Catalytic cracking

Date: March 28, 2021.

IUTT University.
Supervised by: Ali Abbas.

Fluid catalytic cracking (FCC) is the most important conversion process used in petroleum
refineries. It is widely used to convert the high-boiling hydrocarbon fractions of petroleum crude
oils into more valuable gasoline, olefinic gases and other products. Cracking of petroleum
hydrocarbons for conversion of heavy fractions into lighter fractions was originally done by
thermal cracking which has been almost completely replaced by fluid catalytic cracking because it
produces more gasoline with a higher octane rating. It also produces byproduct gases that are more
olefinic, and hence more valuable, than those produced by thermal cracking.

The feedstock to an FCC is usually that portion of the crude oil that has an initial boiling point of
340 °Celsius (C) or higher at atmospheric pressure and an average molecular weight ranging from
about 200 to 600 or higher. The FCC process vaporizes and breaks the long-chain molecules of the
high-boiling hydrocarbon liquids into much shorter molecules by contacting the feedstock, at high
temperature and moderate pressure, with a fluidized powdered catalyst.
A catalyst is a substance added to a chemical reaction that facilitates or accelerates the reaction;
but when the reaction is complete, the catalyst comes out just like it went in. In other words, the
catalyst does not change chemically. It promotes reactions between other chemicals. The feed to
the cat cracking process is usually straight run heavy gas oil and flasher tops. The boiling range cat
feed can be anywhere in the 650 F to 1,100 F range. Heat is required to make the process go;
temperatures in the cat cracker where the cracking takes place can be about 900‫؛‬F or higher, even
approaching 1,100‫؛‬F depending on the catalyst and the results desired. The heat can partly come
from preheating the feed in a heat exchanger and always from the hot catalyst that has just gone
through a heat-generating chemical reaction to clean it up. The process is designed to promote
cracking in a specific way.

The object is to convert the heavy cuts to gasoline. Ideally, all the product coming out of the cat
cracker would be in the gasoline range, but the technology is not that good. During the cracking
process, lots of phenomena occur:
As the large molecules crack, there is not enough hydrogen to go around, so some small amounts
of carbon form coke, which is virtually pure carbon atoms stuck together. As the large molecules
break up, a full range of smaller molecules from methane on up are formed. Due to the deficiency
of hydrogen, many of the molecules are olefins (those double-bonded paraffins). Where the large
molecules in the feed are made up of several aromatic or naphthene rings stuck together, smaller
aromatic or naphthenic compounds plus some olefins can result.
Finally, the large molecules, made up of several aromatic or naphthenic rings plus long side chains,
are likely to crack where the side chains are attached. The resulting molecules, though lower in
carbon count, may be denser or heavier; that is, their specific gravity is higher and their API gravity
is lower.
 They also tend to have higher boiling temperatures. Ironically these molecules can form a product
heavier than the feed. The products of cat cracking are the full range of hydrocarbons, from
methane through to residue, plus coke. Three main parts make up the cat cracking hardware: the
reaction section, the regenerator, and the fractionator.

Flow diagram and process description.

The modern FCC units are all continuous processes which operate 24 hours a day for as much as
two to three years between shutdowns for routine maintenance.
There are a number of different proprietary process designs that have been developed for modern
FCC units. Each design is available under a license that must be purchased from the design
developer by any petroleum refining company desiring to construct and operate an FCC of a given
design.

The feedstock to the FCC are primarily in the heavy vacuum gas oil range; boiling ranges are 640
°F (338 °C) (10%) to 980 °F (527 °C) (90%), and typical feedstock properties are shown in Table
8.2. The gas oil is limited in end point by maximum tolerable metals, although the new zeolite
catalysts have demonstrated higher metals tolerance than the older silica-alumina catalyst.

The process has considerable flexibility; apart from processing the more conventional waxy
distillates to produce gasoline and other fuel components, feed stocks ranging from naphtha to a
suitably pre-treated residuum is successfully processed to meet specific product requirements.

Figure 1. FCC type configurations.

FCC units are preceded by basic separation processes such as atmospheric and vacuum distillation
units, catalytic reacting processes such as hydrotreatment; minor additional units, pumps,
stabilization towers, etc. Downstream, FCC units supply products, mainly to the gasoline pool, but
also to other units that require light hydrocarbons. Finally, when there is a high level of sulfur in
the gasoline, FCC units supply feedstock for hydrodesulfurization processes. Therefore, FCC units
interact very little with downstream processes and some with upstream processes.

In the early stages of FCC, the catalyst used was aluminum trichloride (AlCl3) solution. Later, the
catalyst was amorphous silica alumina. It had superior properties as high thermal and attrition
stability, high activity and optimal pore structure. Recently, the catalysts are zeolite-based, which
are mixtures of crystalline alumina silicates, active alumina, silica-alumina, clay and rare-earth
oxides. These catalyst types are more active, more stable, form less coke and have higher acidity.
Since zeolite crystals are too active for practical use in the reactors, the zeolite is diluted with
porous silica-alumina material. The dilution also has the benefit of increasing the pore diameter,
which allows bigger size molecules to enter the pores and be cracked.

In the FCC, an oil feed composed of heavy hydrocarbon molecules is mixed with the catalyst and
enters a fluidized bed reactor. The long molecules react on the surface of the catalyst and are
cracked into lighter products molecules (e.g., gasoline), which leave the reactor from the top.
During the cracking process, carbon and other heavy uncracked organic materials are deposited on
the surface of the catalyst and very quickly reduce its activity resulting in its deactivation. The
catalyst is regenerated in the regenerator by burning coke with air blown into the regenerator. The
regenerator operates at a temperature of ~1382 °F (~715 °C) and a pressure of ~35 psig (~2.41
barg). The regenerated catalyst then moves through a control valve to the base of the riser to
complete the cycle. Abrasion resistant refractory linings are used throughout the
reactor/regenerator system to prevent erosion due to the catalyst. The combustion of the coke is
exothermic, which produces a large amount of heat that is partially absorbed by the regenerator
catalyst, which in turn provides further heat required for the vaporization of the feedstock and the
endothermic cracking reactions that take place in the catalyst riser. This makes the FCC system a
largely heat-balanced system.
The hot catalyst (at about 1319 °F [715 °C]) leaving the regenerator flows into a catalyst withdrawal
well where any entrained combustion flue gases are allowed to escape and flow back into the upper
part of the regenerator. The flow of regenerated catalyst to the feedstock injection point below the
catalyst riser is regulated by a slide valve in the regenerated catalyst line.

Table 1 – Table of catalytic cracking.

The hot flue gas exits the regenerator after passing through multiple sets of two-stage cyclones that
remove the entrained catalyst from the flue gas. The amount of catalyst circulating between the
regenerator and the reactor is about 5 kg per kg of feedstock, which is equivalent to ~ 4.66 kg per
liter of feedstock.

Process variables
Feed quality. The cracking reaction is very complex, and lots of data are available to predict yields
from feed with different characteristics. The gravity of the feed and the paraffin, naphthene, and
aromatic contents are
important indicators.

Reaction temperature. The higher the temperature, the more cracking will occur; but at some
point, the generation of C4 and lighter gases will really take off, at the expense of the cat gasoline
or cat light gas oil.

Feed rate and recycle rate. The yields will suffer at higher feed rates since the contact with the
catalyst will be diluted. So refiners watch the fresh feed rate and the volume of fractionator bottoms
being recycled.

Time of day and temperature. Cat crackers have better yields at night
than during the day. Are the swing shift and graveyard shift operators better?
Do tinkering engineers come around only during the day and mess things.
The main products separated from a catalytic cracker unit are: LPG, gasoline, light cycle oil, and
residual oil. The main properties of these are as follows:

Figure 2- catalytic cracking.

a. C3–C4 LPG product: The C3–C4 LPG product obtained usually contains H2S and mercaptans
impurities. These are removed first by alkanolamine and caustic soda (NaOH) extraction before
feeding to the C3–C4 splitter. The C3 product exiting from the splitter contains ~ 70% propene and
can be used either as a petrochemical feedstock or by alkylation or catalytic polymerization can be
upgraded into motor gasoline. Similarly, the C4 product obtained from the splitter contains 50%
butane and 35% isobutene, which can be either blended directly into motor gasoline or can be
upgraded by alkylation or catalytic polymerization.

b. Gasoline: Catalytically cracked gasoline is characterized by a rather high research octane number
(RON) but a low motor octane number (MON). Octane number improvement can be obtained by
operating at high reactor temperatures and by selecting a catalyst having low hydrogen transfer
activity to minimize olefin saturation. The malodorous mercaptan sulfur compounds are removed
generally by UOP Merox treatment.

c. Light Cycle Oil: Light Cycle Oil (LCO) is characterized by a very high aromatic content (mainly
two-ring naphthene derivatives) and this is the reason for its high density (940–980 kg/m3 at 15
°C). As its cetane index is very low (15–25), it becomes a poor quality diesel fuel-blending
component. Thus, it is most often used as a diluent in heavy oil blends. It is also possible to upgrade
LCO into gasoline by hydrocracking.

d. Residual Oil: The residual oil is also characterized by its high aromatic content and density
(1050–1100 kg/m3 at 15 °C). Most often this material is blended directly into fuel oil or used as
feedstock for carbon black or needle coke manufacture.

Catalyst
Zeolite
Zeolite, or more properly, zeolite Y, is the key ingredient of the FCC catalyst. It provides product
selectivity and much of the catalytic activity. The catalyst’s performance depends largely on the
nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism,
and properties is essential in choosing the “right” catalyst to produce the desired yields.

Zeolite structure:
consists of a silicon or aluminum atom at the center of the tetrahedron, with oxygen atoms at the
four corners. The properties of the zeolite play a significant role in the overall performance of the
catalyst. Understanding these properties increases our ability to predict catalyst response to changes
in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic
properties under the hostile conditions of the FCC operation. The reactor/regenerator environment
can cause significant changes in chemical and structural composition of the zeolite. In the
regenerator, for instance, the zeolite is subjected to thermal and hydrothermal deactivation. In the
reactor, it is exposed to feedstock contaminants such as vanadium and sodium. Various analytical
tests determine zeolite properties.
Figure 3 Silicon/aluminum-oxygen tetrahedron.
Zeolite is sometimes called a molecular sieve. It has a well-defined lattice structure. Its basic
building blocks are silica and alumina tetrahedral (pyramids). Each tetrahedron)
These tests supply information about the strength, type, number, and distribution of acid sites.
Additional tests can also provide information about surface area and pore size distribution. The
three most common parameters governing zeolite behavior are as follows:
• Unit cell size
• Rare earth level
• Sodium content

Cracking Reactions
1- Thermal Cracking
This is the primary process of converting low-value feedstocks into lighter products. Thermal
cracking processes are employed in vacuum distillation unit (VDU), visbreaker unit (VBU) and
delayed coking unit (DCU) for cracking or residual hydrocarbons. Thermal cracking is a function
of temperature and time. The reaction occurs when hydrocarbons in the absence of a catalyst are
exposed at high temperature in the range of 800–1,200 °F (425–650 °C). The initial step in the
chemistry of thermal cracking is the formation of free radicals by splitting the C–C bond. A free
radical is an uncharged molecule with an unpaired electron. The rupturing gives two uncharged
species that share a pair of electrons.

Eq.1
 Eq.2

The primary reactions are those involving the initial carbon-carbon bond scission and the
intermediate neutralization of the carbonium ion involving the following steps:

Eq.3

2- Isomerization reactions
Isomerization reactions occur frequently in catalytic cracking, infrequently in thermal cracking. In
both, breaking of a bond is via beta-scission. However, in catalytic cracking, carbocations tend to
rearrange to form tertiary ions. Tertiary ions are more stable than secondary and primary ions; they
shift around and crack to produce branched molecules. (In thermal cracking, free radicals yield
normal or straight chain compounds.)

Some of the advantages of isomerization are:

• Higher octane in the gasoline fraction. Isoparaffins in the gasoline boiling range have higher
octane than normal paraffins.
• Higher-value chemical and oxygenate feedstocks in the C3/C4 fraction. Isobutylene and
isoamylene are used for the production of methyl tertiary butyl ether (MTBE) and tertiary amyl
methyl ether (TAME). MTBE and TAME can be blended into the gasoline to reduce auto
emissions.
• Lower cloud point in the diesel fuel. Isoparaffins in the light cycle oil boiling range improve
the cloud point.

3- Hydrogen transfer reactions

Hydrogen transfer is more correctly called hydride transfer. It is a bimolecular reaction in which
one reactant is an olefin. Two examples are the reaction of two olefins and the reaction of an olefin
and a naphthene.

In the reaction of two olefins, both olefins must be adsorbed on active sites that are close together.
One of these olefins becomes a paraffin and the other becomes a cyclo-olefin as hydrogen is moved
from one to the other. Cyclo-olefin is now hydrogen transferred with another olefin to yield a
paraffin a cyclodi-olefin. Cyclodi-olefin will then rearrange to form an aromatic. The chain ends
because aromatics are extremely stable. Hydrogen transfer of olefins converts them to paraffins
and aromatics.
A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth
forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth
protects those acid sites. Because hydrogen transfer needs adjacent acid sites, bridging these sites
with rare earth promotes hydrogen transfer reactions.
Hydrogen transfer reactions usually increase gasoline yield and stability. The reactivity of the
gasoline is reduced; because hydrogen transfer produces fewer olefins. Olefins are the reactive
species in gasoline for secondary reactions; therefore, hydrogen transfer reactions indirectly reduce
“overcracking” of the gasoline.
Some of the drawbacks of hydrogen transfer reactions are:
• Lower gasoline octane,
• Lower light olefin in the LPG,
• Higher aromatics in the gasoline and LCO, and
• Lower olefin in the front end of gasoline.

4- Dehydrogenation
Under ideal conditions, i.e., a “clean” feedstock and a catalyst with no metals, cat cracking does
not yield any appreciable amount of molecular hydrogen. Therefore, dehydrogenation reactions
will proceed only if the catalyst is contaminated with metals such as nickel and vanadium.

5- Coking
Cat cracking yields a residue called coke. The chemistry of coke formation is complex and not very
well understood. Similar to hydrogen transfer reactions, catalytic coke is a “bimolecular” reaction.
It proceeds via carbenium ions or free radicals. In theory, coke yield should increase as the
hydrogen transfer rate is increased. It is postulated that reactions producing unsaturates and
multiring aromatics are the principal coke-forming compounds. Unsaturates such as olefins,
diolefins, and multi-ring polycyclic olefins are very reactive and can polymerize to form coke.

FCC Products

The cat cracker converts less valuable gas oil feedstock to a more valuable product. A major
objective of most FCC units is to maximize the conversion of gas oil to gasoline and LPG, though
recently the trend has been in maximizing diesel production. The typical products produced from
the cat cracker are:
• Dry gas (hydrogen, methane, ethane, ethylene)
• LPG (propane, propylene, isobutane, normal butane, butylenes)
• Gasoline
• LCO
• HCO (in few FCC units)
• Decanted (or slurry) oil
• Combustion coke.
 1- Dry gas

Dry gas is defined as the C2 and lighter gases that are produced in the FCC unit. Often the fuel gas
stream leaving the sponge oil or secondary absorber tower is also referred to as “dry gas” despite
its containing H2S, inert gases, and C3 components. Once the gas is amine-treated for the removal
of H2S and other acid gases, it is usually blended into the refinery fuel gas system. Depending on
the volume percent of hydrogen in the dry gas, some refiners will recover this hydrogen using
processes such as cryogenics, pressure-swing absorption, or membrane separation. This recovered
hydrogen is typically used in hydrotreating processes.

Dry gas is an undesirable by-product of the FCC unit; excessive yields load up the WGC, limiting
the unit’s feed rate and/or severity. The dry gas yield correlates with the feed quality, thermal
cracking reactions, concentration of metals in the feed, and the amount of post-riser nonselective
catalytic cracking reactions. The primary factors which contribute to the increase of dry gas
production are as follows:
• Increase in the concentration of metals (nickel, copper, vanadium, and so on) on the catalyst
• Increase in reactor or regenerator temperatures
• Increase in the residence time of hydrocarbon vapors in the reactor
• Decrease in the performance of the feed nozzles (for the same unit conversion)
• Increase in the aromaticity of the feed.

2- LPG
The overhead stream from the debutanizer or stabilizer tower is a mix of C3’s and C4’s, usually
referred to as LPG. It is rich in propylene and butylenes. These light olefins play an important role
in the manufacture of RFG. Depending on the refinery’s configuration, the cat cracker’s LPG is
used in the following areas:
• Chemical sale, where the LPG is separated into C3’s and C4’s. The C3’s are sold as refinery or
chemical grade propylene. The C4 olefins are polymerized or alkylated.
• Direct blending, where the C4’s are blended into the refinery’s gasoline pool to regulate vapor
pressure and to enhance the octane number. However, new gasoline regulations require reduction
of the vapor pressure, thus displacing a large volume of C4’s for alternative uses.
• Alkylation, where the olefins are reacted with isobutane to make a very desirable gasoline
blending stock. Alkylate is an attractive blending component because it has no aromatics or sulfur,
low vapor pressure, low end point, and high research and motor
octane ratings.
The LPG yield and its olefinicity can be increased by:
• Changing to a catalyst which minimizes “hydrogen transfer” reactions
• Increasing unit conversion
• Decreasing residence time, particularly the amount of time product that the vapors spend in the
reactor housing before entering the main column
• Adding ZSM-5 catalyst additive.
An FCC catalyst containing zeolite with a low hydrogen transfer rate reduces resaturation of the
olefins in the riser. As stated in Chapter 6, primary cracking products in the riser are highly olefinic.
Most of these olefins are in the gasoline boiling range; the rest appear in the LPG and LCO boiling
range. The LPG olefins do not crack further, but they can become saturated by hydrogen transfer.
The gasoline and LCO-range olefins can be cracked again to form gasoline-range olefins and LPG
olefins. The olefins in the gasoline and LCO range can also cyclize to form cycloparaffins. The
cycloparaffins can react through H2 transfer with olefins in the LPG and gasoline to produce
aromatics and paraffins. Therefore, a catalyst which inhibits hydrogen transfer reactions will
increase
olefinicity of the LPG.

The conversion increase is accomplished by manipulating the following operating conditions:

• Increasing the reactor temperature: Increasing the reactor temperature beyond the peak
gasoline yield results in overcracking of the gasoline and LCO fractions. The rate of
production and olefinicity of the LPG will increase.
• Increasing feed/catalyst mix zone temperature: Conversion and LPG yield can be increased by
injecting a portion of the feed, or naphtha, at an intermediate point in the riser Splitting or
segregating the feed results in a high mix-zone temperature, producing more LPG and more olefins.
This practice is particularly useful where the reactor temperature is already maximized due to a
metallurgy constraint.
• Increasing catalyst to oil ratio: The catalyst to oil ratio can be increased through several
knobs including reducing the FCC feed preheat temperature and optimizing the stripping and
dispersion steam rate, and by using a catalyst that deposits less coke on the catalyst.
Gasoline Traditionally, the FCC gasoline has always been the most valuable product of a cat
cracker unit. FCC gasoline accounts for about 35 vol% of the total US gasoline pool. Historically,
the FCC has been run for maximum gasoline yield with the highest octane.

3- Gasoline yield

For a given feedstock, gasoline yield can be increased by:

• Increasing the catalyst to oil ratio by decreasing the feed preheat temperature
• Increasing catalyst activity by increasing fresh catalyst addition or fresh catalyst activity
• Increasing gasoline end point by reducing the main column top pumparound rate and/or
overhead reflux rate
• Increasing reactor temperature (if the increase does not over-crack the already-produced
gasoline)
• Lowering carbon on the regenerated catalyst.

Gasoline quality
The key components affecting FCC gasoline quality are as follows:
• Octane
• Benzene
• Sulfur.
 a- Octane

Factors affecting gasoline octane are:

A. Operating conditions
1. Reactor temperature: As a rule, an increase of 18 F (10 C) in the reactor temperature
increases the RON by 1.0 and MON by 0.4. However, the MON contribution comes from
the aromatic content of the heavy end. Therefore, at high severity, the MON response to the reactor
temperature can be > 0.4 per 18 F.
2. Gasoline end point: The effect of gasoline end point on its octane number depends on the
feedstock quality and severity of the operation. At low severity, lowering the end point of a
paraffinic feedstock may not impact the octane number; however, reducing gasoline end point
produced from a naphthenic or an aromatic feedstock will lower the octane.
3. Gasoline Reid vapor pressure (RVP): The RVP of the gasoline is controlled by adding C4’s,
which increase octane. As a rule, the RON and MON gain 0.3 and 0.2 numbers for a 1.5 psi (10.3
kPa) increase in RVP.

B. Feed quality
1. API gravity: The higher the API gravity, the more paraffins in the feed and the lower the octane.
2. K-factor: The higher the K-factor, the lower the octane.
3. Aniline point: Feeds with a higher aniline point are less aromatic and more paraffinic. The higher
the aniline point, the lower the octane.
4. Sodium: Additive sodium reduces unit conversion and lowers octane.

C. Catalyst
1. Rare earth: Increasing the amount of rare earth oxide (REO) on the zeolite decreases the octane.
2. Unit cell size: Decreasing the unit cell size increases octane.
3. Matrix activity: Increasing the catalyst matrix activity increases the octane.
4. Coke on the regenerated catalyst: Increasing the amount of coke on the regenerated
catalyst lowers its activity and increases octane.
b- Benzene
Most of the benzene in the gasoline pool comes from reformate. Reformate, the high-octane
blending component from a reformer unit, comprises about 30 vol% of the gasoline pool.
Depending on the reformer feedstock and severity, reformate contains 3-5 vol% benzene. FCC
gasoline contains 0.5e1.3 vol% benzene. Since it accounts for about 35 vol% of the gasoline pool,
it is important to know what affects the cat cracker gasoline benzene levels. The benzene content
in the FCC gasoline can be reduced by the following:
• Short contact time in the riser and in the reactor dilute phase
• Lower catalyst to oil ratio and lower reactor temperature
• A catalyst with less hydrogen transfer.
 c- Sulfur
The major source of sulfur in the gasoline pool comes from FCC gasoline. Sulfur in FCC
gasoline is a strong function of the feed sulfur content. Hydrotreating the FCC feedstock reduces
sulfur in the feedstock and consequently in the gasoline. Other factors which can lower sulfur
content are:
• Lower gasoline end point
• Lower reactor temperature
• Increased matrix activity of the catalyst
• Increase in the catalyst activity and hydrogen transfer properties
• Increase in catalyst to oil ratio
• Increase in the use of m