CHAPTER I
INTRODUCTION
1.1
General
the epoxy resin technology had its genesis in research
conducted in the United States and Europe just before the World
War-II. The first series of resins-reaction products of
epichlorohydrin and bisphenol-A was produced commercially in
1947. In ten years a production volume of some 30 million pounds
was achieved and this being tripled six years later.
In the late 1950s, a number of new epoxy resins, different in
kind from the earlier diglycidyl ethers, were introduced and by the
end of 1960 at least 25 distinct types of resins were commercially
available. At this point the term epoxy resins became generic. It is
now applied to a wide family of materials. The epoxy resins can
duplicate the performance of most other thermosetting plastics and
exceeds their performance in variety of specialized applications. In
recent polymer age, epoxy resins have gained increasing
importance due to their wide range of applications in many fields
such as coatings, electrical, electronics, casting resins, dipping
compounds, moulding powders and reinforced plastic industries. Because of their
high strength, versatility and excellent
adhesion to variety of surfaces, epoxy resins have gained wide
acceptance by diverse users. They have revolutionized joining and
fastening technology in some industries. Epoxides are used to bond
metals, glass, ceramics, wood, many plastics, concrete and other
surfaces.
A number of properties have led to rapid growth of epoxy
resins and their use in wide range of industries /3,4/.
1. Versatility: The epoxy resins are probably most versatile of
the contemporary plastics. The basic properties can be modifies in
many ways: by blending of resins types, by selection of curing
agents, and by the use of modifiers and filler.
2 Easy Cure: Epoxy resins cure quickly and easily between
temperature range from 5°C to 150°C, depending on the selection of
curing agent.
1
Chapter 1
3. Low Shrinkage: One of the most advantageous properties
of the epoxy resins is their low shrinkage during cure. Phenolic
casting resins, which evolved water, reveal high shrinkage, as do
the acrylic and polyester resins, which must rearrange and reorient
1
considerably in the liquid and semigelled phase. Epoxy resins react
with very little rearrangement and with no volatile by-products
being evolved.
4. High adhesive strength: Because of the chemical make up,
chiefly the presence of polar hydroxyl and ether groups, the epoxy
resins are excellent adhesives. The resins cure with low shrinkage,
so that the various surface contacts set up between the liquid
epoxy-resin formulation and the adherends are not disturbed
during cure. Adhesive strengths, without the need for either upon
the time or high pressures, are perhaps the best obtainable in the
contemporary plastic technology.
5. High mechanical properties: The strength of properly
formulated epoxy resins usually surpasses that of the other types
of the casting resins. This is probably in part a result of their low
shrinkage, which minimizes stresses that otherwise would weaken
mechanical structure.
6. High electrical insulation: Epoxy resins are excellent
electrical insulators.
7. Good chemical resistance: The chemical resistance of the
cured epoxy resin depends considerably on the curing agent used.
Selectively outstanding chemical resistance can be obtained by
specification of proper materials. Overall, most epoxy resins
possess extremely high resistance to caustics and good to excellent
resistance to acids.
8. Low creep: The cured epoxy resins, maintain their shape
under the prolonged stresses.
9. 100% solid: Unlike the phenolic and some other resinous
adhesives, the epoxies cure without releasing water of other
2
Chapter 1
condensation by-product. This makes it possible to bond the epoxy
resins at only contact pressure or with no pressure at all.
EPOXY RESINS
Epoxy resins comprise a group of crosslinkable materials,
which all possess the same type of reactive functional group, the
epoxy or oxirane group (I). Their chemistry and technology have
been reported in number of texts /1,6/.
The non epoxy part of the molecule may be aliphatic,
cycloaliphatic or highly aromatic hydrocarbon or it may be nonhydrocarbon
and
possibly polar. It
may also contain unsaturation.
The
capability of the
epoxy ring
to
2
react
with
a variety
of
substrates
imparts
versatility
to
the
resins.
Treatment
with
curing
agents
gives
insoluble
and
intractable
thermoset
polymers.
In
order
to
facilitate
processing
and modify
cured resin
properties,
other
constituents
such
as fillers, solvents,
diluents,
plasticizers,
accelerators,
etc
may be
included
in
the compositions.
To meet the different requirements of broad sweep of
applications /1, 5-16/, epoxy resins of many types are available in
combination with the variety of curing agents which contribute the
essential versatility to the epoxy system. In the following parts of
the section, commercially important epoxy resins are discusse
3
CURING AGENTS
Optimum performance properties are obtained by crosslinking the epoxy resins into a three dimensional insoluble and
infusible network. The resin is treated with a curing agents or
hardener. Its choice depends on processing methods, curing
conditions and the physical and chemical properties desired.
Curing agents are either catalytic or co reactive. A catalytic
curing agent functions as an initiator for epoxy resin
homopolymerization, whereas the co-reactive curing agent acts as
a comonomer in the polymerization process.
CATALYTIC CURING AGENTS
Catalytic curing agents can be used for homopolymerization,
as a supplemental curing agent with polyamines or polyamides, or
as accelerator for anhydride cured systems. The widely used
catalytic curing agents are as follows.
(1)
Lewis bases
Lewis bases such as tertiary amines are used for the
homopolymerization of epoxy resins. Cure with primary or
secondary amines gives tertiary amines, which function as catalyst
for homopolymerization. Tertiary amines such as triethylamine,
benzyldimethyl amine and Dimethyl diamino phenol have found
uses in adhesive and coating applications/18/. They are also used
