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).

Question

Accepted Answer

The correct answer is 4.2 to 4.6. GATE 2025 official key: 4.2 to 4.6. With mG/L = 24×0.1/3 = 0.8, the Colburn equation gives N_OG = [1/(1−0.8)]·ln[(1−0.8)(0.008/0.001)+0.8] = 5×ln(2.4) ≈ 4.38.