Chemistry and technology of petroleum

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Chemistry and technology of petroleum

• By Dr. Dang Saebea

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Catalytic Reforming andIsomerization
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Introduction

• Catalytic reforming of heavy naphtha and
  isomerization of light naphtha constitute a very
  important source of products having high octane
  numbers which are key components in the
  production of gasoline.

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Catalytic Reforming

• Catalytic reforming is the process of
  transforming hydrocarbons with low octane numbers
  to aromatics and iso-paraffins which have high
  octane numbers.
• It is a highly endothermic process requiring
  large amounts of energy.

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Role of Reformer in the Refinery and Feed
Preparation

• The catalytic reformer is one of the major units
  for gasoline production in refineries.
• It can produce 37 wt of the total gasoline pool.
  
• Other units
• - fluid catalytic cracker (FCC)
• - alkylation unit
• - isomerization unit

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Octane Number

• An octane number is a measure of the knocking
  tendency of gasoline fuels in spark ignition
  engines.
• The octane number of a fuel is determined by
  measuring its knocking value compared to the
  knocking of a mixture of n-heptane and isooctane
  (2,2,4- trimethyl pentane).

zero octane
Pure n-heptane
Pure isooctane
100 octane
Example an 80 vol isooctane mixture has an
octane number of 80.
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Main feed stock

• The straight run naphtha from the crude
  distillation unit is hydrotreated to remove
  sulphur, nitrogen and oxygen which can all
  deactivate the reforming catalyst.
• The hydrotreated naphtha (HTN) is fractionated
  into light naphtha (LN)
• - light naphtha is mainly C5C6 hydrocarbons
• - heavy naphtha (HN) is mainly C7C10
  hydrocarbons.

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Main feed stock

• Environmental regulations limit on the benzene
  content in gasoline.
• If benzene is present in the final gasoline, it
  produces carcinogenic material on combustion.
• Elimination of benzene forming hydrocarbons, such
  as, hexane will prevent the formation of benzene,
  and this can be achieved by increasing the
  initial point of heavy naphtha.
• These light paraffinic hydrocarbons can be used
  in an isomerization unit to produce high octane
  number isomers.

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Product from catalytic reforming
FEED PRODUCT
Paraffins 30-70 30-50
Olefins 0-2 0-2
Naphthenes 20-60 0-3
Aromatics 7-20 45-60
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The role of the heavy naphtha (HN) reformer in
the refinery
Hydrogen, produced in the reformer can be
recycled to the naphtha hydrotreater, and the
rest is sent to other units demanding hydrogen.
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REACTIONS

• 4 major reactions are categorized as
• Dehydrogenation of naphthenes to aromatics
• Dehydocyclization of paraffins to aromatics
• Isomerization
• Hydrocracking

Desirable
Undesirable
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Dehydrogenation Dehydrocyclization

• Highly endothermic
• Cause decrease in temperatures
• Highest reaction rates
• Aromatics formed have high B.P so end point of
  gasoline rises
• Favourable conditions
• High temperature
• Low pressure
• Low space velocity
• Low H2/HC ratio

Dehydrogenation
Dehydrocyclization
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Isomerization

• Branched isomers increase octane rating
• Small heat effect
• Fairly rapid reactions
• Favourable conditions
• High temperature
• Low pressure
• Low space velocity
• H2/HC ratio no significant effect

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Hydrocracking

• Exothermic reactions
• Slow reactions
• Consume hydrogen
• Produce light gases
• Lead to coking
• Causes are high paraffin concentration feed
• Favourable conditions
• High temperature
• High pressure
• Low space velocity

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Coke Deposition

• Coke can also deposit during hydrocracking
  resulting in the deactivation of the catalyst.
• Coke formation is favoured at low partial
  pressures of hydrogen.
• Hydrocracking is controlled by operating the
  reaction at low pressure between 525 atm, not
  too low for coke deposition and not too high in
  order to avoid cracking and loss of reformate
  yield.

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Thermodynamics of Reforming Reactions

• The dehydrogenation reactions are the main source
  of reformate product and are considered to be the
  most important reactions in reforming.
• These are highly endothermic reactions and
  require a great amount of heat to keep the
  reaction going.
• The dehydrogenation reactions are reversible and
  equilibrium is established based on temperature
  and pressure.
• It is usually important to calculate the
  equilibrium conversion for each reaction.

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Example

• The Gibbs free energy of the following reaction
  at 500 C and 20 atm is calculated to be 20.570
  kcal/mol.
• Calculate the reaction equilibrium conversion and
  barrels of benzene formed per one barrel of
  cyclohexane.
• The hydrogen feed rate to the reactor is 10,000
  SCF/bbl of cyclohexane.
• Cyclohexane density of 0. 779 g/cm3,
• 1mol H2 379 SCF

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Reaction Kinetics and Catalysts

• The catalyst used for reforming is a bifunctional
  catalyst composed of platinum metal on
  chlorinated alumina.

the centre for the dehydrogenation reaction
Platinum
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Reaction Kinetics and Catalysts

• Iridium (Ir) is added to boost activity,
• Rhenium (Re) is added to operate at lower
  pressures and Tin (Sn) is added to improve yield
  at low pressures.

• The use of Pt/Re is now most common in
  semi-regenerative (SR) processes with Pt/Sn is
  used in moving bed reactors.

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Reaction Kinetics and Catalysts

• Impurities that might cause deactivation or
  poisoning of the catalyst include coke, sulphur,
  nitrogen, metals and water.
• The reformer should be operated at high
  temperature and low pressure to minimize coke
  deposition.

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Process description of catalytic reforming process

• Semi-regenerative Fixed Bed Process
• Continuous Regenerative (moving bed) Process

The old technologies are fixed bed
configuration. Moving bed technology has also
recently been introduced
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Semi-regenerative Fixed Bed Process

• Three reactors

fixed bed of catalyst
All of the catalyst is regenerated in situ during
routine catalyst regeneration shutdowns (6 to 24
months) by burning off the carbon formed on the
catalyst surface
Such a unit is referred to as a semi-regenerative
catalytic reformer (SRR).
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Semi-regenerative Fixed Bed Process
Reactions such as dehydrogenation of paraffins
and naphthenes which are very rapid and highly
endothermic
first reactor
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Semi-regenerative Fixed Bed Process
Reactions that are considered rapid, such as
paraffin isomerization and naphthens
dehydroisomerization, give moderate temperature
decline
second reactor
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Semi-regenerative Fixed Bed Process
slow reactions such as dehydrocyclization and
hydrocracking give low temperature decline.
Third reactor
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Semi-regenerative Fixed Bed Process
The temperature and concentration profile in each
reactor
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Semi-regenerative Fixed Bed Process

• Recycling some of the hydrogen produced.

• At the top of the stabilizer residual hydrogen
  and C1 to C4 are withdrawn as condenser products,
  which are then sent to gas processing,
• Part of the liquid product (C3 and C4) is
  returned from the reflux drum back to the
  stabilizer.

Some light hydrocarbons (C1C4) are separated
from the reformate in the stabilizer.
The main product of the column is stabilized
reformate, which is sent to the gasoline blending
plant.
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Semi-regenerative Fixed Bed Process

• A slight modification to the semi-regenerative
  process is to add an extrareactor to avoid
  shutting down the whole unit during regeneration.
  
• Three reactors can be running while the forth is
  being regenerated.
• This modified process is called the cyclic
  fixed bed process

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Continuous Regenerative (moving bed) Process

• In this process, three or four reactors are
  installed one on the top of the other.

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Continuous Regenerative (moving bed) Process

• The effluent from each reactor is sent to a
  common furnace for heating.

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Continuous Regenerative (moving bed) Process

• The catalyst moves downwards by gravity from the
  first reactor (R1) to the forth reactor (R4).
• The catalyst is sent to the regenerator to burn
  off the coke and then sent back to the first
  reactor R1.
• The final product from R4 is sent to the
  stabilizer and gas recovery section.

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Typical operating conditions of three reforming
processes
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PROCESS VARIABLES
Catalyst type

• Chosen to meet refiners yield, activity and
  stability need
• Primary control of changing conditions or
  qualities in reactor.
• High temp increase octane rating.
• High temp reduce catalyst stability but may be
  increased for declining catalyst activity.
• Pressure effects the reformer yield catalyst
  stability.
• Low pressure increases yield octane

Temperature
Pressure
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PROCESS VARIABLES

• Low space velocity favors aromatic formation but
  also promote cracking.
• Higher space velocity allows less reaction time.
• Moles of recycle hydrogen / mole of naphtha
  charge
• Increase H2 partial pressure or increasing the
  ratio suppresses coke formation but promotes
  hydrocracking.

Space velocity
H2 / HC ratio
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Reformer correlations
RONF research octane number of feed
RONRresearch octane number of reformate C5
vol volume percent of reformate yield SCFB
H2standard cubic foot of H2 produced/barrel of
feed K characterization factor (TB)1/3/SG
TB absolute mid-boiling of feed, (R) SG
specific gravity of feed N napthenes vol
and A ? aromatics vol
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Example

• 100 m3/h of heavy naphtha (HN) with specific
  gravity of 0.778 has the following composition A
  11.5 vol, N 21.7 vol and P 66.8 vol is to
  be reformed to naphtha reformate of RON 94.
• Calculate the yields of each product for that
  reformer.

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Solution
The material balance for the reformer is
presented in the following table
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Isomerization of Light Naphtha

• Isomerization is the process in which light
  straight chain paraffins of low RON (C6, C5 and
  C4) are transformed with proper catalyst into
  branched chains with the same carbon number and
  high octane numbers.
• Light naphtha from the hydrotreated naphtha (HTN)
  C580 C is used as a feed to the isomerization
  unit.

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Isomerization Reactions

• Isomerization is a reversible and slightly
  exothermic reaction
• The conversion to iso-paraffin is not complete
  since the reaction is equilibrium conversion
  limited. It does not depend on pressure, but it
  can be increased by lowering the temperature.
• However operating at low temperatures will
  decrease the reaction rate. For this reason a
  very active catalyst must be used.

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Isomerization Catalysts

• Two types of isomerization catalysts
• The standard Pt/chlorinated alumina with high
  chlorine content
• The Pt/zeolite catalyst

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Standard Isomerization Catalyst

• This bi-functional nature catalyst consists of
  highly chlorinated alumina responsible for the
  acidic function of the catalyst.
• Platinum is deposited (0.30.5 wt) on the
  alumina matrix.
• Platinum in the presence of hydrogen will prevent
  coke deposition, thus ensuring high catalyst
  activity.
• The reaction is performed at low temperature at
  about 130 C to improve the equilibrium yield.

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Standard Isomerization Catalyst

• The standard isomerization catalyst is sensitive
  to impurities such as water and sulphur traces
  which will poison the catalyst and lower its
  activity. For this reason, the feed must be
  hydrotreated before isomerization.
• The zeolite catalyst, which is resistant to
  impurities, was developed.

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Zeolite Catalyst

• Zeolites are used to give an acidic function to
  the catalyst.
• Metallic particles of platinum are impregnated
  on the surface of zeolites and act as hydrogen
  transfer centres.
• The zeolite catalyst can resist impurities and
  does not require feed pretreatment, but it does
  have lower activity and thus the reaction must be
  performed at a higher temperature of 250 C (482
  F).

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A comparison of the operating conditions for the
alumina and zeolite processes
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Isomerization Yields

• The reformate yield from light naphtha
  isomerization is usually very high (gt97 wt).
• Typical yields are given in Table

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Example
Light naphtha with a specific gravity of 0.724 is
used as a feed to the isomerization unit at a
rate of 100 m3/h. Find the product composition.
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Solution
Isomerization yields
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The End