water
Article
A Thermodynamical Approach for Evaluating Energy
Consumption of the Forward Osmosis Process Using
Various Draw Solutes
Lan-mu Zeng, Ming-yuan Du and Xiao-lin Wang *
Beijing Key Laboratory of Membrane Materials and Engineering, Department of Chemical Engineering,
Tsinghua University, Beijing 100084, China; [email protected] (L.Z.); [email protected] (M.D.)
* Correspondence: [email protected]; Tel.: +86-10-62794741
Academic Editor: Stephen Gray
Received: 26 December 2016; Accepted: 2 March 2017; Published: 6 March 2017
Abstract: The forward-osmosis (FO) processes have received much attention in past years as an
energy saving desalination process. A typical FO process should inclu de a draw solute recovery
step which contributes to the main operation costs of the process. Therefore, investigating the
energy consumption is very important for the development and employment of the forward osmosis
process. In this work, NH3 -CO2 , Na2 SO4 , propylene glycol mono-butyl ether, and dipropylamine
were selected as draw solutes. The FO processes of different draw solute recovery approaches
were simulated by Aspen PlusTM with a customized FO unit model. The electrolyte Non-Random
Two-Liquid (Electrolyte-NRTL) and Universal Quasi Chemical (UNIQUAC) models were employed
to calculate the thermodynamic properties of the feed and draw solutions. The simulation results
indicated that the FO performance decreased under high feed concentration, while the energy
consumption was improved at high draw solution concentration. The FO process using Na2 SO4
showed the lowest energy consumption, followed by NH3 -CO2 , and dipropylamine. The propylene
glycol mono-butyl ether process exhibited the highest energy consumption due to its low solubility
in water. Finally, in order to compare the equivalent work of the FO processes, the thermal energy
requirements were converted to electrical work.
Keywords: forward osmosis; process simulation; energy consumption; customized model
1. Introduction
The shortage of freshwater has become a main challenge in the 21st century as a consequence
of world population growth, increasing industrial consumption and pollution, development of
agriculture, climate change, etc. [1,2]. The commercialized desalination technologies such as reverse
osmosis (RO), multi-effect distillation (MED), and multi-stage flash (MSF) have been widely used,
which still have the common problem of high energy consumption [3–5]. In recent years, Forward
Osmosis (FO) has received much attention as a low energy cost desalination technology when
compared with the above traditional processes [6–8]. In FO, the water molecules from the feed
solution move to the draw side through a semipermeable membrane as a result of an osmotic pressure
difference across the membrane. As the osmotic pressure of the draw solution drops due to the
dilution, the draw solute recovery step is often necessary to regain the osmotic pressure after the FO
operation [9].
The recovery of draw solutes in an FO process contributes to the major the energy consumption
of the FO process. The selection of draw solutes significantly affects the performance of an FO process.
Over the past decades, various draw solutes have been proposed [10]. One of the most commonly
used draw solute is ammonia–carbon dioxide (NH3 -CO2 ), which was first commercialized by HTI
Water 2017, 9, 189; doi:10.3390/w9030189
www.mdpi.com/journal/water
Water 2017, 9, 189
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Inc., Powell, OH, USA [11–13]. In the NH3 -CO2 FO process, ammonium salts such as ammonium
bicarbonate, ammonium carbonate, and ammonium carbamate are used as draw solutes. After FO, the
ammonium salts in the diluted draw solution can be thermo-decomposed into NH3 and CO2 gases
at elevated temperature, and then the concentrated ammonium salt solution is regenerated from the
mixture of NH3 and CO2 gases. Switchable polarity solvent (SPS), which is capable of changing its
solubility in water by the addition and removal of CO2 , was proposed as a new FO draw solute [14–16].
By heating up the SPS solution, CO2 is driven off and the hydrophobic form of SPS is separated from
the diluted draw solution. Other examples of utilizing low-grade heat in draw solute recovery are
the solutes with a lower critical solution temperature (LCST) point that exhibit liquid-liquid phase
separation with water at high temperature. For example, Nakayama et al. demonstrated that some
types of glycol ethers including di(ethylene glycol) n-hexyl ether (DEH), propylene glycol n-butyl
ether (PB) and di(propylene glycol) n-propyl ether (DPP) can be used as draw solutes [17]. LCST ionic
liquids as draw solutes were reported to be able to treat feed solutions with NaCl concentrations of up
to 1.6 mol·L−1 by Cai et al. [18]. Besides using heat sources, the recovery of draw solutes can also be
achieved by cooling. Zhong et al. employed ionic liquid with an upper critical solution temperature as
the draw solute [19]. In the proposed process by Zhong et al., the FO process is operated at 60 ◦ C while
the draw solute recovery is carried out via cooling to room temperature. From the above discussions, it
can be seen that many types of draw solutes with different recovery schemes are available for the FO
process. So, it is important to evaluate the performance of the FO process for the selection of draw solutes.
For the FO process with NH3 -CO2 draw solute, the modeling results from McGinnis et al. by
chemical modeling software HYSYS showed that the FO process had the lowest equivalent work
requirement among MSF, MED, and RO desalination when a low-grade heat was available [20].
Mazlan et al. compared the energy consumption of FO with nanofiltration (NF) or ultrafiltration (UF)
draw solute recovery and reverse osmosis (RO) [21]. The results suggest that there is practically no
difference in the specific energy consumption of FO-NF (or FO-UF) and RO even if the membrane is
theoretically perfect. The switchable polarity solvent forward osmosis process modelled by Wendt
et al. was found to be a competitive desalination technology that could be applied to a feed solution
with NaCl concentrations of up to 4.0 molal [14]. Park et al. integrated the FO process with the gas
turbine cycle so the recovery step could utilize the waste heat from the turbine to improve the energy
efficiency of the integrated process [22]. According to their simulation results, the FO-gas turbine
system has an advantage on gain output ratio (GOR) and water production capacity compared with
the MSF-gas turbine system. Although there are some existing works on modeling the FO process,
a comprehensive platform is still lacking for the evaluation of the FO performance using different draw
solutes. A directly comparison of the performance of various draw solutes has not been investigated
comprehensively, but this comparison important in the future development of draw solutes.
In the presented work, thermodynamic simulation based on Aspen Plus software was adopted
to model the FO process. Several draw solutes including NH3 -CO2 , sodium sulfate (Na2 SO4 ), and
organic solutes with LCST points were selected. For each draw solute, different recovery methods
were designed with respect to their properties. For example, the distillation column was used for
the recovery of NH3 -CO2 draw solute. The simulations were carried out at different feed and draw
solution concentrations to study the influence of concentrations on the energy consumption of the FO
process using specific draw solutes. The energy consumptions as well as the equivalent works were
calculated to give a basic comparisons among those draw solutes.
2. Materials and Methods
2.1. FO Unit Operation Model
This study was mainly focused on evaluating the energy consumption of the FO process using
certain draw solutes under ideal conditions. Therefore, this work did not involve the analysis of
membrane permeability for reducing the complexity of the simulation. The membrane was assumed to
Water 2017, 9, 189
3 of 17
be theoretically perfect and completely rejecting the solutes, so internal concentration polarization and
the back-leak of draw solutes were not considered. The following equation was applied to estimate
the theoretical minimum energy consumption for an FO process:
πdraw = π f eed + πmin
(1)
where πdraw represents the osmotic pressure of the draw solution side, π f eed is the osmotic pressure of
feed side. πmin is the minimum osmotic pressure difference above which noticeable flow rate could be
observed. The term πmin ensures the osmotic pressure of the draw solution side is always larger than
the feed side.
The mathematical model for the customized FO unit operation was built with the Fortran
programming interface of the Aspen Plus software. The concentrations of inlet streams in the
customized FO model which are the initial concentrations of the feed and draw solutions, were
given by the simulation flowsheet. The concentrations of outlet streams are manipulated by changing
the amount of water migrated from the feed to the draw side, until osmotic equilibrium is reached.
The osmotic pressure was calculated by the activity of water which will be discussed in the next
Section 2.2. Taking the thermodynamic restriction and draw solution recovery into consideration,
the simulation approach with this customized FO unit model can achieve rigorous evaluation of the
energy consumption of the FO process with different draw solutes.
For a better understanding on the influence of draw solutes on the FO performance, the term
“RWATER” was used, as shown below:
RWATER =
moles of water transferred
moles of water in draw solution
(2)
Considering an FO process with draw solution recovery, energy must be used to change the state
of the diluted draw solution, i.e., thermal energy is required for heating up the diluted NH3 -CO2
solution for thermal decomposition. In this process, not only does the transferred water result in energy
consumption, so too does the water in the draw solution. If the FO process is operated at steady-state,
the amount of transferred water would be equal to the quantity of produced water. So RWATER can be
used to evaluate the energy efficiency of the FO process. Higher RWATER values means more energy
is used for desalination rather than being wasted on draw solution.
2.2. Osmotic Pressure
The prediction of osmotic pressure is vital for modeling the FO process. The most commonly
used method in the previous studies is based on the van’t Hoff theory, which the osmotic pressure is
calculated by the following equation:
π = nCRT
(3)
where n is the van’t Hoff factor representing the number of moles of a solute dissociated in the
solution (for example, n = 2 for NaCl and n = 3 for MgCl2 ). C is the molar concentration (mol·L−1 )
of the solution. R is the gas constant which is equivalent to 8.314 J·mol−1 ·K−1 and T is the absolute
temperature (in Kelvin). However, the van’t Hoff equation is only suitable for dilute solutions. When
dealing with concentrated solutions or solutions with multi components, noticeable deviations may be
encountered if the van’t Hoff equation is employed to calculate the osmotic pressure. Besides the van’t
Hoff equation, the osmotic pressure is defined as a function of the activity of water in thermodynamics:
π = − RT/Vw ln aw
(4)
where Vw is the partial molar volume of water and aw represents the activity of water.
This thermodynamic approach allows the determination of osmotic pressure in concentrated electrolyte
solution and even the solution with multiple solvents.
Water 2017, 9, 189
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The activity of water is calculated by thermodynamic models. The Electrolyte-NRTL [23] (eNRTL)
model was employed for electrolyte systems, while the UNIQUAC [24] model was used for systems
containing organic solvents. The eNRTL model, developed by Chen and his coworkers, has been
successfully applied to many electrolyte systems. Detailed description of the eNRTL model can be
found elsewhere [25,26], so only a brief introduction is presented in this paper. The excess Gibbs free
energy in the eNRTL model is expressed as a sum of three terms:
∗ex
G ∗ex
GNRTL
G ∗ex
G ∗ex
= PDH + Born + lc
RT
RT
RT
RT
(5)
where the notation “*” indicates that the excess Gibbs free energy is the unsymmetrical. The subscripted
PDH, Born, and lc represent Pitzer-Debye-Huckel, Born, and local composition contributions, respectively.
