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