lecture-7-general_chlor_alkali_industries.pptx

chlor alkali industry

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

2. 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 Chlor Alkali Industries

3. 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. Caustic Soda – NaOH

4. 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% NaOH Uses – Soaps, textiles, chemicals, petroleum refining, etc. Caustic Soda – N a OH

5. Uses of Caustic Soda

6. 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

7. 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: Energy Changes-- Gibbs equation

8. 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 Heat of Reaction ( ∆H)

9. ∆H is computed in Gibbs Helmholz equation to get E = 2.31 V Voltage Efficiency = E practical ÷E Theoretical ×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

10. 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

11. 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

12. 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

13. 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

14. Asbestos Diaphragm

15. 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

16. 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

17. 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.

18. Membrane Cell

19. Membrane Cell

20. 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

21. 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

22. 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

23. 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

24. 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

25. Mercury Cell

26. MERCURY CELL

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

28. 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

29. 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

30. 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

31. 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 Evaporation and Salt Separation

32. 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

33. 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

34. 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

35. Purification of Caustic soda

36. 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

37. Soda Ash Manufacture Sodium Carbonate

38. 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

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

40. Manufacturing processes Le Blanc Process Solvay Process

41. 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

42. LeBlanc Process Reaction Scheme

43. LeBlanc Process Diagram

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

45. Solvay Process

46. Brine (NaCl) Ammoniated Brine Ammonia Limestone CaCO3 Lime in Kiln Lime Slaker H 2 O CaO Carbonating Tower CO 2 NaCl H 2 O NH 3 Filter Ammonia Recovery NH 3 Ca(OH) 2 NaHCO 3 Product Na 2 CO 3 300 °C Waste by product CaCl 2 Food additive 2. Electrolyte 3. Dehydrating agent NH 4 Cl

47. 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 Reactions

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

49. 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

50. Thank you!