Adsorption, Diffusion, and Reaction Phenomena on FCC Catalysts in

Ind. Eng. Chem. Res. 2004, 43, 4709-4720

4709

Adsorption, Diffusion, and Reaction Phenomena on FCC Catalysts in the CREC Riser Simulator J. A. Atias and H. de Lasa* Chemical Reactor Engineering Centre, Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9

Two typical FCC catalysts with similar acidities and structural properties and different crystallite sizes, CAT-SC (0.4 µm) and CAT-LC (0.9 µm), are studied in a novel CREC riser simulator using 1,3,5-triisopropylbenzene. Catalytic and thermal runs allow for the development of a heterogeneous kinetic model and the assessment of intrinsic kinetic constants and adsorption and diffusional parameters. Analysis of 1,3,5-triisopropylbenzene conversions and product distribution helps establish the influence of intracrystallite diffusion as the controlling step at lower reaction temperatures (350-450 °C). Moreover, product distribution and selectivity differences between the CAT-SC and CAT-LC catalysts provide evidence of cracking reactions affected by hindered diffusion of the reactant and products. 1. Introduction Catalytic cracking is one of the oldest processes in the refining industry, and yet among the most important ones. The process was first introduced toward the end of the 1930s, and the catalyst has evolved considerably ever since.1 Soon, the expected trend regarding catalytic processes will be toward a reduction in costs by improving catalyst activity and, more particularly, catalyst selectivity.2 Typical fluid catalytic cracking catalysts contain H-USY zeolite crystallites supported on an amorphous matrix and binder. H-USY zeolite can be prepared by hydrothermal treatment of Y zeolites, resulting in dealumination of the crystal framework.3 This steaming process yields a number of structural changes that have been well documented in the technical literature. Y zeolites present a three-dimensional network of welldefined micropores, 12.4-Å supercages connected via 7.4-Å windows. The pores in this network can act as reaction channels, allowing the chemical species to reach active sites. Without steaming to generate mesopores, crevices, or fissures, most of the Brønsted acid sites in HY are inaccessible to hydrocarbon molecules and lead to micropore-diffusion-limited cracking reactions.4 Defects generated by steaming favor the diffusion of reactant molecules into and product molecules out of the micropores of the zeolite crystals, as well as promoting an increase in external surface area. Shape-selective reactions, governed by mass transfer within the micropore structure, involve what Weisz called “configurational diffusion”.5 Configurational diffusion occurs when the diameter of the molecules approaches the structural dimensions of the intracrystalline pores.6 The resulting steric restrictions prevent molecules from passing each other inside the zeolite channel.7 The measurement of diffusion in zeolites is a challenging task to which a significant number of contributions in the technical literature have been directed. An interesting review regarding some of the more recent techniques of diffusion measurement and * To whom correspondence should be addressed. Tel.: 519661-2144. Fax: 519-661-3498. E-mail: [email protected].

the main problems associated with the determination and interpretation of molecular diffusion in zeolites was recently presented by Karger.8 The influence of factors such as acidity, type and density of acidic centers, composition gradient, and morphology has become relevant in the tailoring of novel catalysts. As recently suggested,9 one option for improving the catalytic properties of zeolites when dealing with bulky molecules could be to synthesize nanocrystal-sized zeolites, which allows larger ratios of external to internal surfaces to be achieved. Catalytic studies involving catalysts with precisely controlled zeolitic compositions and crystallite sizes are important in studying the relative significance of mass transport and steric constraints. This research is also important in establishing fundamental parameters for designing catalysts for commercially important shapeselective reactions. Also, additional insights into the shape-selective properties of zeolites can be derived from model compound reaction tests that are specifically designed to elucidate the characteristics of the pore structures of the catalysts.6 1,3,5-Triisopropylbenzene (1,3,5-TIPB) is a common model compound used in the study of adsorption, diffusion, and catalytic cracking over a wide variety of zeolites. For instance, Hibino et al.10 measured the adsorption of 1,3,5-TIPB at room temperature with a gravimetric apparatus, finding that 1,3,5-TIPB was unable to penetrate the crystalline structure of mordenites and ZSM-5 zeolites. Hershkowitz and Madiara,11 using a pulsed microbalance reactor, assessed adsorption responses to pulses of 1,3,5-TIPB on LaY zeolite at 400 °C and concluded that the sorbate was excluded from the internal pore structure of the zeolite. O’Connor et al.12 used the cracking of 1,3,5-TIPB between 160 and 270 °C over ZSM-5, mordenites, and β-zeolite as a probe reaction to measure the catalytic activity of the external surface. Zhang et al.13 measured catalytic activities at 250 and 320 °C in the cracking of 1,3,5-TIPB on various catalysts, including HZSM-5, HMCM-41, and mesoporous aluminosilicates, to test their performance. Chen et al.14 studied mesoporous MCM-41 materials using 1,3,5-TIPB cracking, concluding that the reaction on this catalystwas controlled by

10.1021/ie034197q CCC: $27.50 © 2004 American Chemical Society Published on Web 04/24/2004

4710 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

diffusion. Roos et al.15 investigated a group of zeolites and a commercial FCC catalyst and their performance in the catalytic cracking of n-hexadecane and 1,3,5triisopropylbenzene (1,3,5-TIPB). They concluded that the acidic properties and the accessibility to the internal active sites have a significant influence on activity and selectivity. These authors claimed that the access of 1,3,5-TIPB molecules to the intracrystallite acid sites of Y zeolite was not allowed. Comparing samples of H-Y zeolite and FCC catalyst (USY zeolite + active matrix), they also found that the most active sample was the equilibrium catalyst (active matrix + USY zeolite), given the enhanced accessibility of 1,3,5-TIPB molecules to the internal structure of Y zeolites through cavities and fractures created via steaming. Aguiar et al.16 carried out cracking of 1,3,5-TIPB over two calcined REHY zeolites with different crystallite sizes (0.193 and 0.063 µm) at 623 K. These authors explained the difference in the rate of disappearance of 1,3,5-TIPB for the two zeolites by considering that the cracking reaction takes place at the outer surface of the zeolite only. They concluded that only the external surface plays an important role in the cracking of such a bulky alkylaromatic compound. Al-Khattaf and de Lasa17 carried out the catalytic conversion of 1,3,5-TIPB over Y-zeolite catalysts and observed a diffusion-controlled regime at low temperatures (350-450 °C) and a kinetically controlled regime at high temperatures (450-550 °C). Regarding diffusion, Campos et al.18 measured diffusion coefficients in mesoporous samples (MCM-41), finding that the diffusion of bulkier 1,3,5-TIPB was about 1 order of magnitude faster in these materials than in NaX. They explained this result by concluding that the diffusion of 1,3,5-TIPB occurred in the MCM41 cylindrical mesopores. The present contribution studies the catalytic cracking of 1,3,5-TIPB over USY catalysts. Adsorption and diffusion parameters are calculated under reaction conditions, and intrinsic kinetic parameters are evaluated independently. In this respect, the present approach represents a step forward in the study of the adsorption and diffusion of reactants in zeolitic catalysts, ahead of traditional studies at lower temperature and with lower-reactivity zeolites.11 2. Experimental Procedure Thermal and catalytic experiments were performed in a novel CREC (Chemical Reactor Engineering Centre) riser simulator using 1,3,5-triisopropylbenzene (Fluka 92075) as the model compound. This unit is an experimental reactor that uses a batch of catalyst and a batch of fluid, operating isothermally and at constant volume of the reaction mixture. The CREC riser simulator invented by de Lasa19 is a relatively simple well-mixed device that allows for the contact of reacting chemical species with fluidized catalyst throughout a predetermined time. The assumption of well-mixed conditions in the reactor can be supported given the high gas recirculation rate, as reported by Pekediz et al.20 A schematic diagram of the 52-cm3 riser simulator and its components is presented in Figure 1. The CREC riser simulator consists of a catalyst basket bounded by two grids trapping the catalyst and restraining the catalyst mobility within this chamber. An annular space is created between the outer portion of the basket and the inner part of the reactor shell that allows for the

Figure 1. Schematic diagram of the CREC riser simulator (quarter section displaying components under operation).

recirculation of chemical species by rotation of an impeller positioned above the catalyst basket. The rotation of the impeller creates a lower pressure in the center region of the impeller and induces a flow of gas upward through the catalyst chamber. Thus, the impeller provides a fluidized bed of catalyst particles, as well as intense gas mixing inside the reactor. Depending on the reaction conditions, such as hydrocarbon pressure, temperature, and conversion, the predominant cracking reaction mechanism can differ as indicated by Kung et al.4 Thus, it is important to mimic, as in the case of the CREC riser simulator, the relevant reactivity conditions of catalytic cracking by matching the relative pressures of the chemical species, the gassolid contact regime, the reaction time, the temperature, and the level of conversions, with all of this effort ensuring meaningful results. The CREC riser simulator operates in conjunction with a four-port valve that allows the connectionisolation of the reactor and vacuum box and a six-port valve with a sampling loop for collecting samples of the reaction products. Pressure transducers are installed in both chambers (reactor and vacuum box) to monitor the progress of a reaction run. A pressure profile in the CREC riser simulator for the catalytic conversion of 1,3,5-TIPB is shown in Figure 2. As can be seen from Figure 2, before the injection of reactants into the reactor, the vacuum box is maintained at low pressure (∼1 psia), and the reactor is set at atmospheric pressure. At the time of injection of the reactants into the reactor, there is an abrupt increase in pressure due to the rapid reactant vaporization in the reactor (A-B). Immediately thereafter, a less pronounced increase in pressure is observed, evidencing the progress of reactant cracking into different products via catalytic reactions, producing an expansion in the system (B-C). Once the preset reaction time (3, 5, or 7 s) is completed, the reactor and vacuum box are connected via the switching of a four-port valve, producing the sudden withdrawal of hydrocarbons from the reactor to a vacuum box. This withdrawal leads to a sudden decrease in pressure in the reactor and consequent rapid evacuation and stabilization of the pressure in both chambers (C-D). Given the considerable difference in pressures and volumes between the chambers (the vacuum box is 20 times larger in volume than the reactor), most of the reactor contents are evacuated from the reactor and placed in the vacuum box, under conditions such that further reactions are essentially arrested. Once this operation is completed, a product sample collected in the sampling loop of a six-port valve at the

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4711

Figure 3. Typical pressure profile in the CREC riser simulator for 1,3,5-TIPB catalytic and thermal conversion. Figure 2. Pressure profile in the CREC riser simulator during 1,3,5-TIPB catalytic conversion over the CAT-SC catalyst. Catalyst/ 1,3,5-TIPB ) 5, T ) 550 °C, t ) 5 s. Table 1. Propertiesa of Catalysts with Small (CAT-SC) and Large (CAT-LC) USY Crystallites unit cell size (Å) BET surface area (m2/g) external surface area (m2/g) Na2O (wt %) crystallite size (µm) crystallite density (kg/m3)

CAT-SC

CAT-LC

24.28 169 25 negligible 0.4 825

24.28 197 20 negligible 0.9 825

a Properties reported for USY zeolites after being pelletized and exchanged with ammonium nitrate.

time of product withdrawal is subsequently sent to a gas chromatograph for further analysis. A HewlettPackard 5890A GC allows for the quantification of reaction products using a flame-ionization detector (FID) and a 25-m HP-1 capillary column of cross-linked methyl silicone, with an outer diameter of 0.22 mm and an internal diameter of 0.33 µm. The injector and detector temperatures during experiments were set to 320 °C. The column temperature was programmed from 30 °C for 3 min to 235 °C (heating rate ) 15 °C/min) for 1 min to 300 °C (heating rate ) 40 °C/min) for 20 min. Two standard FCC catalyst samples with similar Si/ Al ratios but different crystallite sizes (0.9- and 0.4-µm diameter), were tested. In the present study, these catalysts are referred to as CAT-LC and CAT-SC, respectively. Such a change in crystal size for constant intrinsic catalytic activity, as demonstrated by Atias et al.21 and fully characterized by Tonetto et al.,22 allows for the ready quantitative evaluation of diffusional constraints. Table 1 reports the main catalyst properties. The unit cell size was determined by X-ray diffraction following method ASTM D-3942-80. Surface area was measured using the BET method. CAT-LC and CAT-SC were prepared following the same procedure, and the resulting catalysts displayed similar activities, as previously reported for the catalytic conversion of 1,2,4-TMB,21 and this despite the difference in crystallite size. An inactive matrix of amorphous silica was used in this study to isolate the effect of the dispersed zeolite

crystallites. This allowed for the transport of the reactant and product species through the macropores of the inert matrix supporting the crystallites without any reaction condition and/or diffusional constraints. The reactivities of these USY catalysts, prepared using the various techniques described above, were measured in the CREC riser simulator using the catalytic conversion of 1,3,5-TIPB as a test reaction. These experiments were carried out at different temperatures, with constant catalyst/1,3,5-TIPB ratio of 5 (weight of catalyst ) 0.81 g, weight of reactant injected ) 0.162 g). An average of four experimental runs were performed for each pair of temperatures and residence times, giving a total of 60 experiments per catalyst. 3. Adsorption Phenomena and Assessment of Adsorption Parameters The catalytic cracking of 1,3,5-TIPB takes place on the active sites of the catalyst. Reactant molecules must diffuse through the pore network of the catalyst and then be adsorbed on the surface of the crystallite prior to catalytic conversion. Adsorption of 1,3,5-TIPB is experimentally evidenced by the differences in concentration between the thermal and catalytic experiments. Figure 3 illustrates these dissimilarities with experiments performed under the same conditions, with and without catalyst loaded into the basket. Also shown in Figure 3 are the high rate of adsorption and the almost instantaneous establishment of equilibrium conditions between the concentration of the reactant adsorbed on the surface and the reactant. Moreover, it is desirable to try to obtain a quantitative relation between the amount of a species adsorbed on the solid surface and its corresponding gas-phase partial pressure. This approach not only provides the framework for describing the extent and strength of molecule adsorption on a zeolite surface, but also provides the basis of a method for representing the kinetics of surface-catalyzed reactions in the CREC riser simulator. Figure 3 displays the total pressure profile for a thermal run, with a vaporization period followed by a constant pressure throughout the remaining time. This shows the lack of thermal conversion when no catalyst is loaded in the CREC riser simulator. Moreover, comparing the pressure attained at the end of the

4712 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

vaporization period for a thermal and a catalytic experiment, the difference between pressures represents the fraction of reactant adsorbed on the catalyst. Thus, in a catalytic run, once the total mass of reactant species i is injected and vaporized, this species is also distributed between the gas phase and on the adsorbed phase catalytic mi,inj ) mi,gas + mi,cat

(1)

A similar equation can be considered during a thermal run. In this case, however, the mass of species i injected is equal to the mass of species i in the gas phase thermal mi,inj ) mi,gas

(2)

Thus, by comparing thermal and catalytic runs carried out under the same conditions, the mass of reactants adsorbed on the catalyst can be calculated catalytic thermal - mi,gas mi,cat ) mi,gas

(3)

thermal being the mass of species i for the thermal with mi,gas catalytic run at the time of complete vaporization and mi,gas being the mass of species i in the gas phase for a catalytic reaction at the time of complete vaporization (point B in Figure 2). Equation 3 can also be expressed in terms of pressures, which is the measured variable, by expressing the mass of species i in the gas phase using the ideal gas law. This approximation is adequate given the low total pressures employed in the present study (15-40 psia).

mi,cat )

(Pthermal - Pcatalytic )VMWi i i RT

(4)

Thus, it is possible to calculate the mass of the reactant adsorbed on the catalyst by performing concurrent thermal and catalytic experiments in the CREC riser simulator under the same conditions. The concentration of species i adsorbed on the catalyst surface (qi) can be defined considering the ratio of the number of moles of species i adsorbed, as given by eq 4, to the mass of catalyst

qi )

ni,cat mi,cat/MWi (Pthermal - Pcatalytic )V/RT i i ) ) mc mc mc (5)

Furthermore, 1,3,5-TIPB adsorption at equilibrium, under the relatively low-pressure conditions of FCC, is expected to display a linear isotherm,23 similar to Henry’s law

qi ) KiCi

(6)

with qi given by eq 5 and representing the number of moles of species i adsorbed per unit mass of catalyst, Ki being the adsorption constant of species i, and Ci catalytic [Ci ) mi,gas /(MWiV)] standing for the concentration of species i in the vapor phase. As shown by Atias et al.,23 adsorption of chemical species at equilibrium under the conditions of FCC reactions can be represented using a relationship analogous to Henry’s law. Gas-phase catalytic cracking occurs at temperatures high enough that only a relatively small fraction of adsorption sites are occupied. Thus, the low

Figure 4. Adsorption constants for 1,3,5-TIPB at different temperatures over (0) CAT-SC and (O) CAT-LC. Error ≈ 15% of the value.

coverage of the available surface area within the catalyst validates the use of a linear relationship to model the adsorption of reactant species. Consequently, the adsorption constant of species i can be expressed as a function of the respective masses of species distributed between the catalyst and the gas phase

Ki )

mi,cat/mc

(7)

catalytic mi,gas /V

or in terms of total pressures

Ki )

(Pthermal - Pcatalytic )V i i

(8)

(Pcatalytic - Patm)mc i

Equation 8 allows for the calculation of adsorption constants at the initial conditions of the catalytic reaction (point B in Figure 2) and for this to be done as a decoupled calculation of the kinetic parameters. The assessment of adsorption constants through eq 8 requires pressure profiles of both the thermal and the catalytic runs (obtained as described in Figure 3), under the same conditions. Moreover, if a well-planned number of experiments are performed at various temperatures, the influence of temperature on the adsorption constants can be assessed as follows

Ki ) Ki0 exp

[

(

- ∆Hi 1 1 R T T0

)]

(9)

with Ki0 representing the preexponential factor in units of m3/(kg of catalyst) and ∆Hi being the heat of adsorption in kJ/mol. A centering temperature T0 (723 K) was used to assess parameters with low cross-correlation. Figure 4 reports adsorption constants for 1,3,5-TIPB over two FCC catalysts at various temperatures. Given the exothermic nature of adsorption and the relationship stated by eq 9, the adsorption constants were expected to decrease with temperature. This trend was observed for both catalysts in the 400-550 °C range. However, values assessed at 350 °C showed a dramatic decrease of the adsorption constants compared to the values at 400 °C. This inconsistency indicates the

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4713

Figure 5. Arrhenius plot for 1,3,5-TIPB adsorption over (0) CATSC and (O) CAT-LC. Table 2. Heats of Adsorption and Adsorption Constants for 1,3,5-Triisopropylbenzene over CAT-SC and CAT-LC ∆Hi (kJ/mol) Ki0 × 103 [m3/(kg of crystallite)]a a

CAT-SC

CAT-LC

-66.57 ( 6 19 ( 5

-69.32 ( 6 22 ( 5

T0 ) 450 °C.

inadequacy of eq 8 for evaluating adsorption constants at those temperature levels. The observed anomaly, namely, the decrease of the amount of 1,3,5-TIPB adsorbed in the catalyst as the temperature is decreased from 400 to 350 °C, was attributed to diffusional constraints. In fact, the 1,3,5TIPB has a molecular size close to the size of the window opening of the USY zeolite. Thus, the diffusion of 1,3,5TIPB along the pore network of the crystallites, affected by the continuous interaction between molecules and pore walls, is of the configurational diffusion type. This type of diffusion is known to be a strongly temperatureactivated process. In this particular case, lower temperatures lead to lower 1,3,5-TIPB diffusivities and, according to eq 8, to a lower apparent adsorption constant. Thus, for assessing adsorption constants in the 350-450 °C range over the USY zeolite, an alternative method is required given the diffusivity constraints of 1,3,5-TIPB. Figure 5 displays the Ki Arrhenius plot for 1,3,5-TIPB. A regression was performed, considering the values of adsorption constants at 550, 500, and 450 °C, only when diffusional constraints did not affect the gathered data. A good indication of the adequacy of the method is the similar heats of adsorption obtained for the two crystallite sizes, as reported in Table 2.

from models that exclude chemical reaction. These limitations, acknowledged by Ruthven et al.,24 can be of questionable applicability. Even more, limited data are available for making effective comparisons of the diffusivities of the USY zeolites determined in this study with data from other studies.25 For instance, USY and NaY zeolites are, from a structural point of view, quite different materials with mesopores being much more significant in USY zeolites. Mesopores are formed in USY zeolites as a result of a steaming pretreatment process leading to zeolite dealumination. Thus, one should not be surprised to find larger effective diffusivities in USY than in NaY zeolites. Consequently, there is a need to circumvent this uncertainty by directly assessing effective diffusivities, under reaction conditions, with a model based on governing equations adequate for combined diffusionreaction situations. This approach is, in fact, followed in the present study, and it is believed that this methodology should provide an adequate estimation of the effective diffusivity and intrinsic kinetic parameters. The effectiveness factor and modulus of a solid catalyst are terms that were introduced by Thiele26 to show the effects of particle size, porosity, and diffusivity of a solid particle on reaction rates. If the effectiveness factor of a pellet were unity, the entire solid would be equally accessible for the reactant molecules. However, in the case of the catalytic conversion of 1,3,5-TIPB, 1,3,5-TIPB transport in the USY zeolite structure might have an influence, and the effectiveness factor can be assessed using the following equation

ηss )

tanh(h′) h′

where ηss is the effectiveness factor and h′ is the modified Thiele modulus, which is defined as

h′ )

x

KikintFcrφint Deff

1 aext

Numerous studies on diffusion in zeolites have been reported, however there is still significant uncertainty on the estimation of hydrocarbon effective diffusivities in zeolites. This is particularly true given that the calculated effective diffusivities are in many cases the result of a precarious extrapolation process. It should be noted, as well, that values of effective diffusivities are often based on the results of calculations coming

(11)

where aext ) 3/Rcr is the specific external surface area of the USY crystallite, Deff is the effective diffusivity of 1,3,5-TIPB in the USY intracrystalline network, φint is the intrinsic catalyst activity decay function, and kint is the intrinsic kinetic constant for the first-order catalytic conversion of 1,3,5-TIPB. As discussed in previous sections, the transport of the reactant and product molecules in zeolites takes place in the “configurational diffusion regime”. This regime of diffusion is characterized by a strongly temperatureactivated process, which presents a dependency on temperature of the Arrhenius type as follows

[ (

Deff ) Deff0 exp 4. Modeling Diffusivity and Reactivity Phenomena

(10)

)]

- ED 1 1 R T T0

(12)

Equation 10 is based on several simplifying assumptions, such as (a) uniform site density, (b) small external versus internal surface area, (c) uniform crystallite deactivation, and (d) steady-state conditions. Although assumptions a-c would be considered adequate,22 the lack of applicability of assumption d might introduce error. In fact, unsteady zeolite operation in the CREC riser simulator can be described as follows

4714 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

-

(

| )

∂Ci,in V dCi,ex ) Deff Wcr dt ∂r

r)Rcr

3 RcrFcr

with Ci,ex representing the concentration of 1,3,5-TIPB outside a crystallite and Ci,in being the concentration of 1,3,5-TIPB within the pore network of a crystallite. The above equation hypothesizes that the chemical reaction, with the catalyst matrix being inert, takes place in the USY zeolite crystallites only. As described in the Experimental Section, this is an applicable assumption for the 60-µm catalyst pellets of this study. Moreover, modeling of the cracking reaction, as given in eq 13, has to be complemented by the solution of a partial differential equation in the zeolite crystallite itself

(KiFcr + )

(

)

∂Ci,in Deff ∂ 2 ∂Ci,in + KikintCi,in ) 2 r (14) ∂t ∂r r ∂r

In addition, the modeling of catalytic cracking in the CREC riser simulator requires a number of initial and boundary conditions, which consider instantaneous vaporization (t ) 0, Ci,ex ) Ci,ex|t)0), symmetric concentration profiles inside the crystals (r ) 0, ∂Ci,in/∂r ) 0), and negligible transport limitations around the 60-µm particles and inside the matrix (r ) Rcr, Ci,in ) Ci,ex). Equation 14 expresses the change in concentration along the crystallite (assumed spherical shape) as a consequence of both accumulation and chemical reaction. Although, in principle, both contributions have to be taken into account, considering a worst-case scenario for the catalytic conversion of 1,3,5-TIPB over USY catalysts, Al-Khattaf et al.27 showed that the contribution of the unsteady-state term in eq 14 is significant for reaction times between 0 and 0.5 s only. Consequently, given that the set of catalytic experiments were carried out for reaction times of 3 s and longer, the following conservative hypotheses can be adopted: (a) an unsteady process in the bulk gas phase of the reactor and (b) a quasi-steady-state process inside the zeolite crystallites. This quasi-steady-state approximation within the crystallites validates the use of eq 10 to assess the effectiveness factor. Because the CREC riser simulator is a constantvolume batch reactor unit, operated isothermally, one can postulate a material balance equation similar to eq 13 on the basis of a measured or average 1,3,5-TIPB reaction rate

V dCi ) ηssri Wcr dt

-

(13)

(15)

with V being the volume of the reacting mixture, Wcr the mass of USY crystallite present in the reacting system, and ri the reaction rate if not slowed by pore diffusion (i.e., without diffusion resistance). Because the USY crystallites within the catalyst include a welldefined porous network where most of the active catalytic sites are present, an effectiveness factor, ηss, must be included to account for the effect of the pore diffusion resistance on the overall rate of reaction. Furthermore, assuming a first-order cracking reaction with respect to the adsorbed phase, justified in following sections, and considering that the ideal gas law holds, the design equation for the CREC riser simulator can be expressed as follows

V dyi ) ηssKikintφintyi Wcr dt

(16)

where yi is the mass fraction of species i, kint is the intrinsic kinetic parameter, and φint is the intrinsic decay function that takes into account the deactivation of the catalyst. Coke is formed as a secondary reaction product, and even though its yield is not large, its effect in the deactivation of the catalyst is significant, given the drastic decrease in acidity and loss of activity in the catalyst. Thus, the deactivation function, φi, can be expressed in terms of coke concentration as originally proposed by Froment.28 As a result, a stoichiometric relationship can be established, as demonstrated in AlKhattaf et al.,27 between the amount of reactant and amount of coke produced. This allows for the use of a “reactant conversion” model. This meaningful deactivation function has been successfully tested for the catalytic cracking of 1,3,5-triisopropylbenzene27 and cumene.29 For the case of the catalytic cracking of 1,3,5TIPB, this decay function can be expressed as φint ) exp[-λ(1 - yi)] and eq 16 can be rewritten as follows

V dyi ) ηssKikintyi exp[-λ(1 - yi)] Wcr dt

(17)

with λ being a deactivation parameter. Equation 17 can be further expressed in terms of temperature by substituting eq 9 and the expression

[

kint ) kint0 exp

)]

(

- Eint R 1 1 R T T0

giving

V dyi ) ηssKi0kint0 × Wcr dt

[

exp -λ(1 - yi) +

(

)]

- (∆Hi + Eint R ) 1 1 y (18) R T T0 i

Thus, the catalytic conversion of 1,3,5-TIPB in the CREC riser simulator can be modeled using a set of two equations, namely, eqs 18 and 10, with these equations representing material balances of species i in the bulk phase and within the crystallites, respectively. The present kinetic model does not consider the effect of coke on either diffusion or adsorption processes. Adsorption and diffusion parameters are not assumed to depend on the amount of coke produced. This assumption is based on results published by Atias et al.23 who found that adsorption constants and available surface area remain somewhat constant at low levels of coke. Given that the adsorption parameters (Ki0 and ∆Hi) were already calculated in a previous section of this study, two diffusion parameters (Deff0 and ED) and three intrinsic kinetic parameters (kint0, Eint R , and λ) need to be assessed to fully characterize the adsorptive-diffusive-reactive system. This sequential methodology of parameter estimation leads to adsorption parameters with smaller spans than the intrinsic kinetic parameters because of the propagation of errors. In this respect, the

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4715

Figure 6. 1,3,5-TIPB thermal conversion for 7-s residence time and various temperatures.

following section of this study describes the methodology used to calculate this set of five parameters.

Figure 7. 1,3,5-TIPB conversion at different temperatures over CAT-LC. Catalyst/1,3,5-TIPB ) 5: (]) 3, (0) 5, and (4) 7 s.

5. Catalyst Activity and Selectivity 1,3,5-TIPB catalytic experiments were performed in the CREC riser simulator over FCC catalysts with different crystallite sizes (CAT-SC and CAT-LC). Experiments were carried out at a catalyst/1,3,5-TIPB ratio of 5; residence times of 3, 5, and 7 s; and temperatures of 350, 400, 450, 500, and 550 °C. An average of four experiments were performed for each set of conditions, totalling 60 experiments per catalyst. 5a. Activity of the Catalysts. In principle, both catalytic and thermal effects can influence the overall 1,3,5-TIPB conversion. This significant matter affecting the validity of eq 17 was considered in a preliminary phase of this study. In this respect, a set of kinetic runs was conducted with no catalyst loaded in the reactor at temperatures of 350, 450, and 550 °C and a residence time of 7 s. It was found (Figure 6) that the 1,3,5-TIPB thermal conversions were smaller than 1 wt % for temperatures below 450 °C and ∼6 wt % at 550 °C and 7 s. Given that, at 550 °C and 7 s, the overall conversion with catalyst loaded in the unit is in excess to 87% of that found for thermal cracking, the contribution of thermal conversion was neglected, and eq 17, including only the catalytic conversion, was accepted as a valid description of the chemical changes. Then, the conversion of 1,3,5-TIPB was studied with catalyst now loaded in the CREC riser simulator. Changes of the 1,3,5-TIPB conversion with temperature, at different residence times, over CAT-LC are reported in Figure 7. It can be noticed that the conversion of 1,3,5-TIPB varies significantly for temperatures below 450 °C, ranging from 24.8 to 42.4 wt %, for 7-s catalytic reactions at 350 and 450 °C, respectively. However, for temperatures above 450 °C, the 1,3,5-TIPB conversion increases slightly, going from 42.4 to 45 wt %, for 7-s catalytic reactions at 450 and 550 °C, respectively. Similar observations can be made for the 1,3,5-TIPB conversions attained over CAT-SC (Figure 8), with this stabilization of conversion occurring at 400 °C. Thus, given that the temperature dependency of reactions, in general, is determined by their activation energies, the results reported in Figures 7 and 8 can be explained by considering a change of the controlling reaction step. The change in controlling regimes in this case occurs from diffusionally controlled at low temperatures to

Figure 8. 1,3,5-TIPB conversion at different temperatures over CAT-SC. Catalyst/1,3,5-TIPB ) 5: (]) 3, (0) 5, and (4) 7 s.

kinetically controlled at higher temperatures. This transition of regimes, peculiar to the catalytic conversion of hydrocarbon in zeolites,27 is validated and is in agreement with the results obtained for the adsorption constants, as discussed in a previous section. Figure 9 compares the catalytic conversions of 1,3,5TIPB attained over CAT-SC and CAT-LC at 3 and 7 s. It is apparent that at higher temperatures, the conversions attained with the two catalysts are quite similar. These data confirm that, at high temperatures, the influence of intracrystalline transport process on the overall reaction rate is lower. Therefore, the diffusion of 1,3,5-TIPB molecules within the USY pore network is enhanced at the higher temperatures, with the intrinsic rate determining the overall rate of reaction. As discussed in the Experimental Section, the FCC catalysts of this study were prepared using similar amounts of USY zeolite (∼30 wt %). Moreover, the acidities of the two catalysts were measured by the adsorption of pyridine22 and the catalytic cracking of 1,2,4-TMB21 under conditions free of the effects of diffusional limitations. Both studies concluded that CAT-SC and CAT-LC displayed similar acidities and similar 1,2,4-TMB conversion activities. Thus, acidity variations between CAT-SC and CAT-LC cannot be

4716 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

Figure 9. 1,3,5-TIPB conversion at various residence times and temperatures over CAT-SC and CAT-LC. Catalyst/1,3,5-TIPB ) 5: (]) 3 s with CAT-LC, (4) 7 s and CAT-LC, ([) 3 s with CATSC, (2) 7 s with CAT-SC.

invoked to justify the 1,3,5-TIPB conversion differences observed here (Figure 9) at lower temperatures ( 0.5 s, by

V dyi ) ηssKi0kint0φint × Wcr dt

[

(

)]

-(∆Hi + Eint R ) 1 1 y (19) exp R T T0 i with

φint ) exp[-λ(1 - yi)] ηss ) tanh(h′)/h′

and

/

h′ ) RcrxKikintFcrφint/Deff 3 6a. Assessment of Intrinsic Kinetic Parameters. Regarding the two postulated regimes determining the overall reaction rate, for temperatures greater than 500 °C, the overall rate of conversion is controlled via intrinsic kinetic and is not hindered by 1,3,5-TIPB diffusion within the USY crystallites. Therefore, under these conditions, the effectiveness factor becomes equal to 1, and eq 19 can be simplified to

V dyi ) Ki0kint0 × Wcr dt

[

exp -λ(1 - yi) +

(

)]

- (∆Hi + Eint R ) 1 1 y (20) R T T0 i

Because the ∆Hi and Ki0 parameters were determined independently (refer to section 3), the intrinsic kinetic parameters (kint0, Eint R , λ) of eq 20 can be regressed for each of the catalysts studied using a nonlinear regression of the experimental data of 1,3,5-TIPB conversions at temperatures of 500 and 550 °C and various residence times. The function of residuals is minimized using the

4718 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 3. Intrinsic Kinetic Parameters for 1,3,5-TIPB Catalytic Conversion over USY Catalysts CAT-SC

CAT-LC a

95% CFL

95% CFL kint0 (1/s)b Eint R (kJ/mol) λ a

5.33 73.29 4.98

7.18 37.5 3.97

CAT-SC + CAT-LC

4.89 73.3 6.03

3.87 21.08 2.54

95% CFL 5.09 73.29 5.47

5.31 28.46 3.21

95% confidence limit. b T0 ) 450 °C.

Table 4. Diffusion Parameters for 1,3,5-TIPB Catalytic Conversion over USY Catalysts CAT-SC

CAT-LC

95% CFLa 1.76 Deff0 (m2/s) × 1013 b ED (kJ/mol) 165.89 a

CAT-SC + CAT-LC

95% CFL

3.07

0.82

67.11

113.11

0.2 11.43

95% CFL 0.81

0.47

123.42

26.42

95% confidence limit. T0 ) 450 °C. b

large-scale algorithm based on the interior-reflective Newton method described in Coleman and Li.31 Table 3 reports the intrinsic kinetic parameters obtained for CAT-SC, CAT-LC, and CAT-SC + CAT-LC, along with their corresponding 95% confidence limits. The values obtained for each catalyst showed good consistency, and this agreement was expected given the special care taken to have two catalysts with similar acidities and similar structural properties. Furthermore, it can be noticed that the correlation matrix displayed low cross-correlation between the regressed parameters, indicating proper evaluation of the intrinsic kinetic constants. Moreover, a quick review of the parameters obtained show a catalytic conversion reaction with an intrinsic activation energy in the 70 kJ/mol range. It is interesting to note that this 70 kJ/mol intrinsic activation energy is added (eq 20) to the heat of adsorption of about -68 kJ/mol. This addition yields 2 kJ/mol and explains the low observed activation energies at the higher temperatures (T > 500 °C). 6b. Assessment of Diffusivity Parameters. For the assessment of effective diffusivity parameters, eq 19 was considered at temperatures below 500 °C. Given all the other parameters (kint0, Eint R , λ), Deff0 and ED were determined in independent experiments using eq 19 and nonlinear regression. The function of residuals is minimized using the large-scale algorithm based on the interior-reflective Newton method described in Coleman and Li.31 Table 4 reports the values for these diffusivity parameters, displaying a ∼120 kJ/mol energy of activation, which is consistent with a strongly temperatureactivated diffusional process. Only a few studies have been carried out on the adsorption and diffusion of chemical species in the context of FCC processes.5,23 The results of the present study are in general agreement with the findings of others; however, the importance of the results reported here cannot be denied given that (a) the adsorption parameters are obtained under relevant FCC reaction conditions and (b) the assessment of the adsorption parameters is decoupled from that of the intrinsic kinetic parameters. Therefore, typical limitations in adsorption/diffusion studies using low-activity zeolitess low temperatures, model compounds with low reactivities,24,25 and/or simultaneous regression of numerous parameters32sare overcome.

Figure 13. Residual distribution for nonlinear regression considering three sets of data: (0) CAT-SC, (O) CAT-LC, and (*) CATSC + CAT-LC.

6c. Checking the Adequacy of Conversion Predictions. Figure 13 displays the residual distribution of model-predicted and experimental 1,3,5-TIPB mass fractions for three set of data: CAT-SC, CAT-LC, and CAT-LC + CAT-SC, using the seven model parameters as determined in this study. A normal distribution of residuals was observed, and this validates the adequacy of both the parameter values and the modeling technique adopted. 7. Conclusions The following are the conclusions of the present study (i) A heterogeneous kinetic model for the catalytic cracking of 1,3,5-TIPB including intrinsic kinetic, adsorption, and diffusion phenomena, applicable in the context of the CREC riser simulator, was established. The results point toward a cracking reaction with an intrinsic activation energy of 70 kJ/mol. (ii) Adsorption constants for 1,3,5-TIPB on USY catalyst were assessed under catalytic cracking reaction conditions. The exothermic nature of 1,3,5-TIPB adsorption was confirmed, with a -68kJ/mol heat of adsorption. (iii) Evaluated effective diffusivities in the 10-13 m2/s range with a ∼120 kJ/mol energy of activation suggest a strongly temperature-activated diffusional process, with configurational diffusion dominating the catalyst operation at temperatures below 500 °C. (iv) The formation of intermediates and final products is influenced by species diffusion constraints in the pore network and, as a result, by the size of the crystallite. Acknowledgment We express our appreciation to the Fundacion Gran Mariscal de Ayacucho, which supported J.A.A. with a postgraduate scholarship during the course of this research. We are also very grateful to the Natural Sciences and Research Council of Canada for financial support. Notation aext ) specific external surface area (m-1) Ci ) concentration of species i in the vapor phase (mol/m3) Ci,ex ) concentration of species i outside the USY crystallite (mol/m3) Ci,in ) concentration of species i within the pore network of the USY crystallite (mol/m3)

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4719 Deff ) effective diffusivity of 1,3,5-TIPB in the USY zeolite (m2/s) Deff0 ) preexponential factor for diffusion (m2/s) ED ) activation energy for diffusion (kJ/mol) Eint R ) intrinsic activation energy for reaction (kJ/mol) h′ ) modified Thiele modulus Ki ) adsorption constant for species i [m3/(kg of catalyst)] Ki0 ) preexponential factor in the adsorption constant for species i [m3/(kg of catalyst)] kint ) intrinsic kinetic constant (s-1) kint0 ) preexponential factor for reaction (s-1) mc ) mass of catalyst (kg) mi,inj ) mass of species i injected (kg) catalytic mi,gas ) mass of species i in the gas phase at the moment of complete vaporization in a catalytic run (kg) thermal mi,gas ) mass of species i in the gas phase at the moment of complete vaporization in a thermal run (kg) mi,cat ) mass of species i adsorbed on the catalyst at the moment of complete vaporization (mol) MWi ) molecular weight of species i (kg/mol) ni,cat ) moles of species i adsorbed at the moment of complete vaporization (mol) Pthermal ) partial pressure of species i at the moment of i complete vaporization in a thermal run (psia) Pcatalytic ) partial pressure of species i at the moment of i complete vaporization in a catalytic run (psia) Patm ) atmospheric pressure (14.7 psia) qi ) concentration of species i adsorbed on the surface (mol/ kg) R ) universal gas constant [8.314 J/(mol K)] Rcr ) crystallite radius (m) r ) radial coordinate (m) ri ) kinetic rate of consumption of species i {mol/[(kg of crystallite) s]} t ) time (s) T ) reactor temperature (K) T0 ) centering temperature (723.15 K) V ) reactor volume (m3) Wcr ) mass of crystallites dispersed in the catalyst (kg) yi ) mass fraction of species i in the vapor phase yi,exp ) mass fraction of species i in the vapor phase yi,pred ) mass fraction of species i in the vapor phase Subscripts i ) 1,3,5-TIPB Greek Letters  ) USY crystallite porosity ∆Hi ) heat of adsorption of species i (kJ/mol) ηss ) effectiveness factor Fcr ) density of the USY zeolite crystallite (kg/m3) φint ) intrinsic catalyst activity decay function λ ) deactivation parameter, RC model Abbreviations CAT-LC ) catalyst prepared with large zeolite crystallites CAT-SC ) catalyst prepared with small zeolite crystallites 1,3,5-TIPB ) 1,3,5-triisopropylbenzene 1,3-DIPB ) 1,3-diisopropylbenzene

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Received for review October 23, 2003 Revised manuscript received February 20, 2004 Accepted February 25, 2004 IE034197Q