GATE 2025 CH – Question 55
Gas containing 0.8 mol% component $A$ is to be scrubbed with pure water in a packed bed column to reduce the concentration of $A$ to 0.1 mol% in the exit gas. The inlet gas and water flow rates are 0.1 kmol/s and 3.0 kmol/s, respectively.
For the dilute system, both the operating and equilibrium curves are considered linear. If the slope of the equilibrium line is 24, the number of transfer units, based on the gas side, $N_{OG}$ is ____ (rounded off to 1 decimal place).
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Correct answer: 4.38 to 4.42
Explanation
The absorption factor group is $S = \frac{mG}{L} = \frac{24 \times 0.1}{3.0} = 0.8$. For pure water ($x_2 = 0$), $N_{OG} = \frac{1}{1 - S}\ln\left[(1 - S)\frac{y_1}{y_2} + S\right] = \frac{1}{0.2}\ln\left[0.2 \times 8 + 0.8\right] = 5\ln 2.4 = 4.38$, which is 4.4.