Application of hybrid membrane technology to recover resources from concentrates
Creators: name: Zhang, Xin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400409
subject: Separation technologies

description: The aim of this PhD study is to evaluate and optimize Membrane Distillation Crystallization (MDC) as a sustainable solution for resource recovery from desalination brine. Seawater desalination is increasingly studied to combat freshwater scarcity. However, disposing of the highly saline brine poses several environmental risks. To mitigate this, alternative approaches aiming to recover valuable resources from desalination brine have been conducted. The resources that can be recovered have been studied in Chapter 2, which are minerals and freshwater. The techniques to recover minerals can be divided into pressure-driven techniques, thermal-driven techniques, electro-driven techniques and other techniques while the water recovery techniques employ mainly membrane/thermal integrated hybrid processes. The valuable mineral products have also been reviewed in terms of recovery methods, performance of processes and product quality. The reviewed products are sodium salts (NaCl, NaOH, Na2SO4), lithium salts (LiCl, Li2CO3), magnesium salts (struvite, Mg(OH)2, MgSO4, MgO), calcium salts (CaSO4, CaCO3) and other minerals (U, Rb, Cs). Based on the cost and revenues of each technique, an economic comparison has been conducted along with the cost analysis of operating desalination plants.

MDC is an emerging hybrid thermal membrane technology that synergizes membrane distillation (MD) and crystallization, which can achieve both freshwater and minerals recovery from high concentrated solutions. Due to the outstanding hydrophobic nature of the membranes, MDC has been widely used in numerous fields such as seawater desalination, valuable minerals recovery, industrial wastewater treatment and pharmaceutical applications, where the separation of dissolved solids is required. Despite the fact that MDC has shown great promise in producing both high-purity crystals and freshwater, most studies on MDC remain limited to laboratory scale, and industrializing MDC processes is currently impractical. Chapter 2 summarizes the current state of MDC research, focusing on the mechanisms of MDC, the controls for MD, and the controls for crystallization. Additionally, this chapter categorizes the obstacles hindering the industrialization of MDC into various aspects, including energy consumption, membrane wetting, flux reduction, crystal yield and purity, and crystallizer design. Furthermore, this chapter also indicates the direction for future development of the industrialization of MDC.

Quantitative findings from this research include the development of a new model in Chapter 3 that predicts membrane surface temperatures, permeate flux, and salinity variations. Using the model, impacts of operating parameters such as bulk temperature, flow rate, different membrane materials and feed salinity on the permeate flux as well as the efficiency of driving force were simulated. The results of the simulations indicate that feed temperature, feed flow rate and membrane pore size have the most significant influence on the permeate flux. However, the feed and permeate flow rates have the highest impacts on the temperature polarisation coefficient (TPC) which has the highest value of 0.68 under certain conditions. Besides, the simulations predicted at the saturation point, the seawater and desalination brine have a water recovery of 86.8% and 72.1%, respectively. Finally, the salinity variations with operating time were also simulated and the new model would be vital for the salinity control in desalination processes such as MDC.

Chapter 4 presents experimental results from using a bench-scale MDC to recover high-purity NaCl crystals. In this chapter, crystal samples produced from a bench-scale MDC system have been analysed for crystal morphology, crystal purity and crystal size distribution (CSD). In addition, orthogonal fractional factorial (OFF) experimental design has been introduced to evaluate the influence of various operating conditions on the crystal size and CSD. Feed temperature (40 to 60 °C), crystallization time (60 to 180 min) and the feed composition have been used as the control factors. The results show that the MD in the MDC system maintains a consistent flux of 1.32 to 4.2 kg/m2·h across various feed compositions; higher feed temperatures led to the formation of larger crystal clusters, while longer crystallization time yielded more cubic-shaped crystals; an overall NaCl purity exceeding 99.08% has been achieved throughout all the experiments; and the increased feed temperature and prolonged crystallization time results in a narrower CSD. The result of OFF experimental design identifies feed temperature as the dominant factor affecting crystal size and CSD. However, the presence of impurities in the feed water could pose challenge to the thermal stability and longevity of the membrane.

In conclusion, this study not only confirms the potential of MDC as a sustainable approach for resource recovery from desalination brine but also highlights the critical operational, design, and economic factors that must be addressed for its industrialization. The implications of this research are profound, offering a path towards more sustainable desalination practices and contributing to the broader goal of water security and environmental protection. The quantitative insights and proposed directions for overcoming technological barriers provide a solid foundation for future advancements in the field of membrane technology.