When dioxane is used as solvent the polarity of the medium may be adjusted to the optimal value by adding water.
Acknowledgment The financial support given by Air Products and Chemicals Inc. is acknowledged. We want to thank A. Bjorkman for useful discussions. Literature Cited Cotton, F. A., Wilkinson. G., "Advanced inorganic Chemistry", Interscience, New York, N.Y., 1972. Gritchfield, F. E.,Gibson, J. A., Hall, J. L., J. Am. Chem. SOC., 7 5 , 1991 (1953).
Hjortkjaer, J., Jensen, V. W., hd. Eng. Chem., Prod. Res. Dev., 15, 46 (1976). Kilpi, S., Lindel, E., Ann. Acad. Sci. Fenn., Ser. All, 136 (1967). Landolt-Bernstein, "Elektrische Eigenschaflen I", Vol. 6, p 750, 1959. Mizoroki, T., Nakayama, M., Chem. SOC.Jpn., 39, 1477 (1966). Morris, D. E., Tinker, H. B., J. Organomet. Chem., 49,C53 (1973). Uguagliati, P., Palazzi, A., Deganello, G., Belluco, V., lnorg. Chem., 9, 724 (1970).
Received for review February 2 , 1977 Accepted July 11,1977
Supplementary Material Available: Reaction conditions a n d thermodynamic data ( 5 pages). Ordering information is given o n any c u r r e n t masthead page.
Steam Deactivation Kinetics of Zeolitic Cracking Catalysts Arthur W. Chester' and William A. Stover Mobil Research and Development Corporation, Research Deparfment, Paulsboro, New Jersey 08066
The rate constants for steam deactivation of three commercial FCC catalysts containing zeolite Y as the active component were determined by laboratory steam treatment for different time periods in the temperature range 1240-1550 O F (100% steam, 0 psig, fluidized bed). The temperature dependence could be empirically represented as the sum of two independent first-order decays: kd(T ) = AM exp(-EMIRT) AZ exp(-EZIRT), representing the independent deactivation of matrix (e.g., loss of porosity) and zeolite (e.g., loss of crystallinity) components. The relative stabilities of the three catalysts differ significantly in different temperature ranges. Increasing temperature as a means of increasing catalyst steam deactivation severity can give misleading estimates of overall catalyst stability, since the relative contributions of the two deactivation mechanisms change with temperature. The application of the results to deactivation in FCC units with different operating modes is discussed.
+
Introduction The stability of cracking catalysts toward deactivation is one of the most important catalyst properties. While selectivity, the yield of desirable vs. undesirable properties, has a profound effect on cracking economics, catalyst stability influences total product yield and operating costs. Catalysts with inadequate stability require excessive fresh makeup rates to attain necessary activity levels for optimum operation. While little information is available on deactivation mechanisms and kinetics for modern zeolitic cracking catalysts, a number of studies on deactivation of amorphous (silica-alumina) catalysts have appeared (see Literature Cited). The primary mode of deactivation involves steam-induced loss of surface area by growth of the ultimate gel particles, resulting also in loss of porosity. While amorphous catalysts deactivate thermally as well as hydrothermally, thermal deactivation is a significantly slower process (John and Mikovsky, 1961). Further, Dobres et al. (1966) have shown that the porosity and surface area distributions of equilibrium catalysts are more similar to steam-aged than thermally aged catalysts. Most of the above work dealt with changes in the physical structures of the catalysts, without relating these to changes in catalytic activity. John and Mikovsky (1961) calculated equilibrium catalyst activities assuming a first-order activity decay with some success. The inclusion of zeolites as the active component of modern cracking catalysts introduces a new aspect to formulations of decay mechanisms. In general, cracking catalysts contain
active zeolite and a less active matrix in various combinations; while each component has unique deactivation characteristics (Letzsch et al., 1976),the components may also influence each other. Letzsch et al. (1976) have shown that, like amorphous catalysts, the zeolite component is more strongly deactivated hydrothermally than thermally. Magee and Blazek (1976) estimate catalyst stability by varying steam treatment severity to match equilibrium catalyst activities and other properties. In view of recent innovations in the design and operation of regenerators in fluid catalytic cracking (FCC) processes (Rheaume et al., 1976) and a trend toward high temperature operation, a more quantitative understanding of cracking catalyst deactivation mechanisms is desirable for design and evaluation of suitable catalysts. In the present study, isothermal kinetic steam aging at relatively low temperatures, combined with the results of high temperature steam treatment, is used to derive a more quantitative picture of the steam deactivation kinetics of three commercial FCC catalysts.
Experimental Section The three catalysts used in this study, designated A, B, and C, are all commercially manufactured catalysts; catalyst properties are listed in Table I. Steam treatments were conducted with 100%steam at atmospheric pressure (0 psig) in cylindrical, 3 in. i.d. alonized Inconel vessels. The catalyst samples (500-1500 g) are added Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4 , 1977
285
Table 11. Properties of Wide-Cut Mid-Continent Gas Oil (WCMCGO)
Table I. Catalyst Properties Catalyst A
Pore volume, cm3 of H2Olg Packed density, g/cm3 Surface area, m2lg (calcined 1200
c
R
0.72 0.47 0.54 0.79 336 328
0.44 0.75 169
Semi- Clay synthetic 77 83
Claygel
on, ' I
Catalyst type Relative initial activity
.
79
.
FCC BENCH TEST G L A S S REACTOR
API gravity Sulfur, % wt Nitrogen, % wt Basic nitrogen, ppm Conradson carbon, % wt Aniline point, "F Bromine number Refractive index @ I O "F Pour point, O F Viscosity, kV a210 O F Molecular wt Hydrogen, % wt Specificgravity, 60/60 OF Metals: Ni, ppm V, ppm Fe. nom Distillation, "F IBP 5%vol in 20 30 40 50 60 70 80 ~
Preheat Coils
\
Product Outiet
I
.
.
90
95
29.2 0.51 0.065 152 0.29
181 2.5 1.48852 85 3.55 328 13.06 0.8767 0.1 0.2 32 472 545 578 608 632 665 707 754 796
851 920 958
Chargestock Dispersion [ W i t h N21
Figure 1. Glass reactor for FCC bench scale test unit.
to the vessels at ambient temperature and heated to the desired treatment temperature in air; however, when necessitated by the chemical nature of a catalyst, steam replaced air during its pretreatment period. Temperature was controlled with three thermocouples at different heights in the catalyst bed. Throughout the pretreatment and steam-treatment periods, the catalyst was maintained in a fluidized state. Fine particles (under 40 diameter) present in the fresh catalyst are elutriated during the treatment, resulting in particle size distributions similar to equilibrium catalysts. Catalyst activities were determined in a bench-scale FCC catalyst test unit (Figures 1,2, and 3). The glass reactor is shown in Figure 1. The gas oil charge is vaporized in the upper preheat coils and dispersed through the fluidized catalyst bed (100-180 g) at the bottom. Product is separated from the catalyst with a fritted filter. Liquid product (syncrude) is condensed at 120-130 O F and lighter products isolated in the gas burets (Figures 2 and 3). The s y n crude product is distilled to yield crude gasoline (430 O F end point) and bottoms. After analysis of all products, including carbon-on-catalyst, a material balance is calculated to produce a complete product distribution. Catalyst contact times as low as 0.5 min are possible, although most bench runs are performed at 1-5 min contact times. For the purposes of this study, a wide-cut Mid-Continent gas oil (WCMCGO, Table 11) was cracked over a period of 2.4 min at 935 O F initial temperature. Cracking severity may be altered by changing oil charge rate and/or catalyst quantity. For high temperature steaming (1400-1550 286
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Figure 2. FCC bench scale test unit O F ) activity was determined at 3 C/O, 8.33 WHSV, while for the more active samples from low temperature steamings, a lower severity, 2 C/O, 12.5 WHSV, was used. Activity is defined as conversion (% vol) to material boiling at or below 430 OF.
VENT
Lt-
ILEED N2
ZENITH P LIMP
VENT
"n
REACTOR
U
3
COLD TRAP -45'F
L I G H T GAS RECEIVER VFNT
1i
o
1 I
O F 'WATER BATH
4
AIR
Figure 3. Schematic of FCC bench scale test unit.
