Manufacture of H2SO4 By Contact Process.
“Manufacture of H2SO4 By Contact Process” There are two major processes for the Manufacture of H2SO4 on Commercial-scale:
- Lead Chamber Process.
- Contact Process.
But we are to discuss only the “contact process”.
Principle of contact process:
Sulphur dioxide is obtained by burning of sulphur or iron pyrites. SO2 is converted into SO3 . SO3 So formed is dissolved into sulphuric acid to get oleum or pyrosulphuric acid, which is diluted with water to get sulphuric acid of required concentration.
Conditions for the maximum yield of SO3:
The conversion of SO2 to SO3 is a reversible exothermic process
According to Lechatelier’s principle, the following conditions, should be fulfilled to get the maximum yield of SO3.
Optimum temperature between 400 – 500°C is maintained, so the reaction may occur with a reasonable speed.
High pressure is required to favor the reaction to the forward direction. Mostly the gases are given the pressure of 2 atmospheres.
Excess of Oxygen:
A slight excess of oxygen in the reaction mixture Of SO2 and O2 favours the reaction towards the forward direction. The best results are obtained when the ratio of O2 to SO2 is 3:2.
At 400 – 500°C the rate of reaction is very low because the reaction is exothermic. In order to maintain a reasonable some commonly used catalysts are:
- Fe2O3 with a little amount of CuO.
- Pt – MgSO4.
But now-a-days V2O5 is used because it is cheaper and is not easily poisoned. Anyhow, V2O5 cannot to recovered after use.
Chemical Reactions in H2SO4 Manufacture.
Following are the major chemical reactions.
The plant which is used in the contact process is sketched below in the form of a flow sheet diagram.
The plant which is used for the preparation of H2SO4 is consisted of four major parts:
- Sulphur burners.
- Purification unit.
- Preheater and contact tower
- Absorption tower.
Let us discuss these units one by one.
In sulphur burners, SO2 is produced from sulphur or iron pyrites (FeS2)
The gases which we get from sulphur burners contain
- Sulphur dioxide = 7%
- Oxygen = 10%
- Nitrogen = 83%
But there is a lot of impurities as
- Sulphur dust.
- H2SO4 fog.
All these impurities act as a poison for the catalytic surface. In order to purify SO2 from impurities following stages are necessary.
Heavy dust particles are removed. For this purpose mechanical precipitation is carried out with the help of steam.
The gases are cooled up to 100°C in this unit.
The cooled gases are washed by a spray of water SO2 gas is not absorbed by water at high temperature.
The moisture which is carried by the gases from the scrubbers is removed by concentrated H2SO4. The tower is packed with coke or flint stone. H2SO4 is sprayed from the top.
This contain precipitated Fe(OH)3, which is placed on horizontal shelves. It absorbs As2O3.
A strong beam of light is thrown against the gases in order to check that the gases are absolutely free from dust and As2O3.
First of all the gases are sent to preheater, where the gases are heated up to 400 – 500°C.
The hot gases now enter into an iron cylinder, which is called a contact converter or tower. It is fitted inside with vertical iron tubes. These tubes are packed with platinised asbestos catalyst.
The incoming gases circulated around the hot tubes and SO2 is converted to SO3.
This reaction is highly exothermic. The heat so produced raises the temperature to about 500°C. But the rate of flow of the gases is so adjusted that the temperature of converter is kept around 400 – 500°C.
The SO3 obtained from the contact tower is sent to the absorption tower from the bottom. It meets the descending steam of conc. H2SO4. It forms oleum or fuming sulphuric acid. It is diluted with the calculated quantity of water to get H2SO4 of required concentration