Partial Oxidation of Methane Using Iron Oxide as Donor Denis Barrett School of Chemical Engineering, Cniversity of New South Wales, P.O. Box 1 , Kensington, N.S.W. 2033, Australia
The suitability of iron oxide as an oxygen donor for synthesis gas manufacture from methane has been studied. The results show that reduction of FeO is carried out mainly by Hz and CO and that equilibrium is established in these reactions. The reaction between CHI and HzO or COz is weakly catalyzed by Fe, and this is the rate-controlling step. Experiments with iron ore pellets point to the establishment of an Fe/FeO interface at which reduction reactions take place. Rates of reaction may be increased by the use of a nickel catalyst to accelerate the reaction between CH4 and HtO or COZ.
T h e last two decades have seen a great increase in the amount of light hydrocarbons converted into gaseous mixtures containing carbon monoxide and hydrogen. This partial oxidation process may be performed using oxygen or air; alternatively the hydrocarbon may be reformed using steam or carbon dioxide. Partial combustion is a n efficient process, but unless diluent nitrogen can be tolerated in the product gases, i t requires high-grade tonnage oxygen which is expensive. The reforming processes are strongly endothermic and require the provision of heat from external sources. I n the case of steam reforming, latent heat losses due to uriconverted steam also occur. The reforming reactions are basically partial oxidation processes in which steam and carbon dioxide act as donors of oxygen to hydrocarbons and i t is therefore appropriate to consider other oxides which could provide a donor source. In particular, a donor which remains solid in both its oxidized and reduced state, appears attractive. Ideally a complete process would be:
MO(S) hI(s)
+
'/2
+ CHa CO + 2H2 + M(s) 76 3.76 + 32 N2 hIO(s) + __ 2 N2
-
+
01
Thermodynamic Considerations. In considering metallic oxides for oxidation of hydrocarbons there are two principal methods of operation. I n the first method, the ratio of metallic oxide to hydrocarbon is adjusted so that only the stoichiomet'ric amount of available oxygen required for reaction is provided. Gilliland et al. (1949) have reported on the oxidation of methane by copper oxide. They concluded t h a t the initial reaction is a fast oxidation which converts methane to carbon dioxide and water vapor followed by a slower rate-controlling reforming reaction in which steam and carbon dioxide reform the surplus methane. I n the alternative cases where a n excess of oxygen is present as the metallic oxide, the degree of oxidation will depend upon the equilibrium constants for the various reactions. The reactions which must be considered are:
2M0,
=
+ 202 CHI + CO2 CH4 + HzO
CHI
+ COz + 2H20
2?(/I0,-1 +
+
+ 2H2 CO + 3Hz
2CO
+
0 2
+ Hz +
CO
0 2
'/2
'/z
0 2
+
+
Con
Hz0
If the metallic oxide has a high free energy of formation then the partial pressure of oxygen will be low, only a small amount of methane will be oxidized and carbon monoxide and hydrogen formation will be favored rat'her than water vapor and carbon dioxide. Conversely a metal oxide with a low heat of formation will readily supply oxygen to provide a good conversion of met'hane but mainly to carbon dioxide and water vapor. The compromise between conversion and selectivity of product formation occurs with oxides of intermediate free energies of formation. On the assumption that thermodynamic equilibrium occurs, i t is possible to calculate gas compositions which would obtain due to reaction between methane and various metal oxides; such calculations suggest t h a t of the common oxides only iron in the ferrous state will be a suitable donor. The reactions used in the calculations were:
+ co C02 + n1 MO + Hz H20 + XI 110 + CHI CO + 2H2 + 1 4 310
+
+
+
Thermodynamic data are from Kubachewski et al. (1967) and Spiers (1961). Table I shows results calculated for ferrous oxide (FeO) (wustite) and methane over the temperature range 7001000°C a t atmospheric pressure. The important features of these results are: (a) The conversion of methane is substantially complete. (b) The yield of CO H 2 is in the range 63-66% of the wet product gas, declining with increasing temperature. (c) The ratio of mol CO H2 to mol CH4 converted is about 1.9 to 1 which compares unfavorably with ratios of 2.2-2.8: 1for partial oxidation processes. In a cyclic process, where the oxide is regenerated, the efficiency of the process may be raised by converting some of the sensible heat produced by oxidation of the metal back into potential heat by decomposition of water vapor and carbon dioxide. Equilibrium calculations for equimolar mixtures of methane with water vapor and carbon dioxide in reaction with u-ustite show much more satisfactory yields, as illustrated in Tables I1 and 111.
+
+
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
415
Table
45 '-
I. Equilibrium Compositions for CH4/Fe0 System VOl
CO C02
Hz Hz0 CH4 Ratio Hz:CO % CO H2 Mol (CO Hz)/ mol CHI converted
+
+
%
700'C
800'C
900'C
1000°C
19.8 13.3 46.3 20.0 0.6 2.34 66.1
21.5 11.8 43.7 23.0 0.03 2.04 65.2
22.7 10.6 41.3 25.4 0.004 1.82 64.0
23.7 9.6 39.3 27 4 0.0005 1.66 63.0
1.92
1.89
1.99
1.96 ~
0'
Table II. Equilibrium Gas Compositions for System C H 4 / H 2 0 / F e 0
C02
CO Hz H2O
CHI Hz:CO
+
Ha CO Mol (CO H2)/ mol CHI converted
+
7OO0C
VOl % 8OO0C 9OO0C
9 14 52 22 0 3 66
8 16 49 25 0 3 65
9 9 0 6 59 49 9
2 69
7 17 46 28 0 2 63
9 1 1 9 03 06 2
2 61
9 1 4 6 004 71 5
2 54
1000°C
7 17 44 30 0 2 61
2 8 1 9 0005 58 9
2 47
Table 111. Equilibrium Gas Compositions for System CH4/C02/Fe0 VOl
C02
CO Hz H20
CHI H 2 : CO Hz CO Mol (CO
+
+ Ha)/ mol CH, converted
%
7OO0C
800'C
20.0 29.7 34.7 15.1 0.5 1.17 64.4
14.4 17.7 15.9 34.1 35.6 32.3 29.4 30.9 32.8 19.2 20.6 17.2 0.0004 0.003 0.028 0.83 0.91 1.01 65.0 65.0 65.1
2.62
2.61
900'C
2.60
1000°C
2.60
Theoretically, therefore, i t is possible to produce synthesis gas by reaction of methane with wustite; in practice, however, many questions remain which may be resolved only by experiment. It is unlikely t h a t all reactions will be a t equilibrium and the kinetics of the process may be decisive in the final product gas composition. Wustite is not a naturally occurring substance, but it is produced in the reduction of the higher oxides of iron and many workers have suggested that gas-solid reduction reactions may take place a t a n Fe/FeO interface. McKewan (1960, 1961, 1962) working with dense hematite (Fe203) and magnetite (Fe304) and using hydrogen as reductant concluded that the rate-controlling step was a chemical process occurring a t a n Fe/FeO interface and that the iron layer formed is quite porous and offers negligible impedance to gaseous diffusion. Warner (1964), also using dense hematite and hydrogen, postulated a mixed diffusion-chemical control mechanism with a first-order reversible chemical reaction a t the Fe/FeO interface. Terasawa et al. (1961, 1963) in a study involving 416
20
% Oxygen
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
40 60 Removed
0
Figure 1. Conversion of methane, effect of partial pressure and degree of reduction at 830°C
- 0
-+ -
Feed CHI 1 OO%, N2 0% Feed CHn 72.4%, N2 27.6% Feed CHa 33.8%, N2 66.2%
the reaction between CH, and various metal oxides suggested t h a t reaction with iron oxide took place a t an Fe/FeO interface. Kawasaki et al. (1962) working with porous pellets reported that t h e rate of reduct.ion by hydrogen and carbon monoxide is diffusion controlled. Vnder conditions of diffusion control the gas a t the reaction interface must be a t equilibrium with respect to the oxide level, and the gas diffusing outward must approach equilibrium corresponding t o that of the FeO oxide level. It may therefore be possible to produce synthesis gas by the reaction of methane with iron oxide containing more oxygen than wustite. The experimental program which follows therefore examines first the reactions between methane and wustite and then those bet'ween methane and iron ore pellets. Experimental
Methane-Wustite. The experiment'al program provided information about three groups of reactions, namely: (a) The direct reaction between wustite and methane. (b) The indirect reduction processes due to t'he action of hydrogen and carbon monoxide. (c) The significance of the methane reforming reactions with steam and carbon dioxide. Material. The material used mas prepared from iron ore pellets supplied by the B H P Co. of Australia. The pellets were crushed t,o -6 +8 BSS mesh size (2812-2057 p ) . The porosity, measured by means of a Beckman Air Comparison Pycnometer, was 26.4%. The iron was converted to the ferrous state using an equilibrium mixture of hydrogen and carbon dioxide. Values quoted by Darken and Gurrey (1953) were used to specify the equilibrium mixture. The reduced particles were rapidly quenched after reduction to avoid conversion t,o iron and magnetite and checked by X-ray diffraction, no conversion ivas detected. The atomic Fe:O ratio was calculated using the data of Darken and Gurrey (1945) and was 1 : 1.05. Chemical analysis indicated that the wustite particles contained 66.070 iron, and measured total porosity was42.6%. Equipment. The apparatus used in the experiments had to provide good mixing of the solid particles. I n essence, i t comprises a thin section of n ball mill normal to the axis of rotation in which mixing of the solid and gas occurs due to
Table IV. Reduction of Wustite by ( 1 ) 8.0% Hz 92.0% N, and ( 2 ) 7.1% CO 92.9% Nzat 870°C
Feed Rate in ml min-l
7 0 Wustite H2
reduced
Exit gas composition N2
Feed rate 60 92.0 4.8 91.9 5.2 92.0 5.3 Feed rate 120 4.1 91.9 5.0 92.0 92.0 5.0 60 Feed rate 5.4 92.0 92.0 5.6 120 Feed rate 5.3 92.0 92.0 5.1 60 Feed rate 5.0 92.0 92.0 5.3 Feed rate 120 5.5 92.0 5.0 92.0
0.0
I
I
7.7 20.0
I
24.3
50.0
I
I
54.4 ilv value
KP
HzO
ml min-l 3.2 2.9 2.7 ml min-' 4.0 3.0 3.0 ml min-' 2.6 2.4 ml mind' 2.7 2.9 ml min-' 3.0 2.7 ml min-l 2.5 3.0
PH20
% Wustite
PH,
reduced
0,67 0.56 0.51 0.98 0.60 0.60
7.7
I
I
Exit gas composition
co
NP
2.2 2.4 2 1
60 4.8 4.8 4.6 120
93.0 92.8 93.3
0.46 0.50 0.46
4.5 4.4 60 4.8 4.8 120
93.2 93.5
0.51 0.48
93.2 92.6
0.42 0.54
4.5 60 4.6 4.7 120 4.7 4.7
93.4
0.47
93.1 93.1
0.50 0.47
93.0 92.9
0.49 0.51 0.48 0.49
-
14.2
2.3 2.1 -
0.48 0.43 0.51 0.57
24.3
I
1
2.0 2.6 -
26.5
2.1 -
0.60 0.51 0.45 0.60 0.58 0.59
rotation of the reactor. Under the operating conditions, good mixing of both the solid and bulk gas phases occurred. A detailed description of this reactor has been provided by Barrett. (1971) Results. Preliminary runs involving methane and wustite were made to obtain conditions under which appropriate conversions of methane could be obtained. These mere followed by experiments studying the indirect, reduction react'ions due t o hydrogen and carbon monoxide under conditions similar b u t less favorable for the establishment of equilibrium. The result's are shown in Table IV for three different levels of wustite reduct'ion and indicate the attainment of equilibrium for both reactions. In the reaction between methane and wustite at, the lo^ percentages of methane conversion, the rate of mebhane conversion increased with a n increasing degree of oxide reduction and was approximately proportional to the partial pressure of methane. I n Figure 1 is shown the rat'e of CH, conversion for three different partial pressures of methane and, as the conversion of methane is low, not exceeding 9% of feed CH4 a t 70% reduction, i t is a fair approximation to compare t,he rate of CHd conversion to the feed partial pressure of CH4for various levels of reduction. This ratio is approximately constant and suggests a linear relationship bet,ween rate of CH4 conversion and its part'ial pressure. An explanation for the autocatalytic effect is that t'he free iron acts as a catalyst, probably toward the methane reforming react'ions, thus producing additional amounts of hydrogen and carbon monoxide which then rapidly reach equilibrium with wustite. I n Figure 2 is shown the rate of methane conversion vs. percent oxygen removed a t two different temperatures. If the assumption is made t h a t a t zero reduction the rate of methane decomposition is due solely to the direct methane-wustite reaction, an activat,ion energy of 81,000 caljmol is obtained. Values for the activation energy obtained from rates when 30% and 60% of the oxygen
54.4
I
58.1
Pco, pco
con
2.3 2.1 2_3 2.4
I
I
I
0
/o
20
40
60
3
7'. Oxygen removed Figure 2. Methane conversion. Effect of
- o - Feed CHa 33.8%, - - Feed CHa 33.8%,
temperature
NP 66.2% 870°C Ns 66.2% 830°C
is removed produce values of about 47,000 cal/mol in both cases. The methane-reforming reactions were studied using iron particles produced from t,he complete reduction of w s t i t e particles as catalyst. The results shown in Figure 3 indicate t h a t iron does cat,alyze the reforming reaction aiid thus provides support for the hypothesis that free iron is re5ponsible for the increase in t,he rat,e of methaiie reaction. I n Figure 4 is shown the rate of methaiie coiiversioll by carbon dioxide and steam a t varying partial pressures of niethanc and two different temperatures. The results imply a linear relationship between the rate of methane decompositioll and its partial pressure. l k e r s aiid Camp (1955) have reported that, in the reaction between methane and steam over a nickel catalyst the rate-controlling deli is the thermal deInd. Eng. Chem. Process Des. Develop., Vol. l l , No. 3, 1972
417
F e e d R a t e mls min-' 0
Figure 3. Methane conversion vs. feed rate r,.orming reactions
- 0 - 87OoC, 5-gram catalyst 0 - 91 5'C, 5-gram catalyst - + - 87OoC, 15-gram catalyst - A - 91 5'C, 15-gram catalyst - 0- 92OoC, 15-gram catalyst, 40% CHd, 60% HzO
0
o
I 0'
Pressure
- 0 - Girdler catalyst flow, 47.5mi min-1 - -Insulating firebrick catalyst. Flow, 65 ml min-1 - 0 - No catalyst. Flaw, 47 ml min-1 0
CH4 Atmos.
20
40
I 60
do
removed
5. Conversion of methane vs. feed rate at 930°C Numbers show methane feed rate, ml min-'
41 8
PO
Figure 6. Effect of nickel catalyst on methane conversion at
92OoC, 15-gram catalyst, 40% CH?, 60% HzO 91 5'C, 15-gram catalyst, 37.5% CH4, 62.5 COS 87OoC, 15-gram catalyst, 37.5% CH,, 62.5 COn
% Oxygen Figure
60
Removed
930°C
Figure 4. Methane conversion vs. partial pressure reforming reactions OD-0
40
9
20)
Partial
10
9:Oxygen
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1, No. 3, 1 9 7 2
composition of methane. If a similar system obtains with a n iron catalyst, then it may be expected that the reforming of methane by steam or carbon dioxide will proceed a t similar rates to one another and be linearly related to the partial pressure of methane. An estimate of the temperature response for the methane-carbon dioxide reaction from Figure 4 provides an activation energy of 50,000 cal/mol, which is similar to that obtained in the reduction of wustite when appreciable amounts of iron are present. The effect of varying the methane feed rate a t high conversions of methane with wustite is shown in Figure 5. The absolute rate increases with increasing feed rate-Le., with increasing partial pressure of CHI in the reactor. When allowance is made for the fact that some cracking of methane takes place in the inlet arm of the reactor, the results indicate approximate linearity between the absolute rate of methane conversion and methane partial pressure. If linearity is assumed, an activation energy for the reaction may be calculated by making a Comparison with results obtained for the rate of reaction a t 870°C and on this basis a value of 52,000 cal/mol is obtained when the level of oxide reaction is 60%. This too is similar t o the values obtained a t low levels of methane conversion and also for the methane-carbon dioxide reforming reaction. Further justification for the hypothesis that the reforming reactions are of prime significance in the oxidation of methane by wustite is provided in Figure 6 where the rate of methane conversion n i t h and without a catalyst is shown. The Girdler G-56 catalyst is a commercial steam-naphtha reforming catalyst, and the other catalyst was prepared by impregnation of porous firebrick with nickel. The difference in efficiency between nickel and iron as catalysts is quite marked-e.g., a t a point where 80% of wustite has been reduced, corresponding to a free iron content of 12.5 grams of free iron, the conversion is still less than that produced a t approximately the same feed flow rate by the Girdler catalyst containing 0.65 gram of nickel. The conclusions which may be drawn may be summarized as follows:
Table V. Reduction of Iron Oxide Pellets b y Methane 1
Temp, "C Feed CH4, ml minCatalyst, wt g Exit gas, vol % HI CO
1
870 40 6.0
co coz
26.8 16.0 32.0 25.2 3.7 65.5 36.0 19.1 18.8 26.1 6.5 59.3 40.6 21 .o 15.0 23.4 11.5 60.7 ... ... ...
% 0 2 removed % CH4 converted
... ...
coz
CH4
% 0 2 removed % CHI converted H2
co e02 CH4
% 0 2 removed % CH4 converted H2
co
c02 CH4
% 02 removed % ' CHI converted H2
CH4
...
2
870 40 6.0 21.6 11.9 32.3 34.2 3.3 56.6 31.9 17.0 23.1 28.0 6.3 58.9 37.2 18.8 19.4 24.6 11.9 60.9 43.8 22.0 14.0 20.2 17.0 64.0
3
4
5
870 40 Nil
930 70 Nil
930 70 Nil
... ... , . .
...
... ... 12.0 5.7 15.3 67.0 2.7 23.8 12.0 5.4 17.1 65.5 5.7 28.8 14.1 5.9 16.9 63.1 8.9 26.6
The oxidation of methane in the presence of wustite occurs in two distinct ways, first, as a relatively slow, direct, heterogeneous reaction between wustite and methane and second, in the reaction between methane and water vapor and carbon dioxide. This second type of reaction is faster than the direct reaction. The reduction of the oxide is carried out mainly by carbon monoxide and hydrogen and these reactions are much faster than the reforming reactions. The reforming reactions are therefore essentially t h e rate-controlling step in the overall process. Iron acts as a catalyst for the reforming reactions, but the amount required is quite substantial if high reaction rates are to be obtained. Because equilibrium will occur for the reactions between hydrogen or carbon monoxide with wustite, i t follows t h a t after making allowances for unconverted methane, gasification efficiencies of 60-65~0only may be anticipated. Methane-Iron Ore Pellets. Iron oxides such as hematite or magnetite are unattractive as oxygen donors for methane on thermodynamic grounds unless the main gas-solid reactions take place across an iron-wustite interface. Table V reports eight runs using two different nickel catalysts. The catalyst used in runs 7 and 8 was made using a stronger and less porous support than t h a t for the catalyst in runs 1, 2, and 6 and i t has a much lower activity. All the runs suggest t h a t as the reduction proceeds, the conditions approach those of the methane-wustite system and t h a t there is a decided advantage to be gained from t h e use of a catalyst both i n the quality of the synthesis gas and in the overall rate of reaction. The reduction of hematite and magnetite will therefore be limited by the wustite equilibrium conditions and complete utilization of methane for reduction may he achieved only if carbon
9.6 6.2 22.6 61.6 3.3 31.9 18.0 7.4 13.3 61.3 5.5 25.2 19.8 8.5 13.0 58.7 10.1 26.8 22.4 9.7 12.9 55.0 14.7 29.1
9.2 7.3 26.9 56.5 4.2 37.7 19.4 7.4 14.2 59.0 6.8 26.8 22.0 9.0 13.7 55.3 11.6 29.1 23.6 10.1 12.4 53.9 16.4 29.3
6
930 70 6.0 49.0 27.5 15.0 8.5 6.1 83.3 56.7 27.2 12.1 4.0 11.1 90.8 56.5 28.6 11.3 3.5 22.0 91.8 57.3 27.4 11.8 3.5 32.7 91.6
7
930
a
8.5
930 70 8.5
... ... ... ... ... ... 41.3 17.9 13.0 27.8 7.0 52.6 56.0 26.1 10.2 7.6 16.1 82.5 56.6 27.4 10.2 5.8 25.2 86.7
26.1 15.0 17.1 41.7 2.9 43.5 38.2 19.0 13.7 29.0 10.5 52.9 52.3 25.9 10.2 11.6 19.3 75.6 57.6 27.9 9.4 5.1 28.9 88.0
70
dioxide and water vapor are removed from the effluent gases and the remainder recycled. Conclusions
This investigation has primarily been concerned wit'h the react.ion between porous iron oxide and methane. The coiielusions which may he dra1T-n are that the reduction is mainly carried out by hydrogen and carbon monoxide achieving equilibrium a t a n Fe/FeO interface and that the bulk of the methane reacts by the reforming reactions which are much slower than the reduction reactions. An activation energy of 50,000 for both the reforming reactions over iron and the overall methane-wustit,e reactions suggest a chemical reaction control for the system. Under different physical conditions, such as the use of nonporous iron oxide and/or the inclusion of a nickel catalyst', the relative rates of bhe reforming and reduction reactions could alter greatly and it seems unrealistic to assume that a single kinet'ic model could fit all conditions. h great deal more work is required which takes int,o account the variabilit'y of t'he solid material, possibly involving the preparation of iron oxides of various porosities and sizes. I n -Australia, large deposit,s of iron ore occur inland in relative proximity to natural gas deposit,s. Many of these natural gas deposits are far from urban centers of utilization and a n economic incentive exists to explore all the circumstances surrounding the development of synthesis gas production, direct reduction of iron ore, or a combination of both. Acknowledgment
This paper is based on the P h D thesis of D. Barrett submitted to the School of Chemical Engineering, University of Kew Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
419
South Wales, Australia. The author is grateful to K. S. Basden and J. R. Korman for their valued counsel in the supervision of this work. Literature Cited Akers, W. W., Camp, D. P., AIChE, 4,471-4 (Dee. 1955). Barrett, D., Trans. Inst. Chem. Eng., 49, 80-2 (Jan. 1971). Darken, L. S.,Gurrey, R. W., J . Amer. Chem. SOC.,67, 1398,
(1943). Darken, L. S., Gurrey, R. W., “Physical Chemistry of Metals,” Int. Student Ed., p 219, hIcGraw-Hill-Kogakusha, Tokyo,
Kawasaki, E., Sainscrainte, J., Walsh, T. J., AIChE, 8 (I), 48 (1962). Kubachewski, O., Evans, E. L., Alcock, C. B., “Metallurgical Thermochemistry,” 4th ed., Pergamon, London, 1967. McKewan, W. M., Trans. M e t . SOC.AIME, 218, 2 (1960); 221, 140, (1961); 224, 2, (1962); ibid., p 387. Spiers, H. M., Technical Data on Fuel, British Nat. Committee, World Power Conference, 6th ed., 1961. Terasawa, S., Sakikawa, N., Shiba, T., Bull. Japan Petrol. Inst., 3, 3-13 (March 1961); 5 , 20-6 (March 1963). Warner, N. A., Trans. M e t . SOC.AIME, 230, 163-7 (1964).
1Q.52
Giil%id, E. R., Lewis, W. K., Read, W. A,, Ind. Eng. Chem., 41, 1227-37 (June 1949).
RECEIVED for review August 3, 1971 ACCEPTED February 8, 1972
Development of Continuous-Stirred Gas-Solid Reactors for Studies in Kinetics and Catalyst Eva1uation Vasant R, Choudhary and 1. K. Doraiswamy National Chemical Laboratory, Poona-8, India
Three types of continuous-stirred gas-solid reactors of different designs have been developed: first, a rotating basket-type Reactor I with four cylindrical paddles; second, a fixed cylindrical basket-type Reactor ll with a special impeller rotating around the basket; and third, a rotating tank-type Reactor 111 with a stationary catalyst in a cylindrical basket. Mixing characteristics in these reactors were studied by a tracer technique [step test] and all of them were found to be “perfect mixers” within the applied range of flow rates. Mass transfer studies were also carried out in all the reactors by naphthalene evaporation experiments at various stirring speeds, and appropriate io correlations have been developed. The mass transfer rates in Reactors II and 111 have been observed to be high (approximately two to three times) as compared to Reactor 1. Reactors II and 111 appear to be unique as the actual temperature of the catalyst and/or the temperature along the bed length can be accurately assessed. They can be operated under perfect isothermal conditions. Further, they can be used as single-pellet reactors with stirred tank performances and can be conveniently employed for rapid catalyst evaluations.
G e n e r a l l y , laboratory studies of heterogeneous catalysis are carried out to obtain kinetic models of the surface catalyzed reaction or to evaluate different catalysts. The development of a mathematical model for a gas-solid catalytic reaction requires accurate point reaction rate data. It is essential that the rate data be “true,” Le., uncontaminated by inter- and intraphase heat and mass transfer effects. I n the absence of such intrinsic kinetic data, the extent of diffusional intrusion should be accurately assessed; otherwise the “combined” reaction rate data can yield meaningless rate expressions, and the plant reactor based on such data may perform a t unpredictable levels of activity and yield. These facts must also be considered while evaluating catalysts for commercial use, since catalyst evaluation under the influence of the physical processes occurring simultaneously with the catalytic reaction may lead to rejection of the best catalyst and selection of a relatively poor candidate. Hence the measurement of rate in the absence of diffusional interference (or its isolation from “disguised” rate data) is a n absolute requirement for scale-up. 420 Ind. Eng. Chem.
Process Des. Develop., Vol. 1 1 ,
No. 3, 1972
The continuous-stirred gas-solid reactor (CSGSR) offers a convenient means of meeting this requirement and obtaining accurate rate data. It is, in essence, a reactor with internal recirculation. Provided a given CSGSR has been shown to be a perfect mixer, i t can be used in kinetic studies. Use of this reactor with a high degree of agitation permits one to study the chemical reaction occurring on the catalyst surface without any complicating control of the reaction rate by physical processes, such as gas-solid heat and mass transfer, occurring simultaneously with the catalytic reaction. The CSGSR operates at conditions where “perfect mixing” prevails which ensures a uniform temperature and concentration of the bulk gas throughout the reactor; and the bulk gas concentration and temperature are very close to the concentration and temperature of the gas at the catalyst surface. Since the entire catalyst surface is in contact with the gas of the same composition and temperature, the reaction rate can be calculated directly from the measured difference between the inlet and outlet concentrations, as in case of differential and recycle reactors, by using the simple relation:
