Oxidative-extractive desulfurization of liquid fuel using

Petroleum Science and Technology

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Oxidative-extractive desulfurization of liquid fuel using stannous chloride-acetic acid mixture as catalyst Shailesh Pandey & Vimal Chandra Srivastava To cite this article: Shailesh Pandey & Vimal Chandra Srivastava (2018) Oxidative-extractive desulfurization of liquid fuel using stannous chloride-acetic acid mixture as catalyst, Petroleum Science and Technology, 36:1, 40-47, DOI: 10.1080/10916466.2017.1403451 To link to this article: https://doi.org/10.1080/10916466.2017.1403451

Published online: 19 Dec 2017.

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PETROLEUM SCIENCE AND TECHNOLOGY , VOL. , NO. , – https://doi.org/./..

Oxidative-extractive desulfurization of liquid fuel using stannous chloride-acetic acid mixture as catalyst Shailesh Pandey and Vimal Chandra Srivastava Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India

ABSTRACT

KEYWORDS

Present study reports catalytic oxidative-extractive desulfurization (COEDS) of model oil (dibenzothiophene (DBT) dissolved in iso-octane) using an acid mixture (composed of Lewis acid and organic acid (glacial acetic acid)) as catalyst and inorganic oxysalt, potassium dichromate, as oxidant. A series of acid mixtures were prepared by mixing different amount of Lewis acids (SnCl2 , CaCl2 or CuCl2 ) in specific amount of acetic acid and tested for the removal of sulfur from model oil. SnCl2 , which performed best as a Lewis acid, was used in further studies. Effect of Lewis acid to sulfur (molar) ratio was studied in the range of 1–5. Effects of other parameters i.e. oxidant to sulfur (O/S) molar ratio, temperature and time on desulfurization efficiency were also investigated. Fourier transform infrared (FTIR) spectroscopy of SnCl2 -CH3 COOH mixture, model oil (before COEDS), raffinate and extract layers was carried out to understand the removal mechanism. Apparent activation energy for COEDS process with SnCl2 -CH3 COOH system was calculated as 11.65 kJ/mol. At the optimized conditions of oxidant to sulfur molar ratio (O/S = 2:1) and Lewis acid to sulfur molar ratio (SnCl2 /S = 5:1), maximum 61.3% sulfur removal was observed from model oil containing 1000 ppm of sulfur at 308 K.

catalytic oxidation-extraction; desulfurization; dibenzothiophene; FTIR; kinetic study; lewis acid

1. Introduction Desulfurization of liquid fuels has become the need of the hour so as to control the sulfur oxide emissions (Srivastava, 2012; Kumar, Srivastava, and Nanoti 2017). A number of traditional methods in combination with newer technologies are being researched to meet the future requirement of ultra-low sulfur in the liquid fuels (Jiao et al. 2006; Muzic, Sertic-Bionda, and Adzamic 2011; Kumar et al. 2016; Thaligari, Srivastava, and Prasad 2015). Catalytic oxidative-extractive desulfurization (COEDS) is one such method in which depending upon the catalyst-oxidant system used and reaction conditions applied, majority of the refractory thiophenic sulfur compounds such as dibenzothiophene, DBT (which are not removed in hydrodesulfurization method) are converted into sulfoxides and sulfones, and then extracted from fuel by using specific extractive solvents. Ozone, hydrogen peroxides (H2 O2 ), oxygen, organic peroxides, and inorganic oxidants are the major oxidants used in the desulfurization of liquid fuels. Among them, oxidation capability of H2 O2 is high and can be used more efficiently in desulfurization technologies, however, it cann’t be used at comparatively high temperature. H2 O2 is generally used with some expensive catalysts like polyoxometalate and ionic liquids for better oxidation of sulfur-compounds present in liquid fuel (Lü et al. 2006; Wang et al. 2010). Some inorganic oxidants like KMnO4 and K2 Cr2 O7 having good stability and lower cost, and can be used as oxidants in COEDS process. In addition, some organic acids are also required along with these inorganic oxysalts for better catalysis and oxidation of sulfur compounds. The oxidizability of sulfur compound depends on the acidity of organic acid used. CONTACT Vimal Chandra Srivastava [email protected]; [email protected] Institute of Technology Roorkee, Roorkee , Uttarakhand, India. ©  Taylor & Francis Group, LLC

Department of Chemical Engineering, Indian

PETROLEUM SCIENCE AND TECHNOLOGY

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Desulfurization rate of sulfur compounds is controlled by addition of a specific type of Lewis acid into the organic acid during COEDS treatment. In presence of Lewis acid, organic acids show better extraction characteristics towards sulfur compound (Chen et al. 2012; Gao et al. 2013; Song et al. 2013). Main aim of the present work is to perform COEDS of model oil by using an acid mixture composed of a Lewis acid salt (SnCl2 , CaCl2 or CuCl2 ) and acetic acid as catalyst. Effects of various parameters such as Lewis acid to sulfur molar ratio, oxidant (potassium dichromate) to sulfur molar ratio, time and temperature on the desulfurization efficiency have been studied. Fourier transform infrared spectroscopy (FTIR) spectroscopy has been used to study the sulfur removal mechanism.

2. Experimental 2.1. Materials Iso-octane and glacial acetic acid were purchased from Ranbaxy fine chemicals, New Delhi. DBT was purchased from Spectrochem, Mumbai. SnCl2 , CaCl2 and CuCl2 were purchased from Fisher Scientific, Mumbai. K2 Cr2 O7 , CH2 Cl2 and HCl were purchased from S.D. fine chemicals, Mumbai. All these analytical grade reagents were used as received without further purification.

2.2. Preparation and characterization of catalytic acid mixture In the experiment, the stannous chloride (SnCl2 ) was directly added into the glacial CH3 COOH to prepare a catalytic acidic solution. SnCl2 concentration was varied depending upon the desired molar ratio of SnCl2 to sulfur. To prepare acid mixture, 5 ml of CH3 COOH was taken in beaker, then 0.4266 g of SnCl2 (for the molar ratio of Lewis acid to sulfur ratio of 5:1) was added directly into beaker and magnetically stirred for 30 min to dissolve SnCl2 completely. The SnCl2 /sulfur molar ratio was varied in the range of 1 to 5. Similarly, catalytic acid mixture of CaCl2 and CuCl2 were also prepared.

2.3. Catalytic oxidative-extractive desulfurization experiment The model oil used in the present study was prepared by mixing a certain amount of dibenzothiophene (DBT) in iso–octane. The initial sulfur content in the model oil was 1000 ppm (1000 µg/g). A certain amount of oxidant (K2 Cr2 O7 ) was added into a flask containing the model oil so as to obtain the desired oxidant to sulfur molar ratio. Thereafter, Lewis acid (SnCl2 , CaCl2 or CuCl2 ) was added with vigorous stirring, and after sometime, a certain amount of glacial acetic acid (3 ml/10 ml model oil) was directly added to the flask. This slurry was vigorously stirred at the desired temperature (293, 298, 303 or 308 K) for specific period of time (in range of 5–90 min), after which the sample was kept without stirring for 60 min. Two layers viz. extract (acid layer) and raffinate (oil layer) layers get formed. Total sulfur content in the initial model oil and the top raffinate layer after the experiment was analysed by X-ray fluorescence (XRF) method using Rigaku NEX QC analyzer. To understand the removal mechanism, the sulfur compounds present in the extract layer were analyzed by FTIR analysis. After the experiment was complete, extract phase was separated from the mixture and washed with equal volume of water. To analyse oxidized sulfur compound, a definite amount of dilute HCl (25 ml/100 ml of model oil) solution and dichloromethane (25 ml/100 ml of sample) was added into extract phase and mixed completely using magnetic stirrer. This whole slurry was kept stagnant for 15 min so as to form two phases. Thereafter, CH2 Cl2 phase was separated and washed three times with dilute HCl. Oxidised sulfur compounds were obtained by evaporating the solvent at 313 K and under vacuum. Oxidized DBT was recovered in the form of solid powder; which was used for FTIR analysis by KBr disk technique (Song et al. 2013).

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S. PANDEY AND V. C. SRIVASTAVA

Figure . FTIR analysis of (a) SnCl -CH COOH complex with respect to pure CH COOH, and (b) model oil, raffinate and extract phases.

3. Result and discussion 3.1. Characterization of catalytic acid mixture FTIR spectra of SnCl2 -CH3 COOH mixture and pure CH3 COOH is shown in Figure 1(a). FTIR spectrum of prepared SnCl2 -CH3 COOH acidic mixture shows many peaks corresponding to pure CH3 COOH. In IR spectra of CH3 COOH, one peak at 1722 cm−1 is due to C=O group of pure CH3 COOH; characteristic peak at 1288 cm−1 was due to C-O in acid mixture. Peaks at 2631 cm−1 and 2632 cm−1 are due to O-H group of CH3 COOH. Peaks at 3419 cm−1 and 3415 cm−1 are due to CH3 group in acetic acid. FTIR pattern of SnCl2 -CH3 COOH mixture is similar to that of pure CH3 COOH but intensities of some peaks in SnCl2 -CH3 COOH mixture are greater than that of pure CH3 COOH which indicates enhancement of corresponding functional groups in SnCl2 -CH3 COOH mixture. Mixing a certain amount of Lewis acid (SnCl2 ) with pure acetic acid results in an increase in deshielding effect of the protons on COOH group of acetic acid and accordingly that the positive charge of H atom on COOH group was enhanced. Thereby, the acidity of CH3 COOH increased with increasing the amount of SnCl2 (Song et al. 2013). This enhancement of functional group results in improvement of acidity and helps in improving the catalytic behaviour of CH3 COOH for oxidation and extraction of sulfur compound from model oil. 3.2. Effect of process parameters Three types of catalytic acid mixtures (prepared by adding three Lewis acids namely CaCl2 , CuCl2 and SnCl2 in glacial acetic acid) were used for sulfur removal from model oil at 298 K. Oxidant to sulfur ratio was kept constant at 2. Results are shown in Figure 2a. For Lewis acid to sulfur molar ratio of 5:1, sulfur removal of 61.1%, 43.2% and 36.7% was obtained with SnCl2 , CuCl2 and CaCl2 , respectively. It may be noted, in absence of Lewis acid, only 7.8% removal was achieved using glacial CH3 COOH alone. Thus, Lewis acid acts as catalyst and improves acidity of CH3 COOH which catalyse the reaction during COEDS


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Figure . (a) Effect of Lewis acid (SnCl , CuCl or CaCl ) at various Lewis acids to sulfur molar ratio on sulfur removal, oxidant to sulfur ratio = , temperature =  K and time =  min, (b) Effect of oxidant to sulfur molar ratio on desulfurization efficiency, Lewis acid to sulfur = :, time =  min, temperature =  K, (c) Variation of residual sulphur concentration with time at different temperatures (SnCl /sulfur = :, K Cr O / sulfur = :); experimental data represented by point and the fit of the power-law model is shown by dashed lines.

process. Electronegativity value of Sn, Cu and Ca on Pauling scale are 1.96, 1.90 and 1.00, respectively. Thus, SnCl2 is more acidic than CuCl2 and CaCl2 and thus, the SnCl2 as Lewis acid performed better. Considering the results SnCl2 -CH3 COOH system was further used for studying the effect of parameters and thermodynamics and kinetic studies. COEDS with stannous chloride-acetic acid- potassium dichromate mixture is shown in Figure 3. During COEDS process, DBT first moves from oil phase to acidic phase. In the acidic phase, following reactions occur in the presence of K2 Cr2 O7 , SnCl2 and acetic acid. CH3 COOH + SnCl2 → CH3 COOSnCl2 − + CH3 COOH2+ K2 Cr2 O7 + CH3 COOH2 + → K+ + CrO3 + H2 O CrO3 + DBT → Cr+++ + DBTO2 + H2 O

(1) (2) (3)

Phenomenon of Lewis acids (SnCl2 , CuCl2 and CaCl2 )-CH3 COOH complexation is based on the interaction of Lewis acid salts with hydroxyl (OH) group of CH3 COOH. Such interactions cause decrease in O-H bond strength and release of atom. Complexation between SnCl2 and CH3 COOH results in an increase in acidity of CH3 COOH due to addition of one H atom on COOH group, and CH3 COOH2+ get formed. Deshielding effect on COOH group gets increased due to an increase in the SnCl2 concentration and one more H atom gets attached with COOH group (Song et al. 2013). CH3 COOH2+ reacts with

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Figure . Catalytic oxidative-extractive desulfurization (COEDS) with stannous chloride-acetic acid- potassium dichromate mixture.

K2 Cr2 O7 which was used as oxidant in COEDS system. K2 Cr2 O7 gets converted to CrO3 in presence of SnCl2 -CH3 COOH system. DBT get oxidized to DBTO2 in a catalytic medium of acid mixture and CrO3 . To achieve better removal efficiency, SnCl2 /sulfur molar ratio was varied in range of 1–6. During these experiments, the oxidant molar ratio was kept constant at 2:1, reaction time was 1 h and temperature was 298 K. Sulfur removal at SnCl2 to sulfur molar ratios of 1, 2, 3, 4, 5 and 6 was found to be 16.8, 22.2, 34.2, 58.2 and 61.1% respectively. Thus, sulfur removal increased up to a molar ratio of 5, and thereafter, increase was marginal. To understand the effect of K2 Cr2 O7 concentration on desulfurization efficiency, experiments were carried out at varying oxidant to sulfur molar ratio (in range of 0.5–4). Results are shown in Figure 2b. Only 18.5% removal was achieved for oxidant to sulfur molar ratio of 0.5. An increase in K2 Cr2 O7 to sulfur ratio from 0.5:1 to 4:1, sulfur removal increased up to 58.8%. Further increase in K2 Cr2 O7 amount didn’t affect the desulfurization efficiency. It was observed that K2 Cr2 O7 changed its colour from orange to dark green whenever it was used with SnCl2 -CH3 COOH system. During the reaction, K2 Cr2 O7 might change to Cr(VI) form, and shows better solubility than the anhydrous Lewis-organic acid system. It is known that, in presence of anhydrous H2 SO4 , orange K2 Cr2 O7 changes to dark red CrO3 (Song et al. 2013). Using SnCl2 -CH3 COOH complex, K2 Cr2 O7 is converted to CrO3 but no such conversion was observed on using CH3 COOH alone. During oxidation process, K2 Cr2 O7 was converted to CrO3 and K+ and on reacting with CH3 COOH2 + , thereafter K+ gets attached to CH3 COOSnCl2 − to recover SnCl2 and potassium acetate (Figure 3). 3.3. Characterization of model oil, raffinate phase (upper layer) and extract phase (bottom layer) Characterization of model oil, raffinate phase and extract phase was done by FTIR analysis. In Figure 1b, C-S band stretching (700–750 cm−1 ) corresponding to DBT is observed in both, model oil and raffinate phase. It is seen that the characteristic peak corresponding to C-S stretching gets shifted from 730 to 738 cm−1 after COEDS experiment in the raffinate phase. The peak in raffinate phase is due to residual amount of DBT. It also confirms that DBT in raffinate phase is not oxidised and that the oxidised


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species (sulfones) got transferred into the extract phase. FTIR spectroscopy was employed for further characterization of the oxidation products, sulfones, for which appeared around 1400–1360 cm−1 and 1300–1250 cm−1 . Specific difference observed between the FTIR spectra of extract and raffinate is for the specific absorption peak at 1263 cm−1 , which confirms that the DBT gets oxidized to DBTO2 , during the COEDS process. 3.4. Kinetic study Kinetic study was performed by observing residual sulfur content at different time and temperature. Residual sulfur concentration in extract layer was noted at individual temperatures after various treatment time intervals as shown in Figure 2c. Highest desulfurization efficiency was achieved at 308 K after 90 min experiment time. It is seen that the temperature increases the desulfurization efficiency and the desulfurization rate. Similar results have been reported earlier during oxidative desulfurization of model oil (Kumar, Srivastava, and Badoni 2014; Singh, Srivastava, and Gautam 2016). The change in concentration of CH3 COOOH is nearly insignificant in comparison with that of DBT (Singh, Srivastava, and Gautam 2016; Tam, Kittrell, and Eldridge 1990; Wamg et al. 2006) and assuming nth order reaction with respect to total sulfur content, rate of sulfur oxidation can be written as: dCs = kn Csn (4) dt where, Cs is concentration of sulfur (µg/g), n is the order of reaction and kn is reaction constant for the nth order reaction. By integrating above equation with the limit that at t = 0, Cs = Co = 1000 µg/g (in terms of sulfur) and that at t = t, Cs = Ct , following equation can be obtained: Ct1−n − C01−n = (1 − n) kn t

(5)

Equation 4 can further be integrated with the limit that at t = 0, Cs = Co = 1,000 µg/g (in terms of sulfur) and that at t = t, Cs = Ct to study the first order kinetics for the COEDS of DBT in iso-octane. Following equation is obtained for the first order reaction kinetics: ln

Co = k1 t Ct

(6)

where, k1 represent first order rate constant. Suppose for n = 1, at t = 0, Cs = C1 (concentration of sulfur in the raffinate phase), above equation can be rearranged terms of C1 as follows (Singh, Srivastava, and Gautam 2016): ln

C1 = k1,n t Ct

(7)

Above equations are solved to find a correlation between C0 , C1 and Ct as follows: ln

Co Co = k1,n t + ln Ct C1

(8)

A non-linear regression method was used to solve the equations 5, 6 and 8, by minimizing the average relative error as given by following formula: n 100 Ct,exp,i − Ct,cal,i (9) ARE = n i=1 Ct,exp,i Cexp and Ccat are the experimental and calculated value of sulfur concentrations in ppm. Table 1 represents values of n, k1 , k1 ,n , kn ,C1 and ARE at various temperatures for the three different kinetic model used. Comparison of three kinetic system models was carried out by plotting a graph between experimental values and calculated values of Ct as shown in Figure 4. Data represented in Table 1 and Figure 4 show that the first order kinetic model was least-fit and a comparable fit was observed between the nth order kinetic model and the modified first-order model used previously by Singh, Srivastava, and Gautam

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Table . Kinetic parameters during COEDS with SnCl -CH COOH mixture. Temperature (K)

Rate constant

Reaction order

Power-law model

kn ((µg/g) (−n) /min)

n

. × −

. . . .

. . . .

. . . .

   

. . . .

k,n (min)-

C (µg/g)

. . . .

   

    First-order model     Modified-first order model    

. × − . × − . × −

ARE

k (min)−

Figure . Comparison of experimental and calculated Ct at (a)  K (b)  K (c)  K (d)  K.

. . . .


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(2016). However, minimum ARE was obtained with power law (nth order) model with reaction order of 2.3. Apparent activation energy (Ea ) for COEDS process, calculated by plotting a graph between ln kn versus 1/T, was found to be 11.65 kJ/mol. Li et al. (2012) reported Ea value of 32.5 kJ/mol for a system with C5 H9 NO-SnCl2 ionic liquid. Similarly, Ea values of 26.8 kJ/mol and 31.4 kJ/mol were reported for two systems with the hexadecyltrimethyl ammonium chloride and dodecyltrimethylammonium chloride as phase transfer catalysts (Qiu et al. 2009). Thus, the present system requires less Ea as compared to those in the previous studies.

4. Conclusions This study aimed to investigate performance of the an acid mixture composed of SnCl2 and CH3 COOH as catalyst during catalytic oxidative-extractive desulfurization (COEDS) of model oil using K2 Cr2 O7 as an oxidant. FTIR spectra of SnCl2 -CH3 COOH mixture when compared with that of pure CH3 COOH confirmed improved acidity of CH3 COOH. FTIR spectrum of raffinate phase when compared with that of model oil, confirmed the removal of DBT from model oil. FTIR spectrum of extract phase confirmed presence of DBTO2 , which is an oxidised product of DBT. Optimum Lewis acid to sulfur molar ratio was found to be 5:1 and that the optimum K2 Cr2 O7 to sulfur molar ratio required for oxidation of DBT was found to be 2:1. Apparent activation energy for COEDS process with SnCl2 -CH3 COOH system was calculated as 11.65 kJ/mol with order of reaction being 2.3.

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