There are three types of binary parameters in the eNRTL model: molecule-molecule,
molecule-electrolyte, and electrolyte-electrolyte binary parameters. Here, an “electrolyte” does
not mean the salt dissolved into the solution, but an ion pair composed of a cation and an
anion. For an electrolyte system, the binary parameters are expressed as a and τ. Normally,
the symmetric nonrandom factor parameter a is ranges from 0.1 to 0.5, and often is set to 0.2 for
molecule-electrolyte interaction. While the unsymmetrical binary interaction energy parameter τ is
dependent on temperature:
0
T −T
T
D
(6)
+ ln 0
τ =C+ +E
T
T
T
The reference temperature T 0 is 273.15 K. C, D, and E are adjustable parameters. Thus,
for modeling a draw solution containing electrolytes, the adjustable parameters C, D, and E for
each type of binary interaction energy parameters τ (molecule-molecule, molecule-electrolyte, and
electrolyte-electrolyte) would need to be regressed with respect to experimental thermodynamic data.
Although eNRTL can also be used to calculate the thermodynamic properties of organic solution,
we found that the UNIQUAC model based on group contribution theory was more convenient and
provided better results for calculating the osmotic pressure of water in organic solutions. Besides, the
UNIQUAC model enables us to do a rough estimation of the model parameters from the structure of
organic solvents when there are no experimental data available to do regression. So the UNIQUAC
model was employed for the aqueous propylene glycol mono-butyl ether and dipropylamine solutions.
In the UNIQUAC model, the excess Gibbs free energy is calculated as a sum of a combinatorial
and a residual term:
ex
GUNIQUAC
G ex
G ex
= combinatorial + residual
(7)
RT
RT
RT
And the combinatorial term is expressed as:
ex
Gcombinatorial
=
RT
∑ ni
!"
i
φ
Z
θ
∑ xi ln xii + 2 ∑ qi xi ln φii
i
i
#
(8)
The residual term is expressed as:
ex
Gresidual
=−
RT
∑ ni
!"
i
where:
θi =
∑ qi xi ln ∑ θ j τij
i
!#
(9)
j
qi xi
∑ qj xj
(10)
ri xi
∑ rj xj
(11)
j
φi =
j
θi =
q x
j
(10)
j
j
φi =
Water 2017, 9, 189
ri xi
 rj xj
(11)
5 of 17
j
(
)
= exp
exp aaij +
+ bbij /T
/ T+
+ ccij ln
lnTT +
+ ddijTT ++eeij //T
T 22
ττijij =
ij
ij
ij
ij
ij
(12)(12)
ri areri the
area area
and volume
size size
parameters,
which
are related
to the
of the
qi and
aresurface
the surface
and volume
parameters,
which
are related
to structure
the structure
qi and
molecule. The coordination number Z is set to 10. aij , bij , cij , dij , and eij are the unsymmetrical binary
of the molecule. The coordination number Z is set to 10. aij , bij , c ij , dij , and e ij are the
adjustable interaction parameters between species i and j.
unsymmetrical
interaction
parameters
between
i and
j.
With suitablebinary
modeladjustable
parameters,
the activity
of water
and thespecies
osmotic
pressure
of a certain draw
With
suitable
model
parameters,
the
activity
of
water
and
the
osmotic
pressure
of asolvent)
certain draw
solution can be calculated. Because the molality concentration (moles per weight of
is more
solution can be calculated. Because the molality concentration (moles per weight of solvent) is more
convenient in thermodynamic calculation, the concentration of feed and draw solutions are expressed
convenient in thermodynamic calculation, the concentration of feed and draw solutions are expressed
in molality rather than molarity (moles per volume of solution) in the following section.
in molality rather than molarity (moles per volume of solution) in the following section.
2.3. FO Process Design
2.3. FO Process Design
NH3 -CO2 , Na2 SO4 , and organic solutes were used in this study and different draw solute recovery
NH3-CO2, Na2SO4, and organic solutes were used in this study and different draw solute
schemes were required with respect to their properties. The FO process diagram using NH3 -CO2 as
recovery schemes were required with respect to their properties. The FO process diagram using NH3the draw solute is shown by Figure 1. The original feed and draw solutions were input parameters for
CO2 as the draw solute is shown by Figure 1. The original feed and draw solutions were input
theparameters
customized
FO unit operation model, where the water in the feed side was transferred into the
for the customized FO unit operation model, where the water in the feed side was
draw
side
according
osmotic
Multistage
distillation
columns decomposed
transferred into thetodraw
side pressure
accordingequilibrium.
to osmotic pressure
equilibrium.
Multistage
distillation
NH
completely
and
maximized the water production, but required a heat source of
3 -CO2 more
columns
decomposed
NH3-CO
2 more completely and maximized the water production, but required
higher
temperature
[20]. temperature
Assuming an[20].
available
heat an
source
withheat
the temperature
of more
than 110of◦ C,
a heat
source of higher
Assuming
available
source with the
temperature
a multistage
column was
used for the
decomposition
of the
NH3 -COof
solution.
2 draw
more thandistillation
110 °C, a multistage
distillation
column
was used for
thediluted
decomposition
the diluted
The
distillation
column
with
15 distillation
stages was column
modelled
by the
RadFrac
unit
operation
in the unit
Aspen
NH
3-CO2 draw
solution.
The
with
15 stages
was
modelled
by model
the RadFrac
Plus.
From the
bottom
of the
distillation
column,
water with
low amountcolumn,
of ammonia
operation
model
in the
Aspen
Plus. From
the bottom
of thea distillation
waterand
withcarbonate
a low
was
produced.
The NHand
CO2 gases
the topThe
of the
column
were
amount
of ammonia
carbonate
wasfrom
produced.
NHdistillation
3 and CO2 gases
from
the cooled
top of down
the
3 and
distillation
column
down and
mixed
with water
to regenerate
solution.
and
then mixed
withwere
watercooled
to regenerate
thethen
draw
solution.
The eNRTL
model the
wasdraw
selected
for the
The eNRTL
model was selected
for the calculation
of thermodynamic
properties,
thethe
model
calculation
of thermodynamic
properties,
and the model
parameters were
retrievedand
from
Aspen
parameters
were
retrieved
from
the
Aspen
default
databank.
default databank.
Figure1.1.Aspen
Aspen Plus
Plus flowsheet
flowsheet of
of the
the forward
forward osmosis (FO) process
Figure
process using
using NH
NH33-CO
-CO2 2asasthe
thedraw
drawsolute.
solute.
The outline of the FO process using Na2SO4 as the draw solute is displayed in Figure 2. The
The outline of the FO process using Na2 SO4 as the draw solute is displayed in Figure 2.
Na2SO4 was precipitated out from the diluted draw
solution, in the form of glauber salt, via cooling
The Na2 SO4 was precipitated out from the diluted draw solution, in the form of glauber salt, via cooling
down to 0 ◦ C. The crystallization of the Na2 SO4 was simulated by the Crystallizer model in Aspen
Plus. After crystallization, the solids were separated from the solution via filtration for draw solution
recovery. The clear solution was sent to reverse osmosis (RO) for the production of water. For the
calculation of thermodynamic properties, the eNRTL model parameters in the default databank
were used.
down to 0 °C. The crystallization of the Na2SO4 was simulated by the Crystallizer model in Aspen
down to 0 °C. The crystallization of the Na2SO4 was simulated by the Crystallizer model in Aspen
Plus. After crystallization, the solids were separated from the solution via filtration for draw solution
Plus. After crystallization, the solids were separated from the solution via filtration for draw solution
recovery. The clear solution was sent to reverse osmosis (RO) for the production of water. For the
recovery. The clear solution was sent to reverse osmosis (RO) for the production of water. For the
calculation of thermodynamic properties, the eNRTL model parameters in the default databank were
calculation
Water
2017, 9, 189of thermodynamic properties, the eNRTL model parameters in the default databank were
6 of 17
used.
used.
Figure 2. Aspen Plus flowsheet of the FO process using Na2SO4 as the draw solute.
Figure
AspenPlus
Plusflowsheet
flowsheetof
of the
the FO
FO process using
4 asas
draw
solute.
Figure
2. 2.Aspen
usingNa
Na2SO
the
draw
solute.
2 SO
4 the
Figure 3 demonstrates the Aspen Plus flowsheet of the FO process using organic solutes with an
Figure 3 demonstrates the Aspen Plus flowsheet of the FO process using organic solutes with an
Figure
the Aspen
Plus flowsheet
the FO
process
using organic
with an
LCST point3asdemonstrates
the draw solutes.
The Decenter
model in of
Aspen
Plus
was employed
for thesolutes
simulation
LCST point as the draw solutes. The Decenter model in Aspen Plus was employed for the simulation
LCST
point as the separator,
draw solutes.
The
Decenter
model
in Aspenof
Plus
was employed
the simulation
of a liquid-liquid
which
enables
rigorous
calculation
liquid-liquid
phasefor
separation
by
of a liquid-liquid separator, which enables rigorous calculation of liquid-liquid phase separation by
ofthermodynamic
a
liquid-liquid
separator,
which
enables
rigorous
calculation
of
liquid-liquid
phase
separation
modelling.
After liquid-liquid
separation,
the water
phase was sent
into
the RO by
thermodynamic modelling. After liquid-liquid separation, the water phase was sent into the RO
model for the production
of water.
The organic
phase was
as a new
drawinto
solution.
thermodynamic
liquid-liquid
separation,
therecycled
water phase
the ROThe
model
model for the modelling.
production After
of water.
The organic
phase was
recycled
as awas
newsent
draw solution.
The
UNIQUAC
model
parameters
for
PB
were
regressed
with
respect
to
experimental
liquid-liquid
forUNIQUAC
the production
of parameters
water. The organic
phaseregressed
was recycled
a new to
draw
solution. The
UNIQUAC
model
for PB were
withasrespect
experimental
liquid-liquid
equilibrium
data, as
the
Aspen
databank with
(Aspen
Plus, to
v7.6,
Aspen Technology,
Inc., Bedford,
MA,data,
model
parameters
for
PB
were
regressed
respect
experimental
liquid-liquid
equilibrium
equilibrium data, as the Aspen databank (Aspen Plus, v7.6, Aspen Technology, Inc., Bedford, MA,
does not
have such
parameters.
Unfortunately,
we cannot find
enough
data
to obtain
the vaporasUSA)
the
Aspen
databank
(Aspen
Plus,
v7.6,
Aspen
Technology,
Inc.,
Bedford,
MA,
USA)
does
not
have
USA) does not have such parameters. Unfortunately, we cannot find enough data to obtain the vaporliquid
parameters
for
the
estimation
of
osmotic
pressure;
so
previous
regressed
liquid-liquid
such
parameters.
Unfortunately,
we cannot
enough
data to so
obtain
the vapor-liquid
parameters for
liquid
parameters
for the estimation
of find
osmotic
pressure;
previous
regressed liquid-liquid
parameters
were
used which
may so
cause
someregressed
uncertainty
in the simulation.
Thewere
liquid-liquid
the
estimation
of osmotic
pressure;
previous
liquid-liquid
parameters
used which
parameters
were
used which
may cause
some uncertainty
in the simulation.
The liquid-liquid
parameters
in
the
default
databank
for
dipropylamine
(DP)
were
employed
to
calculate
its
liquidmay
cause some
in the simulation.
The liquid-liquid
parameters
in to
thecalculate
default databank
parameters
in uncertainty
the default databank
for dipropylamine
(DP) were
employed
its liquid-for
liquid equilibrium properties. In addition, the osmotic pressure of DP was calculated by the vaporliquid equilibrium
In addition,
the osmotic
pressure ofequilibrium
DP was calculated
by the
dipropylamine
(DP) properties.
were employed
to calculate
its liquid-liquid
properties.
In vaporaddition,
liquid parameters for a better result.
parameters
better
theliquid
osmotic
pressurefor
ofaDP
wasresult.
calculated by the vapor-liquid parameters for a better result.
Figure 3. Aspen Plus flowsheet of the FO process using organic solutes with a LCST point as the draw
Figure
Aspen Plus
Plus flowsheet
FOFO
process
using
organic
solutes
with awith
LCSTa point
the draw
Figure
3. 3.Aspen
flowsheetofofthe
the
process
using
organic
solutes
LCSTaspoint
as the
solute.
solute.
draw
solute.
3. Results
3.1. FO Process Using NH3 -CO2 as Draw Solute
NH3 -CO2 draw solution is regenerated via thermally decomposing the diluted draw solution
into NH3 and CO2 gases at elevated temperature, then absorbing the gases by water to regenerate a
concentrated draw solution [12]. The dissolved NH3 and CO2 gases in solution present in various
forms including NH4 + , NH2 COO− , HCO3 − , and CO3 2− ions, together with NH3 (0) and CO2 (0) neutral
3. Results
3.1. FO Process Using NH3-CO2 as Draw Solute
NH3-CO2 draw solution is regenerated via thermally decomposing the diluted draw solution
into
and
a
Water NH
2017,3 9,
189 CO2 gases at elevated temperature, then absorbing the gases by water to regenerate
7 of 17
concentrated draw solution [12]. The dissolved NH3 and CO2 gases in solution present in various
forms including NH4+, NH2COO−, HCO3−, and CO32− ions, together with NH3(0) and CO2(0) neutral
molecules. The
The osmotic
osmotic pressure
pressure of
of the
-CO22 solution
solution is
is influenced
influenced by
by the
the distribution
distribution of
of those
those
molecules.
the NH
NH33-CO
species,
which
can
be
affected
by
the
concentrations
of
NH
and
CO
and
also
their
ratio.
An
NH
-CO323 3 and CO
2 2 and also their ratio. An 3NH
species, which can be affected by the concentrations of NH
solution
with a higher NH3 to CO
composed
from ammonia salts like ammonium carbonate and
CO
2 solution with a higher
NH23ratio
to CO
2 ratio composed from ammonia salts like ammonium
ammonium
carbamate
can
provide
higher
driving
forces for
the FO
process
dueFO
to its
higherdue
osmotic
carbonate and ammonium carbamate can provide higher
driving
forces
for the
process
to its
pressure
[27].
However,
the
high
pH
of
those
solutions
with
high
NH
to
CO
ratios
might
become
2
higher osmotic pressure [27]. However, the high pH of those solutions3 with high
NH3 to CO2 ratios
a problem
for athe
membrane.
with
the NH3 to the
CONH
of 1 was preferred
4 HCO3 solutions
2 ratio
might
become
problem
for theNH
membrane.
NH4HCO
3 solutions with
3 to CO2 ratio of 1 was
by
some
other
researchers
because
it
is
less
corrosive
to
the
membrane
[13].
In
this
simulation,
the
preferred by some other researchers because it is less corrosive to the membrane
[13]. In this
performancethe
ofperformance
the FO process
wasFO
theprocess
first concern
sofirst
a high
ratio so
of a
2.4
wasratio
selected.
simulation,
of the
was the
concern
high
of 2.4 was selected.
To
illustrate
the
influence
of
the
feed
concentration
on
the
process
performance,
NaCl
To illustrate the influence of the feed concentration on the process performance, NaCl
−1 . The draw solution was composed of 12 mol·kg−1
concentrations
ranged
from
0.1
to
1
mol
·
kg
concentrations ranged from 0.1 to 1 mol·kg−1. The draw solution was composed of 12 mol·kg−1 NH3
1 CO . The results are presented in terms of RWATER and energy consumption
NH35and
5 mol
kg−
−1 ·CO
and
mol·kg
2. The 2results are presented in terms of RWATER and energy consumption per
per quantity
of produced
water.
It can
seen
Figure
4 thatRWATER
RWATERand
andthe
theenergy
energy consumption
consumption
quantity
of produced
water.
It can
be be
seen
in in
Figure
4 that
clearly
showed
a
reverse
trend
with
an
increase
of
feed
concentration.
RWATER
is
decreased
while
the
clearly showed a reverse trend with an increase of feed concentration. RWATER is decreased
while
energy
consumption
per per
water
increases
withwith
increasing
NaClNaCl
concentration.
WithWith
the increasing
feed
the
energy
consumption
water
increases
increasing
concentration.
the increasing
concentration,
the
osmotic
pressure
differences
across
the
membrane
decreased.
As
a
consequence,
feed concentration, the osmotic pressure differences across the membrane decreased. As a
a lower amount
of water
would
be migrated
draw side
before
pressure
equilibrium
consequence,
a lower
amount
of water
would to
bethe
migrated
to the
drawosmotic
side before
osmotic
pressure
was
reached,
which
leads
to
a
low
RWATER
value.
The
low
RWATER
values
mean
that
more
“extra”
equilibrium was reached, which leads to a low RWATER value. The low RWATER values mean
that
energy
was
needed
to
heat
up
the
draw
solution.
more “extra” energy was needed to heat up the draw solution.
800
Energy
760
RWATER
RWater
15
720
10
680
Energy (MJ/ton)
20
640
5
0.0
0.2
0.4
0.6
0.8
1.0
NaCl (mol/kg)
Figure
concentration on
on the
the FO
FO process
process performance
performance using
using NH
NH3-CO
-CO2 as
Figure 4.
4. The
The effect
effect of
of feed
feed solution
solution concentration
3
2 as
the
draw
solute.
the draw solute.
The FO performance at different NH3-CO2 concentrations is illustrated in Figure 5. The feed
The FO performance at different NH3 -CO2 concentrations is illustrated in Figure 5. The feed
concentration was fixed at 0.5 mol·kg−1 −NaCl,
which has an osmotic pressure of 2.22 MPa. The
concentration was fixed at 0.5 mol·kg 1 NaCl, which has an osmotic pressure of 2.22 MPa.
concentration of CO2 in the draw solution ranged from 1.0 to 10.0 mol·kg−1, and
the NH3 to CO2 ratio
The concentration of CO2 in the draw solution ranged from 1.0 to 10.0 mol·kg−1 , and the NH3 to CO2
was kept at 2.4. It can be seen in Figure 5 that increasing the draw solution concentration would have
ratio was kept at 2.4. It can be seen in Figure 5 that increasing the draw solution concentration would
a very positive effect on improving the energy efficiency. As discussed above, the increasing osmotic
have a very positive effect on improving the energy efficiency. As discussed above, the increasing
pressure difference is the main reason for the improved FO performance at higher draw solution
osmotic pressure difference is the main reason for the improved FO performance at higher draw
concentrations. The osmotic pressure of the NH3-CO2 solution at different concentrations was plotted
solution concentrations. The osmotic pressure of the NH3 -CO2 solution at different
concentrations
in Figure 6. The osmotic pressure of a draw solution containing
2.4 mol·kg−1 NH3 and 1.0 mol·kg−1
was plotted in Figure 6. The osmotic pressure of a draw solution containing 2.4 mol·kg−1 NH3 and
CO2 is 6.20−1MPa, which would generate an osmotic pressure difference of 4.00 MPa when the feed
1.0 mol·kg CO2 is 6.20 MPa, which would generate an osmotic pressure difference of 4.00 MPa when
the feed solution contains 0.5 mol·kg−1 NaCl. By increasing the NH3 and CO2 concentrations to 24.0
and 10.0 mol·kg−1 , an osmotic pressure difference of 59.2 MPa can be obtained.
Water 2017, 9, 189
Water 2017, 9, 189
8 of 17
8 of 17
20
20
1400
1400
15
15
1200
1200
RWater
RWater
10
10
1000
1000
5
5
800
800
Energy
Energy
0
0
0
0
2
2
4
4
6
6
8
8
CO2 (mol/kg)
CO2 (mol/kg)
10
10
Energy(MJ/ton)
(MJ/ton)
Energy
RWATER
RWATER
solution
0.5 mol·kg−1
NaCl. By increasing the NH3 and CO2 concentrations to 24.0 and 10.0
Water
2017,contains
9, 189
of 17
solution
contains
0.5 mol·kg−1 NaCl. By increasing the NH3 and CO2 concentrations to 24.0 and8 10.0
−1
mol·kg −1, an osmotic pressure difference of 59.2 MPa can be obtained.
mol·kg , an osmotic pressure difference of 59.2 MPa can be obtained.
600
600
Figure
5. The
draw
concentration on
on the FO
FO process performance
performance using NH
NH3-CO2 as
Figure
The effect
effect of
of
draw solution
solution
Figure 5.
5. The
effect
of draw
solution concentration
concentration on the
the FO process
process performanceusing
using NH33-CO
-CO22 as
as
the
draw
solute.
the
the draw
draw solute.
solute.
