Computational approach for membrane desalination system development

• Main Author: Mo, Zijing
• Other Authors: She Qianhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Reverse osmosis (RO); System optimizations; Optimization and machine learning; Water treatment technology; Energy efficiency; Dynamic simulation; Brine resource recovery; Sustainable desalination; Semi-closed reverse osmosis; Closed-circuit reverse osmosis
• Online Access: https://hdl.handle.net/10356/181806
• Institution: Nanyang Technological University

Along with surging population growth, the rapid pace of urbanization and industrialization were observed in many developing economies, resulting in a burst in water demand. As predicted by the World Bank Group, by 2030, the world would face a 30% of water shortage if the current trend on demand and supply-side remain unchanged. Therefore, an increased amount of resources should be devoted to the strategic options that can close the gap between water supply and demand. While RO is the most viable solution to closing such gap, it remains energy intensive which hurdle its wide application. The primary objective of this study is to improve energy efficiency in reverse osmosis (RO) designs for desalination, focusing on economically feasible solutions. Despite advancements in alternative RO designs, concerns persist, hindering their widespread adoption. This study addresses these concerns and knowledge gaps to optimize RO system configurations and reduce energy consumption. The study begins with a comprehensive investigation of existing alternative RO designs, highlighting their features, advantages, and disadvantages as delineated in Chapter 2. While some designs, such as Multiple-Stage RO (MSRO), exhibit superior performance in theoretical scenarios, they come with practical hurdles, such as reduced energy recovery efficiency and increased capital costs. Mixing effects compromising efficiency are observed in other designs like Batch RO (BRO), Closed-Circuit RO (CCRO), and CaptuRO. Hybrid RO processes, specifically Split-Feed Osmotically Assisted RO (SF-OARO) and Low-Salt-Rejection RO (LSRRO), show promise in reducing applied pressure for high salinity, higher recovery desalination applications. However, they introduce extra complexities and inefficiencies such as mixing and internal concentration polarization. Chapters 3, 4, 5, and 6 delve into the system optimization of RO designs. Chapter 3 focuses on the efficient hybrid RO design, SF-OARO. Operating parameters are optimized to reduce energy consumption, emphasizing its advantage in high-recovery Brine Volume Minimization (BVM) scenarios. An 18% energy saving and 4.1% cost savings are observed with SF-OARO compared to conventional SSRO at 65% recovery. Despite its superior performance at high recovery, OARO still lacks the ability to reduce overall energy consumption in the typical recovery range (50%) due to significant mixing and ICP effects. To explore potential RO solutions with reduced energy consumption, this study revisits standalone rather than hybrid RO designs due to their simpler transport processes and the avoidance of ICP. An existing advanced RO solution, CCRO, is investigated in Chapter 4, which consists of the more studied closed-loop (CL) period and the less focused plug-flow (PF) period. A time-dependent finite-difference model is developed to comprehensively assess the energy consumption of CCRO. The results indicate that, although varying the applied pressure helps reduce the energy consumption of CCRO compared to conventional designs, intrinsic inefficiencies during the PF period, which were overlooked in previous studies, compromise its practical efficiency. Therefore, compared to a 25% (0.41 kWh/m³) energy saving indicated by theoretical estimation, a reduced saving of 6% (0.15 kWh/m³) is observed by the model for practical CCRO compared to SSRO. To circumvent the intrinsic inefficiencies in existing RO alternatives, a novel Single-Staged Multiple Cycle RO, Semi-Closed Reverse Osmosis (SCRO), is introduced in Chapters 5 and 6. SCRO balances economic practicability and energy efficiency, avoiding over-pressurization without requiring additional stages. Featuring a concise design and flexible operation, SCRO demonstrates promising results in mitigating over-pressurization and outperforms other tested RO processes in low-energy desalination. Results indicate a theoretical 32% (0.53 kWh/m³) and module-wise 9% (0.15 kWh/m³) energy saving with SCRO compared to SSRO, outperforming other RO designs To accurately assess and optimize energy consumption, simulation methods, including both hard and soft computational approaches, are employed. The findings presented in this study offer valuable insights into enhancing RO energy efficiency, paving the way for more sustainable and economically viable desalination processes.