A DRAW SOLUTE FOR A FORWARD OSMOSIS PROCESS

Abstract

A draw solute for a forward osmosis process, the draw solute comprising: a thermally responsive ionic compound having at least one of: a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST), the draw solute being regeneratable from a diluted aqueous draw solution after forward osmosis via one of: liquid-liquid phase separation and solid-liquid phase separation, the draw solute being regeneratable when the diluted aqueous draw solution is at a temperature selected from one of: above the LCST and below the UCST

Claims

1. A draw solute for a forward osmosis process, the draw solute comprising: a thermally responsive ionic compound having at least one of: a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST), the draw solute being regeneratable from a diluted aqueous draw solution after forward osmosis via one of: liquid-liquid phase separation and solid-liquid phase separation, the draw solute being regeneratable when the diluted aqueous draw solution is at a temperature selected from one of: above the LCST and below the UCST.

2. The draw solute of claim 1, wherein the ionic compound comprises an organic cation and an organic anion.

3. The draw solute of claim 1, wherein the ionic compound comprises an organic ion and an inorganic ion, wherein the organic ion is one of: a cation and an anion, and wherein the inorganic ion is the other of: the cation and the anion.

4. The draw solute of claim 2, wherein the cation is one selected from the group consisting of: phosphonium, ammonium, imidazolium, pyridinium, pyrrolidinium, sulfonium, morpholinium and a metallic cation.

5. The draw solute of claim 2, wherein the anion is one selected from the group consisting of: halide, sulfonate, alkylsulfate, tosylate, methane sulfonate, nitride, carboxylate, alkoxide, tetrafluoroborate, hexafluorophosphates, dihydrogen phosphate, tricyanomethanide and bis(trifluoromethylsulfonyl)imide.

6. The draw solute of claim 1, wherein the draw solute comprises a zwitterion.

7. The draw solute of claim 1, wherein the draw solute is amphiphilic.

8. The draw solute of claim 1, wherein a sedimentation phase of one of: the liquid-liquid phase separation and solid-liquid phase separation is directly reusable as the draw solution for the forward osmosis process.

9. The draw solute of claim 1, wherein osmolality of the draw solution increases monotonically and not linearly with molality of the draw solution.

10. The draw solute of claim 1, wherein the thermally responsive ionic compound comprises one of: tetrabutylphosphonium 2,4-dimethylbenzenesulfonate (P.sub.4444DMBS), tetrabutylphosphonium mesitylenesulfonate (P.sub.4444TMBS) and tributyloctyl-phosphonium bromide (P.sub.4448Br).

11. The draw solute of claim 10, wherein the LCST of the thermally responsive ionic compound is in the range of 32° C. to 49° C.

Description

BRIEF DESCRIPTION OF FIGURES

[0020] In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings.

[0021] FIG. 1 is a schematic illustration of FO seawater desalination with three thermally responsive ionic liquids as draw solutes.

[0022] FIG. 2a (inset) is a graph of measured correlation between osmolality and molality of the three draw solutes indicating there is agglomeration besides dissociation of draw solutes.

[0023] FIG. 2a is a graph of profile of osmolality as function of draw solution concentration with predicted osmolality according to a simulated function shown in FIG. 6.

[0024] FIG. 2b is a graph of water flux profiles for P.sub.4444DMBS, and P.sub.4444 TMBS draw solutions at room temperature and for P.sub.4444DMBS draw solution at 14±1° C. against different NaCl concentrations.

[0025] FIG. 2c is a graph of water flux profile at 14±1° C. of 70 wt % P.sub.4444DMBS draw solution with feed solutions of various salinity above seawater.

[0026] FIG. 3a is a graph of LCST as a function of draw solute concentration.

[0027] FIG. 3b is a graph of draw solute concentrations in supernatant and sedimentation after phase separation as a function of temperature (initial concentration fixed at 30 wt %).

[0028] FIG. 3c is a graph of draw solute concentration in supernatant and sedimentation at 13° C. above LCST of corresponding initial concentration.

[0029] FIG. 3d is a graph of concentration of supernatant regenerated in pressurized NF filtration.

[0030] FIG. 4 is a graph of water flux and hydraulic pressure correlation in regeneration process.

[0031] FIG. 5 is a graph of DSC results of the three ionic liquids.

[0032] FIG. 6a is a graph of osmolality-molality simulations for P.sub.4444DMBS at osmolality-measurable concentrations.

[0033] FIG. 6b is a graph of osmolality-molality simulations for P.sub.4444TMBS at osmolality-measurable concentrations.

[0034] FIG. 6c is a graph of osmolality-molality simulations for P.sub.4448BR at osmolality-measurable concentrations.

[0035] FIG. 7 is a graph of water flux profile of P.sub.4448Br with different concentrations with feed salinity at 2000 ppm and the temperature at 14±1° C.

[0036] FIG. 8 is a graph of viscosity profiles of the three draw solutions where P.sub.4448 Br was measured at 16° C. while the others were measured at 23° C.

[0037] FIG. 9 is a composite photograph of phase separation as function of time where water bath temperature was 50° C. and draw solute was P.sub.4444 DMBS at 40%, and wherein white plastic visible under each vial is provided for avoiding direct contact with the hot plate.

[0038] FIG. 10 is a list of cations and anions that may be comprised in the draw solute of the present invention.

DETAILED DESCRIPTION

[0039] Exemplary embodiments of the draw solute for a forward osmosis process will be described below with reference to FIGS. 1 to 9.

[0040] Materials and Instruments

[0041] Tributyloctyl phosphonium bromide was purchased from Wako Pure Chemical Industries. Sodium mesitylenesulfonate, sodium 2,4-dimethylbenzenesulfonate and tetrabutylphosphonium bromide were purchased from Tokyo Chemical Industry CO., Ltd. Anhydrous dichloromethane (>99.8%) and sodium chloride (>99.5%) were purchased from Sigma-Aldrich. All chemicals were used without further purification. A forward osmosis membrane used to study the draw solutes was prepared according to a reported method..sup.26 A nanofiltration membrane with molecular weight cut-off (MWCO) of 270 Da was purchased from Dow FilmTec. The osmolality of the draw solution was measured by a cryoscopic method using OSMOMAT 030, Gonotech. Water flux was measured by a cross-flow setup with flow rate of 4 ml/s, and the effective membrane area was fixed at 15 mm×30 mm orientated in PRO mode throughout this study. Water content in the draw solution was measured by Karl Fischer titration. Viscosity was measured by a Physica MCR 101 rheolometer, Anton Paar.

[0042] Draw Solute Synthesis

[0043] Three exemplary thermally responsive draw solutes were prepared and studied. Their structures are illustrated in FIG. 1.

[0044] Tetrabutylphosphonium 2,4-dimethylbenzenesulfonate (P.sub.4444DMBS) was synthesized through ion exchange reactions: aqueous solutions of tetrabutylphosphonium bromide and slightly excess equal molar of sodium 2,4-dimethylbenzenesulfonate were mixed to form a rough 40 wt % solution and stirred at room temperature for 24 hours. The ionic liquid was extracted by dichloromethane and washed with deionized water for several times. The dichloromethane phase was then put in an evaporator to remove the organic solvent in vacuum (˜1 mbar) for 24 hours at 100° C. Tetrabutylphosphonium mesitylenesulfonate (P.sub.4444TMBS) was prepared using a similar method from tetrabutylphosphonium bromide and sodium mesitylenesulfonate. The third ionic liquid tributyloctyl-phosphonium bromide (P.sub.4448Br) was used as received. P.sub.4444TMBS (.sup.1H, 400 MHz, CDCl.sub.3, δ/ppm relative to TMS): 0.89-0.92 (t, 12H, CH.sub.3), 1.42-1.47 (m, 16H, CH.sub.2), 2.17 (s, 3H, CH.sub.3), 2.24-2.31 (m, 8H, CH.sub.2), 2.66 (s, 6H, CH.sub.3), 6.75 (s, 2H, Ar—H). P.sub.4444DMBS (.sup.1H, 400 MHz, CDCl.sub.3, δ/ppm relative to TMS): 0.72-0.76 (t, 12H, CH.sub.3), 1.23-1.27 (m, 16H, CH.sub.2), 2.00-2.07 (m, 8H, CH.sub.2), 2.11 (s, 3H, CH.sub.3), 2.50 (s, 3H, CH.sub.3), 6.72-6.74 (d, 1H, Ar—H), 6.78 (s, 1H, Ar—H), 7.66-7.68 (d, 1H, Ar—H). At room temperature, P.sub.4444DMBS is a colorless viscous liquid while P.sub.4444TMBS and P.sub.4448Br are in wax form.

[0045] DSC results of the three ionic liquids are shown in FIG. 5 in which the temperature increase/decrease rate was 3° C./min under N.sub.2 sweep. The P.sub.4444DMBS is a colorless viscous liquid at wide temperature range from −50° C. to 150° C. P.sub.4444TMBS and P.sub.4448Br are waxes at room temperature with melting points of ˜80° C. and ˜25° C., respectively. The exothermic peak on P.sub.4448Br at around −3° C. is probably the cold crystallization peak, which is commonly observed in ionic liquids.sup.30, 31. The osmolalities of ionic liquids with concentrations higher than 30 wt % cannot be measured by cryoscopic method with OSMOMAT 030, but can be predicted by extrapolating the measured results in low concentrations.

[0046] FIG. 6 demonstrates the simulations of osmolality-molality correlation of the three draw solutes at low concentrations (<1 mol/Kg, ˜30 wt %). The simulated functions were applied to predict the osmolality-concentration correlation for high concentrations. The conversion formula between molality and weight concentration is:

wt=molality×Mw/[(molality×Mw)+1000]

[0047] where Mw is the molecular weight of draw solutes and wt is the weight concentration. From this prediction, 70 wt % P.sub.4444DMBS is predicted to assume an osmolality of about 5 osmol/Kg as shown in FIG. 2a. While we practically measured that 70 wt % P.sub.4444DMBS can generate a water flux of ˜0.3 LMH from 1.6 M NaCl solution at 14° C., which means that 70 wt % P.sub.4444DMBS should have osmolality higher than 3.2 osmol/Kg at 14° C. Considering that the osmolality was measured and predicted at subzero degree and the osmolality increases with temperature decreases, the prediction should be reliable to undergrid the drawing ability.

[0048] Results and Discussion

[0049] FIG. 1 represents the entire FO desalination processes using an, e.g., 70 wt %, aqueous solution of the thermally responsive ionic liquids as the draw solution. Driven by osmotic pressure difference, desalinated water automatically permeates through the membrane from the feed solution, e.g., seawater, to dilute the draw solution in the FO process. While in the draw solute regeneration process, the diluted draw solution, e.g., 30 wt %, after FO undergoes a liquid-liquid phase separation at a temperature above the LCST of the ionic liquid. The draw solute concentration in the supernatant is much reduced to typically less than 10 wt % and high quality water is obtained by a highly energy efficient low pressure nanofiltration (NF) process (see detailed discussion later concerning energy consumption estimation). Both the retentate in NF and the sedimentation from phase separation are solutions with high ionic liquid (IL) concentrations exceeding 70 wt %, which can be pumped back to be used as draw solution again without the need of any further treatment in the FO process to form a closed loop.

[0050] Draw Solutes Performance in FO Process

[0051] The design of draw solutes of the present invention conforms to the balance in generating high osmotic pressure and ease of regeneration. Unlike traditional ionic liquids which either dissolve (hydrophilic) or repel water (hydrophobic), the thermally responsive ionic liquids of the present invention are amphiphilic. The hydrophobicity from the alkyl groups in cations combining hydrophilicity from suitable anions imparts LCST in aqueous solutions. It is worth noting that the balance between hydrophilicity and hydrophobicity is extremely subtle that tiny molecular structure modification would annihilate LCST. For instance, tetrabutylphosphonium benzenesulfonate (P.sub.4444BS) is highly soluble while tributylhexylphosphonium 2,4-dimethylbenzenesulfonate (P.sub.4446DMBS) is virtually insoluble in water. The unique characteristics of these ionic liquids as draw solutes can also be seen in the inset of FIG. 2a. The osmolality of these ionic liquids draw solutions increases monotonously or monotonically but not linearly with molality, unlike other conventional electrolytes..sup.9, 16 At low concentrations (e.g., <0.1 mol/kg or ˜5 wt %), all the three ionic liquids fully dissociates into relatively ‘free’ cations and anions so that their osmolality is approximately twice as the molality as expected for monovalent electrolyte. As their concentrations increase, the osmolality versus molality plots deviate from the linearity. This is believed to be due to a hydrophobic association of the ionic liquid molecules..sup.27 It also indicates that hydrophobic interaction between P.sub.4448Br molecules is the strongest resulting in the most severe deviation from the ideal linear relationship between osmolality and molality. P.sub.4444DMBS and P.sub.4444TMBS are more hydrophilic and the osmolalities of their 30 wt % (˜0.95 mol/Kg) solutions are 1.34 osmol/Kg and 1.08 osmol/Kg, respectively which are already higher than or approaching that of seawater (1.13 osmol/Kg).

[0052] The osmolality of the draw solutions with higher concentrations cannot be obtained by cryoscopy method. Therefore, the osmolality-molality correlation curves of the three draw solutes were fitted at lower concentrations and extrapolated to higher concentrations (FIG. 2a) with obtained simulated functions (FIG. 6). The calculated osmolality values at higher concentrations presented in FIG. 2a shows that P.sub.4444DMBS and P.sub.4444TMBS can generate very high osmotic pressures. For instance, the osmolality of a 70 wt % solution is 5.4 osmol/Kg for P.sub.4444DMBS and 3.6 osmol/Kg for P.sub.4444TMBS, which is 4.5 and 3 times of seawater's osmolality, respectively. To further investigate this, FO water flux as a function of draw solution concentration was measured with feed solutions of different salinities for P.sub.4444DMBS and P.sub.4444TMBS (FIG. 2b). FO measurement was not carried out using P.sub.4448Br because it has an osmolality less than 1 osmol/kg up to 60 wt % concentration. As expected, P.sub.4444DMBS generates higher water flux than P.sub.4444TMBS with the same feed solution salinity since P.sub.4444DMBS is the most hydrophilic among the three candidates. It is conceivable that both P.sub.4444DMBS and P.sub.4444TMBS can generate reasonable water flux from seawater while the most hydrophobic P.sub.4448Br can only draw water from low salinity feed solution (FIG. 7), which corroborates the prediction in FIG. 2a inset.

