will be vapors of the liquid fuel 22 and typically air the air arrives in the tank 8 through numerous possible paths first and foremost is through the vapor recovery mechanism of an attached fuel dispenser not shown such as the fuel dispensers discussed above air might also be ingested through pressure vacuum valve 15 atop vent pipe 14 if the pressure in the ullage 11 falls to a low level the pressure vacuum valve 15 opens in conventional fashion to permit air to be ingested and avoid potentially dangerous underpressure in the tank 8 as depicted in fig1 the vent pipe 14 forms part of a conduit 7 leading from the ullage 11 to the variable speed vapor pump 17 at the end of the conduit 7 is a membrane module 44 the membrane module 44 has an inlet 48 to the intake conduit 7 and a permeate outlet 46 and a retentate outlet 42 a voc fractionating membrane 18 like those discussed above divides the module 44 so that the inlet 48 and retentate outlet 42 are on one side of the membrane 18 and the permeate outlet 46 is on the opposite side of the membrane the outlet 46 communicates through a second.