Distillation - Principles of Chemical Separations with Environmental Applications - page 102

(a) (b) (c) V

L V L V L

F F F

V V

V

L

L

L

(d) V (e)

V

L L

F F

V V

L L

Figure 4.7 Different types of feeds. q = moles of liquid flow in stripping section
that result from one mole of feed. (a) q > 1: cold liquid feed; (b) q = 1: feed at
bubble point (saturated liquid); (c) 0 < q < 1: feed partially vapor; (d) q = 0:
feed at dew point (saturated vapor); (e) q < 0: feed in superheated vapor.

To plot the feed line, you will need a point in addition to the slope. The y -intercept
could be this point, but an easier one to find is where the feed line intersects the diagonal
at y

x F .
The analysis for the feed stream can be generalized and applied to any side stream,
whether it is a feed or a withdrawal. Withdrawal streams will be discussed later (see Side
streams ,p.111).

= x =

Example 4.3: complete McCabe-Thiele method

Problem:
We desire to use a distillation column to separate an ethanol-water mixture. The
column has a total condenser, a partial reboiler, and a saturated liquid reflux. The
feed is a saturated liquid of composition 0.10 mole fraction ethanol and a flow rate of
250 mol

hr. A bottoms mole fraction of 0.005 and a distillate mole fraction of 0.75
ethanol is desired. The external reflux ratio is 2.0. Assuming constant molar overflow,
find the flowrates, the number of equilibrium stages, optimum feed plate location, and
the liquid and vapor compositions leaving the fourth stage from the top of the column.
Pressure is 1 atm.

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