Prevention of scale formation

W. Smith. The Prevention of Scale Formation by Boiler-Water... 

Step 1 is dependent upon resin type, temperature, and wall thickness. Step 2 depends upon the mold material s thermal properties, porosity, and mold/ cooling layout geometry. Step 3 can be optimized with regard to temperature, fluid flow rate, and prevention of scale formation on the liquid side. The cooling rate of most processes is limited more by the rate of conduction within the plastic than by the rate of conduction in the mold. The cyclic time of a part is usually strongly dependent on its wall thickness. 

A popular type of internal chemical conditioning, precipitation water treatment program. Relies on the careful management of permanent reserve of phosphate ions in the BW to prevent waterside scale formation. 

The objective of the present continuing investigation is to obtain a better knowledge of the mechanism and limits of scale formation on a heated surface to provide a sound basis for developing methods of scale prevention. 

The conventional chemicals used for the prevention or limitation of scale formation (the deposition of inverse solubility salts) include threshold agents, crystal modifiers, dispersants, and surfactants. 

The product from the manufacturing-scale operation is shown in Fig. 10-7b. The prevention of fines formation resulted in an increase in filtration rate of 5x. A low reagent A contamination of 1% was also realized. Using these conditions, scale-up from laboratory to pilot plant to manufacturing, overall 2000x, was successful. The effect of addition time on the amount of the reaction/crystallization actually carried out in the metastable region is shown in Fig. 10-8. 

W. L. Badger and Associates have published a review of literature on the formation and prevention of scale (2). Little is known about the solubilities of the scale-forming components and the crystal phases usually encountered are not those expected on the basis of equilibrium considerations (1). 

During a study of the applicability of "spray" or "fog" evaporation to sea water desalination, it was found that this technique was particularly useful for scale deposition studies. Thus, test conditions are reproducible and heat transfer coefficients are very high, so that the effect of scale formation is readily apparent. Three novel methods for the control of scale deposits on the evaporating surfaces of a spray evaporator were explored. One involves the addition of small quantities of low molecular weight polyacrylic acid to the feed water, which prevents the formation of adherent scale. The methods are applicable under certain conditions to scales formed from sea water containing substantial amounts of calcium sulfate in addition to alkaline scale-forming substances. While spray evaporation appears to be of limited application in water desalination, the scale-control methods developed are probably applicable to other types of evaporator, particularly of the long-tube type. 

Badger Associates, Inc., W. L., Ann Arbor, Mich., Critical Review of Literature on Formation and Prevention of Scale, Office of Saline Water, Rept. 25 (July 1959). 

Most crystalline natural minerals, and many crystals produced industrially, exhibit some form of aggregation or intergrowth, and prevention of the formation of these composite crystals is one of the problems of large-scale crystallization. The presence of aggregates in a crystalline mass spoils the appearance of the product and interferes with its free-flowing nature. More important, however, aggregation is often indicative of impurity because crystal clusters readily retain impure mother liquor and resist efficient washing (section 9.7.2). 

The methacrylamide-MA copolymer has also been studied for purification of industrial waste water/ The disodium salt of MA has been copolymerized with the sodium salt of 2-acrylamido-2-methyl-propanesulfonic acid, using peroxide initiators, to give useful dispersant and deflocculants for water-insoluble compounds of Fe, Ca, Al, and for silt and clay particles/" The copolymers also prevent boiler scale formation, corrosion, etc., without being affected by the hardness of the water. Copolymers prepared in aqueous solutions are claimed to be useful in well wall materials, coatings for microcapsule production, and for paper dry-strength agents. 

The prevention of deposit formation is important in boilers as the deposits (scales, sludges, corrosion products, etc.) act as insulators, causing a loss of heat transfer. The thicker the deposit, the greater the driving force required to penetrate the barrier and higher the temperature of the tubes. There are several methods to control deposits and some important methods are summarized below. The majority of the corrosion products that deposit in the boiler originate in the condensate system, for instance ... 

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

Prevention of Soil Crusting. Acid-based fertilizers such as Unocal s N/Furic (a mixture of urea with sulfuric acid), acidic polymers such as FMC s Spersal (a poly(maleic acid) derivative originally developed to treat boiler scale) (58), the anionic polyacrylamides described previously, as weU as lower molecular weight analogues such as Cytec s Aerotil L Soil Conditioner, have all been used successfully in at least some circumstances to prevent the formation of soil cmsts. It is difficult to prove benefits in the laboratory, and field tests may give variable results depending on local weather conditions. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Conditions that favor dezincification include stagnant solutions, especially acidic ones, high temperatures, and porous scale formation (2). Additions of small amounts of arsenic, antimony, or phosphoms can increase the resistance to dezincification. These elements are, however, not entirely effective in preventing the dezincification of the two-phase (cc—P) brasses because dezincification of the P-phase is not prevented (31). Another area of corrosion concern involves appHed or residual stresses from fabrication that can lead to EIC of brasses in the form of stress-corrosion cracking. 

The importance of being able to predict the effect of a specific gas analysis on the formation of the corrosive scale is essential in the prevention of blade failures. Again, competent manufacturers have successfully developed a means of achieving this through the application of modern computer programs. However, the gas analysis provided must be as accurate as possible if the program and calculations performed are to have any real value. 

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material. 

In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly ehosen. Thus, the addition of normal phosphates to an unsoftened feed water ean eause a dangerous eondition by elogging the makeup line with preeipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the eombination of ealcium or magnesium with negative ions of soap stearates. 

Despite the benefits of blow-down, however, chemical, electrostatic, or electronic treatment of the water is often required to prevent scale formation, corrosion, or biological growth. When treatment is required, or anticipated to be required, the services of a reliable water treatment company should be obtained. ... 

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