Mixing of liquids in Chemical Processing
development
J. HENRY RUSHTON’ lllinois lnsfifufe o f Technology, Technology Center, Chicago 7 6 , 111.
R
E C E N T advances in the field of mixing and agitation have taken place in three main areas. First, progress has been made on a better understanding of the mixing process. Particularly for the mixing of fluids, both liquids and gases, it has been found advantageous to apply principles of fluid mechanics to account for rates of material transfer and thus for rates of mixing. Mixing of fluids, either with or without solids present, is thought to be dependent upon natural or forced convection of parts to be intermingled. Forced convection is brought about by fluid motion, which may derive from a fluid stream or by pulses or vibrations, all of which result in turbulence or disturbances which cause material to move in new paths. Accordingly, much attention has been given to determine how turbulence can be generated and utilized and how forced convection or forced material transfer can be imposed upon a system by means of turbulent flow motion. It is essentially because of this viewpoint that the term “agitation” is less frequently used than heretofore to describe the operation of mixing, since ultimately the object of mixing or agitating is to provide homogeneity and material or heat transfer, regardless of how the agitation is produced. Along with the understanding of the mechanism of material transfer and fluid motion i t becomes possible to relate such motion to a given chemical reaction rate. Thus, this approach, through the mechanics of turbulence and transfer, gives great promise for a basis whereby predictions can be made for the mixing requirements for a particular chemical process. The second area where significant advances have been made is that involving the performance characteristics of rotating equipment used in mixing operations, from the standpoint of mechanics rather than chemistry. The user of equipment supplied by at least one equipment manufacturer (Mixing Equipment Co.) now has available, either in the literature (24) or from the manufacturer, data on power required to rotate various types of mixing impellers in different environments; quantity of liquid displaced at various impeller speeds; hydrostatic heads available; and the differences in flow characteristics between different impeller types. Such information is essential for proper pilot plant and large scale proceqs engineering design. Third, significant developments have been made in the m* chinery available for mixing of liquids. Sound design of shafts, speed reducers, impeller elements, and assemblies is the result of research on critical speeds of overhung shafts such as are commonly used on mixing equipment. The unique requirements for speed reduction equipment for mixers has led to greatly improved mechanical designs offering protection against the unsteady power
* Also, Mixing Equipment Co.,
leads inherent in most rotating mixers, and for wider latitude in speed control. I n addition to the three areas just described, a great deal of work has been done on the mixing of various chemical systems, on methods for calculating the degree of completion of mixes used in series flow, and on many problems of specialized character. These will be mentioned later, but most attention will be focused in the following sections on results of such fundamental nature as to provide the basis for a more comprehensive technology of mixing than has heretofore been available.
136 Mt. Read Blvd., Rochester, N. Y.
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Fluid Motion and Mixing The mechanics of moving fluid streams and the means by whirh they are moved are the fundamental problems of mixing. Mixing is brought about by turbulence and momentum transfer. Turbulence involves fluid motion of a rotary eddy current type and takes place in all directions. Turbulence is defined in terms of intensity and scale; these are functions of fluid properties and boundary conditions. Intensity is the instantaneous velocity of the small elements of eddy or vibratory flow, and the length of path of such flow is called the scale. Eddy viscosity is proportional to the product of intensity and scale, and has the dimensions of kinematic viscosity (square feet per second). Turbulence is brought about by the interaction of momentum, fluid properties, and surfaces, and it is believed that mixing is accomplished by transfer of momentum to and from small masses, thus causing them to move in various directions. The viscosity of a fluid is a basic factor affecting intensity, scale, and the propagation and spread of turbulence. Viscosity is that property of a fluid which represents the internal force that resists deformation of the fluid during flow. Such forces are actually the shear stresses which occur along a plane between two adjacent elements of flow. When a fluid moves at low velocity, such that the resistance to its flow is directly proportional to its velocity, it is said to be flowing under viscous conditions. This occurs at low Reynolds numbers (Lu/v) when the flow is both steady and uniform. Under these conditions there is no deformation of flow owing to fluid shear stresses and thus no material transfer by means of momentum transfer since there is no forced convection in a direction obher than that of the stream flow. Thus, any mixing which might take place is due to molecular diffusion and is independent of the energy imposed to produce the viscous flow. The work necessary to overcome the viscous force which resists the deformation is dissipated as heat through viscous shear and is not transformed into mechanical forces acting in directions different from that of the flow. These viscous flows are stable in that a disturbance imposed on the flow
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INDUSTRIAL AND ENGINEERING CHEMISTRY
will not be propagated but will be damped out. If the velocity of flow (or the Reynolds number) for a viscous flow is increased steadily, a condition will be reached a t which the flow will become unstable to a disturbance. The disturbance may originate from any velocity discontinuity, surface roughness, or from separation. Undulations of the stream flows will result and eddies will develop; they will spread and change the entire flow pattern. Velocities fluctuate in all directions, and momentum is transferred rapidly in various directions; the result is fluid turbulence and mixing. The basic conceDt of mixing caused by turbulence and velocity fluctuBAFFLES atiotis was first proposed by Prandtl and the scale and intensity of turbulence and factors affecting them have been developed during recent years. A c o n c i s e s t a t e m e n t of modern theory of turbuSIDE VIEW lence has been given by Rouse (91). I n a mixing tank such as illustrated by Figure 1, fluid is put in motion .by a rotating impeller. Disturbances arise and turbulence develops from three sources: (1) the high velocity streams from the impeller flowing BOTTOM VIEW alongside low velocity fluid Figure I. Fluid Flow Pattern in the body of the liquid for Turbines with Baffles (velocity discontinuity) resulting in fluid shear; (2) flow along the walls of the container and the impeller blades resulting in surface friction (boundary layer); and (3) flow across baffles or other projections resulting in rapid change in direction of flow (form separation). By far the most important source of turbulence in most liquid mixing operations is that from source 1, namely, fluid velocity discontinuity. The other two sources are of relatively minor importance, although there is a widely held misconception that the “shear” of an impeller rotating in a liquid produces most of the agitation in a mixing tank. It is clear from high speed motion pictures and from trace photographic studies ( 9 7 ) that most of the turbulence in a mixing tank is produced by the high velocity fluid setting up fluid shear forces with adjacent lower velocity streams. Many studies on the behavior of jet flow ( 1 , 9, 8, 2 9 ) further confirm these views.
Blending of liquids and Mixing of Gases Data are available for mixing by the action of streams of fluid pumped through jets into larger fluid bodies-for example, air jets in air (I, 99), gasoline in gasoline (16),water solutions in water ( 5 ) ,and a general survey of the studies relating momentum, material, and heat transfer (16). The spread of a three-dimensional jet (circular cross section) has been evaluated for air, and the principles and values applied to the problem of blending of gasoline by means of the widely used side entering propeller mixer. I t has been shown (see Table I ) that propeller mixing is more efficient in the use of power than mixing by circulating pumps ( 1 3 ) . Theory and data indicate that the kinetic energy of fluid flow in a stream is transferred by fluid shear and momentum to adjacent fluid. The cross flow so produced causes mixing and entrainment of the surrounding fluid into the jet stream. It has been shown that the maximum entrainment per unit of power input for a desired length of circular jet travel occurs when the
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jet diameter is ‘/l&h of the length. For example, if a jet is to entrain for 34 feet, a 2-foot diameter jet will give the highest ratio of liquid entrained per unit of power expended. The amount of entrainment can be estimated by Qd
= p.23
(5)- 1 ] Q
where Q. is the amount of liquid entrained at a distance, 2 , from the jet opening of D Odiameter. Q is the rate of liquid flow from the jet. The equation holds for values of X / D Ogreater than 6. Data available for scale-up of pure jet flow mixing in large tanks have been published (15). Marine-type propellers are frequently used to blend liquids in large tanks and their effectiveness for this operation can be explained by the theory and pcrformnnce of jet flow.
Similitude Application of these basic concepts of turbulence to mixing is recent, but other more widely known fluid mechanics principles have also been reconsidered and used to better advantage in mixing. The principles of similitude are used in fluid mechanics with considerable value; they can also be used in one special area of fluid mechanics, namely, mixing. Perhaps thc greatest utility of similitude relationships is to relate performance on one size of operation to that at another size. The problem of scale-up of pilot chemical operations to plant scale cannot besolvedsuccrssfully unless the principles of similitude are taken into account.
Table 1. Relative Performance for Different Mixing Methods for Gasoline Blending in a 100-Foot Diameter Tank (73) Method of Mixing Side entering propeller, 22-in. diameter Jet, &in. diameter Jet, 2-in.diameter Circulating ,pump and line with &in. diameter jet
Volume Induced b y Propeller or Jet Flow per U n i t of Power. Ratio 1.00
0.28 0.58
0.50
For a reaction to take place a t an interface it is necessary that reactants and products move t o and from the interface. Xccordingly, the reaction rate will be dependent on diffusion unless fluid motion causes forced convection of the reactants. Mixers are often applied t o produce forced convection and t o achieve a rapid rate of reaction, and it is evident that the whole fluid motion regime will be an important variable. If a desired reaction rate is achieved in a pilot-sized operation, it will be necessary to duplicate the same fluid motion pattern and the same motion velocities a t the interface to reproduce the same reaction rate in a larger unit. Thus, it is necessary to reproduce similar dynamic conditions in a large and a small mixing tank if the rates of reaction are to be similar or equal. Dynamic similarity requires that systems be geometrically similar, that the directions of flow at corresponding points be the same, and that the ratio of forces a t corresponding points be equal. Fluid motion-both general flow pattern and turbulence-is dependent on the physical properties of the fluid, the shape, position, and speed of rotation of the impeller, and on the shape of container and its fit3tings. A11 physical boundary conditions can usually be made to be geometrically similar for various sizes of mixing tank operations, but t o achieve equal flow directions and forces i t will also be necessary to consider the fluid properties, particularly the viscosity and specific weight (or density). The requirements for dynamic similarity of fluid motion are based on Newton’s law, ordinarily expressed as Force = mass X acceleration
(2)
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The force is the force of inertia necessary t o provide acceleration t o a mass. Since dynamic similarity is dependent upon equal force ratios at corresponding points, it follows that the basic requirement for dynamic similarity is Equation 2. The forces opposing the inertia force may arise from such fluid properties as viscosity, specific weight, surface tension, or elasticity. If an inertia force is balanced by a viscous force alone, then the flow motion resulting is said t o be controlled by viscosity and is called a viscous flow. By any of a number of techniques it can be shown that for this case, the ratio of the inertia force to the viscous force will serve as a dimensionless parameter and is a requirement or a criterion for dynamic similarity. This ratio is called the Reynolds number and may be written conveniently as
(3) where L is a significant length, u a velocity, and v the kinematic viscosity. When specific weight of a fluid exerts control by balancing all of the inertia force, then the ratio of inertia force to specific weight force (gravity, g) can be used as a requirement for similarity. This ratio is called the Froude number
(4) These and other dimensionless groups have been discussed with reference to mixing elsewhere (24) and detailed analysis will not be repeated here. It is quite possible that both viscous and specific weight forces might affect fluid motion in a mixing operation, and then to achieve dynamic similarity on two different sizes of equipment it would be necessary that the Reynolds numbers be the same a t any scale and that the Froude numbers also be equal a t SIDE VIEW any scale. The specific weight forces (or Froude numbers) affect fluid motion when a liquid surface is not horizontal. A standing wave or a vortex caused by swirling liquid is indicative of the effect. It can be shown (23) that for such a case
n
L, =
(5) where L, is the scale or ratio of size of large unit to small unit, and vI is the kinematic BOTTOM VIEW viscosity ratio of liquid in Figure 2. Flow Pattern withthe large unit to the small out Baffles for Any Shape unit. It is evident from this Impeller that when both viscositv and specific weight (or gravity) play a part in fluid motion, it is not possible t o reproduce that fluid motion with the same liquid on any other scale, since Equation 5 requires that the viscosity must be larger in a larger tank t o get the same motion. A direct result of these considerations of similitude in fluid mechanics applied to mixing is that a pilot plant operation involving a mixer must be operated if possible in such a way that v,2/a
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not more than one fluid property force is predominant. These are the only conditions under which the fluid mechanism can he duplicated. If this can be achieved it will be possible t o scale-up and to predict large scale power requirements and results. If this cannot be achieved, then scale-up cannot be made and large scale results cannot be predicted, unless of course more extensive pilot planting be done a t different sizes to evaluate the interplay of each property force. It has been known for many years that it is usually desirable to use mixers in such a way as t o eliminate appreciable vortexing. Figure 2, for example, illustrates a mixing impeller turning in a liquid without baffles. The vortex prevents rapid mixing (see later section) since very little power can be applied, and it precludes the use of reaction rate data for direct scale-up. When a vortex is present both the viscous and gravity forces are important and Equation 5 states the requirements of dynamic similarity (28) and shows that if the same liquid is used for model and full scale the fluid motion cannot be the same. Hence, if scale-up is to be achieved, vortexes must be eliminated. Vortex formation can be eliminated Ipy the use of baffles (either close to or distant from the impeller) or by proper off-centering of propellers entering either from the side or the top (6, 24).
Performance Characteristics of Mixing Impellers Power necessary to rotate mixing impellers and thereby cause flow and turbulence, the quantity and velocity of fluid discharged by these impellers, and the head developed by mixing impellers have been correlated for many shapes, fluids, and tank shapes and fittings. The power necessary t o rotate an impeller is not only a function of its shape size and speed, but of its position with reference to tank bottom, side, and liquid level, and also to the physical properties of the fluid. There have been many publications relating speed, power, and fluid properties, but the most comprehensive coverage of such material has recently been published (24). Correlations are based on similitude principles. A wide range of types and sizes of mixingimpellers has been studied and can be summarized by reference to Figure 3 and Table 11. Figure 3 is typical for the operating characteristics of a flatblade turbine mixing impeller, shown in Figure 5, as manufactured by Mixing Equipment Co. Curve ABCD results for operation when baffles are placed a t or near the tank wall as illustrated in Figure 1. Curve ABE results for operation when no baffles are present, and swirl and vortex will exist a t high Reynolds numbers. The following general equations are applicable for these curves and relate power, P, speed of rotation, N , impeller diameter, D , liquid viscosity, p, liquid density, p , and the gravity constant, g. For curve AB, viscous flow
For curve CD, fully developed turbulent flow
No equation is recommended for curve BE since the slope of the line is not constant and little practical use can be made of such an equation. K is a constant depending on the impeller shape, size, the number of baffles, and in fact all variables other than those shown in the equation. The value of K is determined from the ordinate of Figure 3. For the case of impellers rotating on the center line in a vertical cylindrical vessel with a flat bottom, a liquid depth equal to tank diameter, impeller diameter one third of tank diameter, and impeller one diameter above tank bottom, the values of K are given in Table I1 for different types of mixing impellers. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
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position described is a common one, widely used in industry. The values of K have been determined for a large range of sizes. For side-entering propellers properly positioned and for topentering propellers without baffles but off-centered to prevent vortexing ( 2 4 ) , the value of K is the same as for the baffled position and the values. of Table I1 apply. Values of K for some other impeller positions and variables are also available ( 2 4 ) .
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and the forces present at corresponding points can be made equal to those at the small scale or larger if desired by increasing the impeller speed without destroying kinematic similarity. This makes it possible not only to achieve dynamic similarity at a change of scale, but to achieve equal flow velocities a t a given point, such as a t a reaction interface, also. From these considerations it is possible to relate a reaction rate coefficient in the form of a dimensionless parameter to the Reynolds number. Figure 4 illustrates the relations which havr been shown to exist for heat transfer correlations in mixing ( 7 , 10, 25) whereby coefficients can be predicted for a change in any of the variables, and 80 is useful for scale-up design. Correlations of the forms of Figure 4 are also useful for mass transfer correlations as indicated. The slope, z,of the line of Figure 4 is called the mixing slope and it can be used together with the relatione of Figure 3 to give the fundamental equations relating a rate transfer coefficient, X-, to the diameter and speed of rotation and power requirement of a mixer in geometrically similar systems ($2) h = KD2~-l,vx
Figure 3.
or
and if h is to be equal in two sized systems 1and 2, then
Power characteristics of a Mixing Impeller
curve ABCD, no vortex present,
or curve BE, vortex
present,
+=
(8)
-
+ A p N W
(a) (D+)" pNaD' and
Table II.
Values of K for Equations 6 and 7
Inipeller with 4 Baffles af Tank Wall, Each 10% Tank Diameter Propeller, square pitch, 3 blades Propeller, pitch of two, 3 blades Turbine, six flat blades Turbine. six curved blades
Equation 6. Viscous Range 41.0 43.5 71.0 70.0
Equation 7 , Turbulent Range 0.32 1.00
6.30
If the mixing slope, z,can be found in one sized system, then the speed of rotation and power required t o duplicate the reaction rate in another sized system can be determined. Thie over-all
4.80
Scale-up Data are available for mixing operations involving heat transfer ( 7 , IO), mass transfer (17, 18), and gas-liquid contacting ( 3 , d ) wherein the foregoing principles and the elimination of vortexing allow them to be directly useful for scale-up. From these and more recent data a general technique has been suggested for correlating mixing data from bench scale or pilot plant studies (22). When mixing is accomplished with baffles or their equivalent so that no vortex is present and dynamic similarity can be achieved at various scales, the power characteristics of the impellers are as shown in Figure 3. If an impeller such as a flat-blade turbine is used, the correlation between power, speed, size, and liquid density and viscosity is as shown in curve A D , Figure 3. ilt high Reynolds numbers (Reynolds number for mixing is DZN/V or D2Np/p), the slope of the line is zero, showing that the inertia forces (or the power number) control. Under such conditions the power number, 4, is constant for a wide range of Reynolds numbers. The flow pattern remains constant for constant power numbers: hence, if the Reynolds number is varied and the kinematic viscosity is held constant, the eddy viscosity (numerator of the Reynolds number) will change. Thus by changing impeller speed or scala of equipment, the power number and flow pattern remain conshant but the velocity of eddy motion will be changed. With such operating characteristics the flow pattern present in a small scale operation can be duplicated a t larger scale with geometric and kinematic similarity,
Log Figure 4.
P D*N~
Correlation of Rate Coefficient, Fluid Properties, and Fluid Motion For heat transfer, $ =
(y )(4)-'
For desorption or other mars transfer operations,
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INDUSTRIAL AND ENGINEERING CHEMISTPY
technique for scale-up is useful only when geometrically similar systems are used and when the power characteristics of the mixer system are as shown in curve CD of Figure 3. For this reason it is recommended that pilot plant experimentation be done under the conditions of line C D of Figure 3, and then relations like Figure 4 can be used directly for scale-up via Equations 8, 9, and 10. Heretofore there was a widely held belief t h a t scale-up could be made on the basis of “equal power per unit volume,” or some arbitrary variation of it. This is not a satisfactory basis for scaleup. It is possible, however, to use the above analysis with the mixing slope, z, and Equations 7 and 9 to relate power per unit volume, the scale-up, and the mixing slope (99). It has been shown that for operations which can be correlated (via., Figure 4), if 5 has a value of 0.75, then for a dimensionally similar scale-up the power per unit volume will be constant. This is the only condition where this is true, If z is greater than 0.75, then less power is required per unit volume for a larger sized unit, whereas values of x less than 0.75 require more power per unit volume on scale-up. It follows that for low values of x, it may prove to be more economical to operate a number of small units rather than one very large unit. Conversely, for systems giving high values of r it may be more economical t o use a large unit rather than a number of small units.
Heat and Mass Transfer Data have been published recently for heat transfer to helical coils for a number of chemicals in mixing tanks (10). Data are also available for heat transfer to vertical tubes in a mixing tank (11, 96). Vertical tubes can be used rather than helical coils for heat transfer during mixing. The vertical tubes provide baffling and thus promote desirable flow characteristics. There are a number of situations where it is desirable to eliminate helical coils with baffles and in such cases the vertical tubes are advantageous. Figure 4 shows the general technique used to correlate data on heat and mass transfer (7, 10, 17, 18). The ordinate in such a plot consists of dimensionless groups, such as the Nusselt and Prandtl for heat transfer and the Colburn and Schmidt groups for mass transfer. In addition, boundary ratios (e.g., tank diameter to impeller diameter) and fluid property ratios (e.g., heat transfer fluid film viscosity to average fluid viscosity) may be added to the ordinate (10). The abscissa is the Reynolds number. Equations are derived from such plots, resulting in the general form
G
K(NR&
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heat transfer data in the literature showing the corresponding powpr requirements are those of Rushton, Lichtman, and Mahony.
Displacement Capacities and Velocities Some data are available for flow from standard Mixing Equipment Co. propellers and flabblade turbines ($6) and more exdata are being obtained. A 6-inch, three-bladed, marineuare-pitch propeller will displace 21.0 cubic feet of water per minute when rotating a t 420 r.p.m. A 6-inch, four flatbladed turbine will displace approximately 7.4 cubic feet of water per minute when rotating a t 100 r.p.m. The flow for geometr rically similar impellers varies directly with the speed and directly with the cube of the impeller diameter. Such data are important for the design of mixing installations for blending and when solids are to be suspended. Further, since power is equal to the product of flow and head if both power and flow data are known, it is possible to compute the head developed by an impeller, and thus obtain the necessary conditions of liquid depth to prevent flashing and cavitation during boiling or crystallizing mixing operations.
Process Applications One of the most important developments in process applications has been the data published on fermentation operations ( 3 , 4 ) . These are the first published results of experimental work in fermentation (gas-liquid mixing) using mixing techniques which have been found to be most effective in large size fermenters. The importance of well-organized pilot plant evaluation of the mixing variables was emphasized inorder tomakelargescaledesign. Performance data for gas-liquid contacting by mixing equipment are available for a varied disk (9) and for a disperser turbine similar to that shown in Figure 5 (16). These data relate power and speed of the mixing impeller together with gas flow rate to the absorption rate coefficient (9) and t o the time of contact (16) between the gas and the liquid. All the data are for operation with baffles as shown in Figure 2. The data have been used successfully for scale-up t o larger, geometrically similar systems.
(11)
The values of x (the mixing slope in the section on scale-up lie between 0.62 and 1 (mostly a t 0.67) for the references cited. The value of the proportionality constant, K , varies for different configurations of heat transfer surface and impeller shape. Also the different experimenters have used different reference surfaces for the sizes (D)in the Nusselt and mass transfer groups and have evaluated different effects of boundary dimensions; accordingly, it is not possible to give comparative values of K based on data now available in the literature. For example, Cummings and West (10) find that their particular coil, jacket, and impeller arrangement gives heat transfer film coefficients appreciably higher than those of Chilton, Drew, and Jebens (7) for a much different arrangement. Values of the constant, K , were 11% higher for the jacket and 16% higher for the coil. The value of K for the vertical tube setup of Rushton, Lichtman, and Mahony (26)appear to be a t least 20% higher than those of Cummings and West based on Cummings and West’s analysis (10). Perhaps a more useful comparison of such data would be that based on power input to the impeller. Uafortunately the only
COURTESY M I X I N O EOUIPMENT C O .
Figure 5.
Flat-Blade Turbine with Stabilizing Ring
The application of mixers and proper mixer technique has recently been made to continuous liquid-liquid extraction. Performance data have been obtained for a compartmented column, shown in Figure 6, in which a component was extracted countercurrently from a light liquid flowing countercurrently upward through a heavier liquid flowing down. The mixing impellers
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Vol. 44, No. 12
liquid film mass transfer coefficient, (moles/hour) (square foot)(moles/cubic foot) L = length, feet N = revolution per second NF~ = Froude number N R= ~ Reynolds number n = an exponent P = power, foot pound/second = exponent of Prandtl group = flow rate &. = entrained flow rate s = exponent of Schmidt group u = velocity, feet/second x = an exponent, also a distance in Equation 1 p = viscosity, pound/foot second or pound/foot hour I, = kinematic viscosity, p / p , equare feet/second $ = a function = power number, Figure 3 p = mass density, pound/cubic foot kL
=
%,
+
Subscript 1 refers to condition 1 2 refers to condition 2 T indicates ratio literature Cited (1)
Albritson, M. L., Dai, 1'. B., Jensen, R. A,, and Rouse, H ,
(2)
Baron, T., and Alexander, L. G., Chem. Eng. Progress, 47, 181
Proc. Am. Soc. Civil Engrs., 74, 1571 (1948). (1951).
Bartholomew, W,H., Karow, E. O., and Sfat, M. R., IND.E N G . CHEM.,42, 1827 (1950). (4) Bartholomew, IT. H., Karow, E. O., Sfat, M. R., and Tilhelm, R. H., Ibzd., 42, 1810 (1950). (5) Binnie, A . >I., Engineering, 153, 503 (1942). (6) Bissell, E. S., Hesse, H. C., Everett, H. J., and Rushton, J. H., (3)
Figure 6.
Continuous Countercurrent Extraction Column Using Turbine Mixers
Left. Light liquid rising mixing impellers not moving Right. Mixing impeller; rotating, light liquid rising, steady operation
Chem. Eng. Progress, 43, 649 (1947).
Chilton, T. H., Drew, T. B., and Jebens, R. H., IND.CNG. CHEM., 36, 508 (1944). (8) Cleeves, V., and Boelter, L. hI. K., Chem. Eng. Progress, 43, 123 (7)
(1947).
Cooper, C. M., Fernstorm, G. A,, and Miller, S. A., IND.EXG. CHEM., 36, 504 (1944). (10) Cumminm. G. H.. and West. A. S.. Ibid.. 42. 2303 (1950). (11) Dunlap, -1.' R., unpublished Ph.D.' thesis, Iilinois Institute of Technology, Chicago, Ill., June 1951. and Piret, E. L., Chem. Eng. Progress, 46, 290 (12) Elridge, J. W., (9)
were of the flat-blade turbine type and were used to create high velocity liquid flows to create liquid shear necessary to form drops of the immiscible liquids and also to recirculate the immiscible mixture during its residence in any one compartment. Separation of the phases occurs at top and bottom of the column, and no separation is allowed between mixing stages. High throughputs can be obtained and over-all efficiencies of 81% have been achieved (20). This use of mixers in extraction differs from another, recently proposed, in which a paddle mixer was used but no baffles were present in the mixing zones and packed sections were placed above and below each mixing section of the tower. Countercurrent flow was maintained and it was deliberately intended that coalescence take place after each stage of mixing of the two immiscible liquids. Performance data for this type are given for two sized columns (28). For flow of reactants through a series of reaction mixing vessels, data and equations are available for prediction of degree of completion of the reaction under various flow and operating conditions (12, 18). These are particularly useful for the case of a firstorder rate reaction, and the data are useful for scale-up since they were taken under baffled conditions ($8). Nomenclature
B
= diffusivity, (moles)(foot)/( hour)(square foot)(moles/cubic
D
= =
foot) impeller diameter Do jet diameter ravitational constant lm coefficient of heat transfer, B.t.u./(hour)(square foot) ( " F.) K = a constant = thermal conductivity, (B.t.u.)(foot)/(hour)(square foot) k ( " F.)
fl
(1950). (13)
Folsom, R. G., and Ferguson, C. K., Trans. Am. Soc.
Necii.
Engrs., 71, 73 (1949). (14) Forstall, W., Jr., and Shapiro, A. H., J . Applied Mechanics, 17, 399 (1950). (15)
Fossett, H., and Prosser, L. E., J . I n s t . Mech. Engrs., 160, S o . 2 , 224, 240, 245 (1949).
Foust, H. C., Mack, D. E., and Rushton, J. H., IXD.ENG. CHEY.,36, 517 (1944). (17) Mack, E. M., and hrarriner, R. E., Chem. Eng. Progress, 45, 545 (16)
(1949).
Mason, J. A., unpublished Chemical Engineering thesis, Illinois Institute of Technology, Chicago, Ill., June 1950. (19) Mason, D. R., and Piret, E. L., IXD.ENG. CHEM.,42, 817 (18)
(1950).
(20)
Oldshue, J. Y . , and Rushton, J. H., Chem. Eng. Progress, 48, 297 (1952).
Rouse, H., "Engineering Hydraulics," New York, John miley & Sons, 1950. (22) Rushton, J. H., Chem. Eng. Progress, 47, 485 (1951). (21)
(23) Ibid., 48, 33, 95 (1952). and Everett, H. J., Ibid., 46, (24) Rushton, J. H., Costich, E. W., 395. 467 (19501. (25) Rushion, J: H.,'Lichtman, R. S., and Mahony, L. H., IND. ENG.CHEM.,40, 1082 (1948). (26) Rushton, J. H., Mack, D. E., and Everett, H. J., Trans. A m , Inst. Chem. Engrs., 42,441 (1946). (27) . . Sachs. J.. unoublished Ph.D. thesis, Illinois Institute of Technology, Chicago, Ill., 1952, ENG. CHEM.,42, 1048 (28) Scheibel, E. G., and Karr, A. E., IXD. (1950). (29) Taylor, J. F., Grimmett, H. L., and Comings, E. W., Chem. Eng. Progress, 47, 175 (1951). RECEIVED for review Xovember 1, 1951. ACCBPTED July 24, 1952. Presented before the XIIth International Congress of Pure and Applied Chemistry, Iiew York, September 1951.
