406
INDUSTRIAL AND ENGINEERING CHEMISTRY
When swollen sections of wood in which the water has been replaced by different organic liquids are dried, the normal shrinkage occurs. This is true even in the case of benzene which causes a negligible swelling of wood. The benzene must thus be held almost entirely by capillary forces and the shrinkage must result from overcoming these capillary forces. This replacement principle Eas been used as a means of depositing nonvolat,ile water-insoluble waxes and resins in the swollen wood structure in order to reduce subsequent shrinking and swelling. The results of this investigation will appear later.
Literature Cited (1) Beiser, W.,KoZZoi&Z., 65,203 (1933). (2) Elliot, C. S., J . Council Sci.Znd. Research, 3, 204 (Nov., 1930). (3) Katz, J. R., Trans. Faraday SOC., 29,279 (1933). (4) Xietler, S. S., J. Phys. C h m . , 36,52 (1932). (5) Martin, W.M., and Gortner, R. A., Ibid., 34,1509 (1930).
(6) Morath, E., inaugural dissertation, Technischen Hochschule Dromstadt, 1932.
VOL. 27, NO. 4
(7) Neale, S.M.,J. TeztdeInst.,20,T373 (1929). (8) Newlin, J. A., and Wilson, T. R. C., U. S. Dept. -igr., Bull. 676 (1919). (9) Pauli, W.,and Handousky, H., Biochem. 2.. 18, 340 (1909); 24,239 (1910). (10) Peck, E.C.,Am. Lumberman, July 14, 1928,p. 52. (11) Proctor, H.R.,and Wilson, J. A,, J. Chem Soc., 109,307(1916). (12) Ritter, G.J., J. Forestry, 28,533 (1930). (13) Schwalbe, C.G.,and Beiser, W., Pupier-fubr., 50, 655 (1933). (14) Sheppard, S. E.,and Newsome, P. T., J . Phgs. Chem., 36, 2306 (1932). (15) Stamm, A. J., IND.ENG.CHEM.,Anal. Ed., 1, 94 (1929). (16) Stamm, A. J., J . Am. Chem. SOC.,56, 1195 (1934). (17) Stamm, A.J., and Seborg, R. M., J.Phys. Chem., 39,133 (1935). (18) Tiemann, H. D., J. Franklin Inst., 188,27 (1919). (19) Tiemann, H. D., Sci.Am., 130,No.5, 314 (1924). (20) Tolman, R. C., and Stearn, A. E., J. Am. C h a . SOC., 40,284 (1918). RECEIVED December 21, 1934. Presented before the Division of Colloid Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
The Drying of Peat A. V. LUIKOV Leninskaya Sloboda 14a, Lod-49, MOSCOW, U. S. S. R.
HE variation with time of the moisture content of a material being dried is conveniently studied by means of drying curves. Sherwood (4)has employed the rate-of-drying curve, which is the amount of moisture evaporated from a unit surface in unit time plotted against the moisture content of the material. In addition, temperature curves (temperature change of the material us. moisture content during the drying process) have been studied (2). Based on these curves, the drying process may be divided into three periods: (1) constant-rate period, (2) unsaturated surface period, and (3) internal diffusion period. The purpose of this paper is to determine the drying process of peat. The experiments were carried out to study the relation between the moisture content of peat, its temperature, and its time of drying, and to investigate the effect of the size of the peat sample on the drying process. The peat used had an initial moisture content of about 700 to 800 per cent on the dry basis and was in the shape of small cubes and parallelepipeds of different sizes. A small peat brick was also subjected to test. Drying was carried out in still air in a thermostat and also in a small circulation drier. The thermostat was heated by an electric furnace and the temperature was controlled by a Hereus thermoregulator. The material to be dried was hung on one of the arms of a special balance resting on the top of the thermostat. At the fulcrum of the balance arm was placed a small mirror upon which fell a narrow beam of light. The changes in weight of the material were calculated by the reflection of the light beam on a vertical scale hung on the wall of the room. The length of the reflection on the 150-cm. scale varied with the weight of the peat being dried. Small weights were added on a special small scale on the balance so that an accuracy to within 0.002 gram was secured, The temperature of the peat at the various stages of dry-
ing was also determined by means of thermocouples imbedded in the sample; the ends of the thermocouples projected along the column of the balance. At the moment the temperature measurement was taken, a compensating wiring connected with FL meter was fixed to the thermocouple terminals, In order to study the shrinkage of peat during the drying process, a series of tests for determining the sizes of peat cubes was carried out. The experimental technic was to prepare simultaneously about ten or eleven small cubes (44 X 44 X 47 mm.) of peat with an initial moisture content of about 700 to 800 per cent on the dry basis. Every 30 to 60 minutes in the process of drying single cubes mere weighed and the volume of each was measured. In order to determine the volume of a single cube, it was covered with a thin coating of paraffin and then ut into water. The volume of paraffin was determined by weigiing the cube before and after coating, and the volume of the cube itself was calculated from the measured water displacement.
