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LECTURE 7 CHLOR-ALKALI INDUSTRIES Soda Ash, Caustic Soda, Chlorine

Chapter 13 Shreve’s Chemical Process Industries, 5th Ed, G. T. Austin

n  Caustic soda, soda ash and chlorine n  Rank close to H2SO4 and NH3 in magnitude of $ value of

use n  Lot of consumption in making other chemicals. n  Uses – Soaps, detergents, fibers and plastics, glass,

petrochemicals, pulp n paper, fertilizer, explosives, solvents and other chemicals

CHLOR ALKALI INDUSTRIES

n  Brittle white solid n  Readily absorbs moisture and CO2 from air n  Sold on basis of Na2O content

n  76% Na2O equivalent to 98% NaOH n  Uses – Soaps, textiles, chemicals, petroleum

refining, etc.

CAUSTIC SODA – NaOH

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n  Previously made by causticization of soda ash with lime

Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3 n  Only 10% NaOH solution obtained n  Electrolysis of brine – most popular method

adopted nowadays.

CAUSTIC SODA – NaOH USES OF CAUSTIC SODA

CAUSTIC SODA - USES

The Pinoy or Filipino dessert (kakanin) called kutsinta uses a bit of lye water to help give the rice flour batter a jelly like consistency. A similar process is also used in the kakanin known as pitsi-pitsi or pichi-pichi (pit-chi-pit-chi) except that the mixture uses grated cassava instead of rice flour.

n  Electrolysis of Brine n  Chlorine at Anode; Hydrogen along with alkali hydroxide at

cathode n  Three types of cell exist:

n  Mercury Cell n  Diaphragm Cell n  Membrane Cell

n  Raw Materials 1. Brine (NaCl) 2. Electricity

MANUFACTURE OF NaOH

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n  Energy consumed in electrolysis is product of current flowing and potential of cell

n  Gibbs Helmholz equation represents the relation between electric energy and heat of reaction:

Energy Changes-- Gibbs equation

n  Found from heats of formation of the components of the overall reaction:

n  This reaction is broken down into following reactions for formation:

HEAT OF REACTION (∆H)

n  ∆H is computed in Gibbs Helmholz equation to get E = 2.31 V

n  Voltage Efficiency = Epractical÷ETheoretical×100 n  Generally range from 60 – 75 %. n  Faraday’s Law: 96,500C of electricity passing

through a cell produce 1 gm.eq. of chemical reactions at each electrode

n  Actually higher – Side reactions

Voltage Efficiency

n  Ratio of theoretical to actual current consumed is current efficiency (≈ 95-97%)

n  Current divided by area on which current acts is current density – high value desirable

n  Product of voltage efficiency and current efficiency is energy efficiency of cell

Current efficiency and Energy efficiency

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n  Ratio of equivalents produced in the cell to equivalents charged

n  Usually about 60 – 65 %. n  Diaphragm cells have very high

decomposition efficiencies n  But encounter difficulties with migration

of hydroxyl ions back to anode à formation of hypochlorite ion

n  At anode, OH- ions give

n  Oxygen formed reacts with graphite anode, decreasing its life

n  In Metal anodes, oxygen does not react.

Decomposition Efficiency

n  Previously mercury was most widely used

n  Health and environmental problems with mercury discharge in nearby waters

n  Improved designs of membrane cells and cheaper purification techniques have reduced cost and improved efficiencies n  Dominate the field nowadays

CELL TYPE

DIAPHRAGM CELLS n  Contain a

diaphragm made of asbestos fibers to separate anode from cathode

n  Allows ions to pass through by migration

n  Graphite anode and cast iron cathode

Asbestos Diaphragm

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n  Diaphragm Permits the construction of compact cells of lowered resistance as the electrodes can be placed close together

n  Diaphragms become clogged with use and should be replaced regularly

n  Diaphragm permits flow of brine from anode to cathode and thus greatly lessens side reactions

n  Cells with metal cathodes rarely get clogged diaphragms and operate for 1-2 years without requiring diaphragm replacements.

