AGING OJ' SILICA-ALUMINA CRACKING CATALYST. I. KINETICS OF STRUCTURAL CHANGES BY HEAT A N D STEAM BY W. G. SCHLAFFER, C. Z. MORGAN AND J. N. WILSON Shell Development Company, Emeryville, California Received November 6 , 1066
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A systematic study has been made of the rate of change in the specific surface area and specific pore volume of silicaalumina cracking catal sts exposed to temperatures in the range 478-950" and steam partial prewures in the range 0-7 :i.tmosphwes. The d e c h e in specific surface area is described aceurately by an empirical equat.ion of the form -dS/dt = kS" where S is the specific surface area, t is time, and n and k are constants at any given set of conditions. These constants vary smoothly with changing temperature and partial pressure of steam. The rate of change in specific pore volume is proportional to the rate of change of specific surface area a t the highest temperatures, but decreases relative to the rate of change of specific surface area as the temperature decreases; changes in the partial pressure of steam a t a given temperature have a sitiall but definite effect on this relationship. The results are discussed in terms of a model for the structure of silicaalumina c h l y s t s and of various possible mechanisms of material transport involved in the aging process. placed in a Pyrex tube 90 cm. long, 9 cm. in diameter having a sintered frit close to the bottom to support the catalyst. Air, dried over drierite, waA passed upward through the column of catalyst powder a t a rate of 14 liters per minute for a eriod of 16 hours. Cagination and Dry Air Aging.-The catalyst was calcined a t 565" in the auartz tube and furnace shown in Fie. 1 . The maximum charge of catalyst was 1000 CC. which, ;hen fluidized by a stream of dry air (4 cc./min./g. catalyst), extended one-half of the length of the furnace. The temperature profile of the bed was maintained to -f2' by means of a Celectray controller. For calcination, the temperature was gradually raised to 200" at, a rate of 2" per minute and then raised to 565" a t a rate of 5" per minute. The time of calcination, 6 hours unless otherwise stated, was considered to start when the catalyst bed reached the temperature desired. For high temperature aging in dry air the same apparatus was used. The calcined catalyst was brought to temperature at a rate of B"/min. in a stream of dry air. Agin in Steam at One Atmosphere.-For aging in one atmospfere steam pressure, water was fed to the bottom of the vertical quartz tube used for the calcinations; from there it is changed to steam and passed through the catalyst column. The water was maintained a t a constant level by the arrangement of the inverted two liter graduate and leveling bulb (G) of Fig. 1 the drain of which waa connected directly to the bottom of the catalyst tube. The drain and air vent of the leveling bulb were of 1 mm. capillary tubin These restrictions revented surging of the water. W i k this arrangement, tge passage of water as steam through the catalyst powder could be held remarkably constant a t 20 g. per hour. Samples were withdrawn periodically by lowering a, quartz thimble suspended t y a platinum wire into the fluidized bed of catalyst. Aging at Less Than One Atmosphere Steam.-For aging a t less than one atmosphere steam pressure, the air humidifier (H) was attached to the bottom of the catalyst tube. The air was saturated with water to any desired level by controlling the temperature of the humidifier. Aging at Greater Than One Atmosphere Steam.-The reactor tube consisted of an enamel lined stainless steel vessel which fitted into the Hevi-Duty furnace; it is shown in Fig. 2. The operation of the pressure steamer is self-evident from the figure. Samples were withdrawn via the decompression chamber (H) connected to a concentric tube leading to the catalyst bed. For all agings the catalyst was heated to the nominal temperature a t a rate of 5" per minute in a fluidizing stream of Catalyst.-The catalyst was a synthetic microspheroidal dry air at 4 cc./min./g. catalyst. After equilibration of the silica-alumina gel commercially manufactured by the temperature, the air flow was discontinued and either water American Cyanamid Company in 1951. It contained or humidified air was allowed to flow to the lower end of the 1 2 . 4 % ~ Altos, . 0 . 0 4 % ~ Fe20!, . 0.02%w..Na20 and 0 . 3 % ~ . catalyst tube by connecting the appropriate apparatus. sulfate based on catalyst weight after ignition to 1100". The zero time sample is that withdrawn when the catalyst After elutriation to remove particles less than 40 p in di- had reached the temperature of the experiment, i.e., just ameter and calcination at 565" to remove the excess water, before the introduction of steam. Before a surface area or the catalyst had a specific surface area of 608 m.2/g. and a pore volume measurement each sample was dried at 500' specific pore volume of 0.73 cc./g. All experimeiits were for one hour in a stream of dry air. Value8 of surface areas made with catalyst that had been pretreated in this way. and pore volumes refer to a gram of catalyst in the condition E1utriation.-Approximat,ely one liter of catalyst was resulting from aging and drying with whatever water re~mains. ( I ) I T . E. Ries, J r . , Adunnres in Calalvsis. IV, 87 ( l Q f i 2 ) .
Introduction The decline in specific surface area of silica-alumina cracking catalyst during its use in the catalytic cracking process is a phenomenon of economic importance and technical interest. It is accompanied by a decline in specific pore volume and in catalytic activity. The most important factors controlling the rate of decline are temperature and partial pressure of steam. Little information concerning the kinetics of this deactivation process is available in the literature. Some relations between the resulting changes in the physical properties of the gels have been reviewed by Ries.' His principal conclusions were the following. (1) During aging a t high temperatures (800l000") in high vaciium the distribution of pore radii (as defined by the Kelvin equation for the relative vapor pressure of liquids in cylindrical capillaries) remains almost unchanged, and the average pore radius (proportional to the ratio of specific pore volume to specific surface area) remains approximately constant. (2) During aging a t lower temperatures (500GOO") in the presencc of steam the average pore radius increases considerably; the distribution of pore radii broadens and the median pore radius increases. The work reported here was undertaken with two objectives. (1) To obtain detailed knowledge of the kinetics of the process and of the dependence of the rate on the temperature and on the partial pressure of steam. (2) If possihle, to relate the changes in the gross physical properties of tJhesystem to the structure of the gel and to the mechanisms of material transport involved in the aging process. Experimental
AGINGOF SILICA-ALUMINA CRACKING CATALYST
June, 1957
715
TABLE1
A Q J NOF ~ SILICA-ALUMINA CATALYST AT VARIOUS TEMPERATURES IN THE PRESENCE OF STEAM Temp., O C . Btesm prescure, atm. (abs.) Time,
hr. 0
.1 $2
.3 .4 .5
.. .. .. .. ..
..
.. ..
.8
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.7 1.0 1.3 1.8 2.0 2.6 3.6 4 10 14 20 48 92 98 120 144 192 196 218 288 360 458 504 672 696 980 Temp., 'C. Bteam pressure, a h . abs.
0 .1 .2 .3 .4 .5 .8
.7 1.0 1.3 1.8 2.0 2.5 3.5 4 7 10
14 20 48 92 98 120 144 182 196
216 288 380 884 458 480 604 652 698 884 984 i3ao
, .
.. ..
... I
.
.
...
... ... ... ... ... .. ... ...
.. .. 637 ..
... ... ... 0.700 ... ...
474 428
.890 .684
..
..
.. ..
380
..
..
380
.. ..
335
..
318
.. ..
.. 4
588
.. .. .. ..
.. ..
..,
...
,675 ..,
... ,871 ...
z A.
...
... , I .
. I .
... ...
...
669
. . . . . .
50.7
..
. . . . . .
523 505
.721
......
488 478 471 435
,888 .682 .679 .870
58.4 57.1 57.7 81.6
62.1
...
...
58.2 84.2
... ...
71.0
... . . I
74.6
...
83.8
... ... ...
*.
..
..
418 405 398 387
..
. . . . . .
...... . . . . . .
,657 .654 .853 .847
62.9 64.6 88.0 68.8
......
..
... ,844 ... .634 . . . ,631 . . .
... 70.8 ... 71.0 . . . 74.5 . . .
321
.628
78.0
.. 365
..
357
..
339
.. .. ..
404
.. ..
418 377
.. 341 ..
278
..
273 281
..
245
.. , .
0.719 ,702 ,895
. .898 ... 5'
...
d7
A.
...
,880 ,642
63.2 08.1
...
.816
...
,812
.609
... .808 ... ...
...
...
326
0.833
77.7
297
.. . I
..
... .817
...
...
147
,808
.593
111
...
...
*. ..
..
188
..
..
.. .. .. .. ..
...
...
.. ..
...
... .608
.597
...
.599 .594
...
8,
VP,
m.'/g.
OO./g.
.. ..
, . .
, . .
.,.
... 83.1 ... ... 111 131 9 . .
143 147
... ... ...
168
...
....... .. ... ... ... ... ... ... ...
... ...
,.
...
... ... ... ...
259
0.603
93.1
..
230
... ... ... ,601 106 ... ... ... ...
20I
,591
.. 170 .. .. ..
... ,686 ... ... ...
..
.. ..
..
. *
,,
.. .. .. .. .. ..
..
..
.. .. .. .. ..
# . .
...
...
... ... ..,
... ... ... ,.,
... ...
...
... .
I
.
... ...
