Why recover vapours?

Gasoline vapours are normally recovered of the following three reasons:

1. To reduce the discharge of environmentally hazardous substances

2. To reduce the safety risks in the distribution and handling of gasoline

3. To reduce the significant losses of valuable energy resources, and hereby economic losses, represented by the vaporization of gasoline.

Adjustment of gasoline types to the surroundings
For the gasoline to vaporize at the right time upon injection into the combustion engine, the vapour pressure of the gasoline must be adjusted to the temperature of the surroundings. An arctic climate requires the gasoline to have a higher vapour pressure than that of a tropic climate, to enable it to vaporize at the lower temperature.

On the other hand, a gasoline suitable for an arctic climate will cause trouble in a tropical climate, vaporizing before injection into the combustion engine and creating vapour pockets in the fuel supply.

Change of RVPs according to temperature
Thus gasoline types differ according to place of use. The vapour pressure of the gasoline is adjusted by changing the blending proportions of the different hydrocarbons in the mixture. More hydrocarbons with a high vapour pressure, like butane and propane, are added to gasoline used in cold climates than in gasoline used in warmer climates. As a result, gasoline with an RVP of 70-90 kPaA is used in colder climates and gasoline with an RVP of 55-70 kPaA in warmer climates.

Loss of gasoline vapours during transfer
Vapours escape to the atmosphere when a gasoline product is transferred from one storage tank to another or from a storage tank to mobile distribution, due to the displacement of the vapour mass in the tank being filled. Vapours also escape to the atmosphere by ´breathing´, i.e. temperature expansion of the liquid, from storage tanks.

Smog gasoline vapours as an environmental hazard
Smog is a well-known pollutant, occurring as a brownish-yellow dimness/haziness, mostly over areas of dense population, and consisting of a chemical compound of gases. The components of smog include ozone, nitrogen oxides, sulphur dioxide, acidic aerosols, gases and particles. These gases are the result of a reaction between various airborne pollutants and strong sunlight. The airborne pollutant constituting up to 90% of all smog in population-dense areas is ozone.

The creation of ozone
Ozone is created by a chemical reaction between VOCs and nitrogen oxides. Nitrogen oxides arise from the combustion of fossil fuels like gasoline, natural gas, fuel oil and coal. VOCs mainly come from the vaporization of flammable liquids like crude oil, gasoline, solvents and organic chemicals.

When air masses are stagnant over densely populated areas, the pollutants are present for extended periods. Sunlight affects these pollutants and transforms them into ozone, which settles in areas close to the ground. The ozone remains in the lower atmosphere until changing weather systems disperse it.

Effects of ozone on living creatures
The effects of ozone on people vary with the exposure. By intense exposure for short periods, the effects on health will include irritation of nose and throat, coughing, breathing pains and reduced lung function. Extended exposure to smog can reduce lung elasticity and the lungs´ ability to resist diseases. Exposure has the same effect on animals as on people. It also damages plant life and crops: losses of up to 20% in yield have been found as a result of ozone pollution.

Effects of ozone on materials
Ozone in the lower atmosphere can even damage man-made materials, especially synthetics. Even low exposure to ozone for a few months will cause rubber products to crack, and continued exposure can cause complete dissolution. Ozone will also destroy the structure of cotton, acetate, nylon, polyester and other textiles.

All these effects considered, it is preferable to reduce ozone production. As VOC is one of the primary causes of ground-level ozone production, vapour recovery offers a relatively simple way to reduce ozone-related damages in the lower atmosphere.

Ozone as a health hazard
Besides contributing to the ozone production and its effects on human, animal and plant life, gasoline also contains numerous substances that constitute a health hazard to people employed in the gasoline distribution. For example is benzene, a component of gasoline, proved to cause cancer. Benzene is also present in gasoline vapours and thus constitutes a serious health threat to the surroundings.

Safety considerations in the distribution and handling of gasoline
Gasoline is explosive in a 1-7% mixture with atmospheric air. Of this reason, caution is required in the distribution and handling of gasoline to ensure that no ignition source may risk igniting the vapours present in the area.

A word of caution about gasoline handling
The distribution of gasoline thus requires great caution, particularly during the loading of tankers, railway carriages and ships, during the filling of underground storage tanks on service stations and the topping up of automobiles.

By sealing the vent openings of storage tanks and mobile tanks for gasoline and guiding the vapours through closed piping systems to a recovery unit, one can practically remove the mentioned safety hazards.  

Economic motivation for the recovery of gasoline vapours
Generally, the recovery of gasoline vapour has a great economic potential. Saturated gasoline vapour at 0°C contains approx. 0.6 kg gasoline per cubic metre, which is equal to 1 litre of recovered gasoline. At 25°C, the saturated vapour contains approx. 2 litres per cubic metre.
Under normal circumstances, a gasoline terminal will have an average recovery potential of approx. 1500 litres of gasoline for every 1000 cubic metres transferred.

Investment in a VRU will soon show profit
The value of the recovered gasoline varies according to whether the recovered product is taxed or not. If the gasoline is taxed, the practice among Western authorities is to refund tax on the recovered product, so the product is not subject to double taxation. This, in particular, makes vapour recovery attractive and it is not unusual for a single vapour recovery unit to have repaid itself in less than one year of service.