Module: 5. Lecture: 28

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1 Module: 5 Lecture: 28 Chlorine (continued) 4. Diaphragm cell process Raw material Basis: 1000kg of Chlorine Salt Sodium carbonate Sulfuric acid Steam Electricity Refrigeration Direct labour 1820kg 35kg 115kg 11445kg 1365kJ 1035kg 22work-h Reaction Manufacture process Block diagram of manufacturing process Diagram with process equipment Animation N P T E L 1

2 Cl2 Raw brine and NaCl with Mg, Ca, SO 4 impurities NaOH, Soda ash H2 Scrubber Caustic solution Purification tank Diaphragm cell Water Cooler Impurities Sulfuric acid (98%) Muds Water Vent Drying & purification Sulfuric acid (70-93%) Steam Steam Salt recycle to process Evaporator Caustic (50%) and water born waste Figure: Manufacture of chlorine by diaphragm cell process The raw salt was mixed with sodium chloride containing impurities of magnesium, calcium and sulfate as impurities. In the purification tank mixture was purified using caustic and soda ash. The purified brine was charged into the diaphragm cell where electrolysis of brine was carried out. Asbestos and lead materials were used in diaphragm cell. Hydrogen gas was evolved during process. Solid waste was removed continuously from cell. The water borne waste containing salt and caustic were removed using evaporation. The chlorine vapour was evolved from the diaphragm cell which was cooled in cooler. The cooled stream passed to drying and purification section, where counter currently sulfuric acid was introduced for drying purpose. Sulfuric acid (70 93%) was recovered from bottom of the section. Chlorine vapour containing some impurities was scrubbed using caustic solution to remove impurities where chlorine was separated out from top. N P T E L 2

3 5. Membrane cell process Cl2 Anode Cathode H2 Membrane NaOH Figure: Membrane Cell Animation Membrane cell used a semi-permeable membrane to separate the anode and cathode compartments. Membrane is porous chemically active plastic sheet that allow Na + ion to pass but rejects the OH ions. While in diaphragm cells, back migration of ion is controlled by the rate of flow of fluids through the diaphragm and this is regulated by careful control of liquid level in the compartments. Several polymers have been developed as membrane. Du Pont has developed per sulfonic acid polymer (Nafion) while Ashai uses a multiple layer membrane of per fluorosulfonic acid polymer. The purpose of membrane is to exclude OH and Cl ions from the anode chamber, thus making the product far lower in salt than that from diaphragm cell. A membrane cell 20 times larger than diaphragm is being offered in Such a cell unit can produce 240 ton of chlorine per year and power consumption is satisfactory reduced below either mercury or diaphragm cells. A bipolar cell unit is capable of producing 20,000 ton per year with a current density of 4 KA/M2. Combination plant using the output of the membrane cells as fed to diaphragm cells might result in considerable cost reduction. Such combinations have been used with mercury cell output feeding the diaphragm cells. N P T E L 3

4 Advantages More concentrated brine can be used Purer and concentrated products (28% NaOH containing 50ppm of NaCl, 40% NaOH product) are produced Saving of energy and transportation cost Low production cost Disadvantages Readily clogged of membrane Pretreatment of brine is required to remove calcium and magnesium salts Engineering aspects Blow gas Some residual which was known as less compressible gas also known as blow gas. Generally an equilibrium mixture of air and chlorine was usually formed and used for making derivatives such as bleaching powder. Bubblecap column can be used to liquefy the chlorine using counter current method. Electrostatic precipitator Electrostatic precipitator is used to remove sulfuric acid mist and a sodium chloride by passing of gaseous chlorine. Then absorption on activated charcoal is also used to remove organic. Anode Platinum-titanium anodes can be used instead of graphite anodes, which are responsible for chlorinated organic compounds in the chlorine product. Atmospheric emissions Emissions of chlorine, carbon dioxide, carbon monoxide, residual gas or hydrogen occur from diaphragm and mercury cell plants. The amounts of emission gases depends upon plant design and operation. If liquid chlorine N P T E L 4

