Agitation in Experimental Rockin =TypeAutoclaves A. N. HOFFXAXY, J. B. 210NTGOMERY, AND J. I measuring t h e rate of hydrogenation of nitrobenzene. The major reactor design factors influencing agitation were the speed of rocking and t h e angle of rocking. Over the ranges studied, t h e first of these factors caused a fourfold change in the rate of hydrogenation of nitrobenzene and t h e second caused a twofold change. The viscositj and surface tension of t h e liquid being agitated were also shown to affect t h e agitation. Because these variables are fixed by t h e material being agitated, rocking autoclaves should be designed to prov:de flexibility of operation, so t h a t t h e various reactions encountered in t h e laboratorj can be carried out a t optimum rates.
V I S U A L EXPERIJIEKTS
The rocking mechanism consisted of a reactor-tube supporting arm mounted on a centered bearing and oscillated in simple harmonic motion with respect to the horizont,al by an adjustable crank arrangement' t,hrough Jvhich the total angle of rocking could be varied between 30 and 120 "-i.e., b e t m e n 15 arid 60 above and belon- the horizontal, respectively. A variable-speed motor provided rocking rates between 9 and 76 oscillations per minute. The reactors were straight, cylindrical glass tubes of various diniensiom (see Table I). The ends were flat. Tube 6, which was fitted with a glass thermocouple ~ d l was , used in the study of all the variables except reactor size and ratio of length to diameter. I n a typical experiment, water was charged into the t,ube, a small amount of 100-mesh charcoal introduced as an aid for observation, and rocking started. The rate of rocking wa5 increased from 9 to 76 oscillations per minute, and the effect of this change in rate on the agitation was observed visually.
TABLE I. REACTOR QPECIFICATIOSS ~~b~
LTEIOGGH rocking-type autoclaves are widely used in the laboratory for bringing liquids into contact n-ith gases and suspended solids, no information relating equipment design with agitation-i.e., the turbulence of mixing of the liquid with a gas or suspended solid as a result of the rocking motion-could be found in the literature. As such information vi-as needed in order to design a rocking-type autoclave which would provide good agitation under widely varying conditions of operation, a study was made of the following factors affecting agitation in this type of equipment:
1. Rocker mechanism a. Rate of rocking b. Angle of rocking 2. Autoclave design a. Rat,io of lengt'h to diameter b. Size 3. Factors independent of equipment a. Free space, or fraction of autoclave filled with liquid b. Physical properties of liquid (1) Viscosity (2) Surface tension
Xo.
Inyide Dimensions, hlm. __ Diameter 330 57
Length 495 154 205 481 479 726
84 19.5 25 61 37
56
Nomirial Ratio Length/Diarnetdr 6 6 8 8 8 13
13
Total Voluine, I11 886 2820 42 100 1320 610 1870
TO VACUUM PUMP
HY E O G E N INTAKE
REACTOR
The qualitative eiTects of these variables on agitation were observed visually, using a glass prototype of the standard autoclave mounted on a rocking mechanism. Slow-motion pictures of t,he rocker aided the visual study of the agitation. The rate of hydrogenation of nitrobenzene in acetic acid solution \\-as used as a yardstick t,o measure quantit,alively the effects of the rate of rocliing and the angle of rocking on ihe agitation. This reaction was chosen because it proceeds rapidly on a palladium catalyst surface a t room temperature and atmospheric pressure. Sufficient catalyst wns used to preclude the rate of chemical reaction as t'he controlling factor. Thus, the rate of hydrogenation was dependent on the rate of hydrogen transfer from the ga.s phase to the catalyst surface, which, in turn, was dependent on the turbulence of the gas-liquid mixing and the effectivenrss of the catalyst dispersion.
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Figure 1. Schematic Drawing of .ipparatus for Measuring Hate of Hj drogenation
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September 1948
1709
INDUSTRIAL AND ENGINEERING CHEMISTRY
For the experiments on the effects of the liquid properties on agitation, viscosities of I, 10, and 100 centipoises were obtained by using water and solutions of 62 and 87% glycerol in water, respectively. Surface tensions between 7 3 (that of water) and 26 dynes per em. were obtained by the addition of Aerosol OT (dioctyl sodium sulfosuccinate) to water. HYDROGENATION EXPERIMENTS
Tube 5 was connected to a calibrated hydrogen reservoir and used to measure the effect of speed and angle of rocking on rate of hydrogenation (see Figure 1). The nitrobenzene was Merck's pure grade, further purified by fractionation through a 20-plate column packed with glass helices, under 50 mm. of mercury pressure. The catalyst, 5% palladium on charcoal, was manufactured by Baker and Company, Newark, N. J. Reagent grade glacial acetic acid was used for the solvent. 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 and hydrogen was introduced. Rocking was started a t a selected angle and speed, and the hydrogen was maintained at. constant pressure (atmospheric) by admitting dibutyl phthalate t o the hydrogen reservoir. The rate of hydrogenation was calculated from the rate of gas absorption. This absorption took place a t a uniform rate until 90 to 957, of the theoretical volume of hydrogen required to reduce the nitrobenzene group had been used, then slowed down, and finally o the theoretical. stopped a t approximately 1 0 0 ~of RATE OF OSCILLATION
At extremely slow rates of oscillation there was little or no turbulence, either within the liquid or between the liquid and the gas. The liquid flowed evenly down the incline caused by the rocking, cushioned itself against the end of the tube, and reversed its even flow as the reactor motion changed direction. The powdered charcoal which was added as a visual aid settled out on the bottom of the tube. As the rate of rocking was increased slightly, turbulence began to occur within the liquid, as evidenced by the uniform dispersion of the charcoal. IIowever, there was still very little gas-liquid mixing. At a somewhat higher rate of rocking a very pronounced gas-liquid turbulence or mixing began to appear. This very effective mixing was caused by the sudden stoppine: of the liquid flow at the termination of each half oscillation, and can best be described as a breaking over of the wave crest of the liquid in the end of the reactor, during which gas was enveloped a n d mixed with the body of the liquid. As the rate of rocking was further increased, this pronounced gas-liquid turbulence (or gas enveloping effect) reached a maximum and gradually decreased until, at very fast rocking rates, the
100 ANGLE OF ROCKING. DEGREES
Figure 3.
120
Rate of Rocking us. Angle of Rocking
breaking over of the wave crest disappeared and the liquid surface remained unbroken except for a few ripples. At these fast rates of rocking beyond which the gas envelopment had ceased, the inertia of the liquid prevented its flow from one end of the reactor t o the other, and the liquid surface remained rssentially parallel t o the long axis of the reactor tube. As the turbulence decreased, the suspended charcoal first concentrated in the center of the reactor, and later, a t the fastest rates of rocking, settled out on the bottom of the reactor. Visual estimates of the oscillation rates a t which gas envelopment started, reached a maximum, and stopped for the different variables studied are shown in Figures 3, 5, 6, and 7.
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0
20
Figure 4.
40 60 80 100 ANGLE OF ROCKING. DEGREES
Kate of Hydrogenation us. Angle of Rocking
The visual observations on the effects of the speed of rocking on the agitation were substantiated by the rate measurements. As the speed of rocking increased, the rate of hydrogenation increased, reached a maximum, and then decreased (Figure 2). For a given rocking angle, the rocking speed at which maximum turbulence was observed visually coincided with the speed a t which the maximum rate of hydrogenation was obtained. For a 30" angle of rocking, the maximum hydrogenation rate a t 35 oscillations per minute was approximately four times the rate a t either 15 or 60 oscillations per minute. ROCKING ANGLE
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Figure 2.
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40 ROCKING
, OSCILLATIONS/MINUTE
Rate of Hydrogenation Rocking
US.
Rate of
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As the angle of rocking was increased from 30 O to 120 ', the pronounced gas-liquid turbulence, or gas envelopment effect, appeared a t slower rates of rocking, and reached a maximum and disappeared at higher rates of rocking (see Figure 3 ) . For example, a t a 120" angle of rocking, maximum turbulence was observed at 52 oscillations per minute, as compared with 35 oscillations per minute a t 30 '. The greater the angle of rocking, the faster was t,he rate of liquid flow froni one end of the reactor t o the other during each half oscillation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1710
Vol. 40, No. 9
volume. ;\laxiiiiuiii turbulrncc 1% a? obscrvcd at liquid voluines of 40 to 60yo,with decreasing tuibulence obseived as the liquid volume a a 5 i i i c i ~ above 50Tc Changing the liquid volume had no rffert on thi> l a t e of locking at which gas cnvelopment appealed (see Figure ti). Honeever, for liquid voluines oi 10 to 50yo, soniemhat faster i a t t r 01 rocking n pie ioquired t o obtain iiiaairriuiri gaq envel opnieiit EFFECT OF PlIYSlC4L PROPEKTILS
T ISCOSITY. An increase in viscosity had little rffect on the rate of rocking a t which the proiiouncrd turbulence or gas envelopment was visually obsri vcd t o start, become maximum, and stop However, t h degree ol tuibulence decieasd as the viscosity incieased. oh ils the suiface tension of the liquid was increased, the gas envelopment was visually observed t o start, reach a maximum, and stop a t higher iales of locking (see Figure 7 ) i.e , a liquid w t h a gieater surface tenLioii requiied a faster rat(. o f I oclnng t o maintain niaximum tu1 bulmrtx.
Figure 5 , Rate of Rocking us, Ratio of Length to U i a m e k r of Reactor
The hydrogenation rate measurenients substantiated the visual observabioiis of the effect cf rocking angle on the agitation. The measured rate of hydrogenation (at the oscillation rat>c selected from Figure 3 as maximum for a given rocking angle) increased with an incrcase in the angle of rocking aiid was directly proportional to the angle as shown in Figure 4. For example, the measured hydrogenation rate obtained at 90" (at 47 cycles per minute) w a p approximately double that a t 30" (at 35 cycles per minute). REACTOR DIMENSIONS
As the length-diameter rat,io of the reactor was decreased, the visually observed speed of rocking required t o produce niaxiniuin turbulence had to be increased. For a given length-diameter ratio, the size of the reactors could be changed without affecting the speeds of rocking at which the gas envelopment started, w-as maximum, and stopped. However, the degree of this turbulcnce for a given length-diameter ratio decreased as the size of the reactor was reduced, and was negligible in the tube of smal.lest capacity studied ( X o , 3).
COXCLU SXONS
The iiiajor reactor design factors influencing agitation \TCIY> ( I ) the speed of rocking, through n-liich the degree of agitation (measured as a rate of hydrogtsnation of nitrobenzene) could increased fourfold, aiid ( 2 ) ihc angle of rocking, through xvhich the degree of agitation could be increased twofold over the range studied. The viscosit,y and surface tension of the liquid being agitated ~ e r ac l ~ oshown to affect the agitation. As these variables arc fixed by the material heing agitated, rocking autoclaves should be designed to provide sufficient flexibility of operation, so that, the various reactioiis eiicountcred in the laboratory can h c i cza,rric:d out at, optimum rates. ACKZTOW LEDGMEYT
Acknowledgment is accorded Stephen Killiams of this lahoratory for the construction of the apparatus. RECEIVED September 2 3 , 1R47. Presented before the Division of Industrial and Engineering Chemistry a t the 112th Meeting of the AMERICAV CHEMIC A L SOCIETY, N F V YOSIC,N. Y .
FREE SPACE
Visual observations of agitation were made in which the volunie of liquid was va.ried between 15 and 757, of the total reactor
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Figure 6.
VOLUME VOLUME
iiauic
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REACTOR
Rate o f Rocking TS. Liquid Volume i n Reactor
lor-nruBE
020
Figure 7.
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L I Q U I D V0O L5 = 5 7 5 U L
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SURFACE T E N S ION, DY NE5 /C M .
80
Rate of Rocking us. Surface Tension of Liquid
