I
J. R. SNYDER', P. F. HAGERTY, and M. C. MOLSTAD University of Pennsylvania, Philadelphia, Pa.
Operation and Performance of Bench Scale Reactors A dashing reactor has definite advantages over a shaken or rocker reactor . . . b b
high rate of gas absorption high useful volume
operation and performance of bench scale reactors are important in the development of new chemical processes, Chemical reactions that offer industrial promise are frequently studied rather thoroughly in bench scale reactors, to explore the useful ranges of pressures, temperatures, and ratios of reactants, or to evaluate catalysts. A large number of industrial reactions involve dispersion of gases in liquids or in liquids containing a solid catalyst. The rate of these reactions depends upon either chemical or physical kinetic processes. When a physical rate process is controlling, the rate of the reaction depends to a great extent on the degree of agitation. Bench scale reactors now in common use differ principally in the methods by which agitation is applied. Consequently, the design of a bench scale reactor in the investigation of a chemical reaction becomes important. Equally important is the proper specification of operating variables.' Four different types of bench scale reactors have been studied and compared. The reactors either shake or rock the entire reaction vessel, or rotate or dash an impeller within the reaction vessel. Two chemical reactions, hydrogenation and oxidation with pure oxygen, were selected for study because a chemical kinetic process was not the controlling mechanism. The effect of the methods of agitation in the four reactors was measured by recording the rate of consumption of the gaseous reactant. The measured rate per unit volume of liquid per unit partial pressure of the gaseous reactant was chosen as the basis for comparison. The hydrogenation reaction was the reduction of nitrobenzene to aniline in an acetic acid solvent and in the presence of a solid catalyst, 5% palladium on charcoal. This reaction was used by Hoffmann, Montgomery, and Moore (5, 6) for the development of rocking and shaking reactors and by Hershberg and others (4)for a rotary reactor. These workers studied the effect of agitation in detail but did not study two T H E
x
1 Present address, Armstrong Cork Co., Lancaster, Pa.
important variables: temperature and catalyst concentration. The oxidation reaction chosen as a development tool in this study had been employed by Cooper, Fernstrom, and Miller (7) for studies on a vaned-disk rotating impeller. More recently Karow, Bartholomew, and Sfat (7) extended this work to the flat-bladed rotating impeller and reported that a sulfite oxidation reaction was suitable for fermentation process scale-up studies, The reaction is the oxidation of an aqueous sodium sulfite solution in the presence of a cupric ion catalyst.
Theory The theory presented serves as the basis for interpretation of the rate data recorded in this study. I t justifies the assumption that the rates of the two chemical reactions selected serve as a performance index by which the several bench scale reactors can be compared. Hydrogenation Reaction. The rate of a chemical reaction catalyzed by a solid depends on the concentration of the reactants at the surface of the catalyst. For the three-phase reaction
the rate of the chemical reaction may be described by a kinetic equation
RT = svrdv
=
to assume that the solid and gaseous phases are always distributed uniformly throughout the liquid phase. Then rr does not depend on the position of dV within the reaction vessel and is a constant with respect to the spatial coordinates. Uniform distribution of the phases is assumed in the discussion which follows, so that
R, = TrV
(2)
where i, is the average value of r,. The diffusional mechanisms by which the gaseous reactant is transported from the gas-phase to the liquid-solid interface a t the catalyst surface in a threehphase reaction can be described by means of several characteristic steps: 1. Diffusion through the gas film between the gaseous phase and gas-liquid interface. 2. Diffusion through the liquid film adjacent to the gas-liquid interface. 3. Diffusion through the liquid bulk located between the liquid film at the gasliquid interface and the liquid film at the liquid-solid interface. 4. Diffusion through the liquid film adjacent to the liquid-solid interface.
To describe the mechanism analytically, the following equations are written, each corresponding consecutively to a diffusional step described above.
Rd
f v f ( C A s , C B S , cus)dV (1)
= ? d v = kGav(P.4 = kLav(CAi
- p~i)v
= X- La m
(CAI
R, = differential rate of chemical reaction
for the whole reactor, gram moles per hour rr = local differential rate of reaction within a differential volume of liquid, dV, gram-moles/(liter)/ (hr.1 V = volume of liquid phase in reactor, liters C A ~C, B ~cup , = liquid phase concentrations of compounds A, B, and U a t solid-liquid interface of the catalyst, gram-moles per liter The functional dependence of r, on the several concentrations a t the solidliquid interface is often complex. Moreover, interfacial concentrations frequently are significantly different from the bulkconcentration because of the presence of diffusional processes. I n order to integrate Equation 1, it is necessary
C A I ) ~
- CA2)V
(5)
-
(6)
-
E
where
(3) (4)
= kh,pw(CAz
CAS)^
where
Rd
differential rate of a diffusional process for the whole reactor, gram moles per hour ?'d = differential rate of a diffusional process averaged over the liquid volume within the reactor, grammoles/(liter) (hr.) ko = mass transfer coefficient for the gas film, gram-moles/(hr.) (sq. cm.) (atm.) av = interfacial area at gas-liquid interface per unit liquid volume, sq. cm. per liter PA, P A ; = partial pressure of compound A in gas phase and at gas-liquid interface, respectively, atm. k L = mass transfer coefficient for liquid film at as-li uid interface, liter/ (sq. c m j (hr? =
VOL. 49, NO. 4
APRIL 1957
689
liquid phase concentrations of component A at gas-liquid interface, at liquid film adjacent to gas-liquid interface, at liquid film adjacent to liquid-solid interface, and at liquid-solid interface, respectively, gram-mole per liter eddy diffusivity in bulk liquid, liter/(cm.) (hr.) average distance in bulk liquid between the two liquid films, cm. average cross-sectional area through which mass transfer occurs in bulk liquid per unit volume of liquid, sq. cm. per liter mass transfer coefficient for liquid film at liquid-solid interface, liter/ (sq. cm.) (hr.) surface area of catalyst particles per gram of catalyst, sq. cm. per gram catalyst bulk density, grams per liter of solution
C A ~ C, A I , CAZ, C A ~=
E
=
xL
=
a,
=
k; = a,
=
pw =
Sometimes the operating conditions under which a reaction is accomplished can be selected to eliminate the effect of one or more diffusional processes. The occurrence of one of the above situations greatly simplifies the analysis of the reaction kinetics. Oxidation Reaction. The oxidation reaction occurs in a liquid phase containing a homogeneous catalyst and can be considered a rapid irreversible reaction between a dissolved gas and a liquid reactant. The theory of simultaneous diffusion and chemical reaction is discussed in detail by Sherwood and Pigford ( 9 ) . The rate in terms of the over-all gas-phase mass transfer coefficient becomes
Rd Assuming that Henry’s law, p = Hc, describes the equilibrium concentration between the gaseous and liquid phases, the rate can be expressed in terms of an over-all mass transfer coefficient. Rd
= ?dY = KGUv(PA - p2,)v
(7)
where
=
?dV = K G U ~ ( P-A 0 ) V
(9)
where
1 1 KGav kGav
1 +
G v
and kL
= liquid film mass transfer co-
efficient, liter/(sq. cm.) (hr.) liquid film mass transfer coefficient in absence of chemical reaction, liter/(sq. cm.) (hr.1 = liquid phase concentrations of CB, CAI component B in bulk liquid and of component A at gasliquid interface, gram moles per liter DA, D B = liquid phase diffusivities of components A and B , sq. cm. per hour
when the partial pressure of the gaseous reactant equals 1 atm. Because the gaseous reactants are pure gases, the gas film is no longer present and the gas-phase mass transfer coefficient becomes infinite. For the hydrogenation reaction
the second term in the parentheses is very small compared to the other terms and can be neglected. Thus, the overall coefficient of the reaction, KGaV, may be controlled by the liquid film at the gas-liquid interface, by the liquid film at the liquid-solid interface, or by both. The effect of the catalyst concentration, pw, on the coefficient, KGav, becomes apparent from Equation l l . By increasing the catalyst concentration the contribution of the liquid film at the liquid-solid interface is minimized in the expression. It is, of course, necessary to assume that the solid catalyst is uniformly distributed throughout the liquid phase as the value of p w is increased. Then Equation 11 reduces to
=
PA8 = H C A ~ Rate equations similar to Equation 6 can be written for the diffusion of the other liquid reactants and products between the liquid bulk and the solidliquid interface. The stoichiometry of the chemical reaction imposes restrictions on these rates:
Rd
= 8 d V = k;a,p,(cAz =
k;’Uwp,[CBz
- C.4,)V -
CB8)Y
(8)
where
kl, kl‘, kl“
= mass transfer coefficients of liquid film at liquid-solid interface for compounds A: B, and U, respectively, liter/(sq. cm.)(hr.)
In writing Equation 8, one assumes as a simplification that the individual liquid film mass transfer coefficients are independent of one another. In the discussion which follows, it is assumed that the magnitude of the individual mass transfer coefficients and the bulk concentrations are such that cBa and cUa are positive or zero as c A s goes to zero. When there is a dynamic balance between the diffusional processes and the chemical process-Le., a quasi-steadystate condition within a bench scale reactor-Equations 2 and 7 can be equated. Theequality is valid only when accumulation or depletion terms can be neglected in the differential material balance for each chemical species. Frequently one of the kinetic processes proceeds so rapidly that its effect on over-all rate process can be neglected in comparison with the other process.
690
Experimental Conditions Common to Both Reactions. Both of the chemical reactions selected for the evaluation of bench scale reactors proceed very rapidly, so that diffusional processes can be presumed to be controlling. Thus, PT,is nearly equal to zero. Parasitic side reactions were insignificant in the hydrogenation and the oxidation, Moreover, the technique of measuring the rate of gas consumption prevented accumulation or depletion of the gaseous reactant within the reaction vessel. Therefore, the rate of gas consumption of the gaseous reactant equals the rate of the chemical reaction. Furthermore, when pure gases are used as gaseous reactants and the vapor pressure ofthesolvent is relatively small, the partial pressure of the gaseous reactant equals the total pressure. The gaseous reactant and solid catalyst are assumed to be distributed uniformly through the liquid phase in the reaction vessels. In view of all these conditions, the magnitude of the over-all coefficient, KGu~, can be calculated by the expression
Thus the over-all mass transfer coefficient equals the absorption rate of the chemical reaction per unit volume of solution
INDUSTRIAL AND ENGINEERING CHEMISTRY
For the oxidation reaction, Equation 9 reduces to a relation of the same form as Equation 12. The rate of reaction is controlled by the liquid film at the gasliquid interface. Thus when a chemical reaction is controlled by diffusional processes, the measurement of KGav, the rate of gas consumption per unit liquid volume per unit pressure of the gaseous reactant, provides a means of recording the effect of agitation in bench scale reactors. Equations 9, 11, and 12 demonstrate that KGav is a function of the interfacial contact areas and several mass transfer coefficients. I n turn, the mass transfer coefficients depend on the scale and intensity of turbulence within the fluid phases. The coefficient K G u ~is also a function of the distribution and flow of the dispersed phases. All of the phenomena mentioned above depend upon the kind and amount of the agitation supplied to the reaction vessel in a very complex manner. Temperature, viscosity of the liquid phase, and the magnitude of the driving forces are also important in determining the performance of bench scale reactors. However, these factors depend principally upon the reaction system itself rather than upon the “degree of agitation” or the method of providing agitation to the reactor. In comparing the performance of several bench scale reactors, the quantities should be held constant, as has been done in this study. One of the primary functions of a bench scale reactor is to optimize the degree of agitation. Therefore, it is
BENCH SCALE REACTORS
Figure 1.
4,
'
Figure 2.
Adams-Voorhees shaker
Rocking reactor with variable-speed drive
desirable to measure this degree of agitation, which is taken to mean the collective contribution of all the diffusional phenomena described above to the magnitude of the absorption coefficient, Kaav. Thus, the magcitude of KoaV for the hydrogenation and the oxidation reactions considered here becomes a suitable basis or performance index for the comparison of bench scale reactors of various designs.
sections of heavy-walled borosilicate glass tubing closed a t the ends by Type 304 stainless steel flanges. Teflon-covered rubber gaskets were used in all glass-to-metal joints. An Adams-Voorhees shaker ( 3 ) shown in Figure 1 was studied (standard model, Parr Instrument Co., Moline,
Procedure and Apparatus
Volume, Reactor Cc. Adams-Voorhees shaker 450
The progress of both hydrogenation and oxidation reactions was determined by measuring the rate of consumption of the gaseous reactant. A constant flow of pure gas (hydrogen or oxygen) was supplied to the reactor by means of a capillary flowmeter ( 2 ) . The supply gas rate was set at a value greater than the anticipated maximum absorption rate, and the gas not consumed by the contents of the reactor was vented to a wet-test meter. The rate of vented gas was calculated from elapsed time measurements and wet-test meter readings, The rate of gas consumption was calculated by difference. The solid and liquid reactants were premixed under an atmosphere of nitrogen and charged to the reactor, which had previously been purged with nitrogen. The temperature of the reactor charge was adjusted, and before agitation was started, the reactor was purged with about 10 volumes of feed gas for each volume of gas space in the reactor. The temperature of the reactor contents was maintained constant during the course of a run by cooling the external walls of the rcactor. The reactor contents were discharged under nitrogen. An analytical determination of the reaction product was made immediately, if desired. The dimensions of all the reaction vessels are recorded in Table I. Most of the vessels were constructed from
Table I.
111.). The shaker agitated the contents
of a stoppered borosilicate glass bottle by swinging it through a 30' arc a t 231 cycles per minute. The effect of placing a rigid baffle in the bottle was investigated (Figure 1, left). A platform shaker was designed to oscillate with an amplitude of 1.5 inches
Dimensions of Reactors I.D., Length, Inches (2.5)
Inches (6)
Platform shaker Rocker
500 995
2.125
Dashed impeller
515
2.125
8.93
1055
3.00
8.93
3930
5
Rotated impeller
Figure 3.
Dashing impeller reactor
17
12
Figure 4.
Remarks Borosilicate glass bottle (I-inch mouth) Erlenmeyer flask Heavy-walled borosilicate glass tubing Heavy-walled borosilicate glass tubing Heavy-walled borosilicate glass tubing Borosilicate glass jar
Rotating impeller reactor
VOL. 49, NO. 4
APRIL 1957
691
Impeller speeds from 500 to 1000 r.p.m. were explored. Several design modifications are possible, as shown in Figure 5. However, data for only the design shown on the left of Figure 5 are reported here [available from Autoclave Engineers, Erie, Pa., under the name Dispersimax (U. S. Patent pending) 1,
-9
TUBE
Results and Discussion LE DISK
UB
'SIX
-
BLADE IMPELLER 20 : 6 I 4 DESIGN
/
Figure 5. Three design variations of g a s pumping impeller
and a t a frequency of 317 cycles per minute. The shaker motion was horizontal and very nearly rectilinear. A 500-cc. Erlenmeyer flask clamped to the shaker platform served as the reaction vessel. The 1-liter rocking reactor (Figure 2) was designed according to the recommendations of Hoffmann, Montgomery, and Moore (5). The reaction vessel was clamped to a rack which was forced to oscillate through an arc of 90' at a frequency of 30 to 60 cycles per minute. The dashing impeller reactor was the product of -4utoclave Engineers, Inc., Erie, Pa. The Magne Dash drive [developed and patented by the Standard Oil Co. (Indiana) J was a solenoid with tM'o coils, each 2.5 inches in diameter by 2.5 inches in length, which were alternately actuated by means of a timer. The dashing frequency could be varied over the range of 0 to 350 qcles per minute. The impeller assembly consisted of an iron core and several impellers attached rigidly to opposite ends of a shaft. The iron core was positioned on the axis of the coils by springs. The impellers were disks or cones with apex directed upward. They had diameters equal to one half the diameter of the vessel and were spaced a t intervals of 1.3 to 1.4 inches. The lowest impeller was positioned so that it could move within 1/4 inch of the reactor bottom. Four impellers were used for both the 0.3-liter and the 1-liter reactors. Two dashing impeller reactors are pictured in Figure 3 : the 1-liter reactor vessel and the drive on the right; the 1-gallon reactor vessel on the left. Experimental data are reported here only for the 0.5-liter and the 1-liter vessels. Similar data for the 1 gallon reactor can be found elsewhere ( 7 7). The Magne Dash drive was modified when used on vessels larger than 0.5 liter by changing the magnetic properties of the iron core, incorporating additional magnetic shielding around the
692
solenoid coils to reduce leakage flux, changing the dimensions of the core. and/or altering the position of the core on the axis of the coils. No dynamic force measurements were made for the dashing impeller. However, static forces caused by the solenoid were measured as a function of the distance the core penetrated a single coil. From these measurements, the average static force exerted by the solenoid over the length of the stroke could be calculated. The average static force on the downstroke differed from that on the upstroke because the core was not positioned symmetrically with respect to the two coils. Table I1 reports the magnitudes of the stroke, the maximum static forceingrams. and theaveragestatic force over the length of the stroke for the several configurations actually used. A rotating impeller reactor of new design was developed, which was capable of providing internal gas recirculation without relying on vortex motion (Figure 4). The gas-pumping impeller consisted of three main parts: a flat-bladed radial-flow turbine, a cover plate, and a hollow shaft or draft tube. The circulating gas entered ports in the rotating hollow shaft. These ports were located above the liquid level. The gas passed downward through the shaft and then radially outward beneath the cover plate. The liquid which was circulated within the reactor vessel by the turbine entrained the gas a t a location adjacent to the periphery of the cover plate. Finally, the gas rose to the surface of the liquid as a well dispersed cloud of bubbles and was recirculated. In the work, the turbine was constructed with six blades having turbinediameter to blade-length to blade-width ratios of 20 to 6 to 4. The blades were mounted radially on a hub with a diameter equal to 0.6 turbine diameter. The turbine diameter was half that of the vessel. The cover plate was mounted over the turbine and had a diameter of either 0.45 or 0.60 turbine diameter.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Hydrogenation Reaction. This reaction involves the combination of nitrobenzene and hydrogen to produce aniline and water. A glacial acetic acid solvent and a 5% palladium-on-charcoal catalyst (all passing 200 mesh) provide a suitable reaction environment. The progress of this reaction may be follox ed by measuring the hydrogen absorption rate. The rate was found to be a function of the reactor agitation only if several other factors were controlled within specified limits. REACTION TEMPERATURE. At 82' F., the hydrogen absorption rate increased about lY0 for each 1' P. rise in temperature. This temperature coefficient corresponds closely to that usually experienced with diffusional operations. All reaction rate data presented in this study were obtained at 82' F. or corrected to this temperature. CATALYST COKCEXTRATION. This factor was studied in several different types of reactors (Figure 6 ) . For catalyst concentrations of less than 1 gram per liter of solution, a small change in the amount of catalyst present greatly af-
--a-
-
-
0
I
I
4
8
I
12
16
Figure 6. Effect of catalyst concentration on hydrogen absorption co-
efficient Initial concentration of nitrobenzene 1 3.0 grams per liter Conversion 5070 0 0.5-liter dashing reactor. Filling ratio 0.40. Frequency 330fminute 1 -gallon rotating impeller reactor. Filling ratio 0.50. Speed 980 r.p.m. V 1-liter rocking reactor. Filling ratio 0.50. Frequency 47fminute
BENCH SCALE REACTORS
a
c
fects the hydrogen absorption rate. In this concentration range the last term of Equation 11 is dominant. The maximum useful concentration so far as the reaction rate is concerned was found to be about 10 grams per liter of solution. I n this work, all rate data are compared on the basis of 4.0 grams of catalyst per liter of solution. When the reaction rate data of Figure 6 were plotted as the reciprocal absorption coefficient, 1 / K ~ a as ~ , a function of reciprocal catalyst bulk density, 1/ p w , straight lines could be placed through the data points, thereby confirming the interpretation of Equation 11. In Figure 6 the curve for the 0.5-liter dashing reactor could be approaching an asymptote, although a maximum value has been indicated. The presence o the maximum can be attributed to nonuniform distribution of catalyst a t these high catalyst concentrations. NITROBENZENE AND ANILINE CONCENTRATIONS. An initial concentration of 13 grams of nitrobenzene per liter of solvent was employed. The reaction rate was not greatly affected by changes in the nitrobenzene concentration. For example, when the aniline concentration was held constant at 5.0 grams per liter, the reaction rate for nitrobenzene concentrations of 6.5 and 13.0 grams per liter differed by only 7%. The presence of aniline had an accelerating effect on the reaction coefficient, KGtav. This fact is contrary to the assumption stated above, that the reaction is completely controlled by diffusional processes. The raie of hydrogen absorption at the end of an experiment was found to be as much as 40Ye greater than the initial rate in some cases. I n light of these findings, it became necessary to obtain the reaction rate at fixed concentrations of nitrobenzene and aniline to eliminate the liquid phase concentration of aniline as a variable. All hydrogen absorption coefficients reported in this study were obtained at 5001, conversion of the nitrobenzene initially present. Hence, only one hydrogen absorption rate per experiment was useful for comparison purposes. MISCELLANEOUS EFFECTS. The effect of pressure was not investigated. All runs were made at atmospheric pressure. Redistilled nitrobenzene was used ; however, technical grade nitrobenzene gkve identical results. No effect of catalyst aging was noted, either on the shelf or during an experiment. Check runs made a t 21-day intervals gave reaction rates differing by a maximum of 2y0. The conversion of nitrobenzene based on net hydrogen absorption agreed within 1% of the aniline yield as determined by bromination (8, 70), for both partial (about 5oY0) and complete conversions of the nitrobenzene initially charged.
Oxidation Reaction. An aqueous 1.ON sodium sulfite solution containing 0.001 mole of copper sulfate catalyst per liter was oxidized by a stream of pure oxygen gas. The sulfite concentration was found to have a negligible effect on the oxygen absorption rate for solutions having concentrations between 0.2 and 1.ON. At 82’ F. the temperature coefficient was slightly greater than a 1% increase in rate for each 1’ F. temperature rise. All rates reported in this work are for a reaction temperature of 82’ F. All runs were made at atmospheric pressure.
Operation a n d Performance of Reactors The designs of the rocking and shaking reactors are conventional, but the designs of the dashing and rotating impeller were selected after considerable experimental effort ( 7 7 ) . The selections were made on a basis of visual observations and measured hydrogenation reaction coefficients. After the physical design of the impellers was considered adequate, performance characteristics of the reactors were determined over the complete range of operating variables. For a reactor having fixed geometrical ratios, only two important operating conditions are independently variable : the filling ratio (volume of liquid per volume of reactor) and the speed of agitation. Effect of Filling Ratio a n d Agitation Speed. SHAKING REACTOR.With this reactor, the only operating variable studied was the filling ratio. The generally recommended filling ratio of about 30% was found to be superior to any larger value. One run a t a lower filling (20%) failed to produce any improvement. The performance of the Adams-Voorhees pendulum shaker was almost identical to that of the platform shaker. The addition of a rigid baffle
increased the oxygen absorption rate 22% and the hydrogen absorption rate
50%. ROCKING REACTOR.For this reactor, rocking frequency and filling ratio influence the gas absorption coefficients. Figure 7 shows the absorption coefficient as a function of the rocking rate a t parametric values of the filling ratio. The oxygen absorption coefficient is not very sensitive to the rocking frequency in the range studied, while the hydrogen absorption coefficient is almost directly proportional to it, except at high filling ratios. The most appropriate filling ratio and rocking frequency for a given chemical reaction depend upon the required performance. If a high absorption rate per unit volume of liquid is desired, a low filling ratio (25y0)and a high rocking frequency (55 cycles per minute) are optimum. If the effect of agitation on reaction rate is to be determined, the data can best be obtained by employing a 25% filling, accompanied by a rocking frequency in the range of 30 to 60 cycles per minute. If a fixed quantity of gas is to be absorbed into this reactor in a minimum time, a 45% filling and rocking frequency of 50 cycles per minute produce the desired results. The necessity for equipping rocking reactors with a variable-speed drive is obvious. DASHINGREACTORS.The effect of dashing frequency and filling ratio on the oxygen and hydrogen absorption coefficients is shown in Figure 8. Under all conditions studied, increasing the dashing frequency increases the gas absorption coefficient. The effect of agitation on a given chemical reaction may be studied conveniently by varying the dashing frequency a t a Soy0 filling VOLUME LIQUID
‘‘0°’ VOLUME REACTOR
VOLUME LIQUID L1oo’ VOLUME REACTOR
I
0
20 ROCKING RATE,
, 40 OYCLESlMlNUTE
I
60
0
20
40
60
ROCKING R A T E , C Y C L E S / M I N U T E
Figure 7. Oxygen and hydrogen absorption coefficients as function of rocking frequency at parametric filling ratio 1-liter reactor. Volume 995 CC. Rocking angle 90’ Leff. Catalyst concentration 0.001 M CuSO4. Initial concentration of NazSOs 0.96N Right. Solid catalyst concentration 4.0 grams per liter. Initial concentration of nitrobenzene 13.0 grams per liter. Conversion 50% VOL. 49, NO. 4
APRIL 1957
693
0
100 200 300 DASHING R A T E , CYCLES / M I N U T E
400
Figure 8. Effect of dashing frequency and filling ratio on absorption coefficients l e f f . Catalyst concentration 0.001 M CuSO4. Initial concentration of NaASOs 0.96N. M a x i mum static thrust 2200 grams, nominal stroke 2.0 inches Vol. liquid/vol. reactor. 0 0.48. A 0.72 Cenfer. Solid catalyst concentration 4.0 grams per liter. Initial concentration of nitrobenzene 13.0 grams per liter. Conversion 50%. M a x i mum static thrust 460 grams, nominal stroke 1.5 inches Vol. liquid/vol. reactor. 0.60 Righf. Solid catalyst concentration 4.0 grams per liter. Initial concentration of nitrobenzene 13.0 grams per liter. Conversion 50%. Maximum static thrust 460 grams (0.5-liter reactor), 930 grams (1 -liter reactor). Nominal stroke 1.5 inches (0.5-liter reactor), 2.0 inches ( 1 -liter reactor). Dashing r a t e 330 cycles per minute
ratio. The useful reactor filling ranged from 40 to 65y0 (Figure 8. right), with a maximum gas absorption rate per unit volume of liquid at about 50Yc;. If a given quantity of gas is to be absorbed into a reactor in a minimum time, the filling should be about 55 to 60$&’,. ROTATING IMPELLER REACTOR.The impeller speed and filling ratio have an effect on the gas absorption coefficient, as shown in Figure 9; where the absorption coefficient is plotted as a function of the filling ratio. The scale at the top is the ratio of the unstirred liquid depth to the inside diameter of the vessel, Z / D . The lower right-hand portions of the curves are extrapolated to the liquid level a t which gas recycle ceases. For a constant filling ratio, the absorption coefficient increases almost in proportion to the square of the impeller speed, if the speed is somewhat above the minimum required for pumping gas. At a constant impeller speed, the gas absorption rate per unit volume of liquid in the reactor is almost inverselv proportional to the filling ratio. The data do not permit a definite conclusion regarding the effect of variation in the cover plate diameter. For the rotating impeller reactor the maximum gas absorption rate per unit volume of liquid, as well as per unit reactor volume, occurs at the highest impeller speed (980 r.p.m.) and the smallest filling (0.40) emploved in this study. As a general rule, the effect of agitation on a given chemical reaction may be investigated to best advantage by varying the impeller speed while maintaining the liquid level equal to one vessel diameter.
694
0
100
200
I
400
300
DASHING RATE, CYCLES/MINUTE
Comparison of Reactors. The performances of the various reactors are compared by both hydrogen and oxygen absorption coefficients in Table 111. The operating conditions assigned for each reactor are those which might be recommended to an average user of the equipment. The relative standings of the reactors demonstrate how “positive” agitation as supplied by dashing and rotating impellers is considerably more effective than “indirect” agitation as obtained by rocking or shaking. The ordinary shaker is not more than half as effective as the best reactors. LVith the exception of the rotating impeller reactor, all reactors retain the same relarive posirions on both the hydrogenation and the oxidation performance scales. From Table I11 the ratio of the hydro-
0.4
I
I
0.5
0.8 0.7 VOLUME Liauio VOLUME REACTOR
gen absorption coefficient to the oxygen absorption coefficient for the several reactors was calculated and arranged in order of ascending values for the reactors, as follows : (Rotating)
