Optimum Performance of Extractive Desulfurization of Liquid Fuels

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Optimum Performance of Extractive Desulfurization of Liquid Fuels using Phosphonium and Pyrrolidinium-Based Ionic Liquids Omar U. Ahmed, Farouq Sabri Mjalli, Talal Al Wahaibi, Yahya Mansoor Al-Waheibi, and Inas Muen AlNashef Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01187 • Publication Date (Web): 05 Jun 2015 Downloaded from http://pubs.acs.org on June 15, 2015

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Industrial & Engineering Chemistry Research

1 2 3

Optimum Performance of Extractive Desulfurization of Liquid Fuels using Phosphonium and Pyrrolidinium-Based Ionic Liquids 4 5 6

a

*a

a

a

Omar U. Ahmed , Farouq S. Mjalli , Talal, Al-Wahaibi , Yahya Al-Wahaibi , Inas M. AlNashef

7

b

9

8 a

Petroleum and Chemical Engineering Department, Sultan Qaboos University, 123, Sultanate of Oman Department of Chemical and Environmental Engineering, Masdar Institute for Science and Technology, Masdar City, Abu Dhabi, United Arab Emirates

10 12

1

b

14

13

Abstract 15 16

Extractive 18

17

Desulfurization

(EDS)

of

thiophene

(T),

benzothiophene

(BT)

and

dibenzothiophene (DBT) in simulated fuel using two phosphonium and two pyrrolidinium 20

ionic liquids was investigated. A set of single-factor-at-a-time experiments was carried out to 21

19

determine factors that significantly affect the EDS process. The single-factor-at-a-time 23

2

experiments indicates that high sulfur removal (SR) can be achieved using long extraction 25

time (>60 min), high temperature, low fuel-to-solvent ratio (1:4) or large number of 26

24

extraction stages (NE = 5). However, the single-factor-at-a-time experiment does not take 28

27

interaction between the factors into consideration and may fail in determining a suitable 30

operating condition. Therefore, a response surface methodology (RSM) based on Box31

29

Behnken design was employed to study and analyze the effects of time, temperature, fuel-to3

32

solvent 35

tetrabutylphosphonium methanesulfonate [P4444][MeSO4] yielded the best result with %SR 36

34

ratio

and

their

interactions

on

the

EDS

process.

The

ionic

liquid,

(DBT, BT, T) of 69%, 62%, 61% at the optimum time, temperature, fuel-to-solvent ratio and 38 39

mixing rate of 15 min, 30 oC, 1:1 and 800 rpm respectively. A similar performance was 40

obtained during the single-factor-at-a-time experiments but at less favorable conditions 41

37

indicating the superiority of the statistical approach used in this work in optimizing the 43

42

liquid-liquid extraction performance. 4 45 46 *

47

Corresponding Author: Email: [email protected]

48 49 50 51 52 53 54 5 56 57 58 60

59

1 ACS Paragon Plus Environment


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1 3

2

1.0 Introduction 5

4

Aliphatic and aromatic sulfur compounds present in refinery streams have both economic and 7

6

environmental consequences. Within the refinery, these compounds can cause corrosion on 9

8

process equipment and likely end up emitted as oxides to the atmosphere. The presence of 10

these compounds in automotive fuels leads to significant reduction in the efficiency of the 12

emission control systems installed in automobiles1. This eventually results in emitting sulfur 14

13

1

and other hazardous gases. Whether in refinery streams or automotive fuels, the reduced and 15

oxidized form of these sulfur compounds have negative impact on the environment2. 17

16

Consequently, strict regulations on the allowable sulfur content in automotive fuels have been 19

18

imposed by countries around the word. 20

Refiners had often used hydrodesulfurization

technology (HDS) to convert these sulfur compounds into H2S using a CoMo or NiMo-based 2

21

catalysts. And the generated H2S from this process is converted to elemental sulfur by the 24

well-known Claus process3. However, a class of aromatic sulfur compounds, known as 25

23

refractory sulfur compounds, is difficult to be removed efficiently and economically using 27

26

conventional HDS. Furthermore, as crude oil gets heavier and sourer the concentration of 29

28

refractory sulfur compounds, which include benzothiophene, dibenzothiophene and their 30

alkyl derivatives, increase. 31 3

32

The strict restriction on the maximum allowable sulfur content in automotive fuel and the 35

inability of the conventional HDS to efficiently remove refractory compounds from 36

34

automotive fuel have led to the growing research activities in the area of alternative 38

37

desulfurization 40

technologies have received a great deal of attention in the scientific community 1, 4, 5. Because 41

39

technologies.

Extractive,

adsorptive

and

oxidative

desulfurization

of the potential simplicity of certain extraction processes, extractive desulfurization (EDS) is 43

42

adjudged as one of the most promising desulfurization technologies under investigation. EDS 45

does not require any catalyst nor consume any material during processing and can be carried 46

4

out under mild operating conditions. 47 49

48

The efficiency of EDS depends on the partitioning of sulfur compounds between the fuel 51

phase and the solvent phase. Therefore, a critical aspect of an efficient EDS process is finding 53

52

50

a suitable solvent. This solvent should preferably be selective towards sulfur, chemically and 54

physically stable and benign to the environment. It is in search of this suitable solvent that 56

Bossmann and co-workers used ionic liquids for the first time as potential solvents for EDS6. 58

57

5

Ionic liquids (ILs) are non-volatile, non-flammable and highly thermally stable organic salts 60

59


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2

1

which are liquid at temperatures below 100 oC 7. ILs are often classified based on their cation 4

3

type. In this regards, the feasibility of different types of ILs as solvents for EDS have been 6

investigated. Imidazolium8-10, pyridinium11, 7

5

12

and thiazolium13,

14

ILs, among others, have

shown promise as potential solvents for EDS. Each of these IL classes possesses a unique set 9

8

of properties and extraction ability. Recent investigations have shown that the extraction 1

ability of any class of IL is affected by not only the cation type but also by the position and 12

10

chain length of the alkyl group attached to the cation. For instance Holbrey and co-workers 14

13

showed that extraction of dibenzothiophene from dodecane varies in the order; 16

dimethylpyridinium > methylpyridinium > pyridinium ≈ imidazolium ≈ pyrrolidinium15. The 17

15

effect of cation and anion of ILs on their extraction ability has also been studied by 19 20

Domańska et al. 16. The presence of aromatic ring in the cation of an IL has been thought to 21

be the reason behind the relative high extraction ability of the imidazolium, pyridinium and 2

18

thiazolium based ILs. The π-π interaction between the cation and the refractory sulfur 24 25

compound is said to be responsible for their respective performances1. However, a work 26

carried out by Wilfred et al. has shown a relationship between specific volume and 27

23

desulfurization efficiency17. The work further showed how an ammonium-based IL, which 29

28

does not contain a ring, performed better than some imidazolium-based ILs. The implication 31

of this is that extraction ability of any IL might depend on many factors related to its 32

30

properties. Therefore, the huge number of possible ionic liquids and their variations has made 34

3

the search for the best IL for EDS a difficult task. For this reason, the feasibility of different 36

classes of ILs are continuously investigated. 37

35

39

38

In addition to the type of IL, conditions under which extraction is carried out also affects the 40

efficiency of an EDS process. Effects of temperature, fuel to solvent mass ratio and time have 42

often been the major parameters studied in EDS18-20. However, much of the studies related to 4

43

41

factors that affect the extraction process are carried out as single-factor-at-a-time 45

experiments. In a single-factor-at-a-time experiment, the combination of factors that give the 47

46

best extraction performance may be missed. This is because interactions between factors 48

cannot be detected and often require large number of experiments21. Different ILs might 50

49

behave differently in response to changes in operating conditions during extraction and 52

51

therefore a better comparison can be made at the conditions where the best performance is 54

53

exhibited by the ILs. Factor interactions can easily be detected and analyzed using statistical 5

design techniques such as the Response Surface Methodology (RSM). 56 57 58 60

59



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1 2

RSM is a set of mathematical and statistical techniques that can be used to optimize a certain 4

3

response by varying the values of the different variables that influence it. Relative to other 6

response surface design, Box-Behnken is efficient and requires fewer number of runs in cases 7

5

involving 3 or 4 variables22. Despite the advantages offered by this statistical approach to 9

8

experimental design and analysis, few works (in the desulfurization field) can be found in 1

literature 22, 23. 12

10

14

13

In this work, the feasibility of two phosphonium and two pyrrolidinium ionic liquids as 15

potential solvents for extractive desulfurization was studied. Phosphonium-based ILs are 17

16

relatively unexplored in the area of deep desulfurization. Although, pyrrolidinium ILs have 19

18

been studied recently, the ILs employed in this work have not been extensively investigated. 20

Effect of operating conditions, such as time, temperature etc. on the EDS were investigated. 2

21

Furthermore, a Box-Behnken design method was used to design, analyze and optimize sulfur 24

23

removal of the four ionic liquids studied. 25 26

2.0 Material and Methods 27 29

28

2.1 Chemicals 30 32

31

Thiophene (99%) and 1-Benzothiophene for synthesis were supplied by Merck while 34

dibenzothiophene (98%) was supplied by Aldrich. Acetonitrile (99.9%, HPLC grade) was 35

3

also supplied by Merck. Iso-dodecane (80%, mixture of isomers tech) was supplied by Alfa 37

36

Aesar. Hexadecane (99%) and toluene (ACS) were supplied by BDH and Honeywell 39

respectively. 40

38

The Ionic liquids Triisobutyl(methyl)phosphonium P-toluenesulfonate

[Pi444,1][Tos] also known as Cyphos IL 106 (98%), and Tetrabutylphosphonium 42

41

methanesulfonate [P4444][MeSO4] (98%), 4

respectively 45

43

while

[BMPyrr][TFSI] 47

46

were supplied by Cytec

1-Butyl-1-methylpyrrolidinium (99%)

and

and Sigma Aldrich

Bis(trifluoromethylsulfonyl)imide 1-Hexyl-1-methylpyrrolidinium

Bis(trifluoromethylsulfonyl)imide [HMPyrr][TFSI] (99%) were supplied Iolitec. 48 50

49

Scheme 1 presents the structure of the cation and anion of the studied ILs, while scheme 2 51

shows the structure of the sulfur compounds employed in this work. All chemicals were used 53

52

as supplied by the manufacturer without further treatment. 54 5 56 57 58 60

59


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1 3

2

2.2 Procedures 5

4

Sulfur-free simulated diesel was prepared by mixing iso-dodecane (51 %), hexadecane (39 7

6

%) and toluene (10 %). Appropriate amount of thiophene (T), benzothiophene (BT) and 9

8

dibenzothiophene (DBT) were dissolved in the sulfur-free simulated fuel in order to obtain a 10

mixture with 500 ppm each of T, BT and DBT. 1 13

12

Extractive desulfurization (EDS) experiments were carried out in a 20 mL screw-capped 15

chromacol vials. A preliminary investigation was carried out prior to the experimental design. 16

14

Factors studied were time (30 min-120 min), temperature (30 oC-80 oC), mass ratio (4:1-1:4), 18

17

number of extraction stages (1-5) and mixing speed (600 rpm-1000 rpm). All experiments 20

were carried out under atmospheric pressure in a thermomixer (Table 1). At the end of each 21

19

EDS experiment, the mixture was allowed to settle for 15 min after which a sample of fuel 23

2

was withdrawn for analysis. Agilent high performance liquid chromatography (HPLC) 25

coupled with variable wavelength detector, details shown in Table 2, was used for 26

24

determining the sulfur content of the simulated fuel before and after EDS experiment. The 28

27

relationship shown as Equation 1 was used to determine the percentage sulfur removal. 29 31

30

% = 32

× 100

1

34

3

Where %SR: % sulfur removal, Co: initial sulfur concentration (ppm), Ct: sulfur 36

concentration after (t) min of experiment. 37

35

39

38

2.3 Experimental Design 40 41

After the preliminary EDS experiments, response surface methodology using Box-Behnken 43

42

design was employed for the optimization of %SR of T, BT and DBT in the studied ILs. 45

Design Expert 7.0 was used for the design, optimization and analysis of the experiment. The 46

4

three independent factors studied were time (15 min-80 min), temperature (30 oC-80 oC) and 48

47

mass fraction (0.5-0.83). The experiment was designed around lesser time and higher fuel to 50

IL mass fraction, as compared to the preliminary investigation, to ensure the optimum is 51

49

obtained while reducing the residence time and higher mass fraction. Typical Coded and 53

52

uncoded levels of the independent variable are shown in Table 3. The following quadratic 5

equation (Equation 2) was used for the optimization. 56

54

57 58 60

59



1 2

= + ∑ + ∑ + ∑ 5

4

3

2

Where Y: response (%SR), Xi: optimized factors and β0, βi, βii and βij are coefficients of the intercept, linear, square and interaction effects respectively. 8

7

6

The optimum response for each sulfur compound and the corresponding levels of the 10

independent variable were determined. F-test and P-value were used to analyze the statistical 1

9

significance of the model and the coefficients. Backward model reduction method was used 13

12

to hierarchically eliminate insignificant model coefficients. 14 16

15

3.0 Results and Discussion 18

17

During the preliminary investigation, the effect of time, temperature, mass ratio and number 20

19

of extraction stages on the %SR of T, BT and DBT were studied as shown below. 2

21

3.1 Effect of Mixing Time 23 25

24

Mixing time is a very important factor for industrial processes in general and the extraction 27

26

process in particular. A process is considered advantageous if the desired goal can be reached 28

in the shortest possible time. This is also true for EDS processes and as such, the effect 30

mixing time on the %SR was investigated and presented in Figure 1. Temperature, mass ratio 31

29

and mixing speed were kept at 30 oC, 1:1 and 600 rpm respectively for all the ILs studied 3

32

except [P4444][MeSO4], where temperature was kept at 60 oC (melting point >58 oC). It can be 35

34

seen that the %SR for all the sulfur compounds studied in the [P4444][MeSO4] increased with 37

36

time and attains a certain maximum level after 60 min of mixing. The %SR remains at the 38

same level even after two hours, which suggests that the IL was saturated with these sulfur 40

compounds. The same phenomenon was observed on all the other three ILs. This indicates 42

41

39

that operating above 60 min will have no significant impact on the degree of desulfurization. 43

Also, it can be seen that the selectivity of the ILs towards the sulfur compounds is in the 45

order; DBT >BT >T. This relationship was observed for all the ILs and can be attributed to 47

46

4

the relative differences in the electron density on the sulfur atoms of the sulfur compounds, 48

which are 5.696, 5.739 and 5.758 for T, BT and DBT respectively24. This suggests that the 50

49

interaction between the sulfur compound and ILs occurs in the relatively electron-deficient 52

51

regions of the ILs. 53 54

Furthermore, the type of IL used also plays a role in the level of sulfur removal. For example, 56

5

after 120 mins, the %SR (DBT, BT, T) was obtained in the order; [P4444][MeSO4] (73%, 57 58 60

59


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1 2

71%, 66%) > [HMPyrr][TFSI] (60%, 56%, 50%) ≈ [Pi444,1][Tos](59%, 57%, 47%) > 4

3

[BMPyrr][TFSI] (54%, 55%, 49%). The effect of alkyl chain length on %SR has been widely 6

reported in the literature1. Therefore the %SR of [HMPyrr][TFSI] was expected to be better 7

5

than that of [BMPyrr][TFSI]. The Phosphonium-based ILs appear to have relatively high 9

8

efficiency despite having short alkyl chain length. Although the %SR of the [P4444][MeSO4] 1

was determined at 60 oC, however, the high temperature is not the only reason for its high 12

10

%SR relative to the others as will be seen later. 13 14 15

3.2 Effect of Temperature 16 18

17

To study the effect of temperature, other conditions were held at 30 min, MR of 1:1 and 20

mixing speed of 600 rpm. Among the four ILs studied, [Pi444,1][Tos] showed the highest 21

19

response to increase in temperature starting from 41% (DBT) at 30 oC to about 62% (DBT) 23 24

at 50 oC, which remain at the same level even at 80 oC. This pattern is similar in the case of 25

BT and T respectively. By increasing the temperature from 60 oC to 80 oC , a slight increase 26

2

in %SR from 39% (T), 50% (BT) and 54% (DBT) to 60%, 61% and 66% respectively were 28

27

observed for the [P4444][MeSO4] IL. However, in the case of both [BMPyrr][TFSI]and 30

[HMPYrr][TFSI], there is an even more slight increase in the %SR from 51% (DBT, 31

29

[HMPyrr][TFSI]) at 30 oC to 55% at 40 oC-50 oC. This was followed by a slight decrease 3 34

down to 50% at 80 oC. This was also the case for T and BT for the two [TFSI]-based 35

pyrrolidinium ILs as seen in Figure 2. In a similar observation made by Mokhtar et al., the 36

32

decrease in %SR at high temperature was assumed to occur due to exothermic nature of 38 39

extraction process at high temperature, governed by the Van’t Hoff law22. Therefore as 40

temperature increases to 50 oC, the viscosity reduces, miscibility increases and the affinity of 41

37

the ILs to the sulfur compounds increases and thereby %SR increases. As the temperature is 43

increased beyond 50 oC, reverse migration occurs thereby decreasing the %SR. 4

42

46

45

At 80 oC, the %SR (DBT, BT, T) was in the order; [P4444][MeSO4] (66%, 61%, 60%) 48

47

>[Pi444,1][Tos](62%, 54%, 51%) > [HMPyrr][TFSI] (50%, 46%, 44%) > [BMPyrr][TFSI] 49

(41%, 43%, 41%). Comparing the %SR order as function of temperature with that of time, it 51

can be concluded that the [Pi444,1][Tos] is more responsive to temperature change as 53

52

50

compared to [HMPyrr][TFSI], which was more responsive to time. It is important to note 54

that although the order DBT > BT > T (except [ BMPyrr][TSI]) was maintained at high 56

temperature, the differences in these values have decreased. 57

5

58 60

59



1 2 3 4 5

3.3 Effect of Fuel to IL mass ratio (MR) 6 8

7

Extraction processes that make use of low solvent amount are more desirable. However, the 10

use of low solvent might be impractical and therefore an optimum amount is normally 1

9

needed. The effect of fuel: IL ratio (MR) was studied and the result is presented in Figure 3. 13

12

As expected, it can be seen that the more the amount of IL, the more the %SR. The %SR 15

(DBT, BT, T) increased from (29%, 25%, 17%) to (60%, 55%, 53%), (35%, 29%, 53%) to 16

14

(91%, 87%, 83%), (22%, 21%, 16%) to (82%, 82%, 78%) and (28%, 24%, 18%) to (86%, 18

17

83%, 79%) for the ILs [Pi444,1][Tos], [P4444][MeSO4] , [BMPyrr][TFSI] and [HMPyrr][TFSI] 20

respectively when MR was reduced from 4:1 to 1:4. Extraction with [Pi444,1][Tos] IL showed 21

19

the least response to the reduction in MR, which increased by 31%. [P4444][MeSO4] , 23

2

[BMPyrr][TFSI] and [HMPyrr][TFSI] increased by 56%, 60% and 58% respectively. 25

Therefore the order in terms of %SR at MR of 1:4 was [P4444][MeSO4] > [HMPyrr][TFSI] > 26

24

[BMPyrr][TFSI] > [Pi444,1][Tos]. It should be pointed out here that a high MR is much more 28

27

desired as this would reduce quantity of solvent required and to be processed in an industrial 30

scale, which in turn reduces cost of running the extraction process. 31

29

3

32

3.4 Effect of Mixing Rate 34 35

The degree of mixing is an important factor in any extraction process. The rate of migration 37

