Oct.. 1922
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
*
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Humidity Equilibria of Various Common Substances’** By Robert E. WilsonS and Tyler Buwa RESEARCH LABORATORY O F APPLIEDCHEMISTRY, MASSACHUSETTS INSTlTUTE OF TECHNOLOQY, CAMBRIDQE, MASS.
The following paper is intended to collect for reference, in a single article. data on the humidity equilibria of various common substances. The paper first discusses the oarious types of humidity equilibrium cumes. Curoes are also gioen for various substances arranged in groups of related materials, as follows: natural textilefibers, artificial textile fibers, pulp and paper fibers. foodstuffs, other organic colloids, absorbents, various forms of carbon, and finely dioided inorganic solids. If the relative humidity is kept constant. there is surprisingly little change in the equilibrium moisture content of most substances ooer the ordinary range of indoor temperatures. There is need, howeoer. of further work on the egect of temperature changes in connection with hot air drying.
KNOWLEDGE of humidity equilibria, or the amount of water which is held by various common substances in contact with air of different humidities, is of very considerable importance from a number of different standpoints. The application of such data to problems involving the drying or humidification of materials is obvious. Again, the physical properties of many substances vary greatly, depending upon their precise moisture content. Proper allowances for gain or loss of moisture are also of great importance in buying and selling textiles, etc., where the value per pound is high and it is not desired to pay for unnecessary moisture. I n spite of these and other important applications of humidity equilibria, the data on the subject are, in general, very meager and scattered. The most extensive are undoubtedly those on textiles and paper, but even these are not in satisfactory shape, and there is frequent doubt as to whether the moisture contents are calculated on the dry or moist basis. It has therefore seemed desirable to assemble the most reliable of the available data, to determine experimentally the eauilibria for many other common substances, and to present these data in a single article to which reference can readily be made. It is obvious that in dealing with such materials, most of which are colloidal in nature and not of definite composition, there is no need to seek excessive refinement in the methods, but the values presented are believed to be correct at least within 0.5 per cent moisture for the particular sample tested, except for a few cases where special difficulties are mentioned.
sent the “relative humidity” of the air with which they are in equilibrium. Curve 2, for mixtures of sulfuric acid in water, is a typical vapor pressure curve for a system comprising water and some other liquid which is miscible therewith in all proportions. The very considerable affiity of the acid for the water is clearly shown by the height of the line, especially at very low humidities. Liquids, such as amyl alcohol, which have much less affinity for water and are only partly miscible therewith, give a different type of curve, shown as No. 5 in Fig. 1. Here, even small amounts of water exert a very high vapor pressure,
A
TYPES OF HUMIDITY EQUILIBRIUM CURVES Before proceeding to a detailed consideration of specific substances, it may be well to consider briefly the various types- of humidity equilibrium curves which are found for different systems. The vapor pressure data for various typical systems are shown in Fig. 1. Throughout this article, equilibrium vapor pressures are plotted in per cent of that of pure water at the same temperature (25’ C.) rather than in millimeters, because the former values change but little with temperature and they repre1
Received May 8, 1922.
* Published as Contribution No, 52 from the Research Laboratory Applied Chemistry, Massachusetts Institute of Technology. * Director, Research Laboratory of Applied Chemistry, M. I. T.
of
only 2.2 per cent of dissolved water being in equilibrium with air of 50 per cent relative humidity. The vapor pressure increases somewhat less rapidly up to 9.8 per cent water, which is the limit of solubility of water in isoamyl alcohol. Any additional water then forms increasing amounts of a separate phase containing a small amount of dissolved isoamyl alcohol. The vapor pressure therefore remains constant (vertical line) over a wide range of water contents, until the alcohol phase entirely disappears. Still a different type of curve is given by the system calcium chloride and water. Solutions of calcium chloride behave very much like those of sulfuric acid, except that the lowering of vapor pressure is somewhat greater, partly due to its lower molecular weight. Eventually a point is reached, however, when the water content drops to 53 per cent, where CaC12.6Hz0 separates out, gjving two phases. From there on, the vapor pressure drops in a stepwise manner, a8 different pairs of hydrates come into existence. A single hydrate is stable, without changing compositions, over a certain range of vapor pressures, giving a horizontal line. When the next higher hydrate comes into existence the vapor pressure becomes constant while the relative amounts of the two phases change, giving a vertical line.
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T H E JOURNAL OF I N D U S T R I A L AND ENGINEERING CHEMISTRY
It will be noted that, in moderately or very humid air, calcium chloride will take up a greater proportion of its weight of water than sulfuric acid, and hence is, from that
F’lO.
