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Файл:Chemical Engineering of Natural Fuels and Carbon Materials. Study Guide
.pdf
7. OIL REFINING PROCESSES IN THE PRESENCE
OF ACIDIC CATALYSTS.
CONVERSION OF CARBOCATIONS
IN CATALYTIC PROCESSES
During the interaction with acids, hydrocarbons form positively
charged ions, called carbocations or carbonium ions.
Carbocations are highly reactive and as radicals undergo
monomolecular decomposition and bimolecular addition and substitution
reactions. An important difference in the chemical properties of
carbocations is an ability to the isomerization of carbon skeleton, hydrideion or methyl anion 1,2-transfer to adjacent (to the neighboring) carbon
atoms is possible.
Carbon skeleton isomerization goes slower than the isomerization
by transfer of a hydride ion. The structure of the tertiary carbocation is the
most stable.
Carbocations decay by C-C bond in beta-position in relation to
positively charged carbon atom. The most difficult is to split methyl-ion
from tertiary carbocation.
In most cases, the isomerization precedes the decay.
Task 7.1.
Write the following carbocations isomerization reactions:
Note. Primary carbocation isomerization into secondary, secondary
to the tertiary is also exothermic. The structure of the tertiary carbocation is
the most stable. The isomerization of the carbon chain by transferring a
methyl anion is associated with the secondary carbocation isomerization
into primary carbocation and, therefore, this process is endothermic and as a
result, carbon skeleton isomerization goes slower than the isomerization by
transfer of a hydride ion.
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Task 7.2.
Write the following carbocations decay reactions:
Task 7.3.
What reaction of primary carbocation decomposition goes easier?
Note. Primary carbocation decay goes easier if secondary and
especially tertiary carbocations are formed. In most cases, the isomerization
should precede decay.
Task 7.4.
Arrange the following proton transfer reactions (may occur in the
interaction of carbocations with alkene molecule) in order of increasing the
easiness of their implementation.
Note. Primary carbocations interaction with alkenes goes easier if
secondary and especially tertiary carbocations are formed. The biggest
energy expenditure is required for the proton transfer of tertiary carbocation
to olefin molecule.
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Task 7.5.
Write the reactions of hydride ion breakoff and indicate which of
them is less difficult to proceed.
Note. Carbocation stabilization by a hydride ion goes easier when
tertiary carbon atom becomes a donor of a hydride ion.
Conversion of various classes of hydrocarbons
in catalytic cracking
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, olefinic gases, and other
products. The catalytic cracking process involves the presence
of acid catalysts, usually solid acids such as silica-alumina and zeolites,
which promote a heterolytic breakage of bonds yielding pairs of ions of
opposite charges, usually a carbocation and a very unstable hydride anion.
Carbon-cations are highly unstable and undergo processes of chain
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rearrangement, C-C scission in beta-position and intra- and intermolecular
hydrogen transfer.
The gasoline produced in the FCC unit has higher octane rating but
is less chemically stable. The FCC LPG is an important source of C
–
3
C4 olefins and isobutane that are essential feeds for the alkylation process
and the production of polymers such as polypropylene.
The feedstock to an FCC is usually that portion of the crude oil that
has an initial boiling point of 340 °C or higher at atmospheric pressure. This
portion of crude oil is often referred to as heavy gas oil or vacuum gas oil
(HVGO).
Fig. 7.1 – Flow diagram of a typical FCC unit
The reactor and regenerator are considered to be the heart of the
fluid catalytic cracking unit. The schematic flow diagram of a typical FCC
unit is given in figure 7.1. 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 by contact and mixing with the very hot powdered
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catalyst from the regenerator. All of the cracking reactions take place in the
catalyst riser within a period of 2–4 seconds. The hydrocarbon vapors
"fluidize" the powdered catalyst and the mixture of hydrocarbon vapors and
catalyst flows upward to enter the reactor.
The reactor is a vessel in which the cracked product vapors are
separated from the so-called spent catalyst by flowing through a set of
cyclones within the reactor and the spent catalyst flows downward through a
steam stripping section to remove any hydrocarbon vapors before the spent
catalyst returns to the catalyst regenerator.
During the process, the catalyst surface becomes covered with coke
which greatly reduces activity and selectivity of the catalyst. This catalyst
flows into a fluidized-bed regenerator where the air is used to burn off the
coke to restore catalyst activity and also provide the necessary heat for the
next reaction cycle. The "regenerated" catalyst then flows to the base of the
riser repeating the cycle.
