chlor-alkali

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Chlor - Alkali Industries – Chlorine, Caustic Soda, Soda Ash ,

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H History 1960-1980- establishment of process industries for raw materials like caustic soda and chlorine. The production of chlorine and caustic soda by the electrolysis of an aqueous solution of sodium chlorine (brine) became very important. 1964 when DuPont recognized the nafion as a separator in chlor alkali cells.

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Chlor - Alkali Industries Products are: Chlorine, Caustic soda and Soda ash

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Chlorine Manufacture

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Chlorine Used for bleaching Increased in importance extremely fast. Largely used in the synthesis of organic chemicals Manufacturing of chlorine is most like have no difference with compare with caustic soda

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Caustic Soda Manufacture NaOH

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Caustic Soda – NaOH Previously made by Causticization of soda ash with lime Na 2 CO 3 + Ca (OH) 2 → 2 NaOH + CaCO 3 Only 10% NaOH solution obtained Electrolysis of Brine – Most popular method adopted nowadays.

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Characteristics: Brittle white solid Readily absorbs moisture and CO 2 from air Sold on basis of Na 2 O content 76% Na2O equivalent to 98% NaOH Uses

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Manufacture Processes NaOH

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Manufacture of NaOH Electrolysis of Brine Chlorine at Anode; Hydrogen along with alkali hydroxide at cathode Three types of cell exist: Mercury Cell Diaphragm Cell Membrane Cell Raw Materials 1. Brine ( NaCl ) 2. Electricity

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Electrolysis of Brine Three important chemicals, NaOH , Cl 2 , H 2 , can be obtained by electrolyzing an aqueous NaCl solution (brine ). This forms the basis of the chlor - alkali industry. The diaphragm cell (also called a Hooker cell) in which the electrolysis is carried out is shown schematically in Fig. 1. At the cathode, water is reduced : Figure 1 Diaphragm or Hooker cell for electrolysis I of brine (schematic). Since chloride ions are removed and hydroxide ions produced by the electrolysis, the electrolyte gradually changes from a solution of sodium chloride to a solution of sodium hydroxide.

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Cell Type Previously mercury was most widely used Health and environmental problems with mercury discharge in nearby waters Improved designs of membrane cells and cheaper purification techniques have reduced cost and improved efficiencies Dominate the field nowadays

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Diaphragm Cells Contain a diaphragm made of asbestos fibers to separate anode from cathode Allows ions to pass through by migration Graphite anode and cast iron cathode

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Asbestos Diaphragm

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Diaphragm Permits the construction of compact cells of lowered resistance as the electrodes can be placed close together Diaphragms become clogged with use and should be replaced regularly Diaphragm permits flow of brine from anode to cathode and thus greatly lessens side reactions Cells with metal cathodes rarely get clogged diaphragms and operate for 1-2 years without requiring diaphragm replacements. Diaphragm Cells

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Major Advantage – Can run on dilute (20%), fairly impure brine Dilute brine produces NaOH 11% ( NaCl 15%) Consumes lot of energy for evaporation For 1 ton of 50% caustic need 2600 kg of water to be evaporated. Some amount of Chloride ion remains and is highly objectionable to some industries (Rayon) Diaphragm Cells– Advantages & Disadvantages

Membrane Cells:

Membrane Cells Use semipermeable membrane to separate anode and cathode compartments. Separate compartments by porous chemically active plastic sheets; that allows sodium ions to pass but reject hydroxyl ions.

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Membrane Cell

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Purpose of membrane is to exclude OH - and Cl - ions from anode chamber Thus making the product far lower in salt than that from a diaphragm cell Membrane cells operate using more concentrated brine and produce purer, more concentrated product (30-35% NaOH containing 50 ppm of NaCl ) Requires only 715 kg of water to be evaporated to produce 1 M ton of 50% NaOH Advantages of Membrane Cell

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Membranes are more readily clogged than diaphragms, so some of savings are lost, bcos of necessity to pretreat the brine fed in order to remove Ca and Mg before electrolysis Disadvantage of Membrane Cells

Mercury Cells:

Operate differently than the other two Cathode is a flowing pool of mercury; graphite anode Electrolysis produces a mercury-sodium alloy (amalgam ) Amalgams is decomposed in a separate vessel as : 2Na.Hg + 2H 2 O → 2 NaOH + H 2 + Hg Mercury Cells

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Mercury Cell

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50% NaOH is produced with very low salt content (30 ppm) No evaporation needed Small loss of mercury to environment poses severe problems. Advantages and Disadvantages of Mercury

Unit Operations and Chemical Conversions:

Brine Purification Brine Electrolysis Evaporation and Salt Separation Final Evaporation Finishing of Caustic Special Purification of Caustic Unit Operations and Chemical Conversions

