To get an idea about the relative volatilities of components we proceed with a
simple fl ash of the outlet reactor mixture at 33 ° C and 9 bar. The selection of the
thermodynamic method is important since the mixture contains both supercritical
and condensable components, some highly polar. From the gas - separation viewpoint
an equation of state with capabilities for polar species should be the fi rst
choice, as SR - Polar in Aspen Plus ™ [16] . From the liquid - separation viewpoint
liquid - activity models are recommended, such as Wilson, NRTL or Uniquac, with
the Hayden O ’ Connell option for handling the vapor - phase dimerization of the
acetic acid [3] . Note that SR - Polar makes use of interaction parameters for C 2 H 4 ,
C 2 H 6 and CO 2 , but neglects the others, while the liquid - activity models account
only for the interactions among vinyl acetate, acetic acid and water. To overcome
this problem a mixed manner is selected, in which the condensable components
are treated by a liquid - activity model and the gaseous species by the Henry law.
Table 10.8 presents a comparison of SR - Polar EOS and Wilson - HOC with Henry
components. The predictions by the two methods are in good agreement, although
surprisingly for the ability of SR - Polar to account for liquid - phase nonideality.
Because by single fl ash only two - thirds of the vinyl acetate passes in liquid phase,
a multistage equilibrium separation is necessary for its advanced recovery, namely
an absorption unit. A suitable solvent is the acetic acid itself. Consequently, after
cooling the reaction gas is treated in countercurrent in the column (C - 1) with acetic
acid fed on the top stage (Figure 10.4 ). To achieve higher recovery a liquid pump -
around with intermediate cooling is employed. The simulation in Aspen Plus ™
[16]indicates that 20 stages are suffi cient for > 99.9% recovery with a pump - around
between stages 4 and 2.