Results Rate Constants. Samples of the three catalysts used in this study were deactivated by steam treating with 100%steam, 0 psig in a fluidized bed at various temperatures (1240-1550 OF). Kinetic Steam Aging. This was performed by steaming each catalyst at 1240,1320, and 1400 O F for 12 h a n d removing samples a t appropriate times (e.g., 1 , 4 , 8 , and 12 h). The activity (conversion, % vol) of each sample was determined a t 2 C/O, 12.5 WHSV in a'fixed fluidized bed. First-order rate constants (day-1) and initial activities were determined from the slope and intercept of log conversion vs. steaming time plots, illustrated in Figure 4 for the deactivation of the three catalysts a t 1400 O F . Deactivation at higher temperatures was measured by steaming for 4 h at 1400,1475, and 1550 O F and testing a t 3 C/O, 8.3 WHSV (because of the lower activities). Rate constants were determined by calculating an initial activity (&) a t 3 C/O by use of the 1400 O F rate constant from the kinetic steam aging data and the activity a t 3 C/O, then using this initial activity for calculating the higher temperature rate constants. The rate constants and initial activities so determined are given in Table 111. The agreement of the initial activities determined independently a t the three different kinetic steam aging temperatures (1240,1320, and 1400 O F ) are excellent, providing a truly temperature-independent measure of actual initial activity. Temperature Dependence. An Arrhenius plot (In k vs. 1/T) for the three catalysts is shown in Figure 5. The curves for A and B are clearly nonlinear, while C is almost linear. (The curves shown are fit empirically to second-order polynomials by linear regression.) The nonlinear temperature dependence suggests that the temperature dependence may be empirically represented by
55
0
25
7
5
5
10
I2 5
lime H w r i
Figure 4. Typical kinetic steam aging data at 1400 "F (100%steam, 0 psig) for catalysts A, B, and C .
the sum of two independent first-order decays
kd(T)= AMe-Er*I/RT+ A ~ ~ - E Z / R T
(1)
The "activation" parameters for eq 1were determined from the rate constant data in Table 111as follows. (a)The experimental constants were "smoothed" by fitting the experimental data to second-order polynomials in the Arrhenius format (Figure 5) and back-calculating rate constants a t four temperatures (usually 1320, 1400, 1475, and 1550 O F ) . (b) The parameters in eq 1were then determined with the NewtonRaphson procedure (see Appendix) for solving simultaneous equations, using the smoothed rate constant-temperature data set and four equations of the form
k, = AMe-EM/RTi+ Aze-Ez/RTi
(i = 1,.. . . , 4 ) (2)
Calculated Arrhenius plots, showing the resolution of the total Ind. Eng. Chern., Prod. Res. Dev., Vol. 16, No. 4, 1977
287
Table 111. Summary of Rate Constantsa and Calculationsb Catalyst B
C
0.08 76.3 0.16 77.6
0.13 84.0 0.26 82.2 0.71 82.4
0.03 78.8 0.18 79.9 0.49 78.3
80.4 82.6
81.0 91.2
79.9 86.7
72.6 0.77
68.9 1.68
50.6 3.23
21.8 7.99
38.3 5.21
25.9 7.25
A k d , 1240 O F Ao (2 C/O), 1240 O F k d , 1320 O F Ao (2 C/O), 1320 O F k d , 1400 O F Ao (2 C/O), 1400 O F
(C) (C)
4 h, 1400 O F , 0 psig Activity, 3 C/O Calcd A0 (3 C/O) 4 h, 1475 O F , 0 psig Activity, 3 C/O k d , 1475 O F 4 h, 1550 O F , 0 psig Activity, 3 C/O k d , 1550 O F
FCC bench tests: 3 C/O, 8.3 WHSV, WCMCGO (935 OF); 2 C/O, 12.5 WHSV, WCMCGO (935 O F ) . Accurate kinetic a k d in day-’. data could not be obtained for catalyst A at 1240 O F because of its high stability. Conclusions in the text regarding its low-temperature stability below 1300 O F must be regarded as extrapolations. Table IV. Steam Deactivation Parameters (Eq.1) Catalyst
“Matrix”
M (day-1) 1X lo3 EM(kcal) 19 A Z (day-’) 2 x 1029 ( E z (kcal) 145
lo5 27 4 x 1015 76 2
.
“Zeolite”
X
2 X 1013 65
3 x 1014 70
0 -
/
A I200
1300 Temperature
I500
1400
F ,AI I
Ti
Figure 5. Temperature dependence for catalyst steam deactivation, 100%steam, 0 psig, for A, B, and C.
rate constant into two independent exponential curves, a total calculated curve and the fit of the experimental data to the calculated curves are shown in Figure 6. The calculated “activation” parameters are given in Table IV.
Discussion The temperature dependence of the deactivation as described by eq l must be regarded as empirical, since it describes a relationship between catalytic activity and a physicochemical process. The conversion of a gas oil to gasoline and lighter products involves many primary cracking steps per active site and is a utilitarian measure of activity. Nevertheless, if care is taken to keep conversions below that a t which gasoline recracking becomes important (about 85% vol for WCMCGO) by changing cracking severity, activities so determined do appear to correlate well with actual activity. Further, since several independent mechanisms may contribute to deactivation, the true temperature dependence would best be described as a sum of many exponential decay terms, as has been proposed by Adams (1963) for amorphous catalysts. The experimental data would limit expansion of eq 1 to more terms, given its accuracy limits of perhaps f 1 % in activity and f 1 0 O F in temperature. The rather good fit of the experimental data to rate constants calculated from eq 1 (Figure 6) suggests that, while the deactivation might be described by many independent 288
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0.03 I 1ZW
I
I
13w
I
15w
14M
Temperature
F
IAs
1’11
B
0 05
I
12M
I 1300 imperalure
1
I
I
1400
l5On
1600
F i A s It11
Figure 6. A, Determination of rate parameters for catalyst A. B, Determination of rate parameters for catalyst B.
mechanisms, these mechanisms may be combined into two groups: a low-temperature group, perhaps due to changes in the amorphous matrix and characterized by A Mand E M ,and a high-temperature group due perhaps to loss of active component (zeolite crystallinity) characterized by A Z and Ez. Deactivation at low temperatures (1100-1350 O F ) can be considered to be dominated by matrix changes, primarily loss of porosity, while deactivation a t high temperatures (13501550 OF) is dominated by hydrothermal loss of zeolite crystallinity. The direct attribution of the parameters of eq 1 to matrix and zeolite exclusively, however, is a t best approximate; matrix-zeolite interactions would certainly influence the A and E parameters. The change of mechanism as temperature is raised produces a “catastrophic decline” in catalyst activity that is particularly well demonstrated by Figure 6 showing the much more rapid loss of activity for A and B at high vs. low temperatures, primarily due to the change in deactivation mechanism. Hightemperature steaming data as a measure of catalyst stability must be used with caution: increasing temperature as a replacement for time can change the mode of deactivation and not give results relevant to low-temperature stability. Relative Catalyst Stability. The parameters determined above do not directly allow stability comparisons. Calculated rate constants for the three catalysts for 1200-1550 O F are compared in Figure 7 . Such data allows relative stability estimates to be made for different temperature ranges, for instance: below 1250 OF, C > A >> B; 1250-1400 O F , A > C > B; above 1400 OF, A > B > C. Catalyst C, while quite stable a t low temperatures, deactivates much more rapidly than catalysts A and B a t higher temperatures. Catalyst A is stable at both low and high temperature ranges; note, however, the rapid increase in deactivation rate above 1500 OF, an excellent example of “catastrophic decline” (Figure 6a). Catalyst B is relatively unstable in all temperature ranges, except above 1500 OF, where its “catastrophic decline” is less severe than A. The low-temperature instability of B is primarily due to lack of zeolitic stability (Figure 6b). Deactivation Rates in Commercial FCC Operation. In contrast to laboratory steam treatments, commercial FCC regenerators are far from isothermal in operation. Spent catalyst enters the regenerator at 850-1000 O F and is rapidly heated to dense bed temperatures of 1150-1350 O F by the regeneration process. Due to afterburning of CO, dilute phase and cyclone temperatures can be 50-250 O F above dense bed temperatures. Thus the rate of catalyst deactivation can be severely influenced by regenerator design and operation: the more catalyst contacting steam at high temperatures, the more severe the deactivation. The actual deactivation rate for a specific catalyst and regenerator can only be calculated if a detailed catalyst-temperature distribution is known. If the catalyst distribution is F ( T ) such that
kc
Day-‘
TinDeriluie
F
Figure 7. Effect of temperature on steam deactivation rate (100% steam, 0 psig) constants for catalysts A, B, and C.
J T m a x F ( Td)T = 1
(3)
Tmin
where Tminand T,,, are, for instance, spent catalyst and cyclone temperatures, the deactivation rate can be calculated as (4) where k d ( T ) is defined by eq 1. Unfortunately, such distribution functions are not generally known and are difficult to obtain. The relationship of measured regenerator temperatures to actual particle temperatures also needs better definition. Effect of Deactivation Temperature on Catalyst Selectivity. The data obtained in this study also allow an examination of the effect of temperature on selectivity. Increased selectivity is defined for cracking catalysts as increased liquid product yield and/or decreased coke yield a t a given conversion. Factors useful for comparisons of selectivity are gasoline efficiency (Cs+ gasoline, % vol/conversion, % vol), coke yield or the ratio of Cs+ gasoline (% vol) to coke (% wt), all at a constant conversion. The data in Table V were chosen from the kinetic steam aging experiments to show the differing selectivities when catalysts are deactivated to a similar activity (73-76% vol conversion) at different temperatures. For a given catalyst, increasing deactivation temperature results in improved selectivity. Further, the relative selectivities of the three catalysts (A > B > C) are the same a t either temperature.
Conclusion Knowledge of low- and high-temperature catalyst stability characteristics, as characterized by the so-called “matrix” and “zeolite” parameters of eq 1, is important in designing and choosing catalysts for different types of regenerator operation.
Table V. Effect of Deactivation Temperature on Selectivity Catalyst ‘Steamingconditions: (100%steam, 0 psig): Temperature, O F Time, h Yields at 2 C/O, 12.5 WHSV: Conversion,% vol Cg gasoline, % vol
+
Coke, % wt C5 + gasoline (% vol)/coke (% wt)
1320 8
73.3 64.8 2.9 22.3
C
B
A
1400 4
1320 8
1400 4
1320 8
1400 2
75.0
73.7 61.3 3.5 17.5
74.4 63.6 3.0 21.2
75.8 64.4 3.8 16.9
75.2 63.4 3.5 18.1 (+7%)
65.4
2.4 27.3 (+22%)
(+21%)
Ind. Eng. Chem., Prod. Res. Dev.,Vol. 16, No. 4, 1977
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In conventional regeneration, substantial quantities of CO are formed during carbon burning, and CO afterburning produces high temperatures in the dilute phase and cyclones. The degree to which high-temperature stability will be required is dependent on the actual temperatures (dependent on the coking characteristics of the catalyst and charge stock), degree of afterburning, and other regenerator characteristics. In modern high-temperature regeneration processes (Rheaume et al., 1976) involving complete oxidation to COz, the additional heat obtained from complete CO oxidation is transferred to the catalyst and regenerator temperatures are 100 O F or more higher than in conventional regeneration. When the oxidation is accomplished thermally, the heat is largely transferred to catalyst in the dilute phase; such processes may require catalysts with good high-temperature stability. In catalytic CO oxidation, where the recently available promoted catalysts are used (Rheaume et ai., 1976), the heat of CO oxidation is transferred to the dense bed, and higher temperature extremes do not occur. In such processes low-temperature (“matrix”) stability may be more important than high-temperature (“zeolite”) stability. The experiments described in this work are perhaps too tedious and time-consuming to apply in evaluating large numbers of catalysts. Nevertheless, the phenomenon described should be considered in evaluating steam deactivation data. Increasing temperature as a means of increasing catalyst aging severity can give misleading estimates of overall stability, due to the differing contribution of the two deactivation mechanisms at different temperatures. Activity losses at both low and high temperatures need be evaluated and related to the type of regeneration of interest.
The determination of the parameters in eq 1 was performed using smoothed rate constant-temperature data (described in the text) and solving by the Newton-Raphson interaction procedure. This procedure involves minimizing a difference vector D, determined as D=A-lF where the vector F is the current value of the functions and A is a matrix of differentials evaluated with current values of the variables
New values of the variables are determined by adding D to the last values. The calculation ends when D is less than some specified value. Initial guesses are determined from the slopes and intercepts of first and last pairs of points in the In h vs. T-l coordinate system.
Literature Cited Adams, C. R., Voje. H. H., J. Phys. Chern., 61, 722 (1957). Adams, C. R., J. Phys. Chem., 67,313 (1963). Ashley, K. D., Innes, W. B., Ind. Eng. Chem., 44, 2857 (1952). Dobrss, R. M., Rheaume, L., Ciapetta, F. G., Ind. Eng. Chem., Prod. Res. Dev., 5, 174 (1966). John, G. S.,Mikovsky, R. J., Chern. Eng. Sci., 15, 161, 172 (1961). Letzsch, W. S.,Ritter, R. E., Vaughn, E. W., Oil Gas J., 74 (4). 130 (Jan 26, 1976). Magee, J. S..Blazek, J. J., in “Zeolite Chemistry and Catalysis”, ACS Monograph 171, J. A. Rabo, Ed., p 615, American Chemical Society, Washington, D.C., 1976. Rheaume, L., Ritter, R. E.,Blazek, J. J., Montgomery, J. A., Oil Gas J., 74 (20), 103 (May 17, 1976); 74 (21), 66 (May 24, 1976). Schlaffer, W. G., Morgan, C. Z., Wilson, J. N., J. Phys. Chem., 61, 744 (1957).
Acknowledgment We are indebted to A. B. Schwartz for advice and support during this research.
Received for review May 13, 1977 Accepted August 1,1977
Appendix Calculation of P a r a m e t e r s f o r Eq 1 from R a t e Data.
Presented at the Fifth North American Meeting of the Catalysis Society, Pittsburgh, Pa., Apr 24-28, 1977.
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