OsmoticPressure
Pressure(MPa)
(MPa)
Osmotic
70
70
60
60
50
50
40
40
30
30
20
20
1 mol/kg NaCl
1 mol/kg NaCl
0.1 mol/kg NaCl
0.1 mol/kg NaCl
10
10
0
0
0
0
2
2
4
4
6
6
CO2 (mol/kg)
CO2 (mol/kg)
8
8
10
10
Figure 6. Osmotic pressure of draw solution at difference CO2 concentrations.
Figure 6.
6. Osmotic
pressure of
of draw
draw solution
solution at
at difference
difference CO
CO22 concentrations.
concentrations.
Figure
Osmotic pressure
3.2. FO Process Using Na2SO4 as Draw Solute
3.2. FO
Using Na
Na2SO
4 as Draw Solute
3.2.
FO Process
Process Using
2 SO4 as Draw Solute
As a draw solute, Na2SO4 would not only generate considerable osmotic pressure, but would
As aa draw
draw solute, Na
Na2SO
SO4 would
only
generate considerable
considerable osmotic
osmotic pressure,
pressure, but
but would
would
would not
notbecause
only generate
2
4 back-leak
also As
have a lowsolute,
draw solute
the SO42−
ion does not easily
penetrate the
FO
also
have
a
low
draw
solute
back-leak
because
the
SO
422−
ion
does
not
easily
penetrate
the
FO
−
also have a[28,29].
low draw
solute
because
SO4
iondecreased
does not at
easily
penetrate the
FO
membrane
Because
the back-leak
solubility of
Na2SOthe
4 was sharply
the temperature
range
membrane
[28,29].
Because
the
solubility
of
Na
2SO4 was sharply decreased at the temperature range
membrane
[28,29].
Because
the solubility
of Na
wascrystallization
sharply decreased
at the
temperature range
of
0 to 32 °C,
it gives
the chance
of recycling
Na22SO
SO44 via
at a low
temperature.
of
0
to
32
°C,
it
gives
the
chance
of
recycling
Na
2SO4 via crystallization at a low temperature.
◦
of 0 to
32 FO
C, it
gives the chance
of recycling
crystallizationisatdisplayed
a low temperature.
2 SO4 via
The
performance
at different
feed Na
solution
concentrations
in Figure 7. The
The FO
performance
atatdifferent
feed
solution
concentrations
is displayed
in Figure
7. The
The
FO
performance
different
feed
solution
concentrations
is
Figure
7.
−1
concentration of draw solution was set 2.9 mol·kg −1, which is close to the displayed
maximum in
solubility
of
concentration
of
draw
solution
was
set
2.9
mol·kg
,
which
is
close
to
the
maximum
solubility
of
−1 , which is close to the maximum solubility of
The
concentration
of
draw
solution
was
set
2.9
mol
·
kg
Na2SO4 in water. Similar to the situation of NH3-CO2, high feed concentration had a negative
Na2SO
4 in water. Similar to the situation of NH3-CO2, high feed concentration had a negative
Na
water.
Similar to the situation
of of
NH
feed concentration
a negative
influence
−1 while
2 SO4 inon
3 -CO2 , high
influence
FO performance.
The value
RWATER
decreased
from 9.77 had
to 1.49
mol·mol
influence
on FO performance.
The
value of RWATER
decreased
from
9.77mol
to ·1.49
mol·mol
while
−
1 while−1energy
on
FO
performance.
The
value
of
RWATER
decreased
from
9.77
to
1.49
mol
energy consumption increased from 138 to 324 MJ·ton−1
when the concentration of feed solution
−1
energy consumption
increased
from
138·ton
to −324
MJ·ton
when the concentration
of changed
feed solution
1 when
consumption
from
138 to
MJ
the
concentration
of feed solution
from
−1 NaCl
−1. Figure
changed
fromincreased
0.1 mol·kg
to324
1 mol·kg
8 shows
the FO performance
at draw
solution
−1 NaCl to 1 mol·kg−1. Figure 8 shows the FO performance at draw solution
changed
from
0.1
mol·kg
−
1
−
1
0.1
mol
·
kg
NaCl
to
1
mol
·
kg
.
Figure
8
shows
the
FO
performance
at
draw
solution
concentrations
−1
−1
concentrations from 1 to 2.9 mol·kg −1, with feed solution of 0.5 mol·kg −1 NaCl. The value of RWATER
concentrations
from
2.9 mol·kg
, with feed
solution
mol·kg
value of RWATER
−10.5
from
1 to almost
2.9 mol
·kg1−1to, with
solution
0.5
mol·kgof
NaCl.
The NaCl.
value The
of RWATER
increased
increased
linearly
withfeed
increasing
Naof
2SO4 concentration, and the energy consumption sharply
increased
almost
linearly
with
increasing
Na
2SO4 concentration, and the energy consumption sharply
almost linearly with increasing Na2 SO4 concentration, and the energy consumption sharply decreased.
decreased.
decreased.
Water 2017, 9, 189
9 of 17
Water 2017, 9, 189
9 of 17
Water 2017, 9, 189
9 of 17
350
10
350
Energy
RWater
RWATER
RWATER
8
300
Energy
300
RWater
8
250
6
250
6
200
4
200
4
150
2
2
150
0
0.0
0
0.0
100
0.2
0.4
0.2
NaCl (mol/kg)
0.4
0.6
0.6
0.8
1.0
0.8
1.0
Energy
(MJ/ton)
Energy
(MJ/ton)
10
100
NaCl (mol/kg)
Figure 7. The effect of feed solution concentration on the FO process performance using Na2SO4 as the
RWATER
RWATER
draw solute.
4.0
500
4.0
3.5
500
450
3.5
3.0
450
400
3.0
2.5
400
350
2.5
2.0
RWater
2.0
1.5
RWater
300
250
1.5
1.0
1.0
0.5
350
300
Energy
Energy
(MJ/ton)
Energy
(MJ/ton)
Figure 7. The effect of feed solution concentration on the FO process performance using Na2 SO4 as the
Figuresolute.
7. The effect of feed solution concentration on the FO process performance using Na2SO4 as the
draw
draw solute.
250
200
200
Energy
0.5
0.0
150
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0.0
150
Na21.8
SO4 2.0
(mol/kg)
0.8 1.0 1.2 1.4 1.6
2.2 2.4 2.6 2.8 3.0
Na2SO4 (mol/kg)
Figure 8. The effect of draw solution concentration on the FO process performance using Na2SO4 as
Figuresolute.
8. The effect of draw solution concentration on the FO process performance using Na2SO4 as
draw
Figure 8. The effect of draw solution concentration on the FO process performance using Na2 SO4 as
draw solute.
draw solute.
3.3. FO Process Using Solvents with LCST Points
3.3. FO Process Using Solvents with LCST Points
An organic
withwith
a low
critical
solution temperature (LCST) point is completely miscible
3.3. FO Process
Usingsolute
Solvents
LCST
Points
organic
with atransition
low critical
solution temperature
(LCST) point
is completely
with An
water
belowsolute
the phase
temperature,
making it possible
to provide
enough miscible
osmotic
withorganic
water
below
phase
transition
temperature,
making
possible
topoint
provide
enough
osmotic
An
solutethe
with
a low
critical
solution
(LCST)
is completely
miscible
pressure
to withdraw
the
water
from
feed
solutionstemperature
[30].
At anitelevated
temperature
above the
LCST
pressure
to
withdraw
the water
from
feed
solutions
[30].
At liquid-liquid
an elevated
temperature
above
the LCST
miscible
solution
would
exhibit
the
phase
separation.
Then
two
with point,
waterthe
below
theorganic-water
phase
transition
temperature,
making
it possible
to provide
enough
osmotic
point,
the
miscible
organic-water
solution
would
exhibit
the
liquid-liquid
phase
separation.
Then
two
liquid
phases
are
formed,
including
the
organic-rich
phase
with
concentrated
draw
solute,
and
the the
pressure to withdraw the water from feed solutions [30]. At an elevated temperature above
liquid
phases
are
formed,
including
the
organic-rich
phase
with
concentrated
draw
solute,
and
the
water-rich
phase.
Therefore,
the
draw
solute
can
be
separated
from
water
by
varying
the
temperature
LCST point, the miscible organic-water solution would exhibit the liquid-liquid phase separation.
water-rich
phase. Therefore,
the draw solute
can besolute
separated
from water
by varying
temperature
for
the liquid-liquid
phase separation.
The draw
recovery
of organic
solutesthe
with
an LCST
Then two liquid phases are formed, including the organic-rich phase with concentrated draw solute,
for theonly
liquid-liquid
phase separation.
Theseparator,
draw solute
recovery
of operate
organic more
soluteseasily
with than
an LCST
point
needs a simple
liquid-liquid
which
would
the
and the water-rich phase. Therefore, the draw solute can be separated from water by varying the
point only columns
needs afor
simple
liquid-liquid of
separator,
would
operate the
more
easily than and
the
distillation
the decomposition
NH3-CO2 which
solution.
Furthermore,
transportation
temperature
forcolumns
the of
liquid-liquid
separation.
The
draw
recovery
organic
solutes
distillation
for the
decomposition
NHit3-CO
2 solution.
Furthermore,
theof
transportation
and
re-condensation
gases
can
bephase
avoided
soofthat
would
makesolute
the
process
operation
more easy.
As with
an LCST
point
onlysolutes
needs
acan
simple
liquid-liquid
which
would
operatemore
more
easily
of
gaseswith
be
avoided
thatto
it separator,
would
make
the
process
As than
are-condensation
result,
organic
LCST
pointssotend
be
promising
draw
soluteoperation
candidates
andeasy.
receive
the distillation
columns
for
the decomposition
oftoNH
-CO2 solution.
Furthermore,
theand
transportation
a result, organic
solutes
with
LCST points tend
be3promising
draw solute
candidates
receive
considerable
research
attention.
considerable
research
attention.
and re-condensation
of gases
can be propylene
avoided so
thatmono-butyl
it would make
operation more
Two organic
solvents,
namely
glycol
ether the
(PB)process
and dipropylamine
(DP), easy.
Two
organic
solvents,
namely
propylene
glycol
mono-butyl
ether
(PB)
and
dipropylamine
(DP),
were
selected
as
the
draw
solutes.
PB
was
once
evaluated
by
Nakayama
et
al.
[17]
and
the
advantages
As a result, organic solutes with LCST points tend to be promising draw solute candidates and
receive
were
selected
as
the
draw
solutes.
PB
was
once
evaluated
by
Nakayama
et
al.
[17]
and
the
advantages
of using glycol
ethers
as draw solutes include low viscosities, low pH values, low volatilities, etc. On
considerable
research
attention.
of using
glycolthe
ethers
draw
solutes
include
lowDP
viscosities,
pH
values,
low
On (DP),
the
contrary,
highaspH
value
of the
aqueous
solutionlow
might
cause
potential
damageetc.
to the
Two
organic
solvents,
namely
propylene
glycol
mono-butyl
ether
(PB)
andvolatilities,
dipropylamine
the
contrary,
the
high
pH
value
of
the
aqueous
DP
solution
might
cause
potential
damage
to
the
membrane, and more caution needs to be taken during the operation because of the volatility and
were selected as the draw solutes. PB was once evaluated by Nakayama et al. [17] and the advantages
membrane,
andSo,
more
to bedraw
takensolution
during the
operation
because
and
toxicity
of DP.
DP caution
is not a needs
very good
in the
real world,
but of
in the
this volatility
case DP was
of using glycol ethers as draw solutes include low viscosities, low pH values, low volatilities, etc.
toxicity of
So, DP is the
not effect
a veryofgood
drawliquid-liquid
solution in the
real world,on
butthe
in FO
this performance.
case DP was
selected
to DP.
demonstrate
different
equilibriums
On the
contrary,
the high pH
aqueous
DPdiagram
solution
cause
potential
damage
selected
demonstrate
thevalue
effectofofthe
different
liquid-liquid
equilibriums
on +the
FO performance.
Figure
9 to
demonstrates
the
liquid-liquid
equilibrium
ofmight
the solvent
water
systems.
Theto the
membrane,
and
more
caution
needs
to
be
taken
during
the
operation
because
of
the
volatility
Figure 9 demonstrates
the liquid-liquid
of the solvent
+ waterofsystems.
The and
feasibility
of using the UNIQUAC
modelequilibrium
to estimate diagram
the thermodynamic
properties
the organic
toxicity
of
DP.
So,
DP
is
not
a
very
good
draw
solution
in
the
real
world,
but
in
this
case
DP
was
selected
feasibility of using the UNIQUAC model to estimate the thermodynamic properties of the organic
to demonstrate the effect of different liquid-liquid equilibriums on the FO performance. Figure 9
demonstrates the liquid-liquid equilibrium diagram of the solvent + water systems. The feasibility
of using the UNIQUAC model to estimate the thermodynamic properties of the organic solutions
Water 2017, 9, 189
10 of 17
Water 2017, 9, 189
10 of 17
was verified
comparing
the calculated
liquid-liquid
equilibrium
data with
literature
data
[31,32].
solutionsby
was
verified by comparing
the calculated
liquid-liquid
equilibrium
data with
literature
data
In addition,
theaddition,
calculated
agree
well agree
with published
results. Itresults.
can beItseen
the that
solubility
of
[31,32]. In
theresults
calculated
results
well with published
can that
be seen
the
of water in the
organic-rich
of the
DP + water
mixture
shows
a greater dependence
watersolubility
in the organic-rich
phase
of the DPphase
+ water
mixture
shows
a greater
dependence
on temperature
Water
2017, 9, 189 than PB, which suggests that larger RWATER values, thus lower 10energy
of 17
temperature
than on
PB,
which
suggests that larger RWATER values, thus lower energy consumption, may be
obtained
may be obtained when using DP.
whenconsumption,
using
DP.
solutions was verified by comparing the calculated liquid-liquid equilibrium data with literature data
[31,32]. In addition, the calculated results agree well with published results. It can be seen that the
solubility of80water in the organic-rich phase of the DP80+ water mixture shows a greater dependence
70
70
on temperature
than PB, which suggests that larger
RWATER values, thus lower energy
60
60 may be obtained when using DP.
consumption,
exp
exp
20
10
o
Temp ( C)
0
Temp ( C)
o
80
70
60
exp
cal
50
40
o
30
-10
30
0.0
20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
cal
50
cal
40
Temp ( C)
o
Temp ( C)
50
0.9
1.0
mole fraction of PB
10
40
80
30
70
20
60
10
50
0
40
-10
30
0.0
20
exp
cal
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
mole fraction of DP
10
0
(a)
(b)
0
-10
-10
Figure 9. Liquid-liquid
equilibrium
0.0 0.1 0.2 0.3
0.4 0.5 0.6 diagram
0.7 0.8 0.9 of
1.0a lower critical solution temperature (LCST) organic
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Figure 9. Liquid-liquid equilibrium
diagram of a lower0.0critical
solution temperature (LCST) organic
of PB
fraction
of DP
solvents + water system;mole
(a)fraction
propylene
glycol n-butyl ether (PB) +mole
water;
(b)
dipropylamine (DP) +
solvents + water system; (a) propylene glycol n-butyl ether (PB) + water; (b) dipropylamine
water.
(a)
(b)
(DP) + water.
9. Liquid-liquid
equilibrium
diagram
of a lower critical
solution
temperature the
(LCST)
organic
ToFigure
investigate
the influence
of feed
concentration
on FO
performance,
temperature
for
solvents
wateratsystem;
(a) propylene
glycol
n-butyl
ether (PB)
+ water;
(b) Figure
dipropylamine
(DP)From
+
separation
was+kept
80 °C. The
results
shown
in Figure
for
PB
and
11the
for DP.
the for
To
investigate
the influence
of
feedare
concentration
on10FO
performance,
temperature
water.
results,
it
can
be
expected
that
the
energy
consumption
of
the
FO
process
using
organic
draw
solute
◦
separation was kept at 80 C. The results are shown in Figure 10 for PB and Figure 11 for DP. From
would increase by increasing the feed concentration, because the osmotic pressure difference is
the results,To
it can
be expected
that the
energy
consumption
the FO process
using organic
investigate
the influence
of feed
concentration
on FOofperformance,
the temperature
for draw
decreased. Comparing the FO performance when using PB as a draw solute with that when using DP,
separation
was kept
80 °C. The results
are concentration,
shown in Figure 10
for PB and
11 for
DP. From
the
solute would
increase
byatincreasing
the feed
because
the Figure
osmotic
pressure
difference
is
it is clearly demonstrated that much more energy is required for the PB draw solute. This can be
results,
it can be expected
that
the energy consumption
of the
FO
process
using
organic
draw
solute
decreased.
Comparing
the
FO
performance
when
using
PB
as
a
draw
solute
with
that
when
using
explained by their difference in liquid-liquid equilibrium behavior. The mole fraction concentration of
increase by increasing
feed concentration,
thefor
osmotic
pressure
difference
is can be
DP, itthe
iswould
clearly
that the
much
isbecause
required
theofPB
draw
solute.
This
PB drawdemonstrated
solution in the organic
rich more
phase energy
is 0.409 at
the
temperature
80 °C.
In addition,
it will
decreased. Comparing the FO performance when using PB as a draw solute with that when using DP,
explained
by their
difference
liquid-liquid
behavior.
mole fraction
concentration
of
reach
osmotic
pressure in
equilibrium
at theequilibrium
concentration
of 0.381,
ifThe
the
rich phase
used
it isthe
clearly
demonstrated
that much more
energy
is required
for the
PBorganic
draw solute.
Thisiscan
be as
◦ C. In addition,
−1 NaCl. Onof
the PB
draw
in
the
organic
rich
phase
is
0.409
at
the
temperature
80
it
will
theexplained
drawsolution
solution
and
the
feed
solution
is
composed
of
0.5
mol·kg
the
contrary,
the
change
by their difference in liquid-liquid equilibrium behavior. The mole fraction concentration of
concentration
for DP
draw
solution
is
0.751attothe0.579
at the ifsame
condition.
The
reachinthe
osmotic
pressure
atphase
thefrom
concentration
of 0.381,
rich
phase
the
PB draw solution
in equilibrium
the
organic
rich
is 0.409
temperature
ofthe
80 organic
°C.
In addition,
ithigher
will is used
−
1
concentration
of
DP
in
the
organic
rich
phase
after
phase
separation
means
it
can
produce
a
higher
reach the
osmoticand
pressure
equilibrium
at the
of 0.381,
if the
rich phase
is used
as
as the draw
solution
the feed
solution
isconcentration
composed of
0.5 mol
·kgorganic
NaCl.
On the
contrary,
the
−1 NaCl.
osmotic
pressure
compared
todraw
that
ofsolution
PB.isThus,
greater
RWATER
value
andthe
a better
FO performance
the
solution
and
feed
solution
composed
of0.751
0.5
mol·kg
On
contrary,
the change
change
in draw
concentration
fortheDP
isafrom
to 0.579
at the
same
condition.
The higher
were
achieved by using
DPdraw
as draw solute.is from 0.751 to 0.579 at the same condition. The higher
in concentration
DP
concentration
of DP inforthe
organicsolution
rich phase after phase separation means it can produce a higher
concentration of DP in the organic rich phase after phase separation means it can produce a higher
osmotic pressure compared to0.20
that of PB. Thus, a greater RWATER value and a better FO performance
osmotic pressure compared to that of PB. Thus, a greater RWATER value and a better FO performance
16000
were achieved
by using
DP DP
as0.18draw
were achieved
by using
as drawsolute.
solute.
Energy
14000
0.14
0.18
16000
12000
RWater
Energy
Energy (MJ/ton)
Energy (MJ/ton)
RWATER
RWATER
RWater
0.16
0.20
0.16
0.12
14000
10000
0.14
0.10
12000
8000
0.12
0.08
10000
0.10
0.06
0.08
0.0
0.06
6000
0.2
0.4
0.6
NaCl (mol/kg)
0.8
1.0
8000
6000
Figure 10. The effect of feed0.0solution
FO process
performance using PB as the
0.2 concentration
0.4
0.6on the0.8
1.0
draw solute.
NaCl (mol/kg)
Figure 10. The effect of feed solution concentration on the FO process performance using PB as the
Figure 10. The effect of feed solution concentration on the FO process performance using PB as the
draw solute.
draw solute.
Water 2017, 9, 189
11 of 17
Water 2017, 9, 189
11 of 17
Water 2017, 9, 189
11 of 17
1.35
RWATER
(mol/kg)
RWATER
(mol/kg)
1.35
1.30
1.30
1.25
RWater
Energy
2450
2400
RWater
Energy
2400
2350
1.25
1.20
2350
2300
1.20
1.15
2300
2250
2250
2200
1.15
1.10
2200
2150
1.10
1.05
Energy
(MJ/ton)
Energy
(MJ/ton)
2450
2150
2100
1.05
1.00
0.0
1.00
0.0
0.2
0.4
0.2
0.4
0.6
NaCl (mol/kg)
0.6
0.8
1.0
0.8
1.0
2100
2050
2050
NaCl (mol/kg)
Figure 11. The effect of feed solution concentration
on the FO process performance using DP as the
Figure 11. The effect of feed solution concentration on the FO process performance using DP as the
draw solute.
drawFigure
solute.11. The effect of feed solution concentration on the FO process performance using DP as the
draw solute.
As the organic rich phase is used as a draw solution, the concentration of draw solution using
organic
solute
with
an
LCST
point
is controlled
bysolution,
the separation
temperature.
temperature
wasusing
As the
rich
phase
is is
used
asasaadraw
theconcentration
concentration
of draw
solution
As organic
the organic
rich
phase
used
draw
solution,
the
ofThe
draw
solution
using
manipulated
to
study
its
effect
on
the
FO
performance
using
DP
as
draw
solute.
It
can
be
seen
from
organic
solute
withwith
an LCST
point
isiscontrolled
the separation
separationtemperature.
temperature.
temperature
organic
solute
an LCST
point
controlled by
by the
TheThe
temperature
was was
the
results
shown
ineffect
Figure
that
the
RWATER value
with
increasing
manipulated
to study
its
effect
the
performance
usingincreases
DPas
asdraw
draw
solute.
It
be
manipulated
to study
its
on12on
the
FOFO
performance
using
DP
solute.
It can
cantemperature.
beseen
seenfrom
from the
However,
the
energy
consumption
was
first
decreased
thenincreases
increasedwith
with increasing
increasing temperature.
temperature,
the
results
shown
in
Figure
12
that
the
RWATER
value
results shown in Figure 12 that the RWATER value increases with increasing temperature. However,
and
achieve
a minimal
value at around
°C.decreased
Although athen
greater
RWATER
be obtained
at higher
However,
the
energy was
consumption
was76first
increased
withcan
increasing
temperature,
the energy
consumption
first decreased
then increased
with
increasing
temperature,
and achieve a
temperatures
because value
of ◦theatmore
concentrated
drawa solution.
More thermal
energy would
be
and achieve a minimal
around
76 °C. Although
greater RWATER
can be obtained
at higher
minimal
value
at around
76 C. Although
a greater RWATER
can be obtained
at higher
temperatures
required
to heat
up the solution.
As aconcentrated
consequence of
thesesolution.
two effects,
an thermal
optimized
temperature
temperatures
because
of the more
draw
More
energy
would for
be
because
of the more
concentrated
draw solution.
More
thermal energy
would
be required
to heat
liquid-liquid
separation
might be
for certain
FO processes
using
draw
solutes
with LCST
required to heat
up the solution.
As found
a consequence
of these
two effects,
an optimized
temperature
for
up the
solution.
As
a
consequence
of
these
two
effects,
an
optimized
temperature
for
liquid-liquid
points.
liquid-liquid separation might be found for certain FO processes using draw solutes with LCST
separation
points.might be found for certain FO processes using draw solutes with LCST points.
3000
3.0
3000
2800
2800
2.5
2.0
2.0
1.5
RWater
2600
RWater
2600
1.5
1.0
1.0
0.5
0.5
0.0
0.0
2400
Energy
2400
Energy
2200
60
70
80
90
100
60
70
o
Temp
80 ( C)
90
100
Energy
(MJ/ton)
Energy
(MJ/ton)
RWATER
RWATER
3.0
2.5
2200
Temp
(oC)FO process performance using DP as the draw
Figure 12. The effect of separation temperature
on the
solute. 12. The effect of separation temperature on the FO process performance using DP as the draw
Figure
Figure 12. The effect of separation temperature on the FO process performance using DP as the
solute.
drawFigure
solute.13 gives a direct comparison of the energy consumptions of the FO processes using
2SO4
different
draw
shows
that the energy
consumptions
of FO processes
PB and Na
Figure
13 solutes.
gives a Itdirect
comparison
of the
energy consumptions
of theusing
FO processes
using
were
more
sensitive
to
feed
concentration
changes.
The
FO
process
using
PB
exhibited
the
highest
SO4
different
shows that the
consumptions
of FO of
processes
PB and
Na2different
Figure
13draw
givessolutes.
a directIt comparison
ofenergy
the energy
consumptions
the FO using
processes
using
energy
consumption
among
the
four investigated
draw
solutes,
whichusing
was about
ten times
the
value
were
more
sensitive
to
feed
concentration
changes.
The
FO
process
PB
exhibited
the
highest
draw solutes. It shows that the energy consumptions of FO processes using PB and Na2 SO4 were
of the NH
3-CO2 process.
Compared
with
NH3-CO2draw
, the Na
2SO4 process
showed
a much
lower
consumption
among
the four
investigated
solutes,
which PB
was
about
ten times
theenergy
value
moreenergy
sensitive
to feed concentration
changes.
The FO process
using
exhibited
the highest
energy
value.
However,
it
should
be
noted
that
the
cooling
operation
is
achieved
normally
with
the
of the NH3-CO2 process. Compared with NH3-CO2, the Na2SO4 process showed a much lower
energy
consumption
among
the
four
investigated
draw
solutes,
which
was
about
ten
times
the
value
of the
utilization
of electric
power which
is a that
high the
grade
energy,
while a low-grade
heat
source was
value. However,
it should
be noted
cooling
operation
is achieved
normally
withused
the
NH3 -CO
Compared
with
NH3 -CO2 , So,
theequivalent
Na2 SO4 process
showed for
a much
lower energy
2 process.
for
the
decomposition
of the which
NH
3-CO
work
is necessary
the comparison
utilization
of electric power
is 2asolution.
high grade energy,
while
a low-grade
heat source
was used
value.for
However,
it
should
be
noted
that
the
cooling
operation
is
achieved
normally
with
the
utilization
of total
energy consumption,
as discussed
later. So, equivalent work is necessary for the comparison
the decomposition
of the NH
3-CO2 solution.
of electric
is a high
grade energy,
of totalpower
energy which
consumption,
as discussed
later. while a low-grade heat source was used for the
decomposition of the NH3 -CO2 solution. So, equivalent work is necessary for the comparison of total
energy consumption, as discussed later.
Water 2017, 9, 189
12 of 17
Water 2017, 9, 189
12 of 17
10000
NH3-CO2
Energy (MJ/ton)
Na2SO4
PB
DP
1000
100
0.0
0.2
0.4
0.6
0.8
1.0
NaCl (mol/kg)
Figure
of of
thethe
FOFO
process
using
different
drawdraw
solutes
as a function
of NaCl
Figure 13.
13. Energy
Energyconsumption
consumption
process
using
different
solutes
as a function
of
concentrations.
NaCl concentrations.
3.4. Equivalent Work for FO
3.4. Equivalent Work for FO
In order to compare the total value of the FO processes using different grades of energy, a
In order to compare the total value of the FO processes using different grades of energy, a method
method of involving the calculation of “equivalent work”, in which the energy consumed was
of involving the calculation of “equivalent work”, in which the energy consumed was assigned an
assigned an electrical work value, was considered. For the FO processes using NH3-CO2 and organic
electrical work value, was considered. For the FO processes using NH3 -CO2 and organic solutes
solutes with LCST points, thermal energy is required for the recovery of draw solutions. In the case
with LCST points, thermal energy is required for the recovery of draw solutions. In the case where
where no low grade heat source is available, fuel energy would be directly charged to the FO process.
no low grade heat source is available, fuel energy would be directly charged to the FO process. So,
So, the equivalent work is associated to the electrical work produced by the fuel energy [31], which
the equivalent work is associated to the electrical work produced by the fuel energy [31], which can be
can be calculated by the following equation:
calculated by the following equation:
W = Q ×η + W
pd
pump
Weqeq= Qdd× η pd
+ Wpump
(13)
(13)
Q means the thermal energy supplied to the FO process, η pd is the overall power plant efficiency,
Qdd means the thermal energy supplied to the FO process, η pd is
the overall power plant efficiency, and
W
is
the
electrical
energy
requirements
for
pumping
work.
and
Wpump is the electrical energy requirements for pumping work.
pump
Figure 14. gives an example where low grade thermal energy from a cogeneration power desalting
Figure 14. gives an example where low grade thermal energy from a cogeneration power
plant [32] is available for the FO process. The steam supplied to the FO process is extracted from the
desalting plant [32] is available for the FO process. The steam supplied to the FO process is extracted
low pressure turbine. In this case, the equivalent work to the thermal energy Qd is relevant to the
from the low pressure turbine. In this case, the equivalent work to the thermal energy Q d is relevant
electrical work produced by the low pressure turbine:
to the electrical work produced by the low pressure turbine:
Weq = Qd × η LP + Wpump
(14)
Weq = Qd ×η LP + Wpump
(14)
where η LP means the low pressure turbine efficiency for producing electrical work. In addition, if
where
the Rankine
low pressure
efficiency
forlow
producing
In addition,
if
η LPthatmeans
assumed
the ideal
cycleturbine
is employed
by the
pressureelectrical
turbine, work.
the equivalent
work
assumed
that the
ideal Rankine
cycle is employed by the low pressure turbine, the equivalent work
can be further
calculated
as:
can be further calculated as:
h − h2
Weq = Qd × 1
× Eturbine + Wpump
(15)
hh1 −− hh3
1
2
Weq = Qd ×
× Eturbine + Wpump
(15)
h3
where h1 is the enthalpy of the steam used hto1 −
provide
process heat input, h2 is the enthalpy of the
steam at the turbine outlet, and h3 is the enthalpy of the steam at the condenser outlet. In this study,
enthalpy of
theassumed
steam used
provide
heat input, h2 is the
enthalpy
of the
where
h1 is the
◦ C. E process
the condenser
temperature
was
to beto35
which
is assumed
turbine is the turbine efficiency
to be 85%.
steam
at the turbine outlet, and h3 is the enthalpy of the steam at the condenser outlet. In this study,
the condenser temperature was assumed to be 35 °C. Eturbine is the turbine efficiency which is
assumed to be 85%.
Water
Water 2017,
2017, 9,
9, 189
189
13
13 of
of 17
17
Wt
We
Qf
Qd
LP Turbine
HP Turbine
FO Process
Figure
Figure 14.
14. Schematic
Schematic diagram
diagram of
of cogeneration
cogeneration power
power desalting
desalting plant.
plant.
2SO
4, electrical
work
is required
for draw
solute
recovery.
The
For the FO
FO process
process using
usingNa
Na
work
is required
for draw
solute
recovery.
2 SO
4 , electrical
equivalent
work
for
the
heat
removal
by
the
refrigerator
system
is
expressed
as:
The equivalent work for the heat removal by the refrigerator system is expressed as:
Weqeq==
W
Qdd
Q
Wpump
++W
pump
COP
COP
(16)
(16)
where
(Coefficient
of Performance)
is theisratio
of cooling
or heating
to energy
consumption.
COP
whereCOP
(Coefficient
of Performance)
the ratio
of cooling
or heating
to energy
consumption.
COP
is
largely
dependent
on
operating
conditions,
like
the
cooling
and
ambient
temperatures,
coolant
COP is largely dependent on operating conditions, like the cooling and ambient temperatures,
medium,
the efficiency
of the heat
pump,
In this
COP
is assumed
to assumed
be 4.5 forto
cooling
coolant medium,
the efficiency
of the
heatetc.
pump,
etc.study,
In this
study,
be 4.5 the
for
COP is
◦
diluted
solution
from
room temperature
to 0 C, and atoglycol
system was used.
cooling draw
the diluted
draw
solution
from room temperature
0 °C, refrigeration
and a glycol refrigeration
system
The results of equivalent work at different draw solution concentrations are displayed in
was used.
Figures
calculations,
the feed
solutions
consisted
of 0.5 mol
kg−1 NaCl.
the
The15–17.
resultsIn
ofthose
equivalent
work atall
different
draw
solution
concentrations
are·displayed
in For
Figures
◦
draw
of NH
boiler
temperature
approximated
at 100−1 NaCl.
C under
pressure
3 -CO2 , theall
15–17.solute
In those
calculations,
the feed
solutionswas
consisted
of 0.5 mol·kg
Forambient
the draw
solute
◦ C were used. Because the temperature for liquid-liquid separation
so
low
grade
steams
of
110
and
130
of NH3-CO2, the boiler temperature was approximated at 100 °C under ambient pressure so low grade
◦ C, a heat source of 90 ◦ C can be used for the FO process
of
DP +of
water
solutions
below
steams
110 and
130 °Cwas
were
used.80Because
the temperature for liquid-liquid separation of DP +
using
a
draw
solute
of
DP.
It
can
be
seen
in
Figure
15 can
thatbe
the
equivalent
work
was using
from 60.5
to
water solutions was below 80 °C, a heat source of 90 °C
used
for the FO
process
a draw
−1 for NH -CO when the heat was directly supplied by fuel energy. When a low grade
−1
117.6
kWh
·
ton
3 in2Figure 15 that the equivalent work was from 60.5 to 117.6 kWh·ton for
solute of DP. It can be seen
◦ C was used, the lowest equivalent work could be 21.1 kWh·ton−1 which suggests that the
steam
of 110
2 when
the heat was directly supplied by fuel energy. When a low grade steam of 110 °C was
NH3-CO
grade
sources
would have
influence
on −1
thewhich
process
value. For
of Na
4,
used, of
theheat
lowest
equivalent
worka significant
could be 21.1
kWh·ton
suggests
thatthe
thecase
grade
of2 SO
heat
−1 if the value of COP was
as
shown
in
Figure
16,
the
minimum
of
equivalent
work
was
12.3
kWh
·
ton
sources would have a significant influence on the process value. For the case of Na2SO4, as shown in
◦ C, was in the
−1 as
4.5.
The16,
equivalent
work of
of DP
using a work
steamwas
of 9012.3
of 55.0
70.6 kWh
Figure
the minimum
equivalent
kWh·ton−1 ifrange
the value
oftoCOP
was·ton
4.5. The
−1 as shown
shown
in Figure
For using
a direct
comparison
of the
work
of thetoFO
processes
different
in
equivalent
work17.
of DP
a steam
of 90 °C,
wasequivalent
in the range
of 55.0
70.6
kWh·tonusing
draw
minimum
equivalent
work
for each draw
the
valuedifferent
for NH3draw
-CO2
Figuresolutes,
17. Forthe
a direct
comparison
of the
equivalent
worksolute
of the(for
FOexample,
processes
using
◦ C heat source) are presented in Figure 18.
process
obtained with
10 molwork
·kg−1for
COeach
110 solute
2 and
solutes, was
the minimum
equivalent
draw
(for example, the value for NH3-CO2
−1 CO2 and
It
can bewas
seenobtained
that while
a lower
temperature
steam
be employed
forpresented
the FO process
using
the
process
with
10 mol·kg
110can
°C heat
source) are
in Figure
18.DP,
It can
equivalent
work
wasa still
larger
than the one
using
-CO2 . The FO
a draw
of
be seen that
while
lower
temperature
steam
canNH
be3employed
for process
the FO using
process
usingsolute
DP, the
Na
SO
exhibited
the
lowest
equivalent
work
among
the
draw
solutes
investigated
in
this
study.
2
4
equivalent
work was still larger than the one using NH3-CO2. The FO process using a draw solute of
The
method the
developed
in this work
provides
an draw
approach
to investigated
quickly estimate
energy
Na2SO
4 exhibited
lowest equivalent
work
among the
solutes
in thisthe
study.
consumption
of andeveloped
FO processin
using
solute,
be helpful
screening
The method
thiscertain
work draw
provides
an therefore,
approach ittowould
quickly
estimateforthe
energy
the
draw
solutes.
However,
it
should
also
be
noted
that
the
assumptions
in
this
study
are
based
on
consumption of an FO process using certain draw solute, therefore, it would be helpful for screening
ideal
conditions.
factors
as also
the membrane
permeability,
long term
stability,
the price
the draw
solutes.More
However,
it such
should
be noted that
the assumptions
in this
studyand
are based
on
of
theconditions.
energy need
to befactors
taken such
into consideration
for more
practical long
operations.
For example,
the
FO
ideal
More
as the membrane
permeability,
term stability,
and the
price
process
using NH
be into
moreconsideration
attractive than
low
grade
heat sources
3 -CO
2 may
2 SO4 if abundant
of the energy
need
to be
taken
forusing
moreNa
practical
operations.
For
example,
the FO
at
cheapusing
pricesNH
are3-CO
available.
process
2 may be more attractive than using Na2SO4 if abundant low grade heat sources
at cheap prices are available.
14 of 17
Water 2017, 9, 189
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Water 2017, 9, 189
14 of 17
14 of 17
14 of 17
Equiv
Equiv
Work
Work
(kWh/ton)
(kWh/ton)
Equiv
Work
(kWh/ton)
Water 2017, 9, 189
120
120
120
110
110
110
100
100
100
90
90
90
80
80
80
70
70
70
60
60
60
50
50
50
40
40
40
30
30
30
20
20
20 0
0
0
110ooC
110oC
130
110ooC
C
130
C
Fuelo
130 C
Fuel
Fuel
2
2
2
4
6
4
6
4 CO2 (mol/kg)
6
8
8
8
CO2 (mol/kg)
CO (mol/kg)
10
10
10
2
Figure 15.
15. Equivalent
work of
of the FO
FO process
using NH
NH3-CO
Figure
Equivalent work
-CO222 as
as the
the draw
draw solute.
solute.
Figure 15. Equivalent
work of the
the FO process
process using
using NH33-CO
as
the
draw
solute.
Figure 15. Equivalent work of the FO process using NH3-CO2 as the draw solute.
Equiv
Equiv
Work
Work
(kWh/ton)
(kWh/ton)
Equiv
Work
(kWh/ton)
30
30
30
25
25
25
20
20
20
15
15
15
10
10
10
1.0
1.0
1.0
1.5
2.0
2.5
1.5
2.0
2.5
(mol/kg) 2.5
1.5 Na2SO42.0
3.0
3.0
3.0
Na2SO4 (mol/kg)
Na2SO4 (mol/kg)
Figure 16. Equivalent work of the FO process using Na2SO4 as the draw solute.
Figure 16.
16. Equivalent
work of
of the
the FO
FO process
process using
using Na
Na2SO
Figure
Equivalent work
SO4 as
as the
the draw
draw solute.
solute.
Figure 16. Equivalent work of the FO process using Na22SO44 as the draw solute.
90ooC
90 oCo
110
oC
90 C
110
C
o
Fuel
110
Fuel C
Fuel
Equiv
Equiv
Work
Work
(kWh/ton)
(kWh/ton)
Equiv
Work
(kWh/ton)
250
250
250
200
200
200
150
150
150
100
100
100
50
5058
5058
58
60
60
60
62
62
62
64
64
64
66
66
66
68 70 72
68 70 72
68
70(ooC)
72
Temp
Temp ( oC)
Temp ( C)
74
74
74
76
76
76
78
78
78
80
80
80
82
82
82
Figure 17. Equivalent work of the FO process using DP as the draw solute.
Figure 17. Equivalent work of the FO process using DP as the draw solute.
Figure
as the
the draw
draw solute.
solute.
Figure 17.
17. Equivalent
Equivalent work
work of
of the
the FO
FO process
process using
using DP
DP as
Water 2017, 9, 189
Water 2017, 9, 189
15 of 17
15 of 17
60
NH3-CO2 (kWh/ton)
50
NH3-CO2
Na2SO4
DP
40
30
20
10
0
Draw Solute
Figure
Figure 18.
18. Equivalent
Equivalent work
work comparison
comparison of
of the
the FO
FO process
process using
using different
different draw
draw solutes.
solutes.
4.
4. Conclusions
Conclusions
The
The energy
energy requirements
requirements of
of the
the FO
FO process
process using
using various
various draw
draw solutes
solutes were
were presented
presented under
under
different
feed
and
draw
solution
concentrations.
It
was
found
that
the
increasing
different feed and draw solution concentrations. It was found that the increasing feed
feed solution
solution
concentrations
concentrations would
would have
have aa negative
negative effect
effect on
on process
process performance.
performance. The
The energy
energy consumption
consumption of
of the
the
FO
process
decreased
with
increasing
draw
solution
concentration.
The
NH
3-CO2 process had the
FO process decreased with increasing draw solution concentration. The NH3 -CO2 process had the
lowest
lowest thermal
thermal energy
energy requirement
requirement among
among the
the three
three FO
FO processes
processes because
because the
the draw
draw solute
solute recovery
recovery
was
achieved
by
heating.
On
the
contrary,
the
FO
process
using
propylene
glycol
mono-butyl
was achieved by heating. On the contrary, the FO process using propylene glycol mono-butyl ether
ether
showed
showed aa relatively
relatively large
large energy
energy consumption
consumption due
due to
to its
its small
small solubility
solubility in
in water.
water. The
The FO
FO process
process
using
using Na
Na22SO
SO44required
requiredthe
thelowest
lowest equivalent
equivalent work,
work, and
and the
the cooling
cooling efficiency
efficiency largely
largely depended
depended on
on
operating
conditions.
operating conditions.
For
development of
of draw
draw solutes,
solutes, the
the NH
NH3-CO
2 system has high potential due to its
For the
the future
future development
3 -CO2 system has high potential due to its
low
thermal
energy
consumption
and
its
ability
to
treat
highly
concentrated
low thermal energy consumption and its ability to treat highly concentrated feed
feed solutions.
solutions. Although
Although
Na
Na22SO
SO44seems
seemsto
tobe
beaa promising
promising draw
draw solute
solute on
on terms
terms of
of the
the equivalent
equivalent work,
work, further
further studies,
studies, such
such
as
as the
the mass
mass transfer
transfer mechanism
mechanism across
across the
the membrane
membrane and
and the
the crystallization
crystallization and
and dissolution
dissolution rate
rate of
of
Na
2SO4 salt, are essential. For the development of draw solutes with LCST points, it is important to
Na2 SO4 salt, are essential. For the development of draw solutes with LCST points, it is important to
find
solubility in
in water
water is
is strongly
strongly dependent
dependent on
on temperature.
temperature.
find an
an organic
organic solute
solute whose
whose solubility
Acknowledgments:
Acknowledgments: The
The support
support of
of the
the National
National Key
Key Technologies
Technologies R&D
R&D Program
Program of
of China
China (No.
(No. 2015BAE06B00)
2015BAE06B00)
is gratefully acknowledged.
Author Contributions:
Contributions: The
The authors
authors contributed
contributed equally
equally to
to this
this work.
work.
Author
Conflicts of Interest: The authors declare no conflict of interest.
Conflicts of Interest: The authors declare no conflict of interest.
References
References
1.
1.
2.
2.
3.
3.
4.
4.
5.
5.
6.
Water Scarcity|Threats|WWF. Available online: http://www.worldwildlife.org/threats/water-scarcity
Water Scarcity|Threats|WWF. Available online: http://www.worldwildlife.org/threats/water-scarcity
(accessed on 14 March 2016).
(accessed on 14 March 2016).
International Decade for Action “Water for Life” 2005–2015. Focus Areas: Water Scarcity. Available online:
International Decade for Action “Water for Life” 2005–2015. Focus Areas: Water Scarcity. Available online:
http://www.un.org/waterforlifedecade/scarcity.shtml (accessed on 14 March 2016).
http://www.un.org/waterforlifedecade/scarcity.shtml (accessed on 14 March 2016).
Morin, O.J. Design and operating comparison of MSF and MED systems. Desalination 1993, 93, 69–109.
Morin, O.J. Design and operating comparison of MSF and MED systems. Desalination 1993, 93, 69–109.
[CrossRef]
Mistry, K.H.; McGovern, R.K.; Thiel, G.P.; Summers, E.K.; Zubair, S.M.; Lienhard, J.H. Entropy generation
Mistry, K.H.; McGovern, R.K.; Thiel, G.P.; Summers, E.K.; Zubair, S.M.; Lienhard, J.H. Entropy generation
analysis of desalination technologies. Entropy 2011, 13, 1829–1864.
analysis of desalination technologies. Entropy 2011, 13, 1829–1864. [CrossRef]
Greenlee, L.F.; Lawler, D.F.; Freeman, B.D.; Marrot, B.; Moulin, P. Reverse osmosis desalination: Water
Greenlee, L.F.; Lawler, D.F.; Freeman, B.D.; Marrot, B.; Moulin, P. Reverse osmosis desalination: Water sources,
sources, technology, and today’s challenges. Water Res. 2009, 43, 2317–2348.
technology, and today’s challenges. Water Res. 2009, 43, 2317–2348. [CrossRef] [PubMed]
Deshmukh, A.; Yip, N.Y.; Lin, S.; Elimelech, M. Desalination by forward osmosis: Identifying performance
limiting parameters through module-scale modeling. J. Membr. Sci. 2015, 491, 159–167.
Water 2017, 9, 189
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
16 of 17
Deshmukh, A.; Yip, N.Y.; Lin, S.; Elimelech, M. Desalination by forward osmosis: Identifying performance
limiting parameters through module-scale modeling. J. Membr. Sci. 2015, 491, 159–167. [CrossRef]
Chung, T.-S.; Li, X.; Ong, R.C.; Ge, Q.; Wang, H.; Han, G. Emerging forward osmosis (FO) technologies and
challenges ahead for clean water and clean energy applications. Curr. Opin. Chem. Eng. 2012, 1, 246–257.
[CrossRef]
Klaysom, C.; Cath, T.Y.; Depuydt, T.; Vankelecom, I.F.J. Forward and pressure retarded osmosis: Potential
solutions for global challenges in energy and water supply. Chem. Soc. Rev. 2013, 42, 6959–6989. [CrossRef]
[PubMed]
Shaffer, D.L.; Werber, J.R.; Jaramillo, H.; Lin, S.; Elimelech, M. Forward osmosis: Where are we now?
Desalination 2015, 356, 271–284. [CrossRef]
Chekli, L.; Phuntsho, S.; Shon, H.K.; Vigneswaran, S.; Kandasamy, J.; Chanan, A. A review of draw solutes in
forward osmosis process and their use in modern applications. Desalination Water Treat. 2012, 43, 167–184.
[CrossRef]
McCutcheon, J.R.; McGinnis, R.L.; Elimelech, M. A novel ammonia—Carbon dioxide forward (direct)
osmosis desalination process. Desalination 2005, 174, 1–11. [CrossRef]
McGinnis, R.L.; Hancock, N.T.; Nowosielski-Slepowron, M.S.; McGurgan, G.D. Pilot demonstration of
the NH3 /CO2 forward osmosis desalination process on high salinity brines. Desalination 2013, 312, 67–74.
[CrossRef]
Kim, Y.; Lee, J.H.; Kim, Y.C.; Lee, K.H.; Park, I.S.; Park, S.-J. Operation and simulation of pilot-scale forward
osmosis desalination with ammonium bicarbonate. Chem. Eng. Res. Des. 2015, 94, 390–395. [CrossRef]
Wendt, D.S.; Orme, C.J.; Mines, G.L.; Wilson, A.D. Energy requirements of the switchable polarity solvent
forward osmosis (SPS-FO) water purification process. Desalination 2015, 374, 81–91. [CrossRef]
Jessop, P.G.; Mercer, S.M.; Heldebrant, D.J. CO2 -triggered switchable solvents, surfactants, and other
materials. Energy Environ. Sci. 2012, 5, 7240–7253. [CrossRef]
Stone, M.L.; Rae, C.; Stewart, F.F.; Wilson, A.D. Switchable polarity solvents as draw solutes for forward
osmosis. Desalination 2013, 312, 124–129. [CrossRef]
Nakayama, D.; Mok, Y.; Noh, M.; Park, J.; Kang, S.; Lee, Y. Lower critical solution temperature (LCST)
phase separation of glycol ethers for forward osmotic control. Phys. Chem. Chem. Phys. 2014, 16, 5319–5325.
[CrossRef] [PubMed]
Cai, Y.; Shen, W.; Wei, J.; Chong, T.H.; Wang, R.; Krantz, W.B.; Fane, A.G.; Hu, X. Energy-efficient desalination
by forward osmosis using responsive ionic liquid draw solutes. Environ. Sci. Water Res. Technol. 2015, 1,
341–347. [CrossRef]
Zhong, Y.; Feng, X.; Chen, W.; Wang, X.; Huang, K.-W.; Gnanou, Y.; Lai, Z. Using UCST ionic liquid as a draw
solute in forward osmosis to treat high-salinity water. Environ. Sci. Technol. 2016, 50, 1039–1045. [CrossRef]
[PubMed]
McGinnis, R.L.; Elimelech, M. Energy requirements of ammonia–carbon dioxide forward osmosis
desalination. Desalination 2007, 207, 370–382. [CrossRef]
Mazlan, N.M.; Peshev, D.; Livingston, A.G. Energy consumption for desalination—A comparison of forward
osmosis with reverse osmosis, and the potential for perfect membranes. Desalination 2016, 377, 138–151.
[CrossRef]
Park, M.Y.; Shin, S.; Kim, E.S. Effective energy management by combining gas turbine cycles and forward
osmosis desalination process. Appl. Energy 2015, 154, 51–61. [CrossRef]
Chen, C.C.; Britt, H.I.; Boston, J.F.; Evans, L.B. Local composition model for excess Gibbs energy of electrolyte
systems. Part I: Single solvent, single completely dissociated electrolyte systems. AIChE J. 1982, 28, 588–596.
[CrossRef]
Abrams, D.S.; Prausnitz, J.M. Statistical thermodynamics of liquid mixtures: A new expression for the excess
Gibbs energy of partly or completely miscible systems. AIChE J. 1975, 21, 116–128. [CrossRef]
Chen, C.C.; Song, Y.H. Generalized electrolyte-NRTL model for mixed-solvent electrolyte systems. AIChE J.
2004, 50, 1928–1941. [CrossRef]
Chen, C.C.; Evans, L.B. A local composition model for the excess Gibbs energy of aqueous electrolyte systems.
AIChE J. 1986, 32, 444–454. [CrossRef]
Water 2017, 9, 189
27.
28.
29.
30.
31.
32.
17 of 17
McCutcheon, J.R.; McGinnis, R.L.; Elimelech, M. Desalination by ammonia–carbon dioxide forward osmosis:
Influence of draw and feed solution concentrations on process performance. J. Membr. Sci. 2006, 278, 114–123.
[CrossRef]
Achilli, A.; Cath, T.Y.; Childress, A.E. Selection of inorganic-based draw solutions for forward osmosis
applications. J. Membr. Sci. 2010, 364, 233–241. [CrossRef]
Zhao, S.; Zou, L.; Mulcahy, D. Brackish water desalination by a hybrid forward osmosis–nanofiltration
system using divalent draw solute. Desalination 2012, 284, 175–181. [CrossRef]
Christensen, S.P.; Donate, F.A.; Frank, T.C.; LaTulip, R.J.; Wilson, L.C. Mutual solubility and lower critical
solution temperature for water + glycol ether systems. J. Chem. Eng. Data 2005, 50, 869–877. [CrossRef]
Al-Karaghouli, A.; Kazmerski, L.L. Energy consumption and water production cost of conventional and
renewable-energy-powered desalination processes. Renew. Sustain. Energy Rev. 2013, 24, 343–356. [CrossRef]
Darwish, M.A.; Al Asfour, F.; Al-Najem, N. Energy consumption in equivalent work by different desalting
methods: Case study for Kuwait. Desalination 2003, 152, 83–92. [CrossRef]
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