(216f) Analogy of Mass Transfer in Absorption and Distillation Columns: Wetted Wall Column Study

The prediction of the packing mass transfer efficiency under distillation conditions out of the absorption data provides results which can be unreliable even if the absorption data are measured using proved systems. This discrepancy can be accounted for the influence of the phase hydrodynamic, different formation of the effective interfacial area or generally unreliable transfer of the mass-transfer coefficients between the distillation and absorption. This study is focused on the analogy of the mass transfer in absorption and distillation columns. The wetted wall column was chosen as the experimental device for this study as it provides identical interfacial area for both, absorption and distillation processes eliminating its influence on the mass transfer. Although there are a lot of papers dealing with the mass transfer characteristics of the wetted wall column, there is only a limited amount of the literature data measured on the absorption systems under the conditions similar to the distillation ones. Especially the liquid side mass transfer coefficient was usually measured on the aqueous systems in turbulent regime of the flow in contrast to the laminar regime common in the distillation columns. In this paper the absorption mass-transfer characteristics (k_G, k_L) of the wetted wall column are shown. The characteristics were measured using the absorption systems with transport properties (characterized by means of the phase Schmidt number) close to the distillation ones. The phase Reynold numbers common in distillation (Re_L=80-800, Re_G=1200-12000) were utilized in these experiments. For the k_G measurements the absorption of sulfur dioxide from air (Sc_G=1.22), helium (Sc_G=2.75) and sulfur hexafluoride (Sc_G=0.45) to the lye was used. The k_L values were determined by the desorption of oxygen from n-heptane and primary alcohols at various temperatures into the stream of nitrogen covering the liquid phase Schmidt numbers from 84 to 2500. The range of the Schmidt numbers enables to evaluate their influence on the mass transfer at the conditions common in distillation systems (Sc_L≈200, ScG≈0.6). In the gas phase the correlation Sh_G∝Sc_G^0.6  was found. The power of 0.6 at the Schmidt number is considerably higher than the power of 1/3 used in the Chilton-Colburn analogy or power of 0.44 found by Gilliland and Sherwood by evaporation of pure liquids into the air stream. In the liquid phase it was observed that Sherwood number of the wavy liquid film is practically independent on the liquid flow rate and depends on the liquid Schmidt number with power of 0.57 which is in good agreement with value of 0.5 observed by most of other authors. However the value of the Sherwood number on organic systems was found to be approx. 2 times higher than predicted from the correlations based on the aqueous systems results which perhaps can be explained with the influence of the surface tension on to the hydrodynamic of the film surface, i.e. waves. The results of absorption experiments enable to predict the separation efficiency, i.e. HETP, of the wetted wall column under distillation conditions. The distillation experiments with the methanol-propanol and ethanol-propanol distillation mixtures will follow. The experimental HETP values will be compared with those predicted out of the absorption data, the “Profile method” will be also used for the calculation of the distillation mass transfer coefficients out of the concentration profiles measured along the column.