The refining processes of diesel vs gasoline begin with the separation of each from crude oil. The separation of diesel and gasoline hydrocarbons from crude oil occurs during the distillation process. Straight-run diesel and straight-run gasolines are the gasoline and diesel products that come out of the distillation column. 

But while flammable, straight-run gasoline is not a finished product. Straight-run diesel is appropriate for off-road use, but not for highway passenger and commercial vehicles. For on-road use, straight-run diesel requires further refinement as well. 

Further refinement of fossil fuels after the distillation process is necessary for almost all fossil fuels. Diesel and residual fuels — those used in boilers and marine engines — can be the exception. But, in most many cases, diesel and residual fuels require further refinement as well.

The Purpose of Crude Oil Refining Processes 

Diesel and gasoline — all fossil fuels and crude oil derivatives for that matter — consist of different hydrocarbons. There are different classes and categories of hydrocarbons. Some hydrocarbons are short and light. Some hydrocarbons are long and heavy. There are straight-chain molecule hydrocarbons and hydrocarbons that branch and loop. 

The ratio of the different types of hydrocarbons in a fossil fuel mixture determines the fossil fuel type. 

For example, diesel is 75 percent alkane hydrocarbons. And, diesel has virtually no olefin hydrocarbons. On the other hand, gasoline can be greater than 10 percent olefin hydrocarbons. But gasoline, too, contains a large percentage of alkane hydrocarbons. However, the alkanes in gasoline are different than those in diesel and other fossil fuels. They are much shorter and lighter. 

The refining processes for gasoline and diesel are different because the hydrocarbon mixtures and mix ratios of gasoline and diesel are different. Additionally, the refining process is also part of the reason diesel and gasoline have different hydrocarbon compositions. Part of the refining process is breaking up large, long-chain hydrocarbons to make new smaller hydrocarbons. 

Different hydrocarbons have different combustion and compression resistance properties. Combustion and compression resistance properties determine for which type of engine a fuel is most appropriate. There are two kinds of combustion engines. 

One type of combustion engine combusts fuel with a spark. The second type of combustion engine compresses fuel until it auto-ignites. Gasoline engines are typically spark-fired engines. Diesel engines are typically compression engines.

The purpose of the refining process is to separate the hydrocarbons for the two types of engines.

Boiling Crude Oil for Distillation

Hydrocarbons — the components in all fossil fuels and biofuels that ignite/burn/combust — release energy when they oxidize. The scientific explanation of the ignition, combustion, and/or the burning of fossil fuels is oxidation. Oxidation, the process by which metals rust, is the same chemical process by which fossil fuels burn.

There are three means by which to oxidize fossil fuels. The first is to expose a fossil fuel to a flame. Spark-fired engines use a spark to ignite the fossil fuels in a combustion chamber. The second means of oxidizing a fossil fuel is with the application of pressure. Compression engines — like those found in diesel engines — condense the space around a fossil fuel until it ignites. 

The third means of igniting fossil fuels is exposure of a fossil fuel to extremely high temperatures. But, before fossil fuel hydrocarbons auto ignite, they vaporize. Crude oil distillation is the process of heating crude oil until the hydrocarbons vaporize, then separating the vapors from the liquid crude.

Boiling Crude Oil to Fractionate Hydrocarbons

It is by boiling crude oil that refineries produced different fossil fuels from a barrel of crude oil. The process of heating crude oil to separate and capture different hydrocarbon vapors is “fractional distillation.” 

While high temperatures will cause fossil fuels to auto-ignite, the temperatures must exceed the flashpoint of the hydrocarbons in a fuel. If the temperature remains below the flashpoints of the different hydrocarbons in crude oil, the oil will merely boil. When it boils, hydrocarbons vaporize. 

Different hydrocarbons types vaporize at different temperatures. A distillation column collects the different hydrocarbon vapors from crude oil at different temperature stages. The vapors collected at different temperature stages constitute different types of fossil fuels. 

Light Hydrocarbons

Smaller, lighter hydrocarbons evaporate at lower temperatures than large, heavy hydrocarbons. As the temperature in a distillation column rises, the first hydrocarbons to separate are the lightest ones. The lightest hydrocarbons in crude oil are those found in liquid petroleum gases (LPGs). Butane and other LPGs are the first fossil fuels to separate from crude oil during the distillation process. LPGs separate at less than 85 degrees Fahrenheit. 

