Experimental study on equilibrium waterfilm concentration in static flash evaporation of aqueous NaCl solution

• An experimental system of static flash evaporation was built.
• Equilibrium waterfilm concentration was significantly influenced by superheat.
• A peak value of waterfilm concentration increment existed.
• Minimum crystallization superheat was affected by initial concentration or separating height.
• A calculating formula of equilibrium waterfilm concentration with an empirical constraint was set up.

Experimental study on equilibrium waterfilm concentration in static flash evaporation of aqueous NaCl solution was presented. Equilibrium waterfilm concentration was defined as the concentration of aqueous NaCl solution in a flash chamber at equilibrium time after each flash. Experiments were carried out with the initial waterfilm concentration varied from 0 to 0.264 (saturation), separating height from 0.115 to 0.415 m, and superheat from 1.8 to 43.4 K. Results indicated that both evaporation and steam-carrying effect influenced the change of equilibrium waterfilm concentration. Equilibrium waterfilm concentration gradually increased with rising superheat until saturation. Meanwhile, minimum crystallization superheat was defined as the minimum superheat that made NaCl crystals separated out from waterfilm during the process of static flash evaporation when the equilibrium waterfilm concentration was in a saturated state. Results showed that the minimum crystallization superheat decreased with the rising initial waterfilm concentration or the decreasing separating height. Finally, a calculating formula of equilibrium waterfilm concentration with an empirical constraint was set up, and the relative error between experimental and calculating equilibrium waterfilm concentrations varied between − 14.6% and 8.7%.