In petroleum refining there is a strong need to “crack” heavy, long chain alkane feedstocks into lighter, shorter chain alkane feedstocks. This can be done in a variety of ways. As mentioned in earlier lessons, one of these ways is through the process of thermal cracking. Another way this can be achieved is through the process of catalytic cracking, which will be the focus of this blog post.

Compared to thermal cracking, catalytic cracking occurs at lower temperatures and pressures, is more selective and flexible, and incorporates a catalyst. Catalytic cracking processes have evolved over the years, and are an exemplary display of chemical engineering. The most recent catalytic cracking technique was developed in 1942 and is called Fluid Catalytic Cracking. Even more recent is the addition of Catalytic Hydrocracking in refineries, which was developed by Chevron in 1958. These two most recent developments are useful in their own way yet very different in many others.

From a feedstock stand point, both catalytic cracking and catalytic hydrocracking use very different compounds. One of hydrocracking’s main advantages over catalytic cracking is its ability to cope with a much wider range of feedstocks. Hydrocracking processes are able to handle the upgrading of heavier crude oil fractions such as heavy vacuum gas oil and vacuum distillation residue. The heaviest fractions of crude oil, heavy vacuum gas oil and vacuum distillation residue, may not be easily processed by catalytic cracking because of potential problems with coking on the catalysts. For this reason, hydrocracking is able to handle much heavier feedstocks than catalytic cracking.

As for the processes themselves, there are many differences as well. The basis of catalytic cracking is carbon rejection, while hydrocracking is a hydrogen addition process. Catalyst cracking uses an acid catalyst, while hydrocracking uses a metal catalyst on acid support. Another differnce is that catalyst cracking is an endothermic process while hydrocracking is an exothermic process.

There are two main processes associated with hydrocracking, and they are; hydrotreating and hydrocracking. Hydrotreating is for the removal of heteroatoms, while hydrocracking is for the increase of the H/C ratio of the hydrocarbons and to decrease their molecular weight. This is done by hydrogenation and cracking, respectively. Hydrocracking is a very versatile process and can be adjusted according to its wide range of feedstocks.

In catalytic cracking, the process is a little different, and has evolved over time. The first process was the McAfee process which was a batch reaction process that involved a lewis acid to be incorporated in the batch. The next process was the first commercial process called the Houdry process which consisted of a continuous feedstock flow with multiple fixed-bed reactors. The incorporation of the reactor was what allowed the process to be used commercially. The process which followed the Houdry process was the Thermafore Catalytic Cracking process which adopted the use of moving-bed catalysts. Finally the last an most recent process is the afore mentioned Fluidized Catalytic Cracking which uses a fluidized bed catalyst. All of the processes were adapted and modified to increase the thermal efficiency of the process and have been increasing in order of appearance.

The last difference between hydrocracking and catalytic cracking is the products which they produce. The products of catalytic cracking can be described using the acronym PIANO, to represent the Paraffins, Iso-paraffins, Aromatics, Naphthenes, and Olefins produced in catalytic cracking. Catalytic cracking’s most important product is high octane gasoline which is a direct result of the branching alkanes produced in the process. As for hydrocracking, it provides a sizable amount of the diesel fuel production. This is due to straight-run light gas oil being a preferred stock for FCC to produce gasoline as the principal product. Catalytic cracking produces more gas and more coke than hydrocracking, but the liquid yield is higher for hydrocracking. Hydrocracking is more desireable in many areas when compare to catalytic cracking, but cost is not one of them as it is much more expensive to run.

References:

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

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

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/

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.