Energy Fuels 2010, 24, 2825–2829 Published on Web 04/23/2010
: DOI:10.1021/ef100064d
Measurement of Diffusion Coefficient of Heavy Oil in Fluidized Catalytic Cracking (FCC) Catalysts Ziyuan Liu, Sheng-Li Chen,* Xiujun Ge, Peng Dong, Jinsen Gao, and Zhiming Xu State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China, and College of Chemistry and Chemical Engineering, China University of Petroleum, Qingdao 266555, People’s Republic of China Received January 20, 2010. Revised Manuscript Received April 8, 2010
Large-pore fluidized catalytic cracking (FCC) catalysts, using polystyrene spheres as templates, were prepared. By adsorption equilibrium experiments, the Freundlich adsorption isotherm of Dagang vacuum residue oil fractions on the prepared FCC catalysts was established. The data of adsorption rate were obtained and the effective diffusion coefficient of the oil fractions in the catalyst particles was calculated to be 10-9-10-8 cm2/s. The effective diffusion coefficient in the large-pore catalyst was 2-3 times larger than that in the conventional catalyst, and it decreased with the molecular weight of the oil fraction. This verified that, because of promotion of the diffusion rate of heavy oil caused by the larger pore size of the catalyst, the catalytic performance of a macroporous catalyst was better than the conventional catalyst.
determine the diffusion behavior of residue fractions in catalyst pores. At the same time, it was found that the heavy fractions, especially the end-cut of the residue, would inhibit catalytic reactivity of the light fractions, so the use of the bulk sample analysis for the entire residue is misleading to determine the reactivity of the residue.9 Thus, it is necessary to cut the residue into several fractions and study their diffusion behavior in the macroporous catalysts to give guidance for designing and preparing catalysts with suitable pores for heavy oil processing. In this research work, FCC catalysts with different pore sizes were prepared with and without monodisperse polystyrene particles as a template. The diffusion experiment of Dagang vacuum residue fractions obtained using supercritical fluid extraction and fractionation (SFEF)10 in the FCC catalyst was performed to calculate the diffusion coefficient of the SFEF fractions and to investigate the influence of catalyst pore size and diameter of oil molecules on the diffusion of residue fractions.
1. Introduction Fluidized catalytic cracking (FCC) is a well-developed process for converting heavier fractions of crude oil to a high-octane-number gasoline component, and residue fluidized catalytic cracking (RFCC) is an extension of the conventional FCC technology with the feedstock from vacuum gas oil to residue. However, because of the large molecular weight of vacuum residue, the catalyst performance is strongly limited by the low diffusion rate of reactants in the FCC catalyst. Residue oil is a colloid system. Asphaltene is the core of a colloidal micelle, the average molecular diameter of which is ∼2-6 nm.1 In residue oil processing, the pore systems of microporous and mesoporous solid catalysts impose limitations on the mass-transfer process, because of the large size of oil molecules.2-6 In our previous work, it was found that macroporous catalysts, prepared using polystyrene spheres as a template, are much more active than those prepared without a template when it is used as the heavy-oil FCC catalyst and the heavy-oil hydrodesulfurization (HDS) catalyst.7,8 Although the reactivity of the large pore catalysts has been verified, information on the diffusivity of the heavy oils in them is still unknown. In the past, because of the lack of appropriate separation techniques to cut deep into residue, it was impossible to
2. Mathematical Model of Diffusion A residue SFEF fraction can be regarded as a pseudocomponent, because the natures of the components that constitute the fraction are similar. The FCC catalyst could be regarded as a sphere to simplify the calculation. For a spherical catalyst in a residue solution, there exists a concentration gradient of residue SFEF fraction along the radial direction of the catalyst sphere, and the residue molecules diffuse into the center of the sphere along the radial direction.11 Assuming that the adsorption equilibrium of oil on the interior surface of the catalyst pore could be reached, an arbitrarily shell thickness dr of the spherical catalyst is taken into consideration, as shown in Figure 1.
*Author to whom correspondence should be addressed. Tel.: 86-01089733396. Fax: 86-010-69724721. E-mail:
[email protected]. (1) Xu, Y.; Koga, Y.; Strausz, O. P. Fuel 1995, 74 (7), 960–964. (2) Zhao, X.; Cheng, W. C.; Rudesill, A. Presented at the NPRA Annual Meeting, San Antonio, TX, 2002; Paper No. AM-02-53. (3) Atias, J. A.; Tonetto, G.; de Lasa, H. Ind. Eng. Chem. Res. 2003, 42, 4162–4173. (4) Atias, J. A.; de Lasa, H. Ind. Eng. Chem. Res. 2004, 43, 4709–4720. (5) Lee, C. K.; Ashtekar, S.; Gladden, L. F.; Barrie, P. J. Chem. Eng. Sci. 2004, 59, 1131–1138. (6) Barrie, P. J.; Lee, C. K.; Gladden, L. F. Chem. Eng. Sci. 2004, 59, 1139–1151. (7) Qi, Y.; Chen, S.; Dong, P.; Xu, K.; Shen, B. J. Fuel Chem. Technol. 2006, 34 (6), 685–689. (8) Chen, S.; Dong, P.; Xu, K.; Qi, Y.; Wang, D. Catal. Today 2007, 125, 143–148. r 2010 American Chemical Society
(9) Yang, C.; Du, F.; Zheng, H.; Chung, K. H. Fuel 2005, 84, 675–684. (10) Wang, R.; Hu, Y.; Xu, Z.; Su, T. Acta Pet. Sin., Pet. Process. Sect. 1997, 13 (1), 53–59. (11) Liu, Z.; Chen, S.; Dong, P.; Gao, J.; Ge, X.; Xu, Z. Energy Fuels 2009, 23, 2862–2866.
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Energy Fuels 2010, 24, 2825–2829
: DOI:10.1021/ef100064d
Liu et al.
Let D p εa þ Ds ∂q F ∂C a
Di 0 ¼
ð6Þ
where Di0 is the effective diffusion coefficient of oil diffusing into a spherical catalyst. Then, eq 5 becomes ! ∂q ∂2 q 2 ∂q 0 ¼ Di ð7Þ þ ∂t ∂r2 r ∂r The initial and boundary conditions are t ¼ 0, q ¼ 0
Figure 1. Sketch of diffusion in a spherical catalyst.
t > 0, q ¼ qi ðconst:Þ r ¼ r0 ∂q t g 0, ¼0 r¼0 ∂r
Suppose that the mass accumulation rate of oil in the shell is Q1, the mass flow of oil diffusing into the spherical shell along the radius direction (r) is Q2, and the mass flow diffusing out of the shell is Q3; then, according to the mass balance, ð1Þ Q1 ¼ Q2 - Q3
where εa is the porosity of the catalyst; Fa the density of the catalyst (given in units of g/cm3); C the concentration of oil solution (given in units of g/mL); and q the adsorbed amount of oil (expressed in terms of g/g-cat). When Q2 is affected by the diffusion rate in the pore and on the surface of catalyst, it can be written as " # ∂C ∂q 2 ð3Þ þ Fa Ds Q2 ¼ ½4πðr þ drÞ Dp εa ∂r r þ dr ∂r r þ dr
where τ¼ u0 ¼ 1 -
Di 0 t r0 2
ω FðtÞ ωþ1
FðtÞ ¼
where Dp and Ds are the diffusion coefficients in the pore and on the surface of the catalyst, respectively (expressed in units of cm2/s). (∂C/∂r)rþdr and (∂q/∂r)rþdr represent the oil concentration and adsorption gradient at the outer surface of the shell along the radial direction, respectively. Similarly, Q3 can be expressed as " # ∂C ∂q 2 ð4Þ Q3 ¼ 4πðrÞ Dp εa þ Fa Ds ∂r r ∂r r
qt q¥
ω is calculated by the following equation: C0 n - C¥ n ω ¼ W0 kð Þ C0 - Cn
