These cooler, peripherally flowing gases, which contain the highest hydrocarbon concentrations, tend to flow peripherally toward and through {HEAD ~ the peripheral or cooler portions} of {TAIL ~ the burn off catalyst} 
of
These cooler , peripherally flowing gases , which contain the highest hydrocarbon concentrations , tend to flow peripherally toward and through { HEAD ~ the peripheral or cooler portions } of { TAIL ~ the burn off catalyst }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL