Vopak and Aker Solutions subsidiary Aker Cool Sorption entered into an agreement Q2 2010 for the provision of a new vapour recovery system for Vopak’s Neckarhaven facility at its Europort terminal close to Rotterdam in the Netherlands. The agreement represents the culmination of a long and detailed review of the requirements and challenges from both partners in the agreement.
The vapour recovery system, due for installation in the first half of 2011, has a number of unique features. It will replace the existing gas engine systems, with a total capacity of 600 m3/hr; which are too small for the current level of demand and have become unreliable as they have aged. This is particularly notable during the summer months when the vapour flows and concentrations lead to insufficient capacity and knocking in the engine, due to the high calorific value of the vapour stream.
The new vapour recovery unit (VRU) will be able to handle vapours resulting from the filling of up to three barges simultaneously, at flow rates of up to 800 m3/hr per barge (2,400 m3/hr in total). The environmental permit for the operation has also been amended, requiring vapour treatment with an emissions limit of 100 mg/Nm3 for VOCs and 1 mg/Nm3 for benzene and that these limits be applied to all products loaded with a vapour pressure greater than 10 kPa at 20ºC.
Dealing with the dock
Vopak’s Neckarhaven facility is used for the loading of barges with gasoline, naphtha, condensates and other products. The products are owned by various different companies, so preventing cross-contamination is vital.
However, this raises a problem with standard VRUs, which return recovered vapours in a continuously circulated absorbent stream. The absorbent is commonly the same product as that being loaded – e.g, VRUs handling gasoline vapours commonly use gasoline as the absorbent – which is drawn from one of the main storage tanks. For ship and barge loading operations it is common for the absorbent stream to be drawn from and returned to the product loading lines. At Neckarhaven this arrangement was not possible and it was necessary to eliminate the requirement to use an absorbent. In addition, the dock facilities are located some 2 km away from the main storage facilities and running dedicated lines for absorbent to and from the various storage tanks would have been expensive and wasteful of tank capacity. There is no room at Neckarhaven to build local tanks to hold absorbents.
An additional constraint attached to Vopak’s selection process was that the VRU system should fulfil one of the following requirements: either full recovery of the vapours to the loaded product, or incineration of the vapours leading to a positive benefit such as power generation.
System design
The VRU proposed by Aker Solutions was able to fulfil all of Vopak’s requirements for the operation. The system uses a traditional front end approach to vapour recovery using activated carbon beds. The hydrocarbons are absorbed on to the activated carbon during a fixed cycle period, before being regenerated through the use of vacuum.
At all times during the operation, one bed is open to the vapour line – in the ‘adsorption mode’ - while the other undergoes regeneration by means of vacuum. Activated carbon is used in the system to adsorb the hydrocarbons in the vapour stream. A mineral-based activated carbon has been selected, which offers an extremely high surface area relative to its volume, allowing the hydrocarbons to be adsorbed on the surface of the carbon in a very thin layer.
The carbon can only adsorb a given mass of hydrocarbon before it approaches saturation.
If saturation of the activated carbon occurs throughout the bed, then the vapours will pass through untreated. Consequently the carbon must be regenerated in order to restore its working capacity, such that it can effectively adsorb hydrocarbons in the following cycles.
In the proposed system, the carbon beds are designed for an effective adsorption cycle time for the design loading vapour profile and hydrocarbon concentrations considered.
Regeneration of the carbon beds takes place in three stages. In the first stage, the bed is evacuated of vapours in the void spaces of the carbon bed vessel. In the second stage, as the pressure in the bed continues to fall it reaches a level at which the hydrocarbons begin to desorb from the surface of the activated carbon.
The bulk of the hydrocarbons are removed during this stage. During the third stage, asmall bleed of purge air is introduced into the carbon bed vessel, polishing the remainder of the hydrocarbons from the activated carbon and thus completing the regeneration process. The emissions from the carbon beds are monitored continuously by the system through the use of a flame ionisation detector (FID) in the vent gas stream.
It is here where the VRU to be installed by Aker Solutions differs from other units. Rather than using an absorber column, the vapours regenerated from the carbon beds are condensed and retained in the system as a hydrocarbon liquid. Vopak proposes to build two tanks to hold the recovered condensate; this will allow one to be in use while the contents of the other are sampled prior to being picked up for off-site disposal. The size of the tanks was determined by the available volume of the road trucks to be used to transport the recovered vapours. It is estimated that Neckarhaven will produce batches of 30 m3 of recovered condensate every few days, with an average of 80 batches each year.
Another key component in the operating permit for the terminal is a high degree of availability. The terminal is allowed a maximum running time without the VRU of 15 hours at a stretch with a maximum of 90 hours per year.
A degree of redundancy is therefore built into the VRU system, which comprises two parallel trains of vacuum pumps and condensers. Each train is able to run independently of the other.
However, as each train provides only half of total capacity, should one train be out of action the operational vapour loading capacity of the system is somewhat reduced from the normal maximum.
In addition to the dual vacuum pump and condenser train approach, individual strategic spare parts will be held at the site in order to ensure the return of the system to full operation as quickly as possible.