Scaling trend of coal chemical saline wastewater in falling film evaporator: experimental solid–liquid equilibrium and numerical simulation

Abstract

It was important to establish the scaling mechanism to slow down scaling, which is one of the important problems faced by saline wastewater treatment with falling film evaporators. In this paper, the phase equilibrium data of ternary system NaCl-CaCl2-H2O at 303.15–373.15 K were measured, and the prediction of the flow characteristics and scaling tendency of salt-containing wastewater in the evaporation tube was investigated by numerical simulation based on the phase equilibrium results. The results showed that the mesostable phase diagram of the ternary system was of simple type, and w(NaCl) and w(CaCl2) showed regular variation with temperature when w(NaCl) in the system was 15–28%. Moreover, the components of the four saline wastewaters (10% NaCl + 9, 10, 11, 12% CaCl2) with less salt content had a faster heating and a faster rate of evaporation of the liquid film in the evaporation tube. Based on the solubility in the experiments and simulations, it was concluded that NaCl crystals were precipitated at tube lengths of 718 mm, 942 mm, 966 mm, and 1,000 mm for four salt concentrations in the evaporation process by comparison, and the reliability of the predicted loci was also demonstrated by a falling film evaporation experimental setup. HIGHLIGHTS Creatively predict scaling site based on phase equilibrium and fluid simulation.; Systematic study of multi-temperature phase equilibrium data for ternary systems.; Fine analysis of the flow characteristics of the brine in the evaporation tube.;

Keywords

• falling film evaporation
• numerical simulation
• phase equilibrium
• saline wastewater
• salt scaling