Page 172 - Separation process engineering

5. When all operating lines have been plotted, step off stages, determine optimum feed plate locations
and the total number of stages. If desired, calculate a fractional number of stages.

Not all of these general steps were illustrated in the previous examples, but they will be illustrated in the
examples that follow.
This problem-solving algorithm should be used as a guide, not as a computer code to be followed exactly.
The wide variety of possible configurations for distillation columns will allow a lot of practice in using
the McCabe-Thiele method for solving problems.

Example 4-5. Distillation with two feeds

We wish to separate ethanol from water in a distillation column with a total condenser and a partial
reboiler. We have 200 kmol/h of feed 1, which is 30 mol% ethanol and is saturated vapor. We also
have 300 kmol/h of feed 2, which is 40 mol% ethanol. Feed 2 is a subcooled liquid. One mole of
vapor must condense inside the column to heat up 4 moles of feed 2 to its boiling point. We desire a
bottoms product that is 2 mol% ethanol and a distillate product that is 72 mol% ethanol. External
reflux ratio is L /D = 1.0. The reflux is a saturated liquid. Column pressure is 101.3 kPa, and the
0
column is well insulated. The feeds are to be input at their optimum feed locations. Find the optimum
feed locations (reported as stages above the reboiler) and the total number of equilibrium stages
required.

Solution

A. Define. Again a sketch will be helpful; see Figure 4-18. Since the two feed streams are already
partially separated, it makes sense to input them separately to maintain the separation that already
exists. We have made an inherent assumption in Figure 4-18. That is, feed 2 of higher mole fraction
ethanol should enter the column higher up than feed 1. This assumption will be checked when the

optimum feed plate locations are calculated, but it will affect the way we do the preliminary
calculations. Since the feed plate locations were asked for as stages above the reboiler, the stages
have been numbered from the bottom up.
Figure 4-18. Two-feed distillation column for Example 4-5