Methods standardization in the measurement of mass-transfer characteristics in packed absorption columns

Abstract

This paper supports the drive to standardize methods used in the measurement of mass-transfer characteristics ( k L a, k G a, a) in packed absorption columns. Using 25 mm Pall rings as packing, Hoffmann et al. [Hoffmann, A., Mackowiak, J.F., Gorak, A., Haase, M., Loning, J.-M., Runiwski, T. and Hallenberger, K., 2007, Standardization of mass transfer measurements. Basis for the description of absorption processes, TransIChemE, Part A, Chem Eng Res Des 85(A1): 40–49] recently proposed various such methods. Their recommendations are assessed in the light of the results reported by other researchers using the same type of packing of different sizes (25 and 50 mm) in different absorption systems. Data differences between these authors are analyzed, and the reasons for them explained. On this basis, absorption systems and measurement procedures are proposed capable of providing consistent data for primary quantities ( k G a, k L a, a). We recommend to use only such absorption systems and under such conditions, in which interfacial mass-transfer resistance is fully concentrated in single, addressed phase. Thus, for the measurement of k L a, we recommend the absorption/desorption of sparingly soluble gases, such as oxygen or carbon dioxide in/from water, i.e. systems in which mass-transfer resistance is fully concentrated in the liquid phase. The absorption of NH 3 in water is not recommended as approx. 50% mass-transfer resistance is concentrated in gas phase and has to be measured and subtracted by independent experiment. Suitable system for the measurement of k G a is the absorption of SO 2 in NaOH solution. For the measurement of the effective interfacial area, we recommend the absorption of CO 2 in aqueous NaOH solution under conditions in which the gas-phase mass-transfer resistance is negligible (i.e. u G ≥ 0.5 m/s, c OH ≤ 1 M). It is emphasized that the values of the physical properties (diffusivity, solubility) used in the evaluation of the area must be the same as those used in the evaluation of the kinetic constant (of the reaction of CO 2 and the OH − ions). To eliminate end-effects, the gas and liquid phases should be sampled directly from the packing rather than from the inlet and outlet pipes.