Hu Lin; Lü Haipeng; Wang Guanghua; Guo Xianhou

(Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education,China University of Mining & Technology, Xuzhou 221116)

Abstract: Oily sludge was recognized as a kind of hazardous wastes, so recycling its high content of petroleum hydrocarbons can contribute to the clean environment. In this study, oily sludge was subjected to continous ultrasonic extraction with an acetone/carbon disul fide (CS2) mixture solvent. Fourier transform infrared spectroscopy (FTIRS), gas chromatography/mass spectrometry (GC/MS), and Quadrupole exactive Orbitrap mass spectrometry (QPEOTMS) were used to analyze the oil phase compounds from oily sludge. The n-alkanes ranging from C8 to C31 hydrocarbons were detected in oil phase. Compared with the analysis results of GC/MS, more heteroatom-containing compounds (HACOCs) of oil phase such as O3, O4, N2Oo and N3Oo (o=0—3) could be detected by using the QPEOTMS analysis. Overall, these findings will potentially contribute to the clean and efficient utilization of oily sludge.

Key words: oily sludge; petroleum hydrocarbon; ultrasonic extraction; heteroatom-containing compounds

1 Introduction

Recently, the oily sludge has been widely existing in the oil field and petrochemical refinery, which is harmful to the environment and human health. Oily sludge is regarded as a kind of hazardous wastes by the environmental protection department of many countries[1].China is a large consumer of crude petroleum, which can produce about five million tons of oily sludge every year. Generally speaking, it is difficult for human being to recover oil phase from the oily sludge, because the oily sludge exists in the form of stable water/oil emulsion[2].As a toxic waste, oily sludge mainly contains water,organic matter, and inorganic ash[3-4]. Generally, organic matters of oily sludge are derived from the petroleum compounds. Compared with other sludge such as sewage sludge, the high added value resulted from utilization of oily sludge is meaningful. Therefore, recycling the high content of petroleum hydrocarbons from oily sludge can contribute to the protection of clean environment.

Solvent extraction can efficiently recover the petroleum compounds from oily sludge. Recently, many characterization methods such as mass spectrometry(MS) and FTIR spectroscopy have been used for the analysis of unknown organic species. As a typical MS technique, the GC/MS technique has been used to analyze the composition of complex organic compounds[5], but its application in the analysis of HACOCs is limited.Due to the fact that GC/MS is not suited for compounds with higher polarity, QPEOTMS can be a better choice to analyze the HACOCs of unknown species. MS coupled with ionization sources has been applied in the characterization of high-temperature coal tar[6], bagasse methanol hydrolysis oil, and nonvolatile components of petroleum[7]. The Orbitrap mass spectrometry can provide high resolution and high-mass accuracy for the HACOCs of unknown organic species.

Many literature reports have focused on the process of separating oil phase from oily sludge. However, little work has reported the oil phase molecular characterization of oily sludge. In this research, solvent extraction was applied to recover oil phase from oily sludge. Acetone/CS2(with a volume ratio of 1:1) mixture solvent was used to separate oil phase from oily sludge under ultrasonic condition. Molecular compositions of separated oil phase were characterized by FTIRS, GC/MS, and QPEOTMS,respectively.

2 Material and Methods

2.1 Sample and experimental

Oily sludge was obtained from the Yanchang petrochemical refinery located in Shanxi, China. The moisture of oily sludge was determined by distillation with petroleum ether (solvent, with a boiling range of 90—120 °C). All commercial analytical reagents were purchased from the Aladdin Chemical Reagent Co., Ltd. The results of proximate analysis and ultimate analysis of oily sludge are shown Table 1. It can be seen from Table 1 that the moisture of oily sludge was 28.12%.

Table 1 The proximate analysis and ultimate analysis of oily sludge

10.0 g of oily sludge were sequentially extracted with 100 mL of acetone/CS2mixture solvent (volume ratio of 1:1) under ultrasonic condition. Solvent extraction was done more than 5 times to extract the organic compounds completely. Thus the oil phase was filtered through a polytetra fluoroethylene membrane filter with a pore size of 0.22 μm to obtain the supernatant, which was then concentrated by a rotary evaporator.

