SULPHURIC ACID (H2SO4)

    On industrial scale, sulphuric acid can be prepared by the following two methods.
 Contact process
 Lead Chamber process
    

CONTACT PROCESS

    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.

DETAILS OF CONTACT PROCESS :

 Following steps are involved in the preparation of H2SO4.
 PREPARATION OF SO2.
 PURIFICATION OF SO2.
 OXIDATION OF SO2.
 ABSORPTION OF SO3.
 DILUTION OF OLEUM.

PREPARATION OF SO2

  SO2 is obtained by burning sulphur or by heating iron pyrite (FeS2) in pyrite burner.

 S + O2 → SO2

 4FeS2 + 11O2 →  2Fe2O3 + 8SO2

PURIFICATION OF SO2

  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.

    ARSENIC PURIFIER:

    Arsenic oxide is a poison for the catalyst. It is removed when the gas is passed over ferric hydroxide.

  As2O3 + 2Fe(OH)3 →  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.

OXIDATION OF SO2 TO SO3

    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  → 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.

ABSORPTION OF SO3 IN H2SO4

    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 →  H2S2O7 (OLEUM)

DILUTION OF OLEUM

    Oleum is now diluted with water to form H2SO4 of required concentration.

H2S2O7 + H2O →  2H2SO4

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