Lecture 6 chlor alkali industries soda ash caustic soda chlorine
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LECTURE 6 Chlor Alkali Industries – Soda Ash, Caustic Soda, Chlorine. Chapter 13 in Shreve’s Chemical Process Industies. 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.

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Lecture 6 chlor alkali industries soda ash caustic soda chlorine

LECTURE 6ChlorAlkali 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 H2SO4 and NH3 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

Chlor Alkali Industries


Caustic soda naoh

  • Previously made by Causticization of soda ash with lime

    Na2CO3 + Ca(OH)2→ 2 NaOH + CaCO3

  • Only 10% NaOH solution obtained

  • Electrolysis of Brine – Most popular method adopted nowadays.

Caustic Soda – NaOH


Caustic soda n a oh

  • Brittle white solid

  • Readily absorbs moisture and CO2 from air

  • Sold on basis of Na2O content

    • 76% Na2O equivalent to 98% NaOH

  • Uses – Soaps, textiles, chemicals, petroleum refining, etc.

  • Caustic Soda – NaOH



    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

    Manufacture of NaOH


    Energy changes gibbs equation

    Energy Changes-- Gibbs equation


    Heat of reaction h

    Heat of Reaction (∆H)


    Voltage efficiency

    • ∆H is computed in Gibbs overall reaction:Helmholz equation to get E = 2.31 V

    • Voltage Efficiency = Epractical÷ETheoretical×100

    • Generally range from 60 – 75 %.

    • Faraday’s Law: 96,500C of electricity passing through a cell produce 1 gm.eq. of chemical reactions at each electrode

    • Actually higher – Side reactions

    Voltage Efficiency


    Current efficiency and energy efficiency

    Current efficiency and Energy efficiency


    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.

    Decomposition Efficiency


    Cell type

    • Previously mercury was most widely used charged

    • 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

    Cell type


    Diaphragm cells
    Diaphragm Cells charged

    • 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



    Diaphragm cells1

    • 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


    Diaphragm cells advantages disadvantages

    • 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 brine

    • 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.




    Advantages of membrane 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


    Advantages of membrane cell1

  • Small, efficient units may cause a revolution in the distribution of the chlor-alkali industry, particularly if efficiencies remain high

  • Advantages of Membrane Cell


    Disadvantage of membrane cells

    Disadvantage of Membrane Cells


    Mercury cells

    • Operate differently than the other two of savings are lost,

    • 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 + 2H2O → 2 NaOH + H2 + Hg

    Mercury Cells


    Advantages and disadvantages of mercury

    • 50% of savings are lost, 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


    Mercury cell
    Mercury Cell of savings are lost,


    Mercury cell1
    MERCURY CELL of savings are lost,


    Unit operations and chemical conversions

    • Brine Purification of savings are lost,

    • Brine Electrolysis

    • Evaporation and Salt Separation

    • Final Evaporation

    • Finishing of Caustic

    • Special Purification of Caustic

    Unit Operations and Chemical Conversions


    Brine purification

    • Ca of savings are lost, , 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 BaCl2.

    • Brine is preheated with other streams to reduce energy requirement.

    Brine Purification


    Brine electrolysis

    Brine Electrolysis


    Evaporation and salt separation

    • 11 % of savings are lost, 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


    Evaporation and salt separation1

    Evaporation and Salt Separation


    Final evaporation

    • 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


    Finishing of caustic

    • Dowtherm single-effect evaporator to 70 – 75% 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


    Special purification of caustic

    • Troublesome impurities in 50% caustic are Fe, single-effect evaporator to 70 – 75% NaCl and NaClO3.

    • Fe removed by treating caustic with 1% CaCO3 and filtration

    • NaCl and NaClO3 may be removed using aq. NH3

    • To further reduce salt content for some uses; caustic is cooled to 20°C as shown in following diagram

    Special Purification of Caustic


    Purification of caustic soda
    Purification of Caustic soda single-effect evaporator to 70 – 75%


    Chlorine and hydrogen

    • Dried Chlorine is compressed to 240 or 550 single-effect evaporator to 70 – 75% 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


    Soda ash manufacture

    Soda Ash single-effect evaporator to 70 – 75% Manufacture

    Sodium Carbonate


    Soda ash

    • Physical single-effect evaporator to 70 – 75%

      • Odourless/hygroscopic; alkaline in nature

      • Mp. 851 °C; M.wt = 106, Density @ 20 °C = 2.53 g/cm3;

    • Chemical

      • Thermal Decomposition at 1000 °C/200 Pa

      • Na2CO3 Na2O + CO2

      • Lethal dose = 4g/kg (rat); 15g/kg human

    Soda Ash


    Uses of soda ash

    • Glass Industry single-effect evaporator to 70 – 75%

    • Water softening agent

    • Baking soda manufacture

    • Paper making

    • In Power generation to remove SO2 from flue gas

    Uses of Soda Ash


    Manufacturing processes

    Manufacturing processes single-effect evaporator to 70 – 75%

    Le Blanc Process

    Solvay Process


    Le blanc process

    • 2 single-effect evaporator to 70 – 75% NaCl + H2SO4 Na2SO4 + 2 HCl

    • Na2SO4 + 2C  Na2S + 2 CO2

    • Na2S + CaCO3 Na2CO3 + CaS

    • Disadvantages

      • Solid Phase

      • Amount of energy

      • CaS pollutant

    Le Blanc Process


    Leblanc process reaction scheme
    LeBlanc Process Reaction Scheme single-effect evaporator to 70 – 75%


    Leblanc process diagram
    LeBlanc Process Diagram single-effect evaporator to 70 – 75%


    Solvay process

    Solvay Process


    Solvay process1
    Solvay single-effect evaporator to 70 – 75% Process


    Brine (NaCl) single-effect evaporator to 70 – 75%

    Ammoniated Brine

    Ammonia

    NaCl

    H2O

    NH3

    Limestone CaCO3

    NH3

    CO2

    Lime in

    Kiln

    Carbonating Tower

    NH4Cl

    Ammonia Recovery

    Filter

    CaO

    H2O

    Ca(OH)2

    Lime Slaker

    NaHCO3

    Waste by product CaCl2

    300 °C

    Product

    Na2CO3

    • Food additive

    • 2. Electrolyte

    • 3. Dehydrating agent


    Reactions

    • Solvay Tower single-effect evaporator to 70 – 75%

      • 2 NH3 + CO2 + H2O  (NH4)2CO3 (exothermic)

      • (NH4)2CO3 + CO2 + H2O  2 NH4HCO3

      • NH4HCO3 + NaCl  NaHCO3 + NH4Cl2

        Middle of Carbonator

  • Lime Kiln

    • CaCO3  CaO + CO2

    • CaO + H2O  Ca(OH)2

  • Calciner

    • 2 NaHCO3  Na2CO3 + CO2 + H2O

  • Ammonia Recovery

    • 2 NH4Cl + Ca(OH)2  CaCl2 + 2 NH3 + 2 H2O

  • Reactions


    Manufacturing steps

    • Brine Preparation single-effect evaporator to 70 – 75%

    • Ammonia Absorption

    • Precipitation of bicarbonate

    • Filtration of bicarbonate

    • Calcination of bicarbonate

    • Recovery of Ammonia

    Manufacturing Steps


    Solvay process2
    Solvay Process single-effect evaporator to 70 – 75%

    • NH3 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; NH3 = 80-90 kg/m3; CO2 = 40-50 kg/m3

    • 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


    Thank you

    Thank you! single-effect evaporator to 70 – 75%


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