Vapour from the loading points pass through the vapour header to the recovery unit.

The vapour header shall be protected from a possible ignition in the VRU. Further more all connected vessels (trucks, railcars, ships, barges, tanks etc) shall also be protected from a detonation coming from the vapour header.

The vapour header shall be fitted with a PVV (Pressure & Vacuum Vent) emergency vent and could be fitted with a controlled emergency vent in case the VRU is shut down. The PVV vent and possible emergency vent should be fitted with endurance burning proof flame arrestors.

Prior to the vapour entering the VRU it must pass through a knock-out vessel to ensure that no gasoline enters the carbon bed (the knock-out vessels, often provided at the loading point are usually sufficient).

The VRU consists of 2 activated carbon beds, the one being connected to the vapour line - "adsorption mode" - while the other undergoes regeneration by means of vacuum. Activated carbon has an extremely high surface area in relation to volume and the hydrocarbons are adsorbed in a very thin layer on the surface of the carbon. The carbon can only adsorb a given amount before it approaches saturation. If this occurs throughout the bed, then the vapours will pass through untreated. Consequently the carbon must be regenerated in order to restore its capacity, so that it can effectively adsorb hydrocarbons in the following cycle.

Bildquelle: John Zink

The regeneration takes place in 2 stages. First the bed is evacuated until the pressure reaches that, at which the hydrocarbons begin to desorb from the carbon. The bulk of the hydrocarbons are removed in this stage. In order to remove the remainder, it is necessary to introduce a small amount of purge air, to complete the regeneration.

The vacuum pump is of the liquid ring type since this limits the temperature rise over the pump.

However, as a result a liquid separator and a liquid cooler are required. The sealing liquid is a mixture of glycol and water. The standard option utilises gasoline as coolant. However alternatives can be offered, if it is thought that this could result in unacceptably high temperatures in the storage tanks.

From the separator the vapour, which is now very rich in hydrocarbons, pass into the absorber column where the bulk of the hydrocarbon is absorbed in a counter flow of gasoline. The small amount of air present, particularly during the purge stage, passes out of the top of the absorber column and results in a small carry over of hydrocarbons, which is returned to the carbon bed, which is in adsorption mode.

The unit is provided with an automatic energy saving function: If vapour loading is low or has stopped, all functions are set on stand by, once both carbon beds have been thoroughly regenerated. When on stand by, the unit is still open for adsorption, but the all pumps are stopped.

The pumps only operate occasionally for short intervals to keep the carbon beds clean and active. When normal load resumes, the plant will automatically restart, with continuous pump operation.