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