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15, 1946.
F. J. EWlNG EI‘AL
2,409,263
DESICCAN'I'
Filed April 1:5, 1943
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BY
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ATTORNEY.
' F. J. EWING ETAL
2,409,263
DESICCANT
Filed April 13, 1943
6 Sheets-Sheet 2
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806EEA . L OVETT
INVENTOR.
2.
BY
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ATTORN EY.
F. J. EWING ' ETAL
2,409,263
DE SIGCANT
Filed April 15, 1943
6 Sheets-Sheet 3
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INVENTOR.
BY
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ATTORN EY.
Oct. '15, 1946.
F. J. EWING EI‘AL
2,409,253
DESICCANT
Filed April 13, 1943
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INVENTbR.
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BY
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ATTORN EY.
Get. 15, 1946.
F. J. EWING ETAL
2,409,263
DESICCANT
Filed April 13, 1943
6 Sheets-Sheet 5
M.
.zE’asé-eA. Lo VET’;
INVENTOR. '
ATTORNEY.
Patented Oct. 15, 1946
2,409,263 "
UNITED STATES “PATENT OFFICE
DESICCANT
Frederick J. Ewing, Pasadena, and Roger A. Lov
ett, East Los Angeles, Calif., assignors to Filtrol
Corporation, Los Angeles, Calif., a corporation
of Delaware
r
1
Application April 13, 1943, Serial No. 482,926
3 Claims. (Cl. 2.52'—269)
This invention relates to adsorbents for vapors,
and particularly to desiccants; that is, adsorbents
for water vapor, and to methods of producing
such adsorbents from natural clays.
We have discovered that we may produce a
highly e?icient adsorbent for water vapor from
montmorillonite clays. We are enabled to pro
duce clays which are highly efficient as desic
cants, particularly in adsorbing vapor from air
of low humidity of about 30% or lower relative
humidity, which are superior to known desic
cants. We have accomplished this by drying the
2
adsorptive eiiiciency, the more useful is the ad
sorbent.
.
Another important consideration is the hard
vness of the desiccant. It should resist abrasion
and crushing. If it is not hard, but 'soft ‘and
friable, it will dust and contaminate the pack
aged material.
,
We have developed an adsorbent which has a
much greater adsorptive ef?ciency at such rela
tive humidities. Our material will,'at tempera
tures of about 80°, absorb from 18 to about 20.5%
of its vweight of water at 40 relative humidity,
desiccants at a controlled rate and to a controlled
from 17 to about 18.5% of its dry weight of water
?nal water content.
from air of 30 relative humidity, from about
Desiccants are now employed for dehydration 15 13.5% to about 15% of its dry weight of Water
of air and/or other gases such as refrigerant
from air of about 20 relative humidity, and from
gases. They are employed in dehydration of air
about 8.5 to 11% of its dry weight from air of
and other gases in closed spaces in order to pre
about 10 relative humidity. “Relative humidity"
vent corrosion, mold, or mildew. This is par
will be hereinafter indicated by the abbreviation
ticularly important in shipment of metallic ob '20 “R- H‘!,
jects such as machine tools, ordnance or other
Due to its greater adsorptive capacity We may
metallic objects which are crated for shipment
use less of our desiccant to obtain the dehydra
or storage. Packaged foods, such as fresh or de
tion required to preserve the package at a rela
hydrated fruits, vegetables, and meat are also
tive humidity of 30 or less.
\
subject to spoilage in humid atmospheres.
Our desiccant is also characterized by the fact
Lumber and wooden objects are subject to warp
that it is hard and not friable and substantially
ing or so-called “dry rot.” Packaged material
non-dusting under ordinary "conditions of han
must be protected from the attack of moisture
dling, packaging, and shipment. It will show
if they are‘ to be subjected, either in shipment
about 1% loss by the standard method of deter
or storage, to hot or humid climates.
30 mining hardness as given in “Army-Navy Aero
In' order to be effective for such'purposes it
nautical Speci?cation,” Speci?cation No. AN
is desirable that the desiccant have a high ad
13-6.
sorptive emciency for water, not only from air
In the following discussion we shall refer to
of high relative humidity, but also from air of
“volatile matter” and “per cent volatile matter,”
low relative humidity. Essentially the objective 35 or its abbreviation “V. M.” By this quantity we
to be obtained is to maintain the atmosphere en
mean the per cent loss as moisture of clay ignited
' closing the object to be protected at a low relative
at a temperature of 1700° F., in accordance with
humidity.
>
the method more fully stated hereinbelow.
It is therefore desirable to incorporate into the
We have found that we can obtain such a des-'
enclosed space, such as a car, ship, warehouse, 40 iccant by controlled drying of montmorillonite
or the package itself, a desiccant which has a
clays of the sub-bentonite type, such as are activ
high adsorptive e?iciency capable of drying the
atable by acid treatment to produce useful. oil
air to a safe humidity. Experience has shown
decolorizing clays and cracking catalysts, Such
that such desiccant should have the ability to
clays are the substantially non-swelling bentonite
adsorb a large amount of moisture from air and 45 clays as distinguished from the swelling type ben
must particularly have this high capacitywhen
tonites whose decolorizing power is not substan
it is contacted with air having a relative humid
tially improved by acid treatment. We have ‘
ity of 30 or less.
found that-such clays, or such clays when activ
Useful adsorbents are such that can absorb
ated by acid treatment, such as is used'in mak
at least‘13% of their weight of water from an 50 ing acid activated montmorillonite clay adsor
atmosphere of 30 relative humidity at a tem
bents and petroleum ‘oil cracking catalysts, are
perature of 75 to 85° F. The adsorptive efficiency
converted to highly e?icient desiccants if theyare
at lower humidities may usefully be at least 10%
dehydrated to a volatile content of from about
by weight at 20 relative humidity and 5% by
weight at 10 relative humidity. The higher the
- ‘5.5 to 8 V.
and most e?cient when brought,
by proper drying procedures, to a V. M. content‘
2,409,263
4
3
of about 5.5 or 6 to 7%. This is somewhat greater
than the theoretical hydroxyl water content of
pure montmorillonite, which is about 5%. It
thus appears that if the dehydration is carried
out under such a condition as to remove sub
stantially all free water, but not to remove the
hydroxyl water content of the montmorillonite,
.
801-1’ F., according to the United States o?lcial
methods speci?ed by the Bureau of Ships ad In
terim Speci?cations issued November 1, 1940, No.
51832 (INT), and Army-Navy Aeronautical Speci
?cation AN-D-6, issued November 20, 1942.
According to the method speci?ed in this bulle
we can obtain a highly e?icient desiccant.
tin, ten gram samples of the clay are weighed into
an adsorption bulb. The bulb is connected to the
a V. M. content of 5.5 to 7% will give the most
a substantially constant value, normally 80° F.
adsorption apparatus which consists of a train
We have found that the rate of heating of
the moist subbentonite clays, that is, the distilla 10 of bottles containing the sulphuric acid whose
concentration is adjusted so that an air stream
tion rate of the moisture in the preparation of
bubbled through these bottles will attain the de
the desiccant, has a material effect on its re
sired humidity. The temperature of the sul
sultant adsorptive efficiency. The removal of
phuric acid solution and the bulb is controlled to
water at a moderate rate in bringing the clay to
e?lcient adsorption.
Periodically, at intervals of one or two hours, the
bulb is weighed, and the process is repeated until
two successive weighings, approximately one hour
apart, do not show a weight variation exceedi g
In order to obtain the most e?icient adsorbents, 20 ten milligrams. The gain in weight divided by
the original weight of the material multiplied by
particularly for water vapor at low relative hu
100 gives the percentage by weight of water va
midities, we desire to cause a relatively slow dis
por that the material will hold in equilibrium
tillation, particularly during the drying of the
with the air at the relative humidity attained in‘
clay from about 8 or 9% V. M. downward, and to
reduce the water content of the clay to the re 25 the saturators, and this quantity is hereinafter
referred to as “adsorption e?‘lciency.”
.
gion of the optimum value at which the maxi
The particle strength or hardness of the ad
mum e?iciency is obtained, and interrupt the
sorbent is determined according to'the method
heating to secure a clay of such optimum water
speci?ed in the above bulletins. This method
content.
'
The process controls which produce our unique 30 consists essentially of exposing the clay to the at
mosphere of a room to permit it to come into
desiccant will be further understood from the fol
equilibrium with the water content of the room.
lowing description taken together with the fol
Then 150 grams of sample are introduced into an
lowing drawings, Fig. 1 to Fig. 5, inclusive, which
8" #16 sieve backed by a #18 sieve and shaken in
show the effect of the various heating variables
on the desiccant e?lciency and illustrate the de 35 an apparatus which has a single eccentric, cir
cular motion of about 290 R. P. M. and a‘ tapping
sirable limits of heating which permit the attain
There appears to be an optimum V. M. content '
for the dried clay produced which gives the great
est adsorptive efficiency.
action of about 150 strokes per minute. The
sieves are shaken for 15 to 20 minutes until not
over 0.05 gram passes through the #18 sieve in
Fig. 1 to Fig. 5, inclusive, are charts showing
40 one minute of continuous sifting. "The fraction
plots of the data hereinafter presented.
ment of the desiccants which constitute a pre
ferred embodiment of our invention.
passing through the #16 sieve and onto the #18
sieve is used for the particle strength test. When
testing larger sire particles, 9. suitable quantity
non-swelling sub-bentonite type produced at
shallbe ground to produce the required fraction
Cheto, Arizona, and-having the following analysis
45
through the #16 sieve onto the #18 sieve.
based on a volatile free basis:
In order to determine this strength, 50 gram
Raw Cnnro-V. F. BASIS
of the sample prepared as above-described are
The clay employed in the following examples
was a montmorillonite clay of the substantially
Silica (SiOz) __________________________ -._ 67.3
Titanium oxide (T102) ________________ __ 0.3
Aluminum oxide (A1203) _______________ __ 19.5
Ferric oxide (F8203) _____~______________ .._
Manganese oxide (MnO) _______________ __
Magnesium oxide (MgO) _______________ .._
Calcium oxide (CaO) __________________ __
1.78
0.80
6.9
3.2
99.8
placed on an 8", #30 sieve which is backed by a
#45 sieve and a retaining pan. Five copper discs
60 of the size and weight of one cent pieces are
placed on the #30 sieve, and the sieves are shaken
for 15 minutes in the above apparatus. The
sample remaining on the #30 sieve and the quan
tity which is contained in the retaining pan are
65 weighed. We use the term “hardness" as the per
cent retained on the #30 sieve. We use the term
“friability” as the per cent falling through the
The original clay so processed has a natural
#45 sieve and retained on the pan. _
In the following examples, the clay was dried
V. M. of about 38 to 42% as mined, and may drop
60 in an oven heated under controlled temperature
to 32 to 3'7 % in transit to the mill.
conditions. The loss in weight of clay at various
In the following examples V. M. or per cent
intervals of time was determined by weighing the
. V. M. is determined as follows:
clay without removing the clay from the oven.
Approximately ?ve grams of sample are placed
The clay was heated in accordance with the tem
in a porcelain crucible and quickly weighed to the
nearest one-tenth milligram. After a preliminary 65 perature schedules hereinafter set forth under
each example.
heating at a low temperature, the crucible is is
Example A.--The clay at room temperature
nited at a temperature 01' 1700" F. for approxi
was introduced into the oven which was main-'
mately twenty minutes. After cooling in a desic
tained at. 350° F. during the entire drying proc
cator, the crucible is again weighed. The loss in
weight divided by the original weight of the clay 70 ess. The clay was held in the oven for four hours
at this temperature. The clay was then removed
multiplied by 100 is the per cent V. M.
to a desiccator and cooled. The volatile matter
The per cent gain in the weight of the clay
content of the cooled clay was 5.66%.
when in equilibrium with air at various relative
Example B.—The clay at room temperature
humidities, herein referred to as “adsorptive e?l
ciency," was determined at a temperature of 75 was introduced into the oven maintained at 400°
2,409,203 I
6
F. during the drying process, and the clay was
TABLE III
held in the oven tor four hours. The clay was
removed to a desiccator and cooled. The, volatile
content of the cooled clay was 5.56%.
' Percentage water vapor adsorption at indicated
'
.Emample C.—The clay at room vtemperature
was introduced into the oven maintained at 500°
F. during the drying'processt and the clay was
held for four hours in the oven. This constituted
a shock heating suddenly to 500° F. The clay
Example
10
E ___________________ __
F _________________________ -_
G_-_ ______________________ ._
20
9. 8 l4. 3
9. 8 15.0
9.8 14.1
40
60
80
19. 5
20. 7
18.9
22. 9
23. 9
21.8
26.8
27.8
26.0
100
35. 6
36. 7
34.6
was removed to a desiccator and cooled. The
V. M. of the cooled clay was 5.52%.
I
'
.The drying curves of Examples vE, F, and G are
Example D.—The clay at room temperature
charted in Fig. I on which is plotted the V, M.
was introduced into the oven maintained at 600°
content of the clay at various times, as given in
F. during the drying process, and the clay held
Table II. . In Fig. 3 we have charted the dehydra
for four hours in the oven. The clay was removed 15 tion curve of the clay. giving the v. M. content of
to a desiccator and cooled. The volatile matter
the clay at the various temperatures as reported
was 5.07%.
I
‘
I
The adsorption emciencies of theselclays are
given in Table I:
'
.
>
inExamples
Table II. H, I. and J.—The clay at room
> I
tem
.
TABLII
. perature was introduced into the oven maintained
, 20 at 350° F.‘ during the drying process.
The cold. >
clay was introduced, rose to the temperature of
Percentage water vapor adso tion
the oven, and then was maintained at that tem- _
at indicatedB. H. m
Example
perature vfor a period of time. The clay was re
'
' moved and cooled.
10
2o
40
60
80
100
as 13.7‘ ‘19.8 214 27.5
35.
8.0 13.1 19.2 23.2
5.6 ‘11.9 17.9. 22.5
27.5
26.6
35.
34.
25.0
34‘.
15
7.8
16.3
20.7
The clay, as in the previous
25 examples, lost some water while being cooled and
the terminal V; M. was determined. The follow
ing TableIV gives the temperature schedule; 1. ‘e..
Gibon-1
theheating rate and the volatile matter content
of the clay at each temperature level and time in-'
30 terval, thus giving the distillation‘ rate. The ter
In the above table, as in other like tables‘here
inafter given, the adsorption e?iciency at any
intermediate value of R. H. may be taken from;
curves drawn for the values of adsorption efn- .
ciency vs._ R. H.
These curves arev'not given.
minal V. M. of the clay was: ‘In Example H,
6.85%; in Example I, 7.15%; and in Example J,
7 15.89%.
35.,
,
herein in order not to multiply the number of
?gures.
Temp., °F.
7
The‘ values so given. however,
may be '
Time, minutes
taken as a disclosure of, the adsorption e?iciencies
at intermediate values of R..H., such as 30 R. H.,
by the plotting of such curves, as will be under
stood by those skilled in the art.
-
"
l
'
TABLE IV
Exam
ple H
ple I
Percent volatile matter
Exam
ple I
Exam
ple H
Exam
ple I
'
Examples E, F, and G.-The clay- at room tem
perature was introduced into the oven at 90° F.
and the oven andv clay were gradually raised in
temperature. Tlie clay was :then femoiéifat the
various times indicated in the following vtable
at the last V, M. recorded under‘ eacl'rexample,
and cooled. During the cooling period some ad
ditional water was lost. The V. M. content of the
cooled clay in Example E was 6.72%; in Example
F it was 5.65%; and inExample- G. it was. 7.61%.
The following Table No. II gives the tempera‘.
ture schedule; i, e., the heating rate of the clay
in each of the examples is given. Wegive also
the volatile content of the clay at each of the 55 The adsorptive‘ ef?ciencies of the. clays of sam
ples H, I, and J at the various values of R. H. are
temperature levels and times.
I given in TableV.
TABLE II
Temp, °' F.
Percent volatile matter .
-
-
‘
‘
TABLE V
_ Percentage water vapor adsorption at indicated
v80
Time, minutes
'
}
--..
-
R. H.
>
.
'
Exam- Exam- Exam- -Exaln- Exam- Exam
ple E
ple F
ple G I ple E
96
138
‘189
90
158
210
32.0
30. 4!
25. 7
32
30'
26
234
256
- 250
234
18. 7
12. 5
18
12
298
'
' ple F
\
304
8.7
311
7.8
77
_7_
6. 5
318
'
332
...... __
ple G
Ema.
8. 6
i
2o
40
9.5
14.3
9.5
9.9
14.0'19.4
14.9 30.3
60-80
19.7 22.6
22.3
23.6
100
21.0
36.2
26.7
28.2
33.7
35.3
The vdrying data of Table IV are charted in Fig.
2. In Fig. 2 is charted the time V. M. curve. From
.
_ 701 Fig. 2 it will be observed that the drying rate of
5.8
samples H, I, and J in going from'32% V. M. to
G .at the various relative humiditiesare given )in
'
'10
6. 1
5. 8
The adsorptive e?lciencies of samples E,-F','_and
Table III.
.
.
7% V. M. has an average value of 1.59% V. M.
loss per minute.
.
-
Examples K, L, and M.—The clay at room
75 temperature was introduced into the oven at a
9,409,283 ‘
7
-
Tut: VIII
temperature of 550° F. The temperature ofthe '
clay rose rapidly. The clay was removed from
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corded in the tablefor each example, and the
V. M. content and adsorption e?iciency deter
The clay in the case of each example was re
mined. The drying data of Table VIII are
charted in Figs. 1 and 3. In Fig. 1 are charted 60 moved irom the oven at the times indicated for
the last value or the V. M. recorded, placed in a
the time V. M. curves and in Fig. 3 the tempera
desiccator and cooled.
ture V. M. curves. In Fig. 1 are charted the
The V. M. content of the cooled clay at the
values for Examples S, T, U, V. The remaining
end 01' the drying process for each of the samples
examples fall closely on these curves. The dry
ing rate of these examples was closely similar to 65 N to V, incmsive, is given in Tame up
that of Examples E, F, and G, giving eil'ect- to the
lower initial V. M. content of the clays of Ex
amples N to V. The average value of the drying
rate for all the Examples E, F, and G, and N to
V, inclusive, are substantially the same, and the 70
average value of all the Examples E, F, G, and
0 to T, inclusive, was 0.47% V. M. per minute.
Table VIII gives the V. M. content and the tem
perature of the clay at various times during the
drying operation oi Examples N to V, inclusive. 76
Tuna IX
Percent volatile matter
18.73
129
9.15
1. 47
6.1
5-89
5.62
5.57
5.76
2,409,268
9
10
vThe adsorptive e?lciencies of the clays, Ex;
at the most rapid rate of 5.9 V. M. per cent per
minute. It win be seen that for each rate of
amples N to V, inclusive are given in Table X.
TABLE X
Percentage water vapor adsorption at
indicated R. H.
Example
10
20
40
60
80
N ................... -_ -2.5
1.5
5.3
8.75
126
3. 3
7.5
9.25
10. 0
10. 1
9. 0
9.5
9.8
9. 1
12.0
14.1
15. 2
l4. 7
13. 75
14.6
15.0
12. 7
16.5
19.2
20. 2
Z). 3
I9. 8
20.0
20.25
16. 2
2125
22.9
24. 3
24. 4
23. 8
24.1
24.5
20. 3
25.0
27.5
29. l
29. 15
28. 25
29.1
29.1
drying observed for the clays here reported, and
at every value of relative humidity, there is an
optimum V. M. content at which the adsorption
e?iciency is at a maximum and that this opti
mum resides in the range or 5.5 to 7% and more
100
closely in the range 01' 5.5 to 6.5% V. M. The ef
fect of departing from this optimum value of the
24.3 10 V. M. content in either direction results in a de
33. 2
crease In the adsorption e?iciency at each value
35.3
39.7
of relative humidity. as is indicated in Table XI.
39. l
41. 79
38. 7
41.6
39.6
In this table the adsorption ei?ciency at the op
timum value is given as the range of adsorption
15 e?lciencies obtained for the range of drying rates
The drying of Examples E, F, and G, and N
there charted. The adsorption e?iciency at the
optimum V. M. is compared with the adsorption
e?lclency of Example D which had been dried to
to V, inclusive, was conducted under approxi
a. V. M. content of 5.07% and Example P which
mately equilibrium conditions. In Fig. 3 we have
charted the V. M. content of the clay at equi 20 had been dried to a V. M. content of 9.75%.
librium at various temperatures. It will be ob
TABLE XI
served that, giving e?’ect to the varying initial
Adsorption emc'iency
V. M. content of the clay entering the drying
process, the drying curves of all these clay
samples are closely similar, showing a plateau at 25
about a V. M. content of 5 to 7%, in which region _
R. H
Example D At gpth um Example P
the change in V. M. is substantially flat begin
ning at about 6% V. M. The knee of the curve
as it enters the plateau curves is in the region of
6 to 7%. It is apparent that the loss of water in 30
the region down to about 7% is of a di?’erent
nature from that occurring in the region of 6%
downward. This type of dehydration curve is
well known to physica1 chemists and it would in
10
20
40
60
80
100
3. 15
7. 8
15. 3
20. 7
25
34
9. 2 to 10. 8
14 50 15. 2
19. 6 to 20.3
22. 9 to 24. 3
E. 5 f0 29. 2
36. 2 to 4D
7. 5
12
16. 5
20. 2
25
36. 3
It will be observed that the drying of the clay
dicate that the montmorillonite crystal is losing 35 to a V. M. lower than about 5.5 to 6.5% has a sub
stantial deleterious e?ect upon the adsorption
emciencies, particularly at the lower values of the
relative humidity. As was said before, it is be
lieved that this results from the fact that in going
its hydroxyl water of constitution when it is
dried below about 6%. While we do not wish to
be bound by any theory of the action of our dry
ing process or of the adsorbent thus produced, it
is important to note that this region of 6 to 7% 40 to a V. M. value below the optimum, we are enter
ing the plateau region of Fig. 3 where losses of
corresponds with the region of optimum adsorp
water of constitution occur and changes in crystal
tion which we have discovered and as will be later
structure result in a destruction of surface ac
described. It appears that the clay, if it is heat
tivity.
‘ed beyond a V. M. content of about 6 to 7 %_, re
While the specific values of the optimum here
sults in a. modi?cation of the crystal structure 45
set forth may vary somewhat with clays from
of the montmorillonite resulting in some loss of
various beds, depending, it is believed, on the
hydroxyl water. It is" of some interest to note
amount of impurities, it will be found that by -
_ that pure montmorillonite having the theoretical
structure (OH)4A14SiaO2o has a hydroxyl water
applying the principles set forth above the limits
content-(i. e. water of constitution) of about 5%.
The additional water which results in a V. M.
content of 6 to 7% may well be 'the'constitutional
' of optimum V. M. content of sub-bentonite type
a small change in V. M. in going below about 6
quality of. the clay, and this will occur‘ at the
knee of ‘the curve of dehydration, such as is
charted in Fig. 3. The exact V. M. percentage
for optimum e?‘iciency may also change with acid
60 treatment of varying degree and intensity em
of montmorillonite clays for the production of
optimum adsorption e?iciencies at each‘ value of
relative humidity may be readily determined. It
water of the impurities associated with the mont
is believed that the optimum will lie in the .region
morillonite in the Cheto clay.‘
_
The sharp decrease in e?lcie'ncy obtained by 55 of about 5 to 8%, depending upon the purity and
to 7%, as hereinafter set forth, supports the view
that the loss of hydroxyl water has a large dele-l
terious effect on the surface, activity and adsorp
tive properties of the clay. '
Figs. 4 and 4a chart the eilect of V. M. con- ,
ployed in acid activation of the clay. This value
tent produced at the various rates of drying
of the optimum ef?ciency may then be deter- -
mined by plotting a chart like Fig. 3 or'Fig; 4, by
observing the change in water content at various
ous values of R. H. In these curves the V., M.
is the terminal V. M. of the cooled clay of Ex 65 temperatures and times during drying, and also
by observing the per cent adsorption at a number
amples A to V, inclusive, and the per cent ad
sorption at each R. H. is that here reported for
of relative humidities su?icient in number to
establish the curves in Fig. 4 for clays of varying
such clay. At each R. H. ‘there is. a series of
upon the adsorption efficiency of the clay at vari
curves. As will be more fully described below,
terminal V. M. content.
'
they represent the adsorption e?iciency of the 70 As has been indicated above and as will appear I
Examples E, F, G, and N to V, inclusive, dried . from Fig.‘ 4, the magnitude of the adsorption
at the slowest rate of about'0.47 V. M. per cent
,e?iciency at all values of relative humidity de
per minute. The Examples H, I, and J are dried
at the intermediate rate of 1.59 V. M. per cent
pends on the rate at which the water has been
removed as well as upon the V. M. content of the.
per minute, and Examples‘ K, L. and M are dried 75 0153? at the terminus or the diving period. As will
2,409,203
11
.
12
as shown in Figs. 4 and 5 from which may be
be observed from Fig. 4, at each value of the V. M.
determined the specific values of ‘the drying rate
content and at each value of relative humidity,
necessary to develop the desired adsorption eiii_
the adsorption humidity of they clay is higher the
cienoies at the various relative humidities to
slower the rate at which the clay has been dried.
The e?ect of the drying rate upon the adsorp 5 which the clay is to be exposed, and by the appli
cation of these principles to .determine the
tion e?iciency at each value of relative humidity
optimum conditions for the drying process to
is also shown in Fig. 5. In Fig. 5 is charted the
obtain the most desirable clay of highest adsorp
adsorption e?iciency of the clay at each value of
tion e?iciency under the conditions under which
‘the relative humidity taken at optimum V. M.
content of the clay at such humidity, to wit, in 10 it is to be employed. It will be found that the
slower the drying rate the higher the adsorption
the region of 5.5 to 6%% against the rate of loss
eillciency at all values of the V. M. content in
of water. Thisrate of loss is taken as the quotient
the range of about 5 to 8% at all values of rela
of the loss of water ‘from the beginning of the
tive humidity, and that for each drying rate
drying process down to the time when the clay
has a V. M. content of 7% divided by the time 15 there is an optimum V. M. content within the
range of about 5 to 8% and more particularly
interval for such loss. This is termed the average
within the range of 5.5 to 7% where the optimum
rate of V. M. loss or the average drying rate. The
V. M. is to be obtained. By choosing the lowest
terminal value of 7% was chosen as representing
practicable drying rate and by choosing the
the break-point or knee of the drying curves
shown in Figs. 1 and 2. At this point there is a 20 optimum V. M. content, the highest emciency ad
sorbent and desiccant can be obtained by the
large change in the rate of V. M. loss. Some other
application of the principles herein set forth.
value close to 7% could be chosen as, for instance,
In order to obtain the best results in the pro
6 to (ii/2%, with a small change in the results
duction of desiccants and adsorbents of highest
attained. In the region of 8% V. M. and lower,
major damage may vresult by overdrying. We 25 adsorption e?lciency, we prefer to employ drying
apparatus which will expose the clay to the
desire to control the drying in that region to
optimum temperature for production of the
obtain a removal of water at a rate suf?ciently
optimum V. M., and will attain the optimum
low to give an optimum adsorption efficiency of
V. M. at the optimum drying rate in such man
high value. The average drying rate in this
region is also included within the scope of the 30 ner as to subject the clay uniformly to such tem
peratures and drying rates. We have found it
term "average rate.”
'
desirable to avoid contact of the clay with gases
It will be observed that the increase in the dry
of excessively high temperature or with hot metal
ing rate affects deleteriously the adsorption em
surfaces under such conditions as might lead to
ciency of the clay dried to its optimum V. M.
content. As is evidenced by the curves of Fig. 5, 35 partial overdrying of the clay or result in ex
posure of the clay to such high temperature as
this e?ect is most pronounced at the relative
will produce excessive drying rates. In other
humidities of 10 and at 60 to ‘100. The clay is
words, while the average temperature conditions
most sensitive to drying rate at such R. ‘H. values.
may nominally be within the range adequate, if
The clay is less sensitive in the region of 20 to,
40 R. H. In fact, a more thansthreefold increase 40 the clay be uniformly heated, it is still possible,
by reason of local hot spots and non-uniform
in going from a drying rate of 0.47% V. M. per
distribution of temperature throughout the clay,
minute to 1.59% V. M. per minute has but a small
for one portion of the clay to be overdried and
e?ect on the adsorption emciency at-2O or 40
another portion be underdried. In like manner
R. H. However, at 10 R. H. and at 60 and 100
it is possible, under such conditions, that part
R. H. major damage is done in going from 0.47%
ofthe clay is subjected to an excessive drying
V. M. to about 1.59 per minute. The decrease in
rate while other parts of the clay are subjected
e?lciency in going to the higher drying rate of
to much lower drying rates. Such a clay when'
5.97% per minute is only moderate. IIThis in
produced will be a mixture of good and poor ad
dicates that the degree of control of the heating
rate which is necessary for the processing of this 50 sorbent with an average value much lower than
that which could be obtained had the clay been
clay will depend upon what level of the humidity
dried more uniformly at the optimum condi
we desire to use for the clay. If we desire to
tions.
operate at 30 R. H. or in the region of 20 to 40
We have found both rotary kilns and tunnel
R. H., we may dry the clay at a drying rate rang- ‘
ing up to about 4% V. M. per minute without 55 type driers satisfactory drying equipment, pro
vided that they be of su?icient capacity to permit
materially a?ecting the adsorption e?lciency of
drying to take place gradually, as described above.
the clay when it is dried to the optimum range '
It is desirable that such drying equipment be
of V. M. However, if we desire that the clay be
of suf?cient size so that the drying load is satis
employed in adsorption in the .region of 10 R. H. .
or in the region of 60 to 100 R. H.,v we will hold 60 ?ed by using gases at relative low temperature.
We have found it useful to carry on our drying
the drying rates to a closer control in the region
operation in a plurality of stages by employing a
under about 2% V. M. per minute, and we will
plurality of driers or tunnel type driers in which
get a greater enhancement if we hold the rate ,.
a plurality of zones of drying at diilercnt tem
under about 1% V. M. per minute.
While the speci?c value of the drying rate to 65 perature and di?erent humidity conditions can
produce the effects herein set forth will change
with clays from different beds, the general prin- '
ciples of the eifect of thedrying rate apply. ;
Those skilled in the art will know how to repeat _.
the tests here set forth to obtain the ‘specific? 70
effects of the various drying rates in producing?
clays with varying -V. M. within the range of
about 5 to 8% and determine the adsorption
emciencies of the clay at each R. H. and V. M.
content and thus obtain a series ‘of curves such 76
be maintained. The ?rst stage of such plural
stage drying process may be operated at a low
temperature level. For. instance, in going down
to about 8 or 10% V. M. to 20% V. M., the clay
may be heated to a temperature of about 250 to
300° F. at a drying rate of about 4% V. M. per
minute down to .5% V. M. or less per minute. In
the second stage the clay is dried to about 5.5 to
7% V. M. at a drying rate of about 4% V. M. per
minute or less ‘at a temperature 'of 300 to 450' 1!‘.
-
2,409,203
13
14
In this second stage the drying rate may be I
controlled to a low rate so that the clay while
drying down through the region where V. M.
‘
~
It will be seen that 'by drying to approxi
mately 6% V. M. we are able to produce a particle
break-down of 1.2 % and less through the 30 mesh
screen and .64% and less through the 45 mesh
change may cause a substantial damage in ad
sorption e?iciency, the drying rate may be closely
controlled. By this separation of the drying
' screen.
By a proper sizing of the'material we can
produce particles having as low as about .72%
through the 30 mesh screen and .45% through
the 45 mesh screen. Such material, experience
process in the two stages, the load imposed on
each drier is reduced, thus permitting more ac
curate control of the drying rate and tempera
has shown, will be non-dusting under ordinary
tures. This permits of ya more accurate control 10 mechanical handling and will‘ not sift through
of the V. M. and drying rate in the second stage
bagging used for containing the material em
to produce the optimum V. M.
ployed for packages in which it is used for con
We have also discovered that by such proce
trolling the humidity for the purpose of avoid
dure, not only will clay of high adsorption ef?
ing damage to the material packaged. A low
ciency be attained, but the clay will also have a 15 friability is also of importance in installations
high hardness value such that it will not dust
requiring the use of the granular desiccant in
under ordinary mechanical handling, as when
large beds through which the ?uid requiring dry
the clay is used as a desiccant in packaging
ing is to be passed. Resistance of the granules
various materials. Because of its high hardness
to crushing and packing permits maintenance of
value, the dry clay will not dust and pass through 20 minimum resistance to ‘gas ?ow in such a bed,
the meshes of the bags in which the desiccant is
and also minimizes mechanical loss of desiccant.
contained. As an example of the nature of the‘
It is entirely unexpected that large granules of
hardness values obtained when the desiccant is
the natural clay, of particle size such as shown
produced in accordance with the principles of
in types a to d, above, would exhibit so great a
our invention as herein set forth, the following 25 degree of physical coherence when dried under
may be taken as illustrative examples:
conditions dictated by the desire of developing
Example -1.—The clay was dried according to
maximum adsorptivity or “porosity.” As a mat
our process to a ?nal V. M. content of 5.9%.
ter of fact, however, the desiccant produced in
When subjected to the hardness test previously
accordance with the present invention is at once
described, .72% of the clay passed through a 30 30 superior to synthetic commercial desiccants such
mesh screen and .45% passed through a 45 mesh
as silica gel or activated alumina, both in terms
of physical hardness and in adsorptivity at low _
relative humidities.
screen.
Example 2.—The clay was dried to a ?nal V. M.
content of 5.8% and its hardness values were 1%
It is to be understood that the foregoing de
through a 30 mesh screen and .53% through a, 45
scription of embodiments of our invention is for
purposes of illustration, and modi?cations may
be made therein without departing from the spirit
mesh screen.
Example 3.—'I'he clay was dried to a ?nal V. M.
content of 6% and then ground and graded into
of the appended claims.
the following grades or types: type a; type b; type
c; and type d. The screen analyses of the various
types were as follows:
i}
Type
Mesh size
(screen number).
Type
+3=0%
a
"""""" "
-—6+8==33. 7%
Mesh size
(screen number)
#6=0. 0%
-3+6=65. 9%
45 bentonite clay which comprises reducing the V. M.
of said clay to a V. M. of about 5.5 to 7%‘ by
-—6+12==62. 1%
b__-...____._..
--8+ =0. 5%
‘
We claim:
1. A desiccant consisting essentially of a native
montmorillonite acid activatable sub-bentonite
clay having a V. M. of about 5.5 to 7%.
.2. The method of producing a desiccant from
native montmorillonite acid activatable sub
—l2+18=37.8%
heating to a temperature insu?icient to modify
the crystal structure of said clay. the reduction
—18+20==0. 1%
—20=9. 2%
of‘V. M. being e?‘ected at an average rate 29
50 greater than about 1% V. M. per minute.
Type
Mesh size (screen
number)
Type
3. The method of producing a desiccant from
Mesh size (screen
number)
native montmorillonite acid activatablev sub
bentonite clay which comprises reducing the V. M.
+2=0.0%
'
+2=-o.o%
of said clay by heating to a V. M. of about 8%
55 and further reducing the V. M. by heating to
—-45==0.0%
-—45=2.5%
within the range of about 5.5 to 7% V. M. at a
rate of less than about 0.5% V. M. per minute,
The index "minus” indicates through the
all of said heating being at a temperature insuf
screen, and the index “plus" indicates retained on
?cient to modify the crystal structure of said clay,
the screen. The several types were subjected to
to produce a desiccant having an adsorption e?l
the hardness test previously described with the
ciency of at least about 17% at 30% relative
following results:
'
,
c
__________ . _
+45=99.2%
.
d ............ . .
+45==97.4%
- humidity.
65
Particle strength percent falling through:
Screen #30 ......................... -.
. 70
l. 2
0. 93
0.87
Screen #46 ......................... .. 0.40
0. 47
0. 53
0. 64
FREDERICK J. EWING.
ROGER A. LOVE-TI‘.
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