LECTURE 6CHLOR ALKALI INDUSTRIES – SODA ASH, CAUSTIC SODA, CHLORINE

Chapter 13 in Shreve’s Chemical Process Industies

• 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

• 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

• 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

USES OF CAUSTIC SODA

• 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 Materials1. Brine (NaCl) 2. Electricity

MANUFACTURE OF NAOH

• 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

• 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)

• ∆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

• 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

• 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

• 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

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 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

MERCURY CELL

MERCURY CELL

• Brine Purification• Brine Electrolysis• Evaporation and Salt

Separation• Final Evaporation• Finishing of Caustic• Special Purification of Caustic

UNIT OPERATIONS AND CHEMICAL CONVERSIONS

• 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

• 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

• 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

• 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

• 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

• 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

• 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

PURIFICATION OF CAUSTIC SODA

• 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

SODA ASH MANUFACTURE

Sodium Carbonate

• Physical– 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

• Glass Industry• Water softening agent• Baking soda manufacture• Paper making• In Power generation to remove

SO2 from flue gas

USES OF SODA ASH

MANUFACTURING PROCESSES

Le Blanc ProcessSolvay Process

• 2 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 DIAGRAM

• Continuous process using limestone, ammonia and NaCl to produce Na2CO3

SOLVAY PROCESS

SOLVAY PROCESS

Brine (NaCl)

Ammoniated Brine

Ammonia

Limestone CaCO3

Lime inKiln

Lime Slaker

H2O CaO

Carbonating Tower

CO2

NaClH2ONH3

FilterAmmonia Recovery

NH3

Ca(OH)2

NaHCO3

ProductNa2CO3

300 °CWaste by product CaCl21. Food

additive2. Electrolyte3. Dehydrating

agent

NH4Cl

• Solvay Tower• 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

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

MANUFACTURING STEPS

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 = 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!