36

of sulfur compounds into the solvent is dependent on the extent of contact surface area. 39

Higher degree of mixing leads to higher contact surface area and therefore results in 40

38

improved %SR. To study the effect of mixing rate, the IL [HMPyrr][TFSI] was used to 42

41

extract the DBT, BT T in fuel at different mixing rate as shown in Figure 4. The %SR (DBT, 4

BT, T) increased from (51%, 49%, 43%) at 600 rpm to (60, 57%, 48%) at 800 rpm, which 45

43

remains the same at 1000 rpm. 46 48

47

3.5 Effect of Number of Extraction Stages (NE) 49 51

50

Figure 5 shows the effect of extraction stages on the %SR for the four studied IL. It can be 52

seen that although the %SR achieved by the different ionic liquids at 30 oC (MR = 1:1, 30 54

53

min and 600 rpm) were modest, deep desulfurization of DBT can be achieved after 5 NE for 56

5

all the ionic liquids tested. For the IL [P4444][MeSO4], this was achieved at NE = 4. 57 58 60

59


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1 3

2

3.6 Optimization of Operating Conditions by the Box-Behnken Design Methodology 5

4

Having carried out preliminary investigation of EDS, Box-Behnken design was employed to 7

6

find the optimum conditions (mixing time, temperature, MR) that yield high %SR. To do this, 9

8

the mixing time span was reduced to 15 min-80 mins, the mass fraction (MR) was increased 10

to 0.5 (1:1) to 0.833 (5:1) while the mixing rate was increased to 800 rpm. The idea behind 12

1

this was to maximize the %SR in the region of low time and high MR in order to increase 14

13

throughput (decreased time) and cost of running the extraction process (increased MR) in an 15

industrial scale. Table 4, S1-S11 shows the experimental data (completely randomized) 17

16

including variables in their coded and uncoded (actual) forms alongside the experimental 19

18

result (response) and their model predicted counterparts. These experimental results were 20

analyzed and fitted to the quadratic model presented as eq (1). Having studied the analysis of 2

21

variance (ANOVA) of DBT, BT, and T in all the four ILs, the backward model reduction 24

23

option was used to hierarchically eliminate insignificant independent variables to yield the 25

relationships presented in Table 5. 26 28

27

Table 6-9 shows the ANOVA table for DBT in the four ILs respectively after model 30

reduction of the quadratic model. Analysis of variance for the predicted model on %SR of BT 31

29

and T in all the four ILs has also been carried out and the equations are available as 3

32

supporting information alongside the model reduced ANOVA table (S12-S19). 34 36

35

Table 6 shows the ANOVA of %SR (DBT) using [HMPyrr][TFSI] after model reduction, the 38

37

F-test value for the model was found to be 1785.28 indicating the model is significant. 39

Furthermore there is F) for model terms should be < 0.05. Model terms 43

42

40

with p-values 0.100 are insignificant. 4

Therefore all the model terms in the model are significant with the exception of X1-time. 46

Because hierarchical backward reduction was carried out, time was included. This is because 48

47

45

the time-temperature and time-mass fraction interactions are in the acceptable range and 49

therefore the coefficients for these interaction terms serve as a correction to the independent 51

variable (time) term. The mass fraction term with an F-value of 11831.08 is by far the most 52

50

significant term followed by temperature with F-value of 641.94. R2 of 0.9993 was obtained 54

53

which indicates that the reduced quadratic model consolidates the experimental data pretty 56

good. The adjusted R2 of 0.9987 is high and very close to the predicted R2 of 0.9969. This 58

57

5

further reinforces the assertion that the model is satisfactory, which can be viewed 60

59



1 2

graphically as in Figure 5A. For the response in case of extraction of BT with 4

3

[HMPyrr][TFSI], the model F-value from the ANOVA (S12) was obtained as 1264.84, which 6

is also significant with p-value of

Optimum Performance of Extractive Desulfurization of Liquid Fuels

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