as accelerators for the curing of anhydride and dicyandiamide
based systems /17/. While imidazoles are used as both hardener
and accelerator /82/.
(2)
Lewis Acids
The most commonly used lewis acids as catalytic curing
agents are boron trifluoride adducts. e.g. Boron trifluoride
monoethylamine (BF
3
NH
2
-C
2
22
H
5
). It is a crystalline material and
cures epoxy resins at 80-100°C. It has been identified as a catalyst,
an accelerator and a crosslinking agent /83-89/.
4
(3)
Photoinitiator
Photoinitiated cationic curing of epoxy resins is a rapidly
growing method for the application of coatings from solvent free or
high solids systems /90/. This method is used for the fabrication
of can coatings /91/ and photoresists /92/. The most efficient and
Chapter 1
effective Photoinitiators for the curing of epoxides are
aryldiazonium salts /93-95/, diaryiodonium salts /96/ and
triaryls ulfonium salts /97/. Upon UV irradiation,
Photoinitiators yield a Lewis acid, which cures the epoxy resins in
the conventional manner.
FILLERS
Fillers are used with epoxy resin systems mainly to reduce
costs /113-114/. They also reduce curing shrinkage, lower the
coefficient of expansion, reduce exotherm and may increase
thermal conductivity. Commercial fillers may be organic and
spheroidal, granular or fibrous in shape. They are chalk powder.
Silica, mica, metal oxide, phenolic micro balloons etc. They are
used in tooling, casting, molding or encapsulating epoxy systems.
ANGE OF APPLICATIONS
Epoxy resins are commercially used in coating and
structural applications. Through the proper selection of resin,
modifier and curing agent, the cured epoxy resin system can be
tailored to specific performance characteristics. The choice depends
on cost, processing and performance requirements. Cured epoxy
resins exhibit excellent adhesion to a variety of substrates;
outstanding chemical and corrosion resistance; excellent electrical
insulation; high tensile, flexural and compressive strength, thermal
stability; a wide range of curing temperatures and low shrinkage
upon cure.
Because of their versatility, the epoxy resins are used in
thousands of industrial applications: Anticorrosion & antifouling
coatings /122-123/, honeycomb structure /124/, for paint brush
bristles and for concrete topping compounds /125/, body solders
and caulking compounds for the repair of plastics and metal boats,
for automotive springs and casting compounds /126/, abrasive &
water lubricated conditions for pump applications /127/,
electrochemical sensing /128/, conducting adhesives as a lead free
alternative in electronic packaging /129/, electro conductive resins
filled with graphite for casting application /130/, electronic
31
5
Chapter 1
packaging /131/, potentiometric sensor for perchlorate ions
/132/, for fabrication of stamping dies, patterns tooling ; caulking
and salient compounds in building and highway construction
applications /133-135/, for cryogenic use /136/, high voltage
composite insulator /137/and in applications where high orders of
chemical resistance are required.
Other applications include potting and encapsulating
compounds /138-141/ and as impregnating resins for aerospace
applications, filament wound structure and tooling fixtures /142143/,
adhesives
for
aircraft
/144,145/,
aerospace
textile
composites
/146/, composite materials in space environment for
development of a moon base /147/, in high performance vehicle
/148/
Epoxy based solutions are used as maintenance and product
finishes, marine finishes, structural steel coatings, tank coatings,
aircraft finishes, automotive primers and furniture finishes
/149,150/. They are also used in decorative floor applications; as
chemical resistant mortar and floor topping compounds
/151,152/, in printing ink, dental and surgical applications /153157/,
for cholesterol
level
lowering
agents
/158/,
for voltammetric
sensors
/159/,
for
humidity
sensors
/160/,
for manufacturing
the
jet-printed
LCD
color
filter
/161/,
in fuel
cells
6
/162/,
for
optical
ammonia
sensor
/163/,
for
marine
primer steel
coating
applications
/164/ and for light weight chemical resistant foams
Coating Application of Resins
The prepared resins were applied on mild steel and tin
panels of required sizes as per IS specifications using DETA and
DDS as the curing agents. Following studies are incorporated in
this chapter.
The coating applications of epoxy resins were studied by
applying the resin systems on the mild steel panel having
dimensions 150 X 50 X 1.25 mm [quality as per IS:513;.1963] with
flat brush confirming to IS:384-1964. The coated mild steel panels
were tested by drying time i.e. tack free time, scratch hardness,
impact resistance, acid and alkali resistance, water and weather
resistance and solvent resistance. For the determination of
flexibility and adhesion, tinned steel panels having dimensions 150
X 50 X 0.315 mm as per Indian Standard Specification [IS: 197 1969]
were
used.
The
flexibility
was
measured
using
two
different
size
of rods
viz., 114"
and 118"
rod by conical mandrel.
EPOXY RESINS
Epoxy is a copolymer; that is, it is formed from two
7
different chemicals. These are referred to as the
"resin" and the "hardener". The resin consists of
monomers or short chain polymers with an epoxide
group at either end. Most common epoxy resins are
produced from a reaction between -epichlorohydrin
and bisphenol-A, though the latter may be replaced
by similar 45 chemicals. The hardener consists of
polyamine monomers, for example
triethylenetetramine (TETA). When these compounds
are mixed together, the amine groups react with the
epoxide groups to form a covalent bond. Each NH
group can react with an epoxide group, so that the
resulting polymer is heavily cross linked, and is thus
rigid and strong.
a. Excellent adhesion to different materials.
b. High resistance to chemical and atmospheric
attack.
c. High dimensional stability.
d. Free from internal stresses.
e. Excellent mechanical and electrical properties.
f. Odorless, tasteless and completely nontoxic.
g. Negligible shrinkage.
1 . 2 Background History
In the early centuries, humans thought that the taste of water is determined by its
purity. The ancient Greek are said to have used evaporation from sea water to obtain drinking
water. The Greeks and Romans are well known for their elaborate water systems. It is said
that the first desalination plant in America turned sea water to drinking water at Fort Zachary
Taylor in Key West Florida as early as 1861, but the use of modern desalination
technology dates from the beginning of the last century.
During World War II, desalination technology was then called as desalting, and was
said to be developed in order to convert saline water into usable water, where supplies for
fresh water were limited. In 1952, the congress passed the ‘‘Saline Water Act’’ to provide
federal support for desalination. During the 1950s and 60s fund was provided by the U.S.
Department of the interior, through the Office of Saline Water (OSW) for the initial
development of desalination technology and for construction of demonstration plants.
Desalination is absolutely a new technology that has developed to reach a very large point
during the latter half of the 20th century, and still continues to undergo technological
improvements at the present time.
By the year 1999, desalination plants have spread throughout the world, with over
more than 11,000 desalination plants in operation, producing over 20 million cubic meters
(approximately six billion US gallons) of water per day. In Western Asia and Middle East
8
Asia exists about 63% of the capacity, North America has about 11% and Europe and North
Africa account for about 7% each of the capacity. The sizes of plants and designs range from
more than 500,000m3/day down to 100m3/day, and low pressure products can produce as
little as a few liters per day for home point of-use application. By the end of the year 2001 a
total of 15,233 large desalting units had been installed, providing a total capacity of 32.4
million
1.3 Necessity
The availability of drinking water is essential for the survival all of mankind.
Adequate amount of water resource is available on our planet but very few of them can be
used for the purpose of drinking. A number of water purification methods have been put forth
by researchers to purify brackish water and sea water. As water purification processes require
some sort of energy source, and with the advent of renewable sources of energy utilization in
various fields, a thorough review of solar powered purification setups is essential.
1.4 Water supply for future purposes
The future of water supply requires adequate sustainability to be able to
effectively supply and support the world’s increasing population. Our global economy needs
a continuous growth. Water is significant to the future, but also one of the major limiting
factor to growth, due to the fast increasing population growth and economic development.
Water resources in many parts of the world are been pushed to their natural limits. In return,
the possibilities of countries and cities to grow, attract investment, meet the primary needs of
populations and ensure environmental protection will be extremely threatened, if there is no
adequate or proper water resource management. Water is a significant resource that enables
life’s growth and increases all human activities. It is now important that every industry,
public and policymakers understand, identify and act.
1 . 5 Objectives
The aim and target of this thesis is to introduce different desalination technologies in
solving water scarcity in the globe, especially in countries where access to clean drinking
water is limited, like the ESCWA member countries and also adopting suitable desalination
technologies in those countries that face water problems.
9
Desalination is one of the main significant factors in substituting water shortage in
different countries. The scope of this thesis is detailed in ex-plaining different desalination
technologies in water treatment from sea-water, brackish water and wastewater in general
level. Then it goes into a more detailed level in introducing desalination technology in
Kuwait and the United Arab Emirates (UAE). At the same time different desalination
technologies will be introduced. A case study reverse osmosis desalination plant in Nuweiba
city of seawater desalination in the Arabian Gulf and will be conducted and suitable
technology will be suggested.
The objectives and study of this thesis seek to examine the various measures that have
been taken by ESCWA countries to upgrade their water resource management and derive
therefrom relevant lessons and specialized knowledge that may be of benefit to the other
countries. Another objective is to contribute to the enhancement of the capacity of member
countries to analyze and evaluate the effectiveness of their water resource management and
identify weaknesses and any measures that should be reformed or strengthened.
1 . 6 Summary
Desalination methods are used to convert saline/brackish water to drinkable
freshwater. Major processes use either thermal energy (conventional distillation) or pressure
energy (Reverse osmosis). Different methods of desalination are discussed and their influence
on overall water production has been highlighted. With the increase in appreciation for a
green technology, desalination methods using renewable/waste energy are drawing
significant attention in recent years. Applying different methods of desalination for coastal
areas in Peninsular Malaysia can be very promising in terms of overall public health and
economy.
10
CHAPTER II
LITERATURE REVIEW
Hefei Zhang et al present a hybrid solar desalination process of the multi effect
humidification dehumidification and the basin type unit. The solar evacuated tube collector
is employed in the desalination system, multi-effect humidification dehumidification
desalination (HDD) process is calculated, and then the water excluded from the multi-effect
HDD process is reused to desalinate in a basin-type unit further ahead. The research proves
that the multi-effect HDD has much room to be enriched.
Guangping Cheng et al proposed a solar desalination process using air humidification and
dehumidification. In order to increase the output of freshwater, the double pass solar air
heater and tubular solar collector are used to heat the air and seawater respectively. The air is
humidified by bubbling in the seawater pool, and dehumidified in the inorganic heat pipe
condenser. The heat transfer performance of the solar air heater with double vacuum glasscovers and double air passes is studied, and the theoretical model of its heat transfer and
the calculation methods are given.
M. Amidpour et al experimentally evaluate and optimize the humidification–
dehumidification desalination process for production of fresh water from brackish water.
Experimental results show that two-stage HD desalination unit can increase thermal recovery
in condensers and hence, reduce thermal energy consumption and investment cost of the unit.
Productivity can also be increased by 20% compared with the single-stage unit.
M. Abd Elkader experimented on a three stage multi-effect humidification (MEH)dehumidification process with energy storage system which was designed, manufactured,
11
installed and outdoor tested in the Faculty of Engineering, Suez Canal University, Port Said,
Egypt. The thermal collection part of the system (three flat plate collectors) has been
designed to provide hot water to the desalination chambers. The investigational test results
showed that, the increase of seawater mass flow rate through the setup from 0.1 liters to 0.13
liters increases the efficiency of the system by 10 %. It can be seen from the results also, that
the use of energy storage increases the output by 13.5%.
Julian Blanco et al have analyzed about the AQUASOL Project whose objective is the
development of a lesser expensive and more energy efficient seawater desalination
technology based on Multi-Effect Distillation process with zero brine discharge. Specific
proposed technological developments (new design of CPC collector and absorption heat
pump, hybridization with natural gas and recovering of salt) are expected to both increase the
energy efficiency of the process and process economy. The expected result would be an
improved MED technology with market potentials and suitable to be applied in the
Mediterranean area and similar locations around the world.
Schafer A. et al. described the design and field-testing of a photovoltaic (PV)-powered
desalination system. The system defined was proposed for use in isolated areas of the
Australian outback, where fresh water is particularly inadequate and it is often essential to
drink high salinity bore water. The system based on a hybrid membrane configuration was
implemented, with an ultra-filtration module for eliminating particulates, bacteria and viruses
and a Nano-filtration or RO membrane for removing dissolved salts. The setup created pure
drinking water from a diversity of feed waters, as well as high salinity (3500 mg/L) bore
water. The specific energy consumption varied from 2 to 8 kW h/m3of disinfected and
desalinated drinking water, reliant on the salinity of the feed water and the experimental setup
working conditions.
Neskakis A. et al. reported on the feasibility of a small Photovoltaic driven RO desalination
plant, with a normal daily production of drinking water with discharge of 0.8-3m3/day. They
examined the effect of feed pressure on produced water quality, on plant productivity and on
total energy consumption. At the feed water pressure of 48 bars, the specific energy
consumption of the project was 16.3 kWh/m\3. Productivity of the project was 124 L/h using
permeate concentration of 450 ppm. When the feed pressure was raised to 63 bar, the specific
energy consumption fell to 15 kWh/m3, and productivity improved to 155 L/h with permeate
concentration of 330 ppm.
12
Schmid J. et al. presented a feasibility study of water desalination in Egyptian deserts and
rural areas, where there is a shortage of fresh water in spite of the presence of large sources of
brackish water areas, using photovoltaic energy as the primary source of energy. They also
proposed an economic design of a PV powered small scale reverse osmosis water
desalination system. It was estimated that the cost of producing 1 m3 of fresh water from the
PV-RO system is approximately $3.73. If the system capacity and the daily range of
operation are raised, the cost of generating fresh water will be diminished in these areas.
CHAPTER III
METHODOLOGY
3 . 1 Introduction
Desalination is a process that is used to recover pure water from saline water using
different types of energy. It is also a process of removing salts from water. Saline water has
been classified to be either seawater or brackish water depending on the salinity and water
sources. There are two main streams of water that desalination produces, the freshwater and a
more concentrated stream (brine). The two main commercial desalination technologies are
those based on thermal and membrane processes. With the rapid improvement in technology,
desalination processes are becoming cost-competitive with other methods that are used to
produce usable water for human consumption and growth. Figure shows worldwide installed
capacity for different desalination processes.
13
Figure 3.1 Worldwide installed capacity for different desalination processes.
3.2Thermal Desalination Processes
Thermal desalination is a process whereby saline water is heated to produce water
vapour and in turn collecting the condense vapour (distillate) to produce pure water. Thermal
desalination is not often used for brackish water desalination. The reason being that it
involves high costs. The thermal processes have, however, been used for sea-water
desalination. Thermal desalination process includes, multi-stage flash desalination (MSF),
multiple-effect distillation (MED), vapour compression (VC) and low temperature
evaporation (LTE). In these processes condensed steam is used to supply latent heat that is
required to vaporize the water. Due to their outstanding high energy demands, this process is
basically used for seawater desalination process, and they are able to produce high purity
water suitable for industrial process applications. Thermal process unit capacities are higher
compared to membrane process and also account for 55% of the total production.
14
Thermal Technologies
VC
RO
ED