Medium Sized Hydrocarbons

The next hydrocarbons to separate are gasoline blending components. The hydrocarbons that make up gasoline blending components separate from crude oil at a temperature range of between 85 degrees and 185 degrees Fahrenheit. 

Naphthas are the fossil fuel hydrocarbons to separate from crude oil as the temperature rises during the distillation process. A wide variety of chemicals, from the chemicals found in paint and toothpaste to those in plastics and carbon fiber, require naphtha. Naphtha’s separate at a temperature range of between 185 and 350 degrees. 

Heavy Hydrocarbons

Kerosene and jet fuel separate from crude oil at a range of between 350 and 450 degrees. Diesel and heating oil separate from crude oil at a range of between 450 and 650 degrees. Heavy gas oil separates from crude oil at between 650 and 1,050 degrees. Residual fuel oil is the last fossil fuel to separate from the crude oil. Temperatures greater than 1,050 are necessary to separate residual fuel oil from crude oil. 

Vacuum — “Atmospheric” — Distillation of Crude Oil

The atmospheric distillation of crude oil is an advanced method of fractional distillation. It is how a distillation column collects the hydrocarbon vapors. Atmospheric distillation separates hydrocarbons with greater precision than a simple fractional distillation. Atmospheric distillation also separates the contaminants in crude oil from the hydrocarbons, sulfur in particular. That means atmospheric distillation allows for the production of low-sulfur diesel, fuel oil, and bunker fuel from high sulfur content crude oil.

Vacuum Distillation Process

The first step of the atmospheric distillation process is the heating of the oil in the “crude unit,” the distillation column. Inside the crude unit, water mixes with the oil. The purpose of the water is to desalinate the crude oil. Sodium chloride is the most common salt found in crude oil. 

The water absorbs the salt. The crude unit heats and vaporizes the saltwater, then removes it via a valve into a low-pressure storage and removal tank. 

It is the movement of vapors from a high-pressure unit — the distillation column — into a low-pressure storage tank from which the process gets the name “vacuum distillation.”

Maverick Engineering Incorporated explains, “The desalted crude then proceeds through several more pre-heat exchangers, absorbing heat from the hot cuts off the atmospheric column once more.” Next, the crude oil moves into a heater where its temperature increases to between 650 and 700 degrees Fahrenheit. At those temperatures, crude oil becomes a mixture of liquid and vapor. The vapor rises, condenses, and drops back down the sides of the atmospheric distillation column. 

Vacuum Distillation Separation of Fossil Fuels

The vaporizing and condensing cycle separates the hydrocarbons far more precisely than simply boiling the crude into infractions. “The highest boiling point liquid condenses on the tray just above the bottom flash zone, and the lowest boiling point liquid condenses at the top of the column.” Once the distillation column fractionalizes the crude oil into its component parts, the hydrocarbons at each level are drawn into storage units.

“Liquid fractions are drawn from the trays and removed based on their boiling point ranges. Light gases (methane, ethane, propane, and butane) pass out the top of the column, while naphtha and straight run gasoline are formed in the top trays. Kerosene, diesel, and atmospheric gas oils are formed in the middle of the column, and residue or fuel oils exit at the bottom of the column.

But, even though the hydrocarbons that make up gasoline and diesel separate from crude oil during the distillation process does not mean the gas and diesel hydrocarbons are ready for distribution. Before distribution, it is necessary to refine distilled diesel and gasoline. 

Post-Distillation Refinement of Gasoline and Diesel

In relation to the refinement of small hydrocarbon, light fossil fuels, the post-distillation process of diesel is simple. In some cases, diesel is ready for distribution almost immediately after the distillation process. “The diesel fuel produced by a refinery is a blend of all the appropriate available streams: straight-run product, FCC light cycle oil, and hydrocracked gas oil. The straight-run diesel may be acceptable as is or may need minor upgrading for use in diesel fuel prepared for off-road use.”

However, on-road diesel requires further refinement. “To meet the 15 ppm sulfur limit, all the streams used to prepare diesel fuel need hydrotreating to lower the sulfur concentration.” In addition to reducing the sulfur count in diesel, adjusting the cetane rating is also necessary.

Adjusting the octane rating of gasoline and cetane rating of diesel is the primary purpose of post-distillation refinement.