2.2 Analytical methods

The in situ Fourier transform infrared spectrometer(NICOLET iS10, USA) was used to analyze the oil phase. About 1.00 mg of oil phase was mixed with 100 mg of dried KBr and then the mixture was measured. The scanning range varied from 4 000 cm-1to 650 cm-1. The composition of oil phase was measured by an Agilent 7890/5975 GC/MS equipped with a capillary column coated with HP-5 at a constant He flowrate of 1.0 mL/min.The column temperature was heated from 60 °C to 300 °C at a temperature increase rate of 5 °C /min and was maintained at 300 °C for 7 min. Data were analyzed by using the MSD ChemStation software.

The Thermo Scientific Exactive QPEOTMS equipped with an atmospheric pressure chemical ionization source in positive ion mode was used to analyze the HACOCs of oil phase. The mass spectra ranged from 100 Da to 1 000 Da at 14 000 resolution. Molecular formulas were limited to a maximum of 50 C, 100 H, 10 O, 5 N, and 2 S atoms. The oil components were usually characterized in terms of DBE (double bond equivalence), carbon number and heteroatom number (Nn, Ooand Ss). DBE was calculated by Eq. (1) for a molecule of CcHhNnOoSs[8].

3 Results and Discussion

3.1 Analysis of the oil phase with FTIRS

As shown in Figure 1, the FTIR spectra of oil phase can be divided into three sections, implying that three types of functional groups appeared in the oil phase. Thus the FTIR spectra of oil phase were mainly concentrated at 3 000—2 800 cm-1, 1 800—1 000 cm-1, and 900—700 cm-1,respectively[9-10]. Strong absorbance of aliphatic C-H stretching bands could be observed at around 2 922 cm-1and 2 852 cm-1. The absorbance identified at around 1 462 cm-1and 1 376 cm-1was also considered as aliphatic C-H stretching bands. Absorption peaks at 856 cm-1and 724 cm-1were assigned to aromatic C-H bending.The peak at 1 625 cm-1was attributed to aromatic C=C functional groups of oil phase. Compared to the intensity of absorption in the aliphatic C-H stretching bands, the absorption peak of aromatic C=C groups was smaller,suggesting that aliphatic compounds of oil phase were abundant.

Figure 1 FTIR spectra of oil phase

3.2 Analysis of the oil phase with GC/MS

The soluble compounds of oil phase were analyzed by GC/MS characterization. The total ion chromatographic patterns of oil phase obtained by GC/MS are shown in Figure 2, the carbon number (CN) of normal alkanes(NAs) increased from 8 to 31, indicating that NAs in oily phase originated from paraffinic compounds of crude petroleum. 203 compounds with molecular weight between 106 Da and 436 Da could be indenti fied in the oil phase. All of the detected compounds were classi fied into NAs, branched alkanes (BAs), cycloalkanes (CAs),alkenes, arenes, esters, ketones, phenols, and nitrogencontaining organic compounds (NCOCs).

Figure 2 Total ion chromatographic patterns of oil phase obtained by GC/MS

As shown in Table 2, the relative content (RC) of NAs was higher than other compounds. Compared to other compounds, BAs and cycloalkanes of oil phase were also enriched. It was acknowledged that polycyclic aromatic hydrocarbons of oily sludge were very harmful to the environment[11-13]. But they played an important role in the refining process of petroleum. The ring number of identified aromatics ranged from 1 to 4 and the most abundant aromatics were two ring compounds. These aromatic compounds included one benzene ring, two benzene rings, three benzene rings, and four benzene rings, coupled with their side chain substituents. As illustrated in Table 2, the oxygen-containing compounds in the oil phase included esters, ketones, and phenols.However, the RC of NCOCs analyzed by GC/MS was very low. Due to the fact that GC/MS characterization was not good at analyzing the HACOCs with higher polarity,the content of HACOCs of oil phase was relatively low.

3.3 Analysis of the oil phase with QPEOTMS

As mentioned in Section 3.2, GC/MS characterization was not suitable for analyzing the HACOCs of oily phase. QPEOTMS characterization was introduced in this research to analyze the HACOCs with polar molecules.Identi fication of HACOCs in oil phase can be achieved at molecular level. The mass spectra of oil phase are shown in Figure 3, denoting that 838 compounds were identi fied by QPEOTMS. It also can be observed that the molecular weight of oil phase ranged from 100 Da to 600 Da. Table 3 shows the RC of HACOCs in the oil phase detected by QPEOTMS. Compared to other compounds, the RC of sulfur-containing organic compounds such as Ss, OoSs,NnSs, and OoNnSswas lower, whereas the sulfur-containing organic compounds were not detected by the GC/MS analysis. The distribution of RC for NnOoclass is exhibited in Table 4. It can be found that RC of N2Ooand N3Oo(o=0—3) class was higher than that of other compounds.

Figure 3 The mass spectra of oil phase obtained by QPEOTMS

According to petroleum chemistry, each double bond or alicyclic ring increases a DBE number and eliminates two hydrogen atoms. Figure 4 provides the iso-abundance plots of DBE versus CN for O3—O4class species in oil phase. The DBE values of 1—8 and CN values of 6—15 in O3and O4class species were observed. As shown inFigure 4, the O3species with DBE values of 5 and 6 might be alkylhydroxybenzoic acid and alkylbenofuran diol,respectively. For the O4class, the DBE values of 5 and 7 could be attributed to alkyldihydroxybenzoic acid and alkylnaphthalene tetraol, respectively. There were O3and O4class species with a DBE of ＜4, implying that these species could be alkylalcohols and alkylcarboxylic acid.

Table 2 Distribution of group compounds in oil phase according to the analysis with GC/MS

As exhibited in Figure 5(a), the DBE values and CN for the N2Oo(o=0—3) class species ranged from 1—14 and 7—26, respectively, indicating that these species contained 1—4 aromatic rings (ARs). The RC of N2, N2O, andN2O2class species with a DBE of ＜ 4 was high, corresponding to the non-aromatic structures. Species with a DBE of 5 and 6 in N2class might correspond to alkyltetrahydro naphthyridine and alkylbenzo imidazole, respectively.Oxygen atoms of N2O, N2O2, and N2O3class species may exist in ether, ketone and ester. As shown in Figure 5(b),N3Oo(o=0—3) class species were mainly distributed in DBE of 1—13 and CN of 6—26, respectively. The nitrogen atoms of N3class species existed in quinoline, amino and imidazole. The oxygen atoms of N3Oo(o=1—3) class species could be assigned to ether, ester and NH2CO-.

Figure 4 Iso-abundance plots of DBE versus CN for O3—O4 class species in oil phase

Table 3 RC of HACOCs in the oil phase detected by QPEOTMS

Table 4 The distribution of RC for NnOo class

Figure 5 Iso-abundance plots of DBE versus CN(a) N2Oo (o=0—3) class species; (b) N3Oo (o=0—3) class species

4 Conclusions

In this research, the oil phase separated from oily sludge was characterized by FTIRS, GC/MS, and QPEOTMS,respectively. Aliphatic C-H, aromatic C-H, and aromatic C=C can be easily observed in oil phase by the FTIRS analysis. Compared to the analysis with GC/MS, more molecular information about HACOCs of oil phase can be obtained by QPEOTMS characterization. These HACOCs were mainly distributed in a DBE of 0—14 and contained 1—4 ARs. The RC of HACOCs such as O3, O4,N2Oo, and N3Oo(o=0—3) was higher than that of sulfurcontaining compounds in oil phase. Overall, using these analysis methods can produce new assessments for waste fuel from oily sludge.

Acknowledgements: This work was supported by the Future Scientists Program of China University of Mining and Technology (2020WLKXJ019).