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Comparison of Attrition Test Methods: ASTM Standard Fluidized Bed vs Jet Cup Rong Zhao,† James G. Goodwin, Jr.,*,† K. Jothimurugesan,‡ James J. Spivey,§ and Santosh K. Gangwal§ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, Department of Chemical Engineering, Hampton University, Hampton, Virginia 23668, and Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709-2194
Attrition resistance is one of the key design parameters for catalysts used in fluidized-bed and slurry phase types of reactors. The ASTM fluidized-bed test has been one of the most commonly used attrition resistance evaluation methods; however, it requires the use of 50 g samplessa large amount for catalyst development studies. Recently a test using the jet cup requiring only 5 g samples has been proposed. In the present study, two series of spray-dried iron catalysts were evaluated using both the ASTM fluidized-bed test and a test based on the jet cup to determine this comparability. It is shown that the two tests give comparable results. This paper, by reporting a comparison of the jet-cup test with the ASTM standard, provides a basis for utilizing the more efficient jet cup with confidence in catalyst attrition studies. Introduction Catalyst attrition resistance is one of the major design criteria in catalyst development. Several types of attrition test methods have been developed and used in the past few decades for evaluating attrition resistance of catalysts for different types of reactors.1,2 Recent research has focused especially on evaluation of fluidizedbed and slurry bubble column reactor (SBCR) catalysts. Catalyst attrition can be especially severe in these types of reactors because of constant catalyst particle movement and collision.2 Among the methods developed for catalyst attrition resistance evaluation,1,2 the fluidized-bed test3,4 (also called the air-jet test) is the most commonly used. By specifying the device configuration and test procedures and conditions, the ASTM fluidized-bed method4 has provided a standard for comparing catalysts that has worked well in the past. The jet-cup test, a recently proposed ASTM method, is a relatively new attrition test method.5 To evaluate its suitability in characterizing catalyst attrition resistance, the jet-cup test has been compared to tests using smaller fluidized beds for fluid catalytic cracking (FCC) catalysts5 and SBCR catalysts.6 These previous comparisons have shown that the attrition mechanisms of these two tests differ somewhat in that the jet-cup attrition process appears to be relatively more fracture dominant.5,6 On the other hand, it has also been concluded that tests using small fluidized beds have lower attrition efficiencies.6 The time on streams required to produce similar amounts of attrition in the small fluidized-bed tests5,6 were, therefore, much longer (around 20 h) than those of the jet-cup tests (1 h). It * To whom correspondence should be addressed. Tel: (412) 624-9642. Fax: (412) 624-9639. E-mail: goodwin@ engrng.pitt.edu. † University of Pittsburgh. ‡ Hampton University. § Research Triangle Institute.
has remained questionable whether the attrition efficiencies of the larger ASTM fluidized bed with relatively high flow rates and of the jet-cup test are similar enough to warrant use of the jet cup as a “standard” method. Because results of a comparison of the jet-cup test with the ASTM fluidized-bed test have never been reported before, it is important that this comparison be made before the jet cup can be used routinely with confidence. In the present study, such a comparison was carried out using two series of spray-dried iron catalysts developed for SBCR use. The results and discussions not only illustrate the comparability of the attrition results measured by the two tests but also provide a better fundamental understanding of these two tests. Experimental Section Catalysts. All of the iron catalysts used in this study were prepared using spray drying. These catalysts had the same ratio of iron, copper, and potassium (100/5/ 4.2 ) Fe/Cu/K) but with different concentrations and types of binder or precipitated SiO2. The iron catalysts were prepared by the precipitation of an aqueous solution containing Fe(NO3)3‚9H2O, Cu(NO3)2‚2.5H2O, and Si(OC2H5)4 (when added) by the addition of ammonium hydroxide. The potassium promoter was added as an aqueous KHCO3 solution to undried, reslurried Fe/Cu/Si precipitate. After addition of binder silica, the catalysts were spray-dried in a Niro spray drier. The detailed catalyst preparation can be found elsewhere.7 The catalysts are denoted in this paper by the SiO2 concentration and type. The notation B stands for binder, whereas P stands for precipitated SiO2. Therefore, a catalyst with a notation of Fe/P(5)/B(12) refers to an iron catalyst which has 5 pbw (parts by weight) of precipitated SiO2 and 12 wt % binder. Fluidized Bed Test. The attrition resistances of the catalysts were first determined using a three-hole airjet attrition tester configured following ASTM-D-575795.4 Fifty grams of catalyst was used for each sample
10.1021/ie990730j CCC: $19.00 © 2000 American Chemical Society Published on Web 03/31/2000
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Table 1. Comparison of Jet-Cup and ASTM Fluidized-Bed Test Results
catalyst
fluidized-bed attrition lossb (wt %) density BET surface (g/mL) areaa (m2/g) 1 h 5h
Fe/P(0)/B(4) Fe/P(0)/B(8) Fe/P(0)/B(10)d Fe/P(0)/B(12) Fe/P(0)/B(16) Fe/P(0)/B(20)
0.83 0.81 1.02 0.90 0.78 0.64
101.3 124.6 95.2 146.2 176.6 158.3
24.4 25.7 7.6 12.8 22.0 34.9
Fe/P(5)/B(12) Fe/P(10)/B(12) Fe/P(15)/B(12) Fe/P(20)/B(12)
0.66 0.62 0.61 0.59
179.4 190.8 216.8 245.0
24.2 31.0 42.1 39.1
jet-cup fines (wt %)c
32.6 35.4 14.6 22.7 30.1 35.0
26.6 21.8 4.8 8.5 18.2 51.6 (run 1) 46.6 (run 2) 37.3 26.6 39.6 33.9 NAe 39.6 NAe 41.3
a Error ) (5%. b Fines carried out of the fluidized bed and collected by the filter: measured using ASTM-D-5757-95. c Fines carried out of the cyclone chamber and collected by the filter: error ) (5%. d Prepared slightly differently than other catalysts. e Attrition was severe enough to plug the tester; weight loss was >40 wt %.
tested. As specified in the ASTM method, the samples were tested under 10 L/min of air flow and the weight loss of fines was recorded at 1 and 5 h of time on stream, respectively. Jet-Cup Test. The attrition resistances of the catalysts were also evaluated using a jet-cup test. Details about the test device and procedures of this test can be found elsewhere.6 In the present study, 5 g of sample was used for each test, and the air flow was controlled at 15 L/min with 60 ( 5% humidity at room temperature. Catalyst Morphology. Particle morphology was obtained for each catalyst (as prepared and after the jet-cup attrition testing) using a Philips XL30 FEG scanning electron microscope (SEM). The samples were coated with palladium before SEM measurements to avoid charging problems. Results and Discussion Each catalyst as prepared and calcined was tested using both the ASTM fluidized bed and the jet-cup tests. Fifteen liters per minute of air flow rate was used for all of the jet-cup tests in the present study to produce similar attrition degrees as generated during the ASTM fluidized-bed tests in this study. The attrition results and some physical properties of the catalysts tested are summarized in Table 1. The particles carried out of the fluidized bed or jet cup and captured by the downstream filter are categorized as “fines”. The weight percentage of fines lost (attrition loss) was calculated using the amount of particles carried out of the fluidized bed or jet cup and collected by a filter and is commonly used as an indication of catalyst attrition resistance. Particle size analysis results showed that fines are generally less than 25 µm in size for the jet-cup test at the conditions used. As can be seen, although the catalysts were prepared similarly, their physical properties, such as densities, BET surface areas, and attrition resistances, varied widely depending on their compositions. For most catalysts, as shown in Table 1, the weight losses of fines during the first hour of time on stream of the two different attrition tests were almost identical. Such similarity between the two sets of attrition results can be even more clearly seen in Figure 1. Only one catalyst
Figure 1. Comparison of the ASTM fluidized-bed and jet-cup test results.
[Fe/P(0)/B(20)] produced results somewhat different, and this difference was found to be reproducible. The similarity between the attrition results clearly shows that the jet-cup test predicts catalyst attrition resistance just as well as the ASTM standard test. Different from the previous comparisons with smaller fluidized-bed systems,5,6 the ASTM fluidized bed was as efficient as the jet cup in terms of weight loss of fines generated in a short period of time. However, the loading of 50 g of catalyst is considered a disadvantage for the application of the ASTM fluidized bed in catalyst development studies. Even if the amount of catalyst required is not an issue, the jet cup can still be used because the production of fines is identical with that of the ASTM fluidized-bed test. In a previous study,5 it was reported that the attrition rate curves (the change in the attrition rate with time) of the proposed jet-cup test and a modified fluidizedbed (smaller in size than the ASTM) test are different. The rate curve of the fluidized-bed test used was found to be time dependent, which has also been reported elsewhere.8 This was related to an abrasion-dominant attrition process of the fluidized-bed test.5,8 On the other hand, for the jet-cup test, a linear rate curve was reported and was related to a fracturing process.5 In the present study, the attrition rate of the ASTM fluidizedbed test was also found to be time dependent, which is clearly shown in Table 1 in that the weight loss of fines did not increase linearly from 1 to 5 h of time on stream. A slight decrease in the attrition rate during the first hour was found for the jet cup in our lab (not shown here). Yet, the decrease in the attrition rate was relatively insignificant and the rate curve was approximately linear. On the basis of previous SEM analysis,6 it is suggested that the attrition process of the jet-cup test exhibits both abrasion and fracture mechanisms, where the abrasion process apparently plays a less important role compared to attrition in the fluidized-bed test. The attrition rate curve itself is not a very important characteristic in the comparison of the relative attrition resistances of different catalysts. Attrition indices have been recommended as quick and easy indications of attrition resistance for both the fluidized-bed and jetcup tests.5 The attrition indices for both of these tests
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Figure 2. SEM micrograph of catalyst Fe/P(0)/B(20) before and after the jet-cup attrition test.
Figure 3. SEM micrograph of catalysts (A) Fe/P(0)/B(16) and (B) Fe/P(5)/B(12) before and after the jet-cup attrition test.
are versions of the accumulated weight percentage of fines lost and, as such, average rates of fines lost. As stated in a previous study,6 the accumulated weight percentage of fines lost is affected by many factors, such as system configuration, particle density and shape, etc. To match the standard attrition measurement as specified by the ASTM fluidized-bed test, the weight percentage of fines lost was still used in this study and is considered to be adequate for our purpose of comparison. As noted above, the results from the two tests were somewhat different for Fe/P(0)/B(20). The jet-cup test was repeated for this catalyst, and the attrition resistance data were found to be reproducible within experimental error (Table 1). The fluidized-bed test, however, was not able to be repeated because of the limited amount of catalyst available. This illustrates the abovementioned disadvantage of the ASTM fluidized-bed test in catalyst development studies. To further examine the accuracy of the results, the catalyst as prepared and the particles remaining in the
chamber after jet-cup testing were evaluated using SEM (Figures 2 and 3). As can be seen, the catalyst particle sizes of Fe/P(0)/B(20) decreased dramatically during attrition testing (Figure 2). This correlates well with the large amounts of fines collected. Compared to the particle size changes of catalysts Fe/P(0)/B(16) and Fe/ P(10)/B(12) before and after jet-cup testing (Figure 3), it is obvious that Fe/P(0)/B(20) (Figure 2) was significantly less attrition resistant. Clearly, the SEM results confirm the differences in attrition resistance indicated by the attrition index (weight percentage of fines lost). The catalyst particle morphologies after both tests were compared using SEM. As can be seen from Figure 4, the catalyst particles of a typical sample [Fe/P(0)/B(10)] after both attrition tests decreased significantly in size. Nearly all of the agglomerates were broken up. The particle morphologies after the different tests differed somewhat, which might be due to slight differences in the attrition mechanisms in the different
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than the others without precipitated SiO2. It is possible that Fe/P(0)/B(20) might have had a much lower fracture resistance compared to the other catalysts. Such a speculation is difficult to prove. Despite this single catalyst, all other attrition results measured by the two tests matched extremely well (Figure 1). Finally, despite the differences seen for Fe/P(0)/B(20), it is clear that the jet cup is able to produce catalyst attrition as well as the ASTM fluidized bed. In addition, it is able to essentially duplicate the degree of attrition in that method, with a greater efficiency in terms of sample weight required. Conclusion In this study, two attrition tests, the ASTM fluidized bed and the recently proposed jet cup, were compared using two series of spray-dried Fe catalysts. The similarity between the two sets of attrition results suggests that the jet-cup test is as adequate as the ASTM fluidized-bed test for catalyst attrition prediction even though there may be some differences in attrition mechanisms in the two systems. Requiring an order of magnitude less of sample for testing, the jet-cup test can especially be considered to be preferable for catalyst development studies and laboratory use. Acknowledgment Financial support by the Department of Energy (Office of Fossil Energy) [under Grant DE-FG2296PC96217] is gratefully acknowledged. Literature Cited
Figure 4. SEM micrograph of catalyst Fe/P(0)/B(10) before and after jet-cup and ASTM fluidized-bed attrition tests: (A) catalyst as prepared; (B) after the jet-cup test; (C) after the ASTM fluidizedbed test.
tests. However, the degree of catalyst attrition is apparently fairly independent of these slight differences. It is not clear what is the exact reason for the differences in the attrition resistance data measured by the two tests for Fe/P(0)/B(20), because the ASTM fluidized-bed test was unable to be repeated. Perhaps, it was due to experimental error. However, besides experimental error, such a difference might have been caused by different attrition processes in the two tests. Clearly, this catalyst was much less attrition resistant
(1) British Materials Handling Board. Particle Attrition, Stateof-the Art Review; Trans Tech Publications: Germany, 1987. (2) Bemrose, C. R.; Bridgwater, J. A. Review of Attrition and Attrition Test Methods. Powder Technol. 1987, 49, 97. (3) Forsythe, W. L., Jr.; Hertwig, W. R. Attrition Characteristics of Fluid Cracking Catalysts. Ind. Eng. Chem. 1949, 41, 1200. (4) ASTM D5757-95: Standard Test Method for Determination of Attrition and Abrasion of Powdered Catalysts by Air Jets; ASTM: Philadelphia, PA, 1995. (5) Weeks, S. A.; Dumbill, P. Method Speeds FCC Catalyst Attrition Resistance Determinations. Oil Gas J. 1990, 88, 38. (6) Zhao, R.; Goodwin, J. G., Jr.; Oukaci, R. Attrition Assessment for Slurry Bubble Column Reactor Catalysts. Appl. Catal. A 1999, 189, 99. (7) Jothimurugesan, K.; Spivey, J. J.; Gangwal, S. K.; Goodwin, J. G., Jr. Effect of Silica on Iron-Based Fischer-Tropsch Catalysts. Studies in Surface Science and Catalysis, Vol. 119: Natural Gas Conversion V; Elsevier Science: New York, 1998. (8) Gwyn, J. E. On the Particle Size Distribution Function and the Attrition of Cracking Catalysts. AIChE Symp. Ser. 1969, 15, 35.
Received for review October 5, 1999 Revised manuscript received February 11, 2000 Accepted February 18, 2000 IE990730J
