SPRING 2019 - Anand Bhartianandbharti.co.in/pre/L20.pdf · Two visbreaking processes are...
Transcript of SPRING 2019 - Anand Bhartianandbharti.co.in/pre/L20.pdf · Two visbreaking processes are...
SPRING 2019
CL 4003 PETROCHEMICALS AND REFINERY ENGINEERING
Lecture 20
Department of Chemical Engineering
Birla Institute of Technology Mesra, Ranchi1
Isomerization of Light Naphtha
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✓ Isomerization is the process in which light straight chain
paraffins of low RON (C6 and C5) are transformed with proper
catalyst into branched chains with the same carbon number
and high octane numbers.
✓ The hydrotreated naphtha is fractionated into heavy naphtha
between 90-190 °C which is used as a feed to the reforming
unit. Light naphtha C5-80 °C is used as a feed to the
isomerization unit.
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Isomerization
There are two reasons for this fractionation:
✓ The first is that light hydrocarbons tend to hydrocrack in the
reformer.
✓ The second is that C6 hydrocarbons tend to form benzene in the
reformer. Gasoline specifications require a very low value of
benzene due to its carcinogenic effect .
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Isomerization
✓ The isomerization reactions are slightly exothermic and the
reactor works in the equilibrium mode.
✓ There is no change in the number of moles and thus the
reaction is not affected by pressure change.
✓ Better conversions are achieved at lower temperature.
✓ Operating the reactor at 130 °C will give good results.
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Thermodynamics of Isomerization
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Thermodynamic equilibrium with and without
recycling normal paraffin
✓ Isomerization is a reversible and slightly exothermic reaction:
✓ The conversion to iso-paraffin is not complete since the reaction
is equilibrium conversion limited.
✓ It does not depend on pressure, but it can be increased by
lowering the temperature.
✓ However operating at low temperatures will decrease the
reaction rate. For this reason a very active catalyst must be
used. 7
Isomerization Reactions
There are two types of isomerization catalysts:
✓ the standard Pt/chlorinated alumina with high chlorine
content, which is considered quite active, and
✓ the Pt/zeolite catalyst.
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Isomerization Catalysts
✓ This bi-functional nature catalyst consists of highly chlorinated
alumina (8–15 wt% Cl2) responsible for the acidic function of the
catalyst.
✓ Platinum is deposited (0.3–0.5 wt%) on the alumina matrix.
✓ Platinum in the presence of hydrogen will prevent coke
deposition, thus ensuring high catalyst activity.
✓ The reaction is performed at low temperature at about 130 °C to
improve the equilibrium yield and to lower chlorine elution.
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Standard Isomerization Catalyst
✓ The standard isomerization catalyst is sensitive to impurities
such as water and sulphur traces which will poison the catalyst
and lower its activity.
✓ For this reason, the feed must be hydrotreated before
isomerization.
✓ The pressure of the hydrogen in the reactor will result in the
elution of chlorine from the catalyst as hydrogen chloride.
✓ For all these reasons, the zeolite catalyst, which is resistant to
impurities, was developed. 10
Standard Isomerization Catalyst
✓ Zeolites are crystallized silico-aluminates that are used to give
an acidic function to the catalyst.
✓ Metallic particles of platinum are impregnated on the surface of
zeolites and act as hydrogen transfer centres.
✓ The zeolite catalyst can resist impurities and does not require
feed pre-treatment, but it does have lower activity and thus the
reaction must be performed at a higher temperature of 250 °C.
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Zeolite Catalyst
✓ The reformate yield from light naphtha isomerization is usually
very high (>97 wt%).
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Isomerization Yields
Light naphtha with a specific gravity of 0.724 is used as a feed to
the isomerization unit at a rate of 100 m3/h. Find the product
composition.
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Example
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Solution
Thermal cracking process
Visbreaking
Delayed coking
Fluid coking and Flexicoking
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Thermal cracking process
✓ Thermal cracking is the cracking of heavy residues under severe
thermal conditions.
✓ The liquid products of this process are highly olefinic, aromatic
and have high sulphur content.
✓ They require hydrogen treatment to improve their properties.
✓ Coking is the process of carbon rejection from the heavy
residues producing lighter components lower in sulphur, since
most of the sulphur is retained in the coke.
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Introduction
✓ The thermal treatment of hydrocarbons follows a free radical
mechanism where cracking reactions take place in the initiation
step.
✓ The reactions in the final step result in the formation of heavy
fractions and products like coke.
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Introduction
Visbreaking
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Visbreaking is a well-established noncatalytic thermal process that
converts atmospheric or vacuum residues to gas, naphtha, gas oil,
and tar (75–85% cracked material of lower viscosity that can be
used as fuel oil).
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Introduction
The conversion of these residues is accomplished by heating the
residue material to high temperatures in a furnace. The material is
passed through a soaking zone, located either in the heater or in
an external drum, under proper temperature and pressure
constraints so as to produce the desired products. The heater
effluent is then quenched with a quenching medium to stop the
reaction.
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Introduction
With refineries today processing heavier crudes and having a
greater demand for distillate products, visbreaking offers a low-
cost conversion capability to produce incremental gas and
distillate products while simultaneously reducing fuel oil viscosity.
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Introduction
The feed to visbreaker can be either
❑ Atmospheric residue (AR)
❑ Vacuum residue (VR)
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Feed Sources
✓ The main reaction in visbreaking is thermal cracking of heavy
hydrocarbons.
✓ The cracking of resin will result in precipitation of asphaltene
forming deposits in the furnace and will also produce unstable
fuel oil.
✓ The cracking severity or conversion is limited by the storage
stability of the final residual fuel.
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Visbreaking Reactions
✓ Paraffinic side chain breaking which will also lower the pour
point;
✓ Cracking of naphthens rings at temperature above 482 °C;
✓ Coke formation by polymerization, condensation,
dehydrogenation and dealkylation.
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Possible reactions in visbreaking
Four products are produced in the visbreaking process:
✓ gases (𝐶4− ),
✓ Naphtha C5 - 166 °C,
✓ gas oil 166–350 °C and
✓ residue or tar 350 °C.
Visbreaking results in an increase of API of 2–5 for the vacuum
residue feed and a reduction of viscosity of 25–75%.
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Product Yield and Properties
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Typical yields of visbreaking process
Visbreaking of Kuwait residue
There are two types of visbreakers:
✓ coil visbreaking, in which thermal cracking occurs in the coil of
the furnace, and
✓ the soak visbreaker, in which cracking occurs in a soak drum.
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Process Description
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Coil Visbreaker
✓ Vacuum or atmospheric residue feedstock is heated and then
mildly cracked in the visbreaker furnace.
✓ Reaction temperatures range from 450 to 480 °C, and operating
pressures vary from as low as 3 bar to as high as 10 bar.
✓ Visbroken 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 visbreaker residue are
most commonly used as quenching streams. 30
Coil Visbreaker
✓ Steam is injected into each heater coil to maintain the required
minimum velocity and residence time and to suppress the
formation of coke in the heater tubes. After leaving the heater
soaking zone, the effluent is quenched with a quenching
medium to stop the reaction and is sent to the visbreaker
fractionator for separation.
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Coil Visbreaker
✓ 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 visbreaker tar (residue).
✓ The gas oil withdrawn from the fractionator is steam-stripped to
remove volatile components and then blended with the
visbreaker bottoms or routed for further processing, such as
hydrotreating, catalytic cracking or hydrocracking.
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Coil Visbreaker
✓ 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 visbreaker bottoms are withdrawn from the fractionator,
heat exchanged with the visbreaker feedstock, mixed with
stripped gas oil (optional) and routed to storage.
