SULFURIC ACID

Although sulfuric acid is produced commercially by two basic processes (chamber or contact), the contact process is employed exclusively by the phosphate industry. There are a number of different types of contact plants, normally referred to by the name of the builder or designer of the plant, in operation throughout the world. Also, contact plants may be further classified according to the raw material source of sulfur used. However, all contact processes consist primarily of the following basic units: sulfur burner, drying tower, converter, absorbing tower, gas cooling equipment, such as waste heat boilers and heat exchangers, etc., as well as required auxiliary equipment. Most phosphate plants use the heat of reaction in the sulfuric acid plants to convert water to steam for the production of electricity from steam driven turbine generators to power the entire chemical complex.

In the contact process the molten sulfur is introduced into a sulfur burner along with combustion air which has been predried by passing through the drying tower containing 93.2 - 99% sulfuric acid. The exit gases from the burner, normally 8-11% sulfur dioxide by volume, are cooled in a waste heat boiler. The cooled sulfur dioxide gas together with predried air then enters the solid catalyst converter where the sulfur dioxide is converted to sulfur trioxide. Several different catalysts are available, however, the most common one in use is vanadium pentoxide. The converter exit gas is cooled by passing through a heat exchanger. The cooled gas stream enters the absorption tower, where the sulfur trioxide is absorbed counter-currently in a circulating stream of high concentration sulfuric acid. In the absorption tower, the sufur dixoide is normally absorbed with an efficiency of essentially 100%. All sulfuric acid plants in the phosphate industry are further equipped with a mist eliminator on the absorber for removal of acid mist and spray from the exit gas stream before release to the atmosphere. The sulfur dioxide combines with the water in the acid to form more sulfuric acid. The acid is withdrawn and stored for use in manufacturing phosphoric acid and other products.