Oil refineries
Sulphur (sulfur)
Limestone and acid rain
The effect of acid rain on limestone

Crude oil contains sulphur. When sulphur is burned, it produces sulphur dioxide. If this gets into the atmosphere, it can cause what is called 'acid rain'.

Acid rain is a broad term to describe the effects of acidic compounds in the atmosphere - even though they don't necessarily fall as rain. Sulphur dioxide (SO2) and nitrogen oxides (NOx - sometimes called 'nox') are released into the atmosphere when we burn fossil fuels (particularly coal). They return to earth on dry dust particles or dissolved in rain drops.

Acid rain is thought to be responsible for damaging large areas of forests and degrading the soil. It also damages materials including limestone and exposed metals.

Therefore, wherever possible, we want to remove sulphur from fuels before we burn them. There are now strict controls in the UK on sulphur and 'nox' emissions from cars. These can be met, in part, thanks to the removal of sulphur from naphtha in the refinery.

Treat gas is used to remove the sulphur from fuel fractions (naphtha, light gas oil and kerosine). The treat gas is mainly hydrogen and is a by-product of other processes.

The hydrogen reacts with the sulphur compounds in the hot hydrocarbons. The reaction produces hydrogen sulphide (which may be familiar to you because of its 'rotten eggs' smell).

The hydrogen sulphide is removed and taken to the sulphur processing plant. Here, it is oxidised to make liquid sulphur. this is solidified and sold to the chemical industry.

The element sulphur.