The objective of catalytic reforming is to convert low octane straight-run naphtha streams into high a high octane, low sulfur reformate which is used as the major blending product for gasoline. To get a higher octane number of gasoline, the aromatic and branched iso-paraffins concentrations have to be increased. Hydrogen is a useful by product produced by catalytic reforming. The hydrogen gas can be used for the hydro treating and hydrocracking processes. The naptha feedstock is usually hydro treated before reforming to protect the catalyst used. The majority of reforming catalysts used in the process contain either platinum, palladium, or bi/tri metallic formulations of platinum with Rhenium, Tin or Iridium supported on alumina. Limits on catalytic reforming capacity in American and European refineries have been placed to regulate the amount of benzene and aromatics sold in gasoline.
The main reactions of interest in catalytic reforming are the dehydrogenation of naphthenes to aromatics, isomerization of straight chain n-paraffins to branched iso-paraffins, dehydroisomerization of alkyl-C5 naphthenes and dehydrocyclization of n-paraffins to aromatics. The aromatics in the feed should remain unchanged under the right conditions. In addition, varying the reactor conditions can control side reactions such as the hydrogenation of aromatics. Considering the reactions are endothermic, the most suitable reactor conditions would be high temperatures, low hydrogen pressures, low space velocity, and low H2/HC ratio. It is important to note that the hydrogen pressure should be high enough to inhibit coke deposition on the catalyst surfaces.
Another objective in catalytic reforming is to inhibit the undesired reaction of hydrocracking in the reactor. Hydrocracking consumes hydrogen in the reaction, thus decreasing the yield of reformate. The reaction is favored at high temperatures and high hydrogen pressures; thus, hydrocracking in the reactor must be taken into consideration when choosing the optimum reactor conditions. Also, it is important to note that the balance between acidic and metallic sites must be controlled to catalyze specific reactions; for example, the platinum surface metals catalyze dehydrogenation reactions compared to the acidic alumina support sites that catalyze isomerization and cracking reactions.
In addition, the process’s coke deposition produced deactivates the coke; hence, the catalysts must be regenerated periodically to maintain a maximum yield. There are 3 types of catalyst regeneration for catalytic reforming and they are: semi-regenerative, cyclic and continuous.
Other reactions have been developed to further increase the octane number. For instance, alkylation is a process whereby light iso-paraffins are combined with C3–C4 olefins, to produce a mixture of higher molecular weight iso-paraffins. Alkylation requires a strong acid catalyst such as sulfuric acid and hydrofluoric acid. However, the latter is preferred because it is less sensitive to temperature fluctuations. Furthermore, the polymerization process combines propenes and butenes to produce olefins, which contribute to a higher octane number, but the process has been largely replaced by alkylation.
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