Collector Grain Research Articles

Detachment of colloids from sediments and sand grains

The mechanisms of colloid transport and attachment have been studied in several fields and a reasonable understanding of the overall process has been developed over the past several decades. However, there is little fundamental understanding of the mechanisms of colloidal particle detachment from larger grains. The ongoing research explores particle detachment from both theoretical and experimental viewpoints. The theoretical studies have developed expressions that may be used to determine the magnitude of forces that would control particle detachment, including electrostatic and van der Waals forces. The results indicate differences when the computations are based on energies of interaction compared with forces of interaction. The initial experimental studies were performed on laboratory scale model systems with well-defined spherical particle suspensions. Latex particles (2 and 5 μm) were attached to a packed column containing glass beads as collector grains. Particle number concentrations were monitored using a laser, time of transition type, particle size analyzer (Brinkmann, Particle Size Analyzer). The detachment of particles during fluidized bed operations was studied in relation to the chemical characteristics of the water. Preliminary experimental results indicated that the detachment of latex particles was (a) independent of ionic strength of solution phase during attachment, but (b) was dependent on the ionic strength during detachment. Detachment efficiency was higher with lower ionic strength waters.

A Stokesian dynamics model for particle deposition and bridging in granular media

This paper presents a general three-dimensional Stokesian Dynamics model that dynamically simulates the motion and deposition of particles as they approach and migrate through a porous medium. The model allows for variations in the fluid velocity, particle size, and forces or torques acting on the particles. The use of a Stokesian dynamics approach provides a unified framework in which no a priori assumptions about the type of filtration need be made. The model is valid for the hydrosol filtration of dilute suspensions. One of the most important and perhaps least understood issues in filtration is the effect of previously deposited particles on the subsequent deposition of particles. As each particle deposits on a grain, it alters the fluid flow field. This distortion in the fluid flow can significantly affect the motion of subsequent particles. The fluid flow field and the particle trajectory therefore dynamically evolve with time and depend on the size and location of the collector grains as well as the deposited particles. The effect of particles lodging in pore throats to cause plugging is twofold. First, particle plugging will change the fluid flow field around the grains that are adjacent to the plugged pore throat. Furthermore, as particles are diverted away from the blocked pore throats, the concentration of particles passing through the open pore throats will increase. This in turn may lead to a rapid buildup of particles on these adjacent collectors until they are also clogged. Consequently if the conditions of deposition are such as to encourage extensive particle bridging, this will eventually lead to a situation where particle bridging occurs across all the pore throats. If this occurs, or near, the face of the filter bed, the particles will tend to deposit on each other and the mode of filtration will change from deep bed filtration to “cake” formation. The model presented in this paper allows us for the first time to dynamically simulate this process of particle deposition, bridging, and cake formation.