An upper outlet permits {HEAD ~ the venting} of gases that percolate from the liquid constituent, a backpressure valve disposed in communication therewith to maintain a positive pressure within {TAIL ~ the vessel} 
An upper outlet permits { HEAD ~ the venting } of gases that percolate from the liquid constituent , a backpressure valve disposed in communication therewith to maintain a positive pressure within { TAIL ~ the vessel }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL