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The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Vanadium(V) oxide (vanadium pentoxide) was formerly employed as a catalyst for the reaction, but as it is susceptible to poisoning by arsenic impurities in the sulfur feedstock, platinum is now preferred despite its much higher cost.

This process was patented in 1831 by the British vinegar merchant Peregrine Phillips. 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.


The process can be divided into three stages:

  1. Preparation and purification of sulfur dioxide
  2. Catalytic oxidation (using vanadium pentoxide catalyst) of sulfur dioxide to sulfur trioxide
  3. Conversion of sulfur trioxide to sulfuric acid

Purification of air and SO2 is necessary to avoid catalyst poisoning (ie. removing catalytic activities). The gas is then washed with water and dried by H2SO4.

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

Sulfur dioxide and oxygen then react in the manner as follows:

2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = −197 kJ mol−1

To increase the reaction rate, high temperatures (450 °C), medium pressures (1-2 atm), and vanadium(V) oxide (V2O5) are used to ensure a 96% conversion. Platinum would be a more effective catalyst, but it is very costly and easily poisoned. The catalyst only serves to increase the rate of reaction - it has no effect on how much SO3 is produced. The mechanism for the action of the catalyst is:

2SO2 + 4V5+ + 2O2- → 2SO3 + 4V4+
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(l) + SO3(g) → H2S2O7(l)

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

Oleum is reacted with water to form concentrated H2SO4.

The average percentage yield of this reaction is around 30%.

H2S2O7(l) + H2O(l) → 2 H2SO4(l)


The next step to the Contact Process is DCDA or Double Contact Double Absorption. 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 get 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 resulting not only achieving high conversion efficiency for SO2 but also enabling production of higher concentration of sulfuric acid H2SO<>4.

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.




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