Investigation on The Enhancement of Harvesting Blue Energy from Saline Water Through Reverse Electrodialysis (RED)

Abstract

The global water crisis is severe, with billions lacking access to clean water and sanitation. The Middle East, including the UAE, faces unique challenges due to its arid climate, population growth, and limited freshwater. To address this, the UAE has implemented a Water Security Strategy focusing on reducing water demand, increasing wastewater reuse, and improving emergency preparedness. In the United Arab Emirates, treated wastewater constitutes only 5% of the total water utilized. The primary sources of water are groundwater and desalinated seawater. Oilfield-produced water (PW), a relatively unexplored resource, presents a potential alternative.

Oilfield-PW represents the largest wastewater stream during oil and gas exploration, drilling, and processing, with an estimated global production of 41 million m3 day-1. The treatment process of the PW may cause environmental pollution and require a considerable amount of energy and resources. However, PW from Arabian Gulf oilfields have a wide salinity range (5 - 269 g/L), offering an opportunity to generate energy through salinity gradient power (SGP). This process, known as SGP, directly converts salt concentration variations into electrical energy. This energy can offset treatment costs, and the treated water can be reused, contributing to water conservation. Reverse Electrodialysis (RED) is one of the promising technologies for generating the SGP.

RED is a technology capable of generating electricity from salinity differences. Furthermore, RED can offer a clean and sustainable energy source while simultaneously treating complex wastewater with high salinity levels. Therefore, this dissertation investigates the feasibility of harnessing energy from the salinity gradient of synthetic oilfield-PW by utilizing RED. Furthermore, low energy efficiency and high membrane costs have limited the use of this technology on a large scale. Ion exchange membranes (IEMs) are key components of RED, enabling the selective transport of ions. These membranes possess fixed anionic or cationic exchange groups that facilitate the selective passage of ions based on their charge. Indeed, developing highly efficient and cost-effective IEMs is critical for scaling up RED to harvest energy from the water salinity gradient. Concurrently, the research is dedicated to developing IEMs exhibiting superior selectivity and efficiency.

Hence, the dissertation is structured into three distinct sections to accomplish these objectives. First, the effect of varying operating conditions of the RED bench scale process to achieve maximum power generation was examined. In addition, the effect of heavy metals (e.g., Ni+2 and Zn+2) and dissolved organic (e.g., phenol) on the performance of the RED was analyzed. Moreover, a theoretical study was conducted to show the feasibility of employing SGP technology to recover energy from the mixing of PW streams on a large scale. Specifically, it explores the potential of integrating SGP with nanofiltration (NF) and reverse osmosis (RO) treatment processes. Findings indicate that mixing PW streams with salinities of 150 g/L and 1 g/L can yield a maximum power output of approximately 8 kWh per barrel of oilfield-PW utilizing a system comprising 1000 cells and a 0.25 m² active membrane area. These results contribute significantly to the advancement of RED technology for sustainable oilfield-PW management.

The second phase of our work focused on developing cation exchange membranes (CEMs) with enhanced selectivity and minimal electrical resistance for optimal power generation. To achieve this, we carefully investigated and optimized the preparation conditions of sulfonated polyethersulfone membranes (SPES), including sulfonation parameters, casting thickness, and polymer concentration. Additionally, we comprehensively explored the impact of blending the SPES with natural polymers like Humic acid (HA). Furthermore, the incorporation of two-dimensional (2D) nanomaterials (e.g., montmorillonite (MMT), and MXene) was investigated. The unique properties of HA, stemming from its phenol and carboxylic acid functional groups, made it a promising candidate. Our findings revealed that the addition of HA significantly improved membrane selectivity and energy conversion efficiency due to hydrogen bonding and electrostatic interactions between its functional groups and the sulfonic groups of the polymer. Furthermore, the incorporation of nanomaterials facilitated the formation of nanofluidic channels, resulting in enhancing the charge selectivity and ion transport of the developed membranes through hydrogen bonding or electrostatic interactions between the polymer and the functional groups of MMT or MXene layers. The fabricated nanocomposite membranes demonstrated superior permselectivity (approximately 98%), energy conversion efficiency, and reduced internal resistance compared to commercial counterparts.

The third phase focused on measuring the effect of heavy metals and organic fouling on the developed membranes' performance. The developed membranes exhibited exceptional resistance to inorganic and organic fouling, surpassing commercial CEMs in monovalent ion selectivity and overall stability. Furthermore, the research presents a viable alternative to costly commercial membranes by offering ion-selective membranes characterized by low cost (50 $/m2), minimal membrane resistivity (0.003 µΩ.m), and high efficiency (49.5%). These technological advancements collectively contribute to sustainable oilfield-PW management by transforming a waste product into a valuable energy source. This research presents the potential to reduce PW’s treatment costs and expand the applications of PW in agriculture and water conservation, addressing both environmental and economic challenges.

Date of Award	17 Dec 2024
Original language	American English
Supervisor	Emad Alhseinat (Supervisor)