n-heptane separation by extractive distillation with

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Toluene/n-heptane separation by extractive distillation with tricyanomethanide-based ionic liquids: experimental and CPA EoS modelling Pablo Navarro, Miguel Ayuso, André M. Palma, Marcos Larriba, Noemí Delgado-Mellado, Julian Garcia, Francisco Rodríguez, Joao A.P. Coutinho, and Pedro Jorge Carvalho Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Industrial & Engineering Chemistry Research

1

Toluene/n-heptane separation by extractive distillation

2

with tricyanomethanide-based ionic liquids: experimental

3

and CPA EoS modelling

4

Pablo Navarroa,*, Miguel Ayusob, André M. Palmaa, Marcos Larribaa,b, Noemí Delgado-

5

Melladob, Julián Garcíab, Francisco Rodríguezb, João A.P. Coutinhoa, Pedro J. Carvalhoa a

CICECO – Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, Aveiro, Portugal. b Department of Chemical Engineering, Complutense University of Madrid, E–28040 Madrid, Spain.

6 7 8 9 10 11

Abstract

12

This work covers the phase equilibrium characterization of systems containing n-heptane,

13

toluene, and two tricyanomethanide-based ionic liquids (ILs), namely 1-ethyl-3-

14

methylimidazolium tricyanomethanide ([C2C1im][TCM]) and 1-butyl-4-methylpyridinium

15

tricyanomethanide ([4-C4C1py][TCM]). Aiming at evaluating these ILs for the n-

16

heptane/toluene extractive distillation, the vapor-liquid-liquid equilibria (VLLE) and vapor-

17

liquid equilibria (VLE) were determined by Headspace - Gas Chromatography (HS-GC) in

18

the whole composition range at temperatures from 323.2 to 423.2 K and solvent to feed (S/F)

19

ratios of 1, 5, and 10. Experimental results were modeled with the Cubic Plus Association

20

(CPA) Equation of State (EoS). ILs’ molecular parameters were regressed through density

21

and heat capacity data and further used to describe the binary and ternary mixtures phase

22

equilibria. Extractive distillation with ILs stands as a powerful approach for the

23

dearomatization of liquid fuels, whereas the combination of HS-GC methodology and CPA

24

EoS has been revealed as an ideal strategy to further exploring this new technology.

25

Keywords: Ionic liquids; aromatic/aliphatic separation; extractive distillation; HS-GC; CPA

26

EoS.

27 28

*

To whom correspondence should be addressed. E-mail address: [email protected] (Pablo Navarro).

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1. Introduction

30

The separation of compounds with close boiling points is one of the most relevant challenges

31

in the oil industry. Accordingly, the design of new separation processes using new

32

configurations and solvents stands as an important issue to enable better technologies and

33

increase purity standards. Among all separations in the oil industry, the aromatic separation

34

from refinery streams, i.e. reformer and pyrolysis gasolines, is one of the most challenging

35

due to the close boiling points of the compounds in these streams, which translates into high

36

energy demanding steps in current-technologies based on volatile organic compounds such as

37

sulfolane [1].

38

Aromatic/aliphatic separation using ionic liquids (ILs) has received special attention by the

39

research community aiming at developing more efficient processes with lower environmental

40

impact at milder conditions [2]. ILs are considered designer solvents due to the wide variety

41

of species that can be synthesized through their anion and cation combinations [3].

42

Furthermore, the ILs design can be tuned aiming at achieving desirable properties for specific

43

separations or applications. Additionally, ILs present outstanding properties that provide an

44

advantage when compared to other conventional solvents, like negligible vapor pressures and

45

wide liquid ranges, the latter due to their low melting points [4], and relatively high thermal

46

stability [5].

47

The development of the aromatic/aliphatic separation by liquid-liquid extraction using ILs has

48

been proved technically feasible, although several limitations, regarding operating costs, have

49

been identified [6-10]. The extractor inability to separate and purify aromatics and aliphatics

50

demands further separation and purification steps at low vacuum conditions [9,10]. The use of

51

an extractive distillation configuration, instead of a liquid-liquid extraction, allows to separate

52

and purify aromatics and aliphatics with proven advantages [11].

53

Extractive distillation using ILs stands out as an alternative to the liquid-liquid extraction with

54

improved aromatic/aliphatic separation [11-17] because of the feasibility of achieving

55

specifications with a single separation unit [18] and no solvent losses (negligible vapor

56

pressure of ILs [11]). Furthermore, the only additional separation required is that of the

57

aromatic/IL separation for the residue stream, but a simple flash distillation unit, operating

58

under vacuum conditions, or a stripping column can be envisioned [9,18-20].


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59

In a recent work, the transferability between these two technologies has been explored. Then,

60

ILs were screened aiming at maximizing the extractive properties (capacity or/and

61

selectivity), the solvent thermal stability and minimizing the viscosity. As reported, the IL

62

capacity stands as the main enhancer for the n-heptane/toluene relative volatility for

63

heterogeneous extractive distillation, whereas selectivity had the leading role for

64

homogeneous extractive distillation. Furthermore, the temperature has an important impact on

65

the vapor/liquid distribution; with low temperatures establishing high capacity mass agents as

66

the best candidates and the opposite trend observed for high temperatures [21].

67

Tricyanomethanide-based ILs were previously reported, within the 323.2 – 403.2 K

68

temperature range, as the most efficient entrainers, leading to the highest n-heptane/toluene

69

mean relative volatility, high thermal stability (round 450 K for long-term scenarios), and the

70

lowest viscosities among all the studied ILs [2,22-24].

71

In this work, the VLE and/or VLLE was evaluated for binary and ternary systems in the

72

whole hydrocarbon composition range for the ionic liquids 1-ethyl-3-methylimidazolium

73

tricyanomethanide, [C2C1im][TCM], and 1-butyl-4-methylpyridinium tricyanomethanide, [4-

74

C4C1py][TCM], at temperatures ranging from 323.2 to 423.2 K, exploring S/F ratios of 1, 5,

75

and 10 on the ternary systems. Experimental data was determined by HS-GC methodology

76

and the CPA EoS was selected as a suitable EoS to describe the phase equilibria of the

77

systems. To obtain a proper selection of a coarse-grained model capable of representing most

78

of the physical features of the compounds, density (ρ) and heat capacity (CP) at 0.1 MPa were

79

used to independently determine the ILs molecular parameters.

80

2. Experimental section

81

2.1. Chemicals

82

The ILs 1-ethyl-3-methylimidazolium tricyanomethanide, [C2C1im][TCM], and 1-butyl-4-

83

methylpyridinium tricyanomethanide, [4-C4C1py][TCM], were acquired from Iolitec GmbH

84

with purities higher than 98 wt. %. The ILs were further purified through a drying procedure

85

under vacuum (50 Pa) and moderate temperature (313 K). Toluene and n-heptane were

86

supplied by Sigma-Aldrich with purities higher than 99.5 wt. % and were kept into their

87

original vessels over 3 Å molecular sieves. Additional details for the chemicals used can be

88

found in Table 1.



89 90

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Table 1. Compound description, supplier, water content, mass fraction purity and supplier of the studied compounds. Chemical

Supplier

Purity in wt. %

Water content in ppm

1-ethyl-3-methylimidazolium tricyanomethanide [C2C1im][TCM]

Iolitec GmbH

98

< 300

1-butyl-4-methylpyridinium tricyanomethanide [4-C4C1py][TCM]

Iolitec GmbH

98

< 300

n-heptane

Sigma-Aldrich

99.7

anhydrous

toluene

Sigma-Aldrich

99.5

anhydrous

91 92

2.2. Determination of VLE/VLLE

93

The VLLE and VLE data were determined by HS-GC technique as fully detailed in previous

94

works [21, 25]. The mixtures were prepared by mass using a Mettler Toledo XS205 balance,

95

with a precision of 10-5 g, in 20 mL sealed glass vials. Each mixture point was prepared in

96

duplicate and the phases allowed to reach equilibrium for a period never smaller than 2 hours.

97

Equilibration time was experimentally checked to be lower than 2 hours by monitoring the

98

evolution of the peak areas with equilibrium time.

99

The vapor phase was sampled by an Agilent 7697A Headspace injector and analyzed with an

100

Agilent 7890A GC. After achieving equilibrium, the vapor phase was analyzed by GC using

101

the response factor method to correct the compositions. For the mixtures presenting two

102

phases, a liquid and a vapor phase, the liquid mole fractions (xi) were determined by mass

103

balance knowing the mixture point compositions (zi) and the vapor phase composition:

104

xi =

zi F − ( piVG RT ) 3

∑( z F − ( pV i

i G

(1)

RT ) )

i=1

105

where F denotes the molar amount of the feed, VG is the headspace volume of the vial, and R

106

is the ideal gas law constant. Partial pressures (Pi) of n-heptane and toluene were obtained

107

together with the vapor phase composition using the relationship between the peak areas of

108

the hydrocarbons (Ai) and the peak areas developed by each hydrocarbon alone in the same

109

conditions (Ai0):

110

pi =

pi0 Ai Ai0

(2)


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111

where pi0 refers to the vapor pressure of each pure hydrocarbon, taken from the literature [26].

112

Thus, the total pressure was calculated as the sum of the partial pressures.

113

For the VLLE measurements, similar to the VLE experiments, the mixture points were

114

prepared in duplicate and the phases were allowed to reach equilibrium for a period never

115

smaller than 2 hours. The mixture gas phase was then sampled with the Agilent 7697A

116

Headspace injector and analyzed using the Agilent 7890A GC. The bottom liquid phase was

117

sampled with an insulin syringe (BD Micro-Fine, 0.3 mL) and analyzed by multiple

118

headspace extraction (MHE) technique. The details and background of the MHE method are

119

widely explained in a previous work [23]. Knowing the compositions of the vapor and IL-rich

120

liquid phase, the composition of the hydrocarbon-rich liquid phase was determined by mass

121

balance.

122

2.3. Determination of heat capacity at constant pressure

123

The measurement of the heat capacity at constant pressure (CP) for the [4-C4C1py][TCM] IL

124

was performed by differential scanning calorimetry using a Mettler Toledo DSC821e. A

125

sample of (25 ± 1) mg was placed in triplicate into 40 µL stainless steel pans and under an

126

inert atmosphere of nitrogen. The IL drying was done in situ to avoid the water impact on the

127

measurements. Further details of the methodology can be found in a previous work [24].

128

3. CPA EoS modelling

129

Kontegeorgis et al. [27,28] proposed a simplified CPA EoS version combining the Soave-

130

Redlich-Kwong (SRK) physical contribution [29,30] together with an association term

131

originally theorized by Wertheim that incorporates intermolecular interaction, i.e. hydrogen

132

bonding and solvation, and is also frequently used in different association models (SAFT-type

133

EoS) [31-33]. The CPA EoS approach is expressed in terms of compressibility factor (Z) as

134

follows:

135

Z = Z phys + Z assoc =

136

where a and b are energy and co-volume parameters, respectively, for the physical term,

137

whereas g refers to a simplified hard-sphere radial distribution, and XAi is the mole fraction of

1 1 ln ∂g  aρ − − 1 + ρ ∑ xi ∑i (1 − X Ai ) ∂ρ  i 1 − bρ RT (1 + bρ ) 2 

(3)



Page 6 of 25

138

component i not bonded at site A for the association contribution. The parameter a is given by

139

a Soave-type temperature-dependent relation [29]:

140

a (T ) = a0 [1 + c1 (1 − Tr 0.5 )]2

141

where a0 and c1, together with the temperature-independent b, are the three parameters needed

142

to define non-association compounds. These parameters are regressed from the properties of

143

the pure component. Here, the three parameters for n-heptane and toluene were taken from

144

literature where they were regressed from their vapor pressures and liquid densities [34]. The

145

non-volatility character of ILs makes the existence of vapor pressures and saturated liquid

146

densities inexistent. Thus, aiming to propose molecular parameters for the studied ILs with

147

sound physical meaning, the CPA parameters were determined by regressing the EoS against

148

atmospheric density and heat capacity data.

149

Regarding the association contribution, here explored for the ILs, XAi is mathematically

150

related to the association strength (∆AiBj) between sites from two different molecules and is

151

defined with the next set of equations:

152

X Ai =

153

   ε AiBj  AiBj ∆AiBj = g ( ρ ) exp   a − 1 bij β  RT   

154

g(ρ ) =

155

where εAiBj and βAiBj are the association energy and volume for a pure compound, respectively.

156

These two association parameters are commonly regressed in the same way as explained

157

before and together with the three cubic parameters comprise the required parameters for a

158

pure association compound. In order to apply CPA EoS to experimental systems, a molecular

159

model, able to represent the basic physical features of the compound, must be proposed.

160

Although, n-heptane and toluene are considered non-associative compounds [34],

161

imidazolium-based ILs have been reported as associative compounds with two association

162

sites (Scheme 2B) [35-38]. Furthermore, Oliveira et al. [37] reported that the alkyl chain

163

length and the cation nature (pyridinium or imidazolium) showed no influence in the

(4)

1

(5)

1 + ρ ∑ j x j ∑ Bj X Bj∆ AiBj

1 1 ; η = bρ 1−1.9η 4

(6)

(7)


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164

association scheme while Maia et al. [38] proposed the same association scheme (scheme 2B)

165

for

166

bis(trifluoromethylsulfonyl)imide

167

bis(trifluoromethylsulfonyl)imide, in their modeling using the CPA EoS; based on these

168

works the 2B scheme was adopted for the ILs evaluated here.

169

Considering mixtures, a and b are calculated following the well-known vdW-1f mixing rules:

170

a = ∑ i ∑ j xi x jaij ; aij = ai aj (1 − kij )

(8)

171

b = ∑i xibi

(9)

172

where kij are the binary interaction parameters. These are the only adjustable binary

173

parameters applied in this work.

174

4. Results and discussion

175

4.1. CPA EoS molecular parameters for tricyanomethanide-based ILs

176

Both heat capacity and atmospheric density data were regressed by CPA EoS; the results are

177

reported in Figure 1 (set 1). Density for both ILs was obtained from a recent work in which an

178

exhaustive comparison with other literature sources was done [39]. On the other hand, heat

179

capacities for [C2C1im][TCM] were taken from literature [24], whereas this is the first work

180

reporting heat capacities for [4-C4C1py][TCM], collecting the new data in Table S1 in the

181

Supporting Information.

182

The regressed CPA EoS molecular parameters allow a good description of the properties with

183

%ARD below 1% for ρ and CP in the corresponding temperature ranges. The IL vapor

184

pressures predicted using the set 1 for the molecular parameters are of 76.3 and 35.4 kPa at

185

443.2 K (near thermal decomposition [22,24]) for [C2C1im][TCM] and [4-C4C1py][TCM] ILs,

186

respectively. However, as reported by Emel’yanenko et al. [40] and Rocha et al. [41], values

187

ranging from 0.05 to 0.2 Pa are expected for imidazolium-based ILs near 450 K. Thus, the

188

CPA EoS molecular parameters from Set 1 will have an important impact in the VLLE

189

description.

other

imidazolium-based

ILs,

namely and

1-ethyl-3-methylimidazolium 1-butyl-3-methylimidazolium


1.10 1.09 1.08 1.07 1.06 1.05 1.04 1.03 1.02 1.01 1.00 0.99

Exptl [C2C1im][TCM] Exptl [4-C4C1py][TCM] CPA (Set 1) CPA (Set 2)

CP /J.g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ρ /gcm-3


2.1

Page 8 of 25

Exptl [C2C1im][TCM] Exptl [4-C4C1py][TCM] CPA (Set 1) CPA (Set 2)

2.0 1.9 1.8 1.7 1.6 1.5 1.4 290

280 290 300 310 320 330 340 350 360 370

300

310

320

330

340

350

360

370

380

T /K

T /K

190 191 192

Figure 1. Density (ρ) and heat capacity (CP) at 0.1 MPa as a function of temperature: experimental and CPA EoS modelling.

193 194

Table 2. CPA EoS molecular parameters, deviations for ρ and CP and critical properties for [C2C1im][TCM] and [4-C4C1py][TCM] ILs IL

[C2C1im][TCM]

[4-C4C1py][TCM]

CPA EoS parameters (Scheme 2B) SET 1

SET 2

p (443 K) /kPa

76.3

a0 / Pa·m6·mol-2

0.4532

-1

0

4

3

10 b / m ·mol

SET 1

SET 2

0.001

35.4

0.001

6.350

0.3707

8.320

1.685

1.804

2.080

2.245

c1

5.985

1.504

6.816

1.362

102 β

1.791

2.791

1.634

2.469

-1

1.293

1.507

1.770

1.535

ρ (0.1 MPa)

0.0950

0.981

0.0657

0.938

CP

0.997

0.377

0.976

0.000100

4

10 ɛ / kJ·mol

ARD %

Critical properties [42] Tc /K

1149.4

1165.0

Pc /MPa

2.460

1.785

w

0.8509

0.9100

195

Therefore, aiming at obtaining a better description of the VLLE of the systems evaluated a

196

second regression of the ILs molecular parameters (Set 2) was performed limiting the IL

197

vapor pressure to be lower than 1 Pa, ensuring thus, the non-volatile character of the ILs in the

198

CPA EoS modelling and keeping a reasonable description for density as detailed below. This

199

approach lead to a deterioration of the density description at atmospheric pressure but

200

improves the description of the CP data. Nonetheless, the deviations found for density are

201

analogous to those reported for CPA EoS in previous works [38]. Energy parameter

202

completely conditions both vapor pressure and temperature dependency on temperature;

203

hence, it is necessary to fulfill an adequate vapor pressure before improving density


Page 9 of 25

205

taken from the literature [42] and average relative deviation (%ARD) obtained from the CPA

206

EoS description of the ρ and CP data are reported in Table 2. However, only those providing

207

negligible vapor pressure were used in the phase equilibria description.

208

4.2. VLE and VLLE for binary systems composed of n-heptane, toluene and

209

tricyanomethanide-based ILs: experimental vs CPA EoS description

210

The VLE for the n-heptane + toluene binary system taken from literature [42] was modelled

211

with the CPA EoS for temperatures ranging from 323.2 to 423.2 K, as depicted in Figure 2.

212

The results show the ability of CPA EoS to describe correctly the binary system within the

213

studied temperatures and compositions range using k12 values with a small temperature

214

dependency as reported in Table 3. p/ kPa

description. Both CPA EoS molecular parameters sets along with estimated critical properties

p/ kPa

204

20 T/ K = 323.2 18

80 T/ K = 363.2

75 70

16

65 14

60

12

55

10

50 0.0

0.2

0.4

0.6

0.8

1.0

x1,y1 p/ kPa

215 p/ kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 T/ K = 403.2 230

0.0

0.2

0.4

0.6

0.8

0.6

0.8

1.0

x1,y1

400 T/ K = 423.2 370

210

340

190

310

170

280 250

150 0.0

0.2

0.4

0.6

0.8

1.0

x1,y1

0.0

0.2

0.4

1.0

x1,y1

216 217

Figure 2. VLE data of n-heptane + toluene binary system. Symbols denote data from ref 42

218

and solid lines the CPA EoS with k12 reported in Table 3.

219

The VLE/VLLE of the binary systems composed of n-heptane or toluene with

220

[C2C1im][TCM] and [4-C4C1py][TCM] were determined in this work, at temperatures

221

between 323 and 423 K, by HS-GC methodology and the data are depicted in Figure 3 and

222

reported in Tables S2 and S3, in the Supporting Information.



Table 3. CPA EoS binary interaction parameters (kij) for the n-heptane (1) + toluene (2) + IL (3) system. k12 [C2C1im][TCM]

[4-C4C1py][TCM]

[C2C1im][TCM]

[4-C4C1py][TCM]

323.2

0.013

0.000

0.040

-0.045

-0.040

363.2

0.010

0.010

0.045

-0.045

-0.035

403.2

0.007

0.020

0.055

-0.035

-0.020

423.2

0.004

0.020

0.060

-0.030

-0.010

450 400 350 300 250 200 150 100 50 0

[C2C1im][TCM]

450 400

k23

300 [C2C1im][TCM] 250 200 150 100 50 0

0.0

225

k13

p/ kPa

T/ K

0.2

0.4

0.6

0.0

0.8 x 1.0 n-heptane

p/ kPa

p/ kPa

223 224

p/ kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

[4-C4C1py][TCM]

350 300

0.2

0.4

0.6

0.8

0.6

0.8

1.0 xtoluene

300 [4-C4C1py][TCM] 250 200

250

150

200 150

100

100

50

50 0

0 0.0

0.2

0.4

0.6

0.8

1.0

xn-heptane

0.0

0.2

0.4

1.0

xtoluene

226 227 228 229 230

Figure 3. VLE/VLLE data of {n-heptane or toluene + IL} binary system. Symbols denote experimental data at , 323.2 K; , 363.2 K; , 403.2 K and , 423.2 K and the solid lines the CPA EoS with ki3 listed in Table 3. {n-heptane or toluene + [4-C4C1py][TCM]} experimental data, at 323.2 K and 363.2 K, were taken from Ref. 22.

231

The shape of the hydrocarbon + IL p-x curves is well-known and consists in the two different

232

regions, namely VLE and VLLE. An increase in pressure is observed as the hydrocarbon

233

mole fraction increases up to the maximum hydrocarbon solubility in the IL (VLE), whereas a

234

VLLE region, with constant pressure, is observed for higher hydrocarbon concentrations. In

235

the wide temperature range analyzed, the liquid-liquid region is weekly dependent on

236

temperature, in line with Hansmeier et al. [43] results. The CPA EoS was used for the

237

experimental phase equilibria description of these binary systems, with the required binary

238

interaction parameters reported in Table 3. CPA EoS correctly describes the n-heptane or

239

toluene + IL systems with a small temperature dependent binary interaction parameter.


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240

4.3. VLE/VLLE for n-heptane + toluene + IL ternary systems: experimental and CPA

241

EoS description

242

The VLE/VLLE was determined for n-heptane + toluene + IL ternary mixtures at 323.2,

243

363.2, 403.2 and 423.2 K. The hydrocarbons binary mixture composition was studied within

244

all composition range, whereas the solvent to feed ratio was evaluated from 1 to 10; as this

245

range covers that of interest for the envisioned separation [21]. The VLE/VLLE data for

246

ternary systems are reported in Tables S4 to S7, in the Supporting Information.

247

4.3.1. VLE/VLLE temperature dependency for S/F = 10. As can be seen in Figure 4 and

248

Figure 5, two behaviors can be identified: for temperatures up to 363.2 K both VLLE and

249

VLE regions are present; while only VLE is observed for the highest temperatures. Aiming at

250

clarifying the phases present, a scheme is shown in inset on the p-xy diagrams where black

251

and gray stand for the liquid phases and white for the gas phase. For temperatures up to 363 K

252

a homogeneous (VLE) region that increases with temperature, is observed for n-heptane mole

253

fractions below its solubility limit. The slope of the p-xy diagram decreases as temperature

254

increase, showing a moderate negative impact in the n-heptane/toluene separation.

255

On the other hand, ternary phase diagrams, depicted in Figure 6, were selected to show the

256

LLE region. As depicted, both ILs present a large immiscibility zone with a hydrocarbon-rich

257

phase with negligible amounts of IL, and an IL-rich phase with a small amount of

258

hydrocarbon, denoting the high capacity and selectivity of the studied ILs. As the temperature

259

increases the liquid-liquid region decreases mostly by increasing the amount of hydrocarbon

260

in the IL-rich phase while the hydrocarbon-rich phase remains free of IL.

261


y1

19

Page 12 of 25

1.0 0.8

14

0.6 0.4

9

0.2

T/ K = 323.2

4 0.0

0.2

0.4

0.6

0.0 0.8

1.0

80

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

x'1,y1

y1

262 p/ kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

p/ kPa


x'1

1.0 0.8

60

0.6 0.4

40

0.2

T/ K = 363.2

0.0

20 0.0

263

0.2

0.4

0.6

0.8

1.0

x'1,y1

x'1

264

265 266 267 268 269

Figure 4. VLE p-xy and y-x diagrams for the n-heptane (1) + toluene (2) + [C2C1im][TCM] (3) ternary system with S/F = 10. Symbols: black denotes IL-rich liquid phase, grey hydrocarbon-rich liquid phase, and white vapor phase. Lines: solid represents the CPA EoS with ki3 reported in Table 3 and dashed denotes several scenarios with different ki3 values.


y1

19

1.0 0.8

15

0.6 11 0.4 7

0.2

T/ K = 323.2

3 0.0

0.2

0.4

0.6

0.0 0.8

1.0

x'1,y1

y1

270 p/ kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


p/ kPa

Page 13 of 25

80 70

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

x'1

1.0 0.8

60 50

0.6

40

0.4

30 0.2

20

T/ K = 363.2

10 0.0

0.2

0.4

0.6

0.0 0.8

1.0

x'1,y1

x'1

271

272 273 274 275 276

Figure 5. VLE p-xy and y-x diagrams for the n-heptane (1) + toluene (2) + [4-C4C1py][TCM] (3) ternary system with S/F = 10. Symbols: black denotes IL-rich liquid phase, grey refers to hydrocarbon-rich liquid phase, and white represents vapor. Lines: solid represents the CPA EoS wth ki3 from Table 3 and dashed denotes several scenarios with different ki3 values.



0.00 1.00

0.00 1.00

0.25

0.25

ep t an

e tan

n-H

ep n-H

0.25

0.50

0.00 1.00

0.75

0.50

0.75

0.25

1.00

T/ K = 363.2

0.50

0.25

1.00 0.00

0.25

0.50

0.00 1.00

0.00 1.00

0.25

0.25

ep t an n-H

ep

tan

e

ne

0.50

0.25

0.50

0.75

[4-C4C1py][TCM]

0.00 1.00

0.50

0.75

0.25

1.00

T/ K = 363.2

0.50

0.25

1.00 0.00

ne

lue

T/ K = 323.2

0.50

lue To

To

n-H

0.75

e

0.75

0.75

0.00

0.00 1.00

0.75

[C2C1im][TCM]

[C2C1im][TCM]

277

ne

0.75

ne

0.50

lue To

lue

T/ K = 323.2

0.50

0.00

0.75

e

0.75

To

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

0.25

0.50

0.75

0.00 1.00

[4-C4C1py][TCM]

278 279 280 281

Figure 6. Ternary LLE diagrams from the VLLE data of n-heptane + toluene + IL ternary system with S/F = 10. Full symbols and solid lines denote experimental data and empty squares and dashed lines are the CPA EoS.

282

As shown the CPA EoS is able to provide an adequate description of the VLE/VLLE using

283

the binary interaction parameters obtained from binary systems. Overall, the CPA EoS

284

description of the p-xy and ternary phase diagrams, at 323.2 and 363.2 K, is quite accurate

285

with small deviation around 3%. Higher deviations are observed, nonetheless, in the p-xy

286

diagrams for temperatures above 403.2 K. These deviations are related to the deterioration of

287

the CPA EoS description with temperature, also observed in the binary systems. The

288

relationship between pressure and S/F ratio, depicted in Figure 7, deteriorates with

289

temperature, implying a pressure overestimation and poor description of the homogeneous

290

phase behavior observed above 403.2 K, in the whole composition range. By contrast, the y-x

291

diagram was properly described.


Page 15 of 25

293

403.2 K may be of interest only for the bottom steps, which are related to toluene high

294

concentration region. In this context, a slight modification of the k23, to properly describe the

295

toluene (2) + IL (3) mixture composition, and the k13, to provide a compensation in the

296

description between the equilibrium pressures and the vapor-liquid compositions (long-dashed

297

lines), allowing to obtain an adequate description of the VLE and VLLE and thus a better

298

process simulation (see Figure 4 and Figure 5). 450 400 350 300 250 200 150 100 50 0

p/ kPa

Considering the temperature profile in an extractive distillation column, temperatures up to

p/ kPa

292

n-heptane + [C2C1im][TCM]

300 toluene + [C2C1im][TCM]

250 200 150 100 50 0

0

10

20

30

40

0

50

10

20

30

40

50

S/F

450 n-heptane + [4-C4C1py][TCM]

400 350

p/ kPa

S/F

299 p/ kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 toluene + [4-C4C1py][TCM] 250 200

300 250

150

200

100

150 100

50

50

0

0 0

10

20

30

40

50

0

10

S/F

20

30

40

50

S/F

300 301 302 303 304

Figure 7. Pressure vs solvent to feed ratio (S/F) diagrams. Symbols denote experimental data at , 323.2 K; , 363.2 K; , 403.2 K; , 423.2 K and solid lines the CPA EoS with kij listed in Table 3. Experimental data for the {hydrocarbon + [4-C4C1py][TCM]} systems, at 323.2 K and 363.2 K, were taken from Ref. 22.

305

4.3.2. VLE/VLLE description as function of S/F ratio at 363.2 K. In order to assess the

306

suitability of the EoS for a wide range of conditions, other S/F ratios were evaluated. VLLE

307

data was determined for S/F ratios of 1 and 5, at 363.2 K. The experimental results are plotted

308

in Figure 8 (VLE, p-xy and y-x diagrams) and Figure 9 (ternary phase diagrams) together with

309

CPA EoS description using only the binary interaction parameters reported in Table 3. As can

310

be seen, the CPA EoS is able to provide a very good predictive description of the phase

311

equilibria.


