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Ap-42, ch 8.11: chlor-alkali8.11 Chlor-Alkali
The chlor-alkali electrolysis process is used in the manufacture of chlorine, hydrogen, and sodium hydroxide (caustic) solution. Of these 3, the primary product is chlorine.
Chlorine is 1 of the more abundant chemicals produced by industry and has a wide variety of industrial uses. Chlorine was first used to produce bleaching agents for the textile and paper industriesand for general cleaning and disinfecting. Since 1950, chlorine has become increasingly important asa raw material for synthetic organic chemistry. Chlorine is an essential component of constructionmaterials, solvents, and insecticides. Annual production from U. S. facilities was9.9 million megagrams (Mg) (10.9 million tons) in 1990 after peaking at 10.4 million Mg(11.4 million tons) in 1989.
There are 3 types of electrolytic processes used in the production of chlorine: (1) the diaphragm cell process, (2) the mercury cell process, and (3) the membrane cell process. In eachprocess, a salt solution is electrolyzed by the action of direct electric current that converts chlorideions to elemental chlorine. The overall process reaction is: In all 3 methods, the chlorine (Cl ) is produced at the positive electrode (anode) and the caustic soda (NaOH) and hydrogen (H ) are produced, directly or indirectly, at the negative electrode (cathode).
The 3 processes differ in the method by which the anode products are kept separate from the cathodeproducts.
Of the chlorine produced in the U. S. in 1989, 94 percent was produced either by the diaphragm cell or mercury cell process. Therefore, these will be the only 2 processes discussed in thissection.
Figure 8.11-1 shows a simplified block diagram of the diaphragm cell process. Water (H O) and sodium chloride (NaCl) are combined to create the starting brine solution. The brine undergoesprecipitation and filtration to remove impurities. Heat is applied and more salt is added. Then thenearly saturated, purified brine is heated again before direct electric current is applied. The anode isseparated from the cathode by a permeable asbestos-based diaphragm to prevent the caustic soda fromreacting with the chlorine. The chlorine produced at the anode is removed, and the saturated brineflows through the diaphragm to the cathode chamber. The chlorine is then purified by liquefactionand evaporation to yield a pure liquified product.
The caustic brine produced at the cathode is separated from salt and concentrated in an elaborate evaporative process to produce commercial caustic soda. The salt is recycled to saturate the Figure 8.11-1. Simplified diagram of the diaphragm cell process.
dilute brine. The hydrogen removed in the cathode chamber is cooled and purified by removal ofoxygen, then used in other plant processes or sold.
Figure 8.11-2 shows a simplified block diagram for the mercury cell process. The recycled brine from the electrolysis process (anolyte) is dechlorinated and purified by a precipitation-filtrationprocess. The liquid mercury cathode and the brine enter the cell flowing concurrently. Theelectrolysis process creates chlorine at the anode and elemental sodium at the cathode. The chlorine isremoved from the anode, cooled, dried, and compressed. The sodium combines with mercury to forma sodium amalgam. The amalgam is further reacted with water in a separate reactor called thedecomposer to produce hydrogen gas and caustic soda solution. The caustic and hydrogen are thenseparately cooled and the mercury is removed before proceeding to storage, sales, or other processes.
Tables 8.11-1 and 8.11-2 are is a summaries of chlorine emission factors for chlor-alkali plants. Factors are expressed in units of kilograms per megagram (kg/Mg) and pounds per ton(lb/ton). Emissions from diaphragm and mercury cell plants include chlorine gas, carbon dioxide(CO ), carbon monoxide (CO), and hydrogen. Gaseous chlorine is present in the blow gas from liquefaction, from vents in tank cars and tank containers during loading and unloading, and fromstorage tanks and process transfer tanks. Carbon dioxide emissions result from the decomposition ofcarbonates in the brine feed when contacted with acid. Carbon monoxide and hydrogen are created byside reactions within the production cell. Other emissions include mercury vapor from mercurycathode cells and chlorine from compressor seals, header seals, and the air blowing of depleted brinein mercury-cell plants. Emissions from these locations are, for the most part, controlled through theuse of the gas in other parts of the plant, neutralization in alkaline scrubbers, or recovery of thechlorine from effluent gas streams.
Table 8.11-3 presents mercury emission factors based on 2 source tests used to substantiate the mercury national emission standard for hazardous air pollutants. Due to insufficient data, emissionfactors for CO, CO , and hydrogen are not presented here.
Figure 8.11-2. Simplified diagram of the mercury cell process.
Table 8.11-1 (Metric Units). EMISSION FACTORS FOR CHLORINE FROM Returned tank car vents (SCC 3-01-008-03) Shipping container vents (SCC 3-01-008-04) Mercury cell brine air blowing (SCC 3-01-008-05) a Reference 4. SCC = Source Classification Code.
Table 8.11-2 (English Units). EMISSION FACTORS FOR CHLORINE FROM Returned tank car vents (SCC 3-01-008-03) Shipping container vents (SCC 3-01-008-04) Mercury cell brine air blowing (SCC 3-01-008-05) a Reference 4. SCC = Source Classification Code.
Table 8.11-3 (Metric And English Units). EMISSION FACTORS FOR MERCURY FROM Ullmann’s Encyclopedia Of Industrial Chemistry, VCH Publishers, New York, 1989.
The Chlorine Institute, Inc., Washington, DC, January 1991.
1991 Directory Of Chemical Producers, Menlo Park, California: Chemical InformationServices, Stanford Research Institute, Stanford, CA, 1991.
Atmospheric Emissions From Chlor-Alkali Manufacture, AP-80, U. S. EnvironmentalProtection Agency, Research Triangle Park, NC, January 1971.
B. F. Goodrich Chemical Company Chlor-Alkali Plant Source Tests, Calvert City, Kentucky,EPA Contract No. CPA 70-132, Roy F. Weston, Inc., May 1972.
Diamond Shamrock Corporation Chlor-Alkali Plant Source Tests, Delaware City, Delaware,EPA Contract No. CPA 70-132, Roy F. Weston, Inc., June 1972.
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