INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1949
any direction in any plane. It derives its name, Zipper, from its method of closure-the belt is closed with zipper teeth during its conveying run. Rollers open or close the zippers at the feed and discharge points, and these feed and discharge points can be made movable to add t o the flexibility of the unit. This unit is so unlike other materials handling units that it offers new possibilities for handling bulk materials. Fragile materials can be conveyed without breakage because the conveyer moves the material in a solid mass without agitation, coarse abrasive materials can be handled, and many corrosive materials can be handled because the conveyer is constructed of rubber. The zipper conveyer is noiseless, has the long life expectancy of rubber belt conveyers, and has power requirements about equal to belt conveyers but less than most types of elevators. Large quantities of material ran be handled with a relatively small cross-section belt because the conveyer operates at comparatively high speeds. Installations of the Zipper conveyer arc now in operation. A new hot materials belt has been announced which, it is claimed, will not char, lose strength, or stretch at temperatures up to 350' F. (11). A conveyer belt with chevron-shaped ribs is said to eliminate any noticeable backslipping of most wet materials on inclines up to 20". By reduction of wear due to backslipping, the belt life is said t o be substantially increased. A vibrating conveyer on which solid materials can be mixed in transit is now being manufactured. The vibrating deck consists of a series of inclined notched plates which cause the intermingling of materials as they fall from one plate to the next. MISCELLANEOUS MATERIALS HANDLING DEVICES
A completely new design for a hopper car unloader has recently been announced (6). The unit is mounted on rubber tires and consists of a cleated belt and chain which can be inserted under the hopper car, either in a track pit or above the rails. The portion of the unloader extending under the hopper car is 8 feet long and only 41/la incher thick, giving it ample clearance and length for the widest hopper bottom car. This unloader handles almost any bulk product from fine sandy material t o large-sized aggregates and coal at capacities up to 3 tons per minute. It eliminates track pits and other expensive equipment usually used to unload hopper cars. An automatic proportioning method for both dry and liquid materials which is adapted t o both volumetric and gravimetric control has been developed (16). This new system permits the exact presetting of total production rate and the individual selection of percentages for the respective ingredients. The system as applied in a general case may make use of a combination of standard volumetric meters, metering pumps, or loss-in-weight
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scale. These units are tied together by a Master Selsyn which is driven a t selectively variable speeds and sets the total production rate. The Master Selsyn drives secondary Selsyns which control the meters, pumps, or scales. The secondary Selsyns act through component ratio controls which determine the percentages for the individual ingredients. Where a pump requires more power than the secondary Selsyn can supply, the motor speed of the pump is varied by a n electronic speed control unit. A differential between the secondary Selsyn and the pump motor actuates the electronic speed control, so that the pump is driven a t a speed t o satisfy the secondary Selsyn. This system has been specifically applied to the blending of basc stocks in the compounding of lubricating oils, but could be extended to almost any materials handling problem involving tho proportioning of various ingredients. MATERIALS HANDLING EXPOSITION
A national exposition covering materials handling equipment is rapidly becoming a n important event in this field. The Second National Materials Handling Exposition which was held in Cleveland in January 1948 had 193 exhibitors and 18 papers were presented at the technical sessions. This exposition provides an excellent way for engineers to examine the latest developments in materials-handling equipment. The Third National Materials Handling Exposition is being held in Philadelphia on January 10 to 14, 1949. LITERATURE CITED
(1) Anon., Byjuc News, 5,No. 17, 1-4 (1947).