Diaphram Cells

n  Major Advantage – Can run on dilute (20%), fairly impure brine

n  Dilute brine produces NaOH 11% (NaCl 15%)

n  Consumes lot of energy for evaporation n  For 1 ton of 50% caustic need 2600 kg

of water to be evaporated. n  Some amount of Chloride ion remains

and is highly objectionable to some industries (Rayon)

Diaphragm Cells– Advantages & Disadvantages

Membrane Cells n  Use semipermeable

membrane to separate anode and cathode compartments.

n  Separate compartments by porous chemically active plastic sheets; that allows sodium ions to pass but reject hydroxyl ions.

MEMBRANE CELL

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MEMBRANE CELL n  Purpose of membrane is to exclude OH- and Cl- ions from

anode chamber n  Thus making the product far lower in salt than that

from a diaphragm cell n  Membrane cells operate using more concentrated brine

and produce purer, more concentrated product n  (30-35% NaOH containing 50 ppm of NaCl)

n  Requires only 715 kg of water to be evaporated to produce 1 M ton of 50% NaOH

ADVANTAGES OF MEMBRANE CELL

n  Because of difficulty and expense of concentration and purification, only large diaphragm cells are feasible

n  Membrane cells produce conc NaOH n  considerable saving in energy (Evaporation) n  and saving in freight (operate to the point of caustic

use) n  Small, efficient units may cause a revolution in the

distribution of the chlor-alkali industry, particularly if efficiencies remain high

ADVANTAGES OF MEMBRANE CELL

n  Membranes are more readily clogged than diaphragms, so some of savings are lost, cost of necessity to pretreat the brine fed in order to remove Ca and Mg before electrolysis

DISADVANTAGE OF MEMBRANE CELLS

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n  Operate differently than the other two n  Cathode is a flowing pool of mercury; graphite

anode n  Electrolysis produces a mercury-sodium alloy

(amalgam) n  Amalgams is decomposed in a separate vessel as: 2Na.Hg + 2H2O → 2 NaOH + H2 + Hg

MERCURY CELLS

n  50% NaOH is produced with very low salt content (30 ppm)

n  No evaporation needed n  Small loss of mercury to

environment poses severe problems.

Advantages and Disadvantages of Mercury

MERCURY CELL

MERCURY CELL

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n  Brine Purification n  Brine Electrolysis n  Evaporation and Salt

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

Unit Operations and Chemical Conversions n  Ca, Fe and Mg compounds plug the diaphragm

n  Precipitation with NaOH is commonly used to remove them

n  Addditional treatment with phosphates is required for membrane cells

n  Sulphates may be removed by BaCl2. n  Brine is preheated with other streams to reduce

energy requirement.

BRINE PURIFICATION

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

n  Monopolar – Cells connected in parallel and low voltage applied to each cell

n  Bipolar – Cells are connected in series and high voltage applied

BRINE ELECTROLYSIS

n  11 % NaOH (Diaphragm cells); 35% (Membrane Cells) are concentrated to 50% NaOH in multiple effect nickel tubed evaporators

n  Salt crystallizes out and recycled n  Concentrated to 73% reduces shipping cost but greatly

increases the shipping and unloading problems n  High m.p of conc material makes steam-heated lines and

steam heating of tank cars necessary. n  Mp for 50% caustic 12°C; for 73%, 65°C.

EVAPORATION AND SALT SEPARATION

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n  Membrane cells produce more concentrated caustic than diaphragm cells

n  Less Evaporation or treatment needed (Membrane cell)

n  Mercury cells produce 50% solution, so no evaporation is needed

EVAPORATION AND SALT SEPARATION n  Cooled and settled 50% caustic may be concentrated in a single-effect evaporator to 70 – 75% NaOH using steam at 500-600 kPa.

n  Strong caustic must be handled in steam-traced pipes to prevent solidification

n  It is run to finishing pots n  Another method – Treating 50% Caustic solution

with Ammonia n  Countercurrent system in pressure vessels n  Anhydrous crystals separate from resulting aq. ammonia

FINAL EVAPORATION

n  Dowtherm heated evaporators – removal of water

n  Product is pumped to discharge the molten material into thin steel drums or into a flaking machine

FINISHING OF CAUSTIC

n  Troublesome impurities in 50% caustic are Fe, NaCl and NaClO3.

