And, after the first catalytic bed, {HEAD ~ the reaction mixture} passes through {TAIL ~ at least one subsequent bed} to promote the other hydroconversion reactions 
passes through
And , after the first catalytic bed , { HEAD ~ the reaction mixture } passes through { TAIL ~ at least one subsequent bed } to promote the other hydroconversion reactions
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH