Physical Properties :
- Molecular formula :H2SO4 , Molecular weight :98.08 gm/mole
- Appearance :Water white slightly viscous liquid
- Boiling point :290 ºC ,Melting point :10 ºC
- Density :1.840 gm/mL (liquid)
- Solubility :Miscible with water in all proportions
- Viscosity :26.7 cP (20ºC)
Contact Process :
Raw Materials :
- 1 ton sulfuric acid (100%)
- Sulfur dioxide or pyrite (FeS2) →670kg
- Air→ 1450–2200 Nm³
- Catalyst→ V2O5
Sources of raw material :
- Sulfur from mines
- Sulfur or hydrogen sulfide recovered from petroleum desulfurization
- Recovery of sulfur dioxide from coal or oil-burning public utility stack gases
- Recovery of sulfur dioxide from the smelting of metal sulfide ores ⇒ 2PbS + 3O2 → 2PbO + 2SO2 ⇐Isolation of SO2 from pyrite.
Reactions : (Temperature = 450 oC, Pressure = 1.2–1.5 atm)
- S + O2 → 2SO3 ΔH = −46.3 kcals
- SO3 + H2O → H2SO4 ΔH = − 31.1 kcals
Burning of sulfur :
- Burning of sulfur in presence of dry air is carried out in sulfur pyrite burner.
- As SO2 is needed for the catalytic oxidation and prevention of corrosion, dry air is used in the combustion process.
- If sulfur contains carbonaceous impurities, the molten material has to be filtered to avoid poisoning the catalyst and forming water from burning hydrogen.
Catalytic oxidation of SO2 to SO3 :
- The catalytic reactor is designed as a four-stage fixed-bed unit. The gas has to be cooled between each step.
- Four passes, together with “double absorption, are necessary for overall conversion of 99.5-99.8% (three passes, 97-98%).
- The temperature rises to over 6000C with the passage of the gas through each catalyst bed.
- The doubled absorption consists of cooling the gases between each bed back to the desired range by sending them through the heat exchanger and then back through the succeeding beds.
- Between the third and fourth beds, the gases are cooled and sent to an absorption tower. This is to shift the equilibrium to the right by absorbing SO3.
- The gases are then sent to the heat exchanger to warm them to 410-430ºC and then on to the fourth catalyst bed.
Hydration of SO3 :
- After the catalytic oxidation process, the resulting SO3 is hydrated by absorption in packed towers filled with 98-99% sulfuric acid.
- This is the H2SO4 azeotrope of minimum total vapour pressure.
- SO2 has a low solubility in 98% H2SO4. At lower acid concentrations, sulfuric acid and SO3 form a troublesome mist and at higher concentrations emissions of SO3 and H2SO4 vapour become significant.
- The acid strength can be adjusted by controlling the streams of H2SO4 to give acid of 91 to 100% H2SO4 with various amounts of added SO3 and water.
- The conversion of sulfur to acid is over 99.5%.