Blending in above ground storage tanks with side-entering agitators

Abstract

The blending of fluids in large, above ground storage tanks is widely practiced in the petroleum industry yet there have been very few studies of this mixing operation. The most demanding application is found when the tank contents have been allowed to stratify creating a light and heavy layer. The blend time in this case is defined as the time taken for the density of the fluid to become axially homogeneous.Wesselingh (1975) measured blend times using brine and water to produce the heavy and light layers and measured conductivity changes to assess the blending process. He only looked at one propeller geometry but did study the effects of several important geometrical system properties, such as the ratio of propeller to vessel diameter and liquid depth to vessel diameter.The study reported here uses Wesselingh’s technique but examines the blending performance of four commercially available propellers. The results show that the Advanced Pitch propeller sold by Philadelphia Mixing Solutions Ltd. and Mixing Solutions Ltd. is significantly more efficient than the competitors’ propellers when compared based on energy usage.These results, and Wesselingh’s, show that there are two operating regimes which are separated by a critical Froude number, FrC. At high Froude numbers (Fr>FrC) the dimensionless blend time is constant, and at lower Froude numbers (Fr<FrC) the dimensionless blend time is inversely proportional to Froude number. The implications for scale-up in both regimes are discussed.