1. Henry
Rushton was born in 1905. He received his 8.5. degree in chemical engineering and his Ph.D. degree a t the University of Pennsylvania. A t the present time he is professor of chemical engineering a t the University of Virginia a t Charlottesville, technical director of the University of Pennsylvania’s Thermodynamics Research Laboratory, general consultant in chemical engineering, and consultant in mixing to the M i x i n g Equipment Company. During the recent war he was a section chief with the O f f i c e of Scientific Research and Development. Rushton has authored a number o f scientific aiticles and published a recent text on process equipment design. He is the chairman of the Chemical Engineering Education Projects Committee o f the American Institute of Chemical Engineers, as well as a member of the American Chemical Society and the Society of Chemical Industry.
J.
Henry Rushton
T
HIS review covers the period between October, 1943, and November, 1945. Although this is an arbitrary choice, it will start a t the point to which A. W. Hixson carried a bibli-
ography on mixing (10) except for one article (20). During this two-year period there have been several definite trends in the development of mixing as a unit operation. -4 considerable amount of attention has been given to the gas-liquid contacting and heat transfer operations and to the adaptation of agitation and mixing equipment to continuous processes. In addition, there has been a welcome increase in the amount of performance data published. This is the most important trend in the field. Attention is being focused on the quantitative aspects of agitation and mixing by the chemical process industries. Many industrial research and development laboratories have installed and are installing experimental and pilot plant equipment, arranged to obtain pertinent data necessary for the proper engineering of large-scale mixing equipment. It is important t o consider and to build into experimental equipment approximately the same geometry found best suited to the economical operation of large-scale equipment-that is, the relations between impeller type and size, tank size and shape, position of impeller in the tank, liquid depth, and type of heat transfer surface must be carefully considered. Further, the mixing equipment manufacturer cannot scale-up with certainty unless there are sufficient experimental data t o show how much power is required and what fluid regime is necessary t o bring about the desired reaction on a large scale. Also, the mixing equipment manufacturer realizes and emphasizes the necessity for better understanding of equipment characteristics and the need for thorough understanding of the fluid flow and turbulence which can be produced throughout a mixing tank (8). Although several general articles have discussed the theory and general applications of equipment, attention has also been called to the types of research and data needed t o establish agitation and mixing on a firm scientific, experimental, and engineering basis. A proposal has been made (2) to set up a committee in cooperation with engineering societies for standardizing “methods of instrumentation and measurement of displacement, velocity, etc.” for evaluating mixer performance and process mixing requirements. Relatively few improvements have been made in mixing equipment, although there have been many adaptations of standard forms of equipment, often in very large size, to newly developed processes, such as penicillin and synthetic rubber manufacture. 12
Hixson ( 10) surveys the literature and discusses the nature and various methods proposed for measuring agitation. He emphasizes the need for a more intensive study of the performance of agitation and mixing devices which will lead to data suitable for a better understanding of the operation. Bissell (8) points out the difficulty in applying mathematical theory of fluid mechanics t o successful field use of mixing equipment. He classifies the types of information required from the user as well as the supplier of agitators. Data necessary for proper adaptation of an agitator to a particular process pertain t o both the particular charauteristics of the fluid system to be mixed and to the propellent and energy-dissipating characteristics of the impeller. There must be complete cooperation between the user and the manufacturer to assure complete success for any agitator adaptation. Rushton (19) amplifies this viewpoint, and calls attention specifically to the function of the pilot plant in the user’s laboratory and to the type of information which is now considered to be required in the pilot plant stage t o assure the best application and scale-up for mixing equipment. Bissell, Miller, and Everett (3) describe typical designs for pilot-plant equipment which are believed to be most useful in securing the necessary information for proper and effective large-scale engineering. They state size and dimension ratios which should be observed for best pilot plant agitation purposes. Miller and Rushton (16)discuss the relation between the stream discharge from a n impeller and the nature of flow required t o produce a desired result in fluid. They conclude that flow characteristics from impellers and fluid flow requirements to establish minimum agitation must be determined before a complete picture is obtained in order to apply agitators most effectively t o the mixing of liquid systems. Bissell ( 1 ) discusses problems incident to applying the proper mixing device t o various mixing applications. He also points out limitations of mixing equipm,ent under various process conditions. Brothman, Wollan, and Feldman (4) develop a theoretical analysis to explain mixer operations in terms of the so-called kinetics of mixing. Their theory is based upon the proposition that mixing is a three-dimensional shuffling operation. KO experimental data are cited. Creene (9) discusses the use of glass-lined mixing equipment. In so doing he proposes the use of the terms “agitative possibility” and “agitative intensity”, and states values for them for glass-lined equipment. APPLICATIONS TO CONTINCOWS PROCESSES. Brothman, Wollan, and Feldman (6) analyze hypothetical cases of continuous mixing operations and develop a number of theoretical equations
