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• Vapor Recovery Around the World
• Retooling the Vapor Recovery System
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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.