7.4 Process parameters

• • Reactor pressure: 4 – 24 barg

• Reactor temperature: 500 – 525°C

• H₂/Hydrocarbon molar ratio: 2 – 3

 

Figure 7.2 Flow sheet of Isomerization of n-paraffin

7.5 Introduction

• • The basic principle of Isomerization is to straight chain alkanes to side chain paraffins.  This enhances the octane number substantially

• For instance, n-pentane has an octane number of 61.7 where as iso-pentane has an octane number of 92.3

• Usually light naphtha is used as a feed stock

7.6 Catalyst

• • Platinum base catalysts are used

• AlCll₃ is used as a promoter for the catalyst

• During reaction, part of the AlCl₃ gets converted to HCl

• Therefore, completely dry conditions shall be maintained to avoid catalyst deactivation and corrosion.

• Catalytic reaction takes place in the presence of hydrogen to suppress coke formation

7.7 Process technology (Figure 7.2)

• • Light naphtha and hydrogen (make up) are totally dried and sent to an isomerisation reactor after pre-heating the feed mixture in a heat exchanger

• Reaction operating conditions: 150 – 200°C and 17 – 28 barg

• Typical conversions: 75 – 80 % for pentanes.

• After reaction, AlCl3 is recovered from the product using condensation or distillation

• The basic principle for AlCl₃ recovery is that at the reactor operating conditions, the AlCll₃ is in volatile conditions and is soluble in hydrocarbons

• After AlCl₃ is recovered from the product, it is sent back to the reactor along with the make- up AlCl₃

• Eventually, the product enters a flash drum where bulkly light ends along with little quantities of HCl are separated from the liquid product.

• The light ends recovered from the flash drum are sent to a HCl absorber where HCl is absorbed into caustic solution to generate the light end gases.  The light end gases can be further used for other processes in the refinery.

• The bottom product then enters a HCl stripper where most of the HCl is stripped from the isomerisation product rich stream. The HCl is recycled back to the reactor to ensure good catalyst activity.  Make-up HCl is added to account for losses

• Subsequently, caustic wash is carried out to remove any trace quantities of HCl

• The isoermized product rich stream is then sent to a fractionators that separates the isomerized product from the unreacted feed.

• The unreacted feed from the fractionators is sent back to the reactor.