as the alcohol content decreases. In Eq. (9), lower values of xl
A are
compensated by the higher values of cl
A. Results from the ideal
model in Fig. 8 show that the evaporation rate increases proportionally
to the highest butanol content, but this trend is reversed
in the UNIFAC-Dortmund-Continuous model and in the experiments
because at high butanol concentrations the activity coeffi-
cients are close to unity, whereas at low concentrations they are
very high (Fig. 3). These results were completed with additional
modelling results for all alcohols (not only n-butanol but for all
alcohols) and the resulting evaporation rates (again averaged along
the first eight hours) are shown in Fig. 9. It can be observed that, for
all alcohols, the mentioned compromise between activity coeffi-
cient and alcohol content leads to a wide region (starting from
10% or 20% contents) in which evaporation losses decrease as the
alcohol content increases. Dotted lines in Fig. 9 indicate unstable
blends [11], for which experimental validation is impossible.
As an intermediate result from the model, Fig. 10 shows, as an
example, that n-butanol evaporates much faster than ideally and
thus, its mole fraction in the liquid phase decreases sharply. The
reason for this is that the mole fraction in the vapour interphase
increases with time (as the alcohol content in the liquid decreases)
rather than decreasing (as predicted by the ideal model), as a consequence
of the increase in the activity coefficient.