GATE 2020 CH – Question 40
The chart gives methanol–water equilibrium at 1 bar. A distillation column at 1 bar must produce 92 mol% methanol from an equimolar saturated-liquid feed. Find the minimum reflux ratio.
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Correct answer: (B) 0.50
Explanation
For a saturated-liquid feed, the q-line is vertical at $x_F=0.5$. The equilibrium chart gives the pinch point $(x,y)=(0.5,0.78)$. At minimum reflux, the rectifying operating line joins that point to $(x_D,x_D)=(0.92,0.92)$, with slope $(0.92-0.78)/(0.92-0.5)=1/3$. Since this slope is $R_{min}/(R_{min}+1)$, $R_{min}=0.5$, B.