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- Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
One book dealing with the fundamentals of thermal and membrane desalination systems and discussing their economical as well as environmental aspects.
With a growing population, climate change and greater water demand, desalination has increasingly become a part of the solution to regional water scarcity - seawater desalination capacity has roughly doubled in the past ten years. Desalination has also begun to receive more attention in academia, with research focusing on improving energy efficiency and system robustness and lowering capital costs.
With this book, an introduction is given to the basics and fundamentals of desalination systems. Both, thermal and membrane desalination systems, are covered and discussed in view of energy, exergy, economic and environmental aspects. In the beginning, Introduction to Desalination: Systems, Processes and Environmental Impacts describes multi effect evaporation, vapor compression and multi-stage flashing. Further chapters deal with common membrane-based separations like reverse osmosis and membrane filtration, forward osmosis, diffusion dialysis and pervaporation as well as thermo-osmosis, electrodialysis and electrodeionization. Subsequently, hybrid systems are discussed, and the economic analysis of such systems and their environmental impact are highlighted. Each chapter contains theoretical and practical examples and concludes with questions and problems for self-study.
* Needed: Desalination has become a part of the solution to regional water scarcity and an introductory book in this field is urgently needed.
* Balanced Approach: Presents the fundamentals of thermal and membrane desalination systems.
* Learning Material: Each chapter includes exercises for self-study and Instructors can find teaching material online.
Introduction to Desalination: Systems, Processes and Environmental Impacts is an important resource for master's students in engineering sciences, lecturers in chemical and mechanical engineering, engineers, environmental chemists, as well as process engineers, engineering scientists in industry, and environmental consultants.
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Table of Contents
Cover
Preface
1 Introduction
1.1 What Is Desalination?
1.2 Aims of Desalination Processes
1.3 Desalination Processes
1.4 Desalination Technologies
1.5 Which Desalination System Is the Best?
1.6 Thermo‐Physical Properties of Water
References
A. Review Questions
Part I: Thermal Desalination Systems
2 Multi‐effect Evaporator (MEE)
2.1 Introduction
2.2 Vaporization
2.3 MEE Processes
2.4 MEE Configurations
2.5 Mathematical Modeling Algorithm for Thermal Systems
2.6 MEE Mathematical Model
2.7 MEE Integrated Auxiliary Devices
2.8 Characteristics of MEE Desalination Systems
2.9 MEE Energy Consumption and Cost
References
3 Multi‐stage Flashing (MSF)
3.1 Flashing Stage
3.2 MSF Once‐Through Configuration MSF–OT
3.3 MSF–Brine Recirculation (MSF–BR)
3.4 MSF with Brine Mixer (MSF–BM)
3.5 Material of Construction
References
A. Review Questions
B. Problems
C. Essay, Design and Open‐Ended Problems
4 Vapor Compression: Thermal Vapor Compression (TVC), Mechanical Vapor Compression (MVC), and Mechanical Vapor Recompression (MVR)
4.1 Thermal Vapor Compression (TVC)
4.2 TVC Mathematical Modeling
4.3 Mechanical Vapor Compression (MVC)
4.4 SEE–MVC Mathematical Modeling
4.5 Mechanical Vapor Recompression (MVR)
4.6 Characteristics of VC Desalination System
References
Part II: Membrane Desalination Systems
5 Pressure Gradient Driving Force: Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF), Microfiltration (MF)
5.1 Semipermeable Membrane: Properties and Modules
5.2 Membrane Modules (Configurations)
5.3 Natural Osmosis Phenomenon
5.4 Reverse Osmosis (RO)
5.5 Membrane Performance
5.6 RO System Components
5.7 RO Advantages and Disadvantages
5.8 RO Performance Using Software
5.9 RO Mathematical Model
5.10 Energy Recovery Device (ERD)
5.11 MF, UF, and NF Membranes: Materials and Applications
References
A. Review Questions
B. Problems
C. Essay, Design and Open‐Ended Problems
6 Electrical Potential Driving Force: Electrodialysis (ED), Electrodialysis Reversed (EDR)
6.1 Electrodialysis
6.2 Electrodialysis Principle
6.3 Conservation of Ionic Mass
6.4 ED Mathematical Modeling
6.5 ED Characteristics
6.6 Advantages and Disadvantages of ED
6.7 Electrodialysis Reversed (EDR)
References
A. Review Questions
B. Problems
C. Essay, Design and Open‐Ended Problems
7 Temperature Gradient Driving Force: Membrane Distillation (MD)
7.1 MD Processes and Configurations
7.2 MD Advantages and Disadvantages
7.3 Characteristics of Hydrophobic Membranes
7.4 Heat and Mass Transfer Models for DCMD
References
8 Concentration Gradient Driving Force: Natural Osmosis, Forward Osmosis (FO), Pervaporation (PV), Dialysis
8.1 Forward Osmosis (FO)
8.2 Pervaporation (PV)
8.3 Dialysis
8.4 Summary: Membrane Desalination Systems
References
Part III: Nonconventional Desalination Systems
9 Renewable Energy and Desalination: Solar, Wind, Geothermal
9.1 Solar Energy
9.2 Calculation of Solar Radiation on Inclined Surface
9.3 Wind Energy
9.4 Geothermal Energy
9.5 Geothermal Well Performance
9.6 Advantages of Geothermal Energy
References
Questions and Problems
10 Hybrid Desalination System
10.1 Case I: Cogeneration–MSF–RO Hybrid Desalination Systems
10.2 Case II: Hybrid SEF–Geothermal Desalination System
10.3 Case III: Hybrid MEE–Solar Desalination System (Adapted from [4])
10.4 Case IV: Hybrid MD–RO Desalination System (Adapted from [5])
10.5 Case V: Hybrid Humidification–Dehumidification Desalination System [6]
References
Essay, Design, and Open‐Ended Problems
Appendix A: Appendix AThermo‐Physical Properties of SeawaterThermo‐Physical Properties of Seawater
Index
End User License Agreement
List of Tables
f04
Table 1 Suggested desalination course plan (14 weeks).
