Studies in Agitation 111. The Simple Agitator as a Classifier A. MCLARENWHITEAND S. D. SUMEFWORD, University of North Carolina, Chapel Hill, N. C. N A P R E V I O U S communication it has been shown that not Only is there
I
The variafion in screen analysis with position studied for sand suspended in water. The Coarse material is found to concentrate beneath the Paddle, while h? material is distributed irregularly through the mass of liquid. The use of the simple agifutor as a classifier is suggested.
a lack of uniformity in the distribution of sand in a tank of water stirred by a simple paddle agitator, but that there is also a variation, with position in the tank, in mean p a r t i c l e size. This latter phenomenon has now been more fully investigated and is the subject of this report.
EXPERIMENTAL PROCEDURE The appsratus used, a tank 52 inches (132.1 cm.) in diameter containing a paddle 25 inches (63.5 cm.) long with lower edge 5-25 inches (13,335 cm.) from the bottom of the tank, has been previous1 described (1,9). Owing to the necessity of obtaining samples &r screen analysis, the size of the samples taken was increased 80 as to yield at least 25 grams and in most Cases 100 grams of sand. The suspension of sand in water was withdrawn through sampling tubes, previously described, into galvanized iron tubs. After settling, the water was siphoned and filtered off, the sand dried and weighed, and a screen analysis run on a Rotap machine with Tyler standard screens. Duplicate runs
FIGURE 1. SCREEN ANALYSES checked within the error inherent in screen analyses. Since lar e volumes of water were withdrawn, the water level in the tan% was maintained constant at the arbitrary value of 2 feet (61 cm.) by the intermittent addition of quantities of fresh water. The withdrawal of 100 grams of sand from a tank containing 30 pounds (13.6 kg.) of sand did not materially affect the sand concentration. At the conclusion of a run, the analyzed sand was returned to the tank. Samples were withd r a m at the following points: (a) 2 inches (5 cm.) from side, 2 inches from bottom; ( b ) 2 inches from side, 14 inches (35.6 cm.)
?;Fbottom;~ ? and~ e( d' ) ,14e inches ) 2 1 ~ n ~ ; ~ ~ from side, 14 inches from bottom.
I n a series of p r e l i m i n a r y experiments, 32-35 mesh sand was mixed in varying proportions with 80-100 mesh sand. It was found that a t agitator speeds of less than 40 r. p. m. for sampling points a, b, and d, the sand contained less than 1 per cent of material coarser than 80 mesh, the amount increasing with agitator speed and with the amount of coarse material added. Sampling point c, beneath the paddle, showed high percentages Of materia1, the amount inCI'eaSing with total coarse material in the tank and with agitator speed. At this point sand containing 80 per cent larger than 80 mesh was readily obtained for a ratio of coarse to fine of o.75 or greater, These results indicated that adequate separation could be obtained by Proper selection of sampling points; therefore, experiments were instituted on the separation of an unsized sand. Thirty-one pounds (14.1 kg.) of sand were added to 220 gallons (833.8 liters) of water, and agitated a t the selected speed until a steady state was attained. Samples were withdrawn a t the points given above for agitator speeds of 27,33,40, and 54 r. p. m. I n Figure 1 are plotted cumulative screen analyses us. logarithm of Tyler screen mesh for runs a t 54, 33, and 27 r. p. in., together with a plot of the screen analysis of the original sand. The sand concentration in grams of sand per 100 cc. of water is given for each sampling point. At high agitator speeds fine material is almost completely removed from beneath the paddle, as shown by curve c of Figure l A , while above the paddle, curve d, little coarse material is present. Near the wall of the tank, curves a and b indicate sand of analysis intermediate between those a t c and d. As the agitator speed is reduced, curves a and b drop markedly, the percentage of coarse sand a t the wall becoming small, as shown in Figure 1B and C. Change in agitator speed does not have an appreciable effect on the screen analysis of the sand a t point d above the paddle. As speed is reduced, curve c for the sand beneath the paddle becomes gradually lower until a t a speed of 27 r. p. m. the sand a t this sampling point contains appreciably more fine material than did the original sand. DISCUSSIONOF RESULTS The screen analyses above represent an equilibrium condition in the tank since very small amounts of sand were withdrawn in comparison to the total amount present. These data are therefore not directly applicable to a continuous process in which sand is removed in several streams as fast as it is fed to the system. The Screen analyses given, if applied t o material balances, indicate incomplete removal of certain sizes of sand. This is particularly noticeable in Figure IC, where fraction c, the coarsest obtained, contains fine material than the original sand. If this particular system were to operate continuously-withdrawing sand at points c and d, for example-the system would build UP
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January, 1934
INDUSTRIAL AND ENGINEERING
coarse material until a steady state obtained in which input equaled outgo, with screen analyses for the new effluents different from those here given, provided the agitator speed is sufficient to keep large particles in motion. By repeated experiment it would be possible to find sampling points in a batch experiment such that material balance considerations for a continuous process would be fulfilled. The particular data given here are merely an indication of the type of separation that may be expected. For a continuous process the total amount of sand present in the tank will adjust itself to a steady state value, depending on the rate of feed, and the screen analyses of the various effluent streams will then, for a given agitator speed, depend on the position of the draw-offs. It is suggested that in separating a material of uniform density into two size fractions, one draw-off should be a t the center of the bottom of the tank. This will remove the largest particles. If the second draw-off is above the paddle, near the center, it will tend t o remove fines only, or, if placed near the wall of the tank, it will contain appreciable amounts
CHEMISTRY
83
of coarse material, with a consequent increase in mean particle size of the effluent from the bottom discharge. If three fractions are desired, the draw-offs should be a t the center of the bottom (coarse), above the paddle near the center (fine), and near the wall of the tank (intermediate). In any event, the agitator speed must be sufficient t o keep the largest particles in motion or there will be a tendency for these t o remain in a layer on the bottom of the container. Since the concentration of solid in liquid is greatest beneath the paddle, comparatively little liquid would be removed at this point; the solid might in fact be allowed to settle into a salt catch. The very fine material above the paddle is found in low concentration, indicating that here large amounts of liquid must be removed.