Shrinkage of Peat during the Drying Process In many materials, such as peat, leather, clay, etc., shrinkage is observed at the start of the drying process. But in some materials-for instance, in wood-shrinkage starts, not a t the beginning of the drying process, but only after the loss of a certain amount of moisture, It is found experimentally that the size of the peat cube (thickness, length, and width) decreases linearly with decrease in moisture content. Since shrinkage of peat is observed at the start and continues during the whole process of drying, then according to the Lewis formula (1): 1 = lO(1 CrW) (1)
+
According to experiments, the linear shrinkage coefficient, a, is 0.001186 or approximately 12.1 X lo-*. The volume of peat may be written:
APRIL, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
At a E 12.10-4, we can neglect the value 3a2W2for the sake of approximation, and then have: --V = -I (1 3aW) v1 B
+
TheFesults of tests with peat are shown in Figure 1; the ratio of V to VI is plotted against the moisture content of the peat. The ratios obtained were confirmed previously (2) by the experimental data given in Figure 1. The surface of evaporation may be written: 17 = Fa(1 ffW)* (3)
407
the same as that of the drying medium. The curves of figure 2 show that the drying process may be divided into three periods. For the third period, the relation between the moisture content and the temperature of the peat may be expressed as follows: T = t, - r"(W - W e ) (4) Under adiabatic conditions of drying, the amount of heat necessary for evaporation and heating of the peat is taken from the surrounding gaseous medium-for example, from
+
Characteristics of the Peat Drying Process From the measurements made, a plot was first prepared of the per cent water in the peat vs. time. Tangents to this curve were then carefully drawn a t several points, the slopes of these tangents were measured with an accurate protractor, and the rate of drying as per cent per hour per sq. cm. was calculated. The area used in calculating the rate was determined from the Lewis formula (Equation 3). Figure 2 gives the rate-of-drying curves (the relationship between the rate of drying in per cent per hour per sq. cm. and moisture content) for three experiments. Under constant conditions (to, t b , v = constant:), the rate of drying of wet peat is constant until the first critical moisture content is reached. Then the drying rate decreases in a straight line with the moisture content until the second critical point is reached (Figure 2) and then falls off along a curve. I n addition to the rate curves, the experimental data were analyzed by plotting the temperature against the moisture content of the peat,. The temperature curve of Figure 3 shows the characteristic form for all experiments. During the preliminary stage of the drying process (Figure 2) the temperature rises rapidly until it reaches the temperature of the wet bulb. This temperature remains constant during the constant rate period. From a certain point corresponding to the first critical point of the drying curve, the temperature rises rapidly and then becomes a straight-line function of moisture content after the second critical point of the drying curve is reached.
YO0
400
PER CENT
600
WATER,DRY BASIS
FIGURE2. FL+~-OF-DRYING CURVES
air. The relation between moisture content and temperature in the drying process can be written (3):
+ (c,mo 6e
8 W T
6T + cw) = h(ta - T.)F 68
(5)
First, or Constant Rate Period I n the constant rate period the temperature is constant throughout the peat and is the same as the temperature of the wet bulb:
T = constant; T, =
tb
For example, in the experiment illustrated in Figure 3, = 55" c. and the temperature shows a region of constancy a t 55" C. Consequently, Equation 5 for the constant rate tb
period will be: FIGURE1. SHRINKAGE OF PEATON DRYINQ
Substituting the expression for F we have:
z =- M ~ F ~1( + dw
I
m
I
m $MOISTURE
The moisture content reaches some value a t the end of the experiment which is considered to be the equilibrium moisture at which the rate of drying is zero. This equilibrium between moisture content of the material and the drying medium is established during the process of continuous drying by which it is impossible to remove the equilibrium moisture under given conditions. The temperature of the mateyial increases with the decrease of moisture, and, as its equilibrium moisture point is reached, the temperature of the material becomes the same as that of the atmosphere. I n the case of the experiment illustrated by :Figure 3, to was 100" and t b 55" C.; the equilibrium moisture may be considered to be close to zero, and the temperature curve intersects the abscissa scale (W = 0) a t approximately 100" C. Consequently, when the equilibrium moisture of the material is reached, its temperature is
w)'
(7)
Integrating Equation 7, we obtain the relation between moisture content and time,
Bearing in mind that we derive
I n Figure 4 the relation between l/(Wl - W ) and l / O is given graphically for drying experiments on peat with an initial moisture content of 700 per cent. The experimental data given in Figure 4 confirm Equation 8 and, consequently, the Lewis formula.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
408
The experimental data of Figure 4 show a series of straight lines with a slope dependent on the conditions. The intersection of these straight lines - with the ordinate axis gives nu-
d:,
merically the value of a If (ab)denotes the portions cut off by these straight lines on the ordinate axis, the coefficient CY will be
For instance, in experiment 59 (Figure 4) ab 800per cent, a n d a = 11.4 X
=
0.0006, Wl
=
The minus sign before Mz is due to the fact that moisture content decreases with increase in time. Substituting the expression for F from Equation 3, 1 diu
&
-
[MI
-
K(w01
- w)lU
+ aW)*
(9)
Thus, if the critical moisture content is known, the rate of drying and consequently the length of the period of drying peat may be calculated by Equation 9. The form of the temperature curve in the second period is determined by integration of Equation 5 which can be obtained by combining with Equation 9.
The Third Period The characteristics of the drying process in this period are determined by the temperature curve. I n order t o study the temperature distribution throughout a section of peat during the process of drying, a series of experiments for determination of the temperature gradient was carried out. In the initial stage of the drying process a comparatively high temperature gradient is observed until the moment the constant rate period begins. Then the temperature becomes uniform throughout the whole peat sample and is the same as the wetTABLE I. EXPERIMENTAL DATAON THE DRYING OF PEAT 0
PO0
$90
700
PER C E N T WATER,ORY B A S I S
Test NO.
FIGURE 3. CHARACTERISTIC TEMPERATURE CURVE
Per Cent Water, Dry Basis Initial, Final,
w1
wz
~i~ Temp., to
* C.
The first critical point may be located as the point of departure of the data from a straight line, since the straight-line relation between these values is observed only in the constant rate period. For instance, in experiment 59 (Figure 4) curvature begins a t w1 - wcl = -0.00192. Since the initial moisture of peat, W1 = 710 per cent, it follows that Wcl = 190 per cent.
The Second Period From the moment the first critical moisture content is reached, the rate of drying is a function of moisture content and consequently cannot be calculated from Equations 6 and
12 14 15 67 68 19 66 24 65 59 13 8
29 37 48 23 4 53 62 40 32 21 46 42 5 30 64 44 35
712
110
73 707 96 862 41 713 45 809 650 80
761 641 651 715 770 650 740 640 670 777 652 651 740 670 615 652 641
112.0 42.2 28.9 60.9 48.6 68.0 3.5 52.0 23.3 17.0 50.8 3.3 29.0 1.3 1.6
5.8 6.4 1.9 5.6 43.0 2.0 15.4 23.0 5.1 1.8 4.2 16.8 4.4 0.0
50
100
~i,. Velocity, 0
Meters/sec. 0 0
0 1 1 4 4 7 7 0
0 0 1 4 7 7
150
0 0 1 4 4 7 7
200
0 0 1 1 4 7
~i~~ ,f Drying,
e
Hours 10 0 10 0 10 0 24 0 6 9 9 0 9 0 11 0 9 0 15 0
11.0 9 0 9.0 5.5 4 0 3.5 9.0 7.0 5.0 2.0 2.5 2.0 1.5 4.0 5.0 3.0 3.5 2.0 2.0
0.0010
I
dJ-W
0.0OPO
G. P
91
0
BETWEEN FIGURE 4. RELATION
0.3
0. Y
c. 1
~ / ( w ~ -AND w ) 1/e
7. Since the rate of drying decreases with moisture content according to a straight-line relationship, this period may be considered to be proved experimentally. From the rate curves (Figure 2) for the rate of drying we can write:
where
-
- M, K = Mi wo1
- We2
bulb temperature. From the first critical point the temperature of the peat rapidly increases, so that the temperature gradient is observed a t the beginning of the second period, but on approaching the second critical point this temperature gradient curve flattens out, and in the third period the temperature curve increases smoothly with the increase of moisture without any appreciable gradient through the sample. Consequently T , = T . Substituting in Equation 5 the expression for T from Equation 4, 1 dw - h ( W - W e ) F T e - _r - l O O C 0 - cw
(10)
Y
The plot of this equation does not give a straight line for t h e dw relation and W which has been confirmed by tests (Figure 2). Thus, during this stage of the drying process of peat, the rate is determined by a critical moisture content, equilibrium moisture, and the coefficient y. It is evident that
APRIL, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
further experiments are necessary t o establish the relationship between these quantities. Analysis of the experimental rate-of-drying curves shows a simple relation between critical points; an increase in the fist critical moisture content is always accompanied by an increase in the second critical moisture content and vice versa, the difference being always constant. Figure 2 shon-s that with peat samples of similar thickness the difference between critical moisture contents remains constart under any drying conditions: wel - w C 2= constant Table I gives the experimental data.
Nomenclature a y c co
F Fa Fl
h 1 lo mo
= = = = = = = = = = =
,MI =
linear shrinkage coefficient (Equation 1) constant coefficient for given ronditions specific heat of water, cal./gram/' C. sperific heat of dry peat, cal./gram/" C. surface of evaporation of peat, sq. cm. surface of evaporation of dry peat, sq. em. initial evaporation surface of peat, sq. cm heat transfer coefficient, cal./sq. cm./' C. /hr. linear dimension of peat, cm. linear dimension of dry peat, cm. dry weight of peat, grams drying rate of first critical point numerically equal t o
L'ALCHIMISTE B\-
Paul Delaroche
No. 52 in the Berolzheimer series
of Alchemical and Historical Repro-
ductions brings a new artist to the series. Delaroche 'was born in Paris in 1797, studied at the Ecole des Beaux Arts, and under Watelet and
Cros. His work was largely historical in concept. Heinrich Heme called him
the leader of the historical school. In 3832 Delaroche was elected a Membre de l'lnstitute and shortly thereafter he w a s appointed Professor at the Ecole des Beaux Arts. He died in Paris in 1856. The original painting from which our reproduction mas ehotographed is in the Walker ollection In London.
.A detailed list of the first thirty-six reprodurtions in the series, together with full particulars for obtsinlng photographic copies of the originals appeared in our issue for January 1934,'page 112. A aupplementary list ifNOH.37 t o 48 ia in our January, 1935, issue, g&ge 102. No. 49 is in the January. 1935. issue, page 86, No. 50 ia in the February 1935 issue page 204, and No. 51 in the M&hj&ue, Gage 314.
409
the drying rate in the constant rate period, grams/sq. cm./hr. M, = drying rate of second critical point, grams/sq. cm./hr. T = latent heat of evaporation, cnl./gram I' = mean temp. of peat, O C. T , = surface temperature of peat, ' C. t, = atmospheric temperature, C. t b = wet-bulb temperature, C. e = time, hours u = velocity of air, meters/sec. V = volume of p a t , cc. V o = volume of dry peat, cc. Vl = initial volume of peat, cc. ET = moisture content of peat, per cent of dry weight w = moisture content of peat, grams w = - mo TVe = equilibrium moisture content of peat, per cent W1 = initial moisture content of peat, per cent WC1= moisture content of peat at first critical point, per cent wcl = moisture content at first critical point, grams W,, = moisture content at second critical point, per cent wcz = moisture content at second critical point, grams O
( G )
Literature Cited Lewis, J. I N D .EXG.CHEY.,13, 4 3 0 (1921). (2) Luikov, J . Chem. (Russ.), No. 4, 223 ( 1 9 3 4 ) . (3) Luikov, J . Thermotech. Inst. (Russ.),9 (87), 27 f 1)
(1933). (4) Shenvood. I X D .E N G .Camf.. 21. 12, 976 (1929): 22, 132 (1930).
RECEIVED October 28, 1934.
