How will increasing the cross-sectional area influence the efficiency of a cross current-flow scrubber for air pollution control?

How will increasing the cross-sectional area influence the efficiency of a cross current-flow scrubber for air pollution control?

• A. the eff will increase because the scrubber volume will increase
• B. the eff will decrease because the gas velocity, vg, will decrease
• C. the eff will increase because the gas velocity, vg, will increase
• D. the eff will decrease because the liquid-gas flow ratio will decrease

Answer: (B)

The key idea is that scrubber efficiency in a cross-current system is closely tied to how fast the gas moves. When you increase the cross-sectional area, the same gas flow rate gives a lower superficial gas velocity (vg). In a cross-current scrubber, higher gas velocity promotes better gas–liquid contact: it thins the gas boundary layer, increases turbulence, and boosts the mass-transfer rate from the gas to the liquid. Slowing the gas down reduces these effects, so the rate at which pollutants are transferred to the liquid drops. Even if residence time might be longer with a larger area, the dominant effect is the reduction in gas-phase mass-transfer capability, so overall scrubbing efficiency decreases as vg decreases.

The key idea is that scrubber efficiency in a cross-current system is closely tied to how fast the gas moves. When you increase the cross-sectional area, the same gas flow rate gives a lower superficial gas velocity (vg). In a cross-current scrubber, higher gas velocity promotes better gas–liquid contact: it thins the gas boundary layer, increases turbulence, and boosts the mass-transfer rate from the gas to the liquid. Slowing the gas down reduces these effects, so the rate at which pollutants are transferred to the liquid drops. Even if residence time might be longer with a larger area, the dominant effect is the reduction in gas-phase mass-transfer capability, so overall scrubbing efficiency decreases as vg decreases.