Experimental study on heat and mass transfer of wet air and alkaline waste liquid over packings under Low-Pressure conditions for water purification

The heat and mass transfer characteristics between gas and liquid in packing materials under low pressure are important factors that influence the performance of vacuum humidification and dehumidification wastewater treatment system. However, research on the heat and mass transfer characteristics of wet air and alkaline wastewater on corrugated packing materials under low-pressure conditions is still very scarce. Therefore, this study aims to investigate the coupled effects of ambient pressure and other gas–liquid thermophysical properties on heat and mass transfer characteristics, in order to optimize the performance of wet air systems under low-pressure conditions. This study employed an experimental approach to examine the effects of various parameters, including vacuum degree (0–40 kPa), waste liquid flow rate (0.06–0.18 kg/s), waste liquid temperature (35.0–55.0 °C), inlet air velocity (0.8–1.6 m/s), air temperature (12.0–22.0 °C), and relative humidity (50–95 %) on heat and mass transfer characteristics. Experiments were conducted on corrugated packings, and the chemical oxygen demand and potential of hydrogen values of the separated liquid, as well as the production of clear water and heat and mass transfer coefficients, were measured. The results indicate that an increase in vacuum degree significantly reduces COD and pH of the separated liquid, attributed to the reduction of droplet entrainment. Moreover, higher vacuum levels enhance clear water production by decreasing the partial pressure of water vapor in the air, although an upper limit exists due to the vapor holding capacity of air. However, this increase in water production is accompanied by a reduction in sensible heat transfer efficiency (21.14–23 %), resulting from the decreased air density. The mass transfer coefficient exhibits a non-linear trend with vacuum degree, with waste liquid temperature and flow rate being the key influencing factors. This study highlights the critical role of ambient pressure adjustments in optimizing mass transfer performance. The findings provide valuable insights for improving the efficiency of wet air systems under low-pressure conditions and have significant scientific and practical implications for industrial applications such as wastewater treatment and environmental engineering.