In almost all boiler applications, providing low hardness feedwater for a boiler minimizes boiler scale and keeps chemical cost low. While there are some exceptions, some type of pretreatment equipment is almost always used to reduce the hardness of the make up water.
How It Works: As city or well water passes through a water softener the cationic resin in the softener exchanges (removes) calcium and magnesium hardness and replaces the hardness for sodium. So the calcium in calcium bicarbonate is exchanged for sodium and becomes sodium bicarbonate.
The amount of hardness that can be removed is typically limited to 30,000 grains (17.1 ppm/grain) for each cubic foot of resin in the softener. As a softener "goes hard", hard water starts to show up in the softener effluent. By flushing the softener with a strong sodium chloride brine solution, the ion exchange process is reversed: the harness ions are removed from the resin and replace with sodium from the salt solution. Typically two or more softener tanks are needed so that one tank can be regenerated while the other is in operation.
Advantages: When properly sized and operated, the use of water softeners are very effective in removing harness from boiler make up water. The initial equipment cost was well as the operating cost is reasonable. And softener maintenance is low compared to other softening methods.
Disadvantages: While water softeners are good at removing hardness, they do not reduce the mineral content of the water; nor do they remove silica or alkalinity. For some boiler systems, reducing these compounds is required.
How It Works: A dealkalizer is similar to softener in that it is regenerated with salt. Dealkalizers have two tanks. The first is a regular softener, containing cationic resin. The second tank contains anionic resin which is also regenerated with salt. During regeneration of the second tank, the chloride in the sodium chloride (salt) brine attaches to the anionic resin removing the alkalinity that was being held by the resin. Alkalinity ions are bicarbonate (HCO3-), carbonate (CO3--), and/or hydroxide (OH-).
In order to increase the efficiency of the anion brine tank, a small amount of sodium hydroxide is added to the tank.
Advantages: A dealkalizer not only removes hardness, it also reduces alkalinity for boiler make up water. The reduction in hardness reduces boiler treatment costs and minimizes scale deposits. The reduction in alkalinity minimizes the amount of carbon dioxide (CO2) that would otherwise leave the boiler with the steam. This is particularly important when the make up water contains high alkalinity. High make up water alkalinity increases boiler blowdown requirement. High alkalinity can also make it impossible or impractical to feed enough neutralizing amine which is required to neutralize the CO2 (carbonic acid) in the condensed steam.
Dealkalizers are also used when amines are not acceptable for use in neutralizing the CO2.
Like water softeners, dealkalizers are only slightly more difficult to operate. They are relatively safe for employees to operate although care must be used when adding sodium hydroxide to the anion brine tank.
Disadvantages: The cost of a dealkalizer system is greater in that the water to be treated must first flow though the cationic tank and then the anionic tank. The cost for a dealkalizer system is somewhat more than double that of a plain water softener. A dealkalizer does not reduce the mineral content of the water nor reduce the silica.
Split Stream Softener
How It Works: The water to be processed is split (divided) with some of the water passing though an acid regenerated cation tank and the other water passing through a regular water softener. The water is recombined following the two softener tanks. The blend of acid water to soft water is adjusted to maintain low alkalinity in the blended water.
Now the water is now piped to a decarbonator tower. The water rains down in this tower. A fan blows air up through the tower and out the top. In doing so, the alkalinity that had been in the raw water has been converted to CO2 gas. The decarbonator tower causes the CO2 that is in the blend water to be removed.
Advantages: A split stream softener reduces both the hardness and alkalinity of the water for boiler make up. The dissolved solids (dissolved minerals) in the influent are reduced in proportion to the alkalinity reduction. The less dissolved solids in the make up water, the less blowdown required. Silica is removed in a strong acid cationic softener. The silica reduction is in proportion to the percent of raw water that passes through the acid softener.
Disadvantages: A split steam softener costs more than a dealkalizer. Sulfuric acid, a strong mineral acid, is used and must be handled with care.
The acid content of the acid regenerated softener slowly decreases over time. That means that more acid water and less soft water should be blended to maintain the desired alkalinity in the blend water. The better the control, the longer the runs between regenerations. Also with less
alkalinity and dissolved solids entering the boilers. This means less CO2 in the steam and less boiler blowdown and therefore lower treatment costs.
Weak Acid Cationic Softener: How It Works: The water to be processed first enters a acid regenerated softener that contains a "weak acid" cationic resin. The water then passes through a regular water softener. The weak acid removes only the hardness combined with alkalinity. The hardness tied to sulfates and chlorides are removed by the regular water softener. Just like a split stream softener, the water flow must go through a decarbonator tower in order for the CO2 to be removed.
Advantages: A weak acid softener is easier to operate than a split steam softener. A weak acid softener is often used when the raw water hardness is equal to or less than the alkalinity. A weak acid stream also reduces both the hardness and alkalinity of the water for boiler make up. The dissolved solids (dissolved minerals) in the influent are reduced in proportion to the alkalinity reduction. The less dissolved solids in the make up water, the less blowdown required. There is no blend water adjustment needed for alkalinity control. The hardness reduction is good as is the alkalinity reduction.
Disadvantages: A weak acid softener costs more than a dealkalizer and about the same as a split stream softener. Sulfuric acid is still used and must be handled with care. Silica is not removed by a weak acid softener.
How It Works: Raw water is passed through a strong acid cationic softener and then through a strong base anionic softener. The cation unit is regenerated with either sulfuric acid or hydrochloric acid. The anion unit is regenerated with caustic (sodium hydroxide). In the cation unit, the H+ from the acid attaches to the resin. In the anion unit the OH- from the caustic attaches to the resin. When raw water passes through the first hydrogen unit, the calcium, magnesium, sodium and all other cations stick to the resin pushing off H+. Now this water passes through the anion unit. All the anions (Cl-, SO4-, HCO3-, CO3- and so forth) stick pushing off the OH-. So coming out of the cation unit we get H+ and coming out of the anion we get OH-; these two combine making HOH which is normally write H2O. A demineralizer removes all the mineral (dissolved solids). A demineralizer can also be a single tank called a mixed bed demineralizer. There are now demineralziers available that are regenerated with electricity rather than chemicals.
Advantages: The processed water contains virtually no dissolved solids and no silica. This greatly reduces boiler blowdown requirements and therefore chemical treatment costs. By removing all the alkalinity, it eliminates the make up water as a source of CO2 corrosion in the steam and condensate lines.
Disadvantages: The cost of installing and operating a demineralizer is high. Part of the high cost is the size of the units. For example, the cation
resin is removing all of the cation minerals, not just those from hardness so more resin is needed. Same for the anion resin. Both the acid and caustic are dangerous and must be handled with care. While a demineralizer removes silica, it is also the first contaminant to leak into the finished water when the demineralizer goes "hard". Silica does not increase the conductivity of the water; instead the water must be tested specifically for silica which takes 15-20 minutes.
While there are several types of lime/soda softeners these softeners can be classified as cold or hot. Both type removes both carbonate and non-carbonate (sulfate) harness, iron, manganese and turbidity if present. The cold process softener reduces hardness to about the 30-85 ppm range. A hot process softener reduces hardness to about 8-25 ppm. Also because a hot process softener operates above 200?F, it also reduces dissolved gases, oxygen and CO2.
How They Works: Cold or hot softening works by adding soda ash and lime in proportion to the hardness and alkalinity in the raw water. The lime raises the pH causing the raw water calcium bicarbonate [Ca(HCO3)2] to change to calcium carbonate [CaCO3] which has low solubility and precipitates to the bottom of the reactor. All the magnesium hardness also reacts with lime and precipitates as magnesium hydroxide [Mg(OH)2]. In this process, the magnesium sulfate and chloride hardness also reacts with lime. The magnesium sulfate and chloride hardness is now present as calcium sulfate or chloride.
The soda ash causes the reacts with the calcium sulfate and calcium chloride and is converted to calcium carbonate which also precipitates.
Advantages: A relatively inexpensive way to remove hardness from very large volumes of water. The hot process softener also removes silica, CO2 and a good bid (but not all) oxygen. The more hardness removed, the lower the dissolved solids in the finish water. That in turn means less boiler blowdown and therefore lower boiler treatment costs.
Disadvantages: Very expensive initial equipment costs. A large foot print is required for all the required equipment; particularly for the cold process softener. Both the cold process and hot process softeners require filters following the softeners to remove suspended solids. The water finished water hardness is still too high for boiler make up. Typically the water, after the filters, is further softened with regular softeners.