Agitation Studies in Shaking Autoclaves A. N. HOFFMANN, J. B. MONTGOMERY, AND J. IC. MOORE' Hercules Experiment Station, Hercules Powder Company, Wilmington 99, Del.
I
T h e effects of the rate of shaking and the distance over which the reactor was shaken (stroke length) on agitation in autoclaves, oscillated in a horizontal back-and-forth motion, were studied using glass models mounted on a shaking mechanism. Conclusions as to the degree of agitation were based on hydrogenation rates of nitrobenzene in glacial acetic acid rather than on visual observations. The major reactor design factors influencing agitation were the rate of shaking, the stroke length, and the reactor volume for any given ratio of length to diameter. The maximum hydrogenation rate obtained using the optimum combination of the variables evaluated in this study was approximately three and a half times as fast as the best rate with rocking-type agitation.
OCKING-type and shaking-type agitation are generally used by laboratories for high pressure hydrogenations to furnish the turbulence necessary t o mix the liquid with the gas or suspended solid. Results of an investigation by this laboratory on rocking-type agitation in glass model autoclaves were discussed recently (2). Although it is assumed that the agitation data obtained under atmospheric conditions are not directly transferable t o high pressure reactions, it is believed that they will be indicative of the agitation conditions under pressure. Since little information concerning the type of agitation produced by a horizontal back-and-forth shaking motion could be found in the literature ( I ) , this investigation was undertaken to establish a quantitative relation between the turbulence thus produced and hydrogenation efficiency, as well as to furnish a comparison between this type of agitation with the more commonly used rocking-type agitation. To compare the relative efficiencies of the two types of agitation, a study was made of the following factors affecting agitation and the related turbulence of mixing of the liquid with the gas in the shaking-type equipment: rate of shaking and length of stroke of the shaker mechanism, and length-diameter ratio and size of the autoclave. The factors independent of equipment design, such as free space in the reactor, viscosity, and surface tension of liquid involved, were not investigated because their effects on rocking-type agitation had already been studied ( 2 ) and are assumed to be the same for the shaking-type agitation. The qualitative effects of shaker mechanism and autoclave design were f i s t observed visually; glass model reactors, similar in design to steel autoclaves, were mounted on the shaker mechanism and half filled with water containing a small amount of 100mesh charcoal. The visual observations indicated that the effectiveness of agitation was increased as the speed and stroke length were increased. Since these observations were subject t o inaccuracies a t the higher shaking speeds, actual hydrogenations were performed over the necessary stroke and speed ranges to establish quantitatively the conditions under which optimum agitation was obtained. The rate of hydrogenation of pure nitrobenzene in acetic acid solution over palladium-on-carbon catalyst was used to measure quantitatively the effects of the rate of shaking and the stroke 1
Deceased.
length on agi:ation. This reaction proceeds rapidly on a palladium catalyst surface a t a uniform rate to the complete reduction of the nitro group a t room temperature and atmospheric pressure. A predetermined excess of catalyst was used to preclude the rate of chemical reaction as the controlling factor. Thus, the rate of hydrogenation was dependent on the rate of hydrogen transfer from the gas phase to the catalyst surface, which, in turn, was dependent on the turbulence of the gas-liquid mixing and the effectiveness of catalyst dispersion. VISUAL EXPERIMENTS
The shaker mechanism consisted of the reactor holder mounted on a set of rollers which were, in turn, guided by a double track. The reactor was oscillated in a back-and-forth motion along the cylindrical axis of the reactor in a horizontal plane by means of an adjustable crank arrangement through which the stroke lengths could be varied from 3 to 12 inches. A variable-speed motor provided uniform shaking rates between 10 and 200 strokes per minute. The reactors were strai h t cylindrical glass tubes of various dimensions with flat ends ?Table I). Tube 6, which was fitted with a glass thermowell, was used in the study of all variables except reactor size and length-diameter ratio.
TABLEI. REACTOR SPECIFICATIONS Tube NO. 1
Inside Dimensions, Mm. Length Diam.
Nominal Ratio, Total Vol., Length/Diam. M1.
2 3 4 5 6
7 8
I n a typical experiment water was charged into the tube to half its volume, and a small amount of 100-mesh charcoal, to simulate the catalyst, was introduced as an aid for observing the extent of dispersion. The shaking rate was then increased from 0 to 200 strokes per minute, the limit of the apparatus. The effects of this change in rate on the agitation were observed visually for each stroke length. HYDROGENATION EXPERIMENTS
Tube 6 was connected to the calibrated hydrogen reservoir and used for measuring the effect of the rate of shaking and stroke length on the rate of hydrogenation (Figure 1). The reagents and catalyst (nitrobenzene, glacial acetic acid, and 5% palladium-oncarbon) were the same as those used for the rocking-type agitation so that comparable data would be obtained ( 2 ) . The tube was charged with 645 ml. of glacial acetic acid, 5.00 grams of nitrobenzene, and 0.250 gram of catalyst. The apparatus (Figure 1) was evacuated three times, hydrogen being introduced after each evacuation. Shaking was started over a predetermined stroke length at a set rate, and the hydrogen maintained a t constant pressure (atmospheric) by admitting dibutyl phthalate to the reservoir. Hydrogen was absorbed a t a uniform rate until90 to 95% of the theoretical volume of hydrogen required to reduce the nitro group had been used, then slowed down, and finally stopped a t approximately 100% of the theoretical amount. When a complete rate curve was desired, hydrogenation was continued a t the same rate of shaking until the nitro group was entirely reduced. However, since the hydrogenation rate was constant up t o 90% absorption for any one set of conditions, a series
1683
INDUSTRIAL AND ENGINEERING CHEMISTRY
1684
served over the longer stroke lengths from 8 to 12 inches. As the shaking rate u as further increased, turbulence apparently became more violent and was still increasing as the shaking limit of the apparatus was approached. A11 evidence of the gas-enveloping swirl a t the ends of the reactor had gradually disappeared as thc shaking rate increased. The liquid was broken into fine drops or a spray; very little remained in the liquid layer as it waq hurled from one end of the reactor to the other with the sudden reversal of the reactor’s motion. The visual observations indicated that, bevond the transition calm, turtmlence increased as the rate of shaking increased, within t’he limits of the apparatus] but this was not substantiated by the hydrogenation experiment’s. The hydrogenation rate increased as the shaking rate increased until it reached a maximum and then gradually decreased for any further increases in skiakirig speed (Figure 3). Thus, the eye could not follow the turbulence changes at high shaking speeds. Apparently gas-enveloping action is the key to thc improved hydrogenation rates. T h e n this factor is removed, turbulence, alt’hough apparently more violent., does not furnish the best gas-liquid mixing conditions necessary for optimum hydrogenation rates.
VACUUM
,
I
LEKGTII AND SPEED OF STROKE
Figure 1. Scheniaiic Drawing of Apparatus for Measuring Rate of Hydrogenation
of hydrogenation rates foi several shaking speeds over the same stroke length was obtained from the one experiment by shaking at a predetermined speed for exactly 1 minute. Readings tyerfi taken, the shaking speed waq readjusted, arid the next dctcxi niination obtained. RATE OF
Vol. 41, No. 8
As the stroke was increased from 3 to 12 inches, the shaking spred a t which the gas-envelopment effect appeared t o be maximum decreased and was, therpforc., inversely proportional to the lmgth of stroke (Figure 2 ) . The rate of shaking necessary to produce the maximum hydrogmation rate decie.nscd as the length of stroke was increased and
sn & K IG~ 50
Visual experiments shoved that a t extrenielj slon ,haking rates there was little or no turbulence, either within the liquid or between the liquid and the gas. The charcoal remained on the bottom of the reactor, arid the liquid level rose and fell about 0.5 inch with respect to the horizontal a t the reactor’s ends as the motion was reversed a t the cnd of each halfstroke. As the rate of shaking was slowly increased, turbulence began within the liquid, evidenced by the uniform dispersal of the charcoal. There was still very little gas-liquid mixing. As the shaking rate was further increased, a pronounced gas-liquid turbulence or nlixing appeared. This wa5 similar to the type of turbulence obtained by rocking-type agitation and was caused by the sudden stopping of the liquid flow a t the termination of each half-stroke. I t can best be described as a breaking over of the mave crest of liquid in the end of the reactor, during kvhich gas was enveloped and niixed with the body of the liquid. This swirling effect was observed to start a t 18 to 30 strokes per minute, depending upon the length of stroke. The shaking speed at R-hich the gas envelopment started and was maximum was inversely proportional to the length of the stroke (Figure 2). However, as the rate of shaking was increased over the shorter stroke lengths of 3, 4, and 6 inches beyond the point where the optimum sriirling effect was observed, turbulence disappeared and the catalyst was concentrated in the center of the reactor and scattled out. A similar transition calm was not ob-
w
s-
p W
?9 30 0
z f
20
b W 5
IO LENGTH
Figure 2.
F’iguiw 3.
OF S T R O K E , INCHES
Effect of Length of Stroke on Shaking Kate
RATE OF SHAKING, STROKES/MIN. Rffect of Shaking on Hydrogenation Rate over Several Stroke Lengths
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1949
1685
Figure 4. Effect of Stroke Length on Rate of Shaking for Maximum Hydrogenation
24
Ol
20
0
I
400
I
800
I
I
I
1200 1600 2000 REACTOR VOLUME,ML.
I
I
2400
2800
J
Figure 6.
Maximum Rate of Hydrogenation i n Reactors of Different Sizes
Figure 7.
Comparison of Rocking-Type and ShakingType Agitation
16
.-• LENGTH OF STROKE- INCHES
Figure 5. Effect of Stroke Length on Maximum R a t e of Hydrogenation
was, therefore, also inversely proportional to the length of the stroke (Figure 4). For example, with a 12-inch stroke the maximum hydrogenation rate occurred a t 75 strokes per minute as compared to 200 strokes per minute for a 4-inch stroke. The maximum rate of hydrogenation increased at an accelerated rate as the stroke length was increased from 4 to 12 inches (Figure 5). The maximum hydrogenation rate over a 12-inch stroke was approximately 50y0faster than that obtained over a 4-inch stroke. REACTOR DIMENSIONS
The optimum hydrogenation rate increased and was directly proportional to volume for the reactors with a length-diameter ratio of 8 to 1. However, for reactors with 6-1 and 13-1 lengthdiameter ratios, the initial increases in the optimum hydrogenation rate were relativelyslarge as the total reactor volume was increased from 100 to 800 ml. Further increases in the reactor volumes showed relatively small increases in the optimum hydrogenation rate (Figure 6). For these determinations the reactorfree space was proportionately the same. For each experiment the reactor was filled to half its total volume with the solution to be hydrogenated. COMPARISON OF AGITATION TYPES
Since the hydrogenation conditions for the shakingitype agitation studies as well as the chemical reagents were identical to the conditions and reagents used in the studies on rocking-type agitation (2), a direct comparison of the relative merits of the two types can be drawn. The optimum hydrogenation rate occurred over the longest stroke (12 inches) obtainable on the apparatus a t the surprisingly slow shaking rate of 75 strokes per minute. This rate was approximately three and a half times as fast (Figure 7) as the optimum hydrogenation rate for rocking motion a t a 90" angle and 47 oscillations per minute ( 2 ) . CONCLUSIONS
The major reactor design factors influencing agitation are rate of shaking, stroke length, and reactor volume in any given lengthdiameter ratio.
TIME .MINUTES
The degree of agitation, measured as the rate of hydrogenation of nitrobenzene, is increased approximately fivefold as the rate of shaking is increased from 35 to 75 strokes per minute. The optimum hydrogenation rate is obtained a t 75 strokes per minute. Increasing the stroke length increases the degree of agitation by 50% ever the range studied Since the rate of shaking t o produce optimum agitation conditions varies as the stroke length is changed, the correct combination must be predetermined in order to obtain maximum hydrogenation rate. Since the rate of chemical reaction rather than gas diffusion is the controlling factor with most of the substances which are difficult to hydrogenate, a close study of the types of reactions to be carried out should be made to determine whether increased turbulence of agitation would have any practical value. ACKNOWLEDGMENT
Bcknowledgment is accorded John Ott of this laboratory for the construction of the apparatus. LITERATURE CITED
(1) Bjorknian, A , , TeL. Tid., 76, No. 10, 2 (1946). (2)
Hoffmann, A. N., Montgomery, J. B., and Moore, J. K . , IND. ENG.CHEM.,40, 1708-11 (1948).
RECEIVED June 22, 1948. Presented before the Delaware Section of t h e ANERICANCHEMICAL SOCIETYa t the Delaware chemical symposium, Newark, Del., January 15, 1949.
