quantum yield for removal of pollutant, g-mol/ Einstein ( @ p ) o = contribution involving oxygen (unsensitized effect) (aP)c1= contribution due to free available chlorine = attenuation coefficient (ph = ahC, where ah = absorptivity a t wavelength A), cm-l +p
=
SUBSCRIPTS p = pollutant (measured as TOC) C1 = free available chlorine o = oxygen, as dissolved 0 2 lp = low-pressure lamp hp = high-pressure lamp tot = total literature Cited
Hancil, V., Smith, J. M., Ind. Eng. Chem. Process Des. Develop., 4, 515 (1971).
Meiners, A. F., “Advanced Study of Light-Catalyzed Oxidation for Large-Scale Treatment of Wastewater,” Midwest Research Institute, Kansas City, MO, Rept. on Contract 14-12-531, Federal Water Quality Administration, September 1970. Noyes, W. A,, Jr., Hammond, G. S., Pitts, J. N., Jr., “Advances in Photochemistry,” Vol. 6, Wiley, h’ew York, NY, 1968. Schorr, V., Hancil, V., Boval, B., Smith, J. M.,Ind. Eng. Chem. Process Des. Develop., 4, 509 (1971). “Standard Methods for the Examination of Water and Wastewater Including Bottom Sediments and Sludges,” 12th ed., Amer. Public Health .4ssn., Inc., New York, NY (1965).
RECEIVED for review November 19, 1971 ACCEPTED April 20, 1972 This work was supported by Grant 17020 EVQ of the Environmental Protection Agency.
Effect of Catalyst Particle Size on Selectivity in Butene Dehydrogenation Hervey H. Vogel and Carroll Z. Morgan Shell Development Co., Emeryville, CA 94608
Dehydrogenation of n-butenes io butadiene over an iron oxide catalyst (Shell 205) in the presence of steam at 620°C approximately follows the kinetics of first-order consecutive reactions where selectivity is concerned. Selectivity to butadiene, the intermediate product, depends strongly on conversion level and moderately on particle size. The particle size effect is explained by diffusion resistance within particles. Agreement with a simplified theory is good over the range 0.5-5 mm diam.
T h e important effect of a diffusion limitation on selectivity of conversion to an intermediate product in reaction over a porous solid catalyst was clearly predicted by Wheeler (1951). Since that time many calculations of diffusional effects on both activity and selectivity have been made, but very few experimental examples of selectivity effects have been published. Komiyama and Inoue (1968) give a n interesting example for the hydrogenation of acetylene to ethylene and ethane over a partially poisoned nickel catalyst. Their data show some of the expected features, but require a complex interpretation. Our data for n-butene dehydrogenation over a n iron oxide-type catalyst, Shell 205, furnish a relatively simple example of practical significance. Furthermore, these data can be fitted within the normal range by a simple treatment using only a single adjustable parameter. Dehydrogenation of n-butenes has been used for commercial butadiene production since 1943. The reaction is carried out i n the presence of steam diluent at near atmospheric pressure and 6O0-65O0C (Russell et al., 1946). Initially the Esso 1707 catalyst (Kearby, 1950) based on magnesium oxide was used, but this was displaced in 1945 by the Shell 105 catalyst. The latter consists of 87.9% Fe203, 2.5% Crz03, and 9.6% KzO. 1
454
To whom correspondence should be addressed. Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
The Shell 105 catalyst made possible continuous operation in place of the hourly regenerations required by the Esso 1707 catalyst. The lifetime of the Shell 105 is also much greater. Still later the Shell 205 catalyst, consisting of 62.5% Fe203, 2.2% CrZO3,and 35.3% K2C03,became the favored catalyst for butadiene manufacture because of smaller steam requirement and somewhat better selectivity. Today the Shell 105 catalyst is widely used for dehydrogenation of ethylbenzene to styrene, while Shell 205 is used for butadiene. A rather detailed study of the kinetics of butadiene formation over the Esso 1707 catalyst was published by Beckberger and Watson (1948). A system of 11 reactions was used; the most important were catalytic dehydrogenation of butenes and catalytic destruction of butadiene. We have not made as complete a kinetic study with the Shell 105 or 205 catalysts, but it is evident from our unpublished data that the system of reactions is nearly the same. For the purpose of correlating selectivity effects in the present paper we have restricted our attention to just two reactions: the catalytic dehydrogenation of n-butenes, and the catalytic destruction of butadiene. Experiments carried out at different space velocities with a given small particle size show that selectivity declines as conversion increases, and that the curve is well-fitted by a consecutive reaction system
~~~~~~
~~
~~~~
Table 1.
