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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

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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

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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.

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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.

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Figure 7. The effect of feed solution concentration on the FO process performance using Na2SO4 as the

RWATER RWATER

draw solute.

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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

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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

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exp cal

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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

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Energy


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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.

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Energy

2450 2400

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Energy

2400 2350

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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

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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.

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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%.

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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.

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Equiv Equiv Work Work (kWh/ton) (kWh/ton) Equiv Work (kWh/ton)

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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.


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


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

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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.

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