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Prediction Of Vapor-Liquid Equilibrium With Modified UNIFAC-PR/SRK Group Contribution Equation Of State

Posted on:2004-01-15Degree:MasterType:Thesis
Country:ChinaCandidate:Y W FuFull Text:PDF
GTID:2121360095956877Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Lermite and Vidal(1992) employed Huron-Vidal mixing rule using only the residual part of explicit activity coefficient models(UNIFAC model), and according to it, they proposed the UNIFAC-PR/SRK model which can be used to predict phase equilibrium for polar and non-polar systems. Moreover, they estimated some group interaction parameters for UNIFAC-PR/SRK models. Thus defined model can be applied to vapor-liquid equilibrium of mixtures containing methane, ethane, carbon dioxide, paraffinic and aromatic hydrocarbons, alcohols and ketones. At present, the group interaction parameter matrix isn't perfect, and the prediction deviations are very big for asymmetric systems, e.g. for ethane/n-C36H74 using the UNIFAC-PR model, mean deviations for the calculation of bubble-pressure is 74.0%. In fact, the structure of alkane molecules are very similar to each other, so it is reasonable to assume that the excess Gibbs free-energy appearing in the Vidal mixing rule should be equal to zero for alkane systems. According to this hypothesis, we can get a modified Vidal mixing rule. Both the simplified mixing rule and HV mixing rule composed the modified group contribution of state, namely MUNIFAC-PR/SRK model. The coefficient equation of PR and SRK equation of states can be obtained according to the mixing rules. At the same time, some groups are re-defined, for example, carbon monoxide, acetylene, ethylene, propylene and propane are considered as single group. And it requires the re-estimation of parameters for all the systems because of using infinite reference pressure of the mixing rule. So, about 34 pares of group interaction parameters have been estimated by means of Nelder-Mead method. Predictions for the bubble-pressure and vapor composition of binary vapor-liquid phase equilibrium are performed by means of this modified UNIFAC-PR/SRK model. This calculation includes 11 asymmetric binary systems and 48 polar and apolar binary systems, which totaled 59 binary vapor-liquid equilibrium systems. The modified model yields much better prediction compared to the prediction with the original model for asymmetric systems,the mean deviation of bubble pressure decreased by more than 25 percent. For 48 polar and non-polar systems, the mean deviations of the calculation of bubble-pressure and vapor composition are 4.07% and 3.53% by using UNIFAC-PR model, respectively. And the mean deviations of bubble-pressure and vapor composition are 4.33% and 3.12% by using UNIFAC-SRK model, respectively. Satisfactoryaccuracy is obtained in calculating the bubble-point pressure and the vapor composition of the phases in equilibrium.
Keywords/Search Tags:vapor-liquid equilibrinum, minxing rules, group interaction paramteters, bubble-point pressure, vapor composition
PDF Full Text Request
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