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Electrochemical cooling water treatment: a new strategy for control of hardness, scale, sludge and reducing water usage.

Article Excerpt
INTRODUCTION

An evaporative cooling water system consists of a heat source, cooling tower, recirculating water pump, interconnecting piping, and water to transport heat from the heat source to the environment by evaporation taking place in the cooling tower. Water containing dissolved minerals (make-up water) is added to a cooling tower system to replace evaporative loss and maintain near constant water volume. As cooling tower water evaporates, dissolved minerals from the make-up concentrate and eventually become problematic. Dissolved mineral buildup is controlled through discharge of concentrated system water as bleedoff, subsequently replaced by relatively dilute make-up water. The volumetric ratio of make-up to bleedoff establishes an evaporative concentration factor of dissolved solids, termed cycles of concentration. Evaporation of water to dissipate heat is an ideal means of cooling because each pound of water evaporated carries away almost 1,000 Btu's of heat. The more water can be concentrated without causing mineral deposition or other problems, the more cost and resource efficient the evaporative cooling system becomes.

Fresh water from almost all sources contains some level of dissolved minerals in the form of positively and negatively charged ions. Common positively charged ions include calcium ([Ca.sup.+2]), magnesium (Mg.sup.+2]), sodium ([Na.sup.+1]), and potassium ([K.sup.+1]). Common negatively charged ions include chloride ([Cl.sup.-1]), sulfate ([SO.sub.4.sup.-2]), and bicarbonate ([HCO.sub.3.sup.-1]). These species dissolve in water as rain interacts with the Earth's surface and subsurface over long periods of time. Of these species, calcium and bicarbonate are of chief interest when combating mineral deposition in evaporative cooling water systems. Calcium bicarbonate is quite soluble in water and is the chief troublesome component of what is termed hard water. Its presence in fresh water is the result of slow, low temperature reaction of acidic rain water, containing carbonic acid or dissolved carbon dioxide ([CO.sub.2]), with limestone composed of calcium carbonate (Ca[CO.sub.3]). This process may be summarized as follows:

Ca[CO.sub.3] + [H.sub.2]O + [CO.sub.2] [right arrow] [CO.sup.+2] + 2[HCO.sub.3.sup.-1] (1)

Hard water becomes problematic in cooling water and other energy transfer systems because bicarbonate ion is thermally unstable and easily decomposes to carbon dioxide and carbonate ion according to the following reaction:

2[HCO.sub.3.sup.-1] + heat [right arrow] [CO.sub.2] [up arrow] + [CO.sub.3.sup.-2] + [H.sub.2]O (2)

Note: Upward pointing arrow ([up arrow]) indicates liberated gas as result of reaction.

Bicarbonate ion breakdown proceeds quickly under the conditions typically found with functioning cooling towers: modest water heating from tap water temperature to 95 [degrees]F, agitation that enhances surface release of carbon dioxide, and large air flow that readily accepts the release of [CO.sub.2]. The natural solubility of calcium carbonate in distilled water at room temperature to its constituent ions with sufficient time is approximately ten parts per million (ppm) as Ca[CO.sub.3], or 4 ppm as [Ca.sup.+2] and 6 ppm as [CO.sub.3.sup.-2]. Cooling tower make-up may easily contain from 50 to 300 ppm or more of calcium bicarbonate and is then concentrated several times by evaporation. Of the bicarbonate present, approximately 20 to 40% undergoes thermal decomposition to carbonate ion. This sets the stage for the recirculating cooling tower water to become highly supersaturated with respect to calcium and carbonate ions, either typically reaching concentrations in excess of 100 ppm or more.

Calcium carbonate deposit formation in practical cooling water systems proceeds slowly even though both ions are present far in excess of their theoretical saturation level because the rate of formation of the smallest constituents of deposition, nucleated crystals, can be quite slow. The process of calcium and carbonate ions coming together to begin building calcium carbonate to the point of nucleation occurs throughout the entire volume of cooling water and is...

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