Flow diagram of a delayed coking unit:5 (1) coker fractionator, (2)
coker heater, (3) coke drum, (4) vapor recovery column.
Fluid Coking
• Heated by the produced coke
• Cracking reactions occur inside the heater and the fluidizedbed reactor.
• The fluid coke is partially formed in the heater.
• Hot coke slurry from the heater is recycled to the fluid reactor
to provide the heat required for the cracking reactions.
• Fluid coke is formed by spraying the hot feed on the alreadyformed coke particles. Reactor temperature
• is about 520°C, and the conversion into coke is immediate,
with complete disorientation of the crystallites of product
coke.
• The burning process in fluid coking tends to concentrate the
metals, but it does not reduce the sulfur content of the coke.
• Characteristics of fluid coke:
• high sulfur content,
• low volatility, poor crystalline structure, and low
grindability index.
• Flexicoking, integrates fluid coking with coke
gasification.
• Most of the coke is gasified. Flexicoking gasification
produces a substantial concentration of the metals in
the coke product.
Flow diagram of an Exxon flexicoking unit:5 (1) reactor, (2)
scrubber, (3) heater, (4) gasifier, (5) coke fines removal, (6)
H2S removal.
CATALYTIC CONVERSION PROCESSES
Catalytic Reforming
• To improve the octane number of a naphtha.
• Aromatics and branched paraffins have high octane ratings
than paraffins and cycloparaffins.
• Many reactions: e.g. dehydrogenation of naphthenes and the
dehydrocyclization of paraffins to aromatics.
•
Catalytic reforming is the key process for obtaining benzene,
toluene, and xylenes (BTX).
• These aromatics are important intermediates for the production
of many chemicals.
Reformer Feeds
• heavy naphtha fraction produced from atmospheric distillation
units.
• Naphtha from other sources such as those produced from
cracking and delayed coking may also be used.
• Before using naphtha as feed for a catalytic reforming unit, it
must be hydrotreated to saturate the olefins and to
hydrodesulfurize and hydrodenitrogenate sulfur and nitrogen
compounds.
• Olefinic compounds are undesirable because they are precursors
for coke, which deactivates the catalyst.
• Sulfur and nitrogen compounds poison the reforming catalyst.
• The reducing atmosphere in catalytic reforming promotes forming
of hydrogen sulfide and ammonia. Ammonia reduces the acid
sites of the catalyst, while platinum becomes sulfided with H2S.
•Important is :
–Types of hydrocarbons in the feed.
– Naphthene content
– The boiling range of the feeds
Feeds with higher end points (≈200°C) are favorable because some
of the long-chain molecules are hydrocracked to molecules in the
gasoline range. These molecules can isomerize and dehydrocyclize
to branched paraffins and to aromatics, respectively.
Reforming Catalysts
• Bi-functional to provide two types of catalytic
sites, hydrogenation-dehydrogenation sites
and acid sites.
• platinum, is the best known hydrogenationdehydrogenation catalyst
• Alumina, (acid sites) promote carbonium ion
formation
• The two types of sites are necessary for
aromatization and isomerization reactions.
• Pt/Re catalysts are very stable, active, and selective.
• Trimetallic catalysts of noble metal alloys are also used for the
same purpose.
• The increased stability of these catalysts allowed operation at
lower pressures.
Reforming Catalysts
Reforming Reactions
Aromatization
• The reaction is endothermic i.e. favoured @ higher temp and
lower pressures.
• Effect of temp on the conversion and selectivity:
Catalytic Cracking
• Catalytic cracking (Cat-cracking): To crack lower-value stocks
and produce higher-value light and middle distillates.
• To produce light hydrocarbon gases, which are important
feedstocks for petrochemicals.
• To produce more gasoline of higher octane than thermal
cracking. This is due to the effect of the catalyst, which
promotes isomerization and dehydrocyclization reactions.
• Feeds vary from gas oils to crude residues
• Polycyclic aromatics and asphaltenes peoduce coke.
Catalytic Catalysts
• Acid-treated clays were the first catalysts used.
• Replaced by synthetic amorphous silica-alumina, which is
more active and stable.
• Incorporating zeolites (crystalline alumina-silica) with the
silica/alumina catalyst improves selectivity towards aromatics.
These catalysts have both Lewis and Bronsted acid sites that
promote carbonium ion formation. An important structural
feature of zeolites is the presence of holes in the crystal
lattice, which are formed by the silica-alumina tetrahedra.
Each tetrahedron is made of four oxygen anions with either an
aluminum or a silicon cation in the center. Each oxygen anion
with a (II) oxidation state is shared between either two silicon,
two aluminum, or an aluminum and a silicon cation.
Catalytic Catalysts
Bronsted acid sites in HY-zeolites mainly originate from protons
that neutralize the alumina tetrahedra. When HY-zeolite (X- and
Y-zeolites are cracking catalysts ) is heated to temperatures in
the range of 400–500°C, Lewis acid sites are formed.
Zeolite Catalysts
• Highly selective due to its smaller pores, which allow diffusion
of only smaller molecules through their pores, and to the
higher rate of hydrogen transfer reactions. However, the
silica-alumina matrix has the ability to crack larger molecules.
• Deactivation of zeolite catalysts occurs due to coke formation
and to poisoning by heavy metals.
• Deactivation may be reversible or irreversible.
• Reversible deactivation occurs due to coke deposition. This is
reversed by burning coke in the regenerator.
• Irreversible deactivation results as a combination of four
separate but interrelated mechanisms: zeolite dealumination,
• zeolite decomposition, matrix surface collapse, and
contamination by metals such as vanadium and sodium.
Cracking Reactions
• A major difference between thermal and catalytic cracking is
that reactions through catalytic cracking occur via carbocation
intermediate, compared to the free radical intermediate in
thermal cracking.
• Carbocations are longer lived and accordingly more selective
than free radicals.
• Acid catalysts such as amorphous silica-alumina and
crystalline zeolites promote the formation of carbocations.
The following illustrates the different ways by which
carbocations may be generated in the reactor:
Aromatization Reactions
• Dehydrocyclization reaction. Olefinic compounds formed by
the beta scission can form a carbocation intermediate with
the configuration conducive to cyclization.
Once cyclization has occurred, the formed carbocation can lose a proton,
and a cyclohexene derivative is obtained. This reaction is aided by the
presence of an olefin in the vicinity (R–CH=CH2).
Cracking Process
• Most catalytic cracking reactors are either fluid bed or moving
bed.
• In FCC, the catalyst is an extremely porous powder with an
average particle size of 60 microns.
• Catalyst size is important, because it acts as a liquid with the
reacting hydrocarbon mixture.
• In the process, the preheated feed enters the reactor section
with hot regenerated catalyst through one or more risers
where cracking occurs. A riser is a fluidized bed where a
concurrent upward flow of the reactant gases and the catalyst
particles occurs.
• The reactor temperature is usually held at about 450–520°C,
and the pressure is approximately 10–20 psig.
• Gases leave the reactor through cyclones to remove the
powdered catalyst, and pass to a fractionator for separation of
the product streams. Catalyst regeneration occurs by
combusting carbon
• deposits to carbon dioxide and the regenerated catalyst is then
returned
Typical FCC reactor/regenerator
Isomerization
Reactions leading to skeltal rearrangements over Pt catalysts
Hydrocracking
A hydrogen-consuming reaction that leads to higher gas
production
Hydrdealkylation
A cracking reaction of an aromatic side chain in presence of
hydrogen
Deep Catalytic Cracking
• Deep catalytic cracking (DCC) is a catalytic cracking process
which selectively cracks a wide variety of feedstocks into light
olefins.
• It produces more olefines than FCC.
Hydrocracking Process
• It is a cracking process in presence of hydrogen.
• The feedstocks are not suitable for catalytic cracking because
of their high metal, sulfur, nitrogen, and asphaltene contents.
• The process can also use feeds with high aromatic content.
• Products from hydrocracking processes lack olefinic
hydrocarbons.
• The product slate ranges from light hydrocarbon gases to
gasolines to residues.
• The process could be adapted for maximizing gasoline, jet
fuel, or diesel production.
Hydrocracking Catalysts and Reactions
•
•
•
•
Bifunctional noble metal containing zeolites are used.
This promote carbonium ion formation.
Catalysts with strong acidic activity promote isomerization.
The hydrogenation-dehydrogenation is promoted by catalysts
such as cobalt, molybdenum, tungsten, vanadium, palladium,
or rare earth elements. As with
• catalytic cracking, the main reactions occur by carbonium ion
and beta scission, yielding two fragments that could be
hydrogenated on the catalyst surface.
• The first-step is formation of a carbocation over the catalyst
surface:
The carbocation rearrange, eliminate a proton to produce an
olefin, or crack at a beta position to yield an olefin and a new
carbocation.
-Products from hydrocracking are saturated. i.e. gasolines from
hydrocracking units have lower octane ratings. They have a
lower aromatic content due to high hydrogenation activity.
- Products from hydrocracking units are suitable for jet fuel use.
Hydrocracking also produces light hydrocarbon gases (LPG)
suitable as petrochemical feedstocks.
Hydrocracking Process
• Mostly single stage, with the possibility of two operation
modes. Once-through and a total conversion of the
fractionator bottoms by recyling.
• In once-though operation, low sulfur fuels are produced and
the fractionator bottom is not recycled.
• In the total conversion mode the fractionator bottom is
recylced to the inlet of the reactor.
• In the two-stage operation, the feed is hydrodesulfurized in
the first reactor with partial hydrocracking. Reactor effluent
goes to a high-pressure separator to separate the hydrogenrich gas, which is recycled and mixed with the fresh feed. The
liquid portion from the separator is fractionated, and the
bottoms of the fractionator are sent to the second stage
reactor.
• Hydrocracking reaction conditions vary widely, depending on
the feed and the required products. Temperature and
pressure range from 400 to 480°C and 35 to 170 atmospheres.
Space velocities in the range of 0.5 to 2.0 hr-1 are applied.
Flow diagram of a Cheveron hydocracking unit:29 (1,4) reactors, (2,5)
HP separators, (3) recycle scrubber (optional), (6) LP separator, (7) fractionator.
Hydrodealkylation Process
• Designed to hydrodealkylate methylbenzenes, ethylbenzene
and C9+ aromatics to benzene. The petrochemical demand for
benzene is greater than for toluene and xylenes.
• After separating benzene from the reformate, the higher
aromatics are charged to a hydrodealkylation unit.
• The reaction is a hydrocracking one, where the alkyl side
chain breaks and is simultaneously hydrogenated.
• Consuming hydrogen is mainly a function of the number of
benzene substituents.
• Dealkylation of polysubstituted benzene increases hydrogen
• consumption and gas production (methane).
Hydrotreatment Processes
• Hydrotreating is a hydrogen-consuming process to reduce or
remove impurities such as sulfur, nitrogen, and some trace
metals from the feeds.
• It also stabilizes the feed by saturating olefinic compounds.
• Feeds could be any petroleum fraction, from naphtha to crude
residues.
• The feed is mixed with hydrogen, heated to the proper
temperature, and introduced to the reactor containing the
catalyst.
Hydrotreatment Catalysts and Reactions
• The same as those developed in Germany for coal hydrogenation.
• The cobalt-molybdenum/alumina is an effective catalyst.
hydrodenitrogenation
Alkylation Process
• To produce large hydrocarbon molecules in the gasoline fraction
from small moleucles. (branched hydrocarbons).
• Normally acid catalyzed using H2SO4 or abhydrous HF.
• The product is known as the alkylate.
Some recent research has been devoted to replace the
homogeneous acid catalysts by heterogeneous solid
catalysts employing zeolites and alumina, or zirconia.
Isomerization process
• Small volume but important refinery process.
• Acid catalyzed. To produce branched alkanes.
• Bifunctional catalysts activated by inorganic chelorides are
used.
• Pt/zeolite is a typical isomerization catalyst.
Oligomerization of Olefines (Dimerization)
•
•
•
•
To produce polymer gasoline with high octane number.
Acid catalyzed. By phosphoric or sulfuric acid.
The feed is Propylne-propane or propykene-butane mixture.
The alkane is used as diluent.