Chapter 1
Table 1.1 Energy requirement for main desalination systems.
Table 1.2 Water classification based on salinity content.
Table 1.3 Secondary drinking water regulation [5].
Table 1.4 WHO standards for potable water [5].
Table 1.5 Thermo‐physical properties of typical seawater at 40 000 ppm and 20...
Table 1.6 Standard seawater composition at salinity equal to ≈35 000 ppm.
Chapter 2
Table 2.1 Typical overall heat transfer coefficient in different types of eva...
Table 2.2 Characteristics of MEE systems.
Chapter 3
Table 3.1 Mathematical model assumptions.
Table 3.2 Material of construction in MSF system.
Chapter 4
Table 4.1 Construction materials of the MED–TVC plants.
Table 4.2 Characteristics of thermal and mechanical desalination technologies...
Table 4.3 Desalinated water calculation of 15 000 m
3
/d production plant.
Chapter 5
Table 5.1 Characteristics of membrane types [1].
Table 5.2 Desalination systems corresponding to driving force type.
Table 5.3 Characteristics of membrane modules [3].
Table 5.4 Osmotic pressure for typical feed solutions (25 °C) [4].
Table 5.5 Alshegaya properties [6].
Table 5.6 Specifications of nanofiltration and RO membranes [8].
Chapter 6
Table 6.1 Maximum concentration for different desalination processes.
Table 6.2 Typical operation parameters for electrodialysis.
Table 6.3 Principel properties of ion‐exchange membranes.
Table 6.4 Advantages and disadvantages of ED compared with RO and NF.
Chapter 7
Table 7.1 MD configuration, advantages, disadvantages, and area of applicatio...
Table 7.2 Surface tension and thermal conductivity values for selected hydrop...
Table 7.3 Contact angle values of some membrane selected materials at ambient...
Table 7.4a Different types of fouling observed in MD studies.
Table 7.4b Examples of MD applications.
Chapter 8
Table 8.1 Composition between forward osmosis and reverse osmosis.
Table 8.2 Aqueous solution osmotic pressure (
π
DS
), concentration (
C
DS
), ...
Table 8.3 Development and performance of commercial FO membranes (performance...
Table 8.4 Advantages and disadvantages of PV process.
Table 8.5 Four types of driving forces.
Table 8.6 An overview of various membrane operations.
Chapter 9
Table 9.1 Environmental impacts of burning fossil fuels.
Table 9.2 Comparative costs (SEC) for common renewable desalination systems[4...
Table 9.3 Selected desalination plants that are integrated with renewable ene...
Table 9.4 Sun facts [6].
Table 9.5 Comparison of CSP collecting technologies [9].
Table 9.6 Wind power classification.
Table 9.7 Tip speed ratio design considerations [14].
Table 9.8 Selection of turbine size and weight configuration [14].
Table 9.9 A typical modern 2 MW wind turbine specification [14].
Table 9.10 Comparative of noise levels [16].
Table 9.11 Results of chemical analysis of hot springs at Baransky Volcano (c...
Table 9.12 Characteristics of geothermal power generation systems.
Table 9.13 Emissions and freshwater usage for different energy sources.
Chapter 10
Table 10.1 General features of MSF and RO [1,7,8].
Table 10.2 Flat‐plate solar collector specifications.
Table 10.3 Comparison of the designed system to the MEE‐solar plant.
Table 10.4 Summary of hybrid MD/RO desalination system with brine recycle opt...
Table 10.5 Comparison of HDH cycles [6].
List of Illustrations
Chapter 1
Figure 1.1 Desalination processes based on separation type.
Figure 1.2 Basic principles of desalination processes.
Figure 1.3 Different desalination systems' capabilities based on feed salini...
Figure 1.4 Specific energy consumption using different desalination technolo...
Figure 1.5 Cost of potable water production using different desalination tec...
Figure 1.6 CO
2
released from different desalination technologies measured in...
Chapter 2
Figure 2.1 Boiling, superheating, and condensation processes.
Figure 2.2 Flashing process.
Figure 2.3 MEE‐FF, feed, and vapor flow in same direction.
Figure 2.4 First evaporation stage in MEE.
Figure 2.5 Temperature distribution within evaporation stage (
n
).
Figure 2.6 Modeling and algorithm for arbitrary thermal system.
Figure 2.7 The effect of varying variable during parametric study.
Figure 2.8 The effect of varying variable during parametric study.
Figure 2.9 Stage number 1.
Figure 2.10 Stage number 2.
Figure 2.11
n
Stage.
Figure 2.12 Condenser.
Figure 2.13 MEE‐BF (
n
= 3), feed flow in opposite direction compared with di...
Figure 2.14a MEE‐PF‐cross type, feed, and distillate flow in parallel direct...
Figure 2.14b MEE‐FF desalination system with feed heater.
Figure 2.15 Feed heater configuration.
Figure 2.16 Mixing box configuration.
Figure 2.17 Flash box streams in MEE system.
Figure 2.18 Flash box.
Figure 2.19 Effect of steam temperature on
gain output ratio
(
GOR
).
Figure 2.20 The effect of steam flow rate on produced water.
Figure 2.21 Effect of feed temperature on gain output ratio (GOR).
Figure 2.22 Effect of number of effects on gain output ratio (GOR).
