4. Sulfuric Acid Industries

4.1 Sulfur

  • Occurrence: Elemental sulfur is found in the sedimentary deposits of Earth’s crust. It is found in non-elemental form in sulfates (gypsum), in sulfidic ores (e.g.: iron pyrites and copper, zinc, lead, nickel and cobalt sulfides) and in fossil fuels. The importance of the sulfur deposits in natural gas and crude oil as sources of sulfur compounds is increasing.

  • Frasch Process:

    • Using superheated water and air to recover sulfur from underground deposits.

    • Developed by Herman Frasch in 1902.

  • Claus Process:

    • Conversion of hydrogen sulfide to sulfur. \[\ce{H2S} + \ce{1/2 O2} \rightarrow \ce{S} + \ce{H2O}\]

    • Hydrogen sulfide separated from natural gas by amine scrubbing is a highly odorous, toxic, low boiling gas and for these reasons, it is difficult to store or ship in large quantities. Hence, a Claus unit is closely associated with each amine scrubber to convert the hydrogen sulfide gas product of the scrubber to elemental sulfur.

    • Two reactions (both exothermic) are employed in Claus units. The first is a simple combustion of one-third of the hydrogen sulfide stream in air, carried out in a waste heat boiler to capture the heat evolved as steam. \[\ce{1/3 H2S} + \ce{1/2 O2} \rightarrow \ce{1/3 SO2} + \ce{1/3 H2O}\] The sulfur dioxide output is blended with the remaining two-thirds of the hydrogen sulfide. This gas mixture is then fed to heated iron oxide catalyst beds where sulfur dioxide is reduced and the hydrogen sulfide is oxidized, both to a sulfur product, with further evolution of heat. \[\ce{2/3 H2S} + \ce{1/3 SO2} \rightarrow \ce{S} + \ce{2/3 H2O}\]

  • Finnish Process: Flash smelting is a smelting process for sulfur-containing ores including chalcopyrite. The process was developed by Outokumpu in Finland and first applied at the Harjavalta plant in 1949 for smelting copper ore, to produce copper and sulfur. Heating iron pyrites to 1200\(^\circ\)C in the absence of air gives liquid sulfur and iron(II) sulfide.

4.2 Sulfuric Acid

  • Sulfuric acid is the largest scale inorganic commodity chemical produced in the World.

  • Sulfuric acid (\(\ce{H2SO4}\), oil of vitriol, battery oil) is a colorless, oily liquid, dense, highly reactive, strong dibasic acid, and miscible with water in all proportions.

  • Heat is evolved when concentrated sulfuric acid is mixed with water. For dilution of the acid, as a safety precaution, the acid should be poured into the water rather than water poured into the acid.

  • Oleum (\(\ce{H2S2O7}\)): It is obtained by dissolving \(\ce{SO3}\) in \(\ce{H2SO4}\). The strength of oleum is expressed as percent of free sulfur trioxide. 20% oleum means, that in 100 kg, there are 20 kg of \(\ce{SO3}\) and 80 kg of \(\ce{H2SO4}\).

  • Production of sulfuric acid: The two most common methods for the conversion of sulfur dioxide to sulfuric acid are: (i) Lead chamber process (old process, now obsolete) (ii) Contact Process (the latest, and most used).

Lead Chamber Process

  • It was introduced by Roebuck of England in 1746; now obsolete. It employed nitrogen oxides as the catalyst. Details of the chemistry remain obscure even today, and plant operation is said to be more of an art than a science. A plausible reaction scheme is given below: \[\begin{aligned} \ce{S} + \ce{O2} &\rightarrow \ce{SO2} \\ \ce{2NO} + \ce{O2} &\rightarrow \ce{2NO2} \\ \ce{SO2} + \ce{NO2} &\rightarrow \ce{SO3} + \ce{NO} \\ \ce{SO3} + \ce{H2O} &\rightarrow \ce{H2SO4} \end{aligned}\]

  • One of the disadvantages which led to the demise of lead-chamber process is that it can produce acid of only 78% strength.

Contact Process

  • The contact process was invented by Phillips in England in 1831 but was not used commercially until many years later.

  • In US, during 1900 to 1925 substantial number of contact plants were built, using platinum catalysts.

  • In the middle 1920s, vanadium pentoxide catalysts came into use and have gradually completely replaced platinum.

  • The contact process has been gradually modified to use double absorption (also called as DCDA process – double conversion (or contact) and double absorption), which increases yield and reduces stack emission of unconverted \(\ce{SO2}\).

  • DCDA process: In the first stage (500–600\(^\circ\)C) 80% of \(\ce{SO2}\) is converted; and in second stage (400–450\(^\circ\)C) 97% of \(\ce{SO2}\) is converted.

  • A modern sulfuric acid plant is a cogeneration plant (generates electricity in addition to sulfuric acid). For 1 metric ton of sulfuric acid production, about 1.20 ton of superheated steam is produced (at 35 bar, 400\(^\circ\)C).

  • In the burning of sulfur, the quantity of air is regulated to give between 8 to 12% \(\ce{SO2}\). Oxygen is maintained in excess, as it promotes better conversion of \(\ce{SO2}\) to \(\ce{SO3}\) in the later stages. Waste heat boiler is used to generate steam from the exothermic reaction of sulfur with air.

  • Reactions with vanadium pentoxide (\(\ce{V2O5}\)) catalyst: \[\begin{aligned} \ce{SO2} + \ce{1/2 O2} &\rightleftharpoons \ce{SO3} \\ \ce{SO3} + \ce{H2O} &\rightarrow \ce{H2SO4}\end{aligned}\]

  • The reaction \(\ce{SO2} + \ce{1/2 O2}\rightleftharpoons \ce{SO3}\) is exothermic and its equilibrium constant decreases with increasing temperature (Le Chatelier’s principle). In practice, the gas temperature must be maintained between 400–500\(^\circ\)C to maintain a high reaction rate and also a high conversion equilibrium.

  • The high conversion accomplished in double-absorption plants are not the result of increased catalyst efficiency, but are accomplished by passing the gas through the converter a second time after a majority of the sulfur trioxide has been scrubbed out of the gas, thus equilibrium is disturbed.

  • The optimum concentration of the absorber acid is near the \(\ce{H2O-H2SO4}\) azeotrope, 98.3% \(\ce{H2SO4}\), where the \(\ce{SO3}\), \(\ce{H2SO4}\), and \(\ce{H2O}\) vapor pressures are at their lowest values.

    If oleum is to be produced at the plant, it is made in a separate oleum tower upstream of the absorber.

    Direct reaction of \(\ce{SO3}\) with water tends to produce mists of \(\ce{H2SO4}\) which are difficult to absorb.

  • Recovery of sulfur as sulfuric acid:

    • Single Conversion Single Absorption (SCSA) process: 97-98% recovery; the remainder is lost to the atmosphere as \(\ce{SO2}\).

    • Double Conversion Double Absorption Process (DCDA): 99.8% recovery.

4.3 Sulfur Trioxide (\(\ce{SO3}\))

  • \(\ce{SO3}\) is also called as sulfuric anhydride.

  • Liquid sulfur trioxide is used for sulfonation, especially in the manufacture of detergents.

  • In the past, the difficulty with sulfur trioxide was its instability. However, now stabilized forms of sulfur trioxide are available (for example, Sulfan\(^\circledR\)). Inhibitors such as boron compounds, methane sulfonyl chloride, sulfur, tellurium, and phosphorus oxychloride inhibit crystallization or conversion of \(\ce{SO3}\) to a polymer.

Last modified: Sunday, 12 October 2025, 1:32 PM