Fundamentals of petroleum processing_ lecture7-1.pdf

INSTRUCTOR: OGENE FORTUNATE (M.Tech, BSc)
Email: ogene.fortunate@gmail.com
SCE 4106 :Fundamentals of Petroleum Processing
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Fuel Refining Processes
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SCE 4106: Fundamentals of Petroleum Processing

Fuel Refining Processes
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Catalytic Isomerization: Introduction
Upgrading light hydrocarbon (C4–C6) streams in refineries and gas
processing plants has increased in importance as new regulations
affecting gasoline composition are enacted in many regions of the world.
 These regulations include lead phasedown, benzene minimization, and
oxygen content requirements.
Light-naphtha isomerization plays a key role in meeting octane demands
in the gasoline pool that result from both lead phasedown and increasing
market share for premium gasoline grades.
 In fact, isomerization is the most-economic means available for reducing
the benzene content in gasoline.

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Catalytic Isomerization: Introduction
Isomerization is the transformation of a paraffin to a more branched
paraffin with the same carbon number.
Purpose:
o Improve the octane rating of Light straight run gasoline by converting
n-paraffins to respecting isomers or branched isoparaffins (Eg. n-
pentane has RON of 61.7 and i-pentane has 92.3).
 The octane numbers of the LSR naphtha [C5-180 oF (C5-82 oC)] can be
improved by the use of an isomerization process to convert normal
paraffins to their isomers.
The primary commercial use of the branched isomers of C4, C5, and C6
paraffins is in the production of clean-burning, high-performance
transportation fuels.

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Isomerization Feed Stock
 Light Straight Run (LSR)naphtha feed stock (SRG) with pentanes,
hexanes, & small amounts of heptanes is the feedstock of isomerisation
unit.
o The Feed is often debutanized — ‘Debutanized Straight Run’.
o Sulfur & nitrogen must be removed since catalysts employ an ‘acid
site’ for activity.
o Common for Straight Run Gasoline (SRG) & Naphtha to be
hydrotreated as one stream & then separated.
o Feed must be free of water and other oxygen sources in order to avoid
catalyst deactivation and potential of corrosion problems.

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Isomerization Catalysts
i. Pt on various bases:
o Requires continuous addition of very small amount of organic
chlorides to maintain high catalyst activity.
o Feed to these units must be free of water and other oxygen sources.
 Catalyst life is usually 3 years or more with all these catalysts.
iii. Pt on a novel metal oxide base:
o Higher activity than conventional zeolite catalyst and can be
regenerated.
ii. Molecular Sieve base:
o Tolerates feed saturated with water at ambient temperature.

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Isomerization Catalysts
 An atmosphere of hydrogen is used to minimize carbon deposits on the
catalyst but hydrogen consumption is negligible.
Isomerization Chemistry
 Primary reaction is to convert normal paraffins to isomeric paraffins.
 Olefins may isomerize and shift the position of the double bond.
 Cycloparaffins (naphthenes) may isomerize & break the ring forming an
olefin.

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Isomerization Process Technology
 Reaction operating conditions: 150 -
200oC and 17- 28 barg.
Typical conversions: 75-80 % for
pentanes.
 After reaction, AlCl3 is recovered from
the product using condensation or
distillation.
Light naphtha and hydrogen (make up) are first totally dried and sent to
an isomerisation reactor after pre-heating the feed mixture in a heat
exchanger.

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 The basic principle for AlCl3 recovery is that at the reactor operating
conditions, the AlCl3 is in volatile conditions and is soluble in
hydrocarbons.
 After AlCl3 is recovered from the product, it is sent back to the reactor
along with the make- up AlCl3.
Eventually, the product enters a flash drum where bulky light ends along
with little quantities of HCl are separated from the liquid product.
Light ends recovered from the flash drum are sent to a HCl absorber
where HCl is absorbed into caustic solution to generate the light end
gases. The light end gases can be further used for other processes in the
refinery.

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The bottom product then enters a HCl stripper where most of the HCl is
stripped from the isomerisation product rich stream.
The HCl is recycled back to the reactor to ensure good catalyst activity.
Normally make-up HCl is added to account for losses.
Subsequently, caustic wash is carried out to remove any trace quantities
of HCl.
The isomerized product rich stream is then sent to a fractionators that
separates the isomerized product from the unreacted feed.
The unreacted feed from the fractionators is sent back to the reactor.

Polymerization
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Polymerization
Capital and operating
costs of polymerisation
are much lower than the
alkylation process.
Polymerization is an alternative to alkylation.
Olefin molecules (C3 & C4) are reacted together to form larger olefins
with high-octane product in the gasoline range boiling.
Polymerization produces about 0.7 barrels of polymer gasoline per barrel
of olefin feed as compared with about 1.5 barrels of alkylate by alkylation.

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Polymerization
Reaction is highly exothermic.
Typical reactions include:
Reaction mechanism is similar to that for polymerization of ethylene to
polyethylene, etc. but conversion is low, so the molecules formed are in
the gasoline range and not macromolecules.
The Catalyst is phosphoric acid on an inert support (e.g. phosphoric acid
mixed with kieselguhr-natural clay OR a film of liquid phosphoric acid on
crushed quartz).

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Polymerization Process
Sulphur in the feed poisons the catalyst and increase cat. Consumption.
Small amount of water (350-400
ppm) is added to promote ionization
of the acid before olefin feed is
heated and passed over the
catalyst bed.
The Feed consisting of (propane, propene, butane, butene) is contacted
with amine to remove H2S, caustic washed to remove mercaptons,
water scrubbed to remove caustic and dried passing through molecular
sieve bed.

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Polymerization Process
Both packed bed and tubular reactors can be used for temperature
control (heat of reaction removed in tubular reactors by steam
generation).
The polymerization reaction is highly exothermic and temperature is
controlled either by injecting a cold propane quench or by generating
steam.
Propane and butane in the feed act as diluents and a heat sink to help
control the rate of reaction and rate of heat release.
o Temperature : 175-235 0C (usually 200-220 oC)
o Pressure : 400-1500 psi
o Space velocity : 0.3 gal/lb
Operating Conditions

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Visbreaking
Visbreaking is a mild thermal cracking of vacuum or atmospheric residues
to produce light products and 75–85% cracked material of lower viscosity
that can be used as fuel oil.
Long paraffinic side chains attached to aromatic rings are the primary
cause of high pour points and viscosities for paraffinic base residua.
Visbreaking is carried out at conditions to optimize the breaking off of
these long side chains and their subsequent cracking to shorter molecules
with lower viscosities and pour points.
Typical Feed Stock
 The feed to vis-breaker can be either: Atmospheric residue (AR) or
Vacuum residue (VR).

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Visbreaking
❑The main reaction in vis-breaking is thermal cracking of heavy
hydrocarbons, since resins are holding asphaltene and keep them
attached to the oil.
Subsequent principal reactions
which occur during the vis-
breaking operation are:
Visbreaking Reactions
ii. Cracking of resins to light hydrocarbons (primarily olefins) and
compounds which convert to asphaltenes.
iii. Cracking of Naphthene rings at temperature above 900 oF (480 oC).
i. Cracking of the side chains attached to cycloparaffin and aromatic
rings so that they are removed or shortened to methyl or ethyl groups.

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There are two types of vis-breaker operations:
i. coil and furnace cracking, in which thermal cracking occurs in the
coil of the furnace, and
ii. the soak cracking, in which cracking occurs in a soak drum.
Process Description
Visbreaking
i. Coil and Furnace Cracking
During coil and furnace cracking, feed is introduced into the furnace and
heated to the desired temperature.
In the furnace or coil cracking process the feed is heated to cracking
temperature [885–930°F (474–500°C)] and quenched as it exits the
furnace.

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Process Description
Visbreaking
The vis-broken products are
immediately quenched to
stop the cracking reaction.
The quenching step is
essential to prevent coking
in the fractionation tower.
The gas oil and the vis-breaker residue are most commonly used as
quenching streams.

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Process Description
Visbreaking
After quenching, the effluent is directed to the lower section of the
fractionator where it is flashed.
The fractionator separates the products into gas, gasoline, gas oil and
vis-breaker tar (residue).
The gas oil withdrawn from the fractionator is steam-stripped to remove
volatile components and
then blended with the vis-breaker bottoms or routed for further
processing, such as hydrotreating, catalytic cracking or hydrocracking.

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Process Description
Visbreaking
The un-stabilized naphtha and fuel gas, recovered as overhead
products, are treated and then used as feedstock for catalytic reforming,
blended into finished products or sent to the fuel system.
The vis-breaker bottoms are withdrawn from the fractionator, heat
exchanged with the Visbreaker feedstock, mixed with stripped gas oil
(optional) and routed to storage.
Advantages and applications of coil vis-breaker.
o Coil cracking yields a slightly more stable vis-breaker products,
which are important for some feedstock and applications.
o It is generally more flexible and allows the production of heavy cuts,
boiling in the vacuum gas oil range.

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Process Description
Visbreaking
ii. Soaker Cracking
Some vis-breakers employ a
soaker between the vis-breaker
furnace and the quenching
step, similar to the conventional
thermal cracking processes.
This type of operation is termed
soaker cracking.
Typically during the soaker cracking operation, the feed leaves the
furnace between 800 and 820°F (427–438°C) and passes through a
soaking drum, which provides the additional reaction time, before it is
quenched.

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Process Description
Visbreaking
Coil cracking is described as a high temperature, short residence time
route whereas soaker cracking is a low temperature, long residence
time route. Typical Feed/Product Quality

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THANK YOU

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