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The activity decay of cracking catalysts: chemical and structural deactivation by coke
B. Delmon and G.F.Froment (Eds.) Catalyst Deactivation 1994 Studies in Surface Science and Catalysis, Vol. 88 0 1994 Elsevier Science B.V. All rights reserved.
249
The activity decay of cracking catalysts: chemical and structural deactivation by coke D. Nevicato, I. Pitault, M. Forissier and J. R. Bernard GCnie catalytique des rkacteurs de raffinage, CNRS-ELF, CRES, BP 22, 69360 Solaize, France The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function are discussed in relation to feedstock analysis and change of effective diffusivity with catalyst coke content. 1. INTRODUCTION
The catalyst deactivation by coke is very fast in the cracking reaction of vacuum distillates. The coke is made very early in the FCC riser [l]. The catalyst is composed of zeolite crystallites dispersed within an amorphous matrix. The cracking reactions in the zeolite follows previous cracking in the matrix. Though the diffusion of reactants in the matrix is not considered as the restrictive step of the chemical process [2], Nilson et a1 [3] and Dadyburjor et al [4], suggest that it is not the case in the zeolite crystallite. The cracking reactions may become diffusionally limited when the reaction temperature and the boiling point of the hydrocarbures are higher. Rajagopalan et a1 [5] show clearly that this mass transfer resistance occurs within the zeolite crystals of non steamed catalysts. Moreover, the accessibility of active sites and diffusion rates are reduced by coke, owing to the porosity decrease and pore plugging. Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-71. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. We consider that the deactivation takes place especially in the zeolite crystals. For this purpose, a large fraction of the coke constitutes 3-6 volatile polyaromatic rings which cannot be elsewhere other than in the zeolite [l]. The present work uses a practical method to express the catalyst activity from measured conversions of commercial feedstocks versus catalyst
250
coke content [lo]. The deactivation fimction @ is the multiplication factor of the kinetic constant. It decreases from 1 to 0 when catalyst coke content increases. Here, the behaviour of the semi empirical deactivation function (@) is discussed, by introducing two modes of deactivation: the coke fouling of active sites and the diffusivity decrease by pore plugging. This latter may be related to an effective difhsivity function of coke content. 2. THE DEACTIVATION FUNCTION OF THE CRACKING CATALYST
2.1. Experimental methods Table 1 Analvsis of vacuum distillate feedstocks and reaction conditions density 288°K (kg/m3) KUOP Conradson Carbon (%wght) Sulfur (%wght) N tot. (Yowght) N bas. (ppm) C/H NMR C aromatic (% nb c) molar weight (g/mol)
Aramco 930.1 11.78 0.84 2.750 1090 250 0.605 20.8 494
Montmirail 902.8 12.1 0.45 0.282 800 312 0.570 13.15 432
Nigeria 940.9 11.59 0.6 0.320 1470 765 0.610 17.4 368
Chemical composition (mass spectrometry) Paraffin (%wght) Naphthene (%wght) Aromatic (%wght) Polar (%wght)
18.04 18.26 51.70 12.00
21.55 30.65 39.60 8.20
12.40 34.80 43.50 9.30
67 1 735 805
679 736 807
658 718 779
TBP simulated distillation (“K) 10% 50% 90% Reaction conditions Temperature: Pressure: nitrogen volume flow rate (diluant) feedstock mass flow rate catalyst hold up iniection duration
803°K 1 atmabs 0.67 cmVs 0.02 gls 6g 50 s
25 1 The experimental studies using industrial feedstock are carried out in a modified M A T . (micro activity test) [lo]. The reaction conditions are presented in table 1. The catalyst is NOVA D equilibrium catalyst from Grace Davison Co. The equilibrium catalysts are previously coked under the same reaction conditions to get partially deactivated samples. The method using the conversion versus initial coke content from experiments to determine the deactivation function, is described in [lo]. Three different feedstocks are used (table 1). 2.2.Results At low coke content, pore plugging is still negligible and decay is mainly due to site coverage. Consequently, the variation of deactivation function with coke content is only due to chemical deactivation, and it is proportional to the remaining activity: dachernical
dc
- -F.@chemical
and
When the coke content increases, pore plugging may become important and the diffusion of reactants becomes more limiting. This latter phenomena can be described by introducing a term proportional to the product of the activity ( a ) by the fouling (1-0). Thus, in the general case, the variation of the activity may be written as:
'a
dc
- F.D + E.(I -
a).~
The integration of equation (2), with Q=l at c=O, yields equation (3). @=
(E + F).exp(-( E + F).c)
(3)
F + E.exp( -( E + F).c)
This simple analysis is semi empirical: it is not a description of the diffusion limited reaction within the crystals but allows one to take into account both phenomena, in order to provide kinetic models for FCC reactor description [l 11. Experimental results on the three feedstocks are shown in figure 1, with the deactivation function determined according to the method described in [lo]. Curves are calculated from equation (3) after fitting E and F. These values are reported in table 2. Table 2 Parameters of the deactivation function at 803°K with Nova D catalyst F E
Aramco 0.08 2.89
Nigeria 0.66 0.46
Montmirail 0.26 1.46
252
PHI1 1
0.8 0.6 0.4 0.2 0
0
0.5
1
1.5
2
2.5
% COKE Figure 1. Deactivation function versus initial coke content: (0)Aramco, (A) Montmirail and @)Nigeria.
rl' , 0.8 0.6 0.4 0.2
--...._ ---.....
\.
I 0
1
2
% COKE Figure 2. Chemical deactivation versus initial coke content (legends in fig. 1)
Yo COKE Figure 3. Effectiveness factor ratio versus initial coke content (legends in fig. 1)
An effectiveness factor q' can be defined to describe the diffusional limitation of catalysis by coke fouling: cf, = @chemical.q'
(4)
The diffusional limitation which may also exist in the uncoked catalyst can be characterised by an other effectiveness factor qo. Thus, the real effectiveness factor of the coked catalyst is:
253
q' is calculated from equations (4,1,3), and @)chemical and q' are presented in figures 2 and 3. q'=
(E + F). exp(-E. c) F + E.exp(-( E + F).c)
The effectiveness factor is obtained by the resolution of the mass transfer equations [12] in a spherical pellet with simple assumptions: - quasi stationary regime, - ideal binary gas mixture, - adsorption and desorption are not the limiting step, - the effective diffusivity is constant in the catalyst We assume that the deactivation function is the same for each reaction. So, only the simple cracking reaction of a feedstock lump, to a product lump, was considered. Such a reaction occurring with the molar expansion (m) may be represented as Feedstock (1)
m. Products (2)
(7)
The true kinetic rate equation is given by. = @chemical.kl.-
c,2 c10
In this equation, the influence of total pressure is well represented by the term CI/C1, [13]. The required form of Thiele modulus cp is given as follows:
The effectiveness factor is related to the Thiele modulus for different molar expansion modulus 8 [12]. The molar expansion modulus 8 expresses the intensity of the molar expansion, 8=(m-l).xlR . Two simple cases may be considered, whether the cracking reaction is initially controlled by diffusion of reactants in the uncoked catalyst, or not. First case: no difhsional limitation in the uncoked catalyst (qo=l) The relation between De and coke content has no analytical expression and the results (obtained by computer resolution of equations (6, 9) and q=f(cp)), are presented in figure 4 (computer parameters: rate constants 950, 530, and 540 s-1 for Aramco, Nigeria and Montmirail respectively, 8=5 and zeolite diameter = 3pm).
Second case: strong diffusional limitations in the uncoked catalyst (qo<
254
The substitution of equations (9,ll) into equation (6) produces an asymptotic relation between the effective diffusivity and the coke content (figure 5). Decoked = (r~)2 =
De0
(E+F).exp(-E.c) F + E. exp(-( E + F).c)
De/lYeo
-.'.
' \
0.2 00
0
1
2
% COKE Figure 4. Effective diffusivity versus initial coke content when q=l (legends in fig. 1)
\
',
~.
'.
*.
---._
1
2
% COKE Figure 5. Effective diffusivity versus initial coke content when q<
2.3.Discussion Rajagopalan et a1 [ 5 ] brought experimental evidence that fresh catalyst is diffusionally limited when cracking West Texas heavy gas oil at 773°K. But, it is not clear whether this limitation remains after the steaming, which simulates the hydrothermal deactivation of fresh catalyst to the equilibrium catalyst. Moreover, since the result of activity in M.A.T. is the average performance of a decaying catalyst, it is impossible to determine whether the effectiveness factor of uncoked catalyst is less than 1. The main conclusions that can be drawn from this study are related to the relative importance of site fouling and pore plugging on deactivation, by comparing E and F for different feedstocks. From table 2, Nigeria VGO deactivates more chemically than diffusionally when coking. It is the contrary for Aramco VGO, and the Montmirail VGO is the intermediate. This can be also found by comparing curves in figure 1. The shape of the Aramco VGO curve is close to the one found by using diffusion percolation models [9]. These differences are related to chemical analysis of feedstocks: molecular weights, aromatic
255
and naphthene content and heavy ends, but detailed explanations are lacking. It has been shown that coke in zeolite consists of polyaromatic molecules blocked by their size in the cages [l]. Basic nitrogen plays a role since it can chemically deactivate by itself with less coking, and a chemically deactivated catalyst is then less sensitive to the diffusional deactivation. These observations are refined by figures 2 and 3: the chemical deactivation by Aramco VGO is weak. One may state that coke is concentrated close to the outer surface of zeolite, so that @chemical remains large and allows the diffusional deactivation to take place. Figures 4 and 5 also illustrate these observations by the change of absolute or relative diffusivity of the feedstock with the coke content. The lower decrease of De/Deo for Nigeria feedstock suggests that the coke is more evenly distributed than for Aramco VGO. 3. CONCLUSION
A refined model can be written to describe deactivation by diffusion and fouling within a catalyst pellet or crystal. Nevertheless, it cannot be used for modelling a whole reactor which demands in itself, a complex model to be solved. We propose a simple decay function which can be easily introduced in the kinetic equations of a reactor model. This function is experimentally determined. It has a physical meaning and it allows to describe different behaviours of feedstocks between pure site fouling and strong diffusional limitation by pore plugging. NOTATIONS
catalyst coke content, % weight molar concentrations of feedstock in the zeolite and initially, m ~ l . m - ~ C1, C,, De effective diffusivity, m2.s.l deactivation parameters defined by equations (1) and (2) E, F kl reaction rate constant, s-1 m molar expansion r reaction rate, moi.m".~-l mole fraction of feedstock at catalyst surface XIR @ deactivation function @chemical chemical deactivation function cp Thiele modulus q,, qcoked effectiveness factor of uncoked catalyst, and of coked catalyst rl' effectiveness factor ratio e molar expansion intensity C
ACKNOWLEDGEMENTS
The authors acknowledge D. Gadolet for his efficient technical expertise and D. Schweich for fruitful discussions.
256
REFERENCES P. Turlier, M. Forissier, P. Rivault and J.R. Bernard, Symp. on Advances in FCC, 206th National Meeting, Am. Chem. SOC.,Chicago, 1993. W.O. Haag, R.M. Lago and P.B. Weisz, Chem. SOC.Faraday. Disc., 72 (1982), 317. 2. K. Rajagopalan, A.W. Peters and G.C. Edwards, Applied Catalysis, 23 (1986), 69. 3. D.B. Dadyburjor and A. Bellare, J. Catalysis, 126 (1990), 261. 4. 5 . P. Nilson, F.E. Massorth, J.E. Ottersdtedt, Applied Catalysis, 26 (1986), 175. J.B. Butt, S. Delgado Diaz and W.E. Muno, Journal of catalysis, 37 (1975), 158. 6. 7. C.H. Tan and 0. M.Fuller, Can. J. Chem. Eng., 48 (1970),74. S.C. Reyes and L.E. Scriven, Ind. Eng. Chem. Res., 30 (1991),71. 8. 9. A.O.E. Beyne and G.F. Froment, Chem. Eng. Sci., 48 (1993), 503. 10. M. Forissier and J.R. Bernard, Catalyst Deactivation, C.H. Bartholomew and J.B.Butt editors, Elsevier, Amsterdam, 359,1991. 11. M.P. Martin, C. Derouin, P. Turlier, M. Forissier, G. Wild and J.R. Bernard, Chem. eng. Sci., 47 (1992), 9-1 1,23 19. 12. V.W. Weekman and R. L. Goring, Journal of catalysis, 4 (1965), 260. 13 M. Forissier, M. Formenti and J.R. Bernard, Catalysis Today, 11 (1991), 73. 1.
