Definitions of Volumetric Coefficients and HTUs

we consider gas-liquid mass transfer rates in gas absorption and its reverse operation - that is, gas desorption in packed columns. The gas entering the column from the bottom, and the liquid entering from the top, exchange solute while contacting each other. In case of absorption, the amount of solute transferred from the gas to the liquid per unit sectional area of the column is 

In the case where the equilibrium curve is straight, the logarithmic mean driving potential (which is similar to the log-mean temperature difference used in heat transfer calculations) can be used to calculate the mass transfer rates in the column. The mass transfer rate r (kg or kmol h-1) in a column of height Z per unit cross-sectional area of the column is given by  

the required packed height Z can be calculated using Equation 6.38 with given values of r and the volumetric coefficient KG a or KL a. 

In the general case where the equilibrium line is curved, the mass transfer rate for gas absorption per differential packed height dZ and unit cross-sectional area of the column is given as  

the HTUs and Ka terms are interconvertible, whichever is convenient for use. Since the NTUs are dimensionless, HTUs have the simple dimension of length. Variations of HTUs with fluid velocities are smaller than those of the Ka terms thus, the values of HTUs are easier to remember than those of the Kas. 

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