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AN INNOVATIVE MEMBRANE DISTILLATION-CRYSTALLIZATION PROCESS FOR BRINE TREATMENT
AdvisorHiibel, Sage R
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Water scarcity is an issue that has emerged during recent decades and it is attributed toincreased water consumption due to rapid population growth and to climate change around the world. Desalination technologies have drawn the attention of researchers and industries as a possible solution to this problem. However, the disposal of brine byproduct to surface waters can have negative environmental impacts as brine contains chemicals from the desalination treatment and it increases the salinity of the water bodies in the disposal zone. For this purpose, a novel membrane distillation-crystallization (MDC) system with a six-tray cascading crystallizer was developed that can recover both clean water and solid salts from the desalination brines. This novel MDC system can be utilized for the treatment of reverse osmosis (RO) brine due to the advantages that it has over conventional and other alternative brine treatment technologies. These advantages are primarily related to the six-tray cascading crystallizer, which provides nucleation sites and support for crystals to grow outside the solution/air interface while having a small footprint. In addition, due to the low operating temperatures of the process (40-80 °C), the heating requirements of the system can be met with low-grade waste heat sources. The effectiveness of the current MDC system in recovering clean water and solid salts from highly concentrated solutions was tested with single-salt brines (sodium chloride (NaCl), potassium chloride (KCl), and sodium nitrate (NaNO3)). The comparison of MDC and membrane distillation (MD) under the same operating conditions showed that higher total water recovery and brine volume reductions were achieved with the MDC system. In addition, the crystallizer demonstrated the ability to recover all three solid salts outside of solution on the extended mesh in each tray of the crystallizer. The highest solid salt recovery was observed with NaNO3, which has the strongest temperature-solubility relationship. Once the MDC system was validated, the focus shifted to further exploring the crystal formation on the mesh materials of the crystallizer. The experiments evaluated the effect of material coverage, surface roughness, and the solution/air interface on amount and size of the solid salts recovered. Two common 3D-printed mesh materials, polyactic acid (PLA) and acrylonitrile butadiene styrene (ABS), were tested under stagnant and flowing conditions using a flow-through crystallizer. The experiments with the stagnant NaCl solution setup revealed that increased solid salt recoveries were achieved by combining a material coverage that provides a balance of material support and open space for crystals to form and grow outside the solution/air interface; mesh materials with a material coverage of 52.24% had the highest solid salt recoveries for both material types. Surface smoothness on the PLA meshes did not have any significant effect on solid salt recovery, whereas in the case of the ABS meshes the solid salt recovery decreased compared to the untreated mesh. Finally, the experiments with the flow-through crystallizer showed that by increasing the amount of solution/air interfacial area, the solid recovery increased. A potential mechanism that explains the crystal growth pattern on the mesh from the solution/air interface towards the non-immersed region of the mesh was also developed. In literature, continuous crystallizers and MDC systems have demonstrated the ability to recover solid salts from brines; however there have been limited studies where the focus was on the salt selectivity. Since the crystallizer consists of six trays that operate under different temperatures, its ability to perform selective salt removal was tested for two different brine mixtures. MDC setup was used with a 50% KCl and 50% NaCl mixed salt solution, while the stand-alone crystallizer was tested with a mixture of sodium carbonate (Na2CO3) and KCl with both salts at an initial concentration of 80% of their 20 °C solubility. In addition, a simple model was developed to predict the maximum amount of salts that may precipitate in each tray of the crystallizer based on their solubility at each temperature. Based on the experimental results and the modeling predictions, the crystallizer is able to selectively remove salts whose solubility increases with temperature, such as KCl.