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INDUSTRIAL AND ENGINEERING CHEMISTRY
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.
VOL. 30, NO. 10
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.
-$-$-Trl-
u LIQUID
-
COLUMNS
B
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
OCTOBER, 1938
1123
INDUSTRIAL AND ENGINEERING CHEMISTRY
In Figure 3 the variation of the vacuum space temperature is plotted with cake temperature and drying rate curves to show the relation. This curve is typical of other runs and is the only one shown. The temperature of the internal vapor from the solid was a t a higher temperature than the cake but did not become constant until nearly 30 per cent of the moisture had been removed about 2 hours after the start of the run. During the initial period 60 per cent of the total moisture was removed. When 9 per cent of the moisture had been removed during the falling-rate period, the vapor temperature began t o rise and to approach the steam shelf temperature.
Cake Temperature
FIGURE 2. THREE-SHELF VACUUMDRYER
air drying. Block (1) maintained this fact as far back as 1919, but he assumed that only vapor and no moisture was transferred through the solid.
The general variation of cake temperature is shown in Figure 3. The cake reaches an equilibrium temperature in half an hour. At the start of drying, the top and bottom of the cake were heated slightly faster than the middle, indicating that heat was being transferred into the solid from both the bottom and the top. The temperature throughout the solid remained constant until 7 5 per cent of the water had been removed. At this point, which is about half way through the falling-rate period (on a moisture content-rate curve, rather than time-rate curve), the temperature at the top of the cake began t o rise and indicated that the zone of evaporation was receding into the solid. This temperature rise followed that of the vapor. With the removal of an additional 7 per cent of water, the temperatures a t the middle and bottom of the cake followed an upward trend simultaneously. As the temperature a t the top of the cake rose sooner than a t the middle or bottom, it is obvious that the flow of heat into the cake from the superheated vapor and by radiation exceeded the flow into the cake from the shelf on which the cake rested. Had the top thermocouple been subject t o radiation, 1.00,
I
I
I
I
I
I W
110°
Vapor Temperature The temperature of the vacuum space depended upon the temperature of the vapor passing through it and was necessarily at a higher temperature than the cake, as it received heat from the steam shelf directly above it. The temperature of the vapor, which was actually superheated steam, depended upon radiation, distance between shelves, and contact of the vapor with the shelf. High vapor temperature means a considerable loss of sensible heat. High vacuums give lower vapor temperatures and less loss of heat, but result in an increased load on the pump.
: a
Apparatus Drying was carried on in an experimental three-shelf vacuum dryer provided with usual auxiliary equipment. The dryer (Figure 2) has three welded steel shelves, each 24 X 24 inches; the internal dimensions of the dryer are 25 X 25 X 11 inches high. Heating was accom lished by introducing steam into the shelves. Commercial S i d C e l , a filter aid, was chosen as the drying solid because it is noncompressible, undergoes no chemical change, and remains consistent at high water content (70 per cent moisture, wet basis), and has a change of color on drying. The cake size was 8l/2 X S1/2X l1/*inches. Copper-constantan thermocouples calibrated t o within '0.5O C. were used to determine cake and vacuum chamber temperature. Thermocouples were placed in the top, middle, and bottom of the cakes, and in the steam shelf, vacuum space, and up er wall. i l l experimental data were taken over a 12-hour period, the control temperature was maintained within *0.5' C . , and the vacuum was kept within *"/la inch of mercury. Moisture content-time curves were plotted, and the instantaneous rates of drying were determined from the slope of these curves.
2
looo
z W 0
80"
0 W
s
80 60"
K 3 I-
B 40'
$ W I-
I
le
1
IO
I
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8
6
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I
I
2
0
T I M E I N HOURS
FIGURE3. DRYING RATEAKD CAKEAND VAPOR TEMPERATURE CURVES AT %-INCH VACUUM AND SHELF TEMPERATURE OF 110" c.
a,
0, rate of drying. 0 , vapor temperature; temperature of top of cake; A ,temperhum of middle of cake; X , temperature of bottom of cake
its temperature would have gradually risen from the start of drying instead of remaining constant as indicated in Figure 3. With solids, such as Prussian blue, that adhere more firmly to the drying trays than Sil-0-Cel, heat may be transferred more readily through the trays than from the vapor. Experimental data show this to be true ( I ) . The cake temperature approached the steam shelf temperature. It is particularly significant that temperature differences of more than 20" C. (36' F.) existed within the cake during the falling-rate period ; this is a rather large temperature differ-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1124
130'
120°
VOL. 30, NO. 10
crease in the amount of vapor, and this accounts for the constancy in the wall temperature. Low temperatures at low vacuum are nearly useless because the heat given to the vapor is so small that the walls are not heated above the condensation temperature. The vapor must be heated enough to prevent condensation, but any temperature above this minimum will result in increased radiation and heat loss.
Rate of Drying
0
a
3 0
f
130'
u.
120°
0
54
W >
fn
n
z a
2
0 130' 120° 1100
SHELF TEMPERATURE. The effect of shelf temperature upon the rate of drying is shown in Figure 4. The vacuum was constant in each case at 18, 22, and 26 inches of mercury. The rate of drying was increased approximately 15 per cent per 10'C. (18'F.) rise in shelf temperature a t the lower temperatures. The increase in drying rate was less pronounced at higher temperatures. With increasing vacuum and temperature the constant-rate period was lengthened. VACUUM.The effect of vacuum changes upon the rate of drying is shown in Figure 5. These drying curves are for a constant shelf temperature of 130" C. (266' F.) and vacuums of 18,22, and 26 inches of mercury. The curves indicate that little variation results in the rate of drying when the vacuum is changed. This is true only for relatively high vacuums where there is a large difference between the boiling point of the water a t the dryer pressure and the steam shelf temperature. The rate of drying depends more on the steam shelf temperature than on the vacuum. High vacuums are not necessary for good rates of drying. Conclusions
looo
Vacuum drying is especially adapted to drying substances that decompose or undergo undesirable physical changes at high temperatures. The drying can be accomplished at a vacuum high enough t o give a total pressure corresponding to a boiling point of water below the transition temperature.
0
40 POUNDS
120
80 H20/100
160
200
POUNDS DRY SOLID
FIGURE 4. EFFECTOF SHELFTEMPERATURE ON DRYING RATE A , vacuum 18 inches;
B , 22 inches;
C,26 inches
K
0.80
3 0
5
0.60
0
5 2 0.40 W >
ence for a cake only ll/s inches thick. Excessive internal strains may develop when high shelf temperatures are employed. Unlike air drying, the cake temperature approaches the boiling point of the liquid rather than the wet-bulb temperature during the constant-rate period. Under diminished pressure the slab temperature remains constant during the first period, regardless of the steam shelf temperature. On the contrary, in air drying, where the liquid is usually below its boiling point, an increased temperature of the drying media results in higher cake temperatures. I
Wall Temperature
Condensation must not occur on the walls because the dryer would act merely as a reflux, and no appreciable water removal would be effected. The temperature of the walls depends upon the heat conveyed to them by the water vapor, and remains low until moisture is vaporized a t the start of drying. It is interesting to note that the wall temperature remained constant during the falling-rate period. Although the vapor temperature rose, there was a corresponding de-
%
=
0.20
fn
n
z
3
B
POUNDS H p 0 / 1 0 0
POUNDS DRY SOLID
FIGURE 5. EFFECTOF VACUUM ON DRYINGRATE A SHELFTEMPERATURE OF 130" C. AND VACUUMOF 18 TO 26 INCHES
CURVESAT
Accurate control can easily be obtained by inserting a thermocouple in the top of the drying cake. Any steam pressure can be used. When the falling-rate period is reached and the cake temperature begins to rise, it is necessary to cut off the steam immediately, for the surface temperature rises rapidly (about 15' C. or 27' F. per hour in this investigation). From this point on, the drying must be completed by bleeding steam into the shelf, or better still, by the use of hot water. As the cake approaches the shelf temperature, the water can be used a t the desired temperature.
OCTOBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
For drying materials not affected by temperature, any vacuum above 18 inches of mercury will give a good drying rate. The higher the steam pressure, the greater the drying ratewill be up to a certain point. The rate willincreasewith increasing temperature until the pump is unable to remove vapor as fast as it is formed. Pressure would then build up and reduce the rate. The maximum temperature that can economically be used will depend upon the individual pump. If this temperature is exceeded, the vapor will not be removed so rapidly as it is formed, and the rate of evaporation will be lowered as pressure builds up. I n this investigation approximately 75 per cent of the water was removed in half the drying time. The temperatures within the dryer are exceedingly important in determining the quality of product as well as the rate of evaporation and dryer efficiency. The method of heat flow into the solid is entirely dependent upon the physical properties of the solid. Excessive temperatures a t the start
1125
of drying, although giving a high initial rate of drying, may cause surface hardening and subsequent lowered drying rates. Large temperature differences, usually occurring a t the end of the constant-rate period, set up internal stresses and cause cracking. This is especially important when the constantrate period extends over a long range of moisture, as the temperature may then rise rapidly with the sudden falling off of the rate of evaporation.
Literature Cited (1) Block, Berthold. Chem. A p p . , 6, 57-9 (1919). (2) Ernst, R. C., Ardern, D. I3 , Schmied, 0. K., IND. ENQ.CHEM., 30,1119(1938). (3) Passburg, Emil, British Patent 3432 (Feb. Fabrik, 3,93-5 (1930); Lavett, C. 0..and J. IND.ENQ. CHEM., 13, 600-5 (1921); Chimie et industrie, 13,883-9 (1925).
and Tiller, F. M.,
12, 1906); Chem Van Marle, D. J., Martin, Armand,
RECEIVEDMay 27, 1938.
Evaporation of Water into Quiet Air From a One-Foot Diameter Surface The evaporation from a one-foot-diameter pan of distilled water into quiet air at 53 per cent humidity was measured for water temperatures
between 63" and 93" I?. The surface of the water was at t h e level of the surroundings. In the critical region, corresponding to a water temperature of 69.4" F., t h e buoyant effects of the mixture a t the water surface and far away are equal. The unit evaporation rate above the critical region, i n pounds/(square foot) (hour) may be expressed by the equation: e = -0.024 f 65(cuw- cum) Below t h e critical region the results may be expressed approximately by t h e equation : e = 18.75 (euw - c u m ) where cow is the concentration of water vapor on the gas side of the gas-liquid interface and cum is t h e concentration of the water vapor in the atmosphere far from t h e pan, both i n pounds per cubic foot. A tentative correlation with thermal free convection data has been accomplished. The ratio N u / N u ' = 1.29 for 108 < Gr' x Pr (and Gr' X Pr') < 108 agrees fairly well with t h e results of Hilpert (6) determined for moist clay plates placed vertically. Acceptance of the thermal free convection-diffusional free convection analogy and generalization t o other fluids is not urged until substantiated by further experimental results. USSELT (8)suggested a detailed attack on the problem of diffusion. Heat transfer and diffusion phenomena obey similar field equations, and if the boundary conditions are similar, the solution of the steady-state thermal free-convection system of a particular geometrical configuration should yield a solution of the corresponding system involving diffusion. The purpose of this paper is to present data which will confirm the analogy for the case of evapora-
N
B. F. SHARPLEY Pelton Water Wheel Company, San Francisco, Calif.
L. M. K. BOELTER University of California, Berkeley, Calif.
tion of distilled water, within the temperature limits of 63" and 93" F., from a one-foot-diameter surface into quiet air a t 71" * 1" F. and 50 to 54 per cent relative humidity. The water surface was maintained flush with the surrounding floor, except for the cases for which rims were used on the pan as described below.
Evaporation Apparatus The experiments were conducted in a humidistatically and thermostatically controlled room. The evaporation pan was surrounded by a quieting chamber, open a t the top, of dimensions 5 X 5 X 7 feet high. All instruments were located outside of the chamber; they included a wattmeter to record the power required to maintain the temperature of the water constant, barograph, potentiometer, galvanometer, psychrometer, and evaporimeter. EVAPORATION PAN. The tinned copper evaporation pan, 1 foot in diameter and 6 inches deep, is shown in Figure 1. The evaporation pan was contained within a pan 20 inches in diameter and 9 inches deep, the space between the two pans was packed with hair felt and 85 per cent magnesia. An electrical heating element placed below the evaporation pan supplied thermal energy to the water. Ten copper-constantan thermocouples were placed in the pan a t various depths and diameters; the most important of them, for the purposes of this paper, were the two located at the surface of the water in such a manner as barely t o deflect the water surface. QUIETINGCHAMBER.The evaporation pan was placed centrally in the floor of the quieting chamber (Figure 2). Air supply to the quieting chamber was provided for by placing the chamber upon blocks 1 inch high. The floor of the quieting chamber was 2 inches less in each dimension than
