Aliphatic Extraction

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This paper reviews the aromatic/aliphatic separation wherein the aliphatic hydrocarbon considered is a cyclic hydrocarbon. The separation of aromatics from ...
Ionic Liquids For Aromatic/Aliphatic Extraction Rima C. Gajjar 1, Dr. Sachin P. Parikh2, and Priyank Khirsariya 3 1 V.V.P Engineering College, Rajkot, Gujarat, India. Email: [email protected] 2 Head of Department, Chemical Engineering, L. D College of Engineering, Ahmedabad, Gujarat, India. Email: [email protected] 3 V.V.P Engineering College, Rajkot, Gujarat, India. Email: [email protected]

Abstract. Petrochemical industries encounter many separations out of which the aromatic/aliphatic separation via Liquid-Liquid Extraction is one of the most studied separations. This paper reviews the aromatic/aliphatic separation wherein the aliphatic hydrocarbon considered is a cyclic hydrocarbon. The separation of aromatics from heavy naphtha, pyrolysis gasoline, light naphtha is carried out using an organic and conventional volatile solvent Sulfolane. This paper highlights the use of Ionic Liquids (ILs) which can replace Sulfolane as they are greener solvents with unique characteristics. This work also reviews various aromatic/aliphatic binary systems in which different ILs were employed to selectively extract the aromatic component. Keywords. ionic liquids, separation, aromatic/cycloalkane, extraction

1 Introduction Extraction of the aromatic hydrocarbons from petrochemical streams of light and heavy naphtha, pyrolysis gasoline and naphtha reformate is essential for prevention of cocking, and for production of aromatics. Use of Sulfolane in Shell UOP process for separation of Benzene, Toluene, Ethyl benzene and Xylene (BTEX) from aromatic/aliphatic type of a mixture faces several disadvantages. Sulfolane is an organometallic compound which is volatile and thereby environmentally harmful causing air pollution. The degradation of Sulfolane due to its interaction with oxygen makes it difficult to handle it. Immense research is important to replace the volatile organic solvents. There has been an emerging interest shown by researchers to find an alternative green solvent. Replacing Sulfolane with Ionic Liquids can turn out to be advantageous to selectively extract aromatic hydrocarbons from their mixture with aliphatic hydrocarbons. Ionic Liquids are a class of solvents with different characterstics. ILs are known as molten salts which have their melting point below 100 ⁰C. These are composed of a bulky asymmetric organic

cation and an organic or inorganic anion. ILs are non volatile, have good conductivity, wide liquidus range, good thermal stability, resistance to decomposition. Their physical and chemical properties can be altered by changing the combination of cation and anion, therefore they are also known thereby as designer solvents. Ionic Liquids having negligible vapour pressure do not cause air pollution. These are hence greener class of solvents. 2

Liquid-Liquid Extraction With Ionic Liquids

Use of ILs as solvents for extracting aromatic component from a mixture in naphtha stream, pyrolysis gasoline, and reformer gasoline is immensely studied. The hydrocarbon mixture containing aromatics is mixed with IL and stirred. And the biphasic separation is observed. The extract layer contains the aromatic hydrocarbon and the IL. The raffinate phase contains the aliphatic hydrocarbon. The separation of the IL from the aromatic hydrocarbon can be done by evaporation. Thus the solvent can be recycled and reused. Selectivity and Distribution ratio of IL is high as compared with Sulfolane. Selectivity is defined as the ability of the IL to extract the aromatic component more as compared to the aliphatic component. Distribution ratio of the aromatic hydrocarbon is the ratio of the amount of aromatic hydrocarbon present in the extract phase to that in the raffinate phase. IL shows wider immiscibility region and can provide separation for wide range of feed composition unlike Sulfolane. 3

