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.
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.
By the end of this lesson, you should be able to:
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.
Readings | J. H. Gary, G. E. Handwerk, Mark J. Kaiser, Chapters 7 (Catalytic Hydrocracking) and Chapter 8 (Hydroprocessing and Resid Processing) |
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Assignments | Exercise 6 Quiz 3. Will cover material in Lessons 6 and 7. Check the Syllabus, or Course Calendar for Quiz 3 schedule. |
If you have any questions, please post them to our Help Discussion (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
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.
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 7.3 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.
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.
Hydrocarbons | Thermal Cracking | Catalytic Cracking |
---|---|---|
n-alkanes (e.g., C16) |
C2is major product C1in large quantities C4-C15olefins in moderate abundance |
C3-C6are major light prods C1in small quantities No olefins > C4 |
Aliphatic |
Little aromatization at 500ºC No branched – chain alkanes present |
Significant aromatization Abundance of branched – chain alkanes |
n-0lefins |
Slow double bond isomerization Little skeletal isomerization |
Rapid isomerization C=C–C–C→ C–C=C–C Rapid skeletal isomerization |
Alkylaromatics | ß – scission | |
Naphthenes | Crack more slowly than n-paraffins | Crack at comparable rates with n-paraffins |
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.
Carbenium Ions | ΔHf(relative) (kcal/mol) |
---|---|
C1+ primary |
19.1 |
C2+ secondary |
1.5 |
C3+ tertiary |
0 |
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.
Carbenium Ions | ΔHf(relative) (kcal/mol) |
---|---|
CH3⊕ | 258 |
C2H5⊕ | 225 |
n-C3H7⊕ | 218 |
i-C3H7⊕ | 198 |
n-C4H9⊕primary | 211 |
t-C4H9⊕ tertiary | 174 |
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].
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.
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
Houdry (1936) - a commercial process
Continuous feedstock flow with multiple fixed-bed reactors
Cracking/catalyst regeneration cycles
Catalyst: clays, natural alumina/silica particles
Thermafor Catalytic Cracking (TCC) (1942)
Continues feedstock flow with moving-bed catalysts
Catalyst: synthetic alumina/silica particles
Higher thermal efficiency by process integration
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.
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.
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 NOx 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:
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.
Product |
RON (600 rpm) |
MON (900 rpm) |
---|---|---|
Regular - Premium Gasoline | 90-100 | 80-90 |
Straight Run Gasoline | 60-68 | 60-68 |
FCC Gasoline (light) | 93 | 82 |
FCC Gasoline (heavy) | 95 | 85 |
Solve a problem on the material balance for the regenerator in Fluid Catalytic Cracking Process.
Burning the coke deposited on the catalyst particles generates all the heat necessary for catalytic cracking. Therefore, the coke burning rate is a critical parameter to control the rate of cracking. The composition of dry flue gas from the regenerator of an FCC unit is given in vol% as follows:
N2: 81.6
CO2:15.7
CO: 1.5
O2: 1.2
The dry air flow rate to the regenerator is given as 593 SCMM (standard cubic meters per minute). Considering that a significant portion of coke is carbon, calculate the carbon burning rate in the regenerator in kg/min. Remember: 1 kgmole at STP = 22.4 m3)
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.
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.
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.
Catalytic Cracking - FCC | Hydrocracking |
---|---|
-carbon rejection -endothermic -acid catalyst -more gas -more coke |
-hydrogen addition -exothermic -metal catalyst on acid support -less gas -less coke -costly process ($$$) |
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.
Please take a few minutes to answer the questions below. When you are satisfied with your responses, click Check My Answers to see how well you understood this lesson. These questions will help you study for the next quiz.
Exercise 6: Solve a problem on the material balance for the regenerator in Fluid Catalytic Cracking Process.
Quiz 3. Will cover material in Lessons 6 and 7. Check the Syllabus, or Course Calendar for Quiz 3 schedule.
Solve a problem on the material balance for the regenerator in Fluid Catalytic Cracking Process.
Burning the coke deposited on the catalyst particles generates all the heat necessary for catalytic cracking. Therefore, the coke burning rate is a critical parameter to control the rate of cracking. The composition of dry flue gas (excluding water) from the regenerator of an FCC unit is given in vol% as follows:
Gas | Volume % |
---|---|
N2 | 81.6 |
CO2 | 15.7 |
CO | 1.5 |
O2 | 1.2 |
The dry air (excluding moisture in the air) flow rate to the regenerator is given as 593 SCMM (standard cubic meters per minute). Considering that a significant portion of coke is carbon, calculate the carbon burning rate in the regenerator in kg/min. Remember: 1 kgmole at STP = 22.4 m3)
In previous offering of this course, I have noted a serious weakness in the students' understanding of how to carry out simple mass balances. Here a few reminders and hints to solve this problem:
Once you have a solution to the exercises, you will submit your answers as a PDF by uploading your file to be graded. The MS Word, or Excel files should be saved as a PDF before submitting the exercise. Please Note: Scans of handwritten pages are not acceptable.
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.
You should now be able to:
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.
Readings | J. H. Gary, G. E. Handwerk, Mark J. Kaiser, Chapters 7 (Catalytic Hydrocracking) and Chapter 8 (Hydroprocessing and Resid Processing |
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Assignments | Exercise 6: The dry air flow rate to the regenerator is given as 593 SCMM (standard cubic meters per minute). Considering that a significant portion of coke is carbon, calculate the carbon burning rate in the regenerator in kg/min. Remember: (1 kgmole at STP = 22.4 m3). Quiz 3. Will cover material in Lessons 6 and 7. Check the Syllabus, or Course Calendar for Quiz 3 schedule. |
If you have any questions, please post them to our Help Discussion (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.