Diffusion analysis of cumene cracking over ZSM5 using a jetloop reactor

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Diffusion Analysis of Cumene Cracking over ZSM5 using a Jetloop Reactor P. Schwan, K.P. M/511er and P.J. Henry 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, in which a linear model is used yielding diffusion, adsorption and reaction rate coefficients for reactant and products. Accurate total gas analysis at ls intervals was possible using a new Multi-Ampoule-Breaker technique. The effective intrinsic rate constants found by both methods were in good agreement, while the diffusivity of the steady state experiment was one order of magnitude higher. Benzene adsorbed unexpectedly strongly on the zeolite. 1. INTRODUCTION Typically in porous catalysts diffusion-adsorption behaviour is measured under inert conditions at temperatures well below that at which reactions occur. The extrapolation of these parameters to reaction conditions has become a topic of debate [1]. It is therefore preferred to study catalysts under reaction conditions. Such data can be interpreted by reaction-diffusion-adsorption models, the so-called "Thiele modulus" approach [2]. The model parameters can be determined by using steady state or transient measurements. Thiele's analysis [2], has been applied under steady state conditions by a number of workers (Haag et al. [3], Weisz [4], Voogd and van Bekkum [5], Garcia and Weisz [6], Swabb and Gates [7]) to estimate diffusion in zeolite systems under reaction conditions. It has been shown [3,5,6,8] that if adsorption is accounted for in the Thiele model, the diffusion coefficients measured during hexane cracking over ZSM5 zeolite approximate those extrapolated from low temperatures reasonably well, notwithstanding the fact that different techniques have been used in each case. However, steady state techniques cannot distinguish all physical phenomena i.e. reaction, adsorption and diffusion without the variation of the particle/pellet diameter or some other physical property. It is therefore necessary to make measurements over a range of particle and/or crystal sizes to evaluate the diffusion coefficients for a particular catalyst. In the cases of industrial pellets it is desirable to make measurements on uncrushed samples [9]. Garcia and Weisz [6] have shown that by converting a suitable second order reaction into a pseudo first order reaction it is possible to estimate the diffusion effects without the variation of crystal size. Such an analysis is limited to special reactions in which it can be verified that the variation of the second component has no influence on the diffusion and adsorption behaviour. Transient techniques have been developed [10-14] which allow the estimation of rate parameters using gradientless reactors. A number of workers [11-13] have observed that all model parameters can be derived from transient experiments by zero, first and second

2730 moment analysis. Transient measurements require rapid sampling techniques to obtain good response curves of all the components and thus accurate parameter estimates. This paper investigates the simultaneous estimation of reaction, adsorption and diffusion coefficients for cumene cracking over ZSM-5. Transient data are compared to steady state analysis. 2. E X P E R I M E N T A L The Jet-Loop is an internal recirculation reactor (50ml), which approximates CSTR behaviour [15]. Argon (Ar) (>99.995%) was used as carder gas flowing through the jet at 175 to 500 ml/min (STP). H-ZSM5 extrudates (Sildchemie T4480, Si/AI=25, 50% binder) were used. Reactions were carded out at atmospheric pressure, n-decane (20ml/min, PC10H22 =0. l kPa) was fed to the effluent as internal standard. 2.1 Steady State Cumene diluted in Ar (PC6HI2 = 0.25kPa) was fed with a constant flow of 20 ml/min (STP) into the reactor, while the jet stream was varied. Samples were taken with a syringe and analysed in a GC using an FID. Carbon mass balances were >95%. The extrudates were crushed to three different sizes (diameter = 0.15, 0.085, 0.065 cm). The reactions were carded out for 3 hrs. The extrudates were then calcined at 480~ in air for 12h. Kinetic studies with the powder form (0.5 ~tm) were measured in a plug flow reactor at a constant contact time (WHSV=0.7 g cat/(g hr) ) 2.2 Pulse Experiments 0.5~tl of liquid cumene was injected into an injector port heated to 300~ and flushed with 20ml/min Ar. The effluent gas was analysed with an automated rotating Multi-Ampoule-Breaker system (MAB), where pre-evacuated ampoules were broken, sealed and later analysed in a GC. The fastest sampling time was Is. Fig. 1 shows that the FID and MAB are in excellent agreement. Thus the MAB can be used with confidence to analyse reaction data. Further details regarding the MAB is in preparation.

X

0.1

.

.

.

.

X

.

.

.

.

.

.

.

o (.3 r 0.01

...............

% 0.001

-0

10

\.~

20

30

40

50

t[s] Fig. 1 CSTR response curve at a residence time of 2.2s (o) and 5.8s (A,x) measured by the Multi-Ampoule-Breaker technique comparerd to FID-online ( ~ ) .

60

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3. M O D E L The model is based on the following assumptions: 9 The adsorbent has an uniform bidisperse pore structure 9 The pellets have spherical geometry 9 Reactor behaves like a CSTR 9 Macropore diffusion according to Fick's law, no external film resistance 9 Linear equilibrium 9 First order reaction Macropore mass balance: (1)

~'e - - ~ + (1 - 6 e )z c - - ~ = -(1 - o.'!,)o~ck Rq . + - - ~ - - ~ y Reactor mass balance: - C i ( R Y) -

3Vc.,Di 8Ci I = V.c,r dCi FRy Oy ~Ry,, F dt ky,,

(2)

Equilibrium: (3)

qi = H i C i

The initial conditions are C i ( y < Ry ,t < O) = 0

For the reactant: C r (Ry, t < O) = 1 ;

products: C,(Ry, t < O) = 0

(4) (5)

The boundary conditions of the parabolic PDE are given by the symmetry and the continuity at the boundary. The model was solved using a 9 point collocation and Gears' integration method. The parameters Hi, kR, Oi were estimated by fitting the model to the experimental data using a BFGS minimisation program.

The steady state and transient conversion are equal and can be written as follows ao

Xss - 1

Vc., / F .

_ VrctrF~oCr (t)dt = 1 + V..

kintr (1 -

e'p )~'c r/

, where rl is the effectiveness factor (6)

/ F . ki.tr (1 - s 1, )6 c r/

with the Thiele modulus tp 2 = k R ( 1 - ~ ' e ) ~ oepD R

r/=~-35- (~ocoth ~p- 1)

Ry2

=

kintr(1-6/,)~ 6t, D R

Ry2

and

(7)

(8)

2732 4. RESULTS AND DISCUSSION

In Table 1 the results for steady state and pulse experiments are summarised. The activation energy for the intrinsic rate constant kintr estimated from the powder data of Table 1, was 34.4 kJ/mol. The low value indicates that the adsorption enthalpy is of the same order as the reaction enthalpy, i.e. (ERxn,obs = Ekr + EAds). The diffusion coefficient under steady state conditions was found to be one order of magnitude smaller than that calculated from Knudsen diffusion using an average pore size of 3.8xl 0 -7 cm measured by BET. A tortuosity factor of 4 was used. Furthermore, the diffusion coefficient was approximately constant from 350 to 440~ as expected when Knudsen diffusion is dominant. Fig. 2 and 3 show the pulse experiments with a catalyst loading of 0.2 and 3g respectively. It can be seen that the same model parameters describe both experiments well, although the conversion ranged from 20 to 80%. However, the experimental product curves are significantly steeper in the short time region than the calculated model curves. A reason for this might be that diffusion in the initial part takes place mainly in the bigger pores of the pellets, which could as well explain the large discrepancy between steady state diffusion and Knudsen diffusion. This is reflected by the transient Thiele modulus of 68 compared to 22 at steady state, while the deviation of both intrinsic rate constants was within 10%. In addition the transient diffusivity agrees well to the expected Knudsen diffusion. From eq.(6) same conversions for steady state and transient experiments are expected for a linear system. The pulse experiment, however, showed a substantially lower conversion of 23% opposed to 41% measured at steady state, at 440~ and 0.2g of catalyst. Same conversions are not expected in a non-linear system and thus the validity of the first order rate law, especially in the initial region must be investigated. It is also possible that the lower conversion is due to coking during transient experiment on a fresh catalyst, causing not all of the compounds to exit the system and thus producing errors in the response curve normalisation. It is worth noting that benzene adsorbed strongly even at high temperatures as opposed to propene. The error analysis of the optimisation is beyond the scope of this paper and must be addressed in a future paper. LIST OF SYMBOLS C D F H kR kintr Wear

Concentration [mol/l] in the macropores Diffusion coeffcient [cm2/s] Flow rate [ml/min] Henry adsorption [/] first order rate constant [ 1/s], eq(1 ) kRHR [l/s] Volume of catalyst [ml]

Vrctr

Volume reactor (50ml)

q Xss q) rI Ep ec

Concentration [mol/1] in the crystal conversion Thiele modul eq.(7) effectiveness factor Pellet porosity = 0.32 percentage of cat in solid material

Index i represents component i; Index r represents the reactant REFERENCE: 1. Chen, N.Y., T.F. Degnan(Jr), and C.M. Smith, "Molecular Diffusion and Reaction in Zeolites", VCH, 1994

2733 2. Thiele, E.W., "Relation between catalytic activity and size of particle", Ind. Eng. Chem., 37,916,1939 3. 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 4. Weisz, P.B., "Molecular Diffusion in Microporus Materials 9Formalisms and Mechanisms", Ind. Eng. Chem. Res., 34,2692,1995 5. Voogd, P., and H. van Bekkum, "The Adsorptive and Reaction Limiting Diffusion of 2,3Dimethylbutane in large crystals of Aluminated Silicalite-1", Ind. Eng. Chem. Res., 30,2123, 1991 6. Garcia, S.F., and P.B. Weisz, "Effective Diffusivity in Zeolites 2 : Experimental Appraisal of Effective Shape Selective Catalysis", J. Catal., 142, 691, 1993 7. Swabb, E.A., and B.C. Gates, "Diffusion, Reaction and Fouling in H-Mordenite Crystals", Ind. Eng. Chem. Fund., 11,4,540,1972 8. 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. 9. Sie, S.T., "Advantages, Possibilities and Limitations of Small Scale Testing of Catalysts for Fixed Bed Processes", ACS Symposium Series,634,6-41, 1996 10. Kelly, J.F., and O.M. Fuller, "Parameter Estimation in Heterogeneous Catalytic Reactions", Can J. Chem. Eng., 50,534, 1972 11. Schobert, M.A., and Y.H. Ma, "Isomerisation of Cyclopropane on Synthetic Faujasite by Pulse Technique - I, Mathematical Model", J. Catal. 70, 102, 1981 a 12. Schobert, M.A. and Y.H. Ma, "Isomerisation of Cyclopropane on Synthetic Faujasite by Pulse Technique - II, Experimental Study"J. Catal. 70, 111, 1981b 13. 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 14. 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 15. M611er, 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

Table 1: Model Parameters: Steady State vs Transient

Steady State

T[~ 350 400 425 440

Kintr[1/s]

tp

D [cmZ/s]

H

Cumene 505 820 1020 1250

13 18 19 22

0.023 0.019 0.019 0.018

. . . 3300 530 (3)*

* estimated not measured DKnudsen= 0.002cm2/s

Unsteady State Kintr[1/s] Cumene . . . . . . . . . 0.403 1330 Benzene Propene -

KR[1/s]

D[cm2/s]

0.0017 0.0094 (0.01)*

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1.000

0 ,.Q L_

m 0.100

0

am=

0

,4,,I

0=

I1.1110

L_

0

0.001

v x

0

10

20

~

t[s]

1

30

40

50

Fig. 2. Response curve of the Cumene reaction on 0.2 g of H-ZSM5 extrudates, diameter = 0.15 cm, measured at a flowrate of 290ml/min (STP) at 440~ model, 9 cumene, A benzene, O propene

tO ..Q L_

cg

0

0.1

m

cu 0 c 0 .C2 L. cu

0.01

r

0.001 0

20

40

60

t[s]

80

100

120

Fig. 3. Response curve of the Cumene reaction on 3.0 g of H-ZSM5 extrudates. Notation and model parameters as for Fig. 2.