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

n  NaCl and NaClO3 may be removed using aq. NH3

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

SPECIAL PURIFICATION OF CAUSTIC

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Purification of Caustic soda

In order to produce 1t of 76% NaOH, 879 kg of Chlorine, and 25.2 kg of Hydrogen, the following materials and utilities are required: Salt: 1.6 t Na2CO3 29.2 kg H2SO4 100.5 kg Steam 10.06 kg Electricity 1197 kJ Refrigerants 0.91 t Direct Labor 20.0 work-hrs

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n  Dried Chlorine is compressed to 240 or 550 kPa n  Lower pressure – rotary compressor n  Larger capacities and Pressures – Centrifugal and non-lubricated

reciprocating compressors

n  Heat of compression is removed and gas condensed n  Liquid Cl is stored in small cylinders n  Hydrogen used in making other compounds

n  With Cl à HCl n  Hydrogenation of fatty acids (Soap manufacture) n  Ammonia

CHLORINE AND HYDROGEN

Soda Ash Manufacture

Sodium Carbonate

n  Physical n  Odourless/hygroscopic; alkaline in

nature n  Mp. 851 °C; M.wt = 106, Density @ 20

°C = 2.53 g/cm3;

n  Chemical n  Thermal Decomposition at 1000 °C/200

Pa n  Na2CO3 à Na2O + CO2

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

Soda Ash

n  Glass Industry n  Water softening agent n  Baking soda manufacture n  Paper making n  In Power generation to remove SO2 from flue gas

USES OF SODA ASH

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Manufacturing processes

Le Blanc Process Solvay Process

n  2 NaCl + H2SO4 à Na2SO4 + 2 HCl n  Na2SO4 + 2C à Na2S + 2 CO2 n  Na2S + CaCO3 à Na2CO3 + CaS n  Disadvantages

n  Solid Phase n  Amount of energy n  CaS pollutant

Le BLANC PROCESS

LeBlanc Process Reaction Scheme

LEBLANC PROCESS DIAGRAM

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

Solvay Process Solvay Process

Brine (NaCl)

Ammoniated Brine

Ammonia

Limestone CaCO3

Lime in

Kiln

Lime Slaker H2O CaO

Carbonating Tower

CO2

NaCl H2O NH3

Filter Ammonia Recovery

NH3

Ca(OH)2

NaHCO3

Product

Na2CO3

300 °C

Waste by product CaCl2

1.  Food additive

2. Electrolyte

3. Dehydrating agent

NH4Cl

n  Solvay Tower n  2 NH3 + CO2 + H2O à (NH4)2CO3

(exothermic) n  (NH4)2CO3 + CO2 + H2O à 2 NH4HCO3 n  NH4HCO3 + NaCl à NaHCO3 + NH4Cl

Middle of Carbonator

n  Lime Kiln n  CaCO3 à CaO + CO2 n  CaO + H2O à Ca(OH)2

n  Calciner n  2 NaHCO3 à Na2CO3 + CO2 + H2O

n  Ammonia Recovery n  2 NH4Cl + Ca(OH)2 à CaCl2 + 2 NH3 + 2

H2O

Reactions

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

Manufacturing Steps

Solvay Process n  NH3 Absorber

n  Counter current flow; Baffles tray n  Cooler to remove heat of solution n  Slightly less than atm pressure n  Made of Cast iron n  At exit; NaCl = 260 g/l; NH3 = 80-90 kg/m3; CO2 = 40-50 kg/m3

n  Carbonator n  6 -9 in number; 20-30 m in height n  Exothermic reaction 60 °C n  To reduce solubility of NaHCO3 use cooler at bottom @ 30 °C n  Vacuum Rotary filter at bottom

Thank you!