Hydrocracking is essentially the combination of two processes: hydrogenation and cracking. Therefore, hydrocracking utilizes a bifunctional catalyst. The catalysts (highly active noble metals used for hydrogenation e.g. Pt and Pd) used in hydrocracking are very susceptible to poisoning and great care must be taken to remove sulfur from the feedstock. The process is typically accompanied by hydrotreating in order to remove heteroatom species (e.g. S, N and O). Additionally, while catalytic cracking is a carbon rejection process, hydrocracking is in a hydrogen addition process. Therefore, some complications from coking are avoided during the hydrocracking process. Several factors distinguish these two cracking processes; however, typically both processes are used in order to provide the most optimal yield of products in an economically efficient manner. The advantages of hydrocracking include its ability to handle a wide range of feedstock, as well as the selectivity of its distillates.

The primary objective of both cracking processes is to produce lighter saturated hydrocarbons with reduced molecular weights and boiling points from heavy oils. But due to the fact that aromatic rings cannot be cracked until they are fully saturated with hydrogen, the hydrocracking process allows for the processing of more aromatic feedstock, including the byproducts of catalytic cracking (e.g. light cycle oil). Furthermore, by modifying reactor configurations (e.g. fixed bed, ebullated bed, or expanded bed), catalysts, and hydrogen/carbon ratios, hydrocracking can be highly flexible, with the ability to process both relatively light feedstock as well as heavy vacuum residue into light and middle distillates.

Another difference between hydrocracking and catalytic cracking includes the change in enthalpy; while catalytic cracking is an endothermic process, hydrocracking is an exothermic process. The heat for catalytic cracking is supplied by the regeneration of catalysts. The evolution of the catalytic cracking (from Houdry to Thermafor to the modern fluid catalytic cracking) has continuously improved upon the thermal efficiencies of the process.

Although the hydrocracking process has several advantages over fluid catalytic cracking, hydrocracking is, in comparison, a more costly process. Therefore, it is not exclusively used, and refineries typically operate with both processes in order to produce the most desirable yield of products.

Following the development of a fixed- bed (Houdry process, 1936) and a moving-bed (Thermafor Catalytic Cracking, 1941) catalytic cracking process, fluid-bed catalytic cracking (FCC, 1942) became the most widely used process worldwide because of the improved thermal efficiency of the process and the high product selectivity achieved, particularly after the introduction of crystalline zeolites as catalysts in the 1960s.

Catalytic Cracking processes were developed during the Second World War and became widely used ever since because of their improved thermal efficiency and the high product selectivity to produce gasoline with a higher octane number. Also, catalysts and additives are very significant for the selectivity and flexibility of catalytic cracking; the introduction of zeolite catalysts in 1965 has had a huge impact on the industry processes.

In 1936, the first full scale industrial catalytic process was developed. It was known as the Houdry Catalytic cracking process, which used much less expensive catalysts, such as clay, natural alumina and silica particles. For this process, the gas oil feed stock must be heated to high temperatures and is fed to a fixed-bed reactor containing the catalyst particles. The product stream is sent to the separator to produce gas, gasoline, light cycle oil (LCO), and heavy cycle (HCO) products. A problem that faced this process was the deactivation of the catalyst bed because of coking. Thus, the process included swing reactors to switch between periodically to overcome this issue. After switching, the reactor is stripped with stream to remove the liquid products from the catalyst bed. The coke is then burnt off to reactivate the catalyst bed. A small percentage of the heat generated by burning off the coke could be used to supply heat for catalytic cracking, however the thermal efficiency of this reaction is considered low.

However, more efficient catalytic cracking processes have been developed. Such process include thermafor catalytic cracking and fluid catalytic cracking. Thermofor cracking utilizes a moving catalyst bed. In addition, catalyst particles used in this process are synthetic and thus have consistent and homogenous properties. This process is a slightly more thermal efficient process than the Houdry process. In comparison, the fluid catalytic cracking utilizes a fluidized catalyst bed. This catalytic cracking process is considered the most thermal efficient process. Also, this process enables the production of large quantities of light distillates known as crackate without the addition of hydrogen by burning off the coke.

Catalytic hydrocracking is relatively a recent addition to the petroleum industry. The main reasons for its development are the increasing demand for light and middle distillates, the large quantities of hydrogen as a by-product from catalytic reforming, and the limits imposed on sulfur and aromatics content in motor fuel. Hydrocracking is able to process a wide range of feedstocks. Its main process objective is to decrease the molecular weight and boiling point of the heavy oils from a mostly aromatic feedstock. Through the hydrocracking process, it is possible to convert an aromatic compound to a paraffin compound with out rejecting any carbon. The catalysts used for such reactions include platinum and palladium metals, however, care must be taken as they can be easily poisoned by sulfur.

Source:

Course Website

Wikipedia

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/>.