How to evaluate the mass transfer resistances in dense solid–liquid suspensions

The dissolution of solids in slurry stirred tanks is affected from the solid concentration distribution to different extend, depending on the agitation conditions. In this work, the relationship between solid–liquid mixing and mass transfer is investigated by estimating the characteristic times of different phenomena: liquid mixing, solid dissolution, diffusion of the dissolved species in the vessel volume. The analysis is based on Electrical Resistance Tomography data collected during the simultaneous operations of dissolution and mixing. For dilute solid–liquid systems, the relative importance of the solid dissolution and the liquid mixing depends on the particle size and the impeller speed, while the diffusion in stagnant zones is often negligible. Instead, the diffusion mechanism has an important effect on mass transfer for dense systems, where strong axial gradients of solid concentration are established and the stirred tank volume can be divided into a lower zone containing a cloud of dense mixture and an almost stagnant clear liquid layer on top. Based on original and literature experimental data, a method for the main mass transfer resistance identification depending on the agitation condition, the solid volume fraction distribution and the soluble particle size is suggested.