A COMPARISON OF VARIOUS CUBIC EQUATION OF STATE MIXING RULES FOR THE SIMULTANEOUS DESCRIPTION OF EXCESS-ENTHALPIES AND VAPOR-LIQUID-EQUILIBRIA

Recent new mixing and combining rules for cubic equations of state (EO S) have extended the range of such equations to the accurate descripti on of the vapor-liquid equilibria (VLE) of highly nonideal mixtures. H owever, the simultaneous correlation and/or prediction of vapor-liquid equilibrium (VLE) and liquid mixture excess enthalpies (H-ex) by eith er activity coefficient models or equations of state has been a diffic ult problem in applied thermodynamics. In this communication, we re-ex amine this problem using a modified version of the Peng-Robinson equat ion of state and the two-parameter van der Waals one-fluid, Wong-Sandl er and modified Huron-Vidal mixing rules. For comparison, the direct u se of activity coefficient models is also considered. In each case a t emperature dependence of the model parameters is introduced in an atte mpt to represent simultaneously VLE and H-ex behavior. Four highly non ideal binary mixtures (2-propanol + water, methanol + benzene, benzene + cyclohexane, and acetone + water) are considered. The results indic ate that while all the models can accurately correlate VLE and H-ex da ta separately, attempting to predict the values of one property with p arameters obtained from the other does not give satisfactory results w ith any model. Also, we find that the simultaneous correlation of both VLE and H-ex with the EOS models at one temperature is possible, but extrapolations to other temperatures with parameters obtained in this way did not result in accurate predictions of either VLE or H-ex. The main problem appears to be that the excess free energy (activity coeff icient) models used are not capable of representing both VLE and H-ex over a range of temperatures, and so equations of state that incorpora te these free energy models have the same shortcoming.