Page 149 - Separation process engineering

(4-18)

This result is analogous to the use of q in flash distillation. Since the liquid and vapor enthalpies can be
estimated, we can calculate q from Eq. (4-17). Then

(4-19)

The quality q is the fraction of feed that is liquid. For example, if the feed is a saturated liquid, h = h, q =
F
1, and . Once has been determined, is calculated from either Eq. (4-15) or Eq. (4-5, stage f +
1) or from

(4-20)

Which can be derived from Eqs. (4-15) and (4-19).

Example 4-1. Stage-by-stage calculations by the Lewis method

A steady-state countercurrent, staged distillation column is to be used to separate ethanol from water.
The feed is a 30 wt % ethanol, 70 wt % water mixture that is a saturated liquid at 1 atm pressure.
Flow rate of feed is 10,000 kg/h. The column operates at a pressure of 1 atm. The reflux is returned as
a saturated liquid. A reflux ratio of L/D = 3.0 is being used. We desire a bottoms composition of x =
B
0.05 (weight fraction ethanol) and a distillate composition of x = 0.80 (weight fraction ethanol). The
D
system has a total condenser and a partial reboiler. The column is well insulated.

Use the Lewis method to find the number of equilibrium contacts required if the feed is input on the
second stage from the top.

Solution

A. Define. The column and known information are shown in the following figure. Find the number of
equilibrium contacts required.