Irreversible Deactivation of Zeolite Fluid Cracking Catalyst. 1 X-Ray and Catalytic Studies of Catalyst Aged in an Automated Microcatalytic System for Gas Oil Cracking Nai Y. Chen,' Thomas 0. Mitchell, David H. Olson, and Bruce P. Pelrine MobB Research and Development Corporation, Central Research Division, Princeton, New Jersey 08540
Catalytic cracking catalysts are known to deactivate by the combined action of prolonged exposure to water vapor and of very high temperatures. However, under conditions representing the maximum temperature (1300 O F ) and partial pressure of water (190mmHg) encountered during normal commercial practice, the catalyst deactivation rate observed in the laboratory is much lower than that observed in commercial practice. An accelerated loss of activity was observed in the laboratory only when the temperature exceeded -1500 O F . Since these higher temperatures could be present during unit upsets and/or in the cyclones, the present study indicates that the catalyst deactivation rate could be reduced substantially if high temperature excursions in commercial regenerators could be avoided through proper unit operation and/or modification. X-ray analysis of the catalysts showed that the structure of zeolite Y undergoes a stabilization process at high temperatures; thus it is believed that additional resistance to deactivation could also be incorporated in the fresh make-up catalyst if it were stabilized before addition to the unit.
Introduction The average life span of a zeolite fluid cracking catalyst in a commercial gas oil cracking unit is between 20 and 100 days. Loss of catalyst due to particle attrition used to be the principal reason for adding catalyst to maintain the required inventory for proper operation. However, with improved attrition resistant catalysts, it became apparent that both catalyst make-up and withdrawal are often necessary in order to maintain the unit at a desired steady state of operation. Commercial catalysts undergo irreversible loss of performance with regard to conversion and product selectivity. Among the major causes of loss of selectivity is the build-up of metals such as nickel and vanadium (Cimbalo et al., 1972). This is becoming an ever more serious problem as the trend toward cracking higher boiling feedstocks continues. In addition, the catalyst in commercial operation undergoes irreversible loss of activity which is, among other causes, related to the hydrothermal stability of the catalyst. Under the influence of steam and high temperature, the nonzeolitic matrix undergoes changes in surface area and pore size distribution, and the zeolitic component loses active sites through crystal destruction. It became a practical problem for refiners and catalyst suppliers to evaluate a catalyst in the laboratory and predict its performance in commercial units. A variety of accelerated laboratory tests have been developed (Magee and Blazek, 1976) to simulate commercially equilibrated catalysts. Such stimulated tests often involve a severe steaming treatment of fresh catalyst. It is questionable that the resulting material would in fact duplicate the true composition of a catalyst aged during commercial operation. (It certainly does not duplicate the heterogeneous mixture making up the commercial inventory). In fact, it has been stated (Letzsch et al., 1976) that the only substitute for a commercially equilibrated catalyst is to run a laboratory unit under commercial operating conditions. This costly and time consuming procedure has seldom, if ever, been adopted by any laboratory. Other variables encountered in commercial practice, such as low concentrations of SO2, thermal shocks as the results of cyclic exposure to hydrocarbons and water vapor, and metal contamination, etc., could also have significant effects on catalyst deactivation. The present study was undertaken in an attempt to resolve some of these uncertainties. 244
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
Recently an automated microcatalytic gas oil cracking system was developed in this laboratory (Chen et al., 1977). The system requires less than 5 g of catalyst, and can be left to operate unattended. It is thus particularly suited for studies which require a long time to complete, such as our attempt to simulate deactivation under commercial conditions in the laboratory. In addition to catalytic evaluations, x-ray diffraction studies were also made on the catalysts to determine the relationship of catalyst structure to activity and stability. Experimental Section Catalyst. An ammonium Y-containing cracking catalyst was used in this study. X-ray analysis of the fresh sample showed 23.6% NH4Y zeolite with a unit cell dimension of 24.72 A. After calcining in a thin layer at 970 O F with dry air for 2 h the sample crystallinity dropped to 11.2%with no significant change in unit cell dimensions. This material was termed unstabilized sample in the present study. The fresh sample, after steaming at 1400 O F in 1 atm of steam for 4 h, also showed a lower crystallinity of 10.7%.In addition, a significant contraction of unit cell to 24.35 A was noted. This material was termed stabilized sample in the present study. This terminology is consistent with the known hydrothermal chemistry of NH4Y (Kerr, 1969;McDaniel and Maher, 1976). For comparison purposes, a typical equilibrium catalyst sample obtained from one of Mobil's FCC units was studied. X-ray analysis showed 3.7% of zeolite Y with a unit cell dimension of 24.36 A. Catalytic Test. The apparatus and general procedure of the automated microcatalytic system have been described previously (Chen et al., 1977). Specifically, the cracking reaction was carried out a t 865 O F , 2 catalyst to oil ratio, and 6 WHSV with 5 g of catalyst (30/70 mesh). Three multicycle runs were made: (I) 80 cycles on a stabilized sample with maximum regeneration temperature of 1250 O F and 24 mmHg partial pressure of water vapor; (11) 133 cycles on an unstabilized sample with maximum regeneration temperature of 1300 O F and 190 mmHg partial pressure of water vapor; and (111)650 cycles on a stabilized sample with varying regeneration temperatures up to 1300 O F and partial pressure of water
Table I. Run Conditions Reaction Cycles
Regeneration
Purge
Feed
Temp, "F
Temp, "F
LETGO
865
1150
8 min, Temp, "F
Gas
22 min, Temp, "F
Gas
1150
1250
24 mmH2O in air
1300
190 mmHpO in air
1250 1250
170 mmH2O in air 121 mmHzO and 2,000 ppm SO2 in air 121 mmHzO and 2,000 ppm SO2 in air 191 mmHzO and 2,000 ppm SO2 in air 191 mmHzO in air
Run I 1-80
He Run I1
1-133
LETGO
865
1030
He
1-158 159-221
LETGO LETGO
865 865
980 980
He He
1030 Run 111 1150 980
222-241
LETGO
865
1030
He
1030
1300
242-397
LETGO
865
1030
He
1030
1300
398-472
LETGO
865
1030
473-4
LETGO
865
475-7" 478-499 500-647
LETGO Refinery Refinery
865 865 960
H2O 1030 1300 (steamed catalyst 100%steam, 1022 "F, 64 h in situ) 1030 He 1030 1300 (steamed catalyst 100%steam, 1022 OF, 24 h in situ) 1030 He 1030 1300 1030 191 mm H20 in air 1030 1300 1030 191 mm HzO in air 1030 1300
191 mmH20 in air 191 mmHzO in air 191 mmHzO in air 191 mmHzO in air
NOTE: Mass balance runs under conditions identical to cycles 475-7 were run as cycles 538-9, 591-4, and 649-50 with LETGO. up to 190 mmHg and other variations. A summary of the run conditions is presented in Table I. The required partial pressure of water vapor was obtained by either of the following two methods: (1)the regenerating air was bubbled through a thermostated water saturator set a t the desired temperature; (2) water was pumped into the air line with a positive displacement pump. The air line was heat traced to prevent condensation of water. C h a r g e Stocks. The first two cyclic runs were made using a Light East Texas Gas Oil (LETGO). The third run began with the same feedstock, and switched to a refinery FCC feed after 477 cycles for the purpose of checking its effect on catalyst stability. Between 478 and 650 cycles, several LETGO runs were made to check the catalyst activity. Compositional data on these two feedstocks are presented in Table 11. X-Ray Analysis. X-ray data were obtained on a Siemens x-ray diffractometer system using graphite monochrometed Cu K a radiation. All samples were equilibrated in a constant humidity box a t 50% relative humidity before measurement. Crystallinity measurements were made by comparing peak heights of the h 2 k 2 -k 1 2 = 43 peak (5,3,3]with an NH4Y zeolite. For samples having crystallinity below 4%, planimetered peak areas were measured in place of peak heights. Unit cell dimensions were calculated for most samples, using the high-angle peak having h2 k 2 l 2 = 243. For samples of low crystallinity, d-spacing of the (5,3,3)1'ine was used.
+
+
+
Results a n d Discussion The initial study of 80 cycles on a stabilized sample with regeneration conditions set a t 1250 OF and 24 mmHg of water vapor failed to deactivate the catalyst. The conversion data are shown in Figure 1.The fresh catalyst had an initial activity of 73% conversion and rapidly approached a steady-state conversion of about 69% conversion after 20 cycles. From there on, the steady-state activity remained essentially constant for the next 60 cycles. The failure to deactivate the catalyst was at first attributed to the low partial pressure of water, since commercial experience indicated a partial pressure of water as high as 190 mmHg. In the subsequent experiments, the partial pressure of water was raised, and the regeneration
temperature was raised to as high as 1300 O F , reported to be the maximum temperature in the dense bed of a fluid regenerator. In addition, in the course of the 650 cycle run, other variables were studied including the addition of 2000 ppm of SO2 in the regenerating gas, the use of refinery FCC feed at higher cracking temperatures, and the use of 100%steam instead of helium as the stripping gas between cycles. All of these changes were made with the expectation of identifying the conditions reproducing the result of the commercial experience. The conversion data for the 650 cycle run on the stabilized sample, and the 133 cycles run on the unstabilized sample are summarized in Table 111.Also shown are the results on fresh catalyst and the refinery equilibrium catalyst. Figure 2 presents the data in terms of % conversion/(100 - % conversion) vs. number of cycles. Both runs showed a gradual decrease in activity with no significant or step change in the deactivation rate. Although the total time of exposure to cracking and regeneration after 650 cycles is equivalent to 27 days on stream in a commercial operation, the measured activity a t the end of 650 cycles was still far higher than that of a typical commercial equilibrium catalyst. X-ray data of the samples are presented in Table IV. Significantly lower crystallinity values were noted with samples from the bottom quarter of the catalyst bed. This unexpected finding led to the discovery that the heater winding in the reactor furnace had shifted during the month-long study and produced a hot spot near the bottom of the catalyst bed estimated at 200-300 O F above the controller set point. The larger loss of crystallinity of the bottom sample was probably the result of this exposure to temperature in the range of 1500 O F and above. Thus it appeared that the catalyst would undergo an accelerated deactivation only at temperatures substantially above 1300 "F. This prediction was tested by the following set of experiments. Samples of fresh catalyst were steamed a t various temperatures for 16 h in 190 mm of water in air; the resulting material was then tested in the microcatalytic unit. Portions were also examined by x-ray. These results shown in Table V confirmed the occurrence of an accelerated deactivation process a t above 1400 O F . The unstabilized sample was initially more active than the Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
245
Table I1
Table 111. LETGO Conversion at 865 OF (2C/Q and 6
WMSV)
LETGO Properties Gravity, OAPI Aniline pt., OF Sulfur, % wt Nitrogen, % wt Basic nitrogen, % wt Conradson carbon, % wt Visc. KV @ 210 O F Bromine no. R.I. @ 70 OC MW (Maxwell) Pour point, OF ASTM 5% ASTM 95% Compositions, % wt Paraffins Naphthenes Aromatics Aromatic rings, CA Refinery FCC Feedstock Properties Gravity, OAPI Aniline point, O F Sulfur, % wt Total nitrogen, % wt Basic nitrogen, ppm Conradson carbon, % wt Viscosity, KV 210 O F Bromine no. Hydrogen content, % wt Pour point, OF Molecular weight R.I. @ 70 "C Density & 70 OC Metals, ppm Nickel Vanadium Copper Iron Distillation, O F IBP 5% vol 10%vol 20% vol 30% vol 40% vol 50% vol 60% vol 70% vol 80% vol 90% vol 95% vol Composition, % wt Paraffins Naphthenes Aromatic Aromatic rings, CA
r-----
40
L 10
I
36.7 159 0.13 0.02 0.004 0.02 1.45
CHANGE FCC FEED TO
2
I
I
L
'
0
1
100
1
1
I 300
1
200
1
1
I
1
400
d 600
CYCLES
Figure 2. Experimental result.
Table PV.X-Bay Data Crystalline component Wt% a o , A
0.2 0.5
1300 O F . As a result, the unstabilized sample at the bottom of the bed was substantially destroyed by the high temperatures. Conclusions The most significant finding of this study is the failure to simulate the commercial deactivation experience in the laboratory. It is clear that a t temperatures typical of normal commercial operation, little hydrothermal deactivation occurred; only a t substantially higher temperature did deactivation become significant. It is possible that gas-phase temperatures in regenerator cyclones sometimes could exceed
1400 O F and, during upsets, the dilute phase regenerator temperature can also reach such levels. These results would suggest that the catalyst deactivation rate could be significantly improved if these high-temperature excursions could be avoided. In commercial practice, the make-up catalyst in its ammonium form is added directly to the dense phase of the fluidized regenerator. Although the x-ray data indicate that the zeolite in the equilibrium catalyst is in its stabilized form, it is believed that additional resistance to deactivation could be achieved if the make-up catalyst were prestabilized. Literature Cited Chen. N. Y., Burgess, W. P., Daniels, R . H.. lnd. Eng. Cnem., Prod Res. Dev., 16, 242 (1977). Cimbalo, R. V., Foster, R. L.. Wachtel, S. J.. Oil Gas J., 'PO (20), 112 (1972). Kerr, G. T.. J. Catal., 15, 200 (1969). Letzsch, W. S.,Ritter, R. E., Vaughan, D. E. W., Oil Gas J., 74 (4), 130 (1976). Magee, J. S., Blazek, J. J., "Zeolite Chemistry and Catalysis", Chapter 11, p 639, ACS Monograph 171, J. Rabo, Ed., 1976. McDaniel, C. V., Maher, P. K.. "Zeolite Chemistry and Catalysis", Chapter 4, p 324, ACS Monograph 171, J. Rabo, Ed., 1976.
Received for review January 21,1977 Accepted June 6,1977
irreversible Deactivation of Zeolite Fluid Cracking Catalyst. 2. Hydrothermal Stability of Catalysts Containing NH4Y and Rare Earth Y Nal Y. Chen,' Thomas 0. Mitchell, David H. Olson, and Bruce P. Pelrine Mobil Research and Development Corporation, Central Research Division, Princeton, New Jersey 08540
Three zeolite-containing fluid cracking catalysts, including an NH4Y type and two REY type, have been examined in a series of steaming experiments. The steamed samples were characterized by x-ray diffraction analysis, BET surface area, and microcatalytic gas oil test. Despite the differences in chemical composition and surface area of the matrix, sample crystallinity was found to bear a unique relationship with catalytic activity as defined by the pseudo-second-order rate constant in gas oil cracking. For the three catalysts, this rate constant is linearly proportional to the concentration of zeolite remaining in the sample. Empirical equations have related the loss of sample crystallinity to the steaming conditions. Extrapolations to commercial operating conditions confirm our earlier conclusions that the high rate of catalyst deactivation observed in commercial practice could be the result of localized high-temperature excursions occurring in the regenerator.
Introduction The slow irreversible loss of activity of zeolite fluid cracking catalysts in commercial use has been the subject of many investigations (Magee and Blazek, 1976; Wojciechowski, 1974). It is generally believed that this irreversible loss is related to the hydrothermal stability of the catalyst. Under the influence of steam and high temperature. the nonzeolite matrix changes its pore size distribution and surface area, and the zeolitic component loses its acid sites and crystallinity. The ideal experiment, a commercial run starting with fresh catalyst and without make-up, is clearly impractical. Current procedures (1,etzsch et al., 1976) in simulating commercially equilibrated catalysts, dictated by practicality, have entailed a few brief steaming experiments harsh enough to cause substantial activity losses. Such experiments differ so sig-
nificantly from field conditions that the question of their validity arises. Recently we reported a laboratory study of the long term stability of an ammonium Y containing cracking catalyst under conditions of cyclic cracking and regeneration with the regeneration conditions set at the maximum temperature (1300 O F ) and partial pressure of water (190 mmHg) encountered during normal commercial practice. We found that the catalyst deactivation rate observed in the laboratory was much lower than that observed in commercial practice and that the catalyst experienced a precipitous loss of crystallinity and catalytic activity a t above 1500 O F . It became apparent that the problem of catalyst stability in commercial units is further complicated by the design and operation of each unit. Thus, it would not be sufficient to evaluate a commercial Ind.
Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977 247
