NYIT Energy Management Membrane Distillation Paper

Outline and bibliography report on the topic “MEMBRANE DISTILLATION”. Around 15 – 20 pages.

Attached a sample report by another student and 3 files related to the topic suggested by professor.

Desalination 308 (2013) 186–197
Contents lists available at SciVerse ScienceDirect
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
finding efficient 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 classified 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 difficulties 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 clarified 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 flux 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 flavor and color [7], in medical
field where high temperatures can sterilize biological fluids [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, configurations, 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 flux 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 configurations
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 configuration the heat losses
by conduction through the membrane matrix is higher than other
configuration 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 flux reduction. The use of an air gap configuration 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 configurations of MD.
2.2. Membrane distillation advantages
Membrane
Feed side
Permeate side
(Cold)
(Hot)
Membrane pores
Fig. 1. Principle of membrane distillation.
The benefits 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 benefit 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 μm 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 configurations.
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 efficient,
energy efficient 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 sufficiently 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 flux 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 fluoride (PVDF)
and polytetrafluoroethylene (PTFE, Teflon), available in capillary or
flat-sheet forms, have been used in MD experiments although these
membranes were prepared for microfiltration 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
specified, [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
(μm)
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 (μm)
0.20
0.45
1.00
0.22
0.45
0.2
Porosity
(%)
80
75
70
Flat-sheet polytetrafluoroethylene membranes supported by polypopylene net.
Flat-sheet polyvinylidene fluoride membranes.
Polypropylene capillary membrane: number of capillaries in a membrane module:
40; effective filtration 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 flow and/or the transition between
them [2–6]. The calculated MD flux 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 coefficient 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 conflict 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 satisfied, as a result, in recent
years, the MD research attention has gone into preparing membranes
specifically for the MD applications. For example, Fang et al. 2004
[11], prepared asymmetric flat-sheet membranes from poly (vinylidene fluoride-co-tetrafluoroethylene) by the phase inversion method. Those membranes were tested by DCMD configuration and the
results were compared to PVDF flat-sheet membranes prepared by
the same procedure. Their new membranes exhibited higher flux
than those of the PVDF membranes. They also prepared membranes
from poly(vinylidene fluoride-co-hexafluoro 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 fiber membrane and device
for desalination by VMD and DCMD. The membranes were commercial polypropylene (PP) membranes coated with plasma polymerized
silicone fluoropolymer. Permeate fluxes 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 fiber membranes
for MD. PVDF was used as a host polymer in the dope solution, where
hydrophobic and hydrophilic surfactants were added. A flux as high as
55 kg/m 2.h was achieved using DCMD configuration.
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 satisfies
all the requirements of higher flux 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 filled 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 flux data were
much higher than the commercial PTFE membranes.
189
3. Heat and mass transfer membrane distillation
In MD, the driving force for water vapor migration through the
membrane pores is the temperature difference between the feed/
membrane interface temperature (Tmf) and the permeate/membrane
interface temperature (Tmp). Due to the heat losses in MD process, the
membrane/interface temperatures are different from the bulk temperatures. This could be considered as one of the MD process drawbacks. This temperature difference leads to a decrease from the
theoretical driving force, which is defined as the difference between
the bulk feed temperature (Tbf) and the bulk permeate temperature
(Tbp). This phenomenon is known as temperature polarization. The
temperature polarization coefficient (TPC) is defined as the ratio between the actual driving force and the theoretical driving force [22];
as a result the temperature polarization coefficient is expressed
mathematically as the following:
TPC ¼
T mf −T mp
:
T bf −T bp
ð1Þ
It is impossible to measure the membrane/interface temperatures
experimentally; usually these temperatures are evaluated by performing a heat balance that relates them to the bulk temperatures
[22]. In order to solve this heat balance for membrane interface temperatures, the heat transfer coefficients in the adjoining liquid boundary layers to the membrane should be evaluated. Generally, the
boundary layer heat transfer coefficients are evaluated using empirical correlations for the determination of Nusselt number, and a wide
variety of these correlations is shown in Table 2 [22]. It is worth mentioning that each shown empirical correlation is valid for certain flow
regime and module geometry. In a recent article, Qtaishat et al. [23]
solved the heat balance and evaluated experimentally the membrane
surface temperatures via applying different empirical correlation that
takes into account the temperature variation effect on the physical
properties of both feed and permeate solutions.
3.1. Heat transfer
The following heat transfer analysis considers the DCMD configuration; however the same analysis could be applied to other MD configuration with some modifications. In DCMD, the heat transfer can be
divided into three regions as shown in Fig. 3; that are: (i) heat transfer in the feed boundary layer, Qf; (ii) Combination of both conductive
heat transfer through the membrane and heat transferred because of
water vapor migration through the membrane pores, Qf; (iii) heat
transfer in the thermal permeate boundary layer, Qp.
Table 2
Empirical correlations for evaluating Nusselt number in MD.
Empirical correlation [22]
Flow regime
Nu ¼ 1:86ðRePrÞ =3
Nu = 3.66
Nu = 4.36
Nu = 0.097Re0.73Pr0.13
1
Nu ¼ 1:95ðRePrÞ =3
0:64 1 =3
Nu ¼ 0:13Re Pr
1
Nu ¼ 0:023Re0:8 Pr =3
1
Nu ¼ 0:036Re0:8 Pr =
3 0:14
μ
Nu ¼ 0:027Re0:8 Prc μ bf
Laminar
Laminar
Laminar
Laminar
Laminar
Laminar
Turbulent
Turbulent
Turbulent
Turbulent
1
mf
Nua ¼
ðf =8ÞRePr
 2

1
1:07þ12:7ðf =8Þ =2 Pr =3 −1
Nua ¼
ðf =8ÞðRe−1000
 2ÞPr 
1
1þ12:7ðf =8Þ =2 Pr =3 −1
Turbulent
a
The friction factor, f, in these correlation was estimated by:
f = (0.79 ln(Re) − 1.64)− 2.
190
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
Feed out
Mf, out
Tbf, out
Heat and mass
fluxes
Combining Eqs. (2)–(4), the heat flux can be written as follows:
Permeate in
Mp, in
Tbp, in
0
1−1

1
1
1A

@
Q¼
þ
þ
T bf −T bp :
hf hm þ J w ΔHv
hp
T −T
Tb,f
mf
Tm,f
Permeate
boundary
layer
Feed
boundary
Jw
layer
As a result, the overall heat transfer coefficient (U) for the DCMD
process may be written as:
0
Tm,p
T b,p
Dry pore
Feed in
Mf, in
Tbf, in
Hydrophobic
membrane
Permeate out
Mp, out
Tbp, out
These heat transfer mechanisms can be expressed mathematically
as follows:
Through the feed solution thermal boundary layer:



Q f ¼ hf T bf −T mf :
ð2Þ
ð3Þ
Through the permeate solution thermal boundary layer:



Q p ¼ hp T mp −T bp :
ð4Þ
In the above equations, hf is the feed boundary layer heat transfer
coefficient, hp is the permeate boundary layer heat transfer coefficient. Jw is the permeate flux, Tmf and Tmp are the membrane/feed interface temperature and membrane/permeate interface temperature,
respectively. ΔHv is the latent heat of vaporization, hm is the heat
transfer coefficient of the hydrophobic membrane, which can be calculated from the thermal conductivities of the hydrophobic membrane polymer (km) and air trapped inside the membrane pores (kg).
hm ¼
kg ε þ km ð1−εÞ
δ
ð5Þ
where δ and ε are the thickness and porosity of the hydrophobic
membrane, respectively.
The evaporation efficiency, EE, is defined as the ratio between the
heat transferred because of water vapor migration through the membrane pores and the total heat transferred through the membrane
[22]. Mathematically, the evaporation efficiency is expressed by
Q m;M:T
J w Hv


:
EE ¼
¼
Q m;M:T þ Q m;cond J H þ h T −T
w v
m
mf
mp
ð9Þ
mp
3.2. Mass transfer



J w ¼ Bm pmf −pmp
ð6Þ
ð7Þ
ð10Þ
where pmf and pmp are the partial pressures of water at the feed and
permeate sides evaluated by using Antoine equation at the temperatures Tmf and Tmp, respectively; such as the following
ð11Þ
where P v is the water vapor pressure in Pascal and T is the corresponding temperature in Kelvin. However, the water vapor pressure
decreases with increasing the salt concentration in the feed water
according to Raoult’s law as follows [9]:
v
P i ¼ ð1−xi ÞP
v
ð12Þ
where xi is the weight fraction of salt in water.
Various types of mechanisms have been proposed for transport
of gasses or vapors through porous membranes: Knudsen model, viscous model, ordinary-diffusion model, and/or the combination thereof. The governing quantity that provides a guideline in determining
which mechanism is operative under a given experimental condition
is the Knudsen number, Kn, defined as the ratio of the mean free path
(λ) of the transported molecules to the pore size (diameter, d) of the
membrane; i.e. Kn = λ/d.
In MD, mass transport across the membrane occurs in three regions
depending on the pore size and the mean free path of the transferring
species [22]: Knudsen region, continuum region (or ordinary-diffusion
region) and transition region (or combined Knudsen/ordinary-diffusion
region). If the mean free path of transporting water molecules is large in
relation with the membrane pore size (i.e. Kn > 1 or r b 0.5λ, where r is
pore radius), the molecule-pore wall collisions are dominant over the
molecule–molecule collisions and Knudsen type of flow will be the prevailing mechanism that describes the water vapor migration through
the membrane pores. In this case, the net MD membrane permeability
can be expressed as follows.
K
At steady state, the overall heat transfer flux through the whole
DCMD system, Q, is given by
Qf ¼ Qm ¼ Qp ¼ Q:
mf


