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Mass Transfer
Mass Transfer Operations
GATE 2020
Hard

SO2 is absorbed from air by pure water in a countercurrent packed column at constant pressure. Pure water enters at 2 kmol/s. Gas leaves the top with SO2 mole fraction 0.005; total gas (SO2+air) enters the bottom at 0.05 kmol/s with SO2 mole fraction 0.015. Equilibrium: y*=40x. The solution is dilute, operating line linear. The number of transfer units (NTU) for this column is

  • A0.5
  • B1.0
  • C1.5
  • 2.0

Explanation

SO2 balance: G(yin−yout)=L(xout−xin): 0.05×0.01=2×xout ⟹ xout=0.00025. Bottom: y1=0.015, y1*=40×0.00025=0.01, Δy1=0.005. Top: y2=0.005, x2=0, y2*=0, Δy2=0.005. Since Δy1=Δy2, NTU_OG=(y1−y2)/Δy=(0.015−0.005)/0.005=2.0.

Source: GATE CH 2020 (2020)

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