Analysis of air humidification in a bubble column

Secure, affordable and resource efficient water supply systems will be as significant for future societies as energy systems. Therefore, thermal water treatment systems are necessary in the future. Humidification Dehumidification (HDH) technology, which works like the natural water cycle of the earth, is a promising process. The humidification of air on a warm surface like the ocean, the subsequent ascent of warm and humid air, the cooling in higher regions like the mountains and the related formation of fog or rain are realized in a technical process. However, humidification of air especially in a bubble column has just been insufficiently investigated in science so far. Hence, mathematic expressions for the relation between humidity of air, thermodynamic and geometric boundary conditions as well as physical- chemical properties of the liquid phase are not available. However, these expressions are necessary to fully understand thermal water treatment systems and to increase their efficiency significantly. Therefore, this project aims to establish a fundamental understanding of the humidification of air in a bubble column for thermal water treatment systems. Based on an extensive experimental data set, the aforementioned mathematical expressions will be deduced. To reach the defined goals, two experimental setups with two different fluid systems and two modes of operation will be developed, operated and analyzed within the project. The first fluid system deals with solar desalination. Therefore, salt water serves as liquid phase in the bubble column. The test series focus on the influence of thermodynamic and geometric boundary conditions. The second system covers processes for industrial waste water treatment. Hence, an oil/water-emulsion is applied as raw material and, subsequently, concentrated in a batch-wise process to get water and concentrated oil as product. The batch-wise operation results in an understanding of the influence of changing physical- chemical properties during the concentration process. The semi-empirical correlations for the humidification of air in a bubble column which will be deduced for the first time in this project will help to replace commonly applied assumptions with experimentally validated equations. This is necessary to reduce uncertainties during simulation and technology design, and to increase the efficiency of thermal water treatment systems based on HDH technology in the future.

In this research project, fundamental work was carried out to improve the understanding of the humidification process in a bubble column. The most important application of bubble column humidifiers is the desalination of seawater, which is becoming increasingly important due to the growing water scarcity. Conventional desalination processes are either thermal or membrane based. However, they also can't be operated with moderate production volumes, a low technical effort and powered by renewable energy sources. A process that potentially meets these criteria is humidification-dehumidification (HDH) desalination. It is based on the natural water cycle, in which an air stream is humidified in direct contact with seawater and subsequently cooled in a separate dehumidifier unit, enabling condensation. To increase the efficiency of this process, bubble columns have recently been proposed as a humidifier type. However, a lack of understanding of the humidification process in a bubble column has so far prevented appropriate design and operation of such systems. In this project, measurements of the humidification process were carried out on an experimental setup especially developed and optimized for this purpose. They were conducted for various operational settings, and for continuous and batch-wise operation. For desalination, the influence of the parameters liquid temperature, superficial air velocity, liquid height, and bubble size on both the humidifier efficiency and the system productivity were quantified. It is shown that the largest part of the humidification process takes place at liquid heights below 5cm. A contour plot was deduced based on the parametric measurements predicting humidifier efficiency for different liquid heights and superficial air velocities. It was shown that scientific research should focus on lower liquid heights and superficial air velocities. In addition to the desalination of seawater, the treatment of oily wastewater also represents an application that is increasingly important. Especially in the petrochemical industry, enormous amounts of oily wastewater are produced that can only be treated insufficiently. To investigate this application, the concentration of an oil-water emulsion was investigated. It was shown that the HDH process is insensitive to various liquid feeds and that the humidification is also very pronounced when operated with emulsions. In a batch process, it was possible to increase the oil content of the oil-water emulsion from about 55% to over 96%. It was also shown that the condensate produced has an oil content of less than 15ppm and is suitable for further disposal. A fundamental understanding of the humidification process in a bubble column was established in this project. The great potential of desalination as well as of oily wastewater accumulation using a bubble column humidifier was shown. Based on this research project, bubble column humidifiers can be dimensioned and operated more efficiently in the future.