~~
Test Data for Various Particle Sizes
Phillips butene-2 over 50 cc Shell 205 catalyst a t butenes gas hourly space velocity of 500 (25'C, 760 mm) and H20/C4H8 mol ratio of 12. Outlet pressure atmospheric. Each analysis shown is the average of two, a t 18-26 hr age. 6-8 10-14 16-20 20-28 Particle size 3/8 in. "16 in. '/g in. '/E x '/g mesh mesh mesh mesh Equiv diam, mm 9.5 4.7-5.2 3.2-3.6 3.18 2.4-3.3 1.17-1.65 0.83-1.0 0.59-0.83 Bulk density, g/cc ... 1.03 1.10 1.08 1.05 1.07 1.06 1.03 Temp, 'C 620 640 660 620 640 660 620 640 620 640 620 640 620 640 620 640 620 640 Reactor AP, mm Hg 8 7 12 8 17 18 38 68 Analysis, vol % ' 22.6 28.8 34.8 24.7 30.8 36.7 29.9 36.1 33.2 40.8 32.9 37.8 34.9 39.6 37.5 43.1 35.8 42.4 H2 COZ 4 . 9 7 . 0 9 . 3 4 . 7 6 . 9 9 . 3 5 . 4 7 . 5 5.9 7 . 9 5.6 7 . 1 6 . 0 7 . 6 6 . 2 8 . 8 6 . 3 8 . 3 CHI 2.2 3.5 5 . 3 2.3 3.6 5.3 2.5 3.7 2.4 3.6 2.3 3.2 2.3 3.2 3.0 4.2 2.9 4'4 0 . 0 0 . 1 0 . 8 0 . 2 0 . 5 0 . 8 0.2 0 . 5 0 . 1 0 . 3 0 . 2 0 . 4 0 . 1 0 . 3 0 . 4 0 . 6 0 . 4 0 . 6 CzH4 C3H6 1 . 8 2 . 9 4 . 2 1 . 9 3 . 2 4 . 6 2 . 1 3 . 3 2 . 1 3 . 2 1 . 9 2 . 7 1 . 9 2.8 2 . 5 3.6 2 . 4 3.7 C4H6 10.3 11.8 12.2 12.7 14.0 14.1 1 4 . 1 14.7 14.6 14.5 14.7 15.8 15.9 16.0 16.3 15.8 16.3 15.9 58.2 45.9 33.4 53.5 41.0 29.2 45.8 34.2 41.7 29.7 42.4 33.0 38.8 30.5 34.1 23.9 35.9 24.7 CdHs 18.7 26.5 36.8 22.9 31.7 42.4 27.7 36.9 30.2 39.0 29.8 38.2 33.1 40.4 37.6 47.9 36.3 47.2 Conversion, % 76.4 71.3 63.0 79.7 73.0 65.4 79.8 73.4 80.1 73.2 81.2 77.1 82.0 77.2 79.3 72.0 79.3 72.1 Selectivity, %
hi
ji2
Butenes +Butadiene +COZ
+ cracked products
with k2/kl = n = 0.9. For larger particle sizes the selectivity curves are displaced toward lower values, and the displacement is explained by a measured diffusivity for the catalyst, combined with the above ratio of rate constants. Experimental
Dehydrogenation was carried out over 50 cc of catalyst in a 1-in. id.-type 304 stainless steel tube a t a steam/butenes ratio of 12. The feed was Phillips butene-2, pure grade, but products from the reactor contained all three n-butenes in equilibrium amounts. The butene and steam from vaporized distilled water were fed downflow through a preheating section and over the catalyst bed. l n axial thermocouple well, 3/16 in. o.d., containing four thermocouples, was used except for the 3/8-in. pellets, for which the well was omitted and temperature was based on the setting of the controlling couples in the metal block surrounding the reactor tube. The apparatus was designed to operate unattended overnight. To stabilize catalysts, runs were continued a t least 18 hr before samples were taken. The usual procedure was to take two samples for glc analysis under each operating condition. Averaged results are reported in the tables. Occasional mass spectrometer analyses agreed well with the glc analyses. Catalysts were from commercial production by Shell Chemical Co. The following sizes were used: Extruded a/8-in.cylindrical pellets, about 3/8-1/2 in. long Extruded a/16-in. cylindrical pellets, about 3/16-3/8 in. long Extruded '/8-in. cylindrical pellets, about 1/8-1/4 in. long Extruded 1/8-in, pellets, selected '/8 in. long Granules, 6-8 Tyler mesh, broken from the 3/16-in. pellets Granules, 10-14 Tyler mesh, broken from the 3/16-in. pellets Granules, 16-20 Tyler mesh, broken from the 3/16-in.pellets Granules, 20-28 Tyler mesh, broken from the a/16-in. pellets Dehydrogenation data for eight particle sizes are given in Table I. I n this series, the temperature was varied from 620' to 640°C, and for some sizes to 66OOC to cover a range of conversions. Variation of space velocity would have been preferable. I n other tests made with I/s-in. pellets, the space velocity was varied, and as shown by the points in Figure 1, there was little or no shifting of the conversion-selectivity relationship as temperature was changed from 620' to 640" or
0 620T A 640°C 0 66OOC
t
I
50
I
I
I
I
I
.
Figure 1 Conversion-selectivity relationship for 1/8-in. pellets at 620-660°C Butene gas hourly space velocity, 130-500; H*O/butenes = 12
660°C. This means that the two successive reactions have similar activation energies. The activation energy of the first reaction was calculated to be 30 kcal from 620' and 64OOC data for 1/8-in. pellets, assuming a first-order reaction, and correcting for catalyst effectiveness factors. The effect of particle size on selectivity is shown in Figure 2, where selectivity data of Table I, interpolated to 3570 conversion, are plotted against particle diameter. Quite a pronounced effect is evident, but there is no gain in selectivity for particles smaller than 1.1-mm diam. Percentage selectivity is defined as mol of butadiene produced/100 mol of n-butenes converted. Pressure has a n adverse effect on the selectivity of butadiene formation. But the increased pressure drop in our reactor, for example, 38-mm Hg with 16-20 mesh granules, as compared to a pressure drop of 8 mm with 3/16-in.pellets, could Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
455
IO0
8 E
i
sz 88 2
90 10-14 MESH AND 16-20 MESH
80
%6
;
5/80
w
Z Lu
$
70
m IG
t2
U 60
d
n w
50 2
4
a
6
10
50
0
IO
PARTICLE DIAMETER, mm
Figure 2. Selectivity a t particle size
35% conversion as a function of
Figure 4,
20 30 CONVERSION OF BUTENES, %
50
40
Experimental and calculated selectivities Points exptl; lines colcd
diameter, which is the same as for 3/~6-in.pellets with the usual 3/16-in. well. The selectivity for '/&I.pellets with the I/& well was 74.6% a t 3570 conversion, as compared to 74.7% at 35% conversion for '/&I.pellets with the 3/16-in. well. The conversion level was higher a t the same ghsv with the smaller well, however (41.7% vs. 31.6y0, both a t 64OOC). Thus there was some bypassing, but because of radial mass transfer and the small curvature of the selectivityconversion line, it had little effect on selectivity. Possible effects of bypassing were also estimated from engineering correlations. A formula of Tichacek (1963) relating selectivity loss to dispersion caused by eddies or bypassing was used. The calculated selectivity corrections for eddy diffusion and bypassing have opposite trends as particle size is changed under our test conditions. The net calculated corrections were as follows: Particle size: Correction, yo:
d
50 o
10
20
CONVERSION
Figure 3. granules
40
30
OF BUTENES,
50
%
Conversion-selectivity relationship
for
small
Points exptl; line colcd for n = 0.9,6 = 0.3
have diminished selectivity from the 760 mm total pressure value for the 16-20 mesh granules by only 0.25 percentage points. This is a negligible correction, and it has not been applied. I n commercial reactors, however, with longer beds, pressure drop is a serious limitation to the use of smaller particles. Bypassing a t the reactor wall was considered as a possible source of error. Flow was laminar to transitional (particle Reynolds KO. = 25 for 3/16-i11.pellets), and contact time was less than 0.2 sec. An experiment was done to check on bypassing. For this test, 1/8-in. pellets mere used, and the 3/16-in. diameter thermocouple well was replaced by a well in. in diam. This gave an annular distance two times the pellet 456 Ind.
