Formation and morphology of colloidal deposits in porous media
Wiesner, Mark R.
Doctor of Philosophy
The effects of physical parameters such as fluid velocity, particle size and influent particle concentration on the morphology of colloidal deposits, removal efficiency and head loss development in porous media are investigated. Monte Carlo (MC) simulations of colloid deposition on a one-dimensional permeable surface from a uniform flow field and on an impermeable one-dimensional surface in plane stagnation flow are performed. Simulation results indicate that (i) the morphology of the deposits formed in uniform flow field vary from open porous structure for small particles and low fluid velocities to compact structure with increase in particle size and fluid velocity and, (ii) the shape and structure of deposits formed in stagnation flow strongly depend on particle size and fluid velocity. At low velocities in stagnation flow, large particles form compact deposits while small particles form open porous structures. At high velocities, large particles form unstable pillar-like structure with fewer particles quickly building up the height of the deposit while smaller particles form fewer and more dense columns. Experimental observations of monodispersed latex particles filtered through a bed of spherical glass beads indicated that at high flow rates, influent particle concentration did not appear to have significant effect on the removal efficiency of the packed beds or on the head loss development as a function of retained particle mass. At low flow rates ($<$0.1 cm/sec), most of the head loss was observed to occur in the top section of the bed. At high flow rates, the particle deposition was relatively more uniform along the depth of the bed. The fractal dimensions of the deposits was observed to vary from 1.6 to 2.4 with increasing fluid velocity from 0.002 to 0.4 cm/sec. A window of low fractal dimensions was observed at intermediate flow rates (0.04-0.15 cm/sec). It is hypothesized that in this flow regime, deposits may have attained pillar-like structures, similar to those observed in simulations. The fractal dimensions of such columnar structures are expected to be low.
Environmental science; Civil engineering; Chemical engineering