Influence of External Factors in Catalytic Reactions Dehydrogenation of Ethyl Alcohol with Copper Catalyst T. J. SUEN, T. P. CHIEN, AND P. S. CHU Tung Li Oil Works, Chungkking, China
M
ANY factors affect the activity of a catalyst, such as
of 92 per cent concentration was fed a t a constant rate into the reaction tube. Before reaching the catalyst bed, it was vaporized and preheated by the oil bath, The exit gas was successively passed through water to remove the unreacted alcohol, through sodium bisulfite solution to remove the aldehyde formed, and through sodium hydroxide solution t o remove any sulfur dioxide carried over from the bisulfite. The remaining hydrogen was measured by a wet gas meter, and the degree of reaction calculated. Preliminary experiments showed that the results obtained by this method of analysis check with those from the direct determination of aldehyde (IO). The experimental points given in the tables and graphs are averages of six to ten readings taken a t IO-minute intervals during single runs. These readings were made after steady state had been attained for some time. The maximum variations of a few individual readings amounted to about 10 per cent; but in general, the deviations from the mean were not more than 3 per cent. No check runs were made. However, it must be emphasized that the sequence of the different experiments with different feed rates was taken a t random, usually a slower rate af1,er a greater rate, then another slower rate.
temperature, pressure, method of preparation, previous treatment, degree of subdivision, shape of packing, etc. Of these factors, the two latter have perhaps been least studied, although they are of considerable industrial importance. A survey of the literature reveals very little data on them. To obtain information on the influence of these factors is not so simple as it might a t first appear. The chief difficulty is to keep the other variables constant while one of them is being studied. Most catalytic reactions are accompanied by side reactions which complicate the evaluation of catalyst activity. Catalytic reactions are usually carried out at comparatively high temperatures which are difficult to control within narrow limits. The activity and even the physical shape of the catalyst may change after continued use; for instance, granules may disintegrate. These factors prevent the obtaining of reliable data. I n this work a catalytic reaction was chosen in which most of the obstacles mentioned could be overcome-namely, dehydrogenation of ethyl alcohol with copper catalyst. Previous investigators (1, 3, 9,13) ascertained that dehydrogenation proceeds without secondary reactions a t temperatures below 300" C. The degree of conversion is easily ascertained by measuring the hydrogen evolved. The catalyst can be made firm in structure. Since its activity decreases with age, only a few runs were made with each portion of catalyst.
Effect of Catalyst Shape I n contact catalytic gas reactions, the rate of flow is commonly expressed in terms of space velocity-i. e., volume of gas under standard conditions per unit time per unit volume of catalyst space. I n applying this term only the volume of the catalyst space has been considered ; its geometrical shape has been supposed to be inconsequential. Benton (g) maintained that the same area of contact surface causes the same amount of reaction, regardless of shape. This assumption lacks adequate experimental proof. Some investigators have asserted that the method of packing the catalyst may influence the results considerably, For instance, in methane chlorination Kiprianov and Kusner (6) claimed that a thin layer with a large cross section of carbon catalyst gave a better yield than the same amount of catalyst packed in a smaller tube with a greater length. The experimental results with the same weight of catalyst packed in tubes of different size are given in Table I and Figure 1. Although the experimental points are somewhat scattered, there is no constant change with diameter of the reaction tubes. I n other words, the results confirm the belief that the shape of the catalyst space has no effect on the reaction. However, it must be pointed out that heat is invariably evolved or absorbed in a chemical reaction. The method of packing the catalyst may greatly influence the rate of heat
Experimental Procedure Two series of experiments were performed. The effect of catalyst bulk was studied in one series and the effect of grain size in the other. In the first series identical amounts of catalyst were packed in Pyrex tubes of different diameters. The catalyst was Merck's black copper oxide wire, reduced by hydrogen 10" C. below the reaction temperature before use. The catalyst used in the second series was fused copper oxide, broken into small pieces (7, 8). The granules were carefully screened to different sizes. They were also reduced by hydrogen 10" C. below reaction temperature. The reaction tube was Pyrex glass, 2.8 cm. in inside diameter. I n carrying out the experiments, the reaction tube was fitted into a specially made oil bath; the inlet and outlet ends of the tube were outside of the bath. The temperature at the exit end of the catalyst bed and the temperature of the oil bath were both measured and for most of the runs were the same. When a slight difference existed, the former was taken as the reaction temperature. The maximum temperature variation during the experiments did not exceed 1" C. and was much smaller on the average. Ethyl alcohol 674
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1942
TABLE I. DEHYDROGENATION AT 235' C. IN REACTION TUBES OF DIFFERENT SIZES Inside Diam. of Tube, Cm. 3.3
Catslyst Weight Grams' 150
Catalyet Volume, Co. 63
Rate of Feed, Co./Hr.
Deoom osition,
9.72 24.1
29.0 26.8 22.7 20.8 15.5 26.1 25.4 24.4 21.1 22.6 17.0
50.0
63.6
2.8
150
59
89.1 25.2 34.5 40.0 43.2 47.1 88.2
g/,
675
The rate of formation of acetaldehyde can be expressed by the following equation:
Since
x=xa-y
~~
At equilibrium
+ klxo
Z Y2 ~ - ~IY.
& dt = O = - k
(5)
Subtracting Equation 5 from 4, rearranging, and setting kl/k2 = b,
Y.
+
3
= kdv.
- d ( b + Y)
(6)
Integrating, 2 303
transfer. I n the experiments on the exothermic reaction of chlorination of methane cited above, a thin layer of catalyst evidently facilitated the dissipation of heat more readily than a thick layer, and for that reason probably gave the smoother reaction as reported.
(log
- 2/ -
Y.
(7)
Y.
Since t is inversely proportional to the rate of flow which, in turn, is approximately proportional t o the rate of feed, R, Equation 7 may be put into the following form:
Variation of Conversion with Flow Rate Other things being equal, variation of rate of flow affects only the time of contact. Evidently the variation can be predicted by chemical kinetics. If side reactions take place along with the main reaction, the case is very complicated for mathematical analysis. With a single, one-way reaction the treatment should be simple. The reactioh studied in this work has been found to be without side reactions at the temperatures in question ( I , 3, 9, IS). However, it is reversible. I n the following derivation the classical constant volume rate equation is used for simplicity. Strictly speaking, this is not applicable to flow processes as employed in the present experimental investigation. But the error is not great when the conversion or change of volume i s small. Consider the reversible reaction, CHaCHaOH + CHsCHO Ha (1)
+
The solid curve in Figure 1 represents Equation 8 with a basis of 1 mole of original alcohol, ye = 0.288, and K = 41.2. From Equation 2,
b = 0.288
+ 0.1164 = 0.4044
The curve of Figure 1 agrees with the experimental datu reasonably well. By interpolating Rideal's data ( I I ) , the equilibrium concentration of aldehyde a t 230' C. is 22.5 and that a t 235' C. is 26.2 per cent. The estimated equilibrium concentrations in this work are 18.8 and 28.8 per cent. Due t o difficulties in standardi~ingthe thermometers employed and also in measuring the true catalyst temperature, the agreement is considered to be satisfactory. Since temperatures in each
0 RATE OF FEED-CC./HR.
FIGURE 1. EFFECTO F CATALYST SHAPE
RL/W
FIQURE2. EFFECTO F
PARTICLE
SIZE
OF
CATALYST
INDUSTRIAL AND ENGINEERING CHEMISTRY
676
AT 230’ C. WITH COPPER CATATABLE11. DEHYDROGENATION LYST OF DIFFERENT SIZES
Particle Size, Meah
Av. Linear Dimensions L , Min.
14-20
2G28
Catalyst Rate of Weight W , Feed R ,
E E‘
Deoomposition, yo
18.1 26.8 33.2 47.2
0,145 0.215 0.266 0.379
13.6 11.4 10.7
10.1 13.5 16.8 26.2 27.5 48.5 59.5
0.115 0,155
Grams
Cc./Hr.
1,000
124.6
0.711
62.6
0.193 0.300 0,320 0.559 0.684
14.8
external surface. The latter is not directly measurable but is proportional to W / L ,where W is the total weight and L the average linear dimension of the particles (16). B y plotting the degree of conversion against RLIW, the experimental points come close together and can be represented by a single line, as shown by the solid curve in Figure 2, thus indicating that the decomposition is a function of RL/ W . The following data indicate that the percentages of voids in the catalyst of different mesh size were approximately the same, and therefore slight differences in this variable hardly accounted for differences in activity:
15.1 13.7 11.9 9.50 10.2
7.95 7.48
series of experiments were comparable, the lack of agreement in equilibrium concentrations is not important. I n the above derivation the phenomenon of adsorption has not been taken into consideration. By assuming that the three gases involved-i. e., alcohol, aldehyde, and hydrogenare only slightly adsorbed by the catalyst, according to Langmuir’s theory (6) the amount of gas adsorbed will be directly proportional to the partial pressure. The rate of reaction is directly proportional to the number of adsorbed molecules and, therefore, directly proportional to the partial pressures of the reacting gases. I n other words, the reaction occurring on the surface of the catalyst follows the same kinetic equation that would be followed if the identical reaction took place in a homogeneous system (4). The equations obtained above are therefore not incompatible with the theory of adsorption.