(2) Anon., FZowZine, 7, No.4, 10-13 (1948). (3) Anon., Mech. Engr., 70, 23 (1948). (4) Anon., Westinghouse Engr., 8, No. 1, 12 (1948). (5) Barber-Greene Co., Aurora, Ill., Catalog 76, p. 25. (6) Belke Mfg. Co., Chicago 51, Ill., Bull. 718. (7) Bell Aircraft Go., Buffalo, N. Y., bulletin on Bell prime mover. (8) Clark Bros. Co., Inc., mimeographed technical description of Clark axial flow compressor. (9) Duriron Co., Inc., Dayton 1, Ohio, TentativeBull. 815-ME4. (10) Goodrich, E. D., Mech. Eng., 70, 733-7 (1948). (11) Hewitt Rubber Division, Hewitt Robins, Inc., Buffalo 5 , N. Y . , Bull. B79. (12) Milton Roy Co., 1300 East Mermaid Ave., Chestnut Hill, Philadelphia 18,Pa., bulletin on Constametric pump. (13) Milton Roy Go., Philadelphia, Pa., Bull. 488,1948. (14) Peerless Pump Division, Food Machinery Corp., Los Angeles 31, Calif., Bull. B-2200. (15) Production Aids, Inc., North Hollywood, Calif ., unnumbered bulletin on automatic pallet loader. (16) Propertioneers, Inc., Providence, R. I., Standard Methods 138 (May 15, 1948). (17) Stephens-Adamson Mfg. Co., Aurora, Ill., Bull. 344. (18) U. S.Dept. Commerce, Office of Technical Services, PB 80,358 (19) Wilson Snyder Mfg. Division, Oil Well Supply Co., Braddock, Pa., Bull. W-323-448. RBCEIVED Ootober 16. 1048.
MIXING J. HENRY RUSHI'ON, ILLINOIS
INSTITUTE OF TECHNOLOGY, CHICAGO, ILL.
S
I N C E the last review of the subject of mixing (16) a large number of articles on various phases of mixing technology have appeared in the technical journals. The list at the end of this review and the lists in the previous annual Unit Operations Reviews in INDUSTRIAL AND ENGINEERING CHEMISTRY make up a comprehensive bibliography on mixing, especially mixing in the liquid phase. The new technical articles cover many phases of the subject. Engineering data are presented in many of them. Some are purely descriptive, and a few describe statistical techniques for
sampling and evaluating performance. One gives data for a new combination of mixing and heat transfer. Some describe small laboratory apparatus. Undoubtedly the science of mixing is developing along longsought-for quantitative experimental lines, but art)icles still appear with unsubstantiated equations and arbitrarily defined criteria for mixing. Many installations of mixing equipment have been made in pilot plants during the past year. These have included variable speed drives and have been provided with calibrated impellers of
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
the principal axial and radial flow types. Data are hecoming available from such pilot, plants so that the equipment manufacturer can scalc u p to large plant, size equipment to the great, economic advantage of the user. Quantitative data are now available from equipment. manufacturers on power consumption of many impeller types for a very wide variety of conditions of operation, and the user of equipment is no longer entirely dependent upon manufacturers recommendations unsupported by experimentally proved performance data. I-Iowever, data are as yet not available as a basis for predicting power requirements and time required to reach a given amount of mixing for new operations that have not been tried on a pilot plant or iridustrial scalc. In other words, the equipment manufacturer can predict the mechanical performance and hydraulic regime for various mixing devices in various environments, and can scale up pilot plant perEormancc to industrial proportions, but he is still dependent upon the processor to establish the type of flow and amount and distribution of energy to be supplied by the mixing device in order to guarantee the optimum mixer installation. The variables in the science of niixirig are slowly being evaluated and the so-called “simplified approaches” to mixing problems can be tolerated no longer. A committec has been set up by the American Institute of Chemical Engineers to establish standards for evaluating mixer performance and if possible to set up criteria by which mixing can be measured in various applications. The subcommittee on mixing functions under the general Committee on Testing Techniques and Equipment Performance Standards. It is hoped that through this committee, manufacturers, users, and researchers in the field of mixing may be able t o set up acceptable standards and to correlate t.he present technical information available in the field of mixing. A G I T A T O R SELECTION
An article by Bissell ( 3 ) on the economics of agitator selection discusses materials of construction, their relabive costs, mechanical seals versus stuffing boxes, stuffing boxes as an integral part of the mixing or of the tank, multiple mixer insiallations versus single large installations, blending, and power requirements. There is a discussion of the reason for relatively high costs of some common materials of construction when used as mixing impellers and shafts. Aside from the basic cost differentials between different materials of construction, costs are also dependent upon the fact that standard nuts, bolts, keys, etc., are not available in some alloys; alloys of low tensile strength require larger shafts and fittings than is the case for steel, and coated materials require special handling and often modifications in basic design. In general, an agitator running with stuffing box as an integral part of the unit permits a more reliable installation and operation. Attention is called t o the fact t h a t there is a break point for mixing equipment manufacture just as for other pieces of process equipment, above which advantage can no longer be gained by the use of a single unit instead of several small units. For example, a side-entering mixer a t about 25 t u 30 horsepower is about the top economic size limit. If 50 t o 60 horsepower are required, it will usually pay t o use smaller units. For top-entering low speed turbines, the break point comes at approximately 50 horsepower. These break points are due to a number of factors apart from operating advantages, such as the ability to use partial power by operating only one or several installed units when desired. Lyons (11) discusses the circulation patterns resulting from the use of the common type of mixing impellers in a variety of tank and baffled conditions. H e gives general rules for the use of various types of impellers, baffles, coils, and compartmentation, outlines the field of use for various combinations, and by a number of drawings illustrates the ideas presented.
