Fluid catalytic cracking and hydrocracking produce a variety of heating and transport fuels including diesel and gasoline. Cracking is a secondary part of the crude oil refining process. The first step in the refining process is the separation of fossil fuel hydrocarbons. The distillation process separates crude oil hydrocarbons into small, medium, and large molecule batches. Cracking is the refinement of the extra-large hydrocarbons left over after the distillation process. 

The two most common methods of hydrocarbon cracking — fluid catalytic cracking (FCC) and hydrocracking — are both processes specific to big, long, heavy hydrocarbons. Both follow the distillation process that separates usable hydrocarbons from those that require further refinement. And both make usable transport and heating fuels out of the otherwise use-limited residual fuel that remains after the distillation process. 

Before Hydrocarbon Cracking, Distillation Process Separates Hydrocarbons by Weight

Crude oil distillation refinement produces gas-state fuels, gasoline, diesel, and fuel oils. It also produces the feedstocks for plastics, paints, rubber, adhesive compounds and hundreds of other hydrocarbon-based products. The distillation refining process does not separate all the hydrocarbons from crude oil. There are leftover hydrocarbons. The name of the leftover hydrocarbons is “residuals.”

It is important to note, there are not “diesel hydrocarbons” nor “gasoline” or “propane” hydrocarbons. There are no hydrocarbon categories or types specific to fossil fuel. Many fossil fuels have the same hydrocarbons. That is to say, the refining process of crude oil is not the separation of fuels and synthetics. While it is true that all fossil fuels are made of hydrocarbons, no fuel is the product of a single hydrocarbon category or class. 

All fossil fuels are a mixture of different hydrocarbon categories, classes, sizes, and weights. So, many fossil fuels have the same hydrocarbons. It is a mixture of different types of hydrocarbons that determine the type of fossil fuel.

Distillation Refinement is the Boiling of Crude Oil

The primary component of a crude oil refinery is the distillation column(s). “The first process is known as distillation. In this process, crude oil is heated and fed into a distillation column. As the temperature of the crude oil in the distillation column rises, the crude oil separates itself into different components, called ‘fractions.’ The fractions are then captured separately. Each fraction corresponds to a different type of petroleum product, depending on the temperature at which that fraction boils off the crude oil mixture.”

The distillation column fills with crude oil, then slowly begins heating it. As the temperature of the crude oil rises, hydrocarbons begin separating out. The process by which hydrocarbons separate in a distillation column is evaporation. As the temperature rises, hydrocarbons become vapors. Once vaporized, the hydrocarbons filter into storage tanks. 

Fundamentally, crude oil refinement is the distillation of the different categories and classes of hydrocarbons through the process of boiling. More specifically, refinement is the distillation of differently sized hydrocarbons. But, even more, relevant to the refining process than the size of hydrocarbons are their weights. 

During the refining process, the lightest hydrocarbons evaporate first. The heaviest hydrocarbons evaporate last. Some hydrocarbons are so heavy and dense that they do not evaporate. So heavy are the residual hydrocarbons in crude oil that they will not vaporize. The hydrocarbons that do not vaporize have the label “residuals.”

Rather than vaporizing, if the heat in a distillation column gets high enough, residual hydrocarbons will auto-ignite They are simply left over at the bottom of the distillation column. 

Why Some Hydrocarbons Do Not Distill

Not all the hydrocarbons in crude oil are suitable for use as heating or transport fuels. Even though most of the hydrocarbons in crude oil have a use, there is a small minority — residuals — that are too heavy and dense to have a practical application.

Residual hydrocarbons are so heavy that they have an extremely high viscosity. That means they pour extremely slowly, if at all. Additionally, residuals are primarily made of long, branched, and looped alkanes. That means residual oil is very stable. As a result, though packed with energy, residual hydrocarbons do no oxygenate easily. 

So, not only are residual hydrocarbon fuel oils almost a semisolid, they do not ignite easily. So, residual hydrocarbons are of very little use. Unless that is, the hydrocarbons in residual fuel oil are changed at a molecular level. To be of practical use — use as transport and/or heating fuels, the hydrocarbons must be broken up into smaller hydrocarbon molecules. 

That is what cracking is, the conversion of residual hydrocarbons into smaller, useful hydrocarbons. 

The demand for heating and transport fuels is exceedingly high. So, there is value in converting residual hydrocarbons into hydrocarbons that are appropriate for use as heating and transport fuels. Cracking converts residual hydrocarbons into hydrocarbons with low viscosity and higher) volatility.

The differences between hydrocracking and catalytic cracking pertain to the types of hydrocarbons each produces. The types of hydrocarbons cracked determine types of hydrocarbons FCC and hydrocracking produce. Some hydrocarbons in residual fuel oil produce gasoline when cracked. Others produce gas-state fossil fuels. And still, others, when cracked, produce the hydrocarbons necessary to produce diesel. To understand the difference between hydrocracking and fluid catalytic cracking, it is necessary to understand the differences between hydrocarbons.   

Crude Oil Components at a Glance

The composition of crude oil is almost entirely hydrocarbons, save the trace contaminants that exist. “Crude oil is a naturally occurring, unrefined petroleum product composed of hydrocarbon deposits and other organic materials. A type of fossil fuel, crude oil can be refined to produce usable products such as gasoline, diesel and various forms of petrochemicals.”

Crude oil hydrocarbons are of two categories and four classes. Both categories consist of two classes a piece. The two categories of hydrocarbons are saturated and unsaturated. 

Saturated Vs Unsaturated Hydrocarbons

Saturated hydrocarbons are those that cannot take on any more carbon or hydrogen atoms. They are complete. Because saturated hydrocarbons are complete, they are extremely stable. 

Unsaturated hydrocarbons, on the other hand, are not complete. The carbon and hydrogen atoms in unsaturated hydrocarbons have room for the addition of more carbon and hydrogen atoms along the chain. And, because unsaturated hydrocarbons are incomplete, they are volatile.

PEDIAA.com explains, “The main difference between saturated and unsaturated hydrocarbon is that saturated hydrocarbons contain only single covalent bonds between carbon atoms, whereas unsaturated hydrocarbons contain at least one double or triple covalent bond in the main chain. Saturated and unsaturated hydrocarbons show different characteristics because of these structural differences.”

Both categories of hydrocarbons consist of a pair of hydrocarbon classes. 

Four Classes of Hydrocarbons

There are two hydrocarbon classes within the saturated hydrocarbon category and two classes within the unsaturated category. Alkanes and cycloalkanes are the saturated classes of hydrocarbons. Alkenes and alkynes are the unsaturated classes. Alkynes and alkenes — the two classes within the unsaturated category —have unique molecular-level traits and characteristics. 

It is debatable as to whether there are actually two classes of saturated hydrocarbons. 

Cycloalkanes are simply alkanes that loop. The carbon atoms at each end of the molecular chain bond together. The result is a hydrocarbon ring. While the structure of cycloalkanes is different than that of alkanes, the chemical composition is the same. “Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. They have the general formula \(C_nH_{2n+2}\) and can be subdivided into the following three groups: the linear straight-chain alkanes, branched alkanes, and cycloalkanes. They are also saturated hydrocarbons. Alkanes are the simplest and least reactive hydrocarbon species containing only carbons and hydrogens.”

Cycloalkanes are not always recognized as an independent class of saturated hydrocarbon. So, cycloalkanes are, fundamentally, alkanes. “Alkanes are described as saturated hydrocarbons, while alkenes, alkynes, and aromatic hydrocarbons are said to be unsaturated.”

The categories and classes of hydrocarbons are relevant to fuel types. For example, “Petroleum-derived diesel is composed of about 75% saturated hydrocarbons (primarily paraffin including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H24, ranging approximately from C10H20 to C15H28.”

The significance of Hydrocarbon Categories and Classes to Cracking 

The categories and classes of hydrocarbons are relevant to hydrocracking and catalytic cracking. The reason being, fluid catalytic cracking breaks heavy, dense, saturated hydrocarbons into smaller molecules in order to produce gas-state fossil fuels and medium weight fuels like gasoline. Hydrocracking is a two-step process. Hydrocracking is, first, the process of adding hydrogen atoms to unsaturated hydrocarbon atoms in order to make them saturated. Once the hydrocarbon molecules are saturated, then they are broken into smaller component parts in order to make diesel fuel and fuel oils. 

The Fluid Catalytic Cracking Process

Cracking hydrocarbons is not a new concept. In fact, it has been around since the turn of the last century. “Catalytic cracking process was developed in 1920 by Eugene Houdry for upgradation of the residue was commercialized latter in 1930. Houdry process was based on cyclic fixed bed configuration. There has been continuous upgradation in the catalytic cracking process from its inception of fixed bed technology to latter fluidized bed catalytic cracking (FCC).”

Fluid catalytic cracking is a means by which large, dense, hydrocarbon molecules are broken down. “Fluid Catalytic Cracking Units (FCCUs) are a secondary conversion operation within more complex refineries, and is used to produce additional gasoline, primarily, from the gas oils produced in the atmospheric and vacuum distillation units.” It is a single stage process. Once the hydrocarbons are cracked, they are fed into the distillation column to be separated with the crude oil contents. 

The hydrocracking process, on the other hand, is a several stage process. 

The Hydrocracking Process

The hydrocracking process is gaining popularity because it accomplishes more than just the cracking of hydrocarbons. Additionally, the hydrocracking process removes the contaminants commonly found in residual oil. “Hydrocracking is one of the most versatile processes for the conversion of low-quality feedstocks into high-quality products like gasoline, naphtha, kerosene, diesel, and hydro wax which can be used as petrochemical feedstock. Its importance is growing more as a refiner’s search for low investment option for producing clean fuel. New environmental legislations require increasing and expensive efforts to meet stringent product quality demands.”

Hydrocracking technology is a two-part process. Both steps are necessary for the conversion of heavy, dense hydrocarbons that come from a wide range of refinery feedstocks into high-value naphtha or distillate products. Hydrocracking, of all the cracking technologies, has the capacity to convert the lowest quality feedstocks into high-quality transport and heating hydrocarbons. 

For example, hydrocracking can convert the hydrocarbons found in oil sands bitumen into a usable hydrocarbon. Oil sands bitumen are typical of almost zero value. But with hydrocracking, they can be made into useable hydrocarbon fuels. 

Hydrocracking is a two-stage process combining catalytic cracking and hydrogenation. The hydrogenation portion of hydrocracking is the addition of hydrogen atoms to unsaturated hydrocarbons, aromatic rings. “due to the fact that aromatic rings cannot be cracked until they are fully saturated with hydrogen, the hydrocracking process allows for the processing of more aromatic feedstock, including the byproducts of catalytic cracking (e.g. light cycle oil).”

In other words, the first step in the hydrocracking process is the saturation of unsaturated hydrocarbons. The second step is the cracking of those hydrocarbons in order to make low viscosity fuels

Importance of Cracking

Cracking hydrocarbons in order to increase the total amount of usable fuel in a barrel of crude oil is becoming increasingly important. Each day, the world produces over a million barrels of FCC gasoline, As the world becomes more and more dependent on heavy crude oils like those found in the tar sands of Venezuela and Canada, FCC and hydrocracking will only become more important.