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