778
778
778
883
0.11
0.3
I
1
0.710 ...
...
48.3
... ...
,..
...
, . .
.
...
,..
... , . .
.
I
...
... ,585
...
... ... ,549 ... ,527
...
,522
...
...
571 51 1 513 505 494
..
... ...
487 453 423 410 395
...
...
.. ..
..
79.9
..
...
363 341 328 305 258
... ...
98.0
,,.
123
... 127 ... ...
138
... ... 150 ...
... ... .501 ...
,.. 160
,494 .483 ,484 ,484
189 173 181 197
..,
...
..
225 217 209 191
.. ..
174
..
183 159
..
0.875 ,859 ,665 ,837 ,633
47.3 51.8 51.1 50.4 61.3
. . . . . . ,620 50.9 . . . . . . . . . . . . .593 ,586
52.2 55.4
...... ,578 .588
56.2 57.5
...... ,533 ,620 .502 ,492 ,448
58.8 61.0
,570
..
..
369 347
.. .. 315 .. .. 273 .. 233
84.5 89.1
191
74.1 74.8 78.7 82.3
. . . . . . ,380
87.4
,369 .384
90.6 91.8
......
..
...... ......
152
.388
98.8
. . . . . . ,..
387
..
..
......
..,
0.700 ,836 ,807
81.8
...... .417 .408 ,401 .393
581 492 449
... ...
. . . . . .
187
..
.. 142 136 128
..
124 114 111
..
104 104
...
...
... ,582 ,527
...
...
,497
48.2 51.7 54.1
493 396 343 311 289
...
287
60.9 80.7
*.
...
..
...
228 212
63.1
*.
... 68.9 ...
...
... ... 70.8 ...
,451
77.4
...
...
.482
...
,426 ,400
.*.
...
*..
86.5 95.8
... ...
.379 ,576 ,874
107 111
* 373
120 127 128
...
,383 .355 ,..
.352 ,357
... , . I
... ...
...
...
I19
...
...
135 137
...
...
...
...
...
...
..
0.848 .579 .534 ,616 .506
62.7 58.6 82.3 85.7 70.0
... ... ,489 73.8 ... ... ... ... .481 ,450
80.9 84.Q
192 177
.442 .429
92.1 98.9
148 133 123 111
,413
..
..
..
... ...
.402
... ...
113 121
,397 )20 .390 141
... I
.
.
..
...
...
.. .
..
.. .. .
I
.. .. .. ..
398 173 131 117 104 97
.. .. .. .. .. ..
., I
.
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.
...
...
...
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...
,,. ... ... ...
...
, . .
...
..
..
.. ..
...
... ... ... ... ... ... ,.. .
.
I
... ... ... ... ... ... ... ..,
...
83.2 70.6 80.6 88.2 92.7 04.8 .,.
... ... ... ... ...
~ , .
... ,..
... ... ...
...
...
... ...
...
... ...
... ...
138
...
..
..
...
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,.
117
...
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.
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...
...
I
...
...
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,
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...
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...
...
...
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...
..
...
... ...
..
..
... ...
,
.
, . .
.. .. ..
...
0.627 .303 ?e4 .252 .241 ,231
..
.. ..
I
CT A. ...
... .*.
..
..
..
..
99.3
214
...
, .
152
109
,..
...
, .
,..
...
...
... ... ...
... ... ...
,596
..
..
102
...
7
zA.
, .
89.7 93.3
219
...
..
...
218 182
104
..
..
...
... 74.4 ... 89.3 ...
.600
...
...
...
230
..
..
..
...
.675
.834
OO./E.
... ...
... 58.2 ...
...
m.*/g.
49.6 52.1 63.7
...
VP,
47.2 48.9 49.3
,682 ,878
... ...
s,
1
,608
96
..
..
VP, oo./g.
578
678
136
117 112 107
584
...
78.7
... ... .515 ...
..
50.8 50.2
51.3 52.1
.859
.. 171 .. 165 .. .. 152 ..
.. 125 ..
.734 .735
.899 .892
...
..
. . . . . .
809 599 565
..
...
..
49.9
545 531
...
..
224
0.732
. . . . . . . .
587 602 578 588
...
...
, . .
.. ..
A.
50.2 51.0
. I
..
OC./E.
,704 ,708
...
293
d7
8, m.'/g.
VP,
501 556
..
..
3
,..
...
... ... ...
I
678
1
..
... ...
, . . .
570
...
...
... ,881 ... ... ...
8, m.a/g.
576 0.30
... ... ...
...
... ... ..,
...
... ... ... I
.
.
... ... ... ...
... ...
.. ..
...
