( x ~ yl), , and the graphical determination ofpx,-l is not shown in Figure 4. Continuous-flow operation to obtain the same total extraction is also shown in Figure 4, illustrating the advantage obtained by cyclic operation. I n the special casep = q = l--i.e., the total holdup of each
phase is transferred-Figure
H
- (x,'
L
1
- pa-,)now
[
2 shows that the point x,',y
falls on the operating line, while the
line representing Equation 6 becomes horizontal. in Figure 3, the point x,
[
,+
+
L
- (y,'
H
- q Y,+~),y,'
Similarly,
1
falls on the
operating line, while the line representing Equation 8 becomes vertical. I n both cases, the various concentrations are now
found by drawing steps between the operating line and the equilibrium curve, just as for the continuous-flow case. For cyclic operation, however, there are two steps between the two curves for each stage. Hence, cyclic-flow operation of N stages gives the same separation as continuous-flow operation of 2N stages. This result was shown analytically by Belter and Speaker (1967) for the case of linear equilibrium relationship, but is seen to hold for the more general case treated here. literature Cited
Belter, P. A., Speaker, S. M., IND.END.CHEM.PROCESS DESIGN DEVELOP. 6, 36 (1967). RECEIVED for review May 8, 1967 ACCEPTEDAugust 31, 1967 Supported by the Norsk Hydro Co.
COKE FORMATION KINETICS ON S I L I C A - A L U M I N A C A T A L Y S T Basic Experimental Data Y U l C H l O Z A W A ' A N D K E N N E T H B. B l S C H O F F l The University of Texas, Austin, Tex. The kinetics of coke formation reactions during the cracking of ethylene over a silica-alumina catalyst was studied a t atmospheric pressure and from 350' to 500". The coke had no significant effect on the surface area of the catalyst and the effective diffusivity through the catalyst pores in the range of less than 1 weight % coke. Evaluation of the effectiveness factor for the pertinent catalyst and reaction indicated that pore diffusion offered negligible resistance. A thermogravimetric system was used for continuous measurement of the weight of the coke formed. Plots of the weight per cent of coke on catalyst vs. process time on a loglog scale showed two straight lines, indicating an initial rapid coke formation. HE activity of catalysts declines rapidly because of the Taccumulation of carbonaceous deposits in most high temperature organic reactions using heterogeneous catalysts. This phenomenon is typically observed in the cracking of hydrocarbons over a silica-alumha catalyst. I t is important to characterize the nature of this coke formation process because of its commercial interest and the effect on the catalyst activity and selectivity. Several empirical equations have been presented to relate the weight of coke formed on the catalyst with the process time. Voorhies (1945) showed, for example, that the' carbon weight per cent on catalyst, C, approximately followed a relation C, = A . P . where t is the process time and A and n are constants. T h e values of exponent n were evaluated for different reactions by Rudershausen and Watson (1955), Tyuryaev (1939), and Wilson and Den Herder (1958), who found values ranging from 0.4 to 1.0. Voorhies reasoned that the rate of coke formation was controlled by a diffusion process; the diffusion rate could be taken to be inversely proportional to the weight per cent of carbon, since the coke itself became the diffusion barrier in the catalyst pellet. Similar semiempirical equations were presented by Blue and Engle (1951) and Panchenkov and Lolesnikov (1959). A recent extensive study by Eberly et al. (1966) used regression techniques to relate the weight per cent coke on catalyst as a function of process time and space velocity in the cracking of n-hexadecane.
Present address, Mobil Oil Co., Paulsboro, N. J. Present address, Department of Chemical Engineering, University of Maryland, College Park, Md. 20742
T h e present study was concerned with the over-all kinetics of coke formation reactions on spherical silica-alumina cracking catalyst. T h e surface areas of the fresh and coked catalysts were measured in order to determine the effect of coke deposition on the area. The effect of the coke on diffusivity was also determined to see whether the diffusivity remained constant during coke formation. With these values, the effective diffusivity of the reaction gas through the catalyst pellet was estimated to learn the importance of the pore diffusion process. T h e weight of coke was measured continuously by a thermogravimetric system. Most of the previous workers determined the weight of coke by burning the coke in a furnace and measuring the COn content of the combustion gas absorbed in a caustic solution (Appleby et al., 1962; Blue and Engle, 1951; Eberly et al., 1966; Greensfelder et a/., 1945; Smitkons, 1949 ; Voorhies, 1945). When the thermogravimetric system is used, the weight of coke can be instantaneously determined without interrupting the experiment. T h e effect of process time, feed rate, and temperature on the coke formation was thus studied using ethylene as a reaction gas in order to have a fairly simple reaction. Experimental
T h e Mobil Oil Co. Durabead 1 catalyst used contained 90 weight % of silica and 9.7 weight 7 0 of alumina, and had a surface area of approximately 200 sq. meters per gram. The catalyst was spherical and divided into four groups: VOL. 7
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JANUARY 1 9 6 8
67
n
THERMO-GRAV
\k
00 ~
r
9 9
VACU, VACU,
VACUUM PUMP
v
FURNACE
SAMPLE COIL
He
T.
I
COLUMN, c.
W
L-/l
0
THERMAL
k
ONDUCTIVITY
Figure 1.
Catalyst A
B C
D
Schematic flow diagram
Particle Diameter, M m . d > 4.76 4.76 > d > 3.36 3.36 > d > 2.38 2.38 > d > 2.00
Surface Area Measurement. T h e Brunauer-EmmettTeller (BET) method was used to measure the surface area of coked and fresh catalyst. Nitrogen (99.999% purity) was used as the adsorbate gas. Approximately 0.2 gram of catalyst was placed in a sample bulb and degassed for 20 hours at 400' C. and 10-6 mm. of Hg pressure. A sample valve was then immersed in a liquid nitrogen bath, a small amount of nitrogen was introduced into the bulb at 40- to 60-minute intervals, and the adsorbate pressure was read on a mercury manometer. Effective Diffusivity Measurement. T h e method developed by Weisz (1957, 1962) was used to measure the effective diffusivity of the catalyst pellets. A catalyst pellet was forced into a slightly undersized Tygon tubing so as to separate the two gas flow systems. At one exposed face of the pellet a stream of nitrogen was passed, a t the other a stream of hydrogen. T h e concentration of hydrogen in the nitrogen stream due to diffusion through the pellet was measured by a thermal conductivity cell (Gow-Mac Instrument Co., Newark, N. J.). This method is rapid and simple enough to obtain statistical information on a large number of individual samples. Fifteen Table I. Surface Area of Fresh and Coked Catalyst Wt. 7,Coke Surface Area, on Catabst Sq. Meters/G. 0.0
0.20 0.39 0.58 0.95
68
GAGE
211 208 202 200 213
I b E C PROCESS DESIGN A N D DEVELOPMENT
catalyst beads were taken for measurements from each lot. All coked beads were regenerated at 450' C. and their diffusivities were re-estimated in order to determine the effect of coke on diffusivity. As will be shown in detail (Ozawa and Bischoff, 1968), effectiveness factors based on these diffusivities were essentially unity. Measurement of Weight of Coke. A thermogravimetric balance was used to measure the weight of the catalyst bead increase continuously as the coke formed on the catalyst. A schematic diagram of the apparatus is shown in Figure 1. Approximately 5 grams of catalyst was weighed and placed in a platinum gauze basket, which was hung on the end of the support rod of the balance by a thin wire (Figure 2). Nitrogen gas was passed through the system while the catalyst was heated to the reaction temperature in order to desorb physically adsorbed water. When the water desorption reached an equilibrium, or no further weight decrease was observed, the whole system was evacuated to 1 to 2 mm. of Hg. Ethylene was then introduced into the reactor. As the coke was formed on the catalyst, the recorder pen moved upward gradually, starting from a zero point. The fluctuation of pen due to the reaction gas flow could be adjusted by controlling the recorder gain. The outlet of the reactor was directly connected to a Beckman GC-2 gas chromatograph so that the conversion of reaction gas could be determined by analyzing the effluent gas. T h e adsorption column consisted of two 4foot columns packed with 30- to 60-mesh activated alumina, connected through a dual-column valve. Because of the mixing of the reaction gas in the large dead space of the thermogravimetric system, the conversion of the main reaction was more accurately measured in another reactor (reactor B), which had an exactly similar geometry but no dead space. Ethylene was chosen for a reaction gas because its products over silica-alumina catalyst are in the gaseous phase at room temperature and because of its high tendency to form coke on the catalyst. Pure grade ethylene (99.82 volume % ethylene) from Phillips Petroleum Co. was used.
Table II. Effect of Coke on Catalyst Diffusivitya Before Regeneration After Regeneration at 450’ C.
a
W t . yo Coke Sample on C A T CAT-C 0.395 CAT-C 0.521 CAT-C 0.954 CAT-B (fresh) 0.0 CAT-B 0.201 CAT-B 0.274 CAT-B 0.584 Hydrogen at room temperature.
Di&siuity,
%
U,
sq. cm./sec.
sq. cm ./sec.
0.0104 0.0104 0.0090 0.0217 0.0175 0.0188 0.0174
0.0019 0.0024 0.0019 0.0084 0,0063 0.0056 0.0051
sq. cm./sec.
0.0094 0.0104 0.0086
0.0021 0.0024 0.0020
0.0192 0.0188 0.0196
0.0065 0.0057 0.0061
C.
063
501
0,
sq. cm./sec.
Table 111. Reaction Ttmp.,
Run N O*
I/4 in.
Diyusivity,
Deviation 18.3 23.1 21.1 39 .O 36.0 30.0 29.3
Reaction Conditions Ethylene Flow Rate, Cc./Min. Catalyst 50 .4
70
Deviation 21.6 26.0 23.3 33.9 31 . O 31.1
Particle Diameter, Cm 0.61
.
0
‘f!
Results
Surface Area Measurement. T h e results of surface a r e a measurements over fresh and coked catalysts are shown i n Table I. It is clear that less than 1.0% coke on the catalyst has no significant effect on the surface area. Effective Diffusivity Measurement. T h e effective diffusivities of 15 catalyst beads measured by Weisz’s method are shown in Table 11. T h e arithmetic mean was taken to obtain the average diffusivity. T h e standard deviations were also calculated (Table 11). Less than 1% coke formed on the catalyst has no important effect on the effective diffusivity. Measurement of Main and Coke Formation Reaction Rates. Three sizes of catalyst pellet, CAT-A, CAT-B, and CAT-D, were tested in reactor B to determine the effect of particle size on conversion at a reaction temperature of 500’ C. and an ethylene flow rate of 50 cc. per minute (Figure 3). Reaction conditions for each run are listed in Table 111. No significant effect of particle size on conversion was observed. A series of experiments using CAT-B was conducted under different reaction conditions and the weight of coke was mea-
- 30 0.d. -7
I
Figure 2.
I
I
0
‘u
aD I
I -
Lf
22 a d .
THERM0 WELL
1/4 in. KOVAR JOINT
l
Reactor A
I
1
I
I 0
070
0
073
A
065
I
I
063
I
0
064
Figure 3. Reproducibility of conversion with different catalyst particle sizes
I
I
I
I
I
I
I
10
20
33
40
50
60
70
PROCESS TIME
I
(hiin.)
VOL 7
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JANUARY 1 9 6 8
69
0 01
I
I
I
I
I
I I I I
3
5
10
20
30
40 5 0 6 0 70
J
0.01
20
3
s
PROCESS TIME ( M i d
PROCESS TIME W i n )
0.01
3
s
30 40 50 60
10
20
10
I
I
3
5
0.01
30 4 0 50 60
PROCESS TIME I M i n 1
I
I
I
10
20
30
I
2.0
-
”
”
-
n
092 n
2 Catalyst deactivation Figure 5.
-
$’
B
n
I
8
0
1
I
IO
I
20
I 30
Y
I
40
PROCESS TIME (Mln.)
70
I L E C PROCESS DESIGN A N D DEVELOPMENT
-
091
I
50
+
I
60
l
40 50 60
PROCESS TIME (Min.)
Relation of weight per cent coke on catalyst to process time
Figure 4.
l
I
B. Typical results are shown in Table V. Ethane, which is Table IV.
Reaction Conditions and Parameters in Voorhies Equation
Reaction Temp., Run No.
O
CzH4 Flow Rate, Cc./Min.
c.
n(t
>
70)
REACTOR A 077 096 075 094 097 082 083 098 080 08 1 099
499 499 498 500 450 449 450 400 399 399 350
089 091 092
500 450 400
0.902 0.885 0.913 0.881 0.742 0.820 0.841 0.625 0.736 0.605 0 554
100
75 50 30 75 50 30 75 50 30 50 REACTOR B
Table V.
... ...
50 50 50
...
Typical Results of Product Gas Analysis for Run 091 Reactor B
Time, Min. 5 13 20 30 40 50 60
HP 0.05 0.04 0.04 0.03 0.02
... , . .
CHa 0.15 0.12 0.12 0.12 0.10 0.10
0.11
CzHe 0.58 0.57 0.56 0.54 0.57 0.56 0.56
Cad 98.71 98.99 99.00 99.07 99.07 99.13 99.17
CsHs 0.06 0.04
... ... ... .. . ...
CsH6 0.45 0.30 0.30 0.30 0.25 0.20 0.18
Cd Trace Trace
... . .. . ..
sured a t different process times in reactor A (thermogravimetric system). T h e same reaction was repeated in reactor B in order to determine accurately the effect of coke on the main reaction conversion. Plots of weight per cent of coke on catalyst against process time on log-log scales are shown in Figure 4. T h e data points can be represented by two straight lines intersecting a t 10 minutes, rather than only one line as predicted by Voorhies (1945). This has been noted by other investigators, such as Van Hook and Emmett (1962), and indicates a n initial rapid coke formation. T h e data points often deviate from a straight line, especially for runs conducted a t a lower temperature. The values of the exponent, n, were obtained graphically from these plots when weight per cent of coke, C,, was given by the equation C, = AT' in accordance with Voorhies' expression. T h e experimental conditions and the values of n for t > 10 minutes are shown in Table IV. T h e values of n were closer to 1.0 a t 500' C. and approached the value of 1/2 suggested by Voorhies at 350' C. The effect of the coke on catalyst activity was determined by measuring the conversion of the main reaction in reactor
produced by hydrogenation of ethylene, is a n important product, along with methane from cracking and higher molecular weight compounds from polymerization. The lower the reaction temperature, the more hydrogen is detected in the effluent gas. T h e conversion, plotted against the process time in Figure 5 , indicates the initial rapid deactivation of catalyst activity. Conclusions
The weight of coke formed on catalyst was measured for the cracking of ethylene over a silica-alumina catalyst for various process times. A thermogravimetric technique proved to be of particular value in continuously determining the weight of coke formed on catalyst. A simple semiempirical equation presented by previous workers was found to be not completely satisfactory in relating the weight of coke on catalyst and the process time. Especially at lower temperatures, considerable deviation from the expression C, = A . t n was observed, indicating that the simple theory was not entirely adequate. Further investigations of the kinetics of the coke formation reaction are necessary to have a better expression between weight of coke and the process time. Ac knowledgrnent
The authors express their gratitude to the Mobil Oil Co. for furnishing the cracking catalyst, Durabead I, and conducting the diffusivity measurements. T h e support of the National Science Foundation through Grant NSF GP-865 for part of the work is also gratefully acknowledged. literature Cited
Appleby, W. G., Gibson, J. W., Good, G. M., IND.ENG.CHEM. PROCESS DESIGN DEVELOP. 1,102 (1962). Engle, C. J., Ind. Eng. Chem. 43,494 (1951). Blue, R. W., Eberlv. P. E.. Kimberlin. C. N.. Miller. W.H.. Drushel. H. V.. IND.'ENG.&EM. PROCE'SS DESIGNDEVELOP. 5,'193 (1966). Greensfelder, B. S., Voge, H. H., Good, G. M., Ind. Eng. Chem. 37, 514 (1945). Ozawa, Y., Bischoff, K. B., IND.ENG.CHEM.FUNDAMENTALS 7, 72 (1968). Panchenkov, G. M., Lolesnikov, I. M., Izv. Vysshikh. Uchebn. Zavedenii 9, 79 (1959). Rudershausen, C . G., Watson, C. C., Chem. Eng. Sci. 3, 110 (1955). Schmitkons, G. E., Proc. A m . Petrol. Inst. 3, 29M, 25 (1949). Tyuryaev, M. D., J . Appl. Chem. U S S R 12, 1462 (1939). Van Hook, W. A., Emmett, P. H., J . A m . Chem. SOC.84, 4410 (19621. Voorhies, A., Ind. Eng. Chem. 37, 318 (1945). Weisz, P. B., Z. Phvsik. Chem. (Frankfurt) 11, 1 (1957). Weisz, P. B., Schwartz, A. B., J . CatalyJis 1, 399 (1962). Wilson, J. L., Den Herder, Ind. Eng. Chem. 50, 305 (1958) RECEIVED for review January 16, 1967 ACCEPTEDAugust 8, 1967
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1968
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