Design procedures for solid-liquid extractors and the effect of hydrodynamic instabilities on extractor performance
1979
Spaninks, J.A.M.
<p/>A design method is proposed for countercurrent mass transfer cascades with crossflow stages, and unsteady operated fixed beds in a countercurrent arrangement. The separation performance of these cascades is calculated from mathematical models and compared with a purely countercurrent extractor. The models are based on simple equations describing mass transfer in a single stage fixed bed. The results are presented as concise correlations between the number of true and exterior apparent transfer units.<p/>Asymptotic values of the mass transfer coefficient for transient diffusion processes in cocurrent and countercurrent extractors are calculated for particles with simple geometry. Comparing the extraction efficiency calculated from the exact solutions of the diffusion equations and the values obtained from the asymptotic mass transfer coefficient shows that differences are small under normal operating conditions for countercurrent extraction. The use of the asymptotic values in design calculations is discussed.<p/>Due to the commonly observed viscosity and density gradients in the solvent phase, non-uniform flow of liquid through the layer of solids subjected to extraction can occur. Then some fraction of the solids is not efficiently contacted with the solvent. Experiments show that a concentration gradient rather than a concentration jump in the liquid flowing through an inert packing material reduces the effect of channelling. The effect of mass transfer between the packing and the surrounding liquid on flow stability is studied. The results indicate that channelling can reduce the extraction efficiency in solid-liquid extractors.
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