Study of the Performance of Catalysts for Catalytic Cracking by

Oct 1, 2008 - ... range of 3.0−7.5 g/g using two industrial feeds have coupled to a five-lump kinetic model for a quantitative study of a set of cra...
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Ind. Eng. Chem. Res. 2009, 48, 1163–1171

1163

Study of the Performance of Catalysts for Catalytic Cracking by Applying a Lump-Based Kinetic Model R. Quintana-Solo´rzano,* A. Rodrı´guez-Herna´ndez, and R. Garcı´a-de-Leo´n Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas Norte 152, C. P., 07730 Me´xico DF, Me´xico

In catalytic cracking, catalysts selection is not a trivial task since a catalyst should exhibit a good hydrothermal stability as well as product distributions and qualities according to the necessities of the refinery. Cracking experiments at the bench scale in a fluidized confined-bed reactor at 789 K in the catalyst to oil range of 3.0-7.5 g/g using two industrial feeds have coupled to a five-lump kinetic model for a quantitative study of a set of cracking catalysts. To a gain insight into the catalysts performance, the assessment of raw rate coefficients has been complemented with a reaction pathways analysis. Primary cracking reactions are 1 order of magnitude higher than secondary ones, the gasoil to gasoline conversion being the fastest reaction. Up to 23% of the formed gasoline may undergo secondary cracking, more than 90% of it ending up in LPG. At the investigated conditions, coke formation is practically fully formed out of gasoline via secondary reactions and not from the gasoil. Cracking results are in concordance with catalyst properties, i.e., specific surface areas, and catalyst composition, e.g., the rate of secondary cracking of gasoline to LPG decreases with the zeolite and rare earths content. 1. Introduction Fluid catalytic cracking (FCC), which deals with the conversion of heavy crude oil fractions with a distillation range above 623 K in the presence of a solid acid catalyst composed of Y-zeolite as main active constituent, is still a key process in the modern refining scheme.1 The amount of the most valuable products formed in FCC, i.e., gasoline, liquefied petroleum gas (LPG) light olefins, and diesel, is determined by feed composition, operating conditions, and catalyst formulation. Although the nature of the feed has been identified to be the most important variable for determining product yields and qualities, catalyst plays a crucial role for a final adjustment of the product distributions of the FCC unit.2 The selection of an adequate catalyst for FCC is a very important issue for refineries because of technical and economical reasons. In a technical context, catalyst selection is traditionally supported by a set of catalyst tests for qualifying its hydrothermal stability, metals tolerance, and cracking performance.3 In the regenerator of a FCC unit, the catalyst is subjected to severe deactivating conditions, particularly, a hydrothermal effect due to the presence of water formed during coke combustion at a temperature above 923 K and a metallic effect mainly caused by the vanadium contained in the feed which in turn ends up in the coke of the spent catalyst.4 When a catalyst exhibits a good hydrothermal stability and metals tolerance, its cracking performance defines if it is a real candidate to be tested at a larger, i.e., pilot or semi-industrial, scale. Despite the differences in hydrodynamics with respect to an industrial FCC riser, experiments in bench-scale reactors are widely used for catalyst manufacturers for a preliminary catalyst screening due to their lower cost, higher precision, and quicker response in comparison with circulating pilot plant tests.5,6 The latter are commonly employed for more advanced studies, e.g., catalysts scaling up as well as process dynamics, simulation, and optimization.7 Unfortunately, the fact that most of the catalyst manufacturers essentially perform a qualitative * To whom correspondence should be addressed. Tel.: + 52 55 9175 8516. E-mail: [email protected].

analysis of the generated cracking data may provide a limited picture of the real potential of a catalyst in a first-stage screening. This work shows the advantage of combining kinetic modeling with experimental cracking data for studying the catalytic performance of an in-house-developed catalyst with respect to three commercial references. Experiments have been carried out in a relatively novel fluidized confined-bed reactor at the bench scale. Apart form obtaining the corresponding model parameters for the catalysts, a contribution analysis is carried out so as to gain insight into the catalysts performance. For practical reasons, a relatively simple lumped model considering five lumps albeit sufficient to provide valuable information is used. In catalytic cracking lumping is necessary as working at the molecular level is impossible due to the very large number of species involved. Needless to say that models which consider the underlying chemistry, for instance, single-event microkinetics, are the most advanced for catalytic cracking due to their fundamental character.8,9 However, due to the large number of parameters involved and the necessity of a detailed characterization of the feed, they are not widely used for practical proposes yet. 2. Experimental Procedures 2.1. Experiments. 2.1.1. Catalysts. Three commercial catalysts denoted as Cat-1, Cat-2, and Cat-3 together with catalyst developed at the IMP, Cat-4, were selected for this study. In fact the idea is to study the performance of Cat-4, a catalyst formulated with an in-house-developed active alumina, with respect to that of three available commercial references. Prior to the cracking reaction all the catalysts were subject to hydrothermal deactivation (HtD) with 100% steam at 1005 K for 24 h on the basis of the ASTM-D-4463 method. Although there are more advanced methods, e.g., cyclic metallic deactivation (CMD) and cyclic propylene steaming (CPS), for trying to emulate the properties of a so-called equilibrium catalyst, simple steaming was used for practical reasons. In fact, metals such as nickel and vanadium have not been used to eliminate the contribution of the noncatalytic products, e.g., contaminant coke. Table 1 summarizes the most relevant properties of fresh and steamed catalysts. In relation to catalyst composition, Cat-4 has

10.1021/ie800570z CCC: $40.75  2009 American Chemical Society Published on Web 10/01/2008

1164 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Table 1. Physical and Chemical Properties of the Catalysts Utilized for the Catalytic Cracking Experimentsa property specific surface area (SSA), m2/g: totalb microporousb zeolite to matrix SSA ratio total SSA retention,d % pore volume,c cm3/g rare earths,e wt % alumina,e wt %

Cat-1

Cat-2

Cat-3

Cat-4

264 (198) 196 (137) 2.9 (2.2)

312 (227) 234 (149) 3.0 (1.9)

310 (205) 226 (143) 2.7 (2.3)

290 (240) 227 (177) 3.6 (2.8)

75

73

66

83

0.192 2.9 37.1

0.239 0.8 38.0

0.186 1.3 28.4

0.203 1.9 48.8

a The values between parentheses correspond to catalyst properties after hydrothermal deactivation, while the others, to properties of fresh catalysts. b Via ASTM-D-3663 method. c Via ASTM D-4222 method. d Calculated as the ratio of total SSA of the deactivated to fresh catalyst. e Via ICP with EPA-6010C method. Rare earths expressed in terms of La.

the highest content of alumina (about 49 wt %), whereas Cat-1 reports the highest content of rare earths, close to 3 wt %. The catalyst with the lowest content of alumina is Cat-3, i.e., 28.4 wt %, with a rare earths content of 1.29 wt %. It is well-known that the incorporation of rare earths to the zeolite stabilizes its crystalline structure, improves the hydrothermal stability of the zeolite, and enhances the hydrogen transfer capacity of the catalyst.10 Even though zeolite typically provides most of the activity of cracking catalysts, alumina with particular acid properties and controlled porosity can be incorporated to catalyst matrix for providing active sites for the cracking of heavy hydrocarbons. Concerning the physical properties of the catalysts in the fresh form, Cat-2 and Cat-3 have the highest total specific surface area (SSA) while the lowest corresponds to Cat-1. After steaming, however, the following rank in terms of total SSA is observed: Cat-4 > Cat-2 > Cat-3 > Cat-1. Total SSA is, in fact, an indirect indicator of the catalyst activity. The treatment with steam extracts aluminum from zeolite crystalline structure, which ultimately lowers the total SSA and the zeolite to matrix SSA ratio. In the fresh form and after steaming, Cat-4 also exhibits the highest values of both microporous SSA and zeolite to matrix SSA ratio; in fact, Cat-4 is the catalyst possessing the highest tolerance to HtD, retaining 83% of its original total SSA. In contrast, Cat-2 and Cat-3, the catalysts containing the lowest levels of rare earths, only retain 73 and 62% of total SSA after steaming. 2.1.2. Feeds. The catalytic cracking performance of the catalysts presented in the previous section was studied for the

conversion of two industrial mixtures of gasoil, denoted as feed-1 and feed-2. The average properties of both feeds are presented in Table 2. The fact of using two feeds is aimed at identifying a feed effect on the relative catalyst performance. From the values in Table 2, in particular, the higher K-UOP factor, the higher oAPI (equivalent to lower specific gravity), the lower refraction index, the lower average boiling point, the higher content of naphthenes, and the lower amount of carbon contained in aromatics of feed-1 compared with feed 2, it is expected that the former feed is more susceptible to cracking. 2.1.3. Reaction Conditions and Products Analysis. The catalytic cracking reaction was performed in an ACE-R (advanced catalytic evaluation research model) unit licensed by Kayser Technology and built by Xytel Corp. The use of this unit by catalyst manufacturers and research groups is becoming more common due to its high degree of automation which results in a very good precision and repeatability.5,11,12 The ACE-R unit is equipped with a fluidized confined-bed reactor that is operated at atmospheric pressure and isothermal condition. The cracking reaction was performed at 789 K, a temperature that is in the range of typical riser outlet temperatures of commercial FCC units. The feed container was preheated at 353 K and injected to a constant inlet flow rate equal to 0.020 g/s. The reactor bed mass amounted to 9.0 g of hydrothermally deactivated catalyst for all the runs, while the feed injection time was spanned from 60 to 150 s in order to adjust the catalyst to oil ratio (C/O) which, in turn, ranged from 3 to 7.5 g/g. Varying the C/O was aimed at being able to obtain product distribution profiles as a function of the feed conversion. Notice that since cracking products are accumulated along the time and analyzed afterward, vide infra, the reported feed conversion and product distributions for this reactor are, actually, average values. A typical cracking experiment consists of feed injection/ cracking, catalyst stripping, and spent catalyst regeneration. The injected feed, admixed with nitrogen as diluent, is vaporized while flowing down to the reactor and cracks when it contacts the catalyst bed. Once the reaction is over, the catalyst bed is subjected to stripping with nitrogen flow for removing residual occluded hydrocarbons. The nitrogen/stripped hydrocarbons stream is added to the gaseous cracked products, and the resulting mixture is collected and quantified in a glass bottle by water displacement at atmospheric pressure and room temperature. The liquid product formed during the cracking reactions is collected in a glass receiver cooled at 263 K. The coke deposited on the stripped spent catalyst is removed by combustion with air at 923 K; the amount of coke formed was determined by quantifying the CO2 produced by the in situ

Table 2. Properties of the Feeds Used for the Catalytic Cracking Experiments method specific gravity 298/277 K (°API) boiling point distribution: IBP/FBP, K 10/50/90 (vol %), K average boiling point, K average molecular mass conradson residual carbon, wt % refraction index, 0/20 K-UOP Fe/Na/V/Ni, ppm sulfur, wt % total nitrogen, ppm carbon distribution (ndm), wt %: aromat./napth./paraf. a

ASTM-D-70 ASTM-D-1160 a ASTM-D-2502 ASTM-D-189 ASTM-D-1218 UOP-375 IMP-QA-007 ASTM-D-4294 ASTM-D-3228 ASTM-D-3238 b

feed-1

feed-2

0.9060 (24.68)

0.9182 (22.64)

484/820 600/696/777 681 342 0.33 1.5074 11.85 0.32/0.56/0. 26/0.06 1.88 890

522/842 609/716/799 699 405 0.29 1.5157 11.79 0.29/1.11/0.64/0.14 2.26 1276

22.0/17.2/60.8

24.1/11.9/64.0

Calculated out of the ASTM-D-1160 boiling point distribution. n.d.m. denotes refraction index, density, and molecular mass, respectively.

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1165

2.3. Reactor Model Equations and Parameter Estimation. Parameter estimation was performed by minimizing the weighted sum of squares of the residuals (RSS) between calculated, yˆij, and experimental, yij, yields of the lumps of hydrocarbons considered in the reaction network in Figure 1 Figure 1. Reaction network, including lumps and rate coefficients associated with the various reactions, considered for the kinetic model construction.

nexp nresp

RSS(k) )

∑ ∑ w (y j

Rgasoil ) - (k1 + k2 + k3 + k4)(Cgasoil)2Φ

(1)

Rgasoline ) (k1(Cgasoil)2 - k5Cgasoline - k6Cgasoline)Φ

(2)

RLPG ) (k2(Cgasoil)2 + k6Cgasoline)Φ

(3)

Rdry gas ) k3(Cgasoil)

2Φ

Rcoke ) (k4(Cgasoil)2 + k5Cgasoline)Φ

(4) (5)

An exponential deactivation function, Φ, in terms of the coke content and a deactivation parameter was used to account for coking effect on the reaction rates. To avoid the incorporation of several deactivation functions with the corresponding deactivation coefficients, it is assumed that coke has the same deactivating effect on all the reaction rates: Φ ) exp(-kdCc)

(6)

For each catalyst, the proposed model ultimately contains six rate coefficients, kj, and one deactivation parameter, kd, which were estimated by minimizing the corresponding weighted objective function, cf. infra.

(7)

j

i

combustion of the spent catalyst in a Servomex Mod 1440C. After removing coke, the catalyst was used for the next run. Gaseous products composed of dry gas (i.e., hydrogen and C1-C2), LPG (i.e., C3-C4), and a small amount of noncondensed gasoline (mainly C5-C6) were analyzed by chromatography in a HP-6890 Series 2 GC. Liquid product referred as to syncrude composed of gasoline (308-494 K), light cyclic oil (LCO, 494-616 K), and heavy cyclic oil (HCO, 616 K+) was analyzed in a HP-6890 gas chromatograph (GC) in accordance with the ASTM-D-2887 method for a simulated distillation. Mass recovery ranged between 97 and 102 wt %. 2.2. Reaction Network and Rate Coefficients. The reaction pathways involved in the catalytic cracking of the feeds accounted for five lumps, viz., gasoil (feed), gasoline, LPG, dry gas, and coke, and six irreversible reactions corresponding to four primary and two secondary ones, as is sketched in Figure 1. The structure of this network has been defined on the basis of the available experimental data while trying to keep the number of rate coefficients to be estimated via regression to a tractable value without losing relevant information. Notice that some secondary reactions, i.e., gasoline to dry gas and LPG to dry gas, proposed by others11,13,14 are not in the network because their contribution to the cracking products was assumed to be negligible compared with the formation of dry gas from the gasoil. In fact the observed dry gas appears to be mainly formed via thermal cracking when the feed contacts the hot catalyst while the formation of dry gas out of gasoline and LPG involves primary carbenium ions which are not favored thermodynamically.15 The net rate of formation of the different lumps, Rj, was expressed by power law expressions in terms of the concentration of the lump in the gas phase. The rate of gasoil conversion to any lump of products is proportional to the square of the gasoil concentration, while the rest of the reaction rates are proportional to the reactant concentration as has been conveniently found elsewhere:16,17

k1,k2,... yij)2f min ij - ˆ

For an experiment i, the experimental yield of a lump j is calculated by formed of lump j)i 100 (8) of feed)i In eq 7, k is the vector of kinetic parameters to be estimated, nexp is the number of independent experiments, and nresp is the number of responses, while wj is a weight factor that can be optionally used for tuning the relative importance of the various responses. For practical reasons only rate coefficients are determined because the estimation of activation energies and preexponential factors requires, strictly speaking, an extensive experimental program at different temperatures. ODRPACK 2.01 solver18 was used to obtain the parameters that minimize the objective function by nonlinear ordinary leastsquares for explicit models with an implementation of the Levenberg-Marquardt method. For each catalyst, four independent experiments with five responses for each one were used for the regression. To assess the parameter estimation results, the F-test for the global significance of the regression and the individual confidence intervals based on the t test for the estimates were computed. Simulated yield of the defined lumps in the objective function were calculated from their corresponding concentrations, Cj, which, in turn, were obtained via integration of a set of ordinary differential equations (ODEs) in eq 9, by using the LSODA integrator routine discussed in ref 19. The ACE-R reactor was supposed to behave like a spouted bed reactor;11,20 accordingly, the set of ODEs represented by eq 9 has been derived for a catalytic pseudohomogeneous reactor model without incorporating explicitly concentration and thermal gradients at particle scale: yij )

(mass

(mass

dCj ) Q(Coj - Cj) + εVRRj (9) dt withCj(0) ) Cjo at t ) 0, as the initial condition. Except for the gasoil,Cjo is equal to zero for all the lumps considered in the model. The net rate of formation of a lump j, Rj, is given by the eqs 1-5; vide supra. 2.4. Contribution Analysis. A contribution analysis of the various reactions considered in the model was performed on the basis of the methodology described in the literature.21,22 This methodology is very useful to quantify the relative importance of the different reactions considered in a kinetic model since it is based on reaction rates, and, hence, both kinetic and concentration effects are accounted for. In general, the integral appearance/disappearance (d/a) contribution factor (φ) of the elementary step i toward the (dis)appearance of component j for a given experiment k is defined as the ratio of the rate of (dis)appearance of j resulting from reaction i, rijd/a, to the total rate of (dis)appearance of j at a certain position inside the reactor: εVR

φd⁄a ij )

rdij ⁄ a

∑r

d⁄a ij

i

(10)

1166 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

Figure 2. Experimental cracking performance of the studied catalysts with feed-1: (a) feed conversion as a function of the catalyst to oil ratio, (b) LPG yield versus feed conversion, (c) gasoline versus feed conversion, and (d) coke yield versus feed. Solid lines are simply used for trends.

Integral contribution factors are determined by using the rates integrated over time. The calculated average rates were obtained via integration of the local rates along the time by using the trapezoid formula. 3. Results and Discussion 3.1. Experimental Cracking Data: Qualitative Assessment. Before dealing with the modeling part, a brief qualitative revision of the raw experimental cracking data has been carried out. Figure 2 and Figure 3 contain plotted data of feed conversion versus catalyst to oil ratio as well as plots of gasoline, LPG, and coke yields as a function of feed conversion for the four catalysts tested corresponding to cracking data for feed-1 and feed-2, respectively. Also, to facilitate the comparison of product distributions for the studied catalysts, Table 3 shows product yields interpolated at a constant feed conversion equal to 75 wt %. From a visual inspection of Figure 2 and Figure 3 it is confirmed that gasoline is a primary unstable product as it reaches a maximum and then decreases with conversion, while coke increases further with conversion demonstrating the secondary character of this product. On the basis of the information in Table 3, it is observed that, for a given C/O, the catalysts convert the feed to a different extent; namely, they have a different cracking activity. The catalyst activity ranking evolves as follows: Cat-4 > Cat-2 > Cat-3 > Cat-1, meaning that Cat-4 requires the lowest C/O for converting 75 wt % of feed and Cat-1 the highest. By comparing product distributions at the same feed conversion level, it appears that Cat-4 leads to the highest yield of gasoline, whereas Cat-1 produces the highest yields of LPG and coke. On the contrary, Cat-1, Cat-3, and Cat-4 produce the lowest yield of gasoline, LPG, and coke, respectively. Relating the catalytic behavior of the tested catalysts to the corresponding properties displayed in Table 1, the total SSA

correlates well with the catalyst activity; Cat-4 is in fact the catalyst with the highest total SSA. The alumina contained in the matrix of Cat-4 has an additional positive effect on the feed conversion. Cracking catalysts can be formulated with an active matrix containing alumina with improved acid properties and controlled porosity that enhance the conversion of the large molecules contained in the gasoil. With respect to a high level of gasoline exhibited by Cat-4 this can be explained on the basis of its higher value of microporous SSA in comparison with the other catalysts. Bro¨nsted sites associated to the zeolite in cracking catalysts are responsible for hydrogen transfer reactions.23 The reconversion of primary products, e.g., gasoline, and hydrogen transfer are reactions in competition when a carbenium ion is adsorbed on the zeolite surface. A higher zeolite content in a catalyst increases the number of acid sites responsible for transfer hydrogen and, hence, reduces the possibilities of gasoline overcracking, resulting in higher yields.24 With regard to the cracking behavior of feeds and on the basis of the information in Table 3, feed-1 is slightly more susceptible to cracking compared with feed-2 as the former feed requires a lower C/O to be cracked at a conversion level equal to 75 wt %. In terms of product distributions, the cracking of feed-2 produces, in general, more coke and less gasoline than feed-2, for a given conversion. These results agree with the properties of the feeds presented in section 2.1.2. In fact it is observed that the relative differences in the catalyst performance are feed-dependent and the catalyst ranking might be influenced by the feed nature. The latter effect is in particular observed for dry gas and LPG production. 3.2. Modeling. The kinetic parameters obtained via regression with the corresponding confidence intervals are displayed in Table 4, for feed-1, and in Table 5, for feed 2. Except for k4, corresponding to the rate coefficient of the feed cracking to coke, all the estimated parameters were statistically significant. The

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1167

Figure 3. Experimental cracking performance of the studied catalysts with feed-2: (a) feed conversion as a function of the catalyst to oil ratio, (b) LPG yield versus feed conversion, (c) gasoline versus feed conversion, and (d) coke yield versus feed conversion. Solid lines are simply used for trends. Table 3. Experimental Catalyst to Oil Ratio and Product Yields Interpolated at a Constant Feed Conversion Equal to 75 wt % for the Catalyst Studied with the Two Feeds Cat-1

Cat-2

Cat-3

Cat-4

Feed-1 catalyst to oil ratio, g/g dry gas, wt % LPG, wt % gasoline, wt % coke, wt %

8.6 1.9 20.2 48.0 5.9

4.5 2.2 18.2 50.7 4.7

5.1 2.2 17.9 51.0 4.9

1.8 2.1 19.6 51.2 3.4

5.6 2.4 18.0 49.4 6.1

6.8 2.5 18.2 48.7 6.7

1.9 2.1 16.7 52.2 4.9

Feed-2 catalyst to oil ratio, g/g dry gas, wt % LPG, wt % gasoline, wt % coke, wt %

8.8 2.6 20.0 45.8 7.5

Table 4. Estimated Rate Coefficients and Deactivation Coefficient with Corresponding Confidence Intervals at the 95% Probability Level after Regression of the Cracking Data with Feed-1a k1 × k2 × k3 × k4 × k5 × k6 × kd × Freg

-3

10 10-2 10-1 108 103 102 10-2

Table 5. Estimated Rate Coefficients and Deactivation Coefficient with Corresponding Confidence Intervals at the 95% Probability Level after Regression of the Cracking Data with Feed-2a

Cat-1

Cat-2

Cat-3

Cat-4

0.8 ( 0.3 1.2 ( 1.0 2.7 ( 1.7 8.1 ( 200 1.1 ( 0.5 3.4 ( 3.0 6.9 ( 5.4 2300

1.5 ( 0.9 2.3 ( 2.1 6.1 ( 5.1 4.3 ( 850 1.6 ( 0.9 5.1 ( 3.4 8.6 ( 7.3 1630

1.2 ( 0.5 1.0 ( 0.2 4.4 ( 3.1 1.0 ( 270 1.3 ( 0.6 5.0 ( 3.4 7.1 ( 6.0 1680

1.7 ( 1.2 5.3 ( 4.5 8.0 ( 6.6 1.6 ( 217 1.6 ( 0.8 1.6 ( 1.3 7.9 ( 7.0 1840

a Statistics: Ftab ) 3.01 for 1 - R ) 0.95 with 14 degrees of freedom (nexpnresp - nparam - 1).

matrix of binary correlation coefficients (not shown for the sake of brevity) contains absolute values lower than 0.9, suggesting that there is no correlation between pairs of estimated parameters. Besides, the value of the significance of the regression,

k1 × k2 × k3 × k4 × k5 × k6 × kd × Freg

-3

10 10-2 10-1 108 103 102 10-2

Cat-1

Cat-2

Cat-3

Cat-4

1.2 ( 0.9 2.2 ( 2.0 5.0 ( 4.9 10.5 ( 300 2.2 ( 1.3 5.2 ( 4.4 1.3 ( 0.9 1410

1.3 ( 0.9 1.8 ( 1.2 5.6 ( 4.9 4.4 ( 320 1.9 ( 1.1 4.9 ( 4.0 7.4 ( 6.6 1290

1.0 ( 0.5 1.6 ( 1.0 4.5 ( 3.2 1.3 ( 210 1.7 ( 0.7 3.9 ( 3.0 6.7 ( 5.3 1760

1.4 ( 0.8 1.9 ( 1.8 6.6 ( 5.3 1.5 ( 156 1.8 ( 0.8 4.6 ( 4.0 5.1 ( 5.0 1490

Statistics: Ftab ) 3.01 for 1 - R ) 0.95 with 14 degrees of freedom (nexpnresp - nparam - 1). a

Freg, denotes adequate model predictions independently of the feed cracked. For instance, parity diagrams presented in Figure 4 for the cracking data with feed-1 confirm that the yields simulated by the model are in a good agreement with experimental ones. By comparing the main value of the rate coefficients estimated for feed-1 and feed-2, i.e., Table 4 and Table 5, differences in the relative performance of the catalysts can be detected which have already been observed from the qualitative analysis of the raw experimental cracking data in a previous section. Although a contribution analysis based on reaction rates is, strictly speaking, required to determine in quantitative terms the relative importance of the various steps in the model, vide infra, the magnitude order of the estimated kinetic parameters can provide a preliminary idea of how cracking occurs. For primary reactions, the largest rate coefficients correspond to that of the gasoil to gasoline reaction, denoted by k1, and that of the gasoil

1168 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

Figure 4. Parity plots showing calculated versus experimental cracking yields of the various lumps considered in the proposed kinetic model for the four catalyst studied with feed-1 at 789 K in the C/O range of 3.5-7.5 g/g. Table 6. Relative Rate Coefficients Referred to Cat-1 for the Four Catalysts Studied with Feed-1

k1rel k2rel k3rel k4rel k5rel k6rel kdrel

a

Cat-1

Cat-2

Cat-3

Cat-4

1.0 1.0 1.0 nc 1.0 1.0 1.0

1.9 1.8 2.3 nc 1.4 1.5 1.3

1.5 0.8 1.7 nc 1.1 1.5 1.0

2.1 4.3 3.0 nc 1.4 0.5 1.2

a Since the corresponding rate coefficient was not significant, the associated relative coefficient was not computed (nc).

Table 7. Relative Rate Coefficients for Feed-1 Referred to Feed-2 for the Four Catalysts Studied

rel

k1 k2rel k3rel k4rel k5rel k6rel kdrel

a

Cat-1

Cat-2

Cat-3

Cat-4

0.7 0.6 0.5 nc 0.5 0.7 0.5

1.2 1.3 1.1 nc 0.9 1.0 1.2

0.9 0.8 0.8 nc 0.7 2.1 1.0

1.2 2.8 1.2 nc 0.9 0.4 1.6

a Since the corresponding rate coefficient was not significant, the associated relative coefficient was not computed (nc).

to LPG reaction, denoted by k2. The value of k1 is in fact 1 order of magnitude higher than that of k2. For secondary reactions, k5 is 1 order of magnitude higher than k6 which, in principle, suggests that LPG formation is favored over coke from the gasoline overcracking. The low value of k4, which in fact is systematically nonsignificant, indicates that, at the particular conditions used in this work, coke formation from the feeds is negligible, namely, coke is essentially a secondary product coinciding with the literature.25 Needless to say that, in a commercial unit, most of the primary coke is formed via a thermal mechanism out of

Table 8. Reaction Rates and Some Contribution Factors Calculated for the Cracking of Feed-1 at 789 K and a C/O Equal to 4.5 g/ga rates, g cm-3 s-1. reaction gasoil to gasoline gasoil to LPG gasoil to dry gas summation primary gasoline to coke gasoline to LPG summation secondary

Cat-1

Cat-2 -2

1.3 × 10 1.9 × 10-3 4.2 × 10-4 1.5 × 10-2 7.2 × 10-5 2.2 × 10-3 2.2 × 10-3

Cat-3 -2

1.3 × 10 2.0 × 10-3 5.4 × 10-4 1.6 × 10-2 8.6 × 10-5 2.6 × 10-3 2.7 × 10-3

Cat-4 -2

1.4 × 10 1.2 × 10-3 5.1 × 10-4 1.6 × 10-2 7.8 × 10-5 3.1 × 10-3 3.2 × 10-3

1.2 × 10-2 3.7 × 10-3 5.6 × 10-4 1.6 × 10-2 9.0 × 10-5 9.3 × 10-4 1.0 × 10-3

contribution factors

Cat-1

Cat-2

Cat-3

Cat-4

primary to secondary ratio LPG via primary gasoline reconverted

6.6 47.0 17.9

5.8 42.8 20.7

4.9 27.4 23.1

16.9 79.9 8.5

a The main values of the rate coefficients reported in Table 4 were used for computing the rates.

the condensation of polyaromatics, when the hot catalyst coming from the regenerator contacts the fresh feed.26 Such a phenomenon is present to a significantly lesser extent in the laboratoryscale reactor used since it works isothermally. In addition, comparing the values of k2 and k6, the former being 4 orders of magnitude higher than the latter, apparently most of LPG is formed via primary cracking of gasoil and not via the secondary cracking of gasoline. However, this is to be verified in the contribution analysis taking into consideration that, in the model, the gasoil to LPG conversion was assumed to be a second-order reaction, while the gasoline to LPG reaction is order 1, and, hence, concentration effects are expected to have an important role in the net rate of formation of LPG and gasoline. To facilitate a comparison of the catalyst performance based on the estimated rate coefficients, relative rate coefficients

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1169 Table 9. Reaction Rates and Some Contribution Factors Calculated for the Cracking of Feed-2 at 789 K and a C/O Equal to 4.5 g/ga rates, g cm-3 s-1 reaction gasoil to gasoline gasoil to LPG gasoil to dry gas summation primary gasoline to coke gasoline to LPG summation secondary

Cat-1

Cat-2 -2

1.2 × 10 2.2 × 10-3 5.0 × 10-4 1.5 × 10-2 8.0 × 10-5 1.9 × 10-3 2.0 × 10-3

Cat-3 -2

1.3 × 10 1.8 × 10-3 5.6 × 10-4 1.5 × 10-2 1.0 × 10-4 2.6 × 10-3 2.7 × 10-3

Cat-4 -2

1.3 × 10 1.9 × 10-3 5.6 × 10-4 1.5 × 10-2 9.6 × 10-5 2.2 × 10-3 2.3 × 10-3

1.3 × 10-2 1.8 × 10-3 6.3 × 10-4 1.6 × 10-2 1.1 × 10-4 2.8 × 10-3 2.9 × 10-3

contribution factors

Cat-1

Cat-2

Cat-3

Cat-4

Primary to secondary ratio LPG via primary Gasoline reconverted

7.2 53.5 17.1

5.8 40.7 20.3

6.7 47.3 17.9

5.5 39.7 21.7

a The main values of the rate coefficients reported in Table 5 were used for computing the rates.

calculated from the main parameter values and with respect to a reference catalyst (cat__ref: Cat-1), have been computed via k rel j )

kj kcat_ref, j

(11)

where kjrel is the relative rate coefficient of a given catalyst for reaction j referred to cat__ref. As an example, Table 6 shows the value of the relative coefficients for the studied catalyst using the cracking data with feed-1.The value of the relative rate coefficients for primary reactions, i.e., k1rel, k2rel, and k3rel, denoting the gasoil to gasoline, to LPG, and to dry gas conversion, respectively, for Cat-2, Cat3, and Cat-4 are higher than the corresponding ones for Cat-1. This suggests a lower participation of primary reactions on the reference catalyst compared with the other three catalysts. It is remarkable the fact that for Cat-4 the valus of k2rel and k3rel were up to four and three times higher than the corresponding ones for the reference catalyst. For secondary reactions, the higher value of k5rel (about 40% higher than the reference catalyst) for Cat-2 and Cat-4 suggests that they are more susceptible to forming coke via secondary cracking of gasoline with respect to the other two catalysts. On the other hand, for the gasoline overcracking to LPG, Cat-4 exhibits the lowest value of k6rel (50% lower than that of the reference catalyst), indicating that this catalyst promotes such a reaction to a lesser extent. With respect to the deactivation of the catalysts, the main value of the deactivation coefficient for Cat-1 and Cat-3 is practically the same while the value for Cat-2 and Cat-4 is 30 and 20% higher compared with that of the reference catalyst. From this, it is deduced that, for a given coke content, Cat-1 and Cat-3 are more susceptible to deactivation by coke than Cat-2 and Cat-4. To compare feeds using the obtained kinetic information, relative rate coefficients for all the catalyst referred to feed-2

