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I UNIT 0 P E RAT1O N S REV1 E W
CHEMICAL ENGINEERING REVIEWS
I I
II Mixing I
DI..PISG rhe year foilo\sing
tb1r
prr\.i-
ous revie\\. i.??)? startirig \ v i t h Sepwmber
1956, a series of articles have appcared on mixing., and a large number of short
descriptive notes \sere published drscribing particular mechanical featurrs of mixing equipment. Thrse lattcr a x not referred to here becausc they do not add significantly to the basic undcrstanding of mixing nor are t l i r y quantitalive in nature. Furrhermorc: such material is revie\\-ed authoritati\-rly I>>Pierce ( 7 6 ) . Pierce frequently reviejvs eqtii~Jinent used in the process industries and notes the standard and modern t g i r s as \sell as new developments in rquipineiit design. His most recent rrvicvs o i l dry mixing equipment ( 7 8 ) gi\.es data on sizes and capaciries for sonic of thr ne\\ er types of dry misers. General
It is \sell kno\sn that in c:oiiri~~rious Aois sysrems the actions o f feeders and controllers regulating streani f l r ~ \ v and composition result in concentration fiuctuarions in the flolving streams. Such fluctuations in a pipe line persist for very long. runs of pipe as is evidenced in the transport of liquid petroleum products through long pipe lines. It follo\vs that it is impossible to bring about thorough mixing of two components simply by pumping them through a long pipe line, and, therefore, for complete mixing it is necessary to discharge the pipe line into a receiving tank. If the receiving tank is well
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mised by large srale i l o ~ vinotion. ilieir the entire contents can rf.ac.11 coiiiplctr hornogcneity or "perfect" mising. Gutoff' (6) publishcd tlie first ~ n a t l i c matical analysis stro\sirig ho\s c o n c ~ n tration ~luctuationsi n a tank d u r i n g continuous fio\v through the t a n k can he smoorhecl out and minimized \slien :I mechanical miser is placed i n the tank. His analysis and plot sholsinq ho\s fiuctuations can be minirni7ed by good misiIig are significant far beyond thr dirrct example hc has discussvd. ( Tutofl' assumed that p d r c t mixing could br obtained in a tank, and on this basis shoivs h o ~ sthe fluctuations or coricrntraiion in the feed to thc tank change t o much smaller fluctuations in thr ourlrr strcarn of the tank. 'This s i n o o i h i i i ~of tluctuation is a runction o f t h e sizr o f the taiik and the rate offiuid fwd. and it is possihli: to find the sizr of a tank Ivhich \ s o i l l d a l h v a g i \ ~ nfiois \sit11 a sinusoidal concentrdtion fluctuation of qivrri maqnitude
J. HENRY RUSHTON i s professor of chemical engineering, Purdue University, and technical adviser to Mixing Equipment Co., Rochester, N. Y. He studied a t the University o f Pennsylvania (B.S. and Ph.D.). Rushton is past chairman of the ACS Division of industrial and Engineering Chemistry and a member of the Society o f Chemical Industry. He is president of AlChE and the 1952 recipient o f the William H. Walker Award.
INDUSTRIAL AND ENGINEERING CHEMISTRY
streams having a minimum of concentration fluctuations. I t is frequently difficult to furnish highly refined proportioning feeders and controllers to produce desired small concentration fluctuations. Hence, process designers should consider the advantages of mixing tanks used with proportioning feeders and controllers to achieve minimum concentration fluctuations in continuous flow processes. One of the most difficult problems in mixing involves the scale-up of pilot plant data for large scale design. Jordon (9) gives techniques for scale-up of various operations which are pertinent to petroleum refining. The first section of his article deals with mixing. He has gathered together some information now existing in the literature, condensed it, and presented the results in relatively simple form. Essentially all of the present techniques are mentioned or referred to, but there is no critical analysis of the different techniques nor a statement of the very definite limitations which are inherent in the different methods. Care must be exercised in applying the proposed methods, because it has been found that the fluid regime best-suited for one particular mixing operation may not be the same as the fluid regime required to give optimum results in another mixing operation. For example, if a particular ratio of flow to turbulence in a system is found to be best from pilot plant data, then a definite relation can be found for use in scale-up. But this relation does not serve for another process which may have been found on pilot plant scale to be better served by a different ratio of flow to turbulence. These ideas are now well established and have been referred to in previous reviews (22). This comment should serve as a word of caution in applying the relations shown by Jordon. The relations are all very sound but the limitations for their use are not shown. The possibility of using ultrasonic energy for mixing and agitation in connection with mass transfer and emulsifying has been the subject of considerable speculation. Few, if any, published data give quantitative evaluations of the use of ultrasonics for mixing. Thompson and Sutherland (27) show data on the ultrasonic energy required for some extraction operations. They have also run experiments on the same extractions using mechanical mixing by means of a rotating impeller and show the comparison in terms of time and material extracted for 400-kilocycle insonation. For the mechanical mixing the weight of material extracted is a function of time for two different speeds of a 7.5-mm. diameter mixing impeller. Power is not given for the mixing impeller nor is the impeller described sufficiently to esti-
mate the power input. The mixing impeller running at 1242 r.p.m. gave a slightly higher extraction rate than could be achieved a t maximum insonation. With their impeller running a t 675 r.p.m. they achieved lower rates than for the minimum insonation used.
Power and Mixing Patterns Much work has been done in Japan on the power characteristics of mixing impellers. This has resulted in elaboration of the fluid motion and extends the data published six years ago in this country. One of the Japanese articles, by Inoue (8), is a theoretical consideration of the subject and develops the same reasoning used in previous literature. There are no new data and the relations are developed with a minimum of time and space. Two articles have appeared by Aiba who worked with paddle-type mixing impellers operated in small tanks (50 cm. or less in diameter). The first of these ( 2 ) applied radioisotopes to determine the flow patterns of liquids in a mixing tank. A cobalt 60 salt was dissolved in water and the liquid velocities were approximated by a radiation counter technique. All flow patterns were very similar a t the high Reynolds number used in the experiments, and confirmed in general those which have been found by other techniques. The second article by Aiba (3) was a study of the power requirements for turning the paddle impellers in various aqueous solutions of glycerol. The data are of use for viscous mixing applications. Nagata and collaborators published three articles on the power required to turn mixing impellers. The first article (74) deals entirely with rotating paddles in cylindrical tanks. Relations are developed from free- and forced-vortex theory for paddles rotating in cylindrical tanks without baffles. The vortex is related to the size of a cylindrically rotating zone. Liquids of six different viscosities were used in small but different sized tanks. The effect of various paddle sizes was studied and considerable data have been obtained showing the effect of the various impeller sizes, tank diameter, and viscosity, on the power required to rotate the paddles. The data provide an excellent extension of the relatively few data previously available of rotating paddles in nonbaffled tanks. A second article by Nagata and Yokoyama (72) deals with methods for measuring power consumption for rotatingmixing impellers. Power measurements have been made with a variety of measuring techniques. The authors have tried to determine the advantages and limitations of different types of measuring devices and have made a critical analysis
of the methods they used and those used by other experimenters. They built several types of dynamometers and conclude that errors have appeared in the previous literature, some of which can be traced directly to the torque-measuring device. They found that a device measuring the displacement between two rotating disks in the drive mechanism provided their most reliable technique. Their analysis of the whole problem of power measurements is sound, but they have not covered the strain gage type which is now considered in this country to be the most precise, and certainly by far the best, for large size equipment power measurements. Strain gages can be placed around the rotating mixing shaft and provide accurate and instantaneous power measurements. The third article by Nagata and others (73)is almost entirely with flat paddles (both straight and pitched) operating in baffled vessels. Although the paddle agitator is not as popular for mixing as it was 30 years ago, there is still a small fraction of new equipment of this nature being installed. Certainly these many new data from Nagata are useful in extending the present information. The data cover a wide range of Reynolds number conditions; from the very viscous to the fully developed turbulent range. The effect of blade size and shape is thoroughly discussed and paddles of 2, 4, and 6 blades have been used in the work. The data are correlated in a similar fashion to much of the information in the literature and are, therefore, of direct use. One interesting new set of data shows the change in power requirement for a simple paddle as it is started. The power decrease as a function of time from start of rotation is shown to be a function of the speed of final rotation. Data on turbine-type mixing impellers as used in extractive metallurgical operations are presented by Parker and others (77). Graphs are given for power requirements for curved blade turbines, and for flow, as a function of power input. These data are in line with previous generalized data in the literature for the same type of impeller. A “selection chart” based on a separation of mixing requirements of nine separate categories is given. Such a section chart is intended for use in preliminary estimates for power consumption and size of mixers. From a process viewpoint the authors show data on the holding time commonly encountered in a set of mixers staged in series. Estimation of the size of a mixer is always necessary to produce the desired amount of mixing and agitation. There are no easy rules for determining accurately the amount of power required to accomplish a particular mixing operation in a specified length of time and to a specified degree of homogeneity.
