Diffusion Analysis of Cumene Cracking over ZSM5 using a Jetloop Reactor

Studies in Surface Science and Catalysis 133 G.F. Fromentand K.C. Waugh (Editors) ~) 2001 Elsevier Science B.V. All rights reserved.

465

Diffusion Analysis of Cumene Cracking over ZSM5 using a Jetloop Reactor P. Schwan, P.J. Henry and K.P. Mrller* Department of Chemical Engineering, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa. Cumene is cracked in a recycle reactor over commercial H-ZSM5 extrudates. A Thiele modulus approach is used to determine the diffusion coefficient and the intrinsic rate constant. The results are compared to those obtained from pulse experiments. A linear model for diffusion, adsorption and reaction rate is applied for reactants and products. In contrast to literature it is argued that if the Thiele modulus is greater than five, the system becomes over parameterised. If additionally adsorption dynamics are negligible, only one lumped parameter can be extracted, which is the apparent reaction constant found from steady state experiments. The pulse experiment of cumene is strongly diffusion limited showing no adsorption dynamics of cumene. However, benzene adsorbed strongly on the zeolite and could be used to extract transient model parameters which are compared to steady state parameters. 1. INTRODUCTION In reactions occurring within porous catalysts the reactants have to adsorb and diffuse to and from the active centres. Typically this diffusion-sorption behaviour is measured under inert conditions at temperatures well below that at which reactions occur. It is preferable to study catalysts under reaction conditions. Such data can be interpreted by reaction-diffusionadsorption models, the so-called "Thiele modulus" approach [ 1]. The model parameters can be determined by using steady state or transient measurements. Thiele's analysis [1], has been applied under steady state conditions by a number of workers (Haag et al. [2], Weisz [3], Voogd and van Bekkum [4], Garcia and Weisz [5]) to estimate diffusion in zeolite systems under reaction conditions. However, it is necessary to vary crystallite or pellet size in order to estimate the diffusion coefficient. In the case of zeolite catalysts, this is extremely difficult to accomplish without changing the intrinsic reaction behaviour. Furthermore this method cannot distinguish between adsorption and intrinsic reaction constants. Transient techniques have been developed [6-9] which allow the estimation of rate parameters using gradientless reactors. A number of workers [7,8] claim that all model parameters can be derived from transient experiments by zero, first and second moment analysis. This paper investigates the simultaneous estimation of reaction, adsorption and diffusion coefficients for cumene cracking over ZSM-5. Transient and steady state data are compared. 2. EXPERIMENTAL The Jet-Loop is an internal recirculation reactor (volume = 50ml), which approximates CSTR behaviour [10]. Argon (Ar) (>99.995%) was used as carrier gas flowing through the jet at 175 to 500 ml/min (STP). H-ZSM5 extrudates (Stidchemie T4480, Si/Al=25, 50 vol%

Corresponding author" [email protected]

466 binder) were used. Reactions were carried out at atmospheric pressure, n-decane (20ml/min, 0.1 kPa in argon) was fed to the effluent as internal standard.

2.1

Steady State Cumene (0.25 kPa) diluted in Argon was fed with a constant flow of 20 ml/min (STP) into the reactor, while the flow rate of the jet stream was varied. Samples were taken with a syringe and analysed in a GC. Carbon mass balances were better than 95%. The extrudates were crushed to three different sizes (diameter = 0.15, 0.085, 0.065 cm). The reactions were carried out for 3 hrs. The extrudates were calcined at 480~ in air for 12h between experiments. Kinetic studies with the powder form (0.5 gm) were measured in a plug flow reactor at a constant contact time (WHSV=0.7 g cat/(g hr) ) 2.2

Pulse Experiments

0.5gl of liquid cumene was injected into an injector port heated to X X 300~ and flushed with 20ml/min Ar. For the 0.1 o sorption studies of o benzene, 150 gl of 0.01 ! vapour was injected. The effluent gas was 0.001 analysed with an 0 10 20 30 40 50 60 automated rotating t[s] Multi-Ampoule-Sampler Fig 1 9Benzene blank response curves at a residence time of system (MAS), where 2.2s (o) and 5.8s (A,x) measured by the Multi-Ampoulepre-evacuated ampoules Sampler technique compared to online FID ( ). were broken, sealed and later analysed in a GC. The fastest sampling time was ls. Fig 1 shows that the online FID and MAS are in excellent agreement. Thus the MAS can be used with confidence to analyse reaction data. Further details regarding the MAS is in preparation. 3. MODEL Model assumptions include the following : (i) The adsorbent has an uniform bidisperse pore structure, (ii) The pellets have spherical geometry, (iii) The reactor behaves like a CSTR, (iv) Ideal pulse input, (v) Macropore diffusion is Fickian, (vi) No external film resistance, (vii) Linear equilibrium, (viii) First order irreversible reaction, (ix) The crystals are small (<0.2 ~tm) agglomerates and diffusion resistance in these can be neglected. The following differential mass balances for species i result : Macropore mass balance:

(1) Reactor mass balance:

