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