ð10Þ ð11Þ ð12Þ
ð13Þ
where k and n are parameters of the Freundlich isothermal adsorption equation. Parameters R and β are the two roots of the following equation:
where the partial differential variables (∂C/∂r)r and (∂q/∂r)r are gradients, as in eq 3, but they are located at the interior surface of the shell. Substitute Q1-Q3 in eq 1 with eqs 2-4. When dr f 0, ∂q/∂C can be seen as a constant, εa , ∂q/∂C,12 and
x2 þ 3ωx - 3ω ¼ 0
ð14Þ
u ¼ u0 ðR - βÞ
ð15Þ
pffiffiffi pffiffiffi FuncðR τÞ ¼ 1 þ erfðR τÞ
ð16Þ
pffiffiffi pffiffiffi Funcðβ τÞ ¼ 1 þ erfðβ τÞ
ð17Þ
Here, R > β. Let
∂2 q ∂q ∂2 C ¼ ∂r2 ∂C ∂r2
Define
3
! 7 ∂q 6 6 Dp εa 7 ∂2 q 2 ∂q þ ¼6 þ Ds 7 5 ∂r2 r ∂r ∂t 4 ∂q Fa ∂C
ð8Þ
where qi is the saturated adsorption (given in terms of g/gcat.); r0 is the radius of catalyst particle (given in centimeters). According to the initial and boundary conditions, an analytical solution can be obtained. For the rate equation of diffusion and conduction are similar, Di0 can be conveniently calculated by the Paterson heat conduction approximation equation:12 pffiffiffi 1 fR expðR2 τÞ½1 þ erfðR τÞ u0 ¼ R-β pffiffiffi ð9Þ - β expðβ2 τÞ½1 þ erfðβ τÞg
where Q1 is the mass accumulation rate of oil in the shell (given in units of g/s); Q2 the mass flow of oil diffusing into the shell (given in units of g/s); and Q3 the mass flow of oil diffusing out of the shell (also given in units of g/s). Q1 can be expressed as ∂C ∂q þ Fa ð2Þ Q1 ¼ 4πr2 dr εa ∂t ∂t
∂q ∂q ∂C ¼ , ∂r ∂C ∂r Then, eq 1 can be written as 2
0 e r < r0
and ð5Þ
Thus, eq 9 is changed to pffiffiffi f ðτÞ ¼ u - R expðR2 τÞ FuncðR τÞ pffiffiffi þ β expðβ2 τÞ Funcðβ τÞ ¼ 0
(12) Ide Teppu (Japan). Water Treatment Engineering;Theory and Application (in Jpn.; translated by Z. Zhang); China Architecture & Building Press: Beijing, PRC, 1986; pp 335-338.
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ð18Þ
Energy Fuels 2010, 24, 2825–2829
: DOI:10.1021/ef100064d
Liu et al.
where W0 is the amount of catalyst put into the oil solution (expressed in terms of g/mL); C0 the initial concentration of the oil solution (given in units of g/mL); C¥ the concentration of the oil solution at adsorption equilibrium (also given in units of g/mL); qt the adsorbed oil amount at time t (in units of g/g-cat); and q¥ the adsorbed oil amount at adsorption equilibrium (given in units of g/g-cat). When F(t) is determined by eq 12, ui0 values at different times t can be calculated by eq 11; then, u is known. The parameter τ can be determined by solving transcendental eq 18, and then Di0 is obtained.
Figure 2. SEM images of FCC catalysts: (a) without a template (scale bar = 700 nm) and (b) with a 20 wt % PS template (scale bar = 2 μm).
3. Experimental Section 3.1. Preparation of the Macroporous FCC Catalyst. The methods for the preparation of monodisperse polystyrene (PS) particles and a macroporous FCC catalyst were described by Chen et al.8 The macroporous FCC catalyst was prepared by mixing specific amounts of sodium silicate, sodium aluminate, sulfuric acid, REUSY zeolite, deionized water, and polystyrene particles under stirring. The polystyrene particles were removed from the catalyst by burning in an air atmosphere at 550 C. The prepared FCC catalyst consisted of 85 wt % amorphous Al2O3-SiO2 and 15 wt % REUSY. The preparation of conventional catalyst (without a template) is the same as the macroporous catalyst, except for the absence of a PS particle template. Catalysts with a 20 wt % PS (diameter = 99.8 nm) template and without a template were prepared, and the catalyst particle size was 0.45-0.90 mm. 3.2. Testing of Catalytic Performance of the FCC Catalyst. The catalytic performance of the FCC catalysts was tested in a fixed-bed tubular reactor, which is made of stainless steel, wherein 5.0 g of catalyst was held centrally within the tube furnace at a temperature of 510 C. Daqing atmospheric residue was used as feedstock. The characteristics of the feed and the disposal of the products were described in ref 8. 3.3. Diffusion Experiment. Dagang vacuum residue was used as feedstock and cut into eight fractions, with each fraction being 10 wt % of the feedstock, by SFEF, using n-pentane as a solvent. The eight SFEF fractions were used in the diffusion experiments, which were performed at 30 C. First, to determine the adsorption isotherm, the SFEF fractions were dissolved in n-heptane (analytically pure) to prepare oil solutions of different concentrations (from 0.1 wt % to 2.0 wt %). Approximately 1 g of the catalyst was weighed and put into a bottle containing the oil solution. The bottle was sealed and then vibrated on an HY-4 oscillator at a frequency of 190 min-1 for 12 h to reach saturated adsorption. After that, the catalyst was taken out of the solution, dried at 60 C for 2 h in an oven, and then dried further at 100 C for 1 h in an vacuum drying oven for complete evaporation of n-heptane. The catalyst was weighed and then calcined in a muffle oven at 600 C for 5 h and weighed again after the calcination. The weight difference of the catalyst is the saturated adsorption amount of the oil. To determine the oil amount diffused into catalyst at different times, six sealed bottles with 30 mL of 2 wt % oil solution were prepared. Approximately 1.5 g of catalyst was put into each of the bottles, which were vibrated on an oscillator at a frequency of 190 min-1. The catalyst was taken out of the bottles periodically, and the oil weight that was absorbed on the catalyst at different diffusion times was measured in the same way as described above.
Figure 3. Graph showing the pore size distribution of FCC catalysts.
without a template (Figure 2a), while many macropores are observed on the surface of the catalyst prepared with 20 wt % PS as a template (Figure 2b). The macropores were the voids formed by removing the PS particles from the catalyst. The catalyst prepared with a PS template has many more macropores than that prepared without a template. Figure 3 shows the FCC catalyst pore size distribution, which was obtained using a Micromeritics Model ASAP2020 sorptometer. It is obvious that there are two peaks for the catalyst prepared with a template. The peak located in the range of 40-210 nm corresponds to the macropores, indicating that there were many macropores 40-210 nm in size in the template catalyst; another peak located in the range of 4-6 nm corresponds to the mesopores. The average pore size is 6.8 nm and 4.4 nm for the catalyst with and without a template, respectively. The surface area and pore volume of the two catalysts are listed in Table 1. Table 1 shows that the BET surface area and pore volume of the PS template catalyst are much bigger than those of a catalyst without a template. 4.2. Catalytic Performance of the Catalysts. The cracking catalytic performance of the FCC catalyst was tested on a fixed-bed reactor made of stainless steel, using Daqing atmospheric residue as feed. Under the same reaction conditions, the conversion of the reaction catalyzed by the macroporous catalyst, prepared with a template, was higher than that obtained by the catalyst without a template (9.8% improvement). (See Table 2.) Therefore, the diffusion coefficient of heavy oil would be tested to verify if the good catalytic performance of the macroporous catalyst is due to the promotion of diffusion rate of heavy oil in the large pore size of the catalyst.
4. Results and Discussion 4.1. Characterization of the Catalyst. Scanning electron microscopy (SEM) images (Hitachi Model S-4200) of the FCC catalysts are shown in Figure 2. As seen in Figure 2, no visible macropores are found inside the catalyst prepared 2827
Energy Fuels 2010, 24, 2825–2829
: DOI:10.1021/ef100064d
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Table 1. Physical Properties of Catalysts catalyst BET surface area pore volume
without template 2
118.3 m /g 0.153 cm3/g
with PS template 163.2 m2/g 0.292 cm3/g
Table 2. Catalytic Performance of Prepared FCC Catalystsa catalyst
without template
with PS template
dry gas LPG gasoline diesel >350 C liquid coke conversion mass balance
1.45 wt % 8.58 wt % 24.82 wt % 19.72 wt % 37.87 wt % 5.89 wt % 60.46% 94.8%
1.80 wt % 7.68 wt % 28.03 wt % 21.20 wt % 33.61 wt % 7.68 wt % 66.39% 95.1%
a Conditions: reaction temperature, 510 C; catalyst-to-oil weight ratio, 5; weighted hourly space velocity (WHSV), 10 h-1.
Figure 4. Plotted log q-log C curve of DG-1 SFEF fraction in a catalyst without a template.
Table 3. Average Molecular Weight and Molecular Diameter of SFEF Fractions of Dagang Vacuum Residue
Table 4. Calculation Data of DG-1 SFEF Fraction Diffusing in Catalyst without a Template
fraction
M
dr (nm)
time, t (s)
qt ( 10-3g/g-cat)
F(t)
u0
u
τ
DG-1 DG-2 DG-3 DG-4 DG-5 DG-6 DG-7 DG-8
720 830 900 960 1100 1335 1600 1800
1.38 1.49 1.56 1.61 1.73 1.92 2.12 2.26
300 900 3600 5400 7200
10.13 14.43 20.99 23.73 24.86
0.3067 0.4369 0.6355 0.7184 0.7526
0.9670 0.9530 0.9317 0.9227 0.9191
1.2142 1.1967 1.1698 1.1586 1.1540
0.00819 0.01857 0.04906 0.07055 0.08185
Table 5. Parameters Used in Calculation of DG-1 SFEF Fraction Diffusing in Catalyst without a Template
4.3. Properties of Oil Fractions. The density of Dagang vacuum residue at 20 C is 0.9796 g/cm3, and the average molecular weight is 1008. The average molecular weight and molecular diameter of the vacuum residue SFEF fractions are listed in Table 3. The molecular weight of the oil fractions was obtained by the vapor pressure osmometry method, and the molecular diameter was calculated using the following equation:9 dr ¼ 0:403M 0:537
parameter W0 C0 C¥ r0 ω R β
According to eq 10, the slope (Di0 /r02) of the line in Figure 5 is 1.19 10-5, so the effective diffusion coefficient is 1.36 10-8 cm2/s. The effective diffusion coefficients of the other SEFE fractions diffusing in catalysts with and without a template can be determined the same way. 4.4.2. Comparison of Effective Diffusion Coefficients of Different SFEF Fractions. The saturated adsorption amounts of DG-1, DG-5, and DG-8 SFEF fractions diffusing in a catalyst with and without a template are shown in Figure 6. This figure shows that the adsorption isotherm is of Freundlich type. The saturated adsorption amount of oil in catalyst increased as the molecular weight of the oil fractions increased. The saturated adsorption amount of oil in the catalyst with a template with larger pore size is much greater than that in a catalyst without a template. The effective diffusion coefficients (Di0 ) of DG-1-DG-8 SFEF fractions diffusing in the prepared catalysts versus their molecular size are shown in Figure 7. Figure 7 shows that the effective diffusion coefficient decreases as the diameter of the oil fraction particles in the catalysts increases. The effective diffusion coefficient of Dagang vacuum residue SFEF fractions in a catalyst with 20 wt % PS template was ∼10-8 cm2/s, while it was ∼10-9 cm2/s in the conventional catalyst, and the diffusion coefficient in template-prepared catalyst was 2-3 times larger than those in the conventional catalyst, which were in agreement
ð19Þ
where M is the average molecular weight and dr is the molecular diameter (expressed in A˚ngstroms). 4.4. Diffusion Coefficient of Residue Oil in FCC Catalyst. 4.4.1. Determination of Effective Coefficient. For DG-1 SFEF fraction solution diffusing in FCC catalyst prepared without template, the effective diffusion coefficient was obtained using the following calculation procedures: According to Freundlich isothermal adsorption equation, q ¼ kC n where k and n are constants. The logarithm form of the equation is log q ¼ log k þ n log C
value 0.050 g/mL 0.01368 g/mL 0.01203 g/mL 0.03375 cm 0.1205 0.4471 -0.8086
ð20Þ
ð21Þ
The relationship between log q and log C is shown in Figure 4. Fitting the data as a line, the slope of the line (log k) is 0.2927 and its intercept is 0.9381. Therefore, for eq 20, k = 1.9620 and n = 0.9381. For a 0.01368 g/mL (2 wt %) DG-1 SFEF fraction solution, the weight of oil diffused into the catalyst prepared without a template at different diffusion times was tested, and F(t) was calculated and are shown in Table 4. The values of τ at time t were acquired by solving eq 18. The τ values and parameters used are listed in Tables 4 and 5, respectively. The plot of τ to t is shown in Figure 5. 2828
Energy Fuels 2010, 24, 2825–2829
: DOI:10.1021/ef100064d
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Figure 5. Plot of the τ-t curve of the DG-1 SFEF fraction diffusing in a catalyst without a template.
Figure 7. Effective diffusion coefficients (Di0 ) of Dagang SFEF fractions in a catalyst with a 20 wt % PS template (black squares) and in a catalyst without a template (red circles).
Therefore, from the above discussion, a conclusion could be drawn that, because of the promotion of diffusion rate of heavy oil in the large pore of the macroporous catalyst, the catalytic performance of the macroporous catalyst was improved, compared to that of the conventional catalyst. 5. Conclusions (1) Macroporous fluidized catalytic cracking (FCC) catalysts were prepared using polystyrene (PS) spheres as template. The average pore size of the prepared large-pore catalyst was 5.6 nm, while that of the conventional catalyst was 4.4 nm. (2) The effective diffusion coefficient of Dagang vacuum residue supercritical fluid extraction and fractionation (SFEF) fractions in the macroporous FCC catalyst particles was determined to be 10-9-10-8 cm2/s. (3) The effective diffusion coefficient decreases as the oil fraction diameter increases, and the diffusion coefficient in the large-pore-size catalyst with a template is 2-3 times larger than those in the catalyst without a template. (4) Because of the promotion of diffusion rate of heavy oil caused by the large pore size of the catalyst, the catalytic performance of macroporous catalyst is better than the conventional catalyst.
Figure 6. Saturated adsorption of Dagang SFEF fractions in a catalyst with a 20 wt % PS template ((9) DG-1, (b) DG-5, (2) DG-8) and without a template ((þ) DG-1, () DG-5, (/) DG-8) catalyst).
with the diffusion coefficient values reported by other researchers.13,14 It is obvious that the effective diffusion coefficient of SFEF fractions increased as the catalyst pore size increased.
Acknowledgment. This research work was supported by the National Nature Science Foundation of China (Grant No. 20976192), National Natural Science Foundation for Distinguished Young Scholars of China (Grant No. 20725620).
(13) Shah, Y. T.; Paraskos, J. A. Ind. Eng. Chem. Process Des. Dev. 1975, 14 (4), 368–372. (14) Guin, J. A.; Tsai, K. J.; Curtis, C. W. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 515–520.
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