To analyze the humidification of air in a bubble column, a humidification-dehumidification test setup for thermal desalination has been built. Because the humidifiers are made of transparent acrylic glass, it is possible to capture and analyze bubble sizes. By carrying out measurements where the water temperature, the liquid height, the superficial gas velocity and the bubble size are varied, we want to gain a deeper knowledge of the humidification and the parameters influencing it. Our results will be especially important for thermal desalination plants and for industrial waste water treatment.
As fresh water sources deplete globally, alternative water treatment systems have grown in importance. The most widely used desalination processes, reverse osmosis and multi-stage flash distillation, are technologically demanding and only economically feasible when operated on a large-scale. In remote, off-grid regions small-scale, standalone water treatment solutions are in great demand. The humidification-dehumidification process (HDH) is a promising approach for producing fresh water locally, since it uses renewable energy or waste heat as the heat source. HDH has a low technological demand and can be used for seawater desalination purposes, for water purification and for extracting water out of oil-water emulsions enabling the recycling of the oil. This process is based on earth’s natural water cycle, where air is humidified through contact diffusion. The moist air is subsequently cooled down, forming clouds and eventually, rain.
In recent studies, bubble columns have been used as air humidifiers as they have high heat and mass transfer coefficients and do not encounter fouling since they involve direct contact humidification. To increase the efficiency of bubble column humidifiers, a clearer understanding of the humidification is necessary.
With our setup, we plan to perform measurements with fresh water, seawater and with an oil-water emulsion. In these measurements the liquid temperature, the liquid height, the superficial gas velocity and the bubble size are varied. The bubble size distribution is determined by digital image analysis of high speed and compact system camera captures. The overall productivity of the system is measured via the amount of condensate produced, and the efficiency of the bubble column humidifier is determined by calculating the water vapor content difference between the inlet and the outlet of the humidifier. Based on these measurements, it is planned to deduce semi-empirical correlations describing humidification as a function of the various parameters. These correlations will help improve the design and efficiency of bubble column based HDH-systems for desalination and water purification.
E. Eder, M. Preißinger, Experimental analysis of the humidification of air in bubble columns for thermal water treatment systems, Experimental Thermal and Fluid Science, 115, Elsevier, https://doi.org/10.1016/j.expthermflusci.2020.110063
M. Preißinger, Bubble Columns in Humidification Dehumidification Technology: From a Demonstration Unit to Fundamental Research in Optical Accessible Laboratory Bubble Columns, in: R. Riehl, M. Preißinger, I.W. Eames, M. Tierney (Eds.), HEAT POWERED CYCLES 2018 Conference Proceedings, Bayreuth, 2018: pp. 44–50.
M. Preißinger, Bilge water treatment and desalination based on HDH technology: an experimental investigation of a demonstration plant, Desalination and Water Treatment, Volume 127, 22. May 2018, doi: 10.5004/dwt.2018.22532, http://www.deswater.com/vol.php?vol=127&oth=127%7C0%7CSeptember%20%20%7C2018