Chapter 3
Figure 3.1 Flashing process on
T
–
v
diagram.
Figure 3.2 Flashing stage.
Figure 3.3 MSF–OT configuration.
Figure 3.4 Single flashing stage (
n
= 1).
Figure 3.5 Single flashing stage (
n
= 2).
Figure 3.6 Optimum number of stages in MSF desalination system.
Figure 3.7 MSF–BR configuration.
Figure 3.8 Flashing stage (
n
) with three basic components.
Figure 3.9 Flashing pool in stage
n
.
Figure 3.10 Distillate tray in stage
n
.
Figure 3.11 Tube bundle in stage
n
.
Figure 3.12 (a) Flow sheet of MSF–BR system (IPSEpro® software). (b) Schemat...
Figure 3.13 Temperature loss profile.
Figure 3.14 Distillate and flashing brine flow rate at each stage.
Figure 3.15 Flashing brine salinity.
Figure 3.16 Effect of top brine temperature on the plant gain output ratio....
Figure 3.17 Effect of top brine temperature on the specific thermal energy....
Figure 3.18 Effect of top brine temperature on exergy destruction.
Figure 3.19 Effect of top brine temperature on exergy destruction.
Figure 3.20 MSF–BM desalination plant.
Chapter 4
Figure 4.1 MEE–TVC (
n
= 2) desalination system.
Figure 4.2 TVC schematic diagram.
Figure 4.3 Distribution of pressure and velocity profiles within TVC system....
Figure 4.4 TVC power performance diagram (EnR,
σ
, ER).
Figure 4.5a Optimal gain output ratio at different suction position for equa...
Figure 4.5b Configuration of MEE‐MVC desalination system.
Figure 4.6 Natural gas production by source, 1990–2040 (trillion cubic feet)...
Chapter 5
Figure 5.1 Membrane safe operating window.
Figure 5.2 Fundamentals of membrane configurations, driving forces, structur...
Figure 5.3 Membrane types; MF,UF,NF, and RO.
Figure 5.4 Typical spiral wound module design.
Figure 5.5 Membrane configurations. (a) Plate and frame. (b) Tubular.
Figure 5.6 Natural osmosis and reverse osmosis.
Figure 5.7 Brief historic timeline of the development of reverse osmosis mem...
Figure 5.8 Membrane flux vs. applied trans‐membrane pressure gradient.
Figure 5.9 Typical values of operating range of RO and EDR desalination tech...
Figure 5.10 Transport in pore flow and solution‐diffusion membrane models....
Chapter 6
Figure 6.1 Milestones in the development of ion‐exchange membrane processes....
Figure 6.2 (a) Worldwide installed capacity of thermal and membrane system i...
Figure 6.3 (a) Theoretical and expected current–voltage curve in electrochem...
Figure 6.4 Principle of desalination by electrodialysis in a stack of cation...
Figure 6.5 Differential element of dilute compartment.
Figure 6.6 Experimentally determined current vs. voltage curve measured with...
Figure 6.7 EDR process with negative polarity using cation (C) and anion (A)...
Chapter 7
Figure 7.1 Effective range of membrane processes and applications under four...
Figure 7.2 Transfer of vapor through hydrophobic membrane.
Figure 7.3 Timeline for membrane distillation (MD).
Figure 7.4 Four common configurations of membrane distillation.
Figure 7.5 DCMD desalination system.
Figure 7.6 Basic functional phenomena of the membrane distillation process: ...
Figure 7.7 Conduction heat and mass transfer through DCMD membrane (
T
1
>
T
2
)...
Figure 7.8 Heat transfer resistances analog in MD.
Figure 7.9 Mass transfer resistances in MD.
Figure 7.10 Types of flows: (a) Knudsen diffusion (
d
≪
L
), (b) steady flow (...
Figure 7.11 Surface fouling (external) and pore blocking (internal).
Chapter 8
Figure 8.1 Direction of permeate flux for RO, PRO, FO, and AFO.
Figure 8.2 FO industrial applications.
Figure 8.3 FO with other industrial system arrangement.
Figure 8.4 Schematic illustration of forward osmosis process with membrane a...
Figure 8.5 Osmotic pressure vs. draw solution concentration.
Figure 8.6 Schematic of ammonia–carbon dioxide FO desalination system.
Figure 8.7 Correlation of water flux, feed recovery, membrane area, and pump...
Figure 8.8 FO and RO integration for seawater desalination.
Figure 8.9 The effect of feed dilution and enhancing membrane permeability o...
Figure 8.10 Flow patterns in a spiral‐wound module for FO, with the feed sol...
Figure 8.11 (a) Concentrative internal CP and (b) dilutive internal CP acros...
Figure 8.12 Milestones in the development of pervaporation.
Figure 8.13 Vacuum pervaporation process.
Figure 8.14 The choice of membrane with respect to the size of particles enc...
Figure 8.15 Sorption–desorption in pervaporation.
Figure 8.16 Comparison between diffusion and natural osmosis process. (a) Di...
Figure 8.17 Conceptual diagram of the hemodialysis treatment for
chronic kid
...
Figure 8.18 Membrane blood fluxes.
Figure 8.19 Schematic representation of ND with a three‐compartment membrane...
Chapter 9
Figure 9.1 World energy consumption by fuel type, 1990–2040 (quadrillion Btu...
Figure 9.2 Possible technological combinations of three types of renewable e...
Figure 9.3 Integrated solar desalination systems.
Figure 9.4 Temperature and concentration profiles of solar pond: UCZ, upper ...
Figure 9.5 Heat balance for UCZ (surface zone).
Figure 9.6 Heat balance for LCZ (strong zone).
