Simulation on vapor-liquid equilibrium of CO2-[emim][Tf2N] in flow state and depressurization of its refrigeration cycle based on Aspen Plus
Abstract To investigate the depressurization mechanism of the CO2 vapor compression refrigeration cycle with an ionic liquid loop, a vapor-liquid equilibrium (VLE) model of CO2-[emim][Tf2N] in the flow state was established based on Aspen Plus, and the prediction of VLE in the supercritical state was carried out. Furthermore, the effects of CO2 inlet pressure, absorber temperature, CO2 inlet mole fraction and CO2 inlet flow rate on the depressurization amplitude of absorber were analyzed through the absorption-desorption process simulation. The results showed that with the increase of CO2 inlet pressure in the supercritical state, the depressurization amplitude increased but its growth rate slowed down, which indicated that the effect of pressure as a driving term of mass transfer decreased in the supercritical state. The depressurization amplitude decreased with the increase of absorber temperature, and decreased rapidly with the increase of CO2 inlet mole fraction. In this sense, adding an auxiliary desorber at the absorber inlet is beneficial to the system depressurization. Due to the limited absorption capacity of quantitative [emim][Tf2N] in the absorber, with the continuous increase of CO2 inlet flow rate, the depressurization amplitude first dropped sharply, then flattened, and later dropped rapidly. Because the CO2 flow rate in an actual system is proportional to the system cooling capacity, the proper mass flow ratio of CO2-[emim][Tf2N] should be selected to reconcile the cooling capacity and depressurization amplitude.