Transfer Coefficients for Packed Towers

When a packed tower is operated in the usual manner as a countercurrent absorber or stripper for transfer of solute between the gas and liquid, the rate of solute transfer can be computed from measured values of the rate of gas and liquid flow and the bulk concentrations of solute in the entering and leaving streams. As explained in Chap. 5, because of the impossibility of measuring solute concentrations at the gas-liquid interface, the resulting rates of mass transfer can be expressed only as overall coefficients, rather than as coefficients for the individual fluids. Further, since the interfacial area between gas and liquid is not directly measured in such experiments, the Jlux of mass transfer cannot be determined, but instead only the rate as the product of the flux and the total interfacial area. By dividing these rates by the volume of the packing, the results appear as volumetric overall coefficients, K a, KyO, K a, Fqj o, 

The individual fluid mass-transfer coefficients ky, F ) and the interfacial area a which make up these overall volumetric coefficients are differently dependent upon fluid properties, flow rates, and type of packing. The overall volumetric coefficients are therefore useful only in the design of towers filled with the same packing and handling the same chemical system at the same flow rates and concentrations as existed during the measurements. For general design purposes, the individual coefficients and the interfacial area are necessary. 

Another approach to obtaining pure gas-phase coefficients is to make measurements when a pure liquid evaporates into a gas. Here there is no liquid resistance since there is no concentration gradient within the liquid. The resulting volumetric coefficients kyO and F a, however, do not agree with those obtained in the manner first described above. The reason is the different effective interfacial areas, as explained below. 

Holdup refers to the liquid retained in the tower as films wetting the packing and as pools caught in the crevices between packing particles. It is found that 

When absorption or desorption of a solute occurs, involving transfer of a solute between the bulk liquid and gas, the liquid of the static holdup rapidly comes to equilibrium with the adjacent gas, and thereafter its interfacial surface contributes nothing to mass transfer except as it is slowly replaced. For absorption and desorption, therefore, the smaller area offered by the moving holdup is effective. When evaporation or condensation occurs with the liquid phase a single, pure component, however, the area offered by the total holdup is effective since the liquid then offers no resistance to mass transfer. 

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ESTIMATION OF MASS-TRANSFER COEFFICIENTS FOR PACKED TOWERS... 

Sec. 10.8 Estimation of Mass-Transfer Coefficients For Packed Towers... 

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