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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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Introduction:In context to refinery processes for petroleum,
cracking is defined as decomposition induced by elevated
temperatures (>350 C, >660 F), whereby the higher molecular
weight constituents of petroleum are converted to lower molecular
weight products. Cracking reactions involve carbon-carbon bond
rupture & are thermodynamically favored at high
temperatures.
Although cracking basically involves converting large oil
molecules into low boiling materials, during the actual process
some smaller molecules may combine to give a product of higher
molecular weight.
A number of products may be formed during cracking of the
petroleum feedstock such as gasoline, coke & fuel oil. Some
material obtained during cracking & having a boiling range
intermediate between gasoline & fuel oil is referred to as
recycle stock, which is recycled back into the cracking equipment
until conversion is complete.
Chemistry of Cracking:Two general types of reaction occur during
cracking:
1. The decomposition of large molecules into small molecules
(primary reactions):
CH3 CH2 CH2 CH3 CH4 + CH3 CH=CH2
Butane methane propeneOr
CH3 CH2 CH2 CH3 CH3 CH3 + CH2=CH2
Butane ethane ethylene2. Reactions by which some of the primary
products react to form higher molecular weight
materials (secondary reactions):
CH2=CH2 + CH2=CH2 CH3 CH2 CH=CH2
Ethylene buteneOr
R CH= CH2 + R CH= CH2 tar, heavy oil, coke, etc.
Methodology of cracking:There are several methods of performing
cracking reactions & are described below:1. Thermal Cracking:
This involves the noncatalytic conversion of higher-boiling
petroleum stocks
into lower-boiling products by application of temperatures above
350 C. From the reaction point of view thermal cracking is a free
radical chain reaction, a free radical being defined as an atom or
group of atoms with an unpaired electron. For free radical
reactions of various kinds involving hydrocarbons, refer page
255-257 of The Chemistry & Technology of Petroleum by James G.
Speight or any standard organic chemistry textbook.
2 Catalytic Cracking : This is nothing but thermal decomposition
similar to thermal cracking except that the cracking process occurs
in the presence of a catalyst, which is not (in theory) consumed in
the process & directs the course of the cracking reactions to
produce more of the desired higher-octane hydrocarbon products.
Nowadays most gasoline fractions are produced by this method
superseding the older thermal cracking process. The chemistry of
catalytic cracking is an ionic process involving carbonium ions
which are hydrocarbon ions having a positive charge on a carbon
atom. For details of chemical reactions involving catalytic
cracking,
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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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refer page 255-257 of The Chemistry & Technology of
Petroleum by James G. Speight or any standard organic chemistry
textbook.
3. Visbreaking: Viscosity breaking or visbreaking is a mild
thermal cracking operation used to reduce the viscosity & pour
point of residual or asphaltic feed stocks. This operation produces
a large amount of partially cracked gas oil which could then be
processed in a conventional catalytic cracking plant.
4. Coking : Coking is again a variation of the thermal cracking
process in which the time of cracking is so long that coke is
produced as the bottom product. It is used for the continuous
conversion of heavy, low-grade oils into lower products. The
feedstock can be material such as reduced crude, straight-run
residua, or cracked residua, & the products are gases, naphtha,
fuel oil, gas oil, & coke. The coke obtained is usually used as
fuel, but also finds specialty uses such as electrode manufacture,
production of chemicals, & metallurgical coke, thus increasing
its value.
5. Hydrocracking: This is an advancement in catalytic cracking
process (>350 C) in which hydrogenation accompanies cracking. It
is characterized by the cleavage of carbon-to-carbon linkages
accompanied by hydrogen saturation of the fragments to produce
lower-boiling products. Relatively high hydrogen pressures (100 to
2000 psi) are required to minimize polymerizations &
condensations leading to coke formation.
6. Hydrotreating : This is again a variation of the catalytic
cracking process, except that, although catalysts are employed for
hydrotreating, there is very little cracking involved & the
process actually is used for selective hydrogen addition to olefins
& aromatics in order to saturate them. Another important
purpose of hydrotreating is removal of sulfur & nitrogen
compounds present in the feedstock by selective hydrogenation. The
temperatures & pressures employed are generally moderate
compared to hydrocracking.
Description of Cracking processes:
1. Thermal Cracking: Thermal cracking of higher-boiling
materials to motor or high-octane gasoline is now becoming an
obsolete process, since these days the requirement of high-octane
& low levels of deleterious sulfur & nitrogen compounds has
proved to be a serious limitation for this process. New units are
now practically not installed & many of the older operating
refineries have either shutdown these units or have gone for
revamping the older units to the more modern catalytic processes.
Nevertheless a brief description of the commercial thermal cracking
processes is given for better understanding The Dubbs Process: This
is a typical application of the thermal cracking process. The
feedstock (reduced crude) is preheated by direct exchange with the
cracked products in the fractionating columns. Cracked gasoline
& heating oil are removed from the upper section of the column.
Light & heavy distillate fractions are removed from the lower
section & are pumped to separate heaters. Higher temperatures
are used to crack the more refractory light distillate fraction.
The streams from the heaters are combined & sent to a reaction
chamber where a certain residence time allows the cracking
reactions to be completed. The cracked products are then separated
in a low-pressure flash chamber where a heavy fuel oil is removed
as bottoms. The remaining cracked products are sent to the
fractionating column.
Low pressures (
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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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Gas
Feedstock (topped crude) Gasoline
Furnace
Oil Residuum Recycle
Mixed-Phase Cracking: Mixed-phase cracking (also called
liquid-phase cracking) is a continuous thermal decomposition
process for the conversion of heavy feedstocks to products boiling
in the gasoline range. The process generally employs rapid heating
of the feedstock (kerosene, gas oil, reduced crude, or even whole
crude), after which it is passed to a reaction chamber & then
to a separator where the vapors are cooled. Overhead products from
the flash chamber are fractionated to gasoline components &
recycle stock, while flash chamber bottoms are withdrawn as a heavy
fuel oil. Coke formation, which may be considerable at the process
temperatures (400 to 480 C), is minimized by use of pressures in
excess of 350 psi.
Gas
Gasoline
Feed Stock
Residuum Mixed-phase thermal cracking
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Vapor-Phase Cracking: Vapor-phase cracking is a high temperature
(545 to 595 C), low-pressure (< 50 psi) thermal conversion
process which favors dehydrogenation of feedstock (gaseous
hydrocarbons to gas oils) to olefins & aromatics. Coke is often
deposited in heater tubes causing shutdowns - relatively large
reactors are required for these units.
Gas
Gasoline
FeedStock
Heavy fuel oil Recycle
Vapor-phase thermal cracking
Selective Cracking: This is a thermal conversion process, which
utilizes optimum conditions of temperature & pressure for
maximum product yield solely depending on the nature of the
feedstock. For example, heavy oil might be cracked at 495 to 515 C
& 300 to 500 psi, while lighter gas oil may be cracked at 510
to 530 C & 500 to 700 psi. It eliminates the accumulation of
stable low-boiling material in the recycle stock & also
minimizes coke formation from high-temperature cracking of the
higher-boiling material. The end result is the production of fairly
high yields of gasoline, middle distillates, & olefinic gases.
Gas
Feedstock (topped crude) Gasoline
Middle Distillate
Residuum Light oil (recycle) Heavy oil (recycle)
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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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Naphtha Cracking: The thermal cracking of naphtha involves the
upgrading of low-octane fractions of catalytic naphtha to
higher-quality material. The process is designed for upgrading
heavier portions of naphtha, which contain uncracked virgin
feedstock & to remove napthenes, as well as paraffins. During
the process, small quantities of heavier aromatics are formed by
condensation reactions, & the product stream contains
substantial quantities of olefins.
2. Catalytic Cracking: As mentioned earlier, catalytic cracking
differs from thermal cracking by use of catalyst & also is a
much more efficient process compared to the non-catalytic thermal
cracking. The catalytic & thermal methods are compared in the
table below:
Sr. No.
Catalytic Cracking Thermal Cracking
1 The gasoline produced by this method has a higher octane
number.
Comparatively lower octane number gasoline produced.
2 The gasoline produced consists largely of isoparaffins &
aromatics, which contribute to the higher octane number & also
are chemically more stable.
Gasoline contains more mono-olefins & diolefins which are
relatively less stable.
3 Catalytic cracking produces substantial quantities of olefinic
gases suitable for polymer gasoline manufacture along with small
amounts of methane, ethane, & ethylene.
Quantity of olefinic gases is smaller in thermal cracking.
4 Catalytic cracking is a more selective cracking process &
gives lesser end products.
Not a very selective process & end products are more.
5 Gives a more economically salable coke. Coke quality is not
very high.6 It has greater capability to accept high-
sulfur feedstock. Also gasoline produced by this method has a
lower sulfur content.
High-sulfur feedstocks can prove to be a limitation.
Catalytic cracking, as a commercial process thus involves
contacting a gas oil fraction with an active catalyst under
suitable conditions of temperature, pressure, & residence time
so that a substantial part (> 50%) of the gas oil is converted
into gasoline & lower-boiling products, usually in a single
pass operation.
A limitation of the catalytic cracking process is the deposition
of carbonaceous material on the catalyst, reducing the catalyst
activity. The removal of the coke or carbonaceous deposit is
therefore an important factor in the design of such units, one
method being the burning of the catalyst bed or layer in the
presence of air for regenerating the catalyst. A brief description
of the various commercial catalytic cracking processes including
the early ones is given below for better understanding: a. Houdry
Fixed-Bed Catalytic Cracking : This was the first of the modern
catalytic processes & went into commercial operation in 1936.
In this fixed-bed process, the catalyst in the form of
small lumps or pellets was made up of layers or beds in several
(four or more) catalyst-containing drums called converters.
Feedstock vaporized at about 450C & under 7 to 15-psi pressure
passed through one of the converters where the cracking reactions
took place. After a short time, deposition of coke on the catalyst
rendered it ineffective, and, using a synchronized valve system,
the feed stream was diverted to the adjacent converter while the
catalyst in the
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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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first converter was regenerated by carefully burning the coke
deposits with air. After about 10 minutes, the catalyst was ready
to go on stream again.
In the present age, fixed-bed processes have been replaced with
the more versatile moving-bed or fluid-bed processes.
Product fractionation
Flue Gas
Air Feedstock (heated) Houdry fixed-bed catalytic cracking
b. Fluid-bed Catalytic Cracking : This is presently the most
widely used catalytic cracking process & is characterized by
the use of a finely divided silica/alumina based catalyst, which is
moved through the processing unit. The catalyst particles are of
such a size that when aerated with air, or hydrocarbon vapor, the
catalyst behaves like a liquid & can be moved through pipes.
Thus, vaporized feedstock & fluidized catalyst flow together
into a reaction chamber where the catalyst, still dispersed in the
hydrocarbon vapors, forms beds in the reaction chamber & the
cracking reactions take place. Because of the even flow
distribution of the catalyst & because of its high specific
heat in relation to the vapors reacting, the entire reaction can be
maintained at a remarkably constant temperature. The cracked vapors
pass through cyclones located in the top of the reaction chamber,
thereby removing the catalyst from the vapors by centrifugal
action. The cracked vapors out of the reaction chamber enter the
fractionating towers where fractionation into light- &
heavy-cracked gas oils, cracked gasoline, & cracked gases takes
place.
Due to the contamination of the catalyst in the reaction chamber
with coke, its activity is reduced, & it has to be regenerated.
Thus the separated spent catalyst flows via steam fluidization from
the reaction chamber to the regenerator vessel, where the coke is
removed by controlled burning. In the course of burning the coke, a
large amount of heat is liberated. Most of this heat of combustion
is absorbed by the regenerated catalyst, & is sufficient to
vaporize the fresh feed entering the reaction chamber.
The fluid-bed catalytic cracking units abbreviated as FCCU are
large-scale processes & unit throughputs are typically in the
range of about 10,000 to 130,000 barrels per day which corresponds
to catalyst circulation rates of 7 to 130 tons per minute. The
large circulation rates of hot, abrasive catalyst constitute a very
significant challenge to the mechanical integrity of the reactor,
the regenerator & their associated internal equipment.
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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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Gases & gasoline
Flue gas
Steam Water Products Light gas Oil
Heavy gas Oil Bottoms
Catalyst Steam make-up Regenerated Air catalyst
Spent catalyst Feedstock
Flowsheet for fluid-bed catalytic cracking
c. Model IV Fluid-Bed Catalytic Cracking Unit : This unit
involves a process in which the catalyst is transferred between the
reactor & regenerator by means of U-bends, & the catalyst
flow
rate can be varied in relation to the amount of air injected
into the spent-catalyst U-bend. Regeneration air, other than that
used to control circulation, enters the regenerator through a grid,
& the reactor & regenerator are mounted side by side. This
design was preceded by the Model III balanced pressure design, the
Model II downflow design, & the original Model I upflow
design.
Flue gas Product fractionation
Feedstock
Regenerated catalyst Steam Air Spent Catalyst
Flowsheet for Model IV fluid-bed catalytic cracking
d. Orthoflow Fluid-Bed Catalytic Cracking: This process uses the
unitary vessel design, which provides straight-line flow of
catalyst & thereby minimizes erosion encountered in
pipe-bends.
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PETROLEUM TECHNOLOGY- Part IIITHE PROCESS & TECHNOLOGY OF
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Commercial Orthoflow designs are of three types: Models A &
C, with the regenerator beneath the reactor, & model B, with
the regenerator above the reactor. In all cases the catalyst-
stripping section is located between the reactor & the
regenerator; all designs employ the heat- balanced principle
incorporating fresh feedrecycle feed cracking.
Flue gas
Air
Product fractionation
Recycle
Steam Feedstock
Air
Model B Orthoflow fluid-bed catalytic cracking process
Product fractionation
Steam
Flue gas
Air
Feedstock Recycle
Model C Orthoflow fluid-bed catalytic cracking unit
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e. Universal Oil Products (UOP) Fluid-bed Catalytic Cracking:
This process is adaptable to the needs of both large & small
refineries, & its important distinguishing features include 1)
elimination of the air riser with its attendant large expansion
joints, 2) elimination of considerable structural steel supports,
& 3) reduction in regenerator & in air-line through use of
15 to 18 psi pressure operation.
Product fractionation
Flue gas Catalyst stripper
Steam
Air Feedstock
Flowsheet for UOP fluid-bed catalytic cracking
f. Shell Two-Stage Fluid-Bed Catalytic Cracking : This two-stage
fluid catalytic process allows greater flexibility in shifting
product when dictated by demand. Thus, virgin feedstock is
first
contacted with cracking catalyst in a riser reactor, that is, a
pipe in which fluidized catalyst &vaporized oil flow
concurrently upward, & the total contact time in this first
stage is of theorder of seconds.
