FCC-Ch-21

554 Adv. in Pet. Engg. I: Refining
21
Fluidized Catalytic Cracking
OMPRAKASH H. NAUTIYAL1*
ABSTRACT
As the heart of a refinery, the Fluid Catalytic Cracking (FCC) unit is
continuously pushed to the limit. Refiners are continually evaluating potential
FCC modifications to increase capacity and improve product yields, as well
as to maximize on-stream factor and mechanical reliability in order to be
more profitable while simultaneously meeting stringent environmental
regulations. The oil crisis of recent times has caused a drastic decrease in the
total consumption of oil and changed the demand pattern for the products of
petroleum refining. The demand for heavier fractions or residual oils has
steadily decreased, making it imperative to convert these into gasoline, diesel
and such lighter fractions. Fluid catalytic cracking (FCC) of these heavier
fractions, however, poses several serious problems, caused mainly by their
much higher hetero-atom concentration, metal contents and coking tendency,
as compared to earlier feed stocks. Several process and catalyst innovations
have been made to tackle these problems. A new generation of FCC catalyst
technology has emerged with tailor-made catalysts for higher structural
stability and attrition strength, more complete CO combustion during
regeneration, reducing SOx
emissions from FCC stacks, enhancing the gasoline
octane number, passivation of the harmful effects of metals like Ni and V
accumulating on the catalyst, etc., These developments contain valuable
lessons for the science and technology of catalysis.
Key words: Liquid fuels, Oil/petroleum, Refining, Technology, Fluid catalytic
cracking
1
Shubh Building, 102, Shivalik II, Canal Road, Chhani Jakat Naka, Vadodara 39002, India
*Corresponding author: E-mail: opnautiyalus@yahoo.com

555Fluidized Catalytic Cracking
INTRODUCTION
The Fluid Catalytic Cracking (FCC) process from Lummus Technology is a
proven technology used to convert gas oils and residual stocks to lighter, higher-
value products such as gasoline. Combining an advanced reaction system design
with an efficient catalyst regeneration system, the process achieves high
conversion and selectivity to light products. While it can be used to maximize
the production of gasoline, the flexibility of the process allows conversion and
selectivity to vary from maximum distillate production at one extreme, to
maximum propylene production at the other. This FCC technology can be
applied fully in grassroots units or partially, as applicable, in the revamp of
existing units to increase throughput and/or residue processing capability,
improve selectivity, and reduce operating costs.
Refineries vary by complexity; more complex refineries have more
secondary conversion capability, meaning they can produce different types of
petroleum products. Fluid Catalytic Cracking (FCC), a type of secondary unit
operation, is primarily used in producing additional gasoline in the refining
process.
Unlike atmospheric distillation and vacuum distillation, which are physical
separation processes, fluid catalytic cracking is a chemical process that uses a
catalyst to create new, smaller molecules from larger molecules to make gasoline
and distillate fuels.
The catalyst is a solid sand-like material that is made fluid by the hot
vapor and liquid fed into the FCC (much as water makes sand into quicksand).
Because the catalyst is fluid, it can circulate around the FCC, moving between
reactor and regenerator vessels (see photo). The FCC uses the catalyst and
heat to break apart the large molecules of gas oil into the smaller molecules
that make up gasoline, distillate, and other higher-value products like butane
and propane.
After the gas oil is cracked through contact with the catalyst, the resulting
effluent is processed in fractionators, which separate the effluent based on
various boiling points into several intermediate products, including butane
and lighter hydrocarbons, gasoline, light gas oil, heavy gas oil, and clarified
slurry oil.
The butane and lighter hydrocarbons are processed further to separate
them into fuel gas (mostly methane and ethane), propane, propylene, butane,
and butene for sale, or for further processing or use. The FCC gasoline must be
desulfurized and reformed before it can be blended into finished gasoline; the
light gas oil is desulfurized before blending into finished heating oil or diesel;
and the heavy gas oil is further cracked in either a hydrocracker (using hydrogen
and a catalyst) or a Coker. The slurry oil can be blended with residual fuel oil
or further processed in the Coker.

Carbon is deposited on the catalyst during the cracking process. This
carbon, known as catalyst coke, adheres to the catalyst, reducing its ability to
crack the oil. The coke on the spent catalyst is burned off, which reheats the
catalyst to add heat to the FCC process. Regeneration produces a flue gas that
passes through environmental control equipment and then is discharged into
the atmosphere.
Process of Fluidized Catalytic Cracking
In the petroleum refining system, an atmospheric distillation column or
reduced-pressure distillation column is used to refine crude oil into gasoline,
kerosene, and lubrication oil. In addition, the petroleum refining system
incorporates an FCC to distill high-octane gasoline and LPG from the heavy
contents of the crude oil. In many refineries the FCC unit serves as the primary
unit, converting, or cracking low-value crude oil heavy ends into a variety of
higher value, light products. In the US, the primary function of the FCC unit
is to produce gasoline. Modern FCC units can process a wide variety of
feedstock and can adjust operating conditions to maximize production of
gasoline, middle distillate olefins (LCO) or light olefins to meet different
market demands. The top gas generated in the fraction column of the FCC
goes through a heat exchanger and is then pumped to a high pressure. The
resulting gas content is transferred to the LPG recovery system and the liquid
content to the gasoline generation system. In this process, it is important to
measure the density (specific gravity) of the gas because the data are
essential as a critical parameter in controlling the operation of the FCC.
In addition to being used to monitor the system and the quality of the
product, this measurement can also help prevent pump pressure surges.
The GD402 Gas Density Meter has been introduced for this explosion
protected application. It features an intrinsically safe and explosion-proof
design, fast response, and a dustproof, anti-corrosive, and flame-proof
construction. GD402 will ensure stable and rapid measurement of gas
density under hazardous conditions. It is capable of displaying specific
gravity and molecular weight readings derived from the density data, and
it will greatly reduce the workload by ensuring continuous and accurate
measurement. Fig. 1 depicts the structure of Fluidized catalytic cracking.
Expected benefits
• Ensures stable and rapid measurement of gas density under hazardous
conditions
• Capable of displaying specific gravity and molecular weight readings
derived from the density data
• Greatly reduces the workload by ensuring continuous and accurate
measurement

Field Data
Process conditions
Measurement point: Outlet of the top fraction column in the FCC
Temperature: 34ºC
Pressure: 75 kPa to 180 kPa
Humidity: Wet
Gas Composition: O2
, N2
, CO, H2
, H2
S, C1 to C5
Dust: None
Measurement Range: 1,600 to 1,800 kg/Nm3
Gas Processing Technology
Patented process technology and proprietary know-how developed by
Lummus Technology are used in more than 200 natural gas plants around
the world. Many of the innovations we have developed, like the use of plate-
fin exchangers and packing in cryogenic columns, remain today as standard
designs in the industry. We have a wide portfolio of patented designs
including deep ethane and propane recovery, NicheLNGSM
, mitigation of
CO2
recovery in NGL processing, and hydrocarbons from refinery streams,
enabling us to expand our technology positions through the natural gas value
chain. Fig. 2 depicts the Petrochemical plant.
Fig. 1: Structure of fluid catalytic cracking (courtesy http://www.yokogawa.com/an/
index.htm)

Fluid catalytic cracking (FCC) is one of the most important conversion
processes used in petroleum refineries. It is widely used to convert the high-
boiling, high-molecular weight hydrocarbon fractions of petroleum crude oil to
more valuable gasoline olifinic gases, and other products. Cracking of petroleum
hydrocarbons was originally done by thermal cracking, which has been almost
completely replaced by catalytic cracking because it produces more gasoline
with a higher octane rating. It also produces by product gases that are more
olefinic, and hence more valuable, than those produced by thermal cracking.
The feedstock to an Fluid Catalytic Cracking is usually that portion of
the crude oil that has an initial boiling point of 340°C or higher at atmospheric
pressure and an average molecular weight ranging from about 200 to 600 or
higher. This portion of crude oil is often referred to as heavy gas oil or
vacuum gas oil (HVGO). The Fluid Catalytic Cracking 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.
In effect, refineries use fluid catalytic cracking to correct the imbalance
between the market demand for gasoline and the excess of heavy, high
boiling range products resulting from the distillation of crude oil.
As of 2006, Fluid Catalytic Cracking units were in operation at 400
petroleum refineries worldwide and about one-third of the crude oil refined
in those refineries is processed in an Fluid Catalytic Cracking to produce
high-octane gasoline and fuel oils.[2][4]
During 2007, the Fluid Catalytic
Cracking units in the United States processed a total of 5,300,000 barrels
(834,300,000 litres) per day of feedstock[5]
and FCC units worldwide processed
about twice that amount.
To maintain the catalyst activity at a useful level, it’s necessary to
regenerate the catalyst by burning off the coke with hot air. As a result, the
catalyst is continuously moved from the reactor to regenerator and back to
reactor. Remaining oil on the catalyst is removed by steam stripping before
catalyst enters the regenerator. The steam supply to the reactor takes place
at a temperature at dry saturated steam. The cracking process produces
carbon (coke) which remains on the catalyst particle and rapidly lowers its
Fig. 2: CB and I petrochemicals plant (courtesy CB&I http://www.cbi.com/technologies/
petrochemicals-technology)

activity. To maintain the catalyst activity at a useful level, it’s necessary to
regenerate the catalyst by burning off this coke with air. Regeneration is a
key part of the FCC process. It’s critical to control the regenerator
temperature carefully to prevent catalyst deactivation by overheating and
to provide the desired amount of burn-off. This is done by controlling the air
flow. A typical air temperature is around 600°C.
Continuously catalyst regenerating makes it possible to manage the
high catalyst coking rate. The constancy of the yields is achieved by catalyst
cycling between reaction and regeneration, which ensures the reactor, is
continuously supplied with freshly regenerated catalyst, and product yields
are maintained at fresh catalyst levels. Catalyst handling valves play an
important role in ensuring proper FCC performance, reliable and accurate
control, on-off and ESD-valve performance is important for total process
efficiency. Each day, several tons of fresh catalysts are added to replace
losses through the cyclones and to maintain the activity of the unit’s inventory
at an acceptable level. A typical temperature here is ambient.
The feedstock is vaporized by the hot regenerated catalyst, the cracking
begins, and the resultant vapour carries the catalyst upward through the
riser. The heat of combustion raises the catalyst temperature to (620–845°C),
and most of this heat is transferred by the catalyst to the oil feed in the feed
riser. The regenerator/reactor cycle continues until catalyst is spent and
removed from process through extraction valve. A typical temperature here
is 760°C. Hot flue gases exit the regenerator through cyclones where catalyst
is separated and recycled to reactor. A typical temperature here is 760°C.
The remaining heavy residual oil, together with any catalyst carryover,
collects at the bottom of the fractionators and recycles back to the reactor
for the catalyst to be used in the reactor. Bottom recycle is used to recover
heat for feed preheat through kettle boilers and exchangers. The fluid is
known as catalyst oil slurry and its control and isolation, due to its highly
abrasive nature and temperature, provide a demanding valve application.
The typical temperature of bottom slurry is 370°C.
The flue gas from the FCC process exiting the regenerator has
significant pressure, temperature and volume, and it is a source of useful
energy that represents an energy cost-saving opportunity to a refinery.
Using an expander could maximize recovery of available energy from the
flue gas. This energy can then be used to drive the compressor that provides
air to the regenerator (the main air blower) or an electric generator.
Flow Diagram and Process Description
The modern Fluid Catalytic Cracking units are all continuous processes
which operate 24 hours a day for as long as 2 to 3 years between scheduled
shutdowns for routine maintenance.

There are several different proprietary 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 a Fluid Catalytic Cracking of a given design.
Fig. 3 shows typical fluid catalytic cracking units in a petroleum refinery.
There are two different configurations for a Fluid Catalytic Cracking
unit: the “stacked” type where the reactor and the catalyst regenerator are
contained in a single vessel with the reactor above the catalyst regenerator
and the “side-by-side” type where the reactor and catalyst regenerator are
in two separate vessels. These are the major Fluid Catalytic Cracking
designers and licensors
Side-by-side configuration:
• CB and I
• Exxon mobil research and engineering (EMRE)
• Shell global solutions
• Stone and webster process technology — currently owned by Technip
• Universal oil products (UOP) — currently fully owned subsidiary
of Honeywell
Stacked configuration:
• Kellogg Brown and root (KBR)
Each of the proprietary design licensors claims to have unique features
and advantages. A complete discussion of the relative advantages of each of
the processes is beyond the scope of this article. Suffice it to say that all of
the licensors have designed and constructed FCC units that have operated
quite satisfactorily.
Fig. 3: A typical fluid catalytic cracking units in a petroleum refinery (courtesy http://
en.wikipedia.org/wiki/fluid_catalytic_cracking)

Reactor and Regenerator
The reactor and regenerator are considered to be the heart of the fluid
catalytic cracking unit. The schematic flow diagram of a typical modern
Fluidized Catalytic Cracking unit in Fig. 1 below is based upon the “side-by-
side” configuration. The preheated high-boiling petroleum feedstock (at about
315 to 430°C) consisting of long-chain hydrocarbon molecules is combined
with recycle slurry oil from the bottom of the distillation column and injected
into the catalyst riser where it is vaporized and cracked into smaller
molecules of vapour by contact and mixing with the very hot powdered
catalyst from the regenerator. All of the cracking reactions take place in
the catalyst riser within a period of 2–4 seconds. The hydrocarbon vapours
“fluidize” the powdered catalyst and the mixture of hydrocarbon vapours
and catalyst flows upward to enter the reactor at a temperature of about
535 °C and pressure of about1.72 brag.
The reactor is a vessel in which the cracked product vapours are: (a)
separated from the so-called spent catalyst by flowing through a set of two-
stage cyclones within the reactor and (b) the spent catalyst flows downward
through a steam stripping section to remove any hydrocarbon vapours before
the spent catalyst returns to the catalyst regenerator. The flow of spent catalyst
to the regenerator is regulated by a slide valve in the spent catalyst line.
Since the cracking reactions produce some carbonaceous material (referred
to as catalyst coke) that deposits on the catalyst and very quickly reduces the
catalyst reactivity, the catalyst is regenerated by burning off the deposited
coke with air blown into the regenerator. The regenerator operates at a
temperature of about 715°C and a pressure of about 2.41 barg.
The combustion of the coke is exothermic and it produces a large amount of
heat that is partially absorbed by the regenerated catalyst and provides the
heat required for the vaporization of the feedstock and the endothermic cracking
reactions that take place in the catalyst riser. For that reason, FCC units are
often referred to as being ‘heat balanced’.
The hot catalyst (at about 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 to 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. The hot flue
gas exits the regenerator after passing through multiple sets of two-stage
cyclones that remove entrained catalyst from the flue gas (Fig. 4).
The amount of catalyst circulating between the regenerator and the
reactor amounts to about 5 kg per kg of feedstock, which is equivalent to
about 4.66 kg per litre of feedstock. Thus, an FCC unit processing 75,000
barrels per day (11,900 m3
/d) will circulate about 55,900 MT per day of
catalyst.

The thermal cracking process functioned largely in accordance with the
free-radical theory of molecular transformation. Under conditions of extreme
heat, the electron bond between carbon atoms in a hydrocarbon molecule can
be broken, thus generating a hydrocarbon group with an unpaired electron.
This negatively charged molecule, called a free radical, enters into reactions
with other hydrocarbons, continually producing other free radicals via the
transfer of negatively charged hydride ions (H”
). Thus a chain reaction is
established that leads to a reduction in molecular size, or “cracking,” of
components of the original feedstock.
Use of a catalyst in the cracking reaction increases the yield of high-
quality products under much less severe operating conditions than in thermal
cracking. Several complex reactions are involved, but the principal
mechanism by which long-chain hydrocarbons are cracked into lighter
products can be explained by the carbonium theory. According to this theory,
a catalyst promotes the removal of a negatively charged hydride ion from
Fig. 4: A schematic flow diagram of a Fluid Catalytic cracking unit as used in petroleum
refineries (courtesy http://en.wikipedia.org/wiki/fluid_catalytic_cracking)

a paraffin compound or the addition of a positively charged proton (H+
) to
an olefin compound. This results in the formation of a carbonium ion, a
positively charged molecule that has only a very short life as an intermediate
compound which transfers the positive charge through the hydrocarbon.
Carbonium transfer continues as hydrocarbon compounds come into contact
with active sites on the surface of the catalyst that promote the continued
addition of protons or removal of hydride ions. The result is a weakening of
carbon-carbon bonds in many of the hydrocarbon molecules and a consequent
cracking into smaller compounds.
Olefins crack more readily than paraffin, since their double carbon-
carbon bonds are more friable under reaction conditions. Isoparaffins and
naphthenes crack more readily than normal paraffins, which in turn crack
faster than aromatics. In fact, aromatic ring compounds are very resistant
to cracking, since they readily deactivate fluid cracking catalysts by blocking
the active sites of the catalyst. The table illustrates many of the principal
reactions that are believed to occur in fluid catalytic cracking unit reactors.
The reactions postulated for olefin compounds apply principally to
intermediate products within the reactor system, since the olefin content of
catalytic cracking feedstock is usually very low. Reactions in fluid catalytic
cracking are shown in Table 1.
Typical modern catalytic cracking reactors operate at 480–550°C (900–
1,020°F) and at relatively low pressures of 0.7 to 1.4 bars (70 to 140 KPa), or
10 to 20 psi. At first natural silica-alumina clays were used as catalysts, but by
the mid-1970s zeolite and molecular sieve-based catalysts became common.
Zeolitic catalysts give more selective yields of products while reducing the
formation of gas and coke.
A modern fluid catalytic cracker employs a finely divided solid catalyst
that has properties analogous to a liquid when it is agitated by air or oil vapours.
The principles of operation of such a unit are shown in the figure. In this
arrangement a reactor and regenerator are located side by side. The oil feed is
vaporized when it meets the hot catalyst at the feed-injection point, and the
vapours flow upward through the riser reactor at high velocity, providing a
fluidizing effect for the catalyst particles. The catalytic reaction occurs
exclusively in the riser reactor. The catalyst then passes into the cyclone vessel,
where it is separated from reactor hydrocarbon products.
As the cracking reactions proceed, carbon is deposited on the catalyst
particles. Since these deposits impair the reaction efficiency, the catalyst
must be continuously withdrawn from the reaction system. Unit product
vapours pass out of the top of the reactor through cyclone separators, but
the catalyst is removed by centrifugal force and dropped back into the
stripper section. In the stripping section, hydrocarbons are removed from
the spent catalyst with steam, and the catalyst is transferred through the
stripper standpipe to the regenerator vessel, where the carbon is burned