Figure 9.7 Different configurations of solar stills. (a) Single‐stage solar ...
Figure 9.8 Various heat transfer modes in convection single slope solar stil...
Figure 9.9 MVC desalination system combined with photovoltaic collectors.
Figure 9.10 Wind turbine.
Figure 9.11 (a) Airfoil: angle of attack (
α
) lift and drag(b) Airfo...
Figure 9.12 Geothermal steam power plant.
Chapter 10
Figure 10.1 Breakdown of total worldwide installed capacity by technology....
Figure 10.2 Process diagram for the SEF‐G system.
Figure 10.3 Thermodynamic model of the SEF‐G system.
Figure 10.4 Process diagram of the hybrid MEE‐FF–solar system.
Figure 10.5 Effect of TBT and numbers of effect on the performance ratio.
Figure 10.6 Effect of TBT and numbers of effect on the specific heat transfe...
Figure 10.7 Effect of TBT and numbers of effect on the specific cooling wate...
Figure 10.8 Design structure option 1 (brine recycling).
Figure 10.9 Design structure option 2 (brine recycling – cold brine).
Figure 10.10 Design structure option 3 (brine recycling after both MD and RO...
Figure 10.11 Design structure option 4 (brine recycling after MD).
Figure 10.12 Design structure option 5 (brine recycling after RO).
Figure 10.13 Performance of option 1 vs. recycle ratio (for variable and fix...
Figure 10.14 Performances of options 3–5 (varying fresh feed makeup).
Figure 10.15 Performances of options 3–5 (fixed fresh feed makeup).
Figure 10.16 Performance of options 2 and 5 at various pressure values (vary...
Figure 10.17 Simplest embodiment of HDH process.
Figure 10.18 Classification of HDH systems based on cycle configurations....
Guide
Cover
Table of Contents
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Introduction to Desalination
Systems, Processes and Environmental Impacts
Fuad Nesf Alasfour
Author
Prof. Fuad Nesf Alasfour
(retired)
Kuwait University
Department of Mechanical Engineering
P.O. Box 5969
13060 Safat
Kuwait
Cover
Image: © NavinTar/Shutterstock
All books published by Wiley‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.:
applied for
British Library Cataloguing‐in‐Publication Data
A catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2020 Wiley‐VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐34357‐7
ePDF ISBN: 978‐3‐527‐81161‐8
ePub ISBN: 978‐3‐527‐81163‐2
oBook ISBN: 978‐3‐527‐81164‐9
Cover Design Wiley
To my beloved
Parents
Wife
Family
and
Students
Preface
The aim of writing Introduction to Desalination textbook is to provide the academic sector with an educational, state‐of‐the‐art desalination teaching book.
The textbook is prepared to be a source of knowledge in the field of desalination and to provide solutions to freshwater capacity need in the world, and it aims to enhance both teaching and learning aspects in engineering educational field.
A comprehensive theoretical foundation for several industrial applications in the field of desalination is presented in a friendly and challenging way, covering basic principles of desalination processes: thermal and membrane system using conventional and renewable energy type, in addition to real engineering examples.
The content tends to integrate the basics of engineering science disciplines under the umbrella of desalination systems, namely, thermodynamics (energy and exergy), fluid mechanics, heat and mass transfer, water chemistry, environmental, and economic, in addition to sustainability aspect.
The textbook includes 10 chapters with materials that are mostly required in university engineering curriculum courses; it aims to develop intuitive understanding of the basics and practices in industrial desalination field. The objectives of teaching desalination course are:
Train and teach student how to model a desalination system, and then analyze, design, and select the proper desalination system in the light of
4E's
(energy, exergy, environment, and economic aspects).
Examine desalination system under different operating conditions and design parameters using sensitivity analysis (parametric study).
Assess and evaluate different conventional and hybrid desalination configurations using conventional and renewable types of energy.
Train student to select one research paper (per topic) from a referred journal for reproduction in the light of 4E's.
The textbook is recommended for senior undergraduate and graduate engineering students during a one‐semester course (14 weeks). Student must have knowledge and background in the following fields: thermodynamics, fluid mechanics, and heat mass transfer as a prerequisite.
There are several sections, examples, and problems that can be considered as advanced types, which are suitable for graduate level course.
On the other side, parts of the textbook can be used by practicing engineers in the field of desalination plant industry. Student needs to download the following software in order to solve examples:
ROSA
Nitto
Engineering Equation Solver
(
EES
)
The textbook's suggested plan in a one‐semester‐course is shown in the following Table 1.
Table 1 Suggested desalination course plan (14 weeks).
Chapter
Topics
Number of weeks
Number of hours (1 wk = 3 h)
1
Introduction
1
3
2
MEE
1.33
4
3
MSF
1.33
4
4
VC
1
3
5
MF, UF, NF, and RO
2
6
6
ED and EDR
1
3
7
MD
1
3
8
FO, PV, and dialysis
1
3
9
Renewable energy
1.33
4
10
Hybrid system
1
3
Σ = 12
+2 wk for quizzes, exams, and project presentation
At the end, special thanks to my students whom I learned from and to engineer Alfadel Taqi for his valuable contribution in software computational calculations. I wish that the textbook will be useful and helpful for both students and faculty members, whom I ask them kindly to contact me in case of any suggestions.
Kuwait2020
Prof. Fuad Nesf Alasfour([email protected])
1Introduction
1.1 What Is Desalination?
Desalination is a process of separating and removing unwanted dissolved salts and minerals from feedwater sources such as seawater, brackish water, or wastewater.
