Mass transfer in packed beds at low Peclet numbers—wrong experiments or wrong interpretations?

Abstract

Pergamon( h,'m~, .I tim/m,'.'rm,/ S: wm e. Vol 52. Nos. 21 22. pp. 3995 41~) For packed beds of spherical particles an :~-value of 3.89 can be calculated on basis of fundamental considerations. However, Sherwood numbers much lower than this minimum value have been observed at Peclct numbers below 100. Several explanations have been proposed for this apparent discrepancy, such as misinterpretation of the experimental results due to unjustified neglection of axial dispersion or wall channeling. In this work, a model that predicts the combined effects of axial dispersion and wall channeling 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 particles arc active. Therefore, in the case of diluted beds other reasons for the apparent drop in mass transfer rate must exist. In the present 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 obtained when large differences cxist between the local concentration, experienced by an active particle and the mixed cup concentration of the whole bed cross-section. (" 1997 Elsevier Science l,td

Effect of Superimposed Pulsating Flow on Liquid-Solid Mass Transfer in Fixed Beds

“…The electrochemical technique cannot be applied to an entire fi xed bed owing to the maldistribution of current and voltage inside the bed. This necessitates the use of a single active particle that may not be typical of the entire bed (Rexwinkel et al, 1997). In the organic solid dissolution technique, errors may be introduced by particle attrition and surface roughness that lead to an exaggerated mass transfer coeffi cient especially at relatively high shear stress.…”

Comparative study of the adsorption of phenol and salicylic acid from aqueous solution onto nonionic polymeric resins

“…The values of V A were estimated by the Le Bas additive method [33], in the case of phenol V A = 108 cm 3 g-mol −1 and V A = 146.8 cm 3 g-mol −1 for salicylic acid. Many correlations have been published for the estimation of k f , most of which are modified versions [34,35] of the well-known Ranz-Marshall equation [36]. These empirical correlations have been used for different systems and may predict quite different values of the film mass transfer [37,38].…”

Dispersion-Concentric Model for Mass Transfer in a Packed Bed with a Countercurrent Flow of Gas and Liquid

“…3 In addition, negligence of the effect of the axial dispersion in the bed could also lead to an underestimation of the mass-transfer coefficient in the bed. [4][5][6] Therefore, some significant deviation between the estimated bed height and the actually required bed height is often encountered in industry. Consequently, a large safety factor is usually incorporated in the design of a packed column.…”