At the C5/C6 splitter, the C5 s are sent out the top of the tower and are recycled back to The beginning of the process unit. The nC5 in the C5 s will be reprocessed while the i C5 will leave the top of the deisopentanizer as finished i C5 product. The C6 and heavier components leave the bottom of the splitter and either go to gasoline blending or to the reformer unit to be made into aromatics.

The isomerization unit
The feed to the isomerization unit in a refinery is typically a light straight run gasoline stream high in C5 and C6 + normal-paraffins. This feed enters the deisopentanizer tower that removes any i C5 already present in the stream. It also removes the i C5 created in the reactors that are returned to the feed as part of the n C5 recycle.

As the i C5 is being sent out the overhead of the deisopentanizer tower, the balance of the feed stream is sent to the isomerization reactor. Hydrgen is also added to the stream to encourage the proper reactions and to help minimize coke formation on the catalyst.

After passing through the reactor, the stream enters a H2 separation unit that removes and recycles the H2 back to the feed of the reactor. The stream then enters a stabilizer tower that removes any light hydrocarbons made during the reactions. These light compounds exit the top of the stabilizer tower and will typically be blended into the refinery fuel gas system. The products stream leaves the bottom of the stabilizer tower and enters a C5 / C6 splitter tower.

Light Naphtha Isomerization
With growth and development of car industries, refining industries have been steadily under pressure of demands for better fuels. In addition, major historic air pollution problems have typically been high levels of impurities arising from the combustion of fossil fuels. New legislations and standards caused the refiners to look for economical solutions meeting new specifications for producing environmentally clean fuels, containing more limited harmful components such as benzene in gasoline and lower contaminants like sulfur in oil derivatives. To improve gasoline octane number several methods, like bellows have been utilized:

• Addition of TEL
• Thermal / Catalytic reforming of naphtha
• Thermal polymerization of olefin light ends to hexenes , heptens & octenes.
• Alkylation of olefins.

With TEL phase out , interest in isomerization of straight run gasoline heightened .Gasoline pool in a refinery may contains different constituents such as FCC naphtha , reformate, cokenaphtha, polygasoline , oxygenates, butane, light naphtha, alkylate and isomerate. Light naphtha comprises typically 10% of the gasoline pool. This fuel cut contains mostly pentanes and hexanes.
Research octane number (RON) of light naphtha is around 70. Isomerization of this constituent can improve its octane number to about 90, so isomerate could be named as a RON booster.

C5/C6 Paraffin Isomerization Reactions
In the presence of proper catalyst and appropriate conditions, skeletal isomerization of normal paraffins of light naphtha take place:
C5/C6 Isomerization Reactions

During isomerization of normal paraffins in the reactor, some other reactions such as hydrocracking, ring opening and benzene saturation in the presence of hydrogen will take place.
1) Hydrocracking :

2) Ring Opening ( Hydrodecyclization ) :

3) Benzene Saturation :

Derescom and partners designs and builds isomerization units for converting normal paraffins to higher-octane isoparaffins for refinery applications. We can process units of any capacity and design them to utilize any process technology available.

The two most common process units we provide are butane (C4) isomerization units and pentane/hexane (C5/C6) units. The C4 unit converts normal butane or a mixture of butanes to isobutane for feedstock to an alkylation unit. The C5/C6 unit converts normal pentane and normal hexane streams—commonly found in straight-run gasoline—-to isopentane and isohexane that can be used for gasoline blending.
A complete EPC services package.
We provide a full array of integrated design-build services for isomerization unit projects. Our capabilities include everything from front-end engineering and design, detail engineering, fabrication and construction to installation, startup and operator training. Our designers and engineers have extensive experience with all process technologies. With expertise spanning the entire oil and gas production and processing cycle, we optimize our projects to provide greater long-term value for our customers. We providing a true EPC solution, we are able to control costs and project quality, reduce the customer's risk and often shorten project schedules. And we maintain a vast global network of engineering, fabrication, procurement and construction resources that can quickly be mobilized to any project site.

Faster, more cost-effective construction. We can errect isomerization units using conventional, "stick-built" construction methods, modular construction; or a combination of the two. We evaluate each project's requirements to determine the best approach for our customers. Modular units are fabricated at our in-house facilities and can be shipped almost anywhere. Most projects benefit from some degree of modularization, which can result in reduced costs and shorter project schedules.