3841
v
:
P ¼ exp 23:328−
T−45
Through the membrane:



¼ hm T mf −T mp þ J w ΔH v :
1−1
1
1
1A
@
U¼
þ
þ
:
hf hm þ J w ΔHv
hp
T −T
In MD process, the mass transport is usually described by assuming a linear relationship between the mass flux (Jw) and the water
vapor pressure difference through the membrane distillation coefficient (Bm) [22]:
Fig. 3. Heat and mass transfer in DCMD.
Qm
ð8Þ
mp
Bm ¼


2 εr 8M 1=2
3 τδ πRT
ð13Þ
Where ε, τ, r, δ are the porosity, pore tortuosity, pore radius and
thickness of the hydrophobic membrane, respectively; M is the molecular weight of water, R is the gas constant and T is the absolute
temperature. The pore tortuosity is usually in the range of 1–2. However, it cannot be measured experimentally directly. It is possible to
evaluate the effective porosity per effective unit length of the
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
membrane (ε/δτ) by performing the gas permeation test that is detailed elsewhere [9].
In MD process, air is always entrapped within the membrane
pores with pressure values close to the atmospheric pressure. Therefore, if Kn b 0.01 (i.e. r > 50 λ), molecular diffusion is used to describe
the mass transport in continuum region caused by the virtually stagnant air trapped within each membrane pore due to the low solubility
of air in water. In this case the following relationship can be used for
the net DCMD membrane permeability.
D
Bm ¼
ε PD M
τδ P a RT
ð14Þ
Where Pa is the air pressure (assumed to be 1 atm), P is the total
pressure inside the pore assumed constant and equal to the sum of
the partial pressures of air and water liquid, and D is the water diffusion coefficient. The value of PD (Pa m 2/s) for water–air can be calculated from the following expression [9,22].
−5 2:072
PD ¼ 1:89510
ð15Þ
T
Finally, in the transition region, 0.01 b Kn b 1 (i.e. 0.5λ b r b 50λ),
the molecules of water liquid collide with each other and diffuse
among the air molecules. In this case, the mass transport takes
place via the combined Knudsen/ordinary-diffusion mechanism
and the following equation is used to determine the water liquid
permeability [22].
C
Bm ¼



