Use of Tracers in Refinery Studies D.
B. TODD
and W. B. WILSON
Shell Development Co., Emeryville, Calif.
Stack Loss of Catalyst from Commercial Catalytic Cracking Units
F A gas fines sampler can be used routinely to follow the daily fluctuations in stack loss of catalyst, if a satisfactory sampling port i s available. Tests on two catalytic cracking units have demonstrated its value.
I N AN accompanying paper ( I ) , the development of a radioactive tracing technique is described for the determination of catalyst mixing patterns in commercial catalytic cracking units. The same tagging technique has been used to measure the loss rate of fresh catalyst from such units. Fresh catalyst is added periodically or continuously to fluid catalytic cracking units in order to maintain the catalyst selectivity and activity at the desired level. Catalyst leaves the unit by direct withdrawal of a portion of the inventory, by entrainment from the reactor, and by entrainment from the regenerator. Catalyst entrained from the reactor generally concentrates in the bottoms of the primary fractionator and may not be lost permanently from the unit since frequently it is returned to the reactor with the slurry oil. Controlled withdrawal is practiced in order to maintain catalyst selectivity and activity, but uncontrolled losses, as from the regenerator stack, may represent an unprofitable operating expense. Also, local ordinances often require that regenerator stack losses be kept below a given level. The determination of the catalyst loss rate from the regenerator is difficult
20
because of the low concentrations and the generally small particle sizes. Refineries frequently determine their overall stack loss rates only by weekly inventory balances. Samples of stack fines frequently have indicated a higher surface area than that for the average inventory catalyst. This is indicative of a preferential loss of fresher catalyst and consequently an uneconomical use of the catalyst. Thus a knowledge of the loss rate of fresh catalyst from an invenrory of equilibrium catalyst becomes desirable as an indication of the net retention and usage of the fresh catalyst. The development of an adequate stack sampler and the use of radioactive tagging techniques have combined to yield valuable information regarding the amount of catalyst lost and the age of that loss. Three tests? relative to either or both of these problems, were performed recently in the Shell Oil Co. refinery at Houston and the refineries of Shell Oil Co. of Canada, Ltd., a t Vancouver and Montreal.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Development of Stack Sampler
The normal functions of a stack sampler involve withdrawal of part of the gas, measurement of the gas flow withdrawn, and recovery of the particles from this gas stream. Ideally, the sampling probe should be designed such that it does not disturb the flow into which it is inserted, and the samples should be withdrawn isokinetically. The withdrawal rate should be easy to measure and control: yet should be fast
enough to prevent deposition of fines along the line between probe and filter, and the temperature should be kept above the dew point. An additional requirement involved in studies employing tagged catalyst is that the filtering element should be easily and rapidly changeable. For the first stack loss test, conducted at the Houston refinery, a stack sampler was developed which incorporated a glass filter paper cylinder (6 inches diameter and 35 inches long) supported on a metal frame and with screen backing. The filter cartridge was housed in a structure affectionately dubbed the “coffin.” The installation is illustrated in Figure 1. This filter arrangement enabled an adequate sample to be collected for counting purposes, although frequent rupturing of the filters before the end of the sampling period prevented a direct measurement of the rate of stack loss. The collected fines accumulated on the inner walls of the glass paper cylinder or in the bottom of the cartridge. A new cylinder had to be made up for each sample. The time necessary to open the coffin and change filter cartridges also meant that sampling could not be continuous. In the later stack loss tests, at Shell Oil of Canada refineries in Vancouver (Shellburn) and Montreal, the filtering system was altered to circumvent these difficulties. The new samplers, schematically represented in Figure 2, used the same type glass filter paper (Mine Safety Appliances, Pittsburgh, Pa., No. 11OGB), but the filter paper was inserted
as a 16-inch diameter flat disk between two flanged canes. The filter paper was supported by a 60-mesh stainless steel screen mounted an a cane pattern metal disk, which in turn was supported by a perforated plate. The flanges could be separated to remove the cane pattern metal disk, screen, filter paper, and sample wiihout dismantling the entire assembly. A parallel sampling system was employed so that one filter could be changed while the other was in use. With this arrangement, samples could be taken continuously, using alternating sample periods as short as 15 minutes. Preparation of Togged Catalyst
I n addition to the other requirements regarding temperature stability necessary for satisfactory tagging (I), the radioactive tracer must not he preferentially adsorbed on the fines and/or outer surfaces of the catalyst panicles and the tagging procedure must not affect the properties of the catalyst. Accordingly, laboratory attrition tests were made on catalysts impregnated in different manners. Impregnation of the catalyst for the Houston test was accomplished using an acidic solution of scandium chloride containing radioactive ScWla, 90 pounds of catalyst being tagged with 35 mc. of scandium-46. This tagging method had no apparent effect an the attrition pmperties. A 15% cumulative attrition plus elutriation to minus 16 micron particles failed to indicate any difference in specific radioactivity (counts/minute/ eram) between the catalvst fines and residue. The isotow used for immeenation of the catalyst'for the Shellburn test was cerium-144, adsorbed from an acidic solution of cerium chloride containing Ce'Wla. One mc. of CeI4' was impregnated on 5 pounds of fresh catalyst. Although this impregnation was not entirely uniform, it was considered acceptable. At Montreal, catalyst was not tagged for the purpose of stack loss rate determinations. The fresh catalysts used in the Houston and Shellburn tests were of the silicaalumina microspheroidal type, Mean particle size was about 75 microns; water content at the time of injection was 15%. -
I
. "
Experimental Procedure
In the Houston test the tagged fresh catalyst was slurried with light gas oil and injected into the cone of the reactor over a period of 24 hours. Samples, usually of 90-minute duration, were taken from both the stack and the slurry oil during the addition period and for several weeks thereafter. The stack samples were taken from a 3-inch diameter probe extending into the center of the stack (Figure l), using the cartridgecoffin type sampler. The end of the
Figures 1 to 4. Top: schematic diagrom of regenerator stack sampling facility, Shell Oil Co,, Houston, Tex.; in circle: cone-type filtering unit; 2nd from top: stack sampling piping, Shellburn regenerator, Shell Oil of Canada, Vancouver, B. C.; and at Montreal regenerator, Shell Oil of Canada (bottom) VOL. 49, NO, 1
JANUARY 1957
21
probe was beveled 30" from horizontal, since there was inadequate space to provide a right-angle bend. This sampling port was not satisfactory for truly accurate sampling, being located immediately above the regenerator closure and immediately below the stack slide valve. The gas flow was metered through a standard ASME long radius flow nozzle en route from the stack probe to the filter housing. The stack samples were diluted to 1 liter with untagged equilibrium catalyst and counted with a dip type scintillation counter, described in detail in (7). Catalyst entrained from the reactor was sampled by taking a slip stream of slurry oil from the fractionator bottoms and passing this stream through a Sparkler filter. The filter cake was washed with toluene, dried, and counted in a well-type scintillation counter. This counter employs a 13/4-inch diameter by 2-inch deep sodium iodide crystal (thallium activated), containing a well into which approximately 5 grams of catalyst in a plastic vial can be placed. The counter is shielded with about 2 inches of lead. At Shellburn, the tagged fresh catalyst was injected in a 1-minute interval into the regenerator dense bed from an injection vessel pressurized with nitrogen. Sampling was continuous from the stack, in intervals of 30 or 60 minutes, for the first 5 hours after injection, and intermittently thereafter for 4 weeks. The cone-type samplers were used, the sample being withdrawn isokinetically from an open ended 1-inch diameter probe inserted axially into a horizontal section of one of the 15-inch diameter stack ells (Figure 3). About 140 grams per hour were collected, providing adequate sample for size and radioactivity determinations. The samples were counted in the well-type scintillation counter. The stack samples at Montreal were taken during a period of addition of a catalyst of different type from that in the inventory. In the process of collecting stack samples, traverses of one radius were made. Figure 4 indicates the arrangement and use of the cone-type samplers for this purpose. The probe consisted of a I-inch diameter movable pipe with a 1-inch hole facing upstream. The probe was located about 4 feet above the regenerator closure and an equal distance below the horizontal section of the 24-inch diameter stack.
4
e
7
I3
3
X
E a
u * 2
B
.d
.," Y
s
0
s $
1 -
w
Cumulative Stack and Slurry Loss
Withdrawal
0
Figure
4 6 Days Since Start of Injection
2
0
8
10
5. Radioactivity balance, stack loss test, Houston catalytic cracking unit
activity based on counting rates in the well counter (3.85 X 1010 c.p.m.). The total radioactivity added equals the sum of the cumulative stack loss at any time plus the instantaneous level in the inventory. The stack loss rate of fresh catalyst was calculated from the radioactivity data shown in Figure 6. Because of occasional difficulties encountered with rupturing of the filter paper and the inability to get a stack traverse, the activity levels indicated for the stack samples may not be entirely correct. The slurry sampling system, however, worked without
1500
.
failure. although with an unknown time lag in the fractionator. Even though the slurry catalyst contributes only about 5% of the total fines loss, the slurry data were used as a guide in interpreting the stack sample data. In Figure 6, the shape of the slurry curve was assumed to be well established and to represent the general shape of the curve of stack loss under these conditions. Consequently, the stack curve shown in Figure 6 omits one point that appears to be in error. The general shape of the curve for slurry fines was reproduced analytically by assuming an exponential decline
Injection Period4
E
M
\
E
a u
.
.-I
1000
a,
>
p
2 G
T S i u r r y Fines
0
*
.,-I
Results
Houston Test. In order to make certain that the radioactivity levels in the stack fines, circulating catalyst, and slurry fines were self-consistent, a balance was made of the radioactivity added to and lost from the unit during a IO-day period starting with the beginning of the 24-hour injection period. This balance is shown in Figure 5>with the total radio-
22
INDUSTRIAL AND ENGINEERING CHEMISTRY
2 'c1
500
2
I
0
0 Figure 6.
1
I
I
3 Days Since Start of I n j e c t i o n 2
I
4
Radiation levels in stack and slurry fines, Houston stack loss test
5
U S E O F T R A C E R S IN R E F I N E R Y S T U D I E S 10
1 10 ° 1
1hr I
0.01
Figure 7.
lweek
lday I
I
I
lmo.
I
I
Shellburn regenerator, radioactivity loss rate
curve for stack loss of an instantaneous addition and by integrating this over the actual 24-hour injection period. An instantaneous loss rate curve could by the same procedure be derived from the experimental data. However, there was some evidence that the rate of addition of tagged catalyst was possibly not uniform over the entire addition period. The stack loss for fresh catalyst continuously added for 24 hours is given in Table I. Assuming a constant addition rate, the cumulative loss at any time during the 30-day period can be approximately represented by C = 6.8
1
I
0.1
1
Figure 8. lative loss
Shellburn regenerator,
Table 1. Cumulative Stack Loss of Fresh Catalyst from Houston Regenerator Time Since Start of Addition,
Cumulative Loss, % Calcd." from Hr . Obsd. 6.8t.0 25 12 4 5.1 24 12 12.0 36 16 14.8 48 18 16.4 120 22 21.8 240 26 26.3 720 34 35.0 On basis at instantaneous injection. 8
a
t0.'6
where C = cumulative loss, yo of catalyst addition and t = hours since instantaneous injection. An accurate representation would require allowance for the material withdrawn during the irregularly scheduled withdrawal periods. One further interesting aspect of stack loss is a comparison between the radiation level of the stack fines as actually
lday 1 week I I I I 10 100 Hours a f t e r Injection
measured and the level expected if only fresh catalyst were lost from the stack. The radiation level of the 10.1 tons of fresh catalyst added concurrently with the 90 pounds of tagged catalyst was about 4000 c.p.m. per gram. Measurements on all counterswere corrected to the 5-gram sodium iodide crystal well-type scintillation counter and corrected for radioactive decay to the date of addition.
lmo I 1 30
fresh catalyst cumu-
Thus, from the radioactivity level (Figure 6), 22% of the stack fines are less than 12 hours old (800 c.p.m. per gram) and 30% are less than 24 hours old (1200 c.p.m. per gram). Shellburn Test. At the time of the Shellburn stack loss test, make-up catalyst was being supplied by the equilibrium catalyst from another refinery. Consequently, the tagged fresh catalyst was injected into an environment of predominantly coarser and heavier catalyst. The location of the sampling port did not permit a traverse of the stack to be made, although isokinetic samples could be taken. A total stack loss rate of 1.58 tons per day was calculated from the samples obtained during the first 3 days from the center of the stack ell. This compares well with the refinery's best estimate of 1.5 tons per day during the same period. Variations in catalyst loading of i5OY0 were noted during successive sampling periods, denoting either a changing flow pattern in the stack or an irregular discharge from the regenerator cyclones.
I
d
/ I
4
I 0.5
/
I
1
Sample Position, Fraction of Radius
Figure 9. Radial concentration profiles, Montreal regenerator stack
Cumulative Weight P e r cent
Figure 10. Size distributions o f stack fines from three Shell regenerators VOL. 49, NO. 1
JANUARY 1957
23
1
The loss rate of radioactivity is plotted venus time over the periods of sampling (Figure 7). The loss rate can be approximated by
Table 111.
I/*
-
where dR/& = hourly loss rate, % of injected radioactivity and t houn after injection. Using the measured inventory loss rate and the dashed line through the data points of Figure 7 and correcting for the slight nonuniformity of impregnation, the cumulative loss of fresh catalyst was calculated as a function of time after injection and is plotted in Figure 8. The data for the firat hundred hours are reasonably well correlated by
\
I
Table II. Catalyst Co :ation rofile-Montreal Regenerator Stac ample
Run
B
Position Fraotioir of Radius
0
0.17 0.35 0.35 0.52 0.70 70 96
24
Collection Rate. Grams)
Av. Dism.
Min.
Microns
0.38 1.19
13 13 I4
1.30 1.08 1.02
.. 13
0.85 0.92 1.84
13 14 15
.
~. . ,
.
.
Yo in sire range Radioactivity, c.p.m./g.
tn
% in size range Radioactivity. c.p.m./g. 4 to 5 Yo in sire range Radioactivity. c.p.m./g. 8 to 8'/3 % in sire range Radioactivity, c.p.m./g. 24 to % in size range Radioictirity, r.p.m./g. 67 to 671/* Yo in sire range Idioaetivity, c.p.m./g.
~~~~~~~
53-62
62-74
>74
47
I1
100
59
4 77
2 84
49 58
9 40
5 60
..
45 95
13 41
5 45
2 61
4 94
48 30
13 28
4 35
2 32
3 86
52 56
17 32
8
282
31
29 62
I1
70
38 27
14 26
29
.. .. ..
3944
44-53
2160
6 170
31 1239
6 124
33 729
2
177
29 523 20
30
plenum chamber into which the secondary cyclones discharge. The erosion of the probe tip also was evidence of preferential wear from one direction. The high catalyst loading at the wall may result from a combination of the centrifugal and wall drag forces. Apparently, however, the length of stack u p to the sampling port was not suffrdently long for much classification with respect to particle size, since the wall sample averaged only 2 or 3 microns larger than the e n t e r sample. No explanation has been found for the maximum and minimum in each curve. The total loss of fines from the stack, obtained from an integration of the profiles in Figure 9, was in good agreement with the best estimate by the refinery staff. P d d e Size Diatributious. The size distributions of representative samples fmm the three regenerator stacks are illustrated in Figure 10. The extremely coarse inventory in the Shellburn unit at the time of the test is reilected in the coarse particles b e i lost. The size distributions were obtained on a Sharples Micromerograph. Several of the Shellburn stack samples were separated into fractions by screening, and the radioactivity levels of these fractions were determined. The radioactivity-size distributions for these samples are given in Table 111. The high radioactivity level on the fine fractions is attributable to the generation of fines by attrition or other mechanism, in addition to the fines originally preeent in the tagged catalyst. Independent tests are planned in which attrition will be studied directly. In these experiments a tagged fraction of coarse catalyst will be injected into the unit, followed by withdrawal of h p l e s from the stack and the dense phase. The radioactivity level on size-fractions of the& samples will give a quantitative measure of the creation of fines in the unit.
INDUSTRIAL AND ENOlNEERlNQ CHEMISTRY.
.
0-39
to 1
Sire vs. Radioactivity
N o m i d Size (by Screening), Microns .
Sample Period; Hours after Injection 1 -7 .__
c = 4.3 10.6 where C cumulative % loss. Although the firat day's loss was 22%, the loss rate dropped rapidly, only 6% being lost during the third day. Extrapolation of the data to 100% losa would imply that no material e n e d in the unit beyond about 30 days. Stack samples taken 3 and 4 weeks after injection were so low in radioactivity that the standard deviation due to the statistical process of decay amounted to 50% of the reported net activity. In the initial loss period, the Ce" activity intmduced in the three catalyst mixing tests ( I ) that preceded this stack 10s test could be ignored. However, the gradual 10s of tagged equilibrium catalyst represented a significant contribution to the radioactivity level of the later stack samples. Montreal Test. The samples taken from the Montreal stack indicate a marked concentration gradient along the radius. The relative loadings at different radial positions are illustrated in Figure 9 and Table I1 for two different sampling periods. The samples were all drawn hkinetically at the superficial stack gas velocity. The low concentration in the center of the stack may be evidence of a residual vortex existing in the
Shellburn Stock Samples-Particle
8
..
..
Summary
The use of radioactive tracers for the determination of the rate of loss of fresh catalyst from catalytic cracking units is desuibed. The isotopes used, Scu and Gel", were strongly adsorbed by the catalyst, and a method of impregnating was used which resulted in a uniform deposition. The evolution of a satisfactory stack fines sampler is outlined, wherein the gas flow is easily measured and all the fines are collected on an adequately supported glass filter paper. The sampler can be used as a routine analytical aid in following the daily fluctuations in stack loss of catalyst if a satisfactory sampling port is available; it is probably more reliable for this purpose than the generally used method of inventory balances. Its use in determining the loss rate of fresh catalyst has been demonstrated in tests on two catalytic cracking units. The cumnlative loss of fresh catalyst from two regenerators is approximated by a power function of time. In a third regenerator, a concentration gradient across the stack radius is shown to exist. Acknowledgment
The authon acknowledge the participation in these tests of many contributors from the Shell research and refinery s&s-from Shell Oil Co.of Canada, Shell Oil Co., and Shell Development Co. Special acknowledgment is due for the contributions of C. W. Bitmer, G. M. Good, V. P. Guinn, and E.Singer.
Literature Cited (1) Singer, E., Todd, D. B., Guinn, V. P., IND. ENO.CUEM.49,11 (1957). R ~ C E N Efor D review December 17, 1955 ACCEPTED May 29,1955 Division of Petroleum Chemistry, 129th Meeting, ACS, D a h , Tex., April 1956
