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