ABSTRACT

CaCO3 deposition on heated surfaces can be ameliorated by chemical, physical or biological methods each with various level of effectiveness. One of the most effective methods is addition of chemical inhibitors, although this can be a high OPEX solution due to the need to continually dose. Here the effects of a range of chemical inhibitors on scale formation were compared using a rapid scaling test, with the best results obtained when polyacrylic acid (PAA) was dosed. Non-chemical treatment methods include the use of magnetic, electronic and electrolytic devices. When comparing the effect of magnetic and electronic pre-treatment, reduction in scaling (51%) was observed only when using magnetic treatment in combination with particulate iron dosing. One option for non-chemical treatment is modification of the scaling surface and in this paper the effect of different surface material and its finish on CaCO3 formation was also investigated.

INTRODUCTION

Fouling of heat transfer equipment by inverse solubility salts, known as scaling or crystallization fouling, is a common problem in domestic, commercial and industrial process where water is used. The primary agents responsible are the carbonates and sulphates of calcium and magnesium, although barium salts, silicate and phosphate scaling are significant in certain areas and industries. Crystallization fouling is probably the most serious single fouling mechanism in heat exchangers (Bansal and Mtfller-Steinhagen, 1993).

The formation of calcium carbonate scales on to a heated surface follows a number of classic steps. The first is the induction period which is the time taken for calcium carbonate to start depositing on the surface. The induction time is effected by level of supersaturation, temperature, flow rate as well as the material and finish of the surface (Epstein, 1983). Before a deposit can form on the surface the foulant must be transported from the bulk solution to the surface at which point it must stick to the surface. Once attached to the surface the scale must be strong enough to withstand erosion and the it will be subjected to ageing which can increase the strength of the scale by recrystallization and dehydration.

Calcium carbonate deposition can be ameliorated by chemical, physical or biological methods. A summary of options available are given in Table 1 along with reported levels of effectiveness. These controls fit into three main categories: those that affect solubility, those that alter the growth mechanisms of the crystals, and those that change the potential of a surface to foul. One of the most effective methods for controlling crystallization fouling is by adding chemical inhibitors to potentially scaling waters and the use of chemicals to control scale build up in boilers dates back to 1821 when it was found that potato starch reduces the rate of scale accumulation (Glater et al., 1980). Commonly used commercial antiscalants are derived from three chemical families: condensed polyphosphates, organophospates, and polyelectrolytes and there are several mechanisms of the scale inhibitory effect including chelation, dispersion and inhibition.

A scale inhibitor is a chemical that can retard or reduce the formation of calcium carbonate crystals and scales and are widely used as a control method in many fields. Impurities such as metal ions and organic molecules greatly influence the rate of precipitation, the crystal structure and the habit. The study of the inhibition effects of metallic cations such as iron, copper and zinc is very important, because of their natural origin in water. Compared with other metals, the effect of zinc is greater; this may be due to the fact that zinc precipitates ma

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