Light products Light products
Middle distillate
Heavy distillate Air
Steam
Regenerated catalyst Feedstock
Flowsheet for Shell two-stage catalytic cracking
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High temperatures 470 to 565 C are employed to reduce
undesirable coke lay-down on catalyst without destruction of
gasoline by secondary cracking. Other operating conditions in the
first stage are a pressure of 16 psi & catalyst-oil ratio of
3:1 to 50:1, & volume conversion ranges between 20 & 70%
have been recorded. All or part of the unconverted or partially
converted gas-oil product from the first stage is then cracked
further in the second-stage fluid-bed reactor. Operating conditions
are 480 to 540 C & 16 psi with a catalyst-oil ratio of 2 to
12/1. Conversion in the second stage varies between 15 & 70%
with an overall conversion range of 50 to 80%.
g. Airlift Thermofor Catalytic Cracking (Socony Airlift TCC
process) : This process is a moving-bed, reactor-over-generator
continuous process for conversion of heavy gas oils into lighter
high-quality gasoline & middle distillate fuel oils. Feed
preparation may consist of flashing in a tar separator to get vapor
feed, & the tar separator bottoms may be sent to a vacuum tower
from which the liquid feed is produced.
The gas-oil vapor-liquid flows downward through the reactor
concurrently with the regenerated synthetic bead catalyst. The
catalyst is purged by steam at the base of the reactor, &
gravitates into the kiln or regeneration is done by the use of air
injected into the kiln. Approximately 70% of the carbon on the
catalyst is burned in the upper kiln burning zone & the
remainder in the bottom burning zone. Regenerated, cooled catalyst
enters the lift pot, where low-pressure air transports it to the
surge hopper above the reactor for reuse.
Regenerated catalyst
(liquid) Feedstock (vapor) Product fractionation Steam
Flue gas
Air (hot)
Air (lift)
Flowsheet for airlift thermofor catalytic cracking
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h. Houdriflow Catalytic Cracking : This is a continuous,
moving-bed process employing an integrated single vessel for the
reactor & regenerator kiln. The charge stock, sweet or sour,
can be any fraction of the crude boiling between naphtha & soft
asphalt. The catalyst is transported from the bottom of the unit to
the top in a gas lift employing compressed flue gas &
steam.
The reactor feed & catalyst pass concurrently through the
reactor zone to a disengager section, in which vapors are separated
& directed to a conventional fractionation system. The spent
catalyst, which has been steam purged of residual oil, flows to the
kiln for regeneration, after which steam & flue gas are used to
transport the catalyst to the reactor.
Feedstock Steam
Products Flue gas
Catalyst Lift
Air
Flowsheet for Houdriflow catalytic cracking
i. Houdresid Catalytic cracking : Houdresid catalytic cracking
is a process that uses a variation of the continuous-moving
catalyst bed designed to get high yields of high-octane gasoline
& light distillate from reduced crude charge.
Residuum cuts ranging from crude tower bottoms to vacuum
bottoms, including residua high in sulfur or nitrogen can be
employed as the feedstock, & the catalyst is synthetic or
natural. Though the equipment employed is similar in many respects
to that used in Houdriflow units, novel process features modify or
eliminate the adverse effects on catalyst & product selectivity
usually resulting when heavy metals iron, nickel, copper, &
vanadium are present in the fuel. The Houdresid catalytic reactor
& catalyst-regenerating kiln are contained in a single vessel.
Fresh feed plus recycled gas oil are charged to top of the unit in
a partially vaporized state & mixed with steam. Refer flowsheet
below:
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Light products
Middle distillates
Feedstock Redu ced crude
Fuel stocks
Recycle
Flowsheet for Houdresid catalytic cracking
j. Suspensoid Catalytic Cracking : This was developed from the
thermal cracking process carried out in tube & tank units.
Small amounts of powdered catalyst or a mixture with the
feedstock & the mixture are pumped through a cracking coil
furnace. Cracking temperatures are 550 to 610 C with pressures of
200 to 500 psi. After leaving the furnace, the cracked material
enters a bubble tower where they are separated into two parts, gas
oil & pressure distillate. The latter is separated into
gasoline & gases. The spent catalyst is filtered from the tar,
which is used as a heavy-industry fuel oil.
The process is a compromise between catalytic & thermal
cracking. Here the catalyst allows a higher cracking temperature
& assists mechanically in keeping coke from accumulating on the
walls of the tubes. The normal catalyst employed is spent clay
obtained from the contact filtration of lubricating oils (2 to 10
lb. per barrel of feed).
Products (gasoline, gases)
Catalyst
Feedstock Heavy fuel oil
Catalyst
Flowsheet for Suspensoid catalytic cracking
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3. Visbreaking: As explained earlier visbreaking is a mild
thermal cracking operation employed for viscosity reduction of
residual or asphaltic feed stocks.
Visbreaking conditions range from 455 to 510C & from 50 to
300 psi at the heating-coil outlet. Liquid-phase cracking takes
place at these low-severity conditions. Besides fuel oil (major
product), material in the gas oil & gasoline boiling range is
also produced. The gas oil produced may be diverted to catalytic
cracking units or used as heating oil.
