Activated carbon has an extremely large surface area and hydrocarbons are adsorbed in a very thin layer on the carbon surface. The carbon can only adsorb a certain amount of hydrocarbons before reaching saturation. Then, the carbon is regenerated in order to restore its capacity, so hydrocarbons can be efficiently adsorbed in the next cycle. The batch of carbon inside the bed will perform with sufficient capacity over the years. Only after a period of seven to ten years, it will be necessary to replace the carbon.
The regeneration takes place in two stages. Firstly, the bed is evacuated until the pressure reaches a value at which hydrocarbons start desorbing from the carbon surface. At this stage, the bulk of hydrocarbons are removed. To remove the remainder, it is necessary to introduce a small amount of purge air.
The vacuum pump used for regeneration will be either liquid seal ring or dry running type, depending on the size of the VRU.
The vapor, which is now very rich on hydrocarbons, passes from the separator to the absorber column where the bulk of hydrocarbons is absorbed in a counter flow of gasoline or crude oil. The small amount of present air, particularly during the purge stage, passes out on top of the absorber column and results in a small carryover of hydrocarbons that returns to the carbon bed operating in adsorption mode.
To maintain a high efficiency in the absorber column, the absorbent should be cooled to approximately 20 C. This is normally done by using sea or fresh water, if available. Also, it is possible to use fin fan air coolers as an alternative. If installation of a cooling system is impractical, the absorber may be slightly pressurized to maintain the absorption properties. Usually, this is done by installing blowers in the line downstream the vacuum pumps and upstream the absorber.
Vapor from the cargo tanks passes through the vapor header to the recovery unit. Since in some cases the vapor may contain sulphurous components in the low boiling range, a prefilter (guard bed) containing an adsorbent (carbon) with high sulphur selectivity is installed to protect the VOC system against DMS, mercaptanes and H2S upstream the VOC recovery system. This makes possible to remove sulphur components in the vapor so that they cannot enter the activated carbon system, thereby reducing the carbons' adsorption capabilities. The guard bed shall be designed for a content of maximum 20 PPM sulphur in the gas flow to the recovery unit. Tapping points (three at minimum) for Draeger tube test of sulphur compositions in the guard bed shall be included to secure a monitoring of the guard bed's condition.
The First Generation VOC Plants for Vysotsk and KaliningradGiven its success in recent years, the CVA system has proved to be the best available technology (or BAT) for both gasoline and crude oil vapor recovery .
Following the campaigning of the CVA system for use on crude oil vapors, an immediate breakthrough was achieved on