Investigation of non-colloidal sedimentation using MRI

Abstract

In order to verify the validity of Kynch theory, instantaneous concentration profiles of the batch sedimentation of non-colloidal hard spheres were measured for various initial suspension concentration from 0.04 to 0.55 using the magnetic resonance imaging (MRI) technique. Measured profiles commonly had two distinct interfaces, the upper one between clarified fluid and settling suspension, and the other lower one between settling suspension and sediment. The upper interface was found to keep spreading due to polydispersity effect, especially for suspensions with low initial concentration. It was observed that the lower interface pattern is highly dependent on the initial concentration. If the pattern is considered to become spread due to the broadening effects of hydrodynamic diffusion and polydispersity, its overall behavior seems to be consistent with Kynch``s prediction. For all initial concentration the sediment was incompressible with packing concentration 0.60 ±0.01. Three transition concentrations separating four regions of different profile patterns were determined by fitting experimental results to Richardson-Zaki formula. The validity of the first (0.16) and the third (0.46) transition concentrations were cross-checked by various criteria based on the analysis of experimental results. The second transition concentration(0.33) determined from the flux curve analysis could not be confirmed experimentally. For the analysis of polydispersity and estimation of gradient diffusivity with particle concentration, a suspension model is proposed to interpret the transient behavior of the interface between the clarified fluid and free-settling suspension. It is assumed that particle radius be represented as a sequence of discrete values, and that particles of the same size constitute a continuous phase. The model equations of coupled nonlinear partial differential equations are solved by numerical methods. A measure of the interface thickness is defined as $c_0...