Manufacture of Sulphuric Acid by Contact Process

Manufacture of Sulphuric Acid by Contact Process

Advantages of Contact Process

Sulphuric acid also known as oil of vitriol or king of chemicals is commercially manufactured by two methods namely lead chamber process and Contact process. Now a days, sulphuric acid is preferably manufactured through catalytic oxidation of sulphur dioxide by contact method which was developed Knietsch in Germany in the early 19th century but began to be used from 1912. The Contact process is preferred over lead chamber process because by contact method, sulphuric acid of highest purity (100%) and of any desired concentration or strength can be prepared directly. Moreover, contact plant needs a small space and cheaper to maintain than the lead chamber process. Thus contact process is much economical than the lead chamber process.

1. Nearly 100% pure H2SO4 can be prepared.

2. Acid of any desired concentration can be prepared.

3. Contact plant needs small space.

4. Plant is cheaper to maintain.

Raw Materials for Process

The starting chemicals for process are:

Basic Principle

The process is based on the formation of sulphur dioxide by burning sulphur or iron pyrite in dry air or gypsum with coke followed by its catalytic air oxidation into sulphur trioxide (SO3) using V2O5 (or Pt) as a catalyst which is first absorbed in 97-98% concentrated sulphuric acid forming oleum or pyrosulphuric acid which is then diluted with calculated amount of water to give sulphuric acid of required concentration. The principle involves following steps:

Conditions for improving yield of SO3

The oxidation of SO2 into SO3 is reversible, exothermic and proceeds with decrease in volume of products. Le-Chatelier’s Principle suggests following conditions for the maximum yield of SO3 from SO2:

Plant Used and Details of Process

The process is carried out in a large plant, which has three main parts involving following steps.

1. Formation of SO2 in Sulphur or pyrite Burner

2. Purification of SO2 in Purification Unit

3. Catalytic air oxidation of SO2 into SO3 in Contact Tower

4. Absorption of SO3 first in oleum and then dilution in Absorption Towers

1. Formation of SO2 in Sulphur or pyrite Burner

Sulphur or iron pyrite is burnt with dry air in pyrite or sulphur burner for getting sulphur dioxide.

2. Purification of SO2 in Purification Unit

Sulphur dioxide contains in addition to O2, a number of impurities such as dust, unburnt sulphur particles, acid fog, N2, NO, NO2, arsenous oxide (As2O3). SO2 and air must be very pure and free from all sorts of impurities, which poison the catalyst; so the mixture of SO2 and air (burner gases) is passed through a series of special dust filters or purification unit for purification.

(a) Electrical Precipitator/Dust Remover

It is a dust catcher whereby the influence of gravity method, dust and sulphur particles are made to settle.

(b) Scrubber Tower/Acid fog Remover

It is packed with Quartz where gases are washed down by spray of water to remove acid fog.

(c) Drying Tower

Here gases are come under spray of conc. H2SO4 (dehydrating agent) to remove moisture, NO, NO2 from burner gases.

(d) Arsenic Purifier

There are number of shelves where ferric hydroxide absorbs arsenous oxide as Ferric arsenate from burner gases.

(e) Tyndall/Testing Box

Impurities of SO2 are further checked by Tyndall Box in Tyndall box.

3. Catalytic oxidation of SO2 into SO3 in Contact Tower/Catalytic Chamber

The washed, dried and purified mixture of sulphur dioxide and air is passed through contact tower or catalytic chamber packed with platinised Asbestos [Ca2Mg5(Si4O4)2(OH)2 or Ca2Mg5Si8O10H2] or solid vanadium pentaoxide (V2O5) promoted by K2SO4 or Pt at 400-500°C under 1.5-1.7 atm. Here catalytic air oxidation of SO2 gives SO3. Under these conditions, equilibrium mixture contains 98% SO3.

The hot gases (SO2 and air) evolved from burning of sulfur ore comes in contact with catalyst bed in the catalyst chamber or oxidation tower, so the name of this process is called contact process.

4. Absorption Towers

SO3 produced is not directly absorbed in water as it is less soluble in water and forms dense fog with water which is difficult to condense. In absorption tower, SO3 from contact tower is absorbed in 97-98% concentrated sulphuric acid in order to avoid fog formation forming a very thick liquid called oleum or pyrosulphuric acid (H2S2O7) which is then diluted with appropriate amount of water to any desired concentration (or strength) of sulphuric acid. The H2SO4 obtained is nearly 100% pure.