Thus, the existing parameters from DDBST can provide accurate predictions for the bubble points of the given system. Similar to the previous systems, some blends in this system have low bubble points due to the presence of ethanol interactions. On the other hand, the presence of water–water and propanol–propanol interactions may account for the high bubble points of some mixtures.
Similar to the previous system, due to the high percentage of ethanol, low bubble points are calculated for some mixtures due to the prevalence of CH2 subgroup interactions. The ethanol–water interaction may still be responsible for the low bubble points and is still the reason for the existence of the ethanol–water minimum boiling azeotrope. On the other hand, some mixes have high bubble points due to the high percentage of water present.
In the UNIFAC Dortmund model, the low bubble points may still be due to the presence of interactions with CH2 subgroups, especially the same CH2-H2O interactions. This may be due to the difference in the strength of OH interactions present in the two types of mixtures. On the other hand, high bubble points may be caused by the prevalence of H2O-H2O interactions due to the high percentage of water in the mixture.
Therefore, the ethanol-ethanol and ethanol-water interactions may still account for the low bubble points of these mixtures.
In general, good predictions are obtained for systems without salt because the UNIFAC Dortmund model shows that the interactions through hydrogen bonds such as OH-OH, OH-H2O and H2O-H2O have high 𝜏𝑚𝑘 values, therefore they are confirmed to be the strong interactions in the mixture. However, caution should be exercised in applying the model to systems with highly polar MSAs, where the effect of the alkyl chain to the OH group in the ethanol molecule is neglected by the model, especially at high concentrations of highly polar MSAs in the mixture. These parameters were used to predict the VLE of ethanol, water and TRIS system, which will be presented in the next section.
As can be seen from the list of values in Tables 5.28–5.29 for the bubble points and vapor composition values, the calculated values are in good agreement with the experimental values. This is also evident from the error values for the various fit metrics in the table, where low error values and high coefficients of determination are obtained. However, if the change in thermodynamic properties in the mixture after the addition of TRIS is observed, as shown in the liquid-vapor composition diagram in Figure 5.20, the UNIFAC Dortmund model may not be a.
This is because the significant change in the properties with the increase in the concentration of TRIS in the mixture cannot be easily seen in the diagram. From the experimental data of Bungay et al. 2011), it was observed that the addition of TRIS in ethanol-water mixture leads to an increase in the bubble point and the salting out of ethanol from the mixture. This is probably due to the high polarity of TRIS containing 3 OH and 1 NH2 groups, which causes it to have strong interactions with water and desalting ethanol in the process.
On the other hand, the addition of TRIS in the UNIFAC Dortmund model causes the existence of CNH2-H2O subgroup interaction, which is the strongest subgroup interaction of water in the mixture based on the 𝜏𝑚𝑘 values. Moreover, this could also be the main cause of the increase in the bubble point of an ethanol-water mixture. Since the bubble point increase, water salting in and ethanol salting out are also predicted by the UNIFAC Dortmund model, this may be the reason why the calculated thermodynamic properties for this system agree well with the experimental results. values.
In the UNIFAC Dortmund model, it was also observed that the estimation of the cmk parameters improved the prediction of the bubble points by reducing their values so that they could be closer to the experimental values. Since the model considered these steric hindrances in the CNH2-OH interaction, this could also have contributed to the good prediction results from the model. It should also be noted that the cmk parameters have been adapted only for this system and have not yet been applied to other systems containing the other CHNH2 and CH2NH2 subgroups in which, like the CNH2 subgroup, they are also included in the CH2NH2 main group.
Revised UNIFAC Dortmund Parameters for Interactions with NaCl and KCl and Prediction Results at VLE
The calculated bubble points using the revised parameters are shown to be closer to the experimental bubble points than those obtained from the previous authors' parameters. This can be supported by the calculated error values and coefficients of determination in tables, where the bubble point deviations resulting from the use of the revised parameters are lower than the parameters of the previous authors. As can be observed, the data points for the revised parameters are closer to the diagonal line than for the parameters from the previous authors.
Therefore, this ion is mainly responsible for the increase in the bubble point of the solution. Due to the resulting deviation of the experimental bubble points, the revised parameters become more reliable for estimation than the previous authors' parameters. Therefore, any of these parameters can be used to accurately predict the bubble points of the system at different pressures.
Similar descriptions of interactions were also obtained using the UNIFAC Dortmund model and the revised parameters of this study. The calculated bubble points using the revised parameters and the parameters of Aznar & Telles (2001) are almost equal to each other, which is why almost equal values of the error values and coefficients of determination from the experimental data are obtained also as shown. in tables 5.40-5.41. Comparison of experimental and calculated bubble points for ethanol (1) + glycerol (3) + NaCl (8) is shown in Figure 5.26.
It can be seen that the calculated bubble points using both the revised parameters of this study and the parameters of Aznar & Telles (2001) are close to the experimental bubble points of the system. However, this is not the case for most of the calculated bubble points at higher temperatures, which even exceed 10 K difference from the experimental bubble points. In general, the UNIFAC Dortmund model using either parameter may not be suitable for predicting bubble points of water and glycerol systems with salt.
Therefore, the poor reliability of the UNIFAC Dortmund model for predicting bubble points can be observed for this system. It can be seen that good prediction results are obtained using the revised parameters and those of Aznar & Telles (2001) and that the calculated values using the revised parameters are closer to the experimental values than those of the previous authors. . Thus, the revised parameters of this study proved to be more reliable for predicting the VLE of both systems than the existing parameters of previous authors.
Using the parameters of Aznar & Telles (2001), it is predicted that the interactions involving ions will be stronger due to higher calculated λ values than those of the revised parameters. This can be verified by the error values and coefficients of determination given in the previous tables, where the error values corresponding to the revised parameters are lower than those of the previous authors' parameters. The comparison of the experimental and calculated bubble points for the remaining quaternary systems containing ethanol, water, propanol and salt is shown in Figures 5.39-5.50.
This is why the revised parameters are more reliable than the parameters of the previous authors.
