Isomerization Process


The UOP’s Penex process is specifically designed for the catalytic isomerization of pentane, hexanes, and mixtures thereof. The reactions take place in the presence of hydrogen, over a fixed bed of catalyst, and at operating conditions that promote isomerization andminimize hydrocracking. Operating conditions are not severe, as reflected by moderate operating pressure, low temperature, and low hydrogen partial pressure requirements.
Ideally, this isomerization catalyst would convert all the feed paraffins to the highoctane- number branched structures: normal pentane (nC5) to isopentane (iC5) and normal hexane (nC6) to 2,2- and 2,3-dimethylbutane. The reaction is controlled by a thermodynamic equilibrium that is more favorable at low temperature.

With C5 paraffins, interconversion of normal pentane and isopentane occurs. The C6-paraffin isomerization is somewhat more complex. Because the formation of 2- and 3-methylpentane and 2,3-dimethylbutane is limited by equilibrium, the net reaction involves mainly the conversion of normal hexane to 2,2-dimethylbutane. All the feed benzene is hydrogenated to cyclohexane, and a thermodynamic equilibrium is established between methylcyclopentane and cyclohexane. The octane rating shows an appreciation of some 14 numbers.

As shown above, light naphtha feed is charged to one of the two dryer vessels. These vessels are filled with molecular sieves, which remove water and protect the catalyst. After mixing with makeup hydrogen, the feed is heat-exchanged against reactor effluent. It then enters a charge heater before entering the reactors. Two reactors normally operate in series.

The reactor effluent is cooled before entering the product stabilizer. In new Penex designs, both the recycle gas compressor and the product separator have been eliminated. Only a slight excess of hydrogen above chemical consumption is used. The makeup hydrogen, which can be of any reasonable purity, is typically provided by a catalytic reformer. The stabilizer overhead vapors are caustic scrubbed for removal of the HCl formed from organic chloride added to the reactor feed to maintain catalyst activity. After scrubbing, the overhead gas then flows to fuel. The stabilized, isomerized liquid product from the bottom of the column then passes to gasoline blending.

Alternatively, the stabilizer bottoms can be separated into normal and isoparaffin components by fractionation or molecular-sieve separation or a combination of the two methods to obtain recycle of the normal paraffins and low-octane methylpentanes (MeC5). Product octanes in the range of 87 to 92 RON, clear, can be achieved by selecting one of the various possible schemes.

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