Propylene, oxygen, and hydrogen are preferably fed to the reactor at or near {HEAD ~ the bottom} of {TAIL ~ the reactor} 
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Propylene , oxygen , and hydrogen are preferably fed to the reactor at or near { HEAD ~ the bottom } of { TAIL ~ the reactor }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL