Hydrogen-to-Oil Ratio and Gas Recycle

Hj/oil ratio is a standard measure of the volume rate of hydrogen circulating through the reaction system with respect to the volume of feed. It is defined by the following relationship  

One relevant aspect of gas recycling is its effect on the gas-liquid equilibrium in the reactor (Hoekstra, 2007). It is typical for most HDT units to operate with partially vaporized hydrocarbon feed. This effect alters gas composition and reaction rates. Increasing H2/oil ratio can be useful to concentrate the heaviest and most refractory compounds (e.g., dibenzothiophenes in gas oil feeds) in the liquid phase, providing them more contact time with the catalyst. However, special care must be taken with excess recycle rate because some of the species in the vaporized fraction might not have access to the active site of the catalyst particle. 

Inside the reactor, H2/oil ratio decreases along each catalyst bed as a result of hydrogen consumption. Interbed hydrogen quenching acts as a make-up source producing a step increase at the entrance of the following catalyst bed. The H2/oil profile along the reactor usually has the inverse shape of axial temperature, as it will be demonstrated in the simulations presented in the next section. 

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