The contact process for sulfuric acid involves burning sulfur with air to form sulfur dioxide, which is then absorbed into sulfuric acid to form oleum and finally reacts with water to form sulfuric acid. Stoichiometry and careful control of reactant quantities are important to reduce costs and improve yields. The process uses a catalyst and produces a highly concentrated form of sulfuric acid. Care must be taken to minimize water and outside air to prevent dilution and corrosion.
The contact process for sulfuric acid is a multi-step reaction starting with the crude sulfur and ending with the concentrated acid. In theory, sulfur can be burned with air to form sulfur dioxide, then reacted directly with water to form sulfuric acid. This process creates a large amount of heat, is not chemically controllable, and as a result is rarely used. The contacting process uses sulfur dioxide to form sulfur dioxide, which is absorbed into the sulfuric acid to form oleum, then a final step causes the oleum to react 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. Sulfur is first burned with air to form sulfur dioxide, one molecule of one sulfur and two atoms of oxygen. The air entering this reaction is bubbled through the concentrated acid to remove the water, as the moisture will form sulfuric acid in the reactor and cause excess heat and corrosion.
Chemical process designers carefully control reactant quantities to keep quantities close to stoichiometric quantities. Stoichiometry is the calculation of the ratios of molecules necessary to provide an optimized reaction without the use of excess materials. Keeping chemical reactions close to the correct ratios will reduce costs and improve yields, often resulting in purer products that require less processing. The contact process for sulfuric acid should be used in this way to control temperatures, as the reactions create a lot of heat which can affect the product and damage equipment.
Once the sulfur dioxide exits the reactor, it goes into a second reaction with more air to form sulfur dioxide, which adds an additional oxygen molecule. The contact process for sulfuric acid could make sulfur trioxide react directly with water, but this reaction is very unstable and difficult to control. The sulfuric acid is mixed with the trioxide molecule, which forms the fuming oleum or 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 helps by allowing the reaction to occur at lower temperatures than would be necessary without it. The oxygen content also needs to be carefully controlled, because the excess air will 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 dilute form, because water can be added later as needed to form the desired strength. The concentrated acid will also absorb water very easily, so care must be taken during manufacturing and shipping to minimize water or outside air, which dilutes the acid and can increase corrosion.
Protect your devices with Threat Protection by NordVPN
