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The contact process is a method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum was originally used as the catalyst for this reaction; however, because it is susceptible to reacting with arsenic impurities in the sulfur feedstock, vanadium(V) oxide (V₂O₅) has since been preferred.^[1]

This process was patented in 1831 by British vinegar merchant Peregrine Phillips.^[2][3][4] In addition to being a far more economical process for producing concentrated sulfuric acid than the previous lead chamber process, the contact process also produces sulfur trioxide and oleum.

In 1901 Eugen de Haën patented the basic process involving combining sulfur dioxide and oxygen in the presence of vanadium oxides, producing sulfur trioxide which was easily absorbed into water, producing sulfuric acid.^[5] This process was improved remarkably by shrinking the particle size of the catalyst (e.g. ≤ 5000 microns), a process discovered by two chemists of BASF in 1914.^[6][7][8]

The process can be divided into four stages:

1. Combining of sulfur and oxygen (O₂) to form sulfur dioxide, then purify the sulfur dioxide in a purification unit
2. Adding an excess of oxygen to sulfur dioxide in the presence of the catalyst vanadium pentoxide at 450 °C and 1-2 atm
3. The sulfur trioxide formed is added to sulfuric acid which gives rise to oleum (disulfuric acid)
4. The oleum is then added to water to form sulfuric acid which is very concentrated. Since this process is an exothermic reaction, the reaction temperature should be as low as possible.

Purification of the air and sulfur dioxide (SO₂) is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried with sulfuric acid.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulfur dioxide and dioxygen then react as follows:

2 SO_2(g) + O_2(g) ⇌ 2 SO_3(g) : ΔH = -197 kJ·mol⁻¹

According to the Le Chatelier's principle, a lower temperature should be used to shift the chemical equilibrium towards the right, hence increasing the percentage yield. However too low of a temperature will lower the formation rate to an uneconomical level. Hence to increase the reaction rate, high temperatures (450 °C), medium pressures (1-2 atm), and vanadium(V) oxide (V₂O₅) are used to ensure an adequate (>95%) conversion. The catalyst only serves to increase the rate of reaction as it does not change the position of the thermodynamic equilibrium. The mechanism for the action of the catalyst comprises two steps:

1. Oxidation of SO₂ into SO₃ by V⁵⁺:

   2SO₂ + 4V⁵⁺ + 2O²⁻ → 2SO₃ + 4V⁴⁺

2. Oxidation of V⁴⁺ back into V⁵⁺ by dioxygen (catalyst regeneration):

   4V⁴⁺ + O₂ → 4V⁵⁺ + 2O²⁻

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H₂SO₄ in the absorption tower to form oleum.

H₂SO₄ + SO₃ → H₂S₂O₇

Note that directly dissolving SO₃ in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H₂SO₄.

H₂S₂O₇ + H₂O → 2 H₂SO₄

This includes the dusting tower, cooling pipes, scrubbers, drying tower, arsenic purifier and testing box. Sulfur dioxide has many impurities such as vapours, dust particles and arsenous oxide. Therefore, it must be purified to avoid catalyst poisoning (i.e.: destroying catalytic activity and loss of efficiency). In this process, the gas is washed with water, and dried by sulfuric acid. In the dusting tower, the sulfur dioxide is exposed to a steam which removes the dust particles. After the gas is cooled, the sulfur dioxide enters the washing tower where it is sprayed by water to remove any soluble impurities. In the drying tower, sulfuric acid is sprayed on the gas to remove the moisture from it. Finally, the arsenic oxide is removed when the gas is exposed to ferric hydroxide.

The next step to the contact process is double contact double absorption (DCDA). In this process the product gases (SO₂) and (SO₃) are passed through absorption towers twice to achieve further absorption and conversion of SO₂ to SO₃ and production of higher grade sulfuric acid.

SO₂-rich gases enter the catalytic converter, usually a tower with multiple catalyst beds, and are converted to SO₃, achieving the first stage of conversion. The exit gases from this stage contain both SO₂ and SO₃ which are passed through intermediate absorption towers where sulfuric acid is trickled down packed columns and SO₃ reacts with water increasing the sulfuric acid concentration. Though SO₂ too passes through the tower it is unreactive and comes out of the absorption tower.

This stream of gas containing SO₂, after necessary cooling is passed through the catalytic converter bed column again achieving up to 99.8% conversion of SO₂ to SO₃ and the gases are again passed through the final absorption column thus achieving not only high conversion efficiency for SO_2, but also enabling production of a higher concentration of sulfuric acid.

The industrial production of sulfuric acid involves proper control of temperatures and flow rates of the gases as both the conversion efficiency and absorption are dependent on these.

• Jim Clark (2002). "The Contact Process". Chemguide.
• "The Contact Process". City Collegiate. 2009. Archived from the original on 2015-03-02. Retrieved 2009-05-01.
• "Absorption Tower". Mining Encyclopedia.