Those Pumps That Have a Rubber Boot Cover That Fits Over Your Gas Tank Filler Tube When You Dispense Gasoline In To Your Car? …

The Colour of Green Applies To Both the Environment and….Money

So how about a discount on fuel price?

Have you ever wondered who gets the financial benefit of those captured gasoline vapor fumes  (hint… it’s probably not the customer) pushed out of your gas tank when you fill up your tank again with gasoline – you know, the same fumes you already paid for at the last fill-up? ( So that rubber boot surrounding the gasoline fill-up nozzle that locks on to your gas cap opening not only captures pollution but also dollar bills.) 

Maybe the green environmental value of the recovered fuel fumes helps pay for the green environmental clean–up collection cost, so maybe a break-even deal…, and then again, at $4 plus per gallon, maybe that green value benefit is greener (think $$) than just dirty laundry clean-up costs.

Technical Explanation and maybe more than you wanted to know…or just enough to make your case: 

So how much green (both environmental and financial) captured fumes are we talking about? 

• Assume a 15 gallon capacity car gas tank. 
• If the empty gas tank contains no air (highly unlikely…but an extreme case assumption)  and only gasoline fumes or vapors (the highest possible vapor amount in the tank) we can apply the ideal gas law (a little chemistry),  PV = nRT,  to estimate the quantity (say in pounds) of gasoline vapor in the empty fuel tank…
  • At P = atmospheric pressure or 14.7 psia (pounds per square inch absolute),
  • R = 10.73 (psia)(ft³)/(^oR)(lb mole) – the ideal gas constant,
  • T = 80^oF (assumed outside temperature) equivalent to   540^oR (degrees Rankin, another way to measure temperature called an ‘absolute’ measurement),
  • V (empty fuel tank volume) = 15 gallons, equivalent to 2 cubic feet (ft³), 
  • n = moles (a chemistry term that is used to determine how many pounds of gasoline vapor is in the  fuel tank). 
• Using the equation PV = nRT, there are about 0.0051 moles of gasoline fumes (n = PV/RT;  (14.7)(2)/(10.73)(540)  in the tank;
• or at a molecular weight (some more chemistry) of about 107 pounds per mole for gasoline, then, 0.54 pounds of gasoline vapor fumes (0.0051×107) are  captured when liquid gasoline fills the tank and displaces (pushes out) the vapor volume. 
• If, however, we assume a more realistic case where there is also some air in the tank and the vapors are in ‘equilibrium’ (yes, more chemistry) with the gasoline (what is referred to as a saturated condition (this means the vapor pressure of liquid gasoline will equal its partial pressure in the air space);
  • The vapor pressure of gasoline at 80^oF is about 7 psi (pounds per square inch),
  • this indicates about 47% (7/14.7)  of the expelled vapors is gasoline (and the other 53% is air)
  • or about 0.26 pounds of gasoline (0.47 x 0.54)  in 15 gallons (equivalent volume) displaced from the tank.  (As a reference, a ‘gallon’ of milk can be interpreted as the ‘weight’ of a gallon jug of liquid milk, or the jug has an air volume capacity of 1 gallon when empty).
• At a density of about 6 lb/gallon for liquid gasoline,
• this means about 0.04 to 0.09 (0.26/6 to 0.55/6);  liquid gallons of equivalent gasoline vapor are recovered by the fill-up station rubber boot per car tank fill up. 
• Or about 17 to 34 cents per fill-up worth of gasoline fumes recovered at a pump price of $4/gallon. 

Probably not enough to fuss about if only one tank, but it does mean from 1 to 2 cents per gallon could be offered as a ‘discount’ for giving the fumes back to the station – the fumes you paid for at the last fill up.  (After all, competing filling stations seem to use that level of value to entice customers:  $3.94 ^9/10 vs. the down the street competitor at $3.95 ^9/10 – good enough for the retailer ought to be good enough for consumers.) 

Typical petrol station underground storage tank capacity is about 10,000 gallons.  So about 30 to 60 gallons [(10000/15) x (0.04 to 0.09)] of gasoline vapor fumes are recovered on each 10,000 gallon storage tank load turn-over  or $120 to $240 worth [$4 x (30 to 60] of recovered fumes per storage tank cycle.  In a year’s time and assuming 200,000 gallons of gasoline sold per year, that’s about $2,400 to $4,800 bucks a year [(200000/10000) x $120 to $240] in ‘free’ fumes. 

Gasoline Vapor Recovery (how it works)

Gasoline stations, as an example, can rely on vacuum-assisted dispensing systems to minimize the release of hydrocarbon vapors to the atmosphere. These systems use a small pump to draw air and vapors from the gasoline dispensing nozzle (captures the fumes displaced from your tank when you fill up). For every gallon (liquid) of gasoline dispensed from the pump, as much as 2 gallons (vapor/fumes/exhaust) of air and gasoline vapor mixture are returned to the underground storage tank. Build-up of captured air in the storage tank leads to atmospheric releases of the clean air from the tank back into the atmosphere.

There are various membrane vapor recovery systems that cleans the released air, for example, for fuel storage tanks of retail gasoline stations, which recovers gasoline vapors and returns them to the storage tank. Hydrocarbon emissions (from filling your car up) is reported to be reduced by 95% to 99% (as claimed by the sellers of capture equipment).

Air and gasoline vapor mixture captured from the car fill-up nozzle (the rubber boot cover), from the gas station dispenser, are collected and sent to the petrol station underground gasoline storage tank. When the pressure in the tank reaches a preset value, a pressure switch activates a small compressor that draws off excess vapor-laden air. A portion of the hydrocarbon vapors condense and is returned to the tank as a liquid. The remaining hydrocarbons in the air/vapor mixture, permeate a membrane and are returned to the tank as concentrated vapor. Air, stripped of 95% to 99% of the hydrocarbons, is vented. In addition to eliminating hydrocarbon emissions, the unit can pay for itself with the value of the recovered gasoline.