Polymer-induced flocculation

Polymer-Induced Flocculation. Polymer-induced flocculation is the most commonly used technique for breaking water-in-oil emulsions in the petroleum industry. This topic will be covered in much more detail in Chapter 9, but will be briefly covered in this section. 

Because of these considerations it is often necessary to deliberately reflocculate the particles but now in a controlled fashion. This can be brought about by the addition of a low level of an adsorbed bridging polymer (see above) or else the use of a fully dissolved nonattached polymer that brings about depletion flocculation. This latter phenomenon occurs due to the exclusion of dissolved polymer from between the particles, resulting in a net attraction between particles. The use of both mechanisms of polymer induced flocculation have been employed by Tadros et al. to promote resuspendability of dispersed particles by forming a loosely flocculated structure. 

Pefferkom, E., StoU, S., Elaissari, H. and Varoqui, R. (1991). Polymer induced flocculation of latex particles aggregation process and related cluster size distributions. Particid. Sci. Technol., 23,76-89. 

Wong, S.S. Teng, T.T. 2006. Treatment of pulp and paper mill wastewater by polyacrylamide (PAM) in polymer induced flocculation. Journal of Hazardous Materials 135 378-388. 

A Felmeisicr. GM Kuchtyak. S Koziol. CJ Felmeistcr. Polymer-induced flocculation of pharmaceutical suspensions. J. Pharm. Sci. 62 2026-2027. 197.3, Carbopol hydrosoiuble resins, BF Goodrich. Cleveland, 1981,... 

R Hogg. P Bunnail. H Suharyono. Chemical and physical variables in polymer-induced flocculation. AIChemE Symposium Solid/Liquid Separation in Industry, Pittsburgh, July 1991... 

Emulsions may be flocculated by the addition of polymers. Excluding those cases where the addition of a polymer affects the van der Waals or electrostatic forces directly (e.g. the addition of polyelectrolytes), the process of polymer-induced flocculation may proceed by two mechanisms, bridging or depletion. These are depicted schematically in Figure 4.3. 

Runkana et al. [23, 25] developed population balance models (PBMs) for polymer-induced flocculation by two well-known mechanisms, simple charge neutralization [23] and bridging [25]. They assumed that polymer adsorption on oppositely charged particle surfaces is very fast and equilibrium conformation is achieved before collisions between particles take place. It was also assumed that polymer adsorbs uniformly and polymer surface coverage and adsorbed layer thickness are the same for all particles. The composite polymer-coated particle radius was estimated by adding adsorbed layer thickness to the soM particle radius. 

This type of mechanism is likely to be partly operative in systems containing inorganic electrolytes as, for example, in the case of aluminium species. Some polyelectrolytes may also induce flocculation by charge neutralisation but the adsorbed polymer may also be able to bridge from one particle surface to another ( polymer bridging ). 

Ideally, it would be desirable to be able to develop quantitative expressions for the interaction energies so that we can deal with coagulation or flocculation, at least in the case of fairly dilute dispersions, the way we did in Sections 13.3-13.4 for electrostatic stabilization. It is possible to develop approximate expressions for interaction energy due to various individual effects such as osmotic repulsion, attraction or repulsion due to the overlap of the tails of the adsorbed (or grafted) polymer layers, interaction of the loops in the layers, and so on (see Fig. 13.15). However, the complicated nature of polymer-induced interactions makes these tasks very difficult. In this section, we merely illustrate some of the issues that need to be considered in developing a fundamental quantitative understanding of polymer-induced forces. In Section... 

Stock composition, kinetics of adsorption and hydrodynamic shear dictate the point at which a cationic polymer is added to a papermaking furnish in order to induce flocculation. Flocculation of cellulose fibers in turbulent flow proceeds very rapidly and is completed in less than two seconds.120-123 Flocks form due to charge interactions through a patch-type or a bridging-type mechanism. However, these flocks will be sensitive to shear force and deflocculation and reflocculation might occur. 

Polymers are often used to stabilize colloidal systems by grafting them on the particle surfaces to provide steric repulsion [1,2], Polymers can also induce flocculation due to either depletion or bridging interactions [3],... 

The adsoiption of polymers on colloids can (1) enhance colloid stability or (2) induce flocculation. ... 

Polyacrylamide (30% hydrolysed) is an anionic polymer which can induce flocculation in kaolinite at very low concentrations. Restabilisation occurs by overdosing, probably by the mechanism outlined in Fig. 7.32. Dosages of polymer which are sufficiently large to saturate the colloidal surfaces produce a stable colloidal system, since no sites are available for the formation of interparticle bridges. Under certain conditions, physical agitation of the system can lead to breaking of polymer-suspension bonds and to a change in the state of the system. 

The classical picture of the role of ions in aqueous polymer solutions is related to the notion that hydrophilic colloids are heavily hydrated in solution. The ions are considered to exert their influence by competing successfully with the polymer for the available molecules of water. Because of the charge on the ions, it might be expected that the ion-dipole interactions would be stronger than the dipole-dipole interactions between the water and the polymer. The classical viewpoint would therefore claim that the ions dehydrate the polymer and so induce flocculation. 

Note that reference to equation (15.60) for the case where the polymer composing the steric layer is different from that in free solution shows that, under some circumstances, AG cannot become negative. Specifically, this requires that be negative in a dispersion medium that is a better than 6-solvent for both of the polymers. A negative value for G corresponds to the free polymer and the stabilizing moieties being compatible (Flory, 1953). If equation (15.60) is correct, it shows that free polymer that differs in chemical composition from the polymer in the steric layers cannot induce flocculation if the two polymers are compatible. No experimental evidence is as yet available to ascertain the validity of this prediction. 

Clarke and Vincent in these studies established the phase diagrams for the free polystyrene-microgel-ethyl benzene systems, two of which are displayed in Fig. 16.2a and b. In all cases, only the stability/instability boun ry is shown. These lines correspond to the locus of the critical volume fraction of free polymer required to induce flocculation (vj ) as a function of the volume fraction of the microgel particles (< 3). 

---