lecture 22-23-24 chloralkali industry

7/23/2019 Lecture 22-23-24 ChlorAlkali Industry

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LECTURE22, 23, 24: CHLORALKALI INDUSTRYSODAASH (SODIUM CARBONATE, NA2CO3)

CAUSTIC SODA (SODIUM HYDROXIDE, NAOH)

CHLORINE (CL2)

CHEMICAL TECHNOLOGY (CH-206)

Department of Chemical Engineering

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SODIUM CARBONATE (NA2CO3)

Sodium carbonate (Na2CO3)also known as

washing soda or soda ash, is a sodium salt of

carbonic acid.

Most commonly occurs as acrystallineheptahydrate, which readily effloresces to form a

white powder, the monohydrate.

Sodium carbonateis domestically well known as

awater softener.

Soda ash is the most important high tonnage, low

cost, reasonably pure, soluble alkali available to

the industries as well to the laboratory.

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SODIUM CARBONATE:MANUFACTURING PROCESSES

1. Leblanc process.

2. Solvays ammonia soda process.

3. Dual process (modified Solvays process)

4. Electrolytic process.

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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS

Raw materialsBasis: 1 ton Sodium carbonate (98% yield)

Common salt 1126kg (sea water, salt lake and

sub soil water )

Sulfuric acid 945kg (contact process )

Lime stone 963kg (mineral calcite or aragonite)

Coke 463kg

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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC

PROCESS

Reactions

NaCl + H2SO4NaHSO4+ HCl (a)

NaHSO4+ NaClNa2SO4+ HCl (b)

Na2SO4+4CNa2S + 4CO (c)

Na2S +CaCO3Na2CO3+ CaS (d)

(Black ash sludge)

CaS + H2O + CO2

CaCO3+ H2S (e)CaS + H2SCa(HS)2 (f)

Ca(HS)2+ CO2+ H2OCaCO3+ 2H2S (g)

H2S + OH2O + S (h)

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(a) (b) (c,d)CaS is

separated


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NaCl is mixed with the conc. H2SO4in equivalent quantities and heated

in a cast iron salt cake furnace by flue gases from adjacent coal of fire.

NaHSO4along with HCl gas is formed.

HCl is passed to tower packed with coke and is absorbed through a spray

of water comes down in the tower.

The paste of NaHSO4is taken out and heated to a high temperature on

the hearth of a furnace along with some more common salt.

NaHSO4is thus converted into sodium sulfate, known as salt cake.

The salt cake is broken or pulverized, mixed with coke and limestone

and charged into black ash rotary furnace consisting of refractory lined

steel shells.The mass is heated by hot combustion gases entering at one end and

leaving at the others.

Themolten porous gray massthus formed known asblack ashis

separated from the calcium sludge and then crushed and leached with

water in absence of air in a series of iron tank.

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The extract containing Na2CO3, NaOH, and other

impurities is sprayed from the top of a tower in

counter current to flow of hot gases from the black ash

furnace.

The sodium carbonate thus obtained is concentratedin open pans and then cooled to get sodium carbonate.

The product is calcined to get soda ash which is re-

crystallized to Na2CO3.10H2O.

The sludge containing mostly CaS is left behind asalkali waste.

The liquor remaining after removal of first batch of

soda ash crystals is purified and then causticized with

lime to produce caustic soda.

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Recovery of sulfur from alkali wasteAlkali waste is charged into cylindrical iron vessels

arranged in series and CO2delivered from lime kilns is

passed through it, the H2S gas thus obtained is then

conduced together with a regulated amount of air in aClaus kiln containing iron oxide as catalyst.

The exothermic reaction proceeds without further

external heat.

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CaS + H2O + CO2CaCO3+ H2S (e)

CaS + H2SCa(HS)2 (f)

Ca(HS)2+ CO2+ H2OCaCO3+ 2H2S (g)

H2S + OH2O + S (h)


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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA

PROCESS

Raw materialsBasis: 1 ton sodium carbonate

Salt 1550kg (sea water, salt lake

and sub soil water)

Limestone 1200kg (mineral calcite oraragonite)

Coke 90kg

Ammonia as a catalyst 1.5kg (Loss)

High pressure steam 1350kgLow pressure steam 1600kg

Cooling water 4000060000kg

Electric power 210 KWH

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PROCESS

Reactions

CaCO3CaO + CO2 H = +43.4kcals

C(s) + O2(g)CO2(g) H = 96.5kcals

CaO(s) + H2O (l)Ca(OH)2(aq) H = 15.9kcals NH3(aq) +

H2O(l)NH4OH(aq) H = 8.4kcals

2NH4OH + CO2(NH4)2CO3+ H2O H = 22.1kcals

(NH4)2CO3+ CO2+ H2O2NH4HCO3

NH4HCO3+ NaClNH4Cl + NaHCO3

2NaHCO3Na2CO3+ CO2+ H2O H = +30.7kcals

2NH4Cl + Ca(OH)22NH3+ CaCl2+ 2H2H = +10.7kcals

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CaCO3

+ 2NaClNa2

CO3

+ CaCl2


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PROCESS

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PROCESS

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PROCESS

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PROCESS

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Preparation and purification of brineSaturated solution of NaCl is used.

Brine contains impurities such as calcium, magnesium and iron

compounds.

To remove calcium sulfate, magnesium and iron salts sodium

carbonate and sodium hydroxide are added.The precipitated carbonates and hydroxide are removed by filtration.

Sometimes sulfate are removed with BaCl2or the hot brine is treated

with OH and CO32ions.

The calcium, magnesium and iron salts from saturated brine may be

precipitated by dilute ammonia and CO2in a series of washingtowers.

The brine is purified by allowing it to settle in vats, as a result of

which precipitated CaCO3, MgCO3, Mg(OH)2and iron hydroxide

settle down and pure brine solution is pumped to the ammonia

absorber tower, where it dissolve NH3with the liberation of heat.


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PROCESS

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Ammoniation of brineThe purified brine is allowed to percolate down the ammonia tower in

which ammonia gas is passed through the bottom in a counter current

fashion.

The brine solution thus takes up the necessary amount of ammonia and

liberates heat.

The gas which escapes solution in the tank is absorbed by the brine falling

down the tower.

Some carbon dioxide is also absorbed by ammonia, as a result of which

some insoluble carbonate is also precipitated.

The ammoniated brine is allowed to settle, cooled to about 30C and

pumped to the carbonating tower.

Carbon dioxide formationLimestone is calcined to get CO2in a lime kiln filled with coke.

As a result of burning of coke necessary heat required for the

decomposition of lime stone is generated.

CaO obtained from the lime kiln is converted into slaked lime and pumped

to the ammonia recovery tower.


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PROCESS

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Carbonation of ammonium brineCO2from the lime kiln is compressed and passed through the

bottom of carbonating tower down which ammoniated brine

percolates.

Carbonating towers operated in series with several precipitation

towers are constructed of cast iron having 2225 m height, 1.6

2.5 m in diameter.

During the precipitation cycle, the temperature is maintained

about 20-25C at the both ends and 45-55C at the middle by

making use of cooling coils.

The tower gradually becomes flooded as sodium bicarbonatecakes on the cooling coils and shelves.

The cooling coils of the foulded tower are shut off.

Then the fresh hot ammoniated brine is fed down the tower in

which NaHCO3are dissolved to form ammonium carbonate

solution.


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PROCESS

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Carbonation of ammonium brineThe solution containing (NH4)2CO3, unconverted NaHCO3

is allowed to fall down a second tower, called making

tower.

The making, towers are constructed with a series of boxesand sloped baffles.

Ammoniated brine and CO2gas (90-95%) from the

bicarbonate calciner is recompressed and pumped to the

bottom of the making tower.

The ammonium carbonate first reacts with CO2to form

ammonium bicarbonate and the latter reacting with salt,

forms sodium bicarbonate.

The heat of exothermic reaction is removed by cooling

coils.


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PROCESS

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FiltrationNaHCO3slurry is then filtered on a rotary vacuum

filter which helps in drying of bicarbonate and in

recovering ammonia.

The filter cake after removal of salt and NH4Cl bywashing with water, sent to a centrifugal filter to

remove the moisture or calcined directly.

During washing, about 10% NaHCO3also passes into

filtrate.The filtrate containing NaCl, NH4Cl, NaHCO3and

NH4HCO3is treated with lime obtained from lime

kiln to recover NH3and CO2.


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PROCESS

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CalcinationNaHCO3from the drum filter is calcined at about 200C in

a horizontal calciner, which is either fired at feed end by gas

or steam heated unit.

The heating being through the shell parallel to the product,

which prevent the formation of bicarbonate lumps.

The hot soda ash form the calciner is passed through a

rotary cooler and packed in bags.

The exit gases (CO2, NH3, steam etc.) are cooled and

condensed to get liquid ammonia; the rich CO2gas is cooled

and returned to the carbonating tower.

The product from the calciner is light soda ash.

To produce dense soda ash, sufficient water is milled with it

to form more mono hydrate Na2CO3.H2O and the mixture is

recycled.


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PROCESS

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Recovery of ammoniaThe NH3is recovered in strong NH3liquor still, consisting of two sections.

The parts above and below the lime inlet is called as heater and lime still,

respectively.

The filtrate obtained from washing of NaHCO3from the pressure type rotary

filter is fed into the heater, where free NH3and CO2are driven off by distillation.Dry lime or milk of lime (slaked lime) obtained from lime kiln is fed through the

lime inlet and mixed with the liquor from the heater.

As the liquor flows down the column, calcium chloride and calcium sulfate are

formed and NH3gas is released.

NH4Cl + Ca(OH)2CaCl2+ 2NH3+ H2O

(NH4)2SO4+ Ca(OH)2CaSO4+ 2NH3+ 2H2OThe liquor from the bottom of the lime still is free from ammonia and contains

unreacted NaCl and largely CaCI2, which is disposed off.

The liquor is, therefore allowed to settle in settling ponds and the clear liquid is

evaporated till the salt separates out and is sold as such for calcium chloride or

further evaporated.


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PROCESS

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Kinetics and thermodynamicsThe overall reaction shows that salt and calcium

carbonate are the only raw materials which are

continuously supplied in the process and that produce

sodium carbonate and calcium chloride

CaCO3+ 2NaClNa2CO3+ CaCl2Overall reaction of ammoniation of brine and then

treatment of carbon dioxide to ammoniated brine is as

under

2NaCl + 2H2O +2NH3+ 2CO22NaHCO3+ 2NH4ClThe above reaction shows the role of ammonia and carbon

dioxide in the process, and also determines the yield of

the final product.


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PROCESS

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Kinetics and thermodynamicsThere so conditions favourable to it are precisely

defined. For these the reaction is divided in to two

steps.

2NH3+ 2CO2+2H2O2NH4HCO3---- (a)2NaCl + 2NH4HCO32NaHCO3+ 2NH4Cl ---- (b)

Reaction (a) is favoured by low temperature because it

requires the dissolution of gas in water, is displaced to

right by virtue of the fact that reaction (b), which

utilizes the product by subtracting it from (a) isdisplaced in the same direction.

Consequently, it is the precipitation of NaHCO3

according to (b) which is the driving force behind the

entire method.


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Above data indicate that precipitation fortunately tends to take place preferentially

with satisfactory yields.

On the basis of data and common ion effect on precipitation of salts, physicochemicalconditions most suitable for the forward step of reaction (b) which causes

precipitation of NaHCO3are as under:To maintain lowest possible temperature in order to lower the solubility off sodium bicarbonate

To maintain the greatest possible concentration of one or both the salts appearing on the

product side of reaction (b) with the aim of lowering still further solubility of sodium

bicarbonate.

The solubilities of the salts at various temperature is as under


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PROCESS

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Kinetics and thermodynamicsThese conditions are nevertheless discerningly applied because they

serve to bring about appreciable increase in the yields of NaHCO3

and permit the most effective use of most costly reagent NH4HCO3

in reaction (b).

Attention is paid to the fact that, if precipitation temperature isalways kept low, the sodium bicarbonate separates in a

microcrystalline form which is with difficult to filter and it is soluble

during subsequent washing on the filter, increase requirement of

NaCl.

Experimentally, the conditions which are most effectively reconcile

the physicochemical aspect of precipitation of sodium bicarbonateeconomically are as under284gm/liter (4.9mole/liter) of NaCl reacting with 76gm/liter (4.5mole/liter)

of NH3instead of equimolecular solution of two reagents.

Relatively high temperature (60-650C) at the start so as to allow the

formation of well-developed NaHCO3crystallization seeds and increasing the

volume of these seeds to decrease the solubility of salt with gradual cooling.


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PROCESS

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Major engineering problemAbsorption units

Making tower

Development of suitable calcining equipment

Filtration unit

Ammonia recovery

Waste disposal


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SODIUM CARBONATE:MANUFACTURING PROCESSES:DUAL PROCESS

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Raw materialsBasis: 1 t Sodium carbonate

Crystalline Salt 1260kg

Ammonia 325kg (HaberBosch Process)

High pressure steam 1350kg

Low pressure steam 100kg

Cooling water 5000080000kg

Electric power 450KWH

Co-product (NH4Cl) 620kg


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Reactions

C + O2CO2

NH3+ H2ONH4+ OH

CO2+ H2OHCO3+ H+

CO2+ OHHCO3

Na++ Cl+ NH4++ HCO3

NH4+Cl+ NaHCO3

2NaHCO3

Na2CO3+ CO2+ H2O


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The liquor from carbonation tower, containing ammonium chloride,

unreacted NaCl and traces of sodium carbonate is ammoniated in

ammonia absorber.

The ammoniated liquor is sent to a bed of washed salt in salt dissolver.

The resulting liquor is gradually cooled to 00C in refrigerating tank unit,

resulting into crystallize out ammonium chloride.The slurry containing ammonium chloride is thickened and NH4Cl is

centrifuged and dried, which is a co-product.

These is the principal modification of dual process in which ammonium

chloride is recovered as co-product rather than liberation of the

contained ammonia for recycle as in the Solvay process.

The liquor obtained after separation of NH4Cl is charged to series of

carbonation towers in which CO2is passed from bottom in the counter

current flow of liquor.

The resulting sodium bicarbonate is thickened into thickener and

centrifuged. It is then calcined into sodium carbonate.


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Major engineering problemSalt purification

Corrosion

Refrigeration Cost


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Advantage of Solvay

processLess electric power

Less corrosion

problemUse of low grade brine

Not a problem of

disposal of co-product

Does not requireammonia plant

Disadvantage of Solvay process

Higher salt consumption

Waste disposal of CaCl2

brine stream

Higher investment in NH3

recovery units than

crystallization unit of NH4Cl

More steam consumption

Higher capacity plant set up

require for economic breakeven operation (100 v/s

55tons/day)

NH4Cl can be used as mixed

fertilizer ingredient which

minimizes the disposal

problem of Duel process.


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SODIUM CARBONATE:MANUFACTURING PROCESSES:ELECTROLYTIC PROCESS

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Raw materialsBasis: 1 t Sodium carbonate (98% yield)

Salt 563kg (sea water, salt lake and sub soil water)

Carbon dioxide 424kg

Reactions

NaClNa++ Cl 2H2O + 2eH2+ OHAt cathode

2H2O + 2eH++ 2OH

Na++ OH NaOH

2NaOH + CO2Na2CO3+ H2OAt Anode

CleCl

Cl + Cl

Cl2


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At cathode (Steel tube)2H2O + 2eH++ 2OH

Na++ OH NaOH

2NaOH + CO2Na2CO3+ H2O

At Anode (Graphite)

CleCl

Cl + ClCl2


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Electrolytic cell consists of a perforated steel tube having a thin

lining of asbestos on the inside.

Brine is placed inside the cathode tube and a graphite rod is

immerged in it acts as anode.

When an electric current is passed, the salt solution undergoes

electrolysis and its ions pass through the diaphragm as a result of

electrical migration.

Hydrogen and caustic soda are formed at the cathode and chlorine

at the anode.

Hydrogen gas is allowed to escape through an opening provided at

the top of the cell.

Chlorine liberated at the anode is led away through a pipe and

compressed into steel cylinders.

The space between the cathode and outer tank is kept full of

steam and Carbon dioxide.


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Sodium ions pass through the asbestos and reach

the cathode, where H+ions and OH Ions are formed

as a result of reduction of water.

Hydrogen escapes through an opening at the top and

Na+ions combine with OH ions to form caustic soda.

Sodium hydroxide is reacted with pressurized CO2

yielding Sodium carbonate which is collected from

bottom of the cell.


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SODIUM CARBONATE:PROPERTIES

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Molecular formula : Na2CO3

Molecular weight : 105.978gm/mole

Appearance : White crystalline solid

Odour : Odourless

Boiling point : 16330CMelting point : 8510C

Density : 2.54gm/mL (Anhydrous)

Solubility : Soluble in water

99%sodium carbonate (58%Na2CO3) is known as light sodaash (solid density 1.86).

Dense soda ash has solid density of 1.91. Both grads (lightly

and dense) are granular.

Na2CO3.10H2O is known as washing soda.


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SODIUM CARBONATE:APPLICATIONS

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Widely used in the manufacture of glass,

Used in manufacture of sodium bicarbonate, caustic

soda,

Used in soap, pulp and paper, textiles industries

Used in petroleum and dyes industries

Used in foods, leather and water softening

industries.

As a photographic film developing agentAs an electrolyte

As a washing soda in household uses.


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SODIUM HYDROXIDE

(NAOH)

CHLORINE (CL)

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SODIUM HYDROXIDE (NAOH)

Sodium hydroxide (NaOH), also known as lye and

caustic sodais a highly caustic metallic base which is

awhite solid available in pellets, flakes, granules,

and as 50% saturated solution.

Caustic soda is produced as co-products by theelectrolysis of brine.

In India 80% caustic soda and more than 95% chlorine

are produced by electrolysis of brine.

Various commercial cells have been developed in order

to keep the anode and cathode products separate fromone another.Diaphragm cell

Mercury cathode cell

Membrane cell

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SODIUM HYDROXIDE (NAOH): TYPES OF

CELLS

Diaphragm cellSubmerged Cells

Cathodes remain submerged in this type of cell.

Graphite is used as anode.

The liquid in the cathode compartment is at low-level inorder to prevent the back flow of OH ions by diffusion, e. g.

Hooker and Townsend cells.

Dry Diaphragm CellsThe diaphragm cells contain a porous asbestos diaphragm

which permits a flow of brine from the anode to cathode and

prevents the mixing of anode product and cathode products.Graphite is used as an anode.Electrolysis starts with dry or empty cathode compartment,

e.g. Nelson, Gibbs and Vorce cells.

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SODIUMHYDROXIDE:DIAPHRAGM


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SODIUM HYDROXIDE: DIAPHRAGM

CELLSUBMERGED CELL: HOOKER CELLS

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Hooker cells are cubic in shape with capacity from 10000 amp to 55000

amp.

It has concrete cover at the base from which flat blades of the graphite

projects upward and act as anodes which is supported vertically by a layer

of lead cast concrete base.

90 graphite anodes, each has dimension of 46163 cm are used.

The cathode consisting of flat steel fingers are supported horizontally from

the side steel frame extending inwards, from two sides so as to fit between

the rows of anode blades.

Concrete cover has inlet for brine and exit pipe for chlorine gas.

This concrete cover also projects the cast lead forming the condenser to the

anodes from attack by cell liquor.The cathode assembly has hydrogen and caustic off takes and the cathode

connection.

The cathode is directly covered with asbestos and forms the diaphragm,

which is completely submerged.Diaphragm is applied by dipping the cathode into a bath of asbestos slurry and the

asbestos is drawn into the screen by applying a vacuum to the hydrogen outlet.

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WorkingA feed of brine between anode and cathode

compartment maintains the separation of anode

products from cathode products.

The brine passed into the anode compartment of thecell through the concrete cover and liberated chlorine

at the anode escapes through the cell cover.

Hydrogen liberates at the steel cathodes and the

weak brine containing caustic soda is withdrawn

through the hollow rectangular channel frames at theside.

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SODIUM HYDROXIDE: DIAPHRAGM CELL

DRY/POROUS DIAPHRAGM CELLS: NELSON CELLS

Reactions

NaClNa++ Cl H = + 97.2kcals

2H2O + 2eH2+ 2OH H = + 68.3kcals

At cathode

2H2O + 2e2H++ 2OH

Na++ OH NaOH H = + 112.0kcals

At Anode

CleCl

Cl + ClCl2

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Nelson cell consists of a perforated steel tube having a thin

lining of asbestos on the inside.

The steel tube acts as the cathode and is suspended in an

outer steel tank.

Brine is placed inside the cathode tube and a graphite rod,

which acts as anode, is immerged in it.

The brine undergoes electrolysis by passing current and ions

of salt are passing through the diaphragm due to electrical

migration.

Sodium ions pass through the asbestos and reach the

cathode, where H+ions and OH ions are formed as a result

of reduction of water.

Hydrogen escapes through an opening at the top and Na+

ions combine with OH ions to form caustic soda, which is

collected at the bottom of the outer tank.

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Hydrogen and caustic soda are formed at the

cathode and chlorine at the anode.

Hydrogen gas is escape through outlet provided at

the top of the cell, while caustic soda is collected at

bottom and withdrawn from time to time.Chlorine liberated at the anode is led away through

a pipe and compressed into steel cylinders.

The space between the cathode and outer tank is

kept full of steam, which acts in two ways.It heats the electrolyte and thus reduces its resistance

Keeps the pores of the asbestos diaphragm clear which

make migration of ions easy.

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SODIUM HYDROXIDE: MERCURY CATHODE CELLS

THE CASTNER KELLNER CELL

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It was introduced by Castner and Kellner in 1892


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Castner Kellner cell consists of large rectangular

tank with a layer of mercury at the bottom and

divided into three compartments by the state

partition which does not touch the bottom of cell.

Movement of eccentric wheel H comforts thecirculation of mercury from one compartment to

another.

Each of the side compartments called A, is fitted

with graphite anodes dipping in brine, whereas aseries of iron roads suspended in the middle

compartment act as cathodes.

The compartment contains a dilute solution of soda.


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When the electric current is passed, the

electrolysis of brine takes in the outer

compartment A.

Chlorine is liberated at the anode and is led away

through an exit provided at the top.Sodium ions are discharged at the mercury layer

which acts as cathode by induction.

It should be noted that H+ion will not be

discharged because of high over potential over themercury.

Na++ eNa (At cathode)


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The liberated sodium atoms dissolved in the mercury to from a

sodium amalgam which comes into the central compartment

due to the rocking motion given to the cell by eccentric wheel H.

In the compartment the Hg layer acts as an anode.

As a result of electrolysis of NaOH solution present in central

compartment, OH ions and Na+ions are formed.The OH ions move to the mercury anode and after getting

discharged react with the sodium atom presents in the

amalgam to form sodium hydroxide.

At the same time, the H+ ions furnished by slight dissociation

of water get discharged as hydrogen which escapes through exitabove the middle compartment.

The caustic soda solution is sufficiently concentrated(above

20%) it is removed periodically and concentrated to get fused

caustic soda.


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SODIUM HYDROXIDE: MEMBRANE CELLS

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A semi-permeable membrane is used to separate the anode and

cathode compartments. Membrane is porous chemically active plastic sheet that allow Na+ion

to pass but rejects the OH ions.

While in diaphragm cells, back migration of ion is controlled by the

rate of flow of fluids through the diaphragm and this is regulated by

careful control of liquid level in the compartments.


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SODIUM HYDROXIDE: MEMBRANE

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Du Pont has developedper sulfonic acid polymer (Nafion)for

membrane while Ashai uses a multiple layer membrane ofper

fluorosulfonic acid polymer.

The purpose of membrane is to exclude OH and Cl ions from

the anode chamber, thus making the product far lower in salt

than that from diaphragm cell.

A membrane cell 20 times larger than diaphragm can produce

240 ton of chlorine per year and power consumption is

satisfactory reduced below either mercury or diaphragm cells.