Solar Thermal



Solar PV



Renewable energy
MSF
MED


Wind





Geothermal





Table 3.1 Thermal desalination technologies which can use energy from renewable
sources.
Parameter
Strengths
RO
MSF & MED
(Membrane based)
(Thermal based )

Lower energy requirements

Relatively simple to operate

Higher water recovery

Capable of producing high
purity water
Weaknesses


Membrane susceptible to

Higher energy requirement
fouling

Lower water recovery
Requirement of thorough
seawater pre-treatment
Table 3.2 Comparison of the three major desalination technology
Figure 3.2 Breakdown of the cost in thermal desalination.
15
3.2.1 Multi Stage Flash Distillation(MSF)
In multi stage flash distillation process, the basic principle is to heat the saline (sea or
brackish) water to a temperature of about 90–120°C using the heat of condensation of the
vapour produced. Then the heated water is then evaporated. The pure water is then obtained
by the condensed vapour produced. The water temperature and pressure increases when
heated in a container, the heated water passes through to another chamber at a lower pressure
which causes vapour to be formed, the vapour is led off and condensed to pure water using
the cold sea water which feeds the first heating stage. The concentrated salt water is then
passed to the second chamber at the same lower pressure as the first stage and more water
evaporates and the vapour is condensed as before. The process is repeated through a series of
chambers until atmospheric pressure is reached. Multi stage flash (MSF) accepts higher
contaminant levels (heavy metals, oil, suspended solids, COD, BOD etc.) in feed sea water. It
also has the capacity of producing distilled quality water product good for power plants, for
industrial processes and many other high purity applications. Figure 23 shows the
schematic diagram of multi-stage flash process.
Figure 3.3 Schematic Diagram of Multi-Stage Flash (MSF) Process.
The main advantages of MSF include the following:

Simple to operate;

Generates high quality water;
16

Marginal costs drop significantly at larger capacities;

Can be semi-operational during cleaning or replacement of
equipment periods, thereby limiting down time;

Few pretreatment requirements;

Does not generate waste from backwash of pretreatment filters.
The main disadvantages of MSF include the following:

High energy consumption compared to RO;

Creates a large amount of air pollution (primarily from high
energy consumption);

Slow response to water demand fluctuations;

High rate of scaling in tubes
3.2.2 Multi Effect Distillation (MED)
MED is an evaporation process going through a series of chambers (also known as
"effects"), with each lower pressure. However, formed in one chamber condenses in the next
chamber with the heat released acting as a heating source. In addition, feed water is sprayed
over the tube bundle on top of each chamber in a typical MED process. As shown in Figure
23, external steam is introduced in the first chamber and feed water evaporates as it absorbs
heat from the steam. The resulting vapour enters through the tube to the second chamber at a
reduced pressure.
The heat released by condensation causes the feed water in the second chamber to
evaporate partly. The process repeats in the third chamber and so on. In each chamber, the
vapour condensing into fresh water inside the tube is then pumped out.
The efficiency of MED can be raised with the addition of a vapour thermocompressor. As indicated in Figure 23, the thermo-compressor extracts part of the steam
generated in the final chamber for recycling use. The extracted steam will be mixed with the
external steam for compression under a high pressure, which then acts as a heating source in
the first chamber.
The main advantages of MED include the following:
17
 Wide selection of feed water;
 High quality of product water with high reliability;
 Less energy consumption than MSF;
 Requires lower temperature operation (reduces scaling and energy costs).
The main disadvantages of MED include the following:

