Information Technology Reference
In-Depth Information
2
4
6
1
DIST1
DIST2
3
5
Figure 14.5
Extractive distillation flowsheet.
ability to change the activity coefficients of the binary system such that the relative
volatility of the lighter component is increased. Typically, the solvent is introduced
near the top of the column and is withdrawn at the bottom along with the heavier
component. The solvent is separated from the heavier component in another column,
and the solvent is recycled. Such a process is shown in Figure 14.5.
When designing such a process, in addition to determining the number of theoretical
stages required and the primary feed location, one must deal with the following issues:
Solvent selection
Solvent use per mole of feed
Stage for introduction of the solvent into the column
There are no hard and fast rules for solvent selection. A list of potential solvents is
given by Van Winkle (1967), and general guidelines for solvent selection based on the
polarities of the components to be separated are given by Wankat (1988).
The initial step in designing a system is to evaluate a proposed solvent. Does
it increase the relative volatility of the lighter component? How much solvent is
required? As an example, Seader and Henley (2006) suggest that aniline would be
a good solvent for the benzene -cyclohexane system. This can be evaluated by cre-
ating a series of
x
-
y
diagrams on a solvent-free basis using the ratio of solvent
to nonsolvents as a parameter. The calculations to generate such data are given at
Examples/ExampleThreeData. These data were transferred to the Excel spreadsheet
Examples/benzenecyclohexaneaniline.xls, where the calculations to produce
x
-
y
data
on an aniline-free basis are given. The results are displayed in Figure 14.6. One may
observe that as the ratio of aniline to cylohexane plus benzene increases, there is a
dramatic effect on the relative volatility of the cyclohexane -benzene system.
Prior to modeling the complete system it is important that each column's design be
undertaken on a stand-alone basis. For the first column, initial calculations to estab-
lish the number of stages, reflux ratio, bottom specification, and feed locations are
