Thermal Cracking Past & Present

Blog 6

Write a post reviewing the significance of thermal cracking in petroleum refining in the past and present.


 

After all of the various physical separations have occurred to a crude oil, such as distillation, deasphalting, and dewaxing, there is a need to now change the composition of the crude oil using chemistry, breaking and creating bonds. The yields of this product after just undergoing the physical changes does not meet the demand required so further chemical separations must be pursued. The earliest discovered method for chemical separation is known as thermal cracking where the chemical bonds are broken through changing temperature.

In the past the large demand for gasoline as a product from crude oil started with the mass produced model T automobile back in the 1920’s. Thermal cracking was the first chemical process introduced to convert heavier hydrocarbons with longer chain paraffins to lighter distillates. Thermal cracking produces shorter straight chain alkanes from longer straight chains. This was the number one method used to obtain gasoline from crude oil back in the day. It does so by using brutal heat, heating the temperatures until the compounds crack and the chemical bonds are broken. This process then delivered the gasoline, with a low octane rating, needed for automobiles at that time.

Thermal cracking proceeds through neutral reactor species called free radicals.  Another use for thermal cracking is to convert the bottom of the barrel into usable products such as fuel oils. Presently thermal cracking is not a significant process in a refinery within the United States because the gasoline that is produced from it would work properly in current automobiles. This is because the current automobiles require a fuel with a higher octane rating. This lead to the introduction of catalytic cracking.

Thermal Cracking

The process of thermal cracking has been around for quite some time. Being discovered back in the late 1800’s, it is a very effective way of breaking down longer straight chain paraffin into shorter chian ones through the use of high temperatures and free radical reactions. The most commonly used cracking methods utilizing free radical reactions are hydrogen abstraction and beta bond scission. Hydrogen abstraction is when a free radical “plucks” away hydrogen from the carbon chain causing it to become shorter. Beta bond scission is when the beta bond, in respect to the position of the free radical, becomes “cut” due to the movement of the free radical along the carbon-carbon chain. After doing some quick research it is apparent that the process of more conventional thermal cracking as we recognize it was invented back in 1912 and operated under temperatures between 700-750F and a constant pressure of 90psi. However more modern techniques of thermal cracking involve what we know as vis breaking and coking. Coking has three main types which are flexi, fluid, and delayed coking. Each has a distinctive process that separates itself from the other two. For example delayed coking always utilizes two drums. Vis breaking occurs when the viscosity of the crude oil is reduced while using high temperatures to accomplish this. Overall the thermal cracking process has become an invaluable part of the petroleum, refining industry and is a growing field that is constantly making minor advancements to get the best result out of its feedstock.

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

 

 

 

Thermal Cracking

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 History and Modern Techniques

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

Significance of Thermal Cracking in Petroleum Refining

Thermal cracking has played a large role in petroleum refining for many years. The first technique of thermal cracking was invented and patented by a Russian engineer in 1891. Ever since its invention, Thermal cracking has been used in the petroleum refining industry to “crack” longer, heavier alkane chains into smaller, lighter alkane chains. This is beneficial to the refining process because it allows a larger yield of lighter products to be created from the heavier less desirable products of refining.

Free radicals, are the mechanisms that allow thermal cracking to be possible. It is because of this free radical chemistry that refineries can use the thermal cracking of gas oil to produce higher yields of low octane number gasoline.

There are three main types of reactions involved in thermal cracking. The three types are initiation, propagation and termination reactions which also occur in that order. During the first step, or the initiation reaction, a single molecule is broken into two free radicals. Then during the propagation reaction one of three types can occur, whether it be hydrogen abstraction, radical decomposition or radical addition, all propagation reactions involve the manipulation of a radical into a different radical. Finally, during the termination reaction, two free radicals are essentially terminated, forming a new, shorter, molecule than the one which was originally initiated.

Thermal cracking produces shorter straight-chain alkanes and olefins but lacks the presence of branched iso-alkanes. It is for this reason that catalytic cracking is highly favored over thermal cracking in the production of high octane gasolines.

References:

1. http://en.wikipedia.org/wiki/Cracking_(chemistry)#History_and_patents

2. https://cms.psu.edu/section/content/default.asp?WCI=pgDisplay&WCU=CRSCNT&ENTRY_ID=F20C6357261A4AE2A750C141B721E8C1

The past and present of Thermal Cracking

Thermal cracking was first developed by William Merriam Burton in 1913 which operated under the temperature 700 F -750 F and pressure at 90 psi. It is probably the first commercialized cracking process. By this time, it is also the history of cracking of heavy crude oil fraction to light fraction started. However, as we learned from lesson 5, thermal cracking produces short straight chain alkanes from longer straight chains found in gas oils and the reactions were governed by the free radicals. The process actually produces gasoline that contain lower octane number than that of gasoline which produced by catalytic cracking. It is due to isomerization of free radicals is not favored. During 1920, the petroleum refining industry was facing a challenge called engine knock. In order to solve the problem, it needed more powerful and stranger engines, but that means it also require gasoline which contain higher octane number. By 1930, gasoline had achieved the octane ring around 60 and 70. The newer and more powerful engine required higher octane gasoline which up to about 100, but thermal cracking could not raise its octane number any higher. It is also the reason that thermal cracking is not common process in U.S refineries. Thermal cracking once was the primary process for distillate fuel production. Even it is not anymore, it still remain as an important process in refineries to produce diesel fuel and ethylene.

The Significance of Thermal Cracking in the Refining Industry

Thermal cracking is a process that manipulates long straight chains found in gas oils and other crude fractions, into shorter straight chain alkanes. The chemistry involves free radical reactions which are the key factor concerning the relatively low octane numbers of gas oils that undergo thermal cracking. Without this process, vacuum distillation residue (VDR) would essentially be a useless byproduct. Thermal cracking allows this residue to be converted into distillate fuels along with its primary goal – coke.

Thermal cracking was initially introduced in the early 1900s in order to produce more motor gasoline and high-octane gasoline for aircrafts. It wasn’t until the 1930s and 1940s, when catalytic cracking was introduced, that the petroleum industry seemed to lose its interest in thermal cracking. Today, there is still a desire for such a process, mainly in countries where the chief petroleum fuel in high demand is diesel fuel. Thermal cracking is also used for VDR with visbreaking and coking processes.

There are two (but technically three) main types of thermal cracking during coking, which include delayed coking and fluid coking. The third is known as flexi-coking, a derivative of fluid coking utilized to maximize the yield of distillate products. There is a notable market for this rejected carbon since coke has economic value as it can be used as fuel or as filler when producing anodes for the electrolysis of alumina.

Supply of Petroleum Fuels in the United States and Petroleum Fuels in Internal Combustion Engines

The most notable change in the supply of petroleum fuels for the United States is the drastic growth in domestic production of both natural gas and crude oil. The development of production in tight formations, or shale formations (particularly the now well-known Marcellus Shale in the Northeastern United States) combined with the technological advances (hydraulic fracturing and directional drilling), has had far-reaching effects, including decreased both dependency on and imports from other countries. The largest change is from Africa, with imports from that region decreasing by 90% from 2010 to 2014. For comparison, 2008 tight oil production accounted for only 12% of US production, while in 2012 the number rose to 35%. By 2019, we can expect half of US oil production to be from tight oil formations. This increased domestic production has also reduced costs, allowed prices to decrease, and making natural gas a viable and inexpensive alternative to coal in the generation of electricity. While increased use of natural gas in the electricity generation sector is partially environmental policy-driven, much of this growth can be attributed to the mechanics of a price competitive market. This change is beneficial, as when comparing these two sources of energy, combustion of natural gas produces far less carbon dioxide, nitrogen oxides, and sulfur emissions than coal. Natural gas also does not contain harmful particulate matter such as mercury. Furthermore, there have also been advances in other renewable sources of energy including wind and solar power. As these sources become more competitive as costs decrease, we can expect renewable electricity generation to account for 16% of total electricity generation in the United States by 2040.

There have been many positive changes regarding the environmental concerns from combustion of petroleum fuels in internal combustion engines. Though the total miles driven and vehicles used have increased (total vehicle miles traveled increasing by 0.9% each year), it has been more than offset by the increased fuel efficiency of engines. Strict regulations and standards set in place have forced manufacturers to develop better and cleaner vehicles. The EIA website states that light-duty vehicle fuel efficiency has increased by nearly 2% each year, and can be expected to reach 37.2 miles per gallon by 2040 from 21.5 mpg in 2012. Additionally, the entire nation has been slowly and gradually moving towards diesel fuels, biofuels, hybrid, and completely electric cars (most notably Tesla, with massive growth in recent years). Overall, I believe that the United States is moving in the right direction with regards to environmental policy and sustainability. However, this is a global issue and countries such as China and India must also follow suit (though India’s new prime minister has stated that the entire country will be moving towards solar powered homes, hoping that each home is powered by the year 2019).

http://ebf301.dutton.psu.edu/2014/05/25/supply-of-petroleum-fuels-in-the-united-states-and-petroleum-fuels-in-internal-combustion-engines/

Significance of Thermal Cracking

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.