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Coil Visbreaker
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Soaker Visbreaker
✓ According to the fundamentals of thermal cracking technology,
the conversion is mainly a function of two operating
parameters, temperature and residence time.
✓ Coil cracking is described as a high temperature, short
residence time route whereas soaker cracking is a low
temperature, long residence time route.
✓ The yields achieved by both options are in principle the same,
as are also the properties of the products.
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Soaker Visbreaker
✓ Both process configurations have their advantages and
applications.
✓ Coil cracking yields a slightly more stable visbreaker products,
which are important for some feedstocks and applications. It is
generally more flexible and allows the production of heavy cuts,
boiling in the vacuum gas oil range.
✓ Soaker cracking usually requires less capital investment,
consumes less fuel and has longer on-stream times.
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Soaker Visbreaker
✓ Two visbreaking processes are commercially available. The first
process is the coil, or furnace, type.
✓ The coil process achieves conversion by high-temperature
cracking within a dedicated soaking coil in the furnace. With
conversion primarily achieved as a result of temperature and
residence time, coil visbreaking is described as a high-
temperature, short-residence-time route.
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COIL VERSUS SOAKER DESIGN
✓ With the coil-type design, decoking of the heater tubes is
accomplished more easily by the use of steam-air decoking.
✓ Coil-type cracking heater produces a stable fuel oil. A stable
visbroken product is particularly important to refiners who do
not have many options in blending stocks.
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COIL VERSUS SOAKER DESIGN
✓ The alternative soaker process achieves some conversion within
the heater. However, the majority of the conversion occurs in a
reaction vessel or soaker which holds the two phase effluent at
an elevated temperature for a predetermined length of time.
✓ Soaker visbreaking is described as a low-temperature, high-
residence-time route.
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COIL VERSUS SOAKER DESIGN
✓ By providing the residence time required to achieve the desired
reaction, the soaker drum design allows the heater to operate at
a lower outlet temperature. This lower heater outlet temperature
results in lower fuel cost.
✓ The main disadvantage is the decoking operation of the heater
and soaker drum. Although decoking requirements of the
soaker drum design are not as frequent as those of the coil-type
design, the soaker design requires more equipment for coke
removal and handling.40
COIL VERSUS SOAKER DESIGN
✓ The customary practice of removing coke from a drum is to cut
it out with high-pressure water. This procedure produces a
significant amount of coke-laden water which needs to be
handled, filtered, and then recycled for use again.
✓ Unlike delayed cokers, visbreakers do not normally include the
facilities required to handle coke-laden water. The cost of these
facilities can be justified for a coker, where coke cutting occurs
every day. However, because of the relatively infrequent
decoking operation associated with a visbreaker, this cost
cannot be justified. 41
COIL VERSUS SOAKER DESIGN
✓ In order to maintain a desired degree of conversion, it is
necessary to stop the reaction at the heater outlet by
quenching.
✓ Quenching not only stops the conversion reaction to produce
the desired results, but will also prevent production of an
unstable bottoms product.
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REACTION PRODUCT QUENCHING
✓ Quenching can be accomplished by using different mediums.
The most frequently used quenching mediums are gas oil,
residue, or a combination of both.
✓ Gas oil is the most prevalent medium used for reaction
quenching. The gas oil quench works primarily by vaporization
and therefore requires a smaller amount of material to stop the
conversion reaction than a residue quench. The gas oil quench
promotes additional mixing and achieves thermal equilibrium
rapidly.
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REACTION PRODUCT QUENCHING
✓ The residue quench operates solely by sensible heat transfer
rather than the latent heat transfer of the gas oil quench.
✓ The gas oil quench is a clean quench and thus minimizes the
degree of unit fouling.
✓ It is believed that the use of a residue quench gives way to
fouling in the transfer line and fractionator. Also the visbreaker
bottoms circuit, from which the residue quench originates, is in
itself subject to fouling.
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REACTION PRODUCT QUENCHING
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Thermal cracking mechanism
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