Higher energy requirements than RO;
 Slow response to water demand fluctuations;
 Lower capacity than MSF
Figure 3.4 Schematic Diagram of Multi-Effect Distillation
3.2.3 Vapour Compression Distillation(VCD)
18
In vapour compression distillation, the steam from seawater and the vapour is then
compressed using a compressor. The temp and pressure of a steam will result of the
compression. The work done in the compressor is changed into heat. The incoming seawater
is used to condenses to distilled fresh water and at the same time the seawater is heated
further producing more steam. (VCD) can also be used in combination with (MED) or by
itself. Vapour compression units are built in different variety of configurations. As a result of
mechanical compressor is used to produce heat for evaporation. Vapour compressor small in
unit capacity, and are mainly used hotels, resorts and industrial applications.
Figure 3.5 Vapour phase compression desalination
3.2.4 LTE Desalination using Waste Heat
Low temperature evaporation desalination unit parts i.e., a heater, separator and
condenser. In the heater shell, vertical tubes are used. Feed seawater enters the unit at the
bottom of the tubes partly evaporates by the time it comes out from the top. When the water
and vapour mixture comes out of the tubes, the vapour rises through the vertical shell, then
enters the horizontal tube bundle kept at the top of the vertical shell and condenses around the
tubes (which are cooled by sea water flowing inside) producing desalinated water pumped
19
out. Due to high energy costs in components that is a major fraction of desalinated water cost,
the use of waste heat as energy input for seawater desalination is then a good option.
Figure 3.6 LTE desalination using a waste heat.
3.2.5 Solar Desalination
Solar desalination is a simple method of technology suitable for small community
level plants considering the economic viability. The heat generated from the sunglass covered
tank causing some to evaporate. The vapour is condensed on a glass cover and the resulting
fresh water is collected. This system of technology is a good alternative when electricity
supply is not available. The most widely used renewable energy source is the sun. This
source of solar energy is unlimited and free. No harmful gases are emitted such as mercury,
nitrogen oxide or Sulphur dioxide is not suitable for large scale water production. The
process requires proper maintenance due to glass cover and also good sealing to avoid vapour
and heat to reduce the effectiveness of the system solar desalination system.
20
Figure 3.7 Diagram of solar distillation system
3.2.6 Vacuum Freeze Desalination(VFD)
Vacuum freeze desalination process can be carried out in different ways. This process
involves three separate steps from obtaining fresh water from saline source: ice separation
from the brine, melting of ice and ice formation by the heat removal of saline water. Almost
all the freezing processes utilizes the same functional components, because they use the same
mechanisms for forming ice and separate from salt water, and melting of vacuum freeze
desalination process has four main components: freezer, washer, melter and heat removal
system. The freezer consists of a vessel in which ice crystals and vapour are formed
simultaneously. The freezing processes usually vary in the apparatus that reheat from the
brine to enable the production of ice crystals that can easily be transferred, removed,
separated, washed and then melted. The heat removed during the freezing process is usually
3transferred to the melter where it is utilized for melting the ice. The design and operating
21
system of vacuum freezer are the two main inputs to producing a high proportion of discrete
ice crystals rather than clumps of ice, so that the amount of seawater trapped between the
crystals formed is minimized. In this process crystal formed is important because fine crystals
are hard to wash. The ice crystal formed are pumped as slurry into the washer where it
separates the ice crystal for the salt water. The washer which is usually the counter current
wash column is utilized as the washer, in which portion of the product water, flowing in a
direction counter to that of the ice motion is used to wash the ice to remove the brine
adhering to the crystal surfaces. In the melter, ice from the washer is melted by transferring
the heat crystallization removed from the brine in the freezer to the melter. This is basically
done by discharging the refrigerant into the melter, where ice picks heat and melts.
Figure 3.8 Vacuum Freeze Desalination.
22
3.3. Membrane desalination processes
3.3.1 Reverse Osmosis (RO)
Figure 3.9 Basic Process of Reverse Osmosis
Reverse osmosis is a pressure driven process where a pressure difference larger than the
osmotic pressure is applied across an appropriate semi permeable membrane. Fresh water
passes from the concentrated salt solution to the dilute one. In practice, the saline feed water
is pumped into a sealed vessel where it is pressurized against the membrane. As a portion of
the water passes through the membrane, the salt content of the remaining feed water rises.
Simultaneously, a portion of this feed water is discharged without passing through the
membrane. The quantity of the feed water discharged to waste in the brine stream varies from
20 to 70% of the feed flow, depending on the salt content of the feed water, pressure, and
type of membrane. A usual salvaged value for a seawater RO system is only 40%. An RO
desalination plant basically comprises of four major systems
(1) Pretreatment
(2) High-pressure pump
(3) Membrane assembly
(4) Post-treatment.
To prolong membrane lifespan, extensive pre-treatment is typically required to condition the
feed seawater.
23
Figure 3.10 Reverse Osmosis Process
Therefore, suspended solids must be extracted and the water pre-treated so that salt
precipitation or microbial development does not ensue on the membranes. Usually, the pretreatment involves, fine filtration and the addition of acid or other chemicals to prevent
precipitation and the growth of microorganisms. The high-pressure pump provides the
pressure required to allow the water to pass through the membrane and have the salts
discharged. Pressurizing the saline water accounts for the greatest energy consumption by the
RO. Subsequently the osmotic pressure, and hence the pressure essential to achieve the
separation is directly linked to the salt concentration, RO is regularly chosen as the method
for purification of brackish water, where only small to intermediate pressures are necessary.
24
The operating pressure varies from 15 to 25 bar (225 to 375 psi) for brackish water and from
54 to 80 bar (800 to 1,180 psi) for sea water using an osmotic pressure of 25 bar. Energy
recovery from the high-pressure brine exiting an RO plant forms an essential part in
decreasing the overall energy consumption for desalination, specifically in large-scale RO
plants for seawater desalination.
Figure 3.11 Breakdown Cost for RO
Table 3.3 Typical effluent properties of (RO) and thermal MSF seawater desalination plants
Parameter
RO plants
Physical properties-
Up to 65,000–85,000 mg/L About 50,000 mg/L +5 to
Salinity Temperature
Ambient
15°C
seawater temperature.
above ambient.
Negatively buoyant
Positively,
Plume density
MSF plants
neutrally
or
negatively
Dissolved oxygen (DO)
If
well
typically,
intakes
below
used: Buoyant depending on the
ambient process, mixing with cooling
seawater DO because of the water from co-located power
low DO content of the source plants and ambient density
water.
If
open
stratification. Could be below
intakes
used: ambient
approximately the same as because
25
seawater
of
salinity
physical
de-
the ambient seawater DO aeration and use of oxygen
concentration.
scavengers
Bio-fouling control additives
and by-productsChlorine
If chlorine or other oxidants Approx. 10–25% of source
are
used
to
biofouling,
control water
these
are feed
dosage,
if
not
typically neutralized before neutralized
the
water
enters
membranes
to
the
prevent
membrane damage.
Halogenated organics
Typically, low content below Varying
harmful levels
composition
and
concentrations, typically trihalo-methanes
Scale control additives
Anti-scalantsAcid (H2SO4)
Not
present
(reacts
with Typically, low content below
seawater to cause harmless toxic levels. Not present
compounds, i.e. water and (reacts
sulfates;
the
acidity
with
seawater
to
is cause harmless compounds,
consumed by the naturally i.e. water and sulfates; the
alkaline seawater, so that the acidity is consumed by the
discharge pH is typically naturally alkaline seawater,
similar or slightly lower than so that the discharge pH is
that of ambient seawater).
typically similar or slightly
Typically low content below lower than that of ambient
toxic levels
seawater)
26
C H A PT E R I V
CASE STUDY
4.1 Abstract
Due to the continuously increasing demand of fresh water in the desert and remote
areas, the development of non-conventional water resources in Egypt is essential. The most
advanced and charming desalination system is the reverse osmosis (RO) system. In this
paper, a 5000 m3/day RO desalination plant in Nuweiba City in Sinai, Egypt is taken as a
case study. The measured data of the plant are recorded during 5 years of its normal
operation. Also, experimental tests are carried out in site to investigate the influence of the
main design and operating parameters on the plant performance.
Desalination is a process removing dissolved minerals from the saline water. Many
technologies have been developed for the sea and brackish water desalination, including
thermal, reverse osmosis (RO), electro dialysis and vapour compression systems. The most
common and widely used process is the reverse osmosis. All desalination processes involve
three liquid streams; the saline feed water, low-salinity product water (permeate), and the
very saline concentrate (brine). The RO plants produce water with salinity from 10 to 500
ppm TDS. The market shares of RO desalination systems have significantly increased in
recent years due to the progress in membrane technology.
The major parameters affecting the RO plant performance are the feed water
temperature, pressure and salinity. The membrane compaction, fouling and maintenance also
affect membrane performance. The influence of operating parameters on the RO plant
performance has taken much attention from researchers [1-5]. The RO desalination system
has also subjected to extensive theoretical work [6-8]. Numerical results have also shown the
effects of operating conditions and concluded that increasing the operating pressure and feed
flow will generally lead to higher water recoveries and salt rejection. However, increasing the
pressure beyond a certain maximum value led to the deterioration of the quality of the
product water.
From the preceding review, it is clear that more experimental work is needed for more
precise evaluation of the system performance. The present case study is carried out to study
the influence of main design and operating parameters on the RO plant performance. The
