SYMPOSIUM ON
DRYING AND AIR CONDITIONING’ Drying Commercial Solids R. C . ERNST, D. B. ARDERN, 0. K. SCHMIED, AND F. M. TILLER University of Louisville, Louisville, Ky.
The air drying of commercial Prussian blue is believed t o be typical of gelatinous solids that adhere firmly to the drying trays. The drying rate curves for this solid do not exhibit the normal constantand falling-rate periods. A rate approaching zero is experienced during the last stage of the constant-rate period and just preceding the falling-rate period. This would lead t o the belief that the capillary forces in the solid were controlling during that interval. Drying in vacuum at lower temperatures and lower vacuum produced drying rate curves in which the zero-rate period was not so pronounced as in air drying. A t higher temperatures and higher vacuum the zero rate period did not exist. This type of solid should be dried on screens or in a rotary dryer. The drying curves show for the first time the relation between air and vacuum drying of solids. OST theoretical work on the mechanism of drying (1,4, 6, 6) has been confined to ideal solids. The drying of industrial materials is not necessarily identical with that of the perfect solids, and some investigators are broadening the accepted drying theory to include other than ideal solids. In this presentation a brief summary of semicommercial experiments performed over a period of years to test rigorously the accepted principles with a solid unlike any
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theoretical solid are discussed. A commercial paint pigment, Prussian blue, was dried under various conditions of humidity, temperature, and vacuum.
Method of Moisture Transfer I n drying a very wet solid, the first period corresponds to the evaporation of a saturated solution, and the rate of drying is largely affected by the solubilitycharacteristicsof the particular solid. Liquid is supplied both by that mechanically present on the surface and by diffusion from within the cake during this constant-rate period. Moisture transfer to the surface occurs by capillary action and diffusion of vapor through the pores of the material. The size of the capillaries, drying temperatures, and external pressure largely determine whether moisture transfer to the surface will be primarily by liquid or vapor movement or a combination of both. Before drying commences, there are a large number of small voids on the surface that contain moisture. As these voids become exhausted after the start of the drying, a transfer of moisture to the surface is begun. As long as the supply of moisture and vapor to the surface is constant, the rate of drying remains constant. When liquid is no longer supplied to the surface in sufficient quantities to keep up the rate of evaporation, the constant-rate period ends. The critical moisture content depends on the factors affecting the diffusion rate. With low diffusional resistance and a slow rate of drying, the constant-rate period is prolonged and the critical moisture content is low; but with a high vaporization rate and large diffusional resistance, the drying rate falls off at high moisture contents (7). When heat is applied internally 1 Fourth Chemioal Engineering Symposium held under the auspices of the Division of Industrial and Engineering Chemistry of the American Chemical Society a t t h e University of Pennsylvania, Philadelphia, Pa., December 27 and 28, 1937. Previous papers in this symposium appeared on pages 384 and 388 of the April issue, on page 506 of the May issue, and on pages 993 to 1010 of the September issue.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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FTGURB 2. DRYINGRATECURVESAT A SHELF TEMPERATURE OF 100-130" c. (212-266O F.) Vacuum: A , 18 inches;
B , 22 inches;
C, 26 inchas
FIGURE1. DRYINGRATECURVES A . 140' F.: 10 and 15 per cent humidity B . 160° F.: 10, 15 and 20 per cent humidity C. 180° F.; 10 and 15 per Oent humidity
to the drying slab and the external pressure is lowered, increased diffusion results because of a greater pressure differential. This tends to lengthen the constant-rate period considerably, as will be shown later. The type of capillaries have a large effect on the liquid diffusion. Solids with firm, closely packed structures have large capillary forces, but materials with loose, fibrous structure exert small tensions (6). The capillaries supply moisture to the surface as soon as vaporization begins, and as water is brought to the surface, it is necessarily replaced by air in the small passages. As water is brought to the exterior, it is necessarily replaced by air. As capillary rise varies inversely with the diameter, the liquid flows in the direction of cspilh i e s having the smallest cross section. When the small internal reservoirs have been depleted, moisture continues coming to the surface until the external capillary diameters equal the internal diameters. Then water will be vaporized in the capillaries and the zone of evaporation will recede into the cake.
Some capillaries have large surface openings and small internal openings; others have their smallest openings in the surface. During the course of drying, the tubes with their smallest openings in the interior draw liquid inward, and then it is brought back to the surface (3). During the constantrate period the inside of the cake is at a low temperature, and little evaporation occurs from the water receciing into the cake. During the falling-rate period and in vacuum drying, considerable amounts of water may be vaporized internally.
Experimental Procedure Commercial Prussian blue, a paint pigment furnished by the Kentucky Color and Chemical Company, was used in the experimental work. The blue was prepared by adding sulfuric acid and ammonium sulfate to ferrous sulfate and then precipitating with sodium ferrocyanide. The resulting precipitate was oxi; dized by sodium chlorate at temperatures ranging from R O O to 100 C. (140' to 212" F.). The pigment was washed by derantation over a period of 3 weeks and was then filtered in a plate-and-frame filter press. Semicommercial batches of 50 pounds containing about 50 per cent moisture were used in the air drying. In the vacuum drying, smaller batches of about 12 pounds were employed. The air drying was carried out in a twelve-tray H-W conditioner. The air was heated by passing i t over steam coils, and the humidity was controlled by steam injection and recirculation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
OCTOBER, 1938
Temperature and humidity conditions of incomin and exhaust air were determined by means of wet- and dry-bulf thermometers. The vacuum drying was effected in a vacuum shelf dryer consisting of a cast-iron shell with three steam-heated shelves. Copper-constantan thermocouples placed in the top, middle, and bottom of the cake were used to determine the cake temperatures. Runs lasting 28 hours were made in the air drying at humidities ranging from 10 to 20 per cent and temperatures from 120" to 140" F. Vacuum drying runs lasted from 8 to 12 hours and were carried out at vacuums of 18 to 26 inches and at shelf temperatures of 100" to 130" C. (212" to 266" F.) The rate of drying curves were obtained by determining the slope of the moisturetime curves and plotting against moisture content.
Vacuum Drying The drying rate curves of the pigment a t varying vacuums and temperatures are plotted on Figure 2. At 18 and 22 inches of mercury there are noticeable breaks in the drying curves which are not so pronounced as in air drying. These discontinuities occur only at the lower shelf temperatures; and as the steam shelf temperature and rate of drying were increased, the shrinkage became much greater. The increasing shelf temperature must not be confused with increase in cake temperature, as the cake temperature depends only on the total pressure within the dryer and not on the shelf temperature during the constant-rate period. Increased shrinkage and crumbling by higher shelf temperature is a function only of the greater rate of moisture removal and not the shelf temperature. The greeter shrinkage caused the cake to crumble and pull away from the tray more rapidly, and the dip in the curve was minimized with increasing temperature. At the highest shelf temperature no break occurred. It was only a t 26 inches of mercury that no cessation of drying was noticed a t the lower temperatures. This gives a conception of the magnitude of the vacuum caused by the solid's adhering to the drying trays. The effect of applying heat internally to a drying slab is strikingly illustrated in the vacuum drying curves. I n each case an increase in shelf temperature is accompanied by lengthened constant-rate period. At 130' C. (266°F.) it is remarkable that the constant-rate period extends over such a great moisture content. Similar results were obtained in the vacuum drying of Sil-0-Cel (3). The heat was supplied through the shelves to the bottom of the cake, and each increase in shelf temperature correspondingly increased the diffusion rate and the amount of water available for evaporation. The variations of cake temperatures are shown on Figure 3. These two sets of curves are typical of the data obtained on the drying in vacuum. Temperatures were recorded a t the top, middle, and bottom of the cake. At a shelf temperature of 120" C. (248" F.) and a vacuum of 18 inches of mercury in Figure 3A the cake temperature rose to a temperature corresponding to the boiling point of water at the total pressure on
Air Drying The rate of drying curves of a few representative runs of Prussian blue are shown in Figure 1. The evaporation rate proceeded normally a t first and at the end of the initial period suddenly decreased. The rate then rose sharply and fell off gradually as it would during the falling-rate period. The blue was exceedingly wet a t the beginning of the runs and firmly adhered to the drying trays. Drying proceeded normally a t first while the surface moisture and internal reservoirs were being exhausted. When most of the cavities were emptied, the only moisture supplied to the surface was that held in the capillaries themselves. The liquid began to rise in the capillaries, and as the bottom of the cake adhered firmly to the tray, air was unable to enter the bottom openings. As the liquid rose, a vacuum was formed in the capillaries and liquid was unable to rise to the surface. With the supply of water diminished, the rate of drying immediately decreased. Some moisture vaporized in the capillaries and the rate never reached zero. The solid which had been shrinking and crumbling pulled away from the bottom of the tray, and air entered the capillaries. With a new supply of moisture a t the surface, the drying rate rose immediately and drying proceeded normally (2). From the suggested mechanism, the break in the curve would occur when the only moisture in the cake was heldin the capillaries. This would occur after all the internal reservoirs had been exhausted at the end of the constant-rate period. This is supported by the rate curves as the diminished drying rates occur a t the end of the constant-rate period. SHELF TEMPERATURE
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FIQURE 3. DRYING RATECURVES AND CAKETEMPERATURES AT A SHELF TEMPERATURE OF 120' Vacuum:
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T I M E I N HOURS
c. (248' F.)
A , 18 inches; B, 26 inches
If the resultant force caused by the vacuum seal could be lessened sufficiently, the break in the drying curve would not occur. It was proposed that in vacuum the effect of the vacuum seal would be minimized, and a t high vacuums no discontinuity would occur.
the dryer. At the end of the constant-rate period the temperature a t the bottom and top of the cake rose before that a t the middle of the slab. The temperature a t the middle reached and passed the temperature a t the top of the cake during the falling-rate period. There was good heat-transfer
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through the bottom of the cake, and the temperature there rose first. The top was being heated by superheated vapor and radiation, and the surface temperature rose before the temperature at the middle. More heat was transferred from the bottom than from the top, and temperature at the middle reached and then passed that on the surface. At the highest vacuum and temperature a different phenomenon was encountered. I n Figure 3B temperature and rate curves at a shelf temperature of 120' C. and a vacuum of 26 inches of mercury are plotted. Similar results were obtained in runs a t the same vacuum and at shelf temperatures of 130" C. The rate of evaporation was very rapid and the moisture was quickly depleted at the surface. Consequently the surface temperature rose, and a considerable amount of water was vaporized internally in addition to the upper surface evaporation. This result is similar to that experienced by Ernst, Ridgway, and Tiller (3) in the drying of Sil-0-Cel. The temperature a t the middle and bottom of the cake surpassed the surface temperature during the fallingrate period.
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Acknowledgment The authors wish to acknowledge the fellowship assistance of the Louisville Oil, Paint, and Varnish Club as aid in carrying out this investigation, to the Kentucky Color and Chemical Company for supplying raw materials, and to S. Miller and E. Musterman for assistance in obtaining data. Literature Cited (1) Ceaglske, N. H., and Hougen, 0. A., Trans. Am. Inst. Chem. Engrs., 33, 283 (1937). (2) Comings, E.W., and Sherwood, T. K., IND.EKQ.CHEM.,26,1096 (1934). (3) Ernst, R. C., Ridgway, J., and Tiller, F., Ibid., 30, 1122 (1938). (4) Newman, A. B., Trans. Am. Inst. Chem. Engrs., 27, 203, 310 (1931). (5) Sherwood, T. K., IND.ENQ.CKEM.,21, 12 (1929); 24, 307 (1932); 25,1134 (1933); 26,1096 (1934). (6) Sherwood, T. K., Trans. Am. Inst Chem. Engrs., 32, 150-68 (1936). (7) Weisselburg, Arnold, Chem. & Met. Eng , 39,427 (1932). RECEIVDD May 27, 1938.
Practical Vacuum Drying R. C. ERNST, J. W. RIDGWAY, AND F. M. TILLER University of Louisville, LouisviIle, Ky.
During the past decade, many investigators have contributed to the theory of air drying ; however, relatively little work has been reported on drying in vacuum (3). Experimental work was undertaken to determine the mechanism of drying at diminished pressure. The effects of varied conditions of temperature and pressure upon a noncompressible solid were studied. Steam shelf ternperature from 100" to 130" C. (212" to 266" F.) and vacuum from 18 to 26 inches of mercucy were employed.
A drying solid may be considered as a complex system of capillaries extending from the surface into the interior of the solid and connecting small voids throughout the cake. Before the liquid reaches the surface, i t may travel many times the thickness of the cake. The surface is connected with comparatively large cavities in the cake which act as liquid reservoirs. Figure 1 shows an idealized solid with capillaries extending from the surface into the interior. The flow of liquid is indicated by solid arrows and of vapor by dotted arrows. The capillaries in an ideal solid are in random distribution; some have their smallest ends in the surface, A , and others have their largest openings in the surface, B.
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ASICALLY vacuum drying is similar to air drying, but there are several fundamental differences. Any pressure below atmospheric falls into the classification of vacuum drying, but in practice it is seldom oarried on under less than 18 inches of mercury. The first stage of vacuum drying is usually, though not necessarily, a constant rate period. As the surface water is removed rapidly, moisture is supplied mostly by capillarity and vapor diffusion. The rate of drying remains constant as long as the total latent heat of the available water is greater than the heat flow into the solid. By available water is meant any water that is free to vaporize, regardless of its position in the cake. The concept of available water is important since a large amount of water may evaporate internally during vacuum drying and diffuse t o the surface as vapor. When the heat flow into the solid just balances the latent heat of evaporating moisture, the critical point is reached, and beyond this, the falling-rate period begins.
INTERNAL RESERVOIRS
FIGURE 1. IDEALIZED SOLIDWITH CAPILLARIES EXTENDING FROM TEE SURFACE INTO THE IKTERIOR
Those capillaries with large surface openings will draw water into the cake, but i t will, in turn, be brought back t o the surface by the capillaries with their smallest openings in the surface. The vapor pressure of the capillary water is lowered because of the concave liquid surface; and in air drying at atmospheric pressure there is little evaporation from the receding liquid columns. I n vacuum drying a t low pressure, however, there is considerably more vaporization from the receding capillaries with subsequent diffusion of vapor t o the surface. Thus, in vacuum, drying is effected across the entire cross section of the solid and not primarily a t the surface as in
