Calculation of catalytic reactors - Sulfur trioxide production

Dihydrogen tetraoxosulfate, of formula H ₂ SO ₄ , more commonly known as sulfuric acid, is obtained by synthesis from a sulfur-containing raw material, which can be either sulfur itself, or H ₂ S gas by-product of certain chemical reactions, or a metal sulfide, or waste acids.

The main industrial natural metal sulfides are :

• pyrite FeS ₂ , used as a source of sulfur, the iron element obtained in oxide form being a by-product;

• blende ZnS ;

• PbS galena;

• chalcopyrite CuFeS ₂ .

The latter three are processed to obtain the corresponding metal, in which sulfur is an element to be eliminated: this elimination is currently carried out in the form of sulfuric acid.

For the manufacture of sulfuric acid, a distinction is made between :

– the production of sulfur dioxide SO ₂ , by combustion of sulfur or H ₂ S in air or by oxidative roasting of metal sulfides or decomposition of waste acids in a furnace; the gas obtained has a SO ₂ content varying from 7 to 12% by volume and an oxygen content varying from 4 to 13% by volume ;

– in the case of metal ore roasting and waste acid decomposition, purification of the gas obtained above;

– a heterogeneous catalysis reaction to oxidize sulfur dioxide SO ₂ to sulfur trioxide SO ₃ ;

– absorption of sulfur trioxide SO ₃ in concentrated sulfuric acid (98% by mass), to gently drive the reaction of one molecule of water and one molecule of SO ₃ leading to sulfuric acid.

The sulfur dioxide oxidation reaction is the key step in the process and is the subject of this article, which presents the elements for calculating reactor catalyst beds. These generally operate at close to atmospheric pressure, so to simplify calculations, pressures are expressed, in what follows, in atmosphere (non-legal unit: 1 atm = 1.013 x 10 ⁵ Pa).