5 was not produced according to a paper mill plant, the plant have no "blow gas" which resulting from liquefaction. Therefore, no chlorine emission takes place. While chlorine emissions vary according to the waste treatment system employed and the chlorine content of the blow gas if liquid chlorine is produced. Operating procedures are established to minimize the potential for fugitive emissions for safety and health point of view. Consequently, the atmospheric emission points are usually known and efforts are made to control the emissions as follow. Chlorine emissions: Blow gas To produce liquid chlorine, chlorine-cell gas was compressed and cooled during this non-condensable gases saturated with chlorine vapour are produced at the discharge of the condenser. Vents from returned tank cars, ton containers, and cylinders In returned tank cars, generally water and other liquids are present. Suction was apply to returned tanks cars as well as to cylinders and ton containers, to ensure a clean empty car before reloading to remove any liquid chlorine remaining in the vessel before inspection and cleaning. The amount of chlorine thus removed varies considerably. The recovered chlorine was usually sent to the chlorine handling system or sometimes it was sent to the caustic scrubber to avoid upsetting their cell operation. Vents from storage tanks, process transfer tanks and tank cars during handling and loading of liquid chlorine Air is used during transferring of chlorine. The amount of chlorine in the vented air was varied and which was greater at higher temperatures. The time required for transfer and the number of transfers made, depend upon the shape of the vessel. Emergency vents Although emissions are vented to the atmosphere, chlorine seals and other sources of infrequent emissions were usually connected to an emergency scrubber. Alarms and electrical line connections are generally provided to permit shutdown or changes in operating procedures to limit the emission. N P T E L 5

6 Air blowing of depleted brine in mercury-cell plants In mercury-cell plants, recycled brine was saturated with chlorine. This was usually vacuum treated, air blown or both to remove residual chlorine before resaturation. Concentrations of chlorine in the vent gas were low and that was economic recovery in water or carbon tetrachloride absorber cannot be obtained. These types of gases were usually used for plant purposes such as water chlorination or for disposal in lime or caustic scrubbers. Mercury-cell endboxes The discharge end box was constructed with a removable cover for servicing in mercury cells. To prevent chlorine gas from entering the cell room end boxes were connected to a common suction header while the covers of the end boxes are opened. Chlorine in the exhaust header can be neutralized using lime or caustic. Other emissions During the use of mercury in mercury-cathode cells, some mercury vapour are produces which can be emitted during cell operations. A Mercury requirement has minimized mercury vapour emissions as the trend toward the use of higher strength amalgams increased. With the newer cells, daily mercury losses have decreased from 0.27kg to less than 1.3kg per 1000kg of installed daily chlorine capacity. The usual range of mercury losses for typical plants is 3%, which was emitted to the surrounding atmosphere. In a double or triple effect evaporator, the weak sodium hydroxide solution was evaporated followed by a washing filter. The 50% solution was sold as such or it can be further concentrated. The vapours from evaporators represent an emission source. These condensed in barometric condensers, so that the only atmospheric emissions of inert gas. Pollution control Sources of wastewater During the manufacture of chlorine and caustic soda or caustic potash usually have the following raw wastes were required from the process, N P T E L 6

7 A solution of NaOCl and NaHCO3 from the scrubbing of chlorine tail gases consists about 3.37kg of dissolved solids per 450kg of chlorine produced, chlorinated organic obtained from the liquefaction of chlorine gas consists about 0.315kg per 450kg of chlorine produced. Brine wastes from the brine purification system consists about 5.5kg of dissolved solids per 450kg of chlorine produced, spent sulfuric acid from the chlorine drying process about 1.9kg per 450kg of chlorine produced. Weak caustic and brine solution from the caustic evaporators using barometric condensers consists about 4.27kg of dissolved solids per 450kg of chlorine produced andweak caustic and brine solution from the caustic filter washdown consists 16.8kg of dissolved solids 450kg of chlorine produced. Currently more than 30% of the industry using anodes which eliminate the lead discharge. The mercury cell process for the manufacture of chlorine and caustic soda or has a similar waste which was obtained in the case of the diaphragm cell. Sources of solid waste The mercury-cell process and the diaphragm-cell process are having the solid wastes which results from the purification of brine. Process requires that calcium, magnesium and sulfates has to be removed in large prior to electrolysis, which was accomplished by precipitating the calcium and magnesium out of solution using caustic and soda ash. Sulfates can be removed by addition of barium chloride. The precipitated compounds are removed from the brine after they settle as sludge. The brine was generally purified by filtration. For satisfactory process it was necessary to remove calcium and magnesium from the brine. Principal sources of solid waste from the mercury cell process are The disposal of solid waste from chlor-alkali plants using mercury cells having some problems because it is necessity to prevent mercury contamination. The mercury can be absorbed into the brine purification sludge and into the graphite. By careful washing, mercury was removed from the sludge and then precipitated as the sulfide from the wash water. The sulfide was subsequently reduced to metallic mercury for reuse. Mercury can be recovered from graphite by either chemical leaching or roasting. N P T E L 7

