Abstract
The kinectics of calcium sulfate dihydrate (CaSO 4 · 2H 2O, gypsum) scale formation on heat exchanger surfaces from aqueous solutions has been studied by a highly reproducible technique. It has been found that gypsum scale formation takes place directly on the surface of the heat exchanger without any bulk or spontaneous precipitation in the reaction cell. The kinetic data also indicate that the rate of scale formation is a function of surface area and the metallurgy of the heat exchanger. A variety of polymeric and nonpolymeric scale inhibitors such as polyacrylates (mol wt 900–250,000), acrylate-based copolymers, polyphosphates (pyro-, tripoly-, and hexametaphosphates), and phosphonates [aminotri(methylenephosphonic acid), 1-hyroxyethylidine 1,1-diphosphonic acid, and 2-phosphonobutane 1,2,4-tricarboxylic acid] have been examined for their effects on the rate of scale formation. The results indicate that the amount of gypsum scale formed on heat exchanger surface is strongly affected by changing the functional group, molecular weight, and concentration of the inhibitor. In addition, amount of gypsum scale formed suggests an optimum effectiveness with molecular of ∼ 2000 for the polyacrylates studied in the molecular weight range 900–250,000. Scanning electron microscopic investigations of the gypsum crystals grown in the presence of polyacrylates show that structures of these crystals are highly modified. A mechanism based upon surface adsorption of inhibitors on the growing gypsum crystals is discussed. The order, in terms of decreasing effectiveness on the rate of gypsum growth, of various scale inhibitors studied is polyacrylic acid (mol wt ∼ 2,000) ⪢ hexametaphosphate > phosphonates.
Published Version
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