Process

The hydrocarbon vapour recovery system is based on the well-known process of adsorption on activated carbon. The saturated carbon is regenerated by means of vacuum. The hydrocarbons desorbed from the carbon are re-absorbed in one of the products present on the storage terminal.

Whilst a tank truck is filled with a product or more products simultaneously, the mixture of air and hydrocarbon vapours is forced out of the compartments and is led through a common pipe to a 4” API vapour coupler.

The hydrocarbon/air mixture usually flows downwards through the carbon beds. The hydrocarbons are rapidly adsorbed in the pores of the activated carbon in the bottom of the bed. The air, stripped from hydrocarbons, leaves the bed through a chimney to atmosphere.

The regeneration of the activated carbon is done by pulling vacuum by means of dry screw vacuum pumps. The engineers of CarboVac have developed this technology specially to overcome the problems of corrosion and abrasion related to the use of glycol based seal liquid in liquid ring vacuum pumps.

The highly concentrated hydrocarbons coming from the vacuum system are brought in direct contact with the absorbent in counter flow on a packed bed of Raschig Super Rings. The rest air, saturated with new hydrocarbons from the absorbent, leaves the column from the top and is led back to the inlet of the VRU.

Design Requirements

The Carbovac VRU is usually designed for a maximum daily loading profile, in order to assure compliance with the emission laws under all circumstances. The loading profile forms the basis for the selection of the most efficient combination of activated carbon volume and capacity of the vacuum system. The daily loading profile is characterized by following periodical loading volumes. Each of these values determines a specific part of the VRU:

• Instantaneous Flow Rate (mainly for pressure drop)

• Volume loaded per Cycle (mainly for the amount of carbon required)

• Volume loaded in a period of 4 hours (mainly for the capacity of the vacuum system)

• Volume loaded per day ( to adjust the optimum combination of vacuum capacity and carbon volume)

• Hydrocarbon concentration (mainly for the capacity of the vacuum system)

• Vapour holder effect (The CarboVac VRU does not need a gasholder to balance out the variations in flow rate and concentration. In fact the activated carbon itself is a far more effective buffer for hydrocarbons than a mechanical vapour holder)

Material Used

Quality and reliability are the prevailing factors for the choice of materials and equipment used in a CarboVac VRU and insures the satisfaction of end user with his equipment on the long term. On top of that, equipment, spare parts and services must be quickly and universally available. CarboVac wants to be an anticipating and reactive VRU supplier. To support these goals we form partnerships with our principal sub-suppliers.

Safety Systems

CarboVac supplies an ATEX Explosion Protection Document with the VRU, as per the European Directive 94/9EU. The basic safety feature of the unit is its entire explosion proof design. The unit resists the pressure of an eventual internal explosion.

Compare VRU…

  • Cheap but lower quality and life expectancy.
  • The initial investment is low, but the operational cost and maintenance cost are high.
  • The energy consumption is nearly twice that of the Dry screw pump.
  • The liquid ring in contact with the vapours causes corrosion and abrasion, and decrease of efficiency.
  • The decreased efficiency of the pumps provokes a sustained insufficient regeneration of the activated carbon and thus reduction of the carbon life time.
  • The degradation of the pumps presents also explosion safety risks.
  • Requirement of lubrication of the metal to metal contact between the vanes and the pump casing.
  • Because part of the regenerated hydrocarbons condenses inside the vacuum pump, the oil film will be damaged and the wear of the pump will largely increase.
  • The increased friction will increase the energy consumption, create heat and increase the risk of explosion.
  • The pumps need to be pre-heated before starting regeneration.
  • Not suitable for the vacuum level required in VRU’s.
  • Due to the very low efficiency of the thermodynamic process the energy consumption is very high.
  • The use of rotary lobe blowers in combination with liquid ring pumps is not recommended.
  • High energy consumption, high maintenance costs and on top of that a very high noise level.
  • The energy consumption is around 50% of the systems using liquid ring vacuum pumps.
  • The use of frequency drives (VFD) improves the energy consumption at vapour loads less than the design loads.
  • The maintenance costs of the CarboVac system are significantly lower.
  • The CarboVac technology is the simplest process available on the market. Simplicity guarantees high availability of the system.
  • No glycol heat exchanger, no glycol circulation and no separator the result is less space requirement.
  • No corrosion and abrasion.
  • The Dry VRU technology consists of only one stage, suitable for any emission level.
  • The Dry Vacuum system is suitable for all types of volatile hydrocarbon components (alcohols, aromatics, etc.)