Literature Cited
Chassain, Y., Ostertag, H., Compt. Rend. 242, 1732-4 (1956); CA 50, 14422h (1956). Clark, H. C., Horsfieldl, A., Symons, M. C. R., J . Chem. SOC. 1961, 7-11. Dinse, K. P., Mobius, K., 2 . Naturforsch. 23a, 695-702 (1968). Funke, W., Chem. Ing. Tech. 35, 336-40 (1963). Ketelaar, J. A. A., Chem. Ing. Tech. 35, 372-6 (1963). Kolthoff, I. M., Belcher, R . , “Volumetric Analysis,” pp. 297f, Vol. 111, Interscience, New York, 1957. Krzesz, Z., Brit. Patent 786,212 (Nov. 13, 1957); CA 52, 7631h (1958). Leader, G. R., Gormhey, J. F., J . Am. Chem. Soc. 73, 5731-3 (1951). Lister, M. W., Garvie, R . C., Can. J . Chem. 37, 156774 (1959). Lynn, S., Rinker, R . G., Corcoran, W. H., J . Phys. Chem. 68, 2363 (1964). MacMullin, R. B., Chem. Eng. Progr. 46, 440-55 (1950). Marshak, E. V., Khim. Nauka i Prom. 2,524-5 (1957); CA 52, 6040d (1958). Mitchell, J., Smith, D. M., “Aquametry,” Vol. V, Interscience, New York, 1948.
Moeller, T., “Inorganic Chemistry, an Advanced Textbook,” p. 534, Wiley, New York, 1952. Rinker, R. G., Gordon, T. P., Mason, D. M., Corcoran, W. H., J . Phys. Chem. 63, 302 (1959). Rinker, R. G., Gordon, T. P., Mason, D. M., Sakaida, R. R., Corcoran, W. H., J . Phys. Chem. 64, 573-81 (1960). Rinker, R. G., Lynn, S., J . Phys. Chem. 72, 4706-7 (1968). Rinker, R . G., Lynn, S., Mason, D. M., Corcoran, W. H., Ind. Eng. Chem. Fundamentals 4, 282-8 (1965). Rougeot, L., Compt. Rend. 222, 1497-9 (1946); CA 40, 6013‘ (1946). Spencer, M. S., Trans. Faraday SOC.63, 2510-15 (1967). Uri, N., Chem. Reus. 50, 375-92 (1952). Vasenko, E. N., Dokl. L’uou Politekh. Inst. 2, 139-43 (1957); CA 55, 11971b (1961). Wang, W. P., Chia, K. S.,Chemistry (Taipei) 1960, 29-33; CA 55, 2327h (1961).
RECEIVED for review August 29, 1968 ACCEPTED July 31, 1969 Work done while R. G. R. was spending a sabbatical year a t the Western Research Laboratories, Dow Chemical Co., Pittsburg, Calif.
METHANATION OF LOW LEVELS OF CARBON MONOXIDE OVER NICKEL CATALYST S. S . R A N D H A V A , E . H . C A M A R A ,
A N D
A M I R A L I
R E H M A T
Institute of Gas Technologg,. Chicago, Ill. 60616 The methanation of low levels ( 5 0 5 , 1090, 3450, 4800, and 9 1 0 0 p.p.m.) of carbon monoxide in hydrogen was studied over a precipitated nickel catalyst ( 5 8 % Ni). Effects of temperature, residence time, and composition of feed gas were investigated. The rate of reaction of carbon monoxide follows the expression -reo = k ( T ) Cc:i. The rate constant of this expression followed the Arrhenius temperature dependence at low temperatures, while at higher temperatures evidence of diffusion control of the reaction rate was found. The secondary reaction resulting in the formation of carbon dioxide was evident as the concentration of carbon monoxide in the feed was increased.
Soois after it was realized that the methanation of carbon monoxide had a commercial significance in gas manufacturing as a possible alternative to oil carbonation, many investigators focused attention on evaluating and establishing the therrnodynamic and kinetic behavior of this reaction over a number of catalysts. Akers and White (1948) studied the kinetics of carbon monoxide methanation, using 20 to 4 5 ‘ ~ carbon monoxide in the feed over a reduced nickel catalyst a t atmospheric pressures. Gilkeson et ai. (1953) reported a detailed study of the synthesis of methane for about the same feed mixture. using stainless steel tubes and steel balls. Nicolai et al. (1946)determined thai kinetics of methanation of carbon monoxide over ruthenium catalyst a t elevated pressures. Extensive work has ‘been reported by the C . S . Bureau of Mines (Karn et a!., 1965),and a detailed survey of the studies in methamtion of carbon monoxide for various
feed mixtures appears in a research bulletin (Dirksen and Linden, 1968). Thus, the kinetics in the methanation of carbon monoxide is well established for reacting mixtures containing fairly high concentrations of carbon monoxide. However, if the concentration of carbon monoxide in the feed is reduced t o a negligible amount, its kinetics could possibly change, since the amount of other components in the reacting mixture will remain practically constant. Reaction of this kind has a practical application, and the knowledge of its kinetics may prove useful. Methanation of carbon monoxide in low concentrations is becoming exceedingly important in the development of economical fuel gas for low-temperature fuel cells. At the energy conversion laboratory of the Institute of Gas Technology, investigations have been under way to remove from commercially available hydrogen anode gas traces of carbon monoxide which would act as a poison in normal VOL. 8 NO. 4 DECEMBER 1969
347
operation. The use of ultrapure hydrogen is prohibitively expensive. Selective methanation of carbon monoxide in a typical gas mixture, containing 20'; CO?, 805 H2, and about 3000 p.p.m. of CO produced by steam reforming and shift of hydrocarbons, would greatly facilitate its use in low-temperature fuel cells The best catalysts for hydrogenation reactions belong to the eighth group of the periodic system: iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium. and platinum. Although the metals of the platinum group are generally active, they are expensive for commercial use. Kickel has been found effective by several investigators (Akers and White, 1948; Dirksen and Linden, 1968) for methanation of carbon monoxide. Methanation of low concentrations of carbon monoxide over ruthenium has been reported (Randhava et al., 1969). In the present study, parts-per-million mixtures of carbon monoxide and hydrogen reacted a t atmospheric pressures in a flow system over the reduced nickel catalyst, and the effects of temperature, feed rate, and feed compositions were investigated. Experimental
The experiments were performed in a flow type unit, which ensured the required flow rates, temperature of the reaction zone, and reproducibility of the results. The flow diagram of the complete system is shown in Figure 1. The premixed mixture of carbon monoxide and hydrogen was passed through the regulator and control valve, and metered through the rotameter, which had previously been calibrated using a wet-test meter. I t then entered the top of the reactor and flowed downward through the reaction zone, which was maintained a t the desired temperature. The reactor was built from stainless steel Type 304 tubing, '/?-inch i.d. by 1-inch 0.d. by 22 inches long. An 11-gage stainless steel Type 304 tube '/.-inch 0.d. by 2 l . 2 inches long was inserted a t both ends and welded to provide the connections t o the reactor. A perforated screen t o support the catalyst within the reactor was welded to a Ii-inch-o.d. thermocouple well and placed 11 inches from the bottom of the reactor. A multipoint thermocouple entering the reactor from the bottom consisted of three Chromel-Alumel thermocouples connected to a Brown tem-
H200UT
8
F E E D G A S SUPPLY MSA L i R A ZEROGAS l o o p m i
@
M S A L I R A SPAN G d S l 1 0 5 0 p p m l
DENSER H 2 0 IN
U
TO V E N T
41
DRY ICE BATH
L;_J
Figure 1 . Equipment and apparatus 348
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
perature recorder. I t was placed inside the bed in such a manner that it measured the temperatures of the bed at intervals. After leaving the reactor. the moisture in the effluent was condensed and the dry gas analyzed for carbon monoxide and methane. This process was conducted continuously until steady-state readings were obtained on the Mine Safety Appliances Co. Lira infrared analyzers (MSA Model 300). They were calibrated at the beginning of each set of data t o read in the required ranges and had an accuracy of i 2 ' ~ of the full scale. Pure hydrogen was used as zero gas. The catalyst used, furnished by the Harshaw Chemical Co. (Xi-0104T). consisted of 5 8 ' ~nickel on kieselguhr in pellet form, l , x 1, inch cylinders. Small alumina pellets were used for catalyst support. The catalyst bed was changed prior to a set of runs with different concentrations of gas. All the premixed and assayed gases consisting of 505, 1090, 3450, 4800, 7500, and 9100 p.p.m. carbon monoxide in hydrogen, the span gases for the Liras, and zero gas were supplied by the Matheson Co. Results
The runs were grouped into six sets, each having a different feed composition. I n each set, runs were made with five different ratios of volume of catalyst to feed gas rate over the temperature interval of 150" to 300'C. The time factor, V c0 (space velocity '), was varied by changing the feed rate while maintaining the same volume of the catalyst. Although there was no noticeable decrease in the catalytic activity of nickel over the range of temperature investigated, a fresh catalyst was used for every new set. Specific runs were repeated t o check the reproducibility of the data. Actual experimental data are not included because of their volume. Calculated data are presented in Figures 2 to 8. The amount of carbon monoxide converted per mole of carbon monoxide in the feed as a function of temperature is shown for each feed gas in Figares 2 to 7. For illustration, the corresponding moles of methane produced per mole of carbon monoxide in the feed are plotted against temperature along the lines of constant time factor for a selected run (Figure 8). Effect of Temperature. Maintaining the same residence time, the percentage conversion of carbon monoxide was measured a t steady state for different reaction temperatures in the range 150" t o 300°C. a t intervals of 25'C. The results given in Figures 2 to 7 indicate the high percentage of equilibrium conversion in the range 200" to 300°C. The rate of conversion is much lower from 150" to 175°C. The effect of change in temperature on the molar conversion of carbon monoxide is particularly pronounced from 175" to 225" C. Methane formation is favored a t all temperatures, increasing with increase in temperature, but a t increased temperature, a noticeable quantity of carbon dioxide is produced. Effect of Residence Time. Keeping the temperature constant, residence time was varied from 0.1 to 0,4 CC. of catalyst per cc. of gas per second. Figures 2 to 8 show that, in general, the conversion of carbon monoxide increases with increase in residence time. In some cases, the residence time was held constant by simultaneously doubling the catalyst volume and gas
O
i I25
I50
175
I
I
I
200
225
250
1 275
300
TEMPERATURE, 'C
Figure 2. Effect of temperature on CO conversion with 505-p.p.m. feed gas
125
I50
175
200 225 TEMPERATURE, "C
250
275
125
150
175
200 225 TEMPERATURE, "C
250
275
300
Figure 4. Effect of temperature on CO conversion with 3450-p.p.m. feed gas
300
Figure 3. Effect of temperature on CO conversion with 1090-p.p.m. feed gas
T E M P E R ATURE, 'C
Figure 5. Effect of temperature on CO conversion with 4800-p.p.m. feed gas VOL. 8 NO. 4 DECEMBER 1 9 6 9
349
'
01 125
I
I
150
175
I
I
225 TEMPERATURE. "C
200
1 250
O
f
i
I 275
300 TEMPERATURE.
Figure 6. Effect of temperature on CO conversion with 7000-p.p.m. feed gas
"C
Figure 8. Effect of temperature on production of methane with 3450-p.p.m. feed gas
flow rate. Very little effect on conversion was noticed as long as the residence time remained the same. Effect of Composition of Feed Gas. In practice, the concentration of impurity (carbon monoxide) is likely to vary in commercial gases, depending upon the gasification procedure adopted. Therefore, it may be helpful t o assess the conversion efficiency of the catalyst for concentrations of carbon monoxide in the feed gas varying from 505 to 9100 p.p.m. a t fixed residence time. Reactivity of the catalyst, evaluated on the basis of molar conversion, decreases continuously with increase in concentration (Figure 9), because the net effect of increase in carbon monoxide is to increase the rate of passage through the catalyst bed, thereby decreasing residence time. Practically no carbon dioxide was formed for the lowest concentration of carbon monoxide used a t any temperature. The formation of carbon dioxide became increasingly evident as the concentration of carbon monoxide in the feed increased (Figure 9). Discussion
The main products of hydrogenation of carbon monoxide over nickel catalysts are methane and carbon dioxide. The reactions responsible for their formation could be:
0 125
150
175
200 225 8 TE M PER ATU RE, C '
250
275
300
Figure 7. Effect of temperature on CO conversion with 9100-p.p.m. feed gas 350
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
of straight lines for all the sets of data (0.7) was used. The slopes of these lines were used to evaluate rate constants h a t various temperatures. As the variation of n is small, it is possible to establish a single rate expression which is valid with a fair degree of accuracy over the entire temperature range 150" to 300" C. Such an expression would be
TEMPERATURE 'C
1 0 175 0 2 2 5 0 2 7 5 A 200 V 2 5 0
where h is a function of temperature given by 0
I000
2000
4000 5000 6000 7000 I N L E T C O CONCENTRATION. ppm
3000
8000
k = k,,e-' '"
9 0 0 0 10,000
Figure 9. Effect of feed gas concentration on CO conversion However, only Reaci-ions 1 and 2 are important (Akers and White. 1948). Rea'ction 1 is considered t o be primary and Reaction 2 is secondary. Dirksen and Linden (1968) have pointed out that the HzO-forming methana1;ion reaction is favored over the CO:-forming reaction by high H,/CO ratios. Since the H,/ CO ratios are extremely high in this investigation, the effects of the CO?-forming reaction (Reaction 2) should be minimal compared to the H20-forming reaction (Reaction 1). The reverse of Reaction 1 does not proceed to any considerable extent a t these temperatures. Therefore we can assume that only the forward step of Reaction 1 is important for kinetic consideration in the hydrogenation of carbon monoxide a t such low concentrations. Past experience has shown (Boudart, 1956; Levenspiel, 1962; Stelling and Krmenstierna, 1958; Weller, 1956) that it is generally possible to express the experimental results by an equation of the type
(10)
The values of h evaluated by the method outlined above were plotted as log h us. l / T (Figure 10). The scatter does not seem excessive. In the lower temperature regions, data follow almost a linear relationship and begin deviating a t higher temperature ranges. This curvature in the Arrhenius plot could be due to either the radial temperature gradients as pointed out by Caretto and Nobe (1966) or the pore diffusion control on the kinetics in those temperature regions. I n our investigation, because of the shape and diameter of the reactor, the radial gradients never exceed lac.,indicating little effect of these gradients on the Arrhenius plot. Further, when the same data are plotted on a rational scale they display a sigmoid characteristic. Therefore, we conclude that there exists a transition from activation control a t low temperatures to diffusion control and mixed situations between. Approximate values of h, and E in Equation 10 were obtained using lower temperatures only. Apparent activation energy E = 15.82 cal. per gram mole.
(5)
-rc.o = k Pnc0PE-
which is simpler to use than an equation of the LangmuirHinshelwood-Hougen-'Watson type. These two types of rate expression can glenerally be approximated t o each other (Boudart, 1956; Weller, 1956). The choice of Equation 5 is justified because carbon monoxide is present in such low concentration that heats of chemisorption may be assumed constant. We can approximate the above rate expression as
or
Because hydrogen is present in such large excess as to be considered essentially constant and the concentration of methane in the product is very small, the total number of moles are unchanged throughout the reaction. Expression 7 when integrated yields
n # 1
(8)
For the various values of n, C&" was plotted as a function of V,co. Thie value of n yielding the best set
00017
aooie
QWIS
00020 00021 I/T
o 0022 00023
00024
(OKI-'
Figure 10. Arrhenius plot for methanation of low concentration CO over Ni catalyst VOL. 8 NO. 4 DECEMBER 1 9 6 9
351
Arrhenius frequency factor h, =
V = volume of catalyst, cc. Xa = moles of A converted or produced per mole of CO in feed
(p.p.m. CO) “.3(cc.gas) log
cc. catalyst-sec.
Conclusions
Methanation of carbon monoxide at very low concentrations over a reduced nickel catalyst in a fixed-bed reactor was investigated. For a feed of carbon monoxide and hydrogen, the rate of reaction can be expressed as
-reo = k
Cc:i
where k follows the Arrhenius temperature dependence at low temperatures. Deviations in the Arrhenius curve a t high temperatures indicated the probable existence of pore diffusion in that region. Nomenclature
B = integration constant CA = concentration of species A in effluent, p.p.m. E = activation energy, cal./ g.-mole k = reaction rate constants k , = Arrhenius frequency factor n = reaction order P , = partial pressure of component A, atm. rA = reaction rate of species A, [ (cc. gas) (p.p.m.)]/cc. catalyst-sec. R = gas constant, cal./g.-mole- K. T = temperature, K. vu = volumetric flow rate of feed gas, cc./sec.
literature Cited
Akers, W. W., White, R. R., Chem. Eng. Progr. 44, 553 (1948). Boudart, M. A., A.I.Ch.E. 2, 62 (1956). Caretto, L. S., Nobe, K., Ind. Eng. Chem. Process Design Develop. 5, 217 (1966). Dirksen, H. A., Linden, H. R., “Pipeline Gas from Coal by Methanation of Synthesis Gas,” Institute of Gas Technology, Res. Bull. 31 (1968). Gilkeson, M. M., White, R. R., Sliepcevich, C. M., Ind. Eng. Chem. 45, 460 (1953). Karn, F . S., Shultz, J. F., Anderson, R. G., IND. ENG. CHEM.PROD.RES. DEVELOP. 4, 265 (1965). Levenspiel, O., “Chemical Reaction on Engineering,” Chap. 14, Wiley, Xew York, 1962. Nicolai, J., d’Hont, M., Jungers, J. C., Bull. SOC.Chim. Belges 55,160 (1946). Randhava, S. S., Rehmat, A., Camara, E. H., Znd. Eng. Chem. Process Design Develop. 8, 482 (1969). Stelling, O., Krusentierna, 0. V., Acta Chem. Scand. 12, 1095 (1958). Weller, S., A.1.Ch.E. J . 2, 59 (1956). RECEIVED for review January 27, 1969 ACCEPTED July 7, 1969 Project sponsored b y the Institute of Gas Technology Basic Research program.
EFFECT OF GRINDING ON NICKEL-KIESELGUHR CATALYST ACTIVITY F R I T Z
P A P M A H L ’
A N D
H O W A R D
F .
R A S E
Department of Chemical Engineering, The University of Texas, Austin, Tex. 78712 A study of hammer grinding and mortar grinding has revealed that grinding may cause a significant change in apparent catalyst activity. Hammer-ground catalyst produced from larger particles of commercial catalyst exhibited activity approximately 22% higher than equal sizes of mortar-ground catalyst and reference material. It is postulated and substantiated by experimental evidence that the lower activities are caused by diffusional resistances resulting from clogging of pores by fines. Implications are discussed relative to activity and effectiveness factor studies in which various-size fractions of catalyst were studied.
IN THE
study of catalyst characteristics and associated rate phenomena it is often advantageous to crush commercial catalyst pellets or extrusions to obtain a range of sizes. Effectiveness factors, for example, can be most readily defined by comparing the rate a t a given conversion observed on the commercial pellet with that determined for a pellet of very small or negligible size. This latter I
Present address, Enjay Chemical Co., Bayway, N. J.
352
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
rate is obtained by extrapolating to zero size, values determined for increasingly smaller sizes. Smaller particles are also often needed for kinetic studies in microreactors in which experiments are designed to eliminate excessive resistance to pore diffusion. For some systems the catalyst is fluidized when such small sizes are required that a packed bed will cause excessive pressure drop. I n these and other uses of ground catalyst, it is usually assumed that the nature of the catalyst and support is
