Why Balance And Recover VOC Vapour?
Balancing and recovering VOC vapour makes good sense
As can be seen from the chart below, a significant amount of gasoline is lost as vapour emitted to the atmosphere, during handling of gasoline in the distribution chain. If there is no control of VOC emission in the gasoline distribution chain, the result is a yearly average total loss of 5.6 litres liquid gasoline per 1,000 litres handled, for North European climatic conditions. If dispatching gasoline by other means than pipeline from the refinery to the depot, another 0.61 litres per 1,000 litres handled, is lost as vapour emitted to the atmosphere.
These losses are easily avoided by balancing and recovering through simple means, which give the associated investments very attractive short-term payback.
The philosophy of balancing is that air can only contain a certain amount of VOC until saturation. Passing VOC saturated air back from one link in the gasoline distribution chain to a previous, will simply avoid evaporation in this link, and hereby avoid the loss. Furthermore, the VOC saturated air balanced back is contained in the previous link, and not emitted to the atmosphere.
Let us look at balancing and recovery of VOC loaded air in the gasoline distribution chain, from the consumer and all way back to the storage terminal:
When the consumer fills up his vehicle with gasoline at the service station, about 1.75 litres of liquid gasoline is lost as evaporated VOC per 1,000 litres pumped through the dispenser. By installing the so-called “Stage II” equipment in the service stations, the VOC saturated air displaced from the consumers fuel tank, passes back to the service stations gasoline storage tanks; the liquid gasoline and the VOC saturated air simply swap their volume i.e. balancing. The Stage II equipment can be either of the passive or the active type, and have a VOC emission reduction efficiency of 60-90%.
With the passive type, the consumer seals a vapour nozzle combined with the liquid filling nozzle, against the fuel tank nozzle on the vehicle, and the VOC loaded air is pressed out of the fuel tank and into a closed piping system, by means of the hydraulic pressure created by filling liquid into the fuel tank. At the same time, the liquid gasoline leaving the service stations gasoline storage tank and being pumped into the vehicle, creates a slight vacuum in the storage tank. This vacuum will together with the pressure in the vehicles fuel tank, overcome the pressure loss in the vapour piping, when the VOC loaded air is passed from the vehicle to the gasoline storage tank.
In the active type Stage II equipment, a vacuum pump installed in the gasoline dispenser is synchronized with the gasoline pump flow rate, drawing the vapour from the vehicle fuel tank back to the service stations gasoline storage tank.
As the VOC loaded air passed to the service station gasoline storage tank is saturated, no further evaporation will take place here, and a loss of 1.4 litres of every 1,000 litres stored, is eliminated from the displacement when filling the storage tank.
When a truck delivers gasoline to a service station, the truck driver will connect two hoses to the nozzles on ground. One for the product to flow to the service station storage tank, and one connected to the vapour vent system of the storage tank. The gasoline flows into the storage tank, creating a pressure, which will cause the VOC loaded air in the storage tank, to pass through the vent manifold, into the connected hose and the compartments of the truck. The vacuum in the compartments of the truck, generated from the liquid flowing out, assists to overcome the pressure drop in the vapour piping, and the balancing of VOC loaded air with liquid gasoline takes place smoothly.
The truck brings VOC saturated air back to the terminal, and when it is filled with product, this VOC air mixture will be pressed into a vapour manifold. Here it will either be balanced into the storage tank from where the liquid is pumped to the truck, or, if the storage tank is equipped with internal floating blankets, passed into a VRU, where the VOC is separated from the air, and recovered. Again, the air is saturated with VOC, so no further evaporation of the gasoline will take place in the trucks compartments during the loading operation, and an evaporation loss of approx. 0.4 litres per 1,000 litres pumped into the truck is avoided. If the VOC saturated air is passed to a VRU at this stage, the recovery outcome will be approx. 1.6 litres per 1,000 litres filled into the truck!
It will in most gasoline distribution depots be economical very attractive to balance the vapour from the loading gantries back to the storage tank. The depot owner/operator will not only avoid to invest in the internal floating blankets, and the expensive maintenance of the seals of these, the investment and operation costs of a VRU will be much lower than if the VRU shall be designed to cope with the peak loading rates in the gantries. Typical the filling rate of the storage tanks are only fractions of the peak loading rates to the gantries, which are the dimensioning factors of a VRU. In both cases the displacement loss of approx. 1.3 litres gasoline per 1,000 litres stored in the tanks, is avoided.
The result of a complete vapour balancing and recovery in the gasoline distribution chain is a recovery of 1.6 litres of gasoline per 1,000 litres handled, and prevention of an evaporative loss of almost 4 litres of gasoline per 1,000 litres handled.