Diesel fuel is the type of fuel used to operate diesel engines, which can be power generators or transport vehicles, such as buses, trucks, trains, and boats. In our day-to-day applications, diesel refers to the fossil fuel-based product, obtained after the purification of crude oil. It is often regarded as petrodiesel or petroleum diesel.
In addition, there are also some alternative sources of diesel, such as algae, vegetable oils, plants, and animal fats. Based on the raw materials, such type of products are known as biodiesel or biomass to liquid diesel. Speaking about the making of petroleum diesel, it is produced by refining crude oil.
Process of Making Diesel Fuel
During mining of fossil fuels, crude oil is extracted from underneath the Earth's surface. This crude oil contains hydrocarbon compounds, which are made up of carbon and hydrogen atoms in specific ratio. The number of carbon and hydrogen atoms characterizes the size, length, and property of hydrocarbon chain.
During the process of refining, groups of hydrocarbon chains with similar molecular size are separated based on the difference in their boiling points.
Once crude oil is extracted, it is sent to oil refineries for separation of the different components. The process of making diesel can be categorized in three basic steps, namely, separation, conversion, and purification.
Separation of diesel fuel is done by performing fractional distillation of the crude oil mixture. In this method, crude oil is filled in a fractional distillation column, which is then subjected to a specific temperature.
The components of the crude oil mixture are separated according to their boiling temperatures. The compounds with low boiling points are present at the top of the column, and those with higher boiling points are settled below.
For example, propane gas and gasoline are distilled first, after which diesel fuel and lubricating oil are separated one after the other.
For breaking heavier fractions of the crude oil, specific catalysts are used that initiate catalytic cracking of the hydrocarbons. With this conversion step, higher amounts of diesel fuel and other components are separated from the crude oil.
The final step in making diesel is purification, which is carried out by reacting the diesel fuel with catalyst and exposing it to hydrogen under controlled conditions. This is how, regular diesel fuel is made. One of the regular diesel fuel facts is that it contains more than 500 parts per million of the element sulfur.
Besides the usual type of diesel, a more refined diesel fuel is introduced in the market, which is called ultra-low sulfur diesel (ULSD). It contains less amount of sulfur per unit volume, hence reduces the emission of sulfur in the atmosphere.
In some countries, stringent rules are imposed on oil refining companies to produce petrodiesel with low sulfur content. In United States, the standard amount of sulfur content in petrodiesel should be equal to or less than 15 parts per million.
The making of ULSD is similar to the regular fractional distillation performed for producing conventional diesel fuel. The only difference is further refining of the separated diesel fuel to reduce sulfur content.
In comparison to gasoline production, making of diesel fuel is less expensive, as it requires less refining steps. However, the high price of diesel fuel is due to the successive filtration stages, which are performed to remove the pollutants.