Page 134 - Separation process engineering

(3-11)

and the enthalpy can be determined (e.g., on the saturated vapor (dew-line) of Figure 2-4 at y = y ). Since
1
the reflux and distillate streams are at the same composition, temperature, and pressure, h = h . Thus,
0 D

(3-12)
Solving for Q we have
c

(3-13)

or, substituting in Eq. (3-9) and then Eq. (3-3),

(3-14)

Note that Q < 0 because the liquid enthalpy, h , is less than the vapor enthalpy, H . This agrees with our
c
D
1
convention. If the reflux is a saturated liquid, H − h = λ, the latent heat of vaporization per mole. With
D
1
Q known we can solve the column energy balance, Eq. (3-5), for Q .
R
c
(3-15a)

or

(3-15b)

or

(3-16)

Q will be a positive number. Use of these equations is illustrated in Example 3-1.
R

Example 3-1. External balances for binary distillation
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 at 40°C. Flow rate of feed is 10,000 kg/h. The
2
column operates at a pressure of 1 kg/cm . 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 = 0.05 (weight fraction ethanol)
B
and a distillate composition of x = 0.80 (weight fraction ethanol). The system has a total condenser
D
and a partial reboiler. Find D, B, Q , and Q .
c
R
Solution

A. Define. The column and known information are sketched in the following figure.