Local Composition Mixing Rules Revisited

CISMONDI DUARTE, MARTIN; MOLLERUP, JØRGEN

Puerto Varas

Conferencia; IX Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design; 2012

Recent works have shown, for the cases of CO2+n-Alkane mixtures, that Cubic Mixing Rules (CMRs) are able to correlate complex phase behavior in wide ranges of temperature and pressure, including critical lines, VLE, LLE and LLVE, which was not possible with Quadratic Mixing Rules (QMRs). Nevertheless, four interaction parameters are used per binary (while it is only two in QMRs) and considering the temperature dependence for the attractive ones, we actually have eight fitting constants per binary. Then, the challenging question is: can we do a similar job with less fitting parameters? One modeling concept that appears worth trying is local compositions since, when having molecules of different chemical nature, their distribution in a dense fluid mixture will not be completely random, but some interactions -or contacts between the corresponding groups or segments- will be statistically preferred to others. Then, we want to investigate how much improvement can be achieved in correlation capacity when the QMR for the attractive parameter is modified according to the concept of local compositions. One important point is that, when considering size asymmetric systems, it should be realized that energetic interactions take place between segments of equivalent size or surface, rather than between entire molecules. This is already implicit in the classical QMRs, which imply random distribution of molecules in the fluid phase, therefore introducing no practical difference respect to a formal treatment based on molecular attractive parameters. Nevertheless, a segment based treatment can make a very important difference when introducing the exponential weight factors in a Boltzmann type distribution law. In this talk the derivation of the new mixing rule will be briefly exposed. Then some preliminary unsuccessful results will be shown, discussing and explaining some effects which may appear as unexpected. Finally, comments on the possible future directions of this investigation will be shared with the audience.