S,D45 3
L7 5
ni,io
1,4
UMW'
WCCID
September 193 9
AGRICULTURE )
FOREST SERVIC E
FOREST PRODUCTS LABORATOR Y
Madison, Wisconsi n
'td'4rTED STATES DEPARVRENT OF
mtion with the University of Wi seman
'-
UNIDIRECTIONAL DRYING OF WOOD l
S
By ,
ERNEST B ATE OMAN, Senior Chemis t
JOHN : P . HOHF, Assistant Chemis t
and
ALFRED J . STAMM, Senior Chemis t
Measurements were made of the rate o f
drying from a single face of small cylinders o f
Sitka s pruce at different temperatures and unde r
different relative humidity and atmospheri c
preseure conditions . Moisture gradients were ,
determined on the s p ecimens prior to tiiercom `L '
plete removal of free watex 'Drying in al l
cases gave weightlosses that varied directl y
wi'th ' the square root of the time . Values fo r
the mean effective diffusion per unit moistur e
gradient were calculated from the rate of dryin g
and the moisture gradients up to the fiber saturation point . The values increase slightl y
with an increase in the relative humidity effective in the drying, and increase to , a greate r
extent with an increase in the drying temperature,a,' decrease, in the atmospheric : pressure ,
and a, decrease in the ' specific gravity of . . the
wood .
The drying of wood is a complicated ' phenomenon which
has thus far defied rigorous theoretical analysis . Most o f
the evidence'- indicates that it is at least in part a diffusion phenomenon . Even . thiis might be questioned, however, o n
the basis of the recent findings of Ceaglske and Hougen (1 )
that the drying of granular nonhygroscopic solids is con trolled entirely by c apillary forces rather than by diffusion .
Tuttle ( g ), Sherwood (6), and . Follmann ( 11) showed that the
moisture gradients obtained in drying wood under definit e
boundary conditions can be theoretically reproduced b y
Fourier analysis methods (2) with a fair degree of accurac y
by assuming that the phenomenon is one ofsimple diffusio n
1
Presented before the Division of Colloid Chemistry at th e
96th Meeting of the American Chemical Society, Milwaukee ,
Wis . Published in Industrial and Engineering Chemistry ,
Vol . 31, No . 9, p . 1150-1154, Se ptember, 1939 .
RI216
over the complete moisture-content range . Hawley (2), however, pointed out that diffusion would not be expected t o
take place above .. the fiber-'saturation point on the basi s
that the fiber-saturat"ion'point
the moisture content i n
equilibrium with unit relative vapor pressure . Further, i n
simple diffusion, the diffusion constant and the diffusivit y
in the Fourier form of the equation (i) should be independen t
of the moisture content . This is ,;-.riot the case for transvers e
drying of wood according to the moistures transfusion measure ments of Martley (1) in which the equilibrium moisture gradients set up under steady-state drying_c,onditions were deter mined .
is
These complications undoubtedly arise from the comple x
nature of the capillary structure of wood (2) . Water is held
with an appreciable reduction in vapor pressure within th e
cell walls of wood as surface-bound and capillary-held wate r
(1) and within the microscopically visible capillary structure with only a small reduction in vapor, pressure . The
fiber-saturation point of wood (the moisture content Lat whic h
the cell walls are saturated but at , which no virtually fre e
water exists in the cell cavities) does -no-t . correspond to a
relative vapor pressure of unity as has been previousl y
assumed, but to a relative vapor pressure : somewhere betwee n
0 .99 and 0 .999 . Consequently it is possible for some movemen t
of water vapor to occur above the fiber-saturation point . On
this basis three different means . of moisture movement through
wood are possible :
as liquid water above, the fiber-saturation point due to capillary . forces1 (b) as bound . water : withi n
the cell walls below the-fiber-saturationpoint due . to mois ture gradients across the cell ;. wa ;l s l and fii(c ). .0.s . water' vapo r
both above and belo w , the fibe r7 s ,t aration point due to' : relative vapor pressure gradients ., Th9 movement of bowed wate r
through the cell walls is- :through transient capillary structure which exists only in the presence of water . This water
might be considered as held in solid solution, Its movemen t
should thus be a diffusion phenomenon . The movement of wate r
vapor should also be in accordance with they diffusion law .
The drying of wood below the fiber-saturation point shoul d
thus be controlled by a combination of these two differen t
types of diffusion .
(a)
Measurement of Rate of Drying
The measurements were made on small cylinders of•Sitk a
spruce heartwood carefully cut with the annual rings in . plane s
parallel to the cylinder axes for tangential drying and wit h
R1216
-2-
S
the fibers in planes parallel to the cylinder axes for longitudinal drying, The cylinders had a diameter of approximatel y
2 .6 cm . ~rnd
1engP~ •of'2' ;6-cm . for the tangential drying an d
5 .0 cm- for longitudinal drying . Most of the measurement s
were made on air-dry wood that was saturated with water b y
alternately applying a vacuum and then a pressure on th e
water-immersed specimens in order to replace the air by water .
This procedure was continued for about a month, in which time
the wood was practically brought to complete saturation .
Sp ecimens that were cut into sections to determine the moisture distribution prior to drying indicated that the moistur e
was uniformly distributed by this soaking method . Measurements were also made on naturally green wood .
In order to minimize side drying, each wood specime n
was inserted in a rubber thumb cot . A glass tube with th e
same external diameter as the specimen was placed on top o f
the specimen within the rubber cot . The glass tube served a s
a guard ring to hold the rubber away from the upper surfac e
of the wood specimen . To prevent the guard ring from appreciably cutting down the surface area of the wood effectiv e
for drying, three small knobs were drawn out on one end o f
the glass cylinder so that it rested on the wood on thes e
points rather than on its total surface area . A rubber co t
containing a soaked specimen and the guard ring was the n
immersed in mercury in a glass weighing bottle and wire d
down so that the mercury level was held about 3 cm . abov e
the surface of the worQd on the outside of the guard ring .
The hydrostatic pressure of the mercury held the rubber co t
firmly against the specimens . This procedure did not entirel y
prevent the loss o f■ water between the rubber and the wood, a s
the following data will show, The measurements, however, wer e
such that .a correction could be made for this loss .
A weighing bottle containing a test specimen was the n
placed in a water-jacketed oven maintained at the desire d
temperature with an accuracy of + 0 .1 0 C . Humidified ai r
was discharged over the' surface of the specimens . Air
=
velocities ranging from 550 to 2,500 liters per hour wer e
used to ensure removal of the moisture from the surface o f
the specimens as" rapidly as it was brought to the surface .
No difference in results over :this range of velocities was ,
observed except in the case of the longitudinal drying fo r
the first few minutes of drying, thus indicating that th e
lower velocity, was adequate in all other cases .
The dry air was obtained by passirz air 'rpq
,cpmpressed air system through six concentrated sulfuric acri d
towers and three calcium chloride tubes in series . Th e
humidified air was obtained by passing air through the wate r
S
R1216
supply end of an air aspirator immersed in water . Thi s
created a stream of very fine air bubbles which were practically saturated with water vapor in spite of the hig h
velocity . The spray was removed in water wash bottles i n
series with the aspirator bottle . The saturated air was the n
reduced to the desired relative humidity by allowing it t o
expand through an accurately controlled expansion valve acros s
which the pressure drop was accurately determined . The
humidity of the expanded air was frequently checked analyti cally, and slight changes in the setting of the expansio n
valve were made to obtain the desired relative humidity .
This simple means of obtaining humidified air at any desire d
relative humidity worked well ; the only drawback was that it
was slightly affected by changes in atmospheric pressure .
The weighing bottle containing the specimen was re moved from the drying oven after different intervals of
time, rapidly capped, weighed, and returned to the oven .
After an appreciable amount of drying had taken place, bu t
before dr , ing had proceeded sufficiently far for all of th e
free wate__' to have been removed from the bottom of the speci men, the crying was discontinued . The specimen was the n
rapidly cut into six to twelve sections perpendicular to th e
moisture gradient ; and center disks were punched from eac h
section . The disks and rings were weighed separately i n
weighing bottles, dried, and again weighed . The thicknes s
of each section was calculated from the original height o f
the soaked cylinder and the fractional dry weight of eac h
section referred to the combined dry weight of
of the
sections . Moisture gradients for•both the inner and oute r
zone of the specimens were then calculated from these data .
all
The foregoing procedure had to be modified in orde r
to dry the specimens under reduced pressure . A small stee l
bomb with an inside diameter the same as that of the weighin g
bottles was used to hold the specimen and the mercury . A
stream of dry air was • drawn in over the surface of the specimen at such a•velocity that a high-vacuum pump was just abl e
to maintain the desired reduced pressure . An interchangeabl e
drying train was placed in the system between the bomb and
the pump . The moisture lost in different intervals of tim e
was determined by the gain in weight of the sorption trai n
rather than by the loss in weight of the speci-en .
The drying of the wood specimens in both the tangentia l
and the longitudinal directions gave a linear relation betwee n
the loss in weight and the square root of the time for eac h
of the drying temperatures, pressures, and relative humidities . This relation is shown in Figure 1 for the tangentia l
R1216
-4-
drying of specimen 1 of Table 1, and for the longitudina l
drying of specimen 1 of Table 4, both of which were originall y
saturated with water . Figure . 2 give's the Same relation fo r
the tangential drying of an initially green s p ecimen (next '
last value in Table 1) . In practically all cases the linear
relation failed to pass through the origin . In the case o f
tangential drying the Initial loss in. weight was slightl y
greater than would be expected and in the case of longitudinal drying it was slightly less . The divergence in th e
case of longitudinal dryin g. was undoubtedly due to the fac t
that the circulation was not quite great enough . during the
first few minutes to remove the water from the surface a s
rapidly as it was brought to the surface . In the case of
tangential drying the divergence may have been due to a sur face film of water which had to be removed before actua l
drying of the specimen occurred .
Figures 3, 4, and 5 give the moisture gradients fo r
the specimens of Figures 1 and 2 . In the case of tangential
drying, the moisture gradient at the side of the specimen i s
lower than that at the center for the higher moisture con tents . This indicates that there was some movement of fre e
water to the circumference of the specimens in the longitudinal direction and consequent leakage between the rubbe r
and the specimen to the surface . A correction was hence mad e
for this extraneous loss as follows : The calculated moistur e
loss was obtained from the difference between the origina l
moisture content and the moisture content of the combine d
disks and rings just after cutting . The'theoretical moistur e
loss that would have occurred if there had been no longitudina l
movement of water towards the circumference of the specime n
was obtained from the difference between the original moisture content of the s p ecimen and the loss that would hav e
occurred if the center gradient had been effective over th e
whole surface area of the specimen . The difference betwee n
these two quantities divided by the former ;,rives the fractio n
by which the moisture loss per unit square root of time shoul d
be reduced to eliminate the side loss . This fraction vario d
from_0 .02 to 0 .23 for the different measurements .
1
Insufficient sections were cut from the' dried soeci mens over the thickness range up to the fiber-saturatio n
point to determine the curvature of the moisture gradient s
with any degree of accuracy . For this reason only the secan t
mean moisture gradient -- that is, a linear gradient between
the boundary moisture-contc ;nt 'conditions below the fiber- saturation point -- was considered rather than the actua l
gradients at each moisture content .
is
It
impossible from existing data to factor out th e
portion of the diffusion that takes place under a vapor pressure gradient . The calculations will hence be made on the
basis that all of the diffusion took place under a moistur e
content gradient . Although this procedure is not strictl y
correct, it has been used by other investigators as a mean s
of determinin g; com parable constants (4, a, 6 8) .
The mean diffusion per unit moisture gradient can b e
expressed in terms of the drying rate and the secant mea n
moisture gradient as follows :
D =
At ;
` (ml- m2)
1
(1 )
where D diffusion per unit moisture gradien t
Q = quantity of water lost in time t
A = effective drying cross sectio n
ml - m2 = change in moisture content over distance 1
The experimental values for each of these quantities and th e
calculated values of D are given, in Tables 1 to 4 . For an y
one temperature and set of boundary conditions not only doe s
the loss of moisture, a, vary directly with the square roo t
of time, but also the distance 1 between the two planes i n
the wood which are at the boundary conditions . The mean diffusion values per unit moisture gradient effective in tangen tial drying to zero percent moisture content boundary condition were cdrrected to a common green s pecific gravity o f
the wood of 0 .365 from a plot of the data for drying to zer o
percent moisture content .
0
DiffusionDat a
The data indicate that under fixed boundary, tempera.
a wood with a definite specifi c
ture, and
gravity gives quite constant D values ., for different values o f
t and 1, and largevariations in the or5.ginal free moistur e
content of the wood . This i.e a, good indication that th e
phenomenon is really one of diffusion . The: data show tha t
°
the mean transverse drying diffusion values per unit .mois t
ture gradient increase with decreasing specific gravities o f
the wood, with increasing relative humidities under whic h
drying takes place, increasing drying temperatures, and de creasing atmospheric pressures . All of these variations are `
in the direction that would . be expected if the phenomeno n
were one of combined bound water and water-vapor diffusion .
The greater .the specific gravity of the Mood, the less the '
capillary volume through which vapor diffusion takes plac e
and the grater she distance across the cell walls throug h
which the bound-water diffusion takes place . Both of thes e
tend to reduce the combined diffusion . An increase in th e
relative humidity under which drying takes place . increase s
the average degree of bwelli1Zg of the wood and hence th e
bound-water diffusion through the cell walls . An increas e
in temperature increases both the diffusion constant of wate r
vapor and of bound water and a decrease in pressure causes a n
increase in the water-vapor diffusion .
pressure-conditions,
It is of interest to note that the movement of fre e
capillary water above the fiber-saturation point does no t
affect the diffusion per unit moisture gradient below th e
fiber-saturation point . In the case of the wood that i s
completely saturated with water, the movement of free wate r
above the fiber-saturation point is reduced to a minimum .
This is shown in the moisture-gradient curve of Figure 3 .
In the case of normally green wood, the movement of fre e
water above the fiber-saturation point is quite appreciabl e
(Figure 5) .
Insufficient longitudinal drying data were obtained t o
draw any conclusions regarding drying in this direction .
•
R1216
Literature Cite d
1 . Ceaglske, N . H ., and Hougen, O . A . Ind . Eng . Chem .
29 :805 (1937) .
2. Hawley, L . F . U . S . Dept . Agric . Tech . Bul . 248 , (1931) .
3.
Ingersoll, L . R ., and Zobel, 0 . J . "Intro^-Lotion to the
Mathematical Theory of Heat Conduction," Boston, Gin n
and Co ., 1913 .
Kollmann, F . Forsch . Gebiete Ingenieurw ., B6 :169 (1935) ;
B7 :113 ( 1 93 6 ) .
5. Martley, J . F . Dept . Sol . Ind . Research (Brit .), Tech .
'
Paper 2 (1926) .
6.
Sherwood, T . K . Ind . Eng . Chem . 21 :12 (1929) .
7.
Stamm, A . J .,' and Hansen, L. A . J . Phys . Chem . 42 :209
(1938) .
8.
Tuttle, F . J . Franklin Inst ., 200 :609 (1926) .
R1216
•
Table 1 .--Tangential drying of bitka spruce under different relative humidity conditions at 40°0, 1
(Fiber-saturation point, m l in grams per gram of dry wood, 0 .294 )
.
Relative
humidity
of drying
air
Moisture
content
:in grams per :
gram of
dry wood
.
in
:equilibrium :
.
.
with
. drying air .
Specific
gravity
of wood
(Gree n
volume)
m2
Cm2 VMin .
0 .000
0 .344
.366
0 .00770
.00792
.00835
.00870
.00802
.00835
.00807
.294
.008499
.345
. 355
.00875
.30
. 3 28
.360
10 .0
.023
.3s6
.359
15 .0
.032
.o6o
50 .0
.084
Cm
1 .10
1 .25
1 .15
1 .15
1 .15
1 .20
1 .25
1 .25
1 .1 5
1 .10
1 .10
Dt
Dt
Cm
VMin .
•
gg x 105
Cm sec .
x 10 5
Cm sec .
0 .0088
.0088
.0088
.0082
- .0085
.0084
.0085
.009 1
.0091
.008 2
.0081
.0080
.0080
0 .384
3c5
.419
.404
.386
.397
.436
.436
:
.382
.384
.
.
.379
.381
.390
.383
.437
.424
.430
. 391
.382
.390
.00683
.00693
.00710
.00674
1 .05
1 .10
1 .15
.90
.0075
.0083
.0081
.0075
.365
l0
.410
.360
.370
.390
.381
.00662
.00652
.00647
.85
.85
.85
.0075
.0075
.0071
.406
.388
.365
.410
1 .15
0 .36 7
•39,
.66
• 374
.368
.392
88
.
.
.
.11
3.0
4
74
:
Av . : .
.# 1
.42
.436
.0079
.0079
.008 0
.0079
.00746
:
.
.
.
:
Mea n
effectiv e
diffusio n
constan t
corrected t o
a woo d
specific
gravit y
of 0 .36 5
.
1 .10
.95
.8 5
.90
.00755 8
.
.
.
Vt
.00816
.00792
0079 4
. 00640
.361
. 3 55
.352
35 .0
1
Mean
effective
diffusion
Constant
.95
.368
.048
.00776
.00779
Distance
divided by :
the square :
rdot of
the time
.0078
.0079
.0082
.358
25 .0
Distance
from
: surface to
: point of
fibe r
: saturation
1
A
Percent
0 .0
Corrected
: moisture loss
per unit
: cross section
divided by
:the square roo t
of the time
.
Av . : .
Av .
-
:
90
.383
84
.382
:
Av .
.37 6
.380
:
:
.343 Av . .:
.376
-
.42 J
.41 4
. 41 7
357
.401
.392
43 0
.425
.380
407
.40 9
.40 7
Av . :
.37 8
.398
75 .0
.137
.363
.oo6oo
.80
.0064
90 .0
.193
.377
6
.00496
0046
.65
.0051
.417
.42 7
00041041
,
. 0
Av . : T
0 .0
.000
.320
.319
.00294
.00297
.38
.84
.0253
.0240
.
cn
.422
.405
.409
.38 5
Av .. .
.36j
.37E
-Initial moisture content of specimens on the basis of the dry weight of the wood, 185 to 274 percent, except
for last two values which are for green wood with an initial moisture content of 50 percent .
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Table 3 .--Tangential drying of water-saturated Sitka spruce under differen t
atmospheric pressures at 40° C . and 0 .0 percent relative humidity l
Atmospheric :Specific : Corrected : Distance : Distance : Mean
:Mean effective
pressure : gravity :moisture loss :
from
:divided by :effective :diffusion con: of wood ; per unit
:surface to :the square :diffusion :stant correct: (green :cross section : point of : root of : constant : ed to a woo d
time
: specifi c
: volume) : divided by
fiber
: gravity o f
:the square
lsaturation :
0 .36 5
:root of time :
9.
A-\~
Hg .
1
Art
1
g
Cm .
-Cm .
Cm. 2 N4I:Iin .
Atmospheric :
.
240
120
62
.374
.
0.00775
.
.00710
.
.00990
.33 4
.
D
: g x 10 5 :
Cm . sec .
g x 10 5
Cm . sec .
0 .3 8
Average from table 1
Atmospheric : 0 .358
48o
Dt
0.80
:
1 .00
.95 r
0 .0086
:
0 .378
.
.37 I
.
.00995 :
.400
:
.4 1
.
,0114
.
.640
:
.6o
.844
:
.83
.358
.
.01085
y
1,15
.
.0137
:
.365
.
.0197
:
.80
.
.0159
:
1 .78
1.78
1
-Initial moisture content of specimens on thq .basis of the dry weight of th e
wood, 201 to 213 percent ; m l - m 2 = 0 .294.
r
Table 4 .--Longitudinal drying of water-saturated Sitka spruce under differen t
relative humidity conditions at 40°C . l
Relative
humidity
of
drying
air
:Specific : Corrected
: Distance :
: gravity :moisture loss :
from
: of wood : per unit
:surface to :
: (green :cross section : point of :
: volume) : divided by :
fiber
.
the square
:saturation :
:root of time :
Q,
AV
Percent
▪
g
Vim .
1
Cm .
Distance
divided
by the n
square
root of
time
0 .0
0 .3)41
0 .0785
0 .0
.341
.0785
•
20 .0
.3 )42
.0720
.
0 .60
20 .0
.
.350
.
.0679
.
50 .0
:
.356
.
.0630
.
50 .0
.
.392
.
.0594
.
0 .36 5
▪
1
Cm .
Min .
in .
: Mean
:Mean effectiv e
effective :diffusion con:diffusion :stant correct: constant : ed .OY a woo d
specifi c
:
gravity o f
0,0169
Dt
:
. 10 5
Dt
x l0
:
Cm . sec .
Cm . sec .
7.5
7 . 1+
1
- 'Initial moisture content of the specimens on the basis of the dry weigh t
of the wood, 225 percent .
R1216
10 ,
O
5
7
6
It
y~y fr
fi i~ "
u)
)
f
/
0
-
--
2
1
Ct
0
•
40
60
100
120
SO
140
Square root of time (minutes )
Figure l .--Moisture loss-square root of time relations for tangential an d
longitudinal drying of water-saturated blocks of Sitka spruce .
(Green volume specific gravities, 0.344 and 0.341, respectively ;
drying at 40° C . and a relative humidity of zero . Dotted line
R1216
represents theoretical curve .)
20
.3
ft .
y'
M
o
//
i
0
10
S
Square root of time (minutes )
2
0
14
12
16
Bigure 2 .--Moisture loss-square root of time relation for tangential
ing of a green block of Sitka spruce ,
(Green-volume• specific gravity, 01319 ; drying at X40-° C . and
relative humidity of zero .)
tire=
:1
.
f'
i
'3
. ;
T
n, t
r
l
4 7
i
'•' fr- , .
ti
i
I
~
rrl
I
H
l '
rI
I!
■
- F
14
23 0
22 0
21 0
Center
Original moisture content -'
-
miml■
20 0
19 0
1g 0
Imo Ism
Sid e
■■■■~
o 130
o
12 0
P+
11 0
10 0
90
1111
II
IIIM
NMI
n
80
70
60
50
Immmr
4-
■=ml■ !
IME
4o
30
20
10
0
-
I
s
0 0 .2 0 .4 0 .6
1 .4 1 .6 1 .8 2,02 .22 .4-2 . 6
Distaaice into wood, fom'ying surface (cm . )
Figure 3 .--Center and side moisture gradients fo r . the tangentia l
ing of a water-saturated block o-f g. i.'tkaspruce ;
(Green volume specific gravity, 0.344 ;:,1 ,dryi ;g t 4.0f;
R1216
and . a relative humidity of zero .) .
90 I
r
i
+
I
I
1
Original moisture content 225 percen t
S5
:MBE
75
65
Ein
° 60
cia )
55
° 50
0
0
45
a)
-
S r
40
a)
c
35
0
0
30
_
FAIR
f
25
20
15
10
0
1
4
5
2
3
Distance into wood from drying surface (cm .) .
Figure IF .--moisture gradient for the longitudinal drying of a water saturated block of Sitka spruce .
(Green volume specific gravity, 0 .341 ; drying at 40° C .
and a relative humidity of zero ; original moisture conR1216
tant, 225 percent .
0
•
55
I
Original moisture conten t
50
45
40
35
30 ~
i
J
25
a
J
20
f{i
~
~
0
5
0
0 .2 0 .4
0.6 0 .8 1,0
1 .2
1 .4 1 .6 1 .8
2 .0 2 . 2
Distance into wood from drying surface (cm . )
2.
4 2.6
Fiire 5 .--Moisture gradient for the tangential drying of a green block o f
Sitka spruce .
(Green volume specific gravity, 0.319 ; drying at 40° O . and a
relative humidity of zero .)