624 Ingenieurbau und Umwelttechnik
Refine
Year of publication
- 2020 (2) (remove)
Document Type
- Article (1)
- Master's Thesis (1)
Institute
Language
- English (2)
Has Fulltext
- yes (2)
Keywords
The humidification-dehumidification process (HDH) for desalination is a promising technology to address water scarcity issues in rural regions. However, a low humidifier efficiency is a weakness of the process. Bubble column humidifiers (BCH) are promising for HDH, as they provide enhanced heat and mass transfer and have low maintenance requirements. Previous studies of HDH-systems with BCHs draw different conclusions regarding the impact of superficial air velocity and liquid height on the humidification. Furthermore, the impact of flow characteristics has never been investigated systematically at all. In this study, an optimized BCH test setup that allows for optical analysis of the humidifier is used and evaluated. Our test setup is validated, since the influence of water temperature on the humidification, which is exponential, is reproduced. Measurements with seawater show that the normalised system productivity is increased by about 56 % with an increase in superficial air velocity from 0.5 to 5 cm/s. Furthermore, the system productivity is increased by around 29 % with an increase in liquid height from 60 to 378 mm. While the impact of superficial air velocity can be traced back to temperature changes at the humidifier and dehumidifier outlets, the impact of liquid height is shown to be caused by a smaller heat loss surface in the humidifier with an increase in liquid height. For the impact of sieve plate orifice diameter, a clear influence on the humidification is not apparent, this parameter needs to be investigated further. Finally, our new test setup allows for analysing the humidification of air (1) in a systematic way, (2) in relevant measurement ranges and (3) in comparison with optical analyses of the flow characteristics.
The humidification dehumidification (HDH) cycle is a process for thermal water treatment. Many studies were carried out investigating operation of an HDH cycle with water and seawater as working liquid. Currently research into other areas of application is limited. Exchanging the working liquid in the humidifier from seawater to a water oil emulsion and investigating its behavioural changes is the basis for the expansion into applications such as bilge water treatment. This master’s thesis covers analysis of the behaviour of an HDH cycle operated with a water oil emulsion. The main elements are (1) proof of concept for operation of the HDH cycle with a water oil emulsion, (2) comparison of measurements and thermodynamic calculations, (3) investigation of the impact of operating parameters and (4) optical analysis of the bubbly flow in water and oil.
Operation of the HDH cycle using water oil emulsion was shown to be feasible with a small change to the setup previously used for investigations with seawater as working liquid. To keep the emulsion from separating into its individual parts, constant movement of the working liquid needs to be ensured. For this a magnetic stirrer was introduced into the bubble column humidifier (BCH) used. In a batch process an oil concentration of >97 % was reached without visible traces of oil in the produced condensate.
Comparison of the measured and thermodynamically evaluated productivity shows that measured productivity is higher. The proposed explanation for this is supersaturation of air at the BCH exit. Further investigation into this phenomenon is needed to confirm this hypothesis.
Influential parameters investigated are (1) liquid temperature, (2) superficial air velocity and (3) sieve plate orifice diameter. Increase of liquid temperature results in an exponential increase in productivity. At superficial air velocities up to 3 cm/s productivity increases with superficial air velocity. For superficial air velocities higher than 3 cm/s productivity plateaus. At low superficial air velocity, an increase of sieve plate orifice diameter results in increasing productivity. Further increase of the sieve plate orifice diameter inverses this phenomenon.
Bubbly flow in water and oil is influenced by the different viscosities of the liquids. Water creates small bubbles of similar size at low superficial air velocities. At superficial air velocities >2 cm/s turbulences start to increase and finely dispersed bubbles are present in the water. Bubbly flow in oil creates larger bubbles at all superficial air velocities. The airflow transitions to plug flow at velocities of 3 cm/s and above.
Result from this master’s thesis can be used for as a basis to broaden the understanding of the HDH cycle and find new areas of applications.