A bipolar cell unit is capable of producing 20,000 ton per yearwith a current density of 4 KA/M2.

Combination plant using the output of the membrane cells as

fed to diaphragm cells might result in considerable cost

reduction.


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SODIUM HYDROXIDE: MEMBRANE

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AdvantagesMore concentrated brine can be used

Purer and concentrated products (28% NaOH

containing 50ppm of NaCl, 40% NaOH product) are

produced.Saving of energy and transportation cost

Low production cost

DisadvantagesReadily clogged of membrane

Pretreatment of brine is required to remove calcium

and magnesium salts


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SODIUM HYDROXIDE (NAOH): PROCESS

USING DIAPHRAGM CELL8/

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Raw materialsBasis: 1 ton of NaOH (76%), 879kg Cl2, 274.7m3H2

Salt 1600kg(sea water, salt lake

and sub soil water)

Sodium carbonate29.2kg(Solvays process, dualprocess or electrolytic process)

Sulfuric acid 100.5kg(Contact process)

Steam 10060kg

Electricity 1197kJ

Refrigeration 910kg

Direct labour 20work-h


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Reaction

NaClNa++ Cl H = + 97.2kcals

2H2O + 2eH2+ OH H = + 68.3kcals

At cathode2H2O + 2eH

++ 2OH

Na++ OH NaOH H = + 112.0kcals

At Anode

CleCl

Cl + ClCl2


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Purification of BrineBrine purification is essential for getting pure caustic soda andto

decrease clogging of the cell diaphragm by insoluble hydroxides formed

during electrolysis.

Brine contains impurities such as calcium, magnesium and iron

compounds.These impurities are removed by adding lime and soda ash, when insoluble

carbonates and hydroxides are precipitated.

Sometimes sulfate are removed with BaCl2or the hot brine is treated with OH

and CO32ions.

After the treatment brine is allow for settling to separate the impurities

and then neutralized with hydrochloric acid.

The saturated brine containing 324gms/liter of NaCl is fed to the cell at600C.

The electrolysis is carried out in diaphragm cells; each cell usually

required 3.0-4.5 volts.

A number of them are put in series to increase the voltage of a given

group.


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Brine electrolysis is carried out with an anode current density of 0.07amp/cm2.

Na+ions formed by electrolysis are moved to the cathode, where H+ions

and OH ions are also formed as a result of reduction of water.

On the other hand Cl ions are directed towards the anode, where they

lose one electron each and form chlorine molecules which liberate aschlorine gas at the anode.

Since the discharge potential of chlorine ions is lower than that of OH

ions, Cl ions are discharge at the anode and OH ions are remain in

solutions.

Similarly the discharge potential of Na+is greater than H+ions hence H+

ions are discharge at the cathode, while Na+ions remain in the solutions.Chlorine attack caustic soda solution, resulting into sodium chloride and

hypochlorite:

2NaOH +Cl2NaCl + NaClO +H2O

To preclude the reaction, it is necessary that NaOH and Cl once formed

do not come in direct contact with one another.


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Evaporation and salt separationAbout 10 to 15% caustic soda solution along with some

unconverted NaCl is obtained after electrolysis.

The decomposition efficiency of the cells being in the

range of only 50%, about half of NaCl remainsunconverted and is recovered by reason of its low

solubility in caustic soda solutions after concentrations.

Hence,the weak caustic soda solution is first

concentrated to 50% in a double or triple effect

evaporator so that NaCl completely separated which is

recycled.

The liquid obtained from the salt separator is 50%

caustic soda solution containing 2% NaCl and 0.1 to

0.5% NaCl on a dry basis.


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Final Evaporation50% NaOH solution is concentrated in huge cast iron pot on open

fire.

Approximately 99% water is removed and molten caustic soda is

formed at 5000C to 6000C.

Now a days these pots are replaced by dowtherm heated evaporators

for caustic evaporation about 50%.

Another method of dehydrating 50% caustic soda is the precipitation

of NaOH.H2O by adding ammonia which also purify the caustic soda.

If 50% caustic soda is treated with anhydrous ammonia in pressure

vessels in a counter current manner, free flowing anhydrous crystals

of NaOH separate out from the resulting aqua ammonia.The hot anhydrous caustic is treated with sulfur to precipitate iron

and then allowed to settle.

Then a centrifugal pump is lowered by crane in the molten NaOH

and the liquid is pumped out in to thin steel drums.


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Purification of caustic soda50% caustic soda solution still contains impurities

such as colloidal iron, NaCl and NaClO.

Iron is removed by treating caustic with 1% by weight

of 300 mesh CaCO3and filtering the resultingmixture through a filter on CaCO3per coat.

Sodium chloride and hypochlorite are removed by

dropping the 50% caustic solution through a column

of 50% NH4OH.


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Chlorine DryingThe hot chlorine evolved from the anode compartment contains much

water vapour.

Therefore, it is cooled to condense most of the water vapour and further

dried in the sulfuric acid scrubber (stoneware tower or stainless steel

tower with acid proof packing).

The dried CI2is compressed between 35 to 80 psi by one of the followingtemperature pressure combination.High pressure (910 atm), water coolingMedium pressure (23 atm), refrigeration at 200CLow pressure (310 cm Hg ), refrigeration at 400C

Rotary compressors with H2SO4seals have been used for liquefaction

process.The heat of compression is progressively removed by water and finally by

refrigeration to about 290C, when all the chlorine should be liquefied.

It is further cooled 450C and the liquid chlorine is led to a steel storage

tank and then filled in steel cylinder of 50-100 kg capacity for sale.


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HydrogenHydrogen evolved at the cathode is either burnt for

boiler fuel or used as hydrogen source.


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LIME SODA PROCESS8/

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Raw materials Basis: 1 ton Sodium hydroxide

Sodium carbonate 1360kg(Solvays process, dual process or

electrolytic process)

Lime 75kg (from mineral calcite or aragonite,

which can be used after removal of clay,slit and sand (silica).)

Water 1000kg

Steam 1225kg

Fuel 13000000 Btu

Electricity 19KWH

Reaction

Na2CO3+ Ca(OH)2NaOH + CaCO3

SO O (AO ) OC SS


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SODIUMHYDROXIDE(NAOH)PROCESS


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Series of causticizer tank or agitator are added with 20% solutionof soda ash (made with weak liquor from a previous stage) and

milk of lime or slaked lime.

The causticizer tanks containing steam line to heat the mixture to

80900C are either fitted with mechanical stirrer or compressed

air as substitute of agitator.After the equilibrium the liquid is allowed to settle for 23 hours .

The clear liquid containing about 10% NaOH is drawn by a swing

pipe.

The sludge is washed in counter current manner with the washing

of the previous operation.

The causticising process is completed in the series of three

agitators.

The mixture of NaOH and CaCO3from the last agitator is charged

to the first Door thickener, which consist of a large shallow

cylindrical tank into which the slurry is fed at the center.



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LIME SODA PROCESS

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The over flow liquid from the first thickener is filtered and filtratecontaining 10-11% NaOH is evaporated to 50 % solution in a triple effect

vacuum evaporator.

The solid CaCO3is gradually settles to the bottom.

The lime sludge from the bottom of the first thickener is washed with the

water.

The filtrate from the next operation is also added to the second thickener,

where the liquor is treated with excess of weak soda solution.

The overflow from the second thickener is used as a weak liquor to make

soda ash solution.

The lye suspension from the thickener is filtered through rotary drum

vacuum filter and passed to a third thickener where it is finally washed

with fresh water to remove any traces of NaOH.

The slug of the filter cake (CaCO3) is return in the lime kiln to from lime.

The caustic soda (11 % strength) contains small amount of NaCl and

Na2CO3.



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LIME SODA PROCESS

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Kinetics and thermodynamicsEffect of temperature

Temperature effects on both equilibrium yield and rate of

reaction.

Concentration of reagents favourable to the process

Na2CO3+ Ca(OH)2NaOH + CaCO3 The equilibrium constant of above reaction is

---- (1) Since calcium carbonate and calcium hydroxide are only slightly soluble, their

solutions are always saturated and concentration of two components in the solution

is therefore constant.

Equation (1) can be written as

---- (2)

[ ][ ][ ] ( )[ ]

232

2

3

OHCaCONa

NaOHCaCOK

c =

[ ][ ]

32

2

CONa

NaOH'kc=



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The yield of NaOH is given by,

---- (3)

The ratio which appears in the denominator of

(3), when use is made of (2) is equal to the other

ratio [NaOH]/K'c. On the basis of this (3) becomes

---- (4)

[ ][ ] [ ] [ ]

[ ]NaOHCONaCONaNaOH

NaOHNaOH

3232 2

1

2+

=

+

=

[ ] [ ]NaOH"K'k

NaOHNaOH

c

c

+

=

+

=

1

1

21

1



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LIME SODA PROCESS

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It is seen from (4) thatthe yield of NaOH ishigh when the concentration of the same

hydroxide at equilibrium is low, i.e. when

starting concentration of sodium carbonate

is small.In practice it is necessary to work with stating

solutions which are not too dilute in order to

avoid excessive cost of concentrating the produced

caustic soda solutions.Generally,solution containing 12-14% of

sodium carbonate are used.

SODIUMHYDROXIDE(NAOH)


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SODIUM HYDROXIDE (NAOH):

PROPERTIES

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Molecular formula : NaOHMolecular weight : 39.997gm/mole

Appearance : White, waxy, opaque crystal

Odour : Odourless

Boiling point : 13880C

Melting point : 3180C (Decompose)

Density : 2.13gm/mL

Solubility : Soluble in waterIt is hygroscopic in nature

SODIUMHYDROXIDE(NAOH)


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SODIUM HYDROXIDE (NAOH):

APPLICATIONS

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It is an important heavy chemical and occupiesamong the basic chemicals position equal in

importance to sulfuric acid and ammonia.

