The contact process for sulfuric acid is a multi-step reaction that begins with raw sulfur and ends with concentrated acid. In theory, sulfur can be burned with air to form sulfuric dioxide, then reacted directly with water to form sulfuric acid. This process creates a great deal of heat, is not chemically controllable, and is rarely used as a result. The contact process uses sulfur dioxide to form sulfur trioxide, which is absorbed into sulfuric acid to form oleum, and then a final step reacts oleum with water to form sulfuric acid.
Sulfur is a yellow mineral found in the ground as deposited layers, often near volcanoes or ancient lava beds. The sulfur is first burned with air to form sulfur dioxide, a molecule of one sulfur and two oxygen atoms. Air entering this reaction is bubbled through concentrated acid to remove any water, because moisture will form sulfuric acid in the reactor and cause excess heat and corrosion.
Chemical process designers carefully control the amounts of reactants to maintain quantities close to the stoichiometric amounts. Stoichiometry is the calculation of the ratios of molecules needed to provide an optimized reaction without use of excess materials. Maintaining chemical reactions near the proper ratios will reduce costs and improve yields, often leading to purer products requiring less processing. The contact process for sulfuric acid must be operated this way to control temperatures, because the reactions create a lot of heat that can affect the product and damage equipment.
Once the sulfur dioxide exits the reactor, it enters a second reaction with more air to form sulfur trioxide, which adds an additional oxygen molecule. The contact process for sulfuric acid could react sulfur trioxide directly with water, but this reaction is very unstable and difficult to control. Sulfuric acid is mixed with the trioxide molecule, which forms oleum or fuming sulfuric acid. Oleum is a very reactive acid containing excess sulfur molecules, but it can be mixed with water in a controlled way to form sulfuric acid while maintaining proper temperatures.
Each reaction step with air is performed in the presence of a catalyst, typically vanadium oxide. The metal catalyst is not consumed in the reaction, but assists by allowing the reaction to occur at lower temperatures than would be required without it. Oxygen content must also be carefully controlled, because extra air does not create additional acid, but will reduce the amount of sulfur dioxide or trioxide because the excess air dilutes the sulfur molecules. The contact process for sulfuric acid will create less acid if extra oxygen is allowed in the process.
Sulfuric acid is sold in a wide variety of strengths, or concentrations, but the contact process for sulfuric acid produces a highly concentrated form. Shipping concentrated sulfuric acid is more economical than a diluted form, because water can be added later as needed to form the desired strength. Concentrated acid will also absorb water very easily, so care must be taken during production and shipping to minimize water or outside air, which dilutes the acid and can increase corrosion.