considered plant has been constructed at Nuweiba City in south Sinai, Egypt and started the
actual production on August 2001. The system is conceived in such manner to conserve
27
energy. The energy rejected from the discharged brine is recovered in the recovery turbine.
The required electrical power for the plant is supplied from the local electrical power
network. The saline water is supplied from 8 beach wells near the coast of Aqaba Gulf. The
salinity of feed sea water is in the order of 44000 ppm. The plant consists of 5 units; each has
a capacity of 1000 m3/day. The following objectives are also studied:
1. The cost analysis of the RO plant.
2. The evaluation of the overall system reliability for long-term automatic operation for a
certain maintenance procedure. The plant is still working under continuous operation without
any significant operational problems.
4.2 Experimental procedure
The plant consists of the following main systems; the intake, the raw water pretreatment unit and cartridge filters, the RO membrane unit and the post treatment system as
illustrated in Fig. 1. The beach wells are constructed according to standards, and are equipped
with submersible pumps. The depth of the well is about 100 meters. A 10" diameter PVC
pipe is introduced in the bore hole.
Figure 4.1 A schematic diagram of RO desalination plant in Nuweiba City
1- Sand filter
2- Carbon filter
3- Raw water tank
4- Feed pump
5- Cartridge filter
28
6- H. P. pump
7- Turbocharger
8- RO membrane
9- Aerated tank
10- Flushing pump
11- Chemicals tank
4.3 Results and discussion
To study the effect of a certain parameter on the system performance, other
parameters are kept constant during the experimental work. Results of the site experimental
work that explain the influence of the main operating parameters are graphically depicted the
effect of the feed water pressure on the productivity is at a feed water temperature 28°C and
salinity 44000 ppm. The productivity increases as the feed pressure increases. The
productivity is expressed as a percentage of the nominal value of the unit (1000 m3/day or
41.67 m3/hr). The productivity increases from 7.2 to 124.8 % corresponding to an increase in
the feed pressure from 41.37 to 72.4 bar respectively. The relation is almost linear between
the feed pressure and the productivity.
The salinity of feed water has a significant influence on the productivity of the RO
plant. As the feed water salinity increases, the productivity of the plant decreases as shown in
Fig. 5. The plant productivity decreases from 120 to 100.8% as the feed water salinity
increases from 15000 to 45000 ppm respectively.
4.4 Economical Analysis
In order to evaluate precisely the RO process, an economical analysis is important.
The analysis is made for one unit (1000 m3/day). In addition to the capital cost, the major
factors that influence the cost are the power consumption, running and maintenance costs.
4.5 Power consumption cost
Using power from local power network (Price of one kWh is 0.22 LE); the following
individual power consumption for each component of the plant is recorded and illustrated in
Table.
29
Table 4.1 Energy requirements
Pump
Power (kW)
kWh/m3
Sea water pump
18
0.432
Filter pump
18
0.432
Additive pumps
1
0.024
Product pump
3
0.072
RO HP pump
285
6.84
Total
725
7.80
30
CHAPTER V
CONCLUSION
1) To achieve sustainable water management and development in this region,
suitable desalination technologies were introduced in this seminar report. In this
region for example, limited scale RO of brackish groundwater units could be
used for remote areas for drinking water and agriculture, large scale desalination
plants for major coastal cities could be used for drinking water to free the natural
water for use in privatization of the desalination industry.
2) From this exhaustive literature review, it is found that various methods are
developed for distillation of water. These methods are subject to the demand of
fresh water, quality of water source and the involved expense. Conventional
Reverse Osmosis systems are currently prevalent domestically but at the cost of
plenty of waste water. Non-conventional water purifiers like solar stills have
unlimited potential but their usage is inadequate due to lesser output rate.
Humidification dehumidification process is the most appropriate option for fresh
water production and combined system for simultaneously hot water production.
The multi-effect distillation method can be used for mass production of fresh
water. The detailed review reveals that there is a need to develop a hybrid
system of water purification which can overcome the limitations of all existing
water purification systems.
3) Although desalination is not yet very popular because of the abundance natural
supply of sweet water. Although it has an excellent prospect for the coastal
areas. It can be very promising with the aid of waste heat/solar energy to get
freshwater from seawater at a much cheaper price. Future planning of water
treatment must focus on exploring different desalination methods to find a better
way to resolve water issues.
4) The RO system is found to be sensitive to the variation in the feed water
temperature, pressure and salinity;
a) Higher feed water temperature increases the plant productivity.
b) Increasing the feed water pressure increases the plant productivity, but
decreases the permeate salinity.
31
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Dossier Water, Planet Water. 1997.
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32