P rocess of De De M ineralization ineralization Plant By Moinul Haque E- Mail: Mail:
[email protected]
Defini De finition tion of DM Plant: De mineralization mineralization is a process process of removing undesir undesirable able minerals minerals and converting converting raw water to Soft Soft water and make make suitable for Industrial purposes, which can be done by the addition of Slaked, or Hydrated, Lime or by an ion-exchange process using zeolite as a water softener. In this topic topic we will discuss discuss about Ion-Exchange Ion-Exchange Process. Process.
How Ion Exchange Exchange Works
Ions are atoms, small particles that are the building blocks for molecules. Ions have a weak electrical charge. The charge may be positive (for cations) or negative (for anions). This positive and negative charge is similar to the north or South Pole of a magnet or the positiv positive e or negative terminal terminal of a car battery. Positively charged sodium ions are commonly used to coat cation exchange resins. Negatively charged chloride or hydroxide ions are commonly used to coat anion exchange resins. Mixed bed resins combine both positive and negative ions. Ion Ion exchange exchange units actually exchange exchange ions from from the resins with those in the water. When water to be treated passes through the ion exchange unit, ions in the water are attracted attracted by by either either a positiv positive e or a negative negative charge to the the ions in the resin resin bed. Since the ions ions from the the water are are usually held more more tightly tightly by the resins than they were were held in in the water, water, they they are, in in effect effect,, removed removed from from the water water in the exchange process. Dematerialized process carried out in several stages .Raw water passes through the following unit 1. Multi Multi Grade Grade Filter Filter 2. Activa Activated ted Carb Carbon on Filter Filter 3. Cation Cation Exchang Exchange e Unit Unit 4. Anion Anion Exch Exchang ange e Unit Unit 5. Mi Mixe xed d Bed Bed Unit Unit
Multi Mult i Grade Grade Filter Fil ter (MGF): (MGF): Water contains three major impurities Physical, Biological, Chemical. Physical impurities troubles in various industrial, commercial and domestic applications hence for better quality physical impurities required to reduce up to application standards. Physical impurity is decreased by filtration, various kind of filtration system available for different application with different sharpness of removal in macro to micron. Principle: Principle : The MGF works on principle of retention and removal of physical impurity in a graded manner through Voids of the filtering media. Process: Process : This is a Stainless Steel inside Rubber Lining (SSRL) pressure vessel, with internal distribution & collection systems. The frontal and inter connecting pipes and requisite ITALIC valves of BRASS construction along with one lot of sand, silex & pebbles. The necessary instruments like inlet / outlet, pressure gauges are provided with the system. Function: The physical impurities of raw water i.e. suspended solids, dirt, dust, & all visible particles are removed in M FILTER up to 50 micron.
Resultant: The system reduces the physical impurity load to less then 2 ppm the system can take load of 20 ppm -25 ppm of suspended solutions. Produ ct Description: Description: Multigrade Multigrade filter filter is a depth filter filter that makes use use of coarse and fine media mixed together in a fixed proportion. This arrangement produces a filter bed with adequate pore dimensions for retaining both large and small suspended particles. This filter performs at a substantially higher specific flow rate than conventional filters. Specific flow rates of 15 30 m/h have been successfully obtained for treating waters
containing 25 50 ppm suspended solids respectively to produce filtrate with less than 5 ppm. Activated Carbon Carbon Filter (ACF): Carbon is a substance that has a long history of being used to absorb impurities and is perhaps the most powerful absorbent known to man. One pound of carbon contains a surface area of roughly 125 acres and can absorb literally thousands of different different chemicals. chemicals. Activated Activated carbon carbon is carbon which has a slight electro-positive charge added to it, making it even more attractive to chemicals and impurities. As the water passes over the positively charged carbon surface, the negative ions of the contaminants are drawn to the surface of the carbon granules. Principle: Activated carbon (AC) is a natural material derived from bituminous coal, lignite, wood, coconut shell etc., activated by steam and other means, and each one have different adsorption properties (e.g. bituminous carbon for high chlorine reduction capacity). Some manufacturers use various blends of carbon to achieve specific water quality and contaminants reduction (e.g. coconut shell carbon for "sweet taste"). Activated carbon surface properties are both hydrophobic and oleophilic; that is, they “hate” water but “love” oil. When flow conditions are suitable, dissolved chemicals in water flowing over the carbon surface “stick” to the carbon in a thin film while the water passes on.
There are two principle mechanisms by which activated carbon removes contaminants from water; adsorption, and catalytic reduction, a process involving the attraction of negativelycharged contaminant ions to the positively-charged activated carbon. Organic compounds are removed by adsorption and residual disinfectants such as chlorine and chloramines are removed by catalytic reduction.
This process is called a d s o r p t i o n . As a result of the adsorption adsorption process, process, activated activated carbon is an effective effective method method in removing chlorine and it's by-products (TTHM's) and volatile organic compounds (carbon based VOC's).
Process: Plastic constitutes a major portion of the materials of construction. The pressure vessel is made of fiber reinforced plastic (FRP), pipes of poly vinyl chloride (PVC) and all the filters have a single multi-port plastic valve operated by a hand lever except the smallest ones where plastic ball valves are used. Function: Activated carbon filters used for water treatment typically contain either either granular granular activated activated carbon (GAC) (GAC) or powdered powdered block carbon. Although both are effective, carbon block filters generally have a higher contaminant removal ratio. The two most important factors affecting the efficiency of activated carbon filtration are the amount of carbon in the unit and the amount of time the contaminant spends in contact with it. The more carbon the better. Similarly, the lower the flow rate of the water, the more time that contaminants will be in contact with the carbon, and the more absorption that will take place. Particle size also affects removal rates. Activated carbon filters are usually rated by the size of the particles they are able to remove, measured in microns, and generally range from 50 microns (least effective) down to 0.5 microns (most effective). Resultant As described described above activated activated carbon carbon is an effective effective method in removing chlorine and it's by-products (TTHM's) and volatile organic compounds (carbon based VOC's). Both, man-made and naturally occurring including among others:
alachlor atrazine benzene carbofuran carbon tetrachloride chlorobenzene 2,4-D dibromochloropropane (DBCP) O, P-dechlorobenzines forms of dichloroethylens
1, 2dechloropropa ne cis-1,3dichloropropyle ne toxaphene chlordane radon lindane simazine PCB's toluene xylenes etc., etc.
Chlorine, benzene, trihalomethane (THM) compounds, radon, solvents and hundreds of other man-made chemicals found in tap water. Some activated carbon filters are moderately effective at removing some, but not all, heavy metals. In addition, addition, densely densely compacted carbon carbon block filters filters mechanically mechanically remove particles down to 0.5 micron, including Giardia and Cryptosporidium, turbidity and particulates. Although some iron, manganese, and hydrogen sulfide will be removed by these higher quality activated carbon filters, a manganese greensand iron reduction filter is generally preferred to remove these contaminants as the effectiveness of carbon filter against iron and manganese is generally short-lived if the contaminant concentration is high. Carbon filters are NOT generally successful at removing dissolved inorganic contaminants or metals such as minerals/salts (hardness or scale-causing contaminants), antimony, arsenic, asbestos, barium, beryllium, cadmium, chromium, copper, fluoride, mercury, nickel, nitrates/nitrites, selenium, sulfate, thallium, and certain radio nuclides. Removing these contaminants requires either a reverse
osmosis water filter system or a distiller (some can also be removed by KDF-55 or manganese greensand). GAC does not not remove sediment sediment / particulate particulate material material very well, so they are often preceded by a sediment filter. Sediment pre-filters also prolong the activate carbon cartridge life by eliminating gross contaminants that would otherwise clog the activated carbon thereby reducing the surface area available for absorption. Carbon block filters are generally better then GAC filters at removing sediment. Bad Bacteria Tastes and & Hydrogen Heavy Arsenic Viruses Odors Odors Chlori Chlorine ne Fluori Fluoride de Sulfide Metals Metals Nitrat Nitrates es Radon Radon Sedime Sediment nt Iron Iron VOC's VOC's ** to = Effectively Removes = Significantly Reduces Minimal or No Removal
=
C a ti ti on o n a n d A n i on o n E xc xc ha h a n ge ge U n it it : D.M. plants consists of cation unit and anion unit placed one after other in seris . The cation unit is charged with strong acid cation resin & anion unit is charged with strong base unit. D. M. Plants remove remove all the anions & cations from from the water. water. Conductivity of the treated water is in the range of 0 to 40 ms per cm. The cation resin is regenerated with Hydrochloric acid & anion resin resin with Caustic Caustic solution. solution. Then the the cation unit is rinsed with feed water and anion unit with decationised water till the acceptable water quality is achieved. Cation Cation Exchange Units (Softeners) How Cation Exchange Exchange W orks: orks : Cation exchange resins are usually coated with positively charged sodium ions. When water containing dissolved cations