3 τδ πRT 1=2 τδ P a RT
þ
2 εr 8M
ε PD M
−1
ð16Þ
4. Solar collecting technologies coupled with membrane
distillation
191
determined by their material properties. All cell materials lose efficiency as the operating temperature rises. The high temperature has
negative effect on the electrical output of the PV module, especially
the dominant crystalline Si based cells, where their conversion efficiency degrades by about 0.4–0.5% per degree rise in temperature
[25]. Tan et al. [26] performed high temperature–humidity tests on
performance degradation of PV cells. It was found that the degradation is directly related to the passivation integrity, and the inception
of moisture causes a significant degradation in the short circuit current and maximum power output.
The tracking flat PV system is one of the methods to increase the
PV power generation. The increase of solar energy capture due to
sun tracking is region by region depending on the local meteorological conditions. Abu-Khader et al. [27] performed an experimental investigation on the effect of using two-axis sun-tracking systems on
the electrical generation of a flat photovoltaic system to evaluate its
performance under Jordanian climate. It was experimentally found
that there was an increase of about 30–45% in the output power for
the North–South axes-tracking system compared to the fixed one.
PV electricity generation costs currently lies between 0.24 and
$0.72/kWh, according to the system type and the solar irradiation.
Such costs are expected to descend to the $0.13–0.31/kWh range
[29].
Power conditioning equipment (e.g. charge controller, inverters)
and energy storage batteries may be required to supply energy to a
desalination plant. Charge controllers are used for the protection of
the battery from overcharging. Inverters are used to convert the direct current from the photovoltaic module system to alternating current. The electricity produced can be used to power pumps for
desalination, mostly for membrane technologies. The photovoltaic
technology connected to a reverse osmosis (RO) system is commercial nowadays. However, the high cost of PV cells is still one of the
major challenges facing the widespread use of this technology.
4.2. Solar thermal
Solar collectors can be used to provide the heat (Solar Thermal) or
electrical energy (Solar Photovoltaic) requirements to operate a
membrane distillation system. The main solar technologies that
could be coupled with membrane distillation are briefly reviewed
below.
4.1. Solar photovoltaic
Photovoltaic (PV) cells are key components of PV applications that
convert solar energy into electricity through the transfer of electrons.
PV can be thought as a direct current (DC) generator powered by the
sun. At present, there are three generations of PV cells: crystalline silicon (c-Si) technologies (1st generation), amorphous silicon thin-film
(TF) technologies (2nd generation) and Nano-PV technologies (3rd
generation). Crystalline silicon are mature and reliable technologies
currently dominating the PV market (about 82% of global cell production in 2009) [23]. The conversion efficiency of c-Si lies between 15%
and 18% [24]. The TF technologies are currently the main alternative
to c-Si (17% market share in 2009) [23]. In addition, thin film (TF)
PV technologies are presently the lowest-cost to manufacture. The
production cost of cadmium telluride (CdTe) thin film module is currently the least; $0.76/Wp [23]. However, scarcity of key component
materials has been highlighted as a potential barrier to both large
scale deployment and reductions in TF technology cost. In particular,
major concerns have been raised for indium and tellurium availability
and potential risks for the TF PV technologies that utilize them, i.e.
cadmium telluride (CdTe) and copper indium gallium (di) selenide
(CIGS) [28].
The photovoltaic cell photo current is directly proportional to the
solar intensity. The performance of the solar cell depends on the cell
temperature. Solar cells work best at low temperatures, as
Solar collectors are well-known devices which are usually used to
absorb and transfer solar energy into a collection fluid. The thermal
energy can be achieved in solar stills, collectors, or solar ponds.
Solar collectors are usually classified according to the temperature
level reached by the thermal fluid in the collectors (Table 3) [29].
Low temperature collectors are those operating in the range below
80 °C while medium temperature collectors are those operating in
the range from 80 to 250 °C. Low temperature collectors provide
low-grade heat that is not useful to serve as a heat source for conventional desalination distillation processes but is of interest for membrane distillation process. Medium temperature collectors can be
used to provide heat for thermal desalination processes by indirect
heating with a heat exchanger. Evacuated tube collectors produce
temperatures of up to 200 °C and thus can be used as an energy
source for thermal desalination processes [30].
High temperature collectors such as parabolic troughs or dishes or
central receiver systems can concentrate the incoming solar radiation
onto a focal point, from which a receiver collects the energy using a
heat transfer fluid. The high thermal energy content can be used directly in thermal desalination processes or can be used to generate
electricity using a steam turbine. Sun tracking can improve the collector efficiency. Large-scale desalination applications require large collector areas.
A solar pond is a body of liquid which collects solar energy by absorbing direct and diffuse sunlight and stores it as a heat. Salt gradient
solar ponds (SGSP) rely on a salt solution (the salts most commonly
used are NaCl and MgCl2) of increasing concentration with depth to
suppress natural convection. Warm concentrated brine at the bottom
of the pond is prevented from rising to the surface and losing its heat
because the upper portion of the pond contains less salt and is,
192
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
Table 3
Solar energy collectors [29].
Collector type
Concentration
ratio
Solar pond
Flat plate (FPC)
Improved flat plate (IFPC)
Evacuated tube (ETC)
Compound Parabolic
Collectors (CPC)
Parabolic trough (PTC)
Linear Fresnel (LFC)
Parabolic dish reflector (PDR)
Central receiver
1
1
1
1
1–5
15–40
15–40
100–1000
100–1500
Typical temperature
range (°C)
50–100
30–80
80–120
50–190
70–240
70–400
70–290
70–930
130–2700
Tracking
No
No
No
No
No
Single axis
Single axis
Two axes
Two axes
Palenzuela et al. [32] considered the combination of desalination
technology into concentrating solar power (CSP) plants for the
planned installation of CSP plants in arid regions. The authors presented a thermodynamic evaluation of different configurations for
coupling parabolic-trough (PT) solar power plants and desalination
facilities in Abu Dhabi as a case for dry locations in the Middle East
and North Africa (MENA) region.
Since solar insolation is intermittent, a thermal energy storage system should be incorporated to run the desalination process round the
clock. One of the solutions to utilize fluctuating solar energy on a continuous basis is to incorporate thermal energy storage (TES) system.
Three types of TES systems are in commercial use; (1) sensible heat
storage, (2) latent heat storage, and (3) thermo chemical storage systems. The most widely used TES is the sensible heat storage system [33].
4.3. Performance parameters in SPMD
therefore, less dense than the lower portion. Whereas the top temperature is close to ambient, a temperature of 90 °C can be reached
at the bottom of the pond where the salt concentration is highest. A
typical profile of density and temperature within a solar pond is
shown in Fig. 4. Heat is extracted by passing the brine from the storage zone through an external heat exchanger. This heat can be used in
a special organic-fluid turbine to generate electricity, provide energy
for desalination, and to supply energy for space heating in buildings.
Solar ponds have large storage capacity allowing seasonal as well as
diurnal thermal energy storage. The annual collection efficiency of
useful heat for desalination is around10–15%. Larger ponds tend to
be more efficient than smaller ones due to losses at the pond edge.
Solar ponds are particularly suitable for desalination plants as waste
brine from desalination can be used as the salt source for the solar
pond density gradient. Using desalination brine for solar ponds not
only provides a preferable alternative to environmental disposal,
but also a convenient and inexpensive source of solar pond salinity.
Gracia-Roderiquez (2002) [21] reported that solar pond-powered desalination is one of the most cost-effective methods.
Many projects are currently under preparation to make possible
large concentrating solar power (CSP) plant developments in arid regions, such as the Shams 1 solar power station initiative. The Shams 1
CSP will feature 768 parabolic trough collectors over 6,300,000 ft 2 of
land. Shams 1’s parabolic trough collectors collect sunlight and convert it into thermal energy. The Shams solar power station is being
built in the city of Madinat Zayed, located 120 km south west of
Abu Dhabi, in the United Arab Emirates (UAE). Construction of
phase 1 of the solar project, Shams 1, commenced in July 2010 and
is expected to be completed by 2012. Upon completion, Shams 1
will be the first solar farm in the Middle East and the largest concentrated solar power (CSP) plant in the world. The project is estimated
to cost $600 m [31].
Fig. 4. Typical salt gradient solar pond.
The gained output ratio (GOR) and the thermal recovery ratio
(TRR) of the system are the most important performance parameters
used in thermal desalination processes as well as in solar powered
membrane distillation processes. The GOR is the ratio of thermal energy required to produce distillate water to the actual thermal energy
consumed in the feed side. Mathematically, the GOR is calculated
from:
GOR ¼
md ΔH v
mh CpðT hi −T ho Þ
ð17Þ
where md is the distillate flow rate (kg/h), λ the latent heat of vaporization (J/kg), mh the feed flow rate (kg/h), Cp the feed specific heat
(J/kg K), Thi, Tho the feed temperatures (in K) at the module inlet
and outlet.
The TRR is the theoretical energy needed for distillate produced
divided by the total thermal energy input. In the SPMD, the total thermal energy input is the solar energy incident on the solar collector. As
such, the TRR can be defined as:
TRR ¼
md ΔHv
AI
ð18Þ
where A is the solar collector area (m 2), and I is the global irradiation
(W/m 2). The TRR of a SPMD plant is measure of its efficiency to produce distillate.
5. Coupling membrane distillation with solar energy collectors
Coupling membrane distillation modules with solar energy collectors has been of interest for many researchers over the world because
MD can tolerate fluctuating and intermittent operating conditions as
well as it requires low grade thermal energy. Two alternative configurations of coupling solar energy with MD are illustrated in Fig. 5.
The solar-assisted MD desalination system (Fig. 5a) comprises solar
thermal collectors which feeds hot water to the MD module. The
heat is supplied to the MD module either directly or through a heat
exchanger. Electricity needed is either supplied from the electric
grid or from an auxiliary diesel generator to drive all pumps and
other electrically powered devices. The solar stand-alone MD desalination system (Fig. 5b) is similar to the solar-assisted MD desalination system in all aspects except that solar powered PV collectors
integrated with direct current (DC) battery cells and electric current
inverters are used instead of the diesel generator to supply the necessary electricity. Membrane distillation modules were coupled with
flat plate collectors, vacuum collectors, solar ponds, solar stills, and
parabolic troughs as detailed below.
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
5.1. Coupling with vacuum or flat plate collectors
The first publication in this field came from Australia where Hogan
et al. [34] from the University of New South Wales described a
0.05 m 3/day system using a 3 m 2 flat plate solar collectors. Hollow
fiber membrane distillation module with heat recovery was used
in their study. The authors reported that the thermal and electrical
energy consumption was 55.6 kWh/m3. The calculated flux of 17 liters per day per square meter of collector area was comparable to
that reported for solar MSF and ME plants.
As reported by Thomas [35] a solar-powered membrane distillation system was installed by the Water Re-use Promotion Center in
Tokyo, Japan, in 1994. Flat plate module and a 12 m 2 field of vacuum
