ABSTRACT

Horizontal tube falling film evaporators are commonly used in multiple-effect distillation 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.

摘要

水平管降膜蒸发器常用于海水淡化的多效蒸馏装置。蒸发器管上的水垢形成很大程度上取决于薄膜流动特性。水平管上的液膜厚度在试验台上用高分辨率光学测微计测量。此外，在中试规模的水平管降膜蒸发器中用人造海水研究了结垢。研究了聚氧化烯三嵌段共聚物表面活性剂在不同润湿速率下对降膜流动和结垢形成的影响。管子底部的膜厚比顶部的膜厚高一个数量级。最薄的薄膜形成在管侧。平均膜厚度随着润湿速率的增加而增加。管子顶部的水垢形成比底部的强。在管侧测量最高氧化皮厚度。水垢质量和水垢层厚度随着润湿率的降低而增加。随着最小和平均膜厚的增加和表面波在表面活性剂的存在下大部分被阻尼，传质阻力更高，因此水垢形成减少。