ISOMERIZATION

Submitted by:

Navaneeth Krishnan A R

B130592CH

B Batch

Introduction

Isomerization is the process of conversion of a chemical with a given molecular formula to another compound with the same molecular formula but a different molecular structure, such as from a straight-chain to a branched-chain hydrocarbon or an alicyclic to an aromatic hydrocarbon. Examples include the isomerization of ethylene oxide to acetaldehyde (both C2H4O) and butane to isobutane (both C4H10).

Eg,

Isomerization in petroleum refinery

Isomerization is a process in petroleum refining that converts n-butane, n-pentane and n-hexane

into their respective isoparaffins of substantially higher octane number. The straight-chain

paraffins are converted to their branched-chain counterparts whose component atoms are the

same but are arranged in a different geometric structure. Isomerization is important for the

conversion of n-butane into isobutane, to provide additional feedstock for alkylation units, and

the conversion of normal pentanes and hexanes into higher branched isomers for gasoline

blending. Isomerization is similar to catalytic reforming in that the hydrocarbon molecules are

rearranged, but unlike catalytic reforming, isomerization just converts normal paraffins to

isoparaffins.

Butane isomerization produces feedstock for alkylation. Aluminum chloride catalyst plus

hydrogen chloride are universally used for the low-temperature processes. Platinum or another

metal catalyst is used for the higher-temperature processes. In a typical low-temperature process,

the feed to the isomerization plant is n-butane or mixed butanes mixed with hydrogen (to inhibit

olefin formation) and passed to the reactor at 230°-340° F and 200-300 psi. Hydrogen is flashed

off in a high-pressure separator and the hydrogen chloride removed in a stripper column. The

resultant butane mixture is sent to a fractionator (deisobutanizer) to separate n-butane from the

isobutane product.

Pentane/hexane isomerization increases the octane number of the light gasoline components n-

pentane and n-hexane, which are found in abundance in straight-run gasoline. In a typical

C5/C6 isomerization process, dried and desulfurized feedstock is mixed with a small amount of

organic chloride and recycled hydrogen, and then heated to reactor temperature. It is then passed

over supported-metal catalyst in the first reactor where benzene and olefins are hydrogenated.

The feed next goes to the isomerization reactor where the paraffins are catalytically isomerized

to isoparaffins. The reactor effluent is then cooled and subsequently separated in the product

separator into two streams: a liquid product (isomerate) and a recycle hydrogen-gas stream. The

isomerate is washed (caustic and water), acid stripped, and stabilized before going to storage. 

Process Diagram for Isomerization of Butane

Process description :-

A typical process (see flow diagram) would comprise of liquid feedstock (light naphtha cut having pentane, hexane etc.) generated by fractionating normal naphtha cut in a naphtha splitter. The light naphtha is combined with the recycle gas/ fresh gas mixture. The resultant combined reactor feed is routed to a feed/ effluent heat exchanger, where it is heated and completely vaporised by the effluent of the reactor. The vapourised combined reactor feed is further heated to the reactor inlet temperature in a heater. The hot feed enters the reactor at the top and flows downwards through the catalyst bed, where isomerization of pentanes, hexane and other normal paraffins takes place generating high octane components. Being exo-thermic reaction, the temperature rise heat release is controlled by a cold quench gas injection into the reactor. Reactor effluent is cooled and subsequently separated in the product separator into two streams: a liquid product (isomerate) and a recycle gas stream returning to the reactor via the recycle gas

compressor.

An Example :

The octane numbers of the LSR naphtha [C5-180oF (C5-82oC)] can be improved by the use of an isomerization process to convert normal paraffins to their isomers.This results in significant octane increases as n-pentane has an unleaded (clear) RON of 61.7 and isopentane has a rating of 92.3. In once-through isomerization where the normal and iso compounds come essentially to thermodynamic equilibrium, the unleaded RON of LSR naphtha can be increased from 70 to about 82–84. If the normal components are recycled, the resulting research octane numbers will be about 87–93 RONC.

Reaction parameters – temperature and catalyst

Isomerization yield is increased by: 1. High temperature (which increases reaction rate) 2. Low space velocity 3. Low pressure

Reaction temperatures of about 200–400oF (95–205oC) are preferred to higher temperatures because the equilibrium conversion to isomers is enhanced at the lower temperatures.

