- Title
- Freezing of supercooled drops of salt solutions for desalination applications
- Creator
- Barma, Milan Chandra
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis is concerned with freezing of small, supercooled drops of NaCl solutions with the aim of reducing the concentration of salts in water. The main objectives of this study are to gain an improved understanding of ice nucleation and freezing of small volumes of saline solution, and fully evaluate the performance of a drop-based Freeze Desalination (FD) process. This work is motivated by the exigent need to improve the production rate and salt removal efficiency of conventional FD process. To gain a fundamental understanding the study starts with the investigation of ice nucleation and freezing temperature of supercooled drops of saline solution (SDSS). Experiments were conducted by varying drop diameter and salt concentration in the ranges of 7.9 – 17 mm and 0 – 6 wt%, respectively. To preclude contamination or any disturbance from the surroundings, each drop was suspended at the interface of two immiscible liquid layers (1, 2-Dichloroethane and Paraffin oil) with specific gravities higher and lighter than the drop, respectively. It was found that the freezing of SDSS was stochastic and occurred at a range of temperatures; this feature was more pronounced for smaller drops and higher NaCl concentrations. The probability of freezing a drop increased with decreasing temperature or salt concentration. However, the median freezing temperature of SDSS increased towards bulk solution freezing temperature when increasing the drop size. For a higher NaCl concentration, the freezing temperature increased with a greater rate as drop size increased. The freezing temperature reached a plateau for drops larger than 13.5 mm; hence, the drop diameter 13.5 mm may be viewed as the critical size that defines whether a discrete volume of salt solution can be considered as a drop or the bulk solution. The freezing temperature of SDSS mainly depends on salt concentration and ice nuclei particles contained in the drop. However, it proves tedious (or sometimes not practically viable) to characterise the detailed information of ice nuclei particles in bulk water, let alone in drops. A theoretical model framework was thus developed based on the existing water activity theory of salt solution (Koop et al., 2000, Nature 406: 611) to predict the freezing temperature of SDSS as a function of salt concentration (𝐶𝑠) and drop size (𝐷𝑑), which are handy information and can be readily obtained in the operation of practical systems. The main assumption is that as the size of relatively large drops decreases, only the number concentration of ice nuclei changes. The horizontal shift of water activity (𝑎𝑤) for a given drop size was calculated based on the measured freezing temperature of water drops of the same size. The freezing temperature (𝑇𝑓) of SDSS was subsequently predicted using a family of curves in the 𝑇 − 𝑎𝑤 plot. Good agreements were obtained for relatively large drops and low salt concentrations. For small drops and/or high salt concentrations, the model’s assumption no longer holds as ice nuclei types in the drop also vary as drop size reduces. In such cases, the freezing temperature of SDSS can no longer be described merely by drop size and salt concentration. Therefore, due care is needed when applying the model to small drops or high salt concentrations, quantitatively described as a region that satisfies 𝐷𝑑 < −0.1𝐶𝑠^2 + 1.5𝐶𝑠 + 6.5. In the second phase of the study, the parametric investigation of drop-based FD process was conducted by freezing SDSS over a wide range of operating conditions. Effects of system key parameters including freezing temperature, freezing time, drop size and salt concentration on the desalination performance were examined. Partial freezing was achieved when SDSS were frozen at a relatively high temperature (> -7.5 °C). The ice recovery (%) was linearly proportional to the freezing time but inversely proportional to the freezing temperature. However, the salinity reduction (%) reduced linearly as the ice recovery (%) increased, with drops of higher salt concentrations showing the least salinity reduction (%). The higher ice growth rate experienced by such drops is believed to be responsible for the observed lower salinity reduction (%). The extended filtration duration effectively reduced the salt concentration in product ice but at the cost of the ice recovery (%). It was found that irrigation water was produced from 3.5 wt% NaCl solution after four consecutive freezing-melting cycles. The drop-based FD process, performance-wise, was found competitive with other crystalliser-based FD processes. However, to retain a number of suspended drops at the liquid interface requires due care and the separation of liquids after extracting ice particles is the major challenge. As ice crystallisation progresses not on a fixed surface but at the liquid interface, the presented drop-based FD process has the potential to run continuously.Therefore, in the last phase of the study, the desalination performance of the drop-based FD process in an axisymmetric tube was investigated emulating the continuous operation mode. Specifically, a SDSS was generated by injecting salt solution into a vinyl tube that was pre-filled with paraffin oil. The injected salt solution has a much greater volumeequivalent diameter than the tube diameter, thereby must adapt its shape to fit the confined space to form a slug. The spontaneous freezing temperature of the SDSS inside the tube was generally under -10 ˚C, at which ice nucleation in a drop often leads to formation of spongy ice, which is not desired for the FD process. Therefore, external stimulation via applying power ultrasound was employed, aiming to induce ice nucleation and freeze the SDSS at a relatively high temperature and provide a closer control on an otherwise stochastic phenomenon of nucleation. The performance of the ultrasound-assisted freeze desalination process was examined in terms of salinity reduction (%) and ice recovery (%). The salinity reduction (%) was found to increase as ultrasound irradiation temperature and coolant temperature were increased. Similarly, the conditions required for greater salinity reduction were found to adversely affect the recovery of ice. Moreover, six consecutive freezing-separation-melting cycles were required to produce potable water through ultrasound assisted drop based FD from a 3.5 wt% salt solution. The performance of the ultrasound-assisted FD ranked highly amongst the existing crystallizer-based FD processes. The study provided data pivotal to the successful design and operation of drop-based FD process. For example, the freezing temperature of SDSS dictates the design (capacity) of the cooling system. The map of freezing state identifies the temperature range required for achieving partial freezing of saline drops that proves to be the pre-requisite for dropbased FD processes. The research on the cycles of freezing-separation-melting to produce irrigation or potable water provide the critical data to inform the practical operation of drop-based FD processes. Moreover, the theoretical model framework could be adopted as a powerful tool for predicting the freezing temperature of SDSS for larger drops for design and operational purposes. Future work including the development and application of advanced model techniques (e.g. Molecular Dynamics simulation) for understanding the ice structure growth and evolution at micro- or mesoscales, optimisation of the system performance (e.g. variation of salinity reduction versus the amount of washing water), and scale-up of the system for large-scale industrial applications, is also recommended.
- Subject
- freeze desalination; drops; freezing temperature; partial freezing
- Identifier
- http://hdl.handle.net/1959.13/1494532
- Identifier
- uon:53824
- Rights
- Copyright 2021 Milan Chandra Barma
- Language
- eng
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