Video: FSC 432 Lesson 7 (5:00)

Overview

Catalytic conversion processes became important in petroleum refining after the Second World War. Catalytic cracking has been developed to produce high yields of gasoline with high octane # from high-boiling stocks using catalysts. As different from thermal cracking, catalytic cracking.

• uses a catalyst;
• takes place at lower temperature and lower pressure;
• is more selective and flexible.

One particular catalytic cracking process, Fluid Catalytic Cracking (FCC), has captured universal acceptance in the refining industry because of its feed flexibility, ability to modify product yields through minor changes in the process operating conditions. FCC is used to produce high-octane gasoline mainly from straight-run atmospheric gas oil and light vacuum gas oil (LVGO) [1]. This process involves breaking up long chains of n-alkanes into shorter chains of branched alkanes (isoalkanes), cycloalkanes (naphthenes), and aromatics by using acidic catalysts. In addition to high-octane gasoline, catalytic cracking produces LPG, cycle oils, and olefin-rich light hydrocarbons (C3, C4). The olefins are used as petrochemical feedstocks, or as reactants in alkylation and polymerization reactions, to produce higher molecular weight branched alkanes and olefins to contribute to the high-octane gasoline pool.

Hydrocracking processes have been introduced for upgrading heavier crude oil fractions such as heavy vacuum gas oil (HVGO) and vacuum distillation residue VDR. The heaviest fractions of crude oil, HVGO and VDR, may not be easily processed by FCC because of potential problems with excessive coking on the catalysts. For upgrading these high-boiling and aromatic-rich feedstocks, hydrogen is introduced in the hydrocracking process, along with bi-functional catalysts systems, to keep coking under control while upgrading the heavy fractions to light and middle distillates.

Learning Outcomes

By the end of this lesson, you should be able to:

• distinguish the chemistry of catalytic cracking from chemistry of thermal cracking and illustrate the formation of carbocations and IUPAC terminology for classification of carbocations;
• categorize the formation of different carbocations on active sites of cracking catalysts and assess the classification of acid sites (Lewis vs Bronsted) on catalyst surfaces;
• compare, with examples, how the product yields and composition obtained from catalytic differ from those from thermal cracking;
• analyze the thermodynamics of carbocation formation and evaluate how ionic chain reactions produce hydrocarbons with high octane numbers;
• appraise the historical evolution of catalytic cracking processes and formulate the driving forces that have shaped this evolution in reactor design and catalyst development;
• locate the hydrocracking process and hydroprocessing in the refinery flow diagram, illustrate hydrocracking processes and evaluate different process objectives.

What is due for Lesson 7?

This lesson will take us less than one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignments below can be found on the Assignments page within this lesson.

As opposed to thermal cracking governed by free radicals, catalytic cracking proceeds through the formation of ionic species on catalyst surfaces, and produces shorter, but branched-chain (not straight-chain) alkanes by cracking the long straight-chain alkanes. The formation of branched-chain alkanes, or iso-alkanes, leads to the production of gasoline with high octane numbers. This is the fundamental reason why catalytic cracking has replaced thermal cracking as the central process in a refinery geared to maximize gasoline production. A high octane number of gasoline is needed for current spark-ignition engines to run at high compression ratios without knocking. High compression ratios in spark-ignition engines translate to high power and high efficiency.

Figure 7.1 introduces the two types of ionic species, carbocations, that are active in catalytic cracking reactions as carbenium, and carbonium ions, using the IUPAC terminology. Carbocations are the positively charged ions made from hydrocarbons. Figure 7.1 shows that removing a hydride ion (H^-, a hydrogen atom with an additional electron) from an alkane (e.g., methane) produces carbenium ions (path 1a). Also, adding a proton (H⁺, a hydrogen atom without the electron) to an olefin (e.g., ethylene) can produce carbenium ions, as shown in path 1b.

Figure 7.1. Definition and formation of carbocations from hydrocarbons.

Click here for a text alternative to Figure 7.1 above

Formation of Carbocations

IUPAC terminology

-Carbenium ions: CH₄ (arrow) H^- + CH3⁺ (alkane – hydride ion)

-C₂H₄ + H⁺ (arrow) C₂H₅⁺. (olefin + proton)

-Carbonium Ions: CH₄ + H⁺ (arrow) CH4

KEY: H^- (hydride ion), H⁺ (proton)

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Analogous to the terminology used for free radicals, C⁺H₃ is called methyl carbenium ion, and C₂⁺H₅ is called an ethyl carbenium ion. Carbonium ions are produced by adding a proton to an alkane, say methane, as shown in Figure 7.1. The resulting ion C⁺H₅ is called methanium. Note that there is some confusion in the literature about naming the carbocations. Carbenium ions used to be called carbonium ions in some sources, including your textbook [2]. All references to carbonium ions in Section 6.3 Cracking Reactions in the textbook should be corrected as carbenium ions.

Carbocations are formed from hydrocarbons on two different acid sites: Bronsted acid sites and Lewis acid sites. You should remember that Bronsted acid sites donate protons, while Lewis acid sites accept electrons to form carbocations from hydrocarbons. Figure 7.2 illustrates how an olefin (e.g., ethylene, C=C) produces an ethyl carbenium ion (C⁺₂H₅) by reacting with a proton donated from Bronsted acid site. Alternatively, also seen in Figure 7.2, a Lewis acid site accepts an electron (or a hydride ion, H^-) from an alkane (e.g., ethane, C-C) to produce the same ethyl carbenium ion (C⁺₂H₅). These two reactions that take place on the acid sites of catalysts, along with the formation of carbonium ions by protonation of hydrocarbons on Bronsted sites, function as the initiation steps in the ionic chain reactions that lead to the products obtained from catalytic cracking.

Figure 7.2. Formation of carbocations on different acid sites of catalysts.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Figure 7.3 compares the distribution of products from thermal cracking (free radical chain reactions) and catalytic cracking (ionic chain reactions). Short chain paraffins constitute the principal products in both cases, with one important difference – an abundance of iso-alkanes (branched-chain alkanes) in catalytic cracking products. One can also note in Figure 7.3 that catalytic cracking products contain higher concentration of aromatic compounds. High octane number of gasoline produced by catalytic cracking can be attributed to high concentrations of i-alkanes and relatively more abundant aromatics present in the crackate (catalytic cracking product). Having no olefins larger than butylene (C₄) from catalytic cracking processes, also distinguishes catalytic cracking products from thermal cracking products obtained from gas oil.

Figure 7.3. Differences in the distribution of hydrocarbons obtained from thermal and catalytic cracking reactions.

Click here for an alternative text description of Figure 7.3 above

Products of Thermal and Catalytic Cracking

Thermal Cracking – Free radical reactions

P araffins

O lefins

N aphthenes

A romatics

Catalytic Cracking – Carbocation reaction

High-octane gasoline because of branching in alkanes

P araffins

I -paraffins

A romatics

N aphthenes

O lefins

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Table 7.1 compares the products of thermal cracking and catalytic cracking of different type of hydrocarbons. Notably, high yields of C₁ and C₂ gaseous products (methane, ethane, and ethylene) from thermal cracking are contrasted with high yields of C₃- C₆, with small quantities of methane and essentially no olefins heavier than butylene, from catalytic cracking. Significant for the octane number of the gasoline fraction from the catalytic cracking of aliphatic hydrocarbons are the abundance of i-alkanes and significant concentration of aromatic compounds (BTX) that increase the octane number.

As discussed in Lesson 6, the slow isomerization of free radicals (moving the unpaired electron from an edge atom to the interior atoms) results in the production of shorter straight-chain alkanes and straight-chain olefins in thermal cracking, thus leading to low octane numbers of the gasoline product. In contrast to free radicals, the isomerization of carbocations is very fast because of the thermodynamic driving force, shown in Table 7.2. One can see in Table 7.2 that the isomerization of a primary propyl carbenium ion to a secondary propyl carbenium ion releases (19.1- 1.5) = 17.6 kcal/mol. This is a very large thermodynamic driving force for the isomerization of a primary ion to a secondary ion, and further to a tertiary ion, with even a larger driving force. Isomerization of the secondary propyl ion to the tertiary propyl ion, releases 1.5 kcal/mol of energy. It is, therefore, clear that the initiation and propagation of carbocations in catalytic cracking chain reactions on the catalyst surfaces will be dominated by the formation of secondary and tertiary carbocations. The reactions of these carbocations lead to the formation of branched-chain alkanes and olefins with high octane numbers.

Another important feature of carbocation formation is the differences in the enthalpy of formation which favors the formation of carbocations > C₃ versus C₁ and C₂ ions (Table 7.3), because C₁ and C₂ ions are primary ions. This explains the low yields of C₁ and C₂ species obtained from catalytic cracking.

Figure 7.4 illustrates the ionic chain reactions that govern catalytic cracking of hydrocarbons. The initiation step includes the formation of a carbonium ion by proton donation from a Bronsted acid site and/or the formation of a carbenium ion through hydride ion abstraction by a Lewis acid site. In a propagation step, the carbonium ion goes through cracking to produce an alkane product and a carbenium ion, while the carbenium ion produced on the Lewis acid site goes through a β-scission to produce an olefin product and another carbenium ion. In additional propagation reactions, carbenium ions (secondary, or tertiary) react with alkanes to produce i-alkane products and other carbenium ions, which can go through isomerization reactions generating more stable ions. Finally, in termination steps, carbenium ions donate a proton to restore a Bronsted acid site and produce an olefin as final product, or they abstract a hydride ion to restore a Lewis acid site producing an i-alkane product, and the ionic chain reaction continues. Other reactions during catalytic cracking include dehydrocyclization and dehydrogenation reactions to produce aromatic compounds. One should note that thermal cracking reactions also take place during catalytic cracking because of the sufficiently high temperatures used in the process. Some claim that initial thermal cracking of alkanes to produce olefins should also be considered as an initiation step in ionic chain reactions [2].

Figure 7.4. Carbocationic chain reactions in catalytic cracking.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Increasing demand for gasoline, along with the need to produce high-octane gasoline for increasingly more powerful spark ignition engines, led to the development and maturation of catalytic cracking processes just before and during World War II. 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.

The list below shows a timeline for the development of the catalytic cracking processes. The evolution of catalytic cracking processes is an exemplary showcase in chemical engineering for discussing the advancement of reactor configuration, driven by energy conservation and process kinetics. The evolution of these processes is discussed in the following subsections.

Historical Time-Line for Catalytic Cracking Processes

1. McAfee (1915)
   • Batch reactor catalytic cracking to produce light distillates
   • Catalyst: A1Cl3 – A Lewis acid, electron acceptor
   • Alkane – electron(abstracted by A1Cl3)→ a carbocation⁽⁺⁾→ ionic chain reactions to crack long chains
2. Houdry (1936) - a commercial process
   • Continuous feedstock flow with multiple fixed-bed reactors
   • Cracking/catalyst regeneration cycles
   • Catalyst: clays, natural alumina/silica particles
3. Thermafor Catalytic Cracking (TCC) (1942)
   • Continues feedstock flow with moving-bed catalysts
   • Catalyst: synthetic alumina/silica particles
   • Higher thermal efficiency by process integration
4. Fluid Catalytic Cracking (FCC) (1942)
   • Continuous feedstock flow with fluidized-bed catalysts
   • Catalyst: synthetic alumina/silica+zeolites (1965)

The first catalytic cracking process was developed as a batch process (McAfee, 1915) shortly after the development of a thermal cracking process. The process used Lewis acid catalysts (e.g., AlCl₃) for cracking. These catalysts were expensive and corrosive. In addition to these impediments, use of a batch reactor in the McAfee process did not allow large-scale commercialization of this process. The first full-scale commercial process, the Houdry Catalytic Cracking, used much less expensive catalysts, such as clays, and natural alumina and silica particles. Figure 7.5 shows the configuration of the Houdry Catalytic Cracking process. For cracking, gas oil feed was heated to 800°F and fed to a fixed-bed reactor packed with the catalyst particles. Cracking products are sent to a fractionator to be separated into gas, gasoline, light cycle oil (LCO) and heavy cycle oil (HCO) products.

Video: Houdry Catalytic Cracking (5:02)

Text description of the Houdry Catalytic Cracking image.

Long Description (Houdry Catalytic Cracking Process Diagram)

This diagram illustrates the flow and operation of the Houdry catalytic cracking process, an early continuous commercial method for converting gas oil into lighter hydrocarbon products using fixed-bed reactors.

At a high level, the process shows feed preparation, reaction in multiple reactors, product separation, and catalyst regeneration occurring in a cyclic sequence across several vessels.

Overall process flow (left to right):
Gas oil feed is first heated in a furnace to approximately 800°F. The heated feed then flows into one of several vertical fixed-bed reactors packed with alumina/silica catalyst pellets. Cracked hydrocarbon products exit the reactor system and are sent to a fractionator, where they are separated into gas, gasoline, light cycle oil (LCO), and heavy oil fractions.

Feed and heating section (left side):

• A heater unit is shown at the lower left, labeled “Heater, T = 800°F.”
• Gas oil enters the heater and is raised to reaction temperature before being directed into the reactor system.

