Coke Formation and Its Relationship to Cumene Cracking 1
C. J. PLANK AND D. M. NACE Research and Development Department, Socony Mobil Laboratories, Paulsboro, N . J .
T
HE cracking, or dealkylation, of cumene has been recognized (7-9) as a good simple measure of cracking catalyst activity. The reaction is ideal because it is uncomplicated by a significant amount of side reactions. The present authors set out to develop a screening test utilizing cumene cracking, which would be satisfactory for the rating of experimental cracking catalysts. Where a single variable was modified in preparing a series of catalysts, the test rated the catalysts well not only as to activity but also as to coke formation. I n developing a satisfactory test some valuable information was obtained regarding coke formation and its relation to the rate of cumene cracking. I n fact, without a recognition of the effects described here, proper control of a cumene test would be impossible. It is largely with these data concerning coke formation and cracking inhibition that the present paper is concerned. EXPERIMENTAL
Cumene was charged into the top of a vertical fixed-bed isothermal reactor by means of a positive displacement pump. The temperature of the charge within the pump was 60' zk 2' F. The borosilicate glass reactor tube, 1.25 inches in inside diameter, was filled with quartz chips except for a 20.0-cc. catalyst section in the center of the tube. The quartz and catalyst were separated by a thin layer of quartz wool. The temperature was measured and controlled to i 5' F. by means of a thermocouple in the center of the catalyst bed. Liquid product was collected in a trap held a t 150' F. to limit the solubility of propylene to a negligible amount. The gaseous products passed through a water-cooled condenser held at 60" F., and then through a wettest meter. I n a typical run nitrogen was passed through the reactor for 15 minutes while the catalyst was brought to temperature, The initial catalyst and preheat temperatures were 10' to 20' F. above the desired operating temperature to balance the small endothermic heat of reaction. Flow of cumene was started. The volume of gas evolved was recorded every 2 minutes for the first 10 minutes, then a t 5-minute intervals. I n early experiments the rate of gas evolution was determined by water displacement. However, as a wet-test meter gave equivalent results, this method was, therefore, used in the experiments described here. The catalyst activity decreased rapidly a t the start of each run to a fairly constant level, as exemplified in Figure 1. At the end of the run, the reactor tube wa8 flushed immediately with nitrogen a t 200 cc. per minute for 45 minutes. The catalyst bed was heated during that period t o 950' F. This long purge with nitrogen was found necessary to give a completely reproducible carbonaceous deposit (coke) on the catalyst. Coke determination was carried out in place after the purge. Dry air was passed at 150 cc. per minute over the catalyst for 90 minutes a t 1000" to 1050" F. The regeneration gases were passed through a copper oxidize tube a t 950' F. to oxidize carbon monoxide and then through separate absorption tubes containing Drierite and Ascarite. The amount of carbon in the coke was calculated from the weight of carbon dioxide picked up by the Ascarite tube. Cumene. Eastman pure cumene and Phillips pure grade were used with further purification.
2374
Early in the work it was discovered that two different batches of so-called "pure" cumene gave greatly different cracking rates and coke yields. Mass spectrometric examination of the two batches showed that one had less than 0.5% impurities, while the other had about 3.5% impurities, mostly ethylbenzene and cumene hydroperoxide. Furthermore, when about 2 % cumene hydroperoxide was added to the purer of these two batches, the cracking results corresponded well with those obtained on the other batch of cumene. It was found that essentially all of the impurities of a polar type could be removed from the cumene by treatment with silica gel. An adsorption column 2 inches in inside diameter and containing 1000 cc. of 6- t o 16-mesh desiccant grade silica gel was used. At a 2 to 1 volume ratio of cumene to silica gel only about 0.1 % ' cumene hydroperoxide remained after treatment of the bad batch of cumene just discussed. The cumene so purifjed cracked well, forming almost no coke. I n fact, after a 60-minute run a t about a 70y0conversion level the coke on catalyst was usually less than 0.1%. As result, the use of cumene represents an excellent way to separate the cracking and coke formation reactions in the study of cracking catalysts. Catalysts. Silica-alumina catalysts, both extruded and bead types, of various activities were used. The different activities were obtained by treatment of the fresh catalysts with steam. Except where otherwise noted catalyst samples were crushed to 8- to 14-mesh before use. Catalyst activities were determined by the CAT-A test ( 4 ) , a standard gas oil cracking test. METHOD OF RATE DETERMINATION
To be sure that cumene dealkylation is a clean-cut reaction even at high conversions, the gas samples from a series of preliminary experiments were analyzed by mass spectrometry. Both the thermal and the catalyzed reactions were studied. The analytical results are summarized in Table I. The specificity is even better than expected from the results obtained by Greensfelder (5) a t 932" F. Even at 1000" F. the amount of thermal reaction is less than 4%. The two important thermal reactions are dehydrogenation and demethylation, occurring to about the same extent. Liquid mass spectrometer analyses on a
Table I.
Gaseous Products from Cumene Cracking
A. Thermal Cracking (Mineral Quartz Packing, LHSV Temp., F. (i5 9 1000 1050 Conversion. vol. yo 4" 9 Hn,vol. % 36 45 35 39 CH4. vol. % Ca, vol. 7 0 11 9 cz C4 cs,vol. % 18 7
= 2)
+ +
B . Catalytic Cracking (Silica-Aluminab, 2 to 6 LHSV) 950 1000 Temp., F. (f5O) 800 850 900 Conversion vol. % 30-50 40-60 50-65 55-70 50-80 Hz CH4,'vol. % 1 1 2-1 4-3 10-8 Ca,vol. % 95-97 94-96 93-96 90-94 84-88 Cn C4, vol. % 4-2 5-3 5-3 6-3 6-4
+ +
a
Approximate results, due to high nitroaen dilution of small gas sample. catalysts, CAT-A activity (4)= 22 and 31.
b Two
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1955
few of the products showed equivalent production of methylstyrene and styrene corresponding to the hydrogen and methane in the gases. 100
80
-
I
s
I I
\
- PURIFIED -----
CUMENE CUMENE t CUMENE HYDROPEROX'DE ( S I L I C A - A L U M I N A CATALYST )
1
\
Y
If coke were the catalyst deactivator, one would expect the coke formation in the runs of Figure 1 t o be inversely related to the catalyst activity. I n particular, one would expect that a steady-state cracking activity could be reached only if coke formation essentially stopped. As seen from Figures 2 and 3, such is not the case. The rate of coke formation is rapid at the start of the run, then decreases with time, and finally becomes constant. The linear portion of the curves coincides with the period over which the catalyst shows constant activity. The data for silica gel are included t o show the very low rate of coke formation for a noncatalytic material of high area. Thus, over a substantial period, coke on catalyst increases markedly without leading to a decrease in activity for cracking cumene containing a given concentration of coke former. However, changing the concentration of coke former changes both the coke level and the cracking rate. From this it is concluded that the inhibitor is the coke former, not the coke. x
0
1
I
10
20
Figure 1.
I
I
30 40 T I M E (MINUTES)
1
I
J
x)
M,
i0
Change of cracking rate with time
I n the catalytic runs very little thermal conversion occurs a t F. and below. Furthermore, between 800' and 900' F. the gas averages about 95% C3's. Some propane (1 to 3%) is produced from the propylene, owing to the hydrogen exchange activitg of the catalyst. The same phenomenon was encountered by Greensfelder and Voge (9). The gas production thus represents a good direct measure of the conversion of cumene to propylene and benzene. The reaction rate drops quickly to a value which remains essentially constant over the last 30 minutes of a 60-minute run (Figure 1). Therefore, the average rate of gas evolution during the last 30 minutes waa taken as the measure of the steady-state cracking
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10.'
2,0
i 0 = SIO,-AI,O,
[~BM?/G)
0
(212Mf/G)
= SIO,-AI,O,
0 = SiO, GEL (487M?/G)
900'
10
0
20
30 TIME
Figure 2.
40
M
80
(MINUTES)
Coke formation of purified cumene
rate. RESULTS AND DISCUSSION
Variation of Catalyst Activity and Coke Formation with Time. I n early exploratory experiments cumene inadvertently containing variable amounts of impurities was cracked over a series of catalysts. An apparent correlation between catalyst activity and coke on catalyst was found, similar t o that shown by Blanding ( 1 ) in the case of gas oil cracking. As a result, the first assumption made from these data was that coke is the catalyst deactivator. This is the assumption usually made with regard to catalyst deactivation in gas oil cracking. Shankland and Schmitkons ( 7 ) and later Blanding (I), for example, have discussed the effect of coke on the activity of catalysts toward cracking gas oils. However, in the case of cumene cracking the experiments described here show that coke, as such, is not the catalyst deactivator. Cumene properly purified cracks with very little accompanying coke formation. I n fact, the more thorough the purification the less the coke production and the greater the ease with which the cumene may be cracked. This discovery represented the first indication that coke might not have a direct relationship to catalyst activity in this particular reaction. The data shown in Figures 1, 2, and 3 emphasize this conclusion. I n Figure 1 are shown plots of cracking rate us. time of run. Two catalysts widely different in surface area were tested with purified cumene and a sample of purified cumene plus cumene hydroperoxide. The typical rapid attainment of the steady-state cracking rate is noted in each case. The time to reach this steady state is definitely less in the case of the pure cumene.
I TIME
Figure 3.
(MINUTES\
Coke formation of cumene containing hydroperoxide
Thus, the initial rapid drop in cracking activity is probably due to the rapid attainment of equilibrium adsorption of the inhibitor on the catalyst. At the same time the rate of coke formation is most rapid. When equilibrium absorption of the inhibitor has been attained, the cracking activity becomes essentially constant, but coke formation continues a t a constant rate. Then it may be assumed that a constant proportion of the active sites is being occupied by inhibitor, with the coke occupying
Vol. 47, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
2376
Table 11. Primary Data on Cumene Cracking in Presence of Inhibitors 1.
A. Silica-Alumina a t 800" F. Extruded Silica-Alumina, 30 AI-CAT-A'", Ro
R
,!
f=
cc.Pmin. Imidazole
P O
4.23 x 10-4 2.12 x 10-4 1.06 X 10-4 0.53 x 10-4
5.0 14.5 21.0 29.5
PO
0.04 0.13 0.18 0.26
1.86 X 0.93 X 0.47 X 0.235 X 0.117 X
10-8 lo-' 10-3 lo-*
0.04 0.115 0.20 0.27
3.72 2.34 1.86 0.93 0.47
X X X X X
10-8 10-3 10-8 lo-' 10-8
7.0 11.0 13.5 21.0 27.5
0.02 0.05 0.13 0.20
7.46 3.73 1.86 0.93
X 10-8 X 10-0
Indole 6.0 12 0 16.5
2.5 6.7 15.3 25.6 33.5
10-8
Quinaldine
1. fG
0.02 0.06 0.13 0.22 0.285
10-4
10-4
4.5 13.5 24.0 32.0
7.43 x 3.72 X 1.86 X 0.93 X
1010-3 10-8 10-8
Piperidine 2.5 5.6 15.I 24.0
n-Butylamine 3.0 14.7 X IO-* 11.5 7.46 X 10-8 24.0 3.73 x 10-1 32.0 1.87 X 10-8
X 10-8 X 10-8
B. Various Catalysts a t 900' F. with Styrene Silica-Alumina Beads, 29 AI Catalyst, Ro = 75.0 Co./Min. RP, P O cc./min. f= 0.08 0.06 0.04 0.02
Pyridine
9.3 x 4.65 X 2.33 X 1.17 X
10-4 10-4
50.0 Cc./Min. RP eo./&. Quinoline
5
37.0 43.5 49.5 60.0
2. Silica-Alumina Pellets. 31 0.06
0.08 0.06 0.04 0.02
0.09 0.115 0.18 0.23
AI Catalyst, Ro
= 92.0 Co./Min.
26.0 35.0 47.0 60.0
0.24 0.315 0.415 0.52
3. Silica-Alumina Crushed, 20 AI Catalyst, Ro = 72.0 Cc./Min. 0.078 29.0 0.265 0.055 0.305 34.0 0,009 0.49 57.0
0.05 0.10 0.14 0.21
a5.o
0.34 0.39 0.435 0.52
4. Silica-Magnesia Crushed, 35 AI Catalyst, 18.0 0.08 21.0 0.06 25.5 0.04 32.0 0.02
0.025 0.10 0.20 0.27
Ro = 43.0 Cc./Min. 0.165 0.19 0.225 0.275
2. Silica-Alumina Bead Catalyets5 Styrene on 35 AI Catalyst, 2,l2Sq. M./G., Cumene Hydro eroxlde on 18 AI Catalyst. RQ = 67.0 Cc./Min. 46 Sq. M . / 8 . , 4.74 x 10-2 15.5 0,;s 1 75 x 1 0 - 2 2.85 X 10-2 24.5 0.: 1.93 X 10-2 31.5 0.3 1.0 x lo-' 39.5 Catalyst crushed to 8-to 14-mesh Tyler. as evolution P average rate for last 30 minutes of 60-minute run, smoothed t o nearest 0.5 cc./minute. A t 100dconversion of pure cumene, rate = 118 cc./minute (calod.).
b Rate of
essentially none of the active centers. And yet, as the silica gel experiments showed, coke formation requires the presence of active sites of the catalyst-that is, something is happening to the inhibitor on the catalyst which is converting it to coke without increasing catalytic inhibition, Linking coke formation to the inhibitor seems justified by the fact that coke formation is negligible when highly purified cumene is cracked. Suggested Mechanism. The data which have been presented may be explained very well by a reaction scheme involving the competition of cumene and coke-forming molecules for cracking sites. This mechanism has also been independently derived and verified in these laboratories by studies using a differential reactor (6, 11) (Schwab type). The cumene, C, and the inhibitor, P, compete for absorption on the active sites, A . Absorbed cumene then cracks and some of the adsorbed inhibitor polymerizes and further reacts by hydrogen exchange to produce coke. If CA and PA represent cumene and inhibitor, respectively, absorbed on the acid sites, the reaction system may be described as follows. Assuming the reverse of the cracking reaction is negligible. ka
(c)f ( A )e( C A ) -+(C6H6) + (CaHd + A k.1
-
kz
(P)
k4 ke ( + P ) (+PI + ( A )e ( P A ) e. (PA) k7
k6
coke
N = total concentration of acid sites = (CA)
d(PA)/dt = 0 = & ( P A ) - k l ( P ) ( A )
(PA)= (k4/ks)(P)(A) (CA)
The assumption then may be made that A