Increasing Cetane Rating of Diesel and Octane Rating of Gasoline

The cetane rating of diesel is the equivalent of the octane rating in gasoline, but the objective is the opposite. “Cetane number is an indicator of the combustion speed of diesel fuel and compression needed for ignition. It is an inverse of the similar octane rating for gasoline.”

The purpose of both cetane and octane rating is to change the compression resistance of a fuel. As mentioned earlier, fossil fuels will ignite/combust/burn when placed under enough pressure. In the case of compression engines, the pressure is how an engine combusts a fossil fuel. In a spark-fired engine, however, compressing ignition is a malfunction.

Compression Engines and Cetane Rating

For a compression engine to run efficiently, the fuel that powers it must combust at the appropriate time. The compression resistance of a fuel determines when it combusts in a compression engine.  Manipulating the cetane rating of diesel allows engineers to assure diesel will combust at the appropriate pressure. 

Diesel fuel with a high cetane rating combusts at a lower temperature than straight-run diesel. High cetane diesel is generally used in cold climates. Cetane boosters include nitrates, nitroalkanes, nitro-carbonates, and peroxides.

Spark-Fired Engines and Octane Rating 

Spark-fired engines have the opposite problem. Diesel engines require diesel with lower compression resistance to operate in cold conditions. Gasoline engines, because they are spark-fired, must avoid compression ignition. 

Today’s gasoline engines generate tremendous pressure inside the engine cylinder. High octane gasoline resists auto combustion under pressure. The higher the octane rating of a fuel, the greater the compression resistance. The higher the cetane rating of a fuel, the lower it’s compression resistance.

Hydrocracking and catalytic cracking are how refineries manipulate octane and cetane ratings. In the refining processes of diesel vs gasoline, hydrocracking is for diesel. Catalytic cracking is for gasoline

Hydrocracking Diesel Fuel in a Refinery

Hydrocracking is the process of breaking large, long-chain molecule hydrocarbons into smaller hydrocarbons. The purpose of breaking hydrocarbon chains into smaller chains is to change their flashpoints and compression resistance. “In a refinery, the hydrocracker upgrades VGO through cracking while injecting hydrogen. This yields a high volume of high-quality diesel and kerosene product. The hydrocracker is particularly valuable in a refinery that is trying to maximize diesel production and reduce residual fuel oil.”

The process has two stages. Each stage occurs in a separate reactor vessel. In the first stage, a hydrotreating catalyst saturates aromatics — one of the two types of unsaturated hydrocarbons — with hydrogen. At this stage, the reactor vessel also removes the sulfur and nitrogen contaminants from the straight-run diesel. In the second stage, a different reactor vessel breaks now-saturated hydrocarbons into smaller parts and, again, saturates any unsaturated molecules or molecule chains with hydrogen.

The result of hydrocracking is highly refined, contaminant-free diesel with a lower compression resistance than straight-run diesel.

But again, the refining processes of diesel vs gasoline are different. The gasoline version of hydrocracking is catalytic cracking. 

Fluid Catalytic Cracking (FCC) Gasoline 

While the distillation of crude oil produces gasoline, gasoline can also be the product of the residual fuel left over after the distillation process. It is possible to make gasoline from residual fuel using a process called fluid catalytic cracking. “In refining, the FCC is the most common unit used to upgrade heavier distillation cuts to light products. The FCC takes VGO and similar intermediate streams and cracks them using heat in the presence of a catalyst. The primary product is FCC gasoline, which is used in gasoline product blending. The FCC is particularly valuable in a refinery that is trying to maximize gasoline production over residual fuel oil.”

The process of FCC is one in which heat and a catalyst to break the long-chain molecules present in residual fuel into smaller molecules chains. Because the gasoline originates from a very high-octane fuel, FCC gasoline generally has high octane. It also has low sulfur. 

Octane Vs. Cetane Principal Difference between Gasoline and Diesel Refinement

Removing contaminants and producing the cleanest, highest quality fuel is the purpose of all fossil fuel refining processes. But, there is another purpose, at least with respect to gasoline and diesel.

Octane and Cetane Ratings Purpose of Post-

The purpose of the refining process is to control the compression resistance of a fuel. Because spark-fired gasoline engines require high octane fuels and compression diesel engines require high cetane fuel, the refining processes are different. The point of post-distillation refinement of gasoline is to increase the compression resistance of gasoline. The point of post-distillation refinement of diesel is to decrease compression resistance.