Hydrodesulfurization feedstock composition

There is, however, one aspect of feedstock properties that has not yet been discussed fully and that is feedstock composition. This particular aspect of the nature of the feedstock is, in fact, related to the previous section where the influence of various feedstock types on the hydrodesulfurization process was noted, but it is especially relevant when residua and heavy oils from various sources are to be desulfurized. 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally platinum or platinum—rhenium on an alumina base. The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions. Nonplatinum catalysts are used in regenerative processes for feedstocks containing sulfur, although pretreatment (hydrodesulfurization) may permit platinum catalysts to be employed. 

The composition of the various feedstocks may, at first sight, seem to be of minor importance when the problem of the hydrodesulfurization of heavy oils and residua comes under consideration. However, consideration of the variation in process conditions that were outlined in the previous section (Table 6-6) for different feedstocks (where the feedstocks are relatively well-defined boiling fractions of petroleum) presents some indication of the problems that may be encountered where the feedstocks are less well defined. Molecular composition is as important as molecular weight (or boiling range). Such is the nature of the problem when dealing with various residua and heavy oils which are (to say the least) unknown in terms of their chemical composition. In fact, the complexity of these particular materials (Chapter 3) has allowed little more than speculation as to the molecular structure of the constituents. 

Nevertheless, there have been some successful attempts to define the behavior of residua (and heavy oils) during the hydrodesulfurization process in terms of physical composition, which has also led to the development of process modifications to suit various heavy feedstocks. This line of investigation arose because of the tendency, over the years, to classify all residua (and heavy oils) as the same type of refinery discard which was, effectively, useless for further processing. The only exception is the production of asphalt from certain residua. It was only when... 

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