Effects of Film Flow and a Surfactant on Scale Formation in Falling Film Evaporators for Seawater Desalination

Abstract

Horizontal tube falling film evaporators are commonly used in multiple-effect distillation (MED) plants for seawater desalination. Scale formation on the evaporator tubes strongly depends on film flow characteristics. The thickness of the liquid film on a horizontal tube was measured in a test rig with a high-resolution optical micrometer. Furthermore, scale formation was studied with artificial seawater in a horizontal tube falling film evaporator at pilot plant scale. The effects of a polyoxyalkylene triblock copolymer surfactant on falling film flow and scale formation were investigated at different wetting rates. Film thicknesses on the bottom of the tube exceed film thicknesses on the top by an order of magnitude. The thinnest film is formed on the tube sides. The mean film thickness increases with increasing wetting rate. The scale formation on the top of the tube is stronger than that at the bottom. The highest scale thickness was measured at the tube sides. Scale mass and scale layer thickness increase with decreasing wetting rate. As the minimum and the mean film thickness increase and surface waves are mostly damped in presence of the surfactant, the mass transfer resistance is higher and, thus, scale formation is reduced.