W. G. RC~ILAFFER,C. Z. MORGAN A N D J. N. WILRON
710
AOINQ OF Tcmp., OC. Time, Iir.
0
.2 .3 .5
.I
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1.0 1.5 2 3 4 5 6 7 24
7RS
z,
TABLE I1 SILICA-ALUMINA CATALYST IN 847
A.
st
lb,
m.a/g.
oa./g.
A.
0.630 ,628 ,631 .623 ,619 ,622
51.7 51.7 53.1 53.0 51.9 53.G
484
0.598
49.4
468 400
,614 ,611
52.5 53.1
441,451 446 405,408 406,437
,560 .552 .544 ,547
50.2 49.5 53.5 53.9
407
.608
52.1
410,419
.534
53.5
8,
1.n
m.a/u. 494 480 475 470 477 4G5
oa./u.
. . . . . . . .
. . . . . . . .
. . . . . . . .
457 431
,601
.593
52.0 55.0
..
*.
436
..
..
.. ..
402,405
d',
......
S, m.a/x. 384
.. .. ..
...... .564 51.7 ...... ......
......
. . . . . . ,511
50.7
317 305 292,310 295 282 282 269
*.
STREAM OF DRYAIR
ROO VP. co./a.
a7 A.
0.495
51.5
...... ...... ...... ......
..
280
A
,395 .378 ,.378 ,388 ,364 ,380 ,345 .341
Vol. 61
49.8 49.6 50.3 53.6 51.6 53.9 49.3 50.7
......
940
8.
VP,
cc./g.
m.l/g.
lz
A.
. . . . . . . .
. . . . . . . . 180 169 165 159 154 143 133 119 114 110 103
,
0.266 .262 ,256 ,249 ,232 .225 ,223 ,191 .1R3 ,174 .154
59.1 62.0 62.1 62.6 60.3 62.0 67.1 64.2 64.2 63.3 60.0
. . . . . . . .
9, m.t/g. 139
950
2,
VD,
co./g.
A.
0.205
59.0
........
........
111
.167
60.2
104 97 92 87 82
.148 ,137 ,125
56.9 56.5 54.3 52.9 54.6
. . . . . . . . ,116 .112
. . . . . . . . . . . . . . . . 72 ,094 52.2 . . . . . . . .
I t is quite evident that both temperature and steam pressure exert powerful influences on the decline of surface area. This is shown clearly in Table 111, by the half-life, h/$, defined as the time necessary to reduce the surface area to 304 m.2/g. HALF LIFE
F
Fig. 1.--Apparatus for aging in steam a t less than one atniosphere pressure: A, removable well for recording t,hermocouplcs; B, quartz tube, 80 cm. long, 7 om. diameter; C, gel charge; D, slotted quartz plate with Refrasil cloth and platinum; E, model M-30245 Hevi-Duty .funace; F, water reservoir, 2-liter graduate; G, water leveling bulb; H, heating tape; I, asbestos insulation of air humidifier; J, sintered glass frit; K, air inlet; L, controlling thermocouples to celectray ; M, controlling and indicating thermocouples. Specific pore volumes were determined by the method of Benesi, Bonnar and Lee8 but using n-heptane and dibutyl hthalate instead of carbon tetrachloride and cetane. gpecific surface areas were measured by the BET method with nitrogen. For brevity, specific surface area (m.e/g.) and APecific pore volume (cc./g.) will be referred to hereafter simply as surface area and pore volume, respectively.
Results and Conclusions Decline of Surface Area.-The data for agings at 478, 576, 678, 778 and 863" in one atmosphere of steam, a t 576" and 0.3, 3 and 7 atmospheres of steam, a t 778" in 0.11 and 0.3 atmosphere of steam and a t 785, 847, 900, 940 and 950" in a stream of dry air are recorded in Tables I and 11. When the log surface area is plotted versus log time, as shown in Fig. 3, almost straight lines are obtained. In most. cases there is a perceptible downward curvature, which, in the case of the agings in steam, becomes progressively less marked at higher temperatures. (2) H.A, Benesi, 1963 (1956).
OC.
A ing con f i tions a h . (abs) steam
478 576 576 576 576 678 778 778 778 785 847 863 900 940 950
1 0.3 1 3 7 1 0.11, 0.3 1 Dry air Dry air 1 Dry air Dry air Dry air
1,
c
R. U.'Bonnar and C. F. Lee, A n a l .
Chem., 27,
TABLE 111 AREA UNDER CONDITIONS
OF SURFACE
VARIOUS
AOINO
1112,
hr.
740 3000 60 3.2 0.2 4 21 2.3 0.37 10" 108
0.02 2
