Retardation Effect of Basic Nitrogen Compounds on Hydrocarbons

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Retardation Effect of Basic Nitrogen Compounds on Hydrocarbons Catalytic Cracking in Coker Gas Oil and Their Structural Identification Ze-kun Li, Gang Wang,* Quan Shi, Chun-ming Xu, and Jin-sen Gao State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China ABSTRACT: The basic nitrogen compounds in coker gas oil (CGO) narrow fractions were enriched, and their influences on hydrocarbons during fluid catalytic cracking (FCC) were investigated. The results show that the content of basic nitrogen compounds has influence on hydrocarbons cracking during CGO FCC reaction, but it is not as obvious as reported before. Furthermore, the compositional and structural identification of basic extracts by positive-ion electrospray Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) shows that basic nitrogen compounds in CGO include N, N2, NO, N2O1, and NS class species. The N1 class species centered at 9 < DBE < 13 with a carbon number ranging from 20 to 24 is the most abundant, and it is a key for CGO’s retarding performance. The effect of structure and composition of basic nitrogen compounds is much more obvious than that of content, and it is stronger with the increase of their rings plus double bond equivalence (DBE).

1. INTRODUCTION In recent years, delayed coking technology has been one of the key ways for residue conversion and processing in the refining industry with the worldwide trend of increasing supply of heavier crude, deterioration of its quality, and rising demand of light fuel products. To our best knowledge, the capacity of coking has gained more than 30 wt % in world heavy oil processing.1 Coker gas oil (CGO), a major intermediate product of delayed coking process that thermally cracks low-grade heavy feedstocks into high-value distillates as well as coke and gas, usually accounts for 20-30 wt % of the total feedstock,2 and therefore its upgrading is becoming increasingly important for oil refiners all over the world with the ever-increasing demand for energy.3,4 Because of its properties, CGO is usually hydrotreated before being used as a feedstock to fluid catalytic cracking (FCC) units or directly treated by hydroprocessing.4 In view of the high capital and operational costs of hydrotreating processes,5-7 nowadays there is an increasing interest in using FCC to process CGO.8-10 It has been known for a long time that nitrogen compounds, especially basic nitrogen compounds, can heavily decrease the catalytic cracking performance of CGO.3,11-17 However, the findings obtained from vacuum residual (VR) FCC showed that, despite its much higher contents in both nitrogen compounds (about 0.52 wt %) and basic nitrogen compounds (about 0.16 wt %) than those in ordinary CGO (the content is 0.34 and 0.11 wt %, respectively), VR usually gave a blending ratio of 80 wt %, while the blending ratio of CGO was only less than 20 wt %.4 This result indicates that neither the total nitrogen content nor the basic nitrogen content is directly related to the retarded FCC performance of CGO, suggesting that a detailed study on the composition and structure of nitrogen compounds and their individual effects on FCC performance is necessary, as pointed out by Fu who investigated the prohibiting effects of nitrogen compounds on CGO FCC performance by using model nitrogen compounds.11 However, r 2011 American Chemical Society

despite the large amount of work that has been done with nitrogen compounds, it has either failed to consider the difference between model compounds and real basic nitrogen compounds in CGO, or just used a full range of CGO as feedstock. Moreover, one of the difficulties encountered in understanding the effects of individual nitrogen compounds on FCC performance is the determination of the composition and structure of numerous nitrogen compounds. To the best of our knowledge, most of the gas chromatography/mass spectrometry (GC-MS)based methods available for analyzing compounds in crude oil are only applicable to molecules with molecular weight no more than 300 Da due to their low volatility,18 while the result of other elemental analysis of many crude oils shows that there is 90 wt % nitrogen species present in VR, most of which are out of the identifying ability of GC-MS.19 Most recently, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has become a frequent method for probing the composition of complex mixtures.20-23 Additionally, the high selectivity of electrospray ionization (ESI) has been coupled with FT-ICR MS for a petroleum system, and more than 17 000 elemental compositions in crude oil have been revealed.24 However, there is little work focus on the structural information of basic nitrogen compounds in CGO, let alone structural identification of basic nitrogen compounds from the view of deactivating effect on hydrocarbon catalytic cracking. In this article, basic nitrogen species in the narrow cut of CGO were selectively separated by solvent extraction. The retardation effects of different kinds and contents of basic nitrogen compounds on hydrocarbons catalytic cracking in CGO FCC performance were investigated by the contrast experiments of CGOs before and after solvent extraction, and the vacuum gas oil Received: October 18, 2010 Accepted: January 27, 2011 Revised: January 14, 2011 Published: March 01, 2011 4123

dx.doi.org/10.1021/ie102117x | Ind. Eng. Chem. Res. 2011, 50, 4123–4132

Industrial & Engineering Chemistry Research

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Figure 1. Experimental scheme.

(VGO) was blended with different amounts of model compounds in a confined fluidized bed reactor over a commercial equilibrium catalyst. Additionally, to reveal the structural effects of basic nitrogen compounds on hydrocarbons catalytic cracking, the comprehensive compositional information of basic nitrogen compounds in feed CGO, HCl extracted oil, and raffinate oil, including class (number of N, O, and S), type (DBE), and carbon number, was determined by ESI FT-ICR MS.

Table 1. Properties of Daqing (DQ) VGO and Dagang (DG) CGO properties density (20 °C), kg/m

3

Conradson carbon residue (CCR), wt % molecular weight

2.1. Experimental Scheme. The Dagang CGO (DG CGO)

was divided into narrow fractions by a true boiling point (TBP) distillation method (ASTM D 2892) during research. A narrow cut was chosen as feeds according to the property analysis and FCC reaction performance. HCl aqueous solutions with different concentrations were used as extract agents to enrich basic nitrogen compounds for the study, and the ESI FT-ICR MS was used to characterize the structure of basic nitrogen compounds in the samples; then for the oils before and after acid wash, the analysis of saturates, aromatics, resins, and asphaltenes (SARA), basic nitrogen content, and the FCC performance were determined. The total scheme of the research is represented in Figure 1. 2.2. FCC Reaction Performance. 2.2.1. Feedstocks and Catalysts. The Daqing (DQ) vacuum gas oil (VGO) was presented as contrast feeds, and its properties are shown in Table 1. To compare the light, middle, and heavy aromatics in the DQ VGO and DG CGO, the separation method of six chemical components based on SARA was used to obtain the contents of light, middle, and heavy aromatics. As compared to DQ VGO, DG CGO is characterized by low saturates and higher nitrogen (especially for basic nitrogen), middle, and heavy aromatics and residue contents. The physicochemical properties and FCC performances of each cut from DG CGO are listed in Table 2. Composition analysis of narrow cuts shown in Table 2 proved that the 425-450 °C cut represented about 20 wt % of the total yields. The basic nitrogen content for 425-450 °C cut is the highest among all the cuts with a content of 1300 μg g-1. The SARA analysis shows that the saturate contents in 425-450 and 450-475 °C cuts are lower than the other narrow fractions, and correspondingly their aromatics and resin contents are highest among narrow fractions; especially the resin content is 2 times as high as other cuts. In addition, the conversion, yields of liquid

801 0.05 426

elemental composition, wt % C

2. EXPERIMENTAL SECTION

DQ-VGO

DG-CGO 910 0.12 308

83.88

85.93

H

15.59

11.69

S

0.46

0.78

N

0.02

basic N, μg g-1

203

H/C ratio

0.54 1180

2.23

1.63

83.00 7.50

60.84 6.87

middle-aromatic (wt %)

3.50

13.49

heavy-aromatic (wt %)

4.50

10.11

resin (wt %)

1.30

8.69

components analysis saturate (wt %) light-aromatic (wt %)

asphaltene (wt %) ASTM, °C IBP

242.0

192.8

10% 50%

378.0 427.0

338.3 393.4

70%

446.0

418.2

95% FBP

487.2 497.0

products, and light oil for the 425-450 °C cut are lower than those of the other narrow cuts, except for the yields of liquid products and light oil of the 450-475 °C cut. Considering its properties and FCC performance, therefore, the 425-450 °C cut was chosen as the experimental feed to investigate the key factors that inhibit the cracking behavior of CGO. A commercial, Y zeolite-based equilibrium FCC catalyst LBO16 obtained from North China Petrochemical Co. of CNPC was employed. Its physicochemical properties are listed in Table 3. During all the tests, the mass balances were all between 97 and 100 wt % of the feed. In this work, conversion is defined as the sum of dry gas, liquid petroleum gas, gasoline, and coke; the total liquid products are referred to the sum of liquid petroleum gas, 4124

dx.doi.org/10.1021/ie102117x |Ind. Eng. Chem. Res. 2011, 50, 4123–4132


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Table 2. Properties and FCC Performancea of Dagang CGO Narrow Fractions distillation rangeb (°C) 375-400 yield of fractions (wt %)

400-425

17.84

425-450

20.00

450-475

19.51

475-500

11.08

500þ

4.02

4.51

molecular weight

297

318

340

365

396

418

density (20 °C), kg/m3

886

901

923

928

939

955

H/C ratio

1.66

1.66

1.65

1.65

1.66

1.66

total N, wt %

0.403

0.467

0.529

0.538

0.542

0.520

basic N, wt %

0.122

0.126

0.130

0.126

0.123

0.118

SARA analysis saturate (wt %) aromatic (wt %)

77.82 17.58

75.33 19.25

63.27 25.25

61.96 26.27

70.08 22.80

67.83 23.93

resin (wt %)

4.05

5.12

10.11

11.24

5.85

6.26

asphaltene (wt %)

0.55

0.31

1.37

0.53

1.27

1.98

conversiona (wt %)

70.06

66.66

60.48

61.40

61.60

72.98

yield of liquid productsa (wt %)

61.18

58.18

53.06

49.75

53.78

63.43

yield of light oila (wt %)

49.51

44.67

41.83

39.64

42.52

48.07

The FCC performance was carried out in a confined fluidized bed reactor with reaction temperature of 510 °C, CTO of 5.0, and WHSV of 15 h-1. b The distillation range listed in the table does not include IBP-350 and 350-375 °C, which yield 20.69 and 2.35 wt %, respectively. a

Table 3. Properties of Catalyst metal content (μg g-1) microactivity index

surface area (m2 g-1)

pore volume (cm3 g-1)

apparent bulk density (g cm-3)

Ni

V

Na

66

102

0.28

0.90

12 465

431

2700

gasoline, and diesel yields; the sum of yields of gasoline and diesel is defined as the light oil; and the selectivity is defined as yield/ convesion. 2.2.2. Product Analysis. The FCC reaction was performed in a confined fluidized bed reactor. Details about this system have been described elsewhere.4 The gas products were analyzed by an Agilent 6890 gas chromatograph to measure the volume percentage of H2, N2, and C1 to C6 hydrocarbons. Collected liquid products were weighted and then analyzed by simulated distillation carried out on another Agilent 6890 gas chromatograph according to the ASTM D 2887 method. The amounts of gasoline, diesel, and heavy oil were quantified considering the temperatures range of IBP to 205, 205-350, and 350þ °C, respectively. Coke content on catalysts was measured by means of a coke analyzer equipped with a thermal conductivity detector (TCD). 2.2.3. Catalysts Characterization. The FT-IR of pyridine adsorption was conducted by the FT-IR spectrometer (BIORAD, FTS 3000) equipped with an in situ cell containing CaF2 windows to determine the acid sites of the catalysts. The superficial shape of catalysts was characterized through SEM by Cambridge S-360 electron microscopy. The scanning electron microscopy was vacuumed 25-30 min until the vacuum degree reached above 1 10-4 Torr, the sample was put into a sample cell and high voltage was given to reach 20 kV, and then the instrument was adjusted to get the best pictures. 2.3. Structural Characterization. 2.3.1. Extraction of Nitrogen Bases. A four-step procedure was used to extract basic nitrogen species from 425-450 °C cut of DG CGO. First, 200 mL of dilute HCl aqueous solution and about 400 g of 425450 °C cut from DG CGO were mixed in a 1000 mL separating funnel. After that, the mixture was stirred vigorously for 20 min,

and then the aqueous phase was separated and collected. This process was repeated three times. Next, the three extracted aqueous phases were gathered up and basified with hydroxyl sodium (NaOH) solution to a pH between 8 and 9, followed by extraction with dichloromethane (CH2Cl2). Finally, CH2Cl2 was removed under vacuum, and basic nitrogen fractions of the CGO were obtained. A similar basic nitrogen extraction procedure had been reported by Shi.25 The basic nitrogen contents were determined by UOP 269 method. Also, the analysis of saturates, aromatics, resins, and asphaltenes (SARA) was obtained according to the procedure described by Liang26 and Xun.27 2.3.2. Sample Preparation for ESI FT-ICR MS Analysis. Basic nitrogen samples were prepared for ESI analysis by dissolving 10 mg of sample in 1 mL of toluene; 2 μL of the solution mixture was removed and then diluted with 1 mL of toluene/methanol (1:1, v/v) solution. All solvents used were analytical reagent grade. 2.3.3. ESI FT-ICR MS Analysis. CGO samples were analyzed with a Bruker apex-ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet. Sample solutions were infused via an Apollo II electrospray source at 150 μL/h by a syringe pump. Typical conditions for positive ion formation were emitter voltage, -2.5 kV; capillary column introduce voltage, -3.0 kV, capillary column end voltage, 320 V. Ions were accumulated for 0.1 s in a hexapole with 2.4 V DC voltages and 300 Vp-p radio frequency (RF) amplitude. The quadrupole (Q1) was optimized at m/z 250 to obtain a broad range for ion transfers, and octopoles were operated at 5 MHz at a peak-to-peak RF amplitude of 400 Vp-p, in which ions accumulated for 1 s. Flight time of ions to analyze pool was 1.3 ms. ICR was operated at a 11.75 db attenuation, mass range was 200-750 Da, acquired data size was 4 M, time-domain data sets were coadded of 64 acquisitions. 4125



3. RESULTS AND DISCUSSION 3.1. Effects of Basic Nitrogen Compounds on CGO FCC Performance. 3.1.1. Quality Improvement of the Tested Charges.

As mentioned previously, CGOs were extracted by HCl aqueous solution to enrich basic nitrogen species; thus samples A (DG CGO of 425-450 °C cut before treatment), B, and C (raffinate oils of A extracted by HCl aqueous solution with concentrations of 0.1 and 0.5 mol/L, respectively) were chosen to discuss effects of basic nitrogen compounds on CGO FCC performance. The basic nitrogen contents and SARA of CGOs before and after being extracted by HCl aqueous solution are presented in Figure 2. Analysis of Figure 2 makes it clear that the nitrogen removal procedure results in a sharp decrease of basic nitrogen content and a slight change in SARA properties. The basic nitrogen contents dropped from 1300 to 720 μg 3 g-1, while there is a tiny increase of 2.5 wt % for saturate, and a decrease of 3 wt % for the sum of resin and asphaltene is observed. This indicates that acid removal treatment is not only effective but also selective. A slight increase of aromatic can be attributed to concentration effect: the aromatic concentration will be higher if they do not precipitate.28 Moreover, the change of resin and asphatene contents indicates that basic nitrogen compounds in CGO are partly concentrated in resin and asphaltene. The physicochemical properties of the initial sample seem to be maintained after acid treatment, with the obvious exceptions of the basic nitrogen contents and classes. This means

Figure 2. The basic nitrogen contents and SARA contents of oils extracted and unextracted by HCl (“A” refers to oil before extracted; “B” and “C” refer to raffinate oils extracted by HCl with concentrations of 0.1 and 0.5 mol/L, respectively).

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that all differences of CGO samples during FCC reactions, which will be reported in the following, are exclusively due to the presence of basic nitrogen compounds. 3.1.2. FCC Performance of CGO Before and After Being Extracted. Figure 3 shows the conversion and product distribution for FCC reaction of oils before and after being extracted at a reaction temperature of 500 °C, catalyst-to-oil (CTO) of 6.0, and weight hourly space velocity (WHSV) of 20 h-1 in a confined fluidized bed reactor. By analyzing Figure 3a, one comes to the conclusion that the basic nitrogen removal has noticeable effects on feed conversion and selectivity of gasoline. The feeds B and C (raffinate oils of A extracted by HCl aqueous solution with concentrations of 0.1 and 0.5 mol/L, respectively) crack much more easily than does feed A (DG CGO of 425-450 °C cut before treatment), resulting in increases of more than 13 and 16 wt % in feed conversion as well as increases of about 2 and 3 wt % in selectivity of gasoline, respectively. As was already reported in previous literature, the basic nitrogen compounds can poison the Brønsted acid sites or Lewis acid sites during catalyst cracking reactions.29-33 In fact, basic nitrogen concentrations of feeds B and C are lower than that of feed A after treatment, which are partly responsible for the deactivation of catalysts during FCC reaction. These compounds poison the catalyst by preferentially interacting with the acid sites severely, and so a low conversion has been achieved by feed A during FCC process. Apart from the discussion of effects on conversion, it is also equally important to assess if basic nitrogen compounds have any effects on the product distribution of the cracking network reaction. The results in Figure 3b clearly show that yields of heavy oil, coke, and dry gas all decreased at different degrees for washed oils, especially for the yield of heavy oil with a dramatic drop from 27.59 to 11.27 wt %. These evolutions become significant when a higher concentration of HCl aqueous solution was employed. It is not surprising when one considers that the extraction by HCl aqueous solution has reduced the content and types of basic nitrogen compounds in CGO, so the poisoning effects of them on catalysts were weakened, and therefore adsorptions for other easy-cracking hydrocarbons were strengthened. From basic extracts obtained by two acids of various concentrations presented above (feeds B and C), it is also interesting to discover that only a difference in basic nitrogen contents of 80 μg g-1 results in an obvious improvement of FCC performance for extracted oil. These results suggest, therefore, the content of basic nitrogen compounds is probably not the only key that inhibits the CGO FCC performance. There may be a major

Figure 3. The FCC performances for oils before and after HCl treatment (“A” refers to oil before extracted; “B” and “C” refer to raffinate oils extracted by HCl with concentrations of 0.1 and 0.5 mol/L, respectively). 4126



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Figure 4. Contrastive tests for different basic nitrogen compounds. (DQ-VGO1 and DG-VGO2 refer to the DQ VGO, which is blended with 2-methy quinoline to reach different contents of basic nitrogen contents. DGZ**-CGO refers to the washed oil of the 400-425 °C cut of DG CGO.)

difference in species and structures of basic nitrogen compounds extracted by two acids of different concentrations that are responsible for the FCC performance discrepancy. 3.1.3. Contrastive Tests of Model Compounds and Basic Nitrogen Species. The contrastive tests of DQ VGO blended with different contents of 2-methy quinoline and real CGOs were carried out to obtain more detailed insight into the key for CGO’s poor FCC performance. Figure 4a shows the different basic nitrogen contents of the tested samples. Figure 4b shows the conversion and product distribution of the four tested samples at a reaction temperature of 500 °C, CTO of 6.0, and WHSV of 20 h-1. The results, shown in Figure 4, present that, for a given basic nitrogen species (indicated by DQ-VGO1 and DQ-VGO2), the deactivation depends almost exclusively on the nitrogen contents. The addition of 2-methy quinoline has obvious effects on the conversion and product distribution during FCC reactions. The oils containing lower basic nitrogen compounds are cracked much easier than those with higher basic nitrogen compounds. One fact could explain this result: because the catalyst used for FCC test in this research is an ultrastable Y-type zeolite, there was a saturation amount for it to hold basic nitrogen compounds, the adsorption opportunities of basic nitrogen compounds on active matrix decreased with the increase of additives content, and as a result the catalytic cracking performance was retarded. Moreover, works by Corma et al.17demonstrated that each basic molecule must cause a decrease in the acid strength of the neighboring acid sites besides poisoning one acid site. The extent of this reduction in acid strength is obviously related to the basicity of the poison, which can also explain the observed results. On the other hand, by analyzing the basic nitrogen contents and FCC performances of DQ-VGO1, DG-CGO, and DGZ**CGO in Figure 4, it is observable that the contents of basic nitrogen compounds are not proportional to the poisoning effects in FCC reactions. The conversion and yields of liquid products and light oil are on a high level, 80.48, 73.53, and 60.91 wt %, respectively, when 2-methy quinoline is added to DQVGO to reach a basic nitrogen content of 2000 μg g-1(DQVGO1), whereas the conversion (73.51 wt %), liquid products (66.36 wt %), and light oil (54.66 wt %) for DG-CGO are all lower than those of DQ-VGO1 even if it is only with a basic nitrogen content of 1180 μg g-1, which was nearly equal to onehalf of that in DQ-VGO1. What is more, there is only a small difference in conversion, liquid products, and light oil between DQ-VGO1 and the washed oil (DGZ**-CGO) despite that the basic nitrogen content of the latter (270 μg g-1) is almost 1/8 of

Table 4. Amounts of Acid Sites Determined by Pyridine Adsorption for Catalysts with or without Being Coked catalysts CatE

initial acid sites, au

Catvc

rest acid sites, au

Catcc

amount of acid sites (μmol g-1) 12.1 7.7

loss ratio,a %

36

rest acid sites, au loss ratio,a %

1.4 88

Loss ratio = (initial acid content - rest acid content)/initial acid content 100 (CatE refers to catalysts before reaction; Catvc refers to coked catalysts after reacting with VGO; Catcc refers to coked catalysts after reacting with CGO). a

that in DQ-VGO1. The reason is that the structures and species of basic nitrogen compounds in CGO, as compared to 2-methy quinoline in VGO, are much more complex as previously presented, and the other components in CGO may also have intereffects with basic nitrogen compounds. It is also commonly accepted that molecules must enter the aperture of catalysts micropore first before the cracking reaction can take place. This also can be explained by their proton affinities, which were determined by measuring the gas-phase proton transfer reaction between a base and the NH4þ ion. In particular, proton affinities correlate with (1) an increase in poisoning with addition of one or two benzene rings to pyridine, and (2) an increase in poisoning with addition of alkylgroups to pyridine. The proton affinities decrease as follows: acridine (233.8 kcal/mol) > quinoline (228.4 kcal/mol) > pyridine (222.8 kcal/mol).34 The results indeed confirm that the structure of basic nitrogen compounds is a much more significant factor in CGO FCC performance than is the basic nitrogen concentration. 3.2. Characterization of Catalysts. It can be inferred from the previous results that the bulky basic nitrogen compounds absorb on the catalysts preferentially due to their aromatic properties, resulting in acid sites decreasing and catalyst pore blocking, and thus restricting the cracking reactions for other components in CGO. To further understand this theory, coked catalysts with a carbon deposition rate of 1% are chosen to discuss the influence of basic nitrogen compounds on FCC catalysts. CatE, Catvc, and Catcc refer to equilibrium catalyst before reaction, and coked catalyst reacted with VGO and CGO, respectively. 3.2.1. Pyridine FT-IR Analysis. Table 4 gives the acid distributions of catalysts before and after interacting with VGO and CGO by pyridine FT-IR method. As compared to the amount of 4127



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Figure 5. SEM morphologies of catalysts before/after reaction with VGO/CGO (CatE refers to catalysts before reaction; Catvc refers to coked catalysts after reacting with VGO; Catcc refers to coked catalysts after reacting with CGO).

Figure 6. Positive-ion electrospray 9.4-T FT-ICR mass spectrum of the 425-450 °C cut of Dagang CGO; insets show the mass scale expanded mass spectra.

acid sites in Table 4, the CGO has a larger effect on the acidic properties. The acid sites loss ratio for VGO was only 36 wt %, but the value for CGO was as high as 88 wt %. This can explain the limited cracking reactions on acid sites for other components due to an amount of bulky basic nitrogen compounds in CGO absorbed on the catalysts preferentially. 3.2.2. SEM Analysis. To clarify the change of catalysts after CGO FCC performance, scanning electron microscopy was used to characterize surface morphology of catalysts before/after reaction with VGO (Catvc) and CGO (Catcc). As shown in Figure 5, as compared to CatE and Catvc, the SEM photograph of Catcc seems to be covered by something like fluffy masses,

indicating that a “shielding effect” was induced by coke deposition due to the aromatic property of basic nitrogen compounds during CGO FCC performance. As a consequence, pore mouth blockage occurred, and the diffusion for reaction molecules is prevented. 3.3. ESI FT-ICR Analysis of Nitrogen Compounds. To understand the chemistry composition behind the effects of basic nitrogen compounds on FCC reactions, the comprehensive composition of 425-450 °C CGO cut, its HCl extracts, and base-free oil were characterized by positive ESI FT-ICR MS. Figure 6 shows the positive ESI FT-ICR MS broadband (m/z 200-550) mass spectra for DG CGO of the 425-450 °C cut. 4128



Figure 7. Broadband positive-ion electrospray 9.4 T FT-ICR mass spectra of Dagang 425-450 °C CGO (a), HCl extract-isolated CGO basic fractions in two acid concentrations ((b) 0.1 mol/L and (c) 0.5 mol/L), and acid washed CGO (d).

The insets show the mass scale expanded segment mass spectra. The achieved resolving power of 490 000 at m/z 330 results in 5333 mass spectral peaks, each with magnitude greater than 6σ of the baseline rms noise. The ultrahigh mass resolving power and high mass accuracy enable the assignment of a unique elemental composition to each peak in the mass spectrum. The molecular weight distributes from m/z 200 to m/z 450, centers at about m/ z 300, with even masses are dominating, which indicates that the nitrogen atom is contained in the molecules.21 Basic species are characterized by class (numbers of N, O, S heteroatoms), type (rings plus double bonds (DBE)), and carbon number distribution. The N, N2, NO, N2O1, and NS class species are assigned in the scal-expanded segment mass spectra and throughout the whole mass range. Positive ESI FT-ICR mass spectra for cut 425-450 °C, basic nitrogen extracts by two different acid concentrations, and washed oil are shown in Figure 7. The most abundant species in DG CGO of the 425-450 °C cut is distributed with m/z value from 220 to 400 and centered at about m/z 300. The mass distribution of basic species shifted to a higher mass range in washed oil, ranging from m/z 220 to 420, centered at about m/z 320. In contrast to the parent CGO, basic fractions exhibited a lower molecular weight distribution, ranging from m/z 200 to 350, and centered at about m/z 270. Relative abundance of heteroatom class species for CGO and its subfractions was shown in Figure 8. Considering the undesirable impact of basic nitrogen compound on FCC reaction,35-38 the N-containing compound is the main part that will be discussed in the following paragraphs. The N is the most dominant species; the abundance of all feeds detected are above 65%. Other classes share a very low abundance, generally 2-22% N2, 1-10% NO, and 1-5% NS species, and the relative abundance of N2 and N2O increases after treatment, but in the original and washed oil, the N2O can hardly be detected. The isoabundance dot-size coded plots are constructed to investigate the differences in DBE and carbon number distribution of the samples to provide more detailed information about molecular composition. Figure 9 illustrates the isoabundance map for DBE versus the carbon number for the N class in samples. The N class in DG CGO of the 425-450 °C cut (Figure 9a) exhibits DBE from 4 to 29, and carbon number from 16 to 32. The highest relative abundances N species exhibit a DBE of 12 and with 20 carbons, consistent in a core structure of nitrogen heterocyclic azapyrene with a C5 alky side chain. N species with DBE of 10-12 and carbon number of 20-25 dominated in the original CGO of cut

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Figure 8. Nitrogen class relative abundances for CGOs Dagang 425450 °C CGO (a), HCl extract-isolated CGO basic fractions in two acid concentrations ((b) 0.1 mol/L and (c) 0.5 mol/L), and acid washed CGO (d).

425-450 °C, which indicates mononitrogen compounds are 34 rings with short alky side chain. Note that the N class isoabundance map of basic fractions (Figure 9b and c) and acid washed oil (Figure 9d) are not obviously different from that of DG CGO of cut 425-450 °C. The distribution of DBEs and carbon number of parts b and c, lower than those of the original CGO (Figure 9a), are most similar, except for relative abundance for different types; this implies that the acid cannot extract large molecular basic nitrogen compounds even with a high acid concentration, even though a higher acid concentration can enrich much more basic compounds. This is analogous to the naphthenic acid extraction in crude oils.39 The distribution of DBE and carbon number of acid washed oil is broader than those of other CGOs, with DBE of 7-15 and carbon number of 2027, and the distinct character of acid washed oil is that the species with a low molecular condensation and a large carbon number exhibits a high abundance. The characterization of the basic nitrogen compounds by means of ESI FT-ICR indicated that it consists of compounds with one to four aromatic rings, and triaromatic compounds serve as the most abundant type. Acid extraction can enrich basic nitrogen compounds with small molecular, and there are still some basic nitrogen compounds in the washed oil. The possible structure of basic nitrogen compounds in CGO detected in this study can be described in Table 5. Note that the identified basic nitrogen compounds are quinoline-derived species; they are characterized by two to four rings with a single nitrogen atom, containing side chains of no more than 5 carbon atoms. As was indicated by the ESI FT-ICR MS results, nitrogen atom was usually performed as a heteroatom united with polycyclic aromatic hydrocarbons with short side chains, which were dominated by alkyquinoline, benzoquinoline, and alkyacridine. The basic extracts obtained by HCl with a lower concentration were likely dominated by alkyquinoline with short side chains, while those obtained by a higher aicd concentration were likely dominated by series of benzoquinoline, acridine, and benzoacridine. The latter has much bigger size, more complex structures, and bigger DBE than the former ones even if the basic nitrogen contents differed a little. Combining previous FCC reaction results, it may be inferred that, therefore, the basic nitrogen compounds with bigger DBEs or more complex structures are stronger poisons during the CGO cracking process despite their low basicity. 4129



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Figure 9. Plots of DBE versus carbon number for N class species from the positive-ion ESI FT-ICR mass spectra of CGOs Dagang 425-450 °C CGO (a), HCl extract-isolated CGO basic fractions in two acid concentrations ((b) 0.1 mol/L and (c) 0.5 mol/L), and acid washed CGO (d). Four possible representative structures illustrate the compounds typically detected as positive ions.

Table 5. Possible Structures of Typical Nitrogen Compounds in CGO

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4. CONCLUSIONS Several important conclusions can be drawn from the results obtained by this research. Basic nitrogen compounds have a strong retardation influence on conversion and product distribution during CGO FCC reaction due to a cumulative effect. The contrastive test shows the structure of basic nitrogen compounds is a much more significant factor in CGO FCC performance than is the basic nitrogen concentration. Besides, interactions with other components of basic nitrogen compounds also have obvious effects on CGO catalytic cracking. The characterization of the equilibrium and coked catalysts indicates that poisonous basic nitrogen compounds can reversibly absorb onto the acid site, resulting in a diminution of acid centers, or act as coke precursors due to their size and aromatic nature, which is partly responsible for the aggravating CGO conversion. The detailed identification of basic nitrogen compounds in original CGO and HCl aqueous solutions washed oil by positive ESI FT-ICR MS shows that acidic extraction separates out basic nitrogen compounds in low molecular weight, with dominate classes of benzoquinoline and benzoacridine, which are characterized by high condensing and with short alkyl side chains. The dominated extracted basic nitrogen species differed with the concentration of HCl aqueous solutions used. The N class species, the most abundant ones, is the key factor for CGO’s poor FCC performance, and the effect of the structure of basic nitrogen compounds on the CGO cracking process is stronger with the increase of their DBE. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: 8610-8973-3775. Fax: 8610-6972-4721. E-mail: wanggang75@ 163.com.

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Retardation Effect of Basic Nitrogen Compounds on Hydrocarbons

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