All about sulphuric acid in industry

 sulphuric acid 

 

  Introduction and Raw Materials :  

                                                           Sulphuric acid is most important sulphur compound in industry. Anhydrous sulphuric acid (monohydrate) H₂SO4, is a heavy, oily liquid which mixes with water in all proportions with liberation of large quantities of heat. The density of H₂SO4, at 0°C is 1.85 g/cm³. It boils at a temperature of 296°C and freezes at -10°C.

Commercially not only the monohydrate is known as sulphuric acid but this name is also applied to its aqueous solutions and also to solutions of sulphur trioxide in monohydrate  also described as oleum.

Oleum fumes when exposed to air due to desorption of SO3. Pure sulphuric acid is a colourless liquid, but the technical product has a dark colour due to impurities present in it. In manufacturing, transporting and using sulphuric acid, the relation between its melting, boiling and concentration are very important.

  

Manufacturing of Sulphuric Acid

Methods of Production:

Classification of processes:

1. Contact process

 

      2.Chamber process 

 

 

Both the processes are based on SO2 Chamber process was developed first (1764) but produced acid is of concentration less than 80%. Contact process yields 98% H₂SO4, and higher which can be diluted if necessary. Chamber process is virtually obsolete.

 

 

 Contact process

Principle :

                      The basis for the manufacture of sulphuric acid by the CONTACT process is the catalytic oxidation of sulphur dioxide to sulphur trioxide and the reaction of SO3 with water.

                               2so2 + o2 → 2so3 

                                          H2o + so3  → H2so4

 

The raw material i.e. sulphur dioxide required in the process could be readily obtained by burning sulphur of iron pyrites. 

Amongst the various steps involved in the process, the oxidation of sulphur dioxide to sulphur trioxide is the most difficult one. It is a reversible process. Hence by applying Le Chatelier's principle, it is possible to establish the working conditions so as to get maximum yield of sulphur trioxide and hence the acid.

 

 Contact process for manufacturing H2SO4:

 Diffusion of reactants to the surface of the catalyst.

(ii) Adsorption of the reactants on the surface of the catalyst.

(iii) Formation of SO3 on the surface of the catalyst.

(iv) De-absorption of SO3 from the catalyst surface.

(v) Diffusion of SO3 away from the catalyst.

 

 process flow diagram :

 

 

             

 Process description:

Air:

          SO2 gas containing 7-10% SO2 and 11-14% O2 is preheated by converter gas, if necessary, and sent to first stage reactors of steel construction. This is the high temperature (500-600°C) stage, contains 30% of total catalyst and converts about 80% of SO₂. 

              The converter product is cooled by heat exchange at 300°C and fed to a second stage where total yield is increased to 97% by operating at 400-450°C for favourable equilibrium.

               High yield product gases are cooled to 150°C by water + air heat exchangers and absorbed in oleum fed at a rate to allow not over a 1% rise in acid strength. Final scrubbing is done with a lower strength (97%) acid.

               Oleum concentrations upto 40% can be made by tower absorption. Higher strength oleum upto 65% is prepared by distilling 20% oleum.

 

 Properties:

Sulphuric acid (molecular weight 98.08, M.P. = 10°C and B.P. = 340°C) is a colourless, heavy oily liquid, miscible with water in all ratios with release of large amount of heat.

It dissolves SO3, to form oleum, H2SO4.nSO3. When n = 1, pyrosulphuric acid is formed  Grades of acid range from 62% H2SO4 to 100% H₂SO in addition 104%, 105% and 114%  H2SO4 containing 20%, 40% and 65% free SO3.
 

 Uses and Economics:

Wide scale and diverse employment of H₂SO4, is due to its chemical activity. It dissolves metallic oxides and most pure metals. It combines avidly with water, hence used for drying gases and concentration of nitric acid.

It finds use in the manufacture of fertilizers, acids and heavy chemicals, dyes and coal-tar products, lacquers and plastics, explosives, textiles, paints and pigments and numerous other products. It is used in refining of oil, in leather tanning and in metallurgy.

Concentrated H2SO4 causes severe skin burns therefore, strict safety regulation must be observed while handling it.

Fertilizer manufacture is the greatest use of H2SO4. Lower grade is used for manufacture of superphosphate and higher grade for that of ammonium sulphate etc. and also for pickling iron and steel before hot dipping and for electroplating. Still higher grades are required for purification of petroleum products, manufacture of nitrogen chemicals, phosphoric acid, etc. Oleums are needed for petroleum, nitroglycerine and dye manufacture.

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