ISOMERIZATION

PROCESS
 INTRODUCTION

• Isomerization is the process by which one isomer is transformed into another one.

• Isomerization leads to production of a compound with another atomic (group)

arrangement but composition and molecular weight do not change.

• In literature isomerization is often called rearrangement, sometimes these are name

reactions (isomerization processes) according to the tradition.

• Isomerization process is aimed at production of high-octane components of

commercial gasoline from low-octane oil fractions by structural change of carbon
skeleton.
 INTRODUCTION
• Isomerization reactions are possible due to isomery – a phenomenon that consists in
existing of several compounds with equal molecular weight, quantitative and
qualitative composition, but with different physical and chemical characteristics. Such
compounds are called isomers.

• For examples, there are 2 isomers of butane, there are 5 basic hexane isomers,17
hexene isomers where as Octane has 18 isomers and tetradecane has as much as 1818
isomers.

• Isomerization feed is a light Naphtha with an IFP of about 30 °C and a FBP of about
145 °C contains most (but not all) of the hydrocarbons with 6 or less carbon atoms.
INTRODUCTION
• There are two main types of isomery: structural and spatial
(stereoisomery).

• There are following types of isomerization in terms of

hydrocarbons. N-butane to isobutane or m-xylene to p-
xylene transforming can be an elementary example of carbon skeleton
isomerization.

• Ring-chained isomerization, for example, propylene to cyclopropane or

methylcyclopentane to cyclohexane, is a particular case of carbon
skeleton isomerization.
• In stereoisomers(spatial) the bond structure is the same, but the
geometrical positioning of atoms and functional groups in space differs.

• Butene-1 to cis-butene-2 can be an example of double bond position

isomerization between carbon atoms. Cis-butene-2 to trans-butene-2
conversion illustrates an example of geometrical (spatial and
configurational) isomerization.
• Isomerization reactions are used extensively for production of both
lower and higher paraffins (isoalkanes).

• Branched C5-C6 paraffins have high octane numbers and are good
motor gasoline components.

• Isopentane and isobutane are valuable feed for synthetic rubber

production.
• Isobutane is also used for alkylbenzene, for high-octane ethers
production, methyl-tret-butyl ether (MTBE) is the most popular of
those. MTBE is a gasoline additive to raise the octane number.

• Higher alkanes isomerization favours decreasing of diesel fuel and

engine oil pour point.
Problem statement

• International and domestic regulations to automotive gasoline

considerably limit the content of benzene, aromatic hydrocarbons,
olefin hydrocarbons and sulfur.

• In 1970s the variants of hydrogenation of the benzene, contained in the

reformate, proceeding without the decrease of product octane number
have been offered.

• However for decrease of the total aromatics content the dilution of

reformate with high-octane nonaromatic components is required. This
situation is complicated by refusal from tetraethyl lead (TEL).

• Thereby the development of isomerization process is one of the

effective methods for solution of this problem. It allows the producing
of commercial gasoline which corresponds to the current and
perspective requirements to the fuels and provides necessary flexibility
of processing.
• The following types of reaction are typical for isomerization
process:

1. paraffin isomerization;

2. opening of naphthenic compounds rings;

3. naphthenes isomerization;

4. benzene saturation;

5. hydrocracking

6. naphthenes transalkylation (Transalkylation is a chemical reaction

involving the transfer of an alkyl group from one organic
compound to another. The reaction is used for the transfer of methyl
and ethyl groups between benzene rings.)
• Paraffins are isomerised into branched fashion where as low
temperature favours creation of more branched isoparaffin
hydrocarbons, however, isomerization rate increases in case of
temperature rise.

• There are other several important reactions besides paraffin

isomerization reaction.

• In the course of gasoline isomerization, rings opening

reaction proceeds faster with temperature rise. For typical conditions in
an isomerization unit reactor, at naphthene rings opening with creation
of paraffin hydrocarbons conversion level is about 20-40%.
• Penex feeds can contain up to 4% benzene. The catalyst will saturate
benzene to cyclohexane. This reaction proceeds very quickly and is
achieved at very low temperatures. The saturation of benzene produces
heat. This heat generation limits the amount of benzene which can be
tolerated in the Penex feed. The platinum function on the Penex catalyst
is responsible for benzene saturation. .

