The methods of cracking chain molecules in petroleum have expanded over the past century just as every other invention and process has been modified for the benefit of the consumer. Thermal cracking was developed one hundred years ago as a way to salvage more useful products (gasoline, naphtha, diesel) from the tower residues and heavy fractions. This was the first commercial conversion process. By using hydrogen abstraction and beta scission, engineers were able to “chop up” the long chain alkanes that are the responsible for the heaviness of the residue into shorter C-H chains. Although the first scientists may not have completely known the chemistry of the reaction, they knew that this process was going to be a crucial part of petroleum refineries, especially with the boom of automobiles in the early 20^th century. The influx of automobiles may have been responsible for the beginning of the use of thermal cracking, but it also may have been its demise. The process is barely ever used for increasing the yield of lighter products since it proved to produce low octane numbers (compared to its more chemically accurate counterpart, catalytic cracking). Thermal cracking is still used in today’s refineries, yet most of the time it is utilized for the production of ethylene.

Thermal cracking was initially developed to attempt to solve an energy source issue for the automobile and for aircrafts. Gasoline is produced by cracking gas oils at high temperature and initially at high pressures for hydrogen abstraction, then at low pressures for the cracking or breaking of the bonds. The heavy and light gas oils are separated to avoid heating the reactive longer alkane chains to maintain coke formations. This was a great means for our country to produce light middle distillates and heavier ends by excessive heating. The US has since discovered catalytic cracking which allows for a conversion process that produces higher yields of gasoline and higher octane number. Although the process of thermal cracking is in the past for the US, other countries still use this conversion process as a principal application of petroleum refining for diesel fuel production.

Visbreaking is known as a mild thermal cracking process which reduces the viscosity of the vacuum distillation residue to produce fuel oil, which is converted to lighter distillates. Thermal severity is a measurement of temperature and time and is an index number that helps describe how viscosity will change in visbreaking. Thermal severity can inform a refinery on how asphaltene and carbon content of feedstocks are characteristics to be aware of when considering possibilities of coking in the visbreaker reactor.

Thermal cracking is the chemical process of converting larger, long straight chain alkanes found in gas oils and other crude oil fractions into shorter straight chain alkanes. These shorter alkane chains are more desired because of their use in transportation fuels like gasoline. The thermal cracking reactions are governed by free radicals. The chain reaction of free radicals starts by breaking the C-C bond in the alkanes. This forms two free radicals. This step is called the initiation reaction. The next step, the propagation reaction produces a short chain alkane and one radial, which continues the chain. The final step is the termination reaction. This process started in the 1900’s as a way to increase the yield of motor gasoline from crude oils. These high-octane fuels were used in aircraft. Catalytic cracking came into use in the 1930’s and 1940’s. Because the catalytic cracking process produced higher yields of gasoline with high octane numbers, thermal cracking is no longer a method for breaking longer chains into shorter chains for gasoline production in modern refineries. In locations where diesel fuels are in high demand thermal cracking is still used. The use of thermal cracking in modern refineries is limited to naphtha cracking of residual fractions like vacuum distillation residue.

Thermal cracking is a process that produces short straight chain paraffin from longer straight chains found in gas oils and other heavier crude oil fractions. The chemistry of thermal cracking involves free radicals that are reactive species with unpaired electrons but have a neutral electronic charge. It is the free radical chemistry that is responsible for producing gasoline with a relatively low octane number.

A Russian engineer named Vladimir Shukov introduced the first thermal cracking method in the Russian Empire in 1891. However, it was much later in 1912 that William Merriam Burton and Robert E. Humphreys designed a similar thermal cracking process which operated under temperature conditions of 700 to 750 °F and an absolute pressure of 90 psi. The advantage of the system they developed was that both the condenser and the boiler were continuously kept under pressure. A few years later in 1921, an employee at the Universal Oil Products Company, C.P. Dubbs, developed a more advanced technique which operated at higher temperatures of 750–860 °F. The design became known as the Dubbs process and was extensively used until the early 1940s.

Modern day techniques of thermal processing include visbreaking and coking. Visbreaking is a mild fom of thermal cracking whereby the viscosity of the heavy crude oil residue is lowered significantly without affecting the boiling point range. Temperatures of about 950° F are used in the distillation column. Visbreaking mostly depends on temperature and time of the reaction. Coking is a severe form of thermal cracking that is used to convert heavy residuals into lighter more useful products and distillates. The most common coking techniques include delayed coking, fluid coking and flexi coking.

Sources:

Wikipedia: http://en.wikipedia.org/wiki/Cracking_(chemistry)

Course Webpage: https://www.e-education.psu.edu/fsc432/content/chemistry-thermal-cracking

Set Laboratories: http://www.setlaboratories.com/therm/tabid/107/Default.aspx

As mentioned in the lesson, the separation process of crude oil provides insufficient yields for the desired products i.e. gasoline. To satisfy the demand for more desirable products, conversion processes are used to enhance the yield. Cracking is the process of breaking down large molecules into smaller ones. Free radicals are the active intermediate species in thermal cracking (as opposed to ions in catalytic cracking). Though radicals are more stable on ternary or secondary carbons, the weakest bond in a compound is broken and the radical is typically produced on a primary carbon. Additionally, due to the fact that beta-bond scission reactions proceed faster than isomerization in a radical during the thermal cracking process, the final product is primarily composed of straight-chain parrafins and negligible amounts of branched-chain paraffins. Straight-chain parrafins have a lower octane rating – or a higher tendency to knock (self ignite) – an undesirable effect in modern gasoline engines. Therefore, presently thermal cracking is rarely used to improve gasoline yields and instead used to convert heavy gas oils into light gas oils with some byproducts of gas, gasoline, and fuel oil. However, it is still used to improve diesel yields (where knocking is desirable) in countries that primarily rely on diesel fuel.