Chapter 12
Contaminant-Metal Deactivation and MetalDehydrogenation Effects During Cyclic Propylene Steaming of Fluid Catalytic Cracking Catalysts Lori T. Boock, Thomas F. Petti, and John A. Rudesill Washington Research Center, Grace Davison, W. R. Grace and Company—Conn., 7500 Grace Drive, Columbia, MD 21044
Recent work on laboratory catalyst deactivation in the presence of Ni and V by cyclic propylene steaming (CPS) has shown that a number of conditions affect the dehydrogenation activity and zeolite destruction activity of the individual metals. These conditions include final metal oxidation state, overall exposure of the metal to oxidation, the catalyst composition, the total metal concentration and the Ni/V ratio. Microactivity data, which show dramatic changes in coke and hydrogen production, and surface area results, which show changes in zeolite stability, are presented that illustrate the effect each of these conditions has on the laboratory deactivation of metals. The CPS conditions which are adjustable, namely final metal oxidation state and overall exposure of the metal to oxidation are used as "variables" which can control the metal deactivation procedure and improve the simulation of commercial catalyst deactivation. In particular, the CPS procedure can be modified to simulate both full combustion and partial combustion regeneration.
One of the challenges in evaluating new FCC catalyst technologies has been in simulating how the catalyst will perform after being deactivated in a commercial FCC unit. As resid processing becomes more prevalent, the issue of metal contaminants and metal deactivation takes on even more importance. In a commercial F C C unit, metal contaminants, particularly Ni and V are deposited on the catalyst from the feedstock. These metals initially have high dehydrogenation activity. They can also react with and destroy the zeolite in the catalyst (1). As the catalyst ages,
0097-6156/96/0634-0171$15.00/0 © 1996 American Chemical Society
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the dehydrogenation activity and the zeolite destruction activity is greatly reduced. However, even the most complex laboratory deactivation procedures take only 2-3 days; thus, these metals are typically much more active than in a commercial unit. Development of a laboratory deactivation procedure that better simulates Ecat (equilibrium catalyst) performance is the objective of this work. Zeolite surface area, microactivity (MA) and M A coke and hydrogen yields are the tools which measure our success. An early attempt to simulate metals deactivation was the introduction of the Mitchell method steam deactivation procedure (2). This procedure involved impregnation of catalysts with Ni and V naphthenates, followed by steaming in the presence of air. While this method was easy to implement and did allow comparison of catalysts in the presence of metal contaminants, both the destruction of the zeolite and the metal dehydrogenation activity was greatly over-predicted (3, 4) in both M A T and riser testing. Cyclic methods which mimic commercial fluid, such as cyclic metal impregnation (CMI) have also been introduced (3) These methods improve on the Mitchell method procedure by exposing the catalyst to both oxidation and reduction conditions and imposing an age distribution on the metals. This greatly reduces the destruction of the zeolite (4), but the metal dehydrogenation activity is still higher than on an Ecat, particularly in M A testing. Additionally, methods such as CMI, which use a feedstock to deposit the metals on the catalyst, are timeconsuming, difficult to implement and have less than satisfactory reproducibility. In an earlier work, we introduced cyclic propylene steaming (CPS) as an alternative to CMI (5). This procedure gave large improvements over the Mitchell method deactivation procedure but is just as easy to implement. Catalyst evaluated by the CPS procedure gave similar yields and activities to the CMI procedure, with much better reproducibility. However, when compared to Ecats, particularly high metal Ecats, CPS still over-predicts M A coke and hydrogen yields. Despite this drawback, this tool is the most promising for catalyst evaluations in the presence of metals. Developments which address the shortcomings and improve on the CPS procedure will be described herein. An additional complication in simulating laboratory deactivated catalysts that match commercial Ecats is the fact that commercial F C C units are operated very differently. One difference which may have dramatic effects on ZSA retention, activity, and coke and hydrogen yields (especially for high metals operations) is full versus partial combustion regeneration. The laboratory deactivation procedures discussed above are all based on mimicking foil combustion regeneration and comparisons are to Ecats from foil combustion units. However, in developing the CPS deactivation procedure, we are targeting the ability to simulate Ecats from both full and partial combustion FCC units.
