FT-ICR MS Analysis of Nitrogen-Containing Compounds in the

In this paper, the distribution of nitrogen (N)-containing compounds of slurry-bed hydrocracking products are investigated. Raw materials of the produ...
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FT-ICR MS Analysis of Nitrogen-Containing Compounds in the Products of Liaohe Atmospheric Residue Hydrocracking Dong Liu,†,* Yue Fu,‡ Wenan Deng,† Quan Shi,§,* Kuijv Ma,∥ Ting Hou,† and Chongchong Wu⊥ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China; China National Offshore Oil Corporation, Beijing, 100010, China; § State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China, ∥ China National Offshore Oil Exploiting Technology Corporation, Tianjin, 300456; ⊥ Research Institute of Petroleum Processing, SINOPEC CORP, China, 100020 ‡

ABSTRACT: In this paper, the distribution of nitrogen (N)-containing compounds of slurry-bed hydrocracking products are investigated. Raw materials of the products were Liaohe atmospheric residue (LHAR), while the hydrocracking reaction was maintained at 430 °C with initial pressure of 8.0 MP for 1 h. The distillation fractions, including 180−350 °C (diesel fuel), 350− 400 °C (VGO), 400−450 °C (VGO), and >450 °C (cracked residue), were analyzed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). It turned out that the main type of N-containing compounds in the products was N1 compounds. The main N-containing compounds in diesel oil were N1 compounds with C17− C23, while in VGO the main N-containing compounds were N1 compounds with C20−C26 and, at the same time, N2 and N2S1 compounds also appeared. In addition, the main compounds in cracked residue were N1 compounds with C24−C28, but the relative abundance of N2 compounds, which mainly centralized in cracked residue, increased. Other types of nitrogen compounds were few in products.

because it can provide evidence to prove that slurry-bed hydrocracking can remove nitrogen compounds effectively and show the conversion discipline of N-containing compounds in a hydrotreatment process. This paper mainly focuses on the main types and distribution of nitrogen compounds before and after slurry-bed hydrocracking reaction of Liaohe atmospheric residue.

Nitrogen (N)-containing compounds tend to exist in the higher boiling fractions and residues. Generally, nitrogen atoms tend to exist in ring structures, and there is almost no aliphatic nitrogen compound in petroleum.1 The main N-containing compounds in oil and its fractions are pyridine derivatives, quinoline derivatives, acridine derivatives, and pyrrolic derivatives. Also, aniline derivatives have been found in oil products. The ring number of N-containing compounds increases with the rise of boiling point of the fractions.2,3 Heavy oil slurry-bed hydrocracking, which is also called suspension-bed hydrocracking in chemical engineering and technology, is a kind of heavy oil conversion process that has a feedstock of feed oil mixed with highly dispersed catalysts or additives and hydrogen.4−9 Despite of low content of N-containing compounds, they have great influence on the process and products of slurry-bed hydroprocessing and are responsible for the poisoning of acidic sites in catalytic fluid processes and hydrocracking zeolite catalysts.10−13 The composition of the N-containing compounds in oil has been a key and difficult problem in the field of petrochemistry for decades. In recent years, the combination of ESI and FT-ICR MS have provided fresh leads in the analysis of polar heteroatomic compounds in heavy oil.14−17 ESI has no ionization effects on the majority of nonpolar and less-polar compounds, such as hydrocarbons. However, in positive mode and negative mode, it can selectively ionize marginal alkali compounds (mainly N-containing compounds) and acid compounds (mainly naphthenic acid). Generally speaking, neutral N-containing hydrocarbon compounds will appear in anion spectra.18−22 Using FT-ICR MS to study the distribution of N-containing compounds in slurry-bed hydrocracking is of great significance © 2011 American Chemical Society

1. EXPERIMENTAL SECTION 1.1. Raw Materials and Equipment. Raw materials used in the experiments were Liaohe atmospheric residue (LHAR) with the physicochemical properties shown in Table 1.

Table 1. Physicochemical Properties of Liaohe Atmospheric Residue ρ20 ν100 (g·cm−3) (mm2·s−1)

wt %(carbon residue)

0.9817 314.8 wt(metal) (μg·g−1)

13.39

wt %(ash)

condensation point (°C)

0.05 26 wt %(element)

nH /nC 1.59

Ni

V

C

H

S

N

88.0

2.16

86.8

11.58

0.39

0.81

Equipment used in the study includes the following: GDW-05 high-pressure reactor (Dalian Tong Chan High Pressure Reactors Manufacturing Co. LIT.) and FT-ICR (Bruker Apex, IV type FTMS). Magnetic field intensity is 7.0 T; ESI source is in positive mode; the feeding speed of the sample is 150 μL/h; the sampling frequency is 1s; Received: September 29, 2011 Revised: November 29, 2011 Published: November 30, 2011 624

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the mass range is 200−1000 u; Superpose spectra 128 times to improve the ratio of signal-to-noise. 1.2. Sample Preparation. Figure 1 shows a flowchart of the sample preparation processes used. They are descibed in more detail in the following paragraphs.

as the sum of the number of double bonds and naphthenic. If the difference between DBE is 1, then the difference between molecules is a double bond or a naphthenic. For example, in N1 compounds, when the DBE is 7, the substance is quinoline and its derivatives.

2. RESULTS AND DISCUSSION 2.1. Comparison of Nitrogen Compounds in Fractions (>350 °C) before and after Reaction. Fractions (>350 °C) of products after reaction were analyzed by FT-ICR, MS which was compared with the analysis result of LHAR. Results are shown in Figure 2.

Figure 1. Sample preparation flowchart. Slurry-Bed Hydrocracking. The reaction of slurry-bed hydrocracking was controlled at a temperature of 430 °C, with an initial pressure of 8.0 MP, for 1 h, with the presence of dispersed catalyst whose active component is NiSx. Distillation. Atmospheric and vacuum distillation were carried out on the hydrocracking products prior to analysis. The distillation products were cut into different distillates by different temperatures. After distillation, five fractions, gasoline (450 °C), were obtained. We calculated the yield of five fractions and analyzed the nitrogen contents in different fractions and the raw material, respectively. The results are shown in Table 2.

Figure 2. High resolution mass spectra (HRMS) and partially enlarged spectra of LHAR and products fraction (>350 °C) after reaction.

It can be seen from Figure 2 that the major range of molecular weight of LHRA is between 300 and 600 and has an relatively average distribution, while after reaction, the molecular weight shifts into the 250−600 rang and mainly focuses on 400 around, which demonstrates that upgrading of AR (>350 °C) happens in the process of hydrocracking. In a partial enlarged drawing, the 12 kinds of nitrogen compounds are all detected before and after reaction when the m/z is between 426.00−426.40. So, it can be concluded that it is very effective to use ESI FT-ICR MS to identify heteroatomic compounds in petroleum, especially in residue. Van Krevelen figures, which show the distribution of heteroatomic compounds, are achieved after data processing of ESI FT-ICR MS, and one is shown in Figure 3.

Table 2. Distribution of Nitrogen of LHAR and Products (wt %)

LHAR products

gasoline

AGO

350−400 °C VGO

400−450 °C VGO

cracked residue

0.13

1.52

2.16 8.28

4.56 12.43

93.28 77.64

It can be concluded from Table 2 that N-containing products convert to light fractions because of the hydrocracking of part of the N-containing compounds in LHAR. However, the principle that the content of N-containing liquid products increases with the rise of boiling point is not changed. In addition, after reaction, N-containing compounds in VGO increases significantly with the rise of boiling point and about 1/5 N-containing compounds in cracked residue converted to fractions lower than 450 °C, which indicates that the cracking of heavy oil is another important reaction besides the combination of macromolecular radicals and hydrogen radicals in the slurry bed hydrogenation process. However, most of N-containing compounds still remain in cracked residue, while fewer than 1/4 are in fractions lower than 450 °C. 1.3. Data Processing and Analytical Methods. Despite of the complexity of polar substances and their derivatives, the types of elements in these compounds are limited, such as C, H, O, N, and S. Hence, FT-ICR MS can determine the formula of these compounds accurately. Van Krevelen figures,17 whose coordinates are atom ratios of elements such as O, S, N, C, and H, can clearly show the elements of a molecule. These can be made into two-dimensional charts or three-dimensional charts, and in a two-dimensional chart, the relative abundance can be distinguished by using different colors. DBE is defined

Figure 3. Distribution before and after reaction. The symbols of N1, N1O1, N1O1S1, N2, N3, etc. indicate their corresponding N-containing compounds with different N, O, S numbers.

