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On
industrial scale, sulphuric acid can be prepared by the following two methods. Contact
process Lead
Chamber process |
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CONTACT
PROCESS
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| Now
a days, sulphuric acid is prepared by contact process all over the world.
Preparation of sulphuric acid by contact process is based upon the catalytic oxidation of SO2 to SO3. |
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DETAILS
OF
CONTACT PROCESS |
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Following steps are involved in the preparation of H2SO4. PREPARATION
OF SO2. PURIFICATION
OF SO2. OXIDATION
OF SO2. ABSORPTION
OF SO3. DILUTION
OF OLEUM. |
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PREPARATION
OF SO2
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| SO2
is obtained by burning sulphur or by heating iron pyrite (FeS2)
in pyrite burner. S + O2 èSO2 4FeS2 + 11O2 è2Fe2O3 + 8SO2 |
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PURIFICATION
OF SO2
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| SO2
contains a number of impurities such as dust particles, Arsenous oxide,
vapours, sulphur etc. These impurities must be removed
otherwise catalyst loses its efficiency (catalyst poisoning). DUST CHAMBER: SO2 is first passed through the dust chamber where steam is spread over the gas to remove dust particles, which settle down. Fe(OH)3 also sprayed over to remove oxides of Arsenic. WASHING TOWER: SO2 is then passed through a washing tower after cooling. Here it is sprayed by water to remove any other soluble impurities. DRYING TOWER: The gas is now dried by passing through drying tower where conc. H2SO4 (dehydrating agent) is sprayed. H2SO4 removes moisture from SO2. TYNDALL BOX (TEST BOX): Arsenic oxide is a poison for the catalyst. It is removed when the gas is passed over ferric hydroxide. As2O3 + 2Fe(OH)3 è 2FeAsO3 + 3H2O. In order to remove traces of As2O3, it is passed through a test box, where a strong beam of light is thrown against the gas. If there is no scattering of light in the box, it indicates that gas is free from As2O3. |
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OXIDATION
OF SO2 TO SO3
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| CONTACT
TOWER: Oxidation of SO2 is carried out in contact tower where V2O5 is filled in different pipes. SO2 here reacts with air (O2) to produce SO3. Under above conditions 98% SO2 is converted into SO3. 2SO2 + O2 è 2SO3 + 45Kcal CONDITIONS NECESSARY FOR MAXIMUM YIELD OF SO3: Oxidation of SO2 is a reversible and exothermic process in which volume of product is less than the volumes of reactants. In order to obtain maximum amount of SO3, according to Le-Chatelier’s Principle following conditions are necessary. CONCENTRATION: Excess of O2. TEMPEATURE: A decrease in temperature favours reaction in forward direction. Optimum temperature for this process is 450oC to 500oC. PRESSURE: Since volumes of reactants are greater than the product (3:2), therefore, according to Le-Chatelier’s Principle a high pressure is favourable. Optimum pressure is about 1.5 to 1.7 atmosphere. USE OF CATALYST: At low temperature, rate of reaction decreases. To increase rate of reaction a catalyst vanadium pentaoxide (V2O5) is used. |
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ABSORPTION
OF
SO3 IN H2SO4 |
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| SO3
is not directly passed in water, because a dense fog of minute particles
of H2SO4 is produced. It is therefore,
dissolved in conc.H2SO4 to form pyrosulphuric acid
(oleum). SO3 + H2SO4 è H2S2O7 (OLEUM) |
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DILUTION
OF OLEUM
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Oleum is now diluted with water to
form H2SO4 of required concentration. H2S2O7 + H2O è 2H2SO4 |
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