8 Principal sources of solid waste from the diaphragm cell process are Brine purification sludge and filtration wastes Deteriorated concrete cell hoods Asbestos from the cell diaphragms Used graphite cell anodes Mastic used in the bonding of carbon anodes Lead from cell bottoms and general industrial trash Control Technology The best practical control technology currently available (BPCTCA) limitations were required the control of specific pollutants, lead and mercury from the wastewater prior to discharge for mercury cell and diaphragm cell categories. While the best available technology economically achievable (BATEA) limitations for both categories prohibit the discharge of process wastewater pollutants. In complying with these limitations, the chlor-alkali industry will have to implement both end of pipe treatment of wastewater and various waste recovery and internal wastewater recycle. For the developed document requires a number of wastewater treatment and pollution abatement measures that are believed to enable the industry to comply with the various effluent limitations. These measures are summarized as follow. Diaphragm Cells BPCTCA Level (1983) - Same as BPCTCA, plus 1. Asbestos and cell rebuilding wastes are filtered or settled in ponds then land dumped 2. Chlorinated organic wastes are incinerated or land dumped 3. Purification muds from brine purification are returned to salt cavity 4. Weak caustic is partially recycled BATEA Level (1983) - Same as BPCTCA, plus 1. Waste sulfuric acid is recycled sold 2. The hypochlorite waste must be catalytically treated and then it is reused 3. All weak brine solution is recycled 4. Conversion to stable anodes N P T E L 8

9 The BPCTCA level in conjunction with a high level of recycle would be sulfuric. Mercury Cells BPCTCA Level (1977) 1. Cell rebuilding wastes are filtered or placed in settling pond then it is used for landfill 2. Chlorinated organic wastes are incinerated 3. Purification muds from brine purification are returned to brine cavity 4. Brine waste streams are partially recycled 5. Mercury effluent is recovered and recycled by curbing, insulation and collection of mercury-contain streams, then it is treated with sodium sulfide BATEA Level (1983) - Same as BPCTCA plus 1. Waste sulfuric acid is recycled or it can berecovered 2. The hypochlorite waste is catalytically treated and then it is reused 3. All weak brine solutions are recycled PROPERTIES Molecular formula : Cl Molecular weight : gm/mole Appearance : Yellow green gas Odour : Distinctive strong odour Boiling point : C Melting point : C Density : 1.56gm/cm 3 Vapour density : 2.48 Solubility : Soluble in water USES As a disinfectant and purifier in plastics and polymers, solvents, agrochemicals and pharmaceuticals As an intermediate in manufacturing other substances where it is not contained in the final product N P T E L 9

10 To purify water supply as the ultimate defense against waterborne microbiological infection As a bleach and disinfectant to bullet-resistant vests, computer hardware, silicon chips and automotive parts In the manufacture of many car components such as nylon for car seatbelts and air bags, for bumpers and mats Essential in the medicines to treat illnesses such as allergies, arthritis and diabetes To sterilise drinking water and to disinfect swimming pools In the manufacture of paper, dyestuffs, textiles, petroleum products, medicines, antiseptics, insecticides, foodstuffs, solvents, paints and plastics, especially PVC To produce chlorates, chloroform, carbon tetrachloride and bromine. As an oxidising agent in substitution reactions in organic chemistry. As a chemical weapon As a reagent in the chemical industry N P T E L 10