Fixed bed reactor assembly having a guard catalyst bed

A method, and a reactor, for hydroprocessing a hydrocarbon feed stream through multistage fixed catalyst beds contained within a single onstream reactor vessel, with a separate catalyst addition and a separate catalyst withdrawal system for the upper or top essentially fixed catalyst bed. The upper or top essentially fixed catalyst bed functions as a guard catalyst bed for removing a major proportion of contaminants (e.g. organometallic constituents) from a hydrocarbon feed stream in order to extend the life of one or more fixed catalyst beds serially disposed underneath the upper or top essentially fixed catalyst bed. Catalyst particles are withdrawn from the essentially fixed catalyst bed by slurrying or liquifying the catalyst particles in a container disposed within the reactor. A method for retrofitting a reactor assembly having two or more fixed catalyst beds in order to place a guard catalyst bed in the upper or top portion of the reactor assembly.