• Hydrocracking reaction is a side reaction. Conversion degree at

hydrocracking depends on feed quality and rigidity of process operating
mode. Molecules with big amount of carbon atoms, such as C7, are
hydrocracked easier than molecules with lesser amount of carbon
atoms. C5-C6 paraffins are also hydrocracked to some degree. In the
result of hydrocracking reaction methane, ethane, propane and butane
are formed.
• Gasoline isomerization process depends on the following parameters:

1. temperature;
2. pressure;
3. feed space velocity;
4. Hydrogen to feed ratio
5. catalyst activity;
6. feed composition and impurities content.
FEED STOCK REQUIREMENT
• To maintain the high activity of the Penex catalyst, the feedstock must be
hydrotreated.

• However, costly pre-fractionation to sharply limit the levels of C6 cyclic and

C7 compounds is not required.

• The Penex system can be applied to the processing of feeds containing up to

15 percent C7 with minimal or no effect on design requirements or operating
performance.

• Generally, the best choice is to operate with lower levels of C7+ material
because these compounds are better suited for upgrading in a reforming
process.

• Charge containing about 5.0 percent or even higher amounts of benzene is

completely acceptable in the Penex chargestock.

• When the feed has extremely high levels of benzene, a Penex-Plus unit is recommended.
FEED STOCK REQUIREMENT
• Sulfur is an undesirable constituent of the Penex feed. However, it is
easily removed by conventional hydrotreating. Sulfur reduces the rate
of isomerization and, therefore, the product octane number. Its effect is
only temporary, however, and once it has been removed from the plant,
the catalyst regains its normal activity.

• Water, other oxygen-containing compounds, and nitrogen compounds

are the only impurities normally found in the feedstock that will
irreversibly poison the Penex catalyst and shorten its life. Fresh feed
and makeup hydrogen are dried by a simple, commercially proven
desiccant system.
• Hydrotreating process is a catalytic process, proceeding in hydrogen
gas using specially matched catalyst. The purpose of the feed pre-
hydrotreating process for the isomerization unit is removal of
substances, deactivating catalyst. Sulfur, oxygen and nitrogen
compounds; metalorganic compounds, containing arsenic, copper, etc.
as well as unsaturated compounds refer to these substances.

• Sometimes reforming units and isomerization units are unified to

single high-octane gasoline production complex. Process scheme of a
particular isomerization unit will depend on an isomerization catalyst
type, which is planned to be charged into a reactor block.
• Zeolite catalysts

• Zeolite catalysts are less active and used at higher operating

temperature compared to another types of catalysts, and consequently
the octane number of isomerizate is low. However they possess high
resistance to impurities in the feed and capability for total regeneration
in the reactor of the unit. The technological scheme of this process is
provided with fire-heaters for heating hydrogen and feed mixture up to
reaction temperature.

• It is necessary high ratio of hydrogen to hydrocarbon feed (along with

isomerization, hydrogen is spent for dearomatization of the feed); that‘s
why compressor for supplying of recycle hydrogen-rich gas and
separator for separation of hydrogen-rich gas are necessary .

• Hysopar catalyst should be marked out among zeolite catalysts; it is the

most progressive in the world catalyst market, because it considerably
exceeds all another catalysts by resistance to impurities in the feed
(available sulfur content is 100 ppm permanently and 200 ppm during
short periods of time)
2. Catalysts based on chlorinated alumina

• Catalysts based on chlorinated aluminum oxide are the most active and
provide high yield and octane number of isomerate.
• It is necessary to say that during isomerization such catalysts loose
chlorine, as a result activity decreases. Therefore, injection of
chlorinated compounds (usually CCL4) into feed is provided to
maintain high catalyst activity.

