Page 231 - Separation process engineering

and cumene (HNK). Feed is 0.125 benzene, 0.225 toluene, 0.375 xylene, and 0.275 cumene.
Recovery of toluene in distillate 99%. Relative volatilities: α ben = 2.25, α = 1.0, α = 0.33, α cum =
tol
xy
0.21.

There is one other case to consider. Suppose we have the four components benzene, toluene, cumene, and
xylene, and we choose cumene as the HK and toluene as the LK. This makes benzene the LNK, but what is
xylene? In this case xylene is an intermediate or sandwich component, which is an NK component with a
volatility between the two key components. Sandwich components will tend to concentrate in the middle
of the column since they are less volatile than the LK in the rectifying section and more volatile than the
HK in the stripping section. Prediction of their final distribution requires a complete simulation.

If the top temperature is too cold and the bottom temperature is too hot to allow sandwich components to
exit at the rate they enter the column, they become trapped in the center of the column and accumulate
there (Kister, 2004). This accumulation can be quite large for trace components in the feed and can cause
column flooding and development of a second liquid phase. The problem can be identified from the
simulation if the engineer knows all the trace components that occur in the feed, accurate vapor-liquid
equilibrium (VLE) correlations are available, and the simulator allows two liquid phases and one vapor
phase. Unfortunately, the VLE may be very nonideal and trace components may not accumulate where we
think they will. For example, when ethanol and water are distilled, there often are traces of heavier
alcohols present. Alcohols with four or more carbons (butanol and heavier) are only partially miscible in
water. They are easily stripped from a water phase (relative volatility >> 1), but when there is little
water present they are less volatile than ethanol. Thus, they collect somewhere in the middle of the
column where they may form a second liquid phase in which the heavy alcohols have low volatility. The
usual solution to this problem is to install a side withdrawal line, separate the intermediate component
from the other components, and return the other components to the column. These heterogeneous systems

are discussed in more detail in Chapter 8.
The differences in the composition profiles for multicomponent and binary distillation for relatively ideal
VLE with no azeotropes can be summarized as follows:

1. In multicomponent distillation the key component concentrations can have maxima.
2. The NK usually do not distribute. That is, HNKs usually appear only in the bottoms, and LNKs only in
the distillate.

3. The NK often go through a plateau region of nearly constant composition.
4. All components must be present at the feed stage, but at that stage the primary distillation changes.

Thus, discontinuities occur at the feed stage.