The Manufacturing Of Sulfuric Acid 1
Sulfuric acid (oil of vitriol, H2SO4) is a colorless, oily liquid, dense, highly reactive, and miscible with water in all proportions. Heat is evolved when concentrated sulfuric acid is mixed with water and, as a safety precaution, the acid should be poured into the water rather than water poured into the acid. Anhydrous, 100% sulfuric acid, is a colorless, odorless, heavy, oily liquid (boiling point: 338oC with decomposition to 98.3% sulfuric acid and sulfur trioxide). Oleum is excess sulfur trioxide dissolved in sulfuric acid. For example, 20% oleum is a 20% sulfur trioxide–80% sulfuric acid mix. Sulfuric acid will dissolve most metals and the concentrated acid oxidizes, dehydrates, or sulfonates most organic compounds, sometimes causing charring.
The manufacture of sulfuric acid by the lead chamber process involves oxidation of sulfur to sulfur dioxide by oxygen, further oxidation of sulfur dioxide to sulfur trioxide with nitrogen dioxide, and, finally, hydrolysis of sulfur trioxide.
S + O2 → SO2
2NO +O2 → 2NO2
SO2+NO2 → SO3+NO
SO3 + H2O → H2SO4
Modifications of the process include towers to recover excess nitrogen oxides and to increase the final acid concentration from 65% (chamber acid) to 78% (tower acid).
The contact process has evolved to become the method of choice for sulfuric acid manufacture because of the ability of the process to produce stronger acid.
S + O2 → SO2
2SO2 +O2 → 2SO3
SO3 + H2O → H2SO4
In the process (Fig. 1), sulfur and oxygen are converted to sulfur dioxide at 1000oC and then cooled to 420oC. The sulfur dioxide and oxygen
enter the converter, which contains a catalyst such as vanadium pentoxide (V2O5). About 60 to 65% of the sulfur dioxide is converted by an exothermic reaction to sulfur trioxide in the first layer with a 2 to 4-second contact time. The gas leaves the converter at 600oC and is cooled to 400oC before it enters the second layer of catalyst. After the third layer, about 95% of the sulfur dioxide is converted into sulfur trioxide. The mixture is then fed to the initial absorption tower, where the sulfur trioxide is hydrated to sulfuric acid after which the gas mixture is reheated to 420oC and enters the fourth layer of catalyst that gives overall a 99.7% conversion of sulfur dioxide to sulfur trioxide. It is cooled and then fed to the final absorption tower and hydrated to sulfuric acid. The final sulfuric acid concentration is 98 to 99% (1 to 2% water). A small amount of this acid is recycled by adding some water and recirculating into the towers to pick up more sulfur trioxide.
Although sulfur is the common starting raw material, other sources of sulfur dioxide can be used, including iron, copper, lead, nickel, and zinc sulfides. Hydrogen sulfide, a by-product of petroleum refining and natural gas refining, can be burned to sulfur dioxide. Gypsum (CaSO4) can also be used but needs high temperatures to be converted to sulfur dioxide. Other uses for sulfuric acid include the manufacture of fertilizers, chemicals, inorganic pigments, petroleum refining, etching, as a catalyst in alkylation processes, in electroplating baths, for pickling and other operations in iron and steel production, in rayon and film manufacture, in the making of explosives, and in nonferrous metallurgy
