Energy & Fuels 1996, 10, 849-854
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Separation and Characterization of FCC Catalysts Using Density Gradient Separation Gary R. Dyrkacz,* Ljiljana Ruscic, and Christopher L. Marshall Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439
William Reagan Amoco Oil Company Research and Development, P.O. Box 3011, Naperville, Illinois 60566-7011 Received December 20, 1995X
Two fluidized catalytic cracking catalysts were separated using isopycnic density gradient methods. For each catalyst, both the fresh and equilibrium materials were separated. Quite different density distributions were found, not only between the equilibrium catalysts but between the fresh catalyst and its corresponding equilibrium version. In addition, different density distributions were found with different separating media, e.g. aqueous sodium polytungstate or CCl4/tetrabromethane. Crystallinity measurement across the density band for one of the fresh catalysts varied by a factor of 3.
1. Introduction Fluidized catalytic cracking (FCC) is the workhorse of the modern oil refinery. Somewhere between onethird and one-half of all oil used by a refinery passes over a cracking catalyst. As such, knowledge of the history of cracking catalysts is extremely important to the fundamental understanding and eventual improvement of the process. In particular, a thorough understanding of both the types and degree of deactivation is crucial to any laboratory work aimed at simulating the change in catalyst activity over time. By its very nature, the constant addition and removal of catalyst from an FCC makes obtaining information regarding the fate of individual particles difficult. The FCC catalyst inventory is made up of particles with a distribution of age; the oldest date from as far back as the initial unit startup, while others have been in the unit for less than a day. Recently, float/sink density separation techniques have been applied to equilibrium cracking catalysts to more fully characterize and understand the changes that occur.1-3 In the original work by Palmer and Cornelius,2 equilibrium catalyst was initially coked using isobutene. Using various mixtures of carbon tetrachloride and tetrabromoethane, the individual particles were separated by age. The age separation of particles was attributed to the amount of coke on the catalyst. Particles with the highest coke levels (most active) were found in the lightest fraction, and those with little or no coke (least active) appeared in the heaviest fraction. * Corresponding author: telephone, (708) 252-7478; FAX, (708) 2529288; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Wilson, W. B.; Good, G. M.; Deahl, T. J.; Brewer, C. P.; Appleby, W. G. Ind. Eng. Chem. Res. 1956, 48, 1982. (2) Palmer, J. L.; Cornelius, E. B. Appl. Catal. 1987, 35, 217-235. (3) Beyerlein, R. A.; Tamborski, G. A.; Marshall, C. L.; Meyers, B. L.; Hall, J. B.; Huggins, B. J. In Fluid Catalytic Cracking II; ACS Symposium Series 452; American Chemical Society, Washington, DC, 1991; pp 109-143.
0887-0624/96/2510-0849$12.00/0
Further work by Beyerlein et al.3 showed that the amount of coke on a catalyst only accounted for the density changes of the youngest fractions (1.8
Figure 1. Separation of catalyst A in sodium polytungstate or CCl4/tetrabromoethane gradients.
g cm-3 range, especially for fine particles. Most of these are halocarbons, such as carbon tetrachloride, tetrabromoethane, and methylene iodide. Unfortunately, these materials are quite toxic and environmentally difficult to dispose of. Tetrabromoethane also has a persistent, obnoxious odor and may cause hypersensitivity in some people. Sodium polytungstate (SPT) has been used in a number of recent noncatalytic materials studies for high-density separations because it is more convenient to use and much less toxic.11-13 Aqueous solutions are clear and only slightly yellow even at high concentrations, so that observing particles is not a problem. The maximum useful solution density is around 3.1 g cm-3. However, the viscosity of the solutions rises rapidly above ∼2.5 g cm-3.14,15 We did not experience gradient preparation difficulties with stock solutions up to ∼2.8 g cm-3. Figure 1 shows density gradient separations of catalyst A using two different media for the separations. The left set of photos is a view of the centrifuge tubes from a separation in a carbon tetrachloride/tetrabromoethane (TBE) gradient. The right set of photos is the same catalyst separated in aqueous sodium polytungstate (SPT). Absolute position comparisons can be made between tubes in the same medium, but not between the different media, because the density ranges and volumes are different for each set. In both media, the equilibrium catalyst shows two well-resolved bands and the fresh catalyst shows a single band. One obvious difference between the sets is the relative position of the fresh catalyst density band. In the organic media, this band quite closely matches with the low-density band in the equilibrium catalyst. However, in the aqueous medium, the fresh catalyst is at a position midway between the two bands. No particle aggregation was observed in either media. This is important, because it immediately says that we are observing the density distribution of the individual particles, and not some averaged agglomerate. From a (11) Plewinsky, B.; Kamps, R. Makromol. Chem. 1986, 185, 1429. (12) Robinson-Cook, S. E. Report 242, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM, 1986. (13) Callahan, J. J. Sedimentology 1987, 57, 765. (14) Hoover, M. D.; Finch, L.; Castorina, B. T. J. Aerosol Sci. 1991, 22, 215-221. (15) Geoliquids Inc., Sodium Polytungstate Data Sheet.
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Figure 2. Gray scale longitudinal profile along the centrifuge tube for the separation of equilibrium catalyst A in CCl4/TBE media. The abscissa is in arbitrary units of length from an arbitrary origin. The ordinate points are related to the gray levels, but the values have been baseline corrected and rescaled.
practical standpoint, it means that no particle dispersing agents are necessary, which can often be a complicated problem to solve. Furthermore, this eliminates the burden of removing additional contaminants, when isolating the particles from the gradient media. To obtain an initial idea of the distribution of material in the gradients, the intensity profiles along the tubes were measured from CCD camera images. An example of such a profile is shown in Figure 2 for the equilibrium catalyst separated in an organic gradient. The noise exhibited in the data is due to discrete particle fluctuations within the gradient. Although this method is a very convenient way to capture the density distribution, there are three problems. First, the abscissa is a function of distance rather than density. This condition could be overcome by introducing standardized beads with accurately known densities to calibrate the gradient. A second problem is that the tubes have a large cross section and are close to the camera lens, which introduces parallax distortion. Some gel scanners can overcome this complication. Finally, the relationship between gray level and mass of material is not known. To more completely document the separations, a dense chase solution was used to literally push the gradients out of the centrifuge tube. The effluent passed through an inline UV/visible absorption monitor flow cell and then through an inline density monitor. Although this method overcomes some of the problems with the image recording, it still suffers from not knowing the relationship between absorbance and mass. Each different material needs to be calibrated, and this is a complicated procedure requiring relatively large scale separations. Thus, the density information is known with good accuracy, but the amount of material may not be. However, unless there are very large changes in particles size or concentration, we usually find there is a fair correspondence. Figure 3 presents the final data obtained for the organic and aqueous separations from separations similar to those displayed in Photo 1. Each of the curves in Figure 3 has been normalized to total area to show the relative distributions of material. In the case of SPT two separate curves are shown for the equilibrium catalyst. This provides some idea of the variation that can be expected from two different separations. Missing from this figure are the data for the fresh catalyst
Figure 3. Density distributions of catalyst A in aqueous sodium polytungstate (SPT) or CCl4/TBE. All densities are at 25 °C. The relative weight is derived by multiplying the absorbance values by the corresponding density values and renormalizing the data. The two curves for the equilibrium catalyst separated using SPT show the reproducibility between successive runs.
separated in CCl4/TBE. In the course of our experiments, we discovered that CCl/TBE separations of the fresh catalyst were irreproducible. Repetitive separations showed wide variations, both in the band density and in the number of bands. We do not understand the reason for these variations, but we suspect it may be due to moisture or to lack of a reproducible sample dispersion technique. However, the variations were not due to sampling inhomogeneity. The containers were well mixed prior to removing a catalyst sample, and we can calculate that the number of particles being separated even in these small-scale separations is roughly 60 000. Typically, the fresh catalyst in TBE indicated a density very near or lower than the lowest density peak of the equilibrium catalyst. Thus, the overall behavior is quite different from the SPT separation, where the fresh material has a peak midway between the twin peaks of the equilibrium catalyst. However, because of the unreliability of the data for the fresh material in TBE, we have chosen not to show it. This problem was not found with the TBE separations of the equilibrium catalyst, or with any of the SPT separations. As already mentioned, in either medium, the equilibrium catalyst is characterized by two well-resolved density bands. However, the shape of the low-density band is different in each case. Whereas the band is nearly symmetrical in SPT medium, it appears to have a definite shoulder in the TBE medium. Both density bands of the SPT separation are also shifted to lower densities. This behavior is most likely due to lower density water filling the catalyst voids instead of the high-density CCl4. A bimodal density distribution was noted by Wilson when a CCl4/TBE fraction was reseparated with water filling the pores.1 Previous sink/float separations of catalysts in CCl4/TBE showed density distributions at lower densities than those observed in our separations
Catalyst Separation by Density Gradient Methods
Energy & Fuels, Vol. 10, No. 3, 1996 853 Table 2. Comparison of Helium Densities with Solution Densitiesa medium sample
He
SPT
TBE
Catalyst A fresh density (g cm-3) spec vol (cm-3/g) equilibrium density (g cm-3) spec vol (cm-3/g)
2.456 0.407
2.433 0.411 (0.9)b
2.754 0.363
2.417 0.414 (13.9)
2.555 0.391 (7.8)
Catalyst B fresh density (g cm-3) spec vol (cm-3/g) equilibrium density (g cm-3) spec vol (cm-3/g)
2.383 0.420
2.150 0.465 (10.8)
2.591 0.386
2.423 0.413 (6.9)
2.444 0.409 (6.0)
a Solution densities are derived by integrating the density distributions in Figures 1 and 2. See Experimental Section. Specific volumes are the reciprocal of the density values. b The numbers in parentheses are percentages of excess specific volumes relative to the corresponding He volumes.
Figure 4. Density distributions of catalyst B in aqueous sodium polytungstate (SPT) or CCl4/TBE. All densities are at 25 °C. Relative weight is derived by multiplying the absorbance values by the corresponding density values and renormalizing the data.
of uncoked catalyst. This reflects the fact that the coke itself has a low density, as expected for short reaction times. Additionally, the coke may prevent solution from accessing part of the zeolite structure.18 The density position of the fresh catalyst in polytungstate solution is surprising. It cannot be directly related to either of the bands in the equilibrium catalyst. Since fresh catalyst is constantly being added to the catalytic cracking unit, it appears that this material is rapidly altered under processing conditions. There are several factors that could contribute to the differences: (1) destruction of zeolite crystallinity and the formation of new phases; (2) rapid deposition of metals on the catalyst particles; and (3) the relative rate of withdrawal and addition of catalyst to the cracking unit. However, without more data, we cannot determine the critical factors; this is beyond the scope of the present work. Figure 4 shows the results of a DGC separation of a second catalyst (B). The distribution patterns are quite unlike those of catalyst A. None of the separations exhibit a distinctive bimodal distribution. However, the shoulder on the high-density side of the TBE separation is reproducible. In the case of the SPT separation, fresh catalyst B has a much narrower band than does catalyst A; the area in catalyst B is ∼2.7 times smaller. Narrow bands, when observed in a nonagglomerating medium, usually indicate that the sample has a higher degree of homogeneity. We suspect that the difference in bandwidth between the fresh catalysts is due to different commercial preparation techniques. Catalysts A and B not only differ in the shape of their density distributions but also have quite different band density positions in different media. Additional insights (16) (a)Post, B. Acta Crystallogr. 1959, 12, 349. (b) Evans, T. H. In Perspectives in Structural Chemistry; Dunitz, J. D., Ibers, J. S., Eds.; Wiley: New York, 1971; Vol. 4, p 1. (17) This value was calculated using the HyperChem modeling package.
into the cause of these differences can be obtained by comparing the apparent solution densities to the corresponding He densities. The small size and low interaction potential of He should provide a good value for the catalysts’ skeletal density. Comparing the He densities in Table 2 with the highest density of all the bands in Figures 3 and 4 shows that they are all below the He values. This indicates that indeed He does penetrate and fill the void volume more completely than either solution can and also that the value is closer to the true skeletal density. In sink/float or density gradient separations where multicomponent fluids are used, the apparent density can be the same, lower, or higher than the He density, depending on what ratio of high- and low-density molecules enter the pore space. Lower apparent densities can arise if none of the fluid species packs into the pores as tightly as helium or if the ratio of low- to highdensity molecules in the catalyst pore is less than in the bulk solution. Conversely, the apparent particle density can be higher than the He density if the highdensity component preferentially enters the pores in excess of the ratio of the components in the bulk solution. In our separations, the He density is just at the highdensity limit of the bands. Thus, low-density water or CCl4 molecules, rather than high-density polytungstate or tetrabromoethane, appear to be preferentially filling the void space of the high-density particles. In the aqueous separations, the reason for this behavior is clear. The opening into the zeolite cages is about 7.6 Å.3 The metapolytungstate anion is ∼12 Å and should not diffuse into the zeolite void space.16a,b Both water and CCl4 (∼6 Å) should have no trouble passing through the zeolite cage windows. The disposition of tetrabromoethane is less clear. It has a van der Waals size of 6.9 Å, which is on the same size order of the entrance window.17 However, the kinetic diameter would be smaller, and thus, it is possible that TBE could diffuse into the zeolite cages. As already explained, if an excess of TBE is trapped by catalyst particles over the ratio of CCl4/TBE found at a point within the gradient, then the apparent density could be above the He density limit. However, whether trapped by binding or by pore filling,
