UNIVERSITA’ DI PISA
Scuola di Ingegneria
CORSO DI LAUREA IN INGEGNERIA ENERGETICA
TESI DI LAUREA MAGISTRALE
Multi-Effect Distillation (MED) plants
for seawater desalination:
thermodynamic and economic improvement
RELATORI:
Prof. Umberto Desideri
Ing. Andrea Baccioli
Ing. Christoph Wieland
CANDIDATA:
Angelica Liponi
Abstract
The growing global demand of fresh water coupled with the increasing interest in renewable energy and in waste heat recovery has resulted in a growing attention to the Multi-Effect Distillation (MED) desalination process because it requires relatively low temperature sources and maximum Top Brine Temperature (TBT) of 70-90°C. The study of MED configurations that could increase the efficiency and reduce desalted water cost of production is an actual field of research. Differently from previous studies present in literature, a Parallel/Cross flow (P/C) MED was studied with variable feed mass flows in each effect to obtain the maximum allowable Recovery Ratio (RR). The maximum allowable RR is related to the need of avoiding scaling problems due to calcium sulfate precipitation (function of the effect temperature and of the feed water salinity). Starting from a base P/C MED, several configurations have been simulated to find the best ones from a thermodynamic and economic point of view. The simulations have been conducted through an Aspen Plus model by varying the Bottom Brine Temperature (BBT) and the TBT. Configurations with a preheater for each effect showed the highest increase of Performance Ratio (PR) relative to the base configuration, in particular for the highest TBT and the lowest BBT (+ 10 % with TBT=75°C and BBT=35°C for a 4 effects-MED) due to the better exploitation of the energy content of desalted water streams. The economic analysis highlighted a strong relation between the thermal energy cost and the cost of water production.
In the case of a thermal energy cost of 1.24 c$/kWh, configurations that improved the PR showed also lower production costs of water (COW) expressed in $ per m3 of desalted; a
linear relation was found between increments of GOR and decrements of COW.
In the case of no thermal energy cost (approximation of the case of waste heat recovery) configurations simpler and less efficient than the base one showed lower COW; higher COW were found for the configurations with more preheaters since on the one hand they had higher PR but on the other hand they required higher capital and maintenance costs.
Acknowledgments
I would like to thank Prof. Umberto Desideri, Ing. Christoph Wieland and Ing. Andrea Baccioli for giving me the opportunity to do my thesis abroad at Technical University of Munich and for their general guidance and their great help, support and insight.
I am also grateful for the love and support of my family. Their patience and encouragement have given me the strength to complete my study.
Table of Contents
Abstract ... i Acknowledgments ... ii List of Figures ... vi List of Tables ... x Nomenclature... xi 1 Introduction ... 1Aim of the thesis ... 1
Outline of the thesis ... 2
1.1 Water resources and uses ... 3
1.2 Water classification ... 6
1.3 Water stress and scarcity ... 8
1.4 Energy - water nexus ... 11
1.5 Seawater desalination ... 13
1.5.1 Seawater properties 13 1.5.2 Main parameters of a desalination process 17 1.5.3 Minimum work and minimum heat of separation 18 2 State of the art ... 23
2.1 Desalination technologies ... 25
2.1.1 Thermal processes 26
2.1.2 Membrane processes 34
2.2 Renewable energy and desalination processes ... 42
2.3 Literature review on MED ... 43
2.3.1 Types of evaporators 43 2.3.2 Layout 47 2.3.3 Feed configuration 48 2.3.4 Recent studies on MED 52 3 Material and methods ... 59
3.1 P/C MED models ... 59
3.2 Aspen Plus and Matlab ... 66
3.3 Energetic and economic analysis ... 71
4 Results and discussion ... 76
4.1 Thermodynamic analysis ... 76
4.1.1 Recovery ratio 76 4.1.2 Gain output ratio 80 4.1.3 Specific heat transfer area 89 4.2 Economic Analysis ... 92
4.2.1 Annualized capital cost 92 4.2.2 Cost of water 96 4.3 Gain output ratio and cost of water ... 101
5 Conclusions ... 107
6 References ... 109
Appendix I: Aspen flowsheets and streams’ and block’s specifications ... 115
List of Figures
Figure 1.1 Distribution of water on Earth ... 3
Figure 1.2 Global use of fresh water ... 4
Figure 1.3 Water withdrawals in energy sector (2014) ... 5
Figure 1.4 Global water demand by sector to 2040 ... 6
Figure 1.5 Per capita total annual renewable water resources per capita in 𝒎𝟑 (2013) ... 9
Figure 1.6 Energy use for various processes in the water sector ... 12
Figure 1.7 Seawater specific heat variations with temperature and salinity, S ... 15
Figure 1.8 Seawater latent heat variations with temperature and salinity, S ... 16
Figure 1.9 Seawater BPE variations with temperature and salinity, S ... 16
Figure 1.10 Solubility limits of saline water (brine from seawater) ... 17
Figure 1.11 Schematic diagram of a work-driven desalination system ... 19
Figure 1.12 Least work of separation as function of recovery ratio ... 20
Figure 1.13 Least heat of separation as function of recovery ratio ... 21
Figure 2.1 Cumulative contracted and online capacity, 1965–2017 ... 23
Figure 2.2 Type of raw water (2018) ... 24
Figure 2.3 Installed capacity by region ... 24
Figure 2.4 Total global installed capacity by technology in 2015 ... 25
Figure 2.5 Once-through MSF ... 27
Figure 2.6 Brine recirculation MSF ... 27
Figure 2.7 Schematic diagram of MED unit ... 29
Figure 2.8 MVCD (a) and TVCD (b) ... 30
Figure 2.9 Humidification-dehumidification systems: (a) open-water closed-air cycle, and (b) open-air closed-water cycle ... 33
Figure 2.10 Major subsystems in a RO system ... 35
Figure 2.11 Ion exchange in ED ... 36
Figure 2.13 Schematic diagram of FO process... 39
Figure 2.14 Different desalination systems capabilities regarding salinity of feed and produced water measured in ppm ... 40
Figure 2.15 Amount of energy required for different desalination technologies ... 40
Figure 2.16 Water production cost for different desalination technologies... ... 41
Figure 2.17 Desalination processes using renewable energy ... 42
Figure 2.18 Elements of two-effect submerged evaporator ... 44
Figure 2.19 Horizontal falling film evaporator ... 45
Figure 2.20 Schematic of a vertical tube falling film evaporator ... 46
Figure 2.21 Plate evaporator ... 46
Figure 2.22 Simple (a) and double (b) Multi-Effect Stacked plants ... 48
Figure 2.23 Forward feed MED configuration ... 49
Figure 2.24 Backward feed MED configuration ... 49
Figure 2.25 Parallel feed MED configuration ... 50
Figure 2.26 Schematic of a parallel/cross flow MED plant ... 50
Figure 2.27 Solubility of calcium sulfate and the temperature concentration profiles... 51
