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Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
Catalyst Deactivation by Pore Structure Changes. The Effect of Coke and Metal Depositions on Diffusion Parameters Brahm D. Prasher, Gerard A. Gabriel, and Yi Hua Ma* Depatfment of Chemical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 0 1609
Diffusion data involving different sized hydrocarbon molecules in three different alumina based pellets are presented. One of the samples has not been used in any reactions, while the two other samples are the same pellets as the unused ones, except that they have been used for the hydrocracking of residuum under varying conditions of high temperature and pressure for several days. The results of the diffusion studies show that the used pellets exhibited severe reduction in intraparticle diffusion characteristics. If intraparticle diffusion influences rate of reaction for hydrocracking, then deactivation can at least partially be explained through the reduction of intraparticle diffusion characteristics, which can be accomplished through severe coke and metal depositions during reaction.
Introduction One of the most practical problems in the study of catalysis is presented by the loss of catalytic activity during reaction within a porous catalyst medium. Understanding deactivation phenomena is difficult because of the existence of multiple possibilities in deactivation mechanisms and also due to a lack of systematization in this field of research. Catalyst decay may be defined as the loss of activity with time that can be induced by several causes. Examples of catalyst decay include sintering of a porous carrier (Anderson, 1956), deposition of reaction residues on active sites (Mills et al., 1950; Maxted, 19511, pore-plugging by poisons in the feed or by poisonous byproducts (Prater and Lago, 1956), the loss of catalyst to products as in certain polymerization reactions (Krylov, 1970), and the loss of catalyst by decomposition such as occurs in enzyme kinetics a t elevated temperatures (Laidler, 1958). Early empirical data presented by Voorhies (1945) in connection with coke deposition in catalysts observed in petroleum processes led a number of workers to conclude that deactivation was intraparticle diffusion controlled. Subsequent mathematical analysis (Carberry and Gorring, 1966) involving external mass transfer and intraparticle diffusion and simultaneous chemical reaction showed that the square root relationship of fraction of catalyst poisoning with time may just be “fortuitous” and that no mechanistic conclusion between reaction or diffusion control may be drawn from empirical relationships cited above. Whether catalyst deactivation through poisoning occurs because of blocking of sites by reactants, impurities, reaction products or just metal and/or coke deposition, it is increasingly realized a t the present time that deactivation may be both diffusion as well as reaction influenced. This has occupied the attention of a number of workers and different analytical approaches to the problem exist (e.g., Wheeler, 1955; Carberry and Gorring, 1966, Khang and Levenspiel, 1973; Masamune and Smith, 1966). While the emphasis on the analysis of the problem by most of the investigators has so far been concentrated on mechanisms based on uniform and pore mouth poisoning, series and parallel deactivation, another aspect of the problem has not been sufficiently dealt with. A number of investigators in the field of diffusion (e.g., Scatterfield and co-workers, 1973; Colton and co-workers, 1975; Prasher and Ma, 1977) report a strong relationship between solute effective diffusivities and ratio of solute diameter to average pore diameter. In a great number of catalytic reactions under severe temperature and pressure conditions, coke deposition and metal deposition 0019-7882/78/1117-0266$01.00/0
present very severe problems resulting in deactivation. In many cases the deactivation of the catalyst bears a direct relationship to the percentage of coke deposited on the catalyst. The deactivation has again been ascribed to the blocking of the reaction sites by poisons, though the mechanisms of the reaction of the deactivation catalyst may vary as has been suggested by the analysis of various investigators cited earlier. One of the possible important variants, viz. the change in the value of diffusivities with reduction or alteration of pore sizes and structure with time, has not been adequately focused on. If the diffusivities of reactants and products are dependent on the relative dimensions and structure of the molecules and the pores, then it follows as deactivation induced by serious coke and metal deposition takes place, the intraparticle effective diffusivity values should undergo radical changes in value with time. The reason this aspect has not been investigated conclusively may partially be explained by the fact that prior to about 1965 there was no adequate reliable diffusion theory in porous media to explain results of investigations of diffusion with reaction (Stoll and Brown, 1974). The existence of reliable evidence available a t this time with regard to the effect of pore structure changes through deposition of impurities on effective diffusivities is fragmentary. Ozawa and Bischoff (1968) conclude that intraparticle diffusivities do not change significantly in the case of ethylene cracking on silica-alumina pellets, but this represents only experiments where coke deposition on catalyst is generally below 1%.Levinter et al. (1967),however, reported losing up to 50% of the surface area through coke deposition, which indicates severe pore blockage and size reduction phenomenon. Stoll and Brown (1974), studying the rate of diffusion of gases in reacting and nonreacting systems, found that the effective diffusivities were generally lower when strongly adsorptive impurities were introduced. This reduction was explained in terms of the differing adsorptive capacity of the diffusing gases as well as the reduction in the pore size for the diffusing solute because of the presence of adsorbed impurities on the wall. Butt (1972) in his review of catalyst deactivation suggests that coke formation under proper conditions can substantially alter physical and transport properties of catalysts as well as their activity. He goes on to state that the apparent contradictions in the results of mass transfer characteristics of different workers stem from differing catalysts and reaction conditions chosen in the different studies. In this study, transient diffusion measurenents for various hydrocarbon solute-solvent systems are made in fresh alu-
0 1978 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
267
Table I. Experimental Values of Diffusion and Equilibrium Partition Coefficients for Different WPI Samples WPI-4 WPI-5 WPI-6 Do X lo5, Deff, Deff, Deff, Solute Solvent T , O C Vmol cm2/s cmz/s K, cm2/s K, cm*/s K, 1. n-Nonane 2. 1,3,5-Triisopropylben-
zene 3. 2,2,4-Trimethylpentane 4. 1,3,5-Triethylbenzene 5. 1-Hexene 6. 7. 8. 9. 10.
1-Methylnaphthalene Cumene 1-Decene m-Xylene n-Heptane 11. Tetralin 12. Mesitylene
Cyclohexane Cyclohexane
25 25
207.2 295.8
0.939 0.758
0.334 0.098
0.779 1.19
0.271 0.095
0.69 0.77
0.132 0.071
0.82 0.77
Toluene Cyclohexane Methylcyclohexane Cyclohexane Cyclohexane Cyclohexane Benzene Toluene Benzene Toluene
25 25 25
185.0 229.2 133.2
1.67 0.884 1.71
7.14 0.272 0.779
0.914 1.06 1.04
0.283 0.075 0.258
0.84 1.20 1.19
0.194 0.056 0.196
0.71 1.17 1.19
25 25 25 25 25 25 25
169.8 162.6 222.0 140.4 162.8 162.4 162.6
1.06 1.09 0.901 1.66 1.80 1.52 1.80
0.204 0.481 0.303 0.562 1.05 0.614 0.716
2.03
0.108 0.162
1.76 0.89
0.093 0.109
1.65 0.94
mina based hydrocracking pellets. Diffusion measurements are also made for the same solute-solvent system in two samples of the same pellets which have been used for a few days for the hydrocracking of residuum. The structure of the pellet used for this study is typical of commercially available hydrocracking catalyst. Materials a n d Experiments Three batches of cylindrically shaped alumina-based hydrocracking material were used for the diffusion studies. WPI-4 is fresh unused hydrocracking pellets having a surface area of 194 m2/g, a pore volume of 0.55 cm3/g, and an average pore radius of 57 A. WPI-5 and WPI-6 are the same pellets as WPI-4, except that they have been used for hydrocracking of residuum for a total of several days a t high temperature and pressure. WPI-5 and WPI-6 are pellets from the same reactor. WPI-5, however, is a sample of pellets from the core of the reactor, while WPI-6 is a sample from the upstream section of reactor. Thus, WPI-6 has been in contact with a greater concentration of metal impurities and polynuclear compounds to be hydrocracked. The two samples of used pellets WPI-5 and WPI-6, have average pore radii of 71.7, and 57.5 A, surface areas of 66.9 and 100 m2/g, and pore volumes of 0.24 and 0.29 cm3/g, respectively. Apparatus a n d Experimental Procedure. Transient diffusion and equilibrium measurements were made on all three samples. The apparatus and experimental procedure employed are similar to those previously reported (Prasher and Ma, 1977). However, in the case of used WPI-5 and WPI-6, the samples were initially contacted repeatedly for several days with warm aromatic and paraffinic light solvents to remove high molecular weight compounds left in the pellet pores during the hydrocracking reactions. Details on the technique and calculations both for the diffusion as well as the adsorption experiments are given in the article by Prasher and Ma (1977). Results a n d Discussion In the diffusion studies with the three samples of pellets, a relatively larger number of diffusion data was obtained in the case of WPI-4 to establish the relationship between the ratio of the restricted diffusivity to the unrestricted diffusivity and A. The following groups of solute-solvent systems were studied both for the diffusion experiments as well as the equilibrium adsorption studies: (1) substituted single and double ring aromatics as solutes and cyclohexane as solvent; (2) paraffinic compounds as solute and cyclohexane and toluene as solvent; (3) straight chain olefins as solute and either cyclohexane or methylcyclohexane as solvent. Table I gives the effective diffusivity data and the equilibrium partition
1.17
1.00 1.27
0.901 1.22 1.23
coefficients for the diffusion and adsorption of solutes using the various solvents for the different samples studied. The effective diffusivities were evaluated from the fractional mass uptake data and time using the solution to the model previously described (Prasher and Ma, 1977). The effective diffusivity is evaluated for a t least five or six samples tapped during the course of the experiment. The values of effective diffusivities obtained are averaged. To show the validity in the assumptions made on the effect of the adsoTption in the model, data on WPI-4, WPI-5, and WPI-6 were plotted in Figure 2. DeffKp/DOand V , when plotted on a semi-log basis gives a linear plot. This is consistent with analogous results reported earlier in another related study (Prasher and Ma, 1977) involving a much wider range solute-solvent system and two different batches of porous material. In the case of the fresh unused WPI-4, if the random-pore model (RPM) is assumed to account for the tortuosity T, the data fall around the same correlation as proposed well within the following semiempirical correlation proposed earlier (Prasher and Ma, 1977). In DeffKpT = a X 3 + b DO where the values of a and b within a 95% confidence limit are -3479.2 f 616.5 and 0.863 f 0.19, respectively. Satterfield et al. (1973), working with a catalyst-solute-solvent system having K, values of approximately 1.0, found a linear relationship in X instead of a cubic relationship. However, they used molecular diameters of comparable sizes as the average pore diameter (i.e., X = 0.15-0.5 vs. X < 0.1 in this study). This seems to indicate that for small values of A, the ratio of the volume of the diffusing molecules to that of the pores is a more appropriate parameter in correlating diffusivity data. The range of K, values obtained in this study varied by more than a factor of 2. The linearity observed in the adsorption isotherms is to be expected due to low solute concentrations involved. The apparent fit of the data with the model for the effective diffusivity indicates that adsorption has a strong effect on diffusion rates, a conclusion in line with other observations (Satterfield et al., 1973; Prasher and Ma, 1977). Figure 1shows a typical result from these diffusion experiments. It shows tremendous differences in the rates of diffusion between the three samples, depending upon their history. If the data for M t / M , equal to 0.5 are examined in the case of all three samples, it may be observed that the time required for this uptake in WPI-5 is three times of that in WPI-4, and over four times in WPI-6. For effective diffusivity data for the various solute-solvent combinations, Table I and Figure 2 generally show these corresponding differences.
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Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978 10
I
I
I
I
,
1.0
0.8
8
s
.-
\
I
Y
0.6
0.2 a
a 0.I
0.4
0.08
A
w,
0.2
WPI-5 W:l5l WPI-6 W: 151 Pellet Length Pellet Diarn
0.06
I
0.06
0.04
0 0
1
2
3 + 112
4
5
6
7
6
9
1
I I
200
v,
Mi", I/2
Figure 1. Fractional uptake vs. time for diffusion of 1-hexene using methylcyclohexane as solvent in three samples.
Figure 2 shows that the reduced diffusivity when plotted on a semi-log basis vs. the molecular volume gives a straight line plot. The lines obtained through a linear regression are distinct and different in each case since the pore structure has undergone radical changes through coke and metal deposition. WPI-5 reduced diffusivity values are generally somewhat four times smaller for solute having smaller V , than in fresh WPI-4. In the case of WPI-6 the reduced diffusivity is over four times smaller than in WPI-4. All these are the result of severe coke and metal deposition that has taken place while in use. The porosity data given earlier bear this out. The structure change found in WPI-5 and WPI-6 through coke deposition is somewhat different in each case. The average pore size (71.7 A) in WPI-5 after coke deposition is actually larger than that of WPI-4 (57 A). Yet the reduced diffusivity for the compounds studied is smaller in WPI-5 by about three to four times. In WPI-6 on the other hand, where the reduced diffusivity is generally even smaller than in WPI-5, the average pore size is the same as in WPI-4. Clearly the reduction in diffusivity values has been accomplished not so much by a change in the average pore diameter as most probably a change in the tortuosity and also most likely by changes in the constrictivity. The deposition of coke on WPI-5 and WPI-6 results in the blocking of pores and also in the reduction of their sizes. Figure 3, the result of mercury porosimetry on the samples, amply provides evidence on this. In this figure, the pore volume of the used pellets is expressed as a percentage of the pore volume of the original unused pellets. In the case of WPI-5, the plot in Figure 3 shows that the smaller pore structures are blocked or plugged to a greater degree than the larger pores. The result of this is presumably not only to increase the diffusion path for the diffusing molecules, but also to decrease relatively the amount of material taken up by the pores having smaller pore radii. Though these two effects in principle counterbalance each other, the effect in this particular instance is to make the intraparticle transport characteristics more favorable as compared to WPI-6. Because of the relatively greater blockage in the smaller pore region, the average pore diameter in WPI-5 is acutally greater than that in the unused pellets (Le., 71.7 vs. 57 A). Kim and Smith (1974) report in the case of sintering the NiO pellets that the tortuosity factor can increase several times because
1
1 IO0
i
300
mligmol
Figure 2. Reduced diffusivity vs. molecular volume.
0 8
I
i 0.6
0.4
0.2
i 0
10
20
I
1 30
I
1 40
60
rp
80 100
200
300
B
Figure 3. dV (used pellets)/dV (unused pellets) vs. rp for WPI samples.
of pore plugging. If it is assumed that pore interconnections can be blocked by depositions of coke or metal resulting in a great increase in the diffusion path, then it is easy to explain that reduced diffusivities can actually decrease even in spite of the increase in average pore size. In WPI-6 on the other hand, the pore blocking has occurred to a greater degree in the region of the larger pore sizes as can be seen from Figure 3. The average pore size in this case is the same as in WPI-4 even though the diffusivity values are even lower than in WPI-5. This case is analogous to shell progressive model in catalyst deactivation (Wheeler, 1955). Stanulonis et al. (1976), in a study on coal liquefaction catalyst aging by use of an electron microprobe, report that the upstream samples from the downstream section exhibited somewhat more uniform coke and metal deposition. The result of pore plugging by coke and metal deposition is not only to increase the diffusion path but it also changes the constrictivity (i.e,, the ratio of the maximum and minimum cross-sectional area of the pore). Increasing the constrictivity of the porous pellets can greatly reduce the diffusivity of molecules (van Brake1 and Heerjes, 1974). The implication of the results presented here seems to suggest that very often, particularly in petroleum processing accompanied by serious coke and metal deposition, the deactivation of the catalyst may not be totally due to the
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
poisoning of active sites. Diffusivity changes with coke deposition can contribute significantly toward deactivation by progressively altering the structure of the porous medium resulting in the simultaneous decrease in intraparticle transport properties with time. For instance for a first- or second-order reaction, especially for a Thiele modulus of greater than 0.5, the effectiveness factor can be reduced anywhere from 25% to 50% by a reduction of the diffusivity of the controlling diffusing species in the reaction. In many catalysts where deactivation has partially or totally been brought about by coke deposition resulting in the reduction of the intraparticle diffusivities, regeneration of catalyst through coke burning should regenerate at least partially the original properties of the catalyst. From this it is implicit that diffusion experiments provide one of the useful tools in trying to diagnose the reasons for catalyst deactivation.