The aim of desalination process is to produce a stream of fresh water (potable) with high quality (purity) according to the standards of World Health Organization (WHO), which state that the accepted maximum limit of total dissolved salts (TDS) in fresh water is 500 ppm (500 mg/l); if we take seawater (called feed or saline) as an example with a salinity of 35 000 ppm, feed can be desalted through two main industrial processes: (i) thermal desalination technologies where feed phase changes through evaporation and condensation processes and (ii) membrane desalination technologies where separation of salts achieved through using semipermeable membrane without feed phase change. In either process, potable water is produced, and salty water (called brine or concentrate) is rejected from desalination system; the rejected brine salinity varies in concentration based on technology, and it can reach up to 90 000 ppm.
Today, desalination industry plays a vital role in society development and economic growth, and worldwide freshwater consumption rate is approximately doubled every 20 years, where the availability of natural water sources is depleted. According to UN report and in the light of global population growth, statistics showed that one‐third of world's population lives under a state of insufficient potable water resources and in communities that suffer from scarcity and water stress. As prehuman body, water is essentially needed for building tissues, blood circulation, and maintaining stable blood temperature, and based on human weight it is recommended that a human drinks 8–14 glasses of water every day. Today desalination industries tend to provide safe drinking water to achieve and maintain sustainable human life and minimize negative environmental impacts. As per industry, there are eight major water‐consuming industrial sectors: power generation, food, pharmaceutical, mining, oil, petrochemical, electronics, and paper.
There are several types of “well‐proven” industrial desalination technologies that have been used in the last seven decades. Note that the major parameters that affect desalination technology performance are feed type and its thermal‐physical characteristics, in addition to the required desalted water quality (purity).
Figure 1.1 shows the classification of desalination systems based on separation processes. Note that such processes can be operated by conventional energy type such as fossil fuel (thermal and electrical) or by renewable energy types such as solar, wind, and geothermal (Chapter 9).
Statistics by IDA for year 2015 shows that there are 18 426 desalination plants worldwide, producing 86.8 × 106 m3/d and serving more than 300 × 106 in 150 countries per day, and such production can provide potable water for municipal, industrial, and agriculture sectors. Today Gulf countries produce around 57% of world desalination capacity [2].
Figure 1.1 Desalination processes based on separation type.
1.2 Aims of Desalination Processes
Several desalination technologies have been invented, developed, and employed during the last seven decades, and the potentials behind such industrial developments are:
Satisfy the global increase in freshwater demand for drinking, and in industrial and agriculture sectors, in addition to hygiene requirements.
Compensate the capacity of limited natural freshwater resources specially in arid and remote areas.
Reduce the values of elevated intensive energy cost of freshwater production.
Reduce the potential levels of emissions and minimize negative environmental impacts on human and climate in terms of greenhouse effect and acid rain.
Provide efficient large size desalination system with the ability of integration with other conventional or renewable energy types.
Achieve water quality based on WHO rules and regulations or other specific industrial standards.
Manage wastewater processes that are associated with municipal and industrial sectors.
1.3 Desalination Processes
There are four main industrial desalination processes that exist today to separate freshwater from feedwater, and separation processes can be achieved via:
Thermal processes
Membrane processes
Chemical processes
Adsorption processes
Figure 1.2 shows the basic principles of general industrial desalination processes.
Figure 1.2 Basic principles of desalination processes.
Feed can be in the form of seawater with high salinity that can reach a value of 45 000 ppm, or it can be in the form of brackish water or even wastewater that has been taken from industry or municipal.
The example of disposal fluid from desalination system can be in the following forms:
Brine fluid (salty water): Which have very high value of salinity that can reach 90 000 ppm in case of thermal desalination system.
Concentrate: Which is a fluid with very high value of salinity in case of membrane desalination system.
Waste: Which is a product in the form of industrial concentrated fluids.
Thick liquor: Product that is produced under specific purposes and applications.
The products from desalination system can be in the following forms:
(a) Drinkable water with salinity less than 500 ppm.
(b) Fresh water.
(c) Fluid with specific required salinity or concentration.
Choosing the proper desalination system (technology) is always a challenge for engineers, because each desalination technology can provide different levels of performance under different circumstances (design and operational conditions). To ensure the performance level for any industrial desalination system, it must be examined in terms of 4E's – energy analysis, exergy analysis, economic analysis, and environmental analysis, in addition to sustainability tendency. Student should remember that selecting desalination system must fulfill the following criteria: low energy consumption, ability to use renewable energy, low cost of water production with high value of quality, and minimum negative environmental impacts, and it must be durable and reliable in operation with minimum required maintenance. Today reverse osmosis (RO) desalination systems present about 65% of worldwide installed capacity, while multi‐stage flashing (MSF) and multi‐effect evaporator (MEE) systems present about 21% and 7%, respectively.
As mentioned before, selecting desalination technology is a great challenge for engineers, and such selection process depends on the evaluation of the following four parameters:
Feed concentration: The first parameter that must be considered during desalination technology selection is feed salinity (concentration); the selected system must be able to handle the required salinity and to produce the required water or fluid quality.
Figure 1.3
shows the capability of different industrial desalination systems as per feed salinity and produced water purity.
Energy consumption: The second parameter that needs to be considered during selection is the value of
specific energy consumption
(
SEC
) that is required to desalt feedwater when electrical and/or thermal energies are used.
Figure 1.4
predicts the amount of SEC (kWh/m
3
) per desalination system.
Cost: The third parameter that must be considered during desalination selection is the cost of freshwater production (
Figure 1.5
).
Emission: The fourth parameter is the level of emitted emissions of CO
2
from desalination that affect the formation of greenhouse phenomenon (
Figure 1.6
).
Figure 1.3 Different desalination systems' capabilities based on feed salinity and produced water purity.
