Technological challenges of seawater desalination: analysis of future opportunities
- Autores: Arturo Buenaventura
- Directores de la Tesis: María de Lourdes García Rodríguez (dir. tes.)
- Lectura: En la Universidad de Sevilla ( España ) en 2019
- Idioma: español
- Número de páginas: 264
- Tribunal Calificador de la Tesis: Francisco José Jiménez-Espadafor Aguilar (presid.), David Tomás Sánchez Martínez (secret.), Baltasar Peñate Suárez (voc.), Julián Blanco Gálvez (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Energética, Química y Ambiental por la Universidad de Sevilla
- Materias:
- Enlaces
- Tesis en acceso abierto en: Idus
- Resumen
Close to 1/3 of the world´s population live in water scarce areas. Over 780 M people are still without access to improved sources of drinking water.
Seawater desalination is part of the solution to these water challenges and has been used now for decades to generate alternative water resources.
In order to make affordable drinking water obtained from seawater desalination, there has been a continuous optimisation of the process looking for more efficient solutions in terms of energy consumption.
In this regard, the use of thermal desalination technologies, such as Multi-Stage Flash distillation (MSF) and Multi-Effect Distillation (MED), has been changed in the last decades to Reverse Osmosis (RO) membrane technology because it requires less energy to desalinate seawater.
However, at the same time there has been a general trend of reviewing all industrial processes under sustainability criteria, that is, looking for more environmental friendly solutions, reducing the energy consumption and CO2 emissions.
In this context, thermal desalination technologies have been considered again because of their conceptual advantage to be coupled with solar thermal energy, thus allowing for solar thermal-powered seawater desalination solutions.
Therefore, assessing the viability of coupling solar thermal energy with seawater thermal desalination technologies has been the first objective of this thesis, analysed in Chapter 1. A key factor in the analysis was the size of the plant, that is, the water production capacity (market opportunities) and the corresponding required power (the size of the solar plant). On top of that, thermal desalination technologies were also compared with reverse osmosis when both coupled with solar thermal-power.
Based on Chapter 1, for coupling with solar thermal energy all thermal desalination technologies where discarded except for very small-capacity seawater desalination systems. Membrane Distillation (MD) technology may be potentially used in small-capacity systems due to the compactness, ability of dealing with highly concentrated saline solutions and its feasibility for operating at partial and variable loads. Seawater desalination based on MD was then analysed in Chapter 2 considering its current trends and future prospects.
The basic concepts of seawater desalination based on MD have been analysed. The existing MD configurations and pre-commercial and commercial MD systems have been reviewed. The performance of those systems have also been studied based on experimental data from tests performed by independent researchers. Having in mind other existing and well proven technologies, such as MED and RO, potential applications of MD technology in seawater desalination have been assessed. While the limited production capacity could be improved by advanced MD configurations, the low energy efficiency of the process may be a real barrier for MD technology. The most promising application of MD technology seems to be in brine concentration systems.
The use of conventional distillation systems (MED, MSF) have been rejected for seawater desalination (Chapter 1) and MD systems have inherent limitations such as low energy efficiency and small production capacity (Chapter 2). Therefore, RO systems are the reference technology for desalination processes in general and, in particular, when considering its combination with solar thermal power.
Having a deep understanding of RO technology is then important (Chapters 3 and 4) and is the technology in which to focus (Chapter 5) in order to improve seawater desalination. For RO Technology, the minimum theoretical Specific Energy Consumption (SEC) required for solvent extraction from standard seawater salt concentration, at recovery rates of 50% and the absolute minimum theoretical SEC required for recovery rate of 0% that is, obtaining only a drop of product water. Besides that, inefficiencies attributable to the status of membrane technology, to pumping inefficiencies, to plant configurations, etc, are calculated. Finally, the option of adopting innovative configurations reported in the literature is assessed in Chapter 3.
The core of the RO technology is the RO membrane, therefore the RO membrane modules are the key components. The design of SWRO membranes have been improved during the last decades coming to a standard design consisting in a module of typically 8 inches diameter and 1 meter long with spiral wounded flat sheets RO membranes. These RO membrane modules are placed inside a Pressure Vessel (PV) where a number of these modules can be installed in series. There are a number of phenomena, such as water and salt permeabilities, scaling, bio-fouling and concentration polarization that are inherent to the RO technology and to the fact that is based on the flow of the dissolvent through a membrane. These phenomena depend on the membrane characteristics, module configuration, operating conditions (pressure and temperature) and the system configuration (recovery rate, number of membranes in serial), and have a direct impact on water production, product quality and energy consumption.
A thorough membrane performance model has been implemented (Chapter 4), including effects of pressure losses and concentration polarization at the feed-blowdown channel. This software calculates the water permeability and salts permeability from experimental data. Alternatively, this calculates salt concentration and flow of permeate from given design parameters of a specific membrane module.
Concerning a membrane serial, criteria of selecting the best membrane type for each position in the pressure vessel, depending on their permeability, have been reviewed. Membrane permeability should increase along the serial of membrane elements, having high rejection low energy elements in the first positions. In Canary Islands, two of those elements are enough to comply the required permeate quality. Besides that, the SEC reduces as the length of the series increases. The limiting factor is the product quality.
Finally, chapter 5 deals with a thorough analysis of innovative configurations with high prospects to achieve SEC decreasing. Not only configuration proposed in the literature are analysed, but also a patent pending innovation is proposed.
