Sulphuric acid industrial production by contact process technology

Sulphuric acid also called as oil of vitriol, hydrogen sulphate, vitriol brown oil, matting acid, battery acid. Formula of sulphuric acid shows the combination of two hydrogen, one sulphur and four oxygen atoms.

Brief process description of contact process:

  • Sulphur powder is melted and burned in presence of oxygen to produce sulphur dioxide gas.
  • The heat from the hot gases recovered to produce steam in a boiler.
  • Three converter filed with catalyst are arranged in series where oxygen and sulphur dioxide are reacted to form sulphur trioxide and heat produced from the reaction is removed by the heat exchanger arranged in between them.
  • Sulphur trioxide produced form the converter is fed to economizer to heat the boiler feed water.
  • In oleum tower SO3 is absorbed by the oleum circulation and unabsorbed SO3 is absorbed in the absorber.
  • Water is added to the acid tank to produce H2SO4 by reacting with oleum.

Process flow diagram of sulphuric acid manufacturing by contact process technology:

simplified process flow diagram of sulphuric acid manufacturing by contact process

Strategy followed to maintain high conversion and reaction rate in converter:

The reaction between sulfur dioxide and oxygen is done in 4-bed arrangement converter. The chemical reaction is exothermic and depends on control of reaction temperature. To achieve maximum conversion and reaction rate, optimum temperature range found from 400 to 500oC. To maintain this temperature at catalyst contact each outlet stream from the beds are passed to economizer coils. Excess heat is removed and temperature is controlled by this heat exchange system strategy.

Inlet and outlet stream temperature control by intermediate heat exchanger approach value to obtain maximum conversion:

Converter catalyst beds Inlet temperature, oC Outlet temperature, oC Heat exchanger cooling, oC Conversion per pass through beds
1st Bed 400 600 600 to 450 75%
2nd Bed 450 500 500 to 420 90%
3rd Bed 420 480 480 to 410 95%
4th Bed 410 430 Passed to preheater/ economizer 98%

Technologies in industrial production of sulfuric acid:

Processes based absorption tower operation:

  • Single contact
  • Double contact absorption
  • Wet contact process

Double absorption tower: Operating pressure and temperature directly affect the rate of absorption of SO3 in oleum. Concentration of recirculation sulfuric acid must be maintained at 98% otherwise the absorption rate drop. The complex condition of operation for better absorption has bottleneck variables.

  • High pressure favors absorption
  • Low temperature increase absorption

The optimized condition for absorption of SO3 with sulfuric acid

  • Column temperature: 65-70
  • Concentration of absorbent: 98%
  • Column pressure: 10-15 mm-Hg

Technology breakout in contact process design:

  • Chemico design
  • Monsanto design
  • Low ignition catalyst
  • Internal heat exchange systems (Energy recovery)

Engineering equipment options in designing:

Acid coolers models applicable to use in the contact process designing:

  • Cascade coolers
  • Shell and tube heat exchangers
  • Plate type heat exchange system
  • Open air coolers

Acid towers selection:

  • MOC of towers:
    1. Cast iron
    2. Carbon steel Lead/Teflon lining
    3. Stainless steel
  • Types of packing:
    1. Triple spiral rings
    2. Berl saddles
    3. Ceramic saddles

Area of scientific research/conclusions on contact process:

Effect of catalyst on SO3 formation:

Catalyst Process difficulties/ bottlenecks
Platinum catalyst Quickly deactivated due to sulfur poisoning
Copper oxide Low conversion
Iron oxide Pore blockage
Chromium oxide Low conversion
Vanadium pentoxide Long life, 98% conversion
Magnesium sulfate Need platinum support
Ruthenium Effective only in last bed of reactor