Chapter 11
Sodium Deactivation of Fluid Catalytic Cracking Catalyst Xinjin Zhao and Wu-Cheng Cheng Washington Research Center, Grace Davison, W. R. Grace and Company—Conn., 7500 Grace Drive, Columbia, MD 21044
The mechanism of FCC catalyst deactivation by sodium is addressed in this paper. In commercial units, sodium is found to deactivate the matrix surface area significantly but no significant trend was observed for the effect of sodium on the zeolite surface area of equilibrium catalysts. On the other hand, the effect of sodium is more pronounced on zeolite and much less severe on matrix surface area in the typical laboratory deactivation protocol. The differences are explained by the mobility of sodium on catalysts. Significant interparticle migration of sodium is observed both commercially and in laboratory deactivated catalysts. The loading of sodium is associated with the available sites of the particular fraction of that catalyst. In commercial units, sodium preferentially migrates to the freshly added catalyst due to its greater availability of exchange sites. The effect of catalyst sodium and feed sodium are simulated in the laboratory and their effect on catalyst activity and cracking yields are discussed.
Sodium on fluid cracking catalyst, FCC, comes from the raw materials used in the catalyst manufacturing process as well as salt contamination in the feedstock. Sodium can deactivate cracking catalysts by poisoning the acid sites on the matrix and zeolite and by promoting sintering of silica-alumina (1). Sodium can act synergistically with vanadium to accelerate the destruction of zeolite (2). The relationship between cracking activity and sodium level of zeolites is complicated and seems to depend on the type of feedstock and catalyst
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pretreatment conditions . Sodium addition drastically reduces the activity of dealuminated Y-zeolite (USY) for alkane cracking (3-5). This is consistent with the notion that cracking of small alkanes requires strong Bronsted acid sites, and the addition of sodium effectively neutralizes these sites. Several authors have found that sodium addition does not decrease the activity of USY for gas oil cracking, a much more facile reaction. However, sodium addition on USY does influence gasoline composition and product selectivity of gas oil cracking (5-8). Differences in activity and selectivity have been observed for sodium added to zeolite before and after hydrothermal deactivation, implying that sodium on fresh catalyst and sodiumfromfeedstock behave differently (7). Investigations of sodium poisoning of amorphous silica-alumina catalysts show that activity loss is not a linear function of sodium content, being more rapid at low Na concentration and leveling off at higher Na concentration. This suggests that the strongest and most accessible sites are poisoned by Na first (1,9). Physical blocking of adjacent sites has also been proposed (10). The present day FCC catalyst consists typically of a USY zeolite in a silica-alumina matrix. The matrix can have a range of surface area and cracking activity (11). At the regenerator temperature, sodium has a solid state diffusion coefficient of 10" to 10 cm s (12) and is expected to move easily within a catalyst particle. The distribution of Na between zeolite and matrix and its effect on the stability of each component under these conditions is of interest. Furthermore, in the presence of steam, interparticle migration of volatile Na species is expected. The mechanism of this process has not been investigated. A greater understanding of these processes will aid catalyst manufacturers in designing a more sodium-tolerant catalyst. In this paper we have examined commercial equilibrium catalysts (Ecat) to look for correlations between zeolite and matrix area stability of various catalyst families as a function of sodium. We have performed density separation of Ecat samples to measure the sodium distribution as a function of catalyst age. Finally, we have revisited the issue of whetherfreshcatalyst sodium is equivalent to feedstock sodium in its influence on zeolite stability, catalytic activity and selectivity. 6
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2
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Experimental Methods Commercially deactivated FCC Ecats of varying matrix types and containing a wide range of sodium were characterized by t-plot surface area (ASTM D436585) to determine the effect of Na on zeolite and matrix area stability. The Ecats were also examined by electron microprobe (Cameca SX50) to determine the Na distribution within a catalyst particle. Some of the Ecats were separated into eight agefractionsbased on a modified sink/float procedure described in the literature (13,14). Each agefractionwas analyzed by ICP, t-plot and zeolite unit cell size (ASTM D3942-91).
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Sodium Deactivation of FCC Catalyst
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To determine if we could simulate in the laboratory the effect of sodium on commercially deactivated FCC catalysts, we prepared catalysts containing Na in the range of 0.22 to 0.41 wt% by modifying the catalyst washing procedure and deactivated the samples at 1088 K for 4 hours under 1 atm of steam. This steaming procedure is commonly used to prepare deactivated catalysts with physical properties (zeolite and matrix surface areas and unit cell size) that match commercial Ecats. The above procedure of incorporating sodium tofreshcatalyst has an inherent shortcoming. SodiumfromFCC feedstock accumulate on catalysts which have been hydrothermally aged. During hydrothermal aging, the zeolite unit cell size decreasesfromabove 24.50 A to typically lower than 24.30 A, the surface area of both zeolite and matrix decreases and transformation of kaolin clay to metakaolin occurs. To demonstrate this effect we prepared the following two catalysts. Catalyst A was a 50 wt% USY silica sol catalyst, washed and exchanged with ammonium sulfate to 0.49 wt% Na20 on catalyst. Catalyst B was the same catalyst, washed and exchanged with ammonium sulfate to 0.17 wt% Na20 on catalyst. Both catalysts were steamed for 4 hours at 1088 K. Subsequently, the steam-deactivated Catalyst B was impregnated with sodium carbonate, to bring its sodium content to the same level as Catalyst A, and calcined in air for 2 hours at 810 K. These samples, which we shall refer to as Catalyst A' and Catalyst B \ were evaluated by standard microactivity (ASTM D-3907) on a Sour Imported Heavy Gas Oil (SIHGO). Properties of the feedstock are shown in Table 1. To determine if and how rapid the impregnated sodium would redistribute on the catalyst under FCC regenerator conditions, we further steamed Catalyst A' and Catalyst B' for 4 hours at 1088 K. These samples, which we shall refer to as Catalyst A" and Catalyst B" were again evaluated by microactivity. Results and Discussion Effect of Sodium on FCC Ecat Matrix and Zeolite Surface Areas As part of our technical service program, Grace Davison analyzes commercially deactivated equilibrium catalystsfromFCC units worldwide. Due to differences in feed quality and unit operation, equilibrium catalystsfromdifferent FCC units contain varying levels of sodium, even though the sodium onfreshcatalyst may be similar. Figure 1 shows the matrix surface area of commercially aged Ecat for four types of cracking catalyst. Each set of data represents one type of cracking catalyst deactivated in different commercial FCC units. The four types of catalysts were chosen for their wide range of matrix surface areas. Although the fresh catalyst matrix surface area for each catalyst type is similar, the equilibrium matrix surface area decreases by as much as fifty percent with increasing Na. The variation of matrix surface could be attributed to other
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Table 1. Properties of Sour Imported Heavy Gas Oil
API Gravity, @ 289 K Aniline Point, K
22.5 346
Sulfur, wt% Total Nitrogen, wt% Basic Nitrogen, wt%
2.59 0.086 0.034
Simulated Distillation, Vol%, K IBP 490 20 616 40 655 60 696 80 745 FBP 826
n-d-m Analysis Cp Cn Ca
59.5 18.0 22.4
K Factor
11.52
Conradson Carbon, wt%
0.25
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causes, but comprehensive analyses of a vast amount of data showed that sodium level on the equilibrium catalysts had the most significant effect on the matrix surface area. Analyses also showed that the effects of nickel or vanadium on matrix surface areas were minimal for this particular catalyst (Figures 2-3). Similar results were observed for all four catalyst families. These results indicate that sodium is very effective in promoting the sintering of FCC matrices. Figure 4 shows the effect of sodium on Ecat zeolite surface area. It is clear that zeolite surface area does not correlate well with the level of sodium on catalysts. Part of the reason for the poor correlation could be due to the differences infreshcatalyst addition rates for different commercial units. It might also mean that matrix surface area is mostly affected by sodium level, while zeolite surface area is more determined by other factors. Since the equilibrium catalysts werefrommany different commercial units, the severity in the units can have a strong influence on the zeolite stability. A typical fresh FCC catalyst contains about 0.3 wt.% sodium on catalyst. Most of the sodium infreshcatalysts is ion-exchanged on the acid sites of the zeolite. During use, under commercial conditions, the number of acid sites in the zeolite decreases due to dealumination, and the sodium migrates out of the zeolite. Most of the loss in zeolite surface area occurs early in the life of the catalyst, when the zeolite unit cell is high and the sodium is concentrated on the zeolite. Our results suggest that once the zeolite reaches its equilibrium cell size, it is very stable with respect of gradual sodium build-up. Matrix and Zeolite Deactivation for Laboratory Deactivated Catalysts Figure 5 shows the measured zeolite and matrix surface areas of a series of catalysts, with varying exchanged sodium content, after laboratory hydrothermal deactivation for 4 hours at 1088K. Surprisingly, the matrix surface area did not change at all with increasing sodium level. However, the steamed zeolite surface area was significantly reduced with increasing level of sodium. An increase of Na from 0.22 wt.% to 0.41 wt.% reduced the steamed zeolite surface area from 86 m2/g to 62 m2/g. There are several possible explanations for the difference in commercial and laboratory deactivation by sodium. One possibility is that sodium on commercially aged and laboratory impregnated samples resides on different sites of the catalyst. Impregnated sodium on fresh catalyst tends to be concentrated on the zeolite due to its much greater availability of exchange sites, where upon steam deactivation, it destroys the zeolite but not the matrix. Feed sodium builds up gradually on catalyst, causing sintering of the matrix but not the zeolite. Consequently, the observed deactivation effect would be different. Another reason may be that the activation energy for matrix sintering and zeolite destruction are very different. Therefore, the high temperature laboratory deactivation protocol preferentially deactivates zeolite.
DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS
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