The important effect of hydrogen partial pressure is the minimization of coking reactions. If the hydrogen pressure is too low for the required duty at any position within the reaction system, premature aging of the remaining portion of catalyst will be encountered. In addition, the effect of hydrogen pressure on desulfurization varies with the feed boiling range. For a given feed there exists a threshold level above which hydrogen pressure is beneficial to the desired desulfurization reaction. Below this level, desulfurization drops off rapidly as hydrogen pressure is reduced.
As the space velocity is increased, desulfurization is decreased but increasing the hydrogen partial pressure and/or the reactor temperature can offset the detrimental effect of increasing space velocity.
Reactors for endothermic processes (catalytic cracking, reforming, coking) require heat input to maintain the reaction temperature in the cracking zone and is shown on the far right in the endothermic region. Burning coke off the catalyst in the regenerator provides this heat and the recirculating catalyst transfers that energy to the cracking reaction in the riser of the fluid catalytic cracking unit. In the reforming reactor, the dehydrogenation reaction is highly endothermic and requires a reactor system of three to four reactors in series, with inter-stage heating between the reactors because the reaction temperature drop in each stage must be increased so that the reaction rate does not slow down too much. Reactors for thermally neutral processes (such as isomerization processes which involve skeletal rearrangement of the molecular constituents of the feedstock, but no change in the molecular weight) do not cause any cooling or heating of the feed stream. Exothermic processes include hydrotreating, hydrocracking, and alkylation processes. Hydrocracking is highly exothermic owing to aromatic saturation reactions. Although the molecular weight is reduced by the cracking reaction, this is preceded by hydrogenation reactions, for example, aromatic ring saturation, which is necessary before the ring opening can occur. The alkylation process is also quite exothermic because higher molecular weight compounds are formed from isobutane and olefins. Distillate and naphtha hydrotreating also release heat when organo-sulfur and nitrogen compounds (i.e., dibenzothiophene and pyridine) are converted to hydrogen sulfide and ammonia, respectively.