LITERATURE CITED (1) White and Sumerford, IND.ENQ.CHEM.,25, 1026 (1933). (2) White, Sumerford, Bryant, and Lukens, Ibid., 24, 1160 (1932).
RECEIVED August 8, 1933.
Influence of Sodium Carbonate upon the Producer Gas Reaction CLARENCEB. WEISS AND ALFREDH. WHITE,University of Michigan, Ann Arbor, Mich.
Fox and White have shown that the reaction 3CO proceeds to the right Na2C03 2C = 2 N a at temperatures above 800" C., and that the vapor pressure of the reaction products surpasses atmospheric pressure about 1025" C. I n their work the reaction products were cooled rapidly to prevent reversal of the reaction. I n the present study the reaction products are allowed to cool more slowly, as would be the case in a gas producer, and it is found that reversal is rapid and complete in ihe range 750" to 900" C. The study of the application of this reaction io gas producer practice is made in a miniature gas producer consisting of a nickel tube inserted in a n electricfurnace. W h e n using untreated Acheson graphite and dry air at 900" C. and t~time of contact of 2 seconds, the exit gases contain 6.6 per cent carbon monoxide: under similar conditions using graphite impregnated with sodium carbonate, 33.4
+
+
T
HE effect of sodium carbonate in promoting the rate of the reaction of carbon dioxide has been ascribed by Dent and Cobb (1) to an alteration of the nature of the carbon surface exposed to the gas. Fox and White (2) explained the apparent increase in the reactivity of graphite which was impregnated with sodium carbonate as being due to a reaction between the carbon and sodium carbonate which was measurable a t temperatures above 750" C:. and which evolved a continuous stream of gaseous products a t temperaturesabove 1025" C. according to the equation: 2C Na2C03 2Na 3CO
+
+
They held that the vapors of sodium and the carbon monoxide evolved a t the surface of the carbon ruptured the stagnant gas film at the surface of each particle and permitted more ready reaction between the carbon and carbon dioxide.
per cent carbon monoxide is*found. As little as 0.1 per cent sodium carbonate is suflcient to give this result. The decomposition of moistened air is accelerated in a similar way. The sodium carbonate is decomposed in the lower part of the producer where the temperature is high, and is regenerated in the upper and cooler part of the producer where it deposits upon the graphite and is carried down mechanically to the reaction zone. B y using soda, a satisfactory gas composition is obtained at a maxim u m temperature of 925" C., while without soda satisfactory operation is not possible eeen at 1040" C. Part of the soda is carried out of the producer with the fuel ash, and the extent of this loss has not been suficiently studied. If it were riot for this loss, a small amount of soda would last a long time, since it is continuously recycling within the producer. Furthermore, the sodium vapor, after getting out into the main stream of gas, reacted with a molecule of carbon dioxide to form sodium oxide and carbon monoxide, and the sodium oxide reacted with a second molecule of carbon dioxide to regenerate the sodium carbonate. Keumann, Kroger, and Fingas (3) reported that potassium carbonate behaved in a similar manner, and they gave a similar explanation of its behavior. Fox and White cooled the reaction products so quickly that reversal was prevented. The temperature of reversal of this reaction formed the first part of the present study. It was made in an electrically heated nickel tube placed in a furnace with two separate heating elements. The lower half of the tube contained granules of Acheson graphite impregnated with sodium carbonate and was maintained at 1050" C. The upper half was empty and was kept a t 750°C. When
