MASS-TRANSFER IN PACKED-BEDS AT LOW PECLET NUMBERS - WRONG EXPERIMENTS OR WRONG INTERPRETATIONS

Much research has been focused on mass transfer phenomena in packed be ds. For Peclet numbers above 200, empirical relations have been derive d that predict the value of the mass transfer coefficient as a functio n of the Reynolds number and the Schmidt number. These relations are m ore or less similar to the well-known relation that Rant and Marshall derived for mass transfer around a single sphere in an infinite medium Sh = alpha + beta (ReScdelta)-Sc-gamma. For packed beds of spherical particles an alpha-value of 3.89 can be calculated on basis of fundame ntal considerations. However, Sherwood numbers much lower than this mi nimum value have been observed at Peclet numbers below 100. Several ex planations have been proposed for this apparent discrepancy, such as m isinterpretation of the experimental results due to unjustified neglec tion of axial dispersion or wall channeling. III this work, a model th at predicts the combined effects of axial dispersion and wall channeli ng has been developed. With this model, it is possible to explain the results obtained with undiluted beds in which all particles are active in the process of mass transfer. However, such an explanation is not possible for the results obtained with diluted beds in which not all p articles are active. Therefore, in the case of diluted beds other reas ons for the apparent drop in mass transfer rate must exist. In the pre sent investigation, it is demonstrated that the drop again originates from misinterpretation of the experimental results. It is shown, both experimentally and theoretically, that low Sherwood numbers can be obt ained when large differences exist between the local concentration, ex perienced by an active particle and the mixed cup concentration of the whole bed cross-section. (C) 1997 Elsevier Science Ltd.