Chapter Seven:
Solvent Deasphalting
and Thermal Cracking (Bottom of
the Barrel) Processes
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Solvent Deasphalting
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Solvent Deasphalting
Solvent deasphalting is a physical process (extraction) NOT
a thermal cracking (process)
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Propane Deasphalting Flowsheet
Propane/Vacuum residue ratio is 6:1 to 10:1 by vol.
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Solvent Deasphalting
Feed is Vacuum residue/bitumen
Coke-forming tendencies of heavier distillation products are reduced
by removal of asphaltenic materials by solvent extraction.
Liquid propane is a good solvent. Butane, Pentane, Heptane or
mixture of solvents are also commonly used.
Vacuum residue is fed to a counter current deasphalting tower.
Deasphalting is based on solubility of hydrocarbons in propane, i.e.
the type of molecule; Alkanes dissolve in propane whereas
asphaltenic materials (aromatic compounds), ‘coke-precursors’ do
not.
Asphalt is sent for thermal processing.
Deasphalted oil can be used as Lube oil base feedstock (LBFS) or as
feed to FCC unit.
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Solvent Deasphalting
DAO from propane deasphalting has the highest quality but lowest yield.
Mixtures of propane & n-butane is more suitable for higher yield
extraction.
Using pentane may double or triple the yield from a heavy feed, but at the
expense of contamination by metals and carbon residues that shorten the
lifetime of downstream cracking catalysts.
Choice of solvent & extraction conditions are critical.
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Thermal Cracking
Processes:
1. Visbreaking
2. Coking
3. Flexi-coking
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Why Thermal Processes
• Residual fractions (Bottom of Barrel) are the
least valuable streams of a refinery.
• Nearly 45%-50% of the typical crude oils contain
370 °C+ fractions.
• Disposal problems due to stringent
environmental regulations.
• Decreasing demand of fuel oil.
• Gradually increasing demand of middle
distillates.
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Thermal Cracking Reactions
• Thermal cracking reactions take place due to
application of heat
• During the cracking process, large molecules
decompose to smaller (lighter) molecules.
• In thermal cracking generally two types of reactions
take place: 1) Primary reactions by decomposition of
large molecules to smaller molecules, 2) Secondary
reactions by which active products from primary
cracking reactions further crack or react to form other
molecules or polymerize to generate heavy products
(coke).
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Thermal Cracking Reactions
.
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Thermal Cracking Reactions
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Visbreaking
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Typical Coil Visbreaker Layout
• Operating Pressure: 50-300 psig.
• Operating Temperature: 455-520°C.
• Liquid phase cracking process.
• Residue from Atmospheric /Vacuum
distillation units can be used.
• Coil/Furnace type operating at high
temperature and short residence time is used
as a reaction medium.
Feed
• Soaker type can also be used, which
operates at lower temperature and longer
residence time.
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Coking
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Coking
•DELAYED COKING: converts heavy
feedstock to coke, oil & gas. The process
has long residence time of 24 h.
• FLEXICOKING: Flexicoking is a
combination of thermal cracking and coke
gasification/ combustion.
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Objectives of Delayed Coking
• Process heavy residuum to produce distillates,
naphtha & gas oils that may be catalytically
upgraded to more valuable products
• Attractive for heavy residuum that is not
suitable for catalytic processes
• Metal, sulfur and other catalyst poisons
generally end up in coke
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Typical Delayed Coking Unit
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Delayed Coking Technology
• Delayed Coking is one of the most commonly used process that upgrades
residues to a wide range of lighter hydrocarbon gases and distillates
through thermal cracking.
• The byproduct of delayed coking process is petroleum coke.
• The goal for delayed coking operation is to maximize the yield of clean
distillates and minimize the yield of coke.
• Delayed coking technology is preferred for upgrading heavier residues due
to its inherent flexibility to handle even the heaviest residues while
producing clean liquid products.
• The main products of delayed coking operation is hydrocarbon gases (such
as LPG), naphtha, Light Gas Oil (LGO) and Heavy Gas Oil (HGO).
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Operating Variables
Four operating parameters govern the yield
pattern and product quality of Delayed
Coking are:
1) Temperature
2) Pressure
3) Recycle Rate (RR)
4) Velocity Medium
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Effects of Operating Variables
• High temperature:
 Favors cracking
 Increases the Formation of more distillates liquids
 Lowers the yield of coke & hydrocarbon gases
• High pressure:
 Decreases the yield of distillates liquids
 Increases the yield of coke & hydrocarbon gases
• High recycle rate:
 Decreases the yield of distillates liquids
 Increases the yield of coke & hydrocarbon gases
• High residence time:
 Favors coke and gas formation
 Lead to over conversion and the reduction in the production
of distillates.
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Chemical Reactions during Delayed Coking
• There are three types of chemical reaction processes
which occur continuously without any distinct steps in the
coking process:
 Dehydrogenation : The initial reaction in carbonization
involves the loss of hydrogen atom from an aromatic
hydrocarbon and formation of aromatic free radical
intermediate.
 Rearrangement Reactions: Thermal rearrangement usually
leads to formation of more stabilized aromatic ring system
which forms building block of graphite (coke) growth.
 Polymerization of aromatic radicals: Aromatic free radicals get
polymerized in the process of coking reaction.
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Delayed Coking Process
• Delayed Coking is a batch/ continuous process:
Flow through the furnace coil is continuous.
Feed is switched between two or more drums in
batchwise fashion.
• Delayed coker drum cycle length varies from
unit to unit. However, it is typically kept within
16 to 24 hours.
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Feedstock of Delayed Coking
• Delayed Coker can process a wide variety of feedstocks:
1) Feedstock can have considerable amount of metal (Ni + V),
sulfur, resins and asphaltenes
2) Contaminants present in feedstock mostly get eliminated with
coke.
• Most typical feedstock is Vacuum Residue (530 °C+). Atmospheric
residue is also occasionally processed.
• Typical feed contains:
 Sulfur: 5 to 6 w%
 Metals: ~0.1 w%
• Properties of coke mostly depends on feed quality.
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Products of Delayed Coking
• Delayed coker produces desirable liquid products (naphtha and gas oil)
and byproducts hydrocarbon gases (also called coker gases) and solid
coke
• Coker gases stream is sent to the gas plant where C3 and C4 is recovered
as LPG and the lighter end products can be used as fuel gas in the
refinery.
• Produced naphtha contains high olefin content and this stream is usually
sent to hydrotreater for stabilization.
• Light Gas Oil (LGO) is sent to diesel hydrotreater for production of diesel.
• Heavy Gas Oil (HGO) is sent to FCC for production of gasoline and other
valuable distillate products.
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Type of Coke
• Coke formed in Delayed Coker Unit can be
classified into three different types
Sponge Coke
Needle Coke
 Shot Coke
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Sponge Coke
• Sponge Coke is porous, irregular shaped lumps.
• It was named by its sponge like appearance.
• Vacuum Residue with low to moderate asphaltene produces
sponge coke.
• Most of the sponge coke is used as fuel.
• Some sponge coke with low metal and sulfur content (< 2 w%)
can be used to make anodes used in aluminum industries.
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Needle Coke
• It is a premium coke from Delayed Coker named by its needle like
appearance.
• It has microscopic, elongated, needle like structure.
• It has very low coefficient of thermal expansion and electrical
resistance suitable for using as electrodes in steel making.
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Shot Coke
• It is formed from high asphaltene content feedstock present at
high coke drum temperature.
• Shot coke is undesirable product in delayed coking.
• Shot coke is usually blended with sponge coke to use as fuel.
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Modern Trends in Delayed Coker Technology
• Maximize the production of liquid distillate.
• Minimize the production of coke.
• Produce heavy gas oil suitable for downstream catalytic processing.
• Optimize number and size of coke drums.
• Maximize air cooling and minimize water cooling.
• Low operating drum pressure.
• Reduction in Recycle Ratio (10% or lower).
• Shorter coke drum cycle
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Delayed Coking:
Equipment & Operating Modes
• Typical Equipments:
 Heater
 Coke Drum
 Fractionator
 Reflux Drum and Strippers
• Coke Drums run in two batch modes
 Filling
 Decoking
• Other Equipments operate in continuous mode
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Delayed Coking:
Fresh Feed & Furnace
• Fresh feed is sent to the bottom of fractionator before charging to
the furnace
• Feed is heated in Furnace