2-APPARATUS
FOR DETEKMINATION OF
divided solids tend to take up certain definite amounts of water corresponding to the humidity of the air. Typical curves are those for wool and cotton cloth (Nos. 3 and 4 in Fig. 1). Most textile materials and other organic colloids lie beyond these two limits, and finely divided solids in general still lower than this-in an entirely different range from drying agents such as calcium chloride and sulfuric acid. The vapor pressures of these colloidal materials are, of course, rather variable from sample to sample, as contrasted with the definite reproducible equilibria for hydrates and mixtures of pure liquids.
HUMIDITY EQUILIBRIA
standpoint, a better drying agent; but that, for very low humidities, sulfuric acid is markedly superior to calcium chloride. The futility of attempting to get air even 99 per cent dry by the use of calcium chloride is indicated by these curves, although it should be said that they were determined on mixtures of two hydrates and that anhydrous calcium chloride probably adsorbs a very small amount of water a t vapor pressures lower than the equilibrium pressure of the anhydrous salt plus the monohydrate. A curve similar in type, but quite different in location, is No. 6, for sodium sulfate and water. Here, the molecular weight is larger and the vapor pressure lowering per unit weight is therefore less in the dilute solution. The decahydrate begins to separate out of the saturated solution a t a humidity of over 90 per cent, and even the system decahydrateanhydrous salt has an equilibrium pressure above 80 per cent. Here, again, there is probably a slight rounding-off of this curve at the lower end, due to adsorption; but in any case, in spite of its high “capacity,” the absurdity of using sodium sulfate as a drying agent, except for taking care of liquid water or practically saturated air, is apparent from an inspection of this curve. A still different type of curve is given by most of the common colloidal substances with which this article is primarily concerned. Clothing, food, structural materials, and finely
Vol. 14, No. 10
METHOD USEDIN
THE
DETERMINATIONS
The method used in determining the equilibrium water contents of the substances considered in this article is described in detail in another paper by one of the authors,4 and therefore a brief description only will be given here.
The substance is subdivided to present a reasonably large surface area, and placed in a weighed U-tube of about 50-cc. capacity. Air, which has been brought to the desired humidity by bubbling through the sulfuric acid bottles, is passed in a slow stream through the U-tube train, as shown by Fig. 2. Moisture equilibrium is generally reached in from 18 to 96 hrs., except in the case of some materials of a colloidal nature, such as soap and gelatin, which are more difficult of equilibration. Each substance was brought to equilibrium successively in this manner at 15, 30, 50, 70, and 90 per cent relative humidity, after which saturated air was passed through the train for a short time and the same equilibrium points again determined by an approach from the saturated end, i. e., a t 90, 70, 50, 30, and 15 per cent relative humidity. Lastly, the dry weight of the sample was found by passing a stream of air, previously dried with phosphorus pentoxide, through the sample, the U-tube being immersed in a bath a t about 50” C. This speeds up the approach of equilibrium a t the dry end, which is otherwise quite slow, without giving appreciably different results from those obtained by much longer runs a t 25” C. I n general, these dry weights also check up well with the ordinary “oven-dry” weights a t 105” or 110” C., though 4 Robert E. Wilson, “Humidity Control by Means of Sulfuric Acid Solutions with Critical Compilation of Vapor Pressure Data,” THISJOUR18 (1921), 326.
NAL,
Oct., 1922
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEiMISTRY
a few organic colloids give up slightly more water under these conditions.
TABLE I-CALCULATION
OF
EQUILIBRIUM MOISTUREO F CELLULOSE ACETATE SILK
Relative Equilibrium Equilibrium Humidity Wt. of Tube Weight of Per cent and SamDle Sample 15 30
Per cent Average of Excess W,?ter on Up” and over Dry “Down” B%’ Runs Weight 0.97 1.43
2 44 3 56 5 25
50
70 90 70 50 30 15 0
Most of the data presented have been determined in this laboratory, but data from the literature have also been included where they appear to be reasonably reliable. The source of the data for each substance and a brief description of the sample used are given in Table 111. While the data given are believed to be quite accurate for the individual sample tested, most of the materials are colloidal in nature, and the results will undoubtedly vary considerably for different samples of similar material. Indeed, a study of the humidity equilibrium curves is frequently valuable in throwing light on differences in the structure and properties of the same type of material. The method of calculation of equilibrium moisture is best explained, perhaps, by the use of an actual illustrative example; that o€ cellulose acetate silk, for instance, is given in Table I.
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RESULTS The equilibrium moisture for each substance was calculated according to the method of the illustrative example just given. The numerical data recorded in Table I11 are, however, in practically all cases the mean of the two equilibrium determinations; that is, the up run, and down run made on each sample. In Figs. 3 to 10, these data are presented in graphical form, arranged in several groups of related materials. It will be noted that, while it is not possible to predict quantitatively the humidity equilibrium curve of a given substance (except in the case of a few pure compounds), a few generalizations can be made in this respect. These may be summarized according to Table 11.. TABLE I1 PER CENT WATERCONTENTS ON DRYBASIS a t 50 Per cent a t 90 Per cent Relative Relative Humidity Humidity NATURE O F SUBSTANCES I N EACHCLASS 0-4 0-6 Pinely divided inorganic substances, carbon and lampblacks, coke, rubber, cellulose acetate silk 4-8 8-19 .Foodstu5s (except macaroni) paper and pulp fibers, glue, fuller’s earth, linen 8-12 13-26 Artificial and natural textile fibers (except cellulose acetate) macaroni, feathers, wood, soap, catgut, ferhc hydroxide gel 12-28 22-50 Absorbent cotton leather, cigarette tobacco, silica gel, activited charcoal
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T H E J O U R N A L OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 14, No. 10
TABLE 111-EQUILIBRIUM MOISTURE CONTENTS -Per
SUBSTANCE Absorbent cotton Cotton cloth Linen Jute Wool Raw silk Sisal hemp Manila hemp White viscose silk Red viscose silk Nitrocellulose silk Cellulose acetate silk
Kraft paper Bond paper Filter aper Cornel7 wall board White bread Crackers Macaroni Flour Starch Gelatin Leather Catgut Glue Beathers - _ _ ._ . .. Rubber Wood (timber) Soap (Ivory) Tobacco, cigarette
cent of Water on Dry BadPer cent Relative Humidity
DESCRIPTIONon MATERIAL
15 30 Group I-Natural Textile Fibers Sterile absorbent cotton “B & B” 8.9 10.1 2.99 Sheeting 4.56 Table-linen material 2.53 3.60 Average several grades 4.33 6.9 Worsted 6.3 9.0 Cheyennes 5.0 7.1 Strands from rope 4.48 5.6 Strands from rope 4.25 5.6 Group 2-Arti~fcial Textile Fibers Skein of silk 5.6 6.7 Skein of silk 5.3 6.4 Skein of silk 3.95 7.0 Fibrous form 0.97 1.43 Group .?-Paper and Pulp Fibers Pine 4.55 6.3 Fresh unbleached 3.35 5.8 Old &bleached 3.49 5.0 604b. brown kraft 2.50 3.85 “Manifest” bond 2.34 3.71 A. D. L. quantitative paper 2.51 4.21 Single-ply, no adhesive 3.71 5.8 Ward Baking Company “Uneeda” Biscuit Patent Cassava
Group +Foodstuffs 0.99 3.14 2.51 3.32 6.5 8.8 3.55 5.3 2.83 5.4 1.01 2.80 Group 5-Other Organic Colloids 7.0 11.1 6.2 8.6 4.24 5.8
Sole leather Medium grade racquet strings Best grade hide Pillow down Goodyear solid tire Forest Products Laboratory. Average 99-44per cent pure1 Fatima cigarettes
6.0
6.4
50
70
90
20.6 6.7 5.1 10.2 12.2 9.0 8.3 8.5
22.2 9.6 7.0 14.4 17.0 13.3 11.7 11.7
25.8 13.5 10.2 20.2 22.9 19.0 15.1 16.0
9.4 9.0 9.1 2.44
12.9 11.8 11.8 3.56
16.8
7.9 6.3 6.6 5.4 5.1 5.6 7.5
9.5 7.8 8.0 7.0 6.5 7.4 10.3
6.2 4.98 11.7 7.9 7.6 4.92
11.0 8.3 16.2 12.3 8.9 7.6
22.1 19.1
16.0 12.0 7.6 8.1 0.60 9.0 8.4 16.0
20.6 17.3 10.7 10.4 0.74 11.5 18.4 24.5
29.2 21 7 12:5) 12.7 0.99 18.6 23.8 50.1
4.31 12.6 20.2 11.6
22.6 24.0
0.17 0.28 4.65 6.9 3.36 4.62 8.9 10.1 Groub 6-Absorbents of Various Kinds Carbon black Best quality for rubber trade 2:48 3.42 3.85 Made in R. L. A. C. Ferric hydroxide gel 5.2 6.8 8.1 From Davison Chemical Co. . 8.0 12.7 17.3 Silica gel “Wilson” Brand, 5 per cent NaