LECTURE 6 CHLOR ALKALI INDUSTRIES SODA ASH CAUSTIC

LECTURE 6 CHLOR ALKALI INDUSTRIES – SODA ASH, CAUSTIC SODA, CHLORINE Chapter 13 in Shreve’s Chemical Process Industies

CHLOR ALKALI INDUSTRIES • Caustic soda, soda ash and chlorine • Rank close to H 2 SO 4 and NH 3 in magnitude of $ value of use • Lot of consumption in making other chemicals. • Uses – Soaps, detergents, fibers and plastics, glass, petrochemicals, pulp n paper, fertilizer, explosives, solvents and other chemicals

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

CAUSTIC SODA – Na. OH • Brittle white solid • Readily absorbs moisture and CO 2 from air • Sold on basis of Na 2 O content • 76% Na 2 O equivalent to 98% Na. OH • Uses – Soaps, textiles, chemicals, petroleum refining, etc.

USES OF CAUSTIC SODA

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 (Na. Cl) 2. Electricity

ENERGY CHANGES-GIBBS EQUATION • Energy consumed in electrolysis is product of current flowing and potential of cell • Gibbs Helmholz equation represents the relation between electric energy and heat of reaction:

HEAT OF REACTION (∆H) • Found from heats of formation of the components of the overall reaction: • This reaction is broken down into following reactions formation: Net ∆H for the overall reaction results from

VOLTAGE EFFICIENCY • ∆H is computed in Gibbs Helmholz equation to get E = 2. 31 V • Voltage Efficiency = Epractical÷ETheoretical× 100 • Generally range from 60 – 75 %. • Faraday’s Law: 96, 500 C of electricity passing through a cell produce 1 gm. eq. of chemical reactions at each electrode • Actually higher – Side reactions

• Ratio of theoretical to actual current consumed is current efficiency (≈ 9597%) CURRENT EFFICIENCY • Current divided by area on which AND ENERGY current acts is current density – high EFFICIENCY value desirable • Product of voltage efficiency and current efficiency is energy efficiency of cell

DECOMPOSITION EFFICIENCY • Ratio of equivalents produced in the cell to equivalents charged • Usually about 60 – 65 %. • Diaphragm cells have very high decomposition efficiencies • But encounter difficulties with migration of hydroxyl ions back to anode formation of hypochlorite ion • At anode, OH- ions give • Oxygen formed reacts with graphite anode, decreasing its life • In Metal anodes, oxygen does not react.

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

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

ASBESTOS DIAPHRAGM

DIAPHRAGM CELLS • 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– ADVANTAGES & DISADVANTAGES • Major Advantage – Can run on dilute (20%), fairly impure brine • Dilute brine produces Na. OH 11% (Na. Cl 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)

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.

MEMBRANE CELL

MEMBRANE CELL

ADVANTAGES OF MEMBRANE CELL • 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% Na. OH containing 50 ppm of Na. Cl) • Requires only 715 kg of water to be evaporated to produce 1 M ton of 50% Na. OH

ADVANTAGES OF MEMBRANE CELL • Because of difficulty and expense of concentration and purification, only large diaphragm cells are feasible • Membrane cells produce conc Na. OH • considerable saving in energy (Evaporation) • and saving in freight (operate to the point of caustic use) • Small, efficient units may cause a revolution in the distribution of the chlor-alkali industry, particularly if efficiencies remain high

DISADVANTAGE OF MEMBRANE CELLS • 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

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: 2 Na. Hg + 2 H 2 O → 2 Na. OH + H 2 + Hg

ADVANTAGES AND DISADVANTAGES OF MERCURY • 50% Na. OH is produced with very low salt content (30 ppm) • No evaporation needed • Small loss of mercury to environment poses severe problems.

MERCURY CELL

MERCURY CELL

UNIT OPERATIONS AND CHEMICAL CONVERSIONS • • • Brine Purification Brine Electrolysis Evaporation and Salt Separation Final Evaporation Finishing of Caustic Special Purification of Caustic

BRINE PURIFICATION • Ca, Fe and Mg compounds plug the diaphragm • Precipitation with Na. OH is commonly used to remove them • Addditional treatment with phosphates is required for membrane cells • Sulphates may be removed by Ba. Cl 2. • Brine is preheated with other streams to reduce energy requirement.

BRINE ELECTROLYSIS • 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

EVAPORATION AND SALT SEPARATION • 11 % Na. OH (Diaphragm cells); 35% (Membrane Cells) are concentrated to 50% Na. OH 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 steamheated lines and steam heating of tank cars necessary. • Mp for 50% caustic 12°C; for 73%, 65°C.

EVAPORATION AND SALT SEPARATION • Membrane cells produce more concentrated caustic than diaphragm cells • Less Evaporation or treatment needed (Membrane cell) • Mercury cells produce 50% solution, so no evaporation is needed

FINAL EVAPORATION • Cooled and settled 50% caustic may be concentrated in a single-effect evaporator to 70 – 75% Na. OH using steam at 500 -600 k. Pa. • Strong caustic must be handled in steamtraced 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

FINISHING OF CAUSTIC • 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

SPECIAL PURIFICATION OF CAUSTIC • Troublesome impurities in 50% caustic are Fe, Na. Cl and Na. Cl. O 3. • Fe removed by treating caustic with 1% Ca. CO 3 and filtration • Na. Cl and Na. Cl. O 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

PURIFICATION OF CAUSTIC SODA

CHLORINE AND HYDROGEN • Dried Chlorine is compressed to 240 or 550 k. Pa • Lower pressure – rotary compressor • Larger capacities and Pressures – Centrifugal and nonlubricated 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

SODA ASH MANUFACTURE Sodium Carbonate

SODA ASH • 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 = 4 g/kg (rat); 15 g/kg human

USES OF SODA ASH • • • Glass Industry Water softening agent Baking soda manufacture Paper making In Power generation to remove SO 2 from flue gas

MANUFACTURING PROCESSES Le Blanc Process Solvay Process

LE BLANC PROCESS • • 2 Na. Cl + H 2 SO 4 Na 2 SO 4 + 2 HCl Na 2 SO 4 + 2 C Na 2 S + 2 CO 2 Na 2 S + Ca. CO 3 Na 2 CO 3 + Ca. S Disadvantages – Solid Phase – Amount of energy – Ca. S pollutant

LEBLANC PROCESS REACTION SCHEME

LEBLANC PROCESS DIAGRAM

SOLVAY PROCESS • Continuous process using limestone, ammonia and Na. Cl to produce Na 2 CO 3

SOLVAY PROCESS

Brine (Na. Cl) Ammoniated Brine Na. Cl Limestone Ca. CO 3 Lime in Ca. O Lime Slaker NH 3 H 2 O NH 3 Carbonating Tower CO 2 Kiln H 2 O Ammonia Filter Ca(OH)2 Na. HCO 3 300 °C NH 4 Cl Ammonia Recovery Waste by product Ca. Cl 2 Product 1. Food additive Na 2 CO 3 2. Electrolyte

• Solvay Tower REACTIONS • 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 + Na. Cl Na. HCO 3 + NH 4 Cl 2 Middle of Carbonator • Lime Kiln • Ca. CO 3 Ca. O + CO 2 • Ca. O + H 2 O Ca(OH)2 • Calciner • 2 Na. HCO 3 Na 2 CO 3 + CO 2 + H 2 O • Ammonia Recovery • 2 NH 4 Cl + Ca(OH)2 Ca. Cl 2 + 2 NH 3 + 2 H 2 O

MANUFACTURING STEPS • • • Brine Preparation Ammonia Absorption Precipitation of bicarbonate Filtration of bicarbonate Calcination of bicarbonate Recovery of Ammonia

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; Na. Cl = 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 Na. HCO 3 use cooler at bottom @ 30 °C – Vacuum Rotary filter at bottom

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