I
J. E. McEVOY, T. H. MILLIKEN, and G. A. MILLS Houdry Process Corp., Marcus Hook, Pa.
Distribution of Metal Contaminants on Cracking Catalysts As metal contaminants cause serious decrease in catalyst selectivity, and the metals are concentrated on the exterior of the catalyst, it is important to maintain maximum catalyst size
T H E EFFECT of heavy metals contamination of petroleum cracking catalysts has grown in importance with the trend to cracking of residual stocks. Principal offenders are nickel and vanadium, contained in the petroleum and deposited upon the catalyst. Their effects in causing loss of cracking catalyst selectivity have been described ( 4 ) . Solution of this problem requires a knowledge of amounts and distribution of metals accumulated on the catalysts during cracking of a metals-containing charge. This report describes work done in determining the distribution of nickel and vanadium deposited from metalcontaining charges on synthetic silicaalumina beads and pelleted cIay cracking catalysts. Distribution studies were made on catalysts contaminated by vapor phase cracking in small laboratory cracking units and on a catalyst contaminated by mixed phase cracking of a residuum by the Houdresid process in commercial and pilot plant moving bed operation. During cracking a sharp negative gradient in metals concentration occurs from the periphery to the center of the catalyst particle. The radial distribution pattern is independent of the total metal concentration on the catalyst. As the depth of metal penetration from the peripheral surface of the catalyst inward is constant, the size of the catalyst particle is important in a commercial moving bed cracking process, where rubbing attrition (peripheral) leads to a significantly lower equilibrium metal content of the catalyst than would be anticipated if uniform metal distribution occurred in the catalyst pellets.
beads and pellets, so that the metals content could be evaluated as a function of particle diameter. Figure 1 shows the apparatus used to remove increments from spherical beads. Emery paper cemented to the inner surface served as a grinding track on which the beads were spun; a tangential air blast served as a propellant. This equipment was very effective in removing peripheral increments in relatively ghort grinding periods. Rate of surface removal was controlled by varying the air velocity through the grinder or using a finer or coarser grade of emery paper. I n the apparatus used to remove peripheral increments from cylindrical pellets (Figure 2), the pellets were suspended in the center tube by an air blast. A surface grinding action was effected by 60-mesh alumina grit, which
was circulated through the tube by the suspending air stream. Vapor phase cracking runs were made in the laboratory in a small fixed bed cracking unit. T o build u p significant concentrations of metals from the charge stock, a number of cycles were made, using the contaminated charge stock. As the coke laydown from this stock was high, it was necessary to regenerate after a 10-minute on-stream period. A reactor of the design shown in Figure 3 was built, which reduced the time for regeneration, allowing the use of a 1- to 1.5-hour cycle consisting of onstream inert purge, regeneration, and purge. The catalyst was contained in the annular space surrounding the central tube which served as a cooling well, so that heat evolved from carbon burning could rapidly be removed by an air
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Apparatus
I t was necessary to devise apparatus to remove uniform increments from
Figure 1. Apparatus for removing peripheral surfaces from spherical bead catalyst VOL. 49, NO. 5
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Figure 2. Apparatus for removing peripheral increments from cylindrical or irregularly shaped catalyst particles
A
blast through the well. Recorded regeneration maximum was never in excess of 1050 F. Mixed and liquid phase cracking runs were made in a commercial catalytic cracking installation, and the metals concentration on the catalyst was supplemented by mixed phase feed cracking of a metals-containing charge stock in
a pilot plant moving bed reactor at the Houdry Laboratories.
Contamination Procedure Laboratory Contamination Procedure. An 80% cut of San Ardo crude was used as a source of contaminating metals. Its high metal content is noteM~orthy.
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Distribution of nickel and vanadium on synthetic bead and pelleted clay cracking catalyst
INDUSTRIAL AND ENGINEERING CHEMISTRY
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METAL C A T A L Y S T C O N T A M I N A N T S 50
40 a% l3z -
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Charge Stock O
E.P., O P. Specific gravity Total ash, p.p.m. Total nickel, p.p.m. Total vanadium, p.p.m. Balance of ash
405 975 0.94 300 40 53
Al, Fe, Si, M g , Pb, Sn, Cu, Ag, Zn, Na, Ti, Ca,
Mn
This stock was pumped directly into the charged reactor. Liquid hourly space velocity was 1.0 to 1.5. Liquid product was condensed and collected; the gas product was discarded. Onstream time was 10 to 15 minutes, followed by a nitrogen purge and a 45-minute regeneration. Catalyst was removed, mixed, and recharged to reactor after every cycle for the first five cycles, and every two cycles thereafter, to obtain a uniform distribution of the metals on the catalyst. At intervals of 5 , 10, 15, 25, and 35 cycles sufficient catalyst was removed for peripheral grinding and analyses. Commercial a n d Pilot Plant Contamination. Catalyst contamination occurred during commercial cracking of a long-range residuum from Mid-Continent Texas and Louisiana in the moving bed Houdresid process ( 7 ) . The metals content of the catalyst contaminated in commercial operation was supplemented by subsequent pilot plant processing with Ordonez and Boscan residua.
Analysis Nickel and vanadium as a function of particle diameter were measured by determining the total nickel and vanadium present in a sample of the beads
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PERIPHERAL
Fiaure 5. Nickel distribution of synthetic bead cracking catalyst at various cycling levels
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Figure 6. Distribution of nickel and vanadium deposited from commercial and pilot plant operation on clay cracking catalyst
of cycles and initial metals concentration. This is shown in Figure 5 from the smoothed data obtained from Figure 4 , A . This indicates that the distribution pattern is similar to and independent of the total metals concentration. This is extremely significant in prediction of equilibrium metals concentration in commercial operation. The possibility of migration of nickel after deposition through the silica alumina lattice from the pellet periphery to the core was investigated. If such migration occurs, mobility should increase with increasing temperature. This catalyst was heat-treated in air a t 1400" F. for 8 hours and the nickel distribution pattern redetermined. Comparison of distribution patterns before and after heat treatment showed no significant migration. Thus, the distribution is probably a function of the diffusion rate of the organometallic compounds and their rate of cracking. Mixed Phase Feed Operation. The distribution of nickel and vanadium deposited on a clay cracking catalyst in commercial and pilot plant Houdresid operation is shown in Figure 6. Metals distribution is of the same type for mixed phase feed operation as for vapor phase
or pellets prior to incremental removal of the surface. Following grinding, the metals content was again determined; this technique was repeated ' until a series of points was obtained. Metals in oils were determined by an adsorption and spectrographic technique (3). I n this manner the following catalyst samples were investigated : bead and clay contaminated by vapor phase cracking to different levels, and clay contaminated by commercial and pilot plant Houdresid operation. Experimental Results Distribution of Nickel a n d Vanadium Deposited. Vapor Phase Cracking. Figure 4 shows the distribution of nickel and vanadium on a synthetic silicaalumina bead catalyst and on a pelleted clay cracking catalyst, as a function of the weight of catalyst removed by peripheral grinding. The metal content of the residual pellet is shown as a a function of the amount of catalyst particle removed. If the data are drawn so that the per cent of total metals deposited is plotted against the peripheral increment removed, there is a random correlation between number
Correlation Between Degree of Contamination and Cat-A Activity Ni Catalyst Synthetic S i O 4 l 2 O s bead, new, heattreated, 1050O F., 3 hr., air After contamination in laboratory unit using nickel-containing charge After peripheral removal of 4.0 wt. %
Concn., Gas P.P.M. Gravity
Gas,
% '
Coke, Gasoline, Coke Wt. % Vol. yo Factor
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310
0.98
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32.4
3.7
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Figure 7. Metals distribution on catalyst for vapor and mixed phase cracking
operation, although the slope of the concentration gradient is significantly greater in mixed phase feed operation. I t is probable that the higher boiling unvaporized components (containing a higher proportion of metals) are deposited on the peripheral portions of the catalyst particles and because of their molecular size do not diffuse as rapidly through the silica-alumina structure as the molecules of lower molecular weight present in the vapor phase. I t is apparent that removal of a relatively small peripheral increment causes a considerable improvement in product distribution. Further experimental work along these lines was thought to be beyond the scope of this report. Discussion
Comparison of Mixed Phase and Vapor Phase Deposition of Metals. Figure 7 shows for both methods of dep-
2 R A D IUS
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Figure 8. Cross section of quartered catalyst particle, showing nickel and vanadium distribution during commercial residuum operation
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osition the weight per cent of total metals lost for a given peripheral weight increment removed by peripheral attrition of catalyst particles. Removal of 5 weight 70 of the peripheral surface of the catalyst particles results in removal of 40 to 60% of the total metals content of the catalyst contaminated in mixed phase (commercial) operation. From 25 to 35% is removed in a similar increment of catalyst contaminated during vapor phase operations in the laboratory.
Depth of Metals Penetration. For mixed phase feed operation using 4-mm. pellets, calculations show that approximately 44% of the total nickel (195 p.p.m.) and 48yo of the total vanadium (800 p.p.m.) are within 35 microns of the outer surface of the catalyst particle. Over a wide range of concentrations approximately ?&yoof the total metals are in the outer 35 microns of 4-mm. catalyst particles contaminated in vapor phase feed operation. Distribution of nickel and vanadium on a typical catalyst particle contaminated during commercial residuum operation is shown in Figure 8. The outside 5 weight 7 0 of the particle, corresponding to a 35-micron penetration, has a nickel concentration of 1670 p.p.m. (44% of total nickel) and a vanadium concentration of 9280 p.p.m. (58yo of total vanadium). The remainder of the particle contains 110 p.p.m. of nickel and 360 p.p.m. of vanadium. Thus, from 5 to 12 times as much metal is concentrated in the peripheral 5 weight 7 0 as in the remainder of the particle. This is in agreement with another instance of moving bed operation ( 5 ) in which the outside 1% of the bead was reported to contain a metals concentration six times greater than the average of the whole bead. The depth of penetration of metals into small particle cracking catalysts has been estimated (6) by an indirect technique as complete in particles 40 microns in diameter or smaller. The degree of uniform contamination decreased as larger particles were used. Commercial experience with metals poisoning in fluid cracking is reported in detail by Duffy and Hart (2). Because of the consistency in correlating the depth of metals penetration with a constant fraction of the total metals content of the catalyst particles, regardless of the level of contamination, it may be concluded that the depth of penetration is independent of catalyst particle size. This is of particular significance in commercial operation of moving bed catalytic cracking units. Commercial Significance of Peripheral Distribution of Metals. Catalyst losses in commercial moving bed operation are a combination of rubbing or surface-to-surface attrition among the particles and complete fracturing of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
particles from impact within the unit. Rubbing attrition becomes important in processing charge stocks containing metal contaminants, because the peripheral concentration of the metals that results is significantly lowered by this phenomenon, and a high proportion of the metals are eliminated with the fines in the normal course of unit operation. For processing metal-contaminated stocks it is desirable to maintain a maximum catalyst particle size. Calculation shows that 50% of the contaminant metals are in the peripheral 5% of the bulk of a 4.0-mm. particle, or in 90% of the bulk of a 100-micron particle. Thus as the catalyst particle size decreases, the possibility of removing significant concentrations of metals by rubbing attrition is minimized. A striking example of the extent of selective removal of nickel and vanadium in a commercial mixing bed catalytic cracking unit is given by Dart, Mills, Oblad, and Peavy (7). Inspection of the residuum charged in this operation showed that nickel and vanadium totaled 11 p.p.m. During 8 months 802,000 barrels of this charge was cracked over a 575-ton catalyst inventory operating a t an average replacement rate of 2.5 tons per day. The original equilibrium clay cracking catalyst contained 200 p,p.m. of nickel and vanadium. At the end of this operation total metals content was 870 p.p.m.-an increase of 670 p ~ p - m .of nickel and vanadium. However, on the basis of the total metals charged, taking into account the make-up supplied total nickel and vanadium should have been in excess of 2500 p.p.m. Analyses of synthetic crude tower bottoms showed that retention of metals on the catalyst was essentially quantitative. The loss of such a significant fraction of nickel and vanadium is attributed to their selective removal in the fines resulting from rubbing attrition during normal operation of the unit. literature Cited
(1) Dart, J., Mills, G. A,, Oblad, A. G., Peavy, D. C., Oil & Gas J . 54, 123 (May 9, 1955). ( 2 ) D~iffy,B. J., Hart, H. M., Chem. Eng. Progr. 38, 344 (1952). (3) McEvoy, J. E., Milliken, T. H., Juliard, A., Anal. Chem. 37, 1869 (1955). (4) Mills, G. A., IND. ENG. CHEM.42, 182 (1950). ( 5 ) Shambaugh, J. P., Petroleum Rejner 32, No. 2, 126 (1953). (6) Taff, W . O., Hardy, R. L., U. S. Patent 2,651,600 (1953). RECEIVED for review January 7, 1956 ACCEPTEDDecember 18, 1956 Division of Petroleum Chemistry, 129th Meeting, ACS, Dallas, Tex., April 1956. Delaware Valley Meeting-in-Miniature, February 1956.
