Ind. Eng. Chem. Res. 2007, 46, 7927-7935
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Assessment and Modeling of Adsorption Selectivities in the Transport of Mixtures of Hydrocarbons in FCC Catalysts Adolfo M. Avila,† Claudia M. Bidabehere,‡ and Ulises Sedran*,† Instituto de InVestigaciones en Cata´ lisis y Petroquı´mica, INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2654, (S3000AOJ) Santa Fe, Argentina, and Instituto de InVestigaciones en Ciencia y Tecnologı´a de Materiales, INTEMA (FI, UNMdP-CONICET), Juan B. Justo 4302, (B7608FDQ) Mar del Plata, Argentina
Adsorption experiments with up to 10 s contact time in a batch adsorber with a ternary mixture of hexane, decane, and toluene at 250 °C and a binary mixture of hexane and 1-hexene at 160 °C were used to assess equilibrium and apparent adsorption selectivities and diffusion selectivities in an equilibrium commercial FCC catalyst, and to confirm the predictions of a zeolite diffusion-adsorption model based on the MaxwellStefan formulation in combination with an isotherm according to the ideal adsorbed solution theory (IAST). Hexane was the reference hydrocarbon, and the decane and toluene apparent selectivities observed in the mixture were higher than those of the individual cases. The predictions of the apparent adsorption selectivities were close to the experimental values and higher than those for diluted conditions. The stronger adsorption of decane and the interactions between the various species within the zeolite particles contributed positively to decane overall transport in comparison to the other hydrocarbons in the mixture. Such interaction effects can be clearly visualized in the simulation of the transient adsorbate concentration profiles inside the zeolite particles. A similar behavior was observed in the case of the olefin in the hexane/hexene mixture. Introduction Essentially, all catalytic and separation processes based on zeolitic materials are related to the behavior of multicomponent systems, and according to the diversity of new materials, there is a growing need of studying the transport properties of mixtures inside microporous solids. The interest in understanding multicomponent fluid-solid contact focuses on either the experimental information obtained in the lab or the search for adequate models that could be used for macroscopic engineering calculations.1,2 However, despite its importance, the transport of mixtures through zeolitic solids has only recently received more substantial attention. This can be assigned to the difficulty of experimentation with mixtures as well as to the exponential increase in complexity of simulations with a large number of components.3 The mass transport of different molecules within zeolites is significantly influenced by their sorption and diffusion characteristics, therefore impacting on chemical reactions. However, multicomponent mass transfer in catalytic and separation processes is often faced considering transport models that assume homogeneous structures and that are usually associated with diluted solutions. Although some experimental data are available on adsorption of pure hydrocarbons for various zeolites, the same is not true for mixture sorption data. Moreover, diffusion in zeolites depends strongly on the pore diameters, the structure of the pore walls, the interactions between the surface atoms and the diffusing molecules, the configuration of the diffusing molecules, the way the channels are interconnected, and, most importantly in mixtures, on adsorbate-adsorbate interactions.4 In the case of the catalysts used in the process of fluid catalytic cracking (FCC), the knowledge of multicomponent * To whom correspondence should be addressed. Tel.: +54(342)452-8062. Fax: +54(342)453-1068. E-mail:
[email protected]. † Instituto de Investigaciones en Cata ´ lisis y Petroquı´mica. ‡ Instituto de Investigaciones en Ciencia y Tecnologı´a de Materiales.
transport can assist modeling, simulation, and design of either the reaction or the stripping processes. The multicomponent transport is associated with the simultaneous existence of diffusion, adsorption, and reaction. In the riser reactor, the kinetics of hydrocarbon reactions not only depend on the differences in the intrinsic rates but also on the differences of adsorbate concentrations on catalyst particles. In the stripper, the yield and selectivities of hydrocarbons desorbed are the result of not only the existing differences in individual equilibrium adsorption but also in adsorbate-adsorbate interactions among the species. It is to be noted that commercial FCC catalysts are compound, with Y zeolite, amorphous matrix, binder, filler, and additive components present.5 It is the objective of this work to study multicomponent transport phenomena of hydrocarbons on equilibrium, zeolitebased commercial FCC catalysts at a moderate temperature of 250 °C, regarding the experimental information obtained from unsteady-state adsorption processes of simple mixtures of hydrocarbons. Experimental Section Apparatus. The adsorption of hydrocarbons was assessed using a laboratory CREC riser simulator6 as a batch adsorber. The unit, which was designed specifically for studies of FCC issues, was used under a modified configuration of the setup, which is shown in Figure 1. The basic design concept of the unit considers that a small slice of an ideal FCC riser, comprising the mixture of catalyst particles and hydrocarbons that “see” each other while moving along the riser after being put into contact, can be located into a batch device with internal recirculation. In this way, residence time and position along the riser are equivalent to contact time in the laboratory unit. An impeller rotating at very high speed over the chamber that keeps the catalyst between two metal porous plates induces the internal circulation of the gas phase in an upward direction through the chamber, thus fluidizing the catalyst. Before the
10.1021/ie070435w CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007
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Figure 1. CREC Riser Simulator setup configuration.
injection of the hydrocarbons, the batch adsorber is filled with inert nitrogen at atmospheric pressure. When the system is at the desired experimental conditions, the hydrocarbon mixture is fed through an injection port, and immediately after the preset contact time is attained, the closing valve VC opens during a very short time period of 0.1 s and allows one to extract a small portion of the gas phase to a sampling loop under vacuum in valve VM, to be sent afterward to analysis by gas chromatography to determine its composition. It is accepted that this operation does not perturb the catalyst-adsorbate system. After sampling, the selecting valve VS conducts the extraction flow to a hot vacuum chamber to end the evacuation step. All the valves are actuated automatically with high-speed pneumatic devicessthe valves switch in 40 ms. Pressure in the adsorber and the vacuum chamber is recorded instantly with INVENSYS (Honeywell) digital sensors. In the adsorber, an ASDX100 model was used with a 0-100 psi range and 0.040 V/psi sensitivity. An ASDX030 model (0-30 psi range and 0.133 V/psi sensitivity) was used in the vacuum chamber. The accuracy (0.1 psi) of the devices was verified with a Hg column for the pressure readings in the adsorber and a vacuum test gauge in the vacuum chamber. Adsorption Experiments. The experiments of adsorption were done with a ternary mixture of n-hexane (C6), n-decane (C10), and toluene (TOL), with molar relationships 64.2:12.0: 23.8. Such a composition was selected with the aim of obtaining similar adsorbate concentrations on the catalyst for the various hydrocarbons, according to experimental information gathered by Avila et al.7 The mixture was injected at 250 °C over an equilibrium commercial FCC catalyst (specific surface area, 125 m2/g; zeolite load, 19.6%; average particle size, 70 µm; zeolite crystal size, 0.7-1.2 µm; rare earth oxides, 2.94%; Ni+V, 10 900 ppm; MAT activity, 63%). The volumes injected ranged from 0.1 to 0.2 mL, and the catalyst masses ranged from 0.9 to 1 g. The contact time in all the experiments was 10 s. Complementary experiments were performed with a hexane/1hexene mixture with molar relationship (96.9:3.1) at 160 °C. These particular molar ratio and temperature values were selected in order to avoid chemical reactions of the highly reactive olefin. However, isomerization occurs at these conditions, and c- and t-2-hexene isomers were produced. Consequently, all the isomers were considered as the same species and quantified as linear olefins with six carbon atoms per molecule. The sample analyses were done by means of standard capillary gas chromatography on an HP6890Plus GC chromatograph equipped with a flame-ionization detector and a crosslinked methyl silicone column. Calculation of Adsorbate Concentrations. The concentrations of adsorbates were assessed by means of
qji )
(mfeedωfeed - mgpωgp i i ) MimCat
(1)
where mfeed, the mass of hydrocarbons injected, is calculated with the known volume and density of the liquid hydrocarbon. The mass of hydrocarbons in the gas phase mgp at the final contact time was calculated with the ideal gas law and the average molecular weight M h of the mixture. The liquid feed composition ωfeed was known through precise weighing, and i the gas-phase mass fraction concentrations ωgp i were obtained by means of the chromatographic analysis of the corresponding sample. Mass balances were performed after each experiment by comparing the mass of hydrocarbons injected with that recovered after the experiments, with agreements being over (1%. Solution of the Proposed Model. The numerical solution of the partial differential equations of the model proposed for the diffusion-adsorption process was achieved using the method of lines (MOL), which converts partial differential equations into a system of ordinary differential equations.8 The spatial discretization was done according to a second-order nonlinear Galerkin-based method. Both the discretization and the resulting ordinary differential equations (ODEs) represented by matrices were calculated with intrinsic codes in Matlab 6.5.1. The calculation of the thermodynamic correction factor matrix was based on the approach of Krishna and Baur9 for generating a highly efficient code. The zeolite particle diameter was assumed to be 1 µm. Results and Discussion Apparent Adsorption Selectivity. The apparent adsorption constant for a particular component can be defined as
K app ) i
qji Pi
(2)
In the case of individual adsorption processes, K app would be i coincident with the Henry constant provided the isotherm is linear and there is no limitation in the mass transport of the adsorbate inside the solid particle. In a very diluted system, the mass transport process inside the solid catalyst particles can be described by means of the linear driving force concept, LDF,10 and the linear Henry expression for the adsorption isotherm. Consequently, the rate of change of the volume-averaged concentration of a species i in the solid, for conditions under which no chemical reaction proceeds, can be expressed as
dqji D0,i ) 15 2 (qi,Rp - qji) dt R
(3)
p
If it is considered that the system has a low adsorption capacity Vsol (expressed by the parameter R ) K ),11 then the concentraVgp tion of the species i in the gas phase could be considered
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Figure 2. Apparent adsorption selectivities at 250 °C; symbols: decane, pure (9), in mixture (0); toluene, pure (2), in mixture (4). The selectivities for pure hydrocarbons were taken from Avila et al.7
Figure 4. Comparison between simulated (proposed model and Henry isotherm) and experimental values of the apparent adsorption selectivities of decane; temperature, 250 °C. Table 1. Individual Langmuir Isotherm Parameters and Diffusivities for n-Hexane, n-Decane, and Toluene at 250 °C; From Avila et al.7 zeolite hydrocarbon
qi,s (mol/kg)
bi (1/psi)
D0,i × 1011 (cm2/s)
C6 C10 TOL
1.29 0.93 1.60
0.11 3.02 0.31
1.70 0.28 0.79
Table 2. Diffusion Selectivities for Mixture C6/C10/TOL 64.2:12.0:23.8 (molar) at 250 °C hydrocarbon
P (psi)
dif S i,j
C10
17.6 13.7 9.2 17.6 13.7 9.2
0.81 0.86 0.68 0.52 0.65 0.52
TOL
Figure 3. Comparison between equilibrium adsorption selectivities of decane and toluene according to IAST (full lines) and Henry (dotted lines) isotherms and apparent adsorption selectivities; temperature, 250 °C; symbols: decane (0) and toluene (4).
constant in the time period between the injection of the mixture (t0) and the end of the experiment (tf), and eq 3 can be integrated between those limits. Since it is always possible to define a correcting factor Fi relating volume-averaged and external concentrations of the adsorbed hydrocarbon
Fi )
qji qi,Rp
(4)
(
qji D0,i ) 1 - exp -15 2 tf qi,Rp Rp
)
qi,Rp ) Ki Pi
The value of the volume-averaged concentration in the solid is
qji ) Ki Pi Fi
(7)
(8)
Then, selectivities can be defined as follows app S i,j
eq S i,j )
dif S i,j
(6)
0.59
qji Ki Pi Fi ) qjj Kj Pj Fj
(5)
At the outer surface of the particle, because of the inexistence of mass transfer restrictions as secured by the characteristics of mixing in the reactor,12 the concentration of the species i is
0.24
and the relationship between two species i and j is
it results that
Fi )
dif S i,j |dil
)
K app i K app j
(apparent adsorption selectivity)
(9)
Ki (equilibrium adsorption selectivity, Kj also called “separation factor”) (10)
Fi ) ) Fj
( (
D0,i
1 - exp -15 1 - exp -15
Rp2 D0,j Rp2
t t
) )
(diffusion selectivity) (11)
Finally, eq 8 can be rewritten as app eq dif ) S i,j S i,j S i,j
(12)
This means that the apparent adsorption selectivity depends on a factor associated with the equilibrium adsorption processes
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Figure 5. Comparison between simulated (proposed model and Henry isotherm) and experimental values of the apparent adsorption selectivities of toluene; temperature, 250 °C.
and another one associated with the transport of hydrocarbons inside the catalyst particles. The definition of eq 12 is also applicable to simultaneous diffusion and nonlinear adsorption processes under nondiluted conditions. However, in that case, the nonlinear characteristic eq and transport of the process may modify both equilibrium S i,j dif S i,j factors in response to changes of operation variables. This means that, as opposed to a process under diluted conditions, the changes in total gas pressure or concentration may affect app apparent adsorption selectivities S i,j . For instance, equilibrium selectivities can depend on total pressure and concentration accordingly to the nonlinearity of the adsorption isotherm. Also, the differences in the adsorption capacities at saturation, qi,s, may modify equilibrium selectivities with changes in operation variables. Similarly, changes in the diffusion selectivities can be produced according to their dependence on adsorption isotherms and adsorbate-adsorbate interactions, as is shown below. Mixture versus Individual Adsorption Process. The diffusion-adsorption processes of individual hydrocarbons in a FCC catalyst were studied by Avila et al.7 assuming that they
are significant only within the zeolite component. Langmuir and diffusion parameters were obtained and selectivities were analyzed to show that the rank of adsorption constants was decane > toluene > hexane and that zero-coverage diffusivities were ordered hexane > toluene > decane (see Table 1). It can be observed in Figure 2 that neat increases of the apparent adsorption selectivities calculated after eq 9 were observed in the mixture for decane (particularly) and toluene, in comparison to the individual adsorption processes.7 It is interesting to learn about the comparison between apparent adsorption selectivities for decane and toluene in the mixture and the corresponding equilibrium adsorption selectivities. In this sense, an adequate multicomponent adsorption isotherm model is required to describe the equilibrium for the various hydrocarbons over the zeolite. In this work, the equilibrium adsorption selectivities for this ternary mixture were evaluated according to the ideal adsorbed solution theory (IAST),13 by making use of the individual isotherm parameters obtained in the previous work.7 Details of the set of equations describing the IAST model can be found in papers by van den Broeke et al. ,14 Krishna et al. ,15 and, more recently, Yu et al.16 Such an adsorption isotherm model, besides describing the multicomponent equilibrium from pure-component parameters, is able to consider the effects on adsorption selectivities related to the different saturation capacities of the various hydrocarbons. For this particular case of a ternary mixture of hydrocarbons with single-component adsorption isotherms satisfactorily represented by Langmuir models, the IAST model allows one to obtain a relationship between the multicomponent fractional coverages (θi) and the gas-phase partial pressures (Pi). As is shown in Figure 3, the observed apparent selectivities for decane and toluene are lower than those of equilibrium predicted by the IAST isotherm. This model shows the equilibrium selectivity for decane decreasing with total hydrocarbon pressure. On the contrary, it increases slightly with pressure for toluene. Such behavior can be associated with entropic effects arising in multicomponent adsorption in microporous materials of species with different saturation capacities;15 it is to be expected that higher pressures can make smaller molecules (hexane in this case) pack better in the zeolite cages, thus
Figure 6. Transient profiles of adsorbed concentration of hexane in the zeolite particle; individual adsorption process, Pi,0 ) 6.3 psi, mCat ) 0.9285 g.
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Figure 7. Transient profiles of adsorbed concentration of hexane in the zeolite particle; ternary adsorption process, 0.1 mL of mixture C6/C10/TOL 64.2: 12.0:23.8 (molar) with mCat ) 0.9457 g.
Figure 8. Typical pressure profile in an experiment, as recorded (full line) and simulated by the model (dotted line); hexane ) 0.05 mL, temperature ) 250 °C.
increasing their adsorption and inducing a decrease in S eq C10,C6. The opposite would apply to the pair toluene-hexane. It can be noticed in Figure 3 that, as expected, the equilibrium selectivities according to the IAST isotherm tend to the equilibrium selectivities predicted by the Henry model as pressure approximates to zero, that is, in very diluted systems eq S i,j |dil )
Ki qi,sbi ) Kj qj,sbj
(13)
According to eq 12, diffusion selectivities can be obtained with experimental values of the apparent selectivities and those of the equilibrium selectivities calculated from the IAST isotherm. The values of the diffusion selectivities are shown in Table 2, where they are also compared with those values obtained for diluted conditions from eq 11. It can be seen that there is a strong increase in the diffusion selectivity for decane in comparison to diluted conditions. This indicates an enhancement of the overall transport of decane in comparison to the other
hydrocarbons in the mixture. The comparison of individual and mixture equilibrium adsorption isotherms of the various hydrocarbons is shown in the following section. Modeling the Mass Transport within the Catalyst. The multicomponent diffusion-adsorption process in the catalyst particles can be adequately described by a model based on the Maxwell-Stefan formulation,17 considering the zeolitic component of the catalyst as the characteristic space. The assumption of mass transfer resistances located only within the zeolite component of the catalysts was justified by Avila et al.7 by the comparison of the diffusion time constants for hexane in the matrix (τp) and the zeolite (τc), respectively.18 The relationship between the constants τp/τc is