Source: Youssef et al. 2014 [3]. Reproduced with permission of Elsevier.
Figure 1.4 Specific energy consumption using different desalination technologies.
Source: Youssef et al. 2014 [3]. Reproduced with permission of Elsevier.
Figure 1.5 Cost of potable water production using different desalination technologies.
Source: Youssef et al. 2014 [3]. Reproduced with permission of Elsevier.
Figure 1.6 CO2 released from different desalination technologies measured in kg/m3.
Source: Youssef et al. 2014 [3]. Reproduced with permission of Elsevier.
1.4 Desalination Technologies
We will present and explain in this section the two most applied commercial types of desalination technologies: thermal and membrane systems.
1.4.1 Thermal Desalination System
There are several mature and reliable thermal desalination systems that are used today, and the two most conventional thermal (distillation) ones are:
MEE, where feed is heated, thus it evaporates via boiling process, then vapor is condensed after been screened through screen (demister) as potable water.
MSF, where feed is evaporated via flashing process due to sudden drop in pressure, then vapor is condensed after been screened through screen (demister) as potable water.
In addition to MEE and MSF, there are several types of thermal desalination systems that are used under specific conditions such as thermal vapor compression (TVC), mechanical vapor compression (MVC), and mechanical vapor recompression (MVR).
In general, thermal desalination systems are considered as an “old conventional” types, and they are characterized with simplicity in design and operation and even in maintenance. They have the ability to operate with feed salinity ranges from 20 000 to 100 000 ppm, the SEC varies from 4 to 6 kWh/m3 in addition to steam cost, and at the same time thermal desalination can produce high capacity of desalted water with high water purity (low TDS). As per processes, feedwater in thermal system desalination was subjected to their main progressive processes:
Heating: Where feed is heated via steam (thermal load) in first stage or by other type of energy source using its latent heat (
h
fg
).
Evaporation: Feed is evaporated through feed phase change from liquid to vapor within the stages via boiling in case of MEE or via flashing in case of MSF.
Condensation: Freshwater vapor is condensed as potable water via heat exchanger (condenser).
The advantages and disadvantages of thermal system are presented.
Advantages of thermal distillation processes:
Suitable to treat high salinity feedwater such as seawaters.
Reliable and rigid system.
Require minimal conventional pretreatment for feedwater.
Capacity to use low waste thermal heat (low grade) from power plants to save energy.
Economical system if a heat source is available.
Easy in maintenance.
Disadvantages of thermal distillation:
The amount of water production depends on feed thermo‐physical properties (
T
f
,
x
f
, …).
Tube scaling (CaSO
4
).
High value of SEC specially at elevated values of thermal performance.
Example 1.1 Boiling Phenomenon
Steam flows in a nonadiabatic horizontal pipe (control volume) with an inlet conditions of , and exits at where T1 > T2, if pipe outer surface is surrounded by water droplets:
Explain heat transfer phenomenon.
Identify the process type.
Solution
Since
T
1
>
T
2
, it is clear that steam is subjected to heat loss.
Since
, then
. Since
T
1
>
T
2
, then
h
2
–
h
1
is negative, which indicate that heat transfer sign is negative, implying that heat is lost from the system (steam) to the surrounding.
Isobaric process.
Extra activity:
Student can perform the following:
Describe boiling phenomenon in MEE system.
Why kinetic and potential energy terms have been neglected from energy balance (first law of thermodynamics)? Explain.
Explain the process associated with change of water droplet phase that surrounds the exterior pipe wall. What is the type of evaporation process?
Based on real numbers, compare the value of
h
1
against
h
2
. Explain the physical meaning of such difference.
Sketch
T
–
v
,
P
–
v
, and
T
–
s
diagrams showing boiling process.
Example 1.2 Flashing Phenomenon
Adiabatic rigid tank (control mass) is divided into two nonequal volumes: A and B by a flexible membrane. Part A contains H2O liquid at 7 kg, 220 kPa, , and at 25 °C, and part B is at sub‐atmospheric pressure under vacuum state condition with no mass and has a volume of 2 .
If membrane is ruptured by making a hole such that two systems reach a final equilibrium state at 25 °C:
Find:
What is the type of water evaporation process?
Final specific volume.
Final pressure.
Equations that represent exergy destruction (irreversibility).
Solution
Flashing process, where sudden drop in pressure causes water to evaporate via flashing, it is always associated with lightning and heat release.
Hence
P
2
= 3.17 kPa
Entropy generation
Exergy destruction
Extra activity:
Student can perform the following:
Describe the flashing process in MSF system.
Sketch
T
–
v
,
P
–
v
, and
T
–
s
diagrams showing flashing process.
Perform a parametric study to investigate the effect of initial water temperature on final phase. Sketch and explain.
Perform a parametric study to investigate the effect of
V
B
on final phase. Sketch and explain.
Calculate the amount of heat release from flashing process, and explain physically what is the source and cause of such heat.
Perform a parametric study to investigate the effect of
ratio on irreversibility.
Example 1.3 Feedwater Heat Exchanger
An adiabatic heat exchanger is used to heat feedwater (H2O) from 40 to 120 °C.
Find:
ratio.
Heat transfer rate from thermal load (steam) to feedwater.
Exergy destruction (irreversibility) per kg of feedwater.
Solution
Mass balance:
Energy balance (first law of thermodynamics):
or .
Hence
Extra activity:
Student can perform the following:
Perform parametric study to investigate the effect of varying
T
feed
during summer to winter seasons on heat transfer performance. Plot and explain. Take
T
summer
= 35 °C.
Calculate exergy flow rate at all four states. Explain.