Figure 2.28 Comparison of GOR and 𝐺𝑂𝑅 WH and fresh water production of different MED configurations as applied to a 75 °C waste heat source, 𝑚h = 100 𝑘𝑔/𝑠 ... 53
Figure 2.29 Fresh water production, heating medium output temperature and PR of different MED configurations ... 54
Figure 2.30 Schematic of a Boosted MED system for sensible heat sources ... 55
Figure 2.31 Schematic of a flash boosted MED design ... 56
Figure 2.32 Schematic of a distributed boosted MED design ... 57
Figure 3.1 a) Aspen representation and b) schematics of BrineFlash (BrF) version ... 60
Figure 3.2 a) Aspen representation and b) schematics of BrineMix (BrM) version ... 61
Figure 3.3 Schematic diagram of the Base configuration, BrF version. ... 62
Figure 3.4 Schematic diagram of the Base configuration, BrM version. ... 62
Figure 3.6 Schematic diagram of the Pht configuration, BrF version. ... 63
Figure 3.7 Schematic diagram of the Ph1 configuration, BrF version. ... 64
Figure 3.8 Schematic diagram of the Ph1n configuration, BrF version. ... 64
Figure 3.9 Schematic diagram of the Ph1VL configuration, BrF version. ... 65
Figure 3.10 Schematic diagram of the Ph1A configuration, BrF version ... 65
Figure 3.11 Schematic diagram of the Ph1nC configuration, BrF version. ... 66
Figure 3.12 Selection of components in Aspen Plus ... 68
Figure 3.13 Aspen flowsheet for the 1st effect... 69
Figure 3.14 Nassi -Schneiderman flowcharts of the code used with Matlab. ... 70
Figure 4.1 RR of the BrM version ... 76
Figure 4.2 RR of the BrF version with a) N=2 and b) N=3... 77
Figure 4.3 Percentage of feed seawater entering the 1st effect for the Base configuration in a) BrF and b) BrM versions for N=2 ... 79
Figure 4.4 GOR of the models for N=3 ... 82
Figure 4.5 Temperature vs specific duty (Ph1 configuration) ... 84
Figure 4.6 Temperature vs specific duty (Base configuration) ... 85
Figure 4.7 ΔGOR of Ph1nC configuration with N=2 in a) BrF and b) BrM version ... 86
Figure 4.8 Desalted water (green) and feedwater (blue) temperatures vs heat exchanged per kilogram of desalted water in the heat exchangers for the Ph1nC configuration, BrF version (TBT=70°C) ... 87
Figure 4.9 GOR of the models vs TBT for BBT = 40°C ... 88
Figure 4.10 Specific heat transfer areas of the Base models for a) N=2, b) N=3 and c) N=4 ... 89
Figure 4.11 Specific heat transfer areas for a) N=2, b) N=3 and c) N=4 ... 90
Figure 4.12 𝐶𝑐𝑎𝑝, 𝑎𝑛𝑛 of the Base model a) N=2 b) N=3 and c) N=4... 92
Figure 4.13 𝐶𝑐𝑎𝑝, 𝑎𝑛𝑛 and 𝛥𝐶𝑐𝑎𝑝, 𝑎𝑛𝑛 for BBT = 40°C a-b) N=2, c-d) N=3, e-f) N=4 ... 94
Figure 4.14 COW of the Base model (Cth en = 1.24 c$/kWh) for a) N=2, b) N=3 and c) N=4 ... 96 Figure 4.15 COW and ΔCOW (𝐶𝑡ℎ 𝑒𝑛 = 1.24 𝑐$/𝑘𝑊ℎ) for BBT = 40°C a-b) N=2, c-d) N=3, e-f) N=4 ... 97 Figure 4.16 COW of the Base model with 𝐶𝑡ℎ 𝑒𝑛 = 0 for a) N=2, b) N=3 and c) N=4 ... 98 Figure 4.17 COW and ΔCOW (𝐶𝑡ℎ 𝑒𝑛 = 0) for BBT = 40°C ... 99 Figure 4.18 PR vs ΔCOW with 𝐶𝑡ℎ 𝑒𝑛 = 1.24 𝑐$/𝑘𝑊ℎ for a) 2 MED, b) 3 effects-MED, c) 4 effects-MED models ... 102 Figure 4.19 PR vs ΔCOW with Cth en = 0 for a) 2 effects-MED, b) 3 effects-MED, c) 4 effects-MED models ... 103 Figure 4.20 𝛥PR vs 𝛥𝐶𝑂𝑊 with 𝐶𝑡ℎ 𝑒𝑛 = 1.24 𝑐$/𝑘𝑊ℎ for all the models ... 104 Figure 4.21 𝛥PR vs 𝛥𝐶𝑂𝑊 with 𝐶𝑡ℎ 𝑒𝑛 = 0 for all the models ... 105
List of Tables
Table 1.1 Types of water. ... 8
Table 1.2 Water scarcity ... 8
Table 1.3 Water stress ... 10
Table 1.4 Sea water composition ... 14
Table 1.5 Main thermodynamic properties... 14
Table 2.1 Comparison between different feed configurations of MED ... 52
Table 3.1 Approximate values of heat transfer coefficients, h. ... 72
Nomenclature
BBT bottom brine temperature BF backword feed
BPE boiling point elevation BrF BrineFlash
BrM BrineMix
COW cost of water [$/𝑚 ] ED Electrodialysis
EDR Electrodialysis Reversal F water stress ratio
FF forward feed Frz freezing
GOR gain output ratio
HDH humidification- dehumidification maxS maximum salt concentration MD membrane distillation
MED multi-effect distillation MSF multi stage flash
MVCD mechanical VCD NF nanofiltration
N number of effects P/C parallel cross flow PF parallel feed
PR performance ratio
RED renewable energy desalination RO reverse osmosis
sA specific heat transfer area [𝑚 / 𝑘𝑔 ] SD solar distillation
S salt concentration
TBT top brine temperature TDS total dissolved salts TVCD thermal VCD
VCD vapor compression distillation
Δ𝑇 , temperature difference between TBT and BBT
1. Introduction
1 Introduction
Interest in desalination processes is continuously growing due to the increased number of countries affected by water stress and scarcity. This problem is accentuated by ever higher standard of living, the growth of the world population and climate changes.
Desalination is a possible solution to overcome the increasing demand of fresh water and the growing water shortage problem.
Nowadays the main desalination processes are Reverse Osmosis (RO) and Multi-Stage Flash (MSF) desalination, followed by Multi-Effect Distillation (MED).
Among desalination processes, the thermal process of Multi-Effect Distillation (MED) is a promising process. Requiring quite low Top Brine Temperatures (TBT), it is suitable for being coupled with power generation, renewable energies and waste heat recovery. Furthermore, it presents more stability in partial load conditions than the MSF process. In literature, there are many different MED patterns, depending on the combination of heat transfer configurations and flowsheet arrangements used.
Aim of the thesis
In this thesis, different parallel/cross flow (P/C) MED configurations were studied with two, three, and four effects through simulations with the software Aspen Plus for different combinations of TBT and Bottom brine temperature (BBT). The models were different for the way and the entity of heat recovery from streams and for the way the brine from one effect enters in the following: in one version, feed seawater was sprayed from the top of the effect and the brine entered from the bottom and simply partially flashed due to the pressure reduction, in the other version, both feed seawater and brine were mixed and then sprayed from the top of the effect.
1. Introduction
Differently, in this thesis these mass flows were varied in order to reach the maximum recovery ratio (RR) in each effect, this means that brine exiting from each effect was at the maximum allowable salt concentration without calcium sulfate precipitation. The maximum allowable salt concentration is related to the solubility at the temperature of the effect. In order to make a thermodynamic comparison, RRs and Performance Ratios were evaluated for each model and for each combination of BBT and TBT.
The economic evaluation was conducted in terms of desalted water production costs’ differences between each model and a base model.