Reactor section (center):

• Three tall, vertical reactors are shown in parallel, representing “swing reactors.”
• These reactors operate alternately rather than simultaneously on feed.
• Arrows indicate that feed enters one reactor at a time, while others are in different stages of the cycle.
• Inside each reactor, catalyst beds facilitate cracking reactions that break large hydrocarbons into smaller molecules.

Reaction and deactivation:

• During operation, coke deposits form rapidly on the catalyst surface.
• The diagram notes “coke build-up on catalysts in 10 min,” indicating that each reactor can only operate for about 10 minutes before deactivation.

Steam stripping stage:

• After a reactor is taken offline, it is stripped with steam for approximately 5 minutes.
• This removes adsorbed hydrocarbons from the catalyst before regeneration.

Catalyst regeneration stage:

• Hot air is introduced into the spent reactor to burn off coke deposits.
• This regeneration step also takes about 5 minutes.
• The diagram labels this as “Hot air to regenerate catalyst after steam stripping.”
• Combustion produces flue gases, shown exiting the system.

Cyclic (swing) operation:

• The three reactors operate in rotation: one reactor is cracking feed, another is being steam-stripped, and another is undergoing regeneration.
• This staggered cycle allows continuous processing despite the short active life of each catalyst bed.

Product separation (right side):

• Cracked products from the active reactor are routed to a vertical fractionator column.
• The fractionator separates the mixture into:
  • Gas (top outlet)
  • Gasoline (upper side outlet)
  • Light cycle oil (LCO) (mid outlet)
  • Heavy oil (bottom outlet)

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

A series of swing reactors were needed to switch the feed flow from one reactor to another after approximately 10 minutes of operation. The switch to a swing reactor was necessary because of rapid coking on catalysts which, being natural materials, had a wide range of activity. Rapid coking on silica/alumina particles deactivated these catalysts and led to plugging of the reactors. After the flow was switched to another reactor, the isolated reactor was stripped with steam for five minutes to remove the liquid products adsorbed on catalyst particles. After stripping with steam, the deactivated catalysts were regenerated by burning off the coke on catalysts with hot air introduced to the reactor. Catalyst regeneration also takes approximately 5 minutes before the reactor with regenerated catalyst is ready to accept the feed again. By this time, the second reactor would be ready for the 10-minute cycle of steam stripping and catalyst regeneration. Having a third reactor in the plant would help deal with any delays/problems in reactor preparation. Considering that catalytic cracking is an endothermic process, the heat generated from burning the coke off the catalyst could be used partially to heat the catalyst particles for the endothermic reaction. A large portion of the heat in the flue gases from coke combustion was not available for the process. Therefore, the thermal efficiency of the Houdry Process was low.

Thermafor (also referred to as “thermofor” in some sources) Cracking Process was introduced for better integration of thermochemistry (endothermic cracking and exothermic catalyst regeneration) by introducing a moving-bed configuration, rather than a fixed-bed, as shown in Figure 7.6. Catalysts used in this process were synthetic alumina/silica beads that have more homogeneous and consistent properties (e.g., activity) than the natural minerals. Catalysts particles and the feed are introduced from the top of the reactor, and the catalyst particles move downward with gravity as the cracking reactions take place on the catalyst surfaces. Steam is injected from the bottom of the reactor to carry the cracking products to the fractionator for recovery. As the particles move down the reactor, they are deactivated by coke build-up on active sites. The deactivated catalysts removed from the bottom of the reactor are sent to a regenerator unit where the coke on catalysts surfaces are burned off and the heated catalysts particles are recycled to the top of the reactors by bucket elevators. Hot catalyst particles provide most of the heat necessary for the cracking reactions in the reactor. Although the thermal efficiency of TCC is higher than that of the Houdry process, there was still a significant amount of heat loss during the transport of heated catalyst particles by bucket elevators.

Video: Thermafor (3:53)

Text description of the Thermafor Catalytic Cracking (TCC) image.

This diagram illustrates the flow and operation of the Thermafor Catalytic Cracking (TCC) process, introduced in 1942, showing a moving-bed system where catalyst particles continuously circulate between a reactor and a regenerator to improve thermal efficiency.

At a high level, the process shows feed entering a reactor with a downward-moving bed of catalyst particles, products being removed to a fractionator, and deactivated catalyst being regenerated and recycled back to the reactor using bucket elevators.

