Question

1.   The liquid product from a flash distillation has a mole fraction of the more volatile component that is lower than the specified liquid purity. Should we decrease, increase, or hold constant the vapor molar flowrate to meet the specified liquid purity? Draw a sketch and explain why?

2.   If the reflux ratio increases, do the molar flowrates for the distillate, bottoms, reflux, and overhead vapor decrease, stay the same, or increase? Support your answer.

3)1000 kmol/h of feed with 60.0 mol% methanol with the remainder water is distilled in a column with a distillate that is 99.9 mol % methanol and a bottoms that is 1.0 mol% methanol. The external reflux ratio is 1.5.

a.   Find the molar flowrates of the distillate, bottoms, reflux, and overhead vapor.

b.   Find the methanol mol% in the overhead vapor and the reflux.

Answer 1

(2)

When top reflux ratio is increased:

1. Temperature of top tower and bottom tower decreases.
2. Amount of heavy key product in the distillate decreases, that is, better separation.
3. Reboiler duty increases to restore the set point temperature, increasing cost.
4. Vapour velocity through the tray increases, hence flooding.
5. The spray height between the trays increases till incipient flooding point.
6. After further increase in reflux ratio, there is backmixing of liquids between the upper tray and lower tray which decreases separation.