Removal of Basic Nitrogen Compounds from Coker Diesel by Eutectic Ionic Liquid

Liu Jie; Liu Jinbo; Li Wenshen; Long Wenyu

(College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University,Fushun 113001)

Abstract: The eutectic ionic liquid (EIL) tetraethyl ammonium bromide-malonic acid (TEAB-Mal) was synthesized, with its structure characterized by the FT-IR spectroscopy and the 1H NMR spectrometry. The performance for removal of basic nitrogen compounds by EIL was studied using coker diesel as the feedstock. Experimental results showed that the EIL (TEAB-Mal) exhibited a good denitrogenation performance, leading to a 93.6% of basic N-removal efficiency under reaction conditions covering: a temperature of 30 °C, an EIL to oil mass ratio of 1:1, an extraction time of 30 min, and a settling time of 120 min, while the basic nitrogen content in diesel dropped from 580 μg/g to 37 μg/g. In addition, the efficiency for extraction of basic N-compounds could still reach 62.9% at an EIL/oil mass ratio of 1:7 after four recycles of the EIL.

Key words: eutectic ionic liquid; tetraethyl ammonium bromide-malonic acid; denitrogenation; basic nitrogen; coker diesel

1 Introduction

The nitrogen-containing compounds (N-compounds) in fuel oil are frequently classified as basic six-membered heterocyclics (e.g. pyridine, quinoline, etc.) and non-basic five-membered heterocyclics (e.g. carbazole, indole, etc.).It is important to remove N-containing compounds from fuel oils before being used not only out of concerns on environmental pollution resulted from nitrogen oxides emission during combustion[1-2], but also because of their inhibiting influence on hydrodesulfurization (HDS)technology[3-4]. Meanwhile, the presence of basic nitrogen compounds is one of the main reasons that results in the inferior oxidization stability of coker diesel[5]. Selective removal of N-compounds from feed oils prior to HDS can strongly enhance the further deep desulfurization and increase the catalyst cycle length[6]. Nowadays the optimum denitrogenation method in the petroleum refining industry is hydrodenitrogenation (HDN), but the technology requires relatively severe operating conditions.It is necessary to develop the non-hydrodenitrogenation method to reduce the severity of the hydrogenation process. Thus, many alternative technologies, such as the adsorptive[7], extractive[8], and oxidative[9]denitrogenation processes, have been developed extensively. Among these approaches, the extractive denitrogenation seems to be the most attractive one because of its facile operation,and many research results showed that the extractive denitrogenation technology can effectively remove the basic N-compounds from fuel oil[10-11]. However, the conventional solvents used in extractive denitrogenation process are volatile and can hardly be recycled.

Ionic liquids (ILs) can be used as ideal substitutes for organic solvents to improve the environmental friendliness of conventional extraction technology because of their negligible vapor pressure, high chemical and thermal stability, nonflammability, or recyclability[12-14]. In recent years, the extractive denitrogenation process using ILs has received remarkable progress. Wang[15]and Su[16]have found out that the Brönsted acidic ILs such as [Bmim]H2PO4can remove effectively N-compounds from fuel oils. [Bmim]HSO4was found to be capable of removing more than 90% of basic N-compounds from the Fushun oil shale-derived diesel[17]. Chen, et al.[18]and Li, et al.[19]investigated the efficiency for denitrogenation of the ZnCl2-based Lewis acidic ILs, e.g., [Bmim]Br/ZnCl2, [Bmim]Cl/ZnCl2, resulting in very high basic N-compounds removal efficiency. The denitrogenation efficiency of dicyanamide-based ionic liquids was reported by Asumana, et al.[20], showing a moderate efficiency for pyridine removal.

Although the imidazolium-based ILs were found to be effective in removing basic N-compounds, their relative complex synthesis method, possible toxicity, poor biodegradability, and high cost limited their potential use in the industry[21]. Another group of solvents called the deep eutectic solvents (DESs), often considered as IL analogues or also known as the eutectic-based ionic liquids (EILs), have attracted great attention in recent years. DESs are composed of the mixture of a salt and a hydrogen-bond donor (HBD, such as amide, carboxylic acid, polyols, etc.) or a complexing agent which can form a liquid with a freezing point lower than the freezing points of the constituting compounds[22-24]. DESs not only share the advantages of ionic liquids, but also are superior to ionic liquids in terms of toxicity, synthesis process, and raw materials cost, which can bring about enhanced greenness and economy of application cost.In recent years, deep eutectic solvents have been widely used in catalysis[25], electrochemistry[26], separation[27]and desulfurization[28]fields. Nevertheless, to the best of our knowledge, up to this moment, DESs have virtually been rarely adopted in the domain of denitrogenation of fuel oil.This research work was focused on the development of eutectic-based ionic liquid (EIL) for extractive denitrogenation. The EIL, tetraethyl ammonium bromidemalonic acid (TEAB-Mal), was prepared, and its performance for removing basic N-compounds from industrial coker diesel was investigated for the first time.Also, the main conditions that could affect the extraction process were investigated in detail, such as the extraction time, the temperature, the EIL/oil mass ratio, etc. The work could provide a new approach for the removal of basic N-compounds from diesel fuel.

2 Experimental

2.1 Experiment reagent and apparatus

Tetraethyl ammonium bromide (99.0%) and malonic acid(98.0%) were purchased from the Sinopharm Chemical Reagent Co., Ltd.

The experimental devices included: a Cary 600 series FTIR spectrometer (American Agilent Technologies);a nuclear magnetic resonance spectrometer Varian Mercury-Plus 300BB (American Varian); a TSN-5000 series fluorescence nitrogen/sulfur analyzer (Jiangfen Electroanalytical Co., Ltd., China); a rotating viscometer(NDJ-79, Shanghai, China); a constant temperature magnetic heating stirrer DF-101S (Gongyi City Instrument Co., Ltd, China); an automatic potentiometric titrator ZD-2 (A) (Shanghai Dapu Instruments Co.,Ltd., China); and a vacuum oven ZK-82J (Shanghai Experimental Instrument Factory, China)

2.2 Experimental feedstock

The coker diesel used in the present study was collected from the Fushun Petrochemical Company, PetroChina,with its main properties depicted in Table 1.

Table 1 Basic properties of coker diesel

2.3 Synthesis of EIL tetraethyl ammonium bromidemalonic acid

Tetraethyl ammonium bromide and malonic acid were mixed at a molar ratio of 1:1 in a round-bottomed flask.Then the mixture became a transparent liquid under vigorously stirring with a magnetic stirrer at a temperature of 80 °C for 3 h. The chemical equation for the synthesis of TEAB-Mal is presented in Figure 1. The melting point of the eutectic ionic liquid prepared thereby is lower than the room temperature. Moreover, compared with conventional imidazolium-based ionic liquids[29-30], the eutectic ionic liquid possesses the advantages of simpler synthesis process and lower cost.

2.4 Denitrogenation experiment procedure and N-content analysis

Figure 1 Synthesis of TEAB-Mal

In a typical experiment, the coker diesel and the EIL TEAB-Mal were placed in a 50-mL conical flask and were magnetically stirred at a specified temperature. After the extraction was carried out over a specified time, the mixture was subject to settling for a definite duration for phase separation. The basic nitrogen content in the upper oil phase was analyzed by the perchloric acid-glacial acetic acid titration method (SH/T 0162—1992, China).The efficiency for extraction (E, %) of basic N-compounds is determined according to the following formula:

where Ciand Cfare the initial and final basic nitrogen contents in diesel.

After denitrogenation, the EIL layer was washed with petroleum ether for 3—5 times, and was evaporated under vacuum to remove the petroleum ether. The regenerated EIL was used for further extraction of fresh coker diesel under the same operating conditions to investigate its performance for extracting basic N-compounds.

3 Results and Discussion

3.1 Structure characterization and property determination

3.1.1 FT IR analysis

It can be seen from the FTIR spectrum (b) of malonic acid in Figure 2 that the broad peak appeared at the wavenumber of 2 927 cm-1could be assigned to the stretching vibrations of -CH2-. The peaks near 1 700 cm-1and 1 301 cm-1came from the stretching vibrations of C=O and C-O, respectively, while the O-H bending vibration peak appeared at 911 cm-1. Judging from the FTIR spectrum (c) of tetraethyl ammonium bromide, the peaks that appeared at 1 380 cm-1and 1 490 cm-1could be assigned to the C-H deformation vibrations of methyl and methylene groups. The peaks near 1 170 cm-1and 1 000 cm-1came from the stretching vibrations of C-N and C-C, respectively. Obviously, the FTIR spectrum(a) of EIL was slightly different from those of tetraethyl ammonium bromide (c) and malonic acid (b). It was noteworthy that the C=O absorption peak centered at 1 700 cm-1in malonic acid shifted to a higher frequency region upon being treated with tetraethyl ammonium bromide, and meanwhile the O-H bending vibration at 911 cm-1disappeared, implying that the interaction of tetraethyl ammonium bromide and malonic acid occurred,consequently coupled with the formation of new EIL.

Figure 2 FTIR spectra of EIL TEAB-Mal (a), malonic acid(b) and tetraethyl ammonium bromide (c)

3.1.2 1H NMR analysis

1H-NMR (DMSO, 500 MHz), δ=3.239 (10H, m, -CH2-),δ=1.151 (12H, m, -CH3). The hydrogen chemical shift of methylene group in malonic acid and tetraethyl ammonium cation was overlapped.

Figure 3 1H NMR of TEAB-Mal

3.1.3 Viscosity determination

The viscosity of TEAB-Mal at different temperature was determined, with the result shown in Figure 4.

Figure 4 Viscosity- temperature relationship of TEAB-Mal

It can be seen from Figure 4 that the viscosity (η) of TEABMal reduced exponentially with the increase of temperature,e.g.: the viscosity was 336.44 mPa·s at 298.15 K, and became 38.47 mPa·s at 333.15 K. It can be found out further that a linear relationship existed between the natural logarithm of viscosity (lnη) and reciprocal value of temperature (1/T),conforming to the Arrhenius equation[31], i.e., lnη=lnη0+Eη/RT. On the basis of this phenomenon, the viscosity activation energy (Eη) of TEAB-Mal was determined as 51.6 kJ/mol. Compared with the viscosity of [Bmim]HSO4and[Bmim]H2PO4reported in the literature[16,30], the viscosity of TEAB-Mal was lower, which was beneficial to the mass transfer in the EIL extraction process.

3.2 Basic N-removal performance by EIL TEAB-Mal at different extraction time

The basic N-removal performance of the EIL tetraethyl ammonium bromide-malonic acid was investigated using coker diesel as the feedstock under different extraction time, with the results shown in Figure 5. Obviously,the EIL exhibited a better basic N-compounds removal performance, and the basic N-compounds extraction efficiency could reach more than 80% under experimental conditions. By taking quinoline as an example, the possible denitrogenation mechanism of TEAB-Mal is shown in Figure 6.

Figure 5 Basic N-removal efficiency of TEAB-Mal at different extraction time

In the process of removing basic N-compounds with TEABMal, the H+ions provided by malonic acid attacked the lone pair of electrons of the N atom in basic N-compounds such as quinoline, which was followed by a complex reaction between the H+and quinoline, so the basic N-compounds can be removed effectively from diesel.

Figure 6 Mechanism of extractive denitrogenation with TEAB-Mal

In addition, it was found from Figure 5 that the basic N-compounds removal efficiency increased significantly when the extraction time increased from 5 min to 20 min,and the N-compounds extraction efficiency basically did not change after 20 min, which indicated that the extraction process could reach the equilibrium quickly.Upon considering the sufficient contact of N-compounds and EIL, the appropriate extraction time was determined as 30 min in the work, resulting in a basic N-compounds removal efficiency of 80.8%.

3.3 Effect of EIL/oil mass ratio

The effect of EIL/oil mass ratio on denitrogenation was investigated, with the results presented in Figure 7.

Figure 7 Effect of EIL/oil mass ratio on the removal of basic N-compounds

It can be observed that the basic N-removal efficiency increased with the increase of EIL/oil mass ratio. At an EIL/oil mass ratio of 1:10, the extraction efficiency was 76.8%, and became 93.6% at an EIL/oil mass ratio of 1:1,while correspondingly the basic nitrogen content in diesel was reduced from the original value of 580 μg/g to 37 μg/g. Obviously, the greater the amount of TEAB-Mal,the greater the extracting capability and the probability of contact between the EIL and the N-compounds in the diesel. The results showed that an extremely low nitrogen content might be attained by varying the EIL/diesel mass ratio. Upon considering the operating cost, the EIL/oil mass ratio was determined as 1:7 to investigate the influence of other conditions on the denitrogenation performance of EIL in the research work.

3.4 Effect of temperature

Six temperature points from 30 °C to 65 °C were selected to assess the effect of temperature, with the results presented in Figure 8. It can be seen that the basic N- compounds removal efficiency of EIL for coker diesel reduced slightly from 80.8% at 30 °C to 74.9% at 65 °C. Similar results were reported in the literature[15-18]. The possible reason was that the complexation between basic N-compounds and EIL was exothermic, and the increase in temperature did not favor the process along the positive direction. So the temperature was determined as 30 °C in the study.

Figure 8 Effect of extraction temperature on basic N-removal efficiency

3.5 Effect of settling time

The settling procedure was very necessary in order to separate the EIL containing undesirable components(e.g. N-compounds) from diesel after the denitrogenation process. The effect of settling time on the basic N-removal efficiency is shown in Figure 9.

Figure 9 Effect of settling time on basic N-removal efficiency

Obviously, the basic N-removal efficiency increased markedly from 74.5% to 79.8% with the settling time extending from 30 min to 90 min, and subsequently the basic N-removal efficiency tended to level off. In order to make EIL and diesel separate completely, the settling time was determined as 120 min in the study.

3.6 Regeneration of EIL TEAB-Mal

The regeneration of TEAB-Mal was performed by the back extraction of N-compounds with petroleum ether.The regenerated EIL was used for further extraction of fresh coker diesel under the same operating conditions.Table 2 shows the efficiency for removal of basic N-compounds by using the regenerated EIL. It can be seen that a basic N-compounds removal efficiency of 62.9% could be obtained with the regenerated EIL after four cycles. This result was very significant for the applications of EIL because of the facile recycling outcome. Nevertheless, the denitrogenation efficiency of the regenerated EIL decreased as compared with that of fresh EIL. The possible reason was that the N-compounds,which were already dissolved in EIL, could not be reextracted completely by petroleum ether. In view of this,further study on more efficient methods for regeneration of the EIL will be carried out later.

Table 2 Efficiency (E) for removal of basic N-compounds with regenerated TEAB-Mal

4 Conclusions

In summary, we reported a novel method for denitrogenation of diesel using the eutectic ionic liquid(EIL) as extractant. Tetraethyl ammonium bromidemalonic acid (TEAB-Mal) was synthesized and its denitrogenation performance was investigated using coker diesel as the feedstock for the first time. It was observed that the EIL was capable of effectively removing the basic N-compounds from coker diesel and a basic N-compounds removal efficiency of 93.6% was achieved at a temperature of 30 °C, an EIL/oil mass ratio of 1:1, an extraction time of 30 min, and a settling time of 120 min.In addition, the basic N-compounds removal efficiency could still reach 62.9% at an EIL/oil mass ratio of 1:7 after four recycles of the EIL.

Acknowledgements:The authors are grateful for the financial support from the Doctoral Funds of Liaoning Provincial Natural Science Foundation (201601323).