Effect of Particle Size Khere the surface area of the catalyst is readily measurable, the activity is roughly proportional to the contact area ( 1 2 ) . For irregular particle sizes, which are most widely used in industry, it seems also reasonable to expect that the increase in catalytic activity might be a function of the increase in external surface area. However, little inforniation is available to support this assumption. By mathematical deduction, Thiele (14) showed that the activity of a porous granular catalyst below a certain critical size is proportional to its volume but independent of its particle size. If the particle size is much larger than the critical, the activity depends on the total external surface area of the particles. Unfortunately, his mathematical treatment is too complicated to be readily applicable. The experimental results for showing the effect of catalyst particle size are given in Table 11. At the same space velocity a smaller catalyst size tends to bring about a greater degree of decomposition. It is reasonable to expect that a given amount of reactant in contact with the same amount of catalyst surface for the same length of time will bring about the same degree of decomposition. If the external surface is a factor affecting the amount of reaction, then catalysts of different sizes will give the same amount of reaction at the same values of R / A , where R is the rate of feed and A the
Vol. 34, No. 6
Vesh size Length, cm.
8-14 8 8
14-20 8 7
20-28 4 6
28-35 8 7
35-48 9 3
B y taking t inversely proportional to RLIW, Equation 7 was also applied to represent the experimental data shown in Figure 2. With ye = 0.188 the dotted curve is calculated from the equation
The calculated and experimental values do not agree closely, but the trends are similar. The experimental data are open to criticism. As the reaction is reversible at a zero value of RLIW, the slope of the curve should be zero as shown by the dotted curve, instead of infinity as shown by the solid curve. A catalyst with a particle size smaller than 48 mesh offered too much friction, and the reaction could not be carried out smoothly.
Somenclature
b
+ kl/kp = rate constant for forward dehydrogenation reaction
=
ye
kl ks = rate constant for reverse dehvdrogenation reaction K = a constant L = average linear dimension of catalyst particles, mm. R = rate of feed, cc. of liquid alcohol per hour “
Y
t = time W = total weight of catalyst, grams z = moles of ethyl alcohol at time t y = moles of acetaldehyde or hydrogen at time t Subscripts 0 = initial condition e = equilibrium condition
Acknowledgment Thanks are due W. K. Leung, formerly of this factory, who performed some preliminary experiments in the investigation.
Literature Cited Armstrong and Hilditch, Proc. Roy. SOC.(London), A97, 269 (1920). Benton, IND.ENG.CHEM.,19, 497 (1927). Bouvealt, Bull. SOC. chim., 141 3, 119 (1906). Hinshelwood, “Kinetics of Chemical Change in Gaseous Systems”, 3rd ed., pp. 314, 329, London, Oxford Univ. Press, 1933. Kiprianov and Kusner, Oil R: Gas J., 38, No. 36, 49 (1940). Langmuir, J . Am. Chem. SOC.,38, 2221 (1916). Legg and Adam, Brit. Patent 166,249 (1919). Maxted, “Catalysia and I t a Industrial .4pplications”, p. 231, London, J. & A. Churchill, 1933. Palmer, Proc. R o y . Soe. (London), A98, 13 (1920). Parkinson and Wagner, IKD.ENG. CHEM.,ANAL.ED., 6, 433 (1934). Rideal, Proc. Roy. SOC.(London), A99, 153 (1921). Rideal and Taylor, “Catalysis in Theory and Practice”, 1st ed., p. 63, London, Macmillan Co., 1919. Sabatier, Compt. rend., 136, 738, 963 (1903). ESG. CHEM.,31, 916 (1939). Th:ele, IND. Walker, Lewis, McAdams, and Gilliland, “Principles of Chemical Engineering”, 3rd ed., p 253, New York, McGraw-Hill Book Co., 1937.
