Chemical Process Industry (Production of Caustic Soda & Chlorine)


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  • The diaphragm is usually made of asbestos and separates the feed brine (anolyte) from thecaustic-containing catholyte.
  • Chemical Process Industry (Production of Caustic Soda & Chlorine)

    1. 1.  What is electrolysis?? Process of breaking down a compound into its constituent elements by passing electricity through it.  Basically chlorine and caustic soda is produced by electrolysis of salt water (brine).  In this process there are three types of electrolytic cell: * Mercury Cell * Diaphragm Cell * Membrane Cell
    2. 2. • Salt is first dissolved in the dissolution tank. • The obtained saturated brine(salt) is then sent to a purification tank to remove impurities.
    3. 3. • Industrial water is also purified before entering the cell. • This shows the need of electricity in electrolytic cell.
    4. 4. • Electric current flowing through the cell decomposes the brine passing through the narrow space between the electrodes, liberating chlorine gas (Cl2) at the anode and metallic sodium (Na) at the cathode. • The chlorine gas is accumulated above the anode assembly and discharged to the purification process. • As it is liberated at the surface of the mercury cathode and the sodium immediately forms an amalgam (a 'mixture' of two metals)
    5. 5. • The liquid amalgam flows from the electrolytic cell to a separate reactor, called the decomposer, where it reacts with water in the presence of a graphite catalyst to form caustic soda (sodium hydroxide) and hydrogen gas. • The sodium-free mercury is fed back into the electrolyser and reused. The reaction in the electrolyser is: 2 Na+ +2Cl-+ 2 Hg → 2 Na-Hg + Cl2(g) The reaction in the decomposer is: 2 Na-Hg + 2 H2O → 2 Na++ 2 OH- + H2 (g) + 2 Hg
    6. 6. • In the diaphragm cell process, there are two compartments separated by a permeable diaphragm. • Brine is introduced into the anode compartment and flows into the cathode compartment. • Similarly to the Membrane Cell, chloride ions are oxidized at the anode to produce chlorine, and at the cathode, water is split into caustic soda and hydrogen. • The diaphragm prevents the reaction of the caustic soda with the chlorine. • A diluted caustic brine leaves the cell. • The caustic soda must usually be concentrated to 50% and the salt removed. This is done using an evaporative process.
    7. 7. • This technology uses water-impermeable ion-conducting membrane. • The membrane is made of a special resin which permits cations (positive ions) to pass through. • The anode chamber of a membrane electrolytic cell is filled with brine, and the cathode chamber with water. • The brine in the anode chamber contains sodium (Na+) and chloride (Cl-) ions.
    8. 8. • These ions migrate when a current is applied: the positively charged sodium ions pass through the membrane to the cathode chamber, while the negatively charged chloride ions are discharged on the anode surface to form chlorine gas (Cl2). • Water in the cathode chamber partly dissociates into hydrogen (H+) and hydroxide (OH-) ions. • The hydrogen ions capture electrons on the cathode surface to form hydrogen gas (H2). • The hydroxide ions are attracted to the anode, but blocked by the membrane, and react with the sodium ions from the anode chamber to form caustic soda (sodium hydroxide, NaOH).
    9. 9. • The chlorine is washed and cooled to remove salt, and further dehydrated before being delivered or liquefied. • Chlorine is liquefied and stored low temperature. The liquid chlorine from the bulk tank can be used as a feedstock.
    10. 10. Caustic soda is further concentrated in a vaporizer to a concentration of about 50% for delivery.
    11. 11. • In the mercury cell process, 50% caustic soda is obtained directly from the decomposers. • The caustic soda is normally pumped through a cooler, then through a mercury removal system and then to the intermediate and final storage sections. In some cases the caustic is heated before filtration. The most common method for removal of mercury from caustic soda is a plate (or leaf) filter with carbon coat. • In the case of diaphragm and membrane technologies the caustic soda is concentrated by evaporation before final storage.
    12. 12.  It never gets contaminated with any sodium chloride solution due to presence of water-impermeable ion-conducting membrane.  The caustic soda is more quality.  Operates at low voltage.  Energy efficient.  Less environmental impact
    13. 13. Safety  Wear goggles all the time to avoid contact with gases.  Handle the gases with care in loading area and also storage area as it is explosive. Always follow the safety hazards.
    14. 14. Health  Eye irritation  Breathing difficulties  Asthma  Irritates respiratory system  Can be fatal
    15. 15. Environmet Air Pollution Water Pollution
    16. 16. Prevention Treat gases before emission Have a proper disposal of industrial waste Practice to recycle and reuse the gases