467

-Ci(Ry)-

3VcatD---------Li0Ci[ = Vrctr C-----L[ d FRy ~ Ry,t F dt Ry,t

(2)

Equilibrium: qi =HiCi

(3)

The boundary conditions of the parabolic PDE are given by the symmetry and the continuity at the boundary. The steady state and transient conversion are equal and can be written as follows oO

Vcat / F- k intr (1 - e p )I; c F [Cr(t)dt= X ss - 1 - Vrctr ; 1 + Veat / F . kintr (1- IZp)eerl

(6)

where 1"1is the effectiveness factor and with the Thiele modulus given by q)2 = kr(1-~Zp)~zcHr Ry2 = ~:pDr

kintr(1-Cp)~Ze R 2y epDr

3 (~ coth ~o- 1) r/= ~-5-

and

(7)

(8)

The system of PDEs was solved by using 49 collocation points with the LSODE [13] package as Integrator. For the nonlinear multi-response parameter estimation GREGPAK[ 14] was used.

3.3 Parameter uniqueness for reaction with strong diffusion limitation The zero, first and second moment of the transient system can be calculated via the Laplace transform [7,8]. With a Thiele modulus greater than five, e.g. strong diffusion limitation, the hyperbolic functions in the moments equations tend to their asymptotic values. It can be shown theoretically that the moments become linearly dependent and the number of model parameters reduces to two, where : DrHr = constant and kr. If additionally the effect of adsorption on the dynamic response becomes negligible, the number of independent parameters, which are needed to describe the response curve, reduces to one : DrHrkr = constant. The latter case can therefore not extract more parameters than could be obtain from a steady state measurement. 4. RESULTS AND DISCUSSION Table 1 summarises the steady state experiments over the zeolite crystals and pellets. The activation energy for the intrinsic rate constant kintr was 34.4 kJ/mol. The low value indicates that the adsorption enthalpy is of the same order as the reaction enthalpy, i.e. (Em,n,obs = Ek~ + EAds). The diffusion coefficient under steady state conditions was found to be an order of magnitude higher than that calculated from Knudsen diffusion (2x10 3 cm2/s) using an average pore size of3.8x 10-7 cm measured by BET and a tortuosity factor of 4. The estimated

468 diffusion coefficient, within experimental error, was approximately constant from 350 to 440~ as expected for Knudsen type diffusion. Table 1 9 Model Parameters for steady state operation "pellets and crystals T[~ kintr[l/s] q~ Drx 103[cm2/s] 350 505 13 23 400 820 18 19 425 1020 19 19 440 1250 22 18 Table 2 9 Model Parameter Estimates with 95% confidence intervals based on a logarithmic probability function of the parameters 9T= 440 ~ Diameter=0.15cm, F=410ml/min(STP) Low Estimate Estimate High Estimate mass catalyst 0.2g 3.6 Dr x 103 [cmVs] 0.52 1.4 4330 kintr[1/s] 600 1610 q~ 100 mass catalyst 0.5g 3.6 D, x 103 [cmVs] 0.52 1.6 14525 kintr[I/s] 1740 5030 q~ 165 mass catalyst 3.0g 12 Dr x 103 [cmVs] 1.7 4.5 7821 kintr[I/s] 543 2060 q~ 63

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o.oo~4,5 o.oo~5

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The adsorption coefficients of benzene on the ZSM-5 extrudates were determined by the first moment of the pulse experiments [ 10]. As indicated in Fig.2, benzene exhibited a strong adsorption with a high adsorption enthalpy of 130 kJ/mol. This strong adsorption behaviour at zero occupancy is well known from literature [11] and heats of adsorption close to 100 kJ/mol are reported for laboratory ZSM-5 crystals.

i n [1,K]

Fig 2 9Arrhenius plot of the adsorption constant of benzene on ZSM-5 extrudates.

Fig's 3 to 5 show the comparison between experiment and model for the pulse measurements of cumene at different conversions. Propene diffuses very fast and is weakly adsorbed and therefore its response curve depends on the parameters of cumene. Apart from experimental errors of the cumene response curve close to C/Co=10 "3, the cumene response curve shows only one time constant. With these approximations the analysis of the propene response curve reveals that the cumene reaction takes place in the strong diffusion limitation regime with no adsorption

469

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Cu'mene Prq;:lene Benzane

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tbe 0.01

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Fig 3 9Comparison of the pulse experiment vs model fit. Mass of catalyst 0.2g. Xss = 0.20.

dynamics. Thus, as argued in Section 3.1, only one lumped parameter can therefore be estimated from both response curves. In order to extract meaningful physical parameters, additional assumptions have hence to be made. It was therefore assumed that the diffusivities of all three components are, according to Knudsen diffusion, related to each other by the square root of their molecular weights. It was also assumed that the adsorption behaviour of benzene was not affected by the reaction. With these assumptions, using the adsorption

coefficients of benzene from Fig 2, leaves only two parameters viz. kr and Prq;:lene v Benzene Dr , for the non-linear least square fitting of all three concentration curves 0.1 ~ simultaneously. It can be seen from Fig's 3-5 that the model describes the experimental curves well. Notice that the cumene tail is not well represented. 0.01 Blank runs with cumene also show tailing at these concentrations and thus the fit is within the observed errors. 0001 m m, Notice also that with increased mass 0 10 20 30 40 50 60 7O (i.e. increased conversion) the benzene Fig 4" Comparison of the pulse experiment vs tail increases significantly. Blank model fit. Mass of catalyst 0.5g. Xss = 0.59. experiments in Fig 1 show no tail for benzene and thus this represents an 1 ,~. . . . , ,, , excellent response which can be , , ...-,,,r'""en@ , Prqaen@ analyses with confidence. Benzene Furthermore, as the conversion increases it becomes more difficult to 0.1 measure and analyse the response of cumene. The estimated diffusivities are in 0.01 good agreement to the theoretical Knudsen diffusivity. However, the confidence intervals of the parameter 0.001 L , I estimates show a strong variance, and 0 10 20 30 40 50 610 70 thus reveal that the parameters can 1Is] only be estimated with a high degree of Fig 5 9Comparison of the pulse experiment vs inaccuracy, although they are within an model fit. Mass of catalyst 3.0g. Xss = 0.90 order of magnitude of the estimates from steady state data. Surprisingly, the estimated diffusivity from transient experiments is considerably closer to the theoretical Knudsen diffusivity than that of the steady state I

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470 measurements. Another surprising result is that the effectiveness factor is an order of magnitude larger for the transient analysis. From this analysis it must be concluded that the steady state results are more reliable, However, the question still remains as to why the transient data deviate significantly. Unfortunately, the data analysis presented here is currently not reliable enough to make clear statements as to the source of the deviation. Work is in progress to improve the interpretation of the data. LIST OF SYMBOLS C

Concentration [mol/1] in the Vrctr Volume reactor (50ml) macropores Concentration [mol/1] in the crystal D Diffusion coeffcient [cmVs] q conversion F Flow rate [ml/min] Xss Thiele modul eq.(7) H Henry adsorption [/] effectiveness factor kr first order rate constant [ I/s], eq(1) rl Pellet porosity = 0.32 kintr krHr [ l/s] ~:p percentage of cat in solid material Vcat Volume of catalyst [ml] ec Index i represents component i; Index r represents the reactant REFERENCES 1. Thiele, E.W., "Relation between catalytic activity and size of particle", Ind. Eng. Chem., 37,916,1939 2. Haag, W.O., R.M. Lago, and P.B.Weisz, "Transport and Reactivity of Hydrocarbon Molecules in a Shape Selective Zeolite", Faraday Discuss. Chem. Soc., 72,217,1981 3. Weisz, P.B., "Molecular Diffusion in Microporus Materials : Formalisms and Mechanisms", Ind. Eng. Chem. Res., 34,2692,1995 4. Voogd, P., and H. van Bekkum, "The Adsorptive and Reaction Limiting Diffusion of 2,3Dimethylbutane in large crystals of Aluminated Silicalite-l", Ind. Eng. Chem. Res., 30,2123, 1991 5. Garcia, S.F., and P.B. Weisz, "Effective Diffusivity in Zeolites 2 : Experimental Appraisal of Effective Shape Selective Catalysis", J. Catal., 142, 691, 1993 6. Post, M.F.M., "Diffusion in Molecular Zeolite Sieves", Studies in Surface Science and Catalysis, 58, Chpt 11, van Bekkum et al. (Eds.), Elsevier, Amsterdam,1991. 7. Kelly, J.F., and O.M. Fuller, "Parameter Estimation in Heterogeneous Catalytic Reactions", Can J. Chem. Eng., 50,534, 1972 8. Schobert, M.A., and Y.H. Ma, "Isomerisation of Cyclopropane on Synthetic Faujasite by Pulse Technique- I, Mathematical Model", J. Catal. 70, 102, 1981a 9. Park, S.H., and Y.G. Kim, "The Effect of Chemical Reaction on Effective Diffusivity within Biporous Catalysts - I", Chem. Eng. Sci., 39(3), 523, 1984a 10. Miro, E.E., D.R. Ardiles, E.A. Lombardo, and J.O. Petunchi, "Continuous-Stirred Tank Reactor (CSTR) Transient Studies in Heterogeneous Catalysts", J. Catal., 97, 43, 1986 11. Mrller, K.P., and C.T. O'Connor, "The Measurement of Diffusion and Adsorption using a Jetloop Recycle Reactor", Studies in Surface Science and Catalysis, 84B, J. Weitkamp et al. (Eds), Elsevier, Amsterdam, 1204, 1994 12. Karger, J. and Ruthven, D.M., "Diffusion in Zeolites", 1991, p.471, Wiley&Sons, New York 13. Hindmarsh, AC, "ODEPACK, a systematized collection of ode solvers in scientific computing", RS Stepleman et al. (eds.), north-holland, amsterdam, 1983, pp. 55-64. 14. GREGPAK, Stewart and Associates Engineering Software Inc, Madison, Wisconsin, USA