tube collectors were used. Automatic controls start up the desalination system whenever sufficient sunlight is present to provide hot
water and electricity for pumping from the solar collectors and PV
panels. The plant had a maximum productivity of 40 liters per hour.
Four autonomous solar-powered membrane distillation plants
were developed through SMADES EU-funded project [36]. First a
so-called “compact” system was designed and tested to generate
process parameters for the design of the so-called “large” system.
Three compact systems were installed in Jordan, Morocco, and
Egypt. The compact system is simple one loop desalination designed
to produce about 100 l of distilled water per day. As such, no thermal
heat storage tanks, no electrical storage (battery), no complex but a
simple and reliable control was needed. The main components of the
system are: two flat-plate solar collectors with a corrosion-free absorber that can directly be used to heat up the salty water, one spiral
wound membrane distillation module with heat recovery, feed
pumps, PV module with a DC/AC converter, and feed and distillate
storage tanks (Fig. 6). One of the “compact” units was installed in
the city of Irbid in northern Jordan in August 2005 and fed with
brackish water [37,38]. The key design data of the compact system
193
are listed in Table 4. The distillate flow rate was about 120 liters
per day during the summer months, and about 50 liters per day during the cloudy winter days. The distillate conductivity was less than
600 μS/cm.
The large system was installed in the city of Aqaba on the Red Sea
coast and fed with untreated seawater in February 2006 [37,38]. The
system consists of two loops. The desalination loop is operated with
seawater and is separated from the collector loop (operated with
tap water) by a titanium corrosion resistant heat exchanger. This
arrangement allows for the use of economic standard components
in the solar collector without the need of cost-intensive corrosion
resistant materials. Four spiral wound membrane distillation modules
exactly the same as those used in the compact systems were operated
in parallel. A schematic of the setup is shown in Fig. 7. The design
capacity of the Aqaba system was 1 m 3/day. The key design data of
this system are listed in Table 5.
A DC/AC converter was used to convert 24 VDC delivered from the
batteries into 230 V AC. The capacity of the battery storage was
300 Ah. A thermal heat storage vessel was used to store the surplus
energy in order to be used whenever sufficient solar radiation is not
available. Due to natural fluctuations of solar radiation and temperature, the water production rate and energy requirements fluctuated
between 600 and 800 liter per day and 200 and 250 kWh/m 3,
respectively.
During the first month of operation (February 2006), the quality of
produced distillate was very good with a conductivity of less than
10 μS/cm. In March 2006, an increase in the distillate conductivity
was noticed. After a thorough evaluation, it was decided to remove
the deteriorated module and to operate the system with three membrane modules instead of four. The flux obtained varied between 2
and 11 liters per day per meter squared of collector area.
Experimental results from the large system showed a gradual decline of the permeate flux and quality during the first five months of
operation. Heating of seawater to temperatures up to 80 °C caused
scale deposit on the membrane surface. Cleaning the membrane
with dilute formic acid resulted in the dissolution of the deposit on
the membrane surface, and the initial membrane permeability was
restored [39]. Nevertheless, the information related to the membrane
durability in membrane distillation (MD) is still immature. It is documented that the membrane wetting and the scale deposition on the
membrane surface are the most serious problems that make the
membrane unworkable in MD [4]. However, there are many membrane designers considered designing the membranes to avoid or
minimize those drawback effects. Their results were very promising
[13–21].
Wang et al. [40] has recently described the performance of a
solar-heated hollow fiber vacuum membrane distillation (VMD) system for potable water production from underground water. The
Solar
irradiation
Solar
collector
Feed
tank
MD
module
Feed
pump
PV
PV module
Fig. 5. Solar-assisted (a) and stand-alone (b) desalination systems.
Over flow
Background
container
Distillate
Refilling pump
Fig. 6. Schematic drawing of the compact system (one loop desalination system).
194
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
Table 4
Specifications of the compact system.
Table 5
Specifications of the large unit.
Plant capacity (Average)
Membrane area
Solar collectors area
PV-module
Compact system
Design capacity (m3/day)
1
100 l/day
10 m2
5.73 m2
106 Wp
Collector area (m2)
Collector type
Heat storage capacity (m3)
Number of membrane modules
PV (kWp)
PV area (m2)
72
Flat plate
3
4
1.44
14
system has four major components, a solar energy collector, a hollow
fiber membrane module, a permeation condenser and two mechanical pumps (Fig. 8). The area of the solar energy collector is 8 m 2 and
the membrane total area is about 0.09 m 2. The membrane is 0.1 μm
hollow fiber membrane made from polypropylene, with the inner
diameter 371 mm, wall thickness 35 mm, and fiber operative length
0.14 m. The experiment results showed that the pure water flux of
the system could reach 32.2 kg per hour per square meter membrane area.
The performance of a desalination plant based on coupling an airgap membrane distillation module with a solar pond was tested by
Walton et al. [41]. Low grade thermal energy (between 13 and
75 °C) was extracted from the pond and supplied via a heat exchanger
to the membrane module. The membrane area was 2.94 m 2. The
Swedish firm SCARAB (http://www.hvr.se) has built and supplied
the membrane distillation module in addition to the controlling
pumps and heaters.
As shown in Fig. 9, hot brine was pumped from the bottom of the
solar pond and circulated through a heat exchanger to supply heat to
the saline solution. Cold water from the solar pond surface was
passed through another heat exchanger to provide cooling. High
and low temperatures for system operation were obtained by changing
the flow rates for solar pond hot and cold water.
The research included measuring the flux per unit area of membrane surface and conductivity of permeate over a range of feed
water salinities and temperature as well as an assessment of membrane fouling. The permeate flux was fluctuating and reached a maximum of 6 L/m 2.h.
Theoretical calculations, based upon measured results, indicate
that membrane distillation with latent heat recovery is necessary to
make the process being competitive with other thermal technologies
in terms of energy use. Walton et al. (2004) [41] reported that membrane distillation is only competitive relative to reverse osmosis
when low cost heat energy is available and/or when the water
chemistry of the source water is too difficult for treatment with reverse osmosis.
Suareza et al. [42] developed a heat and mass transport model to
evaluate the feasibility of coupling a DCMD module with an SGSP
for sustainable freshwater production in an environment such as
that at Walker Lake. They reported that the coupled DCMD/SGSP
system is capable of providing freshwater for terminal lakes reclamation. The coupled system shown in Fig. 10 was found to produce
water flows on the order of 1.6 × 10 − 3 m 3 per day per m 2 of SGSP
with membrane areas ranging from 1.0 to 1.3 × 10 − 3 m 2 per m 2 of
SGSP.
Mericqa et al. [43] has studied the simulation of coupling VMD
with solar energy to produce distillate from seawater. For this purpose solar collectors (SC) as well as salt gradient solar ponds (SGSP)
were considered. Simulation results showed that VMD/SGSP could
induce marked concentration and temperature polarization phenomena that reduced fluxes because of the difficulty to create turbulence
in the feed seawater when SGSP are used. Using the combination of
VMD/SC was more practical, as they concluded.
5.2. Coupling with parabolic trough collectors
Within the frame of MEDESOL (Seawater Desalination by Innovative Solar-Powered Membrane Distillation System) project the technical feasibility of producing fresh water from seawater by
integrating several MD modules (a multi-stage MD system) for a capacity range 0.5–50 m 3/d will be evaluated. The heat source of the
process will be from an advanced compound parabolic solar concentrator, especially developed to achieve the specific needed range of
temperatures. The seawater heater will include the development of
an advanced non-fouling surface coating, as reported by Gálvez et
al. (2009) [44].
Solar
irradiation
Brine
Collector
feild
Feed
pump
Storage
tank
Heat
exchanger
Distillate
MD modules
Battery
Expansion
vessel
Control
unit
DC
AC
PV
PV
PV array
Fig. 7. Schematic drawing of the large system (two loop desalination system).
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
195
Fig. 8. Flow sheet of the solar-heated MD system for producing potable water.
5.3. Coupling with solar stills
Banat et al. [45] described a solar still-membrane distillation integrated system operated with artificial seawater. Hot water from the
still was circulated into a tubular membrane distillation module before being returned back to the still. As such, distilled water was produced from both the solar still and the membrane distillation module.
The flux of the MD module was four times higher than the flux
obtained from the solar still.
6. Availability and cost
Solar energy can be harnessed for MD desalination by producing
the thermal energy required to drive the evaporation and by producing the electricity required to drive the pumps. The main energy requirement for membrane distillation is thermal energy. Electricity
demand is low and is used for auxiliary services such as pumps, sensors, controllers etc… However, the high cost of PV modules and to
less extent the high cost of solar collectors hinders the use of solar energy on wide scale. Capital costs of MD modules and corrosion resistant heat exchangers are important also. At present, no commercial
MD modules are available and researchers either use modules
designed for other membrane separations or design and build their
specific modules. Therefore, it is difficult to conclude if the SPMD process is really competitive with other solar driven conventional desalination processes.
Very few studies on the cost of solar powered MD desalination
plants have been reported in literature. Kullab and Martin [46] have
presented the cost for a scaled-up solar powered air gap membrane
distillation. Evacuated tube solar collectors were used to supply the
thermal energy. For a yearly production of 24,000 m 3 of pure water,
the cost of water production was estimated at 8.9 $/m 3. Around 70%
of this cost was associated with the solar collectors. Banat and Jwaied
[47] estimated the cost of potable water produced by the stand-alone
compact unit to be 15 $/m 3 and 18 $/m 3 for water produced by the
large unit. The authors pointed out that membrane lifetime and
plant lifetime are key factors in determining the water production
cost. The cost decreases with increasing the membrane and/or plant
lifetime.
Integrating solar power and membrane distillation desalination
plants is not yet a straightforward issue and many technological aspects remain to be discussed. Large seawater SPMD desalination
plants need, obviously, facilities to be located near the sea, where
land cost and availability could be a significant problem. Furthermore,
Fig. 9. Flow schematic of SPMD [41].
196
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
Fig. 10. Coupling of the DCMD module to the SGSP [42].
the solar direct normal irradiance (DNI) is normally lower on areas
close to the sea, which makes concentrating solar power (CSP) plants
most optimal locations to be separated from the coast. Other thermal
desalination technologies such as MED or MSF could also be coupled
with membrane distillation to minimize the production cost. To answer all of these issues, techno-economic analysis is needed to define
the best schemes of the integration of a membrane distillation with
solar energy.
7. Summary
Several small and lab scale plants for MD desalination using solar
energy have recently been tested. The process is deemed suitable to
operate in conjunction with solar energy for small capacities. The
main cost is in the initial investment. However, once the system is operational, it is extremely inexpensive to maintain and the energy has
minimal or even no cost. The availability and cost of MD modules is
still a serious and important issue. People not only in remote regions
but also in urban areas will benefit if low cost stand-alone MD
systems are developed commercially.
Nomenclature
Symbols
A
Solar collector area (m 2)
Bm
net DCMD permeability (s/m)
d
mean pore size (nm)
D
water diffusion coefficient (m 2 s − 1)
EE
evaporation efficiency
f
the friction factor
GOR
The gained output ratio
h
heat transfer coefficient (W m − 2 K − 1)
H
Enthalpy (J/kg)
I
Global irradiation (W/m 2)
Jw
DCMD flux (m/s)
k
thermal conductivity (w m − 1 K − 1)
Kn
Knudsen number
Nu
Nusselt number
M
molecular weight of water (kg mol − 1)
md
Distillate flow rate (kg/h)
mh
Feed flow rate (kg/h)
p
liquid pressure (Pa)
Pv
vapor pressure of water (Pa)
P
total pressure (Pa)
Pa
air pressure (Pa)
Pr
Prandtl number
Q
heat flux (W m − 2)
T
absolute temperature (K)
TPC
Temperature polarization coefficient
TRR
The thermal recovery ratio
r
mean pore radius (nm)
R
gas constant (J mol − 1 K − 1)
Re
xi
Reynolds number
solute mole fraction
Greek letters
δ
total membrane thickness (μm)
ε
porosity (%)
ρ
Density (kg/m 3)
λ
mean free path (nm)
μ
water dynamic viscosity (kg m − 1 s − 1)
τ
tortousity
ΔHv
latent heat of vaporization (kJ/mol)
Superscripts
K
Knudsen
D
molecular-diffusion
C
combined Knudsen/ordinary-diffusion
s
aqueous NaCl solution
References
[1] T. Matsuura, D. Rana, M. Qtaishat, G. Singh, (2011), Recent advances in membrane
science and technology in seawater desalination with technology development in
the Middle East and Singapore, in Water and Wastewater Treatment Technologies, [Ed.-], in Encyclopedia of Life Support Systems(EOLSS),Developed under
the Auspices of the UNESCO, Eolss Publishers, Oxford, UK, [http://www.eolss.net]
[Retrieved September 20, 2011].
[2] K. Lawson, K. Lloyd, Review of membrane distillation, J. Membr. Sci. 124 (1997) 1–25.
[3] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding
membrane distillation separation process, J. Membr. Sci. 285 (1–2) (2006) 4.
[4] M. Khayet, Membranes and theoretical modeling of membrane distillation: a
review, Adv. Colloid Interface Sci. 164 (2011) 56.
[5] A. Alkhudhiri, N. Darwish, N. Hilal, Membrane distillation: A comprehensive
review, Desalination 287 (2012) 2–18.
[6] H. Susanto, Towards practical implementations of membrane distillation, Chem.
Eng. Process. 50 (2011) 139.
[7] V. Calabro, B.L. Jiao, E. Drioli, Theoretical and experimental study on membrane
distillation on concentration of orange juice, Ind. Eng. Chem. Res. 33 (1994)
1803–1808.
[8] K. Sakai, T.K. Ano, T. Muroi, M. Tamura, Effect of temperature concentration polarization on water vapour permeability for blood in membrane distillation, Chem.
Eng. J. 38 (1988) B33–B39.
[9] M. Qtaishat, Design of Novel Membrane for desalination by membrane distillation, PhD Dissertation, University of Ottawa, (2008).
[10] M. Khayet, T. Matsuura, J.I. Mengual, M. Qtaishat, Design of novel direct contact
membrane distillation membranes, Desalination 192 (2006) 105–111.
[11] C. Feng, B. Shi, G. Li, Y. Wu, Preparation and properties of microporous membrane
from poly(vinylidene fluoride-co-tetrafluoroethylene) (F2.4) for membrane distillation, J. Membr. Sci. 237 (2004) 15–24.
[12] C. Feng, R. Wang, B. Shi, G. Li, Y. Wu, Factors affecting pore structure and performance of poly(vinylidene fluoride-co-hexafluoro propylene) asymmetric porous
membrane, J. Membr. Sci. 277 (2006) 55–64.
[13] B. Li, K.K. Sirkar, Novel membrane and device for vacuum membrane
distillation-based desalination process, J. Membr. Sci. 257 (2005) 60–75.
M.R. Qtaishat, F. Banat / Desalination 308 (2013) 186–197
[14] L. Song, B. Li, K.K. Sirkar, J.L. Girlon, Direct contact membrane distillation-based
desalination: novel membranes, devices, larger-scale studies, and a model, Ind.
Eng. Chem. Res. 46 (2007) 2307–2323.
[15] S. Bonyadi, T.S. Chung, Flux enhancement in membrane distillation by fabrication
of dual layer hydrophilic-hydrophobic hollow fiber membranes, J. Membr. Sci.
306 (2007) 134–146.
[16] M. Qtaishat, D. Rana, M. Khayet, T. Matsuura, Preparation and characterization of
novel hydrophobic/hydrophilic polyetherimide composite membranes for desalination by direct contact membrane distillation, J. Membr. Sci. 327 (2009)
264–273.
[17] M. Qtaishat, M. Khayet, T. Matsuura, Guidelines for preparation of higher flux
hydrophobic/hydrophilic composite membranes for membrane distillation, J.
Membr. Sci. 329 (2009) 193–200.
[18] M. Qtaishat, M. Khayet, T. Matsuura, Novel porous composite hydrophobic/hydrophilic
polysulfone membranes by direct contact membrane distillation, J. Membr. Sci. 341
(2009) 139–148.
[19] M. Qtaishat, T. Matsuura, M. Khayet, K.C. Khulbe, Comparing the desalination performance of SMM blended polyethersulfone to SMM blended polyetherimide
membranes by direct contact membrane distillation, Desalin. Water Treat. 5
(2009) 91–98.
[20] M. Qtaishat, D. Rana, T. Matsuura, M. Khayet, Effect of surface modifying macromolecules stoichiometric ratio on composite hydrophobic/hydrophilic membranes characteristics and performance in direct contact membrane distillation,
AIChE J. 55 (12) (2009) 3145–3151.
[21] M. Qtaishat, M. Khayet, T. Matsuura, K.C. Khulbe, Effect of casting conditions on
SMM blended polyethersulfone hydro-phobic/-philic composite membranes:
characteristics and desalination performance in membrane distillation, J. Appl.
Membr. Sci. Technol. 11 (2010) 1–8.
[22] M. Qtaishat, T. Matsuura, B. Kruczek, M. Khayet, Heat and mass transfer analysis
in direct contact membrane distillation, Desalination 219 (2008) 272–292.
[23] C. Candelisea, J. Spiersa, R. Grossa, Materials availability for thin film (TF) PV technologies development: a real concern? Renew. Sustain. Energy Rev. 15 (2011)
4972–4981.
[24] H. Wua, Y. Hou, Recent development of grid-connected PV systems in China,
Energy Procedia 12 (2011) 462–470.
[25] J. Tonui, Y. Tripanagnostopoulos, Performance improvement of PV/T solar collectors with natural air flow operation, Sol. Energ. 82 (2008) 1–12.
[26] C. Tan, B. Chen, K. Toh, Humidity study of a-Si PV cell, Microelectron. Reliab. 50
(2010) 1871–1874.
[27] M. Abu-Khader, O. Badran, S. Abdallah, Evaluating multi-axes sun-tracking system at different modes of operation in Jordan, Renew. Sustain. Energy Rev. 12
(2008) 864–873.
[28] IEA, Technology roadmap, Solar photovoltaic energy in International Energy
Agency Report, 2010, Available at: http://www.iea.org/papers/2010/pvroadmap.
pdf.
[29] S. Kalogirou, Seawater desalination using renewable energy sources, Progr. Energ.
Combust. Sci. 31 (3) (2005) 242–281.
[30] V. Belessiotis, E. Delyannis, Water shortage and renewable energies (RE)
desalination — possible technological applications, Desalination 139 (2001) 133–138.
197
[31] Shams 1 Solar Power Station, United Arab Emirates, www.powertechnology.
Com/projects/shamsonesolarpowerst/(Accessed 2012).
[32] P. Palenzuela, G. Zaragoza, D. Padilla, E. Guillén, M. Ibarra, J. Blanco, Assessment of
different configurations for combined parabolic-trough (PT) solar power and desalination plants in arid regions, Energy 36 (2011) 4950–4958.
[33] V. Gude, N. Nirmalakhandan, S. Deng, A. Maganti, Low temperature desalination
using solar collectors augmented by thermal energy storage, Appl. Energ. 91
(2012) 466–474.
[34] P. Hogan, A. Sudjito, G. Fane, G. Morrison, Desalination by solar heated membrane
distillation, Desalination 81 (1991) 81–90.
[35] K. Thomas, Overview of village scale, Renew. Energy Powered Desalination
(1997) pp. 1–31 (NREL/TP-440-22083, UC Category: 1210 DE 97000240).
[36] M. Rommel, M. Wieghaus, J. Koschikowski, Solar powered desalination: an autonomous water supply, Desalination 3 (2008) 22–24.
[37] F. Banat, N. Jwaied, M. Rommel, J. Koschikowski, M. Wieghaus, Desalination by a
“compact SMADES” autonomous solar powered membrane distillation unit, Desalination 217 (2007) 29–37.
[38] F. Banat, N. Jwaied, M. Rommel, J. Koschikowski, M. Wieghaus, Performance evaluation of the “large SMADES” autonomous desalination solar-driven membrane
distillation plant in Aqaba, Jordan, Desalination 217 (2007) 17–28.
[39] F. Banat, N. Jwaied, Autonomous membrane distillation pilot plant unit driven by
solar energy: experiences and lessons learned, Int. J. Sustain. Water & Environ.
Syst. 1 (2010) 21–24.
[40] X. Wang, L. Zhang, H. Yang, H. Chen, Feasibility research of potable water production via solar-heated hollow fiber membrane distillation system, Desalination
247 (2009) 403–411.
[41] J. Walton, H. Lu, C. Turner, S. Solis, H. Hein, Solar and waste heat desalination by
membrane distillation, Desalination and Water Purification Research and Development Program Report No. 81, College of Engineering, University of Texas at El
Paso, 2004.
[42] F. Suareza, S. Tyler, A. Childress, A theoretical study of a direct contact membrane
distillation system coupled to a salt-gradient solar pond for terminal lakes reclamation, Water Res. 44 (2010) 4601–4615.
[43] J. Mericqa, S. Laborieb, C. Cabassud, Evaluation of systems coupling vacuum membrane distillation and solar energy for seawater desalination, Chem. Eng. J. 166
(2011) 596–606.
[44] J. Gálveza, L. García-Rodríguezb, I. Martín-Mateos, Seawater desalination by an
innovative solar-poweredmembrane distillation system: the MEDESOL project,
Desalination 246 (2009) 567–576.
[45] F. Banat, R. Jumah, M. Garaibeh, Exploitation of solar energy collected by solar
stills for desalination by membrane distillation, Renew. Energy 25 (2002)
293–305.
[46] A. Kullab, C. Liu, A. Martin, Solar desalination using membrane distallation —
technical evaluation case study, International Solar Energy Society Conference,
Orlando, FL, August 2005.
[47] F. Banat, N. Jwaied, Economic evaluation of desalination by small-scale autonomous solar-powered membrane distillation units, Desalination 220 (2008)
566–573.
Desalination 356 (2015) 56–84
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
Membrane distillation: Recent developments and perspectives
Enrico Drioli a,b,c,d,⁎, Aamer Ali b,⁎⁎, Francesca Macedonio a,b
a
National Research Council Institute on Membrane Technology (ITM-CNR), Via Pietro BUCCI, c/o The University of Calabria, Cubo 17C, 87036 Rende, CS, Italy
The University of Calabria, Department of Environmental and Chemical Engineering, Rende, CS, Italy
c
Hanyang University, WCU Energy Engineering Department, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea
d
Center of Excellence in Desalination Technology, King Abdulaziz University, Saudi Arabia
b
H I G H L I G H T S
• A research boom has been observed in the field of MD recently.
• Current developments in MD have been reviewed.
• The future perspectives have also been highlighted.
a r t i c l e
i n f o
Article history:
Received 29 August 2014
Received in revised form 13 October 2014
Accepted 14 October 2014
Available online 28 October 2014
Keywords:
Membrane distillation
Recent developments
Future perspectives
a b s t r a c t
Membrane distillation (MD) has gained significant regard from industrial and academic perspective in recent
years, thus the frequency of publications related to the field has greatly accelerated. New perspectives have
boosted the research activities related to deeper understanding of heat and mass transport phenomenon,
novel applications and fabrication of the membranes specifically designed for MD. New efforts for module fabrication and understanding and control of non-traditional fouling in MD have also been highlighted in the recent
literature. The current review summarizes the important and interesting recent developments in MD from the
perspectives of membrane fabrication, heat and mass transport phenomenon, nontraditional fouling, module
fabrication and applications. The future research directions of interest have also been pointed out.
© 2014 Elsevier B.V. All rights reserved.
Contents
1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . .
Membranes for MD . . . . . . . . . . . . . . . . . . . .
2.1.
Recent trends in membranes for MD . . . . . . . . .
2.1.1.
Use of nanotechnology . . . . . . . . . . .
2.1.2.
Enhancement of hydrophobicity . . . . . . .
2.1.3.
Optimization of features and structural design
2.1.4.
Use of green solvents . . . . . . . . . . . .
3.
Heat and mass transfer . . . . . . . . . . . . . . . . . . .
3.1.
DCMD . . . . . . . . . . . . . . . . . . . . . .
3.2.
AGMD . . . . . . . . . . . . . . . . . . . . . .
3.3.
VMD . . . . . . . . . . . . . . . . . . . . . . .
4.
Module designing for MD . . . . . . . . . . . . . . . . . .
5.
Fouling in membrane distillation . . . . . . . . . . . . . .
6.
Emerging applications of MD . . . . . . . . . . . . . . . .
7.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . .
Symbols and abbreviations . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
⁎ Correspondence to: E. Drioli, National Research Council — Institute on Membrane Technology (ITM-CNR), Via Pietro BUCCI, c/o The University of Calabria, Cubo 17C, 87036 Rende, CS, Italy.
⁎⁎ Corresponding author.
E-mail addresses: e.drioli@itm.cnr.it (E. Drioli), aamer.ali@vscht.cz (A. Ali).
http://dx.doi.org/10.1016/j.desal.2014.10.028
0011-9164/© 2014 Elsevier B.V. All rights reserved.
57
58
66
66
67
69
69
69
70
71
72
73
75
78
79
80
80
E. Drioli et al. / Desalination 356 (2015) 56–84
57
Fig. 1. A conceptual design of 3rd generation desalination scheme (www.globalmvp.org).
1. Introduction
Fresh water scarcity has emerged as a big challenge of the current
era. Growing population, improved living standards, flourishing agricultural sector and industrialization have played a major role in making the
problem worse. It has been estimated that more than one billion people
on the earth don’t have the access to clean fresh water [1]. On the other
hand, conventional energy sources and fresh water reservoirs are becoming scarce quickly. Consequently, a strong need to develop less energy intensive and environment friendly water purification techniques
has emerged. The overall volume of fresh water reservoirs might be
enough to fulfill the current demand but unfortunately the distribution
of these reservoirs does not match with population distribution across
the globe. The ocean accounts for ~ 97% of the global water reserves
whereas only ~ 3% is available as the fresh water the most part of
which in form of glaciers and ice caps (2.06%), and a small part as
ground water (0.90%) and surface/other fresh water resources (0.03%)
[2,3]. Accordingly, seawater and brackish water desalination techniques
have gained the popularity to fulfill the demand. As alternative to 1st
generation thermal based desalination techniques, 2nd generation desalination technologies based on membrane operations (mainly reverse
osmosis (RO)) gained popularity during last two decades or so [4]. Currently, RO accounts for 60% of desalination erections across the globe,
thanks to its order of magnitude which requires less energy than its thermal counterparts. However, desalination technologies have to address
the issue of disposal of produced brine and further decreasing of energy
consumption for their sustainable growth.
To address these limitations, several other techniques are being investigated. These techniques mainly include membrane distillation
(MD), forward osmosis and capacitive deionization and will be incorporated into 3rd generation desalination installations (for instance see
Fig. 1 for the layout proposed by Global MVP, www.globalmvp.org).
Among the main techniques under investigation, MD has gained popularity due to some unique benefits associated with the process. Conventionally, MD is based on a thermal gradient created across a microporous
hydrophobic membrane and possesses the potential to concentrate the
solutions to their saturation point without any significant flux-decline.
At the same time, the process can be driven with waste grade heat including solar energy, geothermal energy and waste grade energy associated
with low temperature industrial streams. The membrane used in MD process allows the passage of vapors only and retains all nonvolatile on
retentate side, thus the product obtained is theoretically 100% pure
from solid or nonvolatile contaminants [5,6]. Due to these attractive benefits, MD has emerged as a potential element of 3rd generation desalination technique to address the inherent drawbacks of conservative RO
process.
Table 1
Merits and demerits of various configurations of MD.
Configuration Pros
DCMD
The easiest and simplest configuration to realize practically, flux is more stable
than VMD for the feeds with fouling tendency, high gained output ratio [8], it
might be the most appropriate configuration for removal of volatile
components [10].
VMD
High flux, can be used for recovery of aroma compounds and related
substances, the permeate quality is stable despite of some wetting, no
possibility of wetting from distillate side, thermal polarization is very low.
Relatively high flux, low thermal losses, no wetting on permeate side, less
fouling tendency.
AGMD
SGMD
Thermal polarization is lower, no wetting from permeate side, permeate
quality independent of membrane wetting
Cons
Flux obtained is relatively lower than vacuum configurations under the
identical operating conditions, thermal polarization is highest among all the
configurations, flux is relatively more sensitive to feed concentration, the
permeate quality is sensitive to membrane wetting, suitable mainly for
aqueous solutions.
Higher probability of pore wetting, higher fouling, minimum selectivity of
volatile components [10], require vacuum pump and external condenser.
Air gap provides an additional resistance to vapors, difficult module designing,
difficult to model due to the involvement of too many variables, lowest gained
output ratio [8].
Additional complexity due to the extra equipment involved, heat recovery is
difficult, low flux, pretreatment of sweep gas might be needed
58
E. Drioli et al. / Desalination 356 (2015) 56–84
Depending on the methods to induce vapor pressure gradient across
the membrane and to collect the transported vapors from the permeate
side, MD processes can be classified into four basic configurations. A
common feature of all the configurations is the direct exposure of one
side of the membrane to the feed solution used. Direct contact membrane distillation (DCMD) has been the most studied mode due to its inherent simplicity [7]. On the other hand, vacuum membrane distillation
(VMD) can be used for high output while air gap membrane distillation
(AGMD) and sweep gas membrane distillation (SGMD) enjoy the benefit of low energy losses and high performance ratio [8–10]. Some new
configurations with improved energy efficiency, better permeation
flux or smaller foot print have been proposed such as material gap
membrane distillation (MGMD) [11], multi-effect membrane distillation
(MEMD) [12], vacuum-multi-effect membrane distillation (V-MEMD)
[13] and permeate gap membrane distillation (PGMD) [13]. A pros and
cons analysis of conventional configurations has been explained in
Table 1.
The slow progress of membrane distillation has been related with
the unavailability of appropriate membranes for MD applications, high
energy consumption with respect to RO, membrane wetting, low flux
and limited investigations carried out on module designing. However,
thanks to the recent and growing extensive research activities carried
out in various areas of MD, the process has become much more attractive due to the availability of better membranes, the possibility to utilize
alternative energy sources and uncertainty about the sustainability of
fossil fuel. Furthermore, new rigorous separation requirements driven
by the new regulations and needs have further highlighted the importance of the field. As a result, a “research boom” has been observed in
various aspects of MD since the last one decade or so. Recently, a lot
or interest in commercialization efforts for MD has been realized
(Table 2). As an example Aquaver company has recently commissioned
the world’s first seawater MD based desalination plant in Maldives. The
plant uses the waste grade heat available from a local power plant and
has the capacity of 10,000 L/day (http://www.aquaver.com).
Recently, memsys have applied a patented concept of integrating vacuum with multi-effects in their module designing for MD. V-MEMD is a
modified form of VMD that integrates the concept of state-of-the-art
multi-effect distillation into the VMD. As a general principle of the process, the vapors produced in each stage are condensed during the subsequent stages. Vapors are generated in steam raiser working under
vacuum by exchanging the heat provided by external source. The vapors
are introduced in the 1st stage where these are condensed by exchanging
the heat with feed via a foil. The vapors generated in the 1st stage are
transported through the membrane and collected on the foil in the 2nd
stage. The flow of different streams in a single stage has been illustrated
in Fig. 2. It has been claimed that these modules have excellent gained
to output ratio which is a crucial parameter for industrial applications
[13]. A condenser is used to condense the vapors generated in the final
stage. The vapor pressure in each stage is less than its preceding stage. A
schematic diagram showing the module fabrication methodology has
been illustrated in Fig. 3.
Starting from the first article by Bodell in 1963, substantial growth in
the field has been observed over time. The progress and advancements
have been reviewed in different review articles [14,5,15,16,6,17]. The
number of scientists and researchers working on MD has increased tremendously in the recent years and a lot of research articles are coming
out each year. Advent of commercialization era for the process has
also contributed significantly in fueling the research in MD. DCMD is
still dominant field for recent research, despite the fact that most commercializing companies are adopting VMD or AGMD for their plants.
The first patent related to MD was issued in 1963. Since then, 12
more patents have been registered as documented by Drioli et al. [18].
That number somehow shows slow progress of MD. The research
momentum recently gained by the technology can be realized by the
8 patents published during 2013 and 2014. These patents cover the
membrane preparation methods, application of MD in integration
with the other processes to achieve complex separations, module designing, configurational modifications to improve process efficiency,
oleophobic membranes, use of the process in steam production, etc. A
list of the patents published recently has been provided in Table 3.
An interesting example of the fast growing of membrane distillation
systems can be found also in the GMVP research program in progress in
Korea in which membrane distillation, valuable resource recovery, and
PRO are the main objectives and goals (Fig. 1). A first MD plant with a
capacity of 400 m3 per day will be realized together with a 200 m3 per
day PRO unit (www.globalmvp.org).
In this work, we aim to review the recent advances in MD technology in terms of membrane development, module design, heat and mass
transport phenomenon, nontraditional fouling and applications. The future research directions of interest have also been pointed out.
2. Membranes for MD
One of the most crucial aspects of the membrane distillation is to
have at disposal membranes with well controlled properties. Moreover,
the final performance of a process is a direct consequence of the structural and physicochemical parameters of the utilized membranes. This
aspect gains relevance when the proposed membrane technology is
based on advanced systems where the use of well-structured and functionalized membranes becomes an imperative. Membrane distillation
performance is intrinsically affected by the structure of the film in
terms of thickness, porosity, mean pore size, pore distribution and geometry. Thus, the successful outcome of the process is reasonably
expected to be depending upon the capability of the membrane to interface two media without dispersing one phase into another and to combine high volumetric mass transfer with high resistance to liquid
intrusion in the pores. The membranes for membrane contactor application have to be porous, hydrophobic, with good thermal stability and
excellent chemical resistance to feed solutions. In particular, the characteristics needed for membranes are as follows:
1. High liquid entry pressure (LEP), is the minimum hydrostatic pressure
that must be applied onto the feed solution before it overcomes the hydrophobic forces of the membrane and penetrates into the membrane
pores. LEP is a 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 and a small maximum
pore size (see Eq. (a))
LEPw ¼
B γL cosθ
dmax
ðaÞ
Eq. (a) has been proposed by Franken et al. [19] on the basis of Laplace
equation. Here B is a geometric factor determined by pore structure
with value equal to 1 for cylindrical pores, γL the liquid surface tension
and θ is the liquid/solid contact angle.
However, as the maximum pore size decreases, the mean pore size of
the membrane decreases and the permeability of the membrane becomes low.
2. High permeability. The flux will “increase” with an increase in the
membrane pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In fact, molar flux through a
pore is related to the membrane’s average pore size and other characteristic parameters by:
N∝

α
r ε
τδ
ðbÞ
[17]where ε is the membrane porosity, τ is the membrane tortuosity,
δ is the membrane thickness, 〈rα〉 is the average pore size for Knudsen
diffusion (when α = 1), and 〈rα〉 is the average squared pore size for
viscous flux (when α = 2).
Table 2
Main suppliers and developers active in commercialization of MD.
Membrane trade name/
manufacturer
Material
Structural characteristic
Application
Fraunhofer Institute for Solar
Energy Systems (ISE)
OEM GE Nylon-hydrophobic
membranes
Liqui-Cel®
TNO
Contact
Fraunhofer Institute for Solar
Energy Systems (ISE),
Heidenhofstr.2, D 79110
Freiburg, Germany
Hydrophobic membrane is a pure
polymer internally supported with an
inert polyester web. It is a supported,
hydrophobic nylon impervious to
aqueous-based solutions making it ideal
for use in venting applications.
GE Nylon-hydrophobic membrane is
available in rolls up to 33 cm (13 in.)
wide, as well as sheets, cut discs and
pleat packs that can be customized to
meet your application and size
requirements. The GE NylonHydrophobic membrane is
manufactured on-site in GE Osmonics
facilities.
Donaldson Company Inc.,
Microelectronics Group
Develops PTFE membrane
-Bag vents
-Bioreactor venting applications
-CO2 monitors
-Fermentation air applications
-Filtering gases to remove particulate
-Insufflation filters
-Lyophilizer venting or inlet air
-Spike vents
-Sterile process gases
-Transducer protectors
-Venting gases from sterile processes
-Venting of sterile tanks
-I.V. filter vents
Liqui-Cel® Membrane Contactors are
used for degassing liquids. They are
widely used for O2 removal from water
as well as CO2 removal from water. They
have displaced the vacuum tower,
forced draft de-aerator, and oxygen
scavengers.
Liqui-Cel®, SuperPhobic® and
MiniModule® Membrane contactors
are used extensively for de-aeration of
liquids in the microelectronics,
pharmaceutical, power, food &
beverage, industrial, photographic, ink
and analytical markets.
Donaldson Company Inc.,
Microelectronics Group
Donaldson Europe B.V.B.A.
Interleuvenlaan, 1
B-3001 Leuven, Belgium
Tel.: 32 16 38 3811
Fax: 32 16 40 0077
e-mail:
LeuvenRD@mail.donaldson.com
TetratecEurope@mail.donaldson.com
Website: www.donaldson.com
http://www.liqui-cel.com/
13800 South Lakes Drive
Charlotte, North Carolina
28273, USA
Tel.: 704 588 5310
Fax: 704 587 8585
E. Drioli et al. / Desalination 356 (2015) 56–84
Donaldson Company Inc.,
Microelectronics Group
Bibliography reference
info-beno@tno.nl
drs. A.E. (Albert) Jansen
Business Developer Separation
Technology
Phone: +31 55 549 39 43
59
(continued on next page)
60
Table 2 (continued)
Membrane trade name/
manufacturer
Material
Structural characteristic
Scarab Development AB
S6/2 MD020CP2N/AkzoNobel
Microdyn
Bibliography reference
Contact
Street: Nybrogatan 12. 2 tr
City: 114 39 Stockholm
Country: Sweden
Telephone: (+46) 8-660 39 70
Fax: (+46) 8-662 96 18
M. Khayet, J.I. Mengual, T. Matsuura, Microdyn-Nadir
Microdyn-Nadir
Microdyn Modulban Gmbh
Porous hydrophobic/hydrophilic
produces polymeric membrane (in
Öhder Straße 28-42289
composite membranes
polyamide, polypropylene) and
Wuppertal, Germany
polypropylene hydrophobic membrane. Application in desalination using
direct contact membrane distillation, Postfach 240252. 42232
applications include water and
Wuppertal
Journal of Membrane Science 252
wastewater treatment, food and
Tel.: +49 (0) 202 26092-0
(2005) 101–113
pharmaceutical industry.
Fax: +49 (0) 202 26092-25
e-mail: sales@microdynnadir.de
Web site: www.microdynnadir.de
Membrana GMBH
W. Albrecht, R. Hilke, K. Kneifel, Th.
Commercial hollow fiber membranes
Weigel, K.-V. Peinemann, Selection of Öhder Straße 28, D-42289
from PP (Membrana
Wuppertal, Germany
microporous hydrophobic
GmbH, Germany) with microfiltration
Postfach 200151, D-42201
membranes for use in gas/liquid
properties are often applied
Wuppertal
contactors: An experimental
in gas/liquid contactors.
Tel.: (0) 202 6099-651
approach, Journal of Membrane
Membrana GMBH
Fax: (0) 202 6099 602
produces polyethylene, polypropylene, Science 263 (2005) 66–76
e-mail: info@membrana.de
polyetersulfon, cellulose and polymeric
Web site:
membranes.
http://www.membrana.com
http://www.pall.com/
Corinne Cabassud, David Wirth,
Hollow-fiber
microe.asp
modules containing Pall-Microza PVDF Membrane distillation for water
desalination:
fibers used for VMD for water
how to chose an appropriate
desalination.
membrane?, Desalination 157
(2003) 307–314
Commercial porous hydrophobic
J.M. Ortiz de Zfirate**, L. Pefia, J.I.
membranes for MD.
Mengual, Characterization of
membrane distillation membranes
prepared
by phase inversion, Desalination 100
(1995) 139–148
Commercial porous hydrophobic
J.M. Ortiz de Zfirate**, L. Pefia, J.I.
membranes for MD
Mengual, Characterization of
membrane distillation membranes
prepared
by phase inversion, Desalination 100
(1995) 139–148
J.M. Ortiz de Zfirate**, L. Pefia, J.I.
These membranes are porous,
hydrophobic and flat sheet. Commercial Mengual, Characterization of
membrane distillation membranes
porous hydrophobic membranes for
prepared
MD.
by phase inversion, Desalination 100
(1995) 139–148
J. Mansouri, A.G. Fane, Osmotic
distillation of oily feeds, Journal of
Membrane Science 153 (1999)
103 ± 120
Invests in development of technology
for water purification, solar power,
poly-generation, recycling and
sustainable systems
PP (polypropylene capillary membrane: Thickness =450 μm; average pore
size =0.2 μm; porosity =70%.
number of capillaries in membrane
module = 40; effective filtration
area = 0.1 m2; inner capillary
diameter = 1.8 mm; length of
capillaries = 470 mm)
Pall-Microza PVDF fibers
Commercial
Gelman polyvinylidene fluoride
(PVDF) membranes
Millipore polytetrafluoroethylene (PTFE)
membranes and polyvinylidene
fluoride
(PVDF) membranes
GVSP: Surface modified polyvinylidene
fluoride (PVDF),
UPVP: Ultra high MW polyethylene
(UHMWPE),
Nominal pore diameter = 0.2 μm,
porosity 80%, thickness 108 μm.
Nominal pore diameter = 0.2 μm,
porosity 80%, thickness 90 μm.
E. Drioli et al. / Desalination 356 (2015) 56–84
Membranes from PP (Membrana
GmbH, Germany) with
microfiltration properties.
Durapore
GVSP (PVDF)/Millipore
UPVP (UHMWPE)/Millipore
Application
Celgard 2500 (PP/PE, Hoechst
Celanese).
Commercial hydrophobic membranes,
porous, hydrophobic and flat sheet
Celgar 2500: Polypropylene (PP/PE),
Nominal pore diameter = 0.05 μm,
porosity 45%, thickness 28 μm.
Celgard Inc. — Membrana
Underlining Performance
Industrial Separations
(a Division of Celgard)
This business develops membrane
contactors SuperPhobic® e Liquicel®
Polypropylene
The memsys technology is based on
vacuum multi effect membrane
distillation. The memsys modules,
called “memDist”, consist of flat sheet
membranes combined with a plate and
frame design made of.
-Desalination
-Wastewater treatment
-Process water (Semi conductor, Boiler
feed water, Food & beverage)
-Aircon-desiccant cooling
-Process engineering (Alcohol
distillation)
-Cooling towers
Aquaver
PTFE
Aquaver membrane distillation systems
(MDS) are modular and compact. They
come in skid-mounted or containerized
units. Each MDS has its own controls
and can operate independently.
Seawater desalination
Cogeneration
Brine treatment
Landfill leachate
Industrial wastewater
Difficult-to-treat waters
SolarSpring produces all MD-modules
in cooperation with the Fraunhofer ISE
in Freiburg. They operate a customized
constructed winding-machine to
produce spiral-wounded MD-Modules.
They can vary many parameters like
channel-length, channel-height,
membrane-material, spacer geometry
and material.
Drinking water
Process and industry
Research
Aquastill
SolarSpring
Europe office:
Erlengang 31
22844 Norderstedt, Germany
Tel.: +49 4052 6108 78
Fax: +49 4052 6108 79
e-mail: Jschneid@celgard.net
web site: www.liquicell.com
www.membrane.com
Production: memsys GmbH
Frauenstr. 7
86830 Schwabmünchen
Germany
Principal: memsys clearwater
Pte. Ltd.
51 Goldhill Plaza
#23-11/12
Singapore 308900
T. +65 (0) 6354 0127
F. +65 (0) 6354 0128
Aquaver
info@aquaver.com
Phone: +31 703 002 570
Oosteinde 114, 2271, EJ
Voorburg, The
Netherlands
Nusterweg 69
6136 KT Sittard
The Netherlands

