In addition, {HEAD ~ a portion} of {TAIL ~ the product gas} is used to reflux the purification zone 
of
In addition , { HEAD ~ a portion } of { TAIL ~ the product gas } is used to reflux the purification zone
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH