Dealkalization of water
Encyclopedia
The dealkalization of water refers to the removal of alkalinity
ion
s from water. Chloride cycle anion ion exchange dealkalizers remove alkalinity from water.
Chloride cycle dealkalizers operate similar to sodium cycle cation water softeners. Like water softeners, dealkalizers contain ion exchange resin
s that are regenerated with a concentrated salt (brine
) solution - NaCl. In the case of a water softener, the cation exchange resin is exchanging sodium
(the Na ion of NaCl) for hardness minerals such as calcium
and magnesium
.
A dealkalizer contains strong base
anion exchange resin that exchanges chloride
(the Cl ion of the NaCl) for carbonate
, bicarbonate
and sulfate
. As water passes through the anion resin the carbonate, bicarbonate and sulfate ions are exchanged for chloride ions.
and are usually preceded by a water softener. Alkalinity is a factor that most often dictates the amount of boiler blowdown. High alkalinity promotes boiler foaming and carryover and causes high amounts of boiler blowoff. When alkalinity is the limiting factor affecting the amount of blowdown, a dealkalizer will increase the cycles of concentrations and reduce blowdown and operating costs.
The reduction of blowdown by dealkalization keeps the water treatment chemicals in the boiler longer, thus minimizing the amount of chemicals required for efficient, noncorrosive operation.
Carbonate and bicarbonate alkalinities are decomposed by heat in boiler water releasing carbon dioxide
into the steam. This gas combines with the condensed steam in process equipment and return lines to form carbonic acid
. This depresses the pH
value of the condensate returns and results in corrosive attack on the equipment and piping.
In general, a dealkalizer is best applied to boilers operating below 700 psi
(48 bar
). In order to justify installation of a dealkalizer on low-pressure boilers, the alkalinity content should be above 50 ppm with the amount of make-up water exceeding 1,000 gallon
s (approx. 4,000 litre
s) per day.
Cooling system make-up will also benefit from reduced alkalinity. The addition of a dealkalizer to a cooling water system will substantially reduce the amount of acid
required to treat the same amount of water.
.
Alkalinity
Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s from water. Chloride cycle anion ion exchange dealkalizers remove alkalinity from water.
Chloride cycle dealkalizers operate similar to sodium cycle cation water softeners. Like water softeners, dealkalizers contain ion exchange resin
Ion exchange resin
An ion-exchange resin or ion-exchange polymer is an insoluble matrix normally in the form of small beads, usually white or yellowish, fabricated from an organic polymer substrate. The material has highly developed structure of pores on the surface of which are sites with easily trapped and...
s that are regenerated with a concentrated salt (brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
) solution - NaCl. In the case of a water softener, the cation exchange resin is exchanging sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
(the Na ion of NaCl) for hardness minerals such as calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
.
A dealkalizer contains strong base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
anion exchange resin that exchanges chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
(the Cl ion of the NaCl) for carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
, bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
and sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
. As water passes through the anion resin the carbonate, bicarbonate and sulfate ions are exchanged for chloride ions.
"Higher capacities can be realized by use of type II rather than type I strong base anion resins. Although bicarbonates are not held as tightly as chlorides on the SBA (strong base anion) resins in the hydroxide form, when the resin is predominantly in the chloride form the pH has been raised by a small addition of caustic to the brine regenerant, there will be a favorable exchange of bicarbonate for the chloride. This exchange works well only with high alkalinity waters (40% to 80%), with capacities of 4 to 10 Kg/CF being obtained. The advantages of SBA resin dealkalization is that low-cost salt is used in place of the acid necessary for the SAC (strong acid cation)and un-lined steel tanks can be used."
Why dealkalize?
Dealkalizers are most often used as pre-treatment to a boilerBoiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
and are usually preceded by a water softener. Alkalinity is a factor that most often dictates the amount of boiler blowdown. High alkalinity promotes boiler foaming and carryover and causes high amounts of boiler blowoff. When alkalinity is the limiting factor affecting the amount of blowdown, a dealkalizer will increase the cycles of concentrations and reduce blowdown and operating costs.
The reduction of blowdown by dealkalization keeps the water treatment chemicals in the boiler longer, thus minimizing the amount of chemicals required for efficient, noncorrosive operation.
Carbonate and bicarbonate alkalinities are decomposed by heat in boiler water releasing carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
into the steam. This gas combines with the condensed steam in process equipment and return lines to form carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...
. This depresses the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
value of the condensate returns and results in corrosive attack on the equipment and piping.
In general, a dealkalizer is best applied to boilers operating below 700 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
(48 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
). In order to justify installation of a dealkalizer on low-pressure boilers, the alkalinity content should be above 50 ppm with the amount of make-up water exceeding 1,000 gallon
Gallon
The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...
s (approx. 4,000 litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
s) per day.
Cooling system make-up will also benefit from reduced alkalinity. The addition of a dealkalizer to a cooling water system will substantially reduce the amount of acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
required to treat the same amount of water.
Alternate method
Although seldom used, a hydrogen cycle cation softener will reduce the alkalinity associated with hardnessHard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...
.