Stagewise Contact in Mixer-Settlers

Hydrodynamic aspects of the operation include the minimum impeller speed needed to ensure complete dispersion of one liquid phase in die other, and the size of droplets produced under a given set of conditions, preferaUy during die process of mass transfer. 

Skelland and Seksaria and Skelland and Ramsay showed that in some cases an inqietler speed of KXK) rpm is inaifficient to ensure conqilete dispersion. Skelland and Ramsay combined thmr own measurements widi those from two other publidied sources to obtain the following correlation for the minimum inqreller speed needed h ensure coinidete dispeision in a batch unit  

C and a are tabulated in their paper for fiat- and curved-blade turbines, disk turbines, thiee-bladed propellers, and pitched-blade turbines as a ftinction of position in the vessel. The average absolute deviation between Eq. (7.S-I) and experimental values was 12.7%, and the lowest value of was provided by the six-flat-blade turbine located centrally in the vessel. 

Skelland and Lee showed that gross uniformity of dispersion was obtained with values of N that exceeded by an average of about 8%. Furthermore, it was found by Skelland and Moeti that Eq. 

Coulaloglott and Tavlaiides compiled a tabulation of published correlations of drop size in agitated liquid sterns, but all the expressions were for the period after steady state had been attained with regard to drop size andjnass transfer. In contrast, the correlation by Skelland and Lee is for the Sauter mean droplet diameter dyi when about 50% of the possible mass transfer has occurred—a significantly different coiidition. For agitation with a centrally locmed six-fiat-blade turbine with radial baffles they obtained 

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The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

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