CHARACTERISTICS OF POROUS CELLULOSE ACETATE MEMBRANES FOR T H E SEPARATION OF SOME ORGANIC SUBSTANCES IN AQUEOUS SOLUTION S. S O U R I R A J A N Dicision of Applicd Chemistry, Xahtional Research Council, Ottawa, Canada
The separation and permeability characteristics of the S & S type porous cellulose acetate membranes have been studied for several organic solutes in aqueous solution. No group of related systems has emerged from the experimental separation data. The effects of the chemical nature of the solute, solute concentration, and operating pressure and temperature on the performance of the membranes are illustrated. TUDILS
on the characteristics of porous cellulose acetate
S membranes for the separation of several inorganic salts in
aqueous solution have been reported ( d > 5, 7-9, 1 7 ) . T h e posihle separation of organic solutes in aqueous solution by flo\v under pressure through porous membranes (7) extends the area of application of this technique in chemical process cngincering especiallp in the fields of solute concentration, ivastc recovery: ivater pollution control, and Jvater renovation. 'This paper summarizes the effects of some of the operating variables on the characteristics of the commercially available S 8k S cellulose acetate ultrafine filter membranes (Type UAsuperdense. supplied by Carl Schleicher and Schuell Co.) for the separation of several organic substances in aqueous solution. A fciv dara for the systems sodium chloride-Lvater and hydrazine-\vater are also included for purposes of comparison. Experimental
T h e apparatiis and the experimental procedure \\ere the same as those reported earlier (S). Cnless other\vise stated, the expcriments \\-ere carried out a t the laboratory temperature. A fe\v experiments \\ere also carried out in the range 10' to 30' C. I n the latter case. the cells a n d solutions Ivere placed in a n insulated box ivhich \vas cooled or \\.armed as ncedcd: a n automatic recorder \vas used to record the temperaturc of the solution floiiing through each of the cells to ivhich thermocouples \\.ere attached through Conax fittings. In each cxperiinent. the mole per cent solute removed and the prodiict rate in grams per 7.6 sq. cm. of effective film area \\ere deterinined a t the preset operating conditions. Here the terms "product" and "product rate" refer to the membranepermeated solutions. '1he relative concentrations of the solute in the feed and product solutions were determined by refractive index measurements using a precision Bausch and Lomb rcfracrometcr capable of giving refractive index data Ivithin a n acciiracy of 0.00003. '1-he separation data are accurate ivithin j=lYc in most systems and xt-ithin i2yc in some systems. 'l'he product rate data are good to a n accuracy of &3yGin all cases. Results and Discussion
Relative Separation of Organic Solutes. By varying the temperature and time of hot \vater treatment described earlier (S): membranes rvith different porous structures, capable of giving different separation and product rate data under the
same experimental conditions of operating pressure, feed concentration, and feed rate, can be obtained. Consequently it is necessary to specify the porous structure of any particular film under study in terms of its separation and product rate data for a suitably chosen reference solution system. Using several membranes preshrunk in hot water at different temperatures, the relative separation data for a number of commonly available organic solutes in aqueous solution were determined; they are plotted in Figures 1 and 2 using ethyl alcohol-water as the reference s p t e m . T h e separation technique was successful in all the systems tested, and the product was enriched in ivater content. \Vhen organic solutes of various molecular sizes are involved, more than one mechanism could control the separation process by the technique employed here. T h e solute molecule might be retained on the film surface purely by ultrafiltration-i,e., by virtue of the fact that its molecular size is bigger than the size of the pores on the film surface-or by the negative adsorption of the solute at the membrane-aqueous solution interface, together Lvith the capillary flow of the preferentially sorbed interfacial fluid through the membrane pores (7). I n the latter case: the pores on the membrane surface need not be smaller, and could be bigger, than the molecules involved. \\-bile both the mechanisms could be operative simultaneously to different cxtents rvith reference to any given membranesolution system, the preferential sorption-capillary flow mechanism is probably the predominant one here, since the average size of the pores on the membrane surface is very likely to be several times bigger than the size of the molecules involved in the solution systems studied in this Lvork. T h e concept of related solution systems and the experimental criteria of such systems have been discussed (8, 9 ) . No group of related systems has emerged from the experimental separation data for the aqueous organic s o h tions studied in this \vork. Figures 1 and 2 may, however, provide a useful basis for the approximate prediction of the relative separation characteristics of any similar S gL S porous cellulose acetate film for the several organic solutes in aqueous solution under the specified experimental conditions. T h e available data cover the range of 8.3 to 3370 EtOH separation for the system 0 . 5 M ethyl alcohol-water; the corresponding S a C l separation data cover the range 34 to 947, for the system 0 . 5 X sodium chloride-water. T h e followVOL. 4
NO. 3
SEPTEMBER
1965
201
+
I t
~
t
A
F E E D SOLUTION MOLALITY ' 0 5M
B
B
F E E D RATE 3 0 c c / m i n u t e O P E R A T I N G PRESSURE 1500 pS.i.g. SYSTEM
+
t [ACETALDEHYDE
A
0
[ A C E T O N E - ti201
0
[ M E T H Y L E T H Y L KETONE-HzO]
0
[ACETIC A C I D - HzO]
+
+
1
I
*
40 -
-Ha01
[PROPIONIC A C I D - H 2 0 ] [BUTYRIC A C I D - H a 0 1 A
?
e
40-
I
+ B
FEED SOLUTiON MOLPLITY:C,5M FEED R A T E ~ 3 0 c c / r n i n u l e OPERATING PRESSURE: 1500p.t.i 9. SYSTEM:
/' 0
f
- H~OJ
[UREA-HzO]
+
0
0
0
-
[HYDRAZINE
.
+
0 Cn-PrOH-H20]
0
[i$o-PrOH~H2O]
20
Io
0 [n-BuOH-HzO!
iI t
w j 8 ~ ~ - ~ ~ ~ ~ - ~ 2 ~ : 0 [Sec-BuOH-H20:
m
[tert-BuOH-HzO]
A [ETHYLENE GLYCOL-HzO] A IGLYCEROL-HzO] t [ N a C 1. HZO] 0
-
I 1
0
I
20 30 MOLE % E t O H REMOVED- SYSTEM [EtOH-HZO!
Figure 1. Relative separation of some organic solutes in aqueous solutions
Table 1. Separation and Permeability Characteristics of an S & S Porous Cellulose Acetate Membrane for Some Organic Solutes in Aqueous Solution
Film No. 147 Operating pressure. 1500 p.s.i.g. Feed solution molality. 0.5M .Mole
Operating Temp., Solute Sodium chloride Ethyl alcohol Ethyl alcohol Isopropyl alcohol n-Propyl alcohol Ethyl alcohol tert-Butyl alcohol sec-Butyl alcohol Ethyl alcohol Isobutyl alcohol n-Butyl alcohol n-Propyl alcohol Ethyl alcohol Acetone Methyl ethyl ketone Ethyl alcohol Acetic acid Urea Ethyl alcohol Hydrazine Ethyl alcohol Acetaldehyde Propionic acid Ethyl alcohol Butyric acid Ethyl alcohol
202
l&EC
yo
Solute
Product
PlVP at
RemoLed in
Rate,
20" C.,
C.
Product
C./Hr
G,/Hr.
24.5 23.5 25 0 26 0 24.5 26.0 25.5 25,5 25.5 25.5 25.5 25.5 25.5 25.5 26.0 27.0 25.0 26.0 23.0 26.0 26.0 24.5 25,5 24.5 20.5 24.0
75 0 18.0 18.4 37 4 20.6 18.4 63 6 33.8 18.4 30.4 8.9 23.1 18.4 16.7 17.7 20.4 10.5 23.5 20.0 24.7 20.0 28.4 13.9 18.4 13.1 20.4
5.67 6.28 6.54 5,75 5.35 6.18 4.91 4.16 5.88 3.38 2.48 3.67 4.52 3.97 2.82 3.89 3.58 4.00 3,85 4.01 3.91 3.77 2.87 3.67 0.99 2.35
7.35 7.32 7.03 6.56 6,66 6.09 6.42 5.74 5.94 5.98 5.00 4.51 4.62 4.41 4.44 3.70 3.96 3.73 3.84 3.76 3.89 3.93 3.89 3.89 3,85 2.59
PRODUCT RESEARCH A N D DEVELOPMENT
I
1
20 30 MOLE % E I O H R E M O V E D - S Y S T E M [EtOH-HzO] IO
Figure 2. Relative separation of some organic solutes in aqueous solutions
ing orders of separation exist consistently under the experimental conditions specified in Figures 1 and 2 : n-PrOH > E t O H > n-BuOH Iso-PrOH > n-PrOH tert-BuOH > sec-BuOH > iso-BuOH > n-BuOH Glycerol > ethylene glycol > n-PrOH Acetaldehyde > E t O H > acetic acid Propionic acid > acetic acid NaCl > any of the above organic solutes T h e above orders may be related to the possible differences in the thickness of the interfacial pure water layers, depending on the chemical nature and molecular structure of organic solutes. From the experimental data plotted in Figures 1 and 2, it does not seem likely that any simple criteria of related systems could emerge for organic solutes in aqueous solution, similar to the criteria of ionic valencies for the relative separation of inorganic solutes in aqueous solutions (8:9 ) . Effect of Organic Solutes on Membrane Permeability. T h e permeability of the cellulose acetate membrane is affected by its contact with aqueous organic solutions. This is illustrated by a typical set of data given in Table I, in which the experiments are listed in the order in which they were performed. T h e experiment with the system 0 . 5 M ethyl alcoholwater was repeated frequently to serve as a reference. T h e permeability of the film for pure Jvater (P\\'P) \vas determined before each experiment. T h e PM'P of the film decreased from 7.35 to 2.5 grams per hour in the course of 25 experiments involving several aqueous organic solutions. This decrease in permeability, however, did not affect the separation characteristics of the film. This is evident from the fact that the extent of E t O H separation remained remarkably constant
FEED RATE:30cc/rninute OPERATING P R E S S U R E i 7 5 0 p.s.i.g. SYSTEM : FILM NO.
90
I
[SUCROSE-HzO]
II.
[GLYCEROL- HzO]
6
[ E T H Y L E N E GLYCOL-HzO]
6
r
4
-
8
7
\-
Jp [iso - P r O H HzO] Y [EiOH-HzO]
IO
PL
2
1
[ACETIC A C I D - H 2 0 ]
-
'PIL [N a C I H20]
129
y 6 0
m
N
c a 2 5
PT
\ 0
W
5 4 LT
I-
u
IC
2 3 0 a LT
2o
t
P 2
PIL
I@'
-1
0
Figure 3.
0.25.
0.50
0.75 .1.00 SOLUTE M O L A L I T Y
1.25
1.50
0
0.25
0,?5 1.00 SOLUTION M O L A L I T Y
0.50
1.25
1.50
Effect of feed concentration on separation and permeability characteristics of membranes for some organic systems
within the experimental error of i27, applicable for this system. T h e preferential sorption-capillary flow mechanism is governed by a surface phenomenon (7, 77). For a given membrane-solution system, it is only the size and distribution of the pores or1 the surface of the film in contact with the solution that determine the degree of separation obtainable under a given set of operating conditions; the bulk structure of the film underneath this surface layer affects only the permeability characteristic of the film! but not its separation characteristic. Studying the effect of film thickness on the separation and permeability characteristics of high flo~v porous cellulose acetate membranes, Loeb and Sourirajan postulated (5) that the top fe\v molecular layers of the film contained the very small pores needed for separation, and the remainder of the film was a spongy mass offering relatively little resistance to fluid flow. This postulate ivas confirmed by Riley, Gardner, and Merten ( 6 ) in electron micrograph studies; they found ( 6 ) that the Loeb-Sourirajan film consisted of a thin dense porous surface layer (thickness 0.25 micron), the remainder of the film (total thickness 100 microns) being a spongy mass having a pore size on the order of 0.1 micron. It is reasonable to assume a similar porous structure for the S & S cellulose acetate films used in this \cork. I t is probable that the various aqueous organic solutions used did not affect the dense surface structure of the film, b u t did affect its spongy bulk structure, thereby changing its permeability characteristic but not its separation characteristic. Effect of Solute Concentration. Figure 3 illustrates the effect of feed Concentration on six different organic solutes in aqueous solution on the separation and product rate charac-
teristics of cellulose acetate membranes. T h e extent of solute separation decreases, but only slo~cly,with increase in feed concentration, while the corresponding product rate decreases more rapidly. Only a part of the latter change could be attributed to the effect of the organic solute on the membrane permeability discussed above. According to Ambard and Trautmann (/), the nonelectrolyte molecules in aqueous solution do not mutually dehydrate each other and their volumes in the hydrated state remain constant at all concentrations; consequently? the solute concentration has no effect o n the filtrability of a nonelectrolyte through an ultrafilter membrane. l h e chemical nature of the membrane surface has no controlling influence in the ultrafiltration mechanism. O n the basis of the preferential sorption-capillary flow mechanism, however, the separation and product rate characteristics of a membrane depend on the extent of the negative adsorption of the solute a t the membrane-solution interface, osmotic pressure of the feed solution, and the floxv properties of the fluids involved, all of \vhich factors may change on varying the feed Concentration. A general theory of mass transport applicable to this separation process has not yet been worked out, b u t Levich has shown (3) that the parameter do,ldc (where o is the surface tension and c is the concentration of the surface active agent) is an important variable affecting mass transfer across interfaces especially in the viscous flow regime. Effect of O p e r a t i n g Pressure. Figure 4 illustrates the effect of operating pressure on the separation and flow characteristics of the cellulose acetate membranes for the system iso-PrOH-HZO. T h e data plotted are typical of those obtained for several of the systems studied in this work. T h e mole VOL.
4 NO. 3
SEPTEMBER 1965
203
SYSTEM: [iso-PrOH-H20]
I
F E E D SOLUTION M O L A L I T Y :
0.5M
F E E D R A T E : 30 c c / r n i n u i e
FILM
060-
W
>
-
z $50 I
-
0, 4-40 VI 0
-
$30-
9 0
z
-
20
I
I 1 500 1000 O P E R A T I N G P R E S S U R E p.s.i.g.
0
1500
5 DATA
FOR FILM 161
PURE WATER
4
.
/'
L
r
CT;;;;
d 3 W
+
4
z2
-I U.
I
0
500
1000
1500
OPERATING PRESSURE p.s.i.g.
Figure 4. E f f t c : of operating pressure on separation and permeability characteristics of membranes for the system iso-PrOH-HsO
per cent iso-PrOH removed increased with increase in operating pressure in the range 500 to 1500 p.s.i.g. T h e Clms used in this work Irere initially subjected to a pure water pressure of 17C0 p.s.i.g. for about 2 hours. For such a film, an increase in operating pressure in the range 500 to 1500 p.s.i.g. may not affect the dense surface structure of the film, b u t will render the spongy bulk structure more dense, thereby reducing the permeability of the film. T h e curvature obtained for the pure water permeability line in Figure 4 can be explained entirely on this basis. T h e extent of the above curvature is different for different films. If the product is treated as a mixture of (1) the feed solution and (2) the preferentially sorbed pure \rater, the effect of pressure on the separation and permeability characteristics of the film depends on the relative extents to Lvhich the flo\v of (1) and (2) are individually affected by the change in the operating pressure and the bulk structure of the film. T h e thickness of the interfacial pure \rater la)-er may also change with pressure (7). Figure 5 illustrates the fact that the effect of operating pressure on the separation and
permeability characteristics of the membranes is different for the different organic systems tested. Even though the mole per cent solute removed increases \vith increase in operating pressure for several systems, the opposite effect is observed in some cases. T h e evidence obtained by Debye and Cleland (2) for both positive and negative deviations from Poiseuille's law for liquids flowing in pores a few times greater than molecular dimensions is particularly significant. T h e experimental data reported in this paper point to the need for detailed studies on the fluid flolr phenomenon through microporous media of the type involved in this separation technique. Effect of Operating Temperature. T h e effect of operating temperature, in the range 10' to 30' C., on the separation and permeability characteristics of the membrane for the systems EtOH-H20, n-PrOH-H?O, ethylene glycol-water, glycerolwater, and hydrazine-water, is illustrated in Figure 6 . I n all the above cases, the extent of solute separation decreases and the product rate increases Ivith increase in operating temperature. T h e magnitudes of these changes, hoirever, are different for different systems. A change in the operating temperature changes the densities and viscosities of both the feed solution and the preferentially sorbed pure \vater, and may also change the osmotic pressure of the feed solution and the thickness of the preferentially sorbed interfacial pure Xvater layer. Consequently the relative permeability of pure \rater with reference to that of the feed solution changes with operating temperature. Figure 6 indicates that for each solution system studied, the separation and product rate data for different temperatures are related. This is illustrated in Figure 6 for the system glycerol-water. I t seems possible to use the experimentally determined characteristic separation and product rate curves to predict quantitatively the effect of operating temperature on the performance of any membrane of the same type. Other Applications. Experimental data in Table I1 illustrate the applicability of the porous cellulose acetate membranes for the concentration of a solution of azeotropie composition, natural maple sap, and industrial lignin solutions. A separate rample of the film \vas used for each experiment.
Conclusions
T h e experimental data presented, together with those reported earlier (4:5, 7-77), establish the technical feasibility of developing this process for the industrial separation of a wide variety of solutes in aqueous or nonaqueous solutions using porous cellulose acetate membranes. T h e separation and permeability Characteristics reported are those for only the S 8i S type UA ultrafine superdense cellulose acetate mem-
Separation of Solutes from Propionic Acid-Water, Maple Sap, and lignin Solutions by Flow under Pressure through Porous S 8 S Cellulose Acetate Membranes Concn. of Solute in Feed Operating W t . 5; Solute Product Solution, Feed Rate, Pressure, Remoued in Rate, w t . c/o Cc. j M i n . P.S. I.G. Product G.jHr. Feed Solution Propionic acid-water (azeotropic composition) 18.7 .. 525 65.0 2.7 Natural maple sap 3.lb 30 1000 100 6.0 Table II.
Brown lignin solution from Lignosol Chemical co. Black lignin solution from Dryden Paper Co. a
204
Experiments carried out under static condition.
10.5 20.1
* Equivalent sucrose solution.
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
..
..
1000 1000
88.5 76.5
3.6 2.2
100
I
* - *
/
.-4
z9-
0
-ij. a83
-ff I
v, v, W
/*
pp
&
/FEEDSOLUTION M O L A L I T Y : 0.5M
p"
FEED R A T E : 3 0 c c / m i n u t e ~ SYSTEMS
In
2
50-
I W
5 -
[iso-BuOH-HpO]
I)
-1
[sec-BuOH-HpO]
30-
+ [tert-BuOH-H20]
(j
20-
+
G
$.
@ [ACETALDEHYDE-H20]
W -I
2
0
[n-BuOH-HzO]
40-
I-
0
a
0 [n-PrOH-HpO] 0 [is0 - P r OH -1i~0-j
0 E W
z ta 6 W
A [EiOH-HpO]
l
7-
[ A C E T O N E -HpO]
[r
4 [ACETICACID-H2q
,.% V 0
eo
[GLYCEROL-H~C~
$
[ETHYLENE G L Y C O L - ~ 2 0 1 2
I
I
1
I
I
1
I
I
IO
20
30
40
50
60
70
80
I 90
I 100
1
t
0 I 2 3 4 5
M O L E % SOLUTE REMOVED AT 750 p.s.i.g.
Figure 5.
PRODUCT R A T E G./ hr AT 20'C OPERATING PRESSURE 750 p.s.i.g.
Effect of operating pressure on separation and permeability characteristics of membranes for some organic systems F E E D SOLUTION MOLALITY 1.0M F E E 0 RATE, 30cc/minute OPERATING PRESSURE ;I500 p,s.i.g. SYSTEM
T E M P E R A T U R E IO'C
0
[EtOH-H20]
-
30'C
0
+ o
[n- P r OH H20]
-
[ i s o - P r OH HpO]
.to
[ E T H Y L E N E GLYCOL-HpO]
8 c-0-
[ G L Y C E R O L - HpO]
[H.Y D R A Z I NE - H 201
A
A
0 SYSTEM [GLY CER 0 L- He01
p
I 2
4
I
6
I
1
8
1
0
PRODUCT RATE G./ hr AT 20°C Figure 6. Effect of operating temperature on separation and permeability characteristics of membranes for some organic systems VOL. 4
NO, 3
SEPTEMBER
1965
205
branes. Other types of cellulose acetate membranes, made by different methods and having different surface and permeability characteristics, may be expected to behave similarly, b u t not identically. A study of the separation and permeability characteristics of different types of membranes using different solution systems may make a significant contribution to the understanding of the controlling parameters of this membrane separation process and the development of quality control techniques in the process of casting superior membranes for a wide variety of industrial applications.
Acknowledgment
T h e author is grateful to I. E. Puddington and G. L. Osberg for their encouragement of this project; to L V . S. Peterson, A. E. McIlhinney, and Teil H. Scheel for their help in building the equipment; and to Lucien Pageau and A. G. Baxter for their valuable assistance in the progress of these investigations.
