Solvent Recovery via Organic Solvent Pressure Assisted Osmosis

Mar 6, 2019 - The organic solvent forward osmosis (OSFO) process suffers from certain drawbacks, such as relatively low solvent flux. One strategy to ...
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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Solvent Recovery via Organic Solvent Pressure Assisted Osmosis Yue Cui and Tai-Shung Chung*

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Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 ABSTRACT: The organic solvent forward osmosis (OSFO) process suffers from certain drawbacks, such as relatively low solvent flux. One strategy to overcome the relatively low solvent flux of OSFO is to utilize an external hydraulic pressure, on top of the osmotic pressure difference across a membrane, to enhance the solvent transport through the membrane. Thus, organic solvent pressure assisted osmosis (OSPAO) has been proposed and demonstrated in this study to effectively recover organic solvents from pharmaceutical products. The OSPAO process was conducted using various solvents including ethanol, IPA, and hexane, with different draw solutes, such as lithium chloride (LiCl) and methyl palmitate. Experimental results show that the solvent flux can be significantly enhanced, while the reverse solute flux (RSF) is slightly reduced with an increase in applied hydraulic pressure from 0 to 1 bar. However, the enhancement in solvent flux by the external hydraulic pressure reduces as the draw solution concentration increases. Hence, it is vital to balance the effects from the draw solution concentration and the applied pressure in OSPAO processes. In addition, OSPAO exhibits impressive rejections of >99% when recovering organic solvents from feed solutions containing different solutes in the range of 1000−10 000 ppm. However, a relatively low hydraulic pressure is recommended to maintain the high rejection when the feed concentration is as high as 20 wt %. In summary, the significantly enhanced solvent flux and the well-maintained high rejection of OSPAO make it an extremely attractive technology for practical solvent recovery in the pharmaceutical industry.

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INTRODUCTION The sequential separation and purification of pharmaceutical products and their intermediates are essential as the syntheses of active pharmaceutical ingredients (APIs) not only involve multiple steps of chemical reactions but also take place in different organic solvents.1 It has been reported that solvent use accounts for approximately 60% of the total energy consumption required to produce an API.2 Thus, it makes good economic as well as environmental sense to recover and reuse the solvents. Solvent recovery is becoming an attractive alternative to the conventional method of incineration. In this context, membrane technology is gaining importance and becoming the forerunner in solvent recovery because of its unique characteristics such as relatively low energy consumption, small footprint, and no phase transformation.3 Since most APIs are temperature-sensitive, athermal separations of pharmaceutical products by means of membrane technology are generally preferred.1d,4 Among various membrane processes, organic solvent nanofiltration (OSN) has been used by the pharmaceutical industry for organic solvent recovery.4a,5 However, the high pressure employed by OSN processes might incur additional operating and maintenance costs. Recently, the feasibility of utilizing organic solvent forward osmosis (OSFO) for the simultaneous concentration of pharmaceutical products and solvent recovery has been demonstrated.6 During this OSFO process, the organic solvent © XXXX American Chemical Society

is transported from the feed side to the draw side due to the osmotic pressure gradient across a semipermeable membrane, while the pharmaceutical product in the feed side is rejected by the membrane and is concentrated. Since the driving force for OSFO is the osmotic pressure gradient across the semipermeable membrane, there is no external hydraulic pressure required. Subsequently, the diluted draw solution can be regenerated by many means such as direct filtration, distillation, and evaporation.6 Given that the draw solutes are neither thermal-sensitive nor reactive, the operating conditions for the regeneration of draw solutions should be less stringent than those required for the direct solvent recovery from pharmaceutical solutions via conventional methods, such as distillation.6,7 The operating temperature range for the solvent recovery process can be greatly wider. Compared to OSN, OSFO may offer advantages such as low fouling tendency, minimal irreversible fouling, and capability to treat highly concentrated feed solutions. OSFO can be employed when the mother liquor is too concentrated for pressure-driven processes (such as OSN) to be utilized.8 However, the OSFO process suffers from certain limitations, such as a relatively low solvent flux.6 Similar to conventional FO processes for water reuse and Received: Revised: Accepted: Published: A

December 10, 2018 March 5, 2019 March 6, 2019 March 6, 2019 DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

NMP and PEG 400 at 70 °C overnight. Subsequently, the wellmixed polymer solution was cooled down to room temperature and degassed through the night. The polymer solution was cast onto a glass plate with a casting knife13a and then immersed in DI water to remove the residual NMP and PEG 400 and form the asymmetric structure. Subsequently, the as-cast flat sheet membrane was cross-linked in a 5% HDA water/IPA (50:50) solution for 24 h. Finally, the cross-linked Matrimid substrate was rinsed thoroughly with pure ethanol and preserved in DI water.13b Interfacial Polymerization of TFC Membranes. The formation of a thin polyamide layer on top of cross-linked Matrimid substrates was achieved via the interfacial polymerization between MPD in an aqueous phase and TMC in an organic phase.14 The cross-linked Matrimid substrate was first immersed in a 2% MPD aqueous solution (containing 0.1% SDS) for 120 s. The excess MPD solution on membrane surface was subsequently removed with filter papers. A 0.1% TMC hexane solution was then deposited on top of the MPDsaturated substrate for 60 s. Finally, the membrane was airdried for 5 min for the completion of the interfacial polymerization. The resultant TFC membrane was thoroughly rinsed with ethanol to remove any residual chemicals and then preserved in the organic solvent (i.e., ethanol, IPA, or hexane) in which it would be tested subsequently. Figure 1 illustrates the membrane fabrication process.

seawater desalination, the relatively low solvent flux of OSFO may be enhanced by applying additional hydraulic pressure across the membrane on top of the osmotic pressure.9 Thus, the organic solvent pressure assisted osmosis (OSPAO) is, for the first time, proposed and demonstrated. Unlike the OSFO process, this process exerts an additional hydraulic pressure on the feed side as an auxiliary driving force and brings about a synergy of solution-diffusion and pore-flow mechanisms. The driving force for OSPAO consists of both the osmotic pressure gradient and the external hydraulic pressure. Similar to the OSFO process, the organic solvent is being transported from the feed side to the draw side, while the pharmaceutical product in the feed side is concentrated in the OSPAO process. Subsequently, the diluted draw solution can be regenerated, and the solvent can be recovered. However, it should be noted that only a minor hydraulic pressure is preferred in the OSPAO process, so it will not significantly increase the operating cost or the fouling tendency.9a,10 To demonstrate OSPAO for organic solvent recovery, a thin film composite (TFC) membrane was adopted as the separation barrier in this study. To guarantee its chemical stability in various organic solvents, a polyamide selective layer was formed on top of a cross-linked substrate via interfacial polymerization.11 This type of composite membranes has shown impressive stability in organic solvents.6,12 Solvent recovery via OSPAO was conducted using different draw and/ or feed solutions in various solvent systems. Special attention was given to compare OSPAO and OSFO (i.e., no external pressure applied) in terms of solvent flux, reverse solute flux (RSF), and solute rejection. In addition, transport mechanisms were analyzed and discussed. The encouraging results from this study indicate that the OSPAO process is a promising alternative to OSFO. It has a great potential for organic solvent recovery in the pharmaceutical industry.

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EXPERIMENTAL SECTION Materials. The polyimide Matrimid 5218 (Vantico Inc.), polyethylene glycol 400 (PEG 400, Mw = 400 g·mol−1, Merck), and N-methyl-2-pyrrolidinone (NMP, >99.5%, Merck) were utilized as polymer, nonsolvent, and solvent, respectively, for the fabrication of the membrane substrate. 1, 6-Hexanediamine (HDA, >98%, Alfa-Aesar) was employed to cross-link the substrate. m-Phenylenediamine (MPD, >99%), sodium dodecyl sulfate (SDS, >99%), and trimesoyl chloride (TMC, >98%) were obtained from Sigma-Aldrich and used in the interfacial polymerization reaction. Methyl palmitate (>97.0%, Tokyo Chemical Industry) and lithium chloride (LiCl, >99%, Sigma-Aldrich) were acquired as the draw solutes. The solvents, namely, ethanol (HPLC-grade), n-hexane (HPLCgrade), and isopropanol (IPA, HPLC-grade), were ordered from Fisher Scientific. Industrial triglycerides from soybean oil (liquid, GIIAVA Singapore) and tetracycline (≥98.0%, SigmaAldrich) were employed as the model feed solutes. The deionized (DI) water used in this study was generated by a Milli-Q ultrapure water system (Millipore, USA). All chemicals were used as received. Fabrication of the Cross-linked Matrimid Substrate. The fabrication of the Matrimid substrate had been described in our previous publications.13 Briefly, the polymer dope used in this study consisted of Matrimid, PEG 400 and NMP in the weight ratios of 20, 16, and 64, respectively. The Matrimid 5218 polymer was first dried overnight at 80 °C in a vacuum oven to remove moisture before being dissolved in a mixture of

Figure 1. Schematic diagram of the membrane fabrication and OSPAO process for solvent recovery.

Solvent Reclamation through OSPAO. As demonstrated in Figure 1, the organic solvent was reclaimed via OSPAO using a lab-scale OSPAO unit similar to a typical forward osmosis (FO) unit for water reuse except a valve was installed at the outlet of the feed side to throttle the flow and adjust the hydraulic pressure across the membrane. In addition, the entire setup consisted of solvent resistance components. Both the draw and feed solutions were circulated counter-currently through the OSPAO cell at a constant volumetric flow rate of 0.2 L·min−1 with the use of pumps. In this study, FO mode (i.e., the selective layer faces the feed solution) was adopted for all the experiments. A hydraulic pressure ranging from 0 to 1 bar was applied on the feed side by tuning the pump and the valve. The whole system was allowed to stabilize for 0.5 h B

DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. (a) Pure solvent flux and (b) reverse solute flux as a function of draw solution concentration in OSPAO (feed solution: pure ethanol; draw solution: LiCl in ethanol).

were employed as the feed and draw solutions, respectively. However, for the hexane system, 20% triglycerides and 50% methyl palmitate were adopted as feed and draw solutions, respectively. The solute rejection, R (%), was employed to quantify the membrane rejection to the feed solute (i.e., tetracycline or triglycerides) under the different operating conditions. It could be considered as the percentage of feed solute that was retained by the membrane, and was calculated according to eq 3:

before any measurements were taken. Subsequently, the reverse solute flux (Js, g·m−2·h−1, gMH) and solvent flux (Jw, L·m−2·h−1, LMH) under different hydraulic pressures were determined. The solvent flux was determined based on eq 1: Jw =

Δm ρΔt A m

(1)

where Δm (g) is the average of the absolute weight loss on the feed side and the absolute weight gain on the draw side, Am is the effective membrane area of 4 cm2, ρ (g·cm−3) is the solvent density, and Δt (h) is the test duration of 2 h. In contrast, the reverse solute flux (RSF, Js) of the draw solution was determined from the concentration increase of the draw solute in the feed solution. The RSF was calculated using eq 2:

Js =

ΔCt V Δt A m

R=1−

Cd × Vd /Vp Cf

(3)

where Cf is the feed solute (i.e., tetracycline or triglycerides) concentration in the feed solution, Cd is the triglycerides or tetracycline concentration in the draw solution at the end of each OSPAO test, Vd is the final volume of the draw solution, and Vp is the volume of the permeate. However, it should be noted that Cd of triglycerides was determined using highperformance liquid chromatography (HPLC, Agilent, USA) coupled with a multiple wavelength detector (MWD). Detection was done at the wavelength of 208 nm. In contrast, the measurement of Cd of tetracycline was completed using a UV−vis spectrophotometer at the wavelength of 366 nm where tetracycline has the strongest absorbance. In addition, measures such as reducing the feed volume or extending the test duration had been taken as well to ensure the accuracy of the data. Determination of Transport Properties in the Membrane. For the OSPAO process, it was assumed that the solvent flux was determined by both the hydraulic pressuredriven flux and the osmotic-driven flux. The driving forces of these two fluxes were the hydraulic pressure difference and the osmotic pressure difference, respectively.9b,15 In addition, these two driving forces were also assumed to be independent of each other. Thus, two driving forces were measured and calculated for a more explicit understanding of an OSPAO process. First, the hydraulic pressure-driven solvent flux Jp (L·m−2· −1 h , LMH) of the TFC membranes were defined as Jw when the draw solution concentration was 0. It was determined by utilizing the above-described OSPAO setup. During the measurements, both the feed and draw solutions were replaced by pure solvents, but other testing conditions remained the same as in the previous sections. Subsequently, the osmoticdriven flux Jπ (LMH) of the TFC membranes in the OSPAO

(2) −1

where V (L) and ΔCt (g·L ) are the changes of the feed solution volume and the solute concentration, respectively. In OSPAO tests, LiCl and methyl palmitate were used as the draw solutes. Since they have different solubility and characteristics in different solvents, their concentrations in draw solutions were not the same. Different analytical methods were also used to determine their concentrations in feed solutions due to the reverse fluxes of draw solutes. For example, the LiCl concentration was determined using a conductivity meter (Metrohm, Switzerland) while the methyl palmitate concentration was detected by a UV−vis spectrophotometer (Libra S32, Biochrom, Ltd., England) at the wavelength of 232 nm. A wavelength of 232 nm was used as methyl palmitate has the strongest absorbance there. It should be noted that prior to the concentration determination calibration curves had been attained for all solutes in their respective solvents. This helped precisely compute the solute concentration in each feed solution. To assess the feasibility of using OSPAO to concentrate pharmaceuticals while recovering the organic solvents, triglycerides, and tetracycline dissolved in hexane and alcohols, respectively, were selected as the model feed solutions. In the ethanol system, tetracycline dissolved in ethanol was adopted as the feed solution, while 2 M LiCl was employed as the draw solution. In order to study the effect of feed concentration on membrane performance, the tetracycline concentration was varied from 1000, 2000, 5000 to 10 000 ppm. In contrast, for the IPA system, 2000 ppm tetracycline in IPA and 2 M LiCl C

DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. (a) Solvent flux and (b) tetracycline rejection as a function of tetracycline concentration in OSPAO (draw solution: 2 M LiCl in ethanol).

process was calculated as the difference between Jw (OSPAO solvent flux) and Jp (hydraulic pressure-driven solvent flux): Jπ = Jw − Jp

solution side. On the basis of our observation, it is highly likely that both the convection and solution diffusion coexist as the transport mechanisms for OSPAO and OSFO processes. The solution diffusion mechanism dominates if no hydraulic pressure is applied (i.e., OSFO process), but the convective flow becomes notable when a hydraulic pressure is applied to the feed side (i.e., OSPAO process). As a result, the enhanced convective ethanol transport suppresses the draw solute transport as they travel in opposite directions, and RSF decreases as the hydraulic pressure increases. Since RSF not only causes the loss of osmotic pressure gradient but also has the potential to contaminate the feed solution (i.e., API solution),17 the reduction of RSF via OSPAO certainly minimize the drawback of OSFO and enhances the safety of the feed stream. In summary, the solvent flux significantly increased without compromising the reverse solute flux when an external pressure is applied. Therefore, compared to the OSFO process (i.e., p = 0), the OSPAO process has a larger throughput with a lower risk brought by RSF. Figure 2 also shows the effects of both LiCl concentration and hydraulic pressure on membrane performance in terms of solvent flux (Jw) and RSF (Js). For example, when no external pressure is applied, the ethanol flux rises from 1.15 LMH to 2.25 LMH with an increase in LiCl concentration from 1 to 4 M because of a higher osmotic pressure difference across the membrane. However, the ethanol flux does not increase proportionally to the draw solution concentration as the internal concentration polarization (ICP) becomes intensified when using a more concentrated draw solution. In addition, RSF also jumps with increasing the LiCl concentration. A similar trend can be observed when a hydraulic pressure of 0.5 or 1 bar is applied. Consequently, a more concentrated draw solution is generally preferred to enhance the efficiency of solvent recovery for the OSPAO process under the same applied pressure. Since the ethanol flux augments with an increase in hydraulic pressure regardless of the draw solution concentration, a higher applied hydraulic pressure is generally preferred to enhance the throughput of OSPAO and to reduce the RSF under the same draw solution concentration. However, it is not possible to blindly increase both the draw solution concentration and applied pressure to increase the throughput of OSPAO. As revealed in Figure 2, the response of solvent flux to the applied pressure slows down when the LiCl concentration increases from 1 to 4 M. This is because the hydraulic pressure difference (Δp) plays a much less role on flux when the osmotic pressure difference (Δπ) becomes larger. To have the same extent of flux increase in OSPAO, one

(4)

A comparison between Jπ and Jπ,p=0 can be utilized to evaluate the effect of hydraulic pressure on osmosis driving force. There are three possible situations: (i) Jπ > Jπ,p=0 suggesting that concentration polarization (CP) is reduced by the hydraulic pressure. (ii) If Jπ ≈ Jπ,p=0, then CP is barely influenced by the hydraulic pressure. (iii) If Jπ< Jπ,p=0, then CP is increased by the hydraulic pressure.9b

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RESULTS AND DISCUSSION The membrane fabrication process and the use of OSPAO processes for the recovery of organic solvents from pharmaceutical solutions are illustrated in Figure 1. Solvent Recovery in Ethanol. Solvent recovery via the OSPAO process is first evaluated in the ethanol system. Pure ethanol and LiCl in ethanol are first adopted as the feed and draw solutions, respectively. Although LiCl is utilized in this study due to its ease of detection, it should be noted that nonhazardous draw solutes such as pharmaceutical excipients should be adopted to avoid any potential risk.6 As illustrated in Figure 2, when 2 M LiCl in ethanol is utilized as the draw solution and there is no external applied pressure, an ethanol flux of 1.75 LMH and a reverse solute flux of 0.54 gMH can be achieved. When a low external pressure is gradually applied on the feed side, the ethanol flux starts to increase. The ethanol flux jumps from 1.75 to 3.24 LMH as the applied pressure increases from 0 to 0.5 bar. A further raise in applied pressure to 1 bar significantly increases the flux to 5.19 LMH. The enhancement of solvent flux can be attributed to the applied hydraulic pressure because it acts as an additional driving force for the ethanol transport across the membrane. However, RSF slightly decreases when an external pressure is applied. RSF drops from 0.54 to 0.4 gMH as the hydraulic pressure rises from 0 to 1 bar. Given that the driving force for the draw solute transport is mainly the concentration gradient of the draw solution across the membrane and that this concentration gradient remains relatively constant throughout the entire OSPAO process, the RSF should be theoretically the same with or without applied hydraulic pressure.15,16 Therefore, the decrease in RSF is probably caused by the membrane compaction under the hydraulic pressure, which results in a higher transport resistance for the draw solute.9a,10b,16 The other possible factor is the enhanced convective flow of ethanol under the hydraulic pressure from the feed to the draw D

DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. OSPAO process in IPA: (a) pure IPA and (b) 2000 ppm tetracycline as feed solutions (draw solution: 2 M LiCl in IPA).

concentration increases from 1000 to 10 000 ppm when Δp = 0. However, the decrease in solvent flux can be easily compensated by the applied pressure. At the feed concentration of 10 000 ppm, the solvent flux increases drastically from 1.58 to 4.20 LMH when Δp increases from 0 to 1 bar. More importantly, the rejection of tetracycline remains constantly higher than 99% regardless of the feed concentration and applied pressures. This suggests that the convective flow does not significantly affect the tetracycline transport even when the feed concentration is up to 10 000 ppm. This impressively high rejection indicates that the OSPAO process is capable of concentrating a pharmaceutical solution up to 10 000 ppm without sacrificing the rejection. Solvent Recovery via OSPAO from IPA and Hexane. As various organic solvents are utilized in the pharmaceutical industry, OSPAO has to be examined in different solvent systems. Thus, OSPAO performance in IPA and hexane is evaluated in this section. Similar to the ethanol system, pure IPA is first utilized as the feed solution using 2 M LiCl in IPA as the draw solution. Figure 4a shows the IPA flux and RSF of LiCl under different hydraulic pressures. An IPA flux of 0.56 LMH and a RSF of 0.24 gMH are obtained when Δp = 0. The IPA flux progressively increases, while the RSF value slightly decreases with an increase in Δp. The IPA flux jumps to 1.47 LMH at Δp = 0.5 bar, it further increases to 2.58 LMH when Δp = 1 bar. In contrast, the RSF value slightly declines from 0.24 to 0.15 gMH when Δp rises from 0 to 1 bar. Subsequently, a new feed containing 2000 ppm tetracycline is adopted to evaluate OSPAO for the simultaneous recovery of solvents and concentration of pharmaceutical products. Similar to the previous tetracycline/ethanol system, Figure 4b displays a relatively lower IPA flux when pure IPA is replaced by the new feed. However, there is still a sizable percentage increase of 300% in IPA flux when Δp increases from 0 to 1 bar. Importantly, there is not any compromise in rejection. The rejection to tetracycline remains higher than 99% under Δp = 1 bar because of the aforementioned reasons such as compacted membrane structure, big solute solvated size, large solubility parameter difference between tetracycline and the membrane, and the counterbalance effects between the convective flow and membrane compaction. Since the IPA flux obtained under OSFO (i.e., p = 0) is relatively low, OSPAO is especially desirable for pharmaceutical systems comprising a solvent like IPA because the solvent flux can be remarkably enhanced without compromising the high rejection. As for the hexane system, 50% methyl palmitate in hexane is adopted as the draw solution. Pure hexane is first utilized as the

must employ a much higher hydraulic pressure if the draw solution concentration is comparatively higher. This, in turn, may result in higher operating and maintenance costs as well as higher fouling tendency. Hence, OSPAO might function better when the draw solution concentration is relatively low and a search for techno-economic balance between the solvent recovery rate and operating/maintenance costs must be conducted for its success in real industrial operations. To demonstrate the capability of OSPAO to recover organic solvents from pharmaceutical products, a model API solution containing 2000 ppm tetracycline (Mw = 444.4 g·mol−1) in ethanol and a 2 M LiCl ethanol solution were employed as the feed and draw solutions, respectively. Figure 3 displays the ethanol flux and membrane rejection to tetracycline under different hydraulic pressures. When Δp = 0, the ethanol flux decreases slightly from 1.75 to 1.64 LMH once the feed is replaced by the model API solution containing 2000 ppm tetracycline due to the loss of osmotic pressure gradient across the membrane. The ethanol flux jumps from 1.64 to 4.40 LMH when the applied pressure increases from 0 to 1 bar. Interestingly, the rejections of tetracycline shown in Figure 3b are always higher than 99% even when Δp = 1 bar. The high rejection of tetracycline when Δp = 0 may arise from several factors: First, the membrane has considerable resistance for tetracycline transport because of its highly crosslinked structure.6 Second, tetracycline may have a reasonably low diffusivity and solubility in the membrane because it has a large solvated size in ethanol and a large solubility parameter difference from the membrane (i.e., 28.7 vs 23 MPa1/2).13a,18 In addition, the RSF may also decelerate the transport of tetracycline into the draw solution. The surprisingly maintained high rejection to tetracycline when Δp = 1 bar is probably due to the counterbalance effects between the convective flow and membrane compaction. In other words, although the applied hydraulic pressure induces a convective flow and facilitates the tetracycline transport, it also results in membrane compaction and retards the solute transport. The two factors may offset each other and bring about a minimal variation on rejection. Therefore, applying a minor pressure on the feed solution would not sacrifice the membrane rejection but significantly improve the solvent flux. This unique characteristic makes OSPAO a promising alternative to OSFO in practical pharmaceutical separations. Figure 3 also shows the ethanol flux and tetracycline rejection at lower and higher feed concentrations. As displayed in Figure 3a, the ethanol flux gradually decreases when the feed concentration increases. For instance, the ethanol flux decreases from 1.74 to 1.58 LMH when the tetracycline E

DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. OSPAO process in hexane: (a) pure hexane and (b) 20% triglycerides as feed solutions (draw solution: 50% methyl palmitate).

ethanol, IPA, and hexane) are employed as feed solutions, while 2 M LiCl in ethanol, 2 M LiCl in IPA, and 50% methyl palmitate in hexane are adopted as draw solutions. Generally, the enhancement trends of solvent flux in different solvents are relatively similar but the enhancement rates follow the order of IPA> hexane > ethanol. More importantly, the percentages of flux enhancement can reach up to around 360%. This surprisingly high flux enhancement is interesting despite the fact that the applied pressure is only up to 1 bar. This applied pressure is much lower than the osmotic pressure gradient across the membrane,6 and theoretically, it should only have a slight effect on solvent flux. Therefore, the applied pressure may play a leveling role in determining the solvent flux in FO processes. Figure 7 shows the hydraulic-driven flux, Jp, and the osmoticdriven flux, Jπ, calculated from eqs 1 and 4, respectively. As revealed, the Jp increases to 1.17, 0.66, and 2.71 LMH for ethanol, IPA, and hexane, respectively, when Δp = 1 bar. Generally, Jp increases proportionally with Δp, but its slopes are different for these three solvents because many physicochemical parameters involved during the solvent transport such as solvated size, viscosity and affinity to the membrane are different. In contrast, the osmotic-driven flux, Jπ, shows an unexpected trend. For all three solvent systems, Jπ apparently increases with an increase in Δp, except that the Jπ of hexane only increases slightly. One possible explanation for the increased Jπ is due to the membrane compaction because a thinner dense layer might provide a shorter path for solvent transport.9a,b,10c,19 Another plausible explanation arises from the subdued CP.9 The overall CP, which consists of both ICP and external concentration polarization (ECP), is a factor that can significantly affect FO performance.6,17a,b,20 The higher the CP, the lower the osmotic-driven flux, Jπ. In any FO/OSFO process under the FO mode, where a pure water/solvent is adopted as the feed solution, a dilutive ICP and a concentrative ECP can be observed inside the porous support and at the outer surface directly contacting with the feed solution, respectively.20a,b,21 In the PAO/OSPAO operations, the ECP will be further reduced due to the reduced solute diffusion toward the feed side, while the ICP becomes intensified due to greater solvent permeation toward the draw side.9a−c,16 If the feed contains mainly water, then ICP is the predominant factor affecting the flux in FO while the ECP tends to be negligible. This is because water has a relatively low viscosity and a smaller molecule weight, which facilitate the diffusion of salts into the feed solution (i.e., water). However, the CP effect

feed solution. Figure 5a illustrates its OSPAO performance as a function of Δp. A hexane flux of 0.82 LMH is acquired when Δp = 0. It gradually reaches 3.13 LMH when Δp = 1 bar, equivalent to a percentage increase of flux at about 280%. In contrast, the RSF value of methyl palmitate is negligible under different hydraulic pressures since its concentration is constantly below the detectable limit of the UV−vis spectrometer. Since the pharmaceutical concentration can be up to 20 wt % in practical industrial applications, a solution containing 20 wt % triglycerides in hexane is subsequently adopted as the new feed. Figure 5b shows its OSPAO performance, inclusive of hexane flux and membrane rejection of triglycerides, under different hydraulic pressures. Compared to the pure hexane system in Figure 5a, the hexane fluxes obtained from the new feed are significantly lower. However, the use of Δp = 1 bar can bring back the hexane flux from 0.65 to 2.51 LMH but the rejection of triglycerides decreases as the pressure increases. When Δp = 0, the rejection is higher than 99%. It remains almost unchanged when Δp = 0.5 bar but drops sharply to 96.6% when Δp = 1 bar. As the loss of pharmaceutical products must be minimized, an API rejection of higher than 99% should be maintained. The noticeable reduction of rejection under Δp = 1 bar indicates that a slightly low Δp (e.g., 0.5 bar) is preferred to maintain a high rejection while achieving a reasonable enhancement in solvent flux when treating a concentrated feed solution. Understanding the Solvent Flux. The enhancements of solvent flux as a function of Δp in different solvent systems are summarized in Figure 6. In this section, pure solvents (i.e.,

Figure 6. Enhancements of solvent flux in different solvent systems (draw solution: 2 M LiCl in ethanol; 2 M LiCl in IPA; 50% methyl palmitate in hexane; feed solution: the respective solvents). F

DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. Hydraulic-driven flux and osmotic-driven flux of OSPAO as a function of applied pressure in (a) ethanol, (b) IPA, and (c) hexane. Experimental conditions: (1) Jp: pure solvent in both draw and feed sides; (2) Jw: feed solution: pure solvent; draw solution: (a) 2 M LiCl in ethanol, (b) 2 M LiCl in IPA, and (c) 50% methyl palmitate in hexane; (3) Jπ = Jw − Jp.

