Improved hydrogen yield in catalytic reforming
A process step that sends higher-boiling C6 hydrocarbons to light tops isomerisation delivers an increase in hydrogen production from naphtha catalytic reforming
The “advanced fuel” technology is a patented invention (US Patent 6207040, European Patent 0914405) and states that the higher-boiling C6 hydrocarbons, including naphthenes, benzene and hexane, are more profitably sent to the light tops isomerisation process rather than to catalytic reforming. The critical full-range naphtha-splitting mode to be constructed in the refinery (see Figure 1) requires negligible investment.
The economic gains of advanced fuel technology1 include:
• A gain of more than 4% on the recombined isomerate-reformate blend volume yield per 100 tonnes of isomerisation-reforming feed.2 (This is mainly explained by the good C6 isomerate octane and, in particular, by the even better C6 blending isomerate octane, plus the C6 isomerate weight yield being much higher and the C6 isomerate density being much lower than the reformate weight yield and density.)
• A gain of around three octane number points on the recombined isomerate-reformate blend (resulting from 5-6 more points of the reformate RON1,3 acquired thanks to the better reformability quality of the reforming feed), exchangeable for an additional gasoline yield.
These results are obtained by managing the full-range naphtha splitting unit in such a way as to determine the two ranges of concentrations (0-4 vol% C7 hydrocarbons in the tops, 0-0.5 vol% C6 hydrocarbons in the bottom, see Figure 1), which define the splitting mode and are the necessary and sufficient condition for performing the advanced fuel technology’s functions.
Efficient separation by distillation in the full-range naphtha splitting unit can be achieved according to different strategies, depending on choices the refiner has to make between highly efficient equipment, which is more capital intensive, and highly efficient operation, which consumes more energy. The development in no way limits freedom of choice in a compromise between these options.
In practice, separation efficiency can be improved by modifying the equipment (for instance, number and type of distillation stages, internals type) and/or the reflux ratio. There is also the option to determine an optimum compromise between improvements in equipment and reflux ratio.
A further important feature of advanced fuel technology is the gain it offers in catalytic reforming hydrogen production/availability. The gain in production of catalytic reforming hydrogen (net of increased use in isomerisation) is estimated in the range of 28% to over 48% based on conventional hydrogen production in catalytic reforming. When removing all or nearly all of the C6 molecules (shifting them to the isomerisation process) from the catalytic reforming feed, it has been demonstrated in the refinery that the yield of catalytic reforming hydrogen increases significantly. This is in agreement with the theory that the reforming feed quality, after C6 removal, becomes much more favourable to reforming’s dehydrogenation reactions with hydrogen production and much less favourable to reforming’s hydrocracking reactions with hydrogen consumption.
We will first analyse the basis of the technology before examining the results of the refinery runs.
First, we can consider the aromatisation catalytic reforming reactions:
1 naphthene ↔1 aromatic + 3 H2 - 48-55 Mcal/kmol (million calories per thousand moles) (1)
1 paraffin ↔ 1 aromatic + 4 H2 - 60-65 Mcal/kmol (2)
It is well known that the delta octane number [C8+ aromatics - C8+ (naphthenes + paraffins)] is much higher (around four times) than for [C6 aromatics - C6 naphthenes]. C6 paraffins cannot be considered because the C6 paraffins that pass through catalytic reforming do not, for practical purposes, increase their octane number. In fact, C6 paraffins either crack or pass through unconverted (although an exception is made for a 10% maximum quota of normal hexane with a RON value of about 26).
The heat absorbed in catalytic reforming by a one-molecule aromatisation reaction, generating one aromatic molecule, is roughly speaking about the same, irrespective of the number of carbon atoms. This means that, with about the same heat absorption, C8+ has a delta octane from the aromatisation of one molecule about four times higher than for a C6 molecule.
Regarding one molecule, with approximately the same heat absorption, C8+ has a delta octane four times higher than C6’s delta octane for a gasoline quantity on average about 40% higher (because the relevant gasoline weight is exactly proportional to the molecular weight, while the relevant gasoline volume is approximately proportional).
The hydrocarbons C6, C7, C8, C9 and so on compete with one another in order to make use of the available heat. When C6 is present, it provides a modest octane increase by making use of a portion of the heat subtracted from the availability of C7+. When C6 is present, in order to increase the available heat, the only option is to increase the catalytic bed temperature.
When C6 is removed, the reformate octane increases automatically due to the strong octane upgrading available for C7+, which is simply a result of the absence of C6. The presence of C6 appears to be very harmful to catalytic reforming performance.