Control flocculation

Wistrom, A. and J. Earrell (1998). Simulation and system identification of dynamic models for flocculation control. Water Science Technol. Proc. 7th Int. Workshop on Instrumentation, Control and Automation of Water and Wastewater Treatment and Transport Syst., July 6-9, 1997, Brighton, England, 37, 12, 181-192. Elsevier Science Ltd., Exeter, England. 

As far as conditioning is concerned, in the case of slow flocculation, controlled conditioning is necessary. If the conditioning is weak, the flocculation rate will be slow, but on the other hand intense conditioning can result in fast relative motion of particles which is also unfavorable for their attachment to each other. 

In the following discussion, pesticides that have been dispersed within the polymer in some way or encapsulated as a solution within a polymeric "skin will be discussed under the "Physically Bonded" topics. These systems are those that physically hold the pesticide within the polymeric matrix. On the other hand, the "Chemically Bonded" systems bind the pesticide or pesticidal ligand to part of the molecule of the polymer itself. Their action is due to either the action of the polymer itself or to the gradual breakdown of the molecule leaving a smaller molecule having pesticidal action. Specialized uses will be found in the sections on "Flocculation Control" and "Spray Drift Control". References that emphasize the incorporation of pesticide-polymers as "Polymeric Devices" and those that control insects and weeds through "Plastic Mulches" will be discussed last. 

Flocculating agents differ from other materials used in the chemical process industries in that their effect not only depends on the amount added, but also on the concentration of the solution and the point at which it is added. The process streams to which flocculants are added often vary in composition over relatively short time periods. This presents special problems in process control. 

In the paper industry, PEO is widely used as a retention aid and pitch control agent in the newsprint industry (118—135). Typically, a phenol formaldehyde-type resin is added to the substrate before the addition of PEO. The chemical that is added before PEO has been referred to as an enhancer. Recent pubHcations on designing enhancers that work with PEO have resulted in expanding the use of PEO in flocculation of several substrates (128,129). 

Some polymers from styrene derivatives seem to meet specific market demands and to have the potential to become commercially significant materials. For example, monomeric chlorostyrene is useful in glass-reinforced polyester recipes because it polymerizes several times as fast as styrene (61). Poly(sodium styrenesulfonate) [9003-59-2] a versatile water-soluble polymer, is used in water-poUution control and as a general flocculant (see Water, INDUSTRIAL WATER TREATMENT FLOCCULATING AGENTs) (63,64). Poly(vinylhenzyl ammonium chloride) [70304-37-9] h.a.s been useful as an electroconductive resin (see Electrically conductive polya rs) (65). 

The hot-water separation process involves extremely compHcated surface chemistry with interfaces among various combinations of soUds (including both silica sand and alurninosilicate clays), water, bitumen, and air. The control of pH is critical. The preferred range is 8.0—8.5, achievable by use of any of the monovalent bases. Polyvalent cations must be excluded because they tend to flocculate clays and thus raise viscosity of the middlings in the separation cell. 

Food. Food-grade calcium chloride is used in cheese making to aid in rennet coagulation and to replace calcium lost in pasteurization. In the canning iadustry it is used to firm the skin of fmit such as tomatoes, cucumbers, and jalapenos. It acts as a control in many flocculation, coagulation systems (37). Food-grade calcium chloride is used in the brewing iadustry both to control the mineral salt characteristics of the water and as a basic component of certain beers (see Beer). 

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

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