y1

Page 16 of 25

1.0

80

0.8 60

0.6 0.4

40

0.2 [C2C1im][TCM], S/F = 1

20 0.0

0.2

0.4

0.6

0.0 0.8

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0

1.0

x'1,y1

x'1

y1

p/ kPa

312

1.0

80

0.8 60

0.6 0.4

40

0.2 [C2C1im][TCM], S/F = 5 0.0

20 0.0

0.2

0.4

0.6

0.8

1.0

x'1,y1

x'1

y1

p/ kPa

313

1.0

80

0.8 60

0.6 0.4

40

0.2 [4-C4C1py][TCM], S/F = 1

20 0.0

0.2

0.4

0.6

0.8

0.0 1.0

x'1,y1

y1

314 p/ kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

p/ kPa


80

x'1

1.0 0.8

60

0.6 0.4

40

0.2 [4-C4C1py][TCM], S/F = 5

0.0

20 0.0

315 316 317 318 319 320

0.2

0.4

0.6

0.8

1.0

0.0

x'1,y1

x'1

Figure 8. VLE p-xy diagrams from the VLE/VLLE data of n-heptane (1) + toluene (2) + IL (3) ternary system at 363.2 K with S/F ratio of 1 and 5. Black symbols denote IL-rich liquid phase, grey refer to IL-free liquid phase, and white symbols the vapor phase. Solid lines represent the CPA EoS prediction.


Page 17 of 25

0.00 1.00

0.25

0.00 1.00

0.25

0.75

0.25

ep tan e n-H

n -H

0.50

S/F = 5

0.50

0.50

0.75

ne

e en

0.50

0.75

lue To

ep ta n

e

0.75

lu To

0.25

S/F = 1 1.00 0.00

0.25

0.50

0.00 1.00 1.00 0.00

0.75

0.25

[C2C1im][TCM]

321

0.50

0.00 1.00

0.25

0.00 1.00

0.25

0.75

ta n e

0.25

ep n -H

0.50

S/F = 5

0.50

0.50

0.75

ne

0.50

0.75

lue To

ep t an e

0.75

ne

n-H

0.00 1.00

0.75

[C2C1im][TCM]

lue To

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25

S/F = 1 1.00 0.00

322

0.25

0.50

0.75

[4-C4C1py][TCM]

0.00 1.00 1.00 0.00

0.25

0.50

0.75

0.00 1.00

[4-C4C1py][TCM]

323 324 325

Figure 9. Ternary LLE phase diagrams of the n-heptane (1) + toluene (2) + IL (3) ternary systems at 363.2 K with S/F ratio of 1 and 5. Black symbols and continued lines denote experimental data. Empty squares and dashed lines are the CPA EoS.

326

Although the dependency of binary interaction parameters with the S/F ratio was evaluated

327

only for ternary systems at 363.2 K, this is implicit in the binary systems for all the

328

temperatures evaluated. Thus, one can claim that CPA EoS is suitable up to an S/F ratio of 10

329

and temperatures ranging from 323.2 to 423.2 K. Further remarks regarding the robustness of

330

the model for its practical implementation in a process simulator are given below (section 4.4)

331

through the analyses of the toluene/n-heptane relative volatilities in the presence of the ILs

332

and ILs’ extractive properties.

333 334



Page 18 of 25

335

4.4. Analysis of n-heptane/toluene relative volatility and ILs extractive properties

336

The tricyanomethanide-based ILs suitability for the n-heptane/toluene separation by extractive

337

distillation was shown in the phase equilibria analysis. However, the IL effectiveness is

338

quantitatively given by the n-heptane (1) relative volatility to toluene (2), which can be

339

calculated from the experimental data as follows:

340

α12 =

341

where K is the K-value, yi and xi denote the vapor and liquid phase mole fractions of

342

component i, respectively. The n-heptane/toluene relative volatility is determined along with

343

experimental VLE/VLLE data reported in Tables S3 to S12, in the Supporting Information,

344

and depicted in Figure 10. As can be seen, two trends can be identified: up to 363 K n-

345

heptane/toluene relative volatility remains, within experimental uncertainty, almost constant

346

within the homogeneous region, followed by an asymptotic decrease as n-heptane mole

347

fraction increases in the heterogeneous region. For temperatures higher than 363 K the n-

348

heptane/toluene relative volatility slightly decreases with increasing n-heptane mole fraction.

349

Furthermore, [C2C1im][TCM] presents a higher toluene/n-heptane selectivity enhancing thus

350

higher separation factors in the homogeneous region as discussed in a previous work [44];

351

however, [4-C4C1py][TCM] stands as a better mass agent in the heterogeneous regions due to

352

its higher toluene distribution ratios as previously reported [21]. The CPA EoS is able to

353

describe the relative volatilities and to mimic the reported effects, standing as an adequate

354

EoS to model these systems.

K1 y1 / x1 = K2 y2 / x2

(10)

355


Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


356 357 358 359 360

Figure 10. Values of n-heptane/toluene relative volatility calculated from experimental (symbol) and CPA EoS (lines) for n-heptane (1) + toluene (2) + IL (3) ternary systems as a function of the free-IL n-heptane mole fraction, x’1, in overall liquid, temperature and S/F ratio.

361

A comparison between calculated extractive properties from experimental data and CPA EoS

362

is presented in Figure 11. Hydrocarbon distribution ratios (Di) and toluene (2) /n-heptane (1)

363

selectivity (S2,1) were calculated as follows:

364

Di =

365

S 2,1 =

366

where wi are the mass fraction of component I and superscripts I and II refer to IL-free liquid

367

phase and IL-rich liquid phase, respectively. Toluene/n-heptane selectivity decreases and

368

toluene distribution ratio is hardly affected by toluene mole fraction. Regarding S/F ratio and

369

T effects, it is found that its impact on the toluene/n-heptane selectivity is low, whereas have

370

almost no influence in the toluene distribution ratios. Although is widely accepted that both

371

temperature and S/F ratio negatively impact in the extractive properties [8,43,45], Hansmeier

372

et al. [43] reported also that temperature hardly affects extractive properties in the toluene

373

separation from n-heptane by liquid-liquid extraction using cyano-based ILs in the

374

temperature range of (313.2 to 348.2) K. Furthermore, Larriba et al. [8] and Navarro et al.

375

[45] showed that both temperature and S/F ratio had a low impact in the extractive properties

376

using cyano-based ILs. Considering literature background, extractive properties calculated in

377

this work are consistent. Moreover, CPA EoS provides a good description for both extractive

378

properties, even considering the high toluene/n-heptane selectivity values for both ILs. In this

379

context, the deviations observed for toluene/n-heptane selectivity are admissible and, keeping

380

in mind a vapor-liquid separation perspective, they will not impact on the extractive

381

distillation column design.

wiII wiI

(11)

D2 D1

(12)



Page 20 of 25

382

383 384 385 386

Figure 11. Values of toluene/n-heptane selectivity and toluene distribution ratio calculated from the LLE data and CPA EoS for n-heptane (1) + toluene (2) + IL (3) ternary systems as a function of the free-IL toluene mole fraction, x’2, in overall liquid, temperature and S/F ratio.

387

Conclusions

388

New VLE/VLLE data for the binary and ternary systems composed of n-heptane, toluene, and

389

two ILs, [C2C1im][TCM] and [4-C4C1py][TCM], were measured in a wide range of

390

temperatures and S/F ratios aiming at evaluation the selected ILs as solvents for the toluene/n-

391

heptane extractive distillation. A VLLE region was observed at low temperatures and high n-

392

heptane concentrations where the IL is not capable to solubilize the whole n-heptane present

393

in the liquid phase. The results allow to establish the adequate conditions, homogeneous or

394

heterogeneous phases, for the extractive distillation.

395

CPA EoS was used to describe the experimental data and new molecular parameters for the

396

studied ILs, defined as associating molecules with scheme 2B, were proposed. CPA EoS was

397

able to simultaneously describe phase equilibria and the density and CP of the pure

398

compounds. Temperature-dependent binary interaction parameters were required to accurately

399

describe both VLLE and VLE regions in a wide range of temperatures (323.2 to 423.2) K and

400

S/F ratios (1 to 10). The proposed molecular parameters, molecular scheme and binary

401

interaction parameters were able to correctly describe the toluene/n-heptane relative

402

volatilities and extractive properties of the ILs.


Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


403

The high separation effectiveness using extractive distillation with tricyanomethanide-based

404

ILs in the whole range of conditions has shown the potential of the selected technology and

405

solvents. The high benzene concentration in real gasoline and naphtha will smooth operating

406

temperatures given in this work to operate under homogeneous conditions, whereas

407

heterogeneous extractive distillation does not seem to be an important drawback as separation

408

factors are high enough to ensure the separation, although the column would have certain

409

inefficient volumes.

410

To further explore the extractive distillation with ILs to separate aromatics from aliphatics in

411

other characteristic and more complex models, the HS-GC and CPA EoS is shown to be a

412

good combination of an experimental-modelling strategy.

413 414

Funding Sources

415

This work was developed in the scope of the project CICECO-Aveiro Institute of Materials,

416

POCI-01-0145-FEDER-007679 (Ref. FCT UID/CTM/50011/2013) funded by FEDER

417

through COMPETE2020 – Programa Operacional Competitividade e Internacionalização

418

(POCI) and by national funds through FCT – Fundação para a Ciência e a Tecnologia. The

419

authors are also grateful to Ministerio de Economía y Competitividad (MINECO) of Spain

420

and Comunidad Autónoma de Madrid for financial support of Projects CTQ2017–85340-R

421

and S2013/MAE-2800, respectively. P.N. and P.J.C. also thank FCT for awarding their

422

postdoctoral grant (SFRH/BPD/117084/2016) and contract under the Investigator FCT 2015

423

(IF/00758/2015), respectively. N.D.-M. also thanks MINECO for awarding her an FPI grant

424

(BES–2015–072855). M.L. thanks Ministerio de Eduación, Cultura y Deporte of Spain for

425

awarding him a José Castillejo postdoctoral mobility grant (CAS17/00018). All authors thank

426

Infochem-KBC because Multiflash program was applied for the CPA EoS calculations.

427 428

Supporting information. Heat capacity of [4-C4C1py][TCM] in Table S1, binary vapor-

429

liquid and vapor-liquid-liquid equilibria in Tables S2 to S3, ternary vapor-liquid and vapor-

430

liquid-liquid equilibria in Tables S4 to S7.

431



432

Page 22 of 25

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n-heptane separation by extractive distillation with

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