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