Eng.
Chem. Process Des. Develop., Vol. 11, No. 3, 1972
10-14 mesh 3.1
3/16
2.1
2.3
3/8
3.3
These corrections were not applied. If they had been, the shape of the curve in Figure 2 would not have been changed significantly. B u t the calculations say that the true selectivity for l/s-in. pellets is 76.8% rather t'han 74.7y0 at 35% conversion. Indeed, in other tests we did observe somewhat higher selectivities for 1/8-in. pellets if they were tested in longer beds than the 10-cm bed used here. An addit'ional effect that contributes to better selectivity with larger beds in a given reactor is reduced thermal cracking.
Discussion
The particle size effect can be explained by a diffusion limitation. A complete calculation would require consideration of the reversibility of the reaction, the endothermicity, the volume change, and the exact kinetics, as well as detailed knowledge of diffusivities and pore struct,ures. We do not have data for such a calculation. However, a simplified approach employing a minimum number of adjustable parameters can be used. The assumption of consecutive first-order reactions fits the shape of the conversion-selectivity relation-
ship for small particles. The k 2 / k l ratio is thus determined. The remaining treatment, outlined below, employs analytical solutions to the equations. The chemical reaction is written as
If x is defined as the fractional conversion of A , and y as the fraction of A converted to products other than B , then for no diffusion effect a t a given point in a reactor, dy/dx
= %(x -
y)/(l
- Z)
=
1- u
+ ab + b ( x - y ) / ( l
+
Temp, ‘C
calcd
10-14 mesh
620 640 620 640 660 620 640 660 620 640 660
0 38 0 46 1 00 1 20 1 43 1 50 1 80 2 14 2 80 3 36 4 00
‘/g
3/8
in. in. in.
- 2)
Here a = 1/(1 - n), b = [di6 coth ( 6 6 )- 1]/[$coth __ (6)- I ] , = R d k l / D e ,R is pellet radius, De is the effective diffusion coefficient in the pellet, and kl is the rate constant per unit volume of catalyst pellet. By integrating the equation for d y l d x , using y = 0 when x = 0, we get y as a function of x and thus can calculate fractional selectivity for the reactor products when x is known. The result is ( x - y ) / x = a(1 z)”x - a ( l - x)/x. The effective diffusivity was obtained by the method of Hoogschagen (1955) in which oxygen diffuses through a pellet a t 25°C into a stream of circulating nitrogen. As a n average for three 8//16-in.pellets, this gave De/D12 of 0.10. By another method, nearly the same values were obtained for ’/g- and 3j16-i~1. pellet’s. The large pores in Shell 205 are four or more times the mean free path, and therefore normal gaseous diffusion was assumed. For Dl2 of butene in steam a t 627”C, a formula of Hirschfelder e t al. (1954) for normal gaseouso diffusion was use:, based on molecular diameters of 3.258 for H 2 0 and 5.45A for n-C4H8and using a temperaThis gave D I 2 = 0.720 cm2/sec, and ture function of T1,5. therefore De = 0.072 cm2/sec was employed. K i t h the aid of the D e value, the Thiele modulus was calculated from a measured reaction rate by the method of Wheeler (1951) and Satterfield (1970). The value of 9 for 3/16-in. pellets at 620°C was 1.5. From t’heformula 4 = R d k z , it was then a simple matter to obtain @ for other particle sizes and other temperatures. The ratio of rate constants, n, mas 0.9 from the selectivity curve for small granules. This curve was essentially that for no diffusion effect, but a small correction (6 = 0.3) was applied since the experimental data for 10-14-mesh granules were used, and these granules were calculated to have 9 = 0.3. The theoretical curve and the experiment’al points are shown in Figure 3. I n Table I1 are given experimental and calculated selectivity values for the four larger particle