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Technical Tips
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SPLIT STREAM DEALKALIZATION
Split stream dealkalization involves the use of two cation exchange systems to soften boiler make up water, one system in the conventional sodium form and one in the "acid" or hydrogen form. The stream of make up water is split and fed through either the sodium unit or the hydrogen unit in a proportion dictated by the relative concentration of strong acid anions (primarily chloride sulfate and nitrate) to bicarbonate anions often termed "M" Alkalinity. The stream passing through the sodium cation unit is softened in the conventional way.
The stream passing through the "acid" or hydrogen form unit is softened and the acid cations are converted to mineral acids.
After the softening process the two streams are mixed and the residual "Free Mineral Acidity" or "FMA" (this is the sulfuric, hydrochloric and nitric acids - H2S04, HCl and HN03) in the acid stream reacts with the residual bicarbonate or "M" Alkalinity in the sodium cation stream to convert it to carbonic acid.
This is how the proportion of water passed through the acid cation unit is calculated. Sulfuric acid has a molecular weight of 98 H2(2) S(32) 04(64) = 98 It is divalent (2 hydrogen ions) Sodium Bicarbonate has a molecular weight of 84 and is monovalent (one sodium ion). So one mole of sulfuric acid neutralizes two moles of sodium bicarbonate.
1 ppm of sulfuric acid acid is equivalent to 1.7 ppm of sodium bicarbonate or 2 ppm of "M" Alkalinity expressed as calcium carbonate. For Hydrochloric acid 1 ppm is equivalent to 2.3 ppm sodium bicarbonate or 2.74 ppm of "M" alkalinity expressed as calcium carbonate.
For example, if your water analysis has: 12 ppm chloride, 10 ppm sulfate and 200 ppm "M" alkalinity 12 ppm chloride is equivalent to 32.4 ppm "M" alkalinity 10 ppm sulfate is equivalent to 20 ppm "M" alkalinity Total 52.4 ppm "M" alkalinity The combined sulfate and chloride content will neutralize 2.6x its concentration of "M" alkalinity. Therefore, passage of 10% of your makeup water stream through the acid cation unit will result in the removal of 12.6% of the "M" alkalinity. If our goal is removal of 75% of the "M" alkalinity the ratio of water passed through the acid cation unit versus the sodium form softener should be 75%/12.6% = 5.95 or 60% of the makeup through the acid cation unit to 40% through the sodium unit. You should check for the nitrate content of the makeup water. If this is significant you will need to calculate the input of the nitric acid present in the acid cation effluent. You should probably use 4% sulfuric acid to regenerate the acid cation unit. You will need to put in a system to handle concentrated sulfuric acid and to dilute it to 4% or you may want to buy a diluted sulfuric acid directly. Diluting concentrated sulfuric acid with water is dangerous - the reaction liberates a lot of heat and must be handled properly. |
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