Phase Separation of Alcohol (1-Propanol, 2 ... - ACS Publications

Article pubs.acs.org/jced

Phase Separation of Alcohol (1-Propanol, 2‑Propanol, or tert-Butanol) from Its Aqueous Solution in the Presence of Biological Buffer MOPS Saidah Altway,†,‡ Mohamed Taha,†,§ and Ming-Jer Lee*,† †

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan ‡ Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS, Keputih, Sukolilo, Surabaya 60111, Indonesia § Occupational Health and Safety (OHS) Office, Directorate of Manpower of Beni-Suef, Beni-Suef, Egypt S Supporting Information *

ABSTRACT: Buffering-out is a new finding phase-separation phenomenon. It can minimize the drawback of the conventional salting-out method, which will cause corrosion of the equipment at high concentrations of ionic salts. To develop this new separation process, phase equilibrium data are essentially needed. In the present study, the density measurements method was used to determine the phase boundaries of solid−liquid equilibrium (SLE), and the cloud-point method was used to determine the phase boundaries of liquid−liquid equilibrium (LLE) and solid−liquid−liquid equilibrium (SLLE). The phase compositions of coexistence phases for alcohol (1-propanol, 2-propanol, or tert-butanol)−water + 3-(Nmorpholino)propanesulfonic acid (MOPS), including at LLE and at SLLE, were then measured at 298.15 K with an analytical method. The experimental tie-line data were also accurately correlated by the nonrandom two-liquid (NRTL) model. A conceptual process flowsheet was also proposed for the recovery of 1-propanol from its aqueous solution with the aid of MOPS.

1. INTRODUCTION Alcohols such as 1-propanol, 2-propanol, and tert-butanol (2methyl-2-propanol) are commonly used as solvents in industry. Each of these organic solvents forms an azeotrope with water,1 as shown in Table S1. A conventional distillation method cannot be applied directly to recover high-purity organic solvents from their aqueous solutions. Among several others, inorganic salts are considered to be auxiliary agents in improving the efficiency of separation processes for such azeotropic aqueous systems.2−9 Among those previous studies, Jurkiewicz4 found that propanol, butanol, pentanol, acetone, methylethyl ketone, or diethyl ketone can be squeezed out of the aqueous solutions to form an extra liquid phase by adding sodium chloride to the aqueous solutions. The induction of liquid−liquid phase splitting by the inorganic salt may be helpful in overcoming the separation barrier for the azeotropic systems; however, the corrosive nature of salt may be one of inherent disadvantage for practical applications. Several studies10,11 showed that various ionic liquids may be applicable to the separation of alcohols from their aqueous solutions. Although ionic liquids are considered to be green media, currently the cost of ionic liquids is still relative higher than that of conventional entraniers or extractants. Taha and Lee12−14 and Taha et al.15,16 reported that liquid−liquid phase splitting can also be induced by introducing biological Good’s buffers17 into various organic aqueous solutions. Because of the © 2017 American Chemical Society

noncorrosive and biocompatible nature of the buffers, these compounds have a high potential to replace the inorganic salts as a green auxiliary agent to recover high pure organic solvents from their aqueous solutions. In the present study, a Good’s buffer of 3-(N-morpholino)propanesulfonic acid (MOPS) is selected for separating 1propanol, 2-propanol, and tert-butanol from their aqueous solutions by taking advantage of the buffering-out effect. MOPS is a zwitterionic amino acid that is capable of possessing both positive and negative charges on the same molecule, but the net charge is zero. The molecular structure of MOPS is illustrated in Scheme 1. Because phase equilibrium data are essentially needed for the development of separation processes, liquid− liquid equilibrium (LLE) and solid−liquid−liquid equilibrium (SLLE) data for the ternary aqueous systems containing one of the alcohols (1-propanol, 2-propanol, and tert-butanol) and MOPS were measured at 298.15 K under atmospheric pressure. The separation factors were calculated from the experimental results and applied to evaluate the buffering-out strength of MOPS for these investigated aqueous systems. A phase diagram Special Issue: Memorial Issue in Honor of Ken Marsh Received: November 16, 2016 Accepted: March 21, 2017 Published: March 29, 2017 2509

DOI: 10.1021/acs.jced.6b00954 J. Chem. Eng. Data 2017, 62, 2509−2515

Journal of Chemical & Engineering Data

Article

the experiments was prepared by a NANO pure-Ultra pure water system, and was distilled and deionized with a resistance of 18.3 MΩ·cm. The purity levels of materials, except for biological buffer MOPS, were confirmed by gas chromatography (GC). Because there are no significant impurities, prepurification of these materials is not needed. Descriptions of all substances are listed in Table 1. 2.2. Experimental Apparatus and Procedures. 2.2.1. Solubility Measurements. The saturated solubility values of MOPS in the liquid phase, including in water and in mixed solvent (water + alcohol), were determined by the results of density measurement at 298.15 K under atmospheric pressure. The experimental procedure has been provided in our previous studies.21−23 The densities were measured with a DMA-4500 vibrating-tube densimeter (Anton Paar, Austria) with a standard uncertainty u(ρ) of 5.10−5 g·cm−3. The densimeter was calibrated with air and degassed pure water. The density of the solution was plotted versus the concentration of MOPS. The densities of unsaturated solutions were fitted to a polynomial, while those of saturated solutions were fitted to a straight line. The solubility value of MOPS was determined at the point of intersection of the two fitted lines, as shown in Figure S1. The standard uncertainty u(wi) of the determined solubility values (wi) was estimated to be 0.005. 2.2.2. Phase Boundary Data. The phase boundaries, including LLE and SLLE phase boundaries, were determined experimentally with the cloud-point method, which has been described in our previous studies.12,13 Known compositions of alcohol and water mixtures were prepared first in a series of glass vials. A trace amount of MOPS was then added to each vial and increased until the mixture became turbid and formed two liquid phases. For the SLLE phase boundary measurement, more MOPS was loaded into each sample vial and increased until solid appeared in the liquid phases. After that, the vials were sealed with a Teflon-coated screw cap and placed in a thermostatic shaker equipped with a water bath (BT-350R, YihDer, Taiwan) at 298.15 K for at least 48 h, and then they were allowed to settle for 8 h. The cloud point can be detected with the naked eye. This procedure was repeated three times for each mixture. The average of MOPS added was recorded for calculating the LLE and SLLE phase boundary composition (binodal curve). The standard uncertainty u(wi) of the LLE and SLLE phase boundaries is estimated to be 0.005 in mass fraction. 2.2.3. Tie Lines. The procedure of LLE tie-line measurement has been reported in a previous study by Lin et al.,24 and the experimental apparatus used in this study was based on Peschke and Sandler.25 The tie lines for LLE and SLLE were measured by using a jacketed equilibrium cell with a volume of 45 cm3 at 298.15 K under atmospheric pressure. Thermostatic water from a water bath (refrigerated thermostat) was circulated to keep the temperature of the equilibrium cell constant with a standard uncertainty u(T) of 0.05 K. In general, seven different feed

Scheme 1. Molecular Structure of MOPS

was also constructed on the basis of the experimental results for each ternary system, and a feasible separation process for the recovery of high-purity 1-propanol from its aqueous solution was suggested. For development of separation process, LLE data are needed to correlate with thermodynamics models. Although predictive models, such as COSMO-RS model, may qualitatively estimate the phase compositions of ternary aqueous systems containing inorganic salts,9 the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT)9 and the PC-SAFT10 models were applied to correlate the LLE of systems containing inorganic salts and ionic liquids, respectively. Among several other activity coefficient models, the NRTL model18 is widely employed to describe the nonideal behavior of each component in aqueous solutions. To the best of our knowledge, the deviations are often obviously large for calculating the phase compositions of two coexistent liquid phases for a type-I LLE ternary system by using the parameters determined from the binary VLE data of the two miscible constituent binaries.19 Therefore, the parameters determined from binary VLE data of water + alcohols may be applicable to simulate the performance of a distillation column, but those values may be not accurate enough to simulate the performance of a decanter or an extractor. Fortunately, modern process simulators, such as Aspen Plus, allow users to use different sets of model parameters for different process units. Practically, users often specify different values of the model parameters to simulate a distillation unit and a decanter (or an extractor).20 To retain the accuracy of the simulation for an extractor, we determined the optimal values of all the binary parameters by fitting the model to the ternary LLE data simultaneously.11 Therefore, the parameters reported in this study are for extractor use only. To simulate the operations of distillation columns and flash drums, we redetermined the model parameters from the ternary VLE data (not including in this article). These two different sets of model parameters have been adopted by the process system engineering group of our research team to conduct the simulation for this new separation process.

2. EXPERIMENTAL SECTION 2.1. Materials. Biological buffer MOPS and 1-propanol were purchased from Sigma-Aldrich (USA) with purity levels better than 0.995 and 0.997 in mass fraction, respectively. 2Propanol and tert-butanol were purchased from Acros (USA) with a purity of 0.995 in mass fraction. Water used throughout Table 1. Descriptions of Substances Studied substances

source

CAS number

purity (mass fraction)

purification method

1-propanol 2-propanol tert-butanol MOPS water

Sigma-Aldrich (USA) Acros (USA) Acros (USA) Sigma-Aldrich (USA) prepared in our lab

71-23-8 67-63-0 75-65-0 1132-61-2 7732-18-5

0.997 >0.995 0.995 >0.995 deionized ultrapure water

no no no no no

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compositions of alcohol−water−MOPS were studied for each system. One of them is for the SLLE measurement. Known quantities of each component were weighed by an analytical balance (R&D model GR-200, Japan) and added directly to the equilibrium cell. The mixtures of alcohol−water−MOPS were stirred with a magnetic stirrer for 24 h (for LLE) and 48 h (for SLLE). Then, the ternary mixture was left at rest for 24 h at 298.15 K to separate into two liquid phases. Samples were taken separately from upper (organic-rich phase) and lower (water-rich phase) phases using a syringe. The samples taken from these two liquid phases were analyzed by a gas chromatograph (GC, 8900, China Chromatography, Taiwan) equipped with a thermal conductivity detector (TCD) and a stainless steel column packed with Porapak QS (80/100 mesh, 2 m length). A filter, a short stainless steel tube packed with glass wool, was placed before the packed column and used to prevent MOPS from entering the packed column and detector. The filter was replaced frequently. Helium with purity of 0.9999 in mass fraction at a 30 mL·min−1 flow rate was used as a carrier gas. Calibration curves were prepared for each system prior to analysis for phase compositions. The area fraction of the organic solvent was plotted as a function of mass fraction of the corresponding component for each system. Five replicated samples were analyzed for each phase. The area fraction of the alcohol (MOPS-free basis) can be determined from the peaks shown by Peak-ABC Chromatography Workstation software. From the area fraction, the mass fractions of the alcohol and water can be calculated from the calibration curves. The compositions of MOPS in the upper and the lower phases were determined by gravimetric analysis. Samples in liquid, about 10 g, were taken from the upper and lower phases. After that, the liquid samples were divided into five bottles. The solutions were placed and evaporated in an oven at 120 °C until the mass of MOPS remained constant. The standard uncertainty u(wi) in the water, alcohol (1-propanol, 2-propanol, or tert-butanol), and MOPS composition determination was estimated to be 0.005. The mass fractions of water, alcohol, and MOPS in the coexisting phases were averaged from five replicated samples.

Table 2. Phase Boundary Data of the 1-Propanol (1)−Water (2)−MOPS (3) System as Mass Fractions (wi) at Temperature T = 298.15 K and P = 101.3 kPaa mass fraction I

T/K

w1

LLE

0.1174 0.2217 0.3206 0.4281 0.5470 0.0742 0.1458 0.2098 0.2962 0.3900 0.5255 0.7121 0.7614 0.8265 0.0000 0.0417 0.8440

SLLE

SLE

w2I

w3I

0.4685 0.5169 0.4803 0.4282 0.3649 0.3567 0.3400 0.3142 0.2963 0.2602 0.2254 0.1784 0.1628 0.1516 0.3918b 0.3753 0.1383

0.4141 0.2614 0.1991 0.1437 0.0881 0.5691 0.5142 0.4760 0.4075 0.3498 0.2491 0.1095 0.0758 0.0219 0.6082b 0.5830 0.0177

a

Standard uncertainties are u(T) = 0.05 K, u(P) = 2 kPa, and u(wi) = 0.005 (LLE), 0.005 (SLLE), and 0.005 (SLE). bTaken from Taha and Lee.12

Table 3. Phase Boundary Data of the 2-Propanol (1)−Water (2)−MOPS (3) System as Mass Fractions (wi) at Temperature T = 298.15 K and P = 101.3 kPaa mass fraction T/K

w1

LLE

0.1780 0.2854 0.3905 0.5150 0.6494 0.1342 0.1429 0.2220 0.2973 0.4047 0.5425 0.7304 0.7862 0.0000 0.0418 0.0883 0.8520

SLLE

3. RESULTS AND DISCUSSION The LLE, SLLE, and SLE phase boundary data were determined for ternary systems of alcohol (1-propanol, 2propanol, or tert-butanol)−water−MOPS as listed in Tables 2−4 for 1-propanol−water−MOPS, 2-propanol−water− MOPS, and tert-butanol−water−MOPS systems, respectively. The LLE and SLLE tie-line data for these three ternary systems are reported in Tables 5−7. The experimental results in this study showed that the addition of MOPS can induce the phase separation of the water-miscible organic solvent. The mechanism of buffer-induced phase separation is the same as that of salt. Zwitterions such as MOPS, like electrolytes, have very large dipole moments and interact electrostatically with solvents and other charged molecules in the solution.26 MOPS has both hydrogen bond donor and acceptor sites. Therefore, MOPS may strongly interact with water molecules through hydrogen bonding and electrostatic interactions, which may squeeze the organic solvent molecules from aqueous solutions to form a new liquid phase. The molecular interactions among water, organic solvents, and buffer and the mechanism of the liquid−liquid phase splitting have been studied with molecular dynamics by Taha and Lee14 and Taha et al.27 The phase diagrams of the 1-propanol−water−MOPS, 2propanol−water−MOPS, and tert-butanol−water−MOPS sys-

I

SLE

w2I

w3I

0.4154 0.4280 0.3905 0.3434 0.2784 0.3382 0.3333 0.3180 0.2973 0.2698 0.2326 0.1825 0.1793 0.3918b 0.3730 0.3530 0.1400

0.4066 0.2866 0.2190 0.1416 0.0722 0.5276 0.5238 0.4600 0.4054 0.3255 0.2249 0.0871 0.0345 0.6082b 0.5852 0.5587 0.0080

a


tems are shown in Figures 1−3, respectively. Each phase diagram is divided into five phase regions separated by SLE, LLE, and SLLE phase boundaries. The SLE phase boundaries separate the one-liquid phase (L) and the solid−liquid phase (S + L) regions at the right and at the left triangles. Actually, the solid solubility data at the left triangle is difficult to be determined by density measurements because the MOPS solubility in a high concentration of alcohol is extremely low. The one-liquid phase (L) and the liquid−liquid phase (2L) 2511



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Table 4. Phase Boundary Data of tert-Butanol (1)−Water (2)−MOPS (3) System as Mass Fractions (wi) at Temperature T = 298.15 K and P = 101.3 kPaa

Table 7. Tie-Line Data of the tert-Butanol (1)−Water (2)− MOPS (3) System as Mass Fractions at Temperature T = 298.15 K and P = 101.3 kPaa

mass fraction

organic phase

T/K

w1I

w2I

w3

LLE

0.0999 0.1397 0.2434 0.3497 0.4566 0.5657 0.6771 0.7896 0.0782 0.1439 0.2130 0.2961 0.3896 0.5148 0.7055 0.7835 0.8457 0.0000 0.0413

0.4397 0.5588 0.5677 0.5242 0.4564 0.3770 0.2902 0.1980 0.3487 0.3355 0.3192 0.2960 0.2597 0.2206 0.1769 0.1496 0.1451 0.3918b 0.3717

0.4604 0.3015 0.1889 0.1261 0.0870 0.0573 0.0327 0.0124 0.5731 0.5206 0.4678 0.4079 0.3507 0.2646 0.1176 0.0669 0.0092 0.6082b 0.5870

SLLE

SLE

I

aqueous phase

T/K

w1I

w2I

w3

LLE

0.5700 0.5789 0.6786 0.7065 0.7604 0.7816 0.8457

0.3827 0.3772 0.2938 0.2701 0.2229 0.2038 0.1451

0.0473 0.0439 0.0276 0.0234 0.0167 0.0146 0.0092

SLLE

I

w1II

w2II

w3II

0.2038 0.1982 0.1445 0.1246 0.0985 0.0960 0.0782

0.5651 0.5589 0.5334 0.5223 0.4694 0.4398 0.3487

0.2311 0.2429 0.3221 0.3531 0.4321 0.4642 0.5731

a

Standard uncertainties are u(T) = 0.05 K, u(P) = 2 kPa, and u(wi) = 0.005.

a


Table 5. Tie-Line Data of the 1-Propanol (1)−Water (2)− MOPS (3) System as Mass Fractions (wi) at Temperature T = 298.15 K and P = 101.3 kPaa organic phase

aqueous phase

T/K

w1I

w2I

w3I

w1II

w2II

w3II

LLE

0.6163 0.6376 0.6764 0.7404 0.6993 0.7723 0.8265

0.3176 0.3026 0.2733 0.2223 0.2506 0.1990 0.1516

0.0661 0.0598 0.0503 0.0373 0.0501 0.0287 0.0219

0.1793 0.1660 0.1440 0.1142 0.1333 0.0957 0.0742

0.5145 0.5103 0.4981 0.4591 0.4801 0.4026 0.3567

0.3062 0.3237 0.3579 0.4267 0.3866 0.5017 0.5691

SLLE

Figure 1. Calculated values from the NRTL model and experimental tie-line data for 1-propanol−water−MOPS (in mass fraction) at T = 298.15 K under atmospheric pressure: ■, phase boundary data; −○−, LLE tie-line data; −△−, SLLE tie-line data; ---, calculated values from the NRTL model.

a


Table 6. Tie-Line Data of the 2-Propanol (1)−Water (2)− MOPS (3) System as Mass Fractions at Temperature T = 298.15 K and P = 101.3 kPaa organic phase

aqueous phase

T/K

w1I

w2I

w3I

w1II

w2II

w3II

LLE

0.6057 0.6330 0.6814 0.7003 0.7152 0.7297 0.7862

0.3013 0.2884 0.2579 0.2439 0.2346 0.2234 0.1793

0.0930 0.0786 0.0607 0.0558 0.0502 0.0469 0.0345

0.2060 0.1870 0.1596 0.1644 0.1549 0.1484 0.1342

0.4254 0.4192 0.4059 0.3871 0.3786 0.3714 0.3382

0.3686 0.3938 0.4345 0.4485 0.4665 0.4802 0.5276


Figure 2. Calculated values from the NRTL model and experimental tie-line data for 2-propanol−water−MOPS (in mass fraction) at T = 298.15 K under atmospheric pressure: ■, phase boundary data; −○−, LLE tie-line data; −△−, SLLE tie-line data; ---, calculated values from the NRTL model.

regions are separated by the LLE phase boundary. As can be seen from Figures 1−3, the closed symbols represent LLE

phase boundary data and the open symbols represent LLE and SLLE tie-line data. The SLLE boundary separates the (2L) and

SLLE a

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Article N

Δ=

2

3

expt calc ∑k = 1 ∑ j = 1 ∑i = 1 (xijk − xijk )

(2)

6N

xcalc ijk

xexpt ijk

where N is the number of tie lines, and are the calculated and experimental mole fractions of component i in phase j on tie line k. The calculated tie-line values from the NRTL model and the experimental tie-line data are compared in Figures 1−3. Tables 8 and S2 report the optimal values of bij Table 8. NRTL Model Parameters for Ternary Systems of Alcohol (1)−Water (2)−MOPS (3)

ln γi =

M

∑k Gkixk

M

+

∑ j

i-j

bija/K

bjia/K

1-propanol (1)−water (2)−MOPS (3)

0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

1-2 1-3 2-3 1-2 1-3 2-3 1-2 1-3 2-3

−231.21 1149.96 1796.80 −249.25 1160.26 1796.80 −180.00 1629.86 1796.80

862.08 431.81 −1425.96 771.52 178.52 −1425.96 774.40 2454.33 −1425.96

tert-butanol (1)−water (2)− MOPS (3)