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Much more work is needed before requirements for adequate suspension of solid particles in large equipment can b e assured.
One of the basic reasons for the difficulty in attempting to establish simple criteria is the fact that the power per unit volume of material to be mixed depends upon the ratio between diameters of mixing impeller to tank. Therefore, the optimum impeller diameter together bvith the knowledge of the flo\v regime best suited for the particular mixing operation should first be determined before po\ver level can be prescribed with precision. If the desired flow regime (flow rate from the impeller, turbulence, and flohv pattern) is known or can be chosen arbitrarily, then from experience, certain levels of power input are kno\vn to give good mixing results. Serner (25) s1ioa.s a plot which covers a \vide range of mixing conditions, fluid properties, and power input, the approximate operating range for various levels of poiver input to achieve “mild,” “vigorous,” and “normal“ agitations for viscous liquids. These proposed operating ranges are taken from Serner‘s long experience in the field of mixing.
liquid-liquid Mixing I n liquid-liquid extraction, mixing impellers are the most common type of equipment used to produce drops of one phase in another and to increase forced convection mass transfer in the continuous phase. Continuous flow multistage extraction columns, using rotating mixing impellers, are growing in importance. Articles have appeared on this subject continuously for the past six years. The latest of these is by Scheibel (2-1) who gives operating performance for an internally baffled multistage extraction column approximately 1 foot in diameter. Scheibel shoivs that internal baffles are of major importance in reducing the height of an equivalent theoretical transfer stage, and thus, reducing the over-all height of this type extraction column. H e has used mixing impellers of a standard flat blade turbine design. The data are not directly compared with others in the literature but comparisons can be easily made. Three different liquid pairs were studied over a \vide range of flow rates. Countercurrent extraction operations have for many years been carried out in the conventional mixer-settler combination units Lvherein rotating mixing impellers have been used in the mixing sections. Some years ago in the Atomic Energy program, several types of very small laboratory size mixer-settler units
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were developed. Some of these tiny experimental units have been scaled up to larger size for industrial use. One of the mixer-settler units is a boxtype design. The mixing impeller being placed in a square cross section chambcr and the adjacent settling chamber is of rectangular design. ‘This shape of mixer and settler alloivs simple construction for a very large number of units in series. Factors effecting the capacity of such units is a subject of an article by Graef and Foster (5). Data are given for throughputs for the benzene-water system. Optimum arrangements’ of the equipment to produce the highest flow rates are shown. A small mixing unit suitable for mi settler type of operations is the pumpmix device described several years ago. Holmes and Schafer (7) comment on the performance of small pilot plant units of this type after experience of several years with them. They also give operating characteristics and shoiv the results of a number of runs made vvith the familiar methylisobutyl ketone--acetic acid ivater system. Data on this system allo\v comparison of the pump-mix device Lvith other continuous floxv extraction equipment. Maximum performance of above 9 5 s ; approach to equilibrium conditions \vere reported. Vishnevsky (19) reported on the cffectiveness of flat paddles used to stir small laboratory autoclaves. H e used three different liquid pairs and extracted a dye from one liquid into another. The greatest rate of extraction occurred when the ratio betbyeen diameters of impeller to tank was between 0.5 and 0.7. This result agrees lvith recent experimental work in our laboratories showing that large ratios of flow to turbulence seem to he advantageous for most liquidliquid extractions.
Gas-liquid Mixing \Vhen liquid is to be contacted by gas, as in fermentation and hydrogenation reactions, liquid depth is sometimes considerably greater than tank diameter. I n such cases two or three mixing impellers may be placed on the same vertical drive shaft. Data for gas-liquid contacting with multiple flat blade mixing turbines in baffled tanks are given by Rushton, Gallagher, and Oldshue (23). Multiple mixing turbines musr be very carefully placed with respect to each other to achieve equal or better oprration than can be achieved bv a single
INDUSTRIAL AND ENGINEERING CHEMISTRY
impelicr. I n fact? more than one irnpeller on a shaft may result in de in operating results of as much as .507; and in 110 case were the authors able to measure benefits of niore than 25% in the over-all absorption cocfficients ivhich \ v u e mrasurrd. Therefore, the use of multiple mixing turbines in a single shafr is dicrated large1~-by mcchanical construction of the mixer. Thc mechanism of mass transfer a t a gas-liquid in terrace during mixing of liquid hcloiv the interface, \vas the subject of ivork by Kishinevskii and Serebryanskii ( / / I ) , Small lahoratory size equipment \\,as used and the rate of absorption of nitrogen, oxygen, and hydrogen \vas measured in distilled ivater. The coefficient of absorption was related to paddle stirrer speed. Abovt. a particular speed the coefficients of absorption \vert’ independent of speed, but beloiv a certain speed the diffusivity of the gas in the liquid was found to be controlling. There are insufficient data ro take into account the possible effects of the Froude number and for the cffect of entrainment of gas. Only one size svstrm \vas used. The data are in gt-nera1 agreement Ivith previous work in this country reported some years ago. One of the major applications of qasliquid contacting is in fermentation processes. Gaden ( . I ) covcring the Xvhole field of fermentation included a section of equipment used to aerate and mix in large scale operations. 1.arious types of fermenters Lvith thr most modern equipment noiv being used for these 011erations are described and illustrated by diagrams. The information is authoritative although no operatinq details are siveri t-xcept the limits of aeration that are normally handled and the ranqe of poiver normally required. Another application for gas-liquid contacting is the oxidation of asphalt. Conventional techniques for asphalt oxidation have used dircct air sparging ivithout the use of a mechanical mixer. Rescorla and othcrs i20) presentcd data for asphalt, on the rffect using a mixing impeller properly positioned Lvith hames in the tank and air introduced through a ring just below the mixing impeller. The required time for asphalt oxidation was materially reduced over that required by conventional means. ?‘he quality of products compared favorably Lvith those made by the usual techniques. Data are given for the treatmcnt of three different crude asphalts. Per-
MlXlN G formance is shown in terms of melting point, penetration, and time required for oxidation.
Solid-liquid Mixing The suspension of solids in liquid by the use of mixers is an ever present problem and few quantitative data are available for the proper sizing and positioning of impellers for solid suspension operations. Rao and Mukherji (79) have worked with small beakers and have reported results on the position and speed of propellers used to suspend solid particles. Distribution of solids at various points in the system and the effect of impeller speeds has been noted. The data are of interest, but unfortunately the equipment size is so small (five- and six-inch diameter) that one cannot assume that the conditions in commercial size tanks are the same. Much more work is needed on this problem before there can be assurance as to requirements for adequate suspension of solid particles in large equipment. Kneule (77) reported on his study of paddles, turbines, and propellers used to dissolve granular materials in vessels sized 3 to 50 liters in capacity. He related impeller position and speed together with the size of the solid particles to the time required for mixing. The time was determined by measuring the temperature change of the system as a function of the heat of solution. The energy dissipated by the impeller as heat was also taken into account. In a series of leaching tanks where water is passed through the set continuously, the time of contact of water with solids can be determined by using radioactive tracers. Turgeon (28) experimented on leaching-type equipment with three units in series and traced the radioactivity of an europium isotope flowing through the system. He found that there were definite peaks in the activity a t different times in the different tanks as the europium was leached. T h e activity decreased exponentially with time in the first cell into which the europium was placed. The technique is suggested as a useful one for studying the residence time of solids in leach tanks. Applications-Paper Stock Mixing. T h e performance of mixers to establish and maintain uniformity of pulp in stock chests for paper manufacture has been studied quantitatively by Oldshue and Gretton (75). For many years the terms ”mild blending,” “mixing,” and “uniformity” were used to designate performance of paper stock mixers. Oldshue and Gretton used the following terminology and defined what they meant by “complete uniformity,” “complete motion,” “partial volume uniformity,”
“motion at. the pump suction,” and “batch blending time.” Data were developed to establish meaning for each of these new definitions. Results are reported from work done in tanks from 1.5- to %foot diameter size. The major source of data was from a 12-foot diameter siock chest in regular paper mill operation using a variable speed mixer with maximum power rating of 40 horsepower. Several types of mixing impellers were used and the ratio of impeller diameter to tank diameter and position of the impeller for optimum results were studied. I t was established that mixer performance is a function of the fluid regime produced in the stock chest. The effect of the mixer variables has been shown by quantitative measurement and in terms of the new definitions relating to uniformity and motion. This article is the most definitive and extensive one which has appeared to date on the subject of paper stock mixers. Mixing Chemicals with Soil. The mixing of small amounts of liquids with large quantities of solids is a problem essentially in the area of solids mixing. Equipment for this purpose resembles that for solids and paste mixing. Smith (26) gives the results of mixing small quantities of liquids with earth and describes the results using small and large scale equipment of both portable and stationary type.
Mixing of Solids The blending of solids has been studied by Adams and Baker (7) who reported on batch blending equipment using granulated polyethylene and blending with a number of other materials. They used a double cone blender, a ribbon blender, a rotating cube, and a twin shell. There seem to be no general correlations to classify the various types of equipment. One general conclusion, however, was that the method of adding minor constituents to a blend greatly influenced the final mixture composition. Some tests were made for mixing in a semimolten state. Data are reported for the mixing of solid particles by Oyama and Ayaki (76). The work was done in a horizontal cylindrical rotating cylinder and results are compatible with others of this type, pulAished in the past few years. Additional work on mixing of dry powders in a V-type mixer is reported by Yano and others (30). They mixed sodium carbonate and poly(viny1 chloride) ih two different sized V-type mixers. The largest was a 2-liter capacity. Optimum speeds, found for the operation, depended upon the ratio of the two solids and the method of changing the mixer. Mixing and compounding of rubber
by means of rolls is a well established technique. Because of the large amounts of energy involved, proper temperature control is difficult to maintain in small scale roll-mills. Roth and others (27) described a small (12-inch length) water cooled roll-mill for small scale rubber mixing. Close temperature control was found to be possible. Temperature data are given on some typical rubber compounding operations.
BIBLIOGRAPHY (1) Adams, J. F. F., Baker, A. G., Trans. Zmt. Chem. Engrs. (London) 33, 24 (1955); Chem. Eng. News, 34, 744 (Feb. 13, 1956). (2) Aiba, S., Chem. Eng. (Japan) 20, 281 . (1956). (3) Zbid., p. 288. (4) Gaden, E., Chem. Eng. 63, 159 (April 1956). Graef, E. R., Foster, S. P., Chem. Eng. Prom. 52. 293 (Julv 1956). GutoB, E. ’B., IND. ENC. CHEM.48, 1817 (1956). Holmes,‘ J. H‘., Schafer, A. C., Chem. Eng. Progr. 52, 201 (May 1956). Inoue, I., Jr., Sci. Res. Znst. (Japan), 49, 217 (1955). Jordon, D. G., Petroleum Refiner 35, 129 (No. 6, 1956). Kishinevskii. M. Kh.. Serebrvanskii. V. T., Zhur. Priklad. Khim.‘29, 27 (1956). Kneule, F., Chem.-Ing.-Tech. 28, 221 l\ -i -9- 5- Ih. )
Na ata, S., Yokoyama, T., Maeda, I% Zbid.,,18, No. 1 (Jan. 1956). Na ata, S.; Yoshioka, ‘N., Yoko ama, Zbid., 17, No. 3 (July 19551 Na ata, S., Yokoyama, T., Mem. Fac. Jng. Kyoto Univ. (Japan) 17, No. 4 (October 1955). O&hue, J. Y., Gretton, A. T., Tappi 39, 378 (June 1956). Oyama, Y., Ayaki, K., Chem. Enn. ‘(Japan)20, 148 (1956). Parker, N. H., Gutzeit, G., Papailias, J. G., Mining Engr. 8, 288 (March 1956). Pierce, D. E., IND.END.CHEM.48, No. 7,65A (1956). Rao, S. R., Mukherji, B. K., Trans. Indian Znst. Chem. Engrs. 7, 63 (19545 ). Rescorla, A. R., Forney, W. E., Blakey, A. R., Frino, M. J., IND. END.CHEM.48, 378 (1956). Roth. F. L.. Decker. G. E.. Stiehler. k.’D., Rubber World ’132, 483 1955); Chem. Eng. Progr. 52, 56 11956). R; shton, J. H., IND.ENC.CHEM.48, 552 (1956). Rushton, J. H., Gallagher, J. B., Oldshue, J. Y., Chem. Eng. Progr. 52. 319 (August 1956). Scheibel, E, G.,A.Z.Ch:E. Journal 2, 74 (March 1956). Serner, H. E., Chem. Eng. 63, 195 (1956). ,----,Smith, J. C., IND. ENC. CHEM.47, 2240 (1956). Thompson, D., Sutherland, D. G., Ibid., 47, Twgeon, 3. C. in2 Met. ing Met. Bull. 52 Vinhnevskv. N. E.. Zhur. Priklad. Khim. 28: 1071 (19551. (30) Yano, T., ’Kanise, I., ‘Tanaka, K., &hem. Eng. (Japan) 20, 156 (1956).
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