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Dechlorination

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    Dechlorination

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    Page 1 of 2 ™® Trademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow Form No. 609-22010-604
    FILMTEC™ Membranes Dechlorinating Feedwater Introduction Chlorine (Cl2) has been used for many years to treat municipal and industrial water and wastewaters as a disinfectant because of its capacity to inactivate most pathogenic microorganisms quickly. The effectiveness of Cl2 is dependent on the Cl2 concentration, time of exposure, and the pH of the water. Chlorine is used for the disinfection of potable water where a residual chlorine concentration near 0.5 mg/L is commonly used. In a water treatment scheme, fouling of water intake lines, heat exchangers, sand filters, etc., may be prevented by maintaining a free Cl2 residual of 0.5-1.0 mg/L. When FILMTEC™ FT30 thin-film composite membrane is used in the reverse osmosis (RO) process, the RO feed must be dechlorinated to prevent oxidation of the membrane. FT30 membrane has a chlorine tolerance of up to 1,000 ppm-hours before noticeable loss of salt rejection is observed. If dechlorination upsets occur and if corrected in a timely manner, membrane damage can be minimized. Definitions and Chemistry Cl2 + H2O 􀀧 HOCl + H+ + Cl- Residual chlorine. Refers to the total amount of chlorine (“combined” and “free available” chlorine) remaining in the water at the time of measurement. Combined available chlorine. Refers to one or more of the family of chlorine-ammonia compounds, called chloramines, resulting from the reaction of chlorine with ammonia compounds present in the water. Free available chlorine. This form is actually hypochlorous acid, hypochlorite ion or a mixture of the two, depending on pH and temperature. Free chlorine is usually present after sufficient chlorine has been added to satisfy the demand of ammonium ions present. Engineering Considerations Chlorine is most commonly available as hypochlorites of calcium and sodium or chlorine gas. Capital cost, operating cost and water chemistry will be the predominate factors in deciding which type of system to use. If the product water from an RO system is chlorinated, care must be exercised to insure that the chlorine does not diffuse back into the membrane. Air breaks, check valves, etc., should be employed appropriately. Chloramines Studies have demonstrated that chlorine reacts with organic compounds present in drinking water to produce a variety of trihalomethanes (THMs). Toxicological investigations have implicated certain THMs as carcinogens. The EPA has established a maximum THM contaminate level of 100 ppb for drinking water. To meet this requirement, many water facilities have sought to reduce levels of THMs. This can be done by using chloramine as a disinfectant. Chloramine does not generate THMs. However, considerable controversy has arisen concerning the efficiency of chloramine disinfection and its potential health effects.
    Page 2 of 2 ™® Trademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow Form No. 609-22010-604
    Chloramines (cont.) In aqueous solution, HOCl reacts with ammonia to form inorganic chloramines in a series of stepwise reactions. HOCl + NH3 􀀧 NH2Cl (monochloramine) + H2O HOCl + NH2Cl 􀀧 NHCl2 (dichloramine) + H2O HOCl + NHCl2 􀀧 NCl2 (nitrogen trichloride) + H2O These reactions are governed primarily by pH and chlorine-to-nitrogen weight ratio. FILMTEC™ FT30 membrane has a 300,000 ppm-hour tolerance for chloramine, which implies that dechlorination is not required. However, since chloramines are formed by adding ammonia to chlorine, it is possible that free chlorine will be present. Since this free chlorine can be damaging to the membrane, dechlorination should still be considered. Dechlorination Residual chlorine can be reduced to harmless chlorides by chemical reducing agents. Discussion follows on two of the most common proven techniques. Activated carbon. Chlorine reacts with activated carbon as follows: C + 2Cl2 + 2H2O → 4HCl + CO2 The activated carbon should be replaced or regenerated periodically. Sodium ****bisulfite. Na2S2O5 + H2O → 2NaHSO3 NaHSO3 + HOCl → HCl + NaHSO4 In theory, 1.37 lb of sodium ****bisulfite will remove 1.0 lb of free chlorine. In practice, however, 3.0 lb of sodium ****bisulfite are normally used to remove 1.0 lb of chlorine from brackish water. For seawater, more sodium ****bisulfite may be required when dissolved oxygen is present. Sodium ****bisulfite has a usual shelf life of 4-6 months under cool, dry storage conditions. The solution life, however, varies with concentration as follows: Solution (wt %) Maximum Solution Life 2 3 days 10 1 week 20 1 month 30 6 months The dechlorination reaction requires mixing to ensure completion. Therefore, static mixers should be utilized and the sodium ****bisulfite injected prior to the cartridge filters. FILMTEC Membranes For more information about FILMTEC membranes, call the Dow Liquid Separations business: North America: 1-800-447-4369 Latin America: (+55) 11-5188-9222 Europe: (+32) 3-450-2240 Pacific: +60 3 7958 3392 Japan: +813 5460 2100 China: +86 21 2301 9000 http://www.filmtec.comNotice: The use of this product in and of itself does not necessarily guarantee the removal of cysts and pathogens from water. Effective cyst and pathogen reduction is dependent on the complete system design and on the operation and maintenance of the system. Notice: No freedom from any patent owned by Seller or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is responsible for determining whether products and the information in this document are appropriate for Customer’s use and for ensuring that Customer’s workplace and disposal practices are in compliance with applicable laws and other governmental enactments. Seller assumes no obligation or liability for the information in this document. NO WARRANTIES ARE GIVEN; ALL IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE ARE EXPRESSLY EXCLUDED.


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