The aim of this thesis was to study P/C MED configurations for different values of the TBT and the BBT and to determine which were the best configurations and operating temperatures from a thermodynamic and economic point of view.
Outline of the thesis
In this chapter, water uses and resources, the shortage problem and the need for water desalination are discussed.
Chapter 2 presents a classification and a short description of the main desalination technologies and a literature review on MED process.
In chapter 3 the different models studied in this thesis are described; the hypothesis and methods used for the simulations and for the thermodynamic and economic analysis are presented.
In chapter 4 the thermodynamic and economic results are presented and discussed separately and then together.
1. Introduction
1.1 Water resources and uses
Although water covers nearly 71% of the surface of the earth (510 · 10 𝑘𝑚 ), only 2.5% of this water is fresh water, suitable for human consumption and use, and 97.5% is saltwater [1].
Nearly 70% of the available fresh water is frozen in glaciers, while most of the remaining 30% is in underground hard-to-reach aquifers and only approximately 0.3 % flows into rivers and lakes for direct use.
Readily accessible fresh water, which is found in rivers, lakes, wetlands and aquifers, accounts for less than 1% of the world’s fresh water supply.
Figure 1.1 Distribution of water on Earth.
Unfortunately, this water is not distributed evenly throughout the earth’s surface and it is not available in sufficient quantities either when or where it is needed.
Fresh water is a need not only for domestic use but also in agriculture, industry and energy supply sectors. Figure 1.2 shows a breakdown of global fresh water use.
1. Introduction
Figure 1.2 Global use of fresh water. Based on data from UN-Water [2].
As we can see, domestic use constitutes a minor part of the fresh water use. It includes drinking water, bathing, cooking, toilet flushing, cleaning, laundry and gardening.
Agriculture sector is by far the largest water-consuming sector.
To produce enough food to satisfy a person’s daily diet requires about 2,000 – 3,000 litres of water. In contrast, about 2–3 litres are required for drinking purposes, and 20 – 300 litres for domestic needs [3].
The second largest water-consuming sector is the energy supply. Figure 1.3 shows a breakdown of water withdrawals in energy sector.
Water withdrawal is the water removed from a source. Water consumption is the volume withdrawn that is not returned to the source (i.e. it is evaporated or transported to another location) and is no longer available for other uses. It is lost in transmission, evaporation, absorption or chemical transformation, or otherwise made unavailable for other purposes as a result of human use. By definition, withdrawals are always greater than or equal to consumption [4].
1. Introduction
Figure 1.3 Water withdrawals in energy sector (2014) [4].
Power generation is the largest source (88%) of energy related water withdrawals. Replacing the depleted fossil fuel by renewable and sustainable energy resources becomes crucial to decrease the carbon footprint and greenhouse gases emission that are the main reasons of global warming and climate change.
It is estimated that the global fresh water demand rose twice as fast as population growth in the last century [5].
In 2014 the global annual abstractions of fresh water amounted to about 4 · 10 m ([4], [2]).
Figure 1.4 shows the global fresh water demand in the year 2014 and the projections for the years 2025 and 2040, it shows also a breakdown by sector.
1. Introduction
Figure 1.4 Global water demand by sector to 2040 [4].
Withdrawals to meet municipal water demand accounted for 13% of the total in 2014 and are projected to rise to 17% in 2040. Three-fifths of the increase comes from three regions: India, Africa and other developing countries in Asia (excluding China) [4].
It can be noted that in 2014, primary energy production and power generation accounted for roughly 10% of total worldwide water withdrawals and around 3% of total water consumption [4].
1.2 Water classification
Classification of various types of water, shown in Table 1.1, is based on the purpose for which the water is used and, as consequence, on its salinity.
The first water grade is set for safe drinking, household purposes, and several industrial applications. This water category has a salinity range of 5 to 1,000 ppm. This type of water is found in rivers and lakes and can be generated by industrial desalination processes. In
1. Introduction
large cities, various levels of water salinity are used, where water with salinity below 150 ppm is used for drinking while higher salinity water of up 1,000 ppm is used for various household applications. This has proved to be more effective, because the average per capita consumption of the low salinity drinking water is limited to 2 liters/day. On the other hand, the per capita consumption rate for other household purposes is 200-400 liters/day, which is used for cooking, washing, cleaning, gardening, and other purposes. At industrial scale, the most stringent water quality is set by the makeup water for boilers and applications related to the electronic industry and pharmaceuticals. The water quality for this application is limited to a maximum salinity of 0.1 ppm. This high degree of purity is achieved through the use of ion exchangers, which operates on low salinity river water or industrially desalinated water. Other industrial applications call for less stringent water quality than those used for boilers. Applications include chemical reactions, dairy and food, washing and cleaning, and cooling [6].
The second water category has a salinity range of 1,000-3,000 ppm. This type of water is suitable for irrigation purposes and industrial cooling. This applies for higher salinity water, which includes brackish and seawater. The salinity range for brackish water is 3,000-10,000 ppm. Seawater average salinity is 35,000 ppm. Water with salinity above 10,000 ppm is termed as high salinity water [6].
The salinity of seawater varies subject to local conditions, where it is affected by ambient and topographical conditions. For example, enclosed seas have higher salinity than open seas and oceans. Also, seas, which are found in areas of high temperatures or that receive high drainage rates of saline water, would certainly have higher degree of salinity. For example, the salinity of the Gulf water near the shore lines of Kuwait, Saudi Arabia, and the United Arab Emirates may reach maximum values close to 50,000 ppm. On the other hand, the salinity of the Gulf water near the Western shores of Florida, USA, may reach low values of 30,000 ppm. This is because of the large amount of fresh water received from rivers and springs in that area [6].
1. Introduction
Table 1.1 Types of water.
Salinity TDS [ppm] Use
<150 drinkable water
< 1,000 household application
1,000-3,000 irrigation, industrial cooling
3,000-10,000 brackish water
>10,000 high salinity water
30,000-50,000 seawater
35,000 standard seawater
1.3 Water stress and scarcity
Definitions of the terms water stress and water scarcity vary in the literature and the terms are often used interchangeably.
Water scarcity usually relates to per capita availability of fresh water resources. Scarcity can be caused by a genuine lack of water (physical scarcity) or by a lack of water infrastructure (economic scarcity), or a combination of both.
Table 1.2 shows the definition of different degrees of water scarcity.
Table 1.2 Water scarcity [2].
The term “strained” in Table 1.2 is usually replaced by “stress”, as in Figure 1.5. This can be confusing because it can be easily muddled with environmental water stress, later defined.
1. Introduction
According to FAO [7], the global annual internal renewable water per capita is 5,829 m3,
but, for example, in Northern Europe (Denmark, Faroe Islands, Finland, Iceland, Norway, Sweden) is 31,500, in Middle East is 1,444 and in Northern Africa is only 256, which is slightly lower than the water threshold crisis.
Figure 1.5 shows the total annual renewable water resources per capita by country.
Figure 1.5 Per capita total annual renewable water resources per capita in 𝒎𝟑 (2013)
[8].
Based upon the investigations conducted by the World Health Organization (WHO), annual water availability of 1,000 m3 per capita constitutes the limit below which it will not be
possible to guarantee an acceptable living standard as well as economic development of a country [9] .