249
The activity decay of cracking catalysts: chemical and structural deactivation by coke D. Nevicato, I. Pitault, M. Forissier and J. R. Bernard GCnie catalytique des rkacteurs de raffinage, CNRS-ELF, CRES, BP 22, 69360 Solaize, France The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function are discussed in relation to feedstock analysis and change of effective diffusivity with catalyst coke content. 1. INTRODUCTION
The catalyst deactivation by coke is very fast in the cracking reaction of vacuum distillates. The coke is made very early in the FCC riser [l]. The catalyst is composed of zeolite crystallites dispersed within an amorphous matrix. The cracking reactions in the zeolite follows previous cracking in the matrix. Though the diffusion of reactants in the matrix is not considered as the restrictive step of the chemical process [2], Nilson et a1 [3] and Dadyburjor et al [4], suggest that it is not the case in the zeolite crystallite. The cracking reactions may become diffusionally limited when the reaction temperature and the boiling point of the hydrocarbures are higher. Rajagopalan et a1 [5] show clearly that this mass transfer resistance occurs within the zeolite crystals of non steamed catalysts. Moreover, the accessibility of active sites and diffusion rates are reduced by coke, owing to the porosity decrease and pore plugging. Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-71. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. We consider that the deactivation takes place especially in the zeolite crystals. For this purpose, a large fraction of the coke constitutes 3-6 volatile polyaromatic rings which cannot be elsewhere other than in the zeolite [l]. The present work uses a practical method to express the catalyst activity from measured conversions of commercial feedstocks versus catalyst
250
coke content [lo]. The deactivation fimction @ is the multiplication factor of the kinetic constant. It decreases from 1 to 0 when catalyst coke content increases. Here, the behaviour of the semi empirical deactivation function (@) is discussed, by introducing two modes of deactivation: the coke fouling of active sites and the diffusivity decrease by pore plugging. This latter may be related to an effective difhsivity function of coke content. 2. THE DEACTIVATION FUNCTION OF THE CRACKING CATALYST
2.1. Experimental methods Table 1 Analvsis of vacuum distillate feedstocks and reaction conditions density 288°K (kg/m3) KUOP Conradson Carbon (%wght) Sulfur (%wght) N tot. (Yowght) N bas. (ppm) C/H NMR C aromatic (% nb c) molar weight (g/mol)
Aramco 930.1 11.78 0.84 2.750 1090 250 0.605 20.8 494
Montmirail 902.8 12.1 0.45 0.282 800 312 0.570 13.15 432
Nigeria 940.9 11.59 0.6 0.320 1470 765 0.610 17.4 368
Chemical composition (mass spectrometry) Paraffin (%wght) Naphthene (%wght) Aromatic (%wght) Polar (%wght)
18.04 18.26 51.70 12.00
21.55 30.65 39.60 8.20
12.40 34.80 43.50 9.30
67 1 735 805
679 736 807
658 718 779
TBP simulated distillation (“K) 10% 50% 90% Reaction conditions Temperature: Pressure: nitrogen volume flow rate (diluant) feedstock mass flow rate catalyst hold up iniection duration
803°K 1 atmabs 0.67 cmVs 0.02 gls 6g 50 s
25 1 The experimental studies using industrial feedstock are carried out in a modified M A T . (micro activity test) [lo]. The reaction conditions are presented in table 1. The catalyst is NOVA D equilibrium catalyst from Grace Davison Co. The equilibrium catalysts are previously coked under the same reaction conditions to get partially deactivated samples. The method using the conversion versus initial coke content from experiments to determine the deactivation function, is described in [lo]. Three different feedstocks are used (table 1). 2.2.Results At low coke content, pore plugging is still negligible and decay is mainly due to site coverage. Consequently, the variation of deactivation function with coke content is only due to chemical deactivation, and it is proportional to the remaining activity: dachernical
dc
- -F.@chemical
and
When the coke content increases, pore plugging may become important and the diffusion of reactants becomes more limiting. This latter phenomena can be described by introducing a term proportional to the product of the activity ( a ) by the fouling (1-0). Thus, in the general case, the variation of the activity may be written as:
'a
dc
- F.D + E.(I -
a).~
The integration of equation (2), with Q=l at c=O, yields equation (3). @=
(E + F).exp(-( E + F).c)
(3)
F + E.exp( -( E + F).c)
This simple analysis is semi empirical: it is not a description of the diffusion limited reaction within the crystals but allows one to take into account both phenomena, in order to provide kinetic models for FCC reactor description [l 11. Experimental results on the three feedstocks are shown in figure 1, with the deactivation function determined according to the method described in [lo]. Curves are calculated from equation (3) after fitting E and F. These values are reported in table 2. Table 2 Parameters of the deactivation function at 803°K with Nova D catalyst F E
Aramco 0.08 2.89
Nigeria 0.66 0.46
Montmirail 0.26 1.46
252
PHI1 1
0.8 0.6 0.4 0.2 0
0
0.5
1
1.5
2
2.5
% COKE Figure 1. Deactivation function versus initial coke content: (0)Aramco, (A) Montmirail and @)Nigeria.
rl' , 0.8 0.6 0.4 0.2
--...._ ---.....
\.
I 0
1
2
% COKE Figure 2. Chemical deactivation versus initial coke content (legends in fig. 1)
Yo COKE Figure 3. Effectiveness factor ratio versus initial coke content (legends in fig. 1)
An effectiveness factor q' can be defined to describe the diffusional limitation of catalysis by coke fouling: cf, = @chemical.q'
(4)
The diffusional limitation which may also exist in the uncoked catalyst can be characterised by an other effectiveness factor qo. Thus, the real effectiveness factor of the coked catalyst is:
253
q' is calculated from equations (4,1,3), and @)chemical and q' are presented in figures 2 and 3. q'=
(E + F). exp(-E. c) F + E.exp(-( E + F).c)
The effectiveness factor is obtained by the resolution of the mass transfer equations [12] in a spherical pellet with simple assumptions: - quasi stationary regime, - ideal binary gas mixture, - adsorption and desorption are not the limiting step, - the effective diffusivity is constant in the catalyst We assume that the deactivation function is the same for each reaction. So, only the simple cracking reaction of a feedstock lump, to a product lump, was considered. Such a reaction occurring with the molar expansion (m) may be represented as Feedstock (1)
m. Products (2)
(7)
The true kinetic rate equation is given by. = @chemical.kl.-
c,2 c10
In this equation, the influence of total pressure is well represented by the term CI/C1, [13]. The required form of Thiele modulus cp is given as follows:
The effectiveness factor is related to the Thiele modulus for different molar expansion modulus 8 [12]. The molar expansion modulus 8 expresses the intensity of the molar expansion, 8=(m-l).xlR . Two simple cases may be considered, whether the cracking reaction is initially controlled by diffusion of reactants in the uncoked catalyst, or not. First case: no difhsional limitation in the uncoked catalyst (qo=l) The relation between De and coke content has no analytical expression and the results (obtained by computer resolution of equations (6, 9) and q=f(cp)), are presented in figure 4 (computer parameters: rate constants 950, 530, and 540 s-1 for Aramco, Nigeria and Montmirail respectively, 8=5 and zeolite diameter = 3pm).
Second case: strong diffusional limitations in the uncoked catalyst (qo<
254
The substitution of equations (9,ll) into equation (6) produces an asymptotic relation between the effective diffusivity and the coke content (figure 5). Decoked = (r~)2 =
De0
(E+F).exp(-E.c) F + E. exp(-( E + F).c)
De/lYeo
-.'.
' \
0.2 00
0
1
2
% COKE Figure 4. Effective diffusivity versus initial coke content when q=l (legends in fig. 1)
\
',
~.
'.
*.
---._
1
2
% COKE Figure 5. Effective diffusivity versus initial coke content when q<
2.3.Discussion Rajagopalan et a1 [ 5 ] brought experimental evidence that fresh catalyst is diffusionally limited when cracking West Texas heavy gas oil at 773°K. But, it is not clear whether this limitation remains after the steaming, which simulates the hydrothermal deactivation of fresh catalyst to the equilibrium catalyst. Moreover, since the result of activity in M.A.T. is the average performance of a decaying catalyst, it is impossible to determine whether the effectiveness factor of uncoked catalyst is less than 1. The main conclusions that can be drawn from this study are related to the relative importance of site fouling and pore plugging on deactivation, by comparing E and F for different feedstocks. From table 2, Nigeria VGO deactivates more chemically than diffusionally when coking. It is the contrary for Aramco VGO, and the Montmirail VGO is the intermediate. This can be also found by comparing curves in figure 1. The shape of the Aramco VGO curve is close to the one found by using diffusion percolation models [9]. These differences are related to chemical analysis of feedstocks: molecular weights, aromatic
255
and naphthene content and heavy ends, but detailed explanations are lacking. It has been shown that coke in zeolite consists of polyaromatic molecules blocked by their size in the cages [l]. Basic nitrogen plays a role since it can chemically deactivate by itself with less coking, and a chemically deactivated catalyst is then less sensitive to the diffusional deactivation. These observations are refined by figures 2 and 3: the chemical deactivation by Aramco VGO is weak. One may state that coke is concentrated close to the outer surface of zeolite, so that @chemical remains large and allows the diffusional deactivation to take place. Figures 4 and 5 also illustrate these observations by the change of absolute or relative diffusivity of the feedstock with the coke content. The lower decrease of De/Deo for Nigeria feedstock suggests that the coke is more evenly distributed than for Aramco VGO. 3. CONCLUSION
A refined model can be written to describe deactivation by diffusion and fouling within a catalyst pellet or crystal. Nevertheless, it cannot be used for modelling a whole reactor which demands in itself, a complex model to be solved. We propose a simple decay function which can be easily introduced in the kinetic equations of a reactor model. This function is experimentally determined. It has a physical meaning and it allows to describe different behaviours of feedstocks between pure site fouling and strong diffusional limitation by pore plugging. NOTATIONS
catalyst coke content, % weight molar concentrations of feedstock in the zeolite and initially, m ~ l . m - ~ C1, C,, De effective diffusivity, m2.s.l deactivation parameters defined by equations (1) and (2) E, F kl reaction rate constant, s-1 m molar expansion r reaction rate, moi.m".~-l mole fraction of feedstock at catalyst surface XIR @ deactivation function @chemical chemical deactivation function cp Thiele modulus q,, qcoked effectiveness factor of uncoked catalyst, and of coked catalyst rl' effectiveness factor ratio e molar expansion intensity C
ACKNOWLEDGEMENTS
The authors acknowledge D. Gadolet for his efficient technical expertise and D. Schweich for fruitful discussions.
256
REFERENCES P. Turlier, M. Forissier, P. Rivault and J.R. Bernard, Symp. on Advances in FCC, 206th National Meeting, Am. Chem. SOC.,Chicago, 1993. W.O. Haag, R.M. Lago and P.B. Weisz, Chem. SOC.Faraday. Disc., 72 (1982), 317. 2. K. Rajagopalan, A.W. Peters and G.C. Edwards, Applied Catalysis, 23 (1986), 69. 3. D.B. Dadyburjor and A. Bellare, J. Catalysis, 126 (1990), 261. 4. 5 . P. Nilson, F.E. Massorth, J.E. Ottersdtedt, Applied Catalysis, 26 (1986), 175. J.B. Butt, S. Delgado Diaz and W.E. Muno, Journal of catalysis, 37 (1975), 158. 6. 7. C.H. Tan and 0. M.Fuller, Can. J. Chem. Eng., 48 (1970),74. S.C. Reyes and L.E. Scriven, Ind. Eng. Chem. Res., 30 (1991),71. 8. 9. A.O.E. Beyne and G.F. Froment, Chem. Eng. Sci., 48 (1993), 503. 10. M. Forissier and J.R. Bernard, Catalyst Deactivation, C.H. Bartholomew and J.B.Butt editors, Elsevier, Amsterdam, 359,1991. 11. M.P. Martin, C. Derouin, P. Turlier, M. Forissier, G. Wild and J.R. Bernard, Chem. eng. Sci., 47 (1992), 9-1 1,23 19. 12. V.W. Weekman and R. L. Goring, Journal of catalysis, 4 (1965), 260. 13 M. Forissier, M. Formenti and J.R. Bernard, Catalysis Today, 11 (1991), 73. 1.