Evaporation Rates of Moisture
from a Wet Material and from a Free Water Surface' A. E. STACEY,JR. Buensod-Stacey Air Conditioning, Inc., New York,N. Y.
control could be varied to suit the load. That part of the heater bank under manual control was also divided into several circuits. During o p e r a t i o n t h e closest temperature control could be obtained by using as g r e a t a n u m b e r of manually controlled heaters as possible and only sufficient automatically controlled heaters to cover the variations in the heat requirements. For the convenience of the operator there were two access doors into the enclosure. One of these was fitted with double glass and located in the side of the enclosure, the other formed the end of the enclosuie. One leg of Equipment a balance was extended PROCESSING CABINETFOR EXPERIMENTS ON DRYING DIFa small openthrough A c a b i n e t w a s deFERENT MATERIALS ing in the top of the veloped specially for this The thermostat is shown at the upper right-hand corner and the h gro cabinet, and from this a type of investigation. It stat just to the left. The balance was located in the center of the ,gam: ber over the name plate and directly in front of the glass door. The tray of the drying maconsisted of a well-insuelectric heaters were controlled from the panel in front. The switches were of the three-way type making a wide variation in selection of lated enclosure 24 inches terial was suspended. heaters possible. Periodic weighings could wide, 30 inches high, and 48 inches long. The enthus be made without closure was supported on an angle-iron stand in which were removing the sample from the cabinet. mounted electric heaters, fan with motor, humidifying nozzles, During the period of making weight determinations, the and water reservoir. The air entered the enclosure a t one fan was stopped so as to eliminate errors which might otherend and near the top, a t high velocity, through 1-inch wise be caused by the rapid air circulation. Through the top nozzles. This method of air distribution caused a high of the enclosure two openings were provided for the insertion of a dry-bulb and a wet-bulb thermometer with which temsecondary air circulation within the cabinet. Close regulation of both temperature and relative humidity was acperature readings were made. An outside air opening was complished by means of an air-operated thermostat and located in the return air chamber in front of the heaters, and a hygrostat. The electric heaters were divided into several a relief air opening was located in the end wall of the enclocircuits so that the number of heaters under automatic sure underneath the nozzles. Both were operated by manually moving a connecting lever. This paper was presented as part of the Symposium on Drying and The circuit of the air through the equipment started from the Air Conditioning, the Fourth Chemical Engineering Symposium, held under return air opening in the bottom of the cabinet, passed through the auspices of the Division of Industrial and Engineering Chemistry of the American Chemical Society, at the University of Pennsylvania. Philathe humidifying chamber, through the electric heaters into delphia, P a . , December 27 and 28, 1937. Previous papers in the symthe fan, and through a conduit to a header from which it was posium appeared in April (pages 384 to 397), May (page 5 0 6 ) , September discharged into the cabinet when it passed over the wet ma(pages 993 to l o l o ) , October (pages 1119 to 1138), and on page 1372 of terial several times before again starting through the circuit. this issue.
U R I N G several years of experience in the laboratories of the Carrier Engineering Corporation, drying schedules for many materials were developed. Test data, typical of materials varying widely in physical properties, were selected as a basis for this paper. It is hoped that these experimental data may prove of some assistance to those who have drying problems in the field. Comparison of the data covering the drying of the different materials was simplified by conducting all experiments with the same air velocity.
D
1385
I
1
115.6.C.
D.4.
I
I
e
I
3
I
4
11.1'
.
58.3.C. W.
0
VOL. 30, NO. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
1386
C . D.B.
33.9. C. W.B.
I
I
IPG. I 6
S
'1
DRYlNG TIME IN HOURS
FIQURE 1. EXPERIMENTAL DATAFOR NEWSPAPER PULP
Test Procedure
WTE OF EVAPORATION KG.-M.EHR. FIGURE 2. DIFFERENT STAQES OF DRYING FOR NEWSPAPER PULP
At the outset all available information in regard to effect of temperature and relative humidity on the material to be tested was obtained as assistance in determining a drying schedule. Many times this information was misleading, as no consideration had been given either to the wet-bulb temperature to which, during drying, the temperature of the material approached or t o the effect of the residual water in the material. The sensitivity of many materials to temperature varied with the moisture content. After some definite point in drying had been reached, much higher temperatures could be used without detrimental effect. After the drying schedule had been determined, the material was loaded into pans. Several thicknesses of materials were used so as to select a period of drying which could best be scheduled to the process of manufacture. For example, in a process of manufacture involving only one 8-hour shift, the dryer might be loaded during the day and be ready to unload the following morning or, in case of a 24-hour turnover, a drying schedule a few hours shorter than 24 would allow for filling and unloading of the dryer. If, however, this could be finished in an hour, there would be no advantage in having an equipment of capacity to dry in 16 hours. As soon as the trays had been filled to the proper thickness with the material, weights were taken. When temperature and humidity conditions in the cabinet were as scheduled, the travs were Dlaced in the cabinet. Weight determinations were made it regular intervals throughout the experiment. The time between readings varied from 2 minutes, when the
TABLEI. RATIOOF EVAPORATION FROM
Class
Material
Size
In. I
Pulp (newspaper), Figures 1 and 2
I1
Litho1 red, Figures 3 and 4
1
Carbon pigment, Figures 6 and 6
0.5 1 1.5
I1
0.25 0.5 1 0.25 0.5 1
DRYING TIME
FIQURE3. EXPERIMENTAL DATAFOR LITHOL RED
A
MATERIAL TO
Temperature Dry-bulb Wet-bulb
c. 71.1 71.1 71.1 115.6 115.6 115.6
IN HOURS
O
a.
A
FREEWATERSERVICE
Moisture Content (Dry Wt.)
%
Test Drying Time Hr. 1.417 3.634 6.66 0.66 1.216 2.75
33.9 33.9 33.9 38.3 38.3 38.3
116.5 12s 125 118 121 126
60
27
127
5.5
71.1 71.1 71:l
30.6 30.6 30.6
107 107 107
4 10
387 356
6.5 14.83 3.5 8 18 2s 38
I1
Stannic tetrachloride sludge, Figures 7 and 8
0.5 1
93.3 93.3
36.7 36.7
I11
Clay, Figures 9 and 10
0.25 0.5 1 1.5 2
115.6 115.6 115.6 115.6 115.6
39.4 39.4 39.4 39.4 39.4
67.5 66.9 68.6 67.5 66.9
. ... . . .
Free Water Surface Time
Drying Ratio
Hr. 0.47 0.985 1.925 0.213 0.422 0.870
%
%
33.2 28.1 29 32.2 34.6 31.6
68.5 56 48.2 51 57.5 52.1
5.86
106.5
2.41 5.24 . N o t completed..
Drying Criteria
54
60 53
61.4 45.3 60
2.96 5.92
45.5 40
58.1 62.9
0.965 1.680 3.32 4.89 6.4
27.5 21 18.5 17.5 16.8
25 27 20.4 21.6 23.3
.. . . . .
DECEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
1387
A
3
d loof.: c)
1
7s
LIl-IOL
HED
FIG. 4
I
RATE OF EVAPORATION KG.-M?- HR.
FIGURE 4. DIFFERENT STAGES OF DRYING FOR LITHOL RED
.002
.001
.003
RATE OF EVAPORATION KG.-M.'- HR.
FIGURE 6. DIFFERENT STAGES OF DRYING FOR CARBON PIGMENT
surface of the material being dried, as measured by an anemometer, was 1.8 meters per second. Because of the relatively large volume of air in circulation, the effective air v e locity greatly exceeded this figure. Since the velocity of the air was constant for all experiments, it has been considered permissible to make comparisons between the rate of evaporation from the material, as recorded in the tests, and that from a free water surface (Table I).
Materials
DRYlNG TIME IN HOURS FIGTJRE 5. EXPERIMENTAL DATAFOR CARBON PIGMENT
rate of evaporation was high, to an hour, when the change was small. Graphs were plotted as the readings were taken, using as coordinates time and moisture loss. This procedure checked the accuracy of the weight readings. When materials crack during drying, there is an increase in drying surface with an increased rate of evaporation which was reflected in the readings. In most cases the tests were extended until no further loss of moisture was noted for a period of a t least 'three readings. Standard dry-weight determinations were made in a Freas oven a t 105" C. Where drying temperatures were higher, the dry weight was obtained from the test itself. The temperature of the material was not always recorded, and these data should be available to make a comparison of the results as given in this paper with experimental data obtained in other laboratories. The data as given, however, should approximate that of commercial practice, since the method of loading the material on trays in compartment dryers is the same in both cases. During these experiments the bottom and sides of the trays were exposed to the air circulation in the cabinet. The heat transmitted through these surfaces to the material increased the rate of evaporation. The effective velocity of the air over these surfaces was unknown, so that this amount of heat could not be calculated. The amount of heat transmitted to the material through these dry surfaces would diminish as the temperature of the material approached the dry-bulb temperature of the air. The velocity of the air parallel to the
Because of the wide variation in physical properties of the materials dried, they were classified in three categories according to some feature related to the rate of drying, as follows : CLASSI. Those composed of cellulose. Newspaper ul was selected as ty ical; it was composed of old paper which [axbeen reduced to p$p form. CLASS11. Those which, in their wet state, formed pastes or sludges. The typical materials selected were Litho1 Red (in cakes which were broken into small pieces before dr ing), carbon pigment (a paste), and stannic tetrachloride sLdge (easily poured). CLASS111. Certain ceramic materials. Clay in the form of a heavy paste was chosen as typical.
Data Experimental data are presented in the form of curves (Figures 1, 3, 5, 7, and 9). The original graphs were plotted' with one coordinate, time, against moisture loss. The rate of evaporation a t any time may be calculated by taking a tangent to these curves. The rate of evaporation a t any period of drying was obtained directly from the difference between readings of weights, divided by the number of minutes between these readings. The results thus obtained are expressed in kilograms per square meter per hour. As a basis for comparison the moisture content was calculated on a dry weight basis, which is the ratio between the moisture present in the material a t any time and the dry weight of the material. A common basis was thus obtained. The rate of evaporation as affected by gradient of vapor pressure was obtained by plotting rate against this gradient. Following one Qf the laws of evaporation, the rate varied with the difference of the vapor pressure of the moisture in the air and that of the moisture in the material. Figures 2, 4, 6, 8, and 10 visualize the different stages of drying. The drying process may be divided into three stages which are shown as A-B, B-C, and C-D, respectively:
INDUSTRIAL AND ENGINEERING CHEMISTRY
1388
VOL. 30, NO. 12
N
i.02
(li
2 W
>
g .01 w a;
0,
I: 0
W T E OF EVAPORATION KG.-MzDRYING TIME IN HOURS
HR.
FIGURE 8. DIFFERENT STAGES OF DRYING FOR STANNIC TETRACHLORIDE
DATAFOR STANNIC TETRAFIGURE 7. EXPERIMENTAL CHLORIDE
1. The evaporation of free water from the exposed surface. The data pertaining to this first stage of the drying cycle may vary, depending largely upon the temperature of the material as related to the drying conditions. If the temperature of the material at the time it is placed in the dryer is lower than the dew point of the air in the dryer, water may be condensed from the air upon the material. 2. The movement of the water included between the particles of the material to the surface by capillarity and possibly by difference of vapor pressure. During the first part of this stage, evaporation proceeds at a constant rate. 3. The movement of the water from within the particles to the surface where it is evaporated. During this stage not only the water above the saturation point of the material particles is removed, but the hygroscopic water as well.
Figures 2, 4,6, 8, and 10 also demonstrate that the rate of moisture removal per square meter per hour approximates the same value for the several depths of loading used in the tests. I n order to have a basis of comparison, the rate of evaporation of water from a free water surface was selected as a criterion. Inasmuch as all of the data involved were taken under the same air velocity, a rate of 0.03 kg. per square meter per hour per 25.4-mm. difference of vapor pressure for a free water surface was selected for use in the comparisons shown in Table I. Another method of studying the effect of the temperature of the air, the relative humidity, and the thickness of the material in the trays was based on the ratio of thk time of drying as calculated from the maximum rate (B-C on Figures 2, 4, 6, 8, and 10) of evaporation derived from experi-
I
FIGURE 9.
I
I
EXPERIMENTAL DATAFOR CLAY
mental data, and on the time of drying as recorded. These ratios for the materials under the different experimental conditions are shown in Table I as Drying Criteria. This method is of value, since it includes both the total time of drying and the maximum rate of evaporation under the conditions of the experiment. From the standpoint of drying, the optimum ratio would be obtained when throughout the drying period the. moisture would reach the surface of the material a t the same rate it might be evaporated. This condition, however, would be impractical since it would greatly increase the time of drying and also demand a constantly changing drying condition. If the moisture is evaporated from the surface a t a greater rate than that a t which it reaches the surface, then a resistance will be built up which further decreases the moisture available for evaporation. This effect is shown in the lower criterion for 26-meter newspaper pulp when dried a t 250' C. It is also evident in the criteria for clay. This ratio is not always a measure of commercial efficiency, owing to the necessity for scheduling the drying period with the other operations of the plant, the cost of space, etc. It does, however, give an indication of possible changes in drying schedules which may be causing imperfections in materials. This is particularly valuable when such materials as wood, ceramic ware, etc., are being dried.
I
RATE OF EVAFORAT ION KG.-MF HR. FIGURE 10. DIFFERENT STAGES OF DRYING FOR CLAY
DECEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
The ratio of the drying time calculated from the evaporation from a free water surface and the actual drying time as recorded in the experiments, corrected to the same vapor pressure, was used as a basis of comparison. These ratios for the different materials are given in Table I. The drying ratio for the material designated as newspaper pulp remains approximately constant fo,r three thicknesses of loading and under two different drying temperatures. This material evidently does not caseharden a t the lower temperature, and the surface condition remains such that moisture from the interior of the material will pass to the surface a t the same rate under varying conditions of moisture content. The materials designated as class I1 have a fine granular structure, which may account to some extent for the drying ratios as shown. Carbon pigment was ground with oil. This treatment, no doubt, has an effect on the drying of this material. From the criteria of this class, it would seem that higher temperatures might be used to advantage. I n some cases, however, where delicate colors are involved, there is a limiting temperature to which these materials may besubjected without change of shade.
1389
The material shown in class I11 distinctly shows a casehardening effect. It is well known that clay will caseharden unless it is dried under proper conditions. The effect on the movement of moisture through the material is reduced approximately 28 per cent in this test. From a commercial angle, however, it might be better to accept the extra time in drying because of the cost of handling. The data presented offer an approximate method for estimating (a) the time of drying of a wide variety of materials possessing physical properties comparable to those of the materials used in these experiments, (b) the effect on the time of drying by means of varying the loading per square foot of tray surface, and (c) the effect of vapor pressure difference on the time of drying.
Acknowledgment The writer wishes to make grateful acknowledgment to the Carrier Corporation for the use of the test data on which this paper is based. RECEIVED April 13, 1938.
ROGER BACON B y Howard Pyle
Roger Bacon, one of the large group of monks who were scientists, lived in England from 1214 to 1292, and studied at Oxford and Paris. He was an omnivorous reader and possessed an encyclopedic mind, wrifing on the most diverse of subjects, and was a great believer in experimentation. This led, among other things, to his independent discovery of gunpowder, which he described in his famous anagram. A close study of Bacon’s life and writings is decidedly worth while. The original etching, from which our reproduction has been made, is owned by Mr. Chester G. Fisher of Pittsburgh, whom we thank for his continued coiiperation. This etching is one of many by that famous American illustrator and painter Howard Pyle. Pyle was born in Wilmington, Delaware, March 5, 1853, and died in Florence, Italy, in 1911. He studied at the Art Students’ League in New York, and in 1907 was elected a member of the National Academy of Design. At his home in Wilmington he later established a school of art, where instruction was free to those who showed promise. His illustrative work was largely a brilliant imitation of the style of Duerer. This is No. 96 in the Berolzheimer series of Alchemical and Historical Reproductions. D. D. Berolzheimer 50 East 41st Street New York, N. Y.
(A complete list of Reproductions 1 to 96 will appear in January 1939 issue.)