Environmental water stress is related to fresh water abstractions. If a region is experiencing water stress this means that fresh water abstractions are occurring at rates higher than natural recharge rates. Consequent reductions in lake and river water levels can have catastrophic
1. Introduction
Depletion of groundwater aquifers is much less visible but can have equally dire consequences including: reduced flow of natural springs with consequent effects on downstream rivers and lakes; increased groundwater salinity owing to ingress of seawater into fresh water aquifers; and lowering of the water table which increases depths of wells and bores with consequent increases in energy required for pumping [2].
Water resource depletion is usually temporary in the sense that recovery occurs when abstractions cease. Ecosystem destruction and increased ground water salinity may however be long term or permanent consequences [2].
A water stress ratio (F) can be defined as the ratio of quantity abstracted divided by quantity of renewable water available.
The definition of different degrees of water stress is shown in Table 1.3.
Table 1.3 Water stress [2].
Water scarcity is a growing problem for large regions of the world. Currently, one fifth of the world's population is facing scarcity in water resources. Another one quarter do have access to water, however they lack proper treatment methods to make it potable. By 2030, this water shortage is expected to affect up to 40% of world inhabitants [10].
Climate change adversely affects the water resources of a region due to the frequent droughts. The long-time droughts would usually happen in the regions with high solar radiation level. Therefore, fresh water shortage is related to the regions with high solar radiation levels such as Middle East and North Africa (MENA) countries. Regional droughts are being exacerbated by climate change and the limited non-renewable groundwater
1. Introduction
resources could not sustain the growing population of MENA countries and their increasingly water and energy intensive lifestyles.
1.4 Energy - water nexus
Both water and energy are essential to society.Water and energy have a mutually dependent relationship.
On the one hand, water has become a pressing issue in the energy production. Conventional energy production (coal, natural gas and nuclear power plants) processes require noteworthy amounts of water (see Figure 1.3). Furthermore, biofuel production is strongly increasing the impact on water resources both in terms of quality and quantity. Moreover, the emerging fracking technology requires water during shale gas production but, more importantly, fracking may pose a risk to water resources.
On the other hand, water management depends heavily on the energy sector: obtaining water, transporting and treating it requires significant amounts of energy. The amount of energy required varies. It is influenced by a range of factors, such as topography, distance, water losses and inefficiencies, and the level of treatment necessary. IEA has estimated energy requirement ranges (in terms of electricity and fuel) for various processes in the water sector (Figure 1.6).
1. Introduction
Figure 1.6 Energy use for various processes in the water sector [4].
It has been estimated that electricity accounts for 5% to 30% of the total operating cost of water and wastewater utilities (World Bank, 2012) [8].
The water sector accounted for 4% of global electricity consumption in 2014 [4].
Today’s water infrastructure is not energy efficient enough. By replacing and modernizing assets new opportunities to increase energy savings emerge. Biogas production from waste water treatment or heat recovery from sewage systems may even lead to energy surplus [11]. This interdependency between water and energy leads to choices in one domain affecting direct and indirectly the other.
Desalination is an intensive energy system independently of the type of process, rending energy cost as the major economic problem. Energy consumption of distillation process is higher than any other in the chemical industry and as a consequence, desalinated water is still costly despite the dramatic decline in cost during the recent years. Its price is higher than transportation of the same amount of natural water from a short distance.
1. Introduction
However, transportation or pumping of water from longer distances may be as expensive or more expensive as desalination systems, depending on the distance and the morphology of the site [12].
Increasing the energy efficiency of desalination processes is therefore an important aim to reduce the energy consumption and lower desalted water production costs.
1.5 Seawater desalination
Finding sufficient fresh water resources has become a top priority in the strategic plans of most governments as it affects the potential for economic growth and social well-being of billions of people [10]. One of the most promising solution to overcome the water shortcoming is desalination.
Desalination is a physical procedure of reducing the contents of dissolved salts from waters, brackish and sea water, or any aqueous salt solution in order to collect low-salt content water for any suitable use, such as drinking, industrial, pharmaceutical, municipal, household water [12] or irrigation.
1.5.1 Seawater properties
The main ions found in seawater include Na+, Ca++, K+, Mg++, (SO4) --, and Cl-; also other ions are present in the seawater, but at much smaller concentrations. The chemical composition of open sea is constant; however, the total dissolved amount of dissolved solids changes subject to local conditions. Table shows typical composition of seawater, which has a total salinity of 36,000 ppm. In addition to the dissolved ions, seawater includes a wide variety of fine suspended matter that include sand, clay, microorganisms, viruses, and colloidal matter [6]. The size of these compounds varies over a range of 0.05 to 0.15 µm.
1. Introduction
Table 1.4 Sea water composition [6].
Compound Composition Mass Percent ppm
Chloride Cl- 55.03 19,810.8 Sodium Na+ 30.61 11,019.6 Sulfate (SO4)-- 7.68 2,764.8 Magnesium Mg++ 3.69 1,328.4 Calcium Ca++ 1.16 417.6 Potassium K+ 1.16 417.6
Carbonic acid (CO3)-- 0.41 147.6
Bromine Br- 0.19 68.4
Boric acid H3BO3- 0.04 14.4
Strontium Sr++ 0.07 25.2
Total 100 36,000
The thermodynamics properties of the water vary depending on its salinity. Thus, using accurate thermodynamic properties for a specific salinity is important to evaluate the performance of a desalination process more accurately.
Table 1.5 shows main thermodynamic properties for both seawater (salinity = 36,000 ppm)
and fresh water (salinity = 0 ppm) at 25 °C [6].
Table 1.5 Main thermodynamic properties [6].
Seawater (36,000 ppm) Fresh water (0 ppm) Density [kg/m3] 1,023.8 997 Specific heat [kJ/kg °C] 3.99543 4.186172 Viscosity [kg/m s] 0.960499 0.891807 Thermal conductivity [W/m °C] 0.608656 0.610584
1. Introduction
The thermophysical properties of saline waters are to a first approximation similar to pure water. Extensive data exist for seawater properties. Some primary effects of salinity in water are to lower the specific heat capacity (by about 5% for seawater relative to pure water), to raise the density (by about 3.5% for seawater), and to lower the vapor pressure (about 2% lower for seawater, and reasonably well described by Raoult’s law).
The variation of specific heat capacity with salinity and temperature is shown in Figure 1.7.
Figure 1.7 Seawater specific heat variations with temperature and salinity, S [13].
When water evaporates from seawater, the latent heat of vaporization is the difference between the vapor’s specific enthalpy, which is the same as that for pure water, and the partial specific enthalpy of water in the seawater solution.
1. Introduction
Figure 1.8 Seawater latent heat variations with temperature and salinity, S [13].
Increasing the salinity of seawater lowers the vapor pressure and hence the boiling temperature of seawater is higher than that of pure water at a given pressure by an amount called the boiling point elevation (BPE) [13]. This effect is shown in Figure 1.9.
Figure 1.9 Seawater BPE variations with temperature and salinity, S [13].
BPE is an important effect to take into account for a precise thermal design of desalination systems.
1. Introduction
Additional significant differences between saline water and fresh water stem from the solubility limits of the dissolved ions, including the precipitation of scale-forming salts, such as CaSO , Mg(OH) , and CaCO , and the outgassing of CO as the raw water’s alkalinity and pH shift during H O removal.
Figure 1.10 shows solubility limits for brine obtained from partial evaporation of seawater. Salt concentration limits decrease with increasing temperatures.
Figure 1.10 Solubility limits of saline water (brine from seawater) [6].
1.5.2 Main parameters of a desalination process
There are several parameters which are used to and evaluate the performance and compare desalination processes:
- Recovery Ratio (RR): the ratio between produced desalted water and seawater 𝑅𝑅 =𝑚̇
1. Introduction
- Gain Output Ratio (GOR): flow rates ratio of the distillate and the heating steam (for thermal processes)
𝐺𝑂𝑅 = ̇
̇ =
̇ ∙ @
̇ with 𝛥ℎ = 2.334 MJ/kg
- Performance Ratio (PR): ratio between the distillate flow rate and the furnished heat 𝑃𝑅 =𝑚̇
𝑄̇
𝑘𝑔 𝑀𝐽
- Specific total heat transfer area (sA): total heat transfer area divided by the distillate flow rate
sA =
̇
- Specific thermal energy: ratio between the required heat and the distillate flow rate 𝑠𝑄̇ = ̇
̇ .
1.5.3 Minimum work and minimum heat of separation
Desalination is a work- and/or heat-driven process that undoes the irreversible mixing of salts into water. This separation process requires the least amount of work when it is done reversibly and uses greater amounts of work when the separation process generates entropy through thermal or mechanical irreversibility. In the design or assessment of any desalination process it is therefore important to determine the least, or reversible, work that will be required to remove some percentage of the water from a saline source.
Figure 1.11 shows a schematic diagram of a desalination system. A saline water stream enters the system, and a purified water stream and a concentrated brine stream leave the system. Work is transferred into the system to effect the separation of salts from the fresh water stream, leaving them in the brine stream.
1. Introduction
Figure 1.11 Schematic diagram of a work-driven desalination system [14].
A saline water stream enters the system, and a purified water stream and a concentrated brine stream leave the system. Work is transferred into the system to effect the separation of salts from the fresh water stream, leaving them in the brine stream. For simplicity, we may consider the inlet and outlet streams to have the same pressure and temperature (this in turn implies that the system exchanges heat with the environment at the system temperature). The first and second laws of thermodynamics applied to this system are:
𝑄̇ − 𝑊̇ = 𝑛 ̇ℎ + 𝑛̇ ℎ − 𝑛 ̇ℎ (𝑛 ̇𝑠̃) + (𝑛 ̇𝑠̃) = (𝑛 ̇𝑠̃) + 𝑄̇ 𝑇⁄ + 𝑆̇
In these equations, 𝑊̇ is the rate at which work is done on the system, 𝑄̇ is the rate at which heat is transferred into the system (which is at temperature 𝑇 ), 𝑛̇ is a molar flow rate, ℎ is the enthalpy of mixture per mole, 𝑠̃ is the entropy per mole, and 𝑆̇ is the rate of entropy generation within the system. These equations may be combined to eliminate the heat transfer rate 𝑄̇; with the introduction of the molar Gibbs energy (𝑔 = ℎ − 𝑇𝑠̃), the work of separation is:
𝑊̇ = (𝑛̇𝑔) + (𝑛̇𝑔) − (𝑛̇𝑔) + 𝑇 𝑆̇
Clearly, irreversibility directly raises the work requirements. In the reversible limit (𝑆̇ = 0), the least work of separation is:
1. Introduction
The least work will depend upon what fraction of the water is extracted to the pure water stream (recovery ratio), and this amount rises steadily as that fraction increases. As shown in Figure 1.12 the least work of separation is a strong function of feed salinity and recovery ratio, and a weak function of the product salinity, with slightly less work required to extract a 500 ppm “pure” stream than a 0 ppm pure stream. At 42% water recovery 𝑅𝑅 = ̇̇ from seawater with a 0 ppm purified stream, the least work is 3.7 kJ/kg. For a 5,000 ppm brackish water, the corresponding least work is about 0.4 kJ/kg.
Figure 1.12 Least work of separation as function of recovery ratio at 𝑻𝟎 = 𝟐𝟓°𝑪 [15].
In the case of thermal desalination, minimum heat of separation can be determined similarly. Heat is delivered at a temperature 𝑇 above the system temperature 𝑇 . The mechanical work input is taken to be zero for this purpose (in practice, however, thermal systems require substantial electrical energy for pumping, in addition to the thermal energy requirements). The first and second laws of thermodynamics lead to the heat of separation:
𝑄̇ = (𝑛̇𝑔) + (𝑛̇𝑔) − (𝑛̇𝑔) + 𝑇 𝑆̇
1 − 𝑇 𝑇⁄
1. Introduction
𝑄̇ = (𝑛̇𝑔) + (𝑛̇𝑔) − (𝑛̇𝑔)
1 − 𝑇 𝑇⁄
The least heat of separation as function of recovery ratio and of 𝑇 is shown in Figure 1.13.
Figure 1.13 Least heat of separation as function of recovery ratio at 𝑻𝟎 = 𝟐𝟓°𝑪 [15].
At 42% water recovery from seawater, the least heat is 17.3 kJ/kg for a typical value of 𝑇 =100°C [14].
Obviously, thermal kilojoules are not directly comparable to electrical kilojoules; indeed, the number of thermal kilojoules required to generate an electrical kilojoule depends upon the temperature at which the thermal energy is available and the generation technology applied. Put differently, a kilojoule of low-temperature thermal energy costs only a fraction of a kilojoule of electrical energy. So, the lower energy requirement of a work-driven process relative to a heat-driven process is not meaningful by itself.
The most efficient, large capacity reverse osmosis plants are within a factor of 3 to 4 of the reversible limit. Thermally driven systems are generally within only a factor of 10 or so [16]. The larger difference for thermal systems is to some extent the result of cost-driven design trade-offs. Specifically, a principal irreversibility in thermal distillation processes is entropy produced when heat is transferred through a finite temperature difference in a heater or
1. Introduction
condenser. Such temperature differences can usually be reduced by employing a larger heat exchanger area, but at the penalty of higher capital cost. As a result, the design value of thermal efficiency may be kept low in order to reduce capital expenditures, thus lowering the overall unit cost of water.
In most thermal desalination systems, the brine and product water may both leave at temperatures above that of the inlet seawater, whereas the least heat calculations above assume equal temperatures. This temperature differential represents a loss of available work and degrades thermal performance [16].
As previously said, seawater desalination is an energy intensive process; irreversibilities within the various system components accentuate the energy requirements. Therefore, to identify and reduce the sources of irreversibility within the systems is very important in order to improve desalination plants’ performance and reduce energy consumption.
2. State of the art
2 State of the art
Desalination capacity has grown rapidly over the past half-century as shown in Figure 2.1. The global online capacity reached 92.5 million of 𝑚 /𝑑 in 2017 [17]. During the period of 2007 to 2017, desalination plant online capacity has almost doubled increasing by around 45 million of 𝑚 /𝑑 [17].
Figure 2.1 Cumulative contracted and online capacity, 1965–2017 [17].
Extra large plants (> 50,000 m3/d) constituted about 50 % of the total capacity (2017) but
only 2.5 % of the total number of plants.
Seawater is the main type of raw water ( > 60 %) and is growing faster than other sources as shown in Figure 2.2 Type of raw water (2018)Figure 2.2. The second most used raw water is brackish water (20.5 %).
2. State of the art
Figure 2.2 Type of raw water (2018) [18].
The largest plants are seawater plants while typical brackish plants are for smaller utility users and industrial self-supply.
The worldwide distribution of desalination capacities (million m3/day) in 2017 is shown in
Figure 2.3.
2. State of the art
The MENA (Middle East and North Africa) region produced approximately 47% of the global capacity.
Figure 2.4 presents a breakdown of the different technologies in use.