Thus, a crude oil residuum is passed through a furnace where it
is heated to a temperature of 480C, the outlet pressure controlled
at about 100 psi. The furnace is designed in a manner such that it
contains a soaking section of low heat density, where the charge
can be held until the visbreaking reactions are completed. The
cracked products are then passed into a flash-distillation chamber.
The overhead material from this chamber is then fractionated to
produce a low-quality gasoline as an overhead product & light
gas oil as bottoms. The liquid products from the flash chamber are
cooled with a gas oil flux & then sent to a vacuum
fractionator. This yields a heavy gas oil distillate & a
residual tar of reduced viscosity.
Gasoline Feedstock Heavy
gas oil
Tar
Light gas oil
Flowsheet for Visbreaking operations
4. Coking: As mentioned earlier coking processes generally
utilize longer reaction times thanthermal cracking processes. To
accomplish this, drums or chambers (reaction vessels) are employed.
Normally two or more such vessels are provided, in order to
simultaneouslydecoke the off-line vessel without interrupting the
semicontinuous type of process. Thevarious type of coking processes
are described below:
a. Delayed Coking : This is a semicontinuous process in which
the heated charge is transferred to large soaking (or coking)
drums, which provide the long residence time needed to allow the
cracking reactions to be completed. The feed to these units is
normally an atmospheric residuum although cracked tars & heavy
catalytic oils may also be used.
The process flow is as follows: The feedstock enters the product
fractionator where it is heated & lighter fractions are removed
as side streams. The fractionator bottoms, including a recycle
stream of heavy product, are then heated in a furnace whose outlet
temperature varies from 480 to 515C. The heated material enters one
of a pair of coking drums where the cracking reactions are
completed. The cracked products leave as overheads, & coke
deposits form on the the inner surface of the drum. Two drums allow
for continuous operation, with one on stream while the other is
being cleaned. The temperature in the coke drum ranges from 415 to
450C
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at pressures from 15 to 90 psi. The overheads from the coking
drum go to the fractionator, where naphtha & heating oil
fractions are recovered. The heavy recycle material is material is
combined with preheated fresh feed & returned to the reactor.
The coke drum is usually on stream for about 24 hours before
becoming filled with porous coke, & the following procedure is
used to remove the coke: 1) the coke deposit is cooled with water;
2) one of the heads of the coking drum is removed to permit the
drilling of a hole through the center of the deposit; & 3) a
hydraulic cutting device, which uses multiple high -pressure jets,
is inserted into the hole, & the wet coke is removed from the
drum. These cleaning operations normally require 24 hours before
the drum can be put into reuse.
Gas Gasoline (naphtha) Gas oil
Feedstock
Coke
Operative Nonoperative
Flowsheet for Delayed coking
b. Fluid Coking : Fluid coking is a continuous process, which
uses the fluidized-solids technique to convert residua, including
vacuum pitches, to more valuable products. The residuum is coked by
being sprayed into a fluidized bed of hot, fine coke particles,
which permit the coking reactions to be conducted at higher
temperatures & shorter contact times than can be employed in
delayed coking. Moreover, these conditions result in decreased
yields of coke with greater quantity of more valuable liquid
product being recovered.
Fluid coking uses two vessels, a reactor & a burner; coke
particles are circulated between these to transfer heat (generated
by burning a portion of the coke) to the reactor. The reactor holds
a bed of fluidized coke particles, & steam is introduced at the
bottom of the reactor to fluidize the bed. The pitch feed at, for
example 260 to 370C is injected directly into the reactor. The
temperature in the coking vessel ranges from 480 to 565C, & the
pressure nearly atmospheric causing the incoming feed to partly
vaporize & partly deposit on the fluidized coke particles. The
material on the particle surface then cracks & vaporizes,
leaving a residue, which dries to form coke. The vapor products
pass through cyclones, which remove most of the entrained coke.
The vapor is discharged into the bottom of a scrubber where the
products are cooled to condense a heavy tar, which contains
substantial quantity of coke dust & is recycled back to the
reactor. The upper part of the scrubber tower is a fractionating
zone from which coker gas
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oil is withdrawn for feeding to a catalytic cracking unit. The
naphtha & gas from the fractionating zone are taken overhead to
condensers.
In the reactor, the coke particles flow down through the vessel
into a stripping zone at the bottom. Steam displaces the product
vapors between the particles & the coke then flows into a
riser, which leads to the burner. Steam is added to the riser to
reduce the solids loading & induce upward flow. The average bed
temperature in the burner is 590 to 650C, & air is added as
needed to maintain the temperature by burning part of the product
coke. The pressure in the burner may range from 5 to 25 psi. Flue
gases from the burner pass through cyclones & discharge to the
stack. Hot coke from the bed is returned to the reactor through a
second riser assembly.
Coke is one of the products of the process, & it must be
withdrawn from the system in order to keep the solids inventory
from increasing. The net coke produced is removed from the burner
bed through a quench elutriator drum, where water is added for
cooling & cooled coke is withdrawn & sent to storage.
During the course of the coking reaction the particles tend to grow
in size. The size of the coke particles remaining in the system is
controlled by a grinding system within the reactor.
Fuel gas Cracked gasoline
Gas oil Flue gas
Coke
By-product coke
Feedstock Air
Coke Steam
Flowsheet for Fluid coking
c. Decarbonizing : The decarbonizing thermal process is designed
to minimize coke & gasoline yields but, at the same time, to
give maximum yield of gas oil. The process is essentially the
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same as the delayed coking process, but lower temperatures &
pressures are employed. For example, pressures range from 10 to 25
psi while heater outlet temperatures maybe 485C & coke drum top
temperatures maybe of the order of 415C.
Gas
Gasoline Gas oil
Feedstock
Flowsheet for Decarbonizing
d. Low-Pressure Coking : Low-pressure coking is a process
designed for a once-through, low pressure operation. The process is
similar to delayed coking except that recycling is not practiced
& the coke chamber operating conditions are 435C, 25 psi.
Excessive coking is inhibited by the addition of water to the
feedstock.
Gas and gasoline
Gas oil
Feedstock Fuel oil
Flowsheet for Low-pressure coking
e. High-Temperature Coking : This is a semicontinuous thermal
conversion process designed forhigh-melting asphaltic residua which
yield coke & gas oil as the primary products. The coke may
further be treated to remove sulfur to produce a low-sulfur coke (
5%), even though the feedstock could have as much as 5% wt/wt
sulfur.The process flow is as follows:
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The feedstock is fed into the pitch accumulator, then to the
heater (370C, 30 psi), & finally to the coke oven where
temperatures may be as high as 980 to 1095C. Volatile materials are
fractionated, and, after the cycle is complete, coke is collected
for sulfur removal & quenching prior to storage.
Gas Gasoline
Gas oil
Coke
Flowsheet for High-temperature coking
5. Hydrocracking: Hydrocracking is similar to catalytic
cracking, with hydrogenation superimposed & with the reactions
taking place simultaneously or sequentially. The purpose
hydrocracking is to convert high-boiling feedstocks to
lower-boiling products by cracking the hydrocarbons in the feed
& hydrogenating the unsaturated materials in the product
streams. The polycyclic aromatics are first partially hydrogenated
before cracking of the aromatic nucleus takes place. Also the
majority of sulfur & nitrogen is converted to hydrogen sulfide
& ammonia. The reaction rates are facilitated by use of
catalysts.
Large quantities of hydrogen sulfide & ammonia are formed
when using high sulfur & nitrogen feedstocks for hydrocracking
units. These are removed by the injection of water in which, under
the high pressure conditions employed, both hydrogen sulfide &
ammonia are very soluble compared with hydrogen & hydrocarbon
gases.
6. Hydrotreating: The purpose of the process is the removal of
sulfur & nitrogen compounds without appreciable alteration in
the boiling range or in other words it is selective hydrogenation
of the feedstock for removal of sulfur & nitrogen with very
little cracking involved.
Hydrotreating catalysts are usually cobalt plus molybdenum or
nickel plus molybdenum in the sulfide forms, impregnated on an
alumina base.
The operating conditions of 1000 to 2000 psi hydrogen pressures
& 370C temperatures are such that appreciable hydrogenation of
aromatics will not occur.
Commercial Processes for Hydrocracking & Hydrotreating:
Since commercial processes for hydrocracking & hydrotreating
operate essentially in the same manner i.e. feedstock is passed
along with hydrogen gas into a tower or reactor filled with
catalyst pellets the commercial processes have not been classified
separately as hydrocracking & hydrotreating. The processing
conditions i.e. the temperature & pressures decide whether a
lot of cracking reactions are taking place along with the
hydrogenation or just removal of nitrogen & sulfur is taking
place.
Hydrofining: This process can be applied to lubricating oils,
naphthas, & gas oils. The feedstock is heated in a furnace
& passed with hydrogen through a reactor containing a suitable
metal oxide catalyst, such as cobalt & molybdenum oxides on
alumina. Reactor operating conditions
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range from 205 to 425C & from 50 to 800 psi, depending on
the feedstock & the degree of treating required..
Higher-boiling feedstocks, high sulfur content, & maximum
sulfur removal require higher temperatures & pressures.
After passing through the reactor, the treated oils cooled &
separated from the excess hydrogen, which is recycled through the
reactor. The treated oil is pumped to a stripper tower where
hydrogen sulfide, formed by the hydrogenation reaction, is remove
by steam, vacuum, or flue gas, & the finished product leaves
the bottom of the stripper tower. In this process the catalyst is
usually not regenerated & is replaced after about a years
use.
This process is used to upgrade low-quality, high-sulfur
naphthas. The sulfur content of kerosenes can be reduced with
improved color, odor, & wick-char characteristics. The tendency
of kerosene to form smoke is not affected since aromatics, which
cause smoke., are not affected by the mild hydrofining conditions.
