A simple distillation column separates a binary mixture of A and B. The relative volatility of A with respect to B is 2. The steady-state composition of A in the vapour leaving the 1st, 2nd and 3rd trays in the rectifying section are 94, 90 and 85 mol%, respectively. For ideal trays and constant molal overflow, the reflux-to-distillate ratio is

Question

Accepted Answer

The correct answer is B, 2.7. GATE 2024 official key: (B). Using equilibrium x=y/(α−(α−1)y) to get x₁=0.8868, x₂=0.8182, and the rectifying operating line y_{n+1}=(L/V)x_n+(D/V)x_D between consecutive trays (with x_D=y1=0.94) gives L/V≈0.729, so L/D=(L/V)/(1−L/V)≈2.7.