Enthalpy estimation for new working fluid mixtures based on ionic liquids for absorption heat pumps.

Enthalpy is a thermodynamic property necessary in every thermal engineering applications such as the design of heat exchangers, power plants, absorption heat pumps and refrigeration, etc. There are different approaches to calculate the enthalpy of mixtures from experimental data. The most apropriate way is to calculate it from experimental data of excess enthalpy and isobaric heat capacity at different temperatures and compositions. However, the excess enthalpy is usually available only at a single temeperature. As alternative, it can be obtained from thermodynamic models such as equations of state and activity coefficient based model, which usually require experimental data of vapor-liquid equilibrium to obtain binary interaction parameters. However, it is well known that the direct calculation of derived properties such as excess enthalpy from experimental data usually leads to significant errors due to thermodynamic inconsistencies. For this reason, accurate data and consistant methods are necessary to obtain suitable results. In this research, first we estimated the excess enthalpy using the Redlich-Kwong-Mathias equation of state and the Non-Two Random Liquids (NRTL) model, and next we compared it with the experimental values. Then, the obtained information was used to determine the effect of this property for the enthalpy calculation. Two ionic-liquid based mixtures, with potential application in absorption heat pumps, have been analyzed: 2,2,2-trifluoroethanol + 1-ethyl-3-methylimidazolium tetrafluoroborate, and water + 1,3-dimethylimidazolium dimethylphosphate.