• As a result, caustic soda washing from organic chloride in special

scrubbers is necessary.

• A big disadvantage is that this type of catalyst is very sensitive to

catalytic poisons (oxygen-containing compounds, water, nitrogen,
sulfur, metals) and requires very accurate feed preparation (picture 2).

• Chlorinated catalysts do not regenerate, and their service life is 3-5

years.
• Catalysts based on sulfated metal oxide

• Catalysts based on sulfated metal oxide (oxide catalysts) have become

popular recently, because they combine high activity and resist to
catalytic poisons action, are able to regenerate. As well as for zeolite
catalysts, there is a need for compressor to supply recycle hydrogen gas
(picture 3), however there is no necessity in chlorine supply, adsorptive
feed treatment and caustic treatment of hydrocarbon gas.

• Picture 3. Isomerization process flow diagram over oxide catalysts

with pentane and hexane recycle.
SULPHUR GUARD BED

• The purpose of the sulfur guard bed is to protect the Penex catalyst
from sulfur in the liquid feed. The hydrotreater will remove most of the
sulfur in the Penex feed.

• The guard bed is loaded with adsorbent, a nickel- containing extrudate

designed to chemisorb sulfur from the liquid feed. The feedstock is
heated to the required temperature for sulfur removal, usually 250°-
(120°C) and passed down flow over the adsorbent. Once sulfur
breakthrough occurs, normally after one year or so of operation, the
guard bed is taken off line and reloaded with fresh adsorbent.

• **Chemisorption is a kind of adsorption which involves a chemical

reaction between the surface and the adsorbate. New chemical bonds
are generated at the adsorbant surface.
LIQUID FEED DRIERS / Make up Hydrogen driers

• The liquid feed driers are used to dry the Penex liquid feed to less than
0.1 ppm H20. The piping is designed so that either drier can be in the
lead or the lag position in series flow operation. Either drier can be
operated individually while the other is being regenerated. The driers
are designed for a 48 hour cycle which includes 24 hours in the lead
position, 7 hours regenerating, and 3 hours cooling and 14 hours in the
lag position. Proper drier operations are essential in the Penex process
since the catalyst is water intolerant. Typically, type 4A molecular
sieves are employed within the driers.

• The water content is continuously monitored with a Parametric

moisture analyzer. This analyzer will always be used to monitor the
lead drier effluent. If the lead drier effluent reaches 1.0 wppm H20
content, it must be taken off line and immediately regenerated.
• EXCHANGER CIRCUIT

