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Kattl, S.S. FhD. Thesls, University of Delaware, Newark, 1984. Kattl, S. S.; Westerman, D. W. B.; Qates, B. C.; Youfigless, T.; Petrakis, L. Ind. Eng. Chsm. Recess Des. Dev. 1984, 23, 773. Krlshnamvthy, S.; Shah, Y. T. Chem. Eng. Commun. 1902, 16, 109. Landa, S.; Kafka. 2.; QalL, V.; Safar, M. Collect. Czech. Chem. Commun. 1969, 3 4 , 3987. Ll, C.-L.; Xu, 2.4.; Gates, 8 . C.; Petraks, L. Ind. Eng. Chem. Process Des. h v . 1985, 24, 92. McIlvried, H. G. Rep.-Am. Chem. Soc.,Div. Petrol. Chem. 1970, 15(1), A33. Petrakis, L.; Ruberto, R. G.; Young, D. C.; Gates, B. C. Ind. Eng. Chem. Process Des. Dev. 1983a, 2.2, 292. Petrakis. L.; Young, D. C.; Ruberto, R. G.; Gates B. C. Ind. Eng. Chem. Process Des. Dev. 1983b, 22, 298. Satterfield, C. N.; Modell, M.; Hltes, R. A.; Dederck, C. J . Ind. Eng. Chem. Process Des. Dev. 1978, 17, 141. SatterfW. C. N.; Cocchetto, J. F. Ind. Eng. Chem. Process Des. Dev. 1*81.20,53. Shabtal, J.; Oblad. A. 0.; Wlser, W. H., paper presented at the Fifth Annual DOE Fossll Enecgy Conference on University Coal Research, Aug 23-24, Loulsvllle KY. 1978.
Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B. Prepr.-Am. Chem. Soc.. Dlv. Petrol. CY”. 1977, 22, 919. Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B.; Mathur, K. N., unpubllshed resutts. Sonnemans, J. Proceedings of the Fifth International Congress on Catalysis, Palm Beach, FL, 1972, p 1085. Stern, E. W. J . Catal. 1979, 57, 390. Sundaram. K. M.; Blschoff. K. B.; Katzer, J. R., unpublished results. Wiser, W. H. U.S. Department of Energy, Quarterly Progress Report DOE/ ETl14700-2, Aug 1980, pp 21-25. Zawadski, R.; Shlh, S. S.;Katzer, J. R.; Kwart, H., unpublished results.
Received f o r review March 21, 1985 Accepted September 13, 1985
Supplementary Material Available: Table containing the identification of compounds in the strong-base fraction by GC/MS (4 pages). Ordering information given on any current masthead page.
Kinetics of Combustion of Carbon and Hydrogen in Carbonaceous Deposits on Zeollte-Type Cracking Catalysts Guang-xun Wang, Shi-xlong Lin, Wel-Jlan Mo, Chun-Ian Peng, and Guang-hua Yang East Chlna Petroleum Institute, Oongying, Sttandong, People‘s Republic of China
The kinetics of combustion of carbon and hydrogen in carbonaceous deposits, or “coke”, on zediitype cracking catalysts at high temperatures up to 800 OC and resldual coke contents down to 0.05% of cracking catalysts were investigated with both the continuous-flow technique and p u b f l o w technique. The problem of a very short reaction period (of the order of seconds) at a high temperature of regeneration was overcome by a high-sensitivity, quickqesponse thermoconductivity ceH, and the problem of strong adsorption of one of the reaction products, water, on the cracking catalyst and on the reactor system was solved by means of a pulse technique and a model reactor
technique. Kinetic equations of the combustion of carbon and hydrogen in coke on the cracking catalysts obtained fit the experimental data satisfactorily.
The mechanism and kinetics of the regeneration of carbonaceous deposits on cracking catalysts has been extensively investigated since the 1940s. Fundamental works had been done by Hagerbaumer and Lee (1947), Pansing (1956), Dart e t al. (1949), Weisz and Goodwin (1966), Massoth (1967), and others. However, their works were concerned mainly with the regeneration of amorphorous bead catalysts a t a comparatively low temperature, mostly below 600 “C,and a rather high residual coke content, >0.5%. The kinetic data and rate expression concerning the burning of hydrogen in the coke appeared very few times in the literature. The introduction of zeolite cracking catalysts with their high activity and thermal stability permits the operation of a regenerator of a cracking unit a t temperatures as high 800 OC and a low residual coke content down to 0.05% after regeneration. Therefore, in the design and operation of this type of modern catalytic cracking unit, it is quite essential to acquire the kinetic data for the combustion of carbon and hydrogen in coke on cracking catalysts. An attempt has been made to throw light on this problem by Wang et al. (1982). The present work is a continuation of that effort. Experimental Work The experimental difficulty of determining the kinetics and the mechanism of carbon and hydrogen combustion in coke a t high temperatures arises from the high speed of reaction; so it is important to develop an in-line detection device which has a quick response to the reaction 0196-4305/86/ 1125-0626$01.50/0
signal and high precision of measurement. In the case of measuring hydrogen reaction velocity, the problem is even more strenuous because the reaction product-water-is readily adsorbed on the reactor wall and on the cracking catalyst. To circumvent this difficulty, special techniques were developed and will be described below. Experimental Technique for Measuring Reaction Kinetics of t h e Combustion of Carbon The flow diagram of the experimental setup for the determination of the combustion kinetics of carbon in coke on the cracking catalyst is shown in Figure 1. A quartz reactor, 7-mm inside diameter, was designed to meet the various strict requirements for a good flow pattern, isothermal reaction, small pressure drop across the catalyst bed, differential oxygen consumption in the reactor, and absence of mass transport limitations between coke and oxygen. In accordance with the aforementioned requirements, a limited amount (several milligrams) of coked cracking catalyst was placed on the sintered quartz plate fused to the reactor wall, and a sufficient quantity of coke-free inert of the same particle size was then added to mix with the coked catalyst as a diluent for the isothermal reaction. Air in the reactor was expelled with a stream of nitrogen, purified by passing through two deoxygenators connected in series. The reactor was then heated by submerging its reaction zone into a fluidized bed filled with silica gel microbeads to ensure even temperature distribution. 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
627
1
Figure 1. Flow diagram for the measurement of the reaction rate of combustion of carbon in coke: (1) air compressor; (2) air purification tube; (3) metallic copper deoxygenator; (4) nitrogen purification tube; (5) pressure control valve; (6) six-port valve; (7) flow control valve; (8) rotameter; (9) 401 trace oxygen remover; (10) nitrogen purification tube; (11) sheath thermocouple; (12) fluidized bed filled with silica gel microbeads; (13) catalyst; (14) sintered quartz plate; (15) CO oxidation tube; (16) TCD; (17) split regulating valve; (18) anhydrous magnesium perchlorate; (19) ascarite; (20) silica gel.
When the preassigned reaction temperature had been reached and kept stable, the regeneration reaction was started by switching the four-port valve to replace the nitrogen stream in the reactor with a purified air or oxygen-nitrogen mixture, free from C02and H20 vapor. The reaction products, after oxidation of CO to C02in a heated CuO tube, were split into two equal streams. One was treated with ascarite and Mg(C104),to remove its C02and water vapor and then led to the reference arm of a highsensitivity, quick-response thermoconductivitycell detector (TCD), while the other, after the desiccation with Mg(C104),to remove its moisture content, was directed to the measuring arm of the detector. Since the absorption tubing and connecting fittings in both paths were made as identical as possible in size and configuration, they should have reached both arms synchronously. The output of TCD was four linear to the concentration of C02 in the tail gas.
Experimental Technique for Measuring the Reaction Kinetics of Hydrogen Combustion in Coke The experimental setup for the present purpose is very much similar to that for the study of carbon combustion except for the water vapor detection devices. Gases emitted from the reactor were separated into two equal streams with a proportionating valve. The stream directed to the reference arm of the TCD was desiccated with a quartz tube packed with Mg(C104)2,while the other passed through a dummy tube without any treatment to the measuring arm. C02remaining in both streams was found to have negligible influence on the response of water vapor in TCD. Among the difficult problems encountered in the experimental study of combustion of hydrogen in coke, especially a t high temperature, the most critical one originates from the strong adsorptive behavior of the reaction product-water. Adsorption interferes with the normal transport of water vapor from the catalyst bed through the detector and seriously distorts the concentration distribution of water vapor in the effluent gas and thus obscures the information of the true kinetics of hydrogen combustion. Materials such as fused silica, Teflon, and internalpolished stainless steel tubing which have a minor tendency to adsorb water, were chosen to construct the reaction system, but some distortion of the effluent diagram was still present. It was found that the coked catalyst itself can adsorb some water even at high temperature. To find
Figure 2. Model reactor system for the correlation of experimental results obtained in the pulse mode and in the continuous mode: (1) pressure regulator; (2) desiccator; (3) rotameter; (4) flow control valve; (5) well-stirred vessel; (6) four-port valve; (7) thermocouple; (8) quartz reactor; (9) fluidized bed for heating; (10) proportionating valve; (11) desiccator filled with anhydrous magnesium perchlorate; (12) dummy tube; (13) TCD; (14) air compressor.
a way out, a novel strategy was adopted in our laboratory. The main points of the new methodology are as follows. 1. The pulse reaction technique was employed to determine the instantaneous rate of hydrogen combustion in coke a t any specific time. A pulse of zero air was injected into the nitrogen stream ahead of the reactor, and the water formed in the pulse was completely swept out by the dry carrier gas nitrogen. The output signals of TCD were integrated to give the amount of hydrogen burnt in the single pulse. Successive pulses and, finally, a continuous flow of air were introduced. The total water observed in the detector is equivalent to the hydrogen associated in coke. As for the water which may exist as an ingredient in the zeolite catalyst, it was found that a part of this sort of water was removed in the preheating stage, and no water vapor would evolve if the temperature was kept constant. Consequently, there should be no discrepancy between the amount of the hydrogen burnt and that of the hydrogen detected in TCD in the form of water. A first-order kinetics with respect to hydrogen content of the coke was found by the pulse technique. 2. In case the kinetics of hydrogen combustion with respect to oxygen is far from first-order, the amount of hydrogen burnt by oxygen in the pulse will depend on the shape of the incoming air pulse, i.e., the distribution of the oxygen concentration in the gas stream. If this were the case, the interpretation of the pulse data would be complicated. In our experiments, three levels of oxygen partial pressure were explored to reveal that the order of reaction with respect oxygen is unity. 3. Now that the commercial catalytic cracking unit and most of the experimentalsetups in the regeneration studies ran on a continuous mode, it was quite essential to ensure that the reaction kinetics found by the pulse-flow technique was valid to account for the result realized in a continuous process. A model reactor was devised, simulating the process of the combustion of hydrogen in coke in the continuous mode. In the simulation system, the reacting and detecting devices were just the same as the pulse reaction system, except that a well-stirred vessel was inserted in the nitrogen line ahead of the four-port valve (see Figure 2), and in the quartz reactor proper, the coked catalyst was replaced with a white catalyst mixed with a small amount of CuO particles of the same size. In this plan, we used hydrogen gas as a reactant in place of the hydrogen in coke and gave the well-stirred vessel the role of a first-order reaction and let the effluent pass through the same detection system. The well-stirred vessel, on receiving an injection of hydrogen, would give a transient concentration of hydrogen in nitrogen to the
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Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
A !
1FI
,
me
5
Figure 4. Response diagram of TCD.
I
1
, 4 0
>
0
? U
1 0
Y
Reaction time c m m )
Figure 3. Comparison of experimental data in continuous mode to that of model reactor: (0 0)model reactor; (-) continuous mode.
reactor in an exponential manner, just as the coke delivered hydrogen to form water in a first-order reaction. The well-stirred vessel in our experiment was so made that the reciprocal of its mean residence time was equal to the rate constant of combustion of hydrogen coke a t a chosen temperature, 600 OC,obtained by the pulse-flow technique. The amount of hydrogen injected was equal to the hydrogen burnt in the continuous-flow mode. As a current of dry nitrogen carrier gas was passed through the well-stirred vessel, it picked up a transient amount of hydrogen and brought it to the reactor where it reacted instantaneously with CuO to form water. The amount of water formed a t every instant should have followed the first-order reaction and should have equalled that formed by coke a t the same moment. Since these equal amounts of water experienced the same conditions of adsorption and flow pattern, it was expected that the effluent diagrams given by TCD in both cases would be identical. This turned out to be true, as shown in Figure 3. A conclusion can be drawn that the process of the combustion of hydrogen in coke in the continuous mode follows first-order kinetics and that the kinetic data acquired through the pulse reaction technique is valid to elucidate the continuous process of the combustion of hydrogen in coke. Catalyst Samples Used in This Research. CRC-1 is a zeolite catalyst ingredient supported on Fuller's earth. A sieve fraction of 76-100 ym in diameter was used. The fresh catalyst was aged with superheated steam at 790 O C for 4 h and then placed in a fluidized reactor and coked with cumene a t 500 "C. The carbon and hydrogen contents on the coked catalysts were 1.75% and 0.05%, respectively. Y-7 is a zeolite catalyst resembling CRC-1 in composition and activity. Y-9 is a zeolite catalyst supported on amorphous aluminum silicate sampled from two commercial FCC units.
Results and Discussion I. Kinetics of Combustion of Carbon in Coke. The response of TCD to C02during the process of combustion of coke is depicted in Figures 4 and 5. It is remarkable that a t the very beginning of the reaction, the rate of C 0 2 as shown on the curve is quite low and increases rapidly up to a peak as the reaction proceeds. Soon after passing the summit, it begins to decline in an exponential manner with the reaction time, displaying the characteristic of a
Figure 5. Response diagram of TCD.
first-order reaction. How to explain this phenomenon is an interesting problem. The phenomenon mentioned above also took place consistently in the experimental works published by many authors who have been engaged in this area of research. However, many authors have simply treated that part of the data after the peak has appeared and gotten the first-order reaction kinetics with respect to carbon
c+
0 2
- co,
while leaving the reaction kinetics in the early period uninterpreted, or even ignored. We tried to elucidate the deviation of the effluent response curve from first-order kinetics at an initial period by assuming some interference from physical processes, such as the nonideal flow pattern of the gas passing through the reaction system, the temperature fluctuation in the catalyst bed, and the suspected CO or COz adsorption. Some experiments were made to ascertain the extent of their influence. The result showed that they could not account for the rate behavior in the initial period of the reaction. Finally, since the shape of the curve gives us some indication of a model of series reactions, we turned to the chemical aspects of this problem. A series-parallel reaction model was proposed by us (1982) to explain the effluent curve as a whole; the results looked satisfactory. As more data were gathered, we have tried to adopt a sequential procedure for optimal discrimination among several rival rate models and a sequential procedure for precise estimation of the parameters in the most probable model (Dumez and Froment, 1976). To do this, we proposed a few of the possible kinetic models: (1)simple reaction model coke .-kCO kinetic equation dtCO
+ COJ
dt
+ COz
= kCo exp(-kt)
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
629
(2) series reaction model
coke
ki
C,O,
k2
CO
0 2
+ C02
kinetic equation d(CO + COZ) k1kzCo =exp(-k2t) - exp(-klt) dt k l - k2
(2)
(3) series-parallel reaction model
+
k$'
02
coke
Cyox
co t c02
t *
Reaction lime ( S I
Figure 6. Effluent COz diagrams (00 mental.
co
t
con 750 I
727 1
700 I
e )theoretical; (-)
Temperature, ' C 670 650 I I
experi-
600 I
kinetic equation d ( C 0 + COJ dt
(4)surface reaction model by Massoth (1967). By an elaborate computer computation it was found that for coked CRC-1 catalyst, the series model and the series-parallel model were more favorable than the simple model and surface reaction model. The parameters of the series reaction model are kl = 6.8
X
1O1O exp
k 2 = 2.3
X
lo4 exp( -
l/(atm min)
(4)
10*E103)
(5)
l / ( a t m min)
A comparison of the model with the experimental data is illustrated in Figure 6. As an approximation, experimental data after the peak have been correlated by linear regression to fit the simple reaction model k = 1.67 X 1O1O exp
(-38'5RxT
lo')
l / ( a t m min)
I
1.00
I 1.03
1
1.06
+k
I
1.09
I 1. 12
I
1. 15
I
Figure 7. Arrhenius plot of carbon and hydrogen combustion rates.
(6)
The maximum deviation of our experimental results from eq 6 was less than 20%. The coke on catalyst Y-7 showed similar behavior as CRC-1, and the coke on Y-9 equilibrium catalysts taken from two commercial FCC units burned slower than CRC-1. The variation of k with temperature follows the Arrhenius law, as shown in Figure 7. 11. Kinetics of Combustion of Hydrogen in Coke. Coked CRC-1 catalyst with 0.05% by weight of hydrogen associated with coke has been explored for its hydrogenburning kinetics in the range of 600-750 "C and 0.110.23-atm oxygen partial pressure. Both the continuous and pulse modes of regeneration have been tried. The establishment of the kinetic equation and the estimation of its parameters was based essentially on the data of the pulse experiments. The validity of the pulse mode as a substitute for the conventional continuous mode has been proved by the model reactor. The reaction rate was found linear to the partial pressure of oxygen, as shown in Figure 8. From these experimental results, it is easy to show that the rate equation for the combustion of hydrogen in coke can be expressed as -dH/dt = k,Po,H
1
0.97
(7)
Figure 8. Effect of oxygen partial pressure.
where H = hydrogen concentration in coke deposited on the catalyst a t time t , w t %, Po, = partial pressure of oxygen in the reaction system, and kH = reaction rate
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Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
Table I. Kinetic Constants for the Combustion of Carbon and Hvdrogen for the CRC-1 Catalyst -~ ~~
reactor temp, "C
kc kH
k,/k,
600
650
3.83 9.20 2.40
12.8 29.8 2.33
700 37.5 85.7 2.29
750 99.3 181 1.82
constant for the combustion of hydrogen in coke. The variation of the rate constants K H with temperature was shown in Figure 7. As can be seen from Figure 7, for temperatures below 700 "C, a straight line was obtained, but for temperatures above 700 "C, this line is concave somewhat downward, which demonstrates that the combustion of hydrogen goes so fast that the diffusion of oxygen may hinder the rate of reaction. Regression of the experimental data gave kH = 2.47 X 1O1O exp(-37.66/RT) (8) for temperatures below 700 "C. For temperatures above 700 "C, kH may be expressed as kH' = (1 - Ue-bir)kH (9) where a and b are empirical constants, u = 2.67 X 1030,and b = 73.4 X lo3. Using our experimental results, it is possible to compare the reaction velocities for the combustion of carbon to the combustion of hydrogen in coke; see Table I. Comparing C with H at time t in the process of combustion C / H = C0/& exp((ks - k,)Po,t) (10)
A plot of the degree of conversion of hydrogen, C Y H , to the degree of conversion of carbon in coke, actat the same time is on Figure 9. Figure 9 indicates that in the temperature range of 600-700 "C of combustion, almost all the hydrogen will be burnt out before the conversion of carbon in coke will have attained 85%. Combining the discussion of the combustion of carbon and hydrogen in coke on the cracking catalyst, a unified kinetic model of regeneration is proposed as
J
11.2
I).
4
.
0 8
0.6
