Improved water-evaporation efficiency with helium sparging in a hot-bubble pilot plant

• Helium sparging in a hot-bubble pilot plant enables evaporation-efficient desalination. • 28% helium in air–helium mixtures triples evaporation efficiency vs using just air. • Molecular-dynamics show helium gas atoms weaken H-bonds near gas/water interface. Helium sparging can greatly improve the evaporation efficiency in sub-boiling desalination processes compared to air sparging, even at relatively low operating temperatures between 40–80 °C. This effect apparently occurs because heated helium gas atoms can more easily penetrate the gas/water interface and weaken the hydrogen bonds in water clusters near the interface, thereby increasing the local evaporation efficiency. This mechanism is supported for the first time by density-functional theory/molecular-dynamics calculations, establishing that small-sized helium molecules can disrupt the hydrogen-bond structure more easily and are better able to penetrate the bulk water. By measuring the water production rate and the conductivity of the NaCl solution before and after bubbling, the increased evaporation efficiencies at different volume percentages of helium-air mixtures and inlet-gas temperatures of 90 °C, 120 °C and 147 °C were obtained. Accurate estimates of the evaporation efficiency and energy demand of the bubble desalination processes were obtained for a large-scale hot-bubble pilot plant (HBPP). Issues associated with the use of helium and its recycling are addressed. The results suggest that only 28 % helium in air-helium mixtures is enough to improve the evaporation efficiency significantly. Compared to using just air, helium sparging in a large-scale HBPP could greatly improve evaporation efficiency (about 3 times) and offer an energy-efficient method for seawater desalination that can be operated at reduced costs on an industrial scale.