Sylvie Willis
Gasoline, diesel fuel, and asphalt are all products of crude oil. Crude oil contains hundreds of different types of hydrocarbons all mixed together, and these hydrocarbons must be separated through refining to produce gasoline, diesel, and other oil-based products. This process, known as fractional distillation, involves heating crude oil in a distillation column and extracting the different hydrocarbon chains as vapour according to their vaporisation temperatures. The resulting products differ in their carbon range, with gasoline representing C5-C8, naphtha/kerosene (C8-C12), and diesel (C10-C15). Diesel fuel is a distillate fuel, meaning it does not contain any uncracked residuum fractions, and it is generally simpler to refine than gasoline. However, additional refining is often required to remove pollutants such as sulfur from diesel fuel.
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What You'll Learn
Gasoline and diesel are separated by fractional distillation
Crude oil is a mixture of hundreds of different types of hydrocarbons, all with different impurities. In order to produce gasoline and diesel, the hydrocarbons must be separated through refining. This is achieved through a process called fractional distillation.
Fractional distillation involves heating crude oil in a distillation column. The different hydrocarbon chains are then extracted as a vapour, according to their vaporisation temperatures, and recondensed. The longer the hydrocarbon chain, the higher the boiling point, and so they can be separated in this way. Gasoline, for example, is made of a mix of alkanes and cycloalkanes with a chain length of between 5-12 carbon atoms.
The primary distillation column operates under atmospheric pressure, and the crude oil feedstock is separated into a number of streams of increasingly higher boiling points, known as straight-run products. Straight-run diesel is an example of one of these products. The material that is too heavy to vaporize in atmospheric distillation is removed from the bottom of the column, known as the "atmospheric bottoms".
In most refineries, the atmospheric bottoms are further fractionated by a second distillation, carried out under vacuum. The quantity and quality of the streams drawn off from distillation depend on the chemical composition of the crude oil. Crude oils also yield proportions of gasoline, diesel, residual fuel oil, and other products.
Distillation is the most important process in a refinery. It is the first step in refining crude oil into fuels, and it is through this process that gasoline and diesel are separated.
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Gasoline is made from straight-run gasoline
Gasoline, or petrol, is a mix of alkanes and cycloalkanes with a chain length of between 5 and 12 carbon atoms. It is produced from mineral oil, which is refined from crude oil. Crude oil contains hundreds of different types of hydrocarbons, which need to be separated through a process called fractional distillation. During this process, crude oil is heated in a distillation column, and the different hydrocarbons are extracted as a vapour according to their vaporisation temperatures before being recondensed.
Straight-run gasoline is a light gasoline fraction obtained as a result of refining crude oil, gas condensate, natural gas, associated petroleum gas, oil shale, coal, and other raw materials. It is a type of fuel defined by its method of production. Straight-run gasoline is obtained by direct distillation of oil with a boiling range of 35-180 degrees Celsius. It is used as a raw material for pyrolysis in the production of ethylene at petrochemical facilities, for blending, and as a product for export.
Straight-run gasoline is a fraction of direct distillation of oil, with 80% boiling away in the range of 80-160 degrees Celsius. It usually consists of short-chain hydrocarbons and is transparent in colour. It is also known as naphtha in Russia, with common names including gasoline stable (BGS), gas condensate distillate light (DGKl), and gasoline for the chemical industry.
Straight-run gasoline is used as a raw material in the production of ethylene, propylene, butadiene, isobutylene, and ammonium. It is also a component of commercial gasolines, jet fuels, and lighting kerosene. Straight-run gasoline does not possess detonation resistance, so it is not used in carburetor and injection internal combustion engines. Instead, it is used in the chemical and petrochemical industries, where naphtha is used to produce olefins in steam-cracking equipment.
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Gasoline is more expensive to refine than diesel
Gasoline and diesel are both produced from mineral oil, but they differ in terms of their refining methods and energy content. While diesel has a higher energy content per litre, gasoline has a higher number of carbon atoms in its chains, which affects the ease of refinement and energy output of the final product.
The process of refining crude oil into useable fuels like gasoline and diesel involves separating the various hydrocarbons present in the crude oil. This separation is achieved through fractional distillation, where the crude oil is heated, and the different hydrocarbons are extracted as vapours according to their vaporisation temperatures. The longer the hydrocarbon chain, the higher its boiling point, and the higher its energy content.
Gasoline is more complex to refine than diesel due to the specific hydrocarbon chains required. Gasoline is made up of alkanes and cycloalkanes with chain lengths between 5 and 12 carbon atoms. This range of chain lengths gives gasoline its characteristic energy content and combustion properties. However, achieving this specific composition involves a more intricate refining process, which contributes to the higher cost of gasoline refinement.
