Contact process

The contact method is a technical process for the production of sulfuric acid by means of a catalyst (e.g., vanadium pentoxide on silica). It has been applied on a large scale, but in the meantime developed into more profitable and environmentally friendly double contact process. Previously also found the lead chamber process and the Vitriolverfahren application.

Process Description

In the first step of the process, sulfur dioxide is produced by burning sulfur. The air required for combustion needs to be sufficiently dried before use in order to otherwise resulting sulfuric acid or sulfurous to avoid corrosion and catalyst deactivation system acid.

The sulfur combustion occurs in the excess air in a furnace with refractory lining to a gas with about 10 to 11% sulfur content. The gas must be cooled to about 410 to 440 ° C, to adjust the temperature for the subsequent step of the catalytic oxidation according to the combustion.

Sulfur dioxide can also be produced by the roasting of sulfidic ores.

The resulting sulfur dioxide is reacted with oxygen in an equilibrium reaction with a platinum or vanadium catalyst (on silica gel SiO) to form sulfur trioxide.

The sulfur trioxide obtained reacts with water to form sulfuric acid.

SO3 has a higher solubility in H2SO4 than in water. This disulphuric created (also called Fuming sulfuric acid or oleum ):

This can then be admixed with water in order to obtain double the amount of the sulfuric acid used.

It is common with most acid systems, that used in the solution of approximately 97 to 99 SO 3 % sulfuric acid and the concentration of sulfuric acid is established by adding water so that no fuming sulfuric acid is formed. In some sulfuric acid plants but also aware of oleum is produced, then that is not diluted with water, but is used for special uses.

It is important for the reaction of the sulfur dioxide to sulfur trioxide with oxygen so that the temperature does not exceed a range of 400-600 ° C.

Catalysis

The key reaction step is the oxidation of sulfur with atmospheric oxygen to sulfur trioxide with the aid of vanadium pentoxide as a catalyst. Vanadium pentoxide is contained in the pores of the diatomaceous earth as the solid support, but dissolved in the active state to an alkali sulphate melt. The melting temperature of alkali sulfate therefore indicates the lower operating limit of the catalyst. Recent catalyst developments put through a cesium doping down this melting point, and thus the lower operating limit.

The reactive species in the catalysis of a complex with the composition [ (VO ) 2O (SO4) 4] 4 -. At this initially stored oxygen, then sulfur dioxide to. In two stages react a total of two molecules of sulfur dioxide with oxygen to form sulfur trioxide.

This sulfur is introduced into sulfuric acid and there is H2S2O7, with water it reacts further to sulfuric acid.

Double contact process

The double contact process for producing sulfuric acid is an evolution of the contact process, but more profitable, environmentally sound and will therefore be used today on a large scale.

In contrast to the simple contact process, the gas is washed SO3 - free with 98% sulfuric acid after passage through three contact hordes in the intermediate absorber and led the remaining SO2 to fourth contact Horde. Here the further conversion occurs to SO3. The conversion achieved is at least 99.8 %.