Home

index.html
SolarSpring GmbH
Hanferstr. 28
79108 Freiburg
Germany
Tel.: +49 (0) 761-610-508-3
Fax: +49 (0) 761-610-508-50
E-mail: contact (at)
solarspring.de
www.solarspring.de
E. Drioli et al. / Desalination 356 (2015) 56–84
memsys/memDist module
J. Mansouri, A.G. Fane, Osmotic
distillation of oily feeds, Journal of
Membrane Science 153 (1999)
103 ± 120
61
62
E. Drioli et al. / Desalination 356 (2015) 56–84
Fig. 2. Schematic illustration of streams in V-MEMD module [13].
Eq. (b) illustrates the importance (in terms of molar flux) of maximizing the membrane porosity and pore size, while minimizing the transport path length through the membrane, (τ δ). In other words, to
obtain a high 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. However, a conflict exists between the requirements of high mass transfer associated with
thinner membranes and low conductive heat losses achievable by
using thicker membranes. In fact, as described in the following sections, thermal efficiency in MD increases gradually with the growing
of membrane thickness and on optimization between the two requirements has to be found.
3. Low fouling problem. Fouling is one of the major problems in the application of porous membranes. Fortunately, in the gas–liquid contactor
applications, the contactors are less sensitive to fouling since there is
no convection flow through the membrane pores. However, in industrial applications, gas and liquid streams with large content of
suspended particles can cause plugging due to the small hollow fiber
diameter. Pre-filtration is necessary in such a case [51].
4. High chemical stability. The chemical stability of the membrane material has a significant effect on its long-term stability. Any reaction between the solvent and membrane material could possibly affect the
membrane matrix and surface structure. Liquid with high load of
acid gases are corrosive in the nature, which make the membrane
material less resistance to chemical attack.
5. High thermal stability. Under high temperatures, the membrane material may not be able to resist to degradation or decomposition.
Changing in the nature of membrane depends on the glass transition
temperature Tg for amorphous polymers or the melting point Tm for
crystalline polymers. Over these temperatures, the properties of the
polymers change dramatically. In Table 4, the Tg for the polymers
commonly used in membrane contactors is reported.
The transition temperature of a polymer is determined largely by its
chemical structure, which includes mainly the chain flexibility and
chain interaction. As it can be seen from Table 4, polytetrafluoroethylene
has a much higher Tg compared to polyethylene and polypropylene. This
contributes to the higher stability and less flexible polyvinyl chain of PTFE
with respect to PE and PP. In general, the factors that increase the Tg/Tm or
the crystallinity of a membrane can enhance both its chemical and
thermal stability. Therefore, in terms of long term stability membrane material with suitable Tg needs to be applied. For operations at
high temperatures, fluorinated polymers are good candidates due to
their high hydrophobicity and chemical stability [20].
Traditionally, the membranes prepared for ultrafiltration and
microfiltration through phase inversion processes have been utilized
Fig. 3. Frame and stages used by memsys. (i) A simple frame, (ii) single stage consisting of welded frames and covering plates, (iii) multiple stages [13].
E. Drioli et al. / Desalination 356 (2015) 56–84
63
Table 3
List of MD-related patents published from October 2011 to August 2014.
Patent
Inventor/s
Remarks
Combined membrane-distillation-forward-osmosis
systems and methods of use
Publication number: US 8029671 B2
Publication date: Oct 4, 2011
Membrane distillation apparatus and methods
Publication number: US 20110272354 A1
Publication date: Nov 10, 2011
Tzahi Y. Cath, Christopher R. Martinetti
Composite membranes for membrane distillation and
related methods of manufacture.
Publication number: WO 2012100318 A1
Publication date: Aug 2, 2012
Applicant: Membrane Distillation Desalination Ltd. Co.
Membrane and method
Producing the same
Publication number: US20120285882 A1
Publication date: Nov 15, 2012
Forward osmotic desalination device using membrane
distillation method
Publication number: US 20130112603 A1
Publication date: May 9, 2013
Solar membrane distillation system and method of use
Publication number: US 8,460,551 B2
Publication date: Jun. 11, 2013
Forward osmosis system comprising solvent separation by
means of membrane distillation
Publication number: US 20130264260 A1
Publication date: Oct 10, 2013
Mohamed Khayet, Takeshi Matsuura, Moh’d
Rasool Qtaishat
Embodiments of the present disclosure provide combined
membrane-distillation/forward-osmosis systems and
methods for purifying a liquid, such as reducing its solute or
suspended solids load.
Methods based on MD for solvent removal, sample
preconcentration and desalination employing hollow fiber
porous hydrophobic membranes with carbon nanotubes are
disclosed
The present invention provides composite membranes for
membrane distillation and related methods of manufacture.
Polyazole membrane for water purification
Publication number: EPA 13155093.1
Publication date: 14.08.2013
Nunes, Suzana, Pereira
Thuwal, Maab, Husnul
Thuwal, Francis, Lijo
Thuwal
Jan Hendrik Hanemaaijer
Method of converting thermal energy into mechanical
energy, and an apparatus therefor
Publication number: US 20140014583 A1
Publication date: Jan 16, 2014
Membrane distillation apparatus
Publication number: US 20140138299 A1
Publication date: May 22, 2014
Somenath Mitra, Ken Gethard
May May TEOH, Na PENG, Tai-Shung Chung
Sung Mo Koo, Sang Jin Lee, Sung Min Shim
Hisham Taha Abdulla El-Dessouky
The invention relates to distillation systems and, more
particularly, to a solar driven membrane distillation system
and method of use.
The invention relates to a system for separating a product
contained as solvent in a solution to be processed, comprising
at least one forward osmosis device through which the
solution to be processed and a draw solution flow, and a
device connected downstream thereof for obtaining the
product.
The method describes a membrane prepared for fluid
purification comprising a polyazole polymer.
Wolfgang Heinzl
The invention relates to a method of converting thermal
energy into mechanical energy wherein a working liquid such
as is evaporated to generate a stream of a working fluid
Pieter Nijskens, Bart Kregersman, Sam
Puttemans, Chris Dotremont, Brecht Cools
Membrane distillation modules using oleophobically and
antimicrobially treated microporous membranes
Publication number: US 20130068689 A1
Publication date: Mar 21, 2013
Vishal Bansal, Christopher Keller
Membrane Distillation Device
Publication number: 20140216916
Publication date: 2014-08-07
Wolfgang Heinzl
for MD applications. Some examples from state-of-the-art literature on
ultrafiltration membranes have been provided in Table 5. These membranes generally have low porosity, limited hydrophobicity, broader
pore size distribution and pore size not engineered for MD requirements.
The thickness of these membranes has been design to withstand relatively high pressure of UF and MF which is not encountered in MD. Accordingly, MD flux for such membranes is low and at the same time
conductive losses are high.
Among membrane parameters, the role of thickness is not straightforward. On one side, low thickness offers less resistance to the mass
transfer, while on the other hand, membranes with low thickness suffer
from more energy losses due to heat flux flowing through conduction
across the membranes [22]. In order to address the thickness issue,
dual and even triple layer membranes have been introduced [23]. This
membrane contains a hydrophobic active layer and a hydrophilic support layer. The support layer provides thermal insulation and ensures
the required mechanical robustness of the membrane while the active
layer retains the liquid. Care must be taken in selection of thickness of
the active layer as too less thickness can allow the passage of the liquid
The present disclosure relates to a membrane comprising a
porous polymer body with a plurality of channels extending
through the polymer body, a method of producing the same
and a water treatment system comprising the membrane
The present invention relates to a fresh water separator using
a membrane distillation method and a forward osmotic
desalination device comprising the fresh water separator
The present invention relates to membrane distillation
apparatus and is more particularly, although not exclusively,
concerned with the production of desalinated water from
seawater
The present invention provides a system for liquid distillation
which includes a vapor permeable–liquid impermeable
microporous membrane having structures defining a plurality
of pores, an oleophobic mater…