Visualization study on NaCl crystal behavior on foamed ceramic surfaces used as hot stream channel of air-gap diffusion distillation

Abstract

Foamed ceramics (FCs) have been utilized in Air-gap diffusion distillation (AGDD) devices for desalination. However, during desalination experiments using 3.5 wt% saline water, precipitation of NaCl crystals on FC surfaces significantly hampers the AGDD device performance. In this paper, the NaCl crystal formation process and the experimental operating and FCs structural parameters effects on NaCl crystal growth are investigated. The results indicate two crystal types (“shell-shaped” and “fungus-shaped”) are formed on FC surfaces due to capillary diffusion and macroscopic fluid flow. Additionally, the “fungus-shaped” crystal growth process exhibits two self-regulation behaviors. Increasing the pore density and flowrate can weaken the crystal coverage rate (φ). However, the impact on crystal growth velocity (vc) is not substantial. The elevated feed temperature leads to an increase in the degree of temperature dispersion (DTD) and results in an augmented φ. The smaller the flowrate, the greater the influence of DTD on φ. Among three materials (SiC, Al2O3, and ZrO2), SiC FC exhibits the highest φ and crystal height (Hmax). The Hmax can reach 5.01–7.69 mm. However, no salt crystals appear on the ZrO2 FC (40 PPI) surface after 10 h of continuous operation. The results provide some guidance for improving the stability of AGDD.