literature Cited (1) Ambard, L., Trautmann, S., “Ultrafiltration,” p. 23, Charles
C Thomas, Springfield, Ill., 1960. (2) Debye, P., Cleland, R. J., J . A p p l . Phys. 30, 843 (1959). (3) Levich, V. G., “Physicochemical Hydrodynamics,” p. 390, Prentice-Hall, Englewood Cliffs, N. J., 1962. (4) Loeb, S., Sourirajan, S., Advan. Chem. Ser., No. 38, 117 (1963). (5) Loeb, S., Sourirajan, S., “Sea Water Demineralization by Means of a Semipermeable Membrane,” p. 22, Department of Engineering, University of California, Los Angeles, Rept. 60-60 (1961). (6) Riley, R., Gardner, J. O., Merten, U., Science 143, 801 (1964). (7) Sourirajan, S., IKD.ENG.CHEM.FUNDAMENTALS 2, 51 (1963). (8) Ibtd., 3, 206 (1964). (9) Sourirajan, S., J . .4ppI. Chem. (London) 14, 506 (1964). (10) Sourirajan, S., Nature 203, No. 4952, 1348 (1964). (11) Yuster, S. T., Sourirajan, S., Bernstein, K., “Sea Water Demineralization bv the Surface-Skimming Process.” Deuartment of Engineeridg, University of Califgrnia, Los Angeles, Rept. 58-26 (1958). RECEIVED for review January 18, 1965 ACCEPTED June 7, 1965 Issued as N. R. C. No. 8609.
EVAPORATION SUPPRESSION B Y MONO= LAYERS ON AQUEOUS SALINE SOLUTIONS F O R E S T A. CHEEVES,’ RUSSELL G. D R E S S L E R , AND W I L L I A M C .
M c G A V O C K
Chemistry Dppartment, Trinity L’niiersity, S a n Antonio, Tex.
Water evaporation rate tests, with and without monomolecular fatty alcohol film covering, were conducted using laboratory-scale, precisely developed equipment. The results of tests on NaCI, CaC12, AIC13, and Alz(SO4)a aqueous solutions of 0 to 50 weight strength showed almost identical behavior, showing an evaporation maximum, with film coverage, at about 3.5y0concentration. Evaporation savings are sufficient to justify application of monomolecular films to inland brackish or saline water bodies as a means of water conservation. Water bodies of 5 to 20% salinity have potential of about 20% saving of normal evaporation loss, compared with 20% to 50% potential for water bodies of 0 to 5% salinity.
7 0
interest in water conservation has prompted many of the evaporation-suppressant possibilities of monomolecular films of hydrophylic substances on surfaces of water. Workers have investigated the action of monomolecular films on fresh water in the laboratory a n d on outdoor reservoirs and have observed characteristics of optimum film compositions, film pressures, \vind and other meteorological effects, etc. ( 7 , 3, 5, 8,7 7 , 72, 7 4 75). Although many such investigations have been made on fresh waters, the authors could find no references to study of possible effects on brackish or saline \raters. A practical process for treating large reservoirs of fresh water Recently, certain has been described in detail (4-6). questions have been asked. Would the presence of the ions from dissolved salts interfere with the formation of a continuous monomolecular film on a \rater surface? Can brackish or saline water bodies be treated effectively by methods proposed for fresh water reservoirs? T h e present investigation was designed to answer these questions. A mixed film formed from a n octadecanol-hexadecanol mixture was selected for use: since its evaporation-suppressant properties had long been established ( 7 , 3, 5, 8). I t was a commercial product consisting of 80% octadecanol (C18), 8% hexadecanol (Cle), 8% eicosanol ( C ~ O2% ) , tetradecanol URRENT
C studies
Present address, Chemistry Department, University of Florida, Gainesville, Fla. 206
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
(C14),and 2% docosanol ( ( 2 2 2 ) . This particular mixture has been used as a comparison standard over past years by the authors and others, and normally exhibits a 50 to 527’ water evaporation saving when used o n fresh water in laboratorycontrolled tests. Experimental
T h e tests were conducted in battery jars 9 inches in diameter, representing a 0.158-sq. meter circular surface area. Four such jars, partially submerged in a constant-temperature water bath, were operated, side by side, which allowed duplicate tests and duplicate controls to be run simultaneously. T h e
Table 1.
Data and Results of Typical laboratory Test (Distilled water, pH 7 ) N o Film W i t h Film Water Water Hook gage loss, Hook gage loss, reading, inches reading, inches Hour X Hour inches X IOF3 inches n 1.847 0 0 1.846 0 14 1 1 839 7 1 1 833 15 2 1 833 6 2 1 818 3 1 804 14 3 1 826 7 14 4 1 818 8 4 1 790 57 28 Totals M’ater savings (calculated on basis of hook gage readings), 52y0