Aromatic/Aliphatic Separation With Ionic Liquids

Aromatic/n-alkane and aromatic/cycloalkane separations via LLE with ILs are an important separation focused on by researchers. A lot of work has been carried out in this field. IL ability to separate cycloalkanes has been tested and reported [Gonza´lez et al., 2010; Calvar et al., 2011; González et al., 2011; Gutierrez et al., 2011; Corderi et al., 2012; Sakal et al., 2014] which clarifies influence of the structures of aliphatic components on the separation power of ILs. As evident from the literature studies and data tabulated in Table 1, the solubility of cycloalkanes is higher than n-alkanes in Ionic Liquids as the ring structure of cycloalkane makes it more interactive with the Ionic Liquids aiding higher miscibility. The size of the immiscibility region increases when the cycloalkane is alkylated [Calvar et al., 2011; Gonza´lez et al., 2010] as the structure then approaches more towards aliphatic type adding to the aliphatic character of the cycloalkane. The selectivity of ionic liquids is higher for selectively extracting aromatics from the mixture of n-alkanes as compared to cycloalkanes as wider immiscibility is observed of the aromatic/n-alkane system with ILs. The size of the immiscibility region is affected by the structure of the Ionic Iiquids as per the choice of anion and cation [Gonza´lez et al., 2010; González et al., 2011; Gutierrez et al., 2011; Calvar et al., 2011] The

literature data of few aromatic/cycloalkane systems along with different ILs tested are listed at various Temperature (T) and atmospheric pressure. Its selectivity (S) and aromatic distribution ratio (D2) are also listed.

Table 1. Aromatic/cycloalkane LLE data from literature. Sr. No

System

Ionic liquid

T (k)

S

D2

0.107

4.47

0.38

0.186

4.47

0.32

0.110

6.50

0.24

0.089

7.86

0.96

Corderi et

0.101

11.88

1.24

al., 2012

0.111

9.08

0.68

Calvar et

0.040

9.78

1.65

al., 2011

298.15

0.030

11.55

2.16

313.15

0.032

9.19

1.84

339.15

0.065

4.04

0.39

0.066

4.92

0.12

González

0.140

7.37

0.09

et al.,

ethylbenzene/Methylcyclohexane

0.143

5.73

0.15

2011

Benzene/ Cyclohexane

0.054

19.85

0.63

Gonza´lez

0.057

39.71

0.63

et al.,

Benzene/ Cyclooctane

0.101

44.50

0.54

2010

Benzene/ Methylcyclohexane

0.045

75.09

1.02

Toluene/Methylcyclohexane

0.058

37.48

0.52

0.066

19.46

0.33

293.15

0.0942

17.76

-

313.15

0.0982

12.78

-

333.15

0.1024

10.26

-

293.15

0.0882

14.86

-

[hmim][NTf2] 1

Toluene/Cyclohexane

[hmim][TfO]

298.15

[bmim][TfO] 2

Toluene/Cyclohexane

3

benzene/Cyclohexene

4

5

6

7

Benzene/Cyclohexane

ethylbenzene/Cyclohexane

Benzene/Methylcyclohexane

Ethylbenzene/

[emim][NTf2] [empy][NTf2] [bmim][MSO] [Bmim][NTf2]

[bmim][FeCl4]

[EMim][ESO4]

[emim][ESO4]

[empy][ ESO4]

298.15

298.15

298.15

298.15

298.15

Methylcyclohexane

8

Raffinate

toluene/ methylcyclohexane

[bmim][TCB]

[hmim][TCB]

composition

Reference

Corderi et al., 2012

Sakal et al., 2014

Gonza´lez et al., 2010

Gutierrez et al., 2011

4

313.15

0.0915

11.03

-

333.15

0.0964

8.75

-

Conclusions

Ionic Liquids are a greener class of solvents which can be used for aromatic/aliphatic separations in replacement of Sulfolane. They provide higher selectivity and distribution ratio with ease of recovery of solvent. This paper states few aromatic/cycloalkane systems and the ILs which are employed at different conditions of temperature and atmospheric pressure. The particular property of tailor making the IL is advantageous as tuning the physical and chemical properties can be easier. More work can be done by applying binary mixture of ILs for the separation of aromatic/aliphatic mixture. Acknowledgments We are thankful to V.V.P Engineering College, Rajkot, Gujarat, India and to Dr. Dharamashi Rabari, Ahmedabad University, Ahmedabad, India for their support during this research. References

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