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 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,
The reaction product vapors flow from the top of the reactor to the
bottom section of the distillation column, the main fractionator, where they
are distilled into the FCC end products of cracked naphtha, fuel oil,
and offgas. After further processing for removal of sulfur compounds, the
cracked naphtha becomes a high-octane component of the refinery's blended
gasolines.
Although the schematic flow diagram above depicts the main
fractionator as having only one sidecut stripper and one fuel oil product,
many FCC main fractionators have two sidecut strippers and produce a light
fuel oil and a heavy fuel oil. Likewise, many FCC main fractionators
produce light cracked naphtha and heavy cracked naphtha. The bottom
product from the main fractionator contains residual catalyst particles which
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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.
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. It is a molecular sieve with a distinctive lattice structure that allows
only a certain size range of hydrocarbon molecules to enter the lattice. The
catalytic sites in the zeolite are strong acids and provide most of the
catalytic activity.
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 zeolite does. The binder and filler
components provide the physical strength and integrity of the catalyst.
Nickel, vanadium, iron, copper and other metal contaminants,
present in FCC feedstocks in the parts per million range, 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. Hydrodesulfurization of
the FCC feedstock removes some of the metals and also reduces the sulfur
content of the FCC products. Certain materials can be used as additives
which can impregnate a catalyst or be added to the FCC feedstock in the
form of metal-organic compounds. Such materials react with the metal
contaminants and passivate the contaminants by forming less harmful
compounds that remain on the catalyst.
Task 7.6.
Write a mechanism for n-octane conversion in catalytic cracking
(show the formation of 2,2,4 - trimethylpentane).
Task 7.7.
Write carbocation mechanism of n-butiltcyclohexsane conversion in
catalytic cracking.
Note. Cracking of cycloalkanes can go in several directions
simultaneously:
- The cleavage of the side chain;
- The ring break by the C-C bond;
- Dehydrogenation to the aromatics.
During dehydrogenation, transfers of hydride ions and protons take
place.
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The more the number of carbon atoms in the molecule of the
cycloalkane is and the higher the length of alkyl group in a ring is, the
easier the process of dehydrogenation is.
Task 7.8.
Write the mechanism of the cycloalkanes, cycloalkenes and arenas
formation, based on the example of 1-heptene transformation in catalytic
cracking.
Task 7.9.
What products can be formed from para-xylene in a catalytic
cracking process?
Note. Polymethylated homologues of benzene undergo dealkylation
slightly, they are prone to isomerization of substituents position.
Task 7.10.
Write the mechanism of n-propylbenzene transformation in catalytic
cracking.
Note. An aromatic ring affinity to a proton is significantly greater
than to an alkyl ion, and as a result, in catalytic cracking dealkylation of
alkyl-substituted arenes takes place.
Task 7.11.
Show diphenylethane transformation in catalytic cracking.
Task 7.12.
What products are produced from tetralin in catalytic cracking?
Conversion of various classes of hydrocarbons
in catalytic reforming
Catalytic reforming is a chemical process used to
convert petroleum refinery naphthas, typically having low octane ratings,
into high-octane liquid products called reformates, which are used in
blending to produce high-octane gasoline. The process also produces
significant amounts of byproduct, hydrogen gas, which is fed into
hydrocatalytic refinery processes such as hydrocracking. A side reaction
is hydrogenolysis, which produces light hydrocarbons of lower value, such
47
as methane, ethane,propane, and butanes. Also, reformate can be the source
of aromatic bulk chemicals such
as benzene, toluene, xylene and ethylbenzenewhich have diverse uses.
Naphtha is the main component of the refinery gasoline (petrol)
product, it is processed through a catalytic hydrodesulfurizer to
remove sulfur-containing hydrocarbons and a catalytic reformer to reform
its hydrocarbon molecules into more complex molecules with a higher
octane rating value. The naphtha is a mixture of a lot of different
hydrocarbon compounds. It has an initial boiling point of about 35 °C and a
final boiling point of about 200 °C, and it
contains paraffin, naphthene (cyclic paraffins) and aromatic hydrocarbons
ranging from those containing four carbon atoms to those containing about
ten or eleven carbon atoms. The naphtha from the crude oil distillation, or
straight-run naphthas, is often distilled to light naphtha containing mostly
the hydrocarbons with six or fewer carbon atoms and heavy naphtha
containing mostly of the hydrocarbons with more than six carbon atoms.
The heavy naphtha has an initial boiling point of about 140–150 °C and a
final boiling point of about 190–205 °C.
The straight-run heavy naphtha is usually processed in a catalytic
reformer because the light naphtha components tend to crack into lower
molecular weight hydrocarbons which are not useful as high-octane
gasoline blending components, and also, the molecules with six carbon
atoms tend to form aromatics, particularly benzene, which amount is
limited.
There are many chemical reactions that occur in the catalytic
reforming process, all of which occur in the presence of a catalyst and
high partial pressure of hydrogen at the temperatures of about 490–520 °C
and from pressures of about 0.5 to 4.0 MPa.
The main catalytic reforming reactions are the dehydrogenation of
naphthenes into aromatics, the dehydrogenation and aromatization of
paraffins to aromatics (commonly called dehydrocyclization). Also,
the isomerization of normal paraffins to isoparaffins and hydrocracking of
paraffins into smaller molecules take place.
The most commonly used type of catalytic reforming unit has
three reactors, each with a fixed bed of catalyst, and all amount of the
catalyst is regenerated in situ during catalyst regeneration which occurs
approximately once a year. There are types of catalytic reformers called
continuous catalyst regeneration (CCR) reformers. Such units are
characterized by continuous in-situ regeneration of part of the catalyst in a
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special regenerator, and by continuous addition of the regenerated catalyst
Naphtha
Preheated Naphtha and
Hydrogen
Recycled gas
Pump
Hydrogen
Recycled gas
Vapor
Steam
Liquid
Reformate
Reboiler
Stabilizer
Reflux
Pump
LPG
Reflux
drum
Off gas
Condense
r
Gas
separ
ator
Cooler
Fired
Heater
Fired
Heater
Fired
Heater
Fixed-bed
Reactor
Compressor
to the operating reactors. CCR versions are UOP's CCR Platformer
process and Axens' Octanizing process.
with a stream of hydrogen-rich recycle gas. The resulting liquid–gas
mixture is heated and fed into the first reactor. The major reactions are
highly endothermic and result in a large temperature decrease between the
inlet and outlet of the reactor. To maintain the required reaction temperature
and the rate of reaction, the vaporized stream is reheated in the furnaces
before it flows through the next reactor.
fired heater with three separate heating coils.
by flowing through the heat exchangers before flowing into the gas
separator.
recycle hydrogen gas line and also the hydrogen-rich gas can be used in
other refinery processes that consume hydrogen, such as
hydrodesulfurization units, hydrotreating unit, and hydrocracker unit.
column, commonly called a stabilization column. The offgas product from
Fig. 7.2 – Catalytic reforming unit flow diagram
The liquid feed is pumped up to the reaction pressure and is joined
Some units have three separate fired heaters and some use a single
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The hot reaction products from the third reactor are partially cooled
Most of the hydrogen-rich gas from the gas separator returns to the
The liquid from the gas separator is routed into a fractionating
the stabilizer contains hydrogen, methane, ethane, propane and butane gases
produced as a result of hydrocracking reactions.
The bottom product from the stabilizer is the high-octane liquid
reformate that will become a component of gasoline. Reformate can be
divided into two or more streams, such as light and heavy reformate. The
light reformate has lower octane and can be used as isomerization feedstock
if this unit is available. The heavy reformate is high in octane and low in
benzene, hence, it is an excellent blending component for the gasoline.
The commonly used catalytic reforming catalysts contain noble
metals such as platinum and rhenium on a silica or silica-alumina support
base, which are very susceptible to poisoning by sulfur
and nitrogen compounds. Therefore, the naphtha feedstock to a catalytic
reformer is always pre-processed in a hydrotreating process which removes
both the sulfur and nitrogen compounds.
The noble metals (platinum and rhenium) are considered to be
catalytic sites for the dehydrogenation reactions and the chlorinated alumina
provides the acid sites needed for isomerization, cyclization, and
hydrocracking reactions.
The activity of the catalyst can be periodically regenerated or
restored by in situ high temperature oxidation of the coke followed by
chlorination.
Task 7.13.
Write methylcyclohexane conversion reaction in catalytic
reforming.
Note. Cycloalkanes during catalytic reforming process can undergo:
- Dehydrogenation to aromatics
- Isomerisation into cyclopentanes
- Hydrogenolysis
- Hydrocracking.
Task 7.14.
Write the reactions of cyclohexane conversion in catalytic
reforming.
Task 7.15.
Write the mechanism of aromatization of gem-substituted dimethyl
cyclohexane.
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