Study on the Catalytic Cracking of Heavy Oil by Proper Cut for Higher

Feb 8, 2012 - To improve the product distribution in the residue fluid catalytic cracking (RFCC) process, two representative heavy oils were, respecti...
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Study on the Catalytic Cracking of Heavy Oil by Proper Cut for Higher Conversion and Desirable Products Haohua Gao, Gang Wang,* Rong Li, Chunming Xu, and Jinsen Gao State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China ABSTRACT: To improve the product distribution in the residue fluid catalytic cracking (RFCC) process, two representative heavy oils were, respectively, cut into five fractions by true boiling point (TBP) distillation and their catalytic cracking performance was tested in a fixed fluidized bed reactor. The results indicated that they appeared to have diverse reaction characteristics and that about 6.3−19.3 wt % fractions with better crackability were still contained in the vacuum residual (VR). On the basis of the above, a feasible cut temperature of heavy oil was determined and the fractions were classified into two groups: the high-quality fraction (HF) group and the poor-quality fraction (PF) group. Compared with heavy oil before being cut, the product distribution of heavy oil after being cut was improved. With the increase in the blend levels of VR in the vacuum gas oil (VGO), the desired products yield of heavy oil after being cut was enhanced gradually, especially under the optimal reaction conditions. To describe the reaction behavior of the PF catalytic cracking, a six-lump kinetic model was proposed. Kinetic constants and apparent activation energies were determined with the improved Marquardt method. The effect test showed that the kinetic model could predict product yields well.

1. INTRODUCTION The increasing demand for high-value petroleum products and the worldwide trend of increasing supply of heavier and inferior crude oil are contributing to the increasing utilization of residue feedstock in refineries.1−4 Fluid catalytic cracking (FCC), which converts or cracks heavier fractions of crude oil into a variety of lighter products, is one of the most important processes in the oil refinery industry.5 Thus, it will play a key role as a conversion process for residues,6 especially in China.7 A typical residue fluid catalytic cracking (RFCC) operation involves blending the residues, such as atmospheric residue (AR) or vacuum residue (VR), deasphalted oil or aromatic extracts, and so on, into the vacuum gas oil (VGO).7,8 Since about a decade ago, the blending ratio has been increasing steadily founded on the process’ versatility and high efficiency.9 It is widely acknowledged that the distillation range and physicochemical properties of residues differ from those of VGO.3,7,8 In contrast to VGO, residues usually have a larger molecular weight and a higher boiling point. They contain more sulfur and nitrogen heteroatoms, and this causes acid site poisoning. Meanwhile, the higher content of contaminant metals such as nickel, vanadium, sodium, and iron also contributes to more contaminant coke and to the irreversible deactivation of catalysts.7 Moreover, residues have a higher content of resins, asphaltenes, and aromatics, resulting in a lower atomic ratio of hydrogen to carbon and a higher Conradson carbon residue (CCR).7,8 It is precisely these differences in physicochemical properties that give rise to the significant difference in reaction performance. However, in a routine RFCC, various feedstocks are premixed and then react in one reaction zone at the same reaction conditions, resulting in the interaction between them, further retarding the reaction.10 Blending residues into VGO has produced good economic gains, but many problems still need to be resolved,11 for © 2012 American Chemical Society

instance, the deterioration in product distribution and quality that results from increasing the proportion of residues in VGO and more “liquid coke” being deposited in the cyclone separator and FCC disengager. These problems ultimately result in the RFCC unit having a short operational cycle. Many innovations and improvements have been made, and these have mainly focused on the feed injection system, the reactionregeneration sections, and catalysts.12−15 However, few published papers16,17 have considered the reaction characteristics of different feedstocks and then optimized the respective reaction conditions. Moreover, catalytic cracking involves a complicated reaction system occurring among a vast number of molecules, thus making it extremely difficult to characterize and describe the inherent kinetics at the molecular level. Therefore, the complex reaction system has usually been studied by lumping large numbers of chemical compounds into several pseudocomponents according to their boiling and molecular characteristics.18 Many lump kinetic models have been proposed (3-lump,19 4lump,20 5-lump,21 6-lump,22 7-lump,23 8-lump,24 10-lump,25 11lump,26 13-lump,27 14-lump,28 etc.). These models mainly focus on the VGO or heavy oil; only a few lump kinetic models for pure VR catalytic cracking have been reported. The present study is an attempt to seek a means of alleviating the interaction in the heavy oil catalytic cracking process in order to achieve higher conversion and more desired product. Toward this end, two selected heavy oils were fractionated into several fractions by true boiling point (TBP) distillation. The catalytic cracking performance of these fractions was investigated in a fixed fluidized bed reactor. On the basis of their reaction performance, a feasible fractionation program could be Received: December 1, 2011 Revised: February 8, 2012 Published: February 8, 2012 1880

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determined. These fractions were classified into two groups. The reaction performance of each group was optimized separately. Contrast experiments of heavy oil before and after being cut were also performed at different reaction conditions. Moreover, to describe the catalytic cracking reaction behavior, a new six-lump kinetic model with detailed product distribution was proposed.

Table 2. Properties of Commercial Equilibrium Catalysts items

2. EXPERIMENTS 2.1. Feedstock and Catalyst. Two kinds of feedstock were used: one was a representative paraffin-based feedstock that was obtained from an RFCC unit of the China National Petroleum Corporation (CNPC) Changqing Company; the other, which was an intermediatebase feedstock, was obtained from an RFCC unit of the China Petroleum & Chemical Corporation (SINOPEC) Jinan Company. The detailed properties of these feedstocks are shown in Table 1.

Changqing heavy oil

0.9075 density (20 °C) (g/cm3) viscosity (80 °C) (mm2/s) 40.10 conradson carbon residue (CCR) 4.70 (wt %) molecular weight (g/mol) 501 Elemental Composition (wt %) C 86.30 H 12.51 S 0.15 N 0.39 SARA Analysis (wt %) saturates 62.05 aromatics 25.91 resins 10.42 asphaltenes 1.62 Distillation Distribution (wt %) IBP−460 °C 25.50 460−480 °C 7.00 480−500 °C 5.90 500−520 °C 6.30 520−540 °C 5.60 540+ °C 49.70

CDC 70 0.14 77 0.95 10650 5290 2630 7820 20.2 52.3 27.5

that, the reactor was heated to 680 °C to burn the coke over the catalysts with oxygen. The total amount of coke was quantified by a CO2 infrared detector after the flue gases had passed through a CO converter and a drier. 2.3. 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. The ideal gas-state equation was used to convert the data to mass percentages. The liquid products collected were weighed and then analyzed by a simulated distillation carried out on another Agilent 6890 gas chromatograph according to the ASTM-2887-D method in order to determine the yield of gasoline (IBP−200 °C), light cycle oil (LCO, 201−350 °C), and heavy cycle oil (HCO, 350+ °C). The conversion was defined as the weight percent of feedstock converted to dry gas (H2, C1, and C2), liquefied petroleum gas (LPG, C3, and C4), gasoline, LCO, and coke. In addition, the light oil was the sum of gasoline and LCO:

Table 1. Properties of Feedstocks items

LVR-60R

microactivity index 75 pore volume (cm3/g) 0.19 surface area (m2/g) 141 packing density (g/cm3) 0.98 Metal Content (μg/g) Ni 2155 V 1419 Na 2487 Fe 3842 Particle Size Distribution (wt %) 0−40 (μm) 12.0 40−80 (μm) 45.8 >80 (μm) 42.2

Jinan heavy oil 0.9375 73.97 8.42 550 86.80 12.54 0.59 0.39 49.11 28.19 20.79 1.91

conversion =

dry gas + LPG + gasoline + LCO + coke % feed (1)

24.63 6.75 5.34 4.00

3. RESULTS AND DISCUSSION 3.1. Reaction Performance of Narrow Fractions. Two heavy oils were cut into five fractions, respectively, using the TBP distillation method (ASTM D-1160); the detailed properties are shown in Table 3. Their catalytic cracking performance was studied in a fixed fluidized bed reactor under the conventional reaction conditions (reaction temperature 500 °C, CTO 6, WHSV 20 h−1). Tables 4 and 5 show the conversion and the product distribution of the Changqing and Jinan narrow cuts, respectively. Clearly, the conversion of these narrow fractions derived from the same heavy oil was similar, whereas the product distribution was different. In the case of the Changqing heavy oil, the cuts of high-quality fraction (HF)1 (IBP−500 °C), A (480−500 °C), B (500−520 °C), and C (520−540 °C) had a similar product distribution. Compared with the product distribution of the fraction of poor-quality fraction (PF)-1 (500 °C−FBP), they had a higher yield of light oil (by about 5 wt %) but a lower yield of coke plus dry gas (by about 7 wt %). A similar phenomenon was also observed in the case of the Jinan heavy oil. Therefore, because of the poor crackability of the 500 °C−FBP cuts (PF-1 and PF-i), they were called the poor-quality fraction (PF) group. The remaining narrow fractions were called the high-quality fraction (HF) group due to their better catalytic cracking performance. The above results can be explained by the differences in their chemical composition. With the increasing of the boiling

59.28

Two commercial equilibrium catalysts of RFCC were obtained from the above refineries and were, respectively, identified as LVR-60R and CDC. The physicochemical properties of the catalysts are given in Table. 2. Compared with the CDC catalyst, LVR-60R has a higher microactivity index, surface area, and pore volume. The metal content is relatively low. 2.2. Experimental Apparatus. The experiments were performed in a fixed fluidized bed reactor system, as shown in Figure 1; details of this system have been described elsewhere.29 It is a batch system operated in a fluidized mode. A new commercial equilibrium catalyst was used at the start of each run. In the experiments, the reaction temperatures ranged from 460 to 520 °C and the weight hourly space velocity (WHSV) ranged from 5 to 40 h−1. The catalyst to oil weight ratio (CTO) was varied from 4 to 12 g/g by keeping the amount of the catalyst constant and changing the rate of the oil feed. Prior to each experiment, steam was circulated in the reactor system at the set reaction temperature for 15 to 20 min. During the reaction step, the liquid products were collected in the corresponding glass receivers located at the exit of the reactor, which was kept at −3 °C by an automated cold bath. The gaseous products were collected in a gas buret by water displacement. The coked catalyst was stripped with steam for 30 min in order to recover entrapped hydrocarbons. After 1881

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Figure 1. Schematic diagram for experimental setup: (1) oxygen; (2) air; (3) constant temperature box; (4) electronic balance; (5) feedstock; (6) oil pump; (7) water tank; (8) water pump; (9) steam generator; (10) preheater; (11) reactor; (12) thermocouple; (13) first condenser; (14) receiver for liquid products; (15) second condenser; (16) cold trap; (17) gas collection bottle; (18) water bottle; (19) gas sample connection; (20) drain sump; (21) CO converter; (22) drier; (23) CO2 infrared detector.

Table 3. Properties of Different Fractions distillation range of (°C) Changqing heavy oil

Jinan heavy oil

IBP−500 (HF-1)

480−500 (A)

500−520 (B)

520−540 (C)

500−FBP (PF-1)

IBP−500 (HF-i)

460−480 (a)

480−500 (b)

500−520 (c)

500−FBP (PF-i)

density (20 °C) (g/m3) viscosity (80 °C) (mm2/s) conradson carbon residue (CCR) (wt %) molecular weight (g/mol)

862.6 9.61 0.13

10.08 0.10

16.75 0.23

18.20 0.45

939.7 172.73 8.02

912.6 15.43 0.56

23.80 0.51

23.61 0.63

30.62 0.93

955.2 1214.90 11.59

C H S N

86.88 13.01 0.12 0.18

saturates aromatics resins asphaltenes Ni content (μg/g) V content (μg/g)

80.20 15.75 4.05

items

363

n-alkane > polycyclic aromatics. Therefore, inconsistencies in the two sequences lead to hydrocarbons with strong adsorption and weak reaction ability preferentially adsorbing the limited number of active sites on the catalyst surface during FCC reactions. They further retard the remaining hydrocarbons with better reactivity and cause a competitive adsorption effect. We found that the polycyclic aromatics and polycyclic cycloalkanes are the key incompatible components in the sequences above.

significant adsorption of reactants taking place on the catalyst surface.24 Due to the differences in the structure, molecular size and weight, and adsorption heat of various types of hydrocarbons,34−37 the adsorbability of each type is different. The order is as follows:38,39 polycyclic aromatics > polycyclic cycloalkanes > alkenes > monocyclic aromatics with alkyl side chains > cycloalkanes > alkanes. However, the cracking rate of various hydrocarbons on active sites is not consistent with adsorption capacity and even contradicts it. The order of reactivity is approximately as follows: alkene > monocyclic 1884

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Figure 3. Effects of WHSV, CTO, and reaction temperature on product distribution and conversion of PF-1 (dotted lines, open symbols) and PF-i (solid lines, solid symbols). Symbols: conversion (□, ■); light oil (●, ○); coke + dry gas (Δ, ▲).

Table 7. Product Distribution of HF and PF at Different Reaction Conditionsa HF-1 items

a

OPC

PF-1 COC

dry gas LPG gasoline LCO HCO coke

1.31 11.04 59.41 15.74 10.00 2.50

1.72 15.13 57.22 13.16 9.27 3.49

yield of light oil (wt %) yield of dry gas + coke (wt %) conversion (wt %)

75.15 3.81 90.00

70.38 5.21 90.73

OPC

HF-i

PF-i

COC

OPC

COC

OPC

COC

Product Distribution (wt %) 1.94 2.40 13.29 14.48 52.55 51.69 15.68 14.42 6.90 7.32 9.65 9.69

1.59 14.44 47.73 19.81 12.42 4.01

2.13 16.05 46.17 18.43 11.75 5.48

2.75 14.57 38.12 17.47 10.85 16.24

2.79 15.40 35.75 17.21 12.83 16.02

67.54 5.60 87.58

64.59 7.61 88.25

55.59 18.99 89.15

52.96 18.81 87.17

68.22 11.59 93.10

66.11 12.09 92.68

OPC represents the optimal reaction condition; COC represents the conventional reaction condition.

oil increased and passed through a maximum at 490 °C, in agreement with serial kinetics. The temperature, as a main process parameter, markedly changed the conversion and product distribution caused by the thermodynamics of the catalytic cracking reactions. Catalytic cracking is an endothermic reaction; therefore, it will be favored by a higher reaction temperature. However, the product distribution of a parallelseries reaction is closely associated with reaction depth, and an excessive conversion caused by high temperature will lead to more undesired products, such as dry gas and coke. Therefore, to obtain a relatively high total yield of light oil product, the optimal reaction temperature was about 490 °C. 3.3.2. Poor-Quality Fraction Group. Figure 3a−c shows the effects of WHSV, CTO, and reaction temperature on the reaction performance of PFs. The effects of operating factors

Another key factor is the nonhydrocarbon compounds, especially the nitrogen-containing compounds.33 As the CTO increased, the number of active sites increased correspondingly. The rest of the hydrocarbons were able to make contact with the active centers freely, thus alleviating the competitive adsorption effect to some extent. The difference between the polyaromatic hydrocarbon and nonhydrocarbon compound content in HF-i and HF-1 resulted in a diverse range of required optimal CTOs for the feedstocks used. Figure 2c shows the conversion and the product distribution data as a function of reaction temperature at a WHSV of 25 h−1 and a CTO of 5 for HF-1 and 6 for HF-i. The conversion gradually increased as the reaction temperature was raised from 480 to 520 °C. As the reaction temperature increased, the coke plus dry gas yield increased continuously and the yield of light 1885

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on PF were similar to those on HF. The optimal reaction conditions of PF-1 were a reaction temperature of 480 °C, a CTO of 7, and a WHSV of 15 h−1; for PF-i, the optimal reaction conditions were a reaction temperature of 480 °C, a CTO of 8, and a WHSV of 15 h−1. Table 7 lists the conversion and product distribution of HF and PF under different reaction conditions. Compared with the conventional reaction conditions, under the optimal reaction conditions, there was no obvious change in the conversion of HF and PF but the product distribution improved. In the case of HF, the light oil yield of HF-1 increased by 4.8 wt % and the coke plus dry gas and the LPG yields declined by 1.4 and 4.1 wt %, respectively; in the case of HF-i, the corresponding product yields increased by 3.0 wt % and decreased by 2.0 and 1.6 wt %, respectively. In the case of PF, the light oil yield of PF-1 and PF-i increased by 2.1 wt % and 2.6 wt % respectively, while the yield of coke plus dry gas and the LPG yields decreased slightly. The experimental results also suggest that the conventional reaction conditions were inappropriate for both HF and PF. Merely, the effects of the operating parameters on the product distribution of HF were more evident than they were on the product distribution of PF. In this research, the temperature of the catalyst in the fixed fluidized bed reactor was only 460−520 °C. When the PF came into contact with the heated catalyst, the majority of the PF could not enter the micropores. They could react with the catalyst active sites only when they were cracked into smaller molecules on the matrix. In the optimal reaction conditions, although the higher CTO provided more activity centers, the effect was still limited due to the properties of PF and the low temperature of the catalyst. Therefore, the mixing stages between catalysts and oil should increase the catalyst−oil mixing heat and the probability of molecular collision between the catalysts and the oil and intensify the mass and heat transfer. The intensified mass and heat transfer effectively enforced the cracking of PF.40,41 3.4. FCC Performance of Heavy Oil before and after Being Cut. To further investigate the reaction performance of heavy oil before and after being cut, some comparative tests were carried out. HF-1 and PF-1 and HF-i and PF-i were used as base oils, and a series of mixtures with a different blending ratio (R0) of PF (30 wt %, 50 wt %, 70 wt %) were blended. R0 was defined as the weight percent of PF to the sum of PF and HF. The product distribution of the mixtures was used to simulate that of heavy oil before being cut, whereas the product distribution of heavy oil after being cut was calculated by the weighted average method according to eq 2: Yyield,i = YHF,i × (1 − R 0) + YPF,i × R 0

Table 8. Product Distribution of Changqing Series Mixtures with Different R0 before and after Being Cut at the Conventional Reaction Conditions (Catalyst: LVR-60R) mixturesa M-1 items dry gas LPG gasoline LCO HCO coke yield of light oil (wt %) yield of dry gas + coke (wt %) conversion (wt %)

before

M-2 after

before

Product Distribution (wt 1.91 1.93 2.09 15.16 14.94 15.00 55.51 55.56 54.80 13.48 13.54 13.40 8.43 8.68 7.77 5.51 5.35 6.94

M-3 after

before

after

%) 2.06 14.81 54.46 13.79 8.29 6.59

2.06 14.96 53.18 13.88 7.26 8.66

2.20 14.67 53.35 14.04 7.90 7.83

68.99

69.10

68.20

68.25

67.06

67.39

7.42

7.28

9.03

8.65

10.73

10.03

91.57

91.32

92.23

91.71

92.74

92.10

a

M-1: 70 wt % HF-1 + 30 wt % PF-1; M-2: 50 wt % HF-1 + 50 wt % PF-1; M-3: 30 wt % HF-1 + 70 wt % PF-1; the same values for M-1, M-2, and M-3 apply to Table 10.

Table 9. Product Distribution of Jinan Series Mixtures with Different R0 before and after Being Cut at the Conventional Reaction Conditions (Catalyst: CDC) Mixturesa M-i items dry gas LPG gasoline LCO HCO coke yield of light oil (wt %) yield of dry gas + coke (wt %) conversion (wt %)

before

M-ii after

before

Product Distribution (wt 2.18 2.33 2.44 15.88 15.86 15.87 42.92 43.04 40.27 17.09 18.06 17.25 12.25 12.07 11.71 9.68 8.64 12.44

M-iii after

before

after

%) 2.46 15.73 40.96 17.82 12.29 10.75

2.87 15.75 36.73 18.22 11.30 15.14

2.59 15.60 38.88 17.57 12.50 12.86

60.01

61.10

57.53

58.78

54.95

56.45

11.86

10.97

14.89

13.21

18.01

15.45

87.75

87.93

88.29

87.71

88.70

87.50

a

M-i: 70 wt % HF-i + 30 wt % PF-i; M-ii: 50 wt % HF-i + 50 wt % PFi; M-iii: 30 wt % HF-i + 70 wt % PF-i; the same values for M-i, M-ii, and M-iii apply to Table 10.

of both light oil and coke plus dry gas increased. This result indicates the weakening of the interaction between HF and PF. 3.4.2. Optimal Reaction Conditions. Table 10 lists the product distribution of heavy oil after being cut under the optimal reaction conditions. It should be noted that the optimal reaction conditions were limited to FFB reactors. Compared with the product distribution of heavy oil before being cut under the conventional reaction conditions (Tables 8 and 9), the product distribution of heavy oil after being cut improved significantly in a similar conversion. In Figure 5, the yield difference values of light oil and coke plus dry gas, before and after being cut, versus R0 are represented. In contrast to the product distribution of heavy oil before being cut, the improvement in the product distribution of heavy oil after being cut under the optimal reaction conditions was more remarkable. In the case of the Changqing

(2)

3.4.1. Conventional Reaction Conditions. Tables 8 and 9 show the conversion and product distribution of heavy oil before and after being cut, respectively, under the conventional reaction conditions. Compared with heavy oil before being cut, there was no significant change in the conversion of feed after being cut; while the product distribution improved, the light oil yield increased by 0.1−1.5 wt %, and the yield of coke plus dry gas decreased by 0.2−2.6 wt %. Figure 4 shows the yield difference values of light oil and coke plus dry gas versus R0. The yield difference value was defined as the product yield of heavy oil after being cut subtracted from the corresponding product yield of heavy oil before being cut. From Figure 4, we can see that, with the increase of R0, the yield difference values 1886

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and 32.54 wt %, respectively), resins (19.87 and 31.66 wt %, respectively), and asphaltenes (1.60 and 2.55 wt %, respectively), which leads to a large amount of refractory polycyclic aromatic hydrocarbons. The high content of nitrogen heteroatoms in PF-1 (0.44 wt %) and PF-i (0.99 wt %) produces more nitrogen-containing compounds with strong toxicity to the acid centers. Under the routine catalytic cracking process, they were premixed and then fed into the same reaction zone. The significant difference in physicochemical properties mentioned above caused a competitive adsorption effect among different hydrocarbons and nonhydrocarbon compounds, which manifested itself in the form of the interaction between HF and PF, and a further retardation effect on the reaction in the FCC process. After being cut, there was no change in the composition of the two feeds, but the contradictions between the different types of hydrocarbons were alleviated to some extent. More saturates were classified as HF; correspondingly, the more refractory hydrocarbons were mainly concentrated in the PF. Thus, the interaction among the HF and PF was weakened because they cracked separately. Therefore, according to the results above, an ideal process may be to divide heavy oil into different hydrocarbon groups according to reactive ability and then separately crack these groups, each matched with special catalysts, under the optimal reaction conditions. Obviously, this is unreasonable and unnecessary from the technical and economic point of view. Therefore, a simple and appropriate fractionation program would be to cut heavy oil into HF and PF according to the difference in reaction performance, preferably directly separating the processing VR and VGO, under optimal reaction conditions.

Figure 4. The yield difference values of light oil and coke plus dry gas versus R0 under conventional reaction conditions.

Table 10. Product Distribution of Mixtures with Different R0 after Being Cut in Optimal Reaction Conditions Changqing series mixtures items

M-1

M-2

M-3

Jinan series mixtures M-i

M-ii

M-iii

Product Distribution (wt %) dry gas LPG gasoline LCO HCO coke

1.50 11.71 57.35 15.72 9.07 4.64

1.63 12.16 55.98 15.71 8.45 6.07

1.75 12.61 54.60 15.69 7.83 7.50

1.93 14.48 44.85 19.11 11.95 7.68

2.17 14.50 42.93 18.64 11.63 10.13

2.40 14.53 41.01 18.17 11.32 12.57

yield of light oil (wt %) yield of dry gas + coke (wt %) conversion (wt %)

73.07

71.68

70.30

63.96

61.57

59.18

6.14

7.70

9.26

9.62

12.29

14.97

90.93

91.55

92.17

88.05

88.37

88.68

4. REACTION KINETICS As more attention has been paid to the lump kinetic model of VGO (similar to HF) and only a few papers about lump kinetic models of pure VR (namely, PF) catalytic cracking have been published, the lump kinetics study in this paper only considered the PF. 4.1. Model Description. Generally speaking, the more lumps a model includes, the more kinetic parameters need to be estimated and, consequently, the more experimental data are required. Thus, it is necessary to establish a simple model that can give the key kinetic information. The aim of this study was to improve the desired product yield and decrease the yield of coke and dry gas. Therefore, the product was divided into five groups according to their carbon number and boiling point range: HCO (350−500 °C), light oil (C5−350 °C), LPG (C3− C4), dry gas (C1−C2), and coke. The feed, PF (500+ °C), can be considered as one lump. A six-lump model with 12 reactions was developed (see Figure 6). 4.2. Mathematical Models. In order to develop the mathematical models, several assumptions were made:18,20,29 (1) instantaneous vaporization of feedstock; (2) plug flow for gas and catalyst and negligible radial dispersion in the reactor; (3) either isothermal or adiabatic reactor; and (4) nonselective catalyst deactivation. A continuity equation in the reactor can be written as follows (eq 3):

Figure 5. The yield difference values of light oil and coke plus dry gas versus R0 under optimal reaction conditions.

series mixtures, the light oil yield increased by 3.2−4.1 wt % while the sum of the coke and dry gas yield decreased by 1.3− 1.5 wt %. In the case of the Jinan series mixtures, the yield of light oil increased by 4.0−4.2 wt %, and the coke plus dry gas yield decreased by 2.2−3.0 wt %. The results show that under the optimal reaction conditions, the interaction between the HF and PF can be further weakened. The essence of interaction is the competitive adsorption effect. By comparing the chemical composition of HFs and PFs (Table 3), it can be seen that HF-1 and HF-i contain more saturates (80.20 and 68.86 wt %, respectively). On the other hand, PF-1 and PF-i have a high content of aromatics (30.94

⎛ ∂ρC i ⎞ ⎛ ∂C ⎞ ⎜ ⎟ + G V ⎜ i ⎟ = − ri ⎝ ∂t ⎠ x ⎝ ∂x ⎠t

(3)

Reaction rate (ri) is proportional to the molar concentration of lump i (ρCi) and the ratio of the catalyst mass density to the 1887

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Table 11. Average Molecular Weight of Six Lumps lumps

average molecular weight (g·mol−1)

PF HCO light oil LPG dry gas coke

662 371a 130 40 16 400b

a

Molecular weight of HCO from Peixoto and Medeiros.42 bMolecular weight of coke lump from Peixoto and Medeiros,42, Xu et al.,23 and Wang et al.29

Figure 6. Kinetic scheme for the six lump model.