Figure 2.4 Total global installed capacity by technology in 2015
RO: Reverse Osmosis; MSF: Multi-stage flash; MED: Multi-effect distillation; ED/EDR: Electrodialysis/Electrodialysis Reversal; NF/SR: Nanofiltration/Sulfate
Removal. GWI [19].
The three main desalination processes are Reverse Osmosis (RO), Multi Stage Flash (MSF), and Multiple-Effect Distillation (MED); they will be dominant and competitive in the future [20].
2.1 Desalination technologies
Desalination processes can be divided in two main categories: thermal processes and membrane processes.
The main thermal processes include MSF, MED, TVCD and MVCD. The main membrane processes are RO, ED and EDR.
65% 21% 7% 3% 2% 2% RO MSF MED ED/EDR NF/SR others
2. State of the art
2.1.1 Thermal processes
Thermal desalination processes involve the heating of saline water and collecting the condensed vapor (distillate) to produce pure water. Multi-Stage Flash (MSF), Multiple-Effect Distillation (MED) and Mechanical Vapour Compression Distillation (MVCD) are the main thermal desalination processes. Other types of thermal desalination processes, i.e. solar stills, humidification-dehumidification and freezing, are not found on a commercial scale and are limited to either experimental types or conceptual designs [6]. MSF and MED systems are often constructed in cogeneration plants where power and water are produced simultaneously. This is convenient because both systems require low pressure heating steam which can be easily extracted from the power plant at fairly low cost. The MVCD system is operated solely on electric power [21].
Freezing desalination processes can be included in the category of thermal processes: instead of an addiction of heat, they involve an extraction of heat.
2.1.1.1 Multi-Stage Flash Distillation, MSF
The MSF desalination is a process based on the principle of flash evaporation.
The main energy requirement is thermal energy. Electrical energy is required for auxiliary services such as pumps, dosifiers, vacuum ejectors.
There are two different types of MSF systems: once-through and brine recirculation (the most widely used) systems.
The once-through type (Figure 2.5) is made of two sections: the brine heater (or heat input) section and the heat recovery section.
2. State of the art
Figure 2.5 Once-through MSF [22].
While, the brine recirculation one (Figure 2.6) has also the heat rejection section.
Figure 2.6 Brine recirculation MSF [22].
An actual seawater MSF plant might consist of 19–30 heat recovery stages and 2–3 heat rejection stages.
The stages have a successively lower temperature and pressure (vacuum) that cause flash evaporation of the hot brine; the vacuum is achieved by a steam ejector driven by high pressure steam or by a mechanical vacuum pump.
The feed seawater moves in heat exchangers through the stages to raise its temperature and to condense the water vapor produced for collection as fresh water in each stage.
2. State of the art
After being preheated in the heat recovery section, the feed water enters the brine heater section to raise its temperature to the top brine temperature (TBT), near to the saturation temperature at the maximum system pressure. The heat in the brine heater is furnished by an external heat source, usually condensing steam from a near power plant or an industrial process. The heated brine water then enters the first stage through an orifice. Because of the consequent pressure reduction, it becomes superheated and a small amount of water flashes. The vapor produced passes through a demister and then into the heat exchanger where it is condensed. This process is repeated through the plant as both brine and distillate streams flash as they enter subsequent stages.
Finally, the distillate produced is pumped into a storage tank while the portion of brine that is not recirculated is discharged into the sea.
In the brine recirculation MSF plant, to enhance the MSF process some of the brine discharge is recirculated and mixed with the incoming seawater in the heat rejection section using a brine recirculation pump. Most MSF plants today are of the brine circulation design which reduces the feedwater and therefore the pumping and chemical requirements compared to the once-through type. The disadvantages of recycle are that more pumps and valves are used. Although the heat rejection section is used to improve the efficiency, for small MSF plants the once-through type is preferred for simplicity.
2.1.1.2 Multi-Effect Distillation, MED
Multiple-Effect Distillation (MED), also called Multi-Effect Evaporation (MEE), is the low temperature thermal process of obtaining fresh water by recovering the vapour of boiling sea water in a sequence of vessels (called effects), each maintained at a lower temperature than the last. Because the boiling point of water decreases as pressure decreases, the vapour boiled off in one vessel is used to heat the next one, and only the first one (at the highest pressure) requires external heat from a fossil-fuel boiler, power-plant waste heat, solar or other sources.
2. State of the art
Figure 2.7 Schematic diagram of MED unit [23].
In this process, vapour is produced by flashing and by boiling, but the majority of the distillate is produced by boiling. Unlike an MSF plant, the MED process usually operates as a once-through system without a large mass of brine recirculating around the plant. This design reduces both pumping requirements and scaling tendencies [24].
As with the MSF plant, the incoming feed water in the MED process passes through a series of heaters but after passing through the last of these, instead of entering the brine heater, the feed enters the top effect. The feed water is sprayed or otherwise distributed onto the surface of the evaporator surface (usually tubes) in a thin film to promote rapid boiling and evaporation after it has been preheated to the boiling temperature on the upper section. The surfaces in the first effect, are heated by Steam from Steam turbines of the power plants or a boiler. The steam is then condensed on the colder heat transfer surface inside the effect to heat. The condensate is recycled to the boiler for reuse. The surfaces of all the other effects are heated by the steam produced in each preceding effect. The steam produced in the last effect is condensed in a separate heat exchanger called the final condenser, which is cooled by the incoming sea water, thus preheating the feed water.
2. State of the art
Both the brine and distillate travel down the plant and flash due to progressive reduction in pressure.
There are many possible variations of MED plants, depending on the combinations of heat-transfer configurations and flowsheet arrangements used. They are described more accurately in 2.3.
2.1.1.3 Vapour Compression Distillation, VCD
VCD process relies on the heat generated by the compression of vapor to evaporate salt water.
Figure 2.8 MVCD (a) and TVCD (b) [25].
In a VCD plant, heat recovery is based on raising the pressure of the steam from a stage by means of a compressor. The condensation temperature is thus increased and the steam can be used to provide energy to the same stage it came from or to other stages [50,159]. As in a conventional MED system, the vapour produced in the first effect is used as the heat input to the second effect, which is at a lower pressure. The vapour produced in the last effect is then passed to the vapor compressor, where it is compressed and its saturation temperature is raised before it is returned to the first effect. The compressor represents the major energy input to the system [26].
VCDs are distinguished in mechanical VCDs or MVCDs (Figure 2.8a) and thermal VDCs or TVCDs (Figure 2.8b) on the basis of the method used to compress water vapor.
2. State of the art
MVCD uses a mechanical compressor. The compressor creates a vacuum in the evaporator and then compresses the vapor taken from the evaporator and condenses it inside of a tube bundle. Seawater is sprayed on the outside of the heated tube bundle where it boils and partially evaporates, producing more vapor.
The main problems associated with the MVCD process are [151]:
- Vapour containing brine is carried over into the compressor and leads to corrosion of the compressor blades.
- There are plant-size limitations because of limited compressor capacities.
In the TVCD, called also steam-jet VCD, a Venturi orifice at the steam jet creates and extracts water vapor from the evaporator, creating a lower ambient pressure. The extracted water vapor is compressed by the steam jet. This mixture is condensed on the tube walls to provide the thermal energy, heat of condensation, to evaporate the seawater being applied on the other side of the tube walls in the evaporator.