At these relatively low temperatures a very active catalyst is necessary to provide a reasonable reaction rate.

The available catalysts used for isomerization contain platinum on various bases.

Some types of catalysts require the continuous addition of very small amounts of organic chlorides to maintain high catalyst activities. This is converted to hydrogen chloride in the reactor, and consequently the feed to these units must be free of water and other oxygen sources in order to avoid catalyst deactivation and potential corrosion problems.

A second type of catalyst uses a molecular sieve base and is reported to tolerate feeds saturated with water at ambient temperature.

A third type of catalyst contains platinum supported on a novel metal oxide base. This catalyst has 150oF (83oC) higher activity than conventional zeolitic isomerization catalysts and can be regenerated.

Zeolite catalyst require higer temperature and provide lower octane boost while chlorinated alumina’s result in higher octane.

Catalyst life is usually three years or more with all of these catalysts. An atmosphere of hydrogen is used to minimize carbon deposits on

the catalyst but hydrogen consumption is negligible. The actual product distribution is dependent upon the type and age of

the catalyst, the space velocity, and the reactor temperature. The pentane fraction of the reactor product is about 75 to 80 wt% iso-pentane, and the hexane fraction is about 86 to 90 wt% hexane isomers .

Isomerization Processes Vapor phase process

Hydrogen used to suppress the dehydrogenation and coking. High yields and high selectivity to high-octane isomeric

forms. Provides moderate (but important) contribution to the gasoline

pool. Catalyst types

Chloride alumina catalyst. Organic chloride deposited on active metal sites. High temperature treatment with carbon tetrachloride. Chlorides are sensitive to moisture – drying of feed &

hydrogen. Make-up essential

Acidic zeolite with noble metal catalyst. Platinum catalyst. Does not require activation by hydrogen chloride.

Advantages and disadvantages Pros

Reforming conditions not appropriate for the paraffinic components in SRG.

Essentially zero benzene, aromatics, & olefins. Very low sulfur levels.

Cons High vapor pressure. Moderate octane levels — (R+M)/2 only 85.

Feedstocks Lightest naphtha feed stock (SRG) with pentanes, hexanes, and small

amounts of heptanes. Feed often debutanized —"Debutanized Straight Run".

Sulfur and nitrogen must be removed since catalysts employ an ‘acid site’ for activity.

Merox Clay treating

Hydrotreating Common for Straight Run Gasoline and Naphtha to be hydrotreated

as one stream and then separated.

Products Products

Isoparaffins and cycloparaffins. Small amounts of light gasses from hydrocracking . Unconverted feedstock.

Increased severity increases octane but also increases yield of light ends.

Yields depend on feedstock characteristics and product octane. Poor quality feeds might yield 85% or less liquid product. Good feeds might yield 97% liquid product.

Isomerization Chemistry Primary reaction is to convert normal paraffins to isomeric paraffins. Olefins may isomerize and shift the position of the double bond.

1-butene coluld shift to a mixture of cis-2-butene and trans-2-butene

Cycloparaffins (naphthenes) may isomerize and break the ring forming an olefin.

Cyclobutane to butene.

Use of iso paraffin produced by isomerization

Hydrocarbon rich in isoparaffins are one of the most imp. Components of modern engine gasolines because they have high octane numbers and low sensitivity.

They are free of sulphur , olefins and aromatics, they are non toxic and no harmful product are formed during their combustion.

Conclusion

The isomerization process is gaining importance in the present refining context due to limitations on gasoline benzene, aromatics, and olefin contents. The isomerization process upgrades the octane number of light naphtha fractions and also simultaneously reduces benzene content by saturation of the benzene fraction. Isomerization complements catalytic reforming process in upgrading the octane number of refinery naphtha streams. Isomerization is a simple and cost-effective process for octane enhancement compared with other octane-improving processes. Isomerate product contains very low sulfur and benzene, making it ideal blending component in refinery gasoline pool. Due to the significance of isomerization to the modern refining industry, it becomes essential to review the process with respect to catalysts, catalyst poisons, reactions, thermodynamics, and process developments.

References

1) http://www.eoearth.org/view/article/153923/2) http://www.nptel.ac.in/courses/103102022/233) Petroleum Refinery James G.H.

Chapter 10 CATALYTIC REFORMING AND ISOMERIZATION page 189-2154) http://www.sciencedirect.com/science/article/pii/S0926860X99003312