C. G. FRYE, H. 1,. PICKERING AND H. C. ECKSTROM Vol. 62 tioned is that of a regular octahedron. The elec- ture and even up to 200” the rate of thermal tronic configurations aside from closed shells are decomposition of PuF6 is not measurable. How5f0,5f and 5f 2. The assignment of the configuration ever, at 280” PUFBrapidly decomposes t o give 1508
5f2 rather than 6d2 to PuFs was made on the basis of paramagnetic susceptibility For PuFB an anomalously low and temperature independent susceptibility was observed and explained on a model based on the spin pairing of the two “f” electrons. Such pairing cannot occur with “d” electrons until at least two electrons have gone into the d shell with their spins unpaired. Recently, Griffith and Orgelll have treated this model in detail theoretically and show it to be a reasonable one. The paramagnetic susceptibility of NpFs has also been measured12 and its magnitude and behavior with temperature can be explained on the basis of its possessing one 5f electron. Chemical Properties.-As pointed out earlier in this paper, the increasing difficulty that is experienced in the preparation of the hexafluoride as one proceeds through the series uranium, neptunium and plutonium is a qualitative measure of their decreasing stability. UF6 has been shown t o be an extremely stable compound toward dissociation into Fz and a lower f l ~ 0 r i d e . l ~At room tempera(10) D. M. Gruen, J. G. Malm and B. Weinstock, J . Chem. P h p . , 24, 905 (1956). (11) J. 9. Griffith and L. E. Orgel, ibid., 26, 988 (1957). (12) B. Weinstock and J. G. Malm, ibid.,27, 594 (1957). (13) J. J. Kats and E. Rabinowitch, “The Cheniistry of Uranium.” NNES, VIII-5, p. 412.
PuFl and F2. The equilibrium constant for the reaction PuF~ $. FI +P u F ~
has been measured a t various temperatures and PuFB has been found to be substantially decomposed in all instance^.^^'^ It is a curious fact that in spite of its thermodynamic instability and reactivity PUFBcan be stored for long periods of time at room temperature with no measurable thermal decomposition. NpFa has been shown to be more stable toward thermal decomposition than PuFB. A 212-mg. sample of NpFe at a pressure of about 900 mm. was heated a t 560” for three hours in a nickel reaction vessel and showed no evidence of thermal decomposition. BrFa converts uranium compounds quantitatively to UF6. However PuFB reacts instantly with BrF3to produce BrF3. I. Sheft’s has shown that NpF3 reacts with BrF3 to produce NpF4 and that NpQz reacts with BrF3 to produce NpF4. Preliminary experiments have shown the NpF6 reacts very slowly with pure BrF3 to form a non-volatile product, presumably NpF4. Like UFBand PuFB, NpF6 is hydrolyzed vigorously in water to give neptunyl ion, Np02++. (14) A. E. Florin, el al., J . Inorg. Nurl. Chem., 8, 368 (1956). (15) ANL-4709, p. 65, December, 1951.
CATALYST KINETICS I N THE HYDROCARBON SYNTHESIS REACTION BY C. G. FRYE, H. L. PICKERING AND H. C. ECKSTROM Pan American Petroleum Corporation, Tulsa, Oklahoma Received M a y 16, I068
I n studying the reaction kinetics of hydrogenation of carbon monoxide over an iron base catalyst to form hydrocarbons and oxygenated hydrocarbons, it was observed that changes in operating conditions changed the surface quality of the catalyst and that this change was reversible. When the operating conditions were changed, a corresponding change in the reaction rate occurred. However the reaction rate continued to change until it approached a steady-state value some time later. Thus, the observed kinehcs is a combination of the ordinary reaction kinetics and the kinetics associated with change in catalyst surface quality. The ordinary reaction kinetics for the reaction may be determined by relating the original steadystate value of the reaction rate with the extrapolated ‘Lzero-time”value after a change in the operating conditions. The catalyst kinetics (change in catalyst surface quality) ma be associated with the time-dependent reaction rate. This phenomenon observed for this particular heterogeneous cata&tic system may occur in other systems as well. I n such cases, if the change in catalyst surface quality is not taken into account, the kinetic observations will yield results either greater or less than those which should be associated with the “true” reaction kinetics.
Introduction The synthesis of hydrocarbons and oxygenated hydrocarbons from carbon monoxide and hydrogen over iron base catalysts has been the subject of much investigation; many of the publications have been reviewed by Storch, e t al.1 The reaction kinetics and mechanism of the reaction for this synthesis process have not as yet been satisfactorily determined. Reaction kinetics normally are studied by evaluating the effects of operation variables such as pressure, temperature and reactant concentrations in terms of the active center and activated adsorption (1) H. H. Storch, N. Golumbic and R. B. Anderson, “The FischerTropsch and Related Syntheses,” John Wiley and Sons, Ina., New York, N. Y., 1951, pp. 464-593.
theories of Taylor, Eyring and others. This type of study presupposes a catalyst system in which the catalytic properties, or catalyst surface quality, do not change with changes in operation variables. It is further assumed that product fouling effects and other irreversible catalyst changes are negligible during the time interval required for any given set of experiments, If the catalytic properties are reversibly changed by operating conditions during the time interval required for any measurements, then the observed kinetics become the combined effects of both catalyst kinetics and reaction kinetics. Boudart2 has discussed a basis for catalyst surface quality variation with reaction conditions. (2) M. Boudart. J . Am. Chem. SOC.,74, 1531 (1952).
Dee., 1958
CATALYST K I N E T I C S I N THE
HYDROCARBON SYNTHESIS
The object of many kinetic studies of the hydrogenation of carbon monoxide over iron-base catalyst has been to determine the rate-controlling step in various reactor systems, the primary and secondary reactions, the effects of operation variables on the reaction, and changes in catalysts during use. During studies of this type, data were obtained which show that changes in operating conditions may change the catalyst and that these changes in catalyst surface quality are reversible. It is the object of this paper to discuss these reversible catalyst changes which were observed during hydrocarbon synthesis with carbon monoxide and hydrogen over an iron base catalyst. Most of these studies were made at carbon monoxide conversions less than 15% to minimize the complications of secondary reactions. Also a t these low conversions, the catalyst system approximates a differential reactor and the catalyst is contacted with gas which undergoes relatively small changes in composition. Experimental
REACTION
1509
ing the reactor. This was accomplished by heating the reactor section downstream from the catal st to prevent product condensation. When operation at 60 conversions below about 15%, the partial pressure of products was sufficiently low to prevent significant product conderisation in the atmospheric pressure sample lines leading to the analytical system. However, a t conversions above 150/,, it was necessary to add diluent (helium was used for this purpose) to the product gas to prevent condensation in sample lines. Analytical System.-Two analytical procedures were used-one with pelleted catalyst reactors at conversions less than 15% and another with capillary reactors. With the pelleted catalyst, COZand HzO concentrations in the tail gas were determined with Drierite and Ascarite weighing tubes. Calculations were made by assuming all converted oxygen was present as COz or HzO, and CO conversion and space velocity values were based on metered tail gas rates. A thermal conductivity method of analyzing for HzO, COZand CO was used to obtain data from the small uantities of product made in the capillary reactors. Tierma1 conductivity analyses were made by removing a constituent from the gas stream and measuring the change in thermal conductivity of the gas. Water removal was accomplished by condensation in a Dry Ice trap, and carbon dioxide was removed by Ascarite. The carbon monoxide analysis was performed by first removing COZand HzO from the stream and then oxidizing CO to COZwith iodine pentoxide. The Two types of fixed-bed catalyst reactor systems were used. One required pelleted catalyst and the other used finely stream was subsequently analyzed for COZ. Concentradivided catalyst. The basic requirement in the reactor tion levels below 2% of HzO, COZ and CO were analyzed system was that studies of the reaction be possible from very by this system. Calibration factors for both HzO and COz low conversion (1-2% CO converted) to high conversion were obtained from Drierite and Ascarite data. The thermal conductivity cell sensitivity was free from levels (95-99% CO converted). The general flow system used in all fixed-catalyst studies flow rate effects up to the highest gas rates used (about 80 consisted of: (1) feed gas storage tanks, manifolded to ml. per minute). The concentrations of COz and HzO permit rapid changes in gas composition to the reactor; in the roduct stream were never greater than 2.0 and 1.8 respectively. Since these concentrations are low, (2) two pressure-control valves to provide a constant mole pressure differential across the feed gas flow control valve signal output from the thermal-conductivity cell was linear and to maintain constant reactor pressure; and (3) feed with concentration. Under the usual operating conditions, gas purification units operated either at storage tank pres- the time required for gas to flow from one side to the other sure or at reactor pressure to remove possible catalyst side of the thermal conductivity cells was about one minute. poisons. The latter consisted of soda lime, AsEarite, Consequently, the analysis of a gas whose composition changed rapidly with time was subject to an error which inDrierite and activated charcoal at both 204 and -78 Pelleted Catalyst Reactors.-The reactors using pelleted .creased in size with the rapidity of the composition change. catalyst consist of a 7/8-inch i.d. stainless steel pipe with a It is estimated that a 0.2% change per minute in the product 6/lr-inch 0.d. thermocouple well in the center. When CO concentrations introduced an error of about 1% in the conversion was less than 15%, a total product sampling bleed water and carbon dioxide analyses. Since the maximum placed inside the thermocouple well permitted the sampling reproducibility error for the thermal conductivity analyses was about 5%, changes in gas composition more rapid than of tail gas before any product was removed by condensation. It was also possible to obtain dry tail-gas samples after the about 1% per minute will introduce errors which are greater gas had assed through a Dry Ice trap. The catalyst was than the reproducibility errors. Reactant Gases.-Electrolytic hydrogen manufactured dia. X l / g ” long) before it was charged to the pelleted by the National Cylinder Gas Company was used to reduce reactor, and cylindrical stainless-steel pellets (I/*’‘ dia. X long) were mixed with the catalyst to serve as a diluent. catalysts in all experiments. All reactant feed gases used in A movable thermocouple was used to measure temperatures the CO and Hz synthesis experiments were made by blendwithin the catalyst bed. A tin-lead alloy bath surrounded ing electrolytic hydrogen with HZ-CO gas mixtures obtained the reactor and temperature was controlled by a Leeds and from reformer operation with natural gas which had no Northrup Speedomax controller-recorder equipped with a determinable uantity of sulfur. Reactant gases of the ratio were then compressed and stored in temperature programming device. The catalyst bed con- desired Hz to tained approximately 5-10 g. of catalytic material diluted tanks. A typical feed gas composition was 72.9% Hz, 24.4% CO, 0.5% CHa and 2.2% Nz. with 10 g. of stainless steel pellets per gram of catalyst. Catalyst.-All catalyst samples were prepared from mill Capillary Reactors.-The “capillary reactors” consisted of a U-shaped section of l / ~ ” i.d. stainless steel tubing scale which was slurry impregnated with K&08 to the equipped with connectors to feed and tail gas systems. desired promoter concentration. Composition of a typical The catalyst bed was positioned in the exit arm of the tube promoted mill scale catalyst was approximately 28% Fes04, so that the inlet arm acted as a preheating section. Pro- 68% FeO, 3% impurities (Cr,Al, Mn, Si, P and 8) and vision was made for introducing diluent helium at a point 1.0% K&Oa by weight. The six different catalysts discussed in this paper are just downstream from the catalyst bed and for removing a flowing sample of product gas for analysis. The catalyst designated A through F. Although all of these catalysts bed was held in place by glass-wool plugs at both ends of the were prepared from the same base material, different bed. The catalyst charge varied from 0.05 to 0.2 g. of iron. designations were necessary since the catalysts (after re The particle-size distribution of the catalyst was such that treatment, etc.) had different kinetic characteristics. $he it would all pass a 100-mesh screen with 20 wt. yopassing a reasons for these kinetic differences (characteristics) are 325-mesh screen. The catalyst was diluted with 80- to 120- unknown, and for the purposes of the discussion which mesh quartz to an iron-quartz weight ratio of 0.1 to 0.3. follows, unimportant. However, the facts that there are The heating furnace was designed to permit removal of the differences and that these differences enter into the over-all furnace from the reactor so that reactor temperature could kinetic behavior of the synthesis reaction are important. Procedure.-In all cases the catalysts were first reduced be changed rapidly. This furnace also was equipped with a Leeds and Northrup Speedomax temperature controller- to or-iron at 371’ and 18.0 atm. pressure. Subsequently, the catalyst wa8 contacted with a HrCO gas mixture and recorder with a temperature programming device. The analytical methods used with the capillary system allowed to reach a steady state under some specific set of required that all products remain in vapor state after leav- operating conditions that would be used as the initial condi-
8,
.
F/8”
80
C. G. FRYE, H. L. PICKERING AND H. C. ECKSTROM
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Vol. 62
paper, the distribution of products other than COZand HzO which are formed from the CO and Hz is unimportant since this distribution does not O 2 1000 800 significantly affect the calculation of the reaction wh 600 rate. -$d400 Temperature.-The results of three illustrative experiments with the capillary reactors are shown in .s Y Y od 200 Figs. 1 and 2 where reaction rate is plotted against time for two temperature changes, 332 to 299" 100 and 299 to 332". The pressure was maintained 0 1 2 3 4 8 12 16 20 24 28 constant a t 18 atm. with a 3:l Hz to CO ratio feed Time, hr. after temp. change from 332 to 299". gas. Fig. 1.-Time-dependent effect of change in temperature on Pressure.-The results of one experiment with reaction rate. pelleted catalyst are given in Table I, and results of another experiment with granular catalyst are s shown in Fig. 3. The experimental conditions are gG4000 2000 as given in the table and the figure. 0 V 3
2000
; ;
2
--. M
c2 1000 800 8
600
-
M
f
Ted period
.. .. ..
100 80
I 140 g 2 20 3
42
2
10
0 2 4 6 8 10 1 2 1 4 Time, hr. after temp. change from 299 to 332". a.-Time-dependent effect of change in temperature on reaction rate. 1000 800 600 400 200
100 80 60 40 20 10
18 24 30 36 42 48 54 Time, hours. 3.-Time-dependent effect of change in total pressure on reaction rate. 0
6
12
tions for the study to be undertaken. Once a change in an oweration variable was made, the effect of this change was observed as a function of time.
Results The catalyst activity is arbitrarily defined as the value of the CO reaction rate (gram-moles CO converted per kilogram of iron per hour, g. mole CO conv./kg. Fe/hr.) extrapolated to zero conversion. Below about 15% CO converted, the reaction rate is essentially independent of conversion level and is therefore numerically equal to the catalyst activity as defined above. For the purpose of this
Reaction rate,
%
co
conv.
g. mole CO Pressure, conv./kg.
atm.
Fe/hr.
Time
Time of pressure
change, hr.
1
11.3 12.2 7.05 6.72 6.40
5.68 6.69 6.32 5.94 5.60
0 2.1 18.3 21.1 41.0
2 3 4 5 6
2.32 2.45 2.84 2.72 2.99
9.14 9.67 11.5 11.1 11.7
43.1 46.1 67.7 72.2 90.6
7 8 9 10 11
3.83 5.03 5.26 4.46 7.25
17.0 22.0 22.8 20.0 32.5
92.7 94.7 97.4 98.1 114.7
12 13 14 15 16 17 18 19 20
5.41 4.95 4.38 4.37 4.65 4.77 4.34 4.02 4.27
22.6 20.7 18.8 18.5 14.3 14.5 13.1 13.4 12.3
116.8 118.0 119.5 120.9 138.5 142.0 144.9 164.2 184.6
21 22 23 24
8.03 186.4 5.25 7.45 188.1 4.92 7.00 189.8 4.63 ti. 13 208.5 4.13 214.5 3.82 5.76 correspond to those shown in Fig. 7.
..
3- 60 1
d
TABLE I
2 200
8
d
n?
za a
+-
42.5
+--91.8
+-
116.1
+--185
Gas Composition.-Two different techniques were used to determine the effect of changes in gas composition on catalyst activity. Figure 4 shows the results obtained with a pelleted catalyst bed following a change in Hz to CO gas ratio. Figure 5 shows corresponding changes in the activity of two different catalysts following changes from high to low conversion in capillary reactors. No attempt was made to duplicate the two different techniques with the two types of reactor systems.
i
Dec., 1958
CATALYST KINETICS IN
THE
HYDROCARBON SYNTHESIS REACTION
1511
Discussion It is well known that the phase system of ironbase catalyst used under hydrocarbon synthesis conditions is complicated. During synthesis, the bulk phase has been found to contain a-iron, Fe304, Hagg iron carbide and another iron carbide. It is also known that by changing operating conditions, e.g., gas composition, the bulk phase can be changed from one compound to another. For example, a catalyst consisting of Hagg iron carbide operating under synthesis conditions a t 18 atm. and 332' with 3:l H2 to CO ratio feed gas can be changed t o FeoOcby changing to a 3:l Hz to COZratio feed gas. For these changes to occur in the bulk phase of the catalytic material it is evident that changes in the "surface" of the catalytic material also have taken place. However, over a limited range of a change in operating conditions a change in the surface of the catalyst is possible with no change in the bulk phase composition. Assume that following a change in an operating variable, a modification in the catalyst surface occurs simultaneously with the change in reaction kinetics produced by the operational variable. Three experimental results may be observed, depending upon the magnitude of the time rate of change of catalyst surface quality: (1) if this time rate of change is very fast (e.g., completed in a few seconds or less), the change in catalyst activity observed will be due t o both changes in reaction kinetics and catalyst surface quality; (2) if it is very slow (e.g., only a slight change in 24 hours), the change in activity observed will be due essentially to a change in reaction kinetics alone; (3) if the rate of change in catalyst surface quality is intermediate (e.g., requires 5 to 50 hours for completion), a rapid change in activity due to the change in reaction kinetics will be observed; this will be followed by a period during which catalyst activity changes with time, due to the rate controlled alterations in surface quality of the catalyst. Data are presented illustrating these effects. Temperature Effects.-The effect on catalyst activity of varying the temperature is shown in Fig. 1 for catalyst A, where a t zero time and 332" steady-state synthesis was occurring with an activity of 951 g. mole CO conv./kg. Fe/Hr. A temperature change from 332 to 299" was made, and two data points were obtained as a function of time; no attempt was made to reach a final steady state. During the next 1.5 hours data points were obtained at 310 and 321". Finally the temperature was returned to 332" and activity observed as a function of time. It can be seen that the final 332" curve is approaching the original steady-state value of 951 g. mole CO conv./kg. Fe/hr. These results indicate that the change in catalyst quality which occurs with change in temperature is reversible for this particular catalyst. For catalyst B, a similar result is shown in Fig. 2. In this experiment following steady-state operations a t 299" the temperature was changed to 332" a t zero time. The catalyst activity approached a steady-state (3) H. C. Eckstrom and W. A. Adcock, J. Am. Chem. Soc., 72, 1042 (1950).
1
1
i
r
-
.
1
,
-1
-1-A
30 40 50 60 70 SO 90 Time, hours. 4.-Time-dependent effect of change in H,/CO ratio on reaction rate. ' 400 I ! 0
101 1 0
10
I 10
20
I 20
I
I
I
1
30 40 50 60 70 SO 90 Time, hours. Fig. 5.-Time-dependent effect of change in CO conversion level on reaction rate. 1000 0 U
3
.
%Mh
Q-M
2< * d
800 600 400 200
100 80 60
I 40 100 400 1000 Pressure, atm. Fig. 6.-Reaction rate us. total pressure.
1
4
10
value considerably lower than the zero time extrapolated value of 2320 g. mole CO conv./kg. Fe/Hr. For catalyst C (Fig. 2), no change in catalyst activity with time following the temperature change from 299 to 332" was observed, so that any change in catalyst surface quality was either too fast or too slow to be observed in the time of measurement. One may infer that the time dependent catalyst activity, which was observed after the temperature was changed, was caused by changes in catalyst surface quality requiring a finite time to arrive a t a new steady state. An activation energy based on steady-state values would reflect both reaction and catalyst kinetic changes. Therefore, the activation energy characteristic of the reaction
1512
C. G. FRYE,H. L. PICKERING AND H. C. ECKSTROM
2
4 6 8 10 20 40 Pressure, atm. Fig. 7.-Reaction rate us. total pressure.
TlHE
Fig. 8.
kinetics is that calculated from reaction rate data evaluated by extrapolation back to the time a t which the temperature change was made. For catalyst A, the activation energy, based on extrapolated reaction rate data is 36.0 kcal./g. mole for catalyst B, it is 24 kcal./g. mole and for Catalyst C, 24 kcal./g. mole. An examination of the results given in Fig. 1 and 2 shows that the energy of activation, based on steady-state values only, is either equal to or less than that based on zero time extrapolated values. The dependency of reaction rate on time of observation may explain, in part, the wide range of activation energy values for the hydrocarbon synthesis reaction reported in the 1iteratu1-e.~ The activation energy values observed in this Laboratory range from 22 to 38 kcal./g. mole. The results were based on zero time extrapolated values from mill scale catalysts. An average value of 29 kcal./g. mole with an average deviation of 3 kcal./g. mole has been observed. Activation energy values have been obtained through a temperature range of 260 t o 360" and usually are based on data obtained a t four different temperatures in this range. Pressure Effects.-The data shown in Fig. 3 (catalyst B) illustrate the effect of change in pressure on reaction rate using the capillary reactor systems. Activity is plotted against time and the experimental points are numbered consecutively. At the start of the experiment the reactor was a t steady-state synthesis conditions at 18.0 atm. and 297"; pressure was lowered to 6.1 atm. and later (4) Ref. 1, p. 539-541.
Vol. 62
increased again to 18.0 atm. The activity did not reach a steady-state value during the time interval the pressure was held a t 6.1 atm. However, when the pressure was returned to 18.0 atm., the activity approached the original value of 602 g. mole CO conv./kg. Fe/hr. If the order of the reaction process itself is to be evaluated, the zero-time extrapolated values for catalyst activity should be used. The results shown in Fig. 3 are replotted in Fig. 6 as logarithm of reaction rate us. Iogarithm of pressure with the experimental points numbered consecutively. Points A and B are the zero-time extrapolated values so that curves (a) and (b) represent the pressure effect on reaction kinetics. The slope 0.9 should represent the order of the reaction. The data points on curves (c) and (d) in Fig. 6 indicate that the change in catalyst quality which occurs with change in pressure is reversible for this particular catalyst. The slope of curve (e) in Fig. 6 reflects the effect of pressure change on both reaction kinetics and changes in catalyst surface quality. This line has a slope of approximately 2. I n Table I data are given which are typical oft the effect on reaction rate resulting from changes in pressure for pelleted catalyst systems. These data are plotted in Fig. 7 as logarithm of reaction rate vs. logarithm of pressure. Points A, B, C and D are the zero-time extrapolated values so that curves (a), (b), (c) and (d) represent the pressure effect on reaction kinetics in the same manner as curves (a) and (b) in Fig. 6. The slopes of these four curvesi (a), (b), (c) and (d) are 1.1,0.7, 0.8 and 0.9, respectively. These values approximate those in Fig. 6 and show that the synthesis reaction is about first order with respect t o total pressure. The slope of curve (e) which joins the steady-state points in Fig. 7 increases as the pressure is increased and has an average value of 1.7. Catalysts were observed to undergo reversible changes similar to those discussed above in all cases where the reaction rate was followed as a function of time after a change in pressure. From these data, it appears that the order of the reaction remains constant even though the slope of the curve joining the steady-state values of logarithm reaction rate us. logarithm pressure is not constant. If changes in catalyst surface quality had not been observed because the change was too rapid, the observed slope would not correspond to the order of reaction as generally defined. Gas Composition Effects.-Two types of experiments were performed to study the time-dependent effect of gas composition changes upon the performance of the catalyst. The first method was to change the H2/C0 ratio of the feed gas when the system was being operated as a differential reactor. The second method was to change space velocity of a constant Hz/CO ratio feed gas so that the system changed from one operating under integral conversion conditions to one under differential conditions. The time-dependent effect of change in Hz/C.O feed gas ratio on catalyst activity is shown In Fig. 4 for the pelleted bed reactor system. The
3
&
Dec., 1958
CATALYST KINETICSIN
THE
HYDROCARBON SYNTHESIS REACTION
H2/C0 ratio was changed from 8.0 to 3.0. Thereafter the catalyst activity increased until a steadystate value of 21.1 g. mole CO canv./kg. Fe/hr. was reached at about 94 hours. This phenomenon is similar to that observed with changes in temperature and pressure. The time-dependent effect of change from integral to differential reactor conditions on catalyst activity is shown in Fig. 5. These data were obtained in the capillary reactor system for two different catalysts. For catalyst C the space velocity was changed from 51 to 4860 g. mole CO/kg. Fe/hr. Thereafter for the next 22 hours no change was observed in the catalyst activity. It is significant that for this same catalyst no time-dependent effect on catalyst activity was observed after a temperature change (see Fig. 2). For catalyst F, the space velocity was changed from 40.4 to 1850 g. mole CO/kg. Fe/hr. As seen in Fig. 5, the catalyst activity thereafter increased until a steadystate value of 111 g. mole CO conv./kg. Fe/hr. was reached about 90 hours after the change to low conversion. This phenomenon is similar to the time-dependent effects observed following a change in temperature or pressure. To generalize, the kinetics of the catalytic hydrogenation of CO under hydrocarbon synthesis conditions over iron-base catalysts operated a t low conversion may be graphically summarized in Fig. 8. In this figure the reactor system is shown as operating a t a steady-state represented by curve (1) with a reaction rate of (i) when a change in an operating variable (pressure, temperature, 'or gas composition) is made a t time zero. (In the illustration the change decreased the reaction rate). The reaction rate, instead of changing to a constant value characteristic of the reaction kinetics, continues to change with time as illustrated by curve (2) and approaches a steady-state value of (f) some time later. When the operating variable is returned to its initial value, the reaction rate changes with time as represented by curve (3) until it approaches the initial value of (i) or one slightly less depending upon how much the catalyst would have normally aged during this time interval. Since the reaction rate changes with time after a
1513
change in operating conditions, and since this change is reversible, it has been concluded that the catalyst surface quality is dependent upon the concentration of reactants and products and the temperature a t the catalyst surface. The catalyst kinetics are represented by the time-dependent curves (2) and (3). It is further concluded that the reaction rate a t the zero-time value of point (A) or (B) represents the value obtained from a catalyst with a surface quality identical with that which gave a reaction rate (i) or (f). Thus, the change in reaction rate from (i) to (A) or from (f) to (B) is that required by the reaction kinetics only. The magnitude of the change in reaction rate with time and the shape of curves (2) and (3) are characteristics for the catalyst and appear to be dependent upon such factors as catalyst preparation, pretreatment and on-stream procedure.
Conclusion The significance of these observations in the general field of catalysis will obviously depend upon the particular catalyst and .reaction involved. In those reactions in which a change in catalyst surface quality takes place very slowly or not a t all following a change in an operation variable, the true reaction kinetics will be directly obtainable from conventional kinetic data. In reactions where a change in catalyst surface quality takes place very rapidly after a change in an operation variable, the observed kinetics 'will reflect both the reaction and the catalyst kinetics. Under these circumstances, no direct separation of the two effects will be possible using the ordinary kinetic approach. Thus, for example, the actual reaction order or activation energy of such a reaction could be either higher or lower than that obtained by direct evaluation of experimental data. In such cases it is evident that no inference of a true reaction mechanism can be drawn directly from the kinetic observations. Finally, in reactions where a change in catalyst surface quality takes place a t a moderate rate, the observed kinetics may be separated into the catalyst kinetics and the reaction kinetics by the zero-time extrapolation technique described in this paper.
