Calculation of catalytic reactors

Dihydrogen tetraoxosulfate, of formula H₂SO₄, more commonly known as sulfuric acid, is obtained by synthesis from a sulfur-containing raw material, which can be either sulfur itself, or H₂S gas by-product of certain chemical reactions, or a metal sulfide, or waste acids.

The main industrial natural metal sulfides are :

• pyrite FeS₂, used as a source of sulfur, the iron element obtained in oxide form being a by-product;

• blende ZnS ;

• PbS galena;

• chalcopyrite CuFeS₂.

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 SO₂, by combustion of sulfur or H₂S in air or by oxidative roasting of metal sulfides or decomposition of waste acids in a furnace; the gas obtained has a SO₂ 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 SO₂ to sulfur trioxide SO₃ ;

– absorption of sulfur trioxide SO₃ in concentrated sulfuric acid (98% by mass), to gently drive the reaction of one molecule of water and one molecule of SO₃ 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 10⁵ Pa).