- Email: [email protected]
Polyethylene glycol as a green solvent for effective extractive desulfurization of liquid fuel at ambient conditions
Fuel 137 (2014) 36–40
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Polyethylene glycol as a green solvent for effective extractive desulfurization of liquid fuel at ambient conditions Effat Kianpour, Saeid Azizian ⇑ Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65167, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t Extraction efficiency / %
effective solvent for EDS for the first time. Extraction performance of PEG is temperature and initial sulfur content independent. The BDT content was reduced from 512 to 10 ppmw only within three extraction cycles. Regenerated PEG could effectively extract DBT from fresh model fuel.
S-compond concentration / ppm
PEG was introduced as a green and 600 512 500 400
100 80 60 40 20 0 200
300
400
600
800 1000 1200 1400
Initial conentration of DBT / ppm
200
130
100 22
0 0
1
2
10
3
Extraction times
a r t i c l e
i n f o
Article history: Received 10 July 2014 Received in revised form 29 July 2014 Accepted 30 July 2014 Available online 10 August 2014 Keywords: Extraction Desulfurization Polyethylene glycol Liquid fuel Benzothiophenic compounds
⇑ Corresponding author. http://dx.doi.org/10.1016/j.fuel.2014.07.096 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
a b s t r a c t Today there are serious regulations to reduce sulfur content of fuels because the SOx produced during the combustion of fuels containing sulfur compounds make the air polluted and have dangerous environmental impacts. With the aim of replacement of the present volatile, flammable and toxic organic solvents or inefficient, corrosive and expensive ionic liquids (ILs), the polyethylene glycol (PEG) was introduced as a green, effective, non-toxic, non-corrosive and also recyclable molecular solvent for extractive desulfurization (EDS) of benzothiophenic compounds from liquid fuel in this work for the first time. PEG shows excellent EDS and it has the higher extraction efficiency for dibenzothiophene (DBT) (76% within 90 s) than those of ILs. Using this extractant, the BDT content was reduced from 512 to 10 ppmw (98%) only within three extraction stages, the minimum number of cycles within shortest time reported up to now, and the deep desulfurization was achieved. Effect of some important parameters including initial concentration of sulfur compound, PEG dosage, time and temperature of extraction on the EDS process was investigated. It was fond that extraction performance of PEG is independent to temperature and initial sulfur content, which is an excellent finding for industrialization. The feasibility of PEG for extraction of different thiophenic compounds was observed in the order of dibenzothiophene > benzothiophene > 4,6-dimethyldibenzothiopene. Finally, the PEG was reused in several cycles and then it was regenerated by adsorption method. The results of the present work hopefully provide useful information for future industrial application of PEG as an efficient green solvent for the EDS of liquid fuels. Ó 2014 Elsevier Ltd. All rights reserved.
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
1. Introduction In the recent years, air pollution has become an increasingly important subject worldwide from the environmental point of view. One of the major sources of air pollution is SOx compounds produced during the combustion of fuels containing sulfur compounds [1–4]. Sulfur oxides can destroy catalytic converters used to reduce CO and NOx emissions from an internal combustion engines and also have side effects such as acid rain and ozone depletion [5–8]. Therefore, many governments in the world have forced their petroleum refinery industries to reduce sulfur content in the fuels to near zero-levels (<10 ppm S) by 2012 [1,7]. Hydrodesulfurization (HDS) process, converting sulfur compounds to H2S and corresponding hydrocarbons at very high temperatures (300–400 °C) and pressures (20–100 atm of H2) [8], is the major industrial technique for the removal of sulfur compounds from the fuels. This classical process not only reduces the octane/ cetane number of fuel due to saturation of olefins, but also has a limited capacity to efficiently eliminate the refractory benzothiophenic compounds, e.g. dibenzothiophene (DBT), while these compounds take a major part of the sulfides [6] in fuels [7–9]. Therefore, to meet low sulfur-level by the HDS process, severe operation conditions and high economic investments are necessary. For this reason, exploration of convenient and energy-saving processes to reduce the sulfur content of hydrocarbon fuels and therefore reduce the production of SOx has become a hot research area. At the present, there are various non-hydrodesulfurization approaches including extraction, adsorption, biodesulfurization, oxidation and membrane separation [8]. But replacement of the HDS technique with the existing technologies needs to improve them. Extractive desulfurization (EDS) process can be carried out at mild and simple conditions and it does not change the chemical structure of the compounds and consequently has no effect on the quality of liquid fuels [5,10]. So, this technique seems to be more appropriate process and it has become the object of active research in recent years. Different organic solvents, such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and 1-methyl-2-pyrrolidinone (NMP) have been used as extractive solvents in desulfurization [11]. Nevertheless, the commonly used solvents are volatile, flammable, toxic and they have serious consequences for the environment [4]. Therefore, it can be of great significance to explore new environmentally friendly and also cheap, effective and recyclable solvents to improve the extractive desulfurization process. Because of low vapor pressure and thermal stability, ionic liquids (ILs) are considered to be ‘‘green solvents’’ to replace the volatile organic solvents [12]. During the past few years, many studies have been done on desulfurization by ILs [1,3,7,12]. But extractive desulfurization by ILs has many problems including: unexplored ecosystem impacts, costly expensive [1,12], high viscosity of ILs (which decreases diffusion coefficient and interphase area and therefore diffusion rate during the extraction process) [4,13] and corrosion damages caused by halogen-containing ILs [3]. Moreover, because of rather low desulfurization efficiency by extraction with IL, either large volumes of ionic liquid for an efficient liquid–liquid extraction process must be used [7] or extraction should be integrates with oxidative desulfurization process [5]. Whereas oxidative desulfurization has several major problems, which hindered the industrial application of extractive oxidative desulfurization, such as decreasing the quality of fuel because of non-excellent selectivity to the organosulfur compound (hydrocarbons can also be oxidized), difficult posttreatment of sulfoxides and sulfones produced during the oxidation process, the reaction’s safety and the high cost of the oxidant [5,14].