sizes. The agreement between experimental and calculated select’ivities is good except for t’he 3/s-in. pellets. These represent a particle size used only as a foundation layer in commercial reactors. The experiments were done some years ago and t.hese particular pellets were not examined for diffusivity; now they are not a t hand. Since the 3/8-in. pellets were less tightly compacted than the 3/16, a. higher De value would be reasonable. X doubling of De to 0.144 cm2/sec would bring the calculated values for the 3/g-in.pellets into close agreement with experiment. Experimental points and calculated lines for selectivity are shown in Figure 4. KOline is given for the 3/8-in. particles because of the lack of a De value. It, should be noted t h a t the calculated lines are not for constant temperature, but connect
$,
Particle size
3/16
where n = k2/kl. If there are diffusion limitations for spherical particles as shown by Weisz and Swegler (1955), the corresponding equation is dy/dx
Table It. Summary Comparing Experimental and Calculated Selectivities Converdon, %
33 40 27 36 48 22 31 42 18 26 36
1 4 7 9 0 9 7 4 7 5 8
%
Selectivity, Exptl Calcd
82 77 79 73 66 79 73 65 76 71 63
0 2 9 4 4 7 0 4 4 3 0
81 77 80 74 66 78 71 63 68 62 54
8 2 5 0 1 6 5 0 5 5 0
the points calculated for 620°, 640°, and 660°C. Calculated lines for conversions varied by varying space velocity a t a fixed temperature would be less steep. Although the above treatment adequat’ely describes selectivity effects, it is far from complete. Act’ually, the butene dehydrogenation reaction is endothermic, reversible, and somewhat inhibited by reaction products. The endothermicity is not serious under our conditions. The 0 parameter (Satterfield, 1970) is quite small ( p 5 0.01), and the calculated maximum AT within pellets is only 0.35”C. From the curves of Reisz and Hicks (1962) and Butt (1966), it’is evident that corrections to our calculated isothermal selectivities Lvould be small. Reversibility was limited in our tests by working at total conversions to butadiene below 50% of equilibrium. Inhibition affects both of the consecutive reactions, and hence the selecbivity-conversion relationship is the same as that for consecutive first-order reactions. I n conclusion it can be stated that butene dehydrogenation over a n iron oxide-type catalyst furnishes an example of diffusional control of selectivity to an intermediate product,. A simplified calculation based on a minimum number of parameters explains the data. It is recognized, however, that the treatment is not exact. Acknowledgment
We wish to thank W.E. Armstrong and Y. Sakauye far their participation, and the Shell Companies for permission to publish. Literature Cited
Beckberger, L. H., Watson, K. M., Chem. Eng. Progr., 44, 229 (1948). Butt. J. B., Chem. E m . Sci..21. 275 (1966). ~, Carberry, J. J., ibid.,i7, 675 (i962). Hirschfelder, J. O., Curtiss, C. F., Bird, R. B., “Ilolecular Theorv of Gases and Liauids.” Wilev. I , Xew York. NY (1954)”. Hoogschagen, J., Ind. Eng. Chem., 47,906 (1955). Kearby, K. K., ibid., 42, 295 (1950). Komiyama, H., Inoue, H., J . Chem. Eng. Jap., 1, 142 (1968). Russell, R. P., Murphree, E. V., Asbury, W.C., Trans. Amer. Inst. Chem. Eng., 42, 1 (1946). Satterfield, C. H., “Mass Transfer in Heterogeneous Catalysis,” pp 147, 165, M I T Press, Cambridge, hl.2 (1970). Tichacek, L. J Amer. Inst. Chem. Eng. J., 9,394 (1963). Wakao, S . ,Fujishiro, S., Kagaku Kogaku, 5 , 63 (1967). Weisz, P.B., Hicks, J. $., Chem. Eng. Sci., 17, 265 (1962). Weisz, P. B., Swegler, E. W., J . Phys. Chem., 59, 823 (1955). Wheeler, A, Advan. Catal., 3, 250, 299 (1951).
RECEIVED for review December 9, 1971 ACCEPTED March 20, 1972 Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
457
