| The mechanism of floc formation in protein precipitation by polyelectrolytes is investigated. A theoretical investigation is done which shows that the ultimate floc size distribution formed in charge neutralization flocculation is independent of its initial distribution. Using the system of lysozyme precipitation by polyacrylic acids and an agitation schedule that induces floc aggregation, breakup and re-aggregation in sequence, the effect of polymer molecular weight is examined first. The mechanism of floc formation is found to be primarily a polymer bridging mechanism when a high molecular weight polymer is used, and a charge neutralization mechanism when a low molecular weight polymer is used. A gradual transition from polymer bridging to charge neutralization is observed with the decrease of polymer molecular weight.; The effect of polymer dosage is investigated next. Floc composition (mass PAA/mass lysozyme) is constant within a small dosage range, then increases over a transitional dosage range, and eventually becomes constant again at large dosages. With a high molecular weight polymer, flocs formed at a small dosage are weak, and at a large dosage are very strong. Results also suggest the importance of the rate of polymer addition in determining the mechanism of floc formation.; The effects of protein concentration, initial mixing intensity, shearing intensity and polymer molecular weight distribution are later examined. With a high molecular weight polymer, larger and stronger flocs are produced at a higher protein concentration or under a strong initial agitation. An increase of shearing intensity induces the breakage of large, weak flocs, but improves the aggregation between small particles as well. A wider polymer molecular weight distribution yields larger flocs when a low molecular weight polymer is used, but produces smaller flocs if a high molecular weight polymer is used.; Finally, the process of floc formation is modeled by population balance equations. Despite various assumptions made, the model is able to predict qualitatively the effects of polymer molecular weight, polymer dosage, protein concentration and shear-rate on the ultimate floc size distribution. |
