Mass Transfer into a Liquid Film Flowing over a Packing Piece

TLDR: In this article, the authors investigated the effect of the Schmidt number on the mass transfer coefficients of Raschig rings and Berl saddles in a column of carbon dioxide and showed that the results closely follow predictions based on the unsteady-state diffusion theory.

Abstract: The main objectives of this work were, first to see whether an unsteady-state diffusion theory was applicable to mass transfer into a liquid film flowing over the surface of a packing piece such as a Raschig ring or a Berl saddle, and, second, to obtain a general correlation for liquid phase mass transfer coefficients of these packings.Absorption of pure carbon dioxide by three solvents, water, methanol and n-butanol, were carried out in a column, where single pieces of Raschig rings or Berl saddles were located as shown in Fig. 1. Figs. 2 and 3 show the values of HL for a Raschig ring in case θ is 45°. When the values of (HL/z)·(Ga1/6/Sc1/2) are calculated from these data and plotted against Re as shown in Figs. 4 and 5, the data points for all runs come on a single line as represented by Eq. (5). These results indicate that mass transfer into a liquid film flowing over the packing pieces closely follows predictions based on the unsteady-state diffusion theory. Figs. 6, 7 and 8, similar to Figs. 4 and 5, show the results given with a Raschig ring (θ=90°), with a short wetted-wall column corresponding to a Raschig ring (θ=0°) and with a Berl saddle (θ=45°), respectively. The best lines through the data in these figures are expressed by Eqs. (6), (7) and (8), respectively. Taking the variation of HL with θ into account, Eqs. (19) and (20), giving the average values of HL for single pieces of Raschig rings and Berl saddles, have been obtained.In order to determine the effect of Schmidt number on HL for packed columns, additional experiments on the absorption of pure solute gases, hydrogen, oxygen and carbon dioxide, by water were carried out in a 7.0cm column packed with 15mm Raschig rings. Experimental results indicate that the values of HL vary as the 1/2 power of the Schmidt number, in agreement with the unsteadystate diffusion theory.