Local composition model activity coefficient prediction

Activity Coefficients Predicted by the Local Composition Model for Aqueous Solutions Used in Flue Gas Desulfurization... 

The goal of this research was to improve activity coefficient prediction, and hence, equilibrium calculations in flue gas desulfurization (FGD) processes of both low and high ionic strength. A data base and methods were developed to use the local composition model by Chen et al. (MIT/Aspen Technology). The model was used to predict solubilities in various multicomponent systems for gypsum, magnesium sulfite, calcium sulfite, calcium carbonate, and magnesium carbonate SCU vapor pressure over sulfite/ bisulfite solutions and, C02 vapor pressure over car-bonate/bicarbonate solutions. 

Since the degree of coupling is directly proportional to the product Q (D/k)in, the error level of the predictions of q is mainly related to the reported error levels of Q values. The polynomial fits to the thermal conductivity, mass diifusivity, and heat of transport for the alkanes in chloroform and in carbon tetrachloride are given in Tables C1-C6 in Appendix C. The thermal conductivity for the hexane-carbon tetrachloride mixture has been predicted by the local composition model NRTL. The various activity coefficient models with the data given in DECHEMA series may be used to estimate the thermodynamic factors. However, it should be noted that the thermodynamic factors obtained from various molecular models as well as from two sets of parameters of the same model might be different. 

Local composition models, as suggested by the expressions for the activity coefficient in multicomponent systems, provide description of multi-component VLE behavior using binary parameters only. These parameters, in turn, can be evaluated from the corresponding binary data and, consequently, multicomponent VLE information - that is very scarce in the literature - can be predicted. The Wohl-type models, on the other hand, require at least ternary parameters, as Eq. 13.11.1 indicates. 

Later, further g -models based on the local composition concept were published, such as the NRTL [14] and the UNIQUAC [15] equation, which also allow the prediction of the activity coefficients of multicomponent systems using only binary parameters. In the case of the UNIQUAC equation the activity coefficient is calculated by a combinatorial and a residual part. While the temperature-independent combinatorial part takes into account the size and the shape of the molecule, the interactions between the different compounds are considered by the residual part. In contrast to the Wilson equation the NRTL und UNIQUAC equation can also be used for the calculation of LLE. 

It took a little over half a century for the next major development to occur. G. Wilson in 1964 introduced the idea of local compositions which led to an expression for y,- bearing his name and later to the NRTL and UNIQUAC expressions by J. Prausnitz and his co-workers (1967 and 1975). The expressions have two main advantages over the van Laar model first, their parameters have a built-in temperature dependency making them very useful in distillation applications (why ) second, they provide successful prediction of multicomponent activity coefficients from binary ones. Finally, based on the UNIQUAC model, Fredenslund and his co-workers (1977) have developed a group-contribution model, UNIFAC, that can be used for the estimation of activity coefficients for a large variety of systems. 

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