Experimental and Theoretical Studies on the Effectiveness of

Article pubs.acs.org/IECR

Experimental and Theoretical Studies on the Effectiveness of Phosphonium-Based Ionic Liquids for Butanol Removal at T = 298.15 K and p = 1 atm Dharamashi Rabari and Tamal Banerjee* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati-781039, Assam, India S Supporting Information *

ABSTRACT: Biobutanol obtained via an acetone−butanol−ethanol (ABE) fermentation process is now considered a potential biofuel. In recent times low density phosphonium cations were found to remove butanol from aqueous solutions. In this regard, low density phosphonium-based ionic liquids (ILs) trihexyl(tetradecyl)phosphonium dicyanamide [TDTHP][DCA] and trihexyl(tetradecyl)phosphonium decanoate [TDTHP][DEC] have been used for the separation of 1-butanol from aqueous solution. Ternary liquid−liquid equilibrium data for IL(1)−1-butanol(2)−water(3) are measured at T = 298.15 K and p = 1 atm. Butanol partition coefficients are obtained in the range of 25−85 and 20−290 for [TDTHP][DCA] and [TDTHP][DEC], respectively. 1H NMR spectra indicates the absence of IL and water in the raffinate and extract phase, respectively. This experimentally confirms that there will be negligible cross-contamination of water and IL in either phase. The separation factor of butanol over water approaches infinity for both systems. A wider spread of binodal curve indicated a higher recovery of butanol at different feed concentrations. The experimental data were compared with excess Gibb’s free energy models, namely the nonrandom two liquid (NRTL) and the universal quasichemical (UNIQUAC) models. NRTL and UNIQUAC gave root mean square deviation (RMSD) values in the range of 0.12−0.14% and 0.48−0.55%, respectively, for both ILs. Further the predictive ability of a statistical mechanical framework was also performed using the well-known COSMO-RS model. The COSMO-RS model gave RMSD values of 18.67% and 16.21% for the systems containing the ILs respectively, [TDTHP][DCA] and [TDTHP][DEC].

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INTRODUCTION Environment regulations motivate scientists and researchers to develop clean technologies for sustainable development. Researchers amplify their efforts to convert separation processes like extraction and distillation into clean technologies. The knowledge of thermodynamic properties like equilibrium data and activity coefficient at infinite dilution is required for adequate separation. Liquid−liquid equilibrium (LLE) data helps in the selection of solvent for the selective separation of a solute component. In addition to this economic aspect, clean technology also considers an ecological aspect. Over the last few decades, eco-friendly solvents like ionic liquids (ILs) have been considered as better replacements for conventional volatile solvents.1,2 ILs are low melting point salts that are a combination of large organic cations and inorganic/organic anions. These solvents are attractive because of their inherent properties such as low vapor pressure, less volatility, high thermal/chemical stability, and wider liquid range. For specific application, these properties can be functionalized with a judicious combination of cation and anion.3−5 Limited conventional energy resources such as fossil fuel makes energy conservation essential. Therefore clean technology considers energy along with the economic and environmental impacts of the process involved. As a consequence, various alcohols like ethanol and butanol are blended with gasoline to conserve conventional energy source. Butanol is superior to ethanol due to its higher calorific value,3 higher hydrophobicity, and lesser flammability. In addition to crude oil, biomass is also a source for butanol production. Biobutanol © XXXX American Chemical Society

is similar to conventional butanol in terms of properties which is obtained from ABE (acetone−butanol−ethanol) fermentation in the presence of bacteria6,7 such as Clostridium acetobutylicum and Clostridium bjerinkci under anaerobic conditions in a proportion 6:3:1 (butanol/acetone/ethanol). In ABE fermentation, cell growth is inhibited when the butanol concentration reaches 10 g/L thereby lowering productivity. When compared to other separation operations like pervaporation, membrane separation, and gas stripping, extraction is more favorable for biobutanol separation from ABE ferment broth.7 As compared to organic solvents, ILs have shown better selectivity and distribution coefficient for butanol separation. Various hydrophobic ILs8,9 have shown better selectivity for butanol separation from aqueous solution and are more economical when compared to hydrophilic ILs10 for extraction of water. Nann et al.11 have used four ILs containing ions such as 1decyl-3-methylimidazolium, 4-decyl-4-methylmorpholinium, bis(trifluoromethylsulfonyl)imide and tetracyanoborate for the extraction of 1-butanol from aqueous solution at 308.15 and 323.15 K and found selectivity of 1-butanol over water ranging from 53 to 144. Davis and Morton III12 investigated ternary Special Issue: Ganapati D. Yadav Festschrift Received: March 4, 2014 Revised: May 8, 2014 Accepted: May 12, 2014

A

dx.doi.org/10.1021/ie500833h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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known peaks18 which subsequently confirmed negligible impurities. 1-Butanol was purchased from Sigma-Aldrich with a reported 99% purity. Density was measured using an Anton Paar DMA-4500 vibrating U-tube densitometer to check the purity of 1-butanol. The density deviation was within ±1%. NMR solvents, dimethyl sulfone oxide-D6 (DMSO-D6) and chloroform-D (CDCl3) were supplied by Merck, Germany. All chemicals with their purity, purification method, and analysis method are reported in Table 1.

LLE data for 1-butanol/water/1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at 298.15 K. Ha et al.13 have reported that bis(trifluoromethylsulfonyl)imide based ILs have better extraction efficiency compared to tetrafluoroborate, trifluoromethanesulfonate, and hexafluorophosphate based ILs at 298.15 and 323.15 K. Chapeaux et al.14 investigated 1-hexyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide as a potential solvent for the separation of alcohols from aqueous solution. Domanska and Krolikowski15 have reported that ILs with a longer alkyl chain at the cation show higher selectivity and distribution ratio for extraction of 1-butanol from water with tetracyanoborate as anion. In our earlier work,16 we have recently investigated a low density IL tetradecyl(trihexyl)phosphonium bis(2,4,4-trimethylpentyl) phosphinate as a potential solvent in the extraction of 1-butanol from water giving a higher selectivity of 80−105. Continuing with our studies we have carried out LLE with hydrobhobic anions namely dicyanamide and decanoate. It should be noted that the anion controls the water solubility;4,17 hence, a study is required to evaluate its performance. Hydrophobic ILs, trihexyl(tetradecyl)phosphonium dicyanamide [TDTHP][DCA] (0.9 g·cm−3) and trihexyl(tetradecyl)phosphonium decanoate [TDTHP][DEC] (0.883g.cm−3) are also lighter than water and thus have been considered in the present work (Figure 1). Further lighter ILs are beneficial as they reduce pumping charges resulting in a reduction of operating cost.

Table 1. Chemicals Purity and Purification Methods chemical name 1-butanol [TDTHP] [DCA] [TDTHP] [DEC] CDCl3 DMSO-D6

source Sigma-Aldrich, Germany Sigma-Aldrich, Germany Sigma-Aldrich, Germany Merck, Germany Merck, Germany

purification method

purity (mole fraction) 0.99

vacuum drying vacuum drying

0.99

analysis method density method 1 H NMR

0.99

1

0.998 0.998

1

H NMR H NMR H NMR

1

Procedure and Analysis. Initially, different ternary mixtures (IL, 1-butanol, and water) of 5 mL were prepared in 15 mL cuvettes. These cuvettes were covered with parafilm tape and then placed in mechanical water bath shaker (Labtech Make) for 8 h at 298.15 K. Thereafter 20 h time was provided for settling. The feed points with homogeneous mixture were rejected as it indicated the feed mixture outside the binodal curve. The heterogeneity was ensured with the absence of single droplet in either phase. The layers were separated by 2 mL syringe and further processed for analysis. A volume of 0.1 mL from each layer was mixed with 0.5 mL of solvent in an NMR tube for characterization using 1H NMR of 11.74 Tesla (400 MHz response of 1H). CDCl3 and DMSOD6 were used as NMR solvents for the extract and raffinate phases, respectively. Thereafter NMR tubes with samples were covered with parafilm to avoid losses and then placed for 1H NMR analysis. Reference peaks were noted at 7.24 and 2.5 ppm for both solvents, that is, CDCl3 and DMSO-D6, respectively (Supporting Information, Figures S1−S4). The accuracy was checked by a similar procedure in our previous work.16,19 In both phases, the concentration of each compound was found from the peak area representing a specific functional group in the compound. Water in CDCl3 gave a peak at ∼1.58 ppm. The extract phase NMR spectra (Supporting Information, Figures S1−S2) confirmed the absence of a peak at 1.58 ppm. Karl Fischer titration (Metrohm Make, 787 KF Titrino model) was also performed to verify the absence of water in the extract phase. In the raffinate phase, water showing a peak at ∼3.8 ppm was utilized for characterization and quantification (Supporting Information, Figures S3−S4). 1-Butanol was characterized by a methylene group (−CH2) attached to a hydroxyl group (−OH) thereby representing a peak at ∼3.4−3.6 ppm. 1H NMR (400 MHz, δ/ppm; CDCl3) for [TDTHP][DCA] and [TDTHP][DEC] were labeled18 as 0.79−0.85 (m, 12 H), 1.2−1.3 (m, 32 H), 1.4−1.5 (m, 16 H) and 2−2.3 (m, 8 H) and 0.79−0.96 (m, 15 H), 1.2−1.3 (m, 46 H), 1.4−1.5 (m, 16 H) and 2−2.3 (m, 10 H) ppm, respectively (Supporting Information, Figures S1− S4). Thus, for the estimation of mole fraction, 48 hydrogen atoms of [TDTHP][DCA] and 62 hydrogen atoms of [TDTHP][DEC] all lying in the range of 1.2−1.5 ppm were taken as reference peaks.

Figure 1. Structure of phosphonium ionic liquids used in this work.

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EXPERIMENT Chemicals and Materials. Trihexyl(tetradecyl)phosphonium dicyanamide and trihexyl(tetradecyl)phosphonium decanoate (purity, >99%, BASF quality) were supplied by Sigma-Aldrich. They were further heated in an oil bath at 353 K for 24 h to remove impurities. To remove trace impurities, this heating bath was attached to a high vacuum line. The 1H NMR peak analysis for both ILs were validated with B



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Table 2. Experimental Tie Line Data for [TDTHP][DCA] (1)−1-Butanol (2)−Water (3) at T = 298.15 K and p = 1 atma extract phase

a

raffinate phase

sr. no.

xIL

x1‑butanol

xwater

xIL

x1‑butanol

xwater

1 2 3 4 5 6 7 8

0.6312 0.2497 0.1667 0.0547 0.0935 0.7313 0.5226 0.3546

0.3688 0.7503 0.8333 0.9453 0.9065 0.2687 0.4774 0.6454

0 0 0 0 0 0 0 0

0.0000 0.0000 0.0002 0.0000 0.0006 0.0000 0.0000 0.0000

0 0.0115 0.0127 0.0169 0.0374 0 0.0055 0.0088

1 0.9885 0.9871 0.9831 0.962 1 0.9945 0.9912

partition coefficient (Kp) 65.24 65.61 55.93 24.24 86.80 73.34

Uncertainty, u (mole fraction) = ±10−3.

Table 3. Experimental Tie Line Data for [TDTHP][DEC] (1)−1-Butanol (2)−Water (3) at T = 298.15 K and p = 1 atma extract phase

a

raffinate phase

sr. no.

xIL

x1‑butanol

xwater

xIL

x1‑butanol

xwater

1 2 3 4 5 6 7 8

0.5237 0.2492 0.1042 0.0651 0.6899 0.3761 0.2958 0.8274

0.4763 0.7508 0.8958 0.9349 0.3101 0.6239 0.7042 0.1726

0 0 0 0 0 0 0 0

0.0002 0.0005 0.0009 0.0010 0.0009 0.0009 0.0004 0.0002

0.0026 0.0073 0.0453 0.0536 0.0023 0.0059 0.0067 0.0006

0.9972 0.9922 0.9538 0.9454 0.9968 0.9932 0.9929 0.9992

partition coefficient (Kp) 183.19 102.85 19.77 17.44 134.83 105.75 105.1 287.67

Uncertainty, u (mole fraction) = ±10−3.

The area obtained for single hydrogen of each compound was then utilized for calculation of the concentration of each compound in the respective phase as per eq 1. xi =

Hi 3 ∑i = 1 Hi

(1)

where Hi denotes the peak area of single hydrogen for the sample(s) and xi is the mole fraction of individual solute molecules.

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RESULTS AND DISCUSSION Experimental Results. Experimental ternary data for two systems namely [TDTHP][DCA] (1)−1-butanol (2)−water (3) and [TDTHP][DEC] (1)−1-butanol (2)−water (3) were measured at 298.15 K and 1 atm (Tables 2 and 3 and Figures 2−5). The extraction effectiveness is represented by the partition coefficient (Kp) and the separation factor (SF) for equilibrium biphasic systems. The partition coefficient and separation factor are calculated by ⎛ xE ⎞ KP = ⎜ bR ⎟ ⎝ xb ⎠ SF =

eq

(xbE/xbR ) (x wE/x wR )

Figure 2. Experimental and NRTL tie lines for the ternary system [TDTHP][DCA] (1)−1-butanol (2)−water (3) at T = 298.15 K and p = 1 atm.

(2)

values are much higher than the 0.7−2.2 obtained by Ha et al.13 for different imidazolium-based ILs containing tetrafluoroborate, trifluoromethanesulfonate, hexafluorophosphate, and bis(trifluoromethylsulfonyl)imide anions. Further the positive sloping of the tie lines indicates butanol favorably partitions into the IL phase. The partition coefficient for [TDTHP][DCA] is twice compared to that of the ILs reported by Nann et al.11 that is, 1-decyl-3-methylimidazolium, tetracyanoborate ([Im10.1][tcb]), 4-decyl-4-methylmorpholinium tetracyanoborate ([Mo10.1][tcb]), 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl) ([Im10.1][ntf2]), and 4-decyl-4methylmorpholinium bis(trifluoromethylsulfonyl)imide

(3)

Here, superscript E and R represent extract and raffinate phase, respectively; xb and xw are the mole fraction of butanol and water, respectively. A large value of the partition coefficient is desirable since it indicates less solvent is required for a particular degree of separation. The partition coefficient varied in the range of 25− 85 and 20−290 for ternary systems, namely, [TDTHP][DCA] and [TDTHP][DEC], respectively (Tables 2 and 3). These C



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Figure 3. Experimental and UNIQUAC tie lines for the ternary system [TDTHP][DCA] (1)−1-butanol (2)−water (3) at T = 298.15 K and p = 1 atm.

Figure 5. Experimental and UNIQUAC tie lines for the ternary system [TDTHP][DEC] (1)−1-butanol (2)−water (3) at T = 298.15 K and p = 1 atm.

Figure 4. Experimental and NRTL tie lines for the ternary system [TDTHP][DEC] (1)−1-butanol (2)−water (3) at T = 298.15 K and p = 1 atm. Figure 6. Comparison of partition coefficients with imidazolium and phosphonium based cations at ambient condition.

([Mo10.1][ntf2]). Further it was also observed that the partition coefficient for [TDTHP][DEC] is seven times higher as compared to that of the same ILs.11 Thus, we can conclude that [TDTHP][DEC] more favorably extracts butanol from aqueous streams. The partition coefficients for the present two systems are compared with literature data11,16 in Figure 6. It should be noted that the ternary diagram is wider than that obtained in the case of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by Chapeaux et al.14 It is an indication of better separation with a wide range of feed concentration. In addition, the solvent requirement or the solvent-feed ratio will be less as compared to imidazoliumbased ILs for a particular degree of separation. The separation factor measures the ability to separate 1butanol from water. Higher values of separation factor indicate better selective separation of 1-butanol. Here the mole fraction of water in the extract phase is nearly zero; hence, the

