Petrochemical Processes

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Flow diagram of a delayed coking unit5 (1) coker
fractionator, (2) coker heater, (3) coke drum,
(4) vapor recovery column.
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Fluid Coking

• Heated by the produced coke
• Cracking reactions occur inside the heater and
  the fluidized-bed 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 already-formed coke particles. Reactor
  temperature
• is about 520C, 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.

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• 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.

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Flow diagram of an Exxon flexicoking unit5 (1)
reactor, (2) scrubber, (3) heater, (4) gasifier,
(5) coke fines removal, (6) H2S removal.
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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.

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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.

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• Important is
• Types of hydrocarbons in the feed.
• Naphthene content
• The boiling range of the feeds
• Feeds with higher end points (200C) 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.

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Reforming Catalysts

• Bi-functional to provide two types of catalytic
  sites, hydrogenation-dehydrogenation sites and
  acid sites.
• platinum, is the best known hydrogenation-dehydrog
  enation catalyst
• Alumina, (acid sites) promote carbonium ion
  formation
• The two types of sites are necessary for
  aromatization and isomerization reactions.

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Reforming 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
Aromatization
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• The reaction is endothermic i.e. favoured _at_
  higher temp and lower pressures.
• Effect of temp on the conversion and selectivity

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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.

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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.

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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 400500C, Lewis
acid sites are formed.
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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.

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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

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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
(RCHCH2).
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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.

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• The reactor temperature is usually held at about
  450520C, and the pressure is approximately
  1020 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

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Typical FCC reactor/regenerator
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Isomerization

• Reactions leading to skeltal rearrangements over
  Pt catalysts

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Hydrocracking

• A hydrogen-consuming reaction that leads to
  higher gas production

Hydrdealkylation
A cracking reaction of an aromatic side chain in
presence of hydrogen
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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.

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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.

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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

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• 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.

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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
  hydrogen-rich 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.

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• Hydrocracking reaction conditions vary widely,
  depending on the feed and the required products.
  Temperature and pressure range from 400 to 480C
  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 unit29
(1,4) reactors, (2,5) HP separators, (3) recycle
scrubber (optional), (6) LP separator, (7)
fractionator.
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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.

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• Consuming hydrogen is mainly a function of the
  number of benzene substituents.
• Dealkylation of polysubstituted benzene increases
  hydrogen
• consumption and gas production (methane).

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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.

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Hydrotreatment Catalysts and Reactions

• The same as those developed in Germany for coal
  hydrogenation.
• The cobalt-molybdenum/alumina is an effective
  catalyst.

hydrodenitrogenation
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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.

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Some recent research has been devoted to replace
the homogeneous acid catalysts by heterogeneous
solid catalysts employing zeolites and alumina,
or zirconia.
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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.