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 38, No. 1
for single-pass and multipass contacting. They define the advantages and disadvantages of these operations. Hulburt (13) presents general equations for applying the characteristics of homogeneous reactions t o continuous-flow chemical systems. These theoretical equations are intended to give a simple and direct means of correlating empirical rate data with activation energies and specific reaction rates. Charts are presented to show the dependence of yield on the process variables for simple reaction mechanisms. MacMullin and Weber (16)discuss various terms used in continuous processing, and analyze critically the advantages and disadvantages for both batch and continuous processing operations. They review previous methods for evaluating continuous operations and compare them with batch operations. The “technical aspects of continuous process systems” are discussed in detail by Olsen and Lyons (18). They survey all types of continuous systems in which mixing equipment is used, and enumerate the types and arrangement of the component parts. Most of the possible combinations of equipment are discussed and illustrated. A critical analysis of the literature pertaining to continuous mixing operations is given. in the form of curves, data taken from operating e substantiate certain of the theoretical formulas and correlations In the literature. Further, they show the inconsistencies in certain other theories which have been published.
EQUIPMENT PERFORMANCE DATA. Stoops and Lovell (SO) report data on the power consumption of two sizes of threebladed marine-type square-pitched propeller agitators. Power data are correlated with speed, physical dimensions, and liquid properties by means of a modified Reynolds number and other dimensionless groups. The data are for unbaffled tanks, and they lie in the turbulent range. Miller and Rushton (16) give power data for ‘two sizes of three-bladed square-pitched marinetype propellers, operating in water in turbulent conditions. Brothman, Wollan, and Feldman (4) present a nomograph “for approximating power requirements of mixers”; this nomograph covers paddles, turbines, and propellers, classified for mild, medium, and violent agitation. Power data and degree of mixing data are reported by Miller and Mann (17) for seven different agitators operating inpingleand two-phase liquid mixtures. The data are correlated by dimensionless groups and are compared with other published data. Degree of mixing is evaluated, and the effects of impeller shape, location, and speed are presented i n tables and plots. The a,uthors demonstrate successful scaling-up from their data to large commercial size installations where there is geometric similarity. A double-film concept is applied t o rate of solution of solids in liquids during agitation by Hixson and Baum (11). The studies were made on benzoic acid pellets in dilute sodium hydroxide solution. A flat-bladed turbine agitator made of glass was used without baffles, and it was found that the effective film thickness was inversely proportional t o impeller speed over the range studied. Heat transfer data were determined by Chilton, Drew, and Jebens ( 6 ) for a jacketed vessel containing a coil of pipe. Heat transfer coefficients were developed for both the coil and the inner sqrface of the tank for various temperatures, agitator speeds, and liquids. The agitator was a simple flat paddle located close t o the bottom of the tank. Correlations of dimensionless groups with a modified Reynolds number are given for data covering wide ranges of Reynolds numbers. The data are for unbaffled tank conditions. Data are also reported by Houlton (13) on heat transfer in the special mixing device known as the Votator; its performance is described, and heat transfer coefficients are given for a considerable range of blade speeds. The data are for water-to-water heat transfer. An article by Cooper, Fernstrom, and Miller (7) gives data on the performance of a vaned disk used for gas-liquid contacting. January, 1946
The effectivenessof the mixing device in a baffled tank was evaluated by the rate of oxidation of dilute sodium sulfite solutions. The effects of power, gas rate, tank dimensions, and geometric relations on the rate of reaction were studied. The data were correlated by a so-called absorption number. The data from the vaned disk can be applied to a flat paddle, and experiments are reported showing agreement in scale-up from a vessel 9.5 inches in diameter t o one 8 feet in diameter. Other data on gasliquid contacting by mixers were reported by Foust, Mack, and Rushton (8). This work covered the contacting of air and water, using a disperser turbine. The time of contact between gas and liquid was evaluated for various rates of gas flow and for different power inputs. A baffled tank was used, with the impeller located a t several different positions. The paper also gave data for the relation between power requirements with and without gas flow. Many of the articles just mentioned wcre presented at a Symposium on Agitation and g in December, 1943,held by the Division of Industqial a ngineering Chemistry of the AMERICANCHEMICAL SOCIETY.
EQUIPMENT. An article b ebler (14) describes the construction and installation of gh-speed agitator to be used inside a n experimental high-pressure vessel. There have been a few items of new equipment introduced during the past two years. Mixing‘Equipment Company has available a stabilizing ring t o be attached to the impeller shaft and located directly below a turbine or other impeller. The purpose of the stabilizer ring is t o prevent shaft whip whenever the surface of the liquid falls below the surface of the impeller. From the same company another device to prevent whip is a fin which can be attached to the lower side of the blades of a propeller. Various pieces of equipment have come into widespread use during the past two years although they were introduced earlier. Those worthy of special note are: the turbo-saturator for gasliquid contacting, by The Turbo Mixer Corporation; the Centricone especially for fibrous suspensions, by The Patterson Foundry & Machine Company; the disk-mounted turbines with straight, curved, or disperser type blades for all types of mixing, including gas-liquid contacting, by Mixing Equipment Company; and the Brumagin impeller for high-speed with self stabilizing characteristics, by Struthers-Wells Company, LITERATURE CITED
(1) Bissell, E.S., Chem. & Met. Eng., 52, No.5, 112 (1945). (2) Bissell, E. S., IND.ENG.CHEM.,36, 497 (1944). (3) Bissell, E. S., Miller, F. D., and Everett, H. J., Ibid., 37, 426 (1945). (4) Brothman, A., Wollan, G. N., and Feldman, S. M., Chem. & Met. Eng., 52, No.4, 102 (1945). (5)Ibid., 52, No.5, 126 (1945). (6) Chilton, T. H.,Drew, F. B., and Jebens, R. H., IND. ENQ. CHEM.,36,510 (1944). (7) Cooper, C . M., Fernstrom, G. A., and Miller, S. A . , I b i d . , 36, 504 (1944). (8) Foust, H.C., Mack, D. E., and Rushton, J. H., I b i d . , 36, 517 (1944). (9) Greene, 0.W., Glass L i n i n g , 15,N o . 1 (1944). (10) Hixson, A. W., IND. ENG.CHEM.,36, 488 (1944). (11) Hixson, A. W., and Baum, 6. J . , Ibid., 36, 528 (1944) (12) Houlton, H.G.,Zbid., 36, 5?42 (1944). (13) Hulburt, H.M., Ibid., 36, 1012 (1944). (14) Kiebler, M.W., Zbid., 37, 538 (1945). (1.5) MacMullin, R.B.,and Weber, M., Jr., Chem. & Met. Eng., 52, No.5,101 (1945). (16) Miller, F. D., and Rushton, J. H., IND. ENQ.CHEM.,36, 499 (1944). (17) Miller, 5. A.,and Mann, C. A., Trans. Am. Inst. Chem. Engrs., 40,709 (1944). (18) Olsen, J. F.,and Lyons, E. J., Chem. & Met. Eng., 52, No.5, 118 (1945). (19) Rushton, J. H.,IND.ENG.CHEM.,37, 422 (1945). (20) Stoops, C. E.,and Lovell, C. L., I b i d . , 35, 845 (1943).
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
13