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Ca , Fe and Mg compounds plug the diaphragm Precipitation with NaOH is commonly used to remove them Addditional treatment with phosphates is required for membrane cells Sulphates may be removed by BaCl 2 . Brine is preheated with other streams to reduce energy requirement. Brine Purification

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3.0 – 4.5 V per cell is used; whichever method is adopted Monopolar – Cells connected in parallel and low voltage applied to each cell Bipolar – Cells are connected in series and high voltage applied Brine Electrolysis

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11 % NaOH (Diaphragm cells); 35% (Membrane Cells) are concentrated to 50% NaOH in multiple effect nickel tubed evaporators Salt crystallizes out and recycled Concentrated to 73% reduces shipping cost but greatly increases the shipping and unloading problems High m.p of conc material makes steam-heated lines and steam heating of tank cars necessary. Mp for 50% caustic 12°C; for 73%, 65°C. Evaporation and Salt Separation

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Cooled and settled 50% caustic may be concentrated in a single-effect evaporator to 70 – 75% NaOH using steam at 500-600 kPa . Strong caustic must be handled in steam-traced pipes to prevent solidification It is run to finishing pots Another method – Treating 50% Caustic solution with Ammonia Countercurrent system in pressure vessels Anhydrous crystals separate from resulting aq. ammonia Final Evaporation

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Dowtherm heated evaporators – removal of water Product is pumped by a C.P that discharges the molten material into thin steel drums or into a flaking machine Finishing of Caustic

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Troublesome impurities in 50% caustic are Fe, NaCl and NaClO 3 . Fe removed by treating caustic with 1% CaCO 3 and filtration NaCl and NaClO 3 may be removed using aq. NH 3 To further reduce salt content for some uses; caustic is cooled to 20°C as shown in following diagram Special Purification of Caustic

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Purification of Caustic soda

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Dried Chlorine is compressed to 240 or 550 kPa Lower pressure – rotary compressor Larger capacities and Pressures – Centrifugal and non-lubricated reciprocating compressors Heat of compression is removed and gas condensed Liquid Cl is stored in small cylinders Hydrogen used in making other compounds With Cl  HCl Hydrogenation of fatty acids (Soap manufacture) Ammonia Chlorine and Hydrogen

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Soda Ash Manufacture Sodium Carbonate

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Physical Odourless /hygroscopic; alkaline in nature Mp . 851 °C; M.wt = 106, Density @ 20 °C = 2.53 g/cm 3; Chemical Thermal Decomposition at 1000 °C/200 Pa Na 2 CO 3  Na 2 O + CO 2 Lethal dose = 4g/kg (rat); 15g/kg human Soda Ash

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Glass Industry Water softening agent Baking soda manufacture Paper making In Power generation to remove SO 2 from flue gas Uses of Soda Ash

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Manufacturing Processes Le Blanc Process Solvay Process

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2 NaCl + H 2 SO 4  Na 2 SO 4 + 2 HCl Na 2 SO 4 + 2C  Na 2 S + 2 CO 2 Na 2 S + CaCO 3  Na 2 CO 3 + CaS Disadvantages Solid Phase Amount of energy CaS pollutant Le Blanc Process

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LeBlanc Process Reaction Scheme

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LeBlanc Process Diagram

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Continuous process using limestone, ammonia and NaCl to produce Na 2 CO 3 Solvay Process

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Reactions Solvay Tower 2 NH 3 + CO 2 + H 2 O  (NH 4 ) 2 CO 3 (exothermic) (NH 4 ) 2 CO 3 + CO 2 + H 2 O  2 NH 4 HCO 3 NH 4 HCO 3 + NaCl  NaHCO 3 + NH 4 Cl 2 Middle of Carbonator Lime Kiln CaCO 3  CaO + CO 2 CaO + H 2 O  Ca (OH) 2 Calciner 2 NaHCO 3  Na 2 CO 3 + CO 2 + H 2 O Ammonia Recovery 2 NH 4 Cl + Ca (OH) 2  CaCl 2 + 2 NH 3 + 2 H 2 O

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Manufacturing Steps Brine Preparation Ammonia Absorption Precipitation of bicarbonate Filtration of bicarbonate Calcination of bicarbonate Recovery of Ammonia

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Solvay Process NH 3 Absorber Counter current flow; Baffles tray Cooler to remove heat of solution Slightly less than atm pressure Made of Cast iron At exit; NaCl = 260 g/l; NH 3 = 80-90 kg/m 3 ; CO 2 = 40-50 kg/m 3 Carbonator 6 -9 in number; 20-30 m in height Exothermic reaction 60 °C To reduce solubility of NaHCO3 use cooler at bottom @ 30 °C Vacuum Rotary filter at bottom

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