These relationships assume that the resistances to mass transfer in the gas and liquid phases occur in series, and the gas-liquid interface does not contribute significant resistance to the mass transfer. If ky is much greater than kx/m, then Ky = m/kx, and the rate of absorption is controlled by the liquid-phase transfer. On the other hand, if ky << kx/m, then the system is gas-phase controlled. To some extent, the value of m or H controls the phase control mechanism. For a soluble system (e.g., absorption of NH3 by water), the value of m is small, and thus the mass transfer is gas-phase controlled. For a CO2/water system, the value of m is large, and the absorption is liquid-phase controlled.

The calculations also require estimating the effective interfacial area a. The area a is generally smaller than the geometric area of the packing because some packing area is not wetted and some area is covered by a stagnant, liquid film that is already saturated with the pollutant (i.e., inactive for mass transfer).

Onda, Takeuchi, and Okumoto (1968) have expressions for the individual mass-transfer coefficients kx and ky as well as the effective interfacial area a, and the de-


the effective interfacial area for mass transfer

wetted area of packing a critical surface tension = 61 dyn/cm for ceramic packing, 75 dyn/cm for carbon steel packing, and 33 dyn/cm for polyethylene packing L/appL apL2/(gft)2 L2/apft,

Ml Pldl M-g Pgdg

Absorption with a chemical reaction in the liquid phase is involved in most applications for gas control. Reaction in the liquid phase reduces the equilibrium partial pressure of the pollutant over the solution, which increases the driving force for mass transfer. If the reaction is irreversible, then y* = 0, and the NTU is calculated as follows:

dy ln

A further advantage of the reaction is the possible increase in the liquid-phase mass-transfer coefficient kx and the effective interfacial area a. Perry and Green (1984) describe the design methods in detail.

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