In an alternative embodiment, {HEAD ~ the average gas superficial velocity} is maintained above {TAIL ~ a minimum level} 
is maintained above
In an alternative embodiment , { HEAD ~ the average gas superficial velocity } is maintained above { TAIL ~ a minimum level }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL