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