I
R. E. DONALDSON, THEODORE RICE, and J. R. MURPHY Process Division, Gulf Research & Development Co., P.O. Box 2038, Pittsburgh 30, Pa.
Metals Poisoning of Cracking Catalysts Accurate evaluation of the effects of metals contamination in commercial fluid-cracking operations can result in improved operating efficiency through proper choice of charge stocks, catalysts, and make-up rate
T H E HISTORY of studies of metals poisoning of cracking catalysts has been one of initial oversimplification and not until recently has it been recognized in the literature that experimental methods which duplicate commercial operation must be used to provide reliable quantitative results. I n early studies, the effects of metals contamination were determined by using catalysts which had been impregnated with metals from salt solutions (5). Although these methods gave good qualitative results, the effects of a given amount of metal on activity and selectivity were considerably greater than when naturally occurring metals poisoned the catalysts. In later studies, oil solutions of metal naphthenates were used to deposit the metals on the catalysts (9). These studies showed that the effect of a given amount of metal depended on the molecular weight of the naphthenate and the molecular weight of the oil used to dissolve the naphthenate. More recently, studies have been reported using the naturally occurring metals in high boiling feedstocks ( 4 ) . However, in these studies, several procedures used differ from normal operation of refinery catalytic cracking units. These differences in operating procedures could have a significant effect on the results obtained. Several years ago, comprehensive studies were undertaken by this company to develop information on the effect of metals contamination of cracking catalysts. From the outset of these studies, operational procedures which simulated commercial operation in all important aspects were used. The more important operational procedures used in these studies which differ from those used in previously reported studies were :
Nonpoisoned equilibrium catalysts were aged in an automatic fluidized fixed-bed unit with charge stocks having high naturally occurring vanadium or nickel contents 0 Fresh catalyst was used as make-up 0 Product distribution data were obtained in a fluid catalytic cracking pilot plant at various metals levels 0
on the catalyst using the same charge stock as used in the aging operation 0 Typical commercial operating conditions were used in both the aging and product distribution operations Studies were made using low- and high-alumina synthetic catalysts and a sulfur-resistant natural catalyst to determine if there was any benefit to be derived in choice of catalyst when charging high metals stocks. Studies were also made to determine the effect of catalyst make-up rate, since this method is the one most often used in the refinery for counteracting the effect of metals poisoning. Finally, a study was made to determine if the relative effects of vanadium and nickel poisoning under typical refinery conditions were the same as had been determined in earlier bench-scale studies. These studies showed : 0 Sulfur-resistant
natural and highalumina synthetic catalysts are more resistant to vanadium poisoning than is low-alumina synthetic catalyst 0 Detrimental effects of vanadium poisoning on product distribution are decreased when fresh catalyst make-up rates are sufficiently high to increase equilibrium catalyst activity 0 Differences in the age of vanadium deposits on sulfur-resistant natural catalyst, caused by variations in charge stock metals content or catalyst make-up rate, had very little effect on the degree of deterioration in product distribution resulting from a given amount of vanadium contamination 0 Nickel in the charge caused 4.5 times the increase in coke yield and 7.9 times the decrease in gasoline yield as did an equal amount of vanadium in the charge
The minor effect of deposit age noted with the sulfur-resistant natural catalyst is in contrast to that previously observed with synthetic catalysts, wherein the detrimental effect of metals decreased considerably as age of deposit increased. The difference between nickel and vanadium is somewhat larger than that shown in previous bench-scale studies and is believed to be a result of
the beneficial effect of fresh make-up catalyst during aging with the vanadium containing charge stocks and the absence of such an eflect during aging with the nickel containing charge stock. Equipment and Procedures Metals deposition on the cracking catalysts was accomplished by longtime aging runs on an automatic fluidized fixed-bed unit using gas oil charge stocks containing relatively large amounts of naturally occurring metals prepared by vacuum distillation of selected reduced crudes. I n each aging run, a sample of nonpoisoned equilibrium catalyst was used as the initial catalyst inventory and a fixed amount of fresh catalyst was added each day. No attempt was made to obtain product distribution data directly from the aging unit. Instead, the catalyst was transferred from the aging unit to a smallscale fluid pilot plant at predetermined periodic intervals of catalyst metals content to obtain accurate product distribution data. Product distribution runs on aged catalysts were made using the same charge stock and catalyst make-up rate as used during the aging periods. Operating conditions for both aging and product distribution runs were also about the same and are given in Table I. During the time the catalyst was out of the aging unit, samples were taken to obtain Kellogg activity data, chemical and spectrographic analyses for metals content, and surface area determination:,. After completing these tests and the product distribution runs, the catalyst was returned to the aging unit for additional metals deposition. The automatic fluidized fixed-bed aging unit used in this study is similar to one described by Viland (70) and was constructed by modifying an automatic fixed-bed pelleted catalyst unit (8) so that a fluid catalyst could be handled under pressure. The modified unit has a catalyst inventory of about 3000 grams and was designed so that major operating variables, such as temperature, pressure, and dispersion steam VOL. 53,
NO. 9
SEPTEMBER 1961
721
Table 1. Operating Conditions for Aging and Product Distribution Runs Are Typical of Those Used in ihe Refinery
Unit Fluidized Smallfixed-bed scale aging fluid Operating conditions Reactor temperature, O F. Reactor pressure, p.s.i.g. Catalyst-to-oil ratio, wt./wt. Space velocity, wt./hr./wt.
I
,
I
I
5
IO
15
20
MPRA GAS OIL CHARGED: GRAMS X IO*
925
92 5
20
17
5.7~1 10: 1 Adjusted to obtain desired conversion
2.5
Figure 1. Catalyst metals content can be calculated by assuming 100% lay-down of metals in the feedstock
-
-
_ _E_ _
Catalyst metals by spectrographic analysis Catalyst metals by chemical analysis Catalyst metals calculated from feedstock metals
0.55-0.65
3.0
rates (which might influence the effects of metal contaminants), could be maintained within typical commercial ranges. As operation of the unit is completely automatic, only 4 hours of operator’s time per 24-hour operating day are required to remove products, add oil to the feed tanks, take samples, and add fresh catalyst make-up. Automatic shutdown and safety devices are incorporated to permit safe operation during the time the unit is unattended. To maintain a constant catalyst make-up rate per day and a constant ratio of catalyst make-up to oil charge, the length of total cycle and the amount of oil charged per cycle were held constant throughout the aging runs. In order to operate in this manner, it was necessary to vary the oil charge rate and time of reaction period to maintain gas oil conversion at about the desired 5070 level, The conversion was determined at periodic intervals by taking a grab sample of liquid product and subjecting it to distillation on a Hempel column to remove a gasoline and lighter fraction. The amount of daily catalyst samples taken was equal to the amount of fresh make-up catalyst, so that a constant catalyst inventory was maintained throughout the aging run. The small-scale fluid unit, used for the product distribution studies, has a catalyst inventory of about 2500 grams and a feed rate of about 5.5 gallons per day (7). Determining product distribution on the small-scale fluid unit rather than on the aging unit appeared desirable for two major reasons-the method of operation is more representative of commercial operation than can be obtained in a cyclic unit; and separation of the cracked products into gas, gasoline, and catalytic gas oil by continuous distillation is carried out more easily and accurately
722
NICKEL
k 0’ 0
Carbon-on-regenerated catalyst, wt. yo on catalyst 0.55-0.65 Dispersion steam, wt. yo of fresh feed
tion, fresh catalysts were heat treated at 1050’ F. and any rough edges were removed by attrition. Thus, there were no significant losses of fines or water during the aging runs and effective catalyst make-up was equivalent to actual catalyst make-up. As a result, the application of the pilot plant findings to any commercial operation can be made on the basis of effective catalyst make-up. Early in this work, it was determined that under the conditions of this investigation-Le., no loss of catalyst as fines and no loss of fresh make-up catalyst-there was excellent agreement between catalyst metals content as determined by spectrographic and chemical analyses and as calculated by assuming 1007, deposition of the metals in the charge on the catalyst. Figure 1 compares determined and calculated catalyst metals contents for a typical aging run. T o eliminate any- discrepancies which might result because of use of a faulty analysis of catalyst metals content, the results of the various runs were compared using the calculated metals content.
f- 3 , 0
than by batch distillation which is required on the aging unit because of alternate reaction and regeneration periods. In making the product distribution runs, space velocity was adjusted to obtain the desired conversion. I n operating commercial catalytic cracking units, significant porri.ons of the fresh make-up catalyst are lost as volatile material and fines. I n addition, previously reported data indicate that the concentration of metals in the fines is about twice that of the bulk of the catalyst ( 3 ) . Of course, the losses in fines will vary for different units depending on the type and number of cyclone separators or precipitators installed to reduce the loss of catalyst as fines. Since duplication of these losses in pilot unit equipment is extremely difficult, several measures were taken to reduce catalyst losses to a minimum in the pilot plant work. The initial catalyst charge and the fresh make-up catalyst were elutriated to remove the fines in the 0- to 40-micron diameter range. I n addi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
Results and Discussion Effect of Metals Deposition on Various Types of Catalysts. Since various types of catalysts may exhibit differences in resistance to metals poisoning, it appeared desirable to obtain information which would aid in the choice of catalyst for refinery situations wherein metals contamination is a seriovs problem. Therefore, an investigation was made to determine the relative effects of metals deposition on low-(1370) and high-(25YG) alumina synthetic catalysts and on a sulfur-resistant natural catalyst. Because of its inherently higher steam and thermal stability, the high-alumina synthetic catalyst had a significantly higher activity than did the low-alumina synthetic catalyst and the sulfur-
HIGH ACT SYN
--LOW ACT SYN. S - R NATURAL
I
15
0
I
0.05
0.10
1
I 0.15
0.rn
I
I
I
.I
0 25 1
1 I
0
0 05 0 10 0 15 INCREASE IN VANADIUM % BY WT
020
025
ON CATALYST
Figure 2. Kellogg activity test unit results indicate that the detrimental effect of vanadium on carbon producing factor is decreased by activity increase due to fresh make-up catalyst
C.R A C K I N0 CAT A L Y S TS resistant natural catalyst. I t is felt that the diffrrences in results obtained with the two synthetic catalysts depend more on differences in activity than in alumina content and, therefore, activity has been used as the term or reference in discussing results. Equ.librium catalysts obtained from commercial fluid cracking units and containing negligible amounts of nickel and vanadium were used as initial catalyst charges. Corresponding fresh catalysts were used as make-up in an amount equivalent on a finesand moisture-free basis to 2% of unit inventory per day or 0.81 pound per barrel of oil charged. The activities, carbon factors, and metals contents of the fresh and equilibrium catalysts are summarized in Table 11. A naphthenic gas oil, derived from Mara crude, which contained 6.7 p.p.m. vanadium and 0.43 p.p.m. nickel was used as charge stock throughout this investigation. Because of the small amount of nickel in the charge, there was no significant build-up of nickel on the catalyst at the 2% make-up rate; therefore, the effects of metals deposited have been related entirely to the increase in catalyst vanadium content. The length of these aging runs was about two months, a t the end of which time, the vanadium content of the catalyst was 70 to 80y0of the equilibrium vanadium content. Figure 2 shows the effect of vanadium deposition as determined by the Kellogg activity test for the various catalysts. The activity of all the catalysts increased during the aging runs, with the lowand high-alumina synthetic catalysts showing a 3.8 to 5.5 numbers greater increase in activity than the sulfur-resistant natural catalyst. Corresponding increases in surface area were noted, indicating that the greater increases in activity for the synthetic catalysts resulted from higher activity and better steam stability of the fresh synthetic make-up catalysts. The fact that both activity and metals content of the catalysts increased during the aging runs shows that under certain conditions of amount and activity of make-up catalyst, and metals content of charge stock, two opposing factors (activity-beneficial, metals-detrimental) which influence product distribution can be generated. Since the largest increase in activity was obtained during the early stages of the aging run, the beneficial effect of increased activity would probably be greatest at the low vanadium levels. The carbon factor plot shows that the degree of deterioration in catalyst selectivity caused by a given increase in vanadium content was greatest for the low-activity synthetic catalyst and about the same for the sulfur-resistant natural catalyst and the high-activity synthetic
Table 11.
Comparison of Properties of Fresh and Equilibrium Catalysts Charged to Aging Runs
Catalyst Description
Kellogg Activity
Carbon Factor
32.9 20.9
2.2
58.5 22.8
... 1.3
Sulfur-resistant natural Fresh Equilibrium Low-activity synthetic Fresh Equilibrium High-activity synthetic
Fresh
67.9
Equilibrium
32.9
53
I
Nickel
...
Q.013 0.017