Conclusions 1. Coke and metal depositions in porous catalysts can seriously alter the intraparticle transport characteristics of catalysts by pore-plugging and also introducing constrictivities in catalyst pores. 2. In reactions where intraparticle diffusion is important, catalyst deactivation may a t least partially be the result of coke and metal deposition. Acknowledgment The authors wish to thank Gulf Research and Development Company for financial support of this research. The authors also wish to express their appreciation to Mr. Charles Keisling, Senior Technical Designer, for his invaluable help in designing and building the equipment used in this study. Nomenclature u = constant in eq 1,also radius of cylindrical pellet b = constantineq 1 D,ff = effective diffusivity of solute, cm2/s K , = equilibrium partition coefficient
269
M t = solute uptake up to time t M , = solute uptake at equilibrium r p = poreradius,hl rs = solute molecular radius, A t = time, s V = pore volume, cm3/g V , = molar volume at boiling point, cm3/g-mol Greek Letters
*
0 = dimensionless time, D,fft/u 6 = porosity, cm3/cm3 7 = tortuosity w = ratio of height to diameter for a cylinder X = r s / r p ,ratio of molecular radius to the average pore radius
Literature Cited Anderson, R. B.,Catalysis, IX, 29 (1956). Butt, J. B., Adv. Chem. Ser., No. 109, 259 (1972). Carberry, J. J., Gorring, R. L., J. Catai., 5, 529 (1966). Colton, C. K., Satterfield, C. N., Lai, C., AlChEJ., 21, 289 (1975). Crank, J., "The Mathematics of Diffusion", Oxford at the Clarendon Press, London, 1975. Khang, S. J., Levenspiel, O., hd. Eng. Chem. fundam., 12, 185 (1973). Kim, K. K., Smith, J. M.. AIChEJ., 20, 670 (1974). Krylov, 0. V., "Catalysis by Non Metals", p 255, Academic Press, New York, N.Y., 1950. Laidler, K. J., "The Chemical Kinetics of Enzyme Action", p 336, Oxford University Press, New York, N.Y., 1958. Levinter, M. I., Panchenkov, G. M., Tanatarov, M. A,, lnt. Chem. Eng., 7, 23 (1967). Masamune, S.,Smith, J. M., AlChEJ., 12, 384 (1966). Maxted, E. B.,Adv. Catal., 3, 129 (1951). Mills, G. A., Boedeker, E. R., Oblad, A. G., J. Am. Chem. SOC., 72, 1554 (1950). Ozawa. Y., Birschoff, K. B., Ind. Eng. Chem. Process Des. Dev., 7, 67 (1968). Prasher, B. D., Ma, Y. H.. AlChEJ., 23, 303 (1977). Prater, C. D., Lago, R. M., Adv. Catal., 8, 283 (1956). Satterfield, C. N., Colton, C. K., Pitcher, W. H., Jr.; AlChEJ., 19, 628 (1973). Stanulonis. J.'J., Gates, B. C., Olson, J. H., AlChEJ., 22, 576 (1976). Stoli, D. R., Brown, L. F., J. Catal., 32, 37 (1974). van Brakei, J., Heertjes, P. M., lnt. J. Heat Mass Transfer, 17, 1093 (1974). Voorhies, A., Jr., Ind. Eng. Chem., 37, 318 (1945). Wheeler, A.. in "Catalysis", Vol. 2, P. H. Emmett, Ed., Reinhold, New York, N.Y., 1955.
Received for review M a r c h 9, 1977 Accepted M a r c h 10,1978