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only a finite amount of TBE could be attached to the catalyst, thus limiting the maximum apparent density that a catalyst particle could reach. Thus, we cannot completely rule out the possibility that the lower density material is absorbing TBE. We can only be certain that the highest density material does not appear to take up significant amounts of TBE. Determining the amount of TBE in the catalyst requires detailed information on the fraction of zeolite and its native pore state, the void space for particles at each density, and the nature and composition of all contaminants. 3.2. Crystallinity Variation in Fresh Catalyst A. As indicated in the Introduction, the density of the catalyst has been correlated with a number of parameters such as catalyst activity, microporosity, and crystallinity. The present density gradient separations do not provide enough material for extensive characterizations. The methodology used in these preliminary studies is more useful for fingerprinting and characterization than for separation and isolation. Density gradient methods can be scaled up, using special rotors or other methods, but this approach is outside the bounds of the current work.7 (Scaleup will be reported in a future manuscript.) The present work is designed to show that density gradient separation is feasible, with both traditional separation media and aqueous polytungstate media. However, the broad band for fresh catalyst A suggested that it was heterogeneous. As a probe of this character we examined the variation of crystallinity with density for fresh catalyst A. The catalyst was separated in a series of gradient runs (nine) done at high loadings and each gradient then fractionated. Fractions falling within designated density ranges were combined, washed, and filtered. Combining of fractions from different separations has the drawback that there will be a certain amount of density overlap between the fractions because not all of the gradients had exactly the same volume or gradient shape. The mismatch of material will be greatest for fractions near the wings of the density bands because a small change in density can mean a large change in mass at one end of a density region. We estimate that there is still probably no more than 10% contamination of a fraction in our worst cases. Enough material was obtained to do limited crystallinity measurements; the data are shown in Table 3. The crystallinity values for catalyst A range over a factor of 2.5, with the larger values found at lower densities. Note that the maximum spread in crystallinity is probably much higher; the available data only represent the midportion of the density distribution with the extremes not being included. The heterogeneity is likely due to the method used to prepare the catalyst. The result suggests that caution must be used in interpreting the density distribution of equilibrium
Dyrkacz et al. Table 3. Fresh Catalyst A: Combined Fraction Density and Zeolite Content Data from Multiple Density Gradient Separations fraction no.
fraction rangea (g cm-3)
average densityb (g cm-3)
fraction weight (mg)
1 2 3 4 5 6 7 8 9
2.180-2.235 2.237-2.281 2.277-2.325 2.313-2.366 2.357-2.406 2.394-2.446 2.437-2.484 2.472-2.588 2.581-2.661
2.220 2.260 2.300 2.340 2.378 2.421 2.461 2.531 2.622
0.4 0.9 2.6 15.2 58.9 93.9 69.9 48.5 8.3
zeolite Yc (%)
23. 16. 10. 8.
a The total range of all individual density fractions making up this super fraction. b The calculated average density of all fractions making up the super fraction. c Values derived from XRD measurements.
catalysts. The studies to date have assumed a homogeneous starting material. Previous work has shown that the zeolite crystallinity is one of the strong driving forces for density changes.3 Although the amounts collected were too small to provide more than one fractional cut, the narrow width of the peak for fresh catalyst B would imply that the particles have less variation in the percent zeolite than catalyst A. Therefore, the density gradient centrifugation technique shows that one major difference between catalyst A and B is in the distribution of percent zeolite which is controlled by the manufacturing techniques. Future work will attempt to separate larger samples in an attempt to quantify the changes in zeolite concentration. 4. Summary Catalysts can be efficiently and relatively easily separated by density gradient techniques. This technique provides clear information on the density distribution of the catalyst. It can be used as a simple fingerprinting method or, with more effort, as a separation method to isolate and study detailed variations in catalysts. As an example, we were able to quickly realize that even starting FCC catalyst particles can show a broad range of zeolite content. In addition, the use of aqueous sodium polytungstate as a density separation medium offers a more convenient fluid for all types of separations. However, the density variations observed with different media are useful in further understanding catalyst heterogeneity. Acknowledgment. G.R.D. would like to acknowledge that this work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract number W-31-109-ENG-38. We also wish to thank C. A. A. Bloomquist for the figures. EF9502618
