Membranes, Molecules and the Science of Permeation

The term “vehicle refueling vapors” refers to the vapors displaced from the vehicle fuel tank during refueling. Storage tank evaporative vapors, on the other hand, are vapors that are created as gasoline undergoes a change from liquid phase to vapor phase. This change must occur to re-establish an equilibrium vapor concentration in the space above the liquid in fixed roof storage tanks (USTs or ASTs).

The vapor space concentration is driven below natural levels by the ingestion of lean vapors or air into the storage tank during vehicle refueling. If the natural equilibrium vapor concentration is momentarily reduced, liquid gasoline will evaporate until the equilibrium concentration level is re-attained.

One gallon of liquid gasoline will expand to approximately 520 gallons of vapor at 40 percent hydrocarbon concentration. Therefore, storage tank pressure will increase rapidly as relatively small amounts of liquid evaporate. This increased pressure can result in vapor emissions from pressure/vacuum relief valves or through any leaks in the vapor piping.

What goes in must come out
Any refueling scenario, with or without ORVR or Stage II systems, which introduces lean vapors or air into gasoline storage tanks will result in the creation of evaporative vapors, as discussed above. The subsequent emission of the evaporative vapors compromises the overall efficiency of whatever vapor recovery systems are used. The four possible refueling scenarios are as follows:

  • No ORVR and no Stage II (uncontrolled stations). In this scenario, refueling will contribute to significant liquid evaporation as atmospheric air is ingested into the storage tank via the vapor vent or breaches in the vapor piping. The air will enter at a volume equivalent to the volume of liquid dispensed, and upon re-equilibration, this air will generate significant emissions.
  • Stage II only (no ORVR). In this scenario, atmospheric air and hydrocarbon vapors enter the storage tank when gasoline is pumped to the automobile. Some typical vacuum-assisted systems obtain high collection efficiencies at the expense of introducing excess air into storage tanks. For example, if 10 gallons of liquid gasoline are pumped to an automobile, the vapor volume returned to the storage tanks may range from 11 to 25 gallons. This excess air/vapor volume will quickly increase storage tank pressure.
  • ORVR only (No Stage II). While the ORVR system may capture the refueling vapors as designed, atmospheric air will enter the storage tank as described in the first scenario above (uncontrolled stations).
  • ORVR and Stage II. As the ORVR system captures vapors displaced from the automobile fuel tank, the storage tanks will be back-filled with atmospheric air.
  Where vacuum-assisted Stage II systems are employed, the storage tanks will be backfilled with atmospheric air at a volume greater than the volume of liquid dispensed. In these cases, the combination of excess gaseous volume and extremely low hydrocarbon concentration will result in rapid storage tank pressurization and subsequent emissions.
   These emissions come through either the vent pipes (vent emissions) or leaks in the vapor piping (fugitive emissions). If the vacuum-assisted systems are disabled during refueling of ORVR-equipped vehicles, the resulting storage tank evaporative losses will be equivalent to those generated at uncontrolled dispensing facilities. This must be the case, since the air ingestion volume will be equal to the volume of liquid dispensed.
  If balance Stage II systems are used, the storage tank evaporative losses will be equivalent to those generated at uncontrolled stations. This, again, is because the air ingestion volume will equal the volume of liquid dispensed.

In summary, no matter what the scenario, the space vacated by pumping gasoline out of the storage tank is replaced by air or hydrocarbons that are equivalent at least to the volume of liquid displaced. In some cases involving Stage II systems and ORVR, the volume of air ingested is greater than the volume of liquid displaced.

Measuring evaporative vapor emissions
Using its “evaporative loss model,” ARID has estimated the magnitude of the evaporative vapor emissions and associated reductions in the overall efficiency of vapor recovery under each of four possible refueling scenarios described above. These estimates are shown in Table 1, below.

The key inputs in the model are gasoline Reid Vapor Pressure (RVP), the storage tank temperature and the air ingestion volume or vapor to liquid ratio (V/L). The data on total emissions and recovery percentages (far right two columns) are applicable only if the systems are not equipped with vent vapor processing units.

With reference to Table 1, note the range of evaporative emissions for a dispensing facility that pumps 100,000 gallons of gasoline per month: from 0.92 to 10.76 tons per year. Also note that the uncontrolled refueling emissions for this same facility are estimated at 5.04 tons per year. Therefore, depending on the V/L, RVP and storage tank temperature, the evaporative emissions can exceed the uncontrolled refueling emissions by up to a factor of two.

If one considers a centralized vacuum-assist system operating at V/L of 2.0, the evaporative losses can exceed the uncontrolled refueling losses by up to a factor of three. For a central vacuum system, the resulting large losses are combusted and do not result in atmospheric emissions. However, the economic value of the combusted material is lost. Also, the generation of combustion by-products, such as carbon dioxide and oxides of nitrogen, contribute to the formation of undesirable “greenhouse” gases.

Also as seen in Table 1 below, the storage tank evaporative emissions exceed the “uncaptured” refueling emissions for every scenario except the uncontrolled, first case (no Stage II and no ORVR). Even if Stage II and ORVR systems are 95 percent efficient in capturing refueling losses, the evaporative losses from the storage tank significantly reduce the capture efficiency of the refueling emissions.

It is important to note that all of the overall recovery efficiency values are well below the 95 percent minimum required by the Clean Air Act Amendments of 1990 for Stage II systems. Moreover, even if 100 percent of vehicles on the road had ORVR systems, the best overall recovery efficiency one can hope to achieve, without using a vent vapor processor, is only 50 percent ([9.48-4.69]) ÷ 9.48).

One such processor involves the use of a membrane system to separate, concentrate and recover hydrocarbons from air/vapor mixtures. Such a system can significantly reduce evaporative vapor emissions, which, as discussed above, are becoming more significant as the population of ORVR-equipped vehicles increases.