Process flow:
Gas oil feed, labeled “feed,” enters at the top of a vertical reactor. Inside the reactor, catalyst particles move downward by gravity while cracking reactions occur. Steam, labeled “steam,” enters near the bottom and carries cracked products out through a line labeled “Products to fractionator.” Spent catalyst exits the bottom, is regenerated with hot air, and is returned to the top of the reactor via a circulation loop labeled “Catalyst particles” and “Bucket elevators.”

Reactor section (left side):

• A tall vertical reactor vessel contains a packed moving bed of catalyst particles.
• The inlet at the top is labeled “feed,” indicating where gas oil enters.
• Catalyst particles also enter at the top and move downward through the reactor.
• As the feed contacts the catalyst, cracking reactions occur along the catalyst surfaces.

Product removal and steam stripping:

• A side inlet near the bottom labeled “steam” introduces steam into the reactor.
• The steam flows upward, carrying vaporized cracking products out of the reactor. 
• These products exit through a side outlet labeled “Products to fractionator,” leading to downstream separation.

Catalyst deactivation and removal:

• As catalyst particles move downward, coke builds up on their surfaces, reducing activity.
• The deactivated catalyst exits the bottom of the reactor and flows into a vessel below.

Regenerator section (bottom left):

• The lower vessel acts as a regenerator. 
• A line labeled “Hot air” enters this unit, where coke deposits are burned off the catalyst.
• This regeneration step restores catalyst activity and heats the particles.

Catalyst circulation loop (right side):

• A vertical loop labeled “Catalyst particles” shows the path of regenerated catalyst being recycled.
• Within this loop, a transport system labeled “Bucket elevators” lifts catalyst particles upward.
• Arrows indicate the upward movement of hot, regenerated catalyst back to the top of the reactor.

The diagram emphasizes continuous operation through a moving-bed design in which catalyst particles flow downward during reaction, are regenerated with hot air, and are mechanically lifted back to the reactor. The circulating hot catalyst supplies heat for the endothermic cracking reactions, improving efficiency compared to earlier fixed-bed processes.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Fluid Catalytic Process, also introduced in 1942, offered an excellent integration of the cracking reactor and the catalyst regenerator that provides the highest thermal efficiency, as shown in Figure 7.7. In FCC, a fluidized-bed (or fluid-bed) of catalyst particles is brought into contact with the gas oil feed along with injected steam at the entrance (called the riser) of the reactor. The hot catalyst particles coming from the regenerator unit evaporate the feed gas oil upon contact in the riser, and the cracking starts as the gas oil vapors and the catalyst particles move upward in the reactor. The temperature of the catalyst particles drops as the evaporation of gas oil and endothermic cracking reactions proceed during the upward movement. Cracking reactions also deposit a significant amount of coke on the catalysts, leading to the deactivation of the catalyst. After removing the adsorbed hydrocarbons by steam stripping, the coked catalyst is sent to the regeneration unit to burn off the coke with air. Heat released from burning the coke deposit increases the temperature of the catalyst particles that are returned to the riser to complete the cycle. Burning off the rejected carbon (coke) in the regenerator provides the energy necessary for cracking without much loss, thus increasing the thermal efficiency of the process. The cracking products are sent to the fractionator for recovery after they are separated from the catalyst particles in the upper section of the reactor [3].

In the reactor, the cracking reactions initiate on the active sites of the catalysts with the formation of carbocations and the subsequent ionic chain reactions produce branched alkanes and aromatic compounds to constitute the crackate (cracked gasoline with high octane number), light olefins, cycle oils, and slurry oil that are sent to the fractionator. A carbon-rich byproduct of catalytic cracking, termed “coke,” deposits on catalyst surfaces and blocks the active sites. FCC is considered a carbon rejection process because the coke deposited on the catalyst surface and eventually burned off for heat is rich in carbon and thus enables the production of large quantities of a light distillate (crackate) in the process without the addition of hydrogen.

Two different configurations of the commercial FCC processes exist depending on the positions of the reactor and the regenerator: they can be side by side or stacked, where the reactor is mounted on top of the regenerator. Major licensor companies that offer FCC processes with different configurations include Kellogg Brown & Root, CB&I Lummus, ExxonMobil Research and Engineering, Shell Global Solutions International, Stone & Webster Engineering Corporation, Institut Francais du Petrole (IFP), and UOP. Figure 7.8 shows examples of Exxon and UOP designs [1,4]. The UOP design of high-efficiency two-stage regenerator units offer advantages of uniform coke burn, higher conversion of CO to CO₂ and lower NO_x emissions among others. Another modification to FCC plants could be the installation of a catalyst cooler, which may provide better control of the catalyst/oil ratio; the ability to optimize the FCC operating conditions, increase conversions, and process heavier residual feedstocks; and better catalyst activity and catalyst maintenance [3].

