Hydrogen sulfide in diesel fuel. Whether is it dangerous?
Appearance and quality characteristics of diesel fuels depend on their physical and chemical composition. It is clear that it is largely determined by the composition of feedstock (oil), and the method used to produce the desired diesel fraction. Besides the quality aspect, there is also an environmental factor. Diesel fuel may comprise of components which combusting emit very dangerous and toxic substances into the atmosphere together with exhaust gases. Moreover, the excess content of certain chemical elements in fuel may cause a malfunction in engine and fuel system. Let’s take sulfur as an example. Its increased concentration intensifies the formation of soot. But it is also not possible to completely eliminate sulfur from diesel fuel that makes great impact on fuel economy. It should be present at least at the minimum level, since it has a beneficial effect on the lubricating properties of oil.
However, today we are not talking about sulphur but about hydrogen sulphide. Hydrogen sulphide – is a gas without specific color, which has a fairly specific odor. The smell can be compared with the smell of rotten eggs. Apart from quite understandable commercial aspects (who will buy diesel fuel with an unpleasant odor?), there are also a number of other things. This gas is poisonous, and its contact with the human body must be eliminated. At high concentrations hydrogen sulfide can react with many metals, which has a negative impact on operation of fuel system and engine. But that’s not all. At certain concentrations in mixtures with air hydrogen sulfide can ignite.
This gas is included in the list of substances, the content of which is governed by the standards for diesel fuel. Other controlled substances are sulfur, mercaptan sulfur and an alkali-soluble acid. Modern technologies produce oil excluding elemental sulfur and hydrogen sulphide in amounts which may cause corrosion to metals. The presence of such chemical elements and compounds is monitored using tests on the copper plate. Only fuel with sulfur and hydrogen sulfide content of not more than 0.0015% and 0.0003% can pass it.
If you smell a peculiar odor coming from diesel fuel, it may indicative of bad oil refining and big risk for you fuel economy. What is to be done if hydrogen sulfide content still exceeds the normal?
Methods for removing hydrogen sulfide
Alkaline cleaning (alkalization) is used to remove hydrogen sulfide, mercaptans and acids from petroleum. Specifically for removing hydrogen sulfide is used a solution of sodium hydroxide (NaOH) or acid soda (Na2CO3).
In practice for alkaline cleaning is used a 10% solution of sodium hydroxide. The temperature of the process for diesel fuel is 80-90°C, for gasoline is 40-50°C, for kerosene 60-70°C. With slight increasing of the operating temperature the risk of water-based emulsions decreases with easy sedimenting. Alkali residues are removed by water washing.
Hydrogen sulfide adsorbents (additive). In recent years, the market started to show the so-called hydrogen sulfide absorbents. Mainly they are liquid products of organic origin. Most often, they are used in containers for storage and transportation in cargo compartments of ships as well as in oil pumping. Absorbers are supplied in barrels, the recommended dosage is – 500 g per tonne of diesel fuel.
Removal of hydrogen sulfide using hydrotreater. Large refineries have industrial lines to remove gaseous hydrogen sulfide. They are a part of complex hydrodesulphurization systems – hydrotreaters. The process itself takes place in a special reactor at 300-400°C and 30-130 kPa atmospheric pressure. Also, catalysts are used (often they are alumina with cobalt and molybdenum molecules).
Sulfuric acid cleaning. It is one of the historically first oil purification methods, first tested back in the nineteenth century. Sulfuric acid treatment destroys hydrogen sulfide and mercaptans , removes resinous compounds, and dissolves sulfide and disulfide solution.
Harmful impurities accumulate in a byproduct – acid sludge, which is separated from the main fractions, followed by washing with water and neutralization by alkali.