TVCDs require availability of steam. The required pressure is between 2 and 10 bar and due to the relatively high cost of the steam, a large number of evaporative-condenser heat recovery effects are normally justified [26].
These units are usually used in small- and medium-sized applications. MVCD capacity ranges between 100 and 3000 m3/d, and TVCD capacity ranges between 10,000 and 30,000
m3/d [23]. The larger units power consumption is about 8 kWh/m3 of product water [20].
2.1.1.4 Other thermal processes
Humidification–Dehumidification (HDH)
These units consist of a separate evaporator and condenser to eliminate the loss of latent heat of condensation. The basic idea in humidification–dehumidification (HD) process is to mix air with water vapor and then extract water from the humidified air by the condenser. The amount of vapor that air can hold depends on its temperature. Some advantages of HD units are the following: low-temperature operations, able to combine with renewable energy sources such as solar energy, modest level of technology, and high productivity rates. Two different cycles are available for HD units: HD units based on open-water closed-air cycle, and HD units based on open-air closed-water cycle. These two options are described below.
2. State of the art
Figure 2.9 Humidification-dehumidification systems: (a) open-water closed-air cycle, and (b) open-air closed-water cycle [25].
Figure 2.9a shows an open-water closed-air cycle. In the process, seawater enters the system, is heated in the solar collector, and is then sprayed into the air in the evaporator. Humidified air is circulated in the system and when it reaches the condenser, a certain amount of water vapor starts to condense. Distilled water is collected in a container. Some of the brine can also be recycled in the system to improve the efficiency, and the rest is removed [27]. Figure 2.9b shows an open-air closed-water cycle, which is used to emphasize recycling the brine through the system to ensure a high utilization of the salt water for fresh water production. As air passes through the evaporator, it is humidified. And by passing through condenser, water vapor is extracted [28] in [25] .
2. State of the art
Figure 2.9 Humidification-dehumidification systems: (a) open-water closed-air cycle, and (b) open-air closed-water cycle [25].
Freezing desalination processes
Extensive work was done in the 1950s and 1960s to develop freezing desalination. During the process of freezing, dissolved salts are naturally excluded during the initial formation of ice crystals. Cooling saline water to form ice crystals under controlled conditions can desalinate seawater. Before the entire mass of water has been frozen, the mixture is usually washed and rinsed to remove the salts in the remaining water or adhering to the ice crystals. The ice is then melted to produce fresh water [29].
Therefore, the freezing process is made up of cooling of the seawater feed, partial crystallization of ice, separation of ice from seawater, melting of ice, refrigeration, and heat rejection.
There have been several processes developed to pilot plant status. These include the triple point, secondary refrigerant, indirect, eutectic, and hydrate processes. The advantages of freezing include a lower theoretical energy requirement, minimal potential corrosion, and little scaling or precipitation. The disadvantage of freezing involves handling ice and water mixtures which are mechanically complicated to move and process. A small number of plants have been built over the past 50 years, but the freezing process has not been commercialized successfully to produce fresh water for municipal purposes [20].
2. State of the art
At this stage, freeze-desalting technology probably has better application in the treatment of industrial wastes than in the production of municipal drinking water [29].
Rapid Spray Desalination
Rapid Spray Desalination (RSD) is a newly developed technology that makes effective use of waste heat generated from gas-fired electrical generating plants, wastewater treatment plants, landfills, marine engines, and other sources, for desalination or for the recovery of dissolved substances.
Water under pressure is nebulized into a moving hot air stream. Because of the extremely high surface area of the water droplets, the water vaporizes instantly and efficiently. By careful control of the system, salt that remains behind is concentrated into a brine for material handling purposes [30].
2.1.2 Membrane processes
Based on the driving principle, membrane desalination processes can be categorized in: - Pressure-driven membrane processes, such as Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF) and Microfiltration (MF).
- Electrical potential-driven membrane processes, such as Electrodialysis (ED). - Temperature-driven membrane processes, such as Membrane Distillation (MD). The two main commercial processes are RO and ED.
2.1.2.1 Reverse Osmosis, RO
Reverse osmosis (RO) is a form of pressurized filtration in which the filter is a semi-permeable membrane that allows water, but not salt, to pass through.
This yields permeated fresh water and leaves a concentrated solution on the high-pressure side of the membrane. It has four subsystems (Figure 2.10): pre-treatment, high-pressure pump, membrane, and post-treatment.
2. State of the art
Figure 2.10 Major subsystems in a RO system [25].
Feed water pre-treatment involves filtration, sterilization, and addition of chemicals to prevent scaling and biofouling. The high-pressure pump generates the pressure needed to force the water to pass through the membrane; therefore, the energy needed is electricity to drive the pumps. The pressure needed for desalination ranges from 17 to 27 bars for brackish water and from 55 to 82 bars for seawater. The membranes are designed to yield a permeate water of about 500 ppm and made in a variety of configurations. Several types of membrane are available in the market, with the two most commonly used ones being spiral-wound and hollow fine fiber. The post-treatment removes gases such as hydrogen sulfide and adjusts pH.
RO is a mature technology and it is the most commonly used desalination technique. Its installed capacity ranges between 0.1 m3/day (used in marine and household applications) to
395,000 m3/day (for commercial applications) [23].
2.1.2.2 Electrodialysis (ED) and Electrodialysis Reversal (EDR)
Electrodialysis (ED) is an electrochemical separation process operating at atmospheric pressure that uses direct electrical current to move salt ions selectively through a membrane, leaving fresh water behind.
It was commercially introduced in the early 1960s, about 10 years before RO. Its development provided a cost-effective way to desalt brackish water [29].
2. State of the art
Figure 2.11 Ion exchange in ED [25].
This system, shown schematically in Figure 2.11, works by reducing salinity by transferring ions from the feed water compartment, through membranes, under the influence of an electrical potential difference. The process utilises a DC electric field to remove salt ions in the brackish water. Saline feedwater contains dissolved salts separated into positively charged sodium and negatively charged chlorine ions. These ions will move towards an oppositively charged electrode immersed in the solution. Special membranes, alternatively cation-permeable and anion-permeable, separate the electrodes so that the centre gap between these membranes is depleted of salts.
In an actual process, a large number of alternating cation and anion membranes are stacked together, separated by plastic flowspacers that allow the passage of water. The streams of alternating flow-spacers are a sequence of diluted and concentrated water which flow in parallel to each other. The ED unit consists of a pre-treatment system, a membrane stack, a low-pressure circulation pump, a direct-current power supply (rectifier or photo-voltaic system), and a post-treatment system.
As the energy requirements of the system are proportional to the water’s salinity, ED is more feasible when the salinity of the feedwater is not more than about 6000 ppm of dissolved solids. Similarly, due to the low conductivity, which increases the energy requirements of
2. State of the art
very pure water, the process is not suitable for water of less than about 400 ppm of dissolved solids.
As the process operates with DC power, solar energy can be used with electrodialysis by directly producing the voltage difference required with photovoltaic (PV) panels [26].
Electrodialysis reversal (EDR) is a modification of the basic ED process. An EDR unit operates on the same general principle as a standard ED plant, except that in EDR, the polarity of the electrodes is switched periodically, reversing the flow through the membranes. The concentrate stream is then converted to the feed stream and the feed stream becomes the concentrate stream. This inhibits deposition of inorganic scales and colloidal substances on the membranes without the addition of chemicals to the feed water [25]. This development considerably enhances the viability of this process because the process is self-cleaning. Instead, when the membranes are operated in the same direction all the time, precipitant can build up on the concentrate sides [23]. EDR requires minimum feed water pre-treatment and minimum use of chemicals for membrane cleaning [25].