separation factor value approaches infinity from eq 3. Also Tables 2 and 3 and Figures 2−5 represent a negligible concentration of IL and water in either phase. It implies both ILs are completely immiscible with water in the ternary system. The separation factor is higher than the values of 80−305 as obtained by our earlier work for trihexyl(tetradecyl) phosphonium bis(2,4,4-trimethylpentyl) phosphinate16 thereby indicating more ability to separate butanol. This confirms the findings of Cascon20 for the IL trihexyl(tetradecyl)phosphonium dicyanamide in which SILM-based pervaporation of 1-butanol indicated high affinity for the alcohol. This is due to the strong hydrogen bonding between the dicyanamide anion and 1butanol. The hydrogen bond energy21 between dicyanamide ion with butanol (60.8 kJ/mol) is greater as compared to that D



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nature which thus necessitates the application of the GA toolbox. Here the function F is maximized with respect to the NRTL or UNIQUAC parameters namely τ ij and A ij , respectively. The GA algorithm specifies the number of population and generation among other parameters. In our work the number of population is 100 indicating 100 possible random solutions or vectors, where each vector represents the set of six parameters, namely, A12/τ12, A13/τ13, A21/τ21, A23/τ23, A31/τ31, A32/τ32. This random solution vector is altered in each iteration (or generation) using two operators, crossover and mutation. So this iteration runs for the number of generations that we specify in the GA toolbox, which is 200 in our case.25 After the iteration or number of generations is complete, the optimization stops and displays the results corresponding to the lowest objective function (results reported in Table 6). For both NRTL and UNIQUAC models (Table 4), the modeling procedures are described in our earlier work.25 For the ternary liquid−liquid system, the equilibrium relation is defined as

with water (53.9 kJ/mol). This is contrary to the measurements by Freire et al.4 where [TDTHP][DCA] and [TDTHP][DEC] were found to be more miscible with water. However, the hydrogen bond between the anions and butanol alters the miscibility of the ILs with butanol and water. It can be seen from Figures 4 and 5 that the amount of water content in [TDTHP][DEC] is greater as compared to that of [TDTHP][DCA]. This is attributed to a larger electronegativity atom in [DEC] thereby forming a strong bond with water.21 Gibb’s Free Energy Models Results. Experimental LLE data were correlated with NRTL22 and UNIQUAC23 models (Table 4). For the UNIQUAC model, volume parameter (r) Table 4. NRTL and UNIQUAC Models equations c ∑ j = 1 τjiGjixj c ∑k = 1 Gkixk

ln γi =

NRTL22 c c ⎡ ∑ τ G x ⎞⎤ Gijxj ⎛ ⎜⎜τij − ic= 1 ij ij i ⎟⎟⎥ + ∑⎢ c ⎢∑ G x ∑k = 1 Gkjxk ⎠⎥⎦ j = 1 ⎣ k = 1 kj k ⎝

Gji = exp(− αjiτji), τji =

gji − gii

Aji

⎛ Aij ⎞ τij = exp⎜− ⎟, ⎝ T⎠

rT =

∑ rkxk , k

θi =

qixi qT

,

c

∑ xjlj

zi =

j=1

θτ j ij

j=1

qT =

∑ qkxk , k

Φi =

rx i i rT

Ki =

z li = (rk − qk) + 1 − rk 2

sr. no.

compund

volume parameter (r)

surface area parameter (q)

1 2 3 4

[TDTHP][DCA]a [TDTHP][DEC]a 1-butanol water

8.37 8.77 3.92 0.92

5.81 5.96 3.67 1.4

=

xiI

γi I γi II

(7)

∑

zi(1 − K i) =0 1 + Ψ(K i − 1)

(8)

subject to, Fzi = E IxiI + RIIxiII

(9)

and

ψ = E I /F

(10)

Here EI and RII represent the flow rate of the extract and raffinate phases, respectively. Equation 8 is nonlinear in nature is first solved for Ψ. Then, the mole fractions in both phases are calculated via eqs 11−12. zi xî I = 1 + Ψ(K i − 1) (11)

objective function (eq 4) was minimized using the Genetic Algorithm (GA)25 by regression of experimental and predicted molefractions.

xî II = K ixî I

(12)

NRTL and UNIQUAC (Table 4) model predicted tie lines overlap with the experimental tie lines (Figures 2−5) indicating lesser deviation. The root mean square deviation (rmsd) representing deviation from experimental data is defined as

c

max : Obj⎛ with respect to A ij ⎞ = − ∑ ∑ ∑ wikl(xikl − xik̂ l )2 ⎜ where i , j = 1,2,3 ⎟ ⎜ ⎟ and j ≠ i ⎝ ⎠

xiII

f (Ψ) =

Reference 24.

II

(6)

Then these partition coefficient values are used to solve the isothermal flash equation,

Table 5. UNIQUAC Volume and Surface Area Structural Parameters for Components

m

xiI + xiII 2

The values of activity coefficients in both phases are used to compute the partition coefficient (Ki, i = 1,2,3) using the equation,

and surface area parameter (q) of both ILs were predicted by a combination of the polarizable continuum model (PCM)24 and the GEPOL algorithm. The detailed procedures for the prediction of these parameters are discussed in our previous work,24 and predicted values are reported in Table 5. The

a

γIIi

where i = 1,2,3. and are predicted using the NRTL/ UNIQUAC model. The model input parameter, feed concentration (zi), is calculated by

⎞ ⎟ c ∑k = 1 θkτkj ⎟⎠

c

∑

(5)

γIi

= RT T UNIQUAC23 ⎛θ ⎞ ⎛Φ ⎞ Φ z ln γi = ln⎜ i ⎟ + qi ln⎜ i ⎟ + li − i xi ⎝ Φi ⎠ ⎝ xi ⎠ 2 c ⎛ + qi⎜⎜1 − ln ∑ θτ j ji − j=1 ⎝

γi Ixî I = γi IIxî II

k=1 l=I i=1

(4)

⎡ m c II (x l − x ̂ l )2 ⎤1/2 ik ⎥ rmsd (%) = ⎢∑ ∑ ∑ ik 100 ⎢⎣ k = 1 i = 1 l = I ⎥⎦ 2mc

The objective function represents the difference between experimental mole fractions and those predicted by either NRTL or UNIQUAC. This function is highly nonlinear in E

(13)



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Table 6. NRTL and UNIQUAC Interaction Parameters for Ternary Systems at T = 298.15 K and p = 1 atm NRTL model parameters

a

i−j

τij

τji

1−2 1−3 2−3

−1.93 15.71 20

10.45 10.47 4.56

1−2 1−3 2−3

−4.77 17.24 20

17.99 5.45 3.28

Obja

UNIQUAC model parameters %rmsdb

Aji/K

Obja

%rmsdb

−155.56 −51.98 −15.75

−1.12 × 10−3

0.48

38.2 −87.58 −58.53

−1.46 × 10−3

0.55

Aij/K

[TDTHP][DCA] (1)−1-Butanol (2)−Water (3) −0.21 −7.24 × 10−5 0.12 1000 1000 [TDTHP][DEC] (1)−1-Butanol (2)−Water (3) −232.11 −8.75 × 10−5 0.14 1000 1000

Calculated by eq 4. bCalculated by eq 13.

where xikl and x̂ilk are the experimental and predicted molefraction for component i in the kth tie line of phase l, respectively. Here m refers to the number of tie lines and c refers to the number of components (viz. three for the present system). NRTL and UNIQUAC models gave rmsd values of 0.12% and 0.48%, respectively, for system containing [TDTHP][DCA]. Similarly for the [TDTHP][DEC] containing system, rmsd values were 0.14% and 0.55% for NRTL and UNIQUAC models, respectively (Table 6). COSMO-RS Prediction Model Results. LLE data were also predicted using the quantum chemical based conductor like screening model for real solvents (COSMO-RS) using the isothermal flash calculation of feed concentration. The methodology and parameters can be found from our previous work.26−28 The deviations for both the systems namely, [TDTHP][DCA] and [TDTHP][DEC] were very high. It should be noted that we did not attempt to change the COSMO-RS parameters as they are global in nature. We have modified the hydrogen-bonding interaction similar to the COSMO-SAC model29 by sandler. The calculation of ΔG̲ *i/ires involves pairwise interactions between segments which are based on contact surfaces of the same size. It is necessary to use an averaging process to obtain a collection of only standard segments, each with an “apparent” charge density distribution (σ) over an area that is larger than that of original charge density (σ*). Therefore, this standard segment surface area is considered to be one of the universal parameters in COSMObased models.

σm =

∑n σn* ∑n

2 rn2reff

( exp(−

2 rn2 + reff

2 rn2reff 2 rn2 + reff

exp −

2 dmn

) )

σm =

∑n σn* ∑n

2 rn2 + reff

2 rn2reff 2 2 rn + reff

( exp(−f

exp −fdecay

2 dmn 2 2 rn + reff 2 dmn

2 decay rn2 + reff

) )

(15)

As discussed above, the expression for the Gaussian type probability (PHB(σ)) is now given by eq 16. ⎛ σ2 ⎞ P HB(σ ) = 1 − exp⎜ − 2 ⎟ ⎝ 2σ0 ⎠

(16)

Here σ0 = 0.007 e/Å2 and σ values are calculated from eq 15. There will be an important difference in σ-profile construction between old and new, that is, COSMO-RS and COSMO-SAC. Earlier charge densities (σ) are divided into 61 discrete bins of σ ranging between −0.03 to +0.03 e/Å2 ; now it is divided into 71 discrete bins of −0.035 to +0.035 e/Å2 . Each bin in the hydrogen-bonding (hb) σ-profile is multiplied (reduced) by its PHB(σ) value, and the portion of the hb σ-profile thus removed is added back into the corresponding bin of the non-hb σprofile. In this way, the overall σ-profile is conserved. From a physical standpoint, this approach allows the model to limit hb interactions to a certain fraction of the available hb donor and acceptor segments. The greater the magnitude of σ for a given segment, the more likely that segment is to exhibit hb. This provides a physically reasonable alternative to merely limiting hb to segments whose σ value exceeds an arbitrary threshold. With this explanation the expressions for hb σ-profile and nonhb σ-profile will be eqs 17−19. pihb (σ ) =

2 rn2 + reff 2 dmn

2 rn2reff 2 2 rn + reff

pinhb (σ ) =

(14)

where reff = (aeff/π)1/2, is the radius of the standard surface segment, rn = (an/π)1/2, is the radius of the segment n and dmn is the distance between segment m and n. σ and σ* are charge density after and before the charge averaging process. However, the pairwise interaction model is based on the assumption of independent segments, thus the charge density averaging process should be consistent with the assumption. For that, the original σ-averaging expression (eq 14) was slightly modified and an empirical parameter fdecay is introduced.29 The value of fdecay is 3.57. Other notations carry the same meaning as previous equations.26−29 The modified expression is given by eq 15.

Aihb(σ ) HB P (σ ) Ai Ainhb(σ ) A hb(σ ) + i [1 − P HB(σ )] Ai Ai

pi (σ ) = pihb (σ ) + pinhb (σ )

(17)

(18) (19)

It should be noted that our COSMO-RS model is merely a reimplementation of the earlier model.29 In our earlier results we have not modified the hydrogen bonding as we assume a cut off density for hydrogen bonding, that is, 0.0084 e/Å2. Thus, in the modification this cut off charge is removed and a Gaussian type distribution is adopted. The predicted tie lines are compared with experimental tie lines in Figures 7 and 8. The COSMO-RS predicted tie line compositions gave RMSD deviation of 18.67% for the system containing [TDTHP][DCA]. Similarly RMSD for the system containing [TDTHP][DEC] was 16.21%. F



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0.12%−0.55% for all the systems. Further the quantum chemical-based model COSMO-RS gave an RMSD of 18.67% and 16.21% for the systems containing the ILs [TDTHP][DCA] and [TDTHP][DEC], respectively.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing 1H NMR spectra in both phases for both ILs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 361 2582266. Fax: +91 361 2582291. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express deep gratitude to the Central Instrumental Facility (CIF) Centre for use of the 1H NMR facility.

Figure 7. Experimental and COSMO-RS predicted tie lines for the ternary system [TDTHP][DCA] (1)−1-butanol (2)−water (3) at T = 298.15 K and p = 1 atm.

Figure 8. Experimental and COSMO-RS predicted tie lines for the ternary system: [TDTHP][DEC] (1)−1-butanol (2)−water (3) at T = 298.15 K and p = 1 atm.

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CONCLUSIONS 1-butanol separation using [TDTHP][DCA] and [TDTHP][DEC] is superior as compared to imidazolium cations separation. The hydrogen bond between anion and butanol influence the IL−water miscibility resulting in two separate phases. The IL phase in the [TDTHP][DEC] system contains more water as compared to IL in [TDTHP][DCA]. This was due to the higher elctronegativity of the oxygen atom, which forms a strong hydrogen bond with water. Both the hydrophobic phosphonium-based anions are lighter than water as compared to imidazolium and ammonium-based ILs, hence reducing pumping cost. The partition coefficient is much higher than values reported by imidazolium-based ILs thereby resulting in reduced material cost due to the requirement of less solvent. The separation factor approaches infinity confirming a wider separation range. The excess Gibb’s free energy models namely NRTL and UNIQUAC gave rmsd values within G

NOMENCLATURE Aij = UNIQUAC interaction parameter between component i and j aeff = area of the standard surface segment, Å2 an = area of the segment n, Å2 c = number of components in the LLE system dmn = distance between segment m and n, Å EI = extract phase flow rate, kmol/h F = feed flow rate, kmol/h fdecay = empirical parameter Gji/gji = average interaction energy for the interaction of molecules of component j with molecules of component i Hi = peak area under NMR spectra of species i Kp/Ki = partition coefficient li = Staverman−Guggenheim combinatorial term parameter m = number of tie lines Obj = objective function PHB(σ) = Gaussian type probability over charge density distribution qi = normalized surface area parameter for the Staverman− Guggenheim combinatorial term R = universal gas constant, JK−1mol−1 RII = raffinate phase flow rate, kmol/h reff = radius of the standard surface segment, Å ri = normalized volume parameter for the Staverman− Guggenheim combinatorial term rn = radius of the segment n, Å SF = separation factor T = temperature, K w = weight factor xIi = mole fraction of component i of phase I in the LLE system x̂Ii = mole fraction of component i of phase I in the LLE system predicted by model Z = coordination number (=10) zi = feed concentration of component i [Im10.1][tcb] = 1-decyl-3- methylimidazolium tetracyanoborate [Im10.1][ntf2] = 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide dx.doi.org/10.1021/ie500833h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX


Article

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[Mo10.1][tcb] = 4-decyl-4-methylmorpholinium tetracyanoborate [Mo10.1][ntf2] = 4-decyl-4-methylmorpholinium bis(trifluoromethylsulfonyl)imide COSMO-RS = conductor like screening model for real solvents COSMO-SAC = conductor like screening model with segment activity coefficient GA = genetic algorithm GEPOL = generating polyhedra rmsd = root mean square deviation [TDTHP][DCA] = trihexyl(tetradecyl)phosphonium dicyanamide [TDTHP][DEC] = trihexyl(tetradecyl)phosphonium decanoate [TDTHP][Phosph] = trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl) phosphinate α = NRTL nonrandomness parameter γi = activity coefficient of component i in a phase (extract or raffinate) predicted using the NRTL/UNIQUAC model σm = apparent charge density distribution of the segment m, e/Å2 σn* = original charge density distribution of the segment n, e/ Å2 θ = area fraction in UNIQUAC equation τij = NRTL interaction parameter between component i and j Φ = segment fraction in UNIQUAC equation Ψ = flow rate ratio of extract to feed

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