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CPS Deactivation Procedure The standard CPS procedure involves impregnating the catalysts with metal naphthenates before steaming, followed by alternate exposure of the catalyst to streams containing oxidizing and reducing gases. The standard redox cycles consist of:
10 minutes 50 wt% nitrogen, 50 wt% steam 10 minutes 50 wt% 5% propylene in nitrogen, 50 wt% steam 10 minutes 50 wt% nitrogen, 50 wt% steam 10 minutes 50 wt% 4000 ppm S02 in air, 50 wt% steam These cycles are repeated up to 30 times to give a total run time of 20 hours. Both CPS deactivated catalysts and Ecats are tested in the microactivity unit (modified A S T M D 3907-87) where activity and yields are measured. Catalyst surface areas (total, zeolite and matrix) are also measured by the t-plot procedure (6)
Comparison of CPS to Ecat Ecats from two different FCC units was compared to the same catalyst that was deactivated by standard CPS at comparable metal levels. Both units were operated in full combustion. The two catalysts were different grades with similar Ni («700 ppm), but different V( 600 vs. 2000 ppm) levels. Figures 1 and 2 display the M A coke and hydrogen yields, at constant conversion versus V levels. At the low V level, the CPS deactivated catalyst produced «5% more coke and «45% more hydrogen than the Ecat, whereas at the high V level, the CPS deactivated catalyst produced 40% more coke and 70% more hydrogen than the Ecat. Since the Ni levels were similar, we conclude that this additional coke penalty is due to V dehydrogenation activity. Thus one weakness of the standard CPS procedure is that it overemphasizes the V dehydrogenation activity compared to the corresponding Ecat as measured in M A testing. Figure 3 shows a graphical representation of the current CPS status summarizing the data shown in Figures 1 and 2 and Table I. It is clear from this figure that the ability to independently control the position of the CPS envelope on both the activity and the coke/hydrogen axes by varying the laboratory deactivation conditions is a desirable objective. Factors that Influence Metals Tolerance Differences Catalysts As any catalyst manufacturer will divulge, there are clear differences in the metals tolerance behavior of different catalyst grades. Catalysts are often designed to be tolerant to a specific metal, such as Davison's Ni tolerant technology. Thus, in addition to the ability to
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6.5
lowV
highV
Figure 1. Comparison of Ecat and CPS Deactivated Catalyst M A Coke Yields vs. V Level, illustrating poor match at high V levels
0.6
low V
high V
Figure 2. Comparison of Ecat and CPS Deactivated Catalyst M A Hydrogen Yields vs. V Level, illustrating poor match at high V levels
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Figure 3. The Current Status of CPS
Table I. Comparison of Ecat and CPS Deactivated Catalyst M A Activity and Catalyst Properties, illustrating poorer match at high V levels HighV LowV Ecat CPS Ecat CPS MA C/O ZSA MSA unit cell size,|x
70 4.5 172 47 24.25
70 5.0 163 43 24.23
65 3.4 152 33 24.23
65 4.9 132 33 24.23
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simulate commercial Ecats, a laboratory deactivation procedure must also be able to resolve differences in the metal tolerance of different catalysts. This is necessary if commercial performance of the catalyst is to be predicted. The standard CPS procedure is able to discern how the different catalysts respond to both Ni and V. A series of experiments were performed comparing a conventional catalyst with a Ni-tolerant catalyst, at varying Ni and V levels. The resulting coke factors (slope of M A coke vs. kinetic activity) were fit to a model and the resulting curves are shown in Figures 4 and 5. The contaminant coke produced by the conventional catalyst from Ni increases rapidly at low Ni levels, only leveling off at very high Ni levels. However, on the Ni-tolerant catalyst, the contaminant coke from Ni is very flat at all Ni concentrations. The V contaminant coke curves are similar for both catalysts, with the conventional catalyst producing more coke from V at high V levels and the Ni tolerant catalyst producing more coke at low V levels. These differences in the contaminant coke produced by the individual metals on the two catalysts result in very different Ni/V dehydrogenation ratios (DHR), also shown on the plots in Figures 4 and 5. At low metal levels, the DHR is high on the conventional catalyst, but it decreases as the coke produced by Ni levels off and the coke produced by V increases. Here again we see the over-emphasis of V activity at high V levels. However, the DHR is always greater than one at the conditions studied. On the other hand, the DHR for the Ni-tolerant catalyst is relatively constant and always less than one. This is due in part to the high Ni-tolerance of the catalyst and in part to the over-emphasis of V by the CPS procedure. An important consequence of this work is that the industry accepted Ni/V dehydrogenation ratio of "4" may not be applicable for all catalysts, and will vary with metal levels and F C C operating conditions.
Metal Oxidation State In a commercial FCC unit, the catalyst is exposed to oxidizing and reducing conditions numerous times. It is this oxidation/reduction which is believed to have a large impact on the deactivation of the metals (4, 7). This is why both CMI and CPS deactivation give improved metal deactivation over the Mitchell Method. Additionally, early patents (8) have suggested oxidizing or reducing a metal containing catalyst after steaming can also have a large affect on coke and hydrogen selectivities. Thus it seems that the final oxidation state of the metals may be an important factor affecting the metal tolerance of catalysts. In order to explore this hypothesis, we examined the oxidation state of the metals on the catalyst after CPS as compared to Ecats using temperature programed reduction (TPR). We found that the metals on the CPS catalysts (both V, and to a lesser extent Ni) were in a higher oxidation state than the metals on Ecat, even for a high excess oxygen F C C regenerator. Based on this result, the standard CPS procedure was
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g.
s
jr.
c o o
0
Coke from V
, , ,, •r'T'T'T • . • 500 1000 1500 2000 2500 3000 3500 4000 Metals Level (ppm Ni or V)
Figure 4, Comparison of Contaminant M A Coke from Ni and V (Conventional Catalyst) - Calculated dehydrogenation ratio (DHR) varies
Figure 5. Comparison of Contaminant M A Coke from Ni and V (Ni-Tolerant Catalyst) - Calculated DHR varies (and is