Outlet temperature about 496 0C
Cracking starts at 426 0C
Endothermic Reactions
Superheating allows cracking reactions to continue in the coke drums
• Velocities in Furnace kept high to:
 Prevent coking in the furnace tubes
 Delay Coking Reactions until the Coke Drums (hence the term
‘Delayed Coking’ is used)
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Coke Drums & Coking
• Flow Direction:





Furnace outlet enters from bottom of the coke drum
Vapor flows up the coke drum
Coke solidifies & precipitates in the drum.
Coking reactions take about three hours for the coke to fully solidify
Lower molecular weight products exit from the top of drum as
vapors – quenched with oil and sent to bottom of fractionator
• Number of Coke Drums
 Typically 2 or 4.
 One set online in a coking step while other set is being decoked
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Typical Delayed Coking Unit
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Coke Drums & Coking
• To increase the throughput capacity
 Large & More Coke Drums are used
 Good control of coke fill height is necessary
• Cycle Time




Trend is to decrease cycle time
Typically 24 Hours cycle time
16 hours cycle is common & preferable
14 hour & 12 hour cycle is being designed
• Primary purpose of reducing cycle time is to increase throughput
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Fractionator Feed
• Coke Drum Overhead is sent to bottom of
fractionator above liquid level
• Fresh feed typically sent to bottom of
fractionator below liquid level
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Fractionator Products
• Top: Liquid Naphtha & Gas light ends.
• Middle: Light and heavy gas oil.
• Bottom: Recycled to coking
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Flexi-coking
• Expensive process that have only a small portion
of the coking market
• Continuous fluidized bed technology where coke
particles are used as the continuous particulate
phase with a Reactor & Burner.
• The coke product is gasified with steam & air to
a low Btu gas containing H2S.
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Simplified Flexi-coking Flowsheet
Heavy residuum feed is introduced into the
reactor vessel where it is thermally cracked.
The heat for cracking is
supplied by a fluidized bed of
hot coke transferred to the
reactor from the heater
vessel.
The vapor products of the
reaction leave the reactor
zone to enter the scrubber
section. Fine coke and some of
the heavy oil particles are
removed from the cracked
products in the scrubber zone
and returned to mix with the
fresh feed entering the reactor.
Feed
Steam
The reactor products subsequently leave the scrubber and
are sent to a fractionating facility. Steam is introduced to
the bottom of the reactor to maintain a fluidized bed of
coke and to strip the excess coke leaving the reactor free
from entrained oil.
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Flexi-coking
• Flexi-coking is an extinctive process. By continuous
recycling of heavy oil stream, almost all the feed is
converted into distillate fractions, refinery gas, and low
Btu gas.
• There is a very small coke purge stream which amounts
to about 0.4–0.8 wt% of fresh feed.
• When suitably hydrotreated the fractionated streams
from the flexi-coker provide good quality products.
Hydrotreated coker naphtha provides an excellent
high-naphthene feed to the catalytic reformer.
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End of Chapter Seven
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