Influence of operating parameters on separation performance of air gap diffusion distillation for recovering ammonia from ammonia-nitrogen wastewater

This study proposes air gap diffusion distillation (AGDD) for recovering ammonia from ammonia nitrogen wastewater. The influences of several key parameters on the AGDD performance have been explored. The findings indicate that increasing the feed temperature improves the concentration factor (CF) and reduces the specific thermal energy consumption for ammonia recovery (STEC-N). Cooling water temperature on ammonia flux is higher than that of water flux. An increase in feed flowrate attenuates the boundary layer, substantially enhancing the ammonia flux relative to the water flux and boosting the CF. The effect of feed concentration on total flux is determined to be insignificant. When the air gap thickness is less than 8 mm, molecular diffusion dominates the mass transfer in the air gap, the water flux and ammonia flux decrease with the increase of the air gap thickness, and the CF reaches a peak of about 5.7 near 5 mm. However, when the air gap thickness is greater than 8 mm, natural convection dominates the mass transfer in the air gap. This weakens the negative impact of increased air gap thickness on total flux. This leads to a certain increase in CF, which is more pronounced at higher temperatures. Finally, a dimensionless equation for the mass transfer within the air gap was determined from temperature measurements at different spatial locations within the AGDD unit.