Kinetic model of nitrobenzene hydrogenation to aniline over industrial copper catalyst considering the effects of mass transfer and deactivation

Applied Catalysis, 59 (1990) 31-43 Elsevier Science Publishers B.V., Amsterdam -

31 Printed in The Netherlands

Kinetic Model of Nitrobenzene Hydrogenation to Aniline over Industrial Copper Catalyst Considering the Effects of Mass Transfer and Deactivation L. PETROV*, K. KUMBILIEVA and N. KIRKOV Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, Sofia 1040 (Bulgaria) (Received 15 June 1989, revised manuscript received 26 October 1989)

ABSTRACT A kinetic model was developed of nitrobenzene hydrogenation to aniline over industrial copper catalyst, considering the effects of mass transfer and deactivation. The model describes experimental data with good accuracy. Based on the model, a rapid method for estimating the working time of industrial catalyst for aniline production is described.

INTRODUCTION

Gas-phase catalytic hydrogenation of nitrobenzene is a principal industrial method for preparation of aniline. A current account of the various methods of aniline manufacture and the catalysts used is given in refs. 1 and 2. Most commonly copper catalysts supported on kieselguhr have been applied. In a number of cases they are promoted by various additives, such as magnesium, calcium, zirconium, thorium, vanadium or chromium [ 3,4]. So far, quite a few kinetic studies of nitrobenzene hydrogenation to aniline have been published [ 5-151. The proposed kinetic models are empirical and are not related to a definite reaction mechanism. Most workers have established that the reaction is of fractional order in nitrobenzene and hydrogen. Some authors [ 91 affirm that the reaction rate is retarded by aniline. However, the majority of investigators have found that nitrobenzene is responsible for the decrease in reaction rate. Tsutsomi and Sada [lo] have suggested a kinetic scheme for the process with a nickel catalyst according to which nitrobenzene and aniline adsorption occurs on one type of site whereas hydrogen is absorbed on a different type of site. Under the conditions used in industry, the process proceeds over catalysts which are exposed to a comparatively fast deactivation. In view of this, the effects of deactivation and mass transfer should be thoroughly considered in

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0 1990 Elsevier Science Publishers B.V.

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making a kinetic model of the process. The effects of diffusion and deactivation on the reaction over a nickel catalyst have been studied [ 16,171 and a model, accounting for these factors, has been suggested. Deactivation of copper catalyst has been studied [l&19] and kinetic equations for deactivated catalyst have been proposed. In a previous work [ 201 a kinetic model of nitrobenzene hydrogenation to aniline has been developed which gives account of the catalyst deactivation taking place in the kinetic region. Under special conditions i.e. high LHSV values of nitrobenzene, low nitrobenzene-to-hydrogen ratios, and a partial deactivation of the catalyst, oscillation of reaction rate, heat production and selectivity has been observed [ 211. In the present study a kinetic model of nitrobenzene hydrogenation to aniline is presented in which the effects of heat and mass transfer and of catalyst deactivation on the reaction rate are considered. EXPERIMENTAL

Catalyst This study was performed with a kieselguhr-supported copper catalyst promoted by chromium and nickel (M 19, Neftochim, Bulgaria). The BET area of the catalyst was 119 m2 g-l while the pore volume and the mean pore radius were 0.19 cm3 g-l and 3.25. 10W7cm, respectively. The effective diffusion coefficient of nitrobenzene in the catalyst pellets D,,at 563 K, measured by a gaschromatographic method, was 2.63 cm2 h-l [ 221. Apparatus Experiments were conducted in an all-glass flow circulating system at atmospheric pressure between 503 and 563 K. The temperature of the catalyst bed was kept constant by means of an electronic thermoregulator with an accuracy of & 1 K. Liquid nitrobenzene was admitted to the reactor by a Gilson model 302 pump, the error of the set flow-rate not exceeding 1%. Hydrogen and argon were monitored in the reactor by a differential flow regulator with an accuracy of 3%, supplied by the Plant for Scientific Instrumentation at the Bulgarian Academy of Sciences. Analysis Analysis of the reaction products was carried out on a Tsvet 104 gas chromatograph equipped with a flame ionization detector which was directly connected with the kinetic system by means of a six-way valve. The circulation loop and the six-way valve, as well as the line to the gas chromatograph, were kept at 433 K. Separation of the reaction products was performed by means of

33

a Z-m column filled with Chromaton N-AW-DMCS treated with 3% of KOH, on which Apiezon L 10% was deposited at 473 K. The flow-rate of the carrier gas, argon, was 30 ml min-I. Procedure The catalyst was placed in the reactor and dried in a stream of argon for two hours at 393 K. After that the temperature was slowly increased in hydrogen flow to 473 K. Further, the catalyst bed was kept at the same temperature for three hours. Finally the temperature of the reactor was raised to 523 K and after three hours the catalyst was ready for use. Catalyst reduction Reduction is of great importance for reliable working of the catalyst. Since it is a highly exothermal process, its correct conduction in industrial reactors is retarded. Differential scanning calorimetry studies have shown that the heat of catalyst reduction is independent of temperature in the 523-673 K range and is equal to 116 cal g-l. Regeneration of a deactivated sample caused a slight increase in the heat of reduction attaining the value of 134 cal g-l [ 231. Catalyst deactivation In order to prepare deactivated samples for a short time, the experiments were conducted at nitrobenzene LHSV values between 2 and 9 h-l. These values are from 6 to 18 times higher than those used with the best industrial catalysts. The feed molar ratio between hydrogen and nitrobenzene was from 3 : 1 to 8: 1 which is far less than that used in industry (from 20: 1 to 50 : 1). Nitrobenzene partial pressure was varied between 4. lo3 and 1.6~10~Pa whereas hydrogen was applied in the 1.6*10’-9.06*105 Pa range. Regeneration of deactivated samples Deactivated samples were regenerated by water vapours to remove coke deposits from the surface and by burning the remaining surface carbonaceous species with oxygen. The following procedure was used. After stopping the feeds of nitrobenzene and hydrogen, the system was passivated in argon flow for one hour at the temperature of reaction. Further, water vapour was admitted to the reactor at 473 K and the temperature was raised slowly to 573 K. This treatment lasted three hours at constant temperature. Finally a mixture of oxygen and argon was passed over the catalyst bed at the same temperature. The initial concentration of oxygen was 1 vol.-%. After the first hour the concentration was gradually increased to 10 vol.-%. Oxygen treatment was applied for a period of three hours. The heat of copper oxidation in the catalyst was dependent on temperature. It was 136,177 and 185 cal g-l at 523,573 and 673 K, respectively [ 231.

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Data processing Data processing as well as computation of the rate constants and discrimination between the kinetic models were performed by means of a software package KINETICS [251. Kinetic model of the reaction in the absence of diffusion limitations and deactivation Fig. 1 shows the dependence of the conversion of nitrobenzene on contact time for different temperatures and compositions of the reaction mixture. The reaction rate versus the partial pressure of nitrobenzene for various temperatures is plotted in Fig. 2 [ 261. Special experiments indicated that the reaction rate was not retarded by the

Fig. 1. Dependence of the degree of conversion of nitrobenzene (NB ) to aniline on contact time, W/F, for NB+hydrogen+argonreactionmixture of (a) 1:1:5 and (b) 1: 1O:Oat (1) 503 K, (2) 523 K and (3) 563 K.

0

E 2

4

6

a

10312

p,,JO,@~

Fig. 2. Relationship between the reaction rate, r, and the partial pressure of nitrobenzene, pnb, for NB-to-H,=l:lOat (1)523Kand(2)563K.

35

reaction products (aniline and water), while nitrobenzene acted as a poison. In all the experimental runs no side products were detected in the exit reaction mixture. The experimental data can be described by the following kinetic scheme in which steps 1 and 5 are reversible [ 261. The other three steps are irreversible: (1) C,H,NO,+

(2) [C,H,NO,K]

[K] = [C6K5N02K] +H20 +H+

[C,H,NOK]

+HzO

(3 ) [ C&H,NOK] + HZ+ [ CBHBNHOHK] (4) [C6H6NHOHK] +H2+

[C,H,NH,K]

(1) +H,O

(5) [ CBHjNH2K] = C6H6NH2 + [K] where [K] are free sites on the catalyst surface and [ RiK ] are various surface intermediates. The rates of the steps indicated in Scheme 1 are given below: r+i=k+&&)

r_1=K_l&

r+3=k+382PH

rx3=0

r+4=k+4&pH

r-,=0

r+5=k+5&

r_-5= k-&pan

(2)

where 0, is the free surface; &, & 8, and 0, are fractions of the surface occupied by the surface intermediates which participate in reaction scheme 1. p&, pan andp, are the partial pressures of nitrobenzene, aniline and hydrogen, respectively. According to the kinetic scheme, hydrogen from the gas phase successively joins the nitrobenzene molecule. Since the suggested scheme is linear, to derive the kinetic equation we applied a method based on the graph theory, proposed earlier by Petrov and Shopov [27]. The following expression was obtained: rO=

k$nbpH 1 +kzPnb

(3)

Data procession by using eqn. 3 gave a good description of the experimental rates with the following values of the kinetic constants: ky = 2.403*103,E, = 43 260 J mol-l, kz = 6.805.10p4, E, = 34 484 J mol-l. The mean deviation of the experimental from calculated values of the reaction rates was 20.3%. Kinetic model of nitrobenzene hydrogenation to aniline considering the catalyst deactivation in the kinetic region So far, a few studied on the deactivation and regeneration of copper catalysts for nitrobenzene hydrogenation to aniline have been published [ 18,19,31].

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However, this problem is of certain practical interest because the common industrial catalysts work for lo-15 days to 3-4 months, after that they must be regenerated. The prevent the effect of diffusion, the experiments were carried out over catalyst grains of size 0.7 mm. Under these conditions the Weisz’ criterion, estimated according to the formula [ 241: R2-r

(41

‘= D,gC

had the value of 1.14. In this formula R is the radius of the catalyst grain in cm, r is the reaction rate 3.74~10~” mol h-’ cmP3, D,ris the effective diffusion coefficient, C is the nitrobenzene concentration near the catalyst surface 3.1*10P7 mol cme3. To consider the catalyst deactivation, separable deactivation kinetics [ 281 of the process was assumed: (51

r=r,-a(t)

where r. is the reaction rate in the absence of deactivation given by eqn. 3, a(t) is a function describing the deactivation process. Experimental data showed that the deactivation was mainly due to nitrobenzene while hydrogen impeded it. Then the a(t) function can be expressed by means of the partial pressures of nitrobenzene and hydrogen. In order to determine the type of the deactivation function, describing the experimental data in the best way, we compared the experimental curves of the dependence of the rate of nitrobenzene hydrogenation to aniline on time with theoretical curves based on different laws of catalyst deactivation. Three models of deactivation were examined suggesting respectively one-site or two-site molecular adsorption as well as dissociative adsorption of nitrobenzene on the surface of the catalyst

h(t)

=K*

h(t)

-_K*

dt

da,(t) dt

!?!?

(6)

&

(7)

’ Pnb

dt

2db =K*

p& 3Pfii

(8)

The integration of eqns. 6, 7 and 8, and substitution of the functions a, (t), u2 (t) and u3 (t) for a(t) in eqn. 5, lead to three different expressions for the current reaction rate: rl(t)=r*{l-k:[(PH/Pnb)r-(PH/Pnb)rOl)

(9)

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B

60

120

180

240

300t,mln

60

120

180

240

30Otpin

Fig. 3. Variations of the experimental (----) and calculated reaction rates eqn. 9 (curve l), eqn. 10 (curve 2), eqn. 11 (curve 3) for reaction mixtures in which the ratios of NB-to-hydrogen-toargonwere (a) 1:7.3:1.4; (b) 1:7.3:X9; (c) 1:7.3:4.5;and (d) 1:7.3:5.9.

where k:,kz and kz are rate constants of the deactivation process. The z and r0 indices refer to the current moment and the initial moment of the reaction, respectively. Further we substituted r, from eqn. 3 for eqn. 5 and for eqns. 9, 10 and 11, and processed the experimental data in accordance with the obtained kinetic models. The best coincidence of experimental with calculated data was realized by the model: r= ;F;zHb

n

(1-k:

[h/pnb)T-

(Pdpnb)sOl)

(12)

for deactivation constant KY = 10.34 (Fig. 3). The accuracy of this model in relation to the experimentally measured reaction rate under the conditions of deactivation was 13.7% [20]. Effects of intraparticle resistance on the process kinetics It is important to examine the effect of the heat and mass transfer in the catalyst grain on the deactivation process and on the kinetics of the reaction of nitrobenzene hydrogenation to aniline. Studies were carried out over catalyst pellets of a cylindrical shape having diameter and height of 6 mm. The concentration gradient between the gas flow and the surface of the pellet was determined by the formula [ 291:

38

dC,=

r-d& S/L.D,~.~- (1-t)

(13)

At 563 K the Sherwood’s criterion, Sh, is equal to 4, d& is the effective diameter of the grain (0,4 cm), t is the porosity of the catalyst bed (0.2)) and r is equal to 4.27*10p3 mol cmp3 h-l. Therefore, the concentration gradient SC, was estimated to be equal to 1.25~10-~. Since the concentration of nitrobenzene in the gas phase was C,= 1.47 mol mp3, the gradient SC, accounts for 0.8% of the volume concentration, i.e. the reaction is not affected by external diffusion resistance. The Weisz’ criterion had the value of 48.14 for the reaction rate of 2.1~10-~ mol h-l cm-3 and nitrobenzene concentration of 2.66*10e7 mol cmp3. This means that the reaction takes place in the region of intraparticle resistance. The reaction depth in the grain under these conditions, determined by the formula [ 291: H= (&.c/r)O.~

(14)

had the value of 0.2 and 0.07 cm at 493 and 563 K, respectively. The temperature gradient along the cross-section of the pellets, 6T,, was estimated according to the formula [ 291: dT, = h. (Co,, - Cnb 1 .D,JJ-L

(15)

At 563 K the reaction heat, h, was equal to 106 kcal mol-I, the effective thermal conductivity of the catalyst grain, L& was equal to 1.32 kcal cm-l h-l degree-l, the difference in nitrobenzene concentration on the surface and in the centre of the grain, (C$, -Crib), was equal to 3.5*10m4 mol m-‘. We estimated 6T, at 0.185 K which means that this gradient will not be further considered. The temperature gradient between the gas phase and the surface of the grains was calculated after the formula [ 291:

(16) where $Jis the thermal conductivity of the hydrogen-nitrobenzene gas mixture at 563 K (2.015.10-3 kcal cm-’ h-l degree-l), Nu is the Nusselt criterion (2.06)) and r is equal to 4.27. lop3 mol cmp3 h-l. For ST, 5.45’ were obtained which means that the heat transfer in the studied system is not important for the reaction. All the above calculations show that under the conditions considered the heat transfer in the system does not significantly affect the process. In order to derive a kinetic equation valid under the conditions of intrapartitle resistance, we used the formula of Pshezhetskii and Rubinstein [ 301:

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(17) where Ref is the effective radius of the catalyst grain, Def is the effective diffusion coefficient of the reacting substance in the grain, Q= 7 rkdr

(18)

XS where rk is reaction rate in kinetic region of eqn. 3, Xs and Xc are the conversion degrees on the surface and in the centre of the grain, respectively. Having in mind that the rate of the process is limited by the nitrobenzene diffusion into the catalyst pores, a kinetic model was obtained which accounts for the effect of diffusion on the reaction rate r (t)g ~(t)d=Iz~lR(2D_g,)*.~{1/12~[~~~-

(llb)ln(l+km,)

I}“.”

(19)

The values of the k, and k2 constants, for which the best description of the experimental data was obtained, are presented in Table 1. We note that eqn. 19 conformed with the experimental data with accuracy 11.8%. Kinetic model of the reaction under conditions catalyst deactivation

of intraparticle

resistance

and

The process of nitrobenzene hydrogenation to aniline occurs over a catalyst whose activity is continuously decreased. In order to account for the effect of deactivation on the kinetics of the process, several steps, describing the deactivation process, should be added to Scheme 1. Experimental data indicate that the catalyst deactivation is due to surface blocking by coke. As was established, nitrobenzene strongly decreased the reaction rate and it is reasonable to assume that it is a precursor of coking. Dvorak et al. [31] have found that 1,3dinitrobenzene, concomitant with the starting nitrobenzene, deactivates the catalyst notably higher than nitrobenzene. In view of this, Scheme 1 can be extended with additional reaction steps: TABLE 1 Values of rate constants from eqns. 19 and 21 Temperature (Kl

k’

k2

k’

493

0.33 0.40 0.46

550 550 550

3.2 2.7 2.4

523 563

(6) [C,H,NO,K]

+ [CGH5NHOHK]+

[RK] +nH,

(20)

(7) m[RK]+coke where [RK] is a species that is a precursor of coke, n is a number which is larger than 2 and m is equal to 2 or larger. On the basis of Scheme 1,complemented by the steps of eqn. 20 and eqn. 18, a kinetic model was derived which considers both the effects of diffusion in the catalyst grain and catalyst deactivation on the reaction rate at a certain moment TddT :

I 63

120

18C

2LO

300= y3-

I 60

120

180

240

300

t

L

60

'20

1.80

2‘f.o

300

t

Fig. 4. Variations of the experimental and calculated (eqn. 21) reaction rates with time at (a) 493 K, (b) 523 K, and (c) 563 K for different reaction mixtures and flow-rates, F: (1) n F = 18 300 cm”/h; (3) x FNB-to-H,-to-Ar= = 22 800 cm3/h; NB-to-H,-to-Ar= 1: 7.3 : 2.9; (2) 0 1: F 7.3 = 20: 5.7. 800 cm3/h, NB-to-H,-to-Ar= 1: 7.3 : 4.5; and

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rddT = h aPHT)0.5

(WM

bnw-

(1/k2)ln(l+b,bz)

l)o.5-z

(21)

R,

where 2 = 1- K* (1 -pnb~/pnbo) , n& 0 is the partial pressure of nitrobenzene at the initial moment of the reaction, p& is the partial pressure of nitrobenzene at the moment, K* is a deactivation constants, whose values are given in Table 1, K, and k, are rate constants the values of which are determined by eqn. 19. Fig. 4 shows experimental and calculated curves of the dependence of the reaction rate on time using the proposed model. The mean error between experimental and calculated values of the reaction rate was 6.3%. DISCUSSION

One important question is why catalyst deactivation in the kinetic and diffusion regions is described by different kinetic models and mechanisms. Comparison of the deactivation model in the kinetic region eqn. 9 with that for the diffusion region eqn. 21 indicated a significant difference between the two equations. The deactivation in the kinetic region is a function of the partial pressures of nitrobenzene and hydrogen whereas, in the diffusion region, it is a function only of the nitrobenzene partial pressure. One possible explanation of this fact is the following. According to eqn. 20 the rate of coking is a function of the concentration of the [ C6H5N02K] and [ C,H,NHOHK] surface species. On the other hand, this concentration is a function of the partial pressure of nitrobenzene. The rate of consumption of these species in the kinetic region in the different directions to aniline and coke should be determined, to a large extent, by the partial pressure of hydrogen. The conversion of the above surface species to aniline is expected to predominate at high partial pressures of hydrogen, while low pressures should enhance the rate of deactivation. The situation with the diffusion is somewhat different. The hydrogen molecules are of very small size and are able to penetrate into the internal parts of the catalyst grain more easily than the nitrobenzene molecules. That is why the concentration gradient of the hydrogen molecules along the radius of the grain is negligible in relation to that of nitrobenzene. In view of this, the concentration of the coke precursors along the radius will be dependent mainly on the rate of nitrobenzene diffusion into the catalyst grain which is proportional to its the gas-phase concentration. Prognosis for lifetime of the industrial catalysts for nitrobenzene hydrogenation to aniline The question how to foresee terms of performance of industrial catalysts on the basis of short-term laboratory experiments is quite difficult. Such measurements demand a lot of time. For these reasons the elaboration of a rapid

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method for evaluation of the term of catalyst performance is of great importance for practice. Based on the above results, rapid method has been proposed [32] for evaluation of the catalyst batch production. According to this method, one-daylong experimental runs are sufficient to predict with high accuracy the work of industrial catalysts for nitrobenzene hydrogenation to aniline. CONCLUSIONS

