal of this paper is to propose a systematic method to minimize the power consumption of in-use refrigeration cycles in gas liquefaction processes by taking into account the performance limitations of refrigeration cycle components and the interactions between the refrigeration cycle and the core process. In this regard, a combination of thermodynamic viewpoints and pinch technology is used as well as considering the above mentioned limitations, to express the multi-stage refrigeration cycles’ power consumption minimization problem as a function of several independent variables. Up to 15% reduction in the specific power consumption is achieved when the proposed method are implemented on the optimization of a typical in-use three-stage refrigeration cycle, used in a propane liquefaction plant.

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In the present study, the optimization process of phenolic compounds extraction and antioxidant activity (DPPH radical-scavenging assay, FRAP test, and oxidative stability index) of Pistacia khinjuk fruits was evaluated using ultrasound waves through response surface methodology (RSM). To optimize the extraction process from the central composite design with three independent variables including time (5, 27.5 and 50 minutes), temperature (25, 45 and 65 ° C) and ethanol ratio in water-ethanol solvent (0, 50 and 100% ) was used. Significant effects of independent variables on the extraction of phenol compound, DPPH radical scavenging power, FRAP test and OSI of P.khinjuk fruit extract, with the regression coefficients of 97.65, 98.09, 98.62 and 98.38 %, respectively, were noted. Among the independent parameters, ethanol concentration was the most effective parameter because two effects of quadratic and linear were observed in the proposed models of different tests. The interaction effect between different paprameters was not observed in the models of this study, too. It was also the strongest model for the phenolic compounds extraction test; because in this test, in addition to the linear effect of independent parameters that were common to all tests, a quadratic effect was also observed in the parameters of time and ethanol concentration. Also, the best antioxidant activity of P.khinjuk fruit was obtained by using the ultrasonic wave for 50 minutes at 65 ºC and with 57.7% v/v ethanol/water solution. Finally, it was found that the results of experimental tests were close to the values predicted by the response level method.