Page 163 - Separation process engineering

= 395, h (0.63, satd liquid) = 65 kcal/kg, and

Since all the molecular weights are at the same concentration, they divide out.

This agrees with Figure 4-10. Despite considerable subcooling, this feed line is also steep. Note that
feed rate was not needed to calculate q or the slope for any of these calculations.

4.5 Complete McCabe-Thiele Method

We are now ready to put all the pieces together and solve a design distillation problem by the McCabe-
Thiele method. We will do this in the following example.

Example 4-3. McCabe-Thiele method

A distillation column with a total condenser and a partial reboiler is separating an ethanol-water
mixture. The feed is 20 mol% ethanol, feed rate is 1000 kmol/h, and feed temperature is 80°F. A
distillate composition of 80 mol% ethanol and a bottoms composition of 2 mol% ethanol are desired.
The external reflux ratio is 5/3. The reflux is returned as a saturated liquid and CMO can be assumed.
Find the optimum feed plate location and the total number of equilibrium stages required. Pressure is
1 atm.

Solution

A. Define. The column is sketched in the figure

Find the optimum feed plate location and the total number of equilibrium stages.

B. Explore. Equilibrium data at 1 atm are given in Figure 2-2. An enthalpy-composition diagram at 1
atm will be helpful to estimate q. These are available in other sources (e.g., Brown et al., 1950, or
Foust et al., 1980, p. 36), or a good estimate of q can be made from Figure 2-4 despite the pressure
difference. In Example 4-1 we showed that CMO is valid. Thus we can apply the McCabe-Thiele
method.

C. Plan. Determine q from Eq. (4-17) and the enthalpy-composition diagram at 1 atm. Plot the feed
line. Calculate L/V. Plot the top operating line; then plot the bottom operating line and step off