An adsorption column having completed the adsorption step and {HEAD ~ another adsorption column} having completed {TAIL ~ the vacuum purge step} 
having completed
An adsorption column having completed the adsorption step and { HEAD ~ another adsorption column } having completed { TAIL ~ the vacuum purge step }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL