Transformation of Nitrogen Compounds through Hydrotreatment of

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Transformation of Nitrogen Compounds through Hydrotreatment of Saudi Arabia Atmospheric Residue and Supercritical Fluid Extraction Subfractions Mei Liu,†,‡ Linzhou Zhang,‡ Suoqi Zhao,*,‡ and Dezhi Zhao† †

College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

‡

ABSTRACT: Atmospheric residue (AR) from Saudi Arabia (SZAR) was subjected to supercritical fluid extraction fractionation (SFEF) and four extractable subfractions (SFEF-1−4) and an unextractable end-cut were obtained. SZAR and SFEF-1−4 were subjected to hydrotreatment (HDT) in a continuous mini fixed-bed reactor. Three commercial catalysts were used to remove the impurity in the feedstock. The composition and structural transformation of nitrogen (N)-containing compounds were investigated. Electrospray ionization (ESI) Fourier transform−ion cyclotron resonance mass spectrometry (FT-ICR MS) was used for molecular characterization of N-containing compounds of SZAR, extractable SFEF subfractions, and HDT products. Results showed that N1 class species with high aromaticity and/or low carbon number exhibited higher catalytic hydrogenation reactivity. N1 class species with lower aromaticity were removed regardless of chain length.

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INTRODUCTION Among current residue upgrading processes, hydrotreatment (HDT) has attracted extensive attention due to the high liquid product yield, good product quality, high operation flexibility, and low environmental emission.1 HDT comprises hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodemetallization (HDM), which remove nonhydrocarbon compounds such as sulfur, nitrogen, and metal-containing compounds, respectively. However, the catalytic treatment of atmospheric residue (AR) in enriched heavy fractions (such as asphaltenes) is hindered because of high coke formation potential.2 N-containing compounds in petroleum are difficult to be refined because of their implication in atmospheric pollution and inhibitive or refractive behavior toward hydroprocessing.3−5 Technical improvement of the HDT process requires an improved understanding of heteroatom species composition.6 However, a preliminary separation treatment is widely applied, considering the inherent complexity of HDT (appropriate feed procurement, characterization, reactor testing, and product analysis). Petroleum is separated into different fractions based on their properties, such as solubility,7−11 polarity,12,13 acidity,14,15 molecular weight (MW), and size.16 Supercritical fluid extraction and fractionation (SFEF) is a flexible solvent separation method used for separation of high boiling components.9 By changing solvent, operating pressure, and temperature, the solubility of supercritical fluids can be adjusted. Therefore, the derived heavy oil fractions exhibit varied MW and polarity, thus facilitating the molecular composition or reaction performance study on heavy petroleum.9,15 N-containing compounds are abundant in heavy oils and caused many adverse effects for production, storage, and transport.17 Identification of N-containing compounds in petroleum especially in heavy oil remains a big challenge because of their numerous structural isomers. In the past, the © XXXX American Chemical Society

study of nitrogen compounds in heavy oil stayed at the macro level. The bulk property measurements and chromatographic separation only provide a simple description for refinery operations. As a result of the limitation of analytical method, there are few reports on the study of reaction mechanism. Advances in Fourier transform−ion cyclotron resonance mass spectrometry (FT-ICR MS) are beneficial for studies on detailed molecular composition of complex heavy fossil fuels and in the field of “petroleomics.”18−24 Nonpolar25−31 (pretreatment is needed) and polar components32−39 in petroleum have been characterized through ESI FT-ICR MS in positive and negative ion modes. Moreover, this technique is used to characterize N-containing compounds33,40,41 and is a proven practical approach for investigating the heteroatom species of petroleum. Molecular-level investigation on petroleum HDT processing remains insufficient. This work aims to provide a detailed molecular characterization of polar heteroatom species (especially basic and neutral N-containing compounds) in AR, SFEF-1−4, and HDT products. AR and four extractable subfractions of AR were hydrotreated over three commercial catalysts in a 30 mL continuous fixed-bed reactor under an common industrially process conditions. SZAR, extractable subfractions, and HDT products were then subjected to positive-ion and negative-ion electrospray ionization (ESI) and FT-ICR MS analyses. Polar heteroatom species were characterized according to class, double bond equivalence (DBE), and relative abundance. As a supporting, bulk property analysis was also conducted to gain more insights of these materials. This study investigated the transformation of basic and neutral N-containing compounds in the HDT process. Received: September 22, 2015 Revised: December 20, 2015

A

DOI: 10.1021/acs.energyfuels.5b02158 Energy Fuels XXXX, XXX, XXX−XXX

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Mass Calibration and Data Analysis. FT-ICR MS was internally calibrated with the N1 class homologous series. Elemental composition for each peak was assigned by a custom software.45 Species with the same heteroatom class and isotopes with different values of DBE and carbon number were searched within a set of ±0.0010 Kendrick mass defect (KMD) tolerance.46,47 Details of data processing are reported in previous studies.20,21

EXPERIMENTAL SECTION

Feedstock and SFEF. SZAR was subjected to SFEF through a previously described method.9,10,42 By SFEF, four extractable subfractions and an unextractable end-cut were obtained. The extraction pressure and yields of SFEF-1−4 are described in a previous study.43 The extractable SFEF subfractions amounted to 83.35 wt % of SZAR. The total of the loss was 1%. HDT Processes. SZAR and SFEF-1−4 were subjected to HDT in a 30 mL mini reactor packed with three commercial catalysts in grading beds. Table 1 shows the properties of the catalysts. The process

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RESULTS AND DISCUSSION Bulk Properties. The bulk properties of feeds and HDT products are listed in Table 2. The concentrations of sulfur and nitrogen of SZAR were 3.44 and 0.29 wt %, respectively. The viscosity of the feedstock was 48.36 mm2·s−1 at 100 °C. After separation by SFEF, the H/C ratios were decreased. Meanwhile, the nitrogen and sulfur contents of the subfractions showed steady growth with increasing extracted pressure. Asphaltene were not detected for all the extractable subfractions, indicating that SFEF exerted a decarburization effect. Figure 3 shows the normal boiling point distribution of SZAR and the extractable SFEF subfractions through hightemperature simulated distillation. The boiling point of the SFEF series increased with the temperature. After HDT, sulfur, and nitrogen conversions for the SFEF-1−4 were higher than those for AR. Sulfur (S)-containing compounds exhibited the highest reactivity among all heteroatom species. In light SFEF subfractions, the HDS and HDN conversions were higher because of the relatively small MW. HDS and HDN reactivities decreased with increasing MW and/or aromaticity of the residue fraction. Decreased HDS and HDN reactivities may be due to increased diffusion resistance and reduced intrinsic reactivity. The sulfur and nitrogen conversions for SZAR were 70.64 and 30.50 wt %, respectively (Figure 4). The sulfur conversion for the three lighter subfractions and the nitrogen conversions of all extractable subfractions were higher than this value. Evidently, SFEF technology increased the efficiency of removing heteroatom compounds. Exploring the transformation and separation of different molecules during SFEF and HDT processes can promote understanding of HDN reactivity. Figure 5 shows the SARA composition of SZAR, SFEF-1−4, and HDT products. Saturates were dominant in SFEF-1. Aromatics were approximately 50 wt % in SFEF-4. Asphaltene

Table 1. Parameters of HDM, HDS, and HDN Catalysts parameter

HDM-11

HDS-33

HDN-41

shape pore volume, cm3·g−1 specific surface area, m2·g−1 particle diameter, mm average pore diameter, nm bulk density, g·cm−3 active species

cylindrical 0.55 145 1.1−1.4 16.7 0.49 Mo−Ni



scheme for HDT is shown in Figure 1. First of all, the catalysts were subjected to presulfide treatment. Then, AR and SFEF 1−4 were subjected to thereactor at moderate conditions separately. The specific operating conditions have been mentioned in the previous article.23 Table 2 shows the properties of AR, SFEF subfractions, and HDT products. Compositional analysis of saturates, aromatics, resins, and asphaltenes (SARA) was also performed. Table 3 shows the material balance of the HDT effluent, which conforms to the typical characteristics of the AR HDT product distribution. ESI FT-ICR MS Analysis. The sample processing methods can be found in previous studies.38 Using ESI ionization source, positive-ion mode was to detect basic N-containing compounds, negative-ion mode was to detect neutral N-containing compounds. The magnetic field intensity was 9.4 T. The typical conditions for negative-ion (or positive-ion) formation included the following: emitter voltage, 4.0 kV (or 3.0 kV); capillary column introduced voltage, 4.5 kV (or 3.5 kV); and capillary column end voltage, 320 V (or 320 V). The key operating parameters for SZAR, SFEF-1−4, and HDT products were 250−1200 mass range and 4 M acquired data size. The operating conditions were optimized based on intensity and distribution of mass spectrometry. Figure 2 shows molecular type that can be ionized by ESI.44

Figure 1. HDT scheme. Reactors 1, 2, and 3 were loaded HDM, HDS, and HDN catalysts, respectively. B


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Energy & Fuels Table 2. Properties of SZAR, SFEF Subfractions, and Their HDT Productsa elemental analysis, wt % SZAR SFEF-1 SFEF-2 SFEF-3 SFEF-4

feed product feed product feed product feed product feed product

υ100b, mm2 s−1

CCRc, wt %

Cd

Hd

Se

48.36 21.74 4.43 3.48 10.27 8.50 16.91 10.24 59.51 21.75

10.99 5.99 0.46 0.09 1.96 0.51 2.90 1.01 9.37 2.05

84.81 86.13 85.60 86.13 85.32 85.91 85.11 86.27 84.49 86.15

11.15 11.96 12.27 13.22 12.01 13.01 11.76 12.97 11.34 12.45

3.44 1.01 1.86 0.11 2.44 0.27 2.73 0.33 3.46 0.75

conversion, % 70.64 94.09 88.93 87.91 61.85

Nf 0.2915 0.2026 0.1104 0.0286 0.1524 0.0386 0.1886 0.0484 0.2727 0.1291

conversion, % 30.50 74.09 74.67 74.34 52.66

H/C 1.5667 1.6545 1.7091 1.8289 1.6773 1.8044 1.6464 1.7915 1.5992 1.7221

a

Samples were taken after a 1 day run. bGBT-11137(Cannon-Finn opaque viscometer). cASTM D189 (Shanghai Yutong Instrument YT-30011). d ASTM D5291 (flash EA 1112 analyzer). eASTM D5453 (Antek 7000 elemental analyzer). fASTM D5762 (Antek 7000 elemental analyzer).

Table 3. Material Balance of the HDT Effluent influent, % feed hydrogen total effluent, % H2S NH3 C1−C2 C3−C4 C5 ∼ 165 °C 165−350 °C 350 °C+ C5+ total

SZAR

SFEF-1

SFEF-2

SFEF-3

SFEF-4

100 1.36 101.36

100 1.08 101.08

100 1.24 101.24

100 1.38 101.38

100 1.42 101.42

2.6 0.11 0.36 0.28 1.98 6.01 90.02 98.01 101.36

1.86 0.1 0.35 0.31 2.82 8.66 86.92 98.4 101.08

2.31 0.14 0.32 0.3 2.5 8.53 87.14 98.17 101.24

2.56 0.17 0.33 0.31 2.4 8.27 87.34 98.01 101.38

2.91 0.18 0.35 0.32 1.85 7.72 88.09 97.66 101.42

Figure 3. Nominal boiling point distribution of SZAR and its extractable SFEF subfractions.

as derived from positive-ion ESI FT-ICR MS analysis. The ionized nitrogen compounds should originate from pyridine derivatives. N2 class species with both pyrrole and pyridine functional groups were distinct. These species were also found to be basic compounds, in which pyridine functional groups function in analysis. The N1 class species were dominant in all feedstock. After HDT, the relative abundance of most N1 class species was enhanced. N1O1 class species were highly stable and did not show significant changes. However, as a consequence of preferential removal of sulfur atoms from N1S1 class species, the relative abundance of N1S1 class species in SZAR and SFEF subfractions significantly decreased. This finding indicated that N1O1 class species in different feedstock may possess different molecular structures; in particular, refractory N1O1 class species may contain furan groups, rather than hydroxyl groups.27,48 The relative abundance of N2 class species was similar between SZAR and SFEF-4 and was almost zero after HDT; hence, N2 with two functional groups were easily removed. For SFEF-3, few N2 compounds were found in the raw material. However, N2 compounds in the HDT product were difficult to transform. Moreover, N1 class exhibited the highest removal rate, thereby significantly increasing N2 compound in the products. Numerous sulfoxide structures were present in SZAR and SFEF-2, reflecting the good hydrogenation activity of these samples. Figure 7 shows the iso-abundance plots of DBE as a function of the carbon number for basic nitrogen species in AR, SFEF subfractions, and HDT products. DBE values and carbon

Figure 2. Molecular type that can be ionized by ESI. Structures are representative; all species contain paraffinic chains that extend from the core molecule.44

was not detected in the extractable subfractions. Analysis of SARA indicated that the content of nonhydrocarbon compounds of the SFEF subfraction increased as the SFEF pressure increasing. After HDT, the total contents of aromatics and resins significantly decreased because of the removal of heteroatom compounds. Molecular Composition Characterized by Positive-Ion ESI FT-ICR MS. Figure 6 shows the relative abundance of nitrogen class species in SZAR, SFEF-1−4, and HDT products, C


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Figure 4. Sulfur and nitrogen content SZAR, SFEF fractions, and their HDT products.

Figure 5. SARA compositional analysis result of SZAR, SFEF subfractions, and their HDT products.

Figure 6. Heteroatom class and type distribution for SFEF fractions and their HDT products, from positive-ion ESI FT ICR mass spectra.

numbers of the most abundant pyridine in the SFEF subfractions gradually increased with the increasing of the extraction pressure. The trends of DBE and carbon number were also varied ss the fraction is getting heavier and heavier. After three-step HDT, the ranges of DBE and carbon numbers of SFEF subfractions were similar to those of SZAR and HDT

products. By contrast, the center of the DBE range of SFEF-1, SFEF-2, and SFEF-3 shifted to lower values. These findings indicated that slightly reduction in DBE values could be due to the double bond saturation. The first step in this reaction was the heterocyclic ring hydrogenation saturation. The N1 class species in hydrotreated samples shifted to lower DBE values D


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Figure 7. Iso-abundance maps for N1 class compounds of SZAR, SFEF fractions, and their HDT products, from positive-ion ESI FT ICR mass spectra.

Figure 8. Heteroatom class and type distribution for HDT products of SZAR and SFEF fractions, from negative-ion ESI FT ICR mass spectra.

Figure 9. Iso-abundance maps for O2 class compounds of SFEF-1 and its HDT products, from negative-ion ESI FT ICR mass spectra.

derivatives compounds49 The minimum DBE value for N1 class species (pyridine derivative compounds) in AR and SFEF subfractions was 4, which refers to the core structure of the pyridine unit. In HDT products, abundant nitrogen compounds with less than 4 DBE were found. According to previous study, these were amines derived from basic and neutral nitrogen compounds.50 During HDT, both the saturation reactions of aromatic nitrogen functional groups and ring-opening reactions

and higher carbon numbers (magical reduction of lot of pyrrolic compounds with short or/and less side chains in red circle in Figure 7). On the contrary, as the SFEF subfraction became heavier, low DBE N1 class species exhibited stronger hydrogenation activity and significantly decreased (SFEF-4). Furthermore, N1 compounds with low MW in each DBE series were readily removed. As amines are rarely present in heavy oil, the basic N-containing compounds are identified as pyridine E


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Figure 10. Iso-abundance maps for N1 class compounds of SFEF fractions and their HDT products, from negative-ion ESI FT ICR mass spectra.

Figure 11. Transformation of DBE = 9, 12, and 15 pyrrolic compounds in SFEF fractions before and after HDT. For each fraction, the DBE distribution was rescaled by its highest point.

could result in the formation of amines.51 However, according to our past experience, it is difficult to determine the accurate basic N1 compound conversion process because of the removal of the original basic nitrogen compounds and the generation of secondary basic products from neutral nitrogen compounds. Molecular Composition Characterized by NegativeIon ESI FT-ICR MS. Negative-ion ESI FT- ICR MS was applied to investigate molecular composition of acidic and neutral compounds in SZAR, SFEF subfractions, and HDT products. The relative abundances of acidic and nonbasic compounds are shown in Figure 8. N-containing compounds, which include pyrrole functional series, as well as N1 and O2 class species, were prevalent in the feedstock. After HDT, the relative abundance of N1 class species increased, suggesting that these species, except for SFEF-1, were refractory to HDT. Carboxylic acids were easily removed during the HDT process. However, the removal of fatty acid and alkyl carboxylic acid, which has low MW, was difficult, and the relative abundances of O2 class species in SFEF-1 product increased (relevant evidence is shown in Figure 9). The relative abundances of O1 and Scontaining compounds (N1S1 and O2S1) declined dramatically.

N2 compounds were not detected in the negative-ion mode, and at least one functional group was predicted as pyridine. Interestingly, N1O1 compounds were detected in both positiveion and negative-ion modes, as evidenced by the simultaneous presence of phenol hydroxyl and basic nitrogen functional groups. Negative-ion mode was detected through the presence of phenolic hydroxyl, whereas positive-ion mode was detected by basic nitrogen functional group. Figure 10 shows the iso-abundance plots of DBE as a function of the carbon number for neutral nitrogen species in AR, SFEF subfractions, and HDT products. N1 compounds in SZAR varied with DBE of 6−21 and carbon numbers of 12−45. The most abundant areas of the species were located at DBE of 12 and carbon number of 27. The DBE value and carbon number progressively changed in the SFEF subfractions. The most important distinction between light and heavy SFEF subfractions was the shift in DBE distribution, that is, SFEF-1 with DBE of 9−16 and SFEF-4 with DBE of 5−21. After HDT, quality center moved to the upper right corner, which made the DBE values lower and carbon numbers much higher. Moreover, Because of the smaller space steric hindrance, pyrrole F


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compounds (N1 class species) with low MW in each DBE series were readily removed. Numerous small-molecule and high DBE N1 compounds (especially DBE = 12 in red circle in Figure 10) were dramatically reduced. Figure 11 shows the relative abundance of neutral N1 compounds as a function of carbon number with DBE values of 9, 12, and 15, which are most likely carbazoles, benzocarbazoles, and dibenzocarbazoles, respectively.52 The DBE distribution data for each feedstock showed a Laplacegauss curve. Evidently, as the SFEF subfraction became heavier, the DBE distribution curve shifted to the right. The most abundant neutral N1 compounds with 9 DBE in SZAR possessed a carbon number of 29, whereas those in the SFEF subfractions presented increasing carbon numbers of 18, 24, 30, and 34. The results shown in Figure 11 indicate that low DBE compounds were readily removed by HDT, regardless of the side chain length. However, with increasing DBE, high DBE compounds with small MW and weak polarity were easily removed or converted. Furthermore, N1 class species with long or multisubstituted side chains were much refractory to HDT.

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CONCLUSION The molecular composition of polar species in SZAR, four extractable SFEF subfractions, and HDT products was investigated using ESI FT-ICR MS. The elemental content of SZAR, extractable subfractions, and HDT products, including S, N, Ni, and V, gradually increased with increasing SFEF pressures. The HDT results showed that light SFEF obtained high nitrogen removal rate under similar HDT conditions. The SFEF process was highly beneficial to nonhydrocarbon hydrogenation reaction, especially for nitrogen removal. The plot of DBE as a function of carbon number for N1 class species indicated that the conversion of less unsaturated cores was unrelated to the side chain length. In addition, N1 class species with more unsaturated cores and/or less carbon numbers exhibited improved hydrogenation reactivities. All N-containing compounds with low MW and polarity in each DBE series were readily removed.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-8973-9015. Fax: +86-10-6972-4721. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. NSFCU1162204 and 21176254). REFERENCES

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