• The dried liquid feed from the feed surge drum is pumped by either of
the two reactor charge pumps through the reactor exchanger circuit on
flow control. The reactor exchanger circuit consists of the cold
combined feed exchanger, the hot combined feed exchanger, and the
reactor charge heater. Prior to the entry of the liquid hydrocarbon into
the cold combined feed exchanger, it combines with the makeup
hydrogen stream. After combining, the mixed hydrocarbon-hydrogen
stream passes through the exchanger circuit in the order previously
mentioned.. After the makeup gas combines with the feed a small
quantity of catalyst promoter (CCI4) is added.
• ISOMERIZATI ON REACTORS
• The reactors are the heart of the process. The operation of them is such that
a reactor will be placed in series with the other reactor. At various times
throughout the unit‘s history it will be possible to have either reactor in the
lead or tail position. Thermocouples are inserted into the catalyst bed of each
reactor to monitor the activity of the catalyst. After exiting the reactor charge
heater, the heated combined stream then flows to the first reactor. Upon
exiting the first reactor, the stream then passes to the hot combined feed
exchanger where the first reactor‘s heat of reaction is partially removed. The
degree of temperature removal can be achieved by adjusting the amount of
exchanger bypassing with a temperature controller.The partially cooled
stream is then routed to the second reactor where the final process reactions
are completed. The reactors are equipped with hydrogen purge lines which
are located at the inlet of each reactor. The hydrogen purge is used to
remove hydrocarbon from a reactor which is to be unloaded or to cool a
reactor during an emergency. Each purge is controlled by a board mounted
flow controller.
• In case of a high reactor temperature emergency the reactors are equipped
with depressuring lines to the flare system. The reactors are depressured
from the outlet of the lag reactor. The depressuring line is equipped with two
motorized valves which can be operated from the control room. After exiting
the second reactor, the stream is then routed to the tube side of the cold
combined feed exchanger. The cold combined feed exchanger tube side
effluent is then routed to the stabilizer on pressure control.
• STABILIZER
• The purpose of this column is to separate any dissolved hydrogen, HCl
and cracked gases (C1, C2, and C3‘s) from the isomerate.The feed to
this column is routed hot directly from the cold before entering the
stabilizer.The column is reboiled by either steam or hot oil. The reboiler
heat input is controlled by a FRC on the heating medium. The stabilizer
column overhead vapor, consisting of the light hydrocarbon
components of the column‘s feed, is routed to an air or water cooled
condenser and then to the stabilizer receiver. To maintain pressure
control on the column, gas is vented on pressure control to the
stabilizer gas scrubber. Liquid is pumped from the receiver on level
control with the stabilizer reflux pump. All liquid from the stabilizer
overhead receiver is refluxed to the column on tray No. 1. Bottoms
product is routed to storage on level control after first being cooled in
the stabilizer bottoms cooler. If the stabilizer bottoms is sent to a
Deisohexanizer it is not cooled, but is charged hot to the column. Part
of the stabilizer bottoms is used for regenerating the driers.
• STABILIZER GAS SCRILIBBER
• The stabilizer off gas flows up flow through the stabilizer gas scrubber
to remove hydrogen chloride. The scrubbed gas leaves the top of the
vessel and goes to fuel gas on backpressure control. The hydrogen
purity is monitored on the scrubbed off gas to determine the moles of
H2 leaving the system for the H2/C:H determination. Make-up caustic
is pumped from the refinery to the reservoir section of the gas
scrubber when caustic addition is required. The caustic in the reservoir
section is pumped by the caustic recirculating pumps to the top of the
scrubbing section of the scrubber where a counter current contact
with the rising acidic gas is made. Caustic is also continuously
circulated to the distributor under the packed section. The flow rate of
the circulating caustic can be monitored by a local flow indicator.
Periodically a portion of the caustic is withdrawn to the refinery spent
caustic facilities as spent caustic. The caustic level in the scrubber is
maintained about 1-2 feet below the distributor under the packed
section.
• SEPARATOR AND COMPRESSOR SECTION

• Reactor effluent exits the reactor section and is partially condensed in

the reactor product condenser. It cools the effluent to about 100°F. The
cooled liquid and gas then separate in the product separator.
Unstabilized liquid product is pressured out of the product separator
on level
• control to the stabilizer section. Recycle gas exits from the separator
and goes through the recycle gas compressor to the cold combined
feed exchanger in the reactor section.

• Recycle gas flow and purity are controlled to maintain hydrogen to

hydrocarbon mole ratio of about 2:1. Dry make-up hydrogen, from the
make-up hydrogen drier section, combines with spillback from the
make-up gas compressor. These gases pass through the make-up gas
cooler and into the makeup gas compressor suction drum. Any
entrained hydrocarbons are knocked out and are manually drained to
an appropriate location. The make-up gas is compressed and combined
with the recycle gas to the reactor section. Make-up gas flow is
controlled by the product separator pressure recorder controller.
• PROCESS FLOW DIAGRAM
• The UOP Penex Unit can be divided into ten sections.
• A. Sulfur Guard Bed
• B. Liquid Feed Driers
• C. Makeup Hydrogen Driers
• D. Feed Surge Drum
• E. Exchanger Circuit
• F. lsomerization Reactors
• G. Stabilizer
• H. Stabilizer Gas Scrubber
• I. Separator and Compressor Section (Recycle Gas Units Only)
Reference

http://nefthim.com/manual/Isomerization-
process/