A study of the improved evaporation efficiency of a bubble column evaporator for water treatment

The bubble column evaporator (BCE) is an effective thermal water treatment method based on the mass and heat transfer from the continuously generated heated gas bubbles to the aqueous solution in the BCE column. In this research project, helium has been chosen as the inlet gas to evaporate water from a 0.5 m NaCl solution in a BCE system. The enhanced evaporation efficiency and the reduced enthalpy of vaporization (△Hvap) achieved by helium sparging demonstrated the potential of using helium to improve the evaporation performance of the BCE, while reducing the energy cost. To further increase the working efficiency of the BCE, we added surfactants in the column solution, and the results show that the arrangement of the surfactant molecules on the interface of the bubbles can effectively increase the efficiency of water-vapour transfer into the bubbles. More water molecules can be drawn into the bubbles via the attraction by the hydrophilic heads facing the column solution side, while obstructing their return into the solution by the layer of hydrophobic tails coating the inside of the bubbles. The introduction of surfactants can therefore result in a “molecule diode effect”, apparently improving evaporation efficiency by enhancing the saturation of water molecules within the dry bubbles. The working mechanism of the helium sparge was then studied by contacting helium flow with the salt solution within a membrane contactor. The helium atoms can be sparged into the counter-flowing salt solution through the microscopic pores on the membrane surface. The electrical conductivity of the salt solution, before and after sparging helium flow, was measured and compared; with the increased conductivity demonstrating that helium sparge can effectively enhance the evaporation efficiency, by facilitating the disruption of hydrogen bonds in the water network. This observation corresponds with the experimental outcome that helium flow can result in a higher evaporation performance in the BCE process, compared with other gases at the same inlet temperature. Based on these laboratory results, the research on the BCE process has moved from the laboratory to a pilot plant. A small pilot plant based on the BCE process has been designed and constructed to treat different types of wastewaters, including synthetic seawater, synthetic sewage water and synthetic piggery effluent. The observed amount of evaporated water vapour for the column solutions demonstrated the potential of the BCE process to treat different types of wastewaters on a large scale. In addition, combustion gas was also tested as the inlet gas, for the first time, to treat 0.5 m NaCl solution in the BCE system. The evaporation efficiency increased by 37%, compared with air at the same inlet temperature, most likely due to the existence of water vapour contained in the combustion gas mixture, which increased the transfer of heat to evaporate the surrounding solution. The water quality analysis for the distilled water demonstrated the potential for using the BCE system to produce high-quality water for irrigation or drinking purposes, depending on the type of column solutions