It is concluded from Figure 3 that the main nitrogen compounds in LHAR are N1 compounds, while other nitrogen 625

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Figure 4. High resolution mass spectra of narrow fractions in products.

Figure 5. Composition of heteroatomic compounds of narrow fraction.

shown in Figure 4. Further processing of Figure 4, elemental compositions of heteroatomic compounds, is shown in Figure 5. (1) Analysis of N-containing compounds of diesel fraction (180−350 °C) in products: As can be seen from Figure 4 and Figure 5, the molecular weight of compounds in the diesel fraction is concentrated from 250 to 350 and the number of compounds is relatively small. The main N-containing compounds are N1, N1O1, and N2 compounds. The relative abundance of N1 compounds accounts for more than 90% of all nitrogen-containing compounds in the diesel fraction. On the other hand, the DBE values of N1 compounds are distributed between 4 and 13. Therefore, after hydrogenation, diesel distillates still do not contain N-containing compounds that have a DBE value less than 4, such as pyrrolic compounds. Compounds with DBE values between 5 and 8 are the major part of the N1 compounds, about 70%. (2) Analysis of N-containing compounds of light wax fraction (350−400 °C) in products: The molecular weight of compounds in light wax fraction is between 250 and 400, and most of them are concentrated from 300 to 350. The main N-containing compounds are N1 and N2 compounds, while N2S1 compounds also appear, and the relative abundance of N1 compounds still accounts for more than 90% of all nitrogen-containing compounds in light wax fraction. The DBE values of the main N1 compounds are distributed between 4 and 15,

compounds such as N1O1, N1O1S1, N1S2, N2, and N2S1 compounds make up a relatively lower percentage. DBE value of N1 compounds is continuously distributed between 4 and 22, almost including all kinds of N1 compounds with different saturation, except low saturation compounds such as pyrrolic nitrogen compounds (DBE = 3). N-containing compounds among them that have a DBE value between 8 and 12 account for half of all N1 compounds. The main types of nitrogen compounds of LHAR hydrocracking fractions higher than 350 °C are N1 compounds and N2 compounds, while the relative abundance of other nitrogen compounds remain almost the same. However, N1O1S1 compounds do not exist in AR after hydrocracking reactions, compared with those in LHAR. The DBE value of N1 compounds is continuously distributed between 4 and 22 and presents a more even distribution after reaction, while the DBE values of N2 compounds are continuously distributed between 6 and 21 and the relative abundance between N-containing compounds with different unsaturations are not significantly different. 2.2. Analysis of Nitrogen Compounds of Different Fraction of LHAR Hydrocracking Products. To study the distribution of N-containing compounds in slurry-bed hydrocracking products of LHAR accurately, all liquid products were separated by atmospheric and vacuum distillation. The product was divided into four fractions: 180−350 °C, 350−400 °C, 400− 450 °C, and >450 °C. High resolution mass spectra of fractions are 626

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Figure 6. Distribution of DBE values and the number of carbons in N1 compounds of a narrow fraction.

N-containing compounds in distillates after hydrocracking is N1 compounds, while N2 compounds increase significantly in VR. The N-containing compounds in diesel oil are mainly N1 compounds with low saturation and a DBE value mostly centered from 5 to 9. In addition, the relative abundance of them is roughly the same. The type and distribution of N-containing compounds of light wax oil is close to that of diesel oil. However, when it comes to heavy wax oil, the distribution of compounds is more even and the type is more complex. Moreover, condensation of these compounds increases in heavy oil. N1 compounds that have a DBE value is within the range 4−13 exist in all fractions. With the increase of the boiling point, nitrogen compounds with low saturation gradually reduce, while high saturation compounds increase. A small part of N-containing compounds (DBE> 15) with high degree of condensation appears only in the vacuum distillate. The distribution of N1 compounds in two wax fractions is almost the same, while N2 compounds only exist in light and heavy wax fractions and VR. N1 compounds with different saturations are all present in VR, and the distribution is elementary average. At the same time a considerable number of diazo compounds also appear in VR (>450 °C), which is more than that of LHAR, which was as a result of condensation and polymerization of N1 compounds under the conditions of high temperature and high pressure. 2.3. Distribution of N 1 Compounds in Liaohe Distillates after Reaction. The relations between DBE and the number of carbons in N1 compounds of different distillates of products after reaction are demonstrated by high resolution mass spectrometry (HRMS). The results, including fractions 180−350 °C, 350−400 °C, 400−450 °C, and >450 °C, are shown in Figure 6. To determine the distribution of nitrogen compounds after reaction, the main N1 compounds in distillates are analyzed. The distributions of N1 compounds in narrow fractions with different DBE values and different carbon numbers are shown in Figure 6. It is seen that, after hydrogenation, compounds with different DBE values and carbon numbers do not show

and N-containing compounds that have a DBE value between 5 and 10 take advantage. Compared with the diesel fraction, N-containing compounds that have a DBE value between 14 and 15 appear. (3) Analysis of N-containing compounds of heavy wax fraction (400−450 °C) in products: The molecular weight range of compounds in heavy wax fraction increases to 300−450. The main nitrogen compounds in this fraction are N1, N1O1, and N2 compounds. Among them, N1 compounds are predominant, while the relative abundance of N2 compounds increases. The DBE value of the main N1 compounds is distributed between 4 and 15, which is the same as in the case of the light wax fraction, but N-containing compounds that have a DBE value between 8 and 12 enjoy an advantage. So, it can be seen that N-containing compounds with a medium condensation degree are the majority. (4) Analysis of N-containing compounds of VR (>450 °C) in products: The types of N-containing compounds in VR are the most complex, including N1, N2, N1O1, N1O2, N1S1, N2S1, and N3 compounds, but the main species are N1, N2, and N1O1 compounds. The most obvious feature is that the relative abundance of N2 compounds increases significantly. So, it can be inferred that, under the conditions of high temperature and high pressure, polycondensation of part of the N-containing compounds happens, and N3 and N2S1 compounds are a result of condensation and coking in the hydrocracking process. The DBE value of N1 compounds lies between 4 and 22, and the gap between the abundance of compounds with different DBE values is very small, presenting an average distribution. On the other hand, the DBE values of N2 compounds are between 6 and 22, and N-containing compounds with different DBE are also distributed averagely. It is obvious that the more complex the types of N-containing compounds in the heavier fractions are, the more even the distribution is. Through the analysis of N-containing compounds of narrow fraction in products, it can be concluded that the main type of 627

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normal distribution. In diesel oil, the number of carbons in N1 compounds is mainly concentrated from 17 to 23, and the types of nitrogen compounds are few and have low saturation ranging from 4 to 10. Especially, the content of compounds that have a DBE value of 6 or 7, such as indole and quinoline, is the largest. As for light wax oil, with the rise of boiling point, the number of carbons in N1 compounds increases to 20−26 and two types of compounds with DBE values of 9 and 10 appear, that is carbazole and acridines. With the increasing number of carbon, the substituent changes from alkyl to condensed ring derivative, and correspondingly, nitrogen compounds change to carbazole and acridines, while in heavy wax oil the number of carbons in N1 compounds increases to 24−28 and the DBE values ascend to 6−12. In VR, N1 compounds are the most complex and are more evenly distributed, and there are almost no nitrogen compounds with a small number of carbons and low saturation.

3. CONCLUSION (1) About 3/4 of the nitrogen compounds of liquid products of slurry-bed hydrocracking exist in distillate higher than 450 °C. The main N-containing compounds are N1 compounds, and at the same time, the polymerization of N 1 compounds makes the abundance of N 2 compounds increase. Nitrogen compounds with a DBE value of 4 to 13 exist in all fractions. The DBE value is mainly concentrated from 5 to 8 in diesel oil and from 5 to 10 in light wax oil; it rises to 8−12 in heavy wax oil. (2) In diesel oil, the major nitrogen-containing compounds are C17∼C23 N1 compounds with a low degree of condensation, such as indole and quinoline. With the rise of the distillation range, the main nitrogen-containing compounds are still N1 compounds. For light wax oil, it is C20∼C26 N1 compounds, and at the same time, carbazoles and acridines appear. When it comes to heavy wax oil, the main nitrogencontaining compounds are C24∼C28 N1 compounds. (3) Nitrogen compounds of distillates lower than 450 °C are all C13∼C30. N2 compounds are mainly concentrated on distillates higher than 450 °C, while other types of nitrogen compounds are few in products. The heavier the fraction is, the more complex the nitrogen compounds are, but the distribution of them with different types and condensation degree is more average.

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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21176259) and supported by the Awarded Foundation for Excellent Young and Middle-Aged Scientists of Shandong Province, China (BS2010NJ024).

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