Sulphuric acid also called as oil of vitriol, hydrogen sulphate, vitriol brown oil, matting acid, battery acid. The formula of sulphuric acid shows the combination of two hydrogens, 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 converters 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:
The 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 the 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. The 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 favours 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:
- Cast iron
- Carbon steel Lead/Teflon lining
- Stainless steel
- Types of packing:
- Triple spiral rings
- Berl saddles
- 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 the reactor|