It is used insoap, rayon, dyes, paper, drugs,

foods, rubber, textiles, chemicals, bleaching,metallurgy and petroleum industries.


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CHLORINE (CL)Using diaphragm cellsDeacons method

Other methods

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CHLORINE (CL): INTRODUCTION

Chlorine (Cl) is a chemical element having atomicnumber 17.

It is the second lightest halogen after fluorine.

The element forms diatomic molecules under STP,

called dichlorine.It has the highest electron affinity and the third highest

electro-negativity of all the elements; for this reason,

chlorine is a strong oxidizing agent.

Chlorine can be manufacture by several methods such

as electrolysis, Deacons, heating of auric acid andplatonic chloride.All methods except electrolysis are costly.

Therefore, chlorine is largely manufacture by electrolysis

process

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CHLORINE (CL): INTRODUCTION

In 1630, Jan Baptist van Helmont was the first who obtainedthe Chlorine gas.

The synthesis and characterization of elemental chlorine

occurred in 1774 by Carl Wilhelm Scheele, who called it

"dephlogisticated muriatic acid air," having thought he

synthesized the oxide obtained from the hydrochloric acid.Because acids were thought at the time to necessarily contain

oxygen, a number of chemists, including Claude Berthollet,

suggested that Scheele's dephlogisticated muriatic acid air

must be a combination of oxygen and the yet undiscovered

element, and Scheele named this new element within this oxide

as muriaticum.

In 1809, Joseph Louis Gay-Lussac and Louis-Jacques proved

that this newly discovered gas was the simple element which

was reconfirmed by Sir Humphry Davy in 1810, who named it

chlorine, from the Greek word chlros meaning "green-yellow."

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CHLORINE (CL): PROCESSES

Using diaphragm cellsChlorine can be obtained as co-product during the manufacture of caustic soda

by electrolysis process.

Deacons methodHCl is partially oxidizes to chlorine by heating of HCl gas with oxygen (air) at

4004500C in presence of porous earthenware impregnated CuCl2as catalyst.

4HCl + O22Cl2+ 2H2O

OR

2CuCl22CuCl + 2Cl2

4CuCl + O22Cu2OCl2

Cu2OCl2+ 2HCl2CuCl2+ H2O

Cl2mixed with unconverted HCI and system is washed with cold water and

dried with conc. H2SO4. This is an old method for manufacture of chlorine and

is not in used now.

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CHLORINE (CL): PROCESSES

Pure chlorine can also be prepared by heatingAuric chloride (AuCl3)or platonic chloride

(PtCl4)in a hard glass tube.

175oC 190oC

2AuCl32AuCl + 2Cl2Au + 3Cl2

375oC 600oC

PtCl4PtCl2+ Cl2Pt + 2Cl2

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CHLORINE (CL): PROPERTIES

Molecular formula : Cl2Molecular weight : 70.906 gm/mole

Appearance : Yellow green gas

Odour : Similar to house hold bleach

Boiling point : -40C

Melting point :1010C

Vapour density : 2.48 (v/s air)

Vapour pressure : 4800mmHg (200C)

In upper atmosphere, chlorine containing molecules such as chlorofluoro-carbons

have been implicated in ozone depletion.

Elemental chlorine is extremely dangerous and poisonous for all life forms

It is necessary to most forms of life, including humans, in form of chloride ions.It is the only acidic gas which turns damp blue litmus red and bleaches it to

white.

It is two and a half times heavier than air. It becomes a liquid at 34 C.

The affinity of chlorine for hydrogen is so great that the reaction proceeds with

explosive violence in presence of light

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CHLORINE (CL): APPLICATIONS

Chlorine is usedfor producingsafe drinking water.

for the manufacture of chlorates and it is important in organic

chemistry, forming compounds suchas chloroform, carbon

tetrachloride, polyvinyl chloride, and synthetic rubber.

indyestuffs, petroleum products, medicines, antiseptics,insecticides, foodstuffs, solvents, paints and plastics.

as anoxidizing agentand in substitution reactions.

Inpaper and pulp, solvents, explosives, plastics,

pesticides and sanitation.

Chlorinated compounds are used mostly for sanitation andtextile processing.

As a common disinfectant, chlorine compounds are used in

swimming pools to keep them clean and sanitary.

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ACKNOWLEDGEMENT

Slides are developed from the following references:Austin G. T., "Shreves Chemical Process Industries",

Fifth edition, Tata McGraw Hill, NY.

Kent J.A.,"Riegel's Handbook of Industrial

Chemistry,CBSPublishers.Gopala Rao M. & Marshall Sittig, "Drydens Outlines of

Chemical Technology for the 21stCentury", Affiliated

East West Press, New Delhi.

Mall I. D., "Petrochemical Process Technology",

Macmillan India Ltd., New Delhi.http://nptel.ac.in/courses/103106108/24

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lecture 22-23-24 chloralkali industry

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