GATE 2020 CH – Question 38
An isothermal countercurrent packed absorber removes SO₂ from air into pure water at constant pressure. Water rate is 2 kmol/s; inlet gas rate is 0.05 kmol/s with yᵢₙ = 0.015 and outlet yₒᵤₜ = 0.005. Equilibrium is y* = 40x. The solution is dilute, the operating line is linear, and water evaporation is negligible. Find its overall gas-phase number of transfer units.
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Correct answer: (D) 2.0
Explanation
With essentially constant molar rates, the solute balance from the column top gives $x=(G/L)(y-y_{out})=0.025(y-0.005)$. Thus $y^*=40x=y-0.005$, so the gas-phase driving difference y − y* is constant at 0.005. Therefore $NTU_{OG}=\int_{0.005}^{0.015}dy/(y-y^*)=(0.015-0.005)/0.005=2.0$, D.