contacts the resin, the cations are “exchanged” for or trade places with the loosely held sodium ions on the resin. In this way the calcium and magnesium ions responsible for hardness are removed from the water and placed on the exchange resin, and the sodium ions from the resin are added to the water. water. This This process process makes makes the water water “soft. “soft.”” The calcium calcium and magnesium, which cause hardness, are reported as grains per gallon (gpg), milligrams per liter (mg/L), or parts per million (ppm).. (ppm).. Eventually a point is reached when very few sodium ions remain on the resin; thus, no more calcium or magnesium ions can be removed from the incoming water. The resin at this this point is said to to be “exhausted” “exhausted” or “spent” “spent” and cannot accomplish further water treatment until it is “recharged” or “regenerated.” This can be done by backwashing with a HCL solution. Note: Approximately 20 mg of sodium are added per gallon for each grain of hardness reduced W hat Cation Cation Exchange Removes. Water softeners softeners exchange exchange calcium and magnesium magnesium with sodium. This exchange occurs as the hard water passes through a resin bed which attracts and holds calcium and magnesium in exchange for sodium. Calcium and magnesium cause hard water, and high levels can scale pipes, water heaters, boilers, and appliances, reducing water flow and efficiency. Cation exchange resins also remove barium, cadmium, copper, iron, manganese, radium, zinc, and other metallic, positively-charged ions. A table of Cation given Below: Formula Cations Na+ K+ Fe2+ Fe3+ Cu+
Name Sodium potassium iron (II) or ferrous iron (III) or ferric copper (I) or
2+
Cu
NH4+ H3O+
cuprous copper (II) or cupric ammonium hydronium
A n io i o n E x ch ch a ng n g e U n it it How Anion Exchange Exchange Works: Works : Anion exchange units have a resin that exchanges chloride or hydroxide for the anions (the negatively charged atoms) that they remove. Most use chloride, which increases the chloride content of water and may cause a salty taste. What Anion Exchange Removes. Anion exchange units can remove nitrate, sulfate, and other negatively charged atoms called anions. Researchers are developing resins to selectively remove nitrate more efficiently than can now be done.
A table of anions given below Formula Anions OHClBrICNS2NH2NO3NO2SO42SO32PO43HPO42H2PO4ClO4ClO3ClO2ClOMnO42SiO44O22-
Name hydroxide chloride bromide iodide cyanide sulfide amide nitrate nitrite sulfate sulfite phosphate hydrogen phosphate dihydrogen phosphate perchlorate chlorate chlorite hypochlorite manganate silicate peroxide
Mixed Bed Ion Exchange Industrial Mixed Bed Deionisers are designed to produce high purity treated water required by the pharmaceuticals and electronic industries. These deionisers can be used as polishing units after two bed deionisers or directly to obtain high purity water. Mixed Bed Deionisers are single column units, filled with strongly acidic cation and strongly basic anion exchange resins mixed together evenly. Dissolved solids in the water are thus
removed, removed, producing producing water of very very high quality - confirming confirming to IP specifications of purified water. The treated water, however is not free from bacteria and pyrogen.
Working principle There are four distinct stages in the operation of an industrial mixed-bed deioniser: · Service/exhaustion · Backwashing · Regeneration · Rinse/remix Servi Se rvice/ ce/ exhaustion exhaustion
Backwashing
Once the resins are exhausted, the bed is backwashed. Backwashing is initiated by introducing a uniform upward flow of water through the resin bed. The backwash step serves two important functions: Firstly, it expands the resin bed releasing any
entrapped particulate matter and resin fines. Secondly, the backwash flow separates the denser cation resin from the lighter anion resin, forming two distinct layers in the vessel.
Regeneration The first stage stage in the process of regeneratio regeneration n involves passing passing a dilute solution solution of acid, usually hydrochloric, hydrochloric, through through the cation cation bed. After After the cation resin has been regenerate regenerated, d, the anion resin is regener regenerated ated by passing passing a dilute solution of caustic caustic (sodium (sodium hydroxide) hydroxide) through through the anion resin resin bed. As a result, result, the cation cation resin is reconve reconverted rted to the hydrogen hydrogen form and the anion anion resin to the hydroxyl form. First stage cation regenerate
Rinse water flow
Spent acid drain
Dilute acid regenerate Second Stage anion regeneration Dilute caustic regenerate
Spent caustic to drain
Rinse water flow
Rinse / Remix
The final stage of regeneration is to rinse the resins of excess regenerate and then remix with air.