were computed; cf. Table 7. In principle, one can say that Cat-2 and Cat-4 are more capable of catalyzing primary reactions with feed-2, i.e., the least crackable feed, as they have relative rate coefficients higher than 1. For secondary reactions, the reconversion of gasoline to coke appears to proceed faster with feed-1 compared with feed-2 independently of the catalyst, which is understandable on the basis of the lower aromatic character of the former feed. Concerning the transformation of gasoline to LPG, except for Cat-3, the relative rate coefficients indicate that this reaction is faster on feed-1. The latter behavior may be explained on the basis of the fact that feed-2, with a higher aromatic character than feed-2, produces more aromatic gasoline, which in turn is less susceptible to undergoing further cracking to lighter fractions. With respect to deactivation by coke, all the relative rate coefficients except that for Cat-1 are higher than 1, which suggests that catalysts deactivate to a higher extent when cracking feed-2. Again, the more aromatic character of this feed explains that fact. 3.3. Contribution Analysis. A contribution analysis, combining kinetic and concentration effects, allows one to assess in quantitative terms the relative importance of the different reactions at particular operating conditions. The values of the integral rates, which are necessary for computing the contribution factors, for the reactions considered in the proposed network (Figure 1) are presented in Table 8, for feed-1, and in Table 9, for feed-2. Notice that since the contribution factors depend on the operating conditions, i.e., temperature and C/O, the rates were calculated at 789 K and a C/O equal to 4.5 g/g, in other words, at the same reaction severity. On the basis of the information contained in Table 8 and Table 9, the value of the rates of the various reactions evolved, in general, as follows: gasoil to gasoline > gasoil to LPG ≈ gasoline to LPG > gasoil to dry gas > gasoline to coke. The gasoil to gasoline cracking rate is 1 and 2 orders of magnitude higher compared with that of gasoil to LPG (and gasoline to LPG) and gasoil to dry gas, respectively. Moreover, the rate of primary reactions is 1 order of magnitude greater than that of secondary ones; such a difference in rates is not to the extent of that exhibited by the corresponding rate coefficients reported in Table 4 and Table 5. From the results displayed in Table 8 and Table 9, the derived contribution factors indicate that gasoil conversion for feed-1 distributes as follows: 74-89% to gasoline, 8-24% to LPG, and only 3-4% to dry gas, while for feed-2 gasoil cracks from 81-85% to gasoline, 11-15% to LPG, and 3-4% to dry gas. The contribution of the gasoil to coke is, as stated in a previous section, negligible. The fact that the percentage of the gasoil converted to dry gas is practically constant confirms that the formation of this product is essentially noncatalytic. For secondary reactions, for feed-1 from 91 to 98% of the gasoline that is reconverted via secondary cracking ends up in LPG and only 2-9% yields coke; on the other hand, for feed-2

Figure 5. Example of a reaction network containing the corresponding contribution factors calculated out of the rates presented in Table 8 and Table 9 for Cat-3: (a) feed-1 and (b) feed-2. Numbers at the arrows indicate the relative rate of lump disappearance, while the thickness of the arrows is proportional to the corresponding rates in logarithmic scale. Reaction rates below 10-6 g cm-3 s-1 are represented by dashed arrows.

1170 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

96% of the reconverted gasoline leads to LPG and 4% to coke. From the computed rates, it was also possible to determine that from 10 to 23% of the gasoline formed out of the gasoil is susceptible to a secondary cracking and that from 38 to 70% of the observed LPG is formed via primary reactions for feed-1, whereas for feed-2 from 17 to 22% of the primary gasoline is further cracked and from 40 to 56% of the LPG is formed from the gasoil. For brevity’s sake, Figure 5 shows an example of the proposed reaction network containing the corresponding computed contribution factors for Cat-3 when cracking feed-1 and feed-2. In this figure, the numbers next to the arrows indicate the relative contribution of the reactions to the gasoil conversion, for primary reactions, and to the gasoline overcracking, for secondary reactions. The thickness of the arrow indicates the relative magnitude of rates in the logarithmic scale. In this figure one can see that more gasoline can be produced from the gasoil out of feed-1 and that the gasoline formed is more susceptible for overcracking. Moreover, more LPG is produced from the feed via the cracking of feed-2. The proportion of the gasoil converted to dry gas from feed-1 and feed-2 is practically the same. 4. Conclusions The combination of bench-scale experiments in a confined fluidized bed reactor using real mixtures of gasoil with kinetics using a five-lump model has been of a great utility to gain insight into the catalytic behavior of a set of commercial FCC catalysts in quantitative terms. In particular, a contribution analysis based on rates, i.e., incorporating both kinetic and concentration effects, has given additional elements for a more realistic catalyst comparison. On the basis of the computed rates, it is determined that gasoil to gasoline conversion is the most rapid reaction while primary cracking reactions are 1 order of magnitude higher than secondary ones and most of the secondary cracking of gasoline produces LPG. At the studied operating conditions, coke formation is found to be essentially a secondary product out of gasoline according to the model, and not from gasoil. The presented cracking results are understandable on the basis of the catalyst properties, in particular the catalyst composition expressed in terms of the zeolite, alumina, and rare earth amounts. Alumina as a source of an active matrix may promote the cracking of the large hydrocarbons in the feed, while the secondary conversion of gasoline to lighter hydrocarbons is inhibited in a catalyst with a higher content of zeolite and rare earths. Acknowledgment The authors thank research project D.00356 (IMP/PEMEXRefinacio´n) for providing financial support. The technical aid from Lilia A. Rodrı´guez-Bolan˜os for processing experimental data is acknowledged. Nomenclature Cc ) catalyst coke content, g/g Cj ) concentration of lump j in the reactor, g cm-3 k1, k2, k3, k4 ) rate coefficients of primary reactions, cm3 (g s)-1 k5, k6 ) rate coefficients of secondary reactions, s-1 kd ) deactivation coefficient, gcat. (gcoke)-1 krel ) relative rate coefficient or deactivation coefficient nexp ) number of independent experiments nparam ) number of parameters to be estimated

nresp ) number of responses per experiment Q ) total volumetric inlet flow rate, cm3 s-1 R ) net rate of formation, g cm-3 s-1 t ) time, s VR ) reactor volume ()100 cm3) yj ) experimental yield of lump j yj ) calculated yield of lump j Greek Symbols  ) reactor void fraction ()0.85) φ ) contribution factor Φ ) deactivation function

Literature Cited (1) Harding, R. H.; Peters, A. W.; Nee, J. R. D. New developments in FCC catalyst technology. Appl. Catal., A 2001, 221, 389. (2) Bollas, G. M.; Vasalos, I. A.; Lappas, A. A.; Iatridis, D. K.; Tsioni, G. K. Bulk molecular characterization approach for the simulation of FCC feedstocks. Ind. Eng. Chem. Res. 2004, 43, 3270. (3) Scherzer, J. Octane-enhancing, zeolitic FCC catalysts: scientific and technical aspects. Catal. ReV.-Sci. Eng. 1989, 31 (3), 215. (4) Tangstad, E.; Myrstad, T.; Spjelkavik, A. I.; Stocker, M. Vanadium species and their effect on the catalytic behavior of a FCC catalyst. Appl. Catal., A 2006, 299, 243. (5) Kelkar, C. P.; Xu, M.; Madon, R. J. Laboratory evaluation of cracking catalysts in a fluid bed: Effects of bed dynamics and catalyst deactivation. Ind. Eng. Chem. Res. 2003, 42, 426. (6) Wallenstein, D.; Seese, M.; Zhaob, X. A novel selectivity test for the evaluation of FCC catalysts. Appl. Catal., A 2002, 231, 227. (7) Bollas, G. M.; Vasalos, I. A.; Lappas, A. A.; Iatridis, D. K.; Voutetakis, S. S.; Papadopoulou, S. A. Integrated FCC riser-regenerator dynamics studied in a fluid catalytic cracking pilot plant. Chem. Eng. Sci. 2007, 62, 1887. (8) Quintana-Solo´rzano, R.; Thybaut, J. W.; Marin, G. B. A single-event microkinetic analysis of the catalytic cracking of (cyclo)alkanes on an equilibrium catalyst in the absence of coke formation. Chem. Eng. Sci. 2007, 62, 5503. (9) Froment, G. F. Single event kinetic modeling of complex catalytic processes. Catal. ReV. 2005, 47, 83. (10) Herna´ndez-Beltra´n, F. J.; Moreno-Mayorga, J. C.; Guzma´n-Castillo, M. L.; Navarrete-Bolanos, J.; Gonza´lez-Gonza´lez, J. M.; Handy, B. E. Dealumination-aging pattern of REUSY zeolites contained in fluid cracking catalysts. Appl. Catal., A 2003, 240, 41. (11) Vieira, R. C.; Pinto, J. C.; Biscaia, E. C.; Baptista, C. M. L. A.; Cerqueira, H. S. Simulation of catalytic cracking in a fixed-fluidized-bed unit. Ind. Eng. Chem., Res. 2004, 43, 6027. (12) Herna´ndez-Beltra´n, F. J.; Quintana-Solo´rzano, R.; Sa´nchez-Valente, J.; Pedraza-Archila, F.; Figueras, F. Effect of highly reactive sulfur species on sulfur reduction in cracking gasoline. Appl. Catal., B 2003, 42, 145. (13) Villafuerte-Macı´as, E. F.; Aguilar, R.; Maya-Yescas, R. Towards modelling production of clean fuels: Sour gas formation in catalytic cracking. J. Chem. Technol. Biotechnol. 2004, 79, 1113. (14) Bollas, G. M.; Lappas, A. A.; Iatrodis, D. K.; Vasalos, I. A. Fivelump kinetic model with selective catalyst deactivation for the prediction of the product selectivity in the fluid catalytic cracking process. Catal. Today 2007, 127, 31. (15) Tiong, S. Acid-catalyzed cracking of paraffinic hydrocarbons. 1. Discussion of existing mechanisms and proposal of a new mechanism. Ind. Eng. Chem. Res. 1992, 31, 1881. (16) Nace, M. D.; Voltz, S. E.; Weekman, V. W. Application of a kinetic model for catalytic cracking: Effects on charge stocks. Ind. Eng. Chem. Process Des. DeVel. 1971, 10, 530. (17) Ancheyta-Jua´rez, J.; Lo´pez-Isunza, F. I.; Aguilar-Rodrı´guez, E.; Moreno-Mayorga, J. C. A strategy for kinetic parameter estimation in the fluid catalytic cracking process. Ind. Eng. Chem. Res. 1987, 36, 5170. (18) Boggs, P. T. ; Byrd, R. H. ; Rogers J. E. ; Schnabel. R. B. ODRPACK V. 2.01 User’s Reference Guide, http://www.netlib.com, 1992. (19) Petzold, L. R.; Hindmarsh, C. LSODA solver for ODE equations, http://www.netlib.com, 1997. (20) Moreno-Montiel, N. Modeling micro-activity reactor for the evaluation of FCC catalysts: Cracking models and deactivation. Ph.D. thesis (in Spanish), UAM-I, Mexico, 2004. (21) Chen, Q.; Hoebink, J. H. B. J.; Marin, G. B. Kinetics of the oxidative coupling of methane at atmospheric pressure in the absence of catalyst. Ind. Eng. Chem. Res. 1991, 30, 2088.

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1171 (22) Quintana-Solo´rzano, R.; Thybaut, J. W.; Galtier, P.; Marin, G. B. Single-event microkinetics for coke formation during the catalytic cracking of (cyclo)alkane/1-octene mixtures. Catal. Today 2007, 127, 17. (23) Wallenstein, D.; Harding, R. H. The dependence of ZSM-5 additive performance on the hydrogen-transfer activity of the REUSY base catalyst in fluid catalytic cracking. Appl. Catal., A 2001, 214, 11. (24) Cumming, K. A.; Wojciechowski, B. W. Hydrogen transfer, coke formation and catalyst decay and their role in the chain mechanism of catalytic cracking. Catal. ReV.-Sci. Eng. 1996, 38 (1), 101.

(25) Guisnet, M.; Magnoux, P. Organic chemistry of coke formation. Appl. Catal., A 2001, 212, 83. (26) Dupain, X.; Makee, M.; Moulijn, J. A. Optimal conditions in fluid catalytic cracking: A mechanistic approach. Appl. Catal., A 2006, 297, 198.

ReceiVed for reView April 9, 2008 ReVised manuscript receiVed July 28, 2008 Accepted July 29, 2008 IE800570Z