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Petrochemical Processes

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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. – PowerPoint PPT presentation

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Title: Petrochemical Processes


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

4
  • 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.

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

7
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.

8
  • 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.

9
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.

10
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
11
  • The reaction is endothermic i.e. favoured _at_
    higher temp and lower pressures.
  • Effect of temp on the conversion and selectivity

12
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.

13
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.

14
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.
15
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.

16
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

17
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18
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19
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20
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).
21
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22
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.

23
  • 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

24
Typical FCC reactor/regenerator
25
Isomerization
  • Reactions leading to skeltal rearrangements over
    Pt catalysts

26
Hydrocracking
  • A hydrogen-consuming reaction that leads to
    higher gas production

Hydrdealkylation
A cracking reaction of an aromatic side chain in
presence of hydrogen
27
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.

28
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.

29
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

30
  • 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.

31
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.

32
  • 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.
33
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.

34
  • Consuming hydrogen is mainly a function of the
    number of benzene substituents.
  • Dealkylation of polysubstituted benzene increases
    hydrogen
  • consumption and gas production (methane).

35
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.

36
Hydrotreatment Catalysts and Reactions
  • The same as those developed in Germany for coal
    hydrogenation.
  • The cobalt-molybdenum/alumina is an effective
    catalyst.

hydrodenitrogenation
37
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.

38
Some recent research has been devoted to replace
the homogeneous acid catalysts by heterogeneous
solid catalysts employing zeolites and alumina,
or zirconia.
39
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.
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