1. LECTURE 6 Chlor Alkali Industries – Soda Ash, Caustic Soda, Chlorine
Chapter 13 in Shreve’s Chemical Process Industies

2. 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

4. 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

5. Readily absorbs moisture and CO2 from air
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

6. Uses of Caustic Soda

7. Chlorine at Anode; Hydrogen along with alkali hydroxide at cathode
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

8. 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:

9. Heat of Reaction (∆H)
Found from heats of formation of the components of the overall reaction:
This reaction is broken down into following reactions for formation:
Net ∆H for the overall reaction results from

10. ∆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: 96,500C of electricity passing through a cell produce 1 gm.eq. of chemical reactions at each electrode
Actually higher – Side reactions
Voltage Efficiency

11. Current efficiency and Energy efficiency
Ratio of theoretical to actual current consumed is current efficiency (≈ 95-97%)
Current divided by area on which current acts is current density – high value desirable
Product of voltage efficiency and current efficiency is energy efficiency of cell
Current efficiency and Energy efficiency

12. 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

13. 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
Cell type

14. Allows ions to pass through by migration
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

15. Asbestos Diaphragm

16. Diaphragms become clogged with use and should be replaced regularly
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

17. 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

18. Use semipermeable membrane to separate anode and cathode compartments.
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.

19. Membrane Cell

20. Membrane Cell

21. 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% NaOH containing 50 ppm of NaCl)
Requires only 715 kg of water to be evaporated to produce 1 M ton of 50% NaOH

22. Advantages of Membrane Cell
Because of difficulty and expense of concentration and purification, only large diaphragm cells are feasible
Membrane cells produce conc NaOH
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
Advantages of Membrane Cell

23. 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
Disadvantage of Membrane Cells

24. 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 + 2H2O → 2 NaOH + H2 + Hg
Mercury Cells

25. Advantages and Disadvantages of Mercury
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

26. Mercury Cell

27. MERCURY CELL

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

29. 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 BaCl2.
Brine is preheated with other streams to reduce energy requirement.
Brine Purification

30. 3.0 – 4.5 V per cell is used; whichever method is adopted
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

31. Evaporation and Salt Separation
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.

32. 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

33. It is run to finishing pots
Cooled and settled 50% caustic may be concentrated in a single-effect evaporator to 70 – 75% NaOH using steam at 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

34. Dowtherm heated evaporators – removal of water
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

35. Special Purification of Caustic
Troublesome impurities in 50% caustic are Fe, 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

36. Purification of Caustic soda

37. Dried Chlorine is compressed to 240 or 550 kPa
Chlorine and Hydrogen
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

38. Soda Ash Manufacture
Sodium Carbonate

39. Odourless/hygroscopic; alkaline in nature
Physical
Odourless/hygroscopic; alkaline in nature
Mp. 851 °C; M.wt = 106, 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

40. Baking soda manufacture Paper making
Uses of Soda Ash
Glass Industry
Water softening agent
Baking soda manufacture
Paper making
In Power generation to remove SO2 from flue gas

41. Manufacturing processes
Le Blanc Process
Solvay Process

42. 2 NaCl + H2SO4  Na2SO4 + 2 HCl Na2SO4 + 2C  Na2S + 2 CO2
Le Blanc Process
2 NaCl + H2SO4  Na2SO4 + 2 HCl
Na2SO4 + 2C  Na2S + 2 CO2
Na2S + CaCO3  Na2CO3 + CaS
Disadvantages
Solid Phase
Amount of energy
CaS pollutant

43. LeBlanc Process Reaction Scheme

44. LeBlanc Process Diagram

45. Continuous process using limestone, ammonia and NaCl to produce Na2CO3
Solvay Process
Continuous process using limestone, ammonia and NaCl to produce Na2CO3

46. Solvay Process

47. Brine (NaCl)
Ammoniated Brine
Ammonia
NaCl
H2O
NH3
Limestone CaCO3
NH3
Lime in
Kiln
CO2
Carbonating Tower
NH4Cl
Ammonia Recovery
Filter
H2O
CaO
Ca(OH)2
Lime Slaker
NaHCO3
Waste by product CaCl2
300 °C
Product
Na2CO3
Food additive
2. Electrolyte
3. Dehydrating agent

48. Solvay Tower Lime Kiln Calciner Ammonia Recovery
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

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

50. Solvay Process
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 = kg/m3; CO2 = kg/m3
Carbonator
6 -9 in number; m in height
Exothermic reaction 60 °C
To reduce solubility of NaHCO3 use cooler at 30 °C
Vacuum Rotary filter at bottom

52. Thank you!