Kinetic models were developed of the reaction of nitrobenzene hydrogenation to aniline over commercial copper catalysts which account for the deactivation process both in the kinetic and diffusion regions. These model catalyst behaviour under different conditions with high accuracy. REFERENCES O.T. Nikolaev and A.M. Yakubson, Aniline, Khimia, Moscow, 1984. V.M. Belousov, T.A. Palczewska and L.V, Botutskaja, Catalysis and Catalysts, Vol. 25, Naukova Dumka, Kiev, 1987, p. 29. C.S. Rohrer, J. Phys. Chem., 56 (1952) 662. H. Keki, S. Iharda and C. Sliepcevich, Ind. Eng. Chem., 52 (1960) 137. K.H. Charda and C.M. Sliepcevich, Ind. Eng. Chem., 52 (1960) 417. E. Echigoya, G. Nagai, K. Morikawa, Bull. Jap. Pet. Inst., 13 (1971) 84. D.N. Rihani and T.K. Narayanan, Ind. Eng. Chem., 4 (1965) 403. 8 J. Pasek and J. Cerny, Chem. Prum., 22 (1972) 436. 9 S. Tsutsumi and H. Ferada, J. Chem. Sot. Jap. Ind. Chem. Sect., 54 (1951) 527. S. Tsutsumi and S. Sada, J. Chem. Sot. Jap. Ind. Sect., 73 (1970) 1074. 10 11 MS. Murthy, P.K. Sechpande and N.R. Kuloor, Chem. Age India, 14 (1963), 653. 12 V.V. Pogorelov and A.I. Gelbstein, Kinet. Katal., 17 (1976) 1497. 13 D. Datta, Fertil. Technol., 13 (1976) 125. 14 J. Cerny, J. Pasek and V. Pexidr, Chem. Prum., 18 (1968) 371. 15 L. Petrov, N. Kirkov, K. Tenchev and D. Shopov, Chemistry and Industry, 57 (1985) 443. 16 F.G. Vigdorovich, V.V. Pogorelov, Yu.K. Kapkov, A.G. Gorelik, A.R. Avetisov and A.I. Gelbstein Kinet. Katal., 21 (1980) 975. 17 F.G. Vigdorovich, Yu.K. Kapkov, V.V. Pogorelov, A.G. Gorelik, P.B. Babkova, A.V. Avetisov and A.I. Gelbstein, in M.G. Slink0 (Ed.), Proc. 3rd All-Union Conf. Kinet. Heterogeneous Catal. React., Kalinin, 1980, p. 68. 18 V. Pexidr, J. Caprianova and P. Nedved, Chem. Prum., 32 (1982) 639. 19 V. Pexidr, Chem. Prum., 33, (1983) 28. 20 L. Petrov, N. Kirkov and K. Kumbilieva, in D. Shopov, A. Andreev, A. Palazov and L. Petrov (Eds.), Proc. 6th Int. Symp. Heterogeneous catalysis, Sofia 1987, Part 1, Bulg. Acad. Sci., Sofia, 1987, p.111. 21 L. Petrov, Ch. Vladov, A. Elyas, N. Kirkov, K. Tenchev, Ch. Bonev, D. Filkova and L. Prahov, J. Mol. Catal., 54 ( 1989) 237. 22 L. Petrov, Zh. Stoyanova, K. Kumbilieva, S. Dancheva, N. Kotsev and D. Shopov, Chemistry and Industry, 59 (1987 ) 64. 23 L. Petrov, K. Tenchev and D. Filkova, unpublished results 24 P.B. Weisz. Z. Phys. Chem. Neue Folge, B 11 (1957) s. 1. 25 L. Petrov, Ch. Maximov and V. Jeliazkov, to be published

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L. Petrov, N. Kirkov and D. Shopov, Kinet. Katal., 26,897,1985 L. Petrov and D. Shopov, React. Kinet. Catal. Lett., 7 (1977) 273. 0. Levenspel, J. Catal., 25 (1965) 265. C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, MIT., Cambridge, Mass., USA, 1970. S.Ya. Pshezhetskii and R.N. Rubinstein, Zh. Phys. Chim., 20 (1946) 1127. B. Dvorak, J. Pasek, P. Pavlas and Z. Hejda, in B. Delmon and G.F. Froment (Eds.), Studies in Surface Science and Catalysis, Vol. 34, Catalyst Deactivation, Elsevier, Amsterdam, 1987 L. Petrov, Ch. Vladov, Ch. Bonev, N. Neshev, L. Prahov, M. Vassileva, D. Filkova and S. Dancheva, Bulgarian Patent 74 945,