On the other hand, diesel fuel has a simpler refining process. Diesel primarily consists of medium-length hydrocarbon chains, which are easier to isolate during fractional distillation. While diesel contains more pollutants that must be removed, the overall refining process is generally less complex than that of gasoline.
The demand for gasoline as a motor fuel also influences its refinement costs. Gasoline is in higher demand than diesel, particularly during certain times of the year, such as summertime and Memorial Day in the United States. This increased demand leads to higher production costs, as refineries optimise their processes to meet market demands. Additionally, the transition to ultra-low-sulfur diesel has resulted in infrastructural complications, further contributing to the higher costs of gasoline refinement.
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Gasoline and diesel have different carbon ranges
Gasoline and diesel are both derived from mineral oil, but they differ in their refining processes and physical properties. One of the key distinctions between the two fuels lies in the number of carbon atoms present in their hydrocarbon chains.
Gasoline, also known as petrol, typically contains hydrocarbons with 4-12 carbon atoms. The boiling range of these hydrocarbons falls between 30°C and 210°C. On the other hand, diesel fuel is composed of hydrocarbons with approximately 12–20 carbon atoms, exhibiting a higher boiling range of 170°C to 360°C. This difference in carbon chain length is significant because it contributes to the unique characteristics of each fuel.
The variation in carbon atom counts leads to differences in the performance and combustion characteristics of gasoline and diesel. Gasoline, with its shorter carbon chains, has a lower boiling range and tends to be more volatile. This volatility makes gasoline suitable for spark-ignition engines, which are commonly found in passenger cars. These engines are generally simpler and more cost-effective than diesel engines.
In contrast, diesel fuel, with its longer carbon chains, has a higher boiling range and is less volatile. This property makes diesel better suited for compression-ignition engines, which are known for their higher efficiency and improved fuel economy. Diesel engines have a more efficient combustion process, resulting in higher fuel efficiency and lower carbon dioxide emissions compared to spark-ignition gasoline engines.
The carbon content also influences the energy density of the fuels. Diesel fuel, due to its higher density, offers a higher volumetric energy density than gasoline. This means that diesel contains more energy per unit volume, contributing to its superior fuel efficiency. However, it's important to note that the specific energy content of the fuel is slightly lower in diesel than in gasoline, with diesel providing 43.1 MJ/kg compared to gasoline's 43.2 MJ/kg.
In summary, the difference in carbon ranges between gasoline and diesel fuels contributes to their distinct characteristics, including volatility, ignition method, engine efficiency, and energy density. These differences play a crucial role in determining the suitability of each fuel for specific applications, such as passenger cars, heavy equipment, or power generation.
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Gasoline and diesel have different densities
The difference in density is due to the different hydrocarbon chain lengths in diesel and gasoline. Crude oil contains hundreds of different types of hydrocarbons all mixed together, and these can be separated by fractional distillation. During this process, crude oil is heated in a distillation column, and the different hydrocarbons are extracted as vapours according to their vaporisation temperatures. The longer the hydrocarbon chain, the higher the boiling point, and so the products with higher boiling points are found lower down the column.
The different hydrocarbons produced by fractional distillation can be used for different purposes. For example, gasoline is used as fuel for Otto engines, which contain a four-stroke piston. Diesel, on the other hand, is used as a fuel for compression-ignition engines. Diesel fuel is a distillate fuel, meaning it does not contain any uncracked residuum fractions.
The density of diesel and gasoline is also important when considering their impact on the environment. For example, diesel fuel, when burned, offers a net heating value of 43.1 MJ/kg, compared to 43.2 MJ/kg for gasoline. The CO2 emissions from diesel are 73.25 g/MJ, slightly lower than for gasoline at 73.38 g/MJ. However, diesel contains more pollutants that must be extracted before it can reach the same levels of emissions as petrol.
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Frequently asked questions
Fractional distillation is a process in which crude oil is heated in a distillation column, and the different hydrocarbon chains are extracted as a vapour according to their vaporisation temperatures and then recondensed.
Crude oil contains hundreds of different types of hydrocarbons all mixed together. To produce petrol, diesel or any other oil-based products, the hydrocarbons have to be separated by refining. Gasoline and diesel fuel are separated from crude oil through the process of fractional distillation.
Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. It is a distillate fuel, meaning it does not contain (uncracked) residuum fractions.
Gasoline is separated from asphalt during the fractional distillation of crude oil. Asphalt is a residual, solid product of distillation, while gasoline is a liquid distillate.
