Vapours from Truck / Rail Car loading
Concentration: it increases when recovery of the vapours is done in service stations into the trucks (stage 2).
Most of the vapours recovered are those from the previous product loaded. As a consequence if a vehicle had transported Gasoline and is now loading Diesel, most of the vapours will be gasoline ones.
Vapours composition: the vapours are mainly composed of Propane and Butane as only the light ends HC molecules vaporized.
During the loading of gasoline and diesel in trucks, the concentration of the vapours may vary between 0 to 50 % Vol. depending of :
- the nature of the products previously loaded.
- the loading station
Theses hydrocarbons are generally composed (% Vol) of :
| C1 | C2 | C3 | C4 | C5 | C6 | C7++ | Benzene | Toluene |
| 0 | 0 | 1.5 | 37 | 22 | 8 | 1.7 | 0.26 | 0.36 |
| – | – | – | – | – | – | – | – | – |
| 2 | 0.45 | 3.8 | 50 | 43 | 12 | 5.4 | 2.6 | 1.8 |
In most of the cases the vapours from the truck loading are directly treated into the VRU.
Sometimes a vapour balancing system is elected. The vapours from the truck go to the storage tank (1m³ of liquid > 1 m³ of vapour). The vapours from tank breathing (thermal and filling) go to the VRU. The VRU is bigger than in the precedent case.
For Gasoline we advise to install internal floating roof and to connect only the truck loading part to the VRU.
Bottom loading
Vapour collector one each compartment > all vapours go to the VRU as a mixture
Bottom Loading
On a truck bottom loading bay a pressure of 55 mbar is generally available at vapour connection.
This pressure shall be sufficient to go through the vapour collector and the VRU. The VRU has a pressure drop of app.
25 mbar. The vapour coupler shall be elected carefully targeting a PD as low as possible to keep the PD from the vapour connection to the connection at main header below 15 mbar.
Top loading with vapour return
Depending on the system use to seal the manhole, on the truck quality, … the pressure available may not be sufficient for the vapours to go through the collecting system without assistance. In that case a blower will be installed at the outlet of the unit (vent where only clean
air). This blower will be driven with a VFD, based on the pressure in the line. It will drag the vapour along the systems up to the outlet.