37
Recently, it has been reported that hydrogen bond formed between the active hydrogen of amine-based protic ILs or ammonium-based deep eutectic solvents as extractant and the sulfur atom of S-containing molecules in fuels accounts for the higher extraction efficiency [4,12]. On the other PEG, as a proton donor solvent [15], not only has a very low vapor pressure (<0.01 hPa at 20 °C for PEG-200) and applicability over a wide temperature range because of high boiling point (b.p. >150 °C) [16] but also, in comparison with the synthesized ILs, it has several advantages such as low viscosity (dynamic viscosity 58–85 mPa s at 20 °C) [16], non-toxicity (which approved by the Food and Drug Administration (FDA), USA) [17–19], corrosion inhibition [20,21], cheap and easily obtained molecular solvent [4]. Based on recently reported works [4,12] and the mentioned characters of PEG, it seems that PEG is a good (green, cheap and effective) option for extractive desulfurization. So these points inspire us to investigate the efficiency of pure PEG, as an immiscible liquid with organic phase, for extractive desulfurization of liquid fuel in the present work. 2. Experimental section 2.1. Materials The details of the analytical grade chemicals used are as follows: PEG-200 (Merck), heptane (Merck, purity >99%), activated granular carbon (Merck, size 850–1000 lm), BT, DBT and DMDBT (Sigma–Aldrich, mass fraction purity of >98%). 2.2. Extractive desulfurization process The initial concentration of model fuels was 500 ppmw except for defined tests and they were prepared by dissolving BT, DBT and DMDBT in to n-heptane, separately. In the all experiments, defined amount of model liquid fuel containing DBT (as a typical sulfur compound) and PEG with volume ratios PEG/fuel 1:1 (excluding for certain experiments) transferred into a 50 ml glass bottle and the system was placed in a stirred thermostatted batch system at desired temperature (±0.1 °C). In a typical kinetic run, the binary mixture was stirred, stopped at desired time intervals and sampling was conducted for further quantification. In the other experiments, the mixture was stirred vigorously for 5 min and then the sulfur content of the model fuel was determined. The quantification of initial and final sulfur content in the fuel phase, using UV/Visible spectrophotometer (PG Instrument Ltd., T-80) at the related kmax [22], allowed the determination of the extraction efficiency (E.E%) based on: E.E(%) = (1 Ct/C0) 100; where C0 and Ct are initial and at any time concentration of sulfur compound (ppmw). To use the spent PEG in the next cycle, the upper organic phase was removed and the used PEG was exposed to the fresh model fuel with fixed volume ratio of PEG/model fuel as 1:1 for the next extraction stage. Regeneration of spent PEG was achieved by adsorption. For this purpose, a bottle containing spent PEG and activated granular carbon as a common adsorbent with the mass ratio PEG/adsorbent 50:1 was placed in a constant temperature shaker (n-BioTEK, NB-304) and shaked at 150 rpm for 24 h and then regenerated PEG was used again in a new EDS experiments. 3. Results and discussion 3.1. Effect of extraction conditions on sulfur removal Extraction process may be affected by some factors including phase ratio of extractant to model fuel, initial sulfur content of
38
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
liquid fuel, extraction time and temperature. Fig. 1 shows the effect of contact time on the extraction efficiency of DBT. Just after 120 s, the equilibrium can be approached with much higher extraction efficiency (about 76%) than the extraction by ILs [3–5]. The low viscosity would probably account for the fast extraction but high extraction capability of PEG can be related to the possibility of bond formation between hydrogen of PEG and sulfur atom in DBT [4,12]. Hereinafter, 5 min was chosen as the extraction time which is sufficient to be sure about equilibrium. For the industrial application of extractant, it is preferable that a lesser amount of PEG be used but with high extraction efficiency. So the effect of volume ratio of PEG to model fuel on the removal of DBT was investigated and the results are presented in Fig. 2. This figure shows as the volume ratio of PEG to fuel was varied from 0.25:1 to 2:1 the extraction efficiency was increased and reached up to 87%. When the volume ratio increases from 1:1 to 1:2, the removal efficiency only increases 10%. So, the volume ratio of 1:1 was selected for the next experiments. To find the effect of the initial concentration of sulfur compounds on extraction process, the extractive desulfurization of DBT by PEG from model fuels containing different concentrations (300–1300 ppmw) of this s-compound was carried out and the results are shown in Fig. 3. As can be seen, the extraction efficiency just partially decreased by increasing the initial sulfur content of model fuel contrary to results obtained in extractive desulfurization by amine-based protic ILs [12]. This interesting finding is very important from the industrial point of view, because PEG can be used to extractive desulfurization of liquid fuel with any (low to high) concentrations of sulfur compound. High extraction capability of PEG would probably account for these observations. Effect of temperature on the extraction of sulfur compounds from the related model fuel by PEG was probed in the temperature range of 288–308 K. Results of a typical experiment are presented in Fig. 4. This figure clearly shows that temperature change has no influence on extraction efficiency. This observation is different from extraction desulfurization processes with some ILs, in which, with the increase of the temperature, the extraction efficiencies increase and then decrease [1]. The temperature dependency of extraction by ILs may be attributed to their high viscosity [1]. Insensitivity to temperature is very beneficial in industrial applications, because extractive desulfurization can be carried out at ambient temperature easily without energy consumption.
Fig. 2. Effect of PEG/fuel volume ratio on extraction of DBT (T = 298 K, t = 5 min).
Fig. 3. Effect of initial concentration on extraction of DBT (volume ratio of PEG/fuel = 1:1, T = 298 K, t = 5 min).
3.2. Effect of S-compound on sulfur removal To study the extraction performance of PEG for different sulfur compounds, the extraction rate of BT, DBT and DMDBT has been
Fig. 4. Effect of temperature on extraction of DBT (volume ratio of PEG/fuel = 1:1).
Fig. 1. Time-course variation of extraction efficiency of DBT (500 ppmw) during extraction via PEG. Experimental conditions: volume ratio of PEG/model fuel = 1:1, T = 298 K.
compared. As represented in Fig. 5, the rate of extraction by PEG decreased as DBT > BT > DMDBT. It is interesting to note that for all sulfur compounds the extraction desulfurization reach to the equilibrium less than 3 min. The removal efficiencies of BT, DBT and DMDBT are 69%, 76% and 43%, respectively. It can be deduce that the extraction of the refractory S-containing molecules depends markedly on the nature of the organo-sulfur molecule specially their molecular sizes, electron density and steric hindrance on sulfur atoms [2,22]. The electron density on the sulfur
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
Fig. 5. Time dependency of BT, DBT and DMDBT concentration variation in fuel phase during the extraction (volume ratio of PEG/fuel = 1:1, T = 298 K). Symbols are the experimental values and solid lines represent the predicted values by first order kinetic model.
atom of these sulfur compounds increases as BT < DBT < DMDBT [2] Therefore, lower electron density on the sulfur atom led to a lower extraction of BT than that of DBT. The lowest extraction efficiency of DMDBT may be related to sterically hindered sulfur atom in DMDBT, caused by the presence of two neighbor methyl groups at 4- and 6-positions of DBT, which was a main obstacle for bond formation between sulfur atom of DMDBT and hydrogen of PEG [2,23]. The experimental kinetic data were best fitted using first-order kinetic model, ln (1 F) = kt where F = (C0 Ct)/(C0 Ce) the C0, Ce and Ct are initial, equilibrium and at any time concentrations, respectively. The obtained rate constants are 0.014, 0.032 and 0.010 s 1 at 25 °C for BT, DBT and DMDBT, respectively. The trend of the obtained rate constants is DBT > BT > DMDBT, which can be attributed to the combination of electron density and also spatial steric hindrance of sulfur atom in the molecule. Solid lines in Fig. 5 represent the predicted values by this model.
39
Fig. 6. DBT concentration after multiple extraction steps. (volume ratio of PEG/fuel = 1:1, T = 298 K, t = 5 min).
Fig. 7. Extraction efficiency of DBT by (a) used PEG at different cycles (volume ratio of used PEG/fuel = 1:1, T = 298 K, t = 5 min) and (b) regenerated spent PEG by granular activated carbon (mass ratio spent PEG/adsorbent 50:1, T = 298 K, t = 5 min).
3.3. Multistage extraction To achieve deep desulfurization, multistage extraction strategy was applied as follow: after single extraction of DBT over 5 minute, the extractant phase was separated and fresh PEG was added in the reactor and this procedure was repeated again and again to find the number of necessary extraction times for reduction of DBT concentration to about 10 ppmw. As can be seen in Fig. 6, PEG exhibits excellent extraction behavior and the sulfur content was reduced (ca 98%) from about 500 to 10 ppmw only within three extraction stages. This interesting situation i.e. using just three cycles and a very short time cannot be achieved by the ionic liquids [12,13]. 3.4. Reuse of PEG and regeneration of spent PEG For industrialization and from the environmental point of view, reuse and regeneration of desulfurizer extractant are important and it was studied here and the results are presented in Fig. 7. It can be seen that PEG can be reused several times by some decrease in extraction capability (Fig. 7a). After five cycles it becomes nearly saturated, loses its extraction capability and must be regenerated. There are different regeneration techniques such as distillation, adsorption and back-extraction processes [7]. Although the first one is the main recycling method for ILs, but this technique is not very economic because of high energy consumption [12]. The used extractant can also be regenerated by washing with organic solvents, such as diethyl ether [4], but this methods is not
eco-friendly. In the present work the spent PEG was regenerated by adsorption technique. From Fig. 7b it is clear that regenerated PEG, by granular activated carbon as a common sorbent (mass ratio PEG/sorbent 50:1), can effectively extract DBT from the spent PEG (with extraction efficiency about 60%). 4. Conclusion In conclusion, polyethylene glycol was introduced as a green, cheap, effective, non-toxic, non-corrosive and also recyclable molecular solvent for extractive desulfurization of liquid fuel for the first time. This solvent has higher extraction efficiency for dibenzothiophene (DBT) (76% within 90 s) than those of ionic liquids (ILs). Using this extractant, the BDT content was reduced from 512 to 10 ppmw (98%) only within three extraction cycles, the minimum number of cycles within shortest time reported up to now, and the deep desulfurization was achieved. Effect of some important parameters including initial concentration of sulfur compound, PEG dosage, time and temperature of extraction on the EDS process was investigated. It was found that extraction performance of PEG is temperature and initial sulfur content independent. This is an excellent finding for industrialization. The feasibility of PEG for extraction of different thiophenic compounds was observed in the order of DBT > BT > DMDBT. Furthermore, the equilibrium was achieved less than 3 min in all experiments. The