An ED plant’s typical capacity ranges from 2 to 145,000 m3/day [23].
2.1.2.3 Membrane Distillation (MD)
Membrane desalination is a promising process, especially for situations where low temperature solar, geothermal, waste, or other heat is available. MD was introduced commercially on a small scale during the 1980s, but it has not demonstrated large-scale commercial success due to the high cost and problems associated with membranes [25]. MD combines both the use of distillation and membranes. In the process (Figure 2.12), saline water is warmed to enhance vapor production, and this vapor is exposed to a membrane that can pass water vapor but not liquid water. After the vapor passes through the membrane, it is condensed on a cooler surface to produce fresh water. In the liquid form, the fresh water cannot pass back through the membrane, so it is trapped and collected as the output of the plant [29].
2. State of the art
Figure 2.12 Membrane Distillation [25].
The main advantages of MD are its simplicity and the need for only small temperature differentials to operate. This has resulted in the use of membrane distillation in experimental solar desalting units. However, the temperature differential and the recovery rate, similar to the MSF and MED processes, determine the overall thermal efficiency for the MD process. Thus, when it is run with low temperature differentials, large amounts of water must be used, which affects its overall energy efficiency [29].
More intensive research and development is needed, both in experimentation and modelling, focusing on key issues such as long-term liquid/vapor selectivity, membrane aging and fouling, feed-water contamination, and heat-recovery optimization. Scale-up studies and realistic assessment of the basic working parameters on real pilot plants, including cost and long-term stability, are also considered to be necessary [31] in [25].
2.1.2.4 Forward Osmosis
Forward osmosis (FO) is an emerging desalination technology that has garnered an increasing amount of attention in recent years. FO is a process that utilizes osmotic pressure to drive water from a contaminated or saline water feed solution across a semi-permeable membrane that retains the dissolved solutes.
2. State of the art
This flux is driven by osmotic pressure generated by a draw solution or osmotic agent. This osmotic separation requires little energy (only the pumping of fluids to the membrane element) as it occurs spontaneously due to the tendency toward thermodynamic equilibrium. The draw solution contains dissolved solutes that are also, in ideal circumstances, retained by the membrane. After osmosis, this diluted solution is sent to a secondary separation process that recovers the solute and recycles it while liberating clean, drinkable water from the draw solute. It is this secondary process that requires some energy. The choice of draw solute, which will be discussed in more detail later, determines the process required and the amount and type of energy needed to run it. A schematic diagram of the process is shown in Figure 2.13.
Figure 2.13 Schematic diagramof FO process [32].
When designed with an appropriate membrane and draw solution, FO promises to enable low cost desalination with improved recovery and fouling resistance. The road to commercialization still contains a number of technical hurdles. Membranes must be designed specifically for FO while retaining the high permselectivity of conventional reverse osmosis membranes. Draw solutes must be designed for high solubility, easy removal, and low toxicity. These challenges, while considerable, have not deterred a substantial worldwide research effort on forward osmosis. Commercialization of FO hinges on continued work in these area while eventual successful demonstration on the pilot scale will secure FO in its place among conventional desalination technologies [32].
2. State of the art
2.1.3 Comparison of the different technologies
A comparison of the different desalination technologies is shown in the following figures. In Figure 2.14 a comparison about maximum salinity of feed water and minimum produced water (salinity).
Figure 2.14 Different desalination systems capabilities regarding salinity of feed and produced water measured in ppm [33].
A comparison of thermal and energy requirements per 𝑚 of produced water volumes for the desalination technologies is shown in Figure 2.15. Among thermal processes, HDH shows the highest request of thermal energy, MED shows lower energy requirements than MSF.
2. State of the art
A comparison of water production cost (COW) of different desalination technologies is shown in Figure 2.15. RO has one of the lowest COW and has a lower COW than MSF and MED.
Figure 2.16 Water production cost for different desalination technologies [33].
2. State of the art
2.2 Renewable energy and desalination processes
The integration of the renewable energy into water desalination systems has become increasingly attractive due to the growing demand for the water and energy, and the reduction of the contributions to the carbon footprint. The intensive investigations on the conventional desalination systems, especially in the oil-rich countries have somewhat overshadowed the progress and implementation of the renewable energy desalination (RED) systems. The economic performance evaluation of the RED systems and its comparison with conventional systems is not conclusive due to many varying factors related to the level of technology, the source of energy availability, and the government subsidy. The small RED plants have a high capital cost, low efficiency and productivity which make RED systems uncompetitive with the conventional ones. However, the selection of the small RED plants for the remote arid areas with small water demands is viable due to the elimination of the high cost of the water transportation, and the connection to the electricity grid [1].
2. State of the art
2.3 Literature review on MED
MED plants can be divided in High Temperature MED (HT-MED), when TBT is higher than 90°C and Low Temperature MED (LT-MED), when TBT is less than 90°C.
Operating at higher temperatures results in a decrease of heat transfer area but the PR is almost independent of the TBT. However, operation at lower temperatures has many advantages. Firstly, it avoids scaling and corrosion problem. TBT is limited to about 75°C with antiscalant treatment, even if it is possible to operate at higher TBT by using acid treatment of the feed water [34].
Furthermore, LT-MED results in lower energy consumption. LT-MED allows to make effective use of low-cost, low-grade heat or even zero cost waste heat.
MED plants can operate alone or in combination with mechanical vapor compression (MED-MVC), thermal vapor compression (MED-TVC), absorption heat pump (MED-ABS) or adsorbtion heat pump (MED-ADS). These combinations can increase the PR.
In recent years, research in thermal desalination has focused on the efficient use of low-temperature waste heat for powering a thermally-activated cycle. The rationale of utilizing waste heat is that if unused otherwise, it would have been purged into the ambient. The cost of thermal input to the cycle is therefore deemed as free energy, i.e., non-payable, and this is akin to evaporation of seawater in oceans and rivers of the natural water cycle, utilizing solar thermal energy of the sun [35].
2.3.1 Types of evaporators
The main components of a MED plant are the evaporators.
There are several types of evaporator construction that have been used in desalination processes: submerged tube evaporators, falling film evaporators (horizontal and vertical tube evaporators), and plate evaporators.
2. State of the art
Submerged tube evaporators
In submerged tube evaporators (Figure 2.18), heating steam condenses on the wall of the tube in the first effect, and releases its latent heat to a thin layer of liquid surrounding the outside surfaces of the tubes. This results in the formation and release of vapour bubbles, which rise through the liquid and are released into the vapour space. The formed vapour is routed to the second effect, where it condenses on the wall of the tube and results in the formation of a smaller quantity of vapour. The vapour released in the second effect can be either routed to another effect or condensed against the feed seawater.
They were used in small industrial desalination units during the first half of the twentieth century. These early units were plagued with rapid fouling and scaling of the outside surface of the tubes. This required lengthy and expensive cleaning procedures of the tube bundle [21].
Figure 2.18 Elements of two-effect submerged evaporator [21].
Another drawback of the submerged tube evaporator is the reduction in the overall heat transfer coefficient, caused by the static head of liquid surrounding the outside surface of the tube. This hinders the formation, growth and release of vapour bubbles.
Falling film evaporators
The falling film evaporator eliminates the drawbacks of the submerged tube evaporator. There are two arrangements for the falling film system which include horizontal or vertical tubes.
