Seven-lump kinetic model for catalytic pyrolysis of heavy oil

Catalysis Communications 8 (2007) 1197–1201 www.elsevier.com/locate/catcom

Seven-lump kinetic model for catalytic pyrolysis of heavy oil Xianghai Meng, Chunming Xu, Jinsen Gao *, Li Li State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping, Beijing 102249, China Received 25 August 2006; received in revised form 24 October 2006; accepted 26 October 2006 Available online 3 November 2006

Abstract A 7-lump kinetic model is proposed to describe the catalytic pyrolysis of heavy oil. The kinetic model contains 15 kinetic constants and one for catalyst deactivation. The experimental data were obtained in a confined fluidized bed reactor. The kinetic constants were estimated by a special program compiled based on the Marquardt’s algorithm. The apparent activation energies were calculated according to the Arrhenius equation. This model fits the experimental data well. The prediction shows that catalytic pyrolysis of Chinese Daqing atmospheric residue should be conducted at low space velocity to produce much ethene and at space velocity around 15 h1 to produce much propene and butene.  2006 Elsevier B.V. All rights reserved. Keywords: Kinetics; Model; Lump; Catalytic Pyrolysis; Heavy oil

1. Introduction Catalytic pyrolysis of heavy oil has attracted great interests in recent years [1,2]. A characteristic of catalytic pyrolysis is that the reactions are coupled catalytically and thermally [3]. The kinetics is important to the good understanding of the catalytic pyrolysis of heavy oil, as well as to the design and simulation of the reactor, to the prediction of the reaction behavior and to the optimization of the operating conditions. Lumping kinetics have been studied widely for fluid catalytic cracking [4,5]. Considering the similarity of catalytic cracking and catalytic pyrolysis, some researchers have tried to describe catalytic pyrolysis by lumping the large number of chemical compounds into groups of pseudocomponents, according to their boiling points and molecular characteristics. Xu [6] presented a 4-lump model for deep catalytic cracking (DCC) technology. In this model, dry gas and coke was considered as one lump. Meng et al. [7] proposed a 5-lump model to describe the catalytic pyrolysis of heavy oil, in which the lump of dry gas and *

Corresponding author. Tel.: +86 10 8973 3993; fax: +86 10 6972 4721. E-mail address: [email protected] (J. Gao).

1566-7367/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.10.036

coke was split into dry gas and coke lumps separately. An 8-lump model was developed by Meng et al. [8] for the catalytic pyrolysis of heavy oil, where the feed heavy oil was split into non-aromatic carbons and aromatic carbons. Wang et al. [9] developed a 16-lump model for the heavy oil contact cracking (HCC) process. This model divided the pyrolysis gas into eight lumps (hydrogen, methane, ethene, ethane, propene, propane, butene and butane), and contained 70 kinetic parameters and one parameter for catalyst deactivation. The main objective of this study is to develop a 7-lump kinetic model for the catalytic pyrolysis of heavy oil. With some assumptions, a mathematical model for the confined fluidized bed reactor will be given, and kinetic parameters will be estimated. And then the variation of product yields with space velocity will be predicted and discussed. 2. Seven-lump kinetic model 2.1. Model description Pyrolysis gas can be divided into liquefied petroleum gas (LPG) and dry gas according to the boiling point and the secondary reactivity. Unlike catalytic cracking, the desired

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X. Meng et al. / Catalysis Communications 8 (2007) 1197–1201

Nomenclature a / e q qb Ci GV kij L Mi M P ri

deactivation constant, h1 deactivation function void volume fraction of fluidized bed gas density, g/cm3 catalyst bed density, g/cm3 concentration of lump i, mol/ggas mass velocity at a cross-section, ggas/(cm2 h) rate constant for the reaction of lump i to lump j, (g/cm3)1 h1 effective reactor length, cm molecular weight of lump i, g/mol average molecular weight of gas mixture, g/mol reaction pressure, Pa reaction rate of lump i, (mol/cm3) h1

products of catalytic pyrolysis are ethene, propene and butene. It was reported that the secondary cracking reactivity of butane was different from that of butane [10]. Therefore, LPG can be considered as two lumps: propene plus butene and propane plus butane. Similarly, dry gas can be split into ethene and other gases (hydrogen, methane, ethane, carbon oxide and carbon dioxide). Gasoline and diesel are by-products of the catalytic pyrolysis of heavy oil, and the analysis shows that the main components in gasoline and diesel are aromatics. Consequently, gasoline plus diesel can be considered as one lump, although they are usually considered as two lumps for catalytic cracking. Coke is a significant by-product since coke yield is an important parameter for the design of regenerator, and its formation mechanism is different from those of other products, so coke alone should be considered as a lump. The reaction network for the catalytic pyrolysis of heavy oil is shown in Fig. 1. An advantage of this model is that it can predict the yields of the desired products (ethane, propene and butene) directly. Another advantage is that only 16 kinetic parameters are used to describe the complicated (Lump1) Heavy oil k13

k15

k12 k23

Propene +butene (Lump3)

k25 k35

Ethene (Lump5)

k24

k36

k45 Propane +butane (Lump4)

k16

k26

k46 k17

VR x X yi

disappearance rate of lump i, (mol/cm3) h1 formation rate of lump i, (mol/cm3) h1 gas constant, J/(mol K) steam-to-oil weight ratio space velocity, h1 reaction temperature, K residence time of catalyst, h stoichiometric coefficient for the reaction of lump j to lump i valid volume of the reactor, cm3 reactor length at x cross-section, cm nondimensional reactor length yield of lump i, wt%

catalytic pyrolysis of heavy oil. One limitation of this model is that the kinetic parameters depend on the feed and catalyst properties. One improvement for the model is that the feedstock can be divided into several lumps, in order to predict the cracking reactivity of various feeds. 2.2. Kinetic model Cracking reactions could be considered as irreversible reactions. In order to simplify the kinetic model, first order reactions were assumed for the cracking reactions. A mathematical model for the confined fluidized bed reactor was used with the assumption of uniform porosity, plug flow and quasi-steady state. A continuity equation in the reactor can be written as Eq. (1) [11,12].     oqC i oC i þ GV ¼ ri ð1Þ ot x ox t For a first order reaction, the disappearance rate of lump i (rid) is proportional to its molar concentration (qCi) and the mass density of catalyst to gas volume (qb/e). rid ¼ k i ðqC i Þ 

(Lump2) Gasoli ne +diesel

k14

rid rif R RSO SW T tc vji

k27 Coke(Lump7)

Fig. 1. Reaction network of the 7-lump model.

Other gases (Lump6)

qb / e

ð2Þ

The formation rate of lump i from lump j (rji) is in direct proportion to the molar concentration of lump j (qCj), the stoichiometric coefficient (vji) and the mass density of catalyst to gas volume. The formation rate of lump i (rif) is the sum of all rji. hX i q rif ¼ ð3Þ vji k j ðqC j Þ  b  / e The reaction rate of lump i (ri) is the subtraction of the disappearance rate from the formation rate. hX i q ð4Þ ri ¼ rif  rid ¼ vji k j ðqC j Þ  k i ðqC i Þ  b  / e From Eqs. (1) and (4), we can get Eq. (5).

X. Meng et al. / Catalysis Communications 8 (2007) 1197–1201

    hX i q oqC i oC i þ GV ¼ vji k j ðqC j Þ  k i ðqC i Þ  b  / ot x ox t e ð5Þ For the confined fluidized bed reactor, the residence time of hydrocarbon is much lower than that of catalyst, so the variation rate of hydrocarbon concentration with time is much lower than that with reactor length. Therefore, the first term on the left-hand side of Eq. (5) could be neglected. According to the definition of the mass velocity at a cross-section (GV) and space velocity (SW) [11] S W qb L ð6Þ GV ¼ e Replacing the reactor length at x cross-section by the nondimensional reactor length (X), Eq. (5) can be re-written as, i dC i 1 hX ¼  vji k j ðqC j Þ  k i ðqC i Þ  / ð7Þ SW dX The mathematical equations of the seven lumps can be written as follows based on the reaction network of the 7-lump model and Eq. (7). dC 1 1  ðk 12 þ k 13 þ k 14 þ k 15 þ k 16 þ k 17 Þ  qC 1  / ¼ dX SW ð8Þ dC 2 1  ½v12  k 12  qC 1 ¼ dX SW  ðk 23 þ k 24 þ k 25 þ k 26 þ k 27 Þ  qC 2   / dC 3 1  ½ðv13  k 13  qC 1 þ v23  k 23  qC 2 Þ ¼ SW dX  ðk 35 þ k 36 Þ  qC 3   / dC 4 1  ½ðv14  k 14  qC 1 þ v24  k 24  qC 2 Þ ¼ dX SW  ðk 45 þ k 46 Þ  qC 4   / dC 5 1  ½v15  k 15  qC 1 þ v25  k 25  qC 2 þ v35  k 35  qC 3 ¼ dX SW þ v45  k 45  qC 4   / dC 6 1  ½v16  k 16  qC 1 þ v26  k 26  qC 2 þ v36  k 36  qC 3 ¼ dX SW þ v46  k 46  qC 4   /

ð9Þ

ð10Þ

ð11Þ

ð12Þ

ð13Þ

Coke is a solid deposit on the surface of the spent catalyst, and its concentration can be calculated through a mass balance.  1 C7 ¼  C1 M 1  C2M 2  C3M 3  C4M 4 1 þ RSO , C 5 M 5  C 6 M 6

M7

ð14Þ

Catalyst deactivation function, / ¼ expða  tc Þ y i =M i 1 þ RSO

Stoichiometric coefficient, vji ¼

Mj Mi

ð17Þ

Assuming the gas in the reactor to be the ideal gas, then: q¼

PM RT

ð18Þ

The average molecular weight of the gas mixture (M) is a variant, which varies with X. The average molecular weight can be calculated by Eq. (19) [11]. Pn 1 i¼0 C i M i M¼ P ¼ Pn ð19Þ n i¼0 C i i¼0 C i Where, ‘‘i = 0’’ is considered as the influence of steam on the average molecular weight. For the proposed 7-lump model, n is equal to six, since coke is not in the gas state. 3. Experimental Chinese Daqing atmospheric residue was used as the feed. The catalyst was CEP-1, a typical catalyst for catalytic pyrolysis process (CPP) technology. The properties of the feed and the catalyst have been published in an earlier paper [3]. The experiments of catalytic pyrolysis were performed in a confined fluidized bed reactor at four temperatures from 600 to 700 C. The space velocity, the weight ratios of catalyst-to-oil and steam-to-oil, the flow rates of feeds and steam varied within 5–20 h1, 6–27, 0.2–1.6, 2–10 g/min and 1.5–6.0 g/min, respectively. For each experiment, variable grams of catalyst were loaded into the reactor with an effective volume of about 580 mL. A variable amount of distilled water was pumped into a furnace to form steam, which was used to fluidize the catalyst. The feedstock pumped by another pump mixed with the steam, heated to approximately 500 C in a pre-heater, and then entered into the reactor, where heavy oil contacted with the fluidized catalyst and reactions took place. The oil gas after reaction was cooled and separated into the liquid sample and the gas sample. An Agilent 6890 gas chromatograph with Chem Station software was used to measure the volume percentage of components in the gas sample. The equation of state for the ideal gas converts the data to mass percentages. The liquid sample was analyzed with a simulated distillation gas chromatogram to get the weight percentage of gasoline, diesel and heavy oil. Coke content on the spent catalyst was measured with a coke analyzer. 4. Results and discussion

ð15Þ 4.1. Kinetic constant determination

Concentration of lump i, Ci ¼

1199

ð16Þ

The kinetic constants of the 7-lump model listed in Table 1 were estimated by a special program compiled in

1200

X. Meng et al. / Catalysis Communications 8 (2007) 1197–1201

Table 1 Kinetic constants of the 7-lump model Parameter

Unit

Reaction temperature, (C) 600

630

660

700

k12 k13 k14 k15 k16 k17 k23 k24 k25 k26 k27 k35 k36 k45 k46 a

(g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 h1

2.64 · 104 3.09 · 104 9.46 · 103 8.05 · 103 5.32 · 103 9.99 · 103 6.86 · 102 8.79 · 102 1.92 · 103 1.65 · 103 2.10 · 103 9.54 · 102 2.13 · 102 1.75 · 102 1.76 · 102 39

4.22 · 104 5.08 · 104 1.19 · 104 1.26 · 104 9.89 · 103 1.27 · 104 1.36 · 103 1.87 · 103 2.86 · 103 2.83 · 103 4.85 · 103 1.58 · 103 4.71 · 102 3.93 · 102 3.71 · 102 50

6.48 · 104 7.63 · 104 1.41 · 104 1.96 · 104 2.04 · 104 1.69 · 104 2.23 · 103 2.55 · 103 5.22 · 103 5.51 · 103 7.63 · 103 3.16 · 103 1.43 · 103 1.07 · 103 1.58 · 103 67

1.02 · 105 1.06 · 105 1.74 · 104 3.18 · 104 4.50 · 104 2.29 · 104 3.46 · 103 3.59 · 103 7.94 · 103 8.57 · 103 1.04 · 104 6.63 · 103 4.59 · 103 2.92 · 103 4.92 · 103 96

Matlab language based on Marquardt’s algorithm. The frequency factors and the apparent activation energies listed in Table 2 were calculated according to the Arrhenius equation. The kinetic constants for the cracking of the feed are much higher than those of gasoline plus diesel, propene plus butene and propane plus butane. This indicates that it is the cracking reactions of the feed that primarily take place for the catalytic pyrolysis of heavy oil. And this also explains that the proportion of the secondary cracking reactions of intermediate products to the total cracking reactions is low. The apparent activation energies for the cracking reactions of the feed are smaller than those of gasoline plus diesel, propene plus butene and propane plus butane. This shows that the secondary cracking of intermediate products is more sensitive to reaction temperature than

the cracking of the feed. The apparent activation energy for the catalytic cracking of hydrocarbon ranges from 42 to 125 kJ/mol, and that for the thermal cracking ranges from 210 to 290 kJ/mol [13]. Most of the apparent activation energies determined in this research are near or above 100 kJ/mol. This explains that thermal cracking plays a significant part for the catalytic pyrolysis of heavy oil, which shows good agreement with the reaction mechanistic pathway for the catalytic pyrolysis of heavy oil [3]. The calculated apparent activation energies are close to those reported in literature [4,8,14], but are higher than those for the catalytic cracking reactions at conventional operating conditions [5,15]. The main reason is that the reaction temperature for the catalytic pyrolysis of heavy oil is about 150 C higher than that for the conventional catalytic cracking. 4.2. Effect test of the 7-lump model

Table 2 Frequency factors and apparent activation energies of the 7-lump model Parameter

Frequency factor Unit

k12 k13 k14 k15 k16 k17 k23 k24 k25 k26 k27 k35 k36 k45 k46 a

Apparent activation energy Value

3 1

1

(g/cm ) h (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 (g/cm3)1 h1 h1

(kJ/mol) 10

1.43 · 10 5.60 · 109 3.41 · 106 5.41 · 109 6.88 · 1012 3.55 · 107 4.82 · 109 5.82 · 108 2.99 · 109 2.30 · 1010 1.14 · 1010 1.93 · 1011 3.17 · 1015 2.07 · 1014 6.47 · 1016 2.98 · 105

96 87 43 97 153 59 114 96 104 119 111 139 221 202 244 65

The comparison of the experimental yields (points) and the model-predicted yields (line) is shown in Fig. 2, for the catalytic pyrolysis of Daqing atmospheric residue at 660 C. The predicted yields are close to the experimental ones. This indicates that the 7-lump kinetic model can fit the experimental data well, and the predicted results are reliable. 4.3. Prediction of product yields with space velocity Fig. 3 shows the predicted variation of product yields with space velocity, keeping reaction temperature, catalyst-to-oil weight ratio, steam-to-oil weight ratio and residence time of catalyst constant at 660 C, 16, 0.6 and 0.0083 h, respectively. As space velocity increases, the yields of ethene, coke and other gases decrease, and those of gasoline plus diesel, propene plus butene and

X. Meng et al. / Catalysis Communications 8 (2007) 1197–1201

35

Predicted yield, wt%

favors much ethene production. But to produce much propene and butene, the space velocity should be about 15 h1.

Gasoline+diesel Propene+butene Ethene Propane+butane Other gases Coke

30 25

5. Conclusions

20 15 10 5

5

10

15 20 25 Experimental yield, wt%

30

35

Fig. 2. Comparison between the experimental yields (points) and the predicted yields (line) at 660 C.

Predicted product yield, wt%

30

References

Gasoline+diesel

10 0

Ethene 0

10

20

30

4

0

b

20

Other gases Coke

10

Propane+butane 0

A 7-lump kinetic model for atmospheric residue catalytic pyrolysis is proposed. The model contains heavy oil, gasoline plus diesel, propene plus butene, propane plus butane, ethene, other gases and coke as lumps. The model has 15 kinetic constants and one for catalyst deactivation. Experimental data obtained in a confined fluidized bed reactor were used to estimate kinetic constants and apparent activation energies. The predicted yields agree well with the experimental results. The prediction shows that catalytic pyrolysis of Daqing atmospheric residue should be conducted at low space velocity to produce much ethene and at space velocity about 15 h1 to produce much propene and butene.

a

Propene+butene

20

1201

0

10

20

30

Space velocity, h

405 40

50 0

-1

Fig. 3. Prediction of product yields as a function of space velocity. Reaction temperature 660 C, catalyst-to-oil weight ratio 16, steam-to-oil weight ratio 0.6, and residence time of catalyst 0.0083 h.

propane plus butane show maxima at about 25, 15 and 5 h1, respectively. Ethene is the end product of cracking reactions, so low space velocity (long reaction time)

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