PREPRATION OF SULFURIC ACID

METHODS OF PRODUCTION
1.Contact process
2.Chamber process
Both processes are based on SO2,chamber process was devloped first(1746) but produced acid concentration less than 80%.Contact process yeilds 98% H2SO4 and higher which can be diluted,if necessary. Chamber process is virtually obsolete.

CONTACT PROCESS
chemical reactions
S(s) + O2(g)  = SO2(g)                                      H= -70.9 Kcal
SO2(g)  + 1/2 O2(g)  = SO3(g)                         H= -23.0 Kcal

Raw materials
SO2 is obtained from the following sources:
1.Sulphur
2.Pyrites
3.CuS,ZnS,PbS,MoS2
4.wate H2SO4

Catalyst
Most widely used catalyst is vanadium pentaoxide dispersed on a porous carrier in pellet form. Platinum catalyst was previously used but suffer from easy poisioning ,fragility,rspid heat deactivation,high initial investment./
*Advantages of V2O5 catalyst
-relatively immune to poisons
-low initial investment and only 5% replacement per year
-requires only 10 kg of catalyst mass containing 7-8% V2O5 per daily ton of 100% acid
*Disadvantages of V2O5 catalyst
Must use dilute SO2 input(7-10%);as catalyst is less active and requires high O2/SO2 to give economic conversion.Larger converters and higher initial investment are nessary.

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 reactor of steel contruction. This is the high tempreature (500-600 celcius) stage, contains 30% of total heat catalyst and converts about 80% of SO2. The convertor product is cooled by heat exchange at 300 celcius and fed to a second stage where total yeild is incresed to 9% by operating at 400-450 celcius for favorable equilibrium.
High yeild product gases are cooled to 150 celcius by water and 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 concentration up to 40% can be made by lower absorption. Higher stength oleum up to 65% is prepared by distilling 20% oleum

MAJOR ENGINEERING PROBLEMS

1. Design of multistage catalytic converter for a highly exothermic reaction: some designs contain  3   to 4 stages rather the conventional two satge operation for large capacity plants.
2. Optimization of space velocity in catalyst chamber: Pumping costs versus fixed charges of reactor.
3. corrosion problem
4. Adaptation of process to various types of gas feeds.
5. Removal of heat of absorption of SO3 in acid .Pipe cooler with water dripping over external surface have been replaced by cast iron pipe with internal fins to promote better heat transfer. Pressure drop must be low so 8 cm stacked packing is often used.