Semester

Spring

Date of Graduation

2002

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Rakesh K. Gupta.

Abstract

This study examines the problem of shear-induced agglomeration of particles such as fillers and pigments in polymer solutions, where polymer molecules adsorb on the surface of particles and form a bridge between them causing agglomeration. The rate of agglomeration is usually obtained by multiplying the rate of collision of particles by the collision efficiency. In a laminar flow field, the collision frequency is readily given by Smoluchowski's expression. If agglomeration takes place by coagulation, the collision efficiency is well studied and the influences of hydrodynamic and colloidal forces are well understood. In case of polymer bridging, the collision efficiency is determined by probabilistic models based on fractional surface coverage of particles by the adsorbing polymer, neglecting the influence of hydrodynamic forces. A new model is required to determine the collision efficiency in case of shear-induced agglomeration of non-colloidal size particles by polymer bridging.;In this work, a new model is developed in terms of dimensionless groups and the fractional surface coverage. The dimensionless groups represent the relative magnitudes of the colloidal, hydrodynamic and steric forces. An expression for the optimum surface coverage needed to achieve the maximum collision efficiency is also obtained. In particular, a model is presented for the case where surface of the particles is completely covered by a polymer layer.;To validate this model, experimental collision efficiencies at various agglomeration conditions are determined by shearing a model suspension in a cone-and-plate device. The model suspension consisted of 4.9 mum diameter spherical hollow glass beads dispersed in an aqueous glycerin solution; polyethylene oxide was used as the flocculant. Using experimental collision efficiencies and non-linear regression analysis, the parameters in the collision efficiency model were determined.;It was found that increasing the shear rate decreases the collision efficiency and it can be expressed as a power law. Increasing the molecular weight and concentration of flocculant also gives higher collision efficiency. Agglomerate growth and the equilibrium agglomerate size were also monitored under different flocculation conditions. It was found that the size to which agglomerates grow depends on the shear rate and this can be expressed as a power law as df,95% ∝ g&d2;-0.258 . In addition, agglomerate breakage studies show that the agglomerate breakage occurs by floc rupture mechanism rather than by the surface erosion of primary particles.

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