Question

10.0 kmol of a 40.0 mol% methanol and 60.0 mol% water mixture is processed in a normal batch distillation system with a still pot that acts as an equilibrium stage and a column with two equilibrium stages (total of three equilibrium contacts). The column has a total condenser, and reflux is a saturated liquid. The column is operated with a varying reflux ratio so that xD is held constant. We desire a final still-pot concentration of 8.0 mol% methanol, and distillate should be 84.0 mol% methanol. Pressure is 1 atm, and CMO is valid.
Required:
a. What initial external reflux ratio, LD, must be used?
b. What final external reflux ratio must be used?
c. How much distillate product is withdrawn, and what is the value of Wfinal?

Answer 1

Answer:

[a]. 0.49.

[2]. 0.536

[c]. 4.15 kmol; 5.84 kmol.

Explanation:

Without mincing words let's dive straight into the solution to the question above.

                                                            [a].

The initial external reflux ratio, LD that must be used = [(0.85 - 0.57)/ 0.85 - 0]/ [ 1 - (0.85 - 0.57)/ 0.85 - 0].

The initial external reflux ratio, LD that must be used = 0.329/ 1- 0.329 = 0.49.

                                                             [b].

The final external reflux ratio that must be used = [ 0.85 - 0.13/ 0.85 - 0]/ [ 1 - 0.85 - 0.13/ 0.85 - 0].

Hence, the final external reflux ratio that must be used =0.847/ 1 - 0.847 = 5.536.

                                                              [c].

The amount of distillate product that is withdrawn:

4 = 0.85 H(t) + 0.8 - 0.08.

H(t) = 4.15 kmol, and the value of Wfinal = 5.84 kmol.