Chemistry and technology of petroleum

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Chemistry and technology of petroleum

• By Dr. Dang Saebea

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Hydroconversion
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Introduction

• Hydroconversion is a term used to describe all
  different processes in which hydrocarbon reacts
  with hydrogen.

To describe the process of the removal of
sulphur, nitrogen and metal impurities in the
feedstock by hydrogen in the presence of a
catalyst.
Hydrotreating
The process of catalytic cracking of feedstock to
products with lower boiling points by reacting
them with hydrogen.
Hydrocracking
aromatics are saturated by hydrogen to the
corresponding naphthenes.
Hydrogenation
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Hydrotreating

• Objectives of Hydrotreating
• 1. Removing impurities, such as sulphur, nitrogen
  and oxygen for the control of a final product
  specification or for the preparation of feed for
  further processing.
• 2. Removal of metals, usually in a separate guard
  catalytic reactor when the organo-metallic
  compounds are hydrogenated and decomposed,
  resulting in metal deposition on the catalyst
  pores.
• 3. Saturation of olefins and their unstable
  compounds.

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Role of Hydrotreating

• HT are located before the reformer, hydrocracker
  and FCC

• They are also needed to adjust the final product
  specification for various streams, such as light
  naphtha, kerosene and low sulphur fuel oils
  (LSFOs).

Role of hydrotreating (HT) in the refinery
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Main role of hydrotreating

• 1. Meeting finished product specification.
• Kerosene, gas oil and lube oil desulphurization.
• Olefin saturation for stability improvement.
• Nitrogen removal.
• De-aromatization for kerosene to improve cetane
  number.

Cetane number is the percentage of pure cetane in
a blend of cetane and alpha-methyl-naphthalene.
The latter matches the ignition quality of
kerosene sample.
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Main role of hydrotreating

• 2. Feed preparation for downstream units
• Naphtha is hydrotreated for removal of metal and
  sulphur.
• Sulphur, metal, polyaromatics and Conradson
  carbon removal from vacuum gas oil (VGO) to be
  used as FCC feed.
• Pretreatment of hydrocracking feed to reduce
  sulphur, nitrogen and aromatics.

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Hydrotreating reactions
1. Desulphurization
a. Mercaptanes
b. Sulphides
c. Disulphides
d. Thiophenes
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2. Denitrogenation
a. Pyrrole
b. Pyridine
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3. Deoxidation
a. Phenol
b. Peroxides
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4. Hydrogenation of chlorides
5. Hydrogenation of olefins
6. Hydrogenation of aromatics
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7. Hydrogenation of organo-metallic compounds and
deposition of metals
Vanadium deposited as vanadium sulphide (V2S3)
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8. Coke formation by the chemical condensation of
polynuclear radicals
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Hydrotreating Processes
The main elements of a hydrotreating process

• The liquid feed is mixed with hydrogen and fed
  into a heater and then fed into a fixed bed
  catalytic reactor.
• The effluent is cooled and hydrogen-rich gas is
  separated using a high pressure separator.

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Hydrotreating Processes

• 3. Before the hydrogen is recycled, hydrogen
  sulphide can be removed using an amine scrubber.
• 4. Some of the recycle gas is also purged
• - To prevent the accumulation of light
  hydrocarbons (C1C4)
• - To control hydrogen partial pressure.

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Hydrotreating Processes

• 5. The liquid effluent for the reactor is
  introduced to a fractionator for product
  separation.

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1. Naphtha Hydrotreating

• To remove the impurities so that the
  hydrotreated naphtha can be introduced to the
  catalytic reformer.
• The expensive platinum based catalyst used in the
  reformer is sensitive to poisoning by such
  impurities.

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1. Naphtha Hydrotreating
recycle compressor
reactor
high and low pressure separators
treated naphtha fractionator.
a feed heater
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1. Naphtha Hydrotreating
H2S scrubber
A catalyst of CoMo on alumina is used.
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2. Middle Distillates Hydrotreating

• Middle distillate is mainly composed of saturated
  paraffins and also some aromatics which include
  simple compounds with up to three aromatic rings.
• Kerosene, jet fuel oil and diesel fuel are all
  derived from middle distillate fractions.

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2. Middle Distillates Hydrotreating
A hydrogen sulphide scrubber and a gas purging
are usually used to improve the quality of
recycled hydrogen.
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3. Atmospheric Residue Desulphurization

• atmospheric residue has a sulphur content and
  metals (Ni V).
• The purpose of this process is to remove most of
  the metals and reduce sulphur content in the
  product to less than 0.5 wt.

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3. Atmospheric Residue Desulphurization
Tlt371 C

• 1. The feed is introduced into the heater where
  steam is injected (to prevent coking) to a
  temperature below 371 C.

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3. Atmospheric Residue Desulphurization

• 2. The heated recycled hydrogen is mixed with
  feed and together, they are introduced into a
  guard reactor.
• 3. The stream leaving the guard reactor is
  quenched with cold recycle hydrogen and
  introduced to the first of the three fixed bed
  reactors.

The main reactions of hydrodemetallization,
hydrodesulphurization, denitrogenation and
aromatic hydrogenation take place in the
reactors.
contains a hydrogenation catalyst similar to that
in the main reactor but usually cheaper.
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3. Atmospheric Residue Desulphurization

• 4. The flow diagram also contains high and low
  pressure separators, recycled hydrogen stream
  with online amine treatment and purge.
• 5. The liquid stream from the separators are send
  to a fractionator to produce naphtha, diesel and
  low sulphur fuel oil (LSFO).

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Reactor

• The catalyst should have wide pores to avoid
  plugging due to metal deposition.
• Due to the fast deactivation of this catalyst,
  usually two reactors are used and the catalyst is
  changed in one of them while the other reactor is
  still online.
• Three to four reactors are usually used with
  different combinations of catalysts to achieve
  desired objectives.

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Hydrogen requirements for hydrotreating are
classified into

• Chemical requirement
• This is the amount of hydrogen required to remove
  impurities such as sulphur, oxygen, nitrogen,
  olefins and organometalic compounds, according to
  the stoichiometry of these reactions.
• Sometimes, it might be required to convert
  aromatics and naphthenes to corresponding
  paraffins.

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Hydrogen requirements for hydrotreating are
classified into

• (2) Hydrogen lost due to the dissolution of
  hydrogen in the hydrocarbons treated.
• (3) Amount of hydrogen lost with the purging of
  light hydrocarbons (C1C4) and hydrogen sulphide
  (if not removed by amine treatment).

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Make-up Hydrogen

• A certain hydrogen partial pressure should be
  maintained in the reactors by recycling
  un-reacted hydrogen and adding a make-up hydrogen
  to compensate for the amount consumed.
• The make-up hydrogen can be calculated by the
  following expression

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Operating Conditions

• The operating conditions of the hydrotreating
  processes
• pressure
• temperature
• catalyst loading
• feed flow rate
• hydrogen partial pressure

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Operating Conditions

• Increasing hydrogen partial pressure improves the
  removal of sulphur and nitrogen compounds and
  reduces coke formation.
• Higher temperatures will increase the reaction
  rate constant and improve the kinetics. However,
  excessive temperatures will lead to thermal
  cracking and coke formation.
• The space velocity is the reverse of reactor
  residence time (y). High space velocity results
  in low conversion, low hydrogen consumption and
  low coke formation.

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The range of operating conditions for
hydrotreating of different feed fractions
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Hydrocracking

• Hydrocracking is a catalytic hydrogenation
  process in which high molecular weight feedstocks
  are converted and hydrogenated to lower molecular
  weight products.
• The catalyst used in hydrocracking is a
  bifunctional one. It is composed of a metallic
  part, which promotes hydrogenation, and an acid
  part, which promotes cracking.
• Hydrogenation removes impurities in the feed such
  as sulphur, nitrogen and metals.
• Cracking will break bonds, and the resulting
  unsaturated products are consequently
  hydrogenated into stable compounds.

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Role of Hydrocracking in the Refinery

• It is mainly used to produce middle distillates
  of low sulphur content such as kerosene and
  diesel.
• If mild hydrocracking is used, a LSFO can be
  produced.
• It has been used to remove wax by catalytic
  dewaxing and for aromatic removal by hydrogen
  saturation.
• This has been applied to the lube oil plants and
  is gradually replacing the old solvent dewaxing
  and aromatic solvent extraction.

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Feeds and Products

• VGO is the main feed for hydrocrakers

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Hydrocracking Chemistry
1. Alkane hydrocracking
2. Hydrodealkylation
3. Ring opening
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4. Hydroisomerization
5. Polynuclear aromatics hydrocracking
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Hydrocracking Catalysts

• The cracking function is provided by an acidic
  support, whereas the hydrogenationdehydrogenation
  function is provided by active metals.

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Hydrocracking Processes

• The following factors can affect operation
  (product quality), yield (quantity), and the
  total economics of the process
• 1. Process configuration one stage (once-through
  or recycle) or two stages
• 2. Catalyst type
• 3. Operating condition (depends on process
  objective)
• - Conversion level
• - Maximization of certain product
• - Product quality
• - Catalyst cycle
• - Partial hydrogen pressure
• - Liquid hourly space velocity
• - Feed/hydrogen recycle ratio

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Process Configuration
Simplified flow diagram of one-stage
hydrocracking process with and without recycle

• In commercial hydrocrackers, a conversion of
  4080 of the feed can be achieved.
• However if high conversion is required the
  product from the bottom of the distillation tower
  is recycled back to the reactor for complete
  conversion.
• This configuration can be used to maximize a
  diesel product, and it employs an amorphous
  catalyst.

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Process Configuration
The catalyst in the first stage has a high
hydrogenation/acidity ratio, causing sulphur and
nitrogen removal
In the second reactor, the catalyst used is of a
low hydrogenation/acidity ratio in which naphtha
production is maximized
Conventional two-stage hydrocracker

• The effluent from the first stage reactor is sent
  to a separator and fractionator.
• The fractionator bottoms are sent to the second
  reactor.

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Two-stage hydrocracking

• A hydrotreatment reactor may be added before the
  first hydrocracker to help in removing sulphur
  and nitrogen compounds from the feed.
• Since H2S and NH3 are separated before entering
  the second hydrocracker, this allows the
  selection of special catalysts in the second
  reactor without the poisoning effect of sour
  gases.
• The two-stage configuration offers more
  flexibility than the single stage scheme. It is
  better suited for heavy feedstocks

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• The End