Dissertation/Thesis Abstract

Theory-Guided Membrane Design for Efficient Desalination
by Deshmukh, Akshay, D.Eng., Yale University, 2019, 480; 27542430
Abstract (Summary)

Tackling water scarcity is one of the most important challenges of the 21st century. Current predictions suggest that over half of the global population will live in water stressed regions by 2050. Desalinating saline water sources, such as seawater and brackish water, is one way to alleviate water stress by augmenting freshwater supplies. However, desalination processes can be energy intensive, with the most efficient seawater desalination processes consuming 2.0 kWh of energy per cubic meter of pure water produced. Maximizing the energy efficiency of desalination process is particularly important given the reciprocal interdependence between water production and energy generation, often termed the water-energy nexus.

This dissertation focuses on developing analytical and numerical models to understand how membrane properties and process design affect the energy efficiency of membrane-based desalination processes, including reverse osmosis, membrane distillation, and forward osmosis. By improving our understanding of the factors that drive improved desalination performance, this work aims to guide future membrane development and quantify the potential for novel desalination processes to lower energy consumption. Reverse osmosis (RO) is currently the most efficient desalination technology, with the energy efficiency of modern RO systems reaching 50%. In RO, saline water is pressurized elevating before being contacted with a semi-permeable membrane. Driven by its chemical potential gradient, water partitions into and diffuses through the membrane while dissolved solutes such as sodium chloride are rejected. While previous reductions in the specific energy consumption (SEC) of RO have been driven by increasing membrane permeability, the impact of further increases will be minimal as other factors limit performance. Consequently, recent studies have focused on the development of novel batch and semi-batch processes that are capable of lowering the average excess driving hydraulic pressure applied across the membrane. By developing analytical and one-dimensional finite element models this work quantifies the potential for these novel process configurations to improve the SEC of RO. It shows that while batch and semi-batch processes are able to notable reductions in SEC, parasitic losses, such as friction in the membrane modules, will negate most energetic savings compared to deploying conventional multi-stage RO processes.

While RO membranes have been highly optimized over the last five decades, some mechanistic aspects of RO membrane formation remain poorly understood. Amongst these is the rough morphology observed in the selective polyamide layer of thin-film composite (TFC) membranes, which can have a root mean squared roughness (~ 100 nm) that is an order of magnitude greater than the film thickness itself (~ 10 nm). Previous studies have suggested that hydrodynamic instabilities may be destabilizing the reaction interface during interfacial polymerization as the polyamide layer forms. In this dissertation, a novel semi-analytical propagator based method is developed to solve linear stability problems in multilayer systems. The propagator formulation is used to perform a linear stability analysis of the Rayleigh-Bénard-Marangoni instability in a two-layer system accounting for surface deflection in addition to both buoyancy and surface tension based forces.

Membrane distillation (MD) is a thermal desalination process that has the potential to treat high salinity waters using low-grade or waste heat. By utilizing a partial vapor pressure difference to drive the evaporation and permeation of water through a hydrophobic membrane from a saline feed stream into a pure permeate stream, MD has the potential to recover high quality water from highly saline brines. However, the low energy efficiency of MD has thus far limited its use. In this dissertation, a numerical model is developed to determine the energetic performance of MD. The analysis presented demonstrates that the exergy efficiency of direct contact membrane distillation is limited to around 10% even in ideal conditions, due to the conduction of heat through the air trapped in the membrane pores. Conductive heat transfer also limits the potential for thinner membranes to improve performance. Furthermore, this work shows that transport through vapor-gap membranes can be limited by interfacial reflection, particularly for thin membranes, severely limiting the water fluxes attainable.

Overall, this work presented in this dissertation, identifies key factors that determine the impact of improvements in membrane and process design. By studying mass and heat transport through a wide range of desalination processes, including reverse osmosis, membrane distillation, forward osmosis, and capacitive deionization, this work aims to guide the future development of energy efficient desalination technologies. Ultimately, by identifying the fundamental causes of inefficiencies in a range of desalination processes this work helps optimize future membrane and process design. By developing novel and versatile semi-analytical methods this dissertation also endeavors to provide a platform for the analysis of the impact of Rayleigh-Bénard-Marangoni instabilities in multilayer systems.

Indexing (document details)
Advisor: Elimelech, Menachem
Commitee: Kim, Jaehong, Zimmerman, Julie
School: Yale University
Department: Chemical and Environmental Engineering
School Location: United States -- Connecticut
Source: DAI-B 81/10(E), Dissertation Abstracts International
Subjects: Environmental engineering, Chemical engineering, Water Resources Management
Keywords: Desalination, Linear Stability, Membrane Distillation, Membranes, Reverse Osmosis, Water-Energy Nexus
Publication Number: 27542430
ISBN: 9798607319861
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