Cracked gas oils with high sulfur content can be converted to
excellent furnace fuel oils & diesel fuel oils by reduction in
sulfur content & by removal of components that form gum &
carbon residues.
Hydrogen
Water (vapor)
Feedstock Product stream
Hydrogen
Flowsheet for Hydrofining Unifining: This is regenerative,
fixed-bed, catalytic process to desulfurize & hydrogenate
refinery distillates of any boiling range. Contaminating metals,
nitrogen compounds, & oxygen compounds are eliminated, along
with sulfur. The catalyst is a cobalt molybdenum-alumina type which
may be regenerated in situ with steam & air. Ultrafining: It is
a regenerative, fixed-bed, catalytic process to desulfurize &
hydrogenate refinery stocks from naphthas through lube stocks. The
catalyst is cobalt-molybdenum on alumina & may be regenerated
in situ using an air-stream mixture. Regeneration requires 10 to 20
hours & may be repeated 50 to 100 times for a given batch of
catalyst; catalyst life is 2 to 5 years depending on the
feedstock.
Autofining: The autofining process differs from other
hydrorefining processes in that an external source of hydrogen is
not required. Sufficient hydrogen to convert sulfur to hydrogen
sulfide is obtained by dehydrogenation of naphthenes in the
feedstock.
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The processing equipment is similar to that used in Hydrofining.
The catalyst is cobalt & molybdenum oxides on alumina, &
operating conditions are usually 340 to 425C at pressures of 100 to
200 psi. Hydrogen formed by dehydrogenation of naphthenes in the
reactor, is separated from the treated oil & is then recycled
through the reactor. The catalyst is regenerated with steam &
air at 200 to 1000 hour intervals, depending on whether light or
heavy feedstocks have been processed. The process is used for the
same purpose, as Hydrofining but is limited to fractions with end
points not higher than 370C.
Feedstock Flue gas (preheated)
Product stream
Heavy fuel oil
Flowsheet for Autofining
Isomax: The Isomax process is a two-stage, fixed-bed catalyst
system which operates under hydrogen pressures from 500 to 1500
psig in a temperature range of 205 to 370C, for example with middle
distillate feedstocks. Exact conditions depend on the feedstock
& product requirements, & hydrogen consumption is of the
order of 1000 to 1600 SCF per barrel of feed processed. Each stage
has a separate hydrogen recycle system. Conversion may be balanced
to provide products for variable requirements, & recycle can be
taken to extinction if necessary. Fractionation can also be handled
in a number of ways to yield desired products.
Recycle Hydrogen Fuel gas hydrogen Fuel gas Feedstock recycle
Fuel gas Butanes
Light gasoline Heavy gasoline Diesel fuel
Bottoms Feedstock recycle
Flowsheet for Isomax hydrocracking process
H-Oil: The H-Oil process is basically a catalytic hydrogenation
technique in which, during the reaction, considerable hydrocracking
takes place. The process is used to upgrade heavy sulfur-containing
crudes & residual stocks to high-quality sweet distillates,
thereby reducing fuel oil yield.
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A modification of H-Oil called Hy-C cracking will convert heavy
distillates to middle distillates & kerosene.
Oil & hydrogen are fed upward through the reactors as a
liquid-gas mixture at a velocity such that catalyst is in
continuous motion. Catalyst of small particle size can be used,
giving efficient contact among gas, liquid, and solid with good
mass & heat transfer. Part of the reactor effluent is recycled
back through the reactors for temperature control & to maintain
the requisite liquid velocity. The entire bed is held within a
narrow temperature range, which provides essentially an isothermal
operation with an exothermic process. Because of the movement of
catalyst particles in the liquid-gas medium, deposition of tar
& coke is minimized, & fine solids entrained in the feed
will not lead to reactor plugging. The can also be added &
withdrawn from the reactor without interrupting the continuity of
the process.
The reactor effluent is cooled by exchange & separates into
vapor & liquid. After scrubbing in a lean-oil absorber,
hydrogen is recycled, and the liquid product is either stored
directly or fractionated prior to storage & blending.
Hydrogen recycle
Hydrogen
Gas recycle
Lean oil
Rich oil
Distillate
Product Feedstock stream
Hydrogen make-up
Flowsheet for H-Oil Process
Unicracking-JHC: This is a fixed-bed catalytic process that
employs a high-activity catalyst with a high tolerance for sulfur
& nitrogen compounds & can be regenerated. The design is
based upon a single-stage or a two-stage system with provisions to
recycle to extinction.
A two-stage reactor system receives untreated feed, make-up
hydrogen, and a recycle gas at the first stage in which gasoline
conversion may be as high as 60% by volume. The reactor effluent is
separated to recycle gas, liquid product, and unconverted oil. The
second-stage oil may be either once through or recycle cracking;
feed to the second stage is a mixture of unconverted first-stage
oil & second-stage recycle.
