Differential absorption lidar (DIAL) measurements of the mechanisms of volatile organic compound loss from external floating roofed tanks

This work describes how an IR DIAL system has been sued both to measure the emission rates of Volatile Organic Compounds (VOCs) from external floating roof tanks, and in addition, to map the concentration of VOCs over the roofs of tanks. The data presented shows concentration iso-surfaces resolved at 0.2 ppm intervals over an operating 50 m diameter tank. This data will be used to validate subsequent models of tank emissions. The data suggests that when the floating roof is sufficiently low inside the tank a re-circulating air flow is established that extends for several meters into the open air above the tank rim. In this instance the emission rate of product from the tank is the rate of bleed off from the re-circulation zone into the free air stream. As the tank roof height is increased, it is expected that a point will be reached where the re-circulating zone shrinks to a small region near the rim and the majority of the tank roof is exposed to the free stream air flow. The existence of a large re-circulation zone when the roof is low is confirmed by the preliminary results of flow measurements above a tank roof.