Vol. 41, No. 1
Several general articles by Serner have appeared recently. Several interesting ideas arc proposed in a so-called simplified approach (18) and charts show power rrqtiiremmtfi and peripheral velocities lor various intensities of mixing. No reference is made to the experimental evidence from which the basic data were derived. The author states that “it is now generally conceded that power input is a suitable criterion for determining the mixing accomplished. and knowing the power it is possible t o calculate the time required to achieve the desired circulation rate, and from that, the mixing time in certain of the less complicated situations.” This reviewer believes that many people with long experience in the field of mixing do not subscribe to the idea that power is a suitable criterion for measuring the amount of mixing accomplished. Moreover, it is the goal of all people interested in the theory of mixing and techniques for obtaining mixing, to be able to predict the time required for mixing for a given power input and a given physical environment. Environment is here used to include fluid properties, tank shape, and location of all intcrnal tank fittings. Several mathematical equations are developed but no data are given for experimental proof of the conclusions reached. Serner arbitrarily states that “a peripherical velocity of 6 feet per second corresponds to mild agitation while 7 feet per second corresponds to medium agitation and 9 feet per second corresponds to violent agitation.” These velocities are said to apply to water and other liquids regardless of tank diameter. The user of equipment should be careful to compare such criteria with the opinions of others (3, 11). Therc have appeared during the past few years engineering experimental data concerning power, fluid displacement, and velocities which are a t considerable variance with those of this paper. A second article by Serner (19)gives charts for viscosity factors, power factors, and pitch velocity corrections for different impellers. References are not given to the fundamental data from which these factors were derived or how they were calculated. Statements regarding power consumption in this article are ut variance with those of other published articles during tho past 10 years as well as articles here reviewed (14, 16).
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I
INFLUENCE O F I N T E R N A L T A N K FITTINGS
The mixing impeller is only one of several important mechanical elements involved in any mixing operation. Fluids are always mixed in some container, and the shape and arrangement of all interior parts of the container and its fittings play an important role in the mixing operation. Any given impeller may produce different results when operated in a liquid if the internal tank fittings are varied. An article on design and utilization of internal fittings for mixing vessels ( 4 ) discusses tank shapes, impeller shapes, baffles, heating coils, and draft tubes, in connection with fluid flow patterns and power requirements. Recommended arrangements for best average performance are given and data are presented for power requirements of propellers and turbines operating with baffles and heating coils which have not heretofore been available t o the chemical engineer. Figures illustrate favorable internal fitting arrangements. Proper positioning for the feed of liquids and gases to mixing impellers is also shown. The effect of heating coil arrangement and spacing and the use of baffles with coils are illustrated by data on the. power response of turbine impellers. Information is given on suitable forms of steady bearings for long impeller shafts, with a brief discussion of compartmentation of mixing vessels. P O W E R CHARACTERISTICS O F M I X I N G IMPELLERS
An article on power absorption in mixers (14) givesdatafor three types of impellers under several conditions of tank set up. The data cover experimental work with 11 different liquids of widely
January 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
varying properties and are correlated in dimensionless group fashion, in the usual method. A table summarizes previous agitator power studies from the work of eight other investigators. All the data are for operations using wall baffles or a deflector ring. Although the equations proposed in this paper t o determine power consumption are somewhat more complicated than those given in articles mentioned above (18,19), it is recommended t h a t they be relied upon rather than the more “simplified” proposals. An alignment chart based upon the author’s own work and work of previous investigators is included for estimating power requirements of mixers for a wide variety of impellers and conditions. A paper presented before the American Institute of Chemical Engineers (26) gave experimental data for 11 commercial type impellers in a wide variety of operating conditions. Correlations were developed which allow comparison of performance characteristics for the various impellers operating in both baffled and unbaffled tanks. It is shown that certain impeller types have power characteristics t h a t allow them to be used in liquids of widely varying viscosity with a minimum chance of overloading driving motors. A method of comparing impeller types as t o suitability for different operations, at least with respect t o power consumption, was developed. The report extends manyfold the data previously available to process engineers interested in the mixing operation. EFFECT O F T A N K BAFFLES ON M I X I N G
An article on internal tank fittings ( 4 )gives data for a flat-blade turbine operated in water with four different baffle widths and with from one to six baWes located at the walls of the tank and projecting into the tank along tank diameters. Comparison is made between numbers of baffles used and the width of baf8es. Most industrial mixing operations wherein baffies are used occur with four baffles either one-tenth or one-twelfth tank diameter and extending the full height of the straight side of the cylindrical tank, It is evident from the data presented not only that such a baffle arrangement is convenient, but that the addition of more baffles has relatively small effect on power consumption. I n the same article the effect of baffles placed inside and outside heating coils is demonstrated and data on power response are given. The effect of baffles on fluid flow pattern is illustrated by drawings resulting from visual observation and from motion pictures of operations conducted in transparent tanks. Other sketches of the effect of baffles on flow pattern are given in another article (11).
An article on effect of baffles on agitator power consumption (28) gives data for a flat paddle operating in water in a cylindrical
tank using flat baflles placed in several different positions. The authors describe “fully baffled” conditions as that amount of baffle surface which will result in the maximum power consumption of the agitator for a given arrangement. The data indicate that the use of four baffles one-tenth to one-twelfth tank diameter as described above as common industrial practice will usually produce fully baffled conditions. Data reported later by others (4, 6) are in general agreement with this article but do not show top limits on power consumption with respect t o baffle width. It is therefore thought advisable to use the conditions of baffle width equal to one-tenth tank diameter with four baffles located on the wall of the tank and placed on diameters as a “standard” condition and to m e this as a reference point rather than t o rofcr t o this situation as a fully baffled condition, for it is easily possible t o increase baffle width and thereby increase power consumption of certain impellers by as much as 50% over the so-called fully baffled conditions.
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H E A T TRANSFER IN M I X I N G VESSELS
Data on heat transfer in mixing vessels have been very meager. At the December 1947 meeting of the Institution of Chemical Engineers held at Manchester, England, a paper was presented on heat transfer in agitated jacketed cast iron vessels (5). D a t a were reported on plant size sulfonation and nitration equipment. Impeller types were propellers and anchor-shaped paddles. The vessels were jacketed and data were taken for batch operations for bothcooling and heating through the jacketed surfaces. Overall heat transfer coefficients were obtained over e, wide range of operating conditions. Data were computed for film coefficients on the mixing side during heating by means of steam by assuming steam coefficients of 2000. Film coefficients on the mixing side during cooling with water as the cooling agent were determined by using water film coefficients determined by calibration runs. Data were correlated by dimensionless groups similar to those published by Chilton, Drew, and Jebens. The authors concluded that over-all heat transfer coefficients for the jacketed cast iron pans used varied with fluid composition, absolute temperature level, the use of baffles in the jacketedspace, and agitator speed (25% increase in agitator speed resulting i n 6 t o 11% increase in over-all heat transfer coefficient), and that the variables involved can be correlated by dimensionless equations involving the Nusselt number, the Reynolds number, the Prandtl number, and a viscosity ratio. Further, if a helical coil with outside surface area equal t o that of the effective jacketed surface were used in place of the jacket surface, rates of heat transfer were increased by 116 t o 133%. Other data on heat transfer surface in mixing vessels have been given (1‘7). Data are presented for heat transfer coefficients from vertical tubes acting as baffles in addition t o heat exchange surface, in a vertical cylindrical tank equipped with standard design flat blade mixing turbines. Data also show proper positioning of impeller for optimum heat transfer rates. The experimental unit consists of a tank 4 feet in diameter, a dynamometer drive for the agitator, and vertical heating and cooling coils. It is believed that the use of vertical tubes as heating surface and also as baffle surface is a new development in mixing technology. Within the past year a number of large commercial sized installations have been put into operation wherein this principle is used. Relatively high heat transfer coefficients can be obtained from vertical heating or cooling pipes placed in banks along diameters of vertical cylindrical mixing vessels. One or more turbine type impellers rotating on a central shaft provide energy for fluid motion. Desirable fluid flow patterns for rapid mixing result, and cross flow of fluid across the pipe heat transfer surfaces results in higher heat transfer coefficients than can be obtained on jacketed surfaces. To be able t o apply heating surfaces in the form of a baffle enables the engineer to develop a piece of equipment having good heat transfer characteristics, good mixing characteristics, easy installation, easy servicing, and well controlled conditions. The heat transfer data were obtained under steady state conditions and can be applied readily to continuous or batch operations. HIGH SPEED PROPELLERS A S DISINTEGRATORS
Propellers and other agitators can be used to disintegrate solids and liquids and disperse them through other miscible or immiscible liquids. A paper on propeller influence in high speed stirring (18) presented data from organic laboratory scale equipment for disintegrating sodium and dispersing it in organic liquids. Calcium chloride and aluminum oxide was disintegrated by the same means. The agitators used were flat-blade propellcrs rotated at 10,000 t o 12,000 revolutions per minute. The work was carried on in three-necked glass flasks having indented sides. The author has found indented flasks useful for disintegration operations and for ordinary organic reactions as well.
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The data and experimental evidence show that the effectiveness of mixing and disintegration of solids is increased by increase in the speed of rotation of the impeller, use of impellers of larger diameter, a blade angle of approximately 4 5 O , and a downward bend of the lower edge of the propeller blade. These conclusions are in agreement with the general ideas that power consumed by impellers is increased for each of the abovementioned changes and that rapid disintegration and mixing are a function of power input.
made. A method has been derived for analyzing the time factors for vessels of various sizes by relating them t o an equivalent simple system. A series of curves calculated from the mathematical expressions developed is given for systems containing up t o 11 equal sized vessels in series. No experimental data are presented but the ideas and plots developed should be useful in determining a basis for the accomplishment of continuous mixing operations, where it is desired to find the amount of shortcircuiting taking place. AIR-LIFT AGITATOR
AGITATION IN AUTOCLAVES