becomes more severe and complicated in ethanol and IPA systems than those in water systems. This is because ethanol and IPA have much higher viscosities (1.10 and 2.04 mPa·s, respectively) and larger sizes (4.5 and 4.7 Å, respectively) than those of water (0.89 mPa·s and 2.75 Å, respectively).22 Thus, the ECP effects in OSFO/OSPAO cannot be neglected. They may play a much more significant role in determining the osmotic-driven flux in OSFO/OSPAO for organic solvent recovery than in FO/PAO for water reuse and desalination. When a hydraulic pressure is applied on the feed solution, the mitigation of ECP might be more pronounced than the intensified ICP. As a result, the overall CP effect is subdued and lead to the enhancement in Jπ. This hypothesis can be verified by the Jπ values of hexane under different hydraulic pressures. Among these three solvents, the hexane system has the smallest increase in Jπ. It only increases slightly when Δp increases to 1 bar. Its overall CP effect is less severe than that in the other two solvent systems because hexane has a relatively lower viscosity (0.33 mPa·s) and a smaller solvent size (4.3 Å) than those of the other two solvents. In other words, the CP effect plays a minor role in the hexane system and mainly the compaction of the membrane contributes to the increase of Jπ.

when the feed concentration is as high as 10 000 ppm. Tetracycline rejections greater than 99% were also obtained for all hydraulic pressures in the IPA system when using 2000 ppm tetracycline and 2 M LiCl as feed and draw solutions, respectively. In the hexane system, a concentrated industrial sample of 20% triglycerides was adopted as the feed and a 50/ 50 methyl palmitate/hexane was employed as the draw solution. The triglycerides rejection of above 99% was first obtained when the hydraulic pressure increased to 0.5 bar, but it dropped to 96.6% when 1 bar was applied. Hence, it is suggested to apply a relatively low hydraulic pressure when a highly concentrated feed solution is adopted. Subsequently, the solvent transport mechanism was also examined. It was found that the CP effects, which could be quite severe in ethanol and IPA systems, might be mitigated by the enhanced flux in the OSPAO process. Consequently, both the osmotic-driven and hydraulic-driven fluxes increase with an increase in the applied hydraulic pressure. In summary, the enhanced solvent flux and the wellmaintained high rejection to feed solutes make the OSPAO process a very attractive technology for solvent recovery in the pharmaceutical industry. Future works will focus on (i) the fouling behavior of the OSPAO process, (ii) system integration with other processes, such as direct filtration processes, and (iii) mathematical model of the transport mechanism.

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CONCLUSION In this work, we have demonstrated the utilization of OSPAO processes for the simultaneous recovery of organic solvents and concentration of pharmaceutical products in various solvent systems, which includes ethanol, IPA, and hexane. The OSPAO process is similar to the OSFO process, except that a minor pressure is applied on the feed side to facilitate the solvent transport. Thus, this process takes advantages of both chemical potential gradient and hydraulic pressure difference to enhance the solvent transport across the membrane. In this demonstration, solvent fluxes of all systems were observed to increase, while their RSF values slightly decreased as the applied hydraulic pressure increases. In addition, the solvent fluxes were found to increase as the draw solution concentration increases. However, the enhancement of solvent flux due to the applied pressure decreased significantly when a more concentrated draw solution was utilized. Thus, it is vital to balance the solvent flux with the hydraulic pressure, as the hydraulic pressure is closely related to the operating/ maintenance costs. In terms of rejection, tetracycline rejections higher than 99% in the ethanol system were acquired for all hydraulic pressures when utilizing 2000 ppm tetracycline and 2 M LiCl as feeds. Moreover, the rejections were not sacrificed

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +65 65166645. Fax: +65 67791936. ORCID

Tai-Shung Chung: 0000-0002-6156-0170 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This research is supported by National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Program for the project entitled, “Development of solvent resistant nanofiltration membranes for sustainable pharmaceutical and petrochemical manufacture” (CRP Award No. NRF-CRP14-2014-01 (NUS grant number: R-279-000466-281)). Thanks are due to GIIAVA Singapore for their assistance on this work. G

DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.8b06115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX