THE DRYING OF AIR CONTINUOUSLY
25 OCT1943
BY AIR-BORNE SILICA GEL
By
Arthur W.
Plummer
B.S. University of Kentucky
1939
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Master of Science
in
Chemical Engineering
from the
Massachusetts Institute of Technology
Signature of Author
Depart ment of Chemical Engineering
Signature of Professor
in Charge of Research
Signature of Chairman
Department Committee on
Graduate School
I
/
2~3
A C K N 0 W L E D G M E N T
The author wishes to acknowledge the great assistance rendered by Professor C. A. Stokes during the experimental and early writeup stage of this investigation.
The
use of apparatus and auxiliary equipment previously built
by him made it poasible to complete the experimental work
at night while on duty at the CWS Development Laboratory.
His analysis and conversion of data from the literature
and his willingness to cooperate in taking readings during
the experiments enabled the author to complete considerably
more work in the small time available.
Suggestions made
throughout the investigation were very helpful and are
hereby gratefully acknowledged.
261435
TABLE OF CONTENTS
Page No.
LOCATION OF TABLES
LOCATION OF FIGURES
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
ii-
SUMMARY
1
INTRODUCT ION
4
PROCEDURE
22
RESULTS
24
DISCUSSION OF RESULTS
27
CONCLUSIONS
44
RECOMMENDATIONS
47
APPENDIX
48
A. Expansion of Procedure
49
B. Summarized Data
59
C. Sample Calculations
62
D. Results:
73
Plots and Tables
E. Location of Original Data
79
F. Calibration Data:
81
Plots and Tables
G. Nomenclature
84
H. Literature Citations-.
86
i.
LOCATION OF TABLES
Table No.
Title
Page No.
Original Data
59 (f)*
Temperature of the Surroundings
During Runs 1-19, Inclusive
60
Data for Comparison of Gravimetric
and Psychrometric Methods of Measuring Humidity
61
IV
Table of Calculated Data and Results
73 (f)
V
Variation in Residual Water Content
of Silica Gel with Time of Reactivation in Air at 178 0C.
74
Comparison of Water Balances Calculated From (1) Difference in
Weight of Inlet and Outlet Gel and
(2) Difference in Humidity of Inlet
and Outlet Air
75
Comparison of Gravimetric and Psychrometric Methods of Determining
the Humidity of the Outlet Air
76
Percent Fines Produced During Use
of Silica Gel
77
Estimated Heat Losses From Column
78
Calibration of 00-550C. Thermometers
Used to Measure the Inlet and Outlet
Air Temperatures (Ice Point, 00C.
Used as Standard).
82
Calibration of Triple Beam Balance
Used in Weighing Inlet and Outlet
Gels
83
I
II
III
VI
VII
VIII
IX
X
XI
*Indicates that Table follows page number given
-iiLOCATION OF FIGURES
Figu re No.
Title
1
2
3
Follo wing Page No.
Equilibrium Data For System,
Silica Gel-Water Vapor
11
A. Integrated Heat of Wetting of
Silica Gel By Liquid Water
15
B. Partial Heat of Wetting of
Silica Gel By Liquid Water
15
A. Integrated Heat of Wetting of
Silica Gel By Liquid Water
15
B.
Partial Heat of Wetting of
Silica Gel By Liquid Water
15
4
Elevation Diagram of Apparatus
58
5
Assembly of Apparatus
58
6
Assembly of Column, Funnel and Flasks
58
7
Sharp-Edge Orifice Flowmeter
58
8
Component Parts of Apparatus
58
9
Residual Water Content of Silica Gel
vs. Temperature of Activation
78
10
Inlet Water Vapor Adsorbed vs.
Air Velocity
78
11
Inlet Water Vapor Adsorbed vs. Mass
Rate of Air Flow
78
Variation in Water Balance Error
With Air Flow
78
Variation in Water Balance Error
With us/ug
78
Variation in Water Balance Error
With L /G
78
Variation in Slip Velocity With
Superficial Air Velocity
78
Variatioh in Silica Gel Concentration in Column With Mass Rate of
Air Flow
78
a2
13
14
15
16
-Iii-
LOCATION OF FIGURES (Cont'd.)
Figure No.
Title
Following Page No.
17
Operating Diagrams for Runs
1-19, Inclusive
78
18
Variation in Adsorption Coefficient, KRHa, With Rate of
Air Flow
78
Variation in Adsorption Coefficient, K a, With Rate of
G61 Feed
78
Variation in (K a)(G/1000) With
Rate of Gel Feea
78
Calibration of Air Orifice Flowmeters
83
19
20
21
-1-
I. SUMMARY
Although most air conditioning is done by refrigeration,
two methods gaining in popularity are absorption and adsorption dehumidification.
The latter has been applied in the
form of static beds of silica gel which do not offer ideal
conditions for adsorption.
Furthermore, methods of design
for such dehumidifiers are more or less empirical.
A system
capable of steady state operation has been investigated and
conventional methods of design for absorption columns proposed for the system.
The drying of air continuously by air-supported silica
gel was investigated by passing air and gel (-28 + 48 mesh)
at known rates and water content into- an insulated glass
column through which the gel was carried by the air.
The
amount of water adsorbed was determined by (1) difference
in weights of the inlet and outlet gel, (2) air rate and
difference in the humidity of the inlet and -outlet air,
(3) gel rate and analysis of the inlet and outlet gels for
water content and (4) calculations employing thermal data,
i.e., air and gel rates, air temperature rise and heat of
adsorption data.
The operating data were correlated in terms of overall
mass transfer coefficients, KRHa and Kga, based on driving
forces of percent relative humidity of the air and useful
water concentration of the gel, respectively.
It was found
that KR1ja decreased slightly with increase in air rate, which
-2is attributable entirely to decrease in
"a" because of
lower gel concentration in the column at the higher air rates.
An equation- derived for Kga involving both air and gel rates
is
(Kga)(G/lOOO) = 0.0013L1 + 0.35
These coefficients were calculated using equilibrium
data determined in the same column by the same method of
operation.
Lower Useful Water Concentrationsat equilibrium
were obtained at a given Percent Relative Humidity than those
reported in the literature.
This was attributed to (1) a
lower rate of approach to equilibrium due to the presence of
air or (2)
loss of heat from the column.
During the experiments it was discovered that a gravimetric method could not be used to measure the humidity of
air in the presence of finely divided solid adsorbent due
to additional adsorption of water vapor in the lines leading
to the drying tubes.
A psychometric method was found to be
satisfactoty.
Percent error in the water balance calculated from thermal data (assuming the average of the other three to b e correct) was attributed to a lower temperature reading at the
top of the column than that corresponding to the water adsorbed.
The low temperature was due to either loss of heat
from the column or insufficient time of contact between gel
and air for the heat effect to become fully developed.
Temperature measurements at a point three inches above
the gel inlet indicated that adsorption occurred almost
instantaneously since this temperature was almost equal to
that at the top of the column.
Recommendations for closer temperature control were
made for determination of equilibrium data in any future investigation.
It was also recommended that operating data
taken on different size gels at various gel and air rates be
correlated in terms of overall adsorption coefficients.
-4II. INTRODUCTION
In the past it has been desired to adjust and control
the humidity and temperature of air to be used in homes, commercial establishments and factories for comfort, and in industrial areas where manufacture or storage of certain materials requires narrdwly defined humidity and temperature conditions.
Although the methods by which this control is ob-
tained are several, most air conditioning is accomplished by
refrigeration.
Two other general methods which are gaining
in popularity, however, are absorption and adsorption dehumidification.
These two methods offer excellent control of the
humidity but may not offer desired temperature regulation.
The latter is obtained by a cooling system installed subsequent to and in series with the dehumidification systems.
Both methods have the advantage that air of approximately the
same temperature and humidity may be obtained even though
the inlet air varies over a considerably wide range provided
the dehumidifying agent is introduced into the system at a
constant rate, temperature and water concentration.
The absorption dehumidification method involves the
use of certain salt solutions which are capable of absorbing
water vapor from the atmosphere when the vapor pressure of
the solution is less than the vapor pressure of water vapor
in the air.
As the concentration of salt decreases and the
temperature increases the vapor pressure of the solution increases until it equals that of the water vapor in the
-5Before this point is
atmosphere and absorption ceases.
reached the solution is withdrawn, concentrated by evaporation and returned to the system.
Usually temperature rise
rather than decrease in concentration determines the point
at which the liquid absorbent shall be withdrawn.
absorption system is efficient,
excellent control over humidity.
The liquid
very flexible and offers
However, a part of the
equipment required must generally be corrosion resistant
(unless an air concentration method is used) since evaporation of water from the salt solution necessarily occurs at
an elevated temperature.
Adsorption dehumidification is accomplished by adsorption of water vapor from the air on the surface of certain
solids such as activated alumina or silica gel.
It is well
known that most solids are capable of adsorbing small quantities of gas or vapor on their surface and that it is retained
by a considerable attractive force.
It follows that any solid
possessing very large surface area exposed to the gas or vapor
should be a good adsorbent.
Activated alumina and silica
gel are known to possess extremely large surface area per
unit weight of solid in the form of submicroscopic pores
throughout the solid body (7, 10, 23); these two have been
used extensively for dehumidification of gases and air.
When
in use for this purpose the large surface area exerts an
attraction for water molecules in the vicinity and adsorbs
and condenses them by a tremendous force of attraction.
The
condensed water is carried by capillary action to the recesses
-6of the individual pores.
According to Lewis, Squires and
Broughton (12) the rate at which the water is transferred
to the innermost parts of the gel particle is the controlling factor in the adsorption.
This action continues until
the adsorbed liquid exerts a vapor pressure equal to that
of the water vapor in the surrounding inert gas, at which
point equilibrium is reached.
These solids have remarkable
adsorptive capacities, for example, normal commercial silica
gel will adsorb 50 percent of its weight of water from saturated air (4).
From this result it has been estimated that
the exposed surface area of one cubic inch of silica gel is
about fifty thousand square feet.
Both of these solids are
used in much the same way commercially.
Silica gel is a dehydrated colloidal gel produced under
closely controlled conditions from sulfuric acid and sodium
silicate solutions.
chemicals.
It is a hard substance inert to most
As in the case of activated alumina, adsorption
of water by silica gel is purely a physical action, i.e., the
particles do not change in shape, size or appearance even at
equilibrium.
When equilibrium is reached, the adsorbed water
may be driven off by heating the gel to at least 300 Fahrenheit
and the same gel can be used again with adsorptive capacity
unimpatred.
Silica gel has been used extensively in industry for
dehumidification of air.
However, its uses have been confined
largely to batch or semi-continuous operations.
The types of
apparatus generally employed in dehumidification systems are
-7-
varied but may be represented schematically by the diagram
below.
.ETAIR
A
rART
RO
DSOR13LC
'
H
TEI
aLI
DRY Alt
ACT IVATING- AIR
OUT
WAIME
AC-IVATIMCr AiI I
1N
It is clear that this system operates on the same general
principle as certain types of catalytic cracking units used
in petroleum refining, i.e., alternately, reaction and
regenerative cycles.
Cell A above dehumidifies air passed
to it while cell B is being reactivated with hot gases.
When
the latter operation is complete and the gel cooled, cell B
is shifted to dehumidify air and the gel in cell A is reactivated.
In this way conditioned air is obtained continu-
ously from a system operated batchwise.
This system is em-
ployed in several small dehumidification units commercially
available (2).
Another system available on the market consists of a
rotating cylindrical bed of silica gel arranged in such a
way that one half the bed dehumidifies air while the other
half is being reactivated by hot gases.
As the gel moves
from the reactivation zone it is cooled and then passes to
the dehumidifying zone.
The two zones are separated by baf-
fles sliding closely on the surface of the cylindrical shell
holding the gel.
publications (2).
This system is described in the manufacturer's
-8These two systems have the disadvantage that the best
gel-water vapor contact is not obtained, inasmuch as a static
bed is employed during both the dehumidification and reactivaIn such a bed the points of contact between the
tion cycles.
gel particles are probably untouched by the humid air and thus
are ineffective in removing water vapor therefrom.
The ideal
gel-water vapor contact would be obtained in a system where
the individual gel particles were moving about in and were
completely and continuously aurrounded with the air to be
dehumidified.
Mass transfer of water vapor could then take
place from the air to the entire surface of the particle and,
furthermore, motion of the particle itself would induce local
turbulence which would improve the transfer.
This could be
realized only in a system where the gel particles were in
continuous motion in the air without striking neighboring
particles.
This condition can be approached closely, how-
ever, by keeping the gel in motioh in the air with only
momentary collisions of the particles.
With a system in
which these conditions are approached the weight of silica
gel required per unit of air for dehumidification to a certain point will be less than that required where a static
bed of gel is used.
Such a system has been designed (13, 14) for utilizing
solid adsorbents such as silica gel for separation of gases.
Here the gel is fed into the gas stream at the bottom of a
colun, is carried upward in the gas and is separated therefrom at the top by a cyclone separator.
As the gel passes to
-9the top the particles move about in random motion sidewise
and "jiggle" in the gas stream, thereby effecting aimost perfect gel-gas contact.
Another system (15) has been designed to give very nearly
the same conditions.
However, here the gel is fed at the
top of the column, flows countercurrent to the gas stream and
is removed at the bottom.
Both of these systems have been
commercially used for dehumidification and separation- of
various gases with widely varying boiling points.
It is the purpose of this investigation to conduct experiments with air-borne silica gel in a small laboratory
column, collecting operating data and correlating it in such
a way that design of larger columns may be accomplished.
Literature Survey
Before proceeding with experimental work efforts were
made to locate in the literature, equilibrium and thermal
data for the system silica gel-water vapor.
Although con-
siderable data were found, they were not readily applicable
to the present problem since in most cases the experiments
were conducted by either passing water vapor through static
beds of silica gel or permitting water as vapor or liquid
to remain in contact with the gel for long periods of time
until equilibrium was obtained.
The present investigation
employed an entirely dynamic method, i.e., both the silica
gel and an air-water vapor mixture were constantly in motion.
Furthermore, in many cases previous experiments were performed
-10-
in the absence of air.
According to other investigators this
will affect greatly the adsorption of water vapor.
Patrick and Cohen (16) have found that the rate of adsorption of water on silica gel is independent of permanent
gas only when the latter is present at a pressure less than
0.5 millimeters of mercury and that the rate is inversely
proportional to the molecular weight and partial pressure of
the inert gas if the latter exceeds 0.5 millimeters.
Patrick and McGavack (19) have shown that in adsorption
of SO2 by silica gel, a pressure of air over the gel'too small
to materially affect the calculated pressure of S02 would
increase by several hours the time required for equilibrium
to be attained.
They found also that a higher degree of
evacuation was required in the system for water vapor than
for S02 in order that equilibrium might be established in the
same time.
Thus, any data obtained by a static method in the absence
of air would not be applicable to the present investigation.
From the standpoint of academic interest, however, data obtained by the several different methods are given in Figure 1
for comparison with data taken during this investigation.
Ray and Ganguly (21) conducted a series of experiments
to test the validity of the Patrick adsorption equation
V = K(P /P)l/n. A discussion of this equation is given in
the original article.
Data were obtained by contacting air-
free water vapor and silica gel until equilibrium was reached.
The gel used was obtained from the Silica Gel Corporation
-11and was teeated carefully to remove all acid and air.
Percent
residual water in the gel was found to be 5.04 per cent (basis
not definitely stated).
The data were recalculated and plotted
on Figure 1 as Percent Useful Water Concentration (Grams Water
Adsorbed per Gram Initial Wet Gel) versus Percent Relative
Humidity.
This method of treatment for equilibrium data,
reported by Dehler (5), tends to bring all data together
which were obtained at different temperatures falling within
a fairly narrow range (40-1000F.).
The data of Ray and
Ganguly do not agree particularly well with other data except
at low Percent Relative Humidity.
Ewing and Bauer (6)
permitted silica gel to stand over
concentrated sulfuric acid out of contact with air at 15,
25, 40 and 60 degrees Centigrade until equilibrium with the
water vapor had been established.
The equilibrium water con-
tent of the gel was determined by Tasting to dryness; results
were expressed as percent water (wet basis).
These data
plotted on Figure 1 are in good agreement with others up to
Useful Water Concentrations of approximately 25 percent.
Above that point however, Ewing and Bauer obtained higher
Useful Water Concentrations at a given Percent Relative
Humidity than did other investigators.
Patrick and Greider (17) obtained equilibrium water
contents of gel used in heat of wetting measurements in the
absence of air.
These agree reasonably well with other data
but the shape of the curve at high Percent Relative Humidity
is quite different from all others (see Figure 1).
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-12Experiments performed with presence of air included those
of Patrick and Opdycke (20) who mixed air saturated with water
vapor with dry air and passed the mixture over static beds
of silica gel prepared three years previously by Patrick and
McGavack (21).
The gel was kept sealed in air tight containers
and analyzed to give a water content of 3.57 percent (dry
basis).
They found that "it
was sometimes necessary to
continue the flow of (water) vapor for so long as three days"
in order to obtain equilibrium and the results indicated that
equilibrium was still not established in some cases.
Figure 1 it
From
is evident that the data of Patrick and Opdycke
agree with other data although they obtained slightly higher
Useful Water Concentration than did other investigators at a
given Percent Relative Humidity.
Considering the fact that the equilibrium data mentioned
above were taken by several techniques on various gels prepared
by different methods, the curves drawn through these data on
Figure 1 fall fairly close together.
It is true that even
though two silica gels are prepared by the same method they
will probably possess a different number of capillaries of
different diameter.
This obviously will cause a difference
in the internal volume, surface area, capillary force exerted
on the adsorbed water and consequently differences in the
equilibrium data of the two gels.
Therefore the agreement
between data taken at random from the literature appears to
be even more remarkable.
As mentioned above none of the data found in the literature
-13was applicable to the present investigation.
However, it
was found that data were available from the Davison Chemical
Corporation on the actual gel used.
These data were obtained
by passing mixtures of water vapor and air through static
beds of silica gel (Designation No. 659528-2000) and after
equilibrium was attained the percent useful water concentration was obtained by blasting the gel to dryness (the initial
water content of the gel, wet basis, was approximately 5 percent).
These data are also given in Figure 1 and are seen
to be in fair agreement with other data.
Even though the latter data were taken on the same gel
as used in this investigation it was felt advisable to obtain
more data on the same gel using the dynamic rather than the
static method.
Throughout these determinations it was felt
unnecessary to control the temperature closely from one experiment to another because of the fact that the method of
plotting tends to bring together all data taken at temperatures
falling within a reasonably narrow range.
The data so obtained
gre also given in Figure 1.
Thermal Data
When water vapor is adsorbed by silica gel a certain
amount of heat is evolved dependent upon the initial water
content of the gel and the temperature at which adsorption
occurs.
Considerable controversy has existed in the past
concerning the cause of this heat evolution.
It is a gen-
erally accepted fact that when water vapor is adsorbed by
solids possessing extremely minute pores condensation occurs
-14and the heat of condensation is given off.
However, the
amount of heat evolved during adsorption is generally somewhat
greater than the latent heat of condensation at the temperature of adsorption.
This additional amount, called the heat
of wetting, is the main subject of controversy.
Lamb and Coolidge (11) have felt that this heat is
evolved aa the result of liquid compression in the capillaries
of the gel and by the forces of molecular attraction.
Harkins and Ewing (8) have called it heat of spreading
and have claimed it is due to changes in surface energy
involved.
In agreement with the latter, Patrick and Grimm (18)
have found it possible to calculate quantitatively the heat
of wetting from changes in the surface energy.
Patrick and Greider (17) have shown by heat of adsorption determinations at 0 degrees Centigrade that the heat of
wetting is probably not due to compression of water at the
surface and have pointed out that if the water were compressed
the principle of LeChatelier would call for absorption rather
than evolution of heat.
They also point out that since the
change in surface energy is negligible from 0 to 25 degrees
Centigrade, if the heat of wetting at equilibrium is the same
at the two temperatures, it is probably due to surface energy.
In order to use temperature increase data in this investigation for calculation of material balances around the
column sources of heat of wetting data were sought.
were found but only the following could be applied.
Several
-15Ewing and Bauer (6) determined the heat of wetting of
several silica gels with different water contents.
Their
data, given as the observed heat evolved per unit weight of
initial wet gel when this gel was permitted to come to
equilibrium with liquid water, were plotted in Figure 2A
to give an integrated heat of wetting curve.
The slopes of
this curve plotted in Figure 2B against the initial water
content of the gel give the partial heat of wetting per unit
weight of water adsorbed.
At an initial water content of
5-6 percent (dry basis) the partial heat of wetting was
approximately 250 Btu per pound of water adsorbed.
Thd data of Patrick and Greider (17) was treated similarly
(Figures 3A and 3B) and gave a partial heat of wetting of
approximately 275 Btu per pound of water adsorbed.
Design of Silica Gel Dehumidifiers
Methods of design for silica gel dehumidifiers in the
past have been largely applied to static beds of gel.
The
methods for the most part have been empirical and have been
based upon assumptions which are probably not entirely true.
For example, one empirical method reported (5)
was used to
calculate the size of the adsorber necessary to dry a certain
volume of air, a definite amount.
It was assumed that at
the end of the adsorption period, (1) the gel in the bottom
of the bed would be in equilibrium with the inlet air and
(2) that that at the top of the bed would be in equilibrium
with the outlet air.
In the first place, to satisfy this
condition the adsorption must necessarily be instantaneous
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-16even when the gel and air passing through are near equilibrium.
Obviously, this is probably not true for the system water
vapor - silica gel due to extremely low driving forces near
equilibrium and it certainly will not be true for the system
encountered above, namely, water vapor - air -
silica gel.
Secondly, the mass of gel calculated to adsorb the required
total amount of water would adsorb rapidly at first but the
rate would decrease as equilibrium were approached and the
humidity of the effluent air would change.
It seems therefore,
that some more rigorous method of design should be projected
for static beds of gel or for some other system into which
fresh gel could be fed and in which a steady state of operation would be reached soon after adsorption began.
The lat-
ter procedure appears to offer more promise inasmuch as the
system investigated during the present experiments is the
type which could be operated under steady state conditions.
Several analogies can be drawn between this system and the
usual type of-absorption column.
When absorbing a gas from a stream of air in a liquid
absorbent, both liquid and gas-laden air are passed at constant rates into a column so arrangedthat the mass transfer
area, i.e., the liquid surface will be essentially constant
and large per unit volume of the column.
After a short per-
iod of operation the column reaches steady state and the
inlet and outlet conditions remain the same so long as the
rates of feed and other factors remain the same.
The operat-
ing data of such a system can usually be correlated in terms
-17-
of mass transfer coefficients which account for the diffftsivity of the gas in air, the temperature and the pressure in
the column.
The equations for such a system are given in
The Principles of Chemical Engineering (22) and are discussed
fully.
It is noteworthy that from equations 5 and 6 on pages
486 and 487, respectively, of this same reference (22)
G(Y
- Y2)
= K'aV(Y* - Y)av..
L(X 2 - Xl) = KLaV(X* - X)av.
(1)
(2)
a value of an overall capacity coefficient Kla or KLa can be
calculated if
the inlet and outlet conditions, liquid and air
rates and equilibrium relatibns for the system are known.
With a value of this coefficient it
is clear that the volume
of a column could be calculated for certain specified inlet
and outlet conditions.
This would be very desirable in the
silica gel type dehumidifier investigated.
The analogies between the above absorption system and
the "jiggling" of silica gel are as follows:
(1) A solid adsorbent is fed to the column at a certain
definite rate.
(2) Humid air is also passed through the column at a
constant rate.
(3) Although the gel concentration is not constant
throughout the column, the overall weight concentration is the same at a given air rate for a given
gel particle size and the gel will probably be
distributed in the same manner throughout the column
during a single and during each successive operation.
Thus, an apparent constant concentration and gel
surface area exist.
(4)
Steady state operation is possible.
(5) Equilibrium data for the system are known or may
be determined.
Thus, it appears that it may be possible to calculate from
sufficient operating data a mass transfer coefficient which
could beused in subsequent calculations to determine the
size of column necessary to adsorb a certain amount of water
with a given gel particle size at a specified air rate.
Since the equilibrium data for the system silica gelwater vapor-air are given in terms of percent relative humidity and useful water concentration it would be desirable
to obtain a coefficient also in terms of relative humidity or
useful water concentration driving forces.
The former could
be written as follows:
KRHa = G(Hl - H2 )
V(RH - RHe)av.
(3)
&nd the latter
K~a = L (UWC2 - UWc 1 )
V(UWCe - UWC)av.
Remembering that
% RH
=
H = [(p)/(n -P)
100 (p)/(ps) and
(Mw/Ma)
(5)
then the percent relative humidity becomes
RH = H (0.62 + H8 )
Hs(0.62 + H)
(6)
RH = (100)(H)/(Hs) approximately
(7)
or
-19Substituting this value in equation (3) there is obtained
a/H
K
RH
= G(Hl - H 2 )
s V(H - He)av.
(8)
which is equivalent to the usual equation given for dehumidification, namely,
G(Hl - H 2 ) = k'aV(H - He)av.
(9)
Thus, the operating data could be used to calculate a value
of K
§
RH
which would be constant for isothermal operation
but would vary~ during normal operation which is essentially
adiabatic.
However, by expressing driving forces in terms
of relative humidity (which seems justified in view of the
fact that the equilibrium data correlate well over a reasonably narrow temperature range using percent RH and useful
water concentration) an overall coefficient of mass transfer
may be calculated which should be constant throughout the
column for a given set of operating conditions.
The coefficient, KSa, defined by equation (4) is immediately equivalent to the usual overall liquid side coefficient, KLa given on page 487 in The Principles of Chemical
Engineering (22) and can be calculated from operating and
equilibrium data available.
As is well known, the path of absorption in the isothermal
case can be rppresented by a straight line on the appropriate
equilibrium diagram.
It is interesting to note that a
similar "operating line" can be drawn for the adsorption column
used during this investigation, even though the operation was
not isothermal.
The equilibrium diagram used is a plot of
-20UWC against RH so that the slope of a straight line on this
plot would be represented by the equation
(UWC)2
(RH)1
-
(UWC1 )
slope
(RH)2
(10)
Remembering that RH = (100)(H)/(Hs) approximately,
(Uwc)2 -
(UWC)1
100 [(H/Hs)1 -
(H/H5 )
=
(11)
slope
If the adsorption were isothermal say at a temperature t1 , then
slope1 = (UWC)
100(Hi
-
(WC)(12)
-
H 2 )/Hs(
1
-
or
(100)(slope1 )/H.
(UWC)2 - (UWC)1
(H 1 - H 2 )
(13)
Now from a water balance around the column
(L 1 /100)
(Uwc)2
--
(UWC)1 j
=
G(H
- H2 )
(14)
or
(UWC)2 - (UWC)l
(Hl - H2)
(100)(G)/(L 1 )
and substituting from equation (13), the slope of the operating
line at the bottom of the column becomes
slope1 = (G)(Hsl)/(L1 )
(15)
and. at the top of the column the slope of the operating line
is
slope 2
In this way,
=
(G)(H
)/(L
1
)
(16)
the operating line can be placed if the inlet and
outlet temperatures,
are known.
5
humidities and useful water concentrations
It is clear also that from equation (4) the overall
transfer coefficient, K a could be obtained by graphical integration
-21of the area between the operating line and equilibrium curve.
With these coefficients, determined (by direct calculation or by graphical integration) at different gel and air
rates for various mesh size gels, they could be used in all
subsequent design calculations.
Summarizing, the purposes of this investigation are
(1) to determine operating data at several air and gel rates
for a given size gel, (2) to determine equilibrium data for
the system silica gel-water vapor-air at the same conditions
under which the operating data were collected and (3) to
correlate the data if possible in some manner which could be
utilized for future design calculations, perhaps in the form
of overall mass transfer coefficients.
-22III. PROCEDURE
Silica gel (used in previous runs) containing a small
percentage of useful water was reactivated by heating in air
at 1780 Centigrade from 2 to 72 hours.
Upon removal from
the oven the gel was permitted to cool to room temperature in
a dessicator after which a portion was weighed out in a long
neck glass flask for the run.
The humidity of the inlet air, taken from the dompressed
air line, was determined by means of a et- and dry-bulb
psychrometer and the flow through the column was adjusted to
the desired quantity (from 1 to 2 cubic feet per minute) as
shown by the pressure difference across a sharp-edge orifice.
Silica gel was fed to the column from the long neck
flask through a simple feed device and the following readings
were taken at five-minute intervals.
(a) Temperatures of the inlet and outlet air.
(b) The air temperature three inches above the gel inlet
point.
(c)
The wet- and dry-bulb temperatures of the outlet air.
(d) Flowmeter reading.
When steady state had been reached in Runs 1-7 a small
stream of air was passed through magnesium perchlorate drying
tubes by means of a siphon arrangement.
At the completion
of these runs the increase in weight of the tubes was determined by weighing on an analytical balance and the humidity
of the outlet air was calculated.
The wet- and dry-bulb
temperature of the outlet air was not measured during these runs.
-23At the completion of all
other runs the outlet gel was
weighed to 0.1 gram and samples of both inlet and outlet gel
were taken for analysis for water content.
Equilibrium runs were made by recirculating the gel in
the column continuously with the air flow adjusted to approximately 2 cubic feet per minute.
'When the inlet and outlet
temperatures of the air were equal or the outlet temperature
remained constant with and without the gel in the column it
was assumed that equilibrium had been reached and samples of
gel were taken for analysis for water content.
The absolute
humidity of the air was determined with a wet- and dry-bulb
psychrometer and the relative humidity calculated from known
saturation pressures of water at the temperature observed
at the top of the column.
The temperature of the inlet air
was controlled (in a given run) by a water to air heat exchanger and the humidity of the inlet air was adjusted by
passing the air through a bubbler designed to humidify at
least 3 cubic feet per minute up to 90 percent relative
humidity at the humidifier..
The calculations performed for each run and a detailed
.description of the operation and construction of the apparatus
used are given in Appendices A and C, respectively.
-24IV. RESULTS
The results obtained during this investigation are
listed below.
Plots and tables of results are located in
Appendix D, Results:
Plots and Tables.
1. The residual water content of the silica gels heated
2-72 hours in air at 178 degrees Centigrade agree well with
other data obtained from the literature (6, 2L).
These
results are plotted on Figure 9.
2. Time of reactivation (2-72 hours) of the silica gel
in air had no effect on residual water content.
Table V
gives the values obtained after various timesof reactivation.
3. Humidity of the effluent air could not be measured
satisfactorily by the gravimetric (drying tube) method without
further provisions for removing gel fines from the air stream.
Comparison of values of Water Adsorbed/(hr)(ft.i)calculated
from data taken by this method, by gel weight difference and
by the psychrometric (wet- and dry-bulb temperature) method
is given in Table VI.
4. The psychrometric method of determining outlet air
humidity agrees with the gravimetric method when silica gel
is not in the colurgn; howeer when gel is "jiggling" the
latter method consistently gives lower values of humidity than
the former.
A comparison of values obtained by the two methods
with and without gel "jiggling" is reported in Table VII.
5. Silica gel fines produced by use of the gel from
Run No. 7 to No. 15 is approximately 10 percent (see Table VIII).
-256. Estimated heat losses from the column are slightly
greater at the lower values of the mass rate of air flow, G,
(Runs 1-13, inclusive).
Table IX gives the estimated heat
loss from the column for Runs 1-19.
7. The air temperature three inches above the gel inlet
point was almost equal to the outlet air temperature for Runs
15-19, inclusive.
8. The percent of the inlet water vapor adsorbed by the
silica gel varied inversely with the superficial air velocity.
Figure 10 gives the relationship which can be represented by
the equation
Percent inlet water vapor adsorbed = 213/(us) 1 .0 3
The same relation is shown on Figure 11 for percent inlet
water vapor adsorbed and G, the mass rate of air flow.
9. The percent error in the value of the Water Adsorbed/
(hr)(ft2) calculated from heat of wetting, air and gel rates
and temperature rise data was greater at higher rates of air
flow, greater at lower values of the ratio u./ug, and went
through a minimum as the value of L /G
increased.
Figures
12, 13 and 14 indicate the manner in which'the percent error
changes with the variables mentioned.
10. The slip velocity, us - ug, increased with increase
in air velocity up to approximately 6.5 feet per second and
then decreased (see Figure 15).
11. The concentration
of silica gel in thle column
varied inversely as the mass rate of air flow, G.
Figure
16 indicates the exponent on G to be approximately -8.4.
-2612. Operating lines for all runs placed on an equilibrium
diagram (see Figure 17) were either very close to straight
lines or were definitely S-shaped.
13. Values of the overall adsorption coefficient, KRHa,
(lbs. of water vapor adsorbed per hour per cubic foot per
unit RH difference) became smaller as the mass rate of air
flow, G, increased.
The change in coefficient with air rate
is given on Figure 18.
14. The overall adsorption coefficient Kga (lbs. of water
vapor adsorbed per hour per cubic foot per unit % UWC difference) increased with increase in the gel feed rate.
The value
at a given gel rate depends on the mass rate of air flow, G,
the higher values being obtained at lower rates of air flow
(see Figure 19).
15. Values of (Kga)(G/1000) plotted against the gel
feed rate, Ll, give a straight line represented by the equation
(Kga)(G/1000) = 0.0013L1 + 0.35
(see Figure 20)
16. Equilibrium data obtained during Runs 20-28, inclusive, indicated that lower values of Percent Useful Water
Concentration were obtained at a given Percent Relative
Humidity than those found in the literature.
Figure 1
gives the curve plotted from the experimental data and
also the data obtained from the literature.
-27V. DISCUSSION OF RESULTS
The residual water content of the reactivated silica
gel when plotted against temperature of activation gives a
point or small spread of points which fall near the curve
obtained when data from the literature are treated similarly
even though the latter were obtained largely under vacuum.
The time of reactivation had no apparent effect on the residual
water content of the silica gel (the water content varying
from 4.71 to 4.88 percent, dry basis, see Table V) probably
because of the fact that the amount of water left on the
silica ge.l even after short time of reactivation is held by
very strong attractive forces which could be overcome only
by elevating the temperature considerably or evacuating the
system.
It might also be possible that the presence of even
small amounts of air would increase the time of transfer
of water from the inner portions of the gel to the surface
and thus preclude the possibility of detecting any effect
of time of reactivation on residual water content.
The
former explanation is probably more nearly the correct one,
however, inasmuch as the amount of water left in the gel may
form a monomolecular layer in the large part of the capillaries and thus be held strongly on the surface or it may
rest in the apex of the conical pores and at this point be
held by strong attractive forces or very high surface tension.
Considerable difficulty was experienced during Runs
1-8, inclusive in measuring the humidity of the outlet air by
passing a small stream through drying tubes containing magnesium
-28perchlorate.
In all cases, the water balances calculated by
usibg the value of the humidity, H2 , obtained by this method
were higher than calculated from the difference in weights
of the inlet and outlet gels.
In other words, the humidity
observed by this method was lower than the probable true
value existing at the top of the column.
This can probably
be explained by the fact that small amounts of gel powder
carried into the air line leading to the drying tubes adsorbed additional water vapor from the air stream thus permitting smaller quantities to reach the tubes.
The increase
in weight of the tubeswould, therefore, be less and the calculated humidity would be lower than that existing at the top
of the column.
This explanation is substantiated by the fact
that deposits of powdery gel were observed in the small air
line leading to the drying tubes.
It
is easy to visualize
how the gel might be transferred to the air line and deposited.
The amount of air withdrawn through the drying tubes was very
small, varying from 5 to 12 liters total, and thus the air
velocity through the tubing was low.
Under these conditions
any gel passing the small glass wool filter in the air line
would settle out before reaching the tubes and would adsorb
additional water.
Beginning with Run 8, attempts to determine the humidity
of the effluent air by #eans of the gravimetric method were
discontinued and the psychrometric method adopted.
Before
proceeding with measurements using this method, however, it
was checked against the gravimetric method with no silica gel
-29"jiggling" in the colunn.
It was found that the two methods
gave the same results, 0.00808 for the gravimetric and 0.00810
for the psychrometric method.
Thus, it was shown that the
latter method would give accurate values of the humidity.
The question immediately arises as to the effect of silica
gel fines in the air stream passing over the wet- and drybulb thermometers, i.e., why will not the silica gel fines
adsorb additional water from the air stream thus causing
low values of the humidity to be observed also by this method.
It is probably true tha.t some additional water vapor is adsorbed
by the gel fines in the air passing to the psychrometer
but the amount of air withdrawn is much greater than that
passed through the drying tubes and thus the velocity in the
air line was sufficiently high to permit (1) no gel deposition
therein and (2) very low time of contact between the air and
gelfines before it reached the psychrometer.
Thus, the amount
of water vapor adsorbed would ptobably be small.
Furthermore,
the gel passing to the psychrometer in the air stream would
strike the wet-bulb of the thermometer, adsorb water, liberate
heat of wetting and thus cause a somewhat higher temperature
to be observed than would be normally.
Obviously, then these
two effects are offsetting and it may be that values of the
humidity very near the true value at the top of the column
were observed.
The magnitude of these effects were not in-
vestigated.
The perdent gel fines produced by using the gel from
Runs 7-15, inclusive was approximately 10 percent.
Inasmuch as
-30the amount of silica gel screened to -28 + 48 initially
was approximately twice the amount used in most runs, the
gel rescreened before Run 7 and after
four runs only.
Run 15 was used in
It is probably true that the "jiggling"
operation produced most of the fines, for it was during this
time that the gel particles were spinning violently in the
air stream, striking each other many times as they passed
through the columb.
Unfortunately, no other data were
taken to indicate how long a given silica gel could be used
before the fines produced prohibited its further use.
is an important point and should be investigated in
This
the
future.
Heat losses from the column were calculated by estimating heat transfer coefficients from appropriate empirical
At the lower rates of flow
equations obtained from (22).
where G was approximately 1000, the Reynolds number, DG/u,
indicated the flow to be in
the streamline region; the inside
film coefficients were calculated using an equation for the
streamline region (equation 18,
page 125 in (22)
).
With
these coefficients, the overall coefficient of heat transfer,
U, was found to be approximately 0.26-0.27 and the heat losses
calculated were found to be greater than at the higher rates
of air flow where G was near 2000.
At the latter rates the
flow through the column was close to turbulent (Re above 2100
and less than 7000) and the values of U were 0.29-0.30.
How-
ever, the mean temperature difference existing for Runs 14-19
(G approx. 2000); thus, the estimated heat losses were greater.
-31Another point to be considered is the fact that at the lower
air rates the gel concentration in the column was greater and
in spite of the fact that the flow according to the Reynolds
number was streamline, it was probably definitely turbulent
due to motion of the gel particles.
Thus, the coefficients
calculated at the low air rates are probably low and the heat
loss from the column is actually greater than that estimated
as given in Table IX.
Furthermore, at the greater gel con-
centrations (lower air velocities) most of the gel was concentrated three inches above the gel inlet point and the air
temperature at that point was almost equal to that at the top
of the column.
Thus, the temperature driving force between
the air and surroundings was somewhat greater over more of
the column than was calculated in the heat loss estimations.
Again, then the heat losses at low values of G were probably
greater than those estimated and may have Peen considerably
greater than the heat losses at higher values of G.
As mentioned in the preceding paragraph, the air temperature at a point three inches above the gel inlet was
almost equal to that at the top of the column.
This seems
to indicate that initial adsorption of water vapor occurs
almost instantaneously inasmuch as the time required for the
gel to traverse this small distance at the higher rates of
air flow is extremely small.
This may be explained at both
high and low air rates by the fact that at the gel inlet
the air velocity is higher than at any point above and due
to the inertia of the gel particles, the slip velocity at that
point is probably greater than it is farther up the column.
At low air rates, in addition to the probable greater slip
velocity the concentration of igel at the bottom of the column
was greater than in the upper portions.
Thus, the conditions
for transfer of water vapor from the air to the surface of
the gel were more nearly ideal than at any other point in
the column.
However, this does not explain how the gel itself was
capable of adsorbing water vapor more rapidly at that point.
This may be explained in the following manner which is in
accord with most theories of adsorption of water vapor by
silica gel.
The residual water on the freshly reactivated
gel probably lies near the apex of the conical pores or
along a portion of the surface thereof in a monomolecular
layer, thereby leaving the inlet to the pore or capillary
relatively free of water and capable of rapid adsorption up
to the point of local saturation (that is, up to the point
where the inlet to the capillaries were filled with condensed
water).
When this condition was reached the rate of transfer
of the water from the surface of the gel to the center became
controlling and not only did the rate of adsorption of water
vapor from the air decrease but the amount of heat eVolved
from that time was the heat of condensation of a very small
amount of adsorption plus the heat of wetting of the gel surface by the total liquid water as the water was transferred
toward the apex by capillary action.
This amount of heat was
relatively small compared with that evolved during the f irst
-33stages of adsorption, i.e., heat of condensation of a large
percentage of the total water vapor adsorbed in the coluon.
Thus, in this way the temperature rise of the air within three
inches above the gel inlet was almost equal to the total rise
throughout the entire length of the column.
The fact that the percent of the inlet water vapor adsorbed by the silica gel decreased as the superficial air
velocity increased may be attributed to the fact that (1) the
gel concentration in the column became progressively smaller,
(thereby decreasing the transfer area) and (2) the time of
contact between the gel and air was progressively less.
Even
though the rate of transfer of water vapor from the air to
the gel may have been equal in all runs,
if the area of
transfer and the time of contact were smaller, the absolute
amount of water vapor adsorbed would be smaller.
However,
since the total amount of water vapor passed through the
column at superficial velocities of 7-8 feet per sedond was
approximately twice that at velocities of 4 feet per second
and the percent inlet water vapor Adorbed was only half
then the absolute amount of water vapor adsorbed was nearly
the same at both velocities.
This appeared to indicate that
the rate of adsorption at the higher velocities was considerably greater per unit area of gel surface.
This is substan-
tiated somewhat by calculations of overall transfer coeffidients discussed later in this section.
The water balances calculated by four independent methods
are in very good agreement when consideration is taken of the
-34fact that the amount of water vapor adsorbed in all runs was
small and that two of the values (gel weight difference and
gel water content determinations) were obtained by taking
the difference between relatively large numbers.
(see Table IV)
that in all
runs except 4,
6,
9,
It is clear
and 17 the
value of the water adsorked/(hr)(ft 2 ) calculated from heat
of wetting, gel and air rate and temperature rise data are
smaller than any other.
This could be doe to (1)
a smaller
indicated temperature rise than that corresponding to the
amount of water vapor adsorbed, (2) to an error in the total
heat effect calculated per pound of water adsorbed or (3)
to errors in measurement of the gel aid air rates.
The former
is the most likely cause for the discrepancy since the temperature rise of the air depends on the transfer of the heat
of adsorption from the gel to the air and if' the time permitted for this to occur is not sufficiently great, the full
temperature rise corresponding to the water adsorbed will not
be observed.
From an examination of Table IV and also Figure
12 it is evident that the percent error in the water balance
calculated from thermal data (assuming the average of the
three other values to e nearly the correct) was greater at
the higher air rates.
This might be due to smaller heat
transfer coefficients at the higher air velocities or to
insufficient time of contact between the gel and air.
The
former seems unlikely although it will be pointed out later
in the discussion that such a condition might exist.
It is
more likely that insufficient time of contact is the main reason
for the larger percent error since it is well known that
particularly in the presence of air the time required for
the full heat of adsorption to be developed and re-transferred
to the air is extended considerably.
Most certaihly then the
smaller the time of contact between the gel and air, the
greater will be the observed percent error.
In an effort to develop a more complete and definitive
picture of the cause for the discrepancy in this water balance,
the percent error was plotted against the ratio of the superficial and gel velocities, us/u i(Figure 13) and also against
the ratio L1 /G (Figure 14).
By the former method the percent
error appeared to be somewhat greater at the lower values of
the ratio, us /u
9
i.e., when the superficial and gel velocities
were nearly equal or approached each other the error was
greater.
This appeared to be a point in favor of lower heat
transfer coefficients since the closer the two velocities,
the lower the slip velocity.
However, it so happens that
only at the higher air rates were the velocities approaching each other and at the higher velocities the time of dontact
was small.
The second method of plotting indicated a minimum
in the percent error as the value of L /G increased.
Examina-
tion of the data in Table IV will indicate that the low values
of Ll/G (0.14-0.20) were obtained with (1)
both G and Ll and (2)
average values of Li.
low values of
high values of G and both high and
The points so calculated placed the
left end of the curve shown in Figure 14.
Obviously, at
high values of G and both average and high values of Ll the
gel concentration is low in the column and the time of contact
between the gel and air is small.
Thus the error observed
for these runs could be attributed to the time factor.
When
both Ll and G were low, the gel concentration in the column
was relatively high and therefore, the gel velocity was low.
Although this means that the time of contact between gel
and.air was relatively great, it also means that if approximately the same degree of turbulence existed as at higher
gel rates, the heat loss to the surroundings was probably
greater and thus a low outlet temperature was recorded.
Other than this, there appears to be no reason to explain
why the error should be greater for runs where the values
of L
and G were both low.
The right end of the curve in
Figure 14 was placed by using values of Ll/G obtained from
low values of G and high values of Ll.
Since the gel con-
centration was approximately the same at equal air velocities
and independent of the gel rate, the slip velocity existing
during these runs was somewhat lower and conditions for heat
transfer not quite as good.
Furthermore, since the gel rate
was high and the gel concentration in the column essentially
unchanged, the gel velocity was greater and the time of
contact between gel and air in the column was becoming progressively smaller.
Again there is a point in favor of both
low heat ttansfer coefficients and low time of contact.
The middle pdrtion of the durve in Figure 14 was placed using
values of L1 /G calculated from low values of G and average
values of Ll.
Under these conditions the slip velocity was
-37intermediate between that existing at low and high values
of L1 /G.
Summarizing, it appeared that (1) when the tiite
of contact between air and gel was great, the heat of adsorption was fully developed but was partially transferred to
the surroundings, (2) when the time of contact was intermediate, the heat effect was developed but heat losses to
the surroundings were not as great and (3)
at low times of
contact the heat effect was not fully developed.
In this
way a minimum in the percent error as Ll/G increased might
be possible.
However, it should be pointed out that the
data are few and that perhaps if more runs had been miade
covering a greater range of gel and air rates the curve could
have been placed more definitely.
Then, too, the fact should
not be overlooked that the average of the water balances
calculated by the other three methods may be in error more
for some runs than for others.
The slip velocity, us - ug, plotted on Figure 15 against
the superficial air velocity appears to go through a maximum
as the air velocity increases.
This has not been explained
satisfactorily as yet.
The concentration of gel in the column decreased rapidly
as the mass rate of flow G increased.
The data plotted on
logarithmic paper (Figure 16) gave a line with slope eoual
to -8.4.
The data at the higher air rates were very difficult.
to obtain inasmuch as the gel concentration was extremely low.
Thus, the value of -8.4 is not particularly reliable, since
all points on the upper portion of the curve fall aknost together
The data are sup-
and do not aid inlacing it definitely.
ported in part by visual observation of the gel concentration in the column during operation.
At low air rates the
column appeared to be almost filled with gel but at values
of G-around 2000 it was difficult to see a single gel particle as it passed through the column.
The difficulty in
measuring the gel holdup in the column at the conclusion of
a run was caused by the inability to shut down the air
momentarily at the right time to permit the gel to fall
from the column through the constriction.
Obviously, at
higher air rates, a very small fraction of a second error
in shutting down the air would cause a large error in the
gel concentration determination.
For example, in Runs 14
and 19 the air valve was closed too late and the column was
swept free of gel; for these runs it was necessary to assume
that the velocity of the gel was the same as that of the air
and calculate the concentration existing in the column.
The
results of the calculation were close to those obtained by
actual measurement.
In an attempt to develop a method of design for such
silica gel drying columns an operating line was calculated
for all runs.
The equation for the slopes of these lines at
the bottom and top of the column is given in the Introduction.
It was hoped that some sort of picture of the adsorption
path in the column -might be obtained.
'curves for all runs.
Figure 17 gives the
In all cases, the curves were very
near straight lines or were definitely S-shaped.
The majority
-39of them were of the S-shape type.
The curves are not to be
taken as representing the actual path of adsorption but only
as an indication of the form of the path because of the fact
that there was no method by which a middle point could be
placed with the data available.
The shape of the curves
seemed to indicate that as the gel entered the column instantaneous adsorption occurs but a slight lag in development of the heat effect caused the Useful Water Concentration
to appear to increase more rapidly than the Percent
Relative Humidity decreases.
As soon as the heat of adsorp-
tion was transferred the Percent Relative Humidity decreased
rapidly but the Useful Water Concentration went up slowly
because the absolute water adsorbed in this part of the column
might have been small; perhaps during this part of the process, the water adsorbed initially was being transferred to
the inner portion of the gel particle by capillary attraction.
The upper portion of the curve has not been explained
unless it may be aaid that the cycle was repeated.
It is
believed that further data are needed in order to place curves
representing the path of adsorption through the column. Such
curves placed properly could be used to calculate transfer
coefficients by graphical integration.
Pursuing the same idea projected in the preceding paragraph the data from the operating runs were used in calculating overall adsorption coefficients from equations 3 and 4
in the Introduction.
Inasmuch as the eduilibrium data gave
a single line over a reasonably narrow temperature range when
-40plotted as Useful Water Concentration against Percent Relative
Humidity, it was thought that perhaps a coefficient of mass
transfer based on a unit difference in Percent Relative Humidity
could be correlated with air or gel rate.
It was found that
such a coefficient, KRHa, calculated from equation (3) decreased as the air rate increased.
Since the gel concentra-
tion in the column decreased rapidly as air rate increased,
the term "a" in KRHa must have decreased rapidly and the value
of KRH must increase rapidly as the air rate increased.
As
mentioned before, the data are few and the gel concentrations
at high air rates are not considered too reliable, therefore,
no definite statement can be made as to the extent of the
increase of KRH with air rate.
However, it is believed that
the values given on Figure 18 plotted against the air rate
can be used for design purposes within the temperature range
covered by the equilibrium data and with this particular
gel size (--28 + 48 mesh).
The overall coefficient, KSa, was also calculated from
equation
19.
(4) and plotted against the gel feed rate in Figure
It was found that the values of Ksa increased as the
gel rate increased and fell along two straight lines one
for the low air rates and the other for the high.
The points
for runs with intermediate air rates fell generally between
these two lines.
The lower values of KSa fell along the
line for the higher air rates.
In order to bring the data
together the values of KSa were multiplied by G/L000 and the
results, (K a)(G/1000) were plotted against the gel feed rate
-41on Figure 20 to give a straight line with slope equal to
0.0013 and an intercept of 0.35.
Thus, for a given value
of G, K a increases as the gel feed rate increases.
The
only explanation for this appeared to be the following; since
at constant G, the gel concentration in the column remained
very nearly the same, the gel velocity and therefore, the
turbulence might be sufficient to cause an improvement in the
mass transfer coefficient.
Referring to Figure 20 it is
clear that the points for Runs 16, 18 and 19 lie below the
straight line drawn through a majority of all points.
Exam-
ination of the water balances calculated for these runs and
reported. in Table IV will show how this can be explained.
Run 16
The water balances calculated from L2 - Ll, G(Hl - H 2 )
and gel water content determinations check well but that
calculated from thermal data is much lower.
This indicates
that the top temperature might be low and that the percent
relative humidity calculated therefrom would be too high.
If this were true, then the UWCe would be too high and the
mean driving force calculated would be high.
This would give
a lower value of K 8 a than that which actually existed during
this run.
Run 18
The water balance calculated from thermal data for this
run is also lower than the others and thus the same reasoning holds as for Run 164
Furthermore, since the water balances
calculated from G(H1 - H2 ) and gel water content determinations
-42check reasonably well but that calculated from L 2 - L
is
high, the air rate, G might be too low or the water content
of the outlet gel, C., may be low (if this is low then UWC2
is low,AUWC is low and the UWC driting force calculated would
be too high).
This would tend to give low values of Kga.
Run 19
Both the water balances from gel water content determinations and thermal data are lower than the other two.
Thus
the same reasoning may be followed for Run 19 as for Run 18.
It is believed that a more reliable correlation could
be obtained if a series of experiments were run using gel
of different sizes and collecting operating data as was
done in the present investigation and using that data to calculate the adsorption coefficients.
several gel and air rates.
This should be done at
For the particular gel size used,
however, it is believed that the values of Kga given on
Figure 20 can be employed for design calculations for this
type of silica gel dehumidifier.
Referring to Figure 1 it is clear that the equilibrium
data obtained during this investigation show consistently
lower values of the Useful Water Concentration at a given
Percent Relative Humidity than do the data taken from the
literature.
This may be attributed to at least two differ-
ent factors, namely, (1) presence of air extended the length
of tite required to reach true equilibrium beyond the actual
duration of the equilibrium runs and (2) the use of large
quantities of gel which while setting in the funnel outside
-43the column cooled down to some temperature below that in
the column and when passed into the column absorbed any
small quantities of heat of adsorption which may have been
evolved, thus preventing any rise in temperature of the
air.which would have been observed normally.
The first fac-
tor was known to exist to such an extent that it would probably have taken three or four days to reach the true equilibrium point.
However, with the system used it was impos-
sible to judge when equilibrium had been reached except by
equality of inlet and outlet temperature or by constancy
of outlet temperature with and without the gel "jiggling"
in the column.
Probably, the presence of air so decreased
the rate of adsorption that small amounts of heat evolved
as the gel passed through the column were' not observed.
The
sedond factor, although shown in some runs to have little
effect by the constancy of outlet temperature with and without
the gel in thb column, must have been a contributing factor,
inasmuch as it was known that the funnel at the top of the
column was generally lower in temperature than the inside
of the column.
It is believed that during any future investigations
undertaken to determine equilibrium data for the system
silica gel-water vapor-air by the "jiggling"method, special
care should be taken to control the temperature of the whole
system or to prevent heat loss to the surroundings from either
the gel or the air.
It is also believed that smaller quantities
of gel should be used and consequently, a greater number of
cycles per unit time.
-44VI. CONCLUSIONS
The following conclusions have been reached as a result
of the data obtained during this investigation.
1. The time of reactivation of silica gel at 178 degrees
Centigrade in air has little or no effect on the residual
water concentration.
2. Residual water contents of freshly reactivated gel
u"ed during the experiments agree well with those obtained
by other investigators, even though reactivation was carried
out in the presence of air at atmospheric pressure.
3. Humidity of air cannot be measured satisfactorily
when a small stream is passed to drying tubes through lines
filled partially with an adsorbent soli4,- nor can the method
be used when the velocity of the air being passed to the
drying tubes is so low that long time of contact is permitted
with the suspended or depositied solid.
However, humidity
of air can be measured satisfactorily by the psychrometric
method under the conditions prevailing during this investigation.
4. Further work should be done in determining the extent
of powdering of silica gel in the type system used.
5. Heat loss from the column was greater at low than
at high rates of air flow.
Future investigators should tkke
steps to prevent this heat loss or else construct a column
which will operate isothermally.
6. The rise in air temperature is almost as great in the
-45three inches above the gel inlet point as throughout the
entire length of the column.
7. The percant of the inlet water vapor adsorbed can
be correlated with rate of air flow.
8. The percent error in the water balance calculated
from thermal data is large at both high and low times of
contact between the gel and air passing through the column
and is relatively small at intermediate times of contact.
It is concluded that this is probably due to the fact that
at large times of contact in the column the opportunity for
heat loss to the surroundings is great even though the full
heat of adsorption is developed, that at intermediate times
of contact the full heat effect is developed but the heat
losses are less and that at low times of contact the heat
effect is not fully developed.
9. The slip velocity passes through a maximum as the air
velocity increases.
The reaeon for this should be investigated.
10. The gel concentration in the column decreases rapidly
as the air rate increases.
Determination of the gel concen-
tration at high air rates is inherently difficult.
Consider-
able work should be done in any future investigation in
determining gel concentrations for different size gels at
various air and gel feed rates.
11. The end conditions of the gel and air can be pictured
on an equilibrium diagram by an operating line which represents the form of the path of adsorption through the column.
The shape of these operating lines is inherently S-shaped.
-4612.
The operating data taken during Runs 1-19 can be
correlated with air and gel feed rates in terms of overall
adsorption coefficients.
These adsorption coefficients can
be used for future design work provided the same size gel
is to be used £nrthe rangd6ofcz:tempsitatures
covered by the
equilibrium data given on Figure 1.
13. Equilibrium data taken during Runs 20-28, inclusive,
give lower Useful Water Concentrations at a given Percent
Relative Humidity than other data taken from the literature.
It is concluded that this is due to the presence of air or
to improper technique employed in taking these data.
It is
believed that extreme care should be taken to prevent heat
losses from the entire
system in all future investigations
undertaken to determine equilibrium data for the system
silica gel-water vapor-air by the "jiggling" method.
Otherwise, steps should be taken to make the system isothermal
in operation.
-47VII. RECOMMENDATIONS
It is believed that if the recommendations list6d below
are followed,
considerable data of Value would be obtained.
1. The extent of powdering of the silica gel when
used in the "jiggling" operation should be determined.
2. For all future investigations in which a system
similar to that used here is proposed, care should be taken
to prevent heat loss to the surroundings or the apparatus
should be constructed for isothermal operation.
3. The change in slip velocity with superficial air
velocity should be investigated so that observed variations
in adsorption coefficients might be more easily explained.
4. The gel concentration in the column should be
determined accurately for different size gels at various air
and gel feed rates.
This should also include a study of
the gel distribution in the column at different air rates.
5. Attempts should be made to correlate all operating
data taken by the "jiggling" method in terms of adsorption
coefficients which could be used for future design calculations.
6. Extreme care should be taken during determinations of
equilibrium data for the system silica gel-water vapor-air
to insure that heat losses to the surroundings by any
mechanism are reduced to a minimum.
If possible the system
should be designed to operate isothermally under controlled
conditions of temperature.
-48-
VIII. A P P E N D I X
-49A. Expansion of Procedure
The description of the experimental procedure employed
in this investigation will be included with a description
of the operation and construction details of the apparatus.
Ini order to clarify the description a cross-sectional elevation diagram of the apparatus (Figure 4 following page 58)
has been included in which the details of the construction
are shown clearly.
The apparatus is pictured whild operating
on air of normal moisture content;
the conditions of the
operation will be discussed in the body of the present section.
Certain advantages are gained by separating the experimental procedure into four phases of operation, namely:
A.
Preparation for Run
B. Operations During Run
C. Operations at Conclusion of Run
D. Operations During Equilibrium Run
A. Preparation for Run
Silica gel (screened to -28 +48 mesh) used in a previous
run was reactivated by heating in air at 1780 Centigrade
until the residual water content dropped to 4.7-4.8 percent
on a dry basis.
The reactivation was carried out in an
electric oven equipped with a bi-metallic temperature control.
The reactivated gel was placed then in the dessicRtor pictured at the extreme right of Figures 4,
allowed to cool to room temperature.
, 6 ane. e and
Calcium chloride was
-50used as the dessicant.
Over the total period of the experi-
mental work the time of reactivation was varied purposely
in order to determine the relation between time of reactivation and residual water content of the silica gel.
The reservoir supplying water to the wet-bulb wick of
the wet- and dry-bulb psychrometer (top left, Figures 4 and
8) was filled to insure thorough wetting of the wet bulb.
Screw clamps 51, S2 and 55 were opened, in the order given,
to permit inlet air to flow over the wet- and dry-bulb.
The
type of psychrometer used consisted of two U-tubes fitted
with fractional thermometers reading from 0-55 Centigrade
by 0.1 degree.
The upper part of each U-tube in which the
thermometers were seated were covered with tinfoil and asbestos tape to prevent heat transfer between the bulb and
the surroundings.
The leg of the U-tube housing the wet-bulb
thermometer was relatively short to permit the use of a short
wick.
This type of psychrometer possessed a particular ad-
vantage in that the air after contacting the wet-bulb flowed
over the water reservoir and cooled it to the wet-bulb temperature; this helped to eliminate the possibility that the
correct temperature was never reached at the wet-bulb.
The
wet- and dry-bulb method of determining humidity was checked
against a gravimetric method employing magnesium perchlorate
as the drying agent.and found to be in excellent agreement
therewith at the humidities encountered in this investigation.
When this check was made the air was not passing through
silica gel in the column.
-51In preparation for Runs 1-7, inclusive, U-tubes containing magnesium perchlorate were weighed accurately on
an analytical balance and placed in an air line leading
from the top of the jiggler column to alfive gallon bottle
previously filled completely with water.
The bottle was
connected by means of water filled glass and rubber tubing
to another five gallon bottle placed at a lower level on a
platform balance,
55 pounds capacity; flow of water to the
lower level was prevented by a screw clamp.
the bottle on the balance was recorded.
The weight of
The drying tubes
and bottles are not shown in Figure 4 since they were eliminated
from the apparatus after
and 8, following page
Run 7; they are shown in Figures 5
58.
A one liter volumetric flask, fitted with aluminum funnel and pinch clamp, Pl, was connected directly to the dessicator outlet and a sample of activated gel withdrawn (out
of contact with moist air) through S13 and Pl.
P1 was closed
immediately, the gel was mixed thoroughly by shaking, and a
sample of gel was taken for determination of moisture content.
The weight of the flask and gel content was determined on a
triple beam balance (previously calibrated against brass
analytical weights) to the nearest 0.1 gram and the flask
was mounted as indicated in Figures 4, 5 and 6.
An empty
one liter volumetric flask was weighed and mounted to receive
the wet gel from the column.
At this point the apparatus
was ready for the beginning of a run.
-52B. Operations During a Run
The wet- and dry-bulb temperatures of the inlet air were
recorded and S5 closed.
Air flow was adjusted to the desired
quantity by adjusting either 81 or 82.
The quantity of air
flow was determined by a sharp edged orifice flowmeter
(Figure 7) constructed of two sections of 14 millimeter
Pyrex glass tubing ground smooth at the pressure tap ends.
A brass circular plate with orifice opening of 0.199 inches
diameter was machined with outer diameter equal to that of
the glass tubing but with a concentric shoulder on one side
to slip inside the tubing and thereby center the orifice
opening exactly.
The two sections of tubing with orifice
plate in place were held together with pressure tubing.
The
orifice was calibrated against a dry test meter over the range
from 0.8 to 3.5 cubic feet of air per minute.
The taps of the orifice were connected directly to a
differential water manometer.
The manometer reading in Figure
4 is 11 centimeters of water which corresponds to an air flow
of approximately one cubic foot per minute.
The air passed
from the flowmeter through a water cooled condenser tube
(used for temperature control) and thence to the column (no
temperature control was used during runs made to obtain operating data).
After adjusting the air flow, pinch clamps P2
and Pl were opened in the order given; the run was timed from
the instant P1 was opened.
The dry (4.7-4.8% water, dry
basis ) gel flowed by gravity through Pl into a feed device
consisting simply of a glass tube constricted at the downstream
-53The
end with minimum elongation and thence into the column.
concentration of gel in the column as pictUred in Figure 4
was characteristic of an air flow of one cubic foot per
minute or a superficial velocity of approximately four feet
per second and was built up in less than five minutes.
The
column itself was construdted of 22 millimeter I.D. Pyrex
tubing.
A Venturi constriction above the air inlet and between
the air and silica gel inlet points converted pressure energy
of the air to kinetic energy, providing an air stream of
high velocity at the silica gel-inlet.
As the silica gel
was whisked into the body of the column and "jiggled" in
the- air stream, water vapor was adsorbed, heat of adsorption
given off'and the air temperature increased.
Felted hair
insulation around the upper and lower portions of the column
together with a dead air space provided by a glass jacket
and annular cardboard ring spacers around the body of the
column eliminated decrease in air temperature due to heat
losses to the surroundings.
Fractional thermometers read-
ing from 00-550 Centigrade by 0.1 degrees recorded the increase in air temperature between the inlet point and the top
of the column.
In both cases, heat transfer between the
thermometer bulb and surroundings was reduced by insulation;
at the inlet point tinfoil and asbestos tape was used while
felted hair was used at the top of the column.
A 0*-2204
Centigrade thermometer recorded the air temperature at a point
three inches above the silica gel inlet.
After jiggling up
the length of the column, the silica gel fell over into a
-54tin funnel and thence to the volumetric flask receiver.
The column was seated tightly in a hole through the
side of the tin funnel by means of heavy rubber tubing.
Light galvanized iron sheet soldered to the top of the funnel
sealed the system effectively.
Holes were cut in the funnel
top for insertion of thermometers and air outlet.
A small
air outlet (through this outlet in Figure 4, the 0*-220O
Centigrade thermometer in the column was supported by copper
wire), directly above the column and protected by glass wool
filter, led to the drying tubes; a larger outlet protected
by copper screen and glass wool filter led to the atmosphere
and in later runs to the wet- and dry-bulb thermometers.
The screen and filter were installed -to eliminate as far as
possible, the gel fines formed during operation.
After reaching steady state (usually less than five
minutes) the following readings were taken at five minute
intervals:
1. Manometer readings (all runs)
2. Inlet air temperature (runs 9-19, inclusive; the drybulb temperature was taken as the inlet temperature
for runs 1-8, inclusive)
3. Temperature three inches abbve the silica gel inlet
(runs 15-19, inclusive)
4. Outlet air temperatures (all runs)
5. Wet- and dry-bulb temperature of outlet air (runs
7-19, inclusive)
In Runs 1-7, inclusive, after steady state was reached,
-55a small stream of outlet air was turned through the drying
tubes and thence to the water filled bottle.
Water flowed
from the upper bottle to that resting on the balance, thereby
drawing air through the drying tubes.
The weight of water
displaced, the temperature of the water in the upper bottle,
the head of water in the tubes connecting the bottles and
the barometric pressure were recorded.
The volume of air
drawn through the drying tubes varied from 5-12 liters per
run.
In these runs, the wet- and dry-bulb temperatures of
the outlet air were not measured.
At the conclusion of all runs (usually 20-80 minutes)
as the last particles of silica gel fell through the feed
device, the air flow was discontinued immediately for a
moment to permit the silica gel holdup in the column to drop
below the air inlet point.
The wet- and dry-bulb temperatures of the inlet air
were determined again.
C. Operations at Conclusion of Run
The operations at the conclusion of a run can best be
described by numerical listing.
1. The empty feed flask and the filled receiver flask
were weighed on the triple beam balance.
2.
The silica gel holdup in the column wazs removed
through Sll and S12 and weighed accurately on an analytical
balance.
3.
In Runs 1-7, inclusive, the magnesium perchlorate
tubes were weighed accurately on an analytical balance.
-564. The time of the run as observed by stopwatch wes
recorded.
5. A sample of outlet silica gel was taken after thorough mixing by shaking, weighed accurately on an analytical
balance, transferred to a crucible, and heated to constant
weight with a Meker burner.
The sample of inlet gel taken
before the run was analyzed for moisture content similarly
at the same time.
D. Operations During Equilibrium Run
Air flow was adjusted as before to approximately two
cubic feet per minute.
Reactivated gel contained in the
flask controlled by Pl, was passed into the column through
Pl; pinch clamp P2 was closed during this operation thereby
holding the gel in the tin funnel.
After all gel had passed
through the column, the feed flask was removed and connection made by rubber tubing from the outlet of the tin funnel
to the gel inlet.
P2 was opened partially permitting the
gel to flow into the column, back to the tin funnel, and
again to the column continuously.
The gel was fed to the
column until the inlet and outlet air temperature remained
constant with and without the gel jiggling.
The equality
of the inlet and outlet temperatures or the constancy of the
outlet temperature indicated that no water vapor was being
adsorbed and therefore, that equilibrium had been reached for
that particular percent relative humidity.
At that point,
duplicate samples of gel were withdrawn from the total quantity after thorough mixing by shaking and the moisture content
-57determined by heating to constant weight with a.Meker burner.
The results were plotted as Percent Useful Water Concentration
(based on the weight of initial wet gel) against the percent
relative humidity (see Figure 1).
In order to control the temperature of the air, a watercooled condenser tube was placed in the air line.
The water
supply was taken from a reservoir into which steam (through
B7) and cold water '(through 56) were introduced. in quantities
depending on the desired temperature; a large part of the
water ran to waste from the reservoir through S8, the remainder flowed to the condenser tube jacket.
In this way,
the temperature of the air could be controlled to 0.20
Centigrade over a considerably long period of time.
With
constant room temperature the control was better over longer
periods of time.
Since it was desired to determine equilibrium data at
very high percent relative humidities it became necessary
to construct an air saturator to handle a large volumetric
flow of air, for the relative humidity of the inlet air
seldom exceeded 35 and was never greater than 40 percent.
Ordinary bubbling apparatus proved unsatisfactory due to low
capacity; a fritted glass plug in a glass column eighteen
inches in length was tried first but the flooding velocity
was reached at low volumetric flow.
In order to approach
saturation closely it was necessary to pass the air as relatively fine bubbles through water and at the same time handle
at least two cubic feet of air per minutes.
A simple device
-58was constructed to meet these requirements.
Two types of B.
& W. Kaolin Brick, No. 28 and 26 were cut in half, one half
of each was carved out so that the remaining walls were one
inch in thickness.
A small hole was cut in the top of the
No. 28 brick and a short length of 12 millimeter glass tubing was sealed therein with Plicene cement.
The two halves
of brick were also sealed together with Plicene cement and
placed in water contained in a heavy Pyrex jar twelve inches
in diameter and twelve inches in height.
A circular steel
plate fifteen inches in diameter with a groove, one eighth
inch in depth cut to fit over the glass jar was sealed
thereon with litharge and glycerine cement.
One huddred
pounds of steel weights were placed on the steel plate.
ELEVATION
DIAGRAM OF APPARATUS
FIG.407-55-C. THERMOMETER
WET- AND DRY-BULB
THERMOMETERS.
OUTLET AIR
VAL VE OPEN
VAL VE CLOSED
-
TINFOIL AND
ASESTOS TAPE
VAL VE PARTIAL LY OPEN
-7&
S9
HEAVY RUBBER
TUBING
TO WET- AND DRYBULB THERMOMETERS
WATER MANOMETER
GLASS WOOL FILTER
OUTLET
AIR
COPPER SCREEN
/
WOOL
TIN
FUNNEL
NSULATION
ALUMINUM
FUNNEL
WATER
WICK
WET-BULB
P2
S14
GLASS JACKET--
MM
PYREX
TUBING
-
STO
PRESSURE
TUBING
S4
SI
S3
-
ANNULAR
CARDBOARD RING
WASTE
pi
REACTIVATED
SILICA GEL
-
--
-
100 POUNDS
WEIGHT
BRASS SHARP-EDGED
ORIFICE PLATE
CIUM
SHLORIDE
--
COLD--WATER
GLASS WOOL
FILTER
S13
STEEL PLATE
STEAM
58
LITHARGE AND
GLYCERINE
)-SC
I LITER VOLUMETRIC FLASK
THERMOMETER
S7
HEAVY
PYREX
-------------------
-
WATER
-
PLICENE
CEMENT
B&W K-28 BRICK
CO
AIRD
WOOL INSULATION
58
JAR
B & W K- 2
6
B RIC-
--
-
-
-
TINFOIL AND
ASBESTOS TAPE
TO~ WASTE
Sil
Sl 2
A W.P
12-17-41
r
WI
L
2:
FIGURE 5
ASSEKBLY OF APPARATUS
r
-
1
,-AG
m
MA
FIGURE 6 ASSEMBLY OF COLUMN,
FUNNEL AND FLASKS
A.
Disassembled
B. Assembled
C. Placed for Operation
FIGURE 7
SHARP-EDGE ORIFICL FLCWETER
I
4"
B.
A. Psychrometer
C.
FIGURE 8
MgC10
4
Dessicr tor
Drying Tubes
COM'PONLNT PARTS OF THi APPARATUS
-59B. Summarized Data
All data taken during Runs 1-28, inclusive are given
in Tables I, II and III of the present section.
Table I
includes all operating data for both normal runs and also
those during which equilibrium data were determined.
At
the bottom of Table I will be found the calibration data
for the various flowmeters used during the experiments.
Calibration curies of this data are to be found in Appendix
F.
Table II gives the surrounding temperature for Runs
1-19, inclusive.
These were used in calculating the heat
losses from the column.
Table III gives the data taken
when the psychrometric method of determining humidity of
air was being ehecked against the gravimetric method.
These
data were taken without silica gel "jiggling" in the column.
Similar data for comparison of the two methods with gel in
the column may be found in Table I under Runs 2 and 7.
TABLE I
ORIGINAL DATA
I
2
3
34.0
12.0
31.1
26.6
24.1
16.1
32.0
20.1
19.9
16.9
1I.0
16.60
oo.
Rum
4
5
16
9
-7
1
3
14
2+.10
17.25
15.55
16.30
1 'A
10
2.1
5
27
s6
I to
10.15
24.15 25.20 20.900 2146
22.1
11.80
'1.90
14 13
13.11
14.30
14.11
12.75
13.10
12.15
16.45
7.45
13.16
392 0.1461
0.1411
0,45
0.11L6
0.6318
a1.g
33.5w
48.50
10.00
15.15
26.33 12.90 32.05 41.0
21.iS
15.45 21.20 1.10
14.40 2(.60
1.31
1i.016
o0.1165 12.6110
17
Is
21
24
It
6
Is
as
INLslAII
TIMPItATUht,*C,
My VUL9
W&T SuwL 1aNPPATUIDrC.
PO4IF6O
6.p e
4.07351
s.0. AIR/mINUII
Y6IIPIIATUSI,*C,
KAmannsthO R*6 0414,tt T,
C
31.0 I8l 26.4
10
41.8
"A10IEgTYA KCACA4.46MTIC'i H,0
+7.7
CUSIC F1IT AIR /MINUITL
41.1
47.4
1.01
.05
TIMtPIIt*u66
OuTIt' As : ovistl ru6
,C-
em se36,
mSA9OIW*YW
OUT6JT.TAI
1'ST.
vuess,
30
49.3
49.5
16.61
3.1
11.1
11.4
49.3
1.%&1 .63
0.9
4904
.111116
6"I
1.11
-.
11.15
49.7
49.3
LO
@0779
6.21 28.43 2.5.42. 25.13 4.06 21.67 23.51.
59.4 61.6 6+.5
67.7
66.
66.6
51.6 GS.3
48. 48.6 34.0 36.4 38.1 41.3 43.7 403
S.15 X2.56
57.6
51.+
46.5
1.00
3.9s
4.01
LGI
1.10
1.07
(.931 1.00
1.11
1.91
30
764
al
23
30
762
12.06
17.0
15.2
14.6
25.3
11.+9
25.45
0.0477
0.0(11
76
26
30
30
14.70
17.00
6..00
13.75 11.40
0.20
765
li.60
"4.4
1.79
24.5
30
30
ST.68
16.70
13.04 12.49
41.1
+64
.3 610.4
3.3
39.16
TS6 36.1
16.11
25.19 26.59 12.14 10.91
26.01
a6.61
23.28
23.26
2696
1901
9.90 10.69
11.44
1+.61
12.00
.113
4.99
0.116
32.12
28.16 27.32
15.93
21.3
11.97
11.14.
34.906 32.37 34.48 33.01 19.35 19.13 34.46
SILICA *"1 IN bAS
1.13
,MS.
404.6 630.3 61.5 634.6 191.l 3SI.9 392.9 312.2 317 194.6 118.9 315.9
413.5 2W1.0
6413.9
639.1
6".6
5-IT.
35-51
67-2.
7s-u4t,-
9.66
to.9
11.7l
632.
81181te3 1.09 1110140
SILICA *I. 110OLUP IN C8LPR
1.7
1i.19
.36
t.+o
314.4 190.4 318.3 16o0.1
51.-51
52
it.11
9-m0 51-5
i.9+
lOl
42-9 19-5
10.17
i1.57
132.6
12.21
S1i9.
26-34
9.33
442.1 4
34.9 . 3+9 4127
466.0 419.7
+-4S
44-4
0.079
0.0
94.1
341.6 429.6
4.6- 1
I.10
47-10 41-9
0.27
0.05
0.200 0T.061
0.09+
WOOL P11.161, .81.
-11
POR ANALYSm51.ifS;:
1T+534
1.1641
1ILST
OUTL9SSAAPVLS*
I
6.0i15
G.9299 6.1106
.3101
6.-I
S.A10
.3.00
7.1741
9WL90
6.6616 6.6143 st.L
9
.6a
9.496+
163 6.6300
6.6616
6.6199
8.6914 6.0837 1.jSS
9.06097
mL
ourTLgT,
9.0421
@2. I4a11.1651
6.6195
61140
10.271
IS.
6.-7013j106051 11.111 9."
491.s A"92e BLAS""a,90%6
SIA
*1L~
.11 7.1
6.f01
7.9144
6.14
OUT36T,9
6.04916.7176
5A0011.29
DE
p""MentTsat RmtqIr6,ArC" W.0
53 , 144
MAWSPEY*A 51AbI46,*i9trCf414,O
OPFL.w,
Poky rgser£Mq
S SCON O
ltAble
s
P1
.7?57 8.2902 .1494
8.1471 6.1007 9.9673 0.910 1.441) 6.1660 0.15
6.450
1
U I
0206 0.
:
9 peg. ATOICACVP3SVA0)*h*A
#.5
.1"6
OUTL9T, SAM K4*2
CA6I DRATION
Ad
I.3
14.10
765
2.8
16.1
Me T.rC.umm'leMo.
,eS093U81,
Toe
12510
.65
T.00
27.9
13.0
,C.
SILICA ORL 90s6ICSe,6MS.
DUV
I.51
11.4
a
eulSi
SILICA 69 116asumI
SlICcA 6SI-TAK
57.
4.90 IA5
1.0 AN5-Sll 8151810
itnpsATM a,*c
.aL
010013 0S4
754
1
11. 9
20.7
GAM 0.01.0
V6S6TULDYApSStArUMS,'C-
SILICA
16.10
,C.
29.7
s
cot
03v IuSLU11AvG4xu1
OUTLET
I
26.69 1197 1.62
766.3
M 06
BSUw ma1n34,WCMUS
VoLung ceSN5is46d
gt.
-ASOOS6g
1.56
AOS009 16 MjCI6O*UWSS,6MI
W36t
.6.4
1,.1
k.07
29.9
605
04 OILACS OT AIR
eAmommm
49.5
1.06
25.15
12.20
mai3oo
WAVSUOiSPLA.SSSVAM,
IMP. WAU
14.1
In.9 9 87. 5.1
1+4.60 21.45 21.50
2 .V
*"0
.09
11.4 97.9
Mso
6.4
17.25
i.
1.37
217.1
2.5
So
s+2 5+.4 55.1
49.5
5S 49.9 498
1.95 1.9
I 1
V%.& 197.9 IS11.
I 5 1 5 1 .5
57.0
60.1
61.9
44.1
41.9
19.I
1.10
2.1*
s
44.1
60.1
+4+9
6.
2.5 I 1.s
7S.0 611.,
2.5
2.5
3a 6 ,.
e
66.6 1-1.+
10
1-31.1
11.0
it
31.1
IS2
1.0
jIG
1
55.0
56.1 51.0
49.6
+9.4
MTC.t
+6.1
O
51.9
46
I
61.6
1a
JIl
IS8.3 I'LS.S 163.9 o2.0 j 2.0
I
a .
2.0 I 2.0
I .0
2.0 2.0
£
g
2 0
64.4
4.3
0
11.0
IFICE
55.9 A1e.6
41.0 121.3
1.90
134.3 1e w.$ 104.' 12..S
1.0
OLO IFLOW
6a4C I
9.0l3 8.532
D.
71-1
174.t
@(.f
5.4
131.7
41.6
IX316
890
, o
a 2 5
0
O.3
-
0.5
19.
1.0
-. 111,.6
2.9
3. -
9.I2.I 714 7.53
0.199
W.1,
7.2197 3.131
mtEs
(SIA P
16
99.5 __.3
.
9.2
.
#&' 10
ups IMsa- IN
46.9
+*.a _4_
L,4
-
-70-
a 5.0 4.5 16.0 1 9.0
9.557
1.40 I:'S.g
10.1
6.0 4(.9
A
9.7 1 .96
.6993
44.9
1.0
5'1.
-.
19.6
'17.4
p.2.
3.7
a e~
+
2. 5 1-7. 5 1 .0
AwP
9/441
94+4
54.1
-60TABLE II
TEM1PERATURE OF THE SURROUNDINGS DURING RUNS 1-19, INCLUSIVE
Run No.
Su r r o u n d i n g
T e m p e r a t u r e
Degrees Centigrade
Degrees Fahrenheit
1
34.0
93.1
2
32.0
89.6
3
31.2
88.2
4
26.6
79.8
5
24.2
75.6
6
26.1
79.0
7
26.4
79.6
8
22.7
72.9
9
22.8
73.0
10
23.8
74.8
11
21.5
70.6
12
21.1
70.0
13
24.1
75.4
144
27.3
81.1
15
20.3
68.5
16
23.4
76.2
17
25.2
77.4
18
20.9
69.6
19
21.5
70.7
-61TABLE III
DATA FOR COMPARISON OF GRAVIMETRIC AND
PSYCHROMETRIC METHODS OF MEASURING HUMIDITY
Gravimetric Method
No. 1
Weight of drying tube + adsorbed water
Weight of drying tubes
Weight of adsorbed water
Total weight of adsorbed water
60.5305 gms.
60.5292
0.0013 gms.
31.81 lbs.
12.25
191.56 lbs.
Temperature of water in bottle
29.4000.
Head of water below gas receiver
26.0 inches
Volume of connecting tubeP.
30.0 ml.
754.0 mm. Hg
Psychrometric Method
Wet-bulb temieratur,
17.35
17.42
17.38
17.20
.
Dry-bulb temperature,
27.75
27.85
27.80
27.50
2
70.2852 gms.
70.2224
0.0627 gms.
0.0741 gms.
Weight of bottle + water
Weight of bottle
Weight of water displaced
Barometric pressure
No.
00.
-62C. Sample Calculations
The calculations performed for Run No. 10 are representative of those performed for all other runs except the
first
seven; the calculations for those runs are the same
as all others except for those performed to determine the
outlet air humidity by the drying tube method.
Run No. 10
INLET AIR
Dry Bulb Temperature = 24.600. = 76.3 0 F.
Wet Bulb Temperature = 14.90.
= 58.8 0 F.
From a humidity chart covering the normal temperature
range at atmospheric pressure,
H1 = 0.0068 lbs. H2 0 vapor/lb. b.d. air
RH, = 31%
s1 = 0.243 Btu/(lb.)(OF.)
Average Air Inlet Temperature = 25.250C.
Temperature,
Time
8:45 p.m.
8:50
8:55
9:00
9:05
9:10
9:15
9:20
9:25
9:30
Average
=
77.4 0 F.
0 0.
25.00
25.10
25.50
25.40
25.52
25.70
25.25
25.15
25.00
24.85
25.25
From the humidity chart again at a temperature of 77.4 0 F.
and 31% RH, the specific volume of the inlet air is
v = 13.60 cu. ft./lb. b.d. inlet air
Rate of inlet air flow:
Manometer Readings, cm. water
Left
Right
Time
8:45 p.m.
8:50
8:55
9:00
9:05
9:10
9:15
9:20
9:25
9:30
Average
49.35
49.00
49.10
49.25
49.40
49.10
48.95
49.55
49.15
49.35
49.22
57.30
57.60
57.50
57.35
57.20
57.50
57.60
57.00
57.45
57.20
57.37
Manometer Differential = 57.3,? - 49.22 = 8.15 cm. water
From Figure 21, 8.15 centimeters of water corresponds
to an air flow of 1.01 cubic feet of air per minute.
Then
1.01
13.60 = 0.0743 lbs. of b.d. inlet air flowing/minute
To convert this quantity to one more useful for design
calculations it is necessary to know the tower cross sectional area.
Since the tower measured 22 mm. I.D., the
cross sectional area of flow is
S=
LTrT(22/25.4)a
/(144)(4)
= 4.09 x 10-3
ft.2
Then the rate of air flow, G, becomes
G ='(o.0743)(60)/(4.09 x 10-3) = 1090 lbs. b.d.
air/(hr)(ft2)
The superficial air velocity, us, at this mass rate of air
flow is
us = (1.01)/(60)(4.09 x 10-3) = 4.15 feet/ second
-64OUTLET AIR
Humidity and temperature of the outlet air:
Time
p.m.
Wet Bulb Temp.
00.
OF.
8:45
8:50
8:55
9:00
9:05
9:10
9:15
9:20
9:25
9:30
Ave.
12.80
12.35
12.75
12.70
12.60
12.80
12.75
12.95
12.95
13.00
12.97
55.04
54.40
54.95
54.90
54.70
55.00
54.95
55.60
55.60
55.40
55.05
Dry Bulb Temp.
0F.
00.
Outlet Temp., OC.
25.30
26.10
26.45
26.70
26.55
26.85
26.85
27.00
27.00
27.05
26.59
31.20
32.60
32.70
33.00
33.30
33.35
33.50
33.25
33.35
33.05
33.02 (91.400F.)
77.60
79.00
79.60
80.00
79.80
80.40
80.40
80.60
80.60
80.70
79.87
H*
0.0044
0.0036
0.0036
0.0035
0.0036
0.0035
0.0035
0.0039
0.0039
0.0038
0.0037
*H lbs. of water vapor per lb. of b.d. air
The values of H above were obtained from a standard
humidity chart.
Using the same chart and the average wet- and
dry-bulb temperatures, the percent relative humidity, RH 2 and
humid heat, S2, were found to be
RH 2 = 11%
s=
0.242 Btu/(lb.)(OF.)
The temperature rise, t.
-
ti, of the air in passing
through the column is 91.4 - 77.4 = 14.04F., and the water
adsorbed is
G(Hi-
H2 ) = 1090(0.0068 - 0.0037) = 3.4 lbs. water
vapor adsorbed/
(hr)(ft2)
The percent of the inlet water vapor adsorbed by the
silica gel is
100(0.0068 - 0.0037)/0.0068 = 45.5%
SILICA GEL MEASUREMENTS
Rate of Gel Flow, Ll and L2 :
-65-
Inlet
Outlet
747.88 gms. 641.07 gms.
335.21
435.68
305.76 gms.
gms.
312.20
Weight of Reservoir + Silica Gel
Weight of Reservoir
Weight of Silica Gel
Silica Gel Holdup in Column:
Weight of Bottle + Silica Gel 32.448 gms.
19.878
Weight of Bottle
Weight of Silica Gel Holdup
Weight of Inlet Gel, gms.
12.57 gms.
312.20
318.33
Weight of Outlet Gel, gms.
Since the elapsed time of the run was 49 minutes and 20
seconds or (49 + 20/60)/(60) = 0.82 hours, L1 and L2 are
L1 = (312.2)/(454)(4.09 x 10-3)(0.82) = 205.3 lbs. of silica
gel/(hr)(fta)
I"
L2 = (318.3)/(454)(4.09 x 10- 3 )(Q.82) = 209.2
and the amount of water adsorbed by the gel is
209.2 - 205.3 = 3.9 lbs. of water adsorbed/(ht)(ft2)
Water Content of the Silica Gel:
The gel was sampled at both the inlet and outlet points
and each sample heated to dryness over a,Meker burner.
Inlet
Weight of Bottle + Fresh Gel
Weight of Bottle
Weight of Fresh Gel
Outlet
27.5593 gms. 26.9279 gms.
20.0199
18.5578
7.5394 gms. 8.3701 gms.
Weight of Crucible + Gel, After Blasting:
Weighing No. 1 36.1571 gms. 35.7438 gms.
Weighing No. 2 36.1557
35.7434
27.9070
28.9631
Weight of Crucible
7.8364 gms.
gms.
7.1926
Weight of Dry gel
Water content of the inlet gel is equal to
C1 = (7.5394 - 7.1926)(100)/(7.1926) = 4.83% (dry basis)
Water content of the outlet gel is equal to
02 = (8.3701 - 7.8364)(100)/(7.8364) = 6.81% (dry basis)
-66The amount of water adsorbed by the silica gel can also
be calculated from the rate of gel feed and the water content
of the inlet and outlet gel.
__
For example,
C2
__
/21 [(C2 - 01)/100=
) + Loo+c2)J
.- 10
lbs. H20 adsorbed/(hr)(ft2)
For Run No. 10
[205.3(l.0 -
4.83
2181
+ 209.2(1.0 -
10
l0+6.8i
(6.81 - 4.83)/1001 = 3.9 lbs. of water vapor
adsorbed/(hr)(fta)
F.E2100;4.83
CALCULATION OF WATER BALANCE FROM THERMAL DATA
Inasmuch as the measured increase in air and silica gel
temperature up the column is caused by evolution of heat
of condensation of the water vapor and heat of wetting of
the gel by the water adsorbed, it is possible to calculate
the quantity of water vapor adsorbed in the column from thermal
data for silica gel and heats of condensation.
The partial
heat of wetting of silica gel, expressed as Btu per pound of
water adsorbed is given in Figures 2B and 3B as a function
of initial water content.
Since the change in water content
of the gel in all runs was small, the partial heat of wetting
at the average water content can be used satisfactorily for
calculating the water balance.
The pertinent data are
G = 1090 lbs. of b.d. air/(hr)(ft2
L*.
av.
=
)
206.7 lbs. of -gel/(hr)(fta)
a* = 0.2 Btu/(lb. of gel)(OF.)
*Lav. represents the arithmetic average (weight) of the inlet
and outlet gels. SPecific heat of 0.2 for b.d. gel can be.
used here since the gel water content is low.
-670 aV
= 5.82%
hw (FigureB)- 250 Btu/lb. of water adsorbed
74 + 91.4)/2 = 84.4 0 F.
tav.hv =
(at 84.4 0 F. ) =-1046 Btu/lb. of water condensed
Sav. = 0.243 Btu/(lb. b.d. air)(OF.)
At = 14.04F.
ha = 250 + 1046 = 1296 Btu/lb. of water vapor adsorbed
Heat Evolution Observed:
(1090)(0.243)(14.0)
(206.7)(0.2)(14.0) =
Total
=
3710 Btu
579 Btu
4289 Btu
Water adsorbed = 4289/1296 = 3.31 lbs./(hr)(ft2)
DETERMINATION OF OUTLET AIR HUMIDITY BY GRAVIMETRIC METHOD
Since the perchlorate drying tube method was not used
in Run No. 10 for determining the humidity of the effluent
air, the data taken during Run No. 3 will be used t6 illustrate
the calculations.
Weight of water and five gallon bottle
Weight of five gallon bottle
Weight of water displaced by air
39.00 lbs.
12.31
26.69 lbs.
Temperature of Water in Bottles = 29.6500.
No. 1
No. 2
Weight of drying tubes and ads. water 60.5292 gms. 70.2224 gms.
70.2033
60.5279
Weight of drying tubes
0.0013 gms. 0.0189 gms.
Weight of adsorbed water
Barometric Pressure, mm Hg = 29.67 inches = 754 mm.
Head of water Lbelow gas receiver bottle = 23 inches = 42.8 mm Hg
3
Volume of connecting tubes = 30 cm
Room temperature = 30'C.
-68Vapor pressure of water at 29.650. = 31.2 mm. Hg
Density of water at 29.65C.
= 0.9957 gms/cm3
Then the pounds of bone dry air passed through the tubes
may be calculated as follows:
(26.69)(454)(1/0.9957)
30][ (75
4
(273/302.7)(1/22,400)(29/454)
- 42.8 - 31.2)/(760)
=
0.02792 lbs.
The humidity of the outlet air then is
(0.0013 - 0.0189)/(454)(0.02792) = 0.00159 lbs. water vapor!
lb. b.d. air.
By wet- and dry-bulb temperature measurements on the inlet air
the humidity was found to be 0.0100 lbs. water vapor/lb. b.d.
air.
The water vapor adsorbed by -the gel per lb. of b.d. air
is then
0.0100 - 0.00159 =- 0.00841 lbs.
and since the b.d. air rate for Run No. 3 was found to be 1110
lbs./(hr)(ft2) the
Lbs. of water vapor adsorbed/(hr)(ft 2 ) = (1110)(0.00841) =
9.33
It will be observed from Table VI that' this value is g'eater
than that obtained from a difference in inlet and outlet gel
weights.
The proposed explanation for this is given in the
Discussion of Results.
CALCULATION OF OUTLET AIR HUMIDITY FOR RUNS 1-7 FROM DIFFERENCE
IN INLET AND OUTLET-GEL WEIGHTS
Since determination of the outlet air humidity by the
magnesium perchlorate drying tube method was found to be unreliable during Runs 1-7 this quantity was calculated from the
difference in the weights of inlet and outlet gel as follows:
-69-
L2 - Li
=
357.5 - 352.0 = 5.5 lbs. of water vapor adsorbed/
(hr)(ft2)
G = 1110 lbs. b.d. air/(hr)(ft 2 )
G(Hl - H2 )
= 5.5
1110(0.0100
-
H2 ) = 5.5
0.0100 -H
=
5.5/1110= 0.00495
=
0.0051 lbs. water vapor/lb.
H2
b.d. air
CALCULATION OF WATER BALANCE FOR RUNS 1-8 USING THEMIAL DATA
Inasmuch as the water content of the silica gel was not
determined experimentally during Runs 1-8 it
was not possible
to calculate a water balance from thermal data directly4.
ever,
How-
subsequent runs indicated that the water content of
freshly activated gel was sufficiehtly constant so that an
initial water content could be assumed for the first eight
runs.
The final water content of the gel could then be cal-
culated from the difference in weight of inlet and outlet gel.
In this way heat of wetting data could be used to calculate
a water balance for these runs.
Run No. 3
Assume the initial water content of the gel to be 4.8
percent (dry basis).
The final water content of the outlet
gel is then (see Table I for original data)
100 E (357.5 - 352.0) + (352.0)(4.8)/(104.8)
/(352.0)
(1.0 - 0.048) = 6.42%
Average water content = (4.8 + P.42)/2 = 5.61%
Heat of wetting (Figure 2B) = 250 Btu/lb. of water adsorbed.
A calculation similar to that on page 66 can now be performed.
-70The result is
Lbs. of water adsorbed/(hr)(ft2) = 3.8
CALCULATION OF GEL CONCENTRATION IN COLUMN FROK GEL FEED RATE
AND AIR VELOCITY ASSUMING ZERO SLIP
Inasmuch as the gel holdup in the column at higher superficial air velocities was very small, measurement thereof
was difficult.
Consequently,
for Runs 14 and 19 the gel con-
centration in the column was calculated from the gel feed rate
and the superficial air velocity assuming the gel velocity
to be equal to the air velocity.
Run No. 14
Gel concentration in column,
lbs./ft 3 = (lbs)(hr)/
(hr)(ft2)(ft)
(hr)/(ft) = (sec/ft)(hr/sec) = (1/u,)(1/3600)
Therefore,
Gel concentration = Ll/(3600)(us)
= 387.0/(3600)(7.92) = 0.0136 lbs/ft3
CALCULATION OF SLIP VELOCITY IN THE COLUMN
The slip velocity in the column is the difference between
the gel and air velocities and may be calculated from the gel
feed rate and gel concentration in the column as follows:
Slip velocity = Air velocity, ft/sec
= U B-
-
b(bs
Ll/(gel concentration)(3600)
Run No. 10
L
Gel concentration
= 205.3 lbs.
=
of gel/(hr)(ft 2 )
4.52 lbs./ft3
us =4.15 ft/sec
gel)(fta)
(h)f2 )( lbs .gel)
-71Slip velocity = 4.15 - 205.3/(4.52)(3600) = 4.14 ft/sec
CALCULATION OF ADSORPTION COEFFICIENTS, KRHa, Kaa AND (Kga)(G/1000)
Run No. 10
KRHa = (G)(AH)/(h)(RH - RHe)M
ARH = 35 - 13 = 22
(RH
RHe)m =
-
(35
- 0) -
(13
-
/ln(35/4) = 31/2.17 = 14.3
9)
G = 1180 lbs. b.d. air/(hr)(ft2)
h = 1.5 feet
H = 0.0035 lbs. water vapor/lb. b.d. air
KRHa = (1180)(0.0035)/(1.5)(14.3) = 0.193 lbs. water vapor
adsorbed/(hr) (ft3)
(unit RH difference)
K a = (L )(AUWC)/(l00)(h)(UWC - UWCe)m
L
=
205.3 lbsl of gel/(hr)(ft2)
AUWC = 1.89% (dry basis)
(UWC
-
UWCe)M = [(11.5 - 0) -
(3.16
-
1.89)1 /ln(11.5/l.27)
= 10.23/2.2 = 4.65
Kga = (205.3X1.89)/(100)(1.5)(4.65) = 0.556 lbs. of water
vapor adsorbed/
(hr) (fts) (unit
UWC difference)
(K a)(G/1000) = (0.556)(1180)/1000 = 0.606
Values of KRHa are plotted against the mass rate of air
flow, G, on Figure 18.
Kga and (Kga)(G/1000) are plotted
against rate. of gel feed, Ll, on Figures 19 and 20, respectively.
CALCULATION OF PERCENT RELATIVE HUMIDITY FOR EQUILIBRIUM RUNS
20-2g. INCLUSIVE
Run No.
20
Temperature in column = 22.9*0.
-72p8 at 22.900.
= 20.95 mm Hg
.H = 0.0048 lbs. of water vapor/lb. b.d. air
p = (29)(760)(H)/(18 + 29H)
p = 5.82 mm Hg
= 27.9% relative humidity
100p/p8
The water content of the outlet gel was calculated as
for Runs 1-19, inclusive.
CALCULATION OF AN OPERATING LINE ON THE EQUILIBRIUM DIAGRAM
FOR A GIVEN RUN
The slope of the operating line at the bottom of the
column for any given run was shown in the Introduction to be
(slope) 1
= GH s5
/ L1
and similarly for the top of the column the slope was shown
to be
(slope), = GHS
Then for Run No.
/ L1
10 where
G = 1090 lbs. of air/(hr)(ft2)
L
= 205.3 lbs. of gel,/(hr)(ft 2
H
= 0.0208 lbs. of water vapor/lb. of b.d. air
H5 1
=
)
0.0323 lbs. of water vapor/lb. of b.d. air
the slopes of the operating line at the top and bottom of the
column are
(slope) 1 = (1090)(0.0208)/(205.3)
(slope) 2
=
= 0.111
(1090)(0.0323)/(205.3) = 0.172
Obviously, since the slope of the operating line at the top
of the column is greater than that at the bottom the curve must
be S-shaped to pass through the points representing the terminal
conditions with the slopes calculated.
The results of such
calculations for Runs 1-19, inclusive are plotted in Figure 17.
j~
D.
Results:
Plots and Tables
Table IV of this section gives all operating data calculated to a more useful form for design purposes.
Also
given are the calculated water balances obtained by four
independent methods, namely, (1) air rate and humidity difference, (2) silica gel weight difference, (3) silica gel
analysis for water content and silica gel rate of flow and
(4) temperature rise and heat of wetting data.
All other
tables given in this section have been mentioned in Results
in the main body of the report.
Similarly, all plots given
here have been mentioned under Results.
TABLE M
TA1LE Of CALCULATED DATA AND R[5ULT5
U,4 010.
4
3
a
1
6
s
0
0
a
it
6
is
1+
IS
I
is
is
17
I
20
14
11
1
as
1
26
As
ILCET AIR:
RATE OfPLOW,6, LS
SUPERFICIAL
ARA/("0XFTa)
V90CIT,frT./SEc.,A,
19,IiZRATUEt,*
hlUMIDIST,M,
LII
1O/LS.
5.0.
RELATIVE MUNSOTV,m%.PScEM
Ait
1096
4.31
93.2
s.& $ILI
6.0110
0e.000
3S
33
r
MuMID5 MEAT, 6,
+.16
a
1100
1010
1066
5090
1160
1190
4.39
4.60
413
#
+.
+.S6
4.1s
+.40
4.1t
19.0
1s.6
19.0
,9.6
72.9
19.9
17.4
72.6
12.2
1,60
1140
1160
1043
+.23
21
36
.&+
0.144
0.S44
155.1
101.6
1.4
0.t4e
6A".
0* 06
0.a*S
100
46
30
06
27
40
a.666
35
OUT"CTAIR.I
A9E
49L
,./L.
HUMI0ITYM5,4L6S.
90.1
04.0
91.1
99.5 'o0.,
04.0
703
2045 AI50
.92
#.a
83..
10
L00
79L
71
10
4.91
s15.3
*6+ 10 .0 4 d0
43
55
TO
0.L43
.143
0.243
0.0&43
94.0
89.6
St T?
13
0.00
O
O650 1910
6S
8+4
+3
05066
10.0.o
30
SS
36
16
0.345
0.141
0.43
96-7
7T6
9.0.
PIAT
1.4
0
1s.3 63.0 77.4
91.1
11.0
1
096
0..640
O.243
II
4.8
91.4
APPRA
-5137
110
0.66
__
o0.6
04
.
SIM
69.9
0
76.9
P6.4
0.011301, 6
I
05
1%4
89.7
13.1
101.1
70.9
0.9 69.9 76.3
0.0 TI.7
81.3 o.6 95.+ 16.1 77.3
INLInIto1,1
AlEt
0.66SS
9.637
0.6644
0.01
S0,
0.41
0.0030
6.6644
0636
0.6
0AO6+
0.6611113
a66
064139
0.241
.S10
0. 141
o41 0.241 0.241 0.141 0.2*+ .145
0.1
.065
06015
It IMs 0 .8
9
1
RLtATiVMussiTY,s,,.aRCISP
SUMID MILAT,
0.41
-39
l40.0616
II
T9MP9RATUS,tg.P
TampaxwTuAs"
0.06f
as
31
0.143
0..L*+ 0.14+ 0.141 0.4+
0.4
0.0074
%14,0
I
16-s 1
4614L
0.14
.ocul
13 I1
0.141
1 0 .645
0.00ST
004
10 16
to I1 11 1o
6.141
0143 0.144
0.0046
.0049
00
20 a L1.9
15
0-001Z
13.1
14.4
11
13
0.00
31.2
6604
+1.7
060 0
15.4
00130
6063+
.osaS
66.0
17.6
79.+
46
46
46
0.143
INLE.T SILCA OIL :II
AAISOPPSSO,L,,S0s.GEh/(SSRXK'T')
rt5ILoSACTIVATION,
MOISTURS COITEST,
3640
154.0
11.1 246.3
205.3 231.6
1+5.6
4.0 3 4.69 3.60
5.0
0oleu tklLTIMs11
4.12 4.1
5.95
3.47
6.0 391.0 321.7 216.5 25.1 is.+
13
13
36
1 3
3
+651 .33
+.1?
4.09
4.01 4.9
36.14 4.41
4.52
+.60 4.76
+.71
3.3
0.04
+.63
------
-------
-------
-------
4L35
NA../V,b
in O
271.6
@ASIS),%
% CPR
E9UILSSRSUMIAnCIUMtH,*---1"
conc.
304.0 119.5 AS.0
3WO
4S.
0.06
0.012 0.4O
0609
%$*.(
T1
+.75
0
13
+.60
0
0
0
0
13
46
0
a
0
0
0
0
0.0111
OUL.3T S6LSC^A 41.
RATE OF FLOv,LAS. OL/(KRXFT "),L&
OiSTRu acow"TEA(0R4
61.1 367.2 357.5 369.3 2G.S 109.0 216.3
UASIS),c 6
P133S3T WAI09AP6N AOM 3ELO
ToEM
RU,111S.
s m*AVaVnO6.616 .t.-
6.1
0.66
,r
0A 59 1 54A7
,.1
4(4,-Ma)
F-t.
65.0
49.0
0-93 0.93
S.5 10.0 14.2
.M
,S..
5.6
66.5 45.0 5.0 132.6 44.3 45.5 52.0 47.3
0.71
0."0
0.01
0.85
1.21 1.20 O.S6 0.81
11.2 11.2
51.3
14.5 5.o
I.2 15.1
19.7 15.3
143 0.143 .14L 0.141
0.um43 0143 0.130 0.243 0.*1 .142 .141
4.3
av-s..cITV AMRsINmG Ia
A CALcULAT69 PON PONS -? , MEASURID0 149.t0AFTMA
T"5S0 K5N6T 8 16 W6.54E6 a I.e iFSET
*cALCuaATEo
6
7.0
5.2
1.8
4.7
POW50 ssUaMCIA.At
ToWa COSS AsEcTI6O - w (11/2S.4)r3A/s4x4) a 4.091
T0wAa VOL~UME * (.5X4.09 x so-')s 6.54x 3 ~ri.
3
RAVES OPAIS NPW (
LOS.
.D.AIR/tMW.)(6O/4.-91-)
[/4.O91
(PT'/M)
#UPS9.PSCSAL AiR VELOCITY 0
. r3
+.5
1.1
4,0
4.7
((4 .La-ss.4,4 .61-566.o4a$L,-c,
(L 4 ,.1)(,tt,)-)/..
.3
2
+6.5
5.5
7.2
6.3
6.05
4
ks"Ls~sU' MU"A"EAT RM2
ELAPSSOTIP&OP
1096.1 340.6 1+9.3 391.0 391.6 13.6 290.0 121.1 113.0 293.1
117.0 149.
7.6
it
MOLDUP)
GEL COAC:ISTRAT0e,16 sM 904.11M x (M5S.GL.
RAT901114ILL. Puslo efns. %.9)/(4r4))(s/oMis OP aus)
4.1
4.0
3.7
49i
6.19
1.3
cO
5.9
1.44
5.0
0.6
3.3
.215
5.95
.1
0.6-
14.6
25.0 2,6.3 45.t
0.4+
0.76
0.78
0.65
0.79
0.79
14.7
6.6
4.9
4.0
214
6.6
5.1
0.243
0.4
243 0.3.411
4.1
3.5
T.
4.6
4.9
-3.7
3.3
6.5
-3.0
4.1
6.6
3.1
3.2
a.3
2.1
6.14S 1.143
-3.9
3.2
+.s
6.0
2.9
4.1
4.1
3.7
3.9
3.4
5.6
F.9
9.0
1.1
3.0
4.0
1.9
+.6
.343
4.4
1.,
32.1
5.7
I.+
14.9*
11.14
9.23 56.54 12.90 19.35
36.10 4.99
30.0 12.3
54.0
0.21
3.4
1.3
6.&0
161IP
10-1
o-'X
6.9
3.5
1A
1.6
6.33
.l,6106XL8.
S.0. AtI/MIlot)
- (4.I)(FTI/M4)
/(+54X415
/+.01
sO-@)
XIO-0),
(o.s9
(O.93flXQs.
)(sPIS.
S'
OGL/
RS
5L'OUP)
OP Mul)
AW
p
Ao/+P
31.15
-74wTABLE V
VARIATION IN RESIDUAL WATER CONTENT OF SILICA GEL
WITH TIME OF REACTIVATION IN AIR AT 178 0 C.
Time of reactivation, hrs.
2
Percent residual water, dry basis
4.85
4.83
4.80
13
4.76
4.72
23
4.80
36
4.83
72
4.75
-75TABLE VI
COMPARISON OF WATER BALANCES CALCULATED FROM
(1) DIFFERENCE IN WEIGHT OF INLET AND OUTLET GEL
and
(2) DIFFERENCE IN HUMIDITY OF INLET AND OUTLET AIR
Run No.
P o u n
a so0
Gravimetric
f
W a t e r
A d s o r b
Gel Wt. Difference
/r)(ft)
Psychrometric
% Error
2
8.6
7.2
3
9.3
5.5
69.0
4
7.9
4.3
84.0
5
9.1
7.0
30.0
6
7.3
4.0
82.5
7
6.9
4.7
Note:
5.6
4.8
19.5
47.0
The percent error above is that for the lbs. of water
adsorbed/(hr)(ft2 ) calculated by the gravimetric method
assuming the values calculated from the gel weight difference to be correct.
-76TABLE VII
COMPARISON OF GRAVIMETRIC AND PSYCHROMETRIC METHODS
OF DETERM1INING THE HUMIDITY OF THE OUTLET AIR
Run No.
A bsolute
Humidity,
Gravimetric
Outlet
Ps ychrometric
-With gel "jiggling" in column2
0.0022
0 .0049
7
0.0017
0.0038
-With no gel "jiggling" in column0.00808
Note:
Units of Absolute Humidity,
of water vapor/lb. b.d. air
0.00810
Outlet Air are lbs.
Air
-77TABLE VIII
PERCENT FINES PRODUCED DURING USE OF SILICA GEL
Silica gel
through #48 screen,
Silica gel
unscreened, gms.
% fines
produced
gms.
63.0
636.6
Note:
This gel was screened before Run No. 7 and used
through Run No. 15 before rescreening.
3
9.9
-78TABLE IX
ESTIMATED HEAT LOSSES FROM COLUMN
Run No.
Heat Losses, Percent
1
5.0
2
4.2
3
4.0
4
4.4
5
4.1
6
4.3
7
4.5
8
4.6
9
6.8
10
5.0
11
4.5
12
4.7,
13
4.5
14
3.7
15
9.5
16
3.7
17
3.8
18
3.9
19
5.8
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T77
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-29E. Location of Original Data
The original data for all runs will be found in
Research Notebook Number 1 on the following pages.
Run No.
Page No.
1
6
2
9-10
3
11-12
4
14-15
5
22-24
6
25-26
7
27-28
8
30-31
9
33
10
34
11
35
12
37
13
39
14
40
15
41
16
43
17
44
18
45
19
46
20
48
21
49
22
50
23
51
-80Run No.
Page No.
24
52
25
53
26
54
27
55
28
56
Miscellaneous original data are found in Research
Notebook Number 1 on the following pgges.
Nature of data
Page No.
Comparison - Psychrometric and
Gravimetric Methods
of Determining Humidity
Resereening Silica Gel
13
29, 42
Calibration Data:
Thermometers
1
Triple Beam Balance
13
Air Orifice Flowmeters
17-21
-81F. Calibration Data:
Plots and Tables
Before any experiments were initiated the thermometers
used for temperature measurement were calibrated at the icepoint.
The data obtained are given in Table X.
Similarly,
the triple beam balance was calibrated against brass -analytical weights during the course of the experiments.
Data
obtained at four points are given in Table XI.
Three different flowmeters were used in Runs 1-19, inclusive.
Calibration data for these flowmeters ranging from
0.5 to 5.0 cubic feet per minute are given in Figure 21.
-82TABLE X
CALIBRATION OF 00-5500. THERMOMETERS USED TO MEASURE THE INLET
AND OUTLET AIR TEMPERATURES (ICE POINT, OCC., USED AS STANDARD)
Thermometer No.
2
Note:
T e m o e r a t u r e, 00.
Trial No.
Trial No. 2
Trial No. 1
0.10
0.10
0.10
0.10
0.10
0.10
0.12
0.15
0.15
3
Calibration trial No. 1 was carried out in a mixture
of ice and water in a towel wrapped beaker.
No.
Trials
2 and 3 were made in a small Dewar flask in a
mixture of ice and water.
-83TABLE XI
CALIBRATION OF TRIPLE BEAM BALANCE USED
IN WEIGHING INLET AND OUTLET GELS
Brass Weight, gms.
Weight by tritole beam balance, gims.
50
50.25
70
70.50
80
80.50
90
90.50
CAWRR
___o
G.
4.
-4-0
2.
o
> 4
o0
______
OP
...
_....
_______-
____
-
-
-
____
0-0000"-
_ _ _
_
_
_
(~
_
_
_
_
_
_
0.
-1
I
A
2" 3
0
6
8
10
10
0
so
4
40
Go
so
m
_
_
_
-84G. Nomenclature
a
square feet of surface area of the silica gel per
cubic foot of tower volume
silica gel concentration in the column, lbs. of gel/
b
(ft3 )
01, 02
water content of the inlet and outlet silica gels,
percent (dry basis)
C
av.
the arithmetic average of the water contents o f the
inlet and outlet silica gels
E
error in the water balance calculated from thermal
data, percent
G
mass rate of air flow, lbs./(hr)(ft 2 )
h
height of column,
ha
heat of adsorption, water vapor on silica gel, Btu/lb.
of
hv
ft.
Tnter vapor adsorbed
latent heat of condensation, Btu/lb. of water vapor
condensed
hy1
heat of wetting of silica gel by liquid water, Btu/lb.
of liquid water adsorbed
H1 , H 2
absolute humidity of inlet and outlet air, lbs. of
water vapor/lb. of b.d. air
H5
absolute humidity of air at saturation, lbs. of water
vapor/lb. of bid. air
KRHa
adsorption coefficient, lbs. of water vapor adsorbed/
(hr) (ft3 ) (unit RH difference)
Kga
adsorption coefficient, lbs. of water vapor adsorbed/
(hr) (ft3 ) (unit % UWC difference)
-85Li, L 2
mass rate of flow of inlet
lbs./(hr)
L
av.1
and outlet silica
gel,
(ft2 )
the arithmetic average of L
and L2
p
partial pressure of water vapor in air, mm Hg
pS
partial pressure of water vapor in air at saturation',
mm Hg
RH1 , RH2 relative humidity of the inlet and outlet air,
percent
RHle
RH2e relative humidity of the air in equilibrium with
silica gel at a given UWC, perdent
L'S s2
humid specific heat of the inlet
and outlet air,
Btu/(lb. of air) (OF.)
sav *
the arithmetic average of s l and S2
S
cross-sectional area of the column,
'ft2
ti, t2 , ts temperature oC thet inletIL andi outle tair.-and,-of-they,
air three inches above the gel inlet point, respectively,
At
temperature increase of the air passing through the
column, or t 2
U
-
tl = At, OF.
the overall coefficient of heat transfer based on the
inside heat transfer area of the column, Btu/(hr)(ft 2 )(OF.)
u
velocity of the gel passing through the column, ft/sec
g
u
superficial velocity of the air, ft/sec
UWc , UWC2 useful water concentration of the silica gel, or
UWC
=
100(C2
-
C 1 )/(100+0), or lbs. of adsorbed water/
lb. of initial wet gel, expressed herein as percent
U0C le, UW
e useful water concentration of the silica gel in
equilibrium with moist air at a given percent relative
humidity, percent
v
specific volume of humid air, ft?/lb. of b.d. air
-86H. Literature Citations
1.
Bartell, F. E. and Almy, E. G.
475 (1932)
2.
Bryant, Silica Gel Dehumidifier, Engineering Bulletin
No. 334 and 336, AlA File. No. 30-F, The Bryanl Heater Co.
3.
Chem. and Met. Report on Conditionibg of Gasea and -Air,
May 1940
4.
Dehler, F. C., Silica Gel Adsorption, reprinted from
Chem. and Met., May 1940
5.
Dehler, F. C., Silica Gel, Its Uses as a Dehydrating
Agent, presented at George Washington University,
Washington, D. C., June 23, 1941, Distributed by the
Davison Chemical Corporation, Silica Gel Department,
Baltimore, Md.
6.
Ewing, D.T. and Bauer, G. T., J. Am. Chem.
59, 1548 (1937)
7.
Fells, H. A.
241 (1925)
8.
Harkins, W. D. and Ewing, D. T.,
Sci.,
, 49 (1920)
9.
Industrial and Commercial Air Conditioning, Committee
Report, 1939, Working Committee of the Committee of
Executives on Air Conditioning, American Gas Association, 420 Lexington Ave., New York City
and Firth, J. B.,
J. Phys. Chem., 36
J. Phys,
Soc.,
Chem.,
29
Proc. Nat. Ac'ad.
10.
-Jones, D. C., J. Phys, Chem., 29,
327 (1925)
11.
Lamb, A. B. and Coolidge, A. S.,
42, 1146 (1920)
12.
Lewis, W. K., Squires, L, and Broughton, G. Industrial
Chemistry of Colloidal and Amorphous Materials, The
MacMillan Co., New York City, 1942.
13.
Miller, E. B., U.S. Patent 1,557,534, March 23, 1926
14.
Miller, E. B., U.S. Patent 1,799,858, April 7, 1931
15.
Miller, E. B., U.S. Patent 1,825, 707, October 6, 1931
16.
Patrick, W. A. and Cohen, L. H., J. Phys. Chem., 41,
437-43 (1937)
J. Am.
Chem.
Soc.,
-8717.
Patrick,. W.A. -and Greider, C. E., J. Phys, Chem., 29,
1035 (1925)
18.
Patrick, W. A. and Grimm, F. V., J. Am.
43, 2144 (1921)
19.
Patrick, W. A. and McGavack, John, J. Am. Chem. Soc.,
42, 946 (1920)
20.
Patrick, W.A. and Opdycke, L. H., J. Phys, Chem., _29,
601 (1925)
21.
Ray, R. C. and Ganguly, P. B., Trans.. Faraday Soc.,
30, 997-1007 (1934)
22.
Walker, W. H., Lewis, W. K., McAdams, W. H. and Gilliland,
E. R., Principles of Chemical Engineering, 3rd Edition,
1937, McGraw-Hill Book Company Inc. New York City, N.Y.
23.
Zigmondy, R.,
Z. anorg. chem., 71,
Chem. Soc.,
356 (1911)