J .
H E N R Y R U S H T O N
MIXING TECHNOLOGY Rapid mixing o f gases, preuiously thought to be a simple problem, attracts attention
ixing of gases and of dry solids
IVI have received marked attention by researchers during the past year. Both of these areas have been of considerable interest in the past, but too little attention has been given to the supposedly simple problem of rapid mixing of gases. Since the last review on mixing (38), the problems of scaling-up of liquid-mixing systems from pilot model size to large industrial equipment have been well reviewed (79), together with practical illustrations of successful scale-ups. Further study of liquid flow patterns in unbaffled cylindrical mixing vessels (47, 48) clearly shows the presence of primary and secondary circulation flows. Velocity distributions were measured and momentum transfer evaluated. They show that both the amount of discharge and the amount of turbulence are the important factors in the performance of a mixing impeller. A new source of power for mixing is proposed by Cropper and Seelig (8) who have applied a nonhomogeneous electrostatic field to layers of hydrocarbon and nonaqueous solvents to achieve mixing for the purpose of solvent extraction. Fine dispersions of solvent droplets can be achieved, and rapid rates of extraction can result. T h e solvent layer must be shallow. Power requirements are comparable with conventional mixing methods in small (2- to 6-inch size) equipment, but are undoubtedly much larger for plant sized applications.
Residence time studies have been made in continuous flow small-scale mixers using liquids and rotating impellers (73),and propellers, paddles, and turbine impellers were used (34) to study the mixing of miscible liquids in turbulent flow. T o determine the most effective way to mix two continuously flowing streams of gas (27), various nozzle forms were used to produce jet flow of one gas in another. Correlations are given for single and multiple jet flows and for different sizes of equipment. Recent findings in aerodynamics have been applied to positioning sharp-edged plates in gas ducts to ensure the generation and persistence of vortices along a chosen path (76). Application of the principles for industrial mixing of gases has been proposed. T h e mixing of a gas flowing in a square duct with another gas entering sidewise through a n orifice has been studied (28). T h e mixing of compressible fluids over wide velocity ranges in a constant area duct has been studied by Kennedy (27). T h e axial mixing of gases during flow through tubular high pressure reactors was measured (5) using helium as a tracer gas; return bends contribute considerably to axial effects in these systems. A novel impeller design using thin triangular plates rotated on arms produces turbulent vortices, which are said to be of advantage as an “air stirrer” ( 3 ) . Considerable attention has been given to the mixing of drops of one
liquid in another. Coalescence frequencies of drops in liquid-liquid mixing tank operations are reported (25), together with a novel technique for obtaining significant new data on this complex phenomenon. A light scattering technique has been used (40) to determine drop size distribution in liquid-liquid mixing for drop diameters between 1 and 50 microns. Such distributions are proposed as a measure of degree of agitation produced by turbine mixing impellers. Long (24) has made an addition to his previous paper on scale-up of a mixer-settler extractor, and was able to find the true driving force for his transfer mechanism and thus achieve better results for scale-up of this type equipment. Additional data on the performance of a rotating disk contactor have been obtained (44, 45). I n the area of liquid-solid contacting, Harriott (78) has presented the most comprehensive data to date on rates of mass transfer from liquid to solid particles in mixing tanks. A wide range of all significant variables including tank size gives results which are well correlated and provide excellent data for scaleUP. An authoritative study of the power requirements to suspend slurries of fibers (74) is presented. Such slurries are non-Newtonian fluids which are difficult to characterize, but performance for fiber size is
(Continued on page 56) VOL. 5 5
NO. 8
AUGUST
1963
55
new
ElECTROlYTlC
BRIOGE
20.1% ACCURACY The. Model RC-18 Conductivity Bridge is a further refinement of the familiar Industrial Instruments' Model RC.16 Conductivity Bridge, an industry standard for more than two decades. Combining an accuracy of + O . l % and a sensitivity of better than k 0 . 0 1 yo,the model RC-18 meets the demand for higher precision conductivity measurements.
* * * * *
F EATU R E S : Range 0.100,OOO ohms. Cathode Ray Oscilloscope Null Detector permits separate resistive and reactive balance.
Oscillator provides both 1000 cps and 3000 cps bridge current. Resistance and capacitance decades with inline readout. Self-contained cabinet or rack and panel mounting.
Write for complete information on the Model RC-18 and accessory conductivity cells and constant temperature baths.
correlated with concentration in equipment varying in size by sixfold. Data have been obtained (7) on the mixing of mineral slurries for the purpose of extracting valuable products. Many such slurries are non-Newtonian in behavior. Marine-type propellers were preferred to maintain suspension, whereas turbine impellers appeared to give better solution rates for the same energy expended. A new design of a mixing impeller (29) is proposed to handle slurries of lithopone; power requirements are given. Mixing gives much more uniform and stable operation of sludge digesters (33), and a design arrangement for such a system is presented. Power required to disperse air in liquid with flat-blade turbine impellers is given (26) for a variety of equipment sizes and for several liquids. The effect of a surfaceactive agent was also studied. Bubble surface and mass transfer in gas-liquid mixing are reviewed (43) and different types of impellers are compared. Mixing by means of gas-lift or sparging for liquid-solid contacting appears to be a function of liquid depth, height to diameter ratio, and solid particle size; linear gas velocity is relatively unimportant (42). Mixing of gas and liquid in countercurrent flow in packed beds was studied (22); correlations were made using P e c k and Reynolds numbers. And another article from the same laboratory (23) extends the previous work to account for different packing sizes. Absorption of carbon dioxide in carbonate-bicarbonate solutions in a packed tower as a function of mixing is reported ( 7 ) . The results appear to agree well with the accepted absorption-coefficient method for correlating such data. The mixing of liquid in the spaces between solid spheres in a packed zolumn has been studied (35).
ELECTROLYTIC COND U C TI VIT Y IS OUR BUSINESS! We invite your inquiry.
09 Commerce Road, Cedar Grove, € s e x County, New Jersey Circle No. 525 on Readers' Service Card
56
INDUSTRIAL
AUTHOR J . Henry Rushton is Professor of Chemical Engineering at Purdue
University and Technical Aduiser to 'he Mixing Equipment Co., Rochester, N . Y . He has authored IHEC's nixing reviews since 7946.
A N D E N G I N E E R I N G CHEMISTRY
Mixing of fluids in a fluidizedsolids bed has been studied (30), and distribution of concentrations has been correlated for both liquids and gases used as fluids, and for a variety of fluidized solids. From the same laboratory (37) are data on the lateral direction mixing of the solids in a liquid fluidized solids bed. The mechanism of mixing vapor passing through plates of a distillation column has been studied (36), and data (32) on the extent of mixing of the liquid on the sieve trays of a gas-liquid contacting unit are given. Eddy coefficients in airwater mixing in sieve-tray columns are reported ( 2 ) where amount and effect of mixing are related directly to the longitudinal eddy diffusivity coefficient. An interesting application of mixing of vapors is in the process of sublimation and condensation as a means of separation of substances. Effect of mixing of different ratios of phthalic anhydride and air in a tubular injection setup is evaluated (6) as are methods and advantages of stirring a melt during the crystal growth process (47). Power requirements to mix viscous liquids by helical and marine-type impellers are compared ( 3 9 ) ; for viscosities higher than 50 cp. the helical impeller requires less power. Mixing parameters are related to the degree of polymerization and to the molecular weight distribution (77) in a continuous flow unit, for a particular reaction order for the monomer and each reaction step. Some pseudo-plastic fluids which require an abnormally long time to blend can be handled in much less time under swirling, vortexing, nonbaffled conditions (75). Blend time was found to be inversely proportional to the square root of the vortex depth for pseudo-plastic fluids. A difficult but important operation is the blending of solids with small amounts of liquid. An excellent review is available (72) for all aspects of the operation including the coating of agglomerates to form balls of material.
(Continued on page 58)
PHIL I PS C ryogene rators
can produce any tem perature from ambient to minus 200 "C quickly simply economically
- 100.. . - 200 "C -- 100.. . - 196 "C 7 0 . . - 200 "C - 5 0 . . . - 190 "C - 5 0 . . . - 190 "C - 5 0 . . . - 200 "C ,
APPLICATIONS INCLUDE : Liquefaction or recondensation of gases : nitrogen, air, argon, oxygen, methane, ethane, fluorine? ethylene (Two models are available with capacities of 7.5 and 30 litres per hour for liquid air) Low-temperature gas separation Low-temperature gas purification and de-hydration Cold chambers for environmental testing and materials treatment Cold source for very low temperature processes in industry Low-temperature cooling for physics research? processing in biology, food preservation? high vacuum techniques, missile development, and in electronics
The Philips PLN 430 installation for the production of 2 5 . . . 30 litres/h of high purity liquid nitrogen
Sole distributors in the U.K. : Research & Control Instruments Ltd., Instrument House, 2 0 7 King Cross Road, London W.C. 1 overseas enquiries : N.V. Philips' Gloeilampenfabrieken, Eindhoven, the Netherlands, Scientific Equipment Department
PHIL I PS Circle No. 515
Cryogenerators
on Readers' Service Card VOL. 5 5
NO. 8
AUGUST
1963
57
St u rteva n t Eq w iprn en t
......................................... .........................................
Reduce, Classify To Micron Size In One Operation With Sturtevant MicronizeP Grinding a n d classifying in one high speed operation, t h e S t u r t e v a n t Micronizer fluid energy g r i n d i n g mill provides f i n e s f r o m 1/4 to 44 microns with no a t t r i t i o n a l h e a t . In p h a i maceziticals, t h e hlicronizer is produci n g procaine-penicillin a n d steroid f i n e s in t h e 5 t o 20 micron r a n g e suitable f o r use a s injectibles . . . I n oil r e s e a i c h , many i n f r a r e d absorbing solids a r e ground to smaller dimensions t h a n t h e wavelength of incident radiation f o r spectrophotometric a n alysis . . . 111 p i g m e n t munzifactui.ing, t i t a n i u m dioxide is being reduced a s small a s 1 / 4 micron with high unif o r m i t y of particle size . . . In giincling abiasives, u l t r a - f i n e pulverization of m a t e r i a l s u p to t h e h a r d n e s s of t u n g s t e n carbide, hitherto impossible with commercially available machines, is achieved in a carbide lined Micronizer. Particles circulating a t s u p e r speed, propelled by t a n g e n t i a l j e t s of compressed air or s t e a m , g r i n d each o t h e r by violent impact within t h e shallow chamber. Simultaneous classification occurs a s centrifugal force keeps over-size m a t e r i a l s in t h e grinding zone; cyclone action in t h e c e n t r a l section collects f i n e s f o r discharge, No moving p a r t s . Now available in ten sizes from t h e 2 ” l a b o r a t o r y model to t h e 42” production mill with c a p a cities r a n g i n g f r o m l a b o r a t o r y samples u p to tonnage quantities per hour, depending upon m a t e r i a l a n d fineness desired. T e s t o r production micron-grinding on a c o n t r a c t basis i s p a r t of S t u r t e v a n t service.
For f u r t h e r infornaation, send Bulletin No. 091.
f o S~t u r t e v a n t
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STURTEVANT
85 years of design leadership in Impact M i l l s , Air Clas. sifiers, Micron-Grinders, Pulverizers, Blenders, Crushers, Grinders. Circle NO. 7 on Readers’ Service Card
58
INDUSTRIAL A N D ENGINEERIN
In recent years increased attention has been directed to the mixing of solids. Extensive studies on the mixing of dry solids in horizontal drum-type mixers are available (4, 9, 70, 7 7 ) . Mixing of solids in rotating cylinders and in double cones was measured ( 4 ) where the only difference in the particles was color. Thus, a diffusive type of solids mixing was evaluated, and this has not heretofore been done. Rates of mixing were measured in a plane perpendicular to the principal circulation, and in the plane of circulation. T h e arithmetic sum of the rates in the two planes was found to be significant in the correlation of data. Mixing and demixing (or unmixing) are discussed ( 9 ) ,and the effects of various operating conditions are enumerated and evaluated (70). Finally, recommendations are made for the best choice of mixer forms for various industrial applications ( I 7). Another study was done (20) where dry antibiotic materials were mixed in double-cone and in twinshell (V-type) units. More rapid mixing rates were achieved with large units than with small units. An article (37) reviews various methods used to mix dry solids, and gives experimental results for centrifugal mixers where solids are thrown by centrifugal force and then allowed to fall back into the bed of solids. Recommendations are made for applications of centrifugal type mixers. And, finally, a review of the mechanisms involved in mixing of dry granular material is available (46).
LITERATURE CITED
(1) Astarita, G., Beek, W. J., Chem. Eng. Sci. 17, 665 (1962). (2) Barker, P. E., Self, M. F., Ibid., 17, 541 (1962). (3) Bryer, D. W.,Brit. Chem. Eng. 7, 332 (1962). (4) Carley-Macauly, K . W‘., Donald, M. B., Chem. Eng. Sci. 17, 493 (1962). (5) Carter, D., Bir, LV. G., Chem. Eng. Progr. 58, 40 (1962). (6) Ciborowski, J., Surgiewicz, J., Brit. Chem. Eng. 7, 763 (1962). (7) Couche, R. A,, Australasian Inst. Mining andMet. Proc. 198, 11 (1961). CHEMISTRY
(8) Cropper, I V . P., Seelig, H. S., Znd. Eng. Chem. Fundamentals 1, 48 (1962). (9) Donald, M. B., Roseman, B., Hril. Chem. Eng. 7,749 (1962). (10) Zbid., 7, 823 (1962). (11) Ibzd., 7, 922 (1962). (12) Fischer, J. J., Chem. En?. 69, 83 (1962). (13) Foraboschi, F. P., Lelli, U., Chim.Znd. 43, 1279 (1961). (14) Gibbon, J. D., .4ttwood, D., Trans. Inst. Chem. Engrs. (London) 40, 75 (1962). (15) Godleski, E. S., Smith, J. C., A.Z.Ch.E. J . 8, 617 (1962). (16) Gould, R. I V . F., Brit. Chem. Eng. 7, 667 (1962). (17) Harada, M., Eguchi, W., Nagata, S., Chem. Eng. (Japan) 26, 583 (1962). (18) Harriott, P., A.1.Ch.E. J. 8, 93 (1962). (19) Holland, F. A., Chem. Eng. 69, 179 (1962). (20) Kaufman, A., Ind. Eng. Cliem. Fundamentals 1, No. 2, 104 (1962). (21) Kennedy, E. D., J. Appl. Mechanics 28, 335 (1961). (22) Kunugita, E., Otake, T., Yamanishi, T., Chem. Eng. (Jajan) 26, 800 (1962). (23) Kunugita, E., Otake, T., Yoshii, K., Ibzd., 26, 672 (1962). (24) Long, R. B., 2nd. Eng. Chem. Fundamentals l, No. 2, 152 (1962). (25) Madden, A. J., Damcrell, G. L., 4.I.Ch.E. J . 8, 233 (1962). (26) Michel, B. J., Miller, S. A., Zbzd., 8, 262 (1962). (27) Miller, D. R., Chem. Eng. Progr. 58, 77 (1962). (28) Miller, E., Wohl, K., A.Z.CI1.E. J . 8, 127 (1962). (29) Molyneux, F., Fluid Handling 140, 240 (1961). (30) Muchi, I., Mamuro, T., Sasaki, K., Chem. Eng. (Japan) 25,747 (1961). (31) Muchi, I., Mukaie, S., others, Zbid., 25, 757 (1961). (32) Mutzcnberg, A , , Chem. Zngr.-Tech. 34, 542 (1962). (33) Myers, H. V., Jr., J. Water Pollution Control Federation 33, 1185 (1961). (34) Prochazka, J., Landau, J., Collection Czech. Chem. Commun. 26, 2961 (1961). (35) Ratcliff, G. A., Reid, K. J., Trans. Znst. Chem. Engrs. (London) 40, 69 (1962). (36) Rukenshtein, E., Zhur. Priklad. Khim. 34, No. 1, 157 (1961). (37) Rumpf, H., Mueller, W., Trans. Znst. Chem. Engrs. (London) 40, 272 (1962). (38) Rushton, J. H., IND.ENG. CHEM.54, No. 8, 59 (1962). (39) Serwinski, M., Blasinski, H., Chem Stosowana 5, 17 (1961). (40) Sullivan, D. M., Lindsey, E. E., 2nd. Eng. Chem. Fundamentals 1, No. 2, 8’7 (1962). (41) Turovskii, B. M., Milvidskii, M . G., Kristallograjya 6, 759 (1961). (42) Tyuryaev, I. Y . , Tsailingol’d, A. L., Builov, A. B., Zhur. Priklad. Khim. 34, No. 3, 558 (1961). (43) Valentin, F. H. H., Preen, B. V., Chem. Zngr.-Tech. 34, 194 (1962). (44) Westerterp, K. R., Landsman, P., Cheni. Eng. Sci. 17, 363 (1962). (45) W-esterterp, K. K., Meyberg, W. H., Ibid., 17, 373 (1962). (46) Willemse, T. W., Clzem. Weekblud 57, 377 (1961). (47) Yamamoto, K., Nagata, S., Chem. Eng. (Japan) 26, 500 (1962). (48) Zbid., 26, 510 (1962).
