Simon A. Weeks
BP Research Centre
Sunbury-On-Thames, U.K.
Peter Dumbill
Crosfield Catalysts
Warrington, U.K.
FCC catalyst attrition resistance can be determined by a method that eliminates the need for particle size measurement or accurate sampling procedures. The method, therefore, speeds attrition-resistance determinations.
The method uses the industry-known jet-cup test to provide data for an attrition index WD, which is the gradient (or slope) of a plot of cumulative weight percent catalyst fines vs. time in hours. The method correlates well with other attrition indices.
Catalyst attrition in an FCCU can cause serious catalyst losses from the unit. Attrition depends upon whether a catalyst is in its fresh or equilibrium state, and on the particular operating conditions of the unit.
The ability to quickly determine the attrition resistance of the catalyst in the unit can allow a timely change to a more attrition-resistant catalyst to decrease losses, along with eliminating particle emissions, fines in the slurry oil, and wear in power-recovery trains.
ATTRITION TESTING
FCCU operators and catalyst manufacturers screen catalysts for attrition resistance by a variety of methods, among which no known published comparisons exiSt.1 2 The two most widely used tests, the jet-cup test and the air-jet test, have been compared.
Both of these tests use a high-velocity air flow to agitate catalyst samples and thereby cause attrition (Figs. 1 and 2). The air-jet test requires about 20 hr to run, and it attrites the catalyst in a fluidization tube.
The jet-cup test apparatus confines most of the sample to a small cup, into which the agitating air flow is introduced (Fig. 3). In both tests, fines (particle of less than 20 micron size) are elutriated into a collection device.
ATTRITION BEHAVIOR
The amount of fines collected from attrition of a typical calcined commercial FCC catalyst sample in the air-jet and jet-cup tests is shown in Figs. 4 and 5. It is evident that the two tests produce a different attrition time dependence.
The air-jet cumulative weight percent fines collected shows a logarithmic dependence on time:
cum wt % fines = ktm
where:
m = Weight of the initial sample
t = Time
k = A constant
The time dependence has been similarly determined by Gwyn. 3
In contrast, the jet-cup test displays a linear dependence of cumulative weight percent fines vs. time--the rate of attrition is independent of time.
The time dependence differences between the tests can be ascribed to the differences in the fundamental modes of attrition of the two tests. In the air-let test, attrition is caused predominantly by particle-to-particle collisions that abrade the catalyst particles.
With time, the attrition rate decreases because the catalyst particles are rounded off by the abrasion.
In the jet-cup test, attrition takes place mainly by the impact of catalyst particles against the Wall of the jet cup. The attrition mechanism is more of a fracturing process than abrasion.
The attrition in the jet cup is thus proportional to the average probability of particle fracture giving a constant rate of attrition, provided the number of catalyst particles impinging on the jet-cup walls remains constant (this condition holds during a typical jet-cup test of 1-hr duration).
ATTRITION INDICES
Attrition resistance, measured by attrition tests, is expressed in terms of attrition indices. The air-jet test is typically run for a fixed length of time.
The jet-cup test apparatus is first calibrated using a catalyst of known attrition resistance. This calibration gives the test run times for the index to be used (total time is about 1 hr).
The attrition resistance of three commercial FCC catalysts was determined using both the air-jet and jet-cup tests. Four catalyst conditions were tested: fresh catalyst as supplied, catalyst calcined at 540 C. for 3 hr, catalyst steamed at 816 C. for 5 hr at 100% steam and ambient pressure, and catalyst equilibrated in an FCCU.
The attrition index for the air-jet, AJI, expresses the air-jet data as:
AJI = (Total wt % fines collected during 20-hr test) divided by (mass of sample before test)
The attrition index for the jet-cup test is the Davison Index DI, developed by Davison Chemical Division of W.R. Grace & Co. 2 The index expresses the jet-cup data as:
DI = {[(c/m x 100) + H - G]/100 - G} x 100
where:
c = Weight percent fines collected
m = Weight of the initial sample
G = Weight of particles less than 20 R in the sample before the test
H = Wt % of particles less than 20 microns left in the sample after the test
G and H were measured using a Coulter counter. 4
Low values of both AJI and DI indicate good attrition-resistant catalysts. The results are shown in Figs. 6 and 7.
It is apparent that both tests are equally able to distinguish between soft and hard catalysts. Provided the samples are pretreated in the same manner, their rankings, in terms of attrition resistance, are independent of the test procedure employed (e.g., attrition resistance of calcined samples of Catalyst C Catalyst B Catalyst A).
The order of attrition resistance of the various catalyst conditions is:
Fresh
[see equation]
The large difference between the attrition resistance of fresh catalysts and equilibrium catalysts demonstrates the importance of measuring the attrition resistance of both fresh and calcined catalysts when predicting directional trends in catalyst losses from an FCCU. Catalysts with poor attrition resistance in the fresh state could yield extremely high catalyst losses from the unit, especially in circumstances where there is a high catalyst replacement rate (for instance in residue processing).
NEW INDEX
Although DI is the most widely used attrition index for the jet-cup test, the index requires the determination of the weight percent of particles less than 20 microns in size. The methods which can be used to determine the particle fraction (Coulter counter 4 and laser diffraction 5 ) are not sufficiently accurate, and they give different answers because they are based on different physical principles.
Furthermore, a significant amount of time is required for sampling (essential to obtain acceptable reproducibility) for attrition, particle size measurements, and in performing the analyses.
These limitations led to the development of the WD index the slope of a plot of cumulative weight percent fines vs. time:
Cumulative wt % fines ~= WD x time (hr) + constant
[see equation]
In general, WD correlates well with DI and gives identical attrition-resistance rankings, obviating the need for particle size analyses (Fig. 8). It has the additional advantage of being determined by linear regression, allowing the reliability of data to be assessed by simple statistical means.
The repeatability of the WD measurement is excellent. For a catalyst with a WD (calcined) attrition index of 9.8, the standard deviation is less than 0.2 of a WD unit. Experience has also shown that WD measurements do not require the degree of sampling accuracy needed for determining DI.
Because the WD index obtained from the jet-cup test accurately ranks catalyst attrition resistance, it is recommended that the procedure be considered for standardization by ASTM. This would allow catalyst users and manufacturers to compare the attrition resistance of FCC catalysts against a common baseline.
REFERENCES
- Bemrose, C. R., and Bridgwater, J., Journal of Powder Technology, Vol. 49, 1987, p. 97.
- "Advances in Fluid Catalytic Cracking," Catalytica, Mountain View, Calif., Part 1, 1987, p. 355.
- Gwyn, J.E., AlChE Journal, Vol. 15, 1969, p. 18.
- "Particle Size Distribution of Catalytic Material by Electronic Counting," ASTM D4438-81, American Society for Testing and Materials, 1981.
- "Particle Size Distribution of Catalytic Material by Laser Light Scattering," ASTM D4464-85, American Society for Testing and Materials. 1985.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