No data have been available on performance of autoclaves as agitators, With a new approach to this problem an article on agitation in experimental rocking type autoclaves (8)is a welcome development, and although mixing is produced in a rocking type autoclave without the use of a rotating impeller, such an operation might well be included in consideration of the subject of mising. The major reactor design factors influencing agitation were determined for the hydrogenation of nitrobenzene. It was found that increased speed of rocking could cause fourfold increase in the rate of hydrogenation by the attendent increase in agitation, and also that the angle of rocking could be increased to increase the rate of agitation, and results were measured wherein a twofold increase in the rate of hydrogenation by this means was obtained. It was found that the viscosity and surface tension of the liquid also affect the agitation, and it was recommended that autoclaves be designed to provide for sufficient flexibility of operation, and free space of the liquid, t o allow for variations in these fluid properties. Reactor dimensions mere studied and it was found that the degree of turbulcnce for a given length-diameter ratio decreased as the size of the reactor decreased. SAMPLING AND STATISTICAL MEASURES
Blender efficiency ( d ) , applying the methods of statistical analysis, is the subject of a recent article. A blending ratio has been defined and a mathematical expression developed for its use, so that the variance of one component from the average may be determined. The blending efficiency as defined is a mathematical measure of the degree to which a mixer can produce a perfect homogeneity. An illustration is given of the use of this principle in following the efficiency and completeness of blending or mixing viscose. Application of such a technique t o other mixing operations may be a useful tool for measurement. Sampling lag and purging time in mixing vessels in series is the subject of another article (9). As a n outgrowth of experimental work in the hydrogenation of coal t o form synthetic liquid fuela, a study has been made as t o best sampling techniques to determine when mixtures have been completed to a desired degree. A theoretical mathematical analysis of the sampling lag or purging time for mixing tanks of various sizes arranged in series has been
Vol. 41, No. 1
drawing shows a n effective method for using an air lift to maintain agitation in a slurry tank during wide variations in liquid level, in a n article on air-lift agitator for varying tank levels and coarse, heavy slurries (7). SMALL LABORATORY APPARATUS
Some articles describing small scale laboratory equipment arc mentioned here simply by title: “Microburet with a n Attached Electromagnetic Agitator” ( I ), “Shaking Device or Multiple Containers” ( I O ) , “Beet Laboratory Pulp Mixer” (ZO), and “Stirrer-Actuated Continuous Ether Extractor” (21). I n addition, Morton and Redman ( 1 3 ) describe small laboratory size equipment. LITERATURE CITED
(1) Allan, J. C., S . African J . M e d . Sci., 11, 157-62 (1946). (2) Beaudry, J. P., Chem. Eng., 5 5 , No. 7, 112 (1948). (3) Biasell, E. S., Chem. I n d s . , 62, 586 (1948). (4) Bisscll, E. S., Hesse, H. C., Everett, H. J., and Rushton, J. H., Chem. Eng. Progress, 43, No. 12, 649 (1947). ( 5 ) Brown, R. W., Scott, R., and Toyne, C., Inst. Chem. Cng. meeting, College of Technology, Manchester, England, Dec. 13, 1947. (6) Chilton, Drew, and Jebens, IND.ENC.CHEM.,36, 510 (1944). (7) Heiser, H. W., Chem. Eng., 55, No. 1, 135 (1948). (8) Hoffmann, A. N., Montgomery, J. B., and Moore, J. K., IND. ENG.CHEM.,40, 1708 (1948). (9) Kandiner, H. S., Chem. Eng. Progress, 44, No. 5, 383 (1948). (10) Laum, S., Farr L., and Mueller, J. H., Proc. SOC.Ezptl. Bid. Med., 68, 99 (1948). (11) Lyons, E. J., Chcm. Eng. Progress, 44, No. 5, 341 (1948). (12) Mack, D. E., and Kroll, A. E., Ibid., 44, No. 3, 189 (1948). (13) Morton, A. A., and Redman, L. M., IND. ENO.CiInM., 40, 1190 (1948). (14) Olney, R. B., and Carlson, G. J , Chem. Eng. P r o p e s s , 43, No. 9, 473 (1947). (15) Rushton, J. H., IND.CNG.CHEM.,40, 49 (1948). (16) Rushton, J. H., Everett, H.J., and Costich, E. W., presented at November 1947 meetmg, Am. Inst. Chein Eng., Detroit, Mich.
(17) Rushton, J. H., Lichtmann, R. S.,and Mahony, L. H., IND.ENG CHEM.,40, 1082-7 (1948). (18) Scrner, H. W., Chem. Eng., 55, 1, 127-9, 136 (1948). (19) Ibad., 55, No. 8, 118 (1948). (20) Smith, P. M., Proc. Am. SOC.Sugar Beet Technol., 4, 637 (1947). (21) Yabowite, M. L., and Meuron, H. S., J . Assoc. Oficial Agr. Chemists, 31, 127 (1948). RECEIVED October 18, 1R48.
END OF FOURTH ANNUAL UNIT OPERATIONS REVIEW (Reprints of this and earlier Unit Operations reviews may be purchased for 50 cents each from the Reprint Department, American Chemical Society, 11 5 5 Sixteenth St., N. W., Washington 4, D.C.)