Resolve example using feedwater as seawater with salinity of 40 000 ppm. Refer to Appendix A for thermo‐physical properties.
1.4.2 Membrane Desalination System
There are several types of membrane technologies that are used today in the market, and the most predominant one is RO; the separation process using RO depends on the process of pressurizing feed fluid against semipermeable membrane, and such pressure causes only water molecules to cross (pass, migrate) through membrane, where salts and unwanted constituents such as minerals are rejected in a form of brine or concentrate. In RO process there is no heat addition during desalination process, hence no feed phase change occurred during separation process; besides RO technology, there are other membrane desalination systems used at small and limited scale such as nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF).
The RO desalination system is characterized with low amount of energy consumption compared with conventional thermal types and with high water recovery, but membrane lifetime is short due to fouling and scaling; unlike thermal system RO can only treat feed salinity from 50 to 46 000 ppm, and the value of SEC varies from 1 to 6 kWh/m3. As per electrical demand an example of 50 mG/d capacity RO plant requires 20–35 MW in case of using feed seawater (sea water reverse osmosis [SWRO] system) and 8–20 MW in case of using brackish feedwater (brackish water reverse osmosis [BWRO] system).
The following are advantages and disadvantages of RO system:
Advantages of RO:
Low energy consumption compared with thermal desalination system.
Small in size and compact.
Low value in investment.
Disadvantages of RO:
Membrane has certain thermal stability limit.
Life of a membrane is limited and short.
Low feedwater can treat only salinity compared with thermal system.
Costly in pretreatment.
Use electrical power to drive system.
Costly in maintenance.
Example 1.4 Reverse Osmosis
A simple one‐stage RO desalination system operates under steady flow; brackish water with feed capacity of 200 m
3
/h and 3000 ppm salinity needs to be desalted to 150 ppm as a potable water.
Find:
Concentrate flowrate
Concentrate salinity
Recovery ratio
Solution
Mass balance:
Assume constant density (incompressible)
Then
Salinity balance:
Recovery ratio:
= 70%
Extra activity:
Student can perform the following:
Derive RR relation as a function of three stream salinities.
Select one practical operational parameter and examine its effect on RR. Plot and explain.
Calculate
salt rejection
(
SR
), where SR is defined as
SR = 100 − SP
SP is salt passage across membrane SP = xp/xf, then perform a proper parametric study. Plot and explain.
Perform parametric study to investigate the effect of feed salinity on concentrate salinity. Plot and explain.
1.5 Which Desalination System Is the Best?
Recall that feed type, characteristics, and quality play a major role in selecting desalination system. In addition potable water purity, type of energy consumption, water production cost, and emissions are also factors that play a role during desalination technology selecting; engineers must evaluate all the mentioned parameters to optimize his or her selection process.
For example, if we take a case of 45 000 ppm feed seawater, thermal system can be used, while in other cases such as brackish water with 8000 ppm salinity, membrane system is recommended. In certain applications using two different combined desalination system, thermal and membrane, which is called hybrid (Chapter 10), is another option of selection.
Table 1.1 shows the range operational energy requirement for main desalination systems.
Table 1.1 Energy requirement for main desalination systems.
Source: Alkaisi et al. 2017 [4]. Reproduced with permission of Elsevier.
MEE
MSF
MEE‐TVC
MVC
RO
ED
Typical unit size (m
3
/d)
5000–15 000
50 000–70 000
10 000–35 000
100–2500
24 000
−145 000
Steam pressure (atm)
0.2–0.4
2.5–3.5
0.2–0.4
—
—
—
Electrical energy consumption (kWh/m
3
)
1.5–2.5
4–6
1.5–2.5
7–12
3–7
2.6–5.5
Thermal energy consumption (kJ/kg)
230–390
190–390
145–390
None
None
None
Electrical equivalent for thermal energy (kWh/m
3
)
5–8.5
9.5–19.5
9.5–25.5
None
None
None
Total equivalent energy consumption (kWh/m
3
)
6.5–11
13.5–25.5
11–28
7–12
3–7
26–5.5
1.6 Thermo‐Physical Properties of Water
1.6.1 Potable Water
Potable water (freshwater) is characterized with salinity less than 500 ppm.
Table 1.2 shows water classification based on salinity, and the standards of WHO for freshwater limit is 500 ppm (0.05 mg/kg).
Table 1.2 Water classification based on salinity content.
Type
Total dissolved salts (TDS)
Freshwater
<1500
Brackish water
1500–10 000
Salt water
>10 000
Seawater
10 000–45 000
Standard seawater
35 000
The chemical concentration in fresh water with 500 ppm are shown in Table 1.3.
Table 1.3 Secondary drinking water regulation [5].
Chemicals
SDWR
Aluminum
0.05–0.2 mg/l
Chloride
250 mg/l
Color
15 color units
Copper
1.0 mg/l
Corrosivity
Non‐corrosive
Fluoride
2.0 mg/l
Foaming agents
0.5 mg/l
Iron
0.3 mg/l
Manganese
0.05 mg/l
Odor
Three threshold odor numbers
pH
6.5–8.5
Silver
0.1 mg/l
Sulfate
250 mg/l
Total dissolved solids (TDS)
500 mg/l
Zinc
5 mg/l
Tables 1.3 and 1.4 show the 2018 Drinking Water Standards and Health Advisories, Secondary Drinking Water Regulations.
Table 1.4 WHO standards for potable water [5].
Constitutes
Concentration (ppm)
Limited values
Max allowed values
Total dissolved salts (TDS)
500
1500
Cl
200
600
SO
4
2+
200
400
Ca
2+
75
100
Mg
2+
30
150
F
−
0.7
1.7
NO
3−
<50
100
Cu
2+
0.05
1.5
Fe
3+
0.10
1.0
NaCl
250
—
pH
7–8
6.5–9
Table 1.5 Thermo‐physical properties of typical seawater at 40 000 ppm and 20 °C.
Density
1.0288 kg/m
3
Specific heat capacity
3.973 kJ/(kg °C)
Boiling point elevation, at 20 °C
0.344 K
Boiling point elevation, at 90 °C
0.565 K
Thermal conductivity
0.601 W/(m K)
Dynamic viscosity
1.089 × 10
−3
kg/(m
2
s)
Kinematic viscosity
10.58 × 10
−7
m
2
/s
Latent heat of vaporization
2355.4 kJ/kg
1.6.2 Seawater
Table 1.5 shows thermo‐physical properties of standard seawater at 40 000 ppm and 25 °C.
Note that the comprehensive thermo‐physical properties of seawater as a function of temperature and salinity are presented in Appendix A.
The major six elements that comprise about 99% of seawater are chlorine (Cl−), sodium (Na+), sulfate (SO42−), magnesium (Mg+2), calcium (Ca+2), and potassium (K+), and standard seawater composition at 35 000 ppm are presented in Table 1.6.
Table 1.6 Standard seawater composition at salinity equal to ≈35 000 ppm.
Chemical ion
Concentration (ppm)
Valence
Total salt content (%)
mmol/kg
Molecular weight
Chlorine Cl
−
19 345
−1
55.03
546
35.453
Sodium Na
+
10 752
+1
30.59
468
22.990
Sulfate
SO
4
2−
2701
−2
7.68
28.1
96.062
Magnesium Mg
2+
1295
+2
3.68
53.3
24.305
Calcium Ca
2+
416
+2
1.18
10.4
40.078
Potassium K
+
390
+1
1.11
9.97
39.098
Bicarbonate HCO
3
−
145
−1
0.41
2.34
61.016
Bromide Br
−
66
−1
0.19
0.83
79.904
Borate
BO
3
3−
27
−3
0.08
0.46
58.808
Strontium Sr
2+
13
+2
0.04
0.091
87.620
Fluoride F
−
1
−1
0.003
0.068
18.998
Σ
x
i
= 35 151 ppm
Note that Cl makes up to 55% of salt in seawater and NaCl makes up to 86% of salt in seawater.
Example 1.5 Heat Exchanger with Feed Seawater
An adiabatic heat exchanger provides thermal load to feed seawater, such that feed seawater can be supplied to MEE desalination system at 70 °C.
Steam stream: 1 kg/s, 120 °C
State 1: saturated vapor
State 2: saturated liquid
Feed seawater stream:
State 3: 20 °C
State 4: 70 °C
xf = 40 000 ppm
Find:
Feed seawater flow rate.
Assume feed is fresh water, find feed flow rate.
If steam (thermal load) is generated by conventional boiler using natural gas as a fuel, find fuel flow rate and emitted amount of CO
2
.
Provide emission table for three types of fuels to generate steam (thermal load).
Solution
Energy balance (first law):
of thermal load (≈11 time of steam flow rate).
Alternative solution
cp is calculated at then
kg) of thermal load.
If feed is fresh water
The of natural gas fuel.
For CO2 emission
Environmental impact of burning fossil fuels.
Fuel
Calorific value (MJ/kg)
CO
2
(kg/kg
fuel
)
CO
2
/energy (kg/MJ)
SO
2
(kg/kg
fuel
)
Coal
26
2.361
0.091
0.018
Fuel oil
42
3.153
0.075
0.040
Natural gas
55
2.750
0.050
0
Extra activity:
Student can perform the following:
Perform parametric study to investigate the effect of
on
. Plot and explain.
Perform parametric study to investigate the effect of
x
f
on
. Plot and explain.
Find the values of the following thermo‐physical parameters at
and
(
x
f
=
40 000 ppm);
boiling point elevation
(
BPE
),
μ
,
v
,
k
,
h
fg
,
c
p
,
u
,
s
,
ρ
. Explain the reason behind the differences.
Find the values of the following thermo‐physical parameters at
under two salinities;
x
f
=
20 000 and 40 000 ppm for; BPE,
μ
,
v
,
k
,
h
fg
,
c
p
,
u
,
s
,
ρ
. Explain the reason behind the differences.
Calculate entropy generation and irreversibility per mass of feedwater flow rate.
Example 1.6 Performance of Feed Pump
Feedwater is pumped from 100 kPa, 30 °C, 1.5 kg/s to 3 MPa, if pump efficiency is 75%:
Find:
Actual work input
Reversible work
Exergy destruction (irreversibility)
Second law efficiency
Solution
Working fluid water:
State 1
then h2 = 131.4 kJ/kg and s2 = 0.4423 kJ/(kg K).
W
rev
= Ψ
2
− Ψ
1
Exergy destruction (irreversibility):
Exergetic efficiency:
Extra activity:
Student can perform the following:
Sketch
T
–
v
,
P
–
v
, and
T
–
s
diagram for actual and isentropic processes.
Resolve example using seawater (
x
= 40 000 ppm) as feed. Refer to Appendix A.
Compare results of feed seawater against water. Explain and comment.
Example 1.7 Exergy Analysis of Rankine Cycle (Review)
Simple Rankine cycle operates with isentropic expansion and isentropic compressor to produce net power of 1750 kW, and steam boiler operates isobarically at 6 MPa pressure; using fuel oil as a fuel, the combustion of fuel produces heat that is added at a source temperature of 800 K, and the produced steam enters steam turbine at 500 °C and 6 MPa to be expand isentropically to 20 kPa as it leaves steam turbine.
