The oxidation of dioxide to sulfur trioxide and its subsequent conversion to sulfuric acid is currently almost exclusively carried out using the contact process, in particular the double contact process. The lead chamber process is no longer important.

The contact process is based on the equilibrium between and its oxidation product sulfur trioxide:



which is only attained sufficiently quickly in the presence of a catalyst. Since this equilibrium shifts in favor of the starting materials with increasing temperature, the process has to be carried out at as low a temperature as possible, this being determined by the operating temperature of the catalyst. A higher sulfur dioxide conversion can be obtained by lowering the concentration of the sulfur trioxide formed (double contact process) or by operating under increased pressure (5 bar) (Ugine-Kuhlmann process).

Divanadium(V) oxide catalysts are currently almost exclusively used. They are fused salts, which in a cold state consist essentially of vanadium sulfate and potassium disulfate, applied to a porous support (kieselguhr or diatomaceous earth). The change in valency state between V⁴⁺- and V⁵⁺-ions is probably a critical step in the catalysis. The lowest operating temperature of this catalyst is generally ca. 420 to 440°C.

Oxidation of sulfur dioxide to sulfur trioxide generally proceeds on classical grid-type catalyst trays. In a contact chamber there are four to five sieve trays, on which the catalyst is spread. Sulfur dioxide-containing gas, whose concentration has been adjusted to 10% with dried air, at 450°C before passing through the first tray, passes from the top to the bottom of the chamber through the catalyst trays. During passage through the first tray the gas is heated to 620°C. Before entering the second tray it must be cooled down to 450°C. In plants operating with sulfur dioxide from roasting processes, in which the purified gas is cold, this cold gas is utilized as a coolant in the heat exchangers, thereby itself being heated to 450°C. In the case of sulfur dioxide-containing gas from sulfur combustion, which is already at 450°C and therefore cannot be utilized as a coolant, water is used in the heat exchangers and steam is produced.

There are two processes for the manufacture of sulfur trioxide from sulfur dioxide on catalyst trays.

In the conventional contact process (single contact process) the reaction gases are passed through the four trays without intermediate absorption and the gas is cooled after each tray to 450°C or 430°C for the lowest tray. After passage through the first tray 60 to 63% of the sulfur dioxide has been converted to sulfur trioxide, after the second tray 89 to 90% and after the fourth tray a maximum conversion of 98% is possible, based on sulfur dioxide.

Higher sulfur dioxide conversions (99.6 to 99.7%) can be attained with the double contact process developed by Bayer A.G. In this process sulfur dioxide is converted into sulfur trioxide, as in the single contact process, but after the third tray the sulfur trioxide is completely removed from the reaction gas (conversion 90 to 93%, based on sulfur dioxide) by absorption in 98.5 to 99% sulfuric acid. This is achieved by cooling the reaction gas to 180 to 200°C with a gas cooler and then feeding it into the bottom of a Raschig ring-filled absorption tower in which sulfuric acid at 60 to 70°C is fed in at the top in countercurrent to the gas. The acid is thereby heated up to 80 to 85°C and is cooled with air- or water-coolers before it is returned to the absorption tower. The resulting sulfur dioxide-containing gas (ca. 0.6 to 1.1 % by volume) is heated up with reaction gas in a heat exchanger and is passed through the fourth and optionally fifth tray, so-called post contact. There it is further converted to sulfur trioxide so that an overall conversion of 99.6 to 99.7% is attained based on sulfur dioxide.

In both contact processes the sulfur trioxide is, after the final passage through the final tray and cooling to 180 to 200°C, absorbed in 98.5 to 99% sulfuric acid. This occurs either in a countercurrent absorber tower or in a jet scrubber in which acid is sprayed from the top in co-current with the reaction gas and is accompanied by conversion of sulfur trioxide into sulfuric acid.

If the gas emerging from the beds is sprayed with oleum in a tower, the absorbed sulfur dioxide does not form sulfuric acid, oleum with a higher sulfur trioxide content is obtained instead.

The production of sulfuric acid from proceeds exothermically in all reaction steps. Per ton of 100% sulfuric acid a total of ca. 5.4 MJ of heat is produced. Most of this is utilized in the production of steam (1.1 t of high-pressure steam e.g. at 40 bar and 400°C per ton of 100% sulfuric acid).

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid).

The emission of sulfur dioxide from sulfuric acid plants is strongly reduced with the double contact process. If all the sulfuric acid manufactured in the Federal Republic of Germany were produced by modern plants using the double contact process, the resulting emission of sulfur dioxide would account for only 0.32% of the total emission from human activities.

The catalytic oxidation of sulfur dioxide to sulfur trioxide can also be carried out in a fluidized bed reactor. The gas to be converted is fed in at the bottom of a fluidized bed containing the catalyst in the form of abrasion resistant beads. The whole fluidized bed can be kept at the required temperature by removing the heat of reaction with a pipe cooler. This isothermal mode of operation enables gases with a higher sulfur dioxide content to be processed and more compact plants to be built.

A variant of the contact process is operated in coking plants, the moist gas catalysis process, in which wet sulfur dioxide from the combustion of hydrogen sulfide is converted into sulfuric acid. In this process hydrogen sulfide, from coking plants, is converted to sulfur dioxide and water with an excess of air:



and the moist sulfur dioxide catalytically oxidized to sulfur trioxide. The high water content of the gas means that only 75 to 78% sulfuric acid can be produced in this process. In coking plants this is generally reacted with the ammonia produced during coking gas purification to form ammonium sulfate. In newer processes the water vapor is largely condensed allowing sulfuric acids with a content of up to 98% to be produced.

Nitrous processes (lead chamber and tower processes) have practically no industrial importance. These processes are wet catalytic processes with nitrosyl hydrogen sulfate as oxidizing agent, which are carried out at temperatures up to 80°C in an aqueous phase and were the main production processes for sulfuric acid up to the 1920's. They have a number of disadvantages over the contact process in particular that only sulfuric acids of up to 78% can be produced. However, the low operating temperature of the nitrous process can be advantageous in particular cases. In a process developed by Ciba-Geigy, sulfuric acid can be produced from gases with low sulfur dioxide contents (0.5 to 3% by volume).