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The fouling resistance curves obtained in the case of plasma treatment at 0.5 m/s depicted similar trend as the 0.1 m/s case shown in Fig. Fouling resistances for 250 ppm hard water under no-treatment and plasma-treated cases with a flow velocity of 0.5 m/s. The final asymptotic fouling resistance at the end of the test was 50% lower than that obtained from baseline test, clearly indicating the beneficial effect of the plasma discharge on the mitigation of mineral fouling.įig. The similar particulate fouling build-up and dislodge process were repeated during the period between 9 and 12 h. At t = 4 h, the fouling resistance showed a significant drop, indicating that large-scale pieces were dislodged due to the shear stress of the water flow. Hence, much faster particulate fouling took place at the first several hours of the test, causing the dramatic increase in the fouling resistance. It was demonstrated in the authors’ previous study that the precipitation of CaCO 3 could be induced by application of pulsed spark discharge in supersaturated hard water and thus creating a significantly greater number of CaCO 3 particles than the untreated water. This type of fouling can easily be removed by shear forces created by flow rather than those deposits produced from the precipitation of mineral ions directly on the solid heat transfer surface, i.e., precipitation fouling. The former refers to the adhesion of suspended particles to the heat transfer surface in the form of soft sludge. Note that there are two different categories of fouling: particulate fouling and precipitation fouling. The fouling resistance had a steep increase to a maximum value in the first 4 h of operation.
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The fouling resistance curves obtained in the cases of the plasma treatment depicted a completely different trend compared to that obtained for the no-treatment case. After that, at t = 16 h, the fouling resistance began to rise as the thickness of the scale layer slowly increased until the end of test, indicating that the deposition rate of the scales was consistently larger than the removal rate during this period because of the slow flow velocity. At t = 15 h, the fouling resistance increased dramatically as the entire surface of copper tube was fully covered by mineral scales. The fouling resistance in the no-treatment case demonstrated a slow increase in the first 14 h of operation. The scale deposition involved the cumulative effect of a direct diffusion of dissolved calcium ions to the heat transfer surface and the deposition of precipitated calcium salt particles due to supersaturated conditions and accelerated precipitation of calcium salts by PWT. Fouling resistances for 250 ppm hard water under no-treatment and plasma-treated cases with a flow velocity of 0.1 m/s. As calcium carbonate and magnesium hydroxide have lower solubility at high temperatures, hot lime softening can decrease magnesium to 2 ppm and calcium to 25 ppm.įig. Sodium carbonate is used to precipitate calcium ions as calcium hydroxide.
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This can be done under a variety of conditions depending upon the quality of the raw water and the purpose of the treatment.Ĭold lime softening uses calcium hydroxide to precipitate magnesium ions as magnesium hydroxide. Lime or sodium aluminate may be added to raw water to precipitate noncarbonate salts of calcium and magnesium. Calcium and magnesium associated with chloride or sulfate are termed permanent or noncarbonate hardness. Calcium and magnesium associated with bicarbonate anions are termed carbonate or temporary hardness as they can be removed by heating. Moderate to high hardness water of 150–500 ppm can be partially softened by precipitation of the offending species.
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Water hardness is a measure of the amount of calcium and magnesium present and is reported as parts per million (as calcium carbonate). Pratima Bajpai, in Biermann's Handbook of Pulp and Paper (Third Edition), 2018 Precipitation Softening