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The use of hydrocarbons as refrigerants has regained the interest of researchers and industries in the last ten years because of increasing restrictions to the use of halogenated fluids. Hydrocarbons do not deplete the ozone layer and allow good cycle performance. Besides this, the need to reduce energy consumption leads to a constant search for optimizing the operation of refrigeration cycles, which are major consumers of electrical energy in many processes industries, especially in the food industry. In this work, two alternatives to approach these issues were combined: the use of hydrocarbon mixtures as refrigerant and the modification of a conventional vapor compression cycle (VCC) by adding a second compression stage and a flash tank for vapor injection between compression stages (FTVI). Using process simulator Aspen HYSYS®, eight different pair of mixed refrigerants involving ethane, propane, isobutane, n-butane and CO2 with different compositions were explored and compared to the performance of a pure refrigerant. FTVI and VCC cycles were compared as well. For each composition tested the cycle was optimized for the highest COP, while attending a fixed refrigeration capacity of 1 kW for a heat exchange fluid temperatures of 8 °C to -4 °C in the evaporator and 25 °C to 35 °C in the condenser. FTVI led to a COP improvement from 4% to 36% when compared to the VCC at the same conditions. Mixed refrigerants presented a better performance than the pure ones for the pairs ethane/propane, propane/isobutane and propane/nbutane. The best results for the tested conditions were obtained for propane/nbutane in the FTVI cycle, with maximum COP (4.9) for the composition 60/40 wt%. Later, a preliminary economic analysis of the cycles was presented and they were then optimized for minimal annual cost. For this second part of the study, the considered refrigeration capacity was 1 MW. The smallest annual cost was obtained for FTVI system, operating with propane/n-butane 20/80 wt% refrigerant mixture.

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Refrigeration systems are major consumers of energy. To reduce this energy consumption, alternative cycles have been studied and one of the most important is the refrigerant injection system. In the last decades, the environmental restraints of refrigerants have increased and refrigerant mixtures have attracted attention. In this context, this work presents the energy and exergy analyses for a conventional vapor compression refrigeration cycle (VCC) and an alternative vapor injection cycle with flash tank (FTVI) using a mixture refrigerant of R290/R600a. The influences of refrigerant mixture composition and expansion ratio in the upperstage valve on energetic and exergetic performance parameters were evaluated. Free expansion and heat transfer were identified as the greatest irreversibilities in VCC and FTVI, respectively, accounting for 26.7% and 23.6% of total exergy inputs in these systems. Higher values of efficiency metrics were observed at around 40% wt of R290 for both cycles. Reasons for the observed behaviors and ways to improve the cycles from the thermodynamic perspective are identified and discussed.