Catalytic cracking and catalytic hydro cracking are two very different processes. Hydro cracking can be considered a more refined process because it is fairly new when considering refinery processes. It was first used commercially in 1958. Catalytic cracking initiates on the catalyst surface where ionic species are formed. This process produces branched chains alkanes from long straight chain alkanes. These branched chains alkanes, also called iso-alkanes, produces gasoline with very high octane #’s. This high octane gasoline is needed for modern day internal combustion engines. The high octane gasoline is produced because of the high concentrations of i- alkanes as well as the aromatics present in the catalytic cracking product. There are 3 main types of catalytic cracking Houdry, Thermafor, and Fluid catalytic cracking. Houdry catalytic cracking was the first continous commercialized catalytic cracking process. This process operates with three different reactors. This is so the process can remain continuous. This has to be done because after 10 minutes of operation there is a significant amount of coke build up in the reactor. The reactors are stripped with team and the coke burnt off before they are operational again. This process was introduced commercially in 1936. The second catalytic process is Thermafor catalytic cracking. This process was introduced in 1942. This process using one reactor and a moving bed of catalysts rather then the fixed bed in the Houdry process. This process has a much higher efficiency then the Houdry process. The Third type of catalytic cracking is Fluid catalytic cracking. This process has the highest theramal efficiency of the three and was also introduced commercially in 1942. The coke buildup is very rich in carbon which allows the production of distillates without the addition of hydrogen. All three of these processes have significant coke build up.
Catalytic hydrocracking is a much more modern process then catalytic cracking. It was first introduced commercially by chevron in 1958. The process objective of hydro cracking is similar to catalytic cracking but yet more refined. Hydrocracking aims to reduce the molecular weight and boiling point of heavy oils to produce saturated hydrocarbons. It has a wider range of feedstock then catalytic cracking. It uses highly aromatic feedstock and distillation residue, a much greater flexibility in feedstock. This process is also a hydrogen addition process unlike catalytic cracking. Catalytic cracking is a carbon rejection process. There is a advantage to hydrogen addition processes because there is no build up of low grade byproducts, such as coke. Although hydro cracking is a much more expensive process then catalytic cracking.

Cracking is the process of breaking straight chain alkanes into smaller straight chains hence the term “cracking.” This process was, and still is, and extremely important process in developing higher quality products such as gasoline that had an ever increasing octane number, something that is very desirable when producing gasoline. The two types of cracking processes are catalytic and thermal. In addition Catalytic cracking can be broken into catalytic cracking and catalytic hydrocracking.

Catalytic hydrocracking uses the same principles of catalytic cracking will also relying heavily on the elevated partial pressure of hydrogen gas. One positive attribute of catalytic hydrocracking is its ability to accept many different feeds that with minor adjustments can have a large effect on desired the product yields. FCC produces high octane gasoline from straight run gas oil. The long chains of n-alkanes in the feedstock are broken up through the process of catalytic cracking into smaller straight chain i-alkanes, or isoalkanes. Other products are cycloalkanes and other aromatic structures. As noted in the lesson, LPG, cycle oils, and olephin-rich light hydrocarbons are very important products of this cracking method as well.

The whole purpose with hydrocracking is the addition of hydrogen to keep the levels of coke productions under control. Without this, it would be much more difficult to introduce heavier crude oil fractions as a feedstock due to the high amounts of coking on the catalysts. So this is why hydrocracking was invented.

Catalytic cracking involves the presence of acid catalysts. This process has the effect of causing asymmetric breakage of bonds. Because of the use of free radicals in many cracking reactions, namely beta bond scission and hydrogen abstraction, the reactions are self-propagating as both free radicals and carbocations are formed. The formation of these two very highly unstable atomic and molecular structures respectively, result in the reaction proceeding until recombination is achieved.

Catalytic and catalytic hydrocracking are two very important processes that dominate the refining industry in their complexity and their yields of production of highly desirable products. These methods are constantly being heavily researched in order to better the products and the methods used in order to obtain them.

Catalytic Cracking process was developed back in the 1920’s by Eugene Houdry to upgrade and put to better use residue using a process based on cyclic fixed bed configuration, which was later commercialized in the 1930’s.¹ Since then the process has been altered and upgraded into its current form as a fluidized bed catalytic cracking (FCC) process.¹ The feedstock for this process is often light gas oil obtained from a vacuum distillation column.¹ The process takes the feedstock and cracks ow value high molecular weight hydrocarbons to more valuable products of low molecular weight. This includes products such as gasoline, LPG Diesel, petrochemical feedstock’s such as propylene, and C4 gases like isobutylene, Isobutene, and butane.¹ The major reactions is the process are cracking, Isomerization, Dethrogeneration, Hydrogen transfer, Cyclization, Condensation, Alkylation, and Dealkylation.¹

Today Fluid Catalytic Cracking provides 50% of all transportation fuel, and 35% pf total gasoline fuel. In the process there are several specific steps that take place. The first step is the reaction step, in which the feedstock reacts with catalyst and cracks into different hydrocarbons.¹ Secondly the regeneration steps takes place, in which the catalyst is reactivated by burning off cook, and recirculated to reactor.¹ The fractionation step is next, in which the cracked hydrocarbon stream is separated into its various products. ¹

As technology progressed and as feed stocks became heavier with large concentrations sulfur, nitrogen, and heavy metals, a new process was in demand to obtain desirable products. Hydrocracking meet that need by providing a more versatile process which could convert low quality feed stock into high quality products like gasoline, naphtha, kerosene, diesel, and hydro wax used as petrochemical feedstock. ¹ This process uses a wide variety of feedstock’s like naphtha, atmospheric gas oil, vacuum gas oils, coke oils, catalytically cracked light and heavy cycle oil, cracked residue, and deasphalted oil.¹ The end products from this feed are high quality products will excellent product quality, and low sulfur content.¹ The feed in this process goes from straight run gas oil, vacuum gas oils, cycle oils, coker gas oils, thermally cracked stocks, solvent deasphalted residual oil, straight run naphtha, cracked naphtha, into desirable products including liquefied petroleum gas, motor gasoline, reformer feeds, aviation turbine fuel, diesel fuels, heating oils, solvent and thinners, lube oil, and FCC feed.¹

When we compare catalytic cracking to hydrocracking we see two very different processes.¹ Catalytic cracking is defined as a carbon rejection process, where Hydro-cracking is hydrogen addition process. Catalytic also uses riser-regeneration-configuration, where hydro uses down flow packed bed.¹ The products for catalytic are LPG’s and gasoline, and for hydro they are kerosene and diesel.¹ Catalytic cracking produces products which are rich in unsaturated components, where hydro cracking results in products with few aromatics, low sulfur and nitrogen content.¹

References

1. Lecture 5: Catalytic Cracking: Fluid Catalytic Cracking and Hydro-cracking
   1. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=0CF0QFjAH&url=http%3A%2F%2Fnptel.ac.in%2Fcourses%2F103107082%2Fmodule6%2Flecture5%2Flecture5.pdf&ei=AZ25U6TABIqhyASrlICAAg&usg=AFQjCNG_5vWWo1ZoBVsPb_ZcB7lBXi3i3A&sig2=XKP7yFZafdnQw2BTKUpb5A&bvm=bv.70138588,d.b2U

Catalytic cracking is a petroleum refining process that dates back to as early as 1915 however it came into prominence during the Second World War. There was a need for and improved process that would provide higher quality and quantity products than the brute force tactics used in thermal cracking. By applying newer, more advanced knowledge of chemistry and chemical reactions petroleum refining could produce better yields of gasoline with higher octane ratings than thermal cracking could ever hope to accomplish.

Catalytic cracking accomplishes this goal by cracking small ionic molecules, called carbocations, off of longer straight chain alkanes. These carbocations can then reattach to an alkane molecule to create an iso-alkane which has the required higher octane ratings needed in today’s society. Catalytic cracking, as noted before, runs on relatively long straight chain alkanes and therefore the feedstocks usually consist of, light cycle oils and potentially heavy gas oils or light vacuum gas oils¹. These types of oils have the size necessary to be able to be cracked while still forming the correct length range of alkane products. The primary goal of catalytic cracking is to increase the quantity of production of higher octane gasoline than could be done from straight run products or thermal cracking, however constituents such as kerosene, LPG, heating oil, and olefins are produced as well².

Catalytic hydrocracking, also known as hydrocracking, is a refinery process that was just added in the last thirty years with a main goal of enhancing catalytic cracking. The process of hydrocracking is typically completed in two main parts: first hydrotreatment must be conducted before the actual hydrocracking can take place. Catalytic hydrocracking can bring in feedstocks such as some atmospheric residue, heavy vacuum gas oil, light cycle oil, and potentially even deasphalted oil which all contain high concentrations of aromatic and heteroatom compounds³. Because these feeds contain such high concentrations, hydrogen must be added first, during hydrotreatment, in order to convert highly stable, unsaturated aromatic compounds to saturated aliphatic compounds. Once this process has taken place the newly formed cycloalkanes are cracked in the presence of more hydrogen to prevent coking. The cracking process, called hydrocracking, forms the necessary alkane molecules needed in catalytic cracking.

Catalytic hydrocracking is beneficial to catalytic cracking not only because it can produce some of its feedstock but also because it provides a way to remove heteroatoms such as nitrogen, sulfur, oxygen, and other metals before they enter the catalytic cracker. If these contaminants were to enter the catalytic cracking system they could potentially poison the catalysts which are needed to run reactions forming the iso-alkanes.

1. “Fluid catalytic cracking.” Wikipedia. Wikimedia Foundation, n.d. Web. 4 July 2014. <http://en.wikipedia.org/wiki/Fluid_catalytic_cracking#Flow_diagram_and_process_description>.
2. “Cracking.” Encyclopedia of Earth. N.p., n.d. Web. 4 July 2014. <http://www.eoearth.org/view/article/151525/>.
3. Meister, Jill , and Roger Lawrence. “Hydrocracking, Processing Heavy Feedstocks to Maximize High Quality Distillate Fuels.” UOP. N.p., n.d. Web. 4 July 2014. <http://www.uop.com/hydrocracking-processing-heavy-feedstocks-maximize-high-quality-distillate-fuels/>.

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.