the solid−liquid−liquid phase (S + 2L) regions. Furthermore, the areas of the (2L) region are in the order of tert-butanol > 1propanol > 2-propanol for the alcohol−water−buffer systems. It will be favorable for the recovery of high-purity organic compounds from their aqueous solutions via liquid−liquid phase splitting if the concentrations of the organic component in the organic-rich phase are greater than their azeotropic composition. This means that the organic compounds can be easily purified from the organic-rich phase by subsequent conventional distillation. Tables 5−7 show that the highest content of alcohols in the organic-rich phase can be found from the SLLE tie-line data. This value is a measure to evaluate the capability of the auxiliary agent (MOPS) to overcome the azeotropic barrier. As seen from those tabulated values, MOPS is favorable for 1-propanol recovery because the composition of 1-propanol in the organic-rich phase at the SLLE (w1I = 0.8265) is substantially greater than its azeotropic composition (0.717). Therefore, high-purity 1-propanol can be recovered from its aqueous solution by taking advantage of liquid-phase splitting induced by MOPS. For 2-propanol and tert-butanol, the compositions of these organic solvents in the organic-rich phase at SLLE (w1I = 0.7862 and 0.8457, respectively) are lower than their azeotropic compositions (0.88 and 0.8824, respectively). Thus MOPS is not favorable to overcoming the separation barrier of azeotropic 2-propanol−water and tertbutanol−water. The NRTL activity coefficient model18 was also utilized to correlate the experimental tie-line data. The NRTL model for multicomponent systems is defined in the following equation M

αij

2-propanol (1)−water (2)−MOPS (3)

Figure 3. Calculated values from the NRTL model and experimental tie-line data for tert-butanol−water−MOPS (in mass fraction) at T = 298.15 K under atmospheric pressure: ■, phase boundary data; −○−, LLE tie-line data; −△−, SLLE tie-line data; ---, calculated values from the NRTL model.

∑ j τjiGjixj

system

and bji, the AAD of alcohol, water, and MOPS in organic-rich and water-rich phases, and the grand AAD (Δ) for the alcohol−water−MOPS systems, respectively. As seen from those tabulated values, the NRTL model correlates experimental tie-line data satisfactorily. The separation factor (S) reveals the capability of an auxiliary agent (i.e., buffer MOPS in this study) to induce liquid-phase splitting for a given mixture system. A higher separation factor represents a stronger buffering-out effect. The separation factor is calculated from the ratio of the distribution coefficient of alcohol (D1) to water (D2) as follows:28 S=

D1 w I/w II = 1I 1II D2 w2/w2

(3)

Superscripts I and II represent the organic and aqueous phases, respectively. Symbols w1 and w2 are the mass fractions of alcohol and water, respectively. The values of the distribution coefficient and the separation factor are listed in Table 9. It can be seen that the values of the separation factor can be as high as 26.1 for 1-propanol−water−MOPS, 26.0 for tert-butanol− water−MOPS, and 11.1 for 2-propanol−water−MOPS. These values indicated that the buffering-out strength of buffer MOPS is in the order 1-propanol > tert-butanol > 2-propanol. Furthermore, the separation factors and the distribution coefficients of alcohols are greater than 1 for all investigated systems. These results indicate that the recovery of alcohols from aqueous solutions by the addition of buffer MOPS is favorable. Figure 4 shows the relationship between the separation factor and the mass fraction of alcohols in the aqueous phase. The lower composition of alcohols in the aqueous phase indicated the greater ability of buffer MOPS to induce the liquid-phase splitting. For the 1-propanol system, the experimental results show that using MOPS as an auxiliary agent the composition of the organic-rich phase is substantially greater than the azeotropic composition of water−1-propanol. Thus, a high purity of 1propanol can be recovered easily from the organic-rich phase with a conventional distillation method. On the basis of the phase diagram prepared from this work, we suggest a feasible

M ⎛ ∑ xτ G ⎞ ⎜τ − k k kj kj ⎟ ij M M ∑k Gkjxk ⎜⎝ ∑k Gkjxk ⎟⎠

xjGij

(1)

where γi and M represent the activity coefficient for component I and the number of components, respectively, and Gij = exp(−αijτij), τij = bij/T, τii = τjj = 0, and αij = αji. The value of αij, the nonrandomness parameter, was fixed at 0.2 for each pair (ij). The optimal values of the binary parameters (bij and bji) were determined by the minimization of the objective function, Δ (grand AAD, average absolute deviation), defined as follows 2513



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Table 9. Distribution Coefficients (Di) and Separation Factors (S) for the Investigated Systems at T = 298.15 K and 101.3 kPa 1-propanol (1)−water (2)−MOPS (3) wII1

D1

D2

0.0742 0.0957 0.1142 0.1333 0.1440 0.1660 0.1793

11.1 8.07 6.48 5.25 4.70 3.84 3.44

0.425 0.494 0.484 0.522 0.549 0.593 0.617 tert-butanol

S

2-propanol (1)−water (2)−MOPS (3) wII1

D1

26.1 0.1342 5.86 16.3 0.1484 4.92 13.4 0.1549 4.62 10.1 0.1596 4.27 8.56 0.1644 4.26 6.48 0.187 3.39 5.58 0.2060 2.94 (1)−water (2)−MOPS (3)

D2

S

0.530 0.602 0.620 0.635 0.630 0.688 0.708

11.1 8.17 7.45 6.72 6.76 4.93 4.15

wII1

D1

D2

S

0.0782 0.0960 0.0985 0.1246 0.1445 0.1982 0.2038

10.8 8.14 7.72 5.67 4.70 2.92 2.80

0.416 0.463 0.475 0.517 0.551 0.675 0.677

26.0 17.6 16.3 11.0 8.53 4.33 4.14

Figure 5. Conceptual process flowsheet for the recovery of 1-propanol from its aqueous solution with the aid of biological buffer MOPS.

of drum F2. Because MOPS is biocompatible, noncorrosive, and nonvolatile, the suggested process can be considered to be a cleaner method for recovering high-purity 1-propanol from its aqueous solution. For 2-propanol and tert-butanol systems, the abovementioned simple process cannot be directly applied because the compositions of the organic-rich phase are no greater than the azeotropic compositions of these two aqueous mixtures. However, MOPS can still induce liquid−liquid phase splitting for these two aqueous solutions. Although 2-propanol and tertbutanol cannot be directly recovered from the organic-rich phase, a process that is similar to the heterogeneous extractive distillation method may be applicable to removing 2-propanol or tert-butanol from its aqueous solution.

4. CONCLUSIONS The LLE, SLE, and SLLE data were measured in this study for ternary systems alcohol (1-propanol, 2-propanol, or tertbutanol)−water−MOPS at 298.15 K under atmospheric pressure. The areas of the liquid−liquid region are in the order of tert-butanol > 1-propanol > 2-propanol for the alcohol−water−buffer systems. The experimental LLE tie-line data were correlated well using the NRTL model. On the basis of the values of the separation factor, the buffering-out strength of MOPS for alcohol−water−MOPS systems are in the order of 1-propanol > tert-butanol > 2-propanol. Furthermore, according to the experimental results, high-purity 1-propanol can be easily recovered from its aqueous solution by taking advantage of liquid−liquid phase splitting induced by adding biocompatible, noncorrosive, nonvolatile buffer MOPS.

Figure 4. Relationship between the separation factor (S) and the mass fraction of alcohols in the aqueous phase (wII1 ).

flowsheet, as illustrated in Figure 5, for the recovery of 1propanol from its aqueous solution. This process contains five units: two distillation columns (T1 and T2), two flash drums (F1 and F2), and an extractor (E). The feed streams (F) of aqueous 1-propanol solution and four recycled streams are all diverted into the extractor E. Liquid−liquid phase splitting occurs in the extractor. The concentration of 1-propanol in the upper phase (I) is greater than the azeotropic composition of water +1-propanol. To remove a trace amount of MOPS from this organic-rich phase, the solution of the upper phase is transferred to the flash drum (F1). After this treatment, the top stream from F1 is buffer-free 1-propanol containing a small amount of water. High-purity 1-propanol can be obtained from the bottom of the subsequent distillation column (T1). Because of the hydrophilic nature of buffer MOPS, the majority of the buffer is dissolved in the lower aqueous-rich phase (II) of the extractor. While MOPS can be totally recycled via distillation column T2 and flash drum F2, water is discharged from the top

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00954. Azeotropic conditions for binary systems water− investigated alcohols, AADs of the LLE data correlations, and an illustrative graph for the determination of solid solubility (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +886 2 2737 6626. Fax: +886 2 2737 6644. E-mail: [email protected]. 2514



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Ming-Jer Lee: 0000-0001-7586-7379 Funding

The authors are grateful for the financial support provided by the Ministry of Science and Technology, Taiwan, through grant no. NSC103-ET-E011-006-ET and a fellowship from National Taiwan University of Science and Technology. Notes

The authors declare no competing financial interest.

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