The cracking (literally “split“) is a key process in the hydrocarbon refining which provides for break the longest chains to transform them into lighter products. The crude oil in fact it is a complex mixture of hydrocarbonswhich have different economic values and are not all usable, for example, as energy sources. For optimize the performance of the crude oil this must be subjected to refining processes aimed at separate molecules of commercial interest: one of the key processes is precisely that of the cracking.
What is hydrocarbon cracking?
The cracking It is a fundamental process in the refining of hydrocarbons, usually carried out after the fractional distillationThe latter separates crude oil into different fractions based on their density. Through cracking, it is possible to increase the yield of oil by breaking the longer chains of hydrocarbons, such as heavy oils, and transforming them into lighter and more sought-after products on the market, such as gasoline. This process therefore allows to optimize the use of extracted resources and to better satisfy the demand for light fuels.
Crude oil, in fact, is made up of a variable mixture of simple hydrocarbonshaving a limited number of carbon atoms, and of complex hydrocarbons, which can contain more than 20 carbon atoms. As the number of carbon atoms increases, the molecules grow, giving rise to long hydrocarbon chains, which can be linear, branched or cyclical. At the same time, however, the energy value of hydrocarbons decreases as their density increases and molecular complexity. The simplest compounds, in fact, have lower boiling points, are more volatile and they burn more easilymaking them ideal as fuels. This explains why gases used to heat homes or as fuels, as well as petrol, are composed of mixtures of hydrocarbons with a number of carbon atoms that varies from 1 to 10. Examples of these include the methane (CH4), The propane (C3H8), The pentane (C5H12) And the octane (C8H18). Conversely, longer hydrocarbon chains have higher boiling points and higher densities, and are therefore less suitable for energy applications.
Consequently, to make this mixture a usable resource, it is necessary to subject it to refining processes, such as cracking, which have the purpose of separating heavy hydrocarbons from light ones and obtaining the maximum yield from the economically valuable compounds.
How does cracking work?
As mentioned, cracking involves the breaking of bonds between carbon atoms within hydrocarbon molecules, leading to the formation of lighter fractions. In the example in the figure below, the cracking of the hydrocarbon moleculeoctane (C8H18) can happen with the breaking of a CC bond and the rearrangement of hydrogen atoms, resulting in the formation of two new distinct compounds: thehexane (C6H14) and theethene (also known as ethylene, C2H4). In this case, ethene is a molecule unsaturated, which has a double bond between two carbon atoms, and belongs to the family of alkenes. On the other hand, octane and hexane, having only single bonds between their carbon atoms, are classified as saturated hydrocarbonsthat is to say alkanes. The cracking process of a hydrocarbon always produces a combination of at least one saturated and one unsaturated compound. The total sum of carbon and hydrogen atoms in the cracked products will always be equal to that of the original molecule.
Furthermore, it is important to emphasize that the breakdown of a hydrocarbon it doesn’t always happen the same way. Factors such as reaction conditions (temperature, pressure), the reaction speed, there presence of catalysts and the initial composition of the hydrocarbon mixture significantly influence the cracking process and its products.
Thermal and catalytic cracking
At the industrial level, there are two main methods for cracking hydrocarbons: thermal cracking and the catalytic cracking.
Thermal cracking
Thermal cracking, developed in the early 1900s, is the older and simpler of the two methods. This process involves breaking the C-C bonds in hydrocarbons through exposure to high temperaturestypically between 450°C and 750°Cand, in some cases, at pressures of up to 70 atm. This methodology has been used for some time to improve the heavy fractions of kerosene and transform heavy oils into lower density gasolines and diesels.
The steam cracking It is a variant of thermal cracking characterized by the use of water vapor and conducted under conditions of high temperatures (generally higher than 850°C), in the absence of oxygen. The reaction is very rapid: in modern ovens, the length of stay of the substances inside can be so short as to be measured in millisecondsall this in order to improve the yield of the desired compounds. This is a versatile technique, applicable to a wide range of hydrocarbon mixtures, such as LPG, naphthas, gasolines or combinations of diesel oils.
Catalytic cracking
The catalytic cracking is perhaps the most effective method for obtaining light alkanes. In this process, the longer hydrocarbon chains are broken by heating in the presence of a catalysta substance that accelerates the reaction and allows operation at milder temperature and pressure conditions. In fact, catalytic cracking takes place at temperatures around 500°C. The zeolitesaluminosilicate minerals composed of aluminum, silicon and oxygen, are among the most commonly used catalysts. Catalytic cracking is the main source of hydrocarbons with a chain length between 5 and 10 atomstypically used in the production of gasoline.
THE’hydrocrackingalso known as hydrogenating crackingis a variant of catalytic cracking that uses catalysts based on noble metals such as palladium and the platinum and other elements such as the molybdenumThe tungstencobalt and nickel. The reaction occurs in a hydrogen-enriched environmentat temperatures up to 425°C and high pressures. In addition to the fragmentation of hydrocarbon molecules, the presence of hydrogen favors the development of hydrogenolysis reactions which remove unwanted elements, such as sulfur and nitrogen, converting them into gases such as hydrogen sulfide and ammonia. Hydrocracking allows the production of a variety of hydrocarbon fractions, including kerosene and jet fuel, naphtha and diesel, starting from dense feedstocks such as heavy gas oils and fuel oils.
Bibliography
