Contact Process
The contact process is the main method used to produce sulfuric acid from elemental sulfur (or sulfur dioxide). There are four main steps. It is named after the contact which the sulfur dioxide and oxygen make with the catalyst.
In summary:
- S(l) + O2(g) -> SO2(g)
- SO2(g) + 1/2O2(g) <-> SO3(g) $\Delta$ = -99 kJ/mol
- SO3(g) + H2SO4(l) -> H2S2O7(l)
- H2S2O7(l) +H2O(l) -> 2H2SO4(l),,
Production of SO2
If the process is started with elemental sulfur, it is first melted then sprayed into an excess of dry air at atmospheric pressure. The air is dried by passage through sulfuric acid, a dehydrating agent. This is done to avoid acid mist and corrosion in downstream pipes.
Relevant Equations: S(l) + O2(g) -> SO2(g)
An excess of air (oxygen) ensures the sulfur reacts completely. The combustion of sulfur generates a lot of heat. It is necessary to cool the gas stream from 1000oC to around 400oC, he optimum temperature for the next step. Heat exchangers are used to remove the excess heat and recycle this heat energy so it can be used to remelt more sulfur or power turbines for electrical energy.
Alternatively, SO2 can be obtained from the smelting of metal oxide ores.
For example:
2CuFeS2(s) + 5O2(g) -> 2Cu(l) + 2FeO(s) + 4SO2(g)
Catalytic Oxidation of SO2 and SO3
Clean, dry sulfur dioxide is the feedstock for this stage. Electrostatic precipitators are used to remove any ash from the stream.
Relevant Equations: SO2(g) + 1/2O2(g) <-> SO3(g) $\Delta$ = -99 kJ/mol
The sulfur dioxide is mixed with air at pressure slightly higher than atmospheric, and is passed through a catalyst tower, called a converter. The tower contains 3 or 4 layers of vanadium oxide catalyst. Unreacted gases are recycled back into the gas stream.
| Equilibrium Considerations |
|---|
| To maximise the yield (force equilibrium to the right) we would have to introduce: |
| * excess oxygen |
| * high pressure (due to 1.5:1 ratio) |
| * low temperatures (as reaction is exothermic) |
| Kinetic Considerations |
| * a catalyst |
| * high temperature |
| * high pressure (increasing frequency of collisions as concentrations increases) |
| Energy Considerations |
| To minimise energy use (thus cost): |
| * atmospheric pressure (as high-pressure containers are expensive) |
| * no catalysts, as cost of researching, producing and using them are high |
| * heating from exothermic reactions (combustion of sulfur) |
| Compromise Conditions |
| A gas pressure between 100 and 200 kPa increases the collision frequency between the reacting gases. They increase the yield by driving the reaction to the right (1.5 moles to 1 mole of gas). These pressures are also sufficiently low to avoid expensive apparatuses. |
| An excess of oxygen will drive the reaction to the product side, increasing the yield. Thus, a 5:1 air:SO2 ratio is used (essentially 1:1 O2:SO2) creating an excess of oxygen. |
| A catalyst is used to increase the rate of reaction, compensating for the lower temperature. Its identity is V2O5 supported on a silica bed. |
| Sulfur trioxide is removed from the reaction mix, just before the last bed, by passing it through an interpass absorption tower (producing oleum). It helps shift the reaction to the right and increases yield. As such the gases removed to the atmosphere contain no more than 0.3% SO2. |
In summary, the conditions for the conversion of SO2 to SO3 are:
- Pressure slightly higher than atmospheric.
- Excess of oxygen (1:1 ratio)
Absorption of SO3
The cooled sulfur trioxide from the final catalyst bed is dissolved in 98% sulfuric acid in the absorption tower. The acid is sprayed over the SO3 and produces an oily liquid called oleum.
Relevant Equations:
SO3(g) + H2SO4(l) -> H2S2O7(l)
This is preferable over directly dissolving sulfur trioxide in water as this forms sulfuric acid mists, due to the large heat of dissolution (130 kJ/mol), which are hard to control and difficult to coalesce. Also it is difficult to separate the sulfuric acid gas from others such as nitrogen.
Conversion of Oleum to H2SO4
In the diluter, water is mixed with oleum to produce 98% sulfuric acid (18M).
Relevant Equations:
H2S2O7(l) +H2O(l) -> 2H2SO4(l),,
It is sold either as concentrated (98%) or diluted acid.