Catalytic Cracking Constraints Analysis and Divisional Fluid Catalytic

Article pubs.acs.org/EF

Catalytic Cracking Constraints Analysis and Divisional Fluid Catalytic Cracking Process for Coker Gas Oil Zekun Li,† Gang Wang,*,† Yindong Liu,‡ Hao Wang,§ Yongmei Liang,† Chunming Xu,† and Jinsen Gao† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China CNPC Research Institute of Petrochemical Technology, Beijing, 100195, China § Petrochina Planning and Engineering Institute, Beijing, 100083, China ‡

ABSTRACT: The influences of the boiling point and fractional composition of coker gas oil (CGO) on the fluid catalytic cracking performance were investigated. Nitrogen compounds and condensed aromatics in CGO were identified by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) and gas chromatography-mass spectrometry (GC-MS), respectively, and their effects were studied systematically. The result shows that the catalytic cracking performance of CGO does not correspond to the boiling point of narrow fractions but rather to the basic nitrogen compounds and condensed aromatics that have high molecular weight and/or high condensation tendencies adversely. On the basis of these results, a divisional fluid catalytic cracking (DFCC) process was proposed where a separate reaction zone was added to reduce the contents and effects of the adverse compounds in CGO during catalytic cracking. The simulation experiments of the DFCC process show that improved conversion and enhanced yield of light oil can be achieved when an appropriate reaction condition is applied to every reaction zone.

1. INTRODUCTION Recently, difficulties have arisen in the petroleum refining industry as a result of increasing demands for light oils and limited availability of heavy crude oils. They have led to greater interest among oil refiners worldwide in the use of the coking process to upgrade heavy oil. Coker gas oil (CGO), which usually amounts to 20−30 wt % of the total coking products from heavy oil upgrading,1 can then be converted into light oil to meet the demands. CGO has been considered difficult to process for several decades due to its characteristic hydrogen deficiency, high aromatic content, and high nitrogen content (especially basic nitrogen content).2−4 In general, hydrotreating and hydrocracking are the processes employed to further upgrade CGO, but they require high capital and operating costs.5,6 In China, the shortage of hydrogen supplies and the lack of necessary processing equipment call for alternative approaches and render fluid catalytic cracking (FCC), that uses CGO directly as feedstock, an economically feasible method.7−9 However, the presence of high nitrogen content in CGO can reduce the activity of the catalysts during catalytic cracking due to the strong adsorption of basic nitrogen compounds toward the acid sites responsible for CGO cracking, which causes the conversion and subsequent blending ratio of CGO to be limited. A great deal of effort has been devoted to enhancing the processability of CGO and the viability of FCC with CGO as a direct feedstock through three major approaches. The first approach focuses on pretreating CGO before using it in FCC to mitigate the adverse effects of nitrogen compounds. Many such pretreating processes have been developed, including (a) hydrotreating,10 (b) adsorption by solid adsorbents,11 (c) immiscible solvent extraction,12,13 and (d) neutralization by acid additives.14 However, these pretreating processes face possible problems of high initial investment, low yield, high © 2012 American Chemical Society

energy consumption, and significant environmental concerns. Some researchers have developed nitrogen-resistant catalysts for FCC, which can be categorized as the second approach. However, this still does not solve the issue of limited blending ratio of CGO.15 The third approach involves the development of new FCC processes. In this respect, Zhang et al.16 introduced a new process based on denitrified catalytic cracking technology (DNCC) to improve the blending ratio of CGO. However, the conversion and the resultant blending ratio of CGO from this process are still quite limited because of the need to ensure smooth catalytic cracking reaction of typical feedstock in the bottom of the riser. Yuan et al.17 investigated the feasibility of upgrading CGO by a two-stage residue fluid catalytic cracking (TSRFCC) and found that the TSRFCC process offers excellent advantages in the catalytic cracking of CGO under the same conversion as conventional FCC. Some nitrogen-resistant processes have also been developed and granted patents. Krug et al.18 proposed a nitrogen-tolerant cracking process, where hydrocarbon feedstock containing basic contaminants is exposed to catalysts for a sufficient time in a precontacting zone under relatively high temperature so as to remove some or all of the basic contaminants at the expense of sacrificing a certain amount of catalysts. Bourgogne et al.19 also attempted to use a fluidized-bed catalytic cracking process to treat feedstock with a high content of basic nitrogen compounds. However, no further progress of these processes seems to have been made, leaving a simple and economically feasible FCC process for upgrading CGO still to be desired. The purpose of this study is to determine the controlling factors of CGO catalytic cracking performance, including the Received: December 4, 2011 Revised: February 29, 2012 Published: February 29, 2012 2281

dx.doi.org/10.1021/ef201893s | Energy Fuels 2012, 26, 2281−2291

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Table 1 shows that, compared to Q-VGO, G-CGO is characterized by a lower amount of saturates and higher amounts of nitrogen (especially basic nitrogen), heavy aromatics, and residue contents. The properties of G-CGO narrow fractions in Table 2 show that the middle narrow fractions of 400−425 and 425−450 °C represent more than 40 wt % of the total yield, while the narrow fractions above 475 °C represent less than 9 wt % of the yield. There are only slight variations in C wt%, H wt%, and C/H ratio among all the narrow fractions analyzed. The total nitrogen content (Nt) and basic nitrogen content (Nb) first increase and then decrease as the narrow fractions move from lower boiling temperatures to higher boiling temperatures and accordingly from lighter molecular weights to heavier molecular weights. As for the (saturates, aromatics, resin, and asphaltenes) SARA analysis, the aromatics increase and the saturates decrease with increasing boiling temperature until 475 °C, after which the aromatics decrease and the saturates increases. 2.2. Catalysts. A commercial, Y zeolite-based equilibrium FCC catalyst obtained from the North China Petrochemical Company of China National Petroleum Corporation (CNPC) was employed in this study. Its physicochemical properties are listed in Table 3.

boiling point and fractional composition of CGO narrow fractions and the contents of nitrogen compounds and condensed aromatics. On the basis of such analyses, a divisional fluid catalytic cracking process (DFCC) having an additional reaction zone for CGO conversion was proposed for reducing the adverse effects of nitrogen compounds and condensed aromatics in the feed. To assess the proposed DFCC process, simulation experiments with Dagang CGO were investigated in a technical pilot-scale riser FCC apparatus (TPSR) and a fixed fluidized-bed reactor.

2. EXPERIMENTAL SECTION 2.1. Feedstock. Dagang CGO (G-CGO) was used as feedstock for the simulative partitioning conversion FCC, and several narrow fractions of G-CGO obtained from true boiling point distillation (TBP) were used to analyze the controlling factors during CGO catalytic cracking. For comparison, the Daqing VGO (Q-VGO) was also used as feedstock. The relevant properties of G-CGO and Q-VGO are listed in Table 1, and the results of the analysis of G-CGO narrow fractions are listed in Table 2.

Table 3. Properties of the Catalyst Employed in This Work

Table 1. Properties of Daqing VGO (Q-VGO) and Dagang CGO (G-CGO) properties −3

density (20 °C), kg·m Conradson carbon residue (CCR), wt % molecular weight elemental composition, wt % C H S N basic N, μg·g−1 H/C ratio SARA analysis, wt % saturate aromatic resin + asphaltene ASTM, °C IBP 10% 50% 70% 95% FBP

Q-VGO

G-CGO

801 0.05 426

910 0.12 308

83.88 15.59 0.46 0.02 203 2.23

85.93 11.69 0.78 0.54 1600 1.63

83.00 15.5 1.30

60.84 30.47 8.69

242.0 378.0 427.0 446.0

metal content, μg·g−1

microactivity index

surface area, m2·g−1

66

102

pore volume, cm3·g−1

apparent bulk density, g·cm−3

Ni

0.28 0.90 12 465 particle size distribution, wt %

V

Na

431

2700

0−20 (μm)

20−40 (μm)

40−80 (μm)

>80 (μm)

0

5

51

44

2.3. Experimental Apparatus. The catalytic cracking experiments for obtaining different narrow fractions were performed in a fixed fluidized-bed reactor. The schematic diagram of this reactor has been reported before and can be easily found in the literature.4 It can be divided into five sections: oil and steam input mechanisms, reaction zone, temperature control, product separation, and collection system. The DFCC to convert CGO individually was implemented with a technical pilot scale riser FCC apparatus (TPSR) that has a throughput mass flow rate of 2.0 kg/h and a catalyst holdup of 12 kg. The plant is composed of a feed injection system, a riser reactor, a catalyst stripper with a disengaging section, a regenerator, and a product recovery system that could be operated with continuous cracking and catalyst regeneration. The details of the pilot plant have been reported elsewhere.20

192.8 338.3 393.4 418.2 487.2

497.0

Table 2. Properties and FCC Performance of Dagang CGO Narrow Fractionsa distillation range,a °C

375−400

400−425

425−450

450−475

475−500

500+

yield of fractions, wt % molecular weight density (20 °C), kg·m−3 C, wt % H, wt % H/C ratio total N (Nt), wt % basic N (Nb), μg·g−1 Nb/Nt ratio SARA analysis, wt % saturate aromatic resin asphaltene

17.84 297 886 86.36 11.98 1.66 0.403 1220 30.27

20.00 318 901 86.96 12.03 1.66 0.467 1260 26.98

19.51 340 923 86.86 11.97 1.65 0.529 1300 24.57

11.08 365 928 86.34 11.90 1.65 0.538 1260 23.42

4.02 396 939 86.64 11.98 1.66 0.542 1230 22.69

4.51 418 955 86.76 11.97 1.66 0.520 1180 22.69

77.82 17.58 4.05 0.55

75.33 19.25 5.12 0.31

63.27 25.25 10.11 1.37

61.96 26.27 11.24 0.53

70.08 22.80 5.85 1.27

67.83 23.93 6.26 1.98

a

The distillation range listed in the table does not include IBP-350 and 350−375 °C, which yields 20.69 and 2.35 wt %, respectively. 2282


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During all of the above tests, the output was measured to be between 97 and 100 wt % of the injected feed, indicative of reasonably good mass balances. In this work, the conversion of the feed is defined as the sum of the weight percentages of dry gas, liquid petroleum gas (LPG), gasoline, and coke produced from cracking. The weight percentage of each product species is defined as its yield, and the percentage of a yield in the total conversion is defined as selectivity. 2.4. Product Analysis. 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 ASTM2887-D method. The amounts of gasoline, diesel, and heavy oil were quantified in three separate temperature ranges: IBP to 205 °C, 206 to 350 °C, and 350+ °C. The coke content deposited on the catalysts in the fixed fluidized-bed apparatus was measured using a coke analyzer equipped with a thermal conductivity detector (TCD), and the coke generated from the reaction in the pilot plant was determined by a CO and CO2 analyzer at the flue gas outlet; the flue gas volume was measured by a flow meter. 2.5. Identification of Nitrogen Species and Condensed Aromatics. The HCl-furfural fractional extraction method was used to extract basic nitrogen compounds and nonbasic nitrogen compounds. HCl aqueous solution was first used to enrich basic nitrogen compounds, and furfural was then used to extract nonbasic nitrogen and condensed aromatics from the HCl aqueous solution raffinate oil. The structural information of heteroatomic compounds in CGO can provide benefits for further study. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), which provides an ultrahigh level of mass resolution and mass accuracy, has been successfully employed to analyze species in heavy petroleum fractions.21 Electrospray ionization (ESI) can be applied to selectively analyze polar compounds, with heteroatom species such as basic nitrogen compounds9−16 being ionized by positive-ion ESI and acids and neutral nitrogen compounds9,11,13 being ionized by negative-ion ESI. For improved analysis capabilities, a positive-ion (or negative-ion) electrospray ionization (ESI) was coupled to a Bruker Apex-Ultra FTICR mass spectrometer equipped with a 9.4 T superconducting magnet. The sample solution was infused by a syringe pump via an Apollo II electrospray source at a flow rate of 180 μL/h. The conditions for positive-ion (or negative-ion) formation were −2.5 kV (or 2.5 kV) emitter voltage, −3.0 kV (or 3.0 kV) capillary column front end voltage, and 320 V (or −320 V) capillary column end voltage. Ions are collected over 0.1 s in a hexapole with a direct-current (DC) voltage of 2.4 V (or −2.4 V) and a peak-to-peak radiofrequency (rf) amplitude of 400 V. The optimized mass for Q1 was 200 Da. Hexapoles of the Qh interface were operated at 5 MHz at a peak-topeak rf amplitude of 400 V, and the ions are collected over 0.2 s (or 0.4 s). The delay was set to 0.8 ms to transfer the ions to an ICR

cell by electrostatic focusing of transfer optics. The ICR was operated at 11.75 db (or 15 db) attenuation, 200−750 Da (or 160−600 Da) mass range, and 4 Macquired data size. The data sets in the time domain were added together from 64 data acquisitions. The analytical parameters and instrument calibration used for our data analyses using the ESI FT-ICR mass spectrometer are taken from Shi et al.22 Moreover, a Thermo-Finnigan Trace DSQ GC-MS coupled with a HP-5MS column (30 m × 0.25 mm × 0.25 μm) was used to analyze the composition of aromatic compounds in the extract fraction and CGOs before and after extraction. The GC oven was maintained at 35 °C for 1 min, increased to 300 at 2 °C/min, and then kept at 300 °C for 10 min. The sample was injected at 300 °C. The electron impact (EI) ionization source was operated under an ionization energy of either 12 or 70 eV. The mass range was set to 35−500 Da at a 1 s scanning period. The ion source temperature was 200 °C, and the ion current was 250 μA.

3. RESULTS AND DISCUSSION 3.1. Constraints Analysis of CGO FCC. The properties of the narrow fractions listed in Table 2 show that their differences can be summarized on the basis of their boiling points and contents of nonhydrocarbon and polar components. The FCC experiments of every narrow fraction were conducted in a fixed fluidized-bed reactor to investigate the constraints of CGO catalytic cracking. 3.1.1. Effect of Boiling Point. Figure 1 presents the FCC performances of CGO narrow fractions at the reaction temperature of 500 °C, catalyst to oil (CTO) ratio of 5, and weight hourly space velocity (WHSV) of 15 h−1 as a function of their boiling points, whose trends coincide with their molecular weights. As can been seen from Figure 1, the conversion, yield of liquid products, and coke production exhibit different dependencies on the boiling points, or equivalently molecular weights, of the CGO narrow fractions. As the molecular weights become heavier at higher boiling points, the conversion and yield of liquid products first decrease and then increase, with the poorest FCC performance registered by the middle fractions resulting from the boiling point ranges of 425−450 and 450−475 °C while the FCC performance improves again with narrow fractions from boiling points above 500 °C. However, the yield and selectivity of coke can be seen to increase constantly and significantly with increased boiling temperatures. Consequently, it can be concluded here that the boiling point of narrow fractions is an important factor for coke production but not a key one for the FCC process.

Figure 1. Effects of narrow fraction on CGO FCC performance. 2283


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Figure 2. Effects of fractional composition on CGO FCC performance.

Figure 3. Effects of CTO ratio on CGO FCC performance at T = 510 °C and WHSV = 15 h−1.

3.1.2. Effect of Fractional Composition. The analysis of fractional composition is another key to a more complete understanding of the FCC performance of different narrow fractions whose contents of basic nitrogen (Nb), total nitrogen (Nt), aromatics, and resin are shown in Figure 2. It can be easily seen that, while the Nb content (Figure 2a) shows only slight variations, the Nt content increases with increasing boiling point, which is consistent with the trend of coke production (Figure 1). This indicates that the structural information of the molecules should be factored into our consideration and understanding. As can be seen from Figure 2b, the contents of aromatics and resin differ greatly from narrow fraction to narrow fraction and reach their highest values with the narrow factions from 425−450 °C and 450−475 °C ranges which also exhibit the poorest FCC performance. It is worth noting that the resin contents in these two narrow fractions are nearly twice those in other narrow fractions. In order to further investigate of the influence of the composition on CGO FCC performance, experiments are conducted under different reaction conditions and the results are presented in Figures 3 to 5 in a manner to facilitate systematic comparison. Figure 3 shows the effect of the CTO ratio on the FCC performance, where it is evident that increasing CTO ratio promotes the conversion sharply for all narrow fractions. While different narrow fractions differ widely in their conversions under lower CTO ratios, their differences become reduced under higher CTO ratios and eventually

diminished with all cases practically converging to the same conversion when the CTO ratio reaches 11. This fact of convergence also suggests that the amounts of components convertible by FCC are almost equal in every narrow fraction and similar initial reaction rates can be reached with all the narrow fractions if the catalyst activities and amounts are provided at relatively high levels. As for the coke production, the narrow fractions from higher boiling points contain more heavier coking precursors and thus lead to higher yields of coke as the CTO ratio and the conversion increase. With the combination of the narrow fraction with the highest boiling points, 475−500 °C, and the highest CTO ratio tested at 11, the yield of coke reaches its maximum value that exceeds 16 wt %. Nevertheless, on the basis of the results presented above, the coke content is not the only factor which can decrease the activity of catalyst. Figure 4 shows the effects of reaction temperature on the FCC performance of different narrow fractions. As the reaction temperature increases, the conversions of all the narrow fractions go up as expected but the differences among them appear to become smaller mainly because the conversions of the 400−425 and 450−475 °C narrow fractions increase more sharply than those of the 375−400 and 475−500 °C narrow fractions. The conversions of the 425−450 °C narrow fraction also increase relatively slowly like those of the 375−400 and 475−500 °C narrow fractions, but more importantly, they are constantly the lowest ones over the entire range of reaction 2284


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Figure 4. Effects of reaction temperature on CGO FCC performance at CTO = 5 and WHSV = 15 h−1.

Figure 5. Effects of weight hourly space velocity on CGO FCC performance at T = 510 °C and CTO = 5.

short reaction time and subsequently promote the formation of coke that decreases the catalytic activities. Perhaps more importantly, the yield of coke decreases across the board with increasing WHSV, indicating that short reaction time can be used in CGO catalytic cracking to offset the coking tendency induced by high CTO ratio. 3.2. Structural Analysis of Nitrogen Compounds and Condensed Aromatics. Besides the macroscale analyses of composition and polarity in every narrow fraction from the aspect of CGO FCC performance, the analysis of the structural information of the nitrogen compounds and condensed aromatics in CGO should be beneficial in providing a deeper understanding of their inhibiting effects. The basic nitrogen compounds, nonbasic nitrogen compounds, and condensed aromatics were concentrated from the 425−450 °C narrow fraction using a HCl-furfural fractional extraction method. The yields of the HCl and furfural fractional extraction were 99.9 and 90 wt %, respectively. The basic nitrogen content and the SARA composition were analyzed, and the data are listed in Table 4. While the HCl extraction does not change the composition of CGO except for the nitrogen contents, the furfural extraction clearly changes the SARA composition. To obtain the relevant structural information at the molecular level, the positive-ion ESI FT-ICR MS, negative-ion ESI FT-ICR MS, and gas chromatography-mass spectrometry (GC-MS) were used to identify the basic nitrogen compounds, nonbasic nitrogen compounds, and condensed aromatics, respectively.

temperatures. These results suggest that under low CTO ratios the employed catalyst exhibits different levels of activity in response to different narrow fractions despite their nearly equal amounts of convertible components and the catalytic activity can be further elevated by the use of higher reaction temperatures. Also from Figure 4b coupled with Figure 2a, the coke load increases with increasing boiling point and increasing content of nitrogen compounds except for the narrow fraction of 375−400 °C due to its high conversion. This result indicates that the nitrogen compounds can form coking centers during CGO FCC reactions to promote the coke production.23 Figure 5 shows the effect of WHSV on the FCC reaction of CGO narrow fractions. As the WHSV increases, the contact time between the reactant and catalysts is shortened and the conversion of each narrow fraction decreases as expected but to different degrees. The conversions of the 375−400 and 400− 425 °C narrow fractions are more strongly influenced by WHSV due to their smaller average molecular weights and lower initial reaction rates. On the other hand, the changes in the conversion of the 475−500 °C narrow fraction which contains more heavy molecules and has a higher initial reaction rate are relatively small as a function of WHSV. The conversions of the 425−450 and 450−475 °C narrow fractions are not only the lowest but also the least influenced by WHSV changes because these two narrow fractions contain the highest amounts of nitrogen compounds and condensed aromatics. With stronger absorption abilities than other components, the nitrogen compounds can absorb on the catalyst within a very 2285


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to be acridines, cycloalkyl-acridines, azapyrene, and benzoacridines,22,24,25 respectively. 3.2.2. Nonbasic Nitrogen Compounds. Similar to Figure 6, Figure 7a shows the negative-ion ESI FT-ICR MS broadband spectrum of nonbasic nitrogen species, while Figure 7b,c shows the nitrogen class relative abundances and plots of DBE vs carbon number for the N class species, respectively. Similarly, the N class nitrogen compounds are the dominant species in all identified nitrogen species of N, NO, N1O2, and NS. The N class species of nonbasic nitrogen in CGO have DBE values between 9 and 16 and carbon numbers between 16 and 30. Here, the N class species with the highest relative abundance has 20 carbon atoms and a DBE value of 12. The possible core

Table 4. Change of Nitrogen Content and SARA in Oils before and after Extraction oil samples basic N (Nb), μg·g−1 SARA analysis, wt % saturate aromatic resin asphaltene

untreated oil 1300 63.27 25.25 10.11 1.37

HCl treated oil 493 65.79 25.78 8.12 0.32

furfural treated oil 335 81.98 14.96 3.06

3.2.1. Basic Nitrogen Compounds. Figure 6a shows the positive-ion ESI FT-ICR mass spectrum of the basic nitrogen species, and Figure 6b shows the relative abundances of the heteroatom nitrogen class species. The species involved in the analyses contain N, N2, N2O, NO, and NS and N is dominated. In order to identify the molecular compositions of the various N class species in CGO, iso-abundance dot-size coded plots are constructed by correlating the distributions of the DBE value and carbon number of the species; the results are shown in Figure 6c; the possible structures for N specie also are depicted in Figure 6c. From Figure 6c, the N class species in the CGO narrow fractions can be interpreted to spread over wide ranges of DBE (5−18) and carbon number (15−35). The N class species with the highest relative abundances have DBE values equal to 10, 11, 12, and 13, which are considered most likely

Figure 8. Total ion chromatogram of nonbasic nitrogen and condensed aromatic extracts (condensed aromatics).

Figure 6. Positive-ion ESI FT-ICR analysis of basic nitrogen extracts (basic nitrogen species).

Figure 7. Negative-ion ESI FT-ICR analysis of nonbasic nitrogen extracts (nonbasic nitrogen species). 2286


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Figure 9. Mass chromatogram of nonbasic nitrogen and condensed aromatic extracts (condensed aromatics).

Figure 10. FCC performance of oil samples before and after treatment.

structure of this compound could be benzocarbazoles,26 the structure shown in Figure 7c. 3.2.3. Condensed Aromatics. The condensed aromatics in extracts were characterized by GC-MS because they are highmolecular-weight and nonpolar compounds. Figures 8 and 9 show the total ion chromatogram and mass chromatogram of the condensed aromatics in CGO, respectively. The polynuclear aromatics were identified to be composed of three or more aromatic rings, and most of them also have three or four nonaromatic rings as well as high condensation degrees. In addition, phenanthrenes, benzonaphenothiophene, pyrene, and benzoanthracenes, each with side chains of 0−3 carbon atoms, were considered to be the predominant aromatics (the possible structure also shown in Figure 9). On the basis of the structural analyses of the nitrogen compounds and condensed aromatics presented above, the information can be summarized as follows: The basic nitrogen

compounds in CGO are abundant with species such as benzoquinoline and benzoacridine, which are characterized by high condensation tendencies and short alkyl side chains. Their N atoms are embedded in stable benzene rings, which hardly allow the ring-opening reactions to take place during catalytic cracking and hence tend to adsorb irreversibly on the catalytic acid centers. The nonbasic nitrogen compounds in CGO usually have higher molecular weights than the basic nitrogen compounds, and benzocarbazoles are the predominant species in this category. In addition, these compounds usually have large molecular diameters and lower diffusivity into catalyst pore and, consequently, tend to accumulate at the surface of catalyst and promote the formation of coke. The condensed aromatics in CGO have three or four benzene rings with phenanthrenes, benzonaphenothiophene, pyrene, and benzoanthracenes being the predominant species. 2287


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Figure 11. Schematic of the proposed DFCC process and a routine FCC process.

aromatics should be removed first in order to obtain better products from the FCC reactions of CGO. As reported in previous studies, refiners have traditionally attempted to deal with high nitrogen feeds in a number of different ways, including (a) hydrotreating, (b) acid treatment to remove basic nitrogen compounds, (c) injection of acid into the feed, (d) changing the process conditions, for example, by increasing reaction temperature or the severity, i.e., the catalyst to oil ratio, (e) blending of multiple feedstock to control the concentration of nitrogen compounds, and (f) development and use of more active catalysts. However, these approaches are all subjected to the restrictions of high capital or recovery rate of feedstock. In order to upgrade CGO properly for obtaining superior quality product without affecting the conversion of normal FCC feedstock and subjecting to the adverse effects of impurities, a novel CGO FCC process was proposed and studied in this work. In this new process that is titled “divisional fluid catalytic cracking (DFCC) process”, a separate reaction zone was added to the conventional FCC riser for “catalytic denitrogen-stiu upgrading” of CGO. Figure 11 shows the schematics of the proposed DFCC process and a routine FCC process. Compared to the routine FCC process with one reaction zone, the riser of the DFCC process is divided into three parts: a routine feedstock reaction zone, a CGO reaction zone, and a mixed reaction zone. More details about this DFCC process are shown in Figure 12. Small portions of the catalysts from the regenerator are used to

They have side chains of 0−3 carbon atoms in length and high condensation tendencies that enable them to be absorbed easily on the catalysts, leaving the pore jammed and eventually leading to the formation of coke. 3.3. Reaction Performance of CGO Before and After Treatment. Figure 10 contrasts the FCC performances of CGO before and after the treatment that removes the basic nitrogen compounds, nonbasic nitrogen compounds, and condensed aromatics. As can be expected from the results presented above, such a treatment provides a remarkable boost to the FCC performance of CGO. This suggests that once the components with adverse effects are removed, the main parts of the hydrocarbons in CGO themselves possess good crackability. It is thus desirable to devise new FCC processes for CGO that are improved with proper pretreating steps. For example, feedstock could be treated by means of solvent extraction or hydrotreating to reduce the contents of adverse components and their effects before being sent into a FCC process. However, drawbacks such as a high investment and/or a low blending ratio cannot be ignored for these processes. A good choice is to achieve enhanced conversion of CGO, improved selectivity of desirable species in the upgraded product, and prolonged use of catalysts during FCC process. 3.4. New Divisional Fluid Catalytic Cracking (DFCC) Process for CGO Conversion. According to the results discussed above, the coking precursors, basic nitrogen compounds, nonbasic nitrogen compounds, and condensed 2288


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Figure 12. Relation between reaction zones in the proposed DFCC process.

Table 5. Effects of Different Operating Conditions on Product Distribution routine FCC process

DFCC test 1

feedstock and test code

CGO

CTO, kg·kg −1 T, °C residence time, s dry gas LPG gasoline diesel heavy oil coke conversion, wt % yield of light oil, wt % yield of liquid product, wt % selectivity of coke, %

6 500 3.0 3.94 16.64 35.71 23.03 15.24 5.43 61.72 58.75 75.39 8.80

3 470 1.3

test 2 6 500 1.5

2.53 11.09 41.83 21.89 16.27 6.31 61.77 63.72 74.81 10.22

6 500 1.3

test 3 6 500 1.5

3.09 15.09 42.67 20.06 12.85 6.71 67.56 62.73 77.82 9.93

9 530 1.3

6 500 1.5 3.96 16.18 43.33 17.99 11.63 7.08 70.56 60.32 76.50 10.03

For Test 1, the catalytic cracking simulated in the CGO reaction zone was carried out at a condition of CTO = 3, 470 °C as the reaction temperature, and 1.3 s as the residence time, while the reaction condition of the CGO reaction zone was changed to CTO = 6, 500 °C, and 1.3 s for Test 2 and CTO = 9, 530 °C, and 1.3 s for Test 3. Compared the reaction conditions of the three tests, the reaction conditions of the CGO reaction zone were remarkably different. Whereas the three tests have significantly different reaction conditions for the CGO reaction zone, they share the same reaction condition for the mixed reaction zone set at CTO = 6, 500 °C as the reaction temperature, and 1.5 s as the residence time. It is worth noting here that in order for Test 1 to achieve the higher reaction temperature and desired catalytic activity in the mixed reaction zone, the low reaction temperature and CTO ratio used in the CGO reaction zone need to be compensated by a routine reaction zone that has a high reaction temperature and a high CTO ratio. In Test 2, the reaction condition of the CGO reaction zone was similar to that of the routine FCC process, which is also a condition capable of cracking most CGO, the reaction condition of the routine reaction zone could be easily adjusted to provide any needed compensation. In Test 3, the CGO reaction has a reaction temperature and a CTO ratio already higher than those of the routine FCC process, the conversion of CGO was mostly completed in this reaction zone and, thus, the reaction condition of the routine reaction zone can be adjusted independently. The results shown in Table 5 confirm that the production distribution from each of the three DFCC processes tested is superior to that of the routine FCC process. The production of

convert the CGO and routine feedstock separately in the CGO reaction zone and routine feedstock reaction zone, respectively. This design allows for many of the nitrogen compounds and impurities to be bound to catalytic acid sites at the expense of a small amount of catalysts and hence eliminated from interfering with the main reaction of the process. Then, the primary reaction products from these two reaction zones merge together to enter the mixed reaction zone for further cracking reactions to proceed. In practice, the catalytic activity of the mixed reaction zone depends on the CTO ratio used in the routine reaction zone, and the level of conversion can be adjusted by controlling the reaction times in the CGO reaction zone and routine reaction zone. 3.5. Simulation Experiments of the DFCC Process. In order to study the DFCC process for CGO catalytic cracking, experiments were carried out in the TPSR and fixed fluidizedbed reactor. For our experiments, three reaction conditions with different operating severity were chosen to simulate the initial conversion in the CGO reaction zone where a small amount of catalysts was sacrificed to bind with adverse components such as basic nitrogen compounds, nonbasic nitrogen compounds, and condensed aromatics. The resultant liquid products were collected and then used as feedstock for simulating the subsequent CGO upgrading in the mixed reaction zone in the fixed fluidized-bed reactor. The partly coked catalysts obtained from the initial cracking of CGO were used for the subsequent reactions. The results for the simulated DFCC process were calculated by a weighted average method and listed in Table 5. 2289


Energy & Fuels gasoline, yield of light oil, and yield of liquid product are all higher than those from the routine FCC process. Moreover, the different reaction conditions of the three FCC processes gave them different specific characteristics and different potential advantages. Compared with the routine FCC process, Test 1 has a nearly identical conversion, a very comparable yield of liquid product, and a noticeable increase in the yield of light oil by 5 wt %. The increases in the conversion, yield of light oil, and yield of liquid product from Test 2 are 5.8, 4.0, and 2.4 wt %, respectively. For Test 3, the conversion increases by a high margin of 8.9 wt %, while the yield of light oil and yield of liquid product increases only by 1.57 and 1.69 wt %. On the basis of these test results, the moderate CGO reaction zone condition and conversion (Test 2) can be stated to be the better design for the proposed DFCC process to produce increased conversion and enhanced yields of light oil and liquid product by relatively large margins.

ACKNOWLEDGMENTS

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REFERENCES

The authors acknowledge the financial support provided by National Science Fund for Distinguished Young Scholars of China (20725620), the National Natural Science Fund (21176252), and the China National Petroleum Science Research Program (2011B-2404-01).

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4. CONCLUSIONS The CGO FCC reaction performance of narrow fractions was found not to correspond to their boiling points, and the middle narrow fraction with the highest contents of nitrogen compounds and condensed aromatics exhibits the lowest conversion because these compounds can absorb on the catalyst active centers responsible for cracking reactions, thereby reducing the catalytic cracking performance. Coke load is found not to be a valid index for evaluating FCC process although its content goes up with increases in the boiling point of narrow fractions. Conversions of different narrow fractions converge to the same high level at high CTO ratios, suggesting the amounts of the convertible components in every narrow fraction to be almost the same. Structural analysis of nitrogen compounds and condensed aromatics by ESI FT-ICR MS and GC-MS indicate that basic nitrogen compounds, nonbasic nitrogen compounds, and condensed aromatics in CGO are highly condensed compounds with three to four benzene rings. It is necessary to remove these compounds first so as to maintain more catalytic active centers for the cracking reactions of other hydrocarbons. The DFCC for CGO was proposed on the basis of the experimental results, where the riser was divided into three reaction zones: routine reaction zone, CGO reaction zone, and mixed reaction zone. The initial cracking of routine FCC feedstock is processed in the routine reaction zone. The compounds with adverse effects in CGO can be adsorbed away by the catalysts in the CGO reaction zone where CGO feed also undergoes initial cracking. The primary reaction products from these two reaction zones merge together to enter the mixed reaction zone for subsequent cracking reactions to proceed. The simulation tests of the DFCC process carried out in a technical pilot scale riser FCC apparatus showed that improved conversion and enhanced yield of light oil of products can be achieved with a typical FCC reaction condition applied to every reaction zone.

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*Tel.: 8610-8973-3775. Fax: 8610-6972-4721. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2290


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Catalytic Cracking Constraints Analysis and Divisional Fluid Catalytic

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