Background of Cracking Process

In petroleum chemistry, cracking is known as the process of breaking down heavy or complex organic hydrocarbon molecules into lighter, much simpler and more functional molecules with the presence of heat and usually pressure. Cracking, also referred to as pyrolysis, is the secondary step in oil refining process. Sometimes, the participation of catalyst is also required throughout the cracking process. The presence of catalyst and the amount of heat supplied (temperature) will determine the rate of cracking process as well as the type of products formed by the end of the process.

In 1913, the first thermal cracking process was developed by William Merriam Burton, which was adapted from the Shukhov Cracking Process found by Vladimir Shukhow in 1891. The process was then diversely enhanced in the year 1920s, in which Eugene Houdry, a French chemist introduced the addition of catalysts which leads to higher-octane products obtained. Catalytic cracking was then further improved in the 1940s by incorporating the use of moving or fluidized beds of powdered catalyst. In the 1950s, there were high demands for jet and automobile fuels therefore, hydrocracking was developed.

Cracking process is one of the most crucial part in the commercial production of gasoline and diesel fuel as it helps in keeping the balance between availability of the various fractions and the demands for those fractions. Some of the most significant products of distillation of crude oil include petrol, naphta, kerosene, liquefied petroleum gas and gas oil.  Since during cracking, bigger hydrocarbons is transformed into various kinds of smaller molecules which plays important roles in various applications, the supply of fuel is increased. At the same time, the demands and supply of the fuel can be balanced. For instance, among the types of oil produced by cracking of petroleum are light oils which refers to gasoline, middle-range oils which are widely used as diesel fuel, residual heavy oils, solid carbonaceous products termed as coke, and gases like methane, ethane, propane and so on.

As of now, there are four main types of cracking methodologies known as Fluid Catalytic Cracking (FCC), Hydrocracking, Steam Cracking, Thermal Cracking and Catalytic Cracking.

1.  Fluid Catalytic Cracking (FCC): FCC is primarily used in petroleum refiners. Initially, it involves the use of low activity alumina catalyst and a reactor where the catalyst particles will be suspended in a rising flow of feed hydrocarbons in a fluidized bed. Later on, the reactions process is improvised in which it utilizes very active zeolite-based catalyst to vaporize the feed and catalyze the cracking reactions.

2. Hydrocracking: One of the catalytic cracking process which requires the presence of hydrogen gas at elevated partial pressure. Normally, this process is aided by bi-functional catalyst that can not only rearrange and break the hydrocarbon chains, but also add the hydrogen to aromatics and olefins.

3. Steam Cracking: This is a petrochemical process that converts saturated hydrocarbons into lighter alkenes and olefins such as ethane and propene. Steam is used to dilute gaseous or liquid hydrocarbon feed before they are briefly heated in a furnace at around 850 ºC. The composition of the feed, the hydrocarbon to steam ratio, the cracking temperature and furnace residence time will determine the products formed.

4. Thermal Cracking: This type of cracking involves implementation of high temperatures and pressures of about 800ºC and 700kPa respectively. A variety of free radicals-based chemical reactions take place during this process including initiation reaction, hydrogen abstraction, radical decomposition, radical addition and termination reactions.

5. Catalytic Cracking: Zeolite catalyst and moderately high temperature of 400-500ºC are used in this process. During this process, carbonaceous products deposits known as coke are produced due to the accumulation of intermediate cations on the catalysts’ active sites. Typically, burning process is employed to remove the deposits and restore catalysts activity.

References:

Britannica, T. E. (2020, June 02). Cracking. Retrieved January 04, 2021, from Encyclopaedia Britannica: https://www.britannica.com/technology/cracking-chemical-process

Cracking and Related Refinery Processes. (2014, September 7). Retrieved from The Essential Chemical Industry - online: https://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html