Chemistry and technology

of petroleum

By Dr. Dang Saebea

Catalytic Reforming and
Isomerization
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.
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.
 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
 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).

Pure n-heptane zero octane

Pure isooctane 100 octane

Example : an 80 vol% isooctane mixture has an
octane number of 80.
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 C5–C6 hydrocarbons
- heavy naphtha (HN) is mainly C7–C10 hydrocarbons.
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.
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

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.
 REACTIONS
4 major reactions are categorized as

Desirable
Dehydrogenation of naphthenes to aromatics
Dehydocyclization of paraffins to aromatics
Isomerization

Undesirable
Hydrocracking
Dehydrogenation & Dehydrocyclization
 Highly endothermic
 Cause decrease in
temperatures
 Highest reaction rates
 Aromatics formed have
Dehydrogenation
high B.P so end point of
gasoline rises

Favourable conditions
 High temperature
 Low pressure Dehydrocyclization
 Low space velocity
 Low H /HC ratio
2
 Isomerization
 Branched isomers
increase octane rating
 Small heat effect
 Fairly rapid reactions

Favourable conditions
 High temperature
 Low pressure
 Low space velocity
 H /HC ratio no
2
significant effect
 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
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 5–25 atm, not too low for
coke deposition and not too high in order to avoid
cracking and loss of reformate yield.
 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.
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
 Reaction Kinetics and Catalysts
 Thecatalyst used for reforming is a bifunctional catalyst
composed of platinum metal on chlorinated alumina.

the centre for the dehydrogenation

Platinum
reaction

chlorinated alumina an acidic site to promote structure

changes
- cyclization of paraffins
- isomerization of the naphthenes.
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.
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.
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
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).

Semi-regenerative Fixed Bed Process

first reactor Reactions such as dehydrogenation of

paraffins and naphthenes which are
very rapid and highly endothermic
Semi-regenerative Fixed Bed Process

second reactor Reactions that are considered rapid, such

as paraffin isomerization and naphthens
dehydroisomerization, give moderate
temperature decline
Semi-regenerative Fixed Bed Process

Third reactor slow reactions such as dehydrocyclization and

hydrocracking give low temperature decline.
Semi-regenerative Fixed Bed Process

The temperature and concentration profile in

each reactor
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
Some light to gas processing,
hydrocarbons  Part of the liquid
(C1–C4) are product (C3 and C4)
separated from is returned from the
the reformate in reflux drum back to
the stabilizer. the stabilizer.
The main product of the column is stabilized reformate, which is sent to the
gasoline blending plant.
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
Continuous Regenerative (moving bed) Process

 In this process, three or four reactors are installed one on the top of the
other.
Continuous Regenerative (moving bed) Process

 The effluent from each reactor is sent to a common furnace for heating.
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.
Typical operating conditions of three
reforming processes
 PROCESS VARIABLES
Catalyst type
 Chosen to meet refiners yield, activity and stability need

Temperature
 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
 Pressure effects the reformer yield & catalyst stability.
 Low pressure increases yield & octane
 PROCESS VARIABLES
Space velocity
 Low space velocity favors aromatic formation but also promote
cracking.
 Higher space velocity allows less reaction time.

H2 / HC ratio
 Moles of recycle hydrogen / mole of naphtha charge
 Increase H2 partial pressure or increasing the ratio suppresses coke
formation but promotes hydrocracking.
The End