Vapor-liquid equilibria in associating systems using equation of state and activity coefficient models

A chemical compound that hydrogen bonds generally exhibits unusual physical properties that are difficult to model theoretically. Also, in mixtures, hydrogen bonds can produce both almost ideal mixtures or very large deviations from Raoult's law. Our purpose is to develop methods for engineering use that can deal successfully with hydrogen bond. We have developed equation of state and activity coefficient models for the associating compounds and mixtures. We summarize this work below.; A simplified SAFT equation of state is developed for nonassociating and associating compounds. For pure compounds, the simplified SAFT equation of state can correlate the vapor pressure and liquid density with errors equal to those of as the original SAFT equation. With only a single binary adjustable parameter, the new equation results in better correlations of vapor-liquid equilibria than the original SAFT equation in most associating mixtures. However, for some mixtures, both the original and simplified SAFT equations of state fail to provide a satisfactory description.; Based on the Wertheim's theory, a UNIQUAC association model has been developed. In this model hydrogen bond is assumed to be a strong interaction force. Compared with the original UNIQUAC model, this UNIQUAC association model produces better correlated results in the binary vapor-liquid equilibrium data for self-association mixtures containing an alcohol or an acid and both activity coefficient models have small errors in pressure and vapor phase mole fraction of binary cross-associating mixtures.; Two UNIFAC association models were developed based upon Wertheim's theory of association. In both models the activity coefficient is the sum of combinatorial, residual and association contributions. The UNIFAC group-contribution model is used for the combinatorial and residual terms. Two different types of association models are considered. The UNIFAC-AG model uses the functional group concept for association, while the UNIFAC-AM model considers association to occur between molecules. For associating mixtures containing acids, alcohols or water, both activity coefficient models provide better predictions of binary vapor-liquid equilibrium than the original UNIFAC model.