* Corresponding author. Fax: # 31-15-2784 452. E-mail address: p.j.m.lebens@stm.tudelft.nl (P.J.M. Lebens) Chemical Engineering Science 54 (1999) 5119}5125 Gas}liquid mass transfer in an internally "nned monolith operated countercurrently in the "lm #ow regime P.J.M. Lebens*, J.J. Heiszwolf, F. Kapteijn, S.T. Sie, J.A. Moulijn Department of Chemical Process Technology, Delft University of Technology, Julianalaan 136, NL-2628 BL Delft, Netherlands Abstract Gas}liquid mass transfer has been determined in internally "nned monoliths (IFM) by desorbing oxygen from a falling water "lm. Measurements were performed for various monolith lengths to quantify the inlet e!ects caused, among others, by the development of the concentration pro"le. The local volumetric mass transfer coe$cients were found to vary from approximately 0.04 (s) at the inlet to about 0.01 (s) in the downstream sections. A theoretical model has been derived, assuming that the water #ow can be treated as a "lm falling down in a corner. By averaging the radial mass transfer perpendicular to the "lm surface, mass transfer can be predicted within reasonable accuracy. On comparing the "lm in the IFM with a "lm along a vertical plane, it was found that in an IFM the concentration pro"le develops faster while mass transfer is less e!ective in the developed sections.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Internally "nned channel; Monolith; Countercurrent #ow; Falling "lm; Liquid saturation; Gas}liquid mass transfer 1. Introduction Recently, a new type of monolithic structure has been developed for gas}liquid countercurrent applications (Sie and Lebens, 1998). The so-called internally "nned mono- liths (IFMs, Fig. 1), which have longitudinal "ns incorporated in the walls of the channels, are designed to reduce the momentum transfer between gas and liquid at a minimal expense of mass transfer. Liquid is distrib- uted at the top of the monolith and #ows down as a falling "lm while gas #ows up through the remaining void space. The #ooding limits of the packing are con- siderably extended by creating separate paths for the gas and the liquid (Lebens, Stork, Kapteijn, Sie & Moulijn, 1999). However, the reduced gas}liquid interaction and the laminar nature of the #ow will likely a!ect the mass transfer rates between the two phases. The purpose of this paper is to experimentally determine and theoret- ically describe the gas-liquid mass transfer occurring in an IFM. 2. Theory 2.1. Hydrodynamics Di!erent #ow regimes, varying from smooth "lm #ow to turbulent wavy #ow, may exist for a falling "lm. According to the criterion reported by Zhang and Giot (1995) for "lm #ow inside vertical tubes, the onset of large disturbance waves occurs at, Re  '400. (1) For the liquid #ow rates inside the IFMs considered here (Re  (75), this implies that the #ow regime is dominated by smooth "lm #ow. The profound in#uence of the liquid distributor on the development of the liquid "lm, how- ever, does not fully exclude the existence of some distur- bance waves (Yih, 1986). For steady uniform laminar #ow the velocity "eld inside the "lm can easily be obtained by solving the momentum balance (Bird, Stewart & Lightfoot, 1960). For a smooth "lm along a vertical plane or tube, this can be done analytically. For more complex geometries, like the IFMs, a numerical method should be applied to derive the two-dimensional velocity #ow "eld. It has been reported previously (Lebens et al., 0009-2509/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 6 5 - 1