Water Desalination Process Comparing with Similar Technologies Project Report Should be specific about the water desalination process while writing with a

Water Desalination Process Comparing with Similar Technologies Project Report Should be specific about the water desalination process while writing with a good research with the files attached. Report should be between 20 to 27 pages.Report should include membrane distillation for desalination. Desalination 308 (2013) 186–197
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Desalination
journal homepage: www.elsevier.com/locate/desal
Desalination by solar powered membrane distillation systems
Mohammed Rasool Qtaishat a,?, Fawzi Banat b
a
b
Department of Chemical Engineering, University of Jordan, Amman, Jordan
Department of Chemical Engineering, The Petroleum Institute, PO Box 2533 Abu Dhabi, UAE
a r t i c l e
i n f o
Article history:
Received 18 October 2011
Received in revised form 14 January 2012
Accepted 22 January 2012
Available online 25 February 2012
Keywords:
Membrane distillation
Desalination
Solar energy
Solar collectors
a b s t r a c t
Membrane distillation (MD) is a hybrid membrane-evaporative process which has been of interest for desalination. MD requires two types of energy, namely, low temperature heat and electricity. Solar collectors and
PV panels are mature technologies which could be coupled to MD process. The interest of using solar powered membrane distillation (SPMD) systems for desalination is growing worldwide due to the MD attractive
features. Small scale SPMD units suitable to provide water for human needs in remote areas where water and
electricity infrastructures are currently lacking have been developed and tested by a number of researchers.
The combination of solar energy with MD has proven technically feasible; however, the cost of produced
water is relatively high compared with that produced from the commercial PV–RO process. The production
of commercial, reliable, low cost and long lasting MD modules will put this process on the front edge of
desalination technologies. The aim of this article is to present the main features of MD along with its basic
principles. Efforts of researchers in coupling MD with solar energy and their cost estimates are reviewed as well.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The demand on fresh water is growing steadily and is becoming
one of the worldwide challenges. The World Health Organization
(WHO) estimates that 20% of the world’s population has inadequate
access to drinking water. Although over two-thirds of the planet is
covered with water, 99.3% of the total water is either too salty (seawater) or inaccessible (ice caps). Since water is potable only when
it contains less than 500 ppm of salt, much research has gone into
?nding ef?cient methods of removing salt from seawater and brackish water. These are called desalination processes. Desalination of
seawater is a promising alternative to compensate for the shortage
of drinking water. Generally, desalination can be accomplished
using a number of techniques. These may be classi?ed under the following categories:Thermal processes that involve phase change such
as Multi-Effect Distillation (MED) and Multi Stage Flash (MSF). Membrane processes that do not involve phase change such as Reverse
Osmosis (RO) and electro dialysis (ED).Hybrid process that involve
both membrane and phase change such as membrane distillation (MD).
The thermal desalination processes depend on the evaporation of
water by the addition of heat provided by the sun or by combustion
processes, this was one of mankind’s earliest forms of water treatment and is still a popular treatment solution. On the other hand,
the development of modern polymeric materials in recent years has
led to the production of membranes which allow the selective
? Corresponding author.
E-mail addresses: mrasool78@yahoo.com (M.R. Qtaishat), banatf@just.edu.jo
(F. Banat).
0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2012.01.021
passage of water in liquid or vapor state or ions and thus providing
the basis for membrane desalination processes. Among those membrane processes, RO is the leading commercial membrane desalination process which requires applying high pressure to overcome the
osmotic pressure.
It is worth mentioning that both, thermal and RO are the leading
desalination processes in the water market [1]. However, those processes suffer from drawbacks and some technical dif?culties which
are: i) They are considered energy intensive either by the heat demand
(i.e. thermal processes) or by the high pressure demand as in reverse
osmosis process, this high energy consumption generates more pollutants and undesired emissions. ii) The scaling and fouling problem is one
of the major challenges that adds to the complexity and cost of those
processes. iii) The membrane cost and its durability in the membrane
processes are still immature subjects that require more research and
development.
These drawbacks affected the economic feasibility of those processes, which necessitates the search for alternative, environment
friendly and sustainable desalination.
Membrane distillation (MD) is a promising new comer to the
desalination processes which can be coupled to low-grade and renewable energy source such as wind and solar energy.
The developments in the use of renewable energy sources (RES)
have demonstrated that it is ideally suited for desalination, when
the demand of fresh water is not too large. The rapid escalation in
the costs of fuels has made the RES alternative more attractive. In certain remote arid regions, this may be the only alternative.
The interdependence of water and energy is increasingly evident
due to their territorial, environmental and economic implications.
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
Innovations in the area of energy supply can improve the economic
viability of prospective desalination plants considerably. Recently,
considerable attention has been given to the use of renewable energy
including solar, wind and geothermal as sources for desalination,
especially in remote areas and islands, because of the high costs of
fossil fuels.
Solar energy can be used for seawater desalination either by producing the thermal energy required to drive the phase-change processes or
by producing the electricity required to drive the membrane processes.
It should be clari?ed that membrane distillation (MD) has not
been yet commercialized for large-scale desalination plant in spite
of its attractive features especially the possibility of coupling to lowgrade source of energy, this is due to the lower ?ux of MD and
some technical problems such as the membrane wetting. However,
much research has gone into developing new membranes for MD
that overcomes those membrane design drawbacks [2–6].
MD applications are not limited only to desalination, since lower
operating temperatures have also made membrane distillation attractive in the food industry where concentrated fruit juices and sugar solutions can be prepared with better ?avor and color [7], in medical
?eld where high temperatures can sterilize biological ?uids [8], and
in the environmental applications such as removal of benzene and
heavy metals from water [3–6].
The purpose of this research paper is to provide a state-of-the-art
review on membrane distillation systems associated with solar energy for seawater and brackish water desalination. This article presents
the membrane distillation principle, con?gurations, mathematical
models and economic feasibility.
2. Membrane distillation process
Membrane distillation (MD) is a hybrid of thermal distillation and
membrane processes. MD is a relatively new process that is being investigated worldwide as a low cost and energy saving alternative to
conventional separation processes such as distillation and reverse osmosis [2–6]. Membrane distillation (MD) process is not commercialized yet for large scale industry. The reason behind this is that MD
process ?ux is lower than the commercialized separation processes.
The principle of membrane distillation is illustrated in Fig. 1. Conventionally, membrane distillation (MD) is a thermally driven process
in which a microporous membrane acts as a physical support separating a warm solution from a cooler chamber, which contains either a
liquid or a gas.
As the process is non-isothermal, vapor molecules (water vapor in
the case of concentrating non-volatile solutes) migrate through the
membrane pores from the high to the low vapor pressure side; that
is, from the warmer to the cooler compartment.
187
Generally, the transport mechanism of MD can be summarized in
the following steps:
• Evaporation of water at the warm feed side of the membrane.
• Migration of water vapor through the non-wetted pores.
• Condensation of water vapor transported at the permeate side of
the membrane.
2.1. Membrane distillation con?gurations
Among membrane distillation processes, variation exists as to the
method by which the vapor is recovered once it has migrated through
the membrane. These alternatives are as follows:
2.1.1. Direct contact membrane distillation (DCMD)
DCMD is the oldest and most widely used process, having liquid
phases in direct contact with both sides of the membrane. The vapor
diffusion path is limited to the thickness of the membrane, thereby
reducing mass and heat transfer resistances. Condensation within the
pores is avoided by selecting appropriate temperature differences
across the membrane.
It is worth mentioning that in DCMD con?guration the heat losses
by conduction through the membrane matrix is higher than other
con?guration due to the existence a continuous contact between
the membrane surfaces and the feed (hot) and permeate (cold)
solutions.
2.1.2. Air gap membrane distillation (AGMD)
AGMD has an additional air gap interposed between the membrane and the condensation surface. This gives rise to higher heat
and mass transfer resistances. Although heat loss by conduction is reduced, the penalty is ?ux reduction. The use of an air gap con?guration allows larger temperature differences to be applied across the
membrane, which can compensate in part for the greater transfer
resistances.
2.1.3. Vacuum membrane distillation (VMD)
The vapor is withdrawn by applying a vacuum on the permeate
side. The permeate-side pressure is lower than the saturation pressure of the evaporating species and the condensation of the permeate
takes place outside the module.
2.1.4. Sweeping gas membrane distillation
The permeating vapor is removed by using an inert gas stream
which passes on the permeate side of the membrane. Condensation
is done externally and involves large volumes of the sweep gas
and vapor stream. Fig. 2 shows the different con?gurations of MD.
2.2. Membrane distillation advantages
Membrane
Feed side
Permeate side
(Cold)
(Hot)
Membrane pores
Fig. 1. Principle of membrane distillation.
The bene?ts of membrane distillation compared to other more
popular separation processes stem from:
• 100% (theoretical) rejection of ions, macromolecules, colloids, cells
and other non-volatiles;
• lower operating temperatures than conventional distillation;
• lower operating pressures than conventional pressure-driven
membrane separation processes;
• reduced chemical interaction between membrane and process
solution;
• less demanding membrane mechanical property requirements;
• reduced vapor spaces compared to conventional distillation processes.
The last bene?t is considered one of the amazing advantages of
MD process, since the large vapor space required in conventional distillation column is replaced in MD by the pore volume of a microporous membrane, which is generally of 100 ?m thick.
188
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
Feed in
Liquid
permeate out
Feed in
Condenser
membrane
membrane
Sweep gas out
Product
Feed out
Liquid
permeate in
Feed out
Sweep gas in
SGMD
DCMD
Feed in
Feed in
Condenser
Permeate
membrane
Coolant out
Air gap
membrane
Condensing
plate
Feed out
Coolant in
Vacuum pump
Feed out
Product
AGMD
VMD
Fig. 2. Membrane distillation con?gurations.
Conventional distillation relies on high vapor velocities to provide
intimate vapor-liquid contact while MD employs a hydrophobic microporous membrane to support a vapor–liquid interface.
As a result, MD process equipment can be much smaller, which
translates to saving in terms of footprint, and the required operating
temperatures are much lower, because it is not necessary to heat
the process liquids above their boiling points. Feed temperature in
membrane distillation typically ranged from 60 to 90 °C, although
temperature as low as 30 °C has been used [1–6]. Therefore, lowgrade, waste and/or alternative energy sources such as solar and geothermal energy can be coupled with MD systems for a cost ef?cient,
energy ef?cient liquid separation system.
2.3. Membrane distillation disadvantages
The main disadvantage of MD process is the drawback of membrane wetting. The wettability of the microporous membranes is a
function of three main factors: the surface tension of the process solution, membrane material and the membrane structure.
To overcome the membrane wetting: the process solution must be
aqueous and suf?ciently dilute. This limits MD for certain applications
such as desalination, removal of trace volatile organic compounds
from wastewater and concentration of ionic, colloids or other nonvolatile aqueous solutions [9].
2.4. Membrane distillation membrane
2.4.1. High liquid entry pressure (LEP)
This is the minimum hydrostatic pressure that must be applied
onto the liquid feed solution before it overcomes the hydrophobic
forces of the membrane and penetrates into the membrane pores.
LEP is characteristic of each membrane and permits to prevent wetting
of the membrane pores. High LEP may be achieved using a membrane
material with high hydrophobicity (i.e. large water contact angle) and
a small maximum pore size. However, as the maximum pore size
decreases, the mean pore size of the membrane decreases and the
permeability of the membrane becomes low.
2.4.2. High permeability
The MD ?ux will “increase” with an increase in the membrane
pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In other words, to obtain a high MD permeability, the surface layer that governs the membrane transport must
be as thin as possible and its surface porosity as well as pore size
must be as large as possible.
In fact, the relationship between the membrane pore size and the
mean free path of migrating molecules determines the dominant diffusion mechanism. In MD, air is trapped within the membrane pores
with pressure values close to the atmospheric pressure if no vacuum
Table 1
Some commercial membranes commonly used in membrane distillation.
Membrane
As a matter of fact, commercial microporous hydrophobic membranes, made of polypropylene (PP), polyvinylidene ?uoride (PVDF)
and polytetra?uoroethylene (PTFE, Te?on), available in capillary or
?at-sheet forms, have been used in MD experiments although these
membranes were prepared for micro?ltration purposes [9]. Table 1
summarizes some of the commercial membranes commonly used in
MD processes together with some of their characteristics [9].
Recently, the desired characteristics for MD membranes have been
speci?ed, [10]. As it is well known, a MD membrane must be porous
and hydrophobic, with good thermal stability and excellent chemical
resistance to feed solutions. The characteristics needed for MD membranes are the following:
Manufacturer
Material
Thickness
(?m)
Gelman
PTFE/PPa
178
Trade name
TF200
TF450
TF1000
GVHP
HVHP
S6/2
MD020CP2N
a
Millipore
PVDF
AkzoNobel
Microdyn
PPc
b
110
140
450
Average pore
size (?m)
0.20
0.45
1.00
0.22
0.45
0.2
Porosity
(%)
80
75
70
Flat-sheet polytetra?uoroethylene membranes supported by polypopylene net.
Flat-sheet polyvinylidene ?uoride membranes.
Polypropylene capillary membrane: number of capillaries in a membrane module:
40; effective ?ltration area: 0.1 m2, inner capillary diameter: 1.8 mm; length of capillaries: 470 mm.
b
c
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
is applied vapor permeates through the porous membrane, as a result
of molecular diffusion, Knudsen ?ow and/or the transition between
them [2–6]. The calculated MD ?ux considering Knudsen mechanism
is higher than that considering the combined Knudsen/molecular diffusion mechanism.
2.4.3. Low thermal conductivity
In MD heat loss by conduction occurs through both the pores and
the matrix of the membrane. The conductive heat loss is greater for
thinner membranes. Various possibilities may be applied to diminish
the conductive heat loss by using:
i) Membrane materials with low thermal conductivities. This
does not necessarily guarantee the improvement of the MD
process because most hydrophobic polymers have similar
heat conductivities; at least the materials have thermal conductivities with the same order of magnitude.
ii) Membranes with high porosity, since the conductive heat
transfer coef?cient of the gas entrapped within the membrane
pores is an order of magnitude smaller than that of the membrane matrix. This possibility is parallel to the need of high
DCMD permeability as the available surface area of evaporation
is enhanced.
iii) Thicker membranes. However, there is a con?ict between the
requirements of high mass transfer associated with thinner
membranes and low conductive heat transfer through the
membrane obtained by using thicker membranes.
MD can be commercialized for large scale industry if the above
listed membrane requirements are satis?ed, as a result, in recent
years, the MD research attention has gone into preparing membranes
speci?cally for the MD applications. For example, Fang et al. 2004
[11], prepared asymmetric ?at-sheet membranes from poly (vinylidene ?uoride-co-tetra?uoroethylene) by the phase inversion method. Those membranes were tested by DCMD con?guration and the
results were compared to PVDF ?at-sheet membranes prepared by
the same procedure. Their new membranes exhibited higher ?ux
than those of the PVDF membranes. They also prepared membranes
from poly(vinylidene ?uoride-co-hexa?uoro propylene) [12] and
found that the DCMD performance of these membranes was better
than that of the PVDF membrane. Li and Sirkar 2005 [13] and Song
et al. 2007 [14], designed novel hollow ?ber membrane and device
for desalination by VMD and DCMD. The membranes were commercial polypropylene (PP) membranes coated with plasma polymerized
silicone ?uoropolymer. Permeate ?uxes as high as 71 kg/m2.h were
achieved. Bonyadi and Chung 2007 [15], used the co-extrusion method
to prepare dual layer hydrophilic/hydrophobic hollow ?ber membranes
for MD. PVDF was used as a host polymer in the dope solution, where
hydrophobic and hydrophilic surfactants were added. A ?ux as high as
55 kg/m 2.h was achieved using DCMD con?guration.
In a series of publications, Qtaishat et al. 2009 and 2010 [16–21],
presented the concept of hydrophobic/hydrophilic composite membranes for MD. It was shown that this type of membranes satis?es
all the requirements of higher ?ux MD membranes as mentioned earlier. Since the very thin hydrophobic layer is responsible for the mass
transfer, on the other hand the thick hydrophilic layer, the pores of
which are ?lled with water, will contribute to preventing the heat
loss through the overall membrane.
The hydrophobic/hydrophilic membrane was prepared by phase
inversion method in a single casting step. A hydrophobic surface
modifying macromolecules (SMMs) was blended with a hydrophilic
base polymer. During the casting step, the SMMs migrated to the
air/polymer interface since they have lower surface energy. Consequently, the membrane top-layer becomes hydrophobic while the
bottom layer becomes hydrophilic. These membrane were proved to
be workable membranes in MD, furthermore, their ?ux data were
much higher than the commercial PTFE membranes.
189
3. Heat and mass transfer membrane distillation
In MD, the driving force for wate…
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