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