Question

It is from the book Elementary Principles of Chemical Processes, 4th Edition, Problem 5.66

The quantity of sulfuric acid used globally places it among the most plentiful of all commodity chemicals. In the modern chemical industry, synthesis of most sulfuric acid utilizes elemental sulfur as a feedstock. However, an alternative and historically important source of sulfuric acid was the conversion of an ore containing iron pyrites (FeS₂) to sulfur oxides by roasting (burning) the ore with air. The following reactions occurred in an oven:

2FeS2(s)+11/2 O2(g)?Fe2O3(s)+4SO2(g)

(1)

SO2(g)+1/2 O2(g)?SO3(g)

(2)

The gases leaving the oven were fed to a catalytic converter in which most of the remaining SO₂ produced was oxidized to SO₃. Finally, the gas leaving the converter was sent to an absorption column where theSO₃ was taken up by water to produce sulfuric acid (H₂SO₄).

(a)The ore fed to the oven was 90.0 wt% FeS₂, and the remaining material may be considered inert. Dry air was fed to the oven in 30.0% excess of the amount required to oxidize all of the sulfur in the ore toSO₃. Eighty-five percent of the FeS₂ was oxidized, and 60% of the SO₂ produced was oxidized to SO₃. Leaving the roaster were (i) a gas stream containing SO₂, SO₃, O₂, and N₂ and (ii) a solid stream containing unconverted pyrites, ferric oxide (Fe₂O₃), and the inert material. Calculate the required feed rate of air in standard cubic meters per 100 kg of ore fed to the process. Also determine the molar composition and volume (SCM/100 kg ore) of the gas leaving the oven.

(b)The gas leaving the oven entered the catalytic converter, which operated at 1.0 atm. Reaction (2) proceeded to equilibrium, at which point the component partial pressures are related by the expression

KP(T)=pSO3pSO2pO20.5

The gases were first heated to 600°C to accelerate the rate of reaction, and then cooled to 400°C to enhance SO₂ conversion. The equilibrium constant KP at these two temperatures is 9.53 atm0.5 and 397 atm0.5, respectively. Calculate the equilibrium fractional conversions of SO₂ at these two temperatures.

(c)Estimate the production rate of sulfuric acid in kg/kg ore if all of the SO₃ leaving the converter was transformed to sulfuric acid. What would this value be if all the sulfur in the ore had been converted?

Exploratory Exercises—Research and Discover

(d)Two of the important factors affecting the utility of a chemical reaction are the maximum extent of the reaction and the rate at which the reaction occurs. Consider these two factors and explain the steps in the converter in which the gas was heated first and then cooled.

(e)Why has elemental sulfur come to be the dominant feedstock in sulfuric acid manufacturing?

Answer 1

(d) The gases are heated to increase the rate of the reaction. However, the second step in the manufacturing of sulfuric acid is a reversible exothermic process. For exothermic process if the temperature is kept high the equlibrium conversion is less. If equilibrium conversion vs temperature curve (can be verified from Elements of Reaction Engineering by Octave Levenspiel) is plotted for the exothermic process it can be seen that cooling between stages enhances the conversion of SO₃.  

(e) Elemental sulfur is a processed sulfur in the elemental form produced from native sulfur or combined sulfur sources and has generally a minimum sulfur content of 99.5%. Due to its high sulfur content as compared to the other sources of sulfur, like sulfide ores that contains impurities and inert material, it is being dominantly used in sulfuric acid production.