According to the reaction network of the six-lump model, the mathematical equations of the kinetic models can be written as follows:

gas volume (ρb/ε), as shown in eq 4. Rate constant ki′ is not a constant and it decreases with catalyst deactivation: ri = k i′(ρC i)

dC1 P MW =− (k1 + k2 + k3 + k4 + k5)C1Φ dX S WH RT

ρb (4)

ε

dC 2 P MW = [v12k1C1 − (k6 + k 7 + k8 + k 9)C2]Φ dX S WH RT

Equation 5 can be deduced from eqs 3 and 4: ρ ⎛ ∂ρC i ⎞ ⎛ ∂C ⎞ ⎜ ⎟ + G V ⎜ i ⎟ = −k i′(ρC i) b ⎝ ∂t ⎠ x ⎝ ∂x ⎠t ε

(13)

dC3 P MW [v13k2C1 + v23k6C2 − (k10 + k11 + k12) = dX S WH RT

(5)

In steady-state fluidized-bed reactors, for gas-phase plug flow, the time partial derivative is zero. We replaced x with the dimensionless length X = x/L. Thus, on the basis of the above assumptions, eq 5 can be rewritten as ρ G V dC i = −k i′(ρC i) b L dX ε

(12)

C3]Φ

(14)

dC4 P MW (v14k3C1 + v24k 7C2 + v34k10C3)Φ = dX S WH RT (15)

(6)

dC5 P MW (v15k4C1 + v25k8C2 + v35k11C3)Φ = dX S WH RT

By definition, when Gv = (SWHρbL)/ε, eq 6 may be written as follows:

(16)

dC i 1 =− k i′(ρC i) dX S WH

C6 = (1 − C1M1 − C2M2 − C3M3 − C4M 4 − C5M5)

(7)

/ M6

Here, assuming the oil gas in the reactor to be the ideal gas, then ρ=

PMW RT

MW =

∑in= 1 C i MWi ∑in= 1 C i

where stoichiometric coefficient vij = MWi/MWj. The concentration of the coke’s lump C6 was calculated through a mass balance. Coking, poisoning, and sintering were the three reasons for catalyst deactivation, coking being the most important one. The catalyst deactivation function (Φ) could be described,7 where the β = 0.93, M = −0.68, as

(8)

=

1 ∑in= 1 C i

Φ = (1 + βCC)−M

(9)

(10)

Therefore, eq 11 can be deduced from eqs 7−10: dC i P MW =− k iC i Φ dX S WH RT

(18)

4.3. Estimation of Kinetic Parameters. A program for determining kinetic constants was compiled in Matlab language. The kinetic constants of the six-lump model at 460, 480, 500, and 520 °C were estimated according to the experimental data of the catalytic cracking PF-1, which are listed in Table 12. The kinetic constants of the PF lump cracking are higher than those of the HCO lump and the light oil lump (gasoline plus diesel). This indicates that the cracking reactions of the feed played an important role in the catalytic cracking process. In addition, it also shows that the proportion of the undesirable secondary cracking reactions (k10, k11, k12) of the intermediate products in the total cracking reactions was low. Table 13 lists the frequency factors and apparent activation energies calculated according to the Arrhenius equation. The activation energies in the present study were mainly in the

where MW is the average molecular weight of oil gas and MWi is the average molecular weight of lump i. The average molecular weights of the lumps in the six-lump model are shown in Table 11. The actual rate constant ki′ equals the product of the intrinsic rate constant ki and the catalyst deactivation function (Φ), which is shown in eq 10, where ki is a constant: k i′ = k i Φ

(17)

(11) 1888

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Table 12. Kinetic Rate Constants reaction temperature (°C) kinetic rate constants ((kg·m−3)−1·h−1)

460

480

500

520

k1 k2 k3 k4 k5 k6 k7 k8 k9 k10 k11 k12

2.5724 17.3967 3.6503 0.3008 3.0344 1.7626 1.4444 0.4895 0.2299 0.0297 0.0313 0.1532

3.7835 22.2267 3.9385 0.3413 3.3891 2.6169 2.0691 0.6767 0.2689 0.0387 0.0416 0.1949

4.2596 24.8894 4.2346 0.3563 3.6130 2.9566 2.2667 0.8078 0.3782 0.0500 0.0521 0.2436

4.9252 30.0234 4.3978 0.3820 3.8413 3.3280 2.6700 0.9349 0.4279 0.0567 0.0634 0.2781

Figure 7. Comparison between the experimental yields (points) and the predicted yields (line) at 480 °C.

was contained in the VR and that determining the cut temperature range to be 500 to 540 °C for Changqing heavy oil and 500 to 520 °C for Jinan heavy oil was reasonable. Compared with the conventional reaction conditions in a fixed fluidized bed reactor, under the operating parameters of a relatively low temperature (490 °C) and CTO (5 and 6), a high WHSV (25 h−1) is favorable to HF. However, under a relatively low temperature (480 °C) and WHSV (15 h−1), a high CTO (7 and 8) can improve the product distribution of PF. Moreover, it also shows that the conventional reaction conditions are inappropriate for both HF and PF. Compared to the product distribution of heavy oil before being cut, the product distribution of heavy oil after being cut improved significantly, with an increase in R0, more light oil, and fewer final products obtained, especially under the optimal reaction conditions. A six-lump kinetic model was developed to describe the reaction pathway of PF. The model contained PF, HCO, light oil, LPG, dry gas, and coke as lumps and had 12 kinetic constants. The rate constants at 460, 480, 500, and 520 °C and the apparent activation energies were calculated. The kinetic constants of the PF lump cracking were higher than those of the HCO lump and the light oil lump. The product yields predicted by the model exhibited good agreement with the experimental data.

Table 13. Frequency Factors and Apparent Activation Energies reactions

frequency factor ((kg·m−3)−1·h−1)

activation energy (kJ·mol−1)

r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12

9911.99 31558.56 46.01 6.21 66.14 5900.55 3210.88 2348.19 1644.02 200.52 364.45 487.80

49.96 45.62 15.43 18.36 18.73 49.06 46.65 51.46 54.31 53.69 57.04 49.13

range of 40−60 kJ·mol−1, which is close to the values reported in the literature23 and lower than those for thermal cracking (210−290 kJ·mol−1).7 This result can be attributed to the catalytic cracking reaction mechanism. The activation energies of the reaction of PF to gas (LPG, dry gas) and coke were lower than those of other reactions. This shows that the PF has the characteristics of favoring the production of coke and gas. Moreover, the apparent activation energies of the cracking reactions of the feed were lower than those of the secondary cracking reactions. This indicates that secondary cracking reactions are more sensitive to temperature. Therefore, a relatively low temperature is favorable to obtaining more liquid product. 4.4. Comparison of Experimental and Predicted Values. Figure 7 shows the comparison of the experimental yields (points) and the model-predicted yields (line) for the catalytic cracking of PF-1 at 480 °C. The predicted yields are close to the experimental ones. This indicates that the six-lump kinetic model fit the experimental data well and that the predicted results are reliable.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 8610-8973-3085. Fax: 8610-6972-4721. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the National Science Fund for Distinguished Young Scholars of China (20906103) and the National Natural Science Foundation (21176252).

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5. CONCLUSIONS By evaluating the catalytic cracking performance of heavy oil narrow fractions, the results of this study indicate that they are greatly different in terms of reaction performance. The fractions can be classified into HFs and PFs according to their reaction performance. Moreover, the results indicate that about 6.3− 19.3 wt % narrow fractions with better reaction performance 1889

NOMENCLATURE E = void volume fraction of fluidized bed ρ = gas density, g·cm−3 ρb = catalyst bed density, g·cm−3 vij = stoichometric coefficient for the reaction of lump i to lump j dx.doi.org/10.1021/ef201895e | Energy Fuels 2012, 26, 1880−1891

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k = rate constant of reaction, (kg·m−3)−1·h−1 L = effective reactor length, cm MW = average molecular weight of lump, g·mol−1 P = reaction pressure, Pa R = gas constant, 8.314 J·mol−1·K−1 SWH = weight hourly space velocity, h−1 t = residence time of oil gas, s T = reaction temperature, K x = reactor length at x cross-section, cm Ci = concentration of lump i, mol/ggas Cc = coke content on catalyst, % X = nondimensional length

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