In the first video below, the animation of an explosion in an FCC unit in 2015 (7:12 minute long) provides a good review of the FCC process, and points out the potential hazards of working with hydrocarbons exposed to high temperatures in refinery units:

Video: 2015 Explosion at ExxonMobil Refinery (7:12)

Video: Fluid Catalytic Cracking (5:08)

Text description of the Fluid Catalytic Cracking (FCC) process image.

This diagram illustrates the flow and operation of the Fluid Catalytic Cracking (FCC) process, introduced in 1942, showing a highly integrated system where a fluidized catalyst circulates continuously between a reactor and a regenerator to achieve high thermal efficiency.

At a high level, the process shows feed entering a reactor riser where it contacts hot regenerated catalyst, cracked products being separated in a fractionator, and spent catalyst being regenerated and recycled in a continuous loop.

Process flow (left to right):
Feed, labeled “Feed (300°F),” enters near the bottom of a vertical reactor section labeled “Reactor.” It contacts hot catalyst coming from a regenerator. The mixture flows upward through a vertical tube labeled “riser (1000°F),” where cracking occurs. The resulting products exit through a line labeled “Products” and are sent to a column labeled “Fractionator,” where they are separated into “Gas,” “Gasoline,” “LCO,” “HCO,” and “Decant oil.” Spent catalyst, labeled “coked catalyst,” is sent to a unit labeled “Regenerator (1300–1400°F),” where it is regenerated with “air,” and then returned as “regenerated catalyst” to the reactor.

Reactor and riser section (center-right):

• The reactor is labeled “Reactor,” with an inlet labeled “Feed (300°F)” near the bottom.
• Steam, labeled “steam,” is also introduced near the base to assist fluidization and transport.
• Hot catalyst enters from the regenerator and mixes with the feed.
• The mixture flows upward through a vertical tube labeled “riser (1000°F),” where rapid cracking reactions occur as the feed vaporizes and contacts the catalyst.

Product separation:

• The cracked hydrocarbon vapors exit the riser through a line labeled “Products.”
• These products enter a tall vertical column labeled “Fractionator.”
• The fractionator separates the mixture into multiple streams labeled along the column: “Gas” at the top, followed by “Gasoline,” “LCO,” “HCO,” and “Decant oil” at lower خروج points.

Catalyst deactivation and transfer:

• As cracking occurs, coke deposits form on the catalyst, producing a stream labeled “coked catalyst.”
• This spent catalyst flows from the reactor area to the regenerator.

Regenerator section (left side):

• The regenerator is labeled “Regenerator (1300–1400°F).”
• A line labeled “air” enters at the bottom of the regenerator, where coke is burned off the catalyst.
• This combustion restores catalyst activity and heats the catalyst to a high temperature.

Flue gas and heat recovery:

• Flue gases exit the regenerator through a line labeled “Flue gas.”
• These gases are sent to a unit labeled “CO Boiler,” where additional heat recovery or combustion occurs.

Catalyst circulation loop:

• Regenerated catalyst, labeled “regenerated catalyst,” flows from the regenerator back to the reactor.
• Arrows indicate this continuous circulation between reactor and regenerator.
• The diagram also highlights “heat” transfer from the regenerator to the reactor via the hot catalyst.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Figure 7.8. Different configurations of the FCC Units [4].

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0Source: Dr. Semih Eser

One of the significant developments in FCC practice was the introduction of zeolite catalysts in 1965. Catalysts and additives play a major role in the selectivity and flexibility of FCC processes. FCC catalyst consists of a fine powder with an average particle size of 60–75 μm and a size distribution ranging from 20 to 120 μm. Four major components make up the catalysts: zeolite, active matrix, filler, and binder. Each of these constituents has a unique role to play, but zeolite is the key component that is more active and selective for high-octane number gasoline production [4]. Table 7.4 compares the octane numbers of some refinery products and FCC gasoline.

Figure 7.9. Components of FCC catalyst [4].

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Catalytic hydrocracking is one of the latest additions to petroleum refining processes, with the first modern commercial unit started up by Chevron in 1958. The interest in hydrocracking has been attributed to the increasing demand for light and middle distillates, the availability of byproduct hydrogen in large quantities from catalytic reforming, and the environmental regulations limiting sulfur and aromatic hydrocarbons in motor fuels [5]. The advantages of hydrocracking include its ability to handle a wide range of feedstocks that may be difficult to process by catalytic cracking and its flexibility in selectivity between light and middle distillates. The principal objective of hydrocracking is to decrease the molecular weight and boiling point of heavy oils to produce saturated hydrocarbons (diesel and jet fuel) from highly aromatic feedstocks (e.g., LCO from FCC) and distillation residua.

The hydrocracking process has two dimensions: Hydrogenation of aromatic rings and cracking of aliphatic compounds, as shown in Figure 7.10, using naphthalene as an example for an aromatic ring system. One should note that the aromatic rings cannot be cracked before they are saturated with hydrogen. With hydrocracking, it is possible to convert an aromatic compound to a paraffinic compound without any loss of carbon, as shown in Figure 7.10. As a hydrogen-addition process, hydrocracking provides high yields of valuable distillates without producing low-grade byproducts (e.g., heavy oils, gas, or coke) as experienced in carbon rejection processes such as coking.

Figure 7.10. Reactions of hydrocracking.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

The two different reaction systems in hydrocracking, hydrogenation and cracking, are supported by bifunctional catalyst formulations, as illustrated in Figure 7.11. Hydrogenation reactions are promoted by the metal component of the catalysts (e.g., Ni, Co, Mo), and the cracking takes place on catalyst support consisting of silica/alumina. Highly active noble metals (e.g., Pt, and Pd) can be used for hydrogenation of hydrocarbons with extremely low sulfur contents as the noble metals are susceptible to sulfur poisoning.

Figure 7.11. Functions of hydrocracking catalysts.

Click here for a text alternative for Figure 7.11 above

Hydrocracking

• Hydrocracking has a dual function (bifunctional) catalyst
  • Metals + acid supports to perform two functions

Hydrocracking Catalysts

• Cracking (acid Support
  • SiO₂ – Al₂O₃
  • Al₂O₃
  • Low zeolite SiO₂ – Al₂O₃
  • High zeolite SiO₂ – Al₂O₃
• Hydrogenation
  • Noble – Pt, Pd
    • Susceptible to S poisoning
  • Non-noble – Ni, Mo, Co, W
    • Sulfided forms (M_xS_y) for high sulfur feeds

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Hydrocracking processes most commonly include two reaction stages: Hydrotreating to remove heteroatom (S, N, O) species and Hydrocracking to increase the H/C ratio of the hydrocarbons in the feeds by hydrogenation and to decrease their molecular weight by cracking. In most cases, the hydrotreating reactor (HT) packed with cobalt-molybdenum catalysts precedes the hydrocracking (HC) reactor typically packed with nickel-tungsten catalysts (for hydrogenation) supported on alumina/silica (for cracking). Figure 7.12 shows different configurations of hydrocracking processes, depending on the heteroatom content of the feeds. For feeds with very low heteroatom contents, hydrocracking without hydrotreating may be applied, but this is very rare. Other process configurations include single stage with dual catalysts, two-stage dual and triple catalysts, as shown in Figure 7.11. The hydrocracking reactions are performed at 300–400°C and 8–15 MPa of hydrogen pressure.

In a refinery, hydrocracking complements catalytic cracking by taking on the more aromatic feedstocks that resist cracking, including the byproducts of FCC, such as light cycle oil (LCO). Hydrocracking can also be used to upgrade residual fractions using different reactor configurations and catalysts depending on the complexity of the upgrading tasks, as shown in Figure 7.12. As shown in Figure 7.13, for hydrocracking a relatively light feedstock (e.g., atmospheric residue), a fixed-bed configuration and relatively large-size catalyst particles can be used. In extreme cases with very heavy vacuum residue, an expanded bed configuration is used, in which very fine catalyst particles are entrained in the feed at high hydrogen pressures (high hydrogen/oil ratio). These extreme reaction conditions are necessary to prevent extensive coking on catalysts that could shut down the process. For intermediate cases, an ebullated (fluidized) bed configuration can be used, as shown in Figure 7.13.

In the United States, hydrocracking of LCO (from FCC) provides a large proportion of the diesel fuel production because straight-run LGO is a preferred stock for FCC to produce gasoline as the principal product. The major licensors of hydrocracking processes include Chevron, UOP, ExxonMobil Research and Engineering, BP, Shell, and BASF-IFP.

Figure 7.12. Configuration of hydrocracking process with respect to feed properties.

Click here for a text alternative to Figure 7.12 above

Hydrocracking Process

• Single Stage
  • Hydrocracking only
    • single catalyst
    • low S, N feeds
    • Hydrotreatment, Hydrocracking
  • dual catalyst
    • high S&N
    • can be done with or without recycling
• Two-Stage (separation of the products between stages 1 & 2)
  • HT & HC
    • dual catalyst
    • very high S + N feeds
  • HT & HC
    • triple catalyst
    • extremely high S, N, metal feed

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Figure 7.13. Hydrocracking of heavy residua.

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Residue Hydrocracking

• increasing H/C for upgrading
• atmospheric or vacuum distillation reside

Hydrocracking of heavy residua

Bed Type	Cat. Size	Particles/cm3
Fixed	1.5 - 3mm	120
Ebullated	0.8 - 3mm	250
Expanded	0.002mm	2.4x109

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Hydrocracking vs. Catalytic Cracking

• Hydrocracking involves C-C bond cleavage to produce lighter HC's
• More liquid yield with HYDRCRC
• More hydrogenated products
  • Cracking is less severe
  • Secondary cracking reactions are minimized by stabilization of active species by hydrogen
• Hydrocracking is more flexible w.r.t. the feedstock
  • Refractory feedstocks (i.e. aromatics) can be processed
  • C-C bond breaking after saturation with hydrogen

Above, we compare catalytic cracking (FCC - a carbon rejection process) with hydrocracking (HYDRCRC) with respect to the major attributes of both projects. Clearly, in a flexible refinery with a wide range of crude oil feedstocks, both processes are needed for the optimum conversion of the crude oil into desirable refinery products.

Take a few minutes to answer the questions below. They will help you study for this week's quiz. When you are ready, click Check to see the solution. Use the arrow to go to the next question. 

Summary

Catalytic processes constitute the core of the petroleum refineries to accomplish a number of conversion and finishing tasks. Catalytic cracking has been developed to produce high yields of gasoline with high octane # from high-boiling stocks using catalysts. Compared to thermal cracking, catalytic cracking takes place at lower temperatures and pressures and proceeds through carbocationic active species produced on acidic sites on catalyst surfaces. Fluid Catalytic Cracking (FCC) has become a universal refining process because of its high efficiency and feed flexibility. This process involves breaking up long chains of n-alkanes into shorter chains of branched alkanes (isoalkanes), cycloalkanes (naphthenes), and aromatics in high yields. Although the main product from FCC is high-octane number gasoline, it also produces LPG, cycle oils, and olefin-rich light hydrocarbons (C3, C4). The olefins are used as petrochemical feedstocks, or as reactants in alkylation and polymerization reactions, to produce higher molecular weight branched alkanes and olefins to contribute to the high-octane gasoline pool. Hydrocracking processes have been introduced for upgrading heavier crude oil fractions such as heavy vacuum gas oil (HVGO) and vacuum distillation residue VDR. The heaviest fractions of crude oil, HVGO and VDR, may not be easily processed by FCC because of potential problems with excessive coking on the catalysts. For upgrading these high-boiling and aromatic-rich feedstocks, hydrogen is introduced in the hydrocracking process, along with bifunctional catalysts systems, to keep coking under control while upgrading the heavy fractions to light and middle distillates.

Learning Outcomes

You should now be able to:

• distinguish the chemistry of catalytic cracking from the chemistry of thermal cracking and illustrate the formation of carbocations and IUPAC terminology for classification of carbocations;
• categorize the formation of different carbocations on active sites of cracking catalysts and assess the classification of acid sites (Lewis vs Bronsted) on catalyst surfaces;
• compare with examples how the product yields and composition obtained from catalytic processes differ from those from thermal cracking processes;
• analyze the thermodynamics of carbocation formation and evaluate how ionic chain reactions produce hydrocarbons with high octane numbers;
• appraise the historical evolution of catalytic cracking processes and formulate the driving forces that have shaped this evolution in reactor design and catalyst development;
• locate the hydrocracking process and hydroprocessing in the refinery flow diagram, illustrate hydrocracking processes, and evaluate different process objectives.

Reminder - Complete all of the Lesson 7 tasks!

You have reached the end of Lesson 7! Double-check the to-do list below to make sure you have completed all of the activities listed there before you begin Lesson 8. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignments below can be found within this lesson.