Catalysts heavy paraffins isomerization

Par-Isom [Paraffin isomerization] A process for isomerizing light naphtha in order to improve the octane number. The proprietary catalyst was developed by Cosmo Oil Company and Mitsubishi Heavy Industries, and the process was developed by UOP. The oxide catalyst is claimed to be more efficient than zeolite catalysts currently used for this process. 

Paraffin isomerization of heavy alkane feeds is often used to alter the cloud or pour point of diesel or lube fractions. Catalysts for this reaction are almost always dual-function catalysts of Pt supported on a one-dimensional zeolite. Using a onedimensional zeolite allows control of the isomerized product to contain few branches, usually methyl branches (Table 12.4). 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

The kinetics of paraffin and naphthene isomerization or cracking can be described by a mathematical equation in which the hydrogen pressure is in the denominator. This would mean that the rate decreases when hydrogen pressure increases. However, the reverse effect is observed for heavy feedstocks. This can be explained by considering all the reactions taking place simultaneously, especially the HDN and HDA reactions, and by knowing that the acidic sites of the catalyst are very sensitive to nitrogen and heavy aromatic compounds. 

When gasoline is the main product required, it is usual to remove the Cs-Ce straight-mn naphtha cut before reforming the 100-160°C fraction. This is because separate isomerization of the Cs-Ce paraffins (Section 6.10) gives an overall improvement in octane number. If aromatics are required, a lower-boiling fraction, in the range 80-120°C, is reformed. The heavy naphthas are not reformed because coke forms more readily and quickly leads to deactivation of the catalyst. 

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