[0053] However, it is worth noting that the cryoscopic method measures the osmolality at subzero degree Celsius, thus the actual osmolality of LCST-type draw solution at room temperature would be lower. Although based on osmolality (FIG. 2a) 30 wt % (0.95 mol/Kg) of P.sub.4444DMBS draw solution should start to draw seawater, experimental data (FIG. 2b) indicate that even at 50 wt % of P.sub.4444DMBS, virtually no drawing is achieved against seawater feed when tested at room temperature (24° C.). It is understandable that lowering the test temperature would result in higher drawing power because of the reduced molecular association and hence higher effective osmotic pressure of the draw solution. In fact, it is believed that this temperature effect is true and applicable to any draw solute having a LCST. When measured at 14±1° C., the water fluxes generated by P.sub.4444DMBS draw solutions were higher than those measured at room temperature. In fact, at 14±1° C., P.sub.4444DMBS draw solutions produce reasonable FO water fluxes against RO brine with 1.2 M NaCl. FIG. 2c also shows that, even at 70 wt %, P.sub.4444DMBS is able to draw water from a NaCl feed solution with 1.6 M or 9.3% salinity (approximately 2.7 times that of seawater). Such high drawing ability is unprecedented among previously reported smart or responsive draw solutes. It is attributable to the unique molecular structures of these ionic liquids which give rise to a delicate balance of characteristics of being ionic while possessing LCST through hydrophobic interaction. More importantly the results indicate the promising potentials of further developing this new class of draw solutes for process water treatment and other applications in addition to seawater desalination via FO.

[0054] Draw Solutes Regeneration

[0055] After the FO process, the diluted draw solution is ready for regeneration process where draw solute is separated from water. FIG. 3a demonstrates that even the most hydrophilic P.sub.4444DMBS has a LCST only less than 20° C. above room temperature, making it convenient to utilize solar energy or waste heat to realize the phase separation. All these three draw solutes have a concentration-dependent LCST. Interestingly, the LCST decreases monotonously when draw solution of P.sub.4444DMBS or P.sub.4444TMBS is diluted from 70 wt % to 30 wt %, which helps minimize thermal energy needed for phase separation before regeneration process.

[0056] When each draw solute's concentration was fixed at 30 wt %, as shown in FIG. 3b, all three draw solutes have more complete phase separation as temperature increases above LCST, namely, higher concentration in the sedimentation while lower concentration in the supernatant. When the temperature is raised about 13° C. above the LCST, the concentrations in both sedimentation and supernatant reach a plateau. Further temperature increase would not tremendously decrease draw solute concentration in the supernatant or increase draw solute concentration in the sedimentation. This is because as shown in FIG. 2a, severely diluted or concentrated draw solution does not have LCST or rather phase separation.

[0057] FIG. 3c demonstrates the phase separation of the three draw solutes with various initial concentrations at 13° C. higher than its corresponding LCST. The sedimentation concentration is quite stable, which is about 70 wt %, 75 wt % and 78 wt % respectively for P.sub.4444DMBS, P.sub.4444TMBS and P.sub.4448Br regardless of initial concentration. However, the supernatant concentrations given in FIG. 3b and FIG. 3c may have been somewhat over-estimated due to the phase-separated but un-precipitated droplets of IL-rich phase in the supernatants (FIG. 8). The actual supernatant concentrations should be lower. For instance, FIG. 3c shows the supernatant concentration is ˜13 wt % for a P.sub.4444DMBS solution with initial concentration of 50 wt % at 50° C. (13° C. above its LCST), but this is impossible because, based on FIG. 3a, it is clear that a 13 wt % P.sub.4444 DMBS cannot be a single phase solution and should have phase separated at 50° C. In order to remove such tiny droplets, the supernatant was isothermally filtered with a 220 nm microfiltration membrane, and it was found that the supernatant concentration is below 8 wt % and 5 wt % for P.sub.4444DMBS and P.sub.4444TMBS, respectively at temperature of 50° C. or above (FIG. 3d). The low concentration in the supernatant minimizes the pressure needed in the NF filtration process to produce purified water.

[0058] Energy Consumption Estimation

[0059] The theoretical minimum energy required to separate solute from solvent is a close function of a solution's osmotic pressure and water recovery:

W.sub.min=V.sub.w*Π*ln(1/(1−Y))/Y  (1)

[0060] where

[0061] V.sub.w is molar volume of water,

[0062] Π is the osmotic pressure,

[0063] Y is the water recovery.

[0064] When the water recovery is zero, the energy needed to remove infinitesimally small amount of water from solution is:

W.sub.0=V.sub.w*Π  (2)

[0065] Currently, RO for seawater desalination has an energy consumption.sup.2 that is above 2 kWh/m.sup.3 with imposed hydraulic pressure of over 60 bar. For the thermally responsive draw solutes of the present invention, their osmotic pressures were reduced below 6 bar with the aid of “free” industrial waste heat or solar energy. As shown in FIG. 3, low hydraulic pressure of 10 bar is sufficient to produce reasonable water flux of 31 LMH (P.sub.4444TMBS) and 18 LMH (P.sub.4444DMBS) with nanofiltration membrane (MWCO ˜270 Da). It is worth emphasizing that in seawater RO, the hydraulic pressure needed increases as seawater is concentrated. However, for separating water from the diluted draw solution, as water is recovered and draw solution is concentrated over 10 wt % (P.sub.4444DMBS) and 5 wt % (P.sub.4444 TMBS), their LCST is regained and phase separation can be achieved with “free” thermal energy. Therefore, theoretically, the hydraulic pressure needed in regeneration would never exceed the osmotic pressure of 10 wt % P.sub.4444DMBS and 5 wt % P.sub.4444 TMBS, and almost all the water in the supernatant would be recovered.

[0066] The osmotic pressure of 10 wt % P.sub.4444DMBS was conservatively substituted into equation 2, and the minimum energy requirement with P.sub.4444DMBS as the draw solute in FO seawater desalination is found to be 0.253 kWh/m.sup.3, only 23% of that needed for seawater RO with 50% recovery (1.09 kWh/m.sup.3). This indicates that FO desalination technology with the novel draw solutes of the present invention is promising to reduce energy consumption for current seawater desalination, if abundant “free” thermal energy could be obtained from the sun or industrial waste heat. The draw solute concentration in the permeate of NF was below 900 mg/l and the water quality was still too bad for direct drinking. Thus the water was further treated with NF (MWCO ˜90 Da) and the permeate has a good quality of total organic carbon (TOC) about 20 mg/L. Since the osmotic pressure of the first NF permeate is so low (˜0.09 bar), industrially pumping pressure (2-3 bar) would be sufficient and we regard the energy consumption in second NF negligible.

[0067] Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations in details of design, construction and/or operation may be made without departing from the present invention. For example, besides the three solutes tetrabutylphosphonium 2,4-dimethylbenzenesulfonate (P.sub.4444DMB S), tetrabutylphosphonium mesitylenesulfonate (P.sub.4444TMBS) and tributyloctyl-phosphonium bromide (P.sub.4448Br) described above, it is envisaged that the draw solute of the present invention may comprise other ionic liquids and organic or organic-inorganic hybrid salts made up of organic/inorganic cations and organic/inorganic anions. The organic/inorganic cations in the ionic liquids/salts may be selected from phosphoniums, ammoniums, imidazoliums, pyridiniums, pyrrolidiniums, sulfoniums, and/or metallic ions etc. The organic/inorganic anions in the ionic liquids/salts may be selected from halides, sulfonates, alkyl sulfates, tosylates, methane sulfonates, nitrides, carboxylates, alkoxides, tetrafluoroborate, hexafluorophosphates, and/or bis(trifluoromethylsulfonyl)imides etc. Examples of the cations (X) and anions (Y) mentioned above are shown in FIG. 10, in which R and R.sub.1-R.sub.3 may independently represent H, OH, a halogen, CN (cyanide), or C.sub.1-8 alkyl group (straight or branched). The thermally responsive draw solutes may also comprise zwitterions compounds. Alternatively to liquid-liquid phase separation as described above, the draw solute may be recovered or regenerated by solid-liquid phase separation due to solubility transition (e.g. at an Upper or Lower Critical Solution Temperature—UCST or LCST of the ionic liquids/salts). Examples of the ionic compound in the draw solute being regenerated at a temperature above its UCST include 1-butyl-3-methylimidazonium tricyanomethanide and Na.sub.2HPO.sub.4.

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