11.4. sulphuric acid

7/28/2019 11.4. Sulphuric Acid

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Objectives:

7.1 Describe the contact process for themanufacture of sulphuric acid.

7.2 Discuss the industrial importance of

compounds of sulphur. 7.3 Assess the impact of sulphuric

industry.


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Sulphuric acid is produced commerciallyusing the contact process. The rawmaterial needed are source of sulphurdioxide, air, water and a catalyst. The mainstages are:

Production of sulphur dioxide

Oxidation of sulphur dioxide to sulphurtrioxide

Hydration of the sulphur trioxide tosulphuric acid.


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This can be produced by: Burning sulphur in an excess of air

S(s)+O2 SO2 (g)

Heating sulphide ores like pyrite in an excess air

4FeS2(s)+1102(g) 2Fe2O3(s)+ 8SO2(g)

Decomposing calcium sulphate in the presenceof coke

2CaSO4(s)+C(s)

2CaO(s)+CO2(g)+2SO2(g)


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A catalyst, ideally vanadium (V) oxide, is needed

for this stage in the process. The oxygen issupplied by the addition of excess air to sulphurdioxide and is purified by an electrostaticprecipitator.

The purified sulphur dioxide combines withoxygen in the presence of this catalyst to form theanhydride of sulphuric acid, sulphur trioxide.

2SO2(g)+O2(g) 2SO3(g) H = -196kJ/ mol


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This reaction is a reversible and theforward one is exothermic one.

It would be ideal, according to LeChateliers Principle, for the conditionsof this process to be low temperatureand high pressure.

2SO2(g)+O2(g)2SO3(g) H = -196kJ/ mol


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But at low temperatures the reaction rate

would be too slow to be economical. And high pressures would result in the sulphur

dioxide to liquefy.

Then,

A catalyst is needed, to increase the rate of

reaction without affecting the position of the

equilibrium.

Vanadium oxide is used with a temperature

of about 450C to produce a fairly high yield

of sulphur trioxide at an acceptable rate.


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Since the reaction is exothermic, as itproceeds, the temperature of the surroundingsystem rises to 600C from 450C. At thistemperature, the yield is reduced toapproximately 70%. Thus to give a yield ofabout 98%, the gases are cooled to about450C between successive layers of catalyst.

A closed water circuit, is usually used as thecooling system in which energy is recovered assteam, this reducing the use of externalheating.

Close to atmospheric pressure is used whichgives an approximate yield of 98%, althoughhigh pressure gives greater yield of sulphur

trioxide in the equilibrium mixture.


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A counter flow system is usually done inthe industry, where water is sprayeddownward over the upward flowing gas.

However, because this will produceuncontrollable, toxic, hard to condenseclouds of sulphuric acid, this method

cannot be used. Concentrated sulphuric acid destroys

skin and flesh and can cause blindness ifeye is contacted.


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INSTEAD, the sulphur trioxide is first dissolved inconcentrated sulphuric acid to form oleum.

H2SO4 (l)+ SO3 (g) H2S2O7 (l)

Then is diluted with water to produce,concentrated sulphuric acid.

H2

S2

O7 (l)

+ H2

O(l)

2H2

SO4 (l)


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Uses ofSulphur:to produce sulphur dioxide to make

sulphuric acid

used in the vulcanization of naturalrubber to improve its elasticity and tensilestrength

( its makes the rubber less sticky and less soluble

in organic solvents)

used as fungicide and as a fumigant


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Uses ofSulphur dioxide (a reducing agent):

used for bleaching wool, straw and paper

and as an antioxidant in food

preservation. Uses of Sulphuric Acid:

used in the manufacture of:

fertilizersdetergents

car batteries

high strength fibres paints

pigments

- petroleum refining and metallurgy


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Advantages of The Contact Process:

Sulphuric acid production reduces the

amount of sulphur dioxide emited into the

atmosphere.

Emission of sulphur dioxide from the

Contact Process is minimized by recycling

unreacted sulphur dioxide. In addition,scrubbing tail gas with a basic solution.

Recycling and regenerating Sulphuric Acid

is another positive impact on the industry.


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Also spent catalyst from the Contactprocess is either disposed in a suitablylicensed landfill site or its vanadiumcontent is recovered and recycled.

Heat energy is also recycled, when heatevolved from the process is used to turnwater into steam, which is in turn used to

generate electricity. The environment is protected, as no

Carbon dioxide is formed because fossilfuel is not used.


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During production, handling and storageof sulphuric acid, there is always a risk ofaccidental leaks which may have a

negative impact on the environment.

Thus precautions and safety measures

must be implemented to reduce theserisk possibilities.

11.4. sulphuric acid

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