40
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
low viscosity would probably account for the fast extraction and high extraction capability of PEG can be related to the possibility of hydrogen bond formation with sulfur atom in s-compound. The PEG was reused five cycles and then spent PEG was regenerated by adsorption method. Regenerated extractant could effectively extract DBT from fresh model fuel with extraction efficiency of 60%. Acknowledgment Financial support from Bu Ali Sina University is gratefully acknowledged. References [1] Dharaskar SA, Wasewar KL, Varma MN, Shende DZ, Tadi KK, Yoo CK. Synthesis, characterization, and application of novel trihexyl tetradecyl phosphonium bis (2,4,4-trimethylpentyl) phosphinate for extractive desulfurization of liquid fuel. Fuel Process Technol 2014;123:1–10. [2] Jiang W, Zhu W, Chang Y, Chao Y, Yin S, Liu H, et al. Ionic liquid extraction and catalytic oxidative desulfurization of fuels using dialkylpiperidinium tetrachloroferrate catalysts. Chem Eng J 2014;250:48–54. [3] Lu X, Yue L, Hu M, Cao Q, Xu L, Guo Y, et al. Piperazinium-based ionic liquids with lactate anion for extractive desulfurization of fuels. Energy Fuels 2014;28:1774–80. [4] Li C, Li D, Zou S, Li Z, Yin J, Wang A, et al. Extraction desulfurization process of fuels with ammonium-based deep eutectic solvents. Green Chem 2013;15:2793–9. [5] Shu C, Sun T, Zhang H, Jia J, Lou Z. A novel process for gasoline desulfurization based on extraction with ionic liquids and reduction by sodium borohydride. Fuel 2014;121:72–8. [6] Lin Y, Wang F, Zhang Z, Yang J, Wei Y. Polymer-supported ionic liquids: synthesis, characterization and application in fuel desulfurization. Fuel 2014;116:273–80. [7] Ferreira AR, Freire MG, Ribeiro JC, Lopes FM, Crespo JG, Coutinho JA. Ionic liquids for thiols desulfurization: experimental liquid–liquid equilibrium and COSMO–RS description. Fuel 2014;128:314–29. [8] Kulkarni PS, Afonso CA. Deep desulfurization of diesel fuel using ionic liquids: current status and future challenges. Green Chem 2010;12:1139–49.
[9] Rodríguez-Cabo B, Rodríguez H, Rodil E, Arce A, Soto A. Extractive and oxidative–extractive desulfurization of fuels with ionic liquids. Fuel 2014;117:882–9. [10] Mokhtar WNAW, Bakar WAWA, Ali R, Kadir AAA. Deep desulfurization of model diesel by extraction with N, N-dimethylformamide: optimization by Box–ehnken design. J Taiwan Inst Chem Eng 2014. http://dx.doi.org/10.1016/ j.jtice.2014.03.017. [11] Ban LL, Liu P, Ma CH, Dai B. Deep extractive desulfurization of diesel fuels by FeCl3/ionic liquids. Chin Chem Lett 2013;24:755–8. [12] Li Z, Li C, Chi Y, Wang A, Zhang Z, Li H, et al. Extraction process of dibenzothiophene with new distillable amine-based protic ionic liquids. Energy Fuels 2012;26:3723–7. [13] Kuhlmann E, Haumann M, Jess A, Seeberger A, Wasserscheid P. Ionic liquids in refinery desulfurization: comparison between biphasic and supported ionic liquid phase suspension processes. Chem Sust Chem 2009;2:969–77. [14] Zhang W, Zhang H, Xiao J, Zhao Z, Yu M, Li Z. Carbon nanotube catalysts for oxidative desulfurization of a model diesel fuel using molecular oxygen. Green Chem 2014;16:211–20. [15] Yang ZZ, He LN, Zhao YN, Yu B. Highly efficient SO2 absorption and its subsequent utilization by weak base/polyethylene glycol binary system. Environ Sci Technol 2013;47:1598–605. [16] Safety Data Sheet, according to the Regulation (EC) No. 1907/2006. Revision Data 1920.1909.2001, Version 1907.1905 www.merck-chemicals.com. [17] Jaiswal A, Ghosh SS, Chattopadhyay A. One step synthesis of C-dots by microwave mediated caramelization of poly (ethylene glycol). Chem Commun 2012;48:407–9. [18] Veronese FM, Pasut G. PEGylation, successful approach to drug delivery. Drug Discovery Today 2005;10:1451–8. [19] Karakoti AS, Das S, Thevuthasan S, Seal S. PEGylated inorganic nanoparticles. Angew Chem Int Ed 2011;50:1980–94. [20] Ashassi-Sorkhabi H, Ghalebsaz-Jeddi N. Inhibition effect of polyethylene glycol on the corrosion of carbon steel in sulphuric acid. Mater Chem Phys 2005;92:480–6. [21] Ashassi-Sorkhabi H, Ghalebsaz-Jeddi N, Hashemzadeh F, Jahani H. Corrosion inhibition of carbon steel in hydrochloric acid by some polyethylene glycols. Electrochim Acta 2006;51:3848–54. [22] Fallah RN, Azizian S. Removal of thiophenic compounds from liquid fuel by different modified activated carbon cloths. Fuel Process Technol 2012;93:45–52. [23] Cychosz KA, Wong-Foy AG, Matzger AJ. Liquid phase adsorption by microporous coordination polymers: removal of organosulfur compounds. J Am Chem Soc 2008;130:6938–9.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Polyethylene glycol as a green solvent for effective extractive desulfurization of liquid fuel at ambient conditions Effat Kianpour, Saeid Azizian ⇑ Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65167, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t Extraction efficiency / %
effective solvent for EDS for the first time. Extraction performance of PEG is temperature and initial sulfur content independent. The BDT content was reduced from 512 to 10 ppmw only within three extraction cycles. Regenerated PEG could effectively extract DBT from fresh model fuel.
S-compond concentration / ppm
PEG was introduced as a green and 600 512 500 400
100 80 60 40 20 0 200
300
400
600
800 1000 1200 1400
Initial conentration of DBT / ppm
200
130
100 22
0 0
1
2
10
3
Extraction times
a r t i c l e
i n f o
Article history: Received 10 July 2014 Received in revised form 29 July 2014 Accepted 30 July 2014 Available online 10 August 2014 Keywords: Extraction Desulfurization Polyethylene glycol Liquid fuel Benzothiophenic compounds
⇑ Corresponding author. http://dx.doi.org/10.1016/j.fuel.2014.07.096 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
a b s t r a c t Today there are serious regulations to reduce sulfur content of fuels because the SOx produced during the combustion of fuels containing sulfur compounds make the air polluted and have dangerous environmental impacts. With the aim of replacement of the present volatile, flammable and toxic organic solvents or inefficient, corrosive and expensive ionic liquids (ILs), the polyethylene glycol (PEG) was introduced as a green, effective, non-toxic, non-corrosive and also recyclable molecular solvent for extractive desulfurization (EDS) of benzothiophenic compounds from liquid fuel in this work for the first time. PEG shows excellent EDS and it has the higher extraction efficiency for dibenzothiophene (DBT) (76% within 90 s) than those of ILs. Using this extractant, the BDT content was reduced from 512 to 10 ppmw (98%) only within three extraction stages, the minimum number of cycles within shortest time reported up to now, and the deep desulfurization was achieved. Effect of some important parameters including initial concentration of sulfur compound, PEG dosage, time and temperature of extraction on the EDS process was investigated. It was fond that extraction performance of PEG is independent to temperature and initial sulfur content, which is an excellent finding for industrialization. The feasibility of PEG for extraction of different thiophenic compounds was observed in the order of dibenzothiophene > benzothiophene > 4,6-dimethyldibenzothiopene. Finally, the PEG was reused in several cycles and then it was regenerated by adsorption method. The results of the present work hopefully provide useful information for future industrial application of PEG as an efficient green solvent for the EDS of liquid fuels. Ó 2014 Elsevier Ltd. All rights reserved.
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
1. Introduction In the recent years, air pollution has become an increasingly important subject worldwide from the environmental point of view. One of the major sources of air pollution is SOx compounds produced during the combustion of fuels containing sulfur compounds [1–4]. Sulfur oxides can destroy catalytic converters used to reduce CO and NOx emissions from an internal combustion engines and also have side effects such as acid rain and ozone depletion [5–8]. Therefore, many governments in the world have forced their petroleum refinery industries to reduce sulfur content in the fuels to near zero-levels (<10 ppm S) by 2012 [1,7]. Hydrodesulfurization (HDS) process, converting sulfur compounds to H2S and corresponding hydrocarbons at very high temperatures (300–400 °C) and pressures (20–100 atm of H2) [8], is the major industrial technique for the removal of sulfur compounds from the fuels. This classical process not only reduces the octane/ cetane number of fuel due to saturation of olefins, but also has a limited capacity to efficiently eliminate the refractory benzothiophenic compounds, e.g. dibenzothiophene (DBT), while these compounds take a major part of the sulfides [6] in fuels [7–9]. Therefore, to meet low sulfur-level by the HDS process, severe operation conditions and high economic investments are necessary. For this reason, exploration of convenient and energy-saving processes to reduce the sulfur content of hydrocarbon fuels and therefore reduce the production of SOx has become a hot research area. At the present, there are various non-hydrodesulfurization approaches including extraction, adsorption, biodesulfurization, oxidation and membrane separation [8]. But replacement of the HDS technique with the existing technologies needs to improve them. Extractive desulfurization (EDS) process can be carried out at mild and simple conditions and it does not change the chemical structure of the compounds and consequently has no effect on the quality of liquid fuels [5,10]. So, this technique seems to be more appropriate process and it has become the object of active research in recent years. Different organic solvents, such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and 1-methyl-2-pyrrolidinone (NMP) have been used as extractive solvents in desulfurization [11]. Nevertheless, the commonly used solvents are volatile, flammable, toxic and they have serious consequences for the environment [4]. Therefore, it can be of great significance to explore new environmentally friendly and also cheap, effective and recyclable solvents to improve the extractive desulfurization process. Because of low vapor pressure and thermal stability, ionic liquids (ILs) are considered to be ‘‘green solvents’’ to replace the volatile organic solvents [12]. During the past few years, many studies have been done on desulfurization by ILs [1,3,7,12]. But extractive desulfurization by ILs has many problems including: unexplored ecosystem impacts, costly expensive [1,12], high viscosity of ILs (which decreases diffusion coefficient and interphase area and therefore diffusion rate during the extraction process) [4,13] and corrosion damages caused by halogen-containing ILs [3]. Moreover, because of rather low desulfurization efficiency by extraction with IL, either large volumes of ionic liquid for an efficient liquid–liquid extraction process must be used [7] or extraction should be integrates with oxidative desulfurization process [5]. Whereas oxidative desulfurization has several major problems, which hindered the industrial application of extractive oxidative desulfurization, such as decreasing the quality of fuel because of non-excellent selectivity to the organosulfur compound (hydrocarbons can also be oxidized), difficult posttreatment of sulfoxides and sulfones produced during the oxidation process, the reaction’s safety and the high cost of the oxidant [5,14].
37
Recently, it has been reported that hydrogen bond formed between the active hydrogen of amine-based protic ILs or ammonium-based deep eutectic solvents as extractant and the sulfur atom of S-containing molecules in fuels accounts for the higher extraction efficiency [4,12]. On the other PEG, as a proton donor solvent [15], not only has a very low vapor pressure (<0.01 hPa at 20 °C for PEG-200) and applicability over a wide temperature range because of high boiling point (b.p. >150 °C) [16] but also, in comparison with the synthesized ILs, it has several advantages such as low viscosity (dynamic viscosity 58–85 mPa s at 20 °C) [16], non-toxicity (which approved by the Food and Drug Administration (FDA), USA) [17–19], corrosion inhibition [20,21], cheap and easily obtained molecular solvent [4]. Based on recently reported works [4,12] and the mentioned characters of PEG, it seems that PEG is a good (green, cheap and effective) option for extractive desulfurization. So these points inspire us to investigate the efficiency of pure PEG, as an immiscible liquid with organic phase, for extractive desulfurization of liquid fuel in the present work. 2. Experimental section 2.1. Materials The details of the analytical grade chemicals used are as follows: PEG-200 (Merck), heptane (Merck, purity >99%), activated granular carbon (Merck, size 850–1000 lm), BT, DBT and DMDBT (Sigma–Aldrich, mass fraction purity of >98%). 2.2. Extractive desulfurization process The initial concentration of model fuels was 500 ppmw except for defined tests and they were prepared by dissolving BT, DBT and DMDBT in to n-heptane, separately. In the all experiments, defined amount of model liquid fuel containing DBT (as a typical sulfur compound) and PEG with volume ratios PEG/fuel 1:1 (excluding for certain experiments) transferred into a 50 ml glass bottle and the system was placed in a stirred thermostatted batch system at desired temperature (±0.1 °C). In a typical kinetic run, the binary mixture was stirred, stopped at desired time intervals and sampling was conducted for further quantification. In the other experiments, the mixture was stirred vigorously for 5 min and then the sulfur content of the model fuel was determined. The quantification of initial and final sulfur content in the fuel phase, using UV/Visible spectrophotometer (PG Instrument Ltd., T-80) at the related kmax [22], allowed the determination of the extraction efficiency (E.E%) based on: E.E(%) = (1 Ct/C0) 100; where C0 and Ct are initial and at any time concentration of sulfur compound (ppmw). To use the spent PEG in the next cycle, the upper organic phase was removed and the used PEG was exposed to the fresh model fuel with fixed volume ratio of PEG/model fuel as 1:1 for the next extraction stage. Regeneration of spent PEG was achieved by adsorption. For this purpose, a bottle containing spent PEG and activated granular carbon as a common adsorbent with the mass ratio PEG/adsorbent 50:1 was placed in a constant temperature shaker (n-BioTEK, NB-304) and shaked at 150 rpm for 24 h and then regenerated PEG was used again in a new EDS experiments. 3. Results and discussion 3.1. Effect of extraction conditions on sulfur removal Extraction process may be affected by some factors including phase ratio of extractant to model fuel, initial sulfur content of
38
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
liquid fuel, extraction time and temperature. Fig. 1 shows the effect of contact time on the extraction efficiency of DBT. Just after 120 s, the equilibrium can be approached with much higher extraction efficiency (about 76%) than the extraction by ILs [3–5]. The low viscosity would probably account for the fast extraction but high extraction capability of PEG can be related to the possibility of bond formation between hydrogen of PEG and sulfur atom in DBT [4,12]. Hereinafter, 5 min was chosen as the extraction time which is sufficient to be sure about equilibrium. For the industrial application of extractant, it is preferable that a lesser amount of PEG be used but with high extraction efficiency. So the effect of volume ratio of PEG to model fuel on the removal of DBT was investigated and the results are presented in Fig. 2. This figure shows as the volume ratio of PEG to fuel was varied from 0.25:1 to 2:1 the extraction efficiency was increased and reached up to 87%. When the volume ratio increases from 1:1 to 1:2, the removal efficiency only increases 10%. So, the volume ratio of 1:1 was selected for the next experiments. To find the effect of the initial concentration of sulfur compounds on extraction process, the extractive desulfurization of DBT by PEG from model fuels containing different concentrations (300–1300 ppmw) of this s-compound was carried out and the results are shown in Fig. 3. As can be seen, the extraction efficiency just partially decreased by increasing the initial sulfur content of model fuel contrary to results obtained in extractive desulfurization by amine-based protic ILs [12]. This interesting finding is very important from the industrial point of view, because PEG can be used to extractive desulfurization of liquid fuel with any (low to high) concentrations of sulfur compound. High extraction capability of PEG would probably account for these observations. Effect of temperature on the extraction of sulfur compounds from the related model fuel by PEG was probed in the temperature range of 288–308 K. Results of a typical experiment are presented in Fig. 4. This figure clearly shows that temperature change has no influence on extraction efficiency. This observation is different from extraction desulfurization processes with some ILs, in which, with the increase of the temperature, the extraction efficiencies increase and then decrease [1]. The temperature dependency of extraction by ILs may be attributed to their high viscosity [1]. Insensitivity to temperature is very beneficial in industrial applications, because extractive desulfurization can be carried out at ambient temperature easily without energy consumption.
Fig. 2. Effect of PEG/fuel volume ratio on extraction of DBT (T = 298 K, t = 5 min).
Fig. 3. Effect of initial concentration on extraction of DBT (volume ratio of PEG/fuel = 1:1, T = 298 K, t = 5 min).
3.2. Effect of S-compound on sulfur removal To study the extraction performance of PEG for different sulfur compounds, the extraction rate of BT, DBT and DMDBT has been
Fig. 4. Effect of temperature on extraction of DBT (volume ratio of PEG/fuel = 1:1).
Fig. 1. Time-course variation of extraction efficiency of DBT (500 ppmw) during extraction via PEG. Experimental conditions: volume ratio of PEG/model fuel = 1:1, T = 298 K.
compared. As represented in Fig. 5, the rate of extraction by PEG decreased as DBT > BT > DMDBT. It is interesting to note that for all sulfur compounds the extraction desulfurization reach to the equilibrium less than 3 min. The removal efficiencies of BT, DBT and DMDBT are 69%, 76% and 43%, respectively. It can be deduce that the extraction of the refractory S-containing molecules depends markedly on the nature of the organo-sulfur molecule specially their molecular sizes, electron density and steric hindrance on sulfur atoms [2,22]. The electron density on the sulfur
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
Fig. 5. Time dependency of BT, DBT and DMDBT concentration variation in fuel phase during the extraction (volume ratio of PEG/fuel = 1:1, T = 298 K). Symbols are the experimental values and solid lines represent the predicted values by first order kinetic model.
atom of these sulfur compounds increases as BT < DBT < DMDBT [2] Therefore, lower electron density on the sulfur atom led to a lower extraction of BT than that of DBT. The lowest extraction efficiency of DMDBT may be related to sterically hindered sulfur atom in DMDBT, caused by the presence of two neighbor methyl groups at 4- and 6-positions of DBT, which was a main obstacle for bond formation between sulfur atom of DMDBT and hydrogen of PEG [2,23]. The experimental kinetic data were best fitted using first-order kinetic model, ln (1 F) = kt where F = (C0 Ct)/(C0 Ce) the C0, Ce and Ct are initial, equilibrium and at any time concentrations, respectively. The obtained rate constants are 0.014, 0.032 and 0.010 s 1 at 25 °C for BT, DBT and DMDBT, respectively. The trend of the obtained rate constants is DBT > BT > DMDBT, which can be attributed to the combination of electron density and also spatial steric hindrance of sulfur atom in the molecule. Solid lines in Fig. 5 represent the predicted values by this model.
39
Fig. 6. DBT concentration after multiple extraction steps. (volume ratio of PEG/fuel = 1:1, T = 298 K, t = 5 min).
Fig. 7. Extraction efficiency of DBT by (a) used PEG at different cycles (volume ratio of used PEG/fuel = 1:1, T = 298 K, t = 5 min) and (b) regenerated spent PEG by granular activated carbon (mass ratio spent PEG/adsorbent 50:1, T = 298 K, t = 5 min).
3.3. Multistage extraction To achieve deep desulfurization, multistage extraction strategy was applied as follow: after single extraction of DBT over 5 minute, the extractant phase was separated and fresh PEG was added in the reactor and this procedure was repeated again and again to find the number of necessary extraction times for reduction of DBT concentration to about 10 ppmw. As can be seen in Fig. 6, PEG exhibits excellent extraction behavior and the sulfur content was reduced (ca 98%) from about 500 to 10 ppmw only within three extraction stages. This interesting situation i.e. using just three cycles and a very short time cannot be achieved by the ionic liquids [12,13]. 3.4. Reuse of PEG and regeneration of spent PEG For industrialization and from the environmental point of view, reuse and regeneration of desulfurizer extractant are important and it was studied here and the results are presented in Fig. 7. It can be seen that PEG can be reused several times by some decrease in extraction capability (Fig. 7a). After five cycles it becomes nearly saturated, loses its extraction capability and must be regenerated. There are different regeneration techniques such as distillation, adsorption and back-extraction processes [7]. Although the first one is the main recycling method for ILs, but this technique is not very economic because of high energy consumption [12]. The used extractant can also be regenerated by washing with organic solvents, such as diethyl ether [4], but this methods is not
eco-friendly. In the present work the spent PEG was regenerated by adsorption technique. From Fig. 7b it is clear that regenerated PEG, by granular activated carbon as a common sorbent (mass ratio PEG/sorbent 50:1), can effectively extract DBT from the spent PEG (with extraction efficiency about 60%). 4. Conclusion In conclusion, polyethylene glycol was introduced as a green, cheap, effective, non-toxic, non-corrosive and also recyclable molecular solvent for extractive desulfurization of liquid fuel for the first time. This solvent has higher extraction efficiency for dibenzothiophene (DBT) (76% within 90 s) than those of ionic liquids (ILs). Using this extractant, the BDT content was reduced from 512 to 10 ppmw (98%) only within three extraction cycles, the minimum number of cycles within shortest time reported up to now, and the deep desulfurization was achieved. Effect of some important parameters including initial concentration of sulfur compound, PEG dosage, time and temperature of extraction on the EDS process was investigated. It was found that extraction performance of PEG is temperature and initial sulfur content independent. This is an excellent finding for industrialization. The feasibility of PEG for extraction of different thiophenic compounds was observed in the order of DBT > BT > DMDBT. Furthermore, the equilibrium was achieved less than 3 min in all experiments. The
40
E. Kianpour, S. Azizian / Fuel 137 (2014) 36–40
low viscosity would probably account for the fast extraction and high extraction capability of PEG can be related to the possibility of hydrogen bond formation with sulfur atom in s-compound. The PEG was reused five cycles and then spent PEG was regenerated by adsorption method. Regenerated extractant could effectively extract DBT from fresh model fuel with extraction efficiency of 60%. Acknowledgment Financial support from Bu Ali Sina University is gratefully acknowledged. References [1] Dharaskar SA, Wasewar KL, Varma MN, Shende DZ, Tadi KK, Yoo CK. Synthesis, characterization, and application of novel trihexyl tetradecyl phosphonium bis (2,4,4-trimethylpentyl) phosphinate for extractive desulfurization of liquid fuel. Fuel Process Technol 2014;123:1–10. [2] Jiang W, Zhu W, Chang Y, Chao Y, Yin S, Liu H, et al. Ionic liquid extraction and catalytic oxidative desulfurization of fuels using dialkylpiperidinium tetrachloroferrate catalysts. Chem Eng J 2014;250:48–54. [3] Lu X, Yue L, Hu M, Cao Q, Xu L, Guo Y, et al. Piperazinium-based ionic liquids with lactate anion for extractive desulfurization of fuels. Energy Fuels 2014;28:1774–80. [4] Li C, Li D, Zou S, Li Z, Yin J, Wang A, et al. Extraction desulfurization process of fuels with ammonium-based deep eutectic solvents. Green Chem 2013;15:2793–9. [5] Shu C, Sun T, Zhang H, Jia J, Lou Z. A novel process for gasoline desulfurization based on extraction with ionic liquids and reduction by sodium borohydride. Fuel 2014;121:72–8. [6] Lin Y, Wang F, Zhang Z, Yang J, Wei Y. Polymer-supported ionic liquids: synthesis, characterization and application in fuel desulfurization. Fuel 2014;116:273–80. [7] Ferreira AR, Freire MG, Ribeiro JC, Lopes FM, Crespo JG, Coutinho JA. Ionic liquids for thiols desulfurization: experimental liquid–liquid equilibrium and COSMO–RS description. Fuel 2014;128:314–29. [8] Kulkarni PS, Afonso CA. Deep desulfurization of diesel fuel using ionic liquids: current status and future challenges. Green Chem 2010;12:1139–49.
[9] Rodríguez-Cabo B, Rodríguez H, Rodil E, Arce A, Soto A. Extractive and oxidative–extractive desulfurization of fuels with ionic liquids. Fuel 2014;117:882–9. [10] Mokhtar WNAW, Bakar WAWA, Ali R, Kadir AAA. Deep desulfurization of model diesel by extraction with N, N-dimethylformamide: optimization by Box–ehnken design. J Taiwan Inst Chem Eng 2014. http://dx.doi.org/10.1016/ j.jtice.2014.03.017. [11] Ban LL, Liu P, Ma CH, Dai B. Deep extractive desulfurization of diesel fuels by FeCl3/ionic liquids. Chin Chem Lett 2013;24:755–8. [12] Li Z, Li C, Chi Y, Wang A, Zhang Z, Li H, et al. Extraction process of dibenzothiophene with new distillable amine-based protic ionic liquids. Energy Fuels 2012;26:3723–7. [13] Kuhlmann E, Haumann M, Jess A, Seeberger A, Wasserscheid P. Ionic liquids in refinery desulfurization: comparison between biphasic and supported ionic liquid phase suspension processes. Chem Sust Chem 2009;2:969–77. [14] Zhang W, Zhang H, Xiao J, Zhao Z, Yu M, Li Z. Carbon nanotube catalysts for oxidative desulfurization of a model diesel fuel using molecular oxygen. Green Chem 2014;16:211–20. [15] Yang ZZ, He LN, Zhao YN, Yu B. Highly efficient SO2 absorption and its subsequent utilization by weak base/polyethylene glycol binary system. Environ Sci Technol 2013;47:1598–605. [16] Safety Data Sheet, according to the Regulation (EC) No. 1907/2006. Revision Data 1920.1909.2001, Version 1907.1905 www.merck-chemicals.com. [17] Jaiswal A, Ghosh SS, Chattopadhyay A. One step synthesis of C-dots by microwave mediated caramelization of poly (ethylene glycol). Chem Commun 2012;48:407–9. [18] Veronese FM, Pasut G. PEGylation, successful approach to drug delivery. Drug Discovery Today 2005;10:1451–8. [19] Karakoti AS, Das S, Thevuthasan S, Seal S. PEGylated inorganic nanoparticles. Angew Chem Int Ed 2011;50:1980–94. [20] Ashassi-Sorkhabi H, Ghalebsaz-Jeddi N. Inhibition effect of polyethylene glycol on the corrosion of carbon steel in sulphuric acid. Mater Chem Phys 2005;92:480–6. [21] Ashassi-Sorkhabi H, Ghalebsaz-Jeddi N, Hashemzadeh F, Jahani H. Corrosion inhibition of carbon steel in hydrochloric acid by some polyethylene glycols. Electrochim Acta 2006;51:3848–54. [22] Fallah RN, Azizian S. Removal of thiophenic compounds from liquid fuel by different modified activated carbon cloths. Fuel Process Technol 2012;93:45–52. [23] Cychosz KA, Wong-Foy AG, Matzger AJ. Liquid phase adsorption by microporous coordination polymers: removal of organosulfur compounds. J Am Chem Soc 2008;130:6938–9.