Hydrogen make-up Gas and light gasoline
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Feedstock
Hydrogen Gasoline recycle
Diesel fuel
Feedstock recycle
Flowsheet for Unicracking-JHC process
Gulf HDS: This is a regenerative fixed-be process to upgrade
petroleum residues by catalytic hydrogenation to refined fuel oils
or to high-quality catalytic charge stocks. Desulfurization &
quality improvement are the primary purposes of the process, but if
the operating conditions & catalysts are varied, light
distillates can be produced & the viscosity of heavy material
can be lowered. Long on-stream cycles are maintained by reducing
random hydrocracking reactions to a minimum, and whole crudes,
virgin, or cracked residua may serve as feedstock.
The catalyst is a metallic compound supported on pelleted
alumina & may be regenerated in situ with air & steam or
flue gas through a temperature cycle of 400 to 650C. On-stream
cycles of 4 to 5 months can be obtained at desulfurization levels
of 65 to 75% & catalyst life may be as long as 2 years.
Hydrogen make-up Hydrogen recycle
Lean Gas Feedstock Diethanolamine Rich Light Gas gasoline Heavy
naphtha
Light gas oil
Heavy gas oil
Lightbottoms
Heavy bottoms
Flowsheet for Gulf HDS processH-G Hydrocracking: This process
may be designed with either a single- or a two-stage reactor system
for conversion of light & heavy gas oils to lower-boiling
fractions. The feedstock is mixed with recycle gas oil, make-up
hydrogen, and hydrogen-rich recycle gas, and then heated &
charged
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to the reactor. The reactor effluent is cooled & sent to a
high-pressure separator where hydrogen-rich gas is flashed off,
scrubbed, then recycled to the reactor. Separator liquid passes to
a stabilizer for removal of butanes & lighter products, &
the bottoms are taken to a fractionator for separation; any
unconverted material is recycled to the reactor. C2 gases
Hydrogen C3, C4 gases Feedstock recycle
(Heavy gas oil) Gasoline
(Recycle) Product Stream
Hydrogen
(Make-up)
Feedstock recycle
Flowsheet for H-G hydrocracking process
Ferrofining: The mild hydrogen-treating process was developed to
treat distilled & solvent-refined lubricating oils. The process
eliminates the need for acid & clay treatment. The catalyst is
a three-component material on alumina base with low hydrogen
consumption & life expectancy of 2 years or more. Process
operations include heating the hydrogen-oil mixture & charging
to a downflow catalyst-filled reactor. Separation of oil & gas
is a two-stage operation whereby gas is removed to the fuel system.
The oil is then stripped to control the flash point, dried in
vacuum, & a final filtering step removes the catalyst fines.
Hydrogen Feedstock Fuel Gas Gas and steam
Product
Flowsheet for Ferrofining processList of know-how suppliers for
Cracking Processes: Most oil majors have developed & modified
technologies to suit their own particular requirements. There have
been many modifications to the
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older technologies & many of these processes are patented
with the patents owned by the worlds leading refining companies.
Some of the major technology licensors are:
Universal Oil Products (UOP): They are one of the leading
technology licensors & provide integrated refinery solutions
including catalytic cracking, hydroprocessing & reforming.
Reliance Petroleum Ltd., the largest single-stream refinery in Asia
has major processing units designed by UOP.
Shell Development Company (Shell): Shell is a leading company
providing technology for visbreaking units & crackers.
ABB-Lummus Global/Chevron: They are among the top technology
& engineering solutions provider for FCC units, Hydrocrackers
& Hydrotreaters.
Stone & Webster Engineering Corporation (S & W): They
specialize in FCC units & have a major share of the world
market.
Institut Francais Petole (IFP): IFP is recognized the world over
for its Hydrotreating technologies for light distillates &
majority of new installations are opting for IFP technology. One of
their specialties is a new selective hydrotreating technology named
Prime G for ultra low sulfur gasoline.
Haldor Topsoe AS (Topsoe): They are leading contenders for
supply of Hydrotreating technology.
Kellogg, Brown & Root Inc. (KBR): Most of the older FCC
units in North America were designed by KBR & they are doing a
lot of revamp/ technology upgradation jobs on their older
units.
BASF: BASF is a Germany based multinational & they have
developed certain commercial Hydroprocessing technologies.
ExxonMobil: They have developed new selective hydrotreating
technologies for ultra low sulfur gasolines called Scanfining,
Octgain 125 & Octgain 220 which shall become very relevant when
international norms for sulfur content in motor fuels are brought
down to 50 ppm.
Some other companies offering Cracking & Hydroprocessing
technologies include CD tech,Petrobras (Brazil), Akzo, Criterion
(mainly catalyst suppliers) etc.
It is important to note that of the various technologies
available, the selection has to be made based basically on three
factors: a) feedstock to be processed b) end-product or its mix
required & c) economics of a particular process vis--vis its
competitive technology.
List of reference books:
1. The Chemistry and Technology of Petroleum James G.
SpeightPublisher: Marcel Dekker, Inc.
2. Petroleum Refinery Engineering W. L. NelsonPublisher:
McGraw-Hill Kogakusha, Ltd.
3. Chemical Process Industries R. Norris Shreve & Joseph A.
Brink, Jr.Publisher: McGraw-Hill International Book Company
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