AGITATION in PILOT PLANTS Everett
S. Bissell, Frank D. Miller,
and Harvey
J.
Everett
M I X I N G EQUIPMENT C O M P A N Y , INC., ROCHESTER, N. Y.
I
I?, FOR purposes of discussion, ~e
agitation is impaired, reduced, or misM o d i f y i n g effects of small containers on mixmay define agitation as a unit operaapplied, t h e consequent. chemical ing action are given. Minimum dimensions are tion, we are here concerned with the change will be affected accordingly. shown to be related to physical characteristics application of a unit operation t o a For simplifying the discussion, of materials. The pilot plant for studying agitaunit process. To be successful, such an the general physical effect,s of agitation is contrasted with the pilot plant requiring installation must be capable of production can be divided into the six groups agitation for carrying on the process. General ing the predicted or expected chemical of Table I. However, in even a simple design data are given for vessel shape, dimenchange or changes in the unit proceds case, such as hydrogenation of a, sions, variable-speed drive, horsepower range, itself, and must be able to be varied and vegetable oil in the presence of a finely impeller selection, method of supporting vessel. controlled so as to enable the optimum divided metallic catalyst, a t least three Photographs show several typical designs of points in the unit process t o be achieved. of these functions of agitation are prespilot plant. Methods are also outlined for obThe characteristics and performance ent-namely, gas dispersion, suspension taining quantitative data and for extending of the agitation equipment must of catalyst in a uniform manner, and Dilot Dlant to commercial scale operation. be capable of being recorded quanthe mat,ter of heat transfer t o be imtitatively so that larger scale equipproved by the circulation of the liquid ment, for commercial size installations, can be designed from the in the tank against the side walls or coils. Let us assume that we are t o consider a pilot plant for a btudy data obtained from the pilot plant operation. I n certain cases, before the pilot plant for the unit process is of this kind. Because of the use of hydrogen gas, it might be set up, it will be desirable or necessary to provide a pilot plant arbitrarily decided that the vessel was t o be kept as small as possible. I n considering the application of a n agitator, however, for the at,udy of agitation as a unit operation so as t,o reduce the it would be found that the dispersion of the hydrogen gas was the number of variables which ~ ~ o i i otherwise ld be necessary in the unit process pilot plant. most difficult or controlling factor. h type of impeller would have to be used which would produce refinement of gas bubbles VESSEL SIZE and also be capable of recirculation of the supernatant gas R-hile commercial plants for a given unit process w-ill generally throughout the entire vessel-in other words, of providin,: the fall Tvithin relatively narrow limits of size, capacity, etc., this, physical conditions necessary for the chemical change. Iuadeunfortunately, is not, true n i t h pilot plants. The greatest variaquate agitation a t this point would result either in incomplete tion in pilot plant design a t present lies in the great range of hydrogenation or prolonged time for hydrogenation, possibly conphysical size. The term “pilot’ plant” has acquired the connotacurrent with excessive amounts of catalyst required. Data derived tion in some quarters of “miniature” so that any extremely small from such results would lead to erroneous conclusions regarding piece of equipment, made of met’al, is apt to be considered pilot the economic factors involved and, if translated t o a large scale, plant, equipment. I t is not rare to encounter autoclaves and rewould result in a totally inefficient commercial process. artion vessels as small as 4 inches in diameter and 6 inches in MINIMUM VESSEL AND IMPELLER SIZE height. I n the absence of any guide or data which illustrate t,he I n the pilot plant unit, it is desirable to use an impellei v,hich difficulties to be encountered in deriving information from reis geonietrically similar to that of the full scale plant in order to sults obtained in such small vesels, it is not surprising that the scale-up the pilot plant results successfully. The considerations physical size of t,he reaction vessel is most frequently decided by needed t o be given a n impeller are: (1) the circulation pattern ot,her factors. Consideration of materials available, safety hazin the vessel, ( 2 ) discharge velocity, (3) discharge capacity, and ards involved, amount of room available, and disposal of end(4) a Reynolds number relation. and by-products play an important part in size determination of Experience has indicated that the ratio of tank diameter to the vessel. impeller diameter should be between 3 and 4. This ratio leaves The purpose of t,his paper is to set, fort!h a desirable minimum a clearance of approximately 1 to 1.5 impeller diameters between size of reaction vessel, together with the reasons involved in arthe impeller and tank wall. I n the usual solutions (under 400 riving a t this size and the handicaps t o be expected in dealing with centipoises) a decreased clearance of impeller to tank wall would smaller vessels. It will be shown that, while the function of mixcause a large energy loss a t the tank wall. More clearance would ing or agitation is primarily to produce physical changes, these result in an insufficient velocity to create turbulence throughout physical changes are invariably associated tvit’h chemical changes the tank. inherent in a unit process; consequently, if the physical effect of
Table 1.
BLENDIXG MISCIBLELIQUIDS
Simple mixing of sol. liquids, as in reducing concn.
Grouping of Agitation Functions According to Physical Effect
MIXINGI m m CIBLE
LIQCIDS
Washing of liquids Extraction Contacting Emulsifying
CRYSTAL SIZE CONTROL Precipitation Evaporation systems
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GASABSORPTIOX AND DISPERSIOX Hydrogenation Aeration Gas scrubbing Chlorination Gas washing
SUSPENSION OF SOLIDE Slurries Suspension of filter aid, activated C, fuller’n earth, crystals while dissolving
HEAT TRANSFER Heating Cooling
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INDUSTRIAL AND ENGINEERING CHEMISTRY
427
to scale-up mixing operations. It is therefore believed that a condition exists (analogous to t h a t expressed by the law of continuity of flow) wherein the resulting fluid velocities a t the tank walls are substantially the same for both the small pilot plant vessel and the selected commercial size, the ratio of impeller diameter to tank diameter being maintained as stated above. The law of continuity of flow is generally stated in the following form for flow in nozzles and Venturis:
A1Vl = AzV2 where A = cross section area of stream V = velocity of stream
*
Again borrowing from the hydraulics theory ( 2 ) ,we find that the discharge rate of similar pumps is proportional t o the product of the discharge area and the discharge velocities. I n other words, rate of discharge for similar impellers that operate a t constant peripheral speed varies as the impeller diameter squared. Whereas the volume of the tank in which a n impeller operates varies as the tank diameter cubed, then with fixed ratios of tank to impeller diameters, smaller tanks should have a decreased rate of circulation. For this reason i t is necessary to set minimum tank and impeller diameters for pilot plant purposes. It has been difficult to obtain satisfactory COURTESY. PATTERSON-KELLEV COMPANY, INC. “scale-up” results for impellers smaller F i g u r e 1. Integral U n i t F i g u r e 2. Pilot Plant than 4 inches in diameter. Using the for High-pressure Pilot Agitator Equipment with 4-inch impeller as a minimum and alP l a n t Work, Equipped w i t h C o u n t e r b a l a n c e d Vessel S u p lowing for a ratio of tank t o impeller A g i t a t o r a n d Special Valve p o r t a n d A g i t a t i n g Device diameter of 3.5, a 14-inch-diameter vessel is found to be the minimum. For batch size convenience, a liquid level depth of 15 inches is This discussion leads into t h e selection of tank proportions. selected to give a 10-gallon batch. The pilot plant vessel When tank proportions are selected with consideration for the should have a space allowance for splashing, foaming, volume agitator, the selection should be based on the stream paths t h a t increase, and overflow connection. Three inches are allowed, give the most uniform velocity distribution. For turbine impelwhich puts the minimum pilot plant vessel as 14 inches in diameter lers, the flow is radial from t h e impeller to the tank wall, then and 18 inches in depth. vertical along the tank wall t o the liquid surface and the tank Mechanical factors which also limit the impeller diameter are: bottom, back to the tank center, and finally to the eye of the im(1) The shaft speed becomes a factor since impellers smaller peller. With this general flow pattern, circular patterns have than 4 inches would have a speed of 900 r.p.m. and above; the the least flow resistance. Square patterns are a good compromise speed would be limited where stuffing boxes are involved and also between the least flow resistance and the use of change of direcwhere higher speeds introduce vibration problems. (2) It betion to create turbulence. The most desirable tank shape is incomes difficult to produce a dimensionally similar impeller in dicated as one having liquid level height equal t o the tank disizes smaller than 4 inches. ameter. Finally, in order t o evaluate an impeller, we use the Reynolds There are few factual data on the discharge velocity of imnumber function S N d 2 / p ,where S is specific pellers. Using the centrifugal pump theory as a basis (a), the gravity, N is impeller speed in r.p.m., d is discharge velocities are proportional t o the peripheral velocities. impeller diameter in inches, and p is visThus, with the lack of positive velocity data, 700 feet per minute cosity in centipoises. SNd2/pis inconsistent has been used as a desirable peripheral velocity applied t o probas far as units of measure are concerned, but lems of mixing immiscible liquids, gas absorption and disnersion, they do contain the necessary dimensions of suspension of solids, and heat transfer where turbulence is dethe classical Reynolds number. This sired throughout the solution. formula is useful inasmuch as i t contains the The tendency has often been to increase the peripheral velocidimensions usually measured. This functies of impellers with increase of vessel size. This has been found tion has been checked by plotting it as log unnecessary. By maintaining a fixed ratio of tank diameter to XNd2/M against log of horsepower/SNW. impeller diameter for similar radial flow turbines, the ratio of the The practicability of this function has been discharge area to the flow area at the tank wall is constant for established by a large number of tests covany size tank and impeller. By maintaining this ratio and fixing ering a wide range of impeller types, sizes the peripheral velocity of the impeller, it has been found possible
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F i g u r e 3. Grease o r Paste M i x i n g K e t t l e w i t h Crank M e c h a n i s m f o r Raising o r Lowering Head, a n d Provision f o r V a r y i n g Speed a n d F u l l Selection of M i x e r Elements COURTESY, BUFFALO FOUNDRY
a
MACHINE COMPANY
4
and speeds, viscosity and density of fluid, and tank sizes and baffling. Our tests have shown that this Reynolds number function is relatively independent of the tank diameter and is remarkably consistent for changes of impeller diameter, speed, density, and viscosity. This function indicates only the flow in the impeller, and is affected by factors such as tank dimensions and baffles only as they influence the flow through the impeller. For Reynolds number in agitation, laminar flow occurs in numbers up to 2 , a transition between 2 and 20, and turbulent flow from 20 up. We have obtained numbers in turbulent range as high as 1,000,000. The Reynolds number alone is no more a measure of agitation than any other factor. It is used in the selection of a favorable diameter and speed to perform the mixing operation. These minimum sizes of impellers and vessels should be closely adhered to. The reaction vessel need not, necessarily, have the same capacity of preceding or following equipment. Material may be accumulated or stored ordinarily so that smaller or larger equipment may precede or follow the reaction vessel. G E N E R A L M E C H A N I C A L REQUIREMENTS
The matter of access to the interior of the vessel is of prime importance. With the weight involved in the minimum size vessel recorded, mechanical aids should be provided. It is possible t o design t h e vessel so that it may alternately: (1) be raised, levered, or tipped for emptying or (2) support the vessel in a fixed position, and raise and lower the head.
Where pressure vessels are involved, there is also the option of utilizing a top-entering or a bottom-entering mixer. The topentering mixer is best supported in a cage construction which carries the drive well above the operator’s level and thus frees the head of the vessel for the installation of connecting nozzles, sight glasses, thermometer wells, etc. Since there will normally be more connections in the head of the vessel than in the body or bottom, it is generally recommended that the head remain fixed and that the vessel be raised or lowered for access. Where heated vessels are involved, flexible tubing or flexible joints can be used for connections to the bottom or body of the vessel. Such items are commercially available and in common use. I t is also suggested that metal equipment (not glass-lined) be used for the first setup so that additional nozzles and connections may easily be welded in place if found desirable. This should be undertaken even where corrosion rates are high, wherever the process has not been fully studied, and where changes in the vessel design must be developed as operation proceeds. It is recommended also that the agitator be mounted centrally for simple transfer of data to commercial scale. Off-center positioning of mixers is most critical in small vessels and should be avoided in pilot plant work. It is suggested that a variable-speed drive is essential for such work. I t is desirable t o provide as wide a speed range as possible with constant horsepower. This enables different types of impellers to be operated a t their optimum peripheral speeds, with horsepower kept on a comparable basis. A satisfactory speed reducer of this type is produced by the Master Electric Company, which has a speed ratio of approximately 9 to 1. At present this must be used with a separate bevel gear reducer of 2 to I ratio. With the vessel sizes recommended, a one-horsepower motor is satisfactory for the drive, and will permit the handling of a wide range of viscosities and specific gravities in the liquids to be agitated. If the pilot plant is expected to produce actual studies of agitafion, provision for accurate power consumption measurements must be made. This would normally involve either supporting the vessel so as to measure the torque transmitted ( 1 ) or use of a differential dynamometer (3). Various speeds and consequent horsepowers may therefore be tested as affecting the unit process itself, and the data thus obtained utilized in dimensioning the commercial vessel and selecting the type, characteristics, and horsepower of the commercial size agitator. For many operations, however, the same results can be observed by securing calibrated impellers from the manufacturer who would be expected to provide speed DS. horsepower curve5 for the impellers in water, together with formulas for correction factors for viscosity and specific gravity. This would require only t h a t the type of impeller and its diameter be recorded together with accurate measurement of actual r.p.m. which should be obtained by tachometer. TYPICAL CONSTRUCTIONS
Figures 1, 2, and 3 are illustrations of pilot plant apparatus involving agitation. Figure 1 is a high-pressure vessel with a fixed-speed stirrer. This is a self-contained and complete element, but does not, provide facilities for variable speed or for raising or lowering either head or vessel, etc. A special valve which does not retain “dead pockets”, as illustrated, is frequently a necessity i n small
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INDUSTRIAL AND ENGINEERING CHEMISTRY
vessels for ease in cleaning and preventing contamination of succeeding batches. Figure 2 shows a self-supporting unit with counterbalance arrangement so that position of vessel can be changed if desired. Space has been gained in the vessel head by using a nozzle attached at an angle instead of the customary normal entry. The design details shown in Figures 1 and 2 can, of course, be combined in a n over-all design which would utilize structural steel cage support, swing joints, and flexible tubing so that the head may remain fixed while the vessel is lowered. The support of the vessel should be at the most comfortable height for the most difficult function involved, including the operations of loading, draining, cleaning, temperature measuring, sampling, and visual observation. Figure 3 shows a complete assembly for handling materials similar to paste, greases, etc. Provision for raising and lowering the mixing head is built into the equipment, and extreme flexibility is obtained by interchanging mixing elements. Speed is apparently vaned by changing flat belt pulleys. I n connection with handling paste, greases, eto., the choice of the mixing elements themselves is very critical. A piece of equipment, as illustrated, is extremely valuable to anyone experimenting in the paste range of materials. In fluid mixing, however, the type of impeller is not critical, the choice most frequently being made on a mechanical limitations basis. Therefore, in fluid mixing it is generally possible to equip the pilot plant with only a single type of impeller, provided this impeller type is capable of being scaled up to the largest anticipated commercial size, and provided the pilot plant is designed to develop the optimum conditions for the impeller through proper selection of size, variable speed, and other factors noted above. A single type or, at most, two types of impellers can be selected Q I ~the basis of final commercial size, physical characteristics of
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material to be handled, and over-all duty requirements of the agitator. Only when all of these factors are unknown at the time of designing the pilot plant is it necessary to make provision for trying out a wide variety of impellers. It is suggested as important that the complete pilot plant for any unit process include, wherever possible, actual equipment representing each unit operation essential in the unit process itself. If a pilot plant is to produce complete.results, both as to the chemical reactions involved ahd as to a study of the mechanical equipment and factors involved, greater attention must be placed on the mechanical equipment than appears to be common practice. For example, many times pumps are omitted from a pilot plant because the quantities handled are small enough to be transferred by buckets or by gravity. Consequently, no information is available for the engineers who design the commercial siae plant. It has also happened that mechanical methods for a unit operation are used in pilot plant work which are incapable of scaling up to commercial size. For example, a rocking tank producing agitation by shaking can hardly be scaled up beyond a relatively small batch size, owing to the massive equipment that would be involved. The authors do not suggest complete standardisation of that portion of the pilot plant involving agitation. They do believe, however, that the factors presented here are important i n unit process pilot plant design, and it is hoped that these factors will provide a general guide for those approaching such problems. LITERATURE CITED
(1) Cooper, C . M., Fernstrom, G . A., and Miller, S. A., IND.EN@.
CHEM.,36, 504-9 (1944).
(2) Gibson, A. H., “Hydraulics and Its Application”, 4th ed., 1930.
(3) Miller, F. D., and Rushton, J. H., IND.ENQ.CRBM.,36, 499 (1944).
OXIDATION of FERROUS SULFATE SOLUTIONS with OXYGEN Kenneth A. Kobe and William Dickey HE oxidation of ferrous sulfate solutions by atmoepheric oxygen has been studied considerably by analytical chemists, who have expressed surprise at the slowness of the reaction (6). They report a variety of conflicting results, probably arising from the differences in experimental methods. I n a physiGal-chemical study of the activation of the oxygen electrode, Lamb and Elder (6) studied a number of variables in this reaction and corrected many of the erroneous conclusions of earlier workers. I n general, they determined the hours required for a n oxidation of 1% of the ferrous ion a t 30” C. Their results will be compared with those obtained in this work.
UNIVERSITY
OF TEXAS, AUSTIN, TEXAS
Industrial studies of this reaction were made by Reedy and Machin (8) who state that the initial concentration of ferrous sulfate makes little difference in the velocity of the reaction. They secured complete oxidation by circulating the solution five times over crushed pyrolusite. Posnjak ( 7 ) determined the hours required for oxidation of 0.1 and 0.5 N ferrous sulfate solutions at room temperature. Agde and Schimmel ( I ) bubbled air or oxygen through ferrous sulfate solutions at various temperatures a n d ’ pressures. As discussed by previous workers (7, 8),
The factors controlling the oxidation of the ferrous iron in a solution OF ferrous sulfate, ferric sulfate, and sulfuric acid have been studied, using both oxygen and air. The percentage oxidation of ferrous sulfate is shown here graphically on curves with respect to time, partial press!re of oxygen, temperature, acid concentration, and catalyst concentration. The oxidation OF ferrous sulFate in acid solution by oxygen is a very difficult reaction, and good conversions to ferric sulfate occur only at quite high temperatures and pressures over a considerable time period.
