Mass Transfer · Distillation · 2 marks · Multiple choice
The equilibrium curves show pressure-dependent azeotropic behaviour for components E and F. Feed is 80 mol% E, and 99.5 mol% E product is desired using the two-column arrangement shown. Column 1 distillate contains 90 mol% E and feeds column 2. Which pressure and product choice is correct?
P1 = 100 kPa, P2 = 20 kPa; E product from column 2 top.
P1 = 100 kPa, P2 = 20 kPa; E product from column 2 bottom.
P1 = 20 kPa, P2 = 100 kPa; E product from column 2 top.
P1 = 20 kPa, P2 = 100 kPa; E product from column 2 bottom.
Correct answer: (D) P1 = 20 kPa, P2 = 100 kPa; E product from column 2 bottom.
Explanation
At 100 kPa, the azeotrope near xE = 0.85 prevents a feed at 0.80 from producing 0.90 distillate, so column 1 must use 20 kPa. At 20 kPa the vapour is enriched in E over the required first-column range. The 0.90 feed to the 100 kPa second column lies on the E-rich side of its azeotrope, where vapour is poorer in E than liquid. High-purity E is therefore obtained as its bottom product. Select D.
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