A modification of the universal quasi-chemical model for correlation of vapor-liquid equilibrium data

This study involves the modification of the Universal Quasi-Chemical model (UNIQUAC) model in order to improve the prediction of the equilibrium data compared to the other conventional activity coefficient models used. The modification takes place by taking the coordination number (z) a fitting parameter and as a second order polynomial function of temperature and employing objective functions on either temperature, vapor composition, activity coefficient, or both temperature and vapor composition difference between the experimental and the predicted VLE data. This modification produces three additional fitting parameters to the original model which is located at the combinatorial term of the model. The values for the coordination number were limited to a low value of 6 and a high value of 12. These were tested on four different systems which were selected based on the different types of interactions present in the system. The model was able to fairly reproduce the equilibrium data for 1-hexane + n-hexene (pairs of non-polar molecules), water, n-propanol, n-butanol (pairs of associating molecules), and non-associating and associating molecules), and methyl ethyl ketone + isopropyl acetate (pairs of non-associating molecules) compared to the other conventional activity coefficients used in literature in terms of absolute average deviation (AAD) except for the system of acetone + ethanol + 2,2,4- trimethylpentane (pairs of non-polar and non-associating, non-polar and associating in which the modified model has similar results with that of the Wilson. The model was also used for data masking in which the parameters derived from the system of 1-hexane + n-hexene at 101.32kPa was used for predicting the VLE at 60kPa and 30kPa. The model was not good at data masking because the coordination number values were lower than 6 which is the lower limit for the value