with a current of air. The high temperature of the regeneration process (675–
785°C, or 1,250–1,450°F) heats the catalyst to the desired reaction temperature
for recon acting fresh feed into the unit. In order to maintain activity, a small
amount of fresh catalyst is added to the system from time to time, and a similar
amount is withdrawn.
The cracked reactor effluent is fractionated in a distillation column. The
yield of light products (with boiling points less than 220°C, or 430°F) is usually
reported as the conversion level for the unit. Conversion levels average about
60 to 70 percent in Europe and Asia and in excess of 80 percent in many catalytic
Table 1: Reactions in fluid catalytic cracking (courtesy Encyclopaedia Britannica) (courtesy
http://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)

cracking units in the United States. About one-third of the product yield consists
of fuel gas and other gaseous hydrocarbons. Half of this is usually propylene
and butylenes, which are important feed stocks for the polymerization and
alkylation processes discussed below. The largest volume is usually cracked
naphtha, an important gasoline blend stock with an octane number of 90 to 94.
The lower conversion units of Europe and Asia produce comparatively more
distillate oil and less naphtha and light hydrocarbons. Fluid catalytic cracking
unit is depicted in Fig. 5.
The light gaseous hydrocarbons produced by catalytic cracking are highly
unsaturated and are usually converted into high-octane gasoline components
in polymerization or alkylation processes. In polymerization, the light
olefins propylene and butylenes are induced to combine, or polymerize, into
molecules of two or three times their original molecular weight. The catalysts
employed consist of H3
PO4
on pellets of kieselguhr, a porous sedimentary
rock. High pressures, on the order of 30 to 75 bars (3 to 7.5 MPa), or 400 to
1,100 psi, are required at temperatures ranging from 175 to 230°C (350 to
450°F). Polymer gasoline’s derived from propylene and butylenes have octane
numbers above 90. The various features during the fluidized catalytic
cracking are shown in the figures 3, 4, 5 and 6. The hydrocarbons, crude oil
(product contents), fractional distillation and fluid catalytic cracking unit
may be understood in an elaborative manner. Crude oil (product contents)
is shown in Fig. 6.
Fig. 5: Fluid catalytic cracking: fluid unit (courtesy Encyclopaedia Britannica) (courtesy
http://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)

The alkylation reaction also achieves a longer chain molecule by the
combination of two smaller molecules, one being an olefin and the other an
isoparaffin (usually isobutane). During World War II, alkylation became the
main process for the manufacture of isooctane, a primary component in the
blending of aviation gasoline.
Two alkylation processes employed in the industry are based upon
different acid systems as catalysts. In sulphuric acid alkylation, concentrated
sulphuric acid of 98 percent purity serves as the catalyst for a reaction that
is carried out at 2 to 7°C (35 to 45°F). Refrigeration is necessary because of
the heat generated by the reaction. The octane number of alkylates produced
range from 85 to 95.
Hydrofluoric acid is also used as a catalyst for many alkylation units.
The chemical reactions are similar to those in the sulphuric acid process,
but it is possible to use higher temperatures (between 24 and 46°C, or 75 to
115°F), thus avoiding the need for refrigeration. Recovery of hydrofluoric
acid is accomplished by distillation. Stringent safety precautions must be
exercised when using this highly corrosive and toxic substance.
The demand for aviation gasoline became so great during World War II
and afterward that the quantities of isobutane available for alkylation feedstock
were insufficient. This deficiency was remedied by isomerization of the more
abundant normal butane into isobutane. The isomerization catalyst is aluminium
chloride supported on alumina and promoted by hydrogen chloride gas.
Fig. 6: Crude oil: product contents (courtesy Encyclopaedia Britannica) (courtesy http://
www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)

Isomerisation
Commercial processes have also been developed for the isomerization of
low-octane normal pentane and normal hexane to the higher-octane is paraffin
form. Here the catalyst is usually promoted with platinum. As in catalytic
reforming, the reactions are carried out in the presence of hydrogen.
Hydrogen is neither produced nor consumed in the process but is employed
to inhibit undesirable side reactions. The reactor step is usually followed by
molecular sieve extraction and distillation. Though this process is an
attractive way to exclude low-octane components from the gasoline blending
pool, it does not produce a final product of sufficiently high octane to
contribute much to the manufacture of unleaded gasoline.
Distillation Column
The reaction product vapours (at 535°C and a pressure of 1.72 brag) flow
from the top of the reactor to the bottom section of the distillation column
(commonly referred to as the main fractionators) where they are distilled
into the FCC end products of cracked naphtha, fuel oil, and off gas. After
further processing for removal of sulphur compounds, the cracked naphtha
becomes a high-octane component of the refinery’s blended gasoline.
Fractional distillation (crude oil column) is depicted in Fig. 7.
The main fractionators’ off gas is sent to what is called a gas recovery
unit where it is separated into butanes and butylenes, propane and propylene,
and lower molecular weight gases (hydrogen, methane, ethylene and ethane).
Some FCC gas recovery units may also separate out some of the ethane and
ethylene.
Although the schematic flow diagram above depicts the main fractionators
as having only one side cut stripper and one fuel oil product, many FCC
main fractionators have two side cut strippers and produce a light fuel oil
and a heavy fuel oil. Likewise, many FCC main fractionators produce light
cracked naphtha and a heavy cracked naphtha. The terminology light and
heavy in this context refers to the product boiling ranges, with light products
having a lower boiling range than heavy products.
The bottom product oil from the main fractionators contains residual
catalyst particles which were not completely removed by the cyclones in the
top of the reactor. For that reason, the bottom product oil is referred to as
slurry oil. Part of that slurry oil is recycled back into the main fractionators
above the entry point of the hot reaction product vapours so as to cool and
partially condense the reaction product vapours as they enter the main
fractionators. The remainder of the slurry oil is pumped through a slurry
settler. The bottom oil from the slurry settler contains most of the slurry oil
catalyst particles and is recycled back into the catalyst riser by combining it

with the FCC feedstock oil. The so-called clarified slurry oiler decant oil is
withdrawn from the top of slurry settler for use elsewhere in the refinery,
as a heavy fuel oil blending component, or as carbon black feedstock.
Depending on the choice of FCC design, the combustion in the
regenerator of the coke on the spent catalyst may or may not be complete
combustion to carbon dioxide CO2
. The combustion air flow is controlled so
as to provide the desired ratio of carbon monoxide (CO) to carbon dioxide
for each specific FCC design.
Regenerator fuel gas
In the design shown in Fig. 1, the coke has only been partially combusted to
CO2
. The combustion flue gas (containing CO and CO2
) at 715°C and at a
pressure of 2.41 brag is routed through a secondary catalyst separator
containing swirl tubes designed to remove 70 to 90 percent of the particulates
in the flue gas leaving the regenerator[8]
. This is required to prevent erosion
damage to the blades in the turbo expander that the flue gas is next routed
through.
Fig. 7: Fractional distillation: crude-oil column (courtesy Encyclopaedia Britannica)
(courtesy http://www.britannica.com/EBchecked/topic/211241/fluid-catalytic-
cracking)

The expansion of flue gas through a turbo-expander provides sufficient
power to drive the regenerator’s combustion air compressor. The electrical
motor generator can consume or produce electrical power. If the expansion
of the flue gas does not provide enough power to drive the air compressor,
the electric motor/generator provides the needed additional power. If the
flue gas expansion provides more power than needed to drive the air
compressor, than the electric motor/generator converts the excess power
into electric power and exports it to the refinery’s electrical system.
The expanded flue gas is then routed through a steam-generating boiler
(referred to as a (CO boiler) where the carbon monoxide in the flue gas is
burned as fuel to provide steam for use in the refinery as well as to comply with
any applicable environmental regulatory limits on carbon monoxide emissions.
The flue gas is finally processed through an electrostatic precipitator
(ESP) to remove residual particulate matter to comply with any applicable
environmental regulations regarding particulate emissions. The ESP removes
particulates in the size range of 2 to 20 microns from the flue gas.
The steam turbine in the flue gas processing system (shown in the above
diagram) is used to drive the regenerator’s combustion air compressor during
start-ups of the FCC unit until there is sufficient combustion flue gas to
take over that task.
Chemistry
Before delving into the chemistry (Fig. 8) involved in catalytic cracking, it
will be helpful to briefly discuss the composition of petroleum crude oil.
Petroleum crude oil consists primarily of a mixture of hydrocarbons with
small amounts of other organic compounds containing sulphur, nitrogen and
oxygen. The crude oil also contains small amounts of metals such as copper,
iron, nickel and vanadium. The elemental composition ranges of crude oil
are summarized in Table 2 and the hydrocarbons in the crude oil can be
classified into three types:
• Paraffin or alkanes: Saturated straight-chain or branched hydrocarbons,
without any ring structures
• Naphthalene or cycloalkanes: Saturated hydrocarbons having one or
more ring structures with one or more side-chain paraffin
• Aromatics: Hydrocarbons having one or more unsaturated ring structures
such as benzene or unsaturated polycyclic ring structures such
as naphthalene or phenanthrene, any of which may also have one or
more side-chain paraffin.
Olefins or alkanes, which are unsaturated straight-chain or branch
hydrocarbons, do not occur naturally in crude oil.
The elemental composition ranges of crude oil are summarized in Table 2.

Table 2: Elemental composition of crude oil
Carbon 83–87%
Hydrogen 10–14%
Nitrogen 0.1–2%
Oxygen 0.1–1.5%
Sulphur 0.5–6%
Metals < 0.1%
Technically, the fluid catalytic cracking process breaks large hydrocarbon
molecules into smaller molecules by contacting them with powdered catalyst
at a high temperature and moderate pressure which first vaporizes the
hydrocarbons and then breaks them. The cracking reactions occur in the
vapour phase and start immediately when the feedstock is vaporized in
the catalyst riser. Fig. 9 is a very simplified schematic diagram that exemplifies
how the process breaks high boiling, straight-chain alkanes (paraffin)
hydrocarbons into smaller straight-chain alkanes as well as branched-chain
alkanes, branched alkenes (olefins) and cycloalkanes (naphthenes). The breaking
of the large hydrocarbon molecules into smaller molecules is more technically
referred to by organic chemists as scission of the carbon-to-carbon bonds.
As depicted in Fig. 8, some of the smaller alkanes are then broken and
converted into even smaller alkenes and branched alkenes such as the
gases ethylene, propylene, butylenes, and isobutylene. Those olefin gases
are valuable for use as petrochemical feed stocks. The propylene, butylenes
and isobutylene are also valuable feed stocks for certain petroleum refining
processes that convert them into high-octane gasoline blending components.
Fig. 8: Carbon: hydrocarbons (courtesy Encyclopaedia Britannica) (courtesy http://
www.britannica.com/EBchecked/topic/211241/fluid-catalytic-cracking)

As also depicted in Fig. 8, the cycloalkanes (naphthenes) formed by the
initial breakup of the large molecules are further converted to aromatics
such as benzene, toluene, and xylenes, which boil in the gasoline boiling range
and have much higher octane ratings than alkanes.
In the cracking process there is also produced carbon that deposits on
the catalyst (catalyst coke). The carbon formation tendency or amount of
carbon in a crude or FCC feed is measured with methods such as Micro Carbon
Residue, Conrad son Carbon Residue or Rams bottom Carbon Residue.
By no means does Fig. 8 include all the chemistry of the primary and
secondary reactions taking place in the fluid catalytic process. There are a
great many other reactions involved. However, a full discussion of the highly
technical details of the various catalytic cracking reactions is beyond the
scope of this article and can be found in the technical literature.
Modern FCC catalysts are fine powders with a bulk density of 0.80 to
0.96 g/cc and having a particle size distribution ranging from 10 to 150 m
and an average particle size of 60 to 100 m. The design and operation of an
FCC unit is largely dependent upon the chemical and physical properties of
the catalyst. The desirable properties of an FCC catalyst are:
• Good stability to high temperature and to steam
Fig. 9: Diagrammatic example of the catalytic cracking of petroleum hydrocarbons
catalysts (courtesy http://en.wikipedia.org/wiki/fluid_catalytic_cracking)

• High activity
• Large pore sizes
• Good resistance to attrition
• Low coke production
A modern FCC catalyst has four major components: crystalline zeolite,
matrix, binder, and filler. Zeolite is the primary active component and can
range from about 15 to 50 weight percent of the catalyst. The zeolite used in
FCC catalysts is referred to as fauja site or as Type Y and is composed
of silica and alumina tetrahedral with each tetrahedron having either
an aluminium or silicon atom at the centre and four oxygen atoms at the
corners. It is a molecular sieve with a distinctive lattice structure that allows
only a certain size range of hydrocarbon molecules to enter the lattice. In
general, the zeolite does not allow molecules larger than 8 to 10 nm (i.e., 80 to
90 Å) to enter the lattice.
The catalytic sites in the zeolite are strong acids (equivalent to 90% H2
SO4
)
and provide most of the catalytic activity. The acidic sites are provided by the
alumina tetrahedral. The aluminium atom at the centre of each alumina
tetrahedral is at a +3 oxidation state surrounded by four oxygen atoms at the
corners which are shared by the neighbouring tetrahedral. Thus, the net charge
of the alumina tetrahedral is -1 which is balanced by a Na+
during the production
of the catalyst. The sodium ion is later replaced by ammonium ion, which is
vaporized when the catalyst is subsequently dried, resulting in the formation
of Lewis and Brønsted acidic sites. In some FCC catalysts, the Brønsted
sites may be later replaced by rare earth metals such as cerium and
lanthanum to provide alternative activity and stability levels.
The matrix component of an FCC catalyst contains amorphous alumina
which also provides catalytic activity sites and in larger pores that allows entry
for larger molecules than does the zeolite. That enables the cracking of higher-
boiling, larger feedstock molecules than are cracked by the zeolite.
The binder and filler components provide the physical strength and
integrity of the catalyst. The binder is usually silica sol and the filler is usually
clay Kaolin).
Nickel, vanadium, iron, copper and other metal contaminants, present in
FCC feed stocks in the parts per million ranges; all have detrimental effects on
the catalyst activity and performance. Nickel and vanadium are particularly
troublesome. There are a number of methods for mitigating the effects of
the contaminant metals:
• Avoid feed stocks with high metals content: This seriously hampers a
refinery’s flexibility to process various crude oils or purchased FCC
feed stocks.

• Feedstock feed pre treatment: Hydro desulfurization of the FCC feedstock
removes some of the metals and also reduces the sulphur content of the
FCC products. However, this is quite a costly option.
• Increasing fresh catalyst addition: some of the circulating equilibrium
catalyst as spent catalyst are withdrawn by FCC units and replaces it
with fresh catalyst in order to maintain a desired level of activity.
Increasing the rate of such exchange lowers the level of metals in the
circulating equilibrium catalyst, but this is also quite a costly option.
• De metallization: The commercial proprietary Demet Process removes
nickel and vanadium from the withdrawn spent catalyst. The nickel
and vanadium are converted to chlorides which are then washed out
of the catalyst. After drying, the de metalized catalyst is recycled into
the circulating catalyst. Removals of about 95 percent nickel removal
and 67 to 85 percent vanadium have been reported. Despite that, the
use of the De metallization process has not become widespread, perhaps
because of the high capital expenditure required.
• Metals passivation: Certain materials can be used as additives which
can be impregnated into the catalyst or added to the FCC feedstock in
the form of metal-organic compounds. Such materials react with the
metal contaminants and passivation the contaminants by forming less
harmful compounds that remain on the catalyst. For example,
antimony and bismuth are effective in passivation nickel and tin is
effective in passivation vanadium. A number of proprietary passivation
processes are available and fairly widely used. The role of catalysts in
conversion process is shown in Fig. 10.
Fig. 10: The role of catalysts in conversion processes (Courtesy CB and I) (courtesy CB&I
http://www.cbi.com/technologies/catalysts-refining-petchem-polymer)

Since World War II the demand for light products (e.g., gasoline, jet, and
diesel fuels) has grown, while the requirement for heavy industrial fuel oils has
declined. Furthermore, many of the new sources of crude petroleum (California,
Alaska, Venezuela, and Mexico) have yielded heavier crude oils with higher
natural yields of residual fuels. As a result, refiners have become even more
dependent on the conversion of residue components into lighter oils that can
serve as feedstock for catalytic cracking units.
As early as 1920, large volumes of residue were being processed
in visbreakers or thermal cracking units. These simple process units basically
consist of a large furnace that heats the feedstock to the range of 450 to
500°C (840 to 930°F) at an operating pressure of about 10 bars (1 MPa), or
about 150 psi. The residence time in the furnace is carefully limited to prevent
much of the reaction from taking place and clogging the furnace tubes. The
heated feed is then charged to a reaction chamber, which is kept at a pressure
high enough to permit cracking of the large molecules but restrict coke
formation. From the reaction chamber the process fluid is cooled to inhibit
further cracking and then charged to a distillation column for separation
into components.
Visbreaking units typically convert about 15 percent of the feedstock to
naphtha and diesel oils and produce a lower-viscosity residual fuel. Thermal
cracking units provide more severe processing and often convert as much
as 50 to 60 percent of the incoming feed to naphtha and light diesel oils.
Coking is severe thermal cracking. The residue feed is heated to about
475 to 520°C (890 to 970°F) in a furnace with very low residence time and is
discharged into the bottom of a large vessel called a coke drum for extensive
and controlled cracking. The cracked lighter product rises to the top of the
drum and is drawn off. It is then charged to the product fractionators for
separation into naphtha, diesel oils, and heavy gas oils for further processing
in the catalytic cracking unit. The heavier product remains and, because of
the retained heat, cracks ultimately to coke, a solid carbonaceous substance
akin to coal. Once the coke drum is filled with solid coke, it is removed from
service and replaced by another coke drum.
Decoking is a routine daily occurrence accomplished by a high-pressure
water jet. First the top and bottom heads of the coke drum are removed.
Next a hole is drilled in the coke from the top to the bottom of the vessel.
Then a rotating stem is lowered through the hole, spraying a water jet
sideways. The high-pressure jet cuts the coke into lumps, which fall out the
bottom of the drum for subsequent loading into trucks or railcars for
shipment to customers. Typically, coke drums operate on 24-hour cycles,
filling with coke over one 24-hour period followed by cooling, decoking, and
reheating over the next 24 hours. The drilling derricks on top of the coke
drums are a notable feature of the refinery skyline.

Cokers produce no liquid residue but yield up to 30 percent coke by
weight. Much of the low-sulphur product is employed to produce electrodes
for the electrolytic smelting of aluminium. Most lower-quality coke is burned
as fuel in admixture with coal. Coker economics usually favour the conversion
of residue into light products even if there is no market for the coke.
Before petroleum products can be marketed, certain impurities must
be removed or made less obnoxious. The most common impurities are sulphur
compounds such as hydrogen sulphide (H2
S) or the mercaptans (“R”SH), the
latter being a series of complex organic compounds having as many as six
carbon atoms in the hydrocarbon radical (“R”). Apart from their foul
odour, sulphur compounds are technically undesirable. In motor and aviation
gasoline they reduce the effectiveness of antiknock additives and interfere
with the operation of exhaust-treatment systems. In diesel fuel they cause
engine corrosion and complicate exhaust-treatment systems. Also, many
major residual and industrial fuel consumers are located in developed areas
and are subject to restrictions on sulfurous emissions.
Most crude oils contain small amounts of hydrogen sulphide, but these
levels may be increased by the decomposition of heavier sulphur compounds
(such as the mercaptans) during refinery processing. The bulk of the
hydrogen sulphide is contained in process-unit overhead gases, which are
ultimately consumed in the refinery fuel system. In order to minimize noxious
emissions, most refinery fuel gases are desulphurized.
Other undesirable components include nitrogen compounds, which poison
catalyst systems, and oxygenated compounds, which can lead to colour formation
and product instability. The principal treatment processes are outlined below.
Sweetening processes oxidize mercaptans into more innocuous disulfides,
which remain in the product fuels. Catalysts assist in the oxidation.
The doctor process employs sodium plum bite, a solution of lead oxide in
caustic soda, as a catalyst. At one time this inexpensive process was widely
practiced, but the necessity of adding elemental sulphur to make the reactions
proceed caused an increase in total sulphur content in the product. It has
largely been replaced by the copper chloride process, in which the catalyst
is slurry of copper chloride and fuller’s earth. It is applicable to both kerosene
and gasoline. The oil is heated and brought into contact with the slurry
while being agitated in a stream of air that oxidizes the mercaptans to
disulfides. The slurry is then allowed to settle and is separated for reuse. A
heater raises the temperature to a point that keeps the water formed in the
reaction dissolved in the oil, so that the catalyst remains properly hydrated.
After sweetening, the oil is water washed to remove any traces of catalyst
and is later dried by passing through a salt filter.
Hydrogen processes, commonly known as hydro treating, are the most
common processes for removing sulphur and nitrogen impurities. The oil is
combined with high-purity hydrogen, vaporized, and then passed over a catalyst

such as tungsten, nickel, or a mixture of cobalt and molybdenum oxides supported
on an alumina base. Operating temperatures are usually between 260and425°C
(500 and 800°F) at pressures of 14 to 70 bars (1.4 to 7 MPa), or 200 to 1,000 psi.
Operating conditions are set to facilitate the desired level of sulphur removal
without promoting any change to the other properties of the oil.
The sulphur in the oil is converted to hydrogen sulphide and the nitrogen
to ammonia. The hydrogen sulphide is removed from the circulating hydrogen
stream by absorption in a solution such as diethanolamine. The solution can
then be heated to remove the sulphide and reused. The hydrogen sulphide
recovered is useful for manufacturing elemental sulphur of high purity. The
ammonia is recovered and either converted to elemental nitrogen and hydrogen,
burned in the refinery fuel-gas system, or processed into agricultural fertilizers.
Molecular sieves are also used to purify petroleum products, since they
have a strong affinity for polar compounds such as water, carbon dioxide,
hydrogen sulphide, and mercaptans. Sieves are prepared by dehydration of
an alumina silicate such as zeolite. The petroleum product is passed through
a bed of zeolite for a predetermined period depending on the impurity to be
removed. The adsorbed contaminants may later be expelled from the sieve
by purging with a gas stream at temperatures between 200 and 315 °C (400
and 600°F). The frequent cycling of the molecular sieve from adsorb to de
sorbs operations is usually fully automated.
PETROLEUM PRODUCTS AND THEIR USES
Gases
Gaseous refinery products include hydrogen, fuel gas, ethane, propane,
and butane. Most of the hydrogen is consumed in refinery desulfurization
facilities, which remove H2
S from the gas stream and then separate that
compound into elemental hydrogen and sulphur; small quantities of the
hydrogen may be delivered to the refinery fuel system. Refinery fuel gas
varies in composition but usually contains a significant amount of methane;
it has a heating value similar to natural gas and is consumed in plant
operations. Periodic variability in heating value makes it unsuitable for
delivery to consumer gas systems. Ethane may be recovered from the refinery
fuel system for use as a petrochemical feedstock. Propane and butane are
sold as liquefied petroleum gas (LPG), which is a convenient portable fuel
for domestic heating and cooking or for light industrial use.
Gasoline
Motor gasoline, or petrol, must meet three primary requirements. It must
provide an even combustion pattern, start easily in cold weather, and meet
prevailing environmental requirements.

In order to meet the first requirement, gasoline must burn smoothly in
the engine without pre mature detonation, or knocking. Severe knocking
can dissipate power output and even cause damage to the engine. When
gasoline engines became more powerful in the 1920s, it was discovered that
some fuels knocked more readily than others. Experimental studies led to
the determination that, of the standard fuels available at the time, the most
extreme knock was produced by a fuel composed of pure normal heptanes,
while the least knock was produced by pure isooctane. This discovery led to
the development of the octane scale for defining gasoline quality. Thus, when
a motor gasoline gives the same performance in a standard knock engine as
a mixture of 90 percent isooctane and 10 percent normal heptanes, it is
given an octane rating of 90.
There are two methods for carrying out the knock engine test. Research
octane is measured under mild conditions of temperature and engine speed
(49°C [120°F] and 600 revolutions per minute, or RPM), while motor octane
is measured under more severe conditions (149°C [300°F] and 900 RPM).
For many years the research octane number was found to be the more
accurate measure of engine performance and was usually quoted alone. Since
the advent of unleaded fuels in the mid-1970s, however, motor octane
measurements have frequently been found to limit actual engine performance.
As a result a new measurement, road octane number, which is a simple
average of the research and motor values, is most frequently used to define
fuel quality for the consumer. Automotive gasolines generally range from
research octane number 87 to 100, while gasoline for piston-engine aircraft
ranges from research octane number 115 to 130.
Each naphtha component that is blended into gasoline is tested
separately for its octane rating. Reformate, alkylates, polymer, and cracked
naphtha, as well as butane, all rank high (90 or higher) on this scale, while
straight-run naphtha may rank at 70 or less. In the 1920s it was discovered
that the addition of tetraethyl lead would substantially enhance the octane
rating of various naphtha. Each naphtha component was found to have a
unique response to lead additives, some combinations being found to be
synergistic and others antagonistic. This gave rise to very sophisticated
techniques for designing the optimal blends of available components into
desired grades of gasoline.
The advent of leaded, or ethyl, gasoline led to the manufacture of high-
octane fuels and became universally employed throughout the world after
World War II. However, beginning in 1975, environmental legislation began
to restrict the use of lead additives in automotive gasoline. It is now banned
in the United States, the European Union, and many countries around the
world. The required use of lead-free gasoline has placed a premium on the
construction of new catalytic reformers and alkylation units for increasing
yields of high-octane gasoline ingredients and on the exclusion of low-octane
naphtha from the gasoline blend.

High-Volatile and Low-Volatile Components
The second major criterion for gasoline is that the fuel be sufficiently volatile
to enable the car engine to start quickly in cold weather is accomplished by
the addition of butane, very low-boiling paraffin, to the gasoline blend.
Fortunately, butane is also a high-octane component with little alternate
economic use, so its application has historically been maximized in gasoline.
Another requirement, that a quality gasoline have high energy content, has
traditionally been satisfied by including higher-boiling components in the
blend.
However, both of these practices are now called into question on
environmental grounds. The same high volatility that provides good starting
characteristics in cold weather can lead to high evaporative losses of gasoline
during refuelling operations, and the inclusion of high-boiling components
to increase the energy content of the gasoline can also increase the emission
of unburned hydrocarbons from engines on start-up. As a result, since the
1990 amendments of the U.S. Clean Air Act, much of the gasoline consumed
in urban areas of the United States has been reformulated to meet stringent
new environmental standards. At first these changes required that gasoline
contain certain percentages of oxygen in order to aid in fuel combustion and
reduce the emission of carbon monoxide and nitrogen oxides. Refiners met
this obligation by including some oxygenated compounds such as ethyl
alcohol or methyl tertiary butyl ether (MTBE) in their blends.
However, MTBE was soon judged to be a hazardous pollutant of
groundwater in some cases where reformulated gasoline leaked from
transmission pipelines or underground storage tanks, and it was banned in
several parts of the country. In 2005 the requirements for specific oxygen
levels were removed from gasoline regulations, and MTBE ceased to be
used in reformulated gasoline. Many blends in the United States contain
significant amounts of ethyl alcohol in order to meet emissions requirements,
and MTBE is still added to gasoline in other parts of the world.
One of the most critical economic issues for a petroleum refiner is
selecting the optimal combination of components to produce final gasoline
products. Gasoline blending is much more complicated than a simple mixing
of components. First, a typical refinery may have as many as 8 to 15 different
hydrocarbon streams to consider as blend stocks. These may range from
butane, the most volatile component, to a heavy naphtha and include several
gasoline naphtha from crude distillation, catalytic cracking, and thermal
processing units in addition to alkylates, polymer, and reformate. Modern
gasoline may be blended to meet simultaneously 10 to 15 different quality
specifications, such as vapour pressure; initial, intermediate, and final boiling
points; sulphur content; colour; stability; aromatics content; olefin content;
octane measurements for several different portions of the blend; and other
local governmental or market restrictions.

Since each of the individual components contributes uniquely in each of
these quality areas and each bears a different cost of manufacture, the
proper allocation of each component into its optimal disposition is of major
economic importance. In order to address this problem, most refiners
employ linear programming, a mathematical technique that permits the rapid
selection of an optimal solution from a multiplicity of feasible alternative
solutions. Each component is characterized by its specific properties and
cost of manufacture and each gasoline grade requirement is similarly defined
by quality requirements and relative market value. The linear programming
solution specifies the unique disposition of each component to achieve
maximum operating profit. The next step is to measure carefully the rate of
addition of each component to the blend and collect it in storage tanks for
final inspection before delivering it for sale. Still, the problem is not fully
resolved until the product is actually delivered into customers’ tanks.
Frequently, last-minute changes in shipping schedules or production qualities
require the re blending of finished gasoline or the substitution of a high-
quality (and therefore costlier) grade for one of more immediate demand
even though it may generate less income for the refinery.
Though its use as an illuminate has greatly diminished, kerosene is still
used extensively throughout the world in cooking and space heating and is
the primary fuel for modern jet engines. When burned as a domestic fuel,
kerosene must produce a flame free of smoke and odour. Standard laboratory
procedures test these properties by burning the oil in special lamps. All
kerosene fuels must satisfy minimum flash-point specifications (49°C, or
120°F) to limit fire hazards in storage and handling.
Jet fuels must burn cleanly and remain fluid and free from wax particles
at the low temperatures experienced in high-altitude flight. The conventional
freeze-point specification for commercial jet fuel is “50°C (“58°F). The fuel
must also be free of any suspended water particles that might cause blockage
of the fuel system with ice particles. Special-purpose military jet fuels have
even more stringent specifications.
Diesel Oils
The principal end use of gas oil is as diesel fuel for powering automobile,
truck, bus, and railway engines. In a diesel engine, combustion is induced
by the heat of compression of the air in the cylinder under compression.
Detonation, which leads to harmful knocking in a gasoline engine, is a
necessity for the diesel engine. A good diesel fuel starts to burn at several
locations within the cylinder after the fuel is injected. Once the flame has
initiated, any more fuel entering the cylinder ignites at once.
Straight-chain hydrocarbons make the best diesel fuels. In order to
have a standard reference scale, the oil is matched against blends of cetane
(normal hexadecane) and alpha methylnaphthalene, the latter of which gives

very poor engine performance. High-quality diesel fuels have cetane ratings of
about 50, giving the same combustion characteristics as a 50-50 mixture of the
standard fuels. The large, slower engines in ships and stationary power plants
can tolerate even heavier diesel oils. The more viscous marine diesel oils are
heated to permit easy pumping and to give the correct viscosity at the fuel
injectors for good combustion.
Until the early 1990s, standards for diesel fuel quality were not particularly
stringent. A minimum cetane number was critical for transportation uses,
but sulphur levels of 5,000 parts per million (ppm) were common in most
markets. With the advent of more stringent exhaust emission controls,
however, diesel fuel qualities came under increased scrutiny. In the European
Union and the United States, diesel fuel is now generally restricted to
maximum sulphur levels of 10 to 15 ppm, and regulations have restricted
aromatic content as well. The limitation of aromatic compounds requires a
much more demanding scheme of processing individual gas oil components
than was necessary for earlier highway diesel fuels.
Fuel Oils
Furnace oil consists largely of residues from crude oil refining. These are
blended with other suitable gas oil fractions in order to achieve the viscosity
required for convenient handling. As a residue product, fuel oil is the only
refined product of significant quantity that commands a market price lower
than the cost of crude oil.
Because the sulphur contained in the crude oil is concentrated in the
residue material, fuel oil sulphur levels are naturally high. The sulphur
level is not critical to the combustion process as long as the flue gases do
not impinge on cool surfaces (which could lead to corrosion by the
condensation of acidic sulphur trioxide). However, in order to reduce air
pollution, most industrialized countries now restrict the sulphur content of
fuel oils. Such regulation has led to the construction of residual desulfurization
units or Cokers in refineries that produce these fuels.
Residual fuels may contain large quantities of heavy metals such
as nickel and vanadium; these produce ash upon burning and can foul burner
systems. Such contaminants are not easily removed and usually lead to
lower market prices for fuel oils with high metal contents.
Olefins
The thermal cracking processes developed for refinery processing in the
1920s were focused primarily on increasing the quantity and quality of gasoline
components. As a by-product of this process, gases were produced that
included a significant proportion of lower-molecular-weight olefins, particularly
ethylene, propylene, and butylenes. Catalytic cracking is also a valuable

source of propylene and butylenes, but it does not account for a very significant
yield of ethylene, the most important of the petrochemical building blocks.
Ethylene is polymerized to produce polyethylene or, in combination with
propylene, to produce copolymers that are used extensively in food-packaging
wraps, plastic household goods, or building materials.
Ethylene manufacture via the steam cracking process is in widespread
practice throughout the world. The operating facilities are similar to gas oil
cracking units, operating at temperatures of 840°C (1,550°F) and at low
pressures of 165 kilopascals (24 pounds per square inch). Steam is added to
the vaporized feed to achieve a 50-50 mixture, and furnace residence times
are only 0.2 to 0.5 second. In the United States and the Middle East, ethane
extracted from natural gas is the predominant feedstock for ethylene cracking
units. Propylene and butylenes are largely derived from catalytic cracking
units in the United States. In Europe and Japan, catalytic cracking is less
common, and natural gas supplies are not as plentiful. As a result, both the
Europeans and Japanese generally crack a naphtha or light gas oil fraction to
produce a full range of olefin products.
Aromatics
The aromatic compounds, produced in the catalytic reforming of naphtha,
are major sources of petrochemical products. In the traditional chemical
industry, aromatics such as benzene, toluene, and the xylenes were made
from coal during the course of carbonization in the production of coke and
town gas. Today a much larger volume of these chemicals are made as refinery
by-products. A further source of supply is the aromatic-rich liquid fraction
produced in the cracking of naphtha or light gas oils during the manufacture
of ethylene and other olefins.
Polymers
A highly significant proportion of these basic petrochemicals are converted
into plastics synthetic rubbers and synthetic fibres. Together these materials
are known as polymers because their molecules are high-molecular-weight
compounds made up of repeated structural units that have combined
chemically. The major products are polyethylene, polyvinyl chloride and
polystyrene, all derived from ethylene, and polypropylene, derived from
monomer propylene. Major raw materials for synthetic rubbers include
butadiene, ethylene, benzene, and propylene. Among synthetic fibres the
polyesters comprised of ethylene glycol and terephthalic acid (made
from xylenes) are the most widely used. They account for about one-half of
all synthetic fibres. The second major synthetic fibre is nylon, its most
important raw material being benzene. Acrylic fibres, in which the major
raw material is the propylene derivative acrylonitrile, make up most of the
remainder of the synthetic fibres.

Power Recovery in Fluid Catalytic Cracker
The combustion fuel gas from the catalyst regenerator of a fluid catalytic
cracker is at a temperature of about 715°C and at a pressure of about 2.4
brag (240 k Pa gauge). Its gaseous components are mostly carbon
monoxide (CO), carbon dioxide (CO2
) and nitrogen (N2
). Although the flue gas
has been through two stages of cyclones (located within the regenerator) to
remove entrained catalyst fines, it still contains some residual catalyst fines.
Figure 5 depicts how power is recovered and utilized by routing the
regenerator flue gas through a turbo expander. After the flue gas exits the
regenerator, it is routed through a secondary catalyst separator containing
swirl tubes designed to remove 70 to 90 percent of the residual catalyst
fines. This is required to prevent erosion damage to the turbo expander.
As shown in Fig. 8, expansion of the flue gas through a turbo expander
provides sufficient power to drive the regenerator’s combustion air compressor.
The electrical motor generator in the power recovery system can consume or
produce electrical power. If the expansion of the flue gas does not provide enough
power to drive the air compressor, the electric motor-generator provides the
needed additional power. If the flue gas expansion provides more power than
needed to drive the air compressor, than the electric motor-generator converts
the excess power into electric power and exports it to the refinery’s electrical
system. The steam turbine shown in Fig. 5 is used to drive the regenerator’s
combustion air compressor during start-ups of the fluid catalytic cracker until
there is sufficient combustion flue gas to take over that task.
The expanded flue gas is then routed through a steam-generating
boiler (referred to as a CO boiler)) where the carbon monoxide in the flue
gas is burned as fuel to provide steam for use in the refinery.
The flue gas from the CO boiler is processed through an electrostatic
precipitator (ESP) to remove residual particulate matter. The ESP removes
particulates in the size range of 2 to 20 micrometers from the flue gas. As a
unit operation being very crucial for the manufacturing of petro products
the power recovery system in a fluid catalytic cracking unit is shown in the
Fig. 11.
Two prominent inorganic chemicals, ammonia and sulphur, are also
derived in large part from petroleum. Ammonia production requires hydrogen
from a hydrocarbon source. Traditionally, the hydrogen was produced from
a coke and steam reaction, but today most ammonia is synthesized from
liquid petroleum fractions, natural gas, or refinery gases. The sulphur
removed from oil products in purification processes is ultimately recoverable
as elemental sulphur or sulphuric acid. It has become an important source
of sulphur for the manufacture of fertilizer.

The most versatile refinery configuration is known as the conversion
refinery. A conversion refinery incorporates all the basic building blocks
found in both the topping and hydro skimming refineries, but it also features
gas oil conversion plants such as catalytic cracking and hydro cracking
units, olefins conversion plants such as alkylation or polymerization units,
and, frequently, coking units for sharply reducing or eliminating the
production of residual fuels. Modern conversion refineries may produce two-
thirds of their output as gasoline, with the balance distributed between
high-quality jet fuel, liquefied petroleum gas (LPG), diesel fuel, and a small
quantity of petroleum coke. Many such refineries also incorporate solvent
extraction processes for manufacturing lubricants and petrochemical units
with which to recover high-purity propylene, benzene, toluene, and xylenes
for further processing into polymers.
Processing Configuration
Each petroleum refinery is uniquely configured to process a specific raw
material into a desired slate of products. In order to determine which
configuration is most economical, engineers and planners survey the local
market for petroleum products and assess the available raw materials. Since
about half the product of fractional distillation is residual fuel oil, the local
market for it is of utmost interest. In parts of Africa, South America, and
Southeast Asia, heavy fuel oil is easily marketed, so that refineries of simple
configuration may be sufficient to meet demand. However, in the United
States, Canada, and Europe, large quantities of gasoline are in demand, and
the market for fuel oil is constrained by environmental regulations and the
availability of natural gas In these places, more complex refineries are
necessary.
Fig. 11: A schematic diagram of the power recovery system in a fluid catalytic cracking
unit Inorganic Chemicals (courtesy http://en.wikipedia.org/wiki/turboexpander)

Fluid catalytic cracking (FCC) plants are used to convert heavy distillates
into lighter ones like gasoline and diesel. The feedstock is primarily vacuum
gas oil, often mixed with refinery residues.
The main products are:
• Gas fraction (mainly C3/C4)
• Liquid fraction
• Coke (solid formation on the catalyst).
FCC units produce sulphur dioxide and nitrogen oxides (NOx) and these
particulates are tightly regulated in the petro chemical companies. This
places refinery operator under pressure to mange NOx emissions and
ensuring, that these impurities do not impair air quality.
Linde offers the various solutions to enrich regeneration air with oxygen
and boost capacity. These include LoTOx technologies to help control
particulates, sulphur dioxide and NOx emissions from the FCC.
LoTOx technology is a patented innovation that uses ozone to selectively
oxidize insoluble NOx to highly soluble species that can be easily removed in
a wet scrubber. The benefits include increased capacity, greater flexibility
in the choice of feeds, increased conversion rates and reduced emissions.
Test plants results explained by Linde Industrial gases is as follows. It may
be seen in Fig. 12.
Fig. 12: Test Plants results by Linde IG
Theory of Heat Balance
Catalytic cracking reactions are endothermic; they create products with
higher heat contents than the reactants and they absorb heat from the
environment. In the cracking of paraffin by the beta scission mechanism,

high molecular weight paraffin is cracked to form a lower molecular weight
olefin and paraffin. Table 3 uses the cracking of normal decane and normal
heptanes as examples of this beta scission mechanism. Lower molecular
weight hydrocarbons and higher molecular weight hydrocarbons both require
approximately equal BTU’s of heat to crack a carbon-carbon bond by beta
scission, but the energy on a per pound basis increases as the molecular
weight of the feed decreases. Thus Table 3 shows a higher endothermic heat
of cracking for heptanes than for decane.
Table 3: Endothermic reactions of scission cracking (Ernest L.Leuenberger and Linda J.
Wilbert)
Reaction 1: N Decane  Npentane + 1 - Pentene
Equation: C10
H22
 N C5
H12
+ C5
H10
Heat of Reaction in
BTU/LB Feed = 249 BTU/LB Feed
Reaction 2: N Heptane  Propane + 1 - Butene
Equation: C7
H16
 C3
H8
+ C4
H8
Heat of Reaction in
BTU/LB Feed = 366 BTU/LB Feed
Table 4: Exothermic reactions of hydrogen transfer (Ernest L. Leuenberger and Linda J.
Wilbert)
Reaction 1: Cyclohexane +  Benzene + 3N Butane
3 C-2 Butene
Equation: C6
H12
+ 3 C4
H8
 C6
H6
+ 3N C4
H10
Heat of Reaction in
BTU/LB Feed = -259 BTU/LB
Reaction 2: Benzene +
3C-2 Butene  Coke + 3 N Butane
Equation: C6
H6
+ 3 C4
H8
 6C + 3 C4
H10
Heat of Reaction in
BTU/LB Feed = -772 BTU/LB
Rare earth exchange in a zeolite catalyst promotes hydrogen transfer
reactions in competition with beta scission. One effect of hydrogen transfer
is to limit the production of C3 and C4 gases by beta scission. When the
number of cracking reactions that form light gases is inhibited, the
endothermic heat of reaction is reduced.
The hydrogen transfer reactions promoted by rare earth exchange also
tend to reduce the endothermic heat of cracking because they are exothermic.
Table 4 gives examples of two such exothermic hydrogen transfer reactions.
In the first example, a naphthenes and lower molecular weight olefins react
to form aromatic and light paraffin. When the olefins saturated by hydrogen
transfer are in the gasoline boiling range, this reaction is responsible for
reducing gasoline octane. The second reaction shows how hydrogen transfer
can form a carbonaceous deposit on the catalyst from a heavy aromatic.
When this type of hydrogen transfer is eliminated, coke make is reduced.

Table 5: Lower Heats of cracking by rare earth exchanged catalysts (Ernest L. Leuenberger
and Linda J. Wilbert)
Catalyst type Heat of cracking BTU/LB Fresh feed
Rare earth exchanged y Faujasite 80
Partially rare earth exchanged y Faujasite 140
Hydrogen exchanged y Faujasite 160
Ultrastable hydrogen exchanged y Faujasite 180
Values for Commercial units Reported by J.L. Mauleon and J.C. Courcelle, Oil and Gas
Journal, October 21, 1985.
FCC octane catalysts maximize the octane of the cracked gasoline by
minimizing the hydrogen transfer reactions that saturate gasoline olefins.
Mauleon et al., observed that reducing hydrogen transfer must also increase
the endothermic heat of cracking. The results of his work, which are
presented in Table 5, show that heat of cracking can be correlated with
catalyst rare earth content. Rajagopalan and Peters observed that reduced
hydrogen transfer reduces coke make may be referred in Table 6.
Table 6: Higher cokes make with rare earth exchange catalysts (Ernest L. Leuenberger
and Linda J. Wilbert)
Catalyst type Rare earth Coke make at
WT% 70% conversion
Rare earth exchanged y Faujasite 3.0 2.1
Partially rare earth exchanged y Faujasite 1.5 2.2
Hydrogen exchanged y Faujasite 0.0 2.1
Values for mat runs on Mid continent feeds reported by K. Rajagopoalan and A.W. preprint
of the A.C.S. division of petroleum chemistry, Vol. 30, No. 3, Page 538 (1985).
Methods of Determining Heats of Cracking
Two different approaches to determining the heat of cracking are possible:
1. Determine the heat absorbed by the cracking reaction through heat
balance methods.
2. Analyze the reaction products and assign each one a heat of combustion,
then add up the heat of combustion of the products. For constant
feedstock, the reaction with the highest product heat of combustion
has the highest endothermic heat of reaction.
The first method was not attempted for laboratory heat of cracking
measurements, but was found useful for commercial data analysis. The second
method was found to be applicable for both laboratory and commercial data
analysis.
The first technique, which is called the heat balance technique for
determining heats of cracking, involves calculating the heat of combustion

of coke in a commercial FCC operation, then subtracting all other heat
requirements. The remaining heat is then assumed to be the endothermic
heat of cracking. The accuracy of this measurement technique depends on
the accuracy of the measurements that determine the coke heat of
combustion and the completeness of the heat balance information recorded
at the commercial plant.
The second technique, which we call the product analysis technique,
was applied by Dart and Oblad in their classic work on measuring heats of
cracking. Each product was assigned a heat of combustion from the literature.
For the liquid products, a correlation from the API data book was used to
determine the heat of cracking from the API gravity and the K factor. This
was a modification of Dart’s procedure, where these liquid heats of
combustion were determined by calorimeter. The heats of combustion used
in the product analysis technique are presented in Table 7.
Table 7: Heat of reactions calculation for heats of combustion (Ernest L. Leuenberger and
Linda J. Wilbert)
Reaction product Heat of combustion BTu/LB
H2
51980
CH4
21580
C2
= 20350
C2
20480
C3
= 19750
C3
19990
C4
= 19480
C4
19670
NC4
19720
Gasoline (58 API, 11.85k) 18780
LCO (24, API, 11.01k) 18030
Bottoms (11 API, 10.64k) 17630
Coke (7% Hydrogen) 16120
Net or Lower heating values are used.
The procedure described in the preceding paragraph is also applicable to
commercial data and was used to confirm the heat balance heat of reaction
calculations.
It was further examined that the heat of cracking and coke make
differences between rare earth exchanged catalysts and FCC octane catalysts.
Our objective is to quantify the heat balance effects and to use the resulting
correlation to model catalyst changes in commercial FCCU’s.
Significance of Heat Cracking and Delta Coke Differences on
Commercial FCCU Operation
When a catalyst change increases the heat of cracking in a commercial FCCU,
the unit must either increase its heat generation or reduce its heat

requirements to stay in heat balance. Since the catalyst change out usually
occurs over a period of several weeks, the changes will be gradual and the
unit will have sufficient time to respond without a crisis. A unit with normal
slide valve controls will respond to the increase in heat requirement by
gradually increasing catalyst circulation. According to one published model
the coke yield will increase proportional to the catalyst circulation raised to
the 65 exponent. Thus higher circulation will increase the coke burned in
the regenerator and bring the unit back into heat balance. If the unit does
not have the air blower capacity to burn more coke, changes must be made
in the unit operation to reduce the heat requirements. The simplified overall
heat balance in Table 8 lists these other requirements as the heat required
to increase the feed temperature to the riser outlet temperature, the heat
to vaporize the feed, and the sensible heat of the air. When no more coke
can be burned, it is most economical to raise the feed preheat in order to
reduce the feed sensible heat requirement. The last resort would be to lower
the feed rate to reduce all three feed enthalpy terms in the heat balance.
The air heat requirement cannot be lowered if the unit is at a coke burning
limit.
Table 8: Simplified overall heat balance (Ernest L. Leuenberger and LInda J. Wilbert)
Heat generated Heat required to Heat required Heat required to Heat required
by coke increase feed to to vaporize crack feed + to increase air
burning = reactor temp + feed + to fuel gas temp
HCOKE
COKE = F CPF
(TREACTOR
– TFEED
)+ FHVAP
+F HCRACK
+ Air CPA
(TFLUE
– TAMB
)
The reduction in delta coke make caused by introducing an octane catalyst
will also create an imbalance in the overall heat balance. Again the standard
FCCU control system would raise catalyst circulation to increase the coke make
back to its original level. This control strategy for handling lower delta coke
will fail only if the catalyst circulation is at its maximum. Alternate strategies
to increase coke make include introducing a feed to the FCCU with a higher
carbon residue such as vacuum reside or slurry recycle. Spraying torch oil into
the regenerator can be used as a last resort.
The increases in catalyst circulation caused by lowering the delta coke and
raising the heat of cracking tend to reduce the regenerator temperature. The
simplified regenerator heat balance in Table 8 shows that the heat transferred
to the reactor plus the air sensible heat is equal to the heat released by burning
coke. The coke burned in the table is represented by the previously sited model.
Coke = K Cat Circ•65
When the heat balance is solved for the regenerator temperature minus
the reactor temperature and the air sensible heat is ignored, this
temperature difference is inversely proportional to the catalyst circulation
raised to the .35 exponent. At constant reactor temperature, the heat balance

Table 9: Projected heat balance effects of FCC octane catalysts (Ernest L. Leuenberger and
Linda J. Wilbert)
Case description Gasoline cat Octave cat at Octane cat at
base case constant activity constant conversion
Temperatures, F
Reactor 968 968 968
Regenerator 1281 1262 1245
Combined feed 300 300 300
Catalyst
Activity 72 72 68
Cat ti OL 7.0 8.0 8.5
Yields
Conversion, vol% 70 72 70
Coke make, Wt% 5.9 6.3 6.2
then requires the regenerator to cool as the catalyst circulation increases.
More complete versions of the simplified coke make kinetics and heat
balances discussed above have been incorporated into an FCCU computer
simulation program. This program was used to model the effects of a 50
BTU per pound increase in the heat of cracking and a 10 relative percent
drop in delta coke make on a commercial FCCU. The results of one simulation
are presented in Table 9. According to the model, if enough octane catalyst
is added to the unit to maintain catalyst activity, then a 20°F loss in
regenerator temperature, a 15 relative percent increase in cat to oil, a 7
relative percent increase in coke make, and a 2 volume percent increase in
conversion would be expected. Correlations show this catalyst would also
increase gasoline octane by 2 RON. A second projection has also been included
where only enough octane catalyst is used to hold conversion constant. This
case is more usual for commercial octane catalyst trials, since the octane
catalysts often do not hold activity in the unit as well as rare earth exchanged
catalysts. For this alternate case, a 35°F drops in regenerator temperature
and a 20 relative percent increase in cat to oil ratio are predicted.
Table 10 shows the heat balance results of an octane catalyst trial that
parallels the simulation model’s predictions. When catalyst P1 replaced a rare
earth exchanged gasoline catalyst, the regenerator temperature equilibrated
70°F lower. The cracked gasoline octane increased by 2.5 RON. The octane
catalyst was not able to hold MAT activity; it dropped by 6 numbers. Even with
that activity loss, the conversion loss was minimized because of a 24 percent
increase in cat to oil ratio. The refiner could have held conversion constant if
he had the coke burning capacity; but he was forced to increase the feed preheat
to keep the coke make constant. Thermal verses catalytic cracking yields on
similar toped feed is presented in Table 11 and comparison of fluid Thermafor,
Houdry and catalytic cracking units may be referred in Table 12.

Table 10: Commercial heat balance data (Ernest L. Leyebverger abd Linda J. Wilbert)
Catak Yst R5 P1
Reo, Wt% 3.9 0.9
Matrix Sa, M2
/Gm 130 117
Fresh unit cell size, A 24.7 24.6
Equilibrium mat activity 75 69
Unit operation
Reactor temperature, F 980 979
Feed temperature, F 430 477
Regen temperature, F 1355 1284
Cat to Oil ratio 5.5 6.8
Conversion, vol% 75 73
Coke make Wt% 5.1 5.1
Table 11: Thermal Vs catalytic cracking yields on similar toped feed
Thermal cracking Catalytic cracking
Wt% vol% wt% vol%
Fresh feed 100.0 100.0 100.0 100.0
Gas 6.6 – 4.5 –
Propane 2.1 3.7 1.3 2.2
Propylene 1.0 1.8 2.0 3.4
Isobutane 0.8 1.3 2.6 4.0
n-Butane 1.9 2.9 0.9 1.4
Butylene 1.8 2.6 2.6 3.8
C5
+ gasoline 26.9 32.1 40.2 46.7
Light cycle oil 1.9 1.9 33.2 32.0
Decant oil – – 7.7 8.7
Residual oil 57.0 50.2 – –
Coke 0 – 5.0 –
Total 100.0 96.5 100.0 102.0
Table 12: Comparison of fluid Thermafor, Houdry catalytic cracking units
FCC TCC HCC
Reactor space velocity 1.1–13.4b
1–3b
1.5–4b
C/O 5–16c
2–7d
3–7d
Recycle/fresh feed, vol 0–0.5 0–0.5 0–0.5
Catalyst requirement, ib/bbi feed 0.15–0.25 0.06–0.13 0.06–0.13
Cat. crclt. rate. ton cat./bbi total feed 0.9–1.5 0.4–0.6 0.4–0.6
On-stream efficiency, % 96–98 – –
Reactor temp., F 885–950c
840–950 875–950
Regenerator temp., F 1200–1500 1100–1200 1100–1200
Reactor pressure, psig 8–30c
8–12 9–12
Regenerator pressure, psig 15–30 – –
Turndown ratio – – 2:1
Gasoline octane, clear
RON 92–99 88-94 88–94
MON 80–85 – –
a
ib/hr/ib.; b
v/hr/v.; c
wt.; d
vol. * One company has operated at 990°F and 40 psig to produce
a 98 RON (clear) gasoline with a C3-650F liquid yield of 120 vol% on feed (once-through);
there was approximately 90% yield of the C5
-650F Product.

REFERENCES
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Alex Hoffmann, C. and Lewis Stein, E. 2002. Gas cyclones and swirl tubes: Principles, design and
operation (1st ed.). Springer. ISBN 3-540-43326-0.
Amos Avidan, A., Edwards, Michael and Owen, Hartley 1990. Mobil research and development.
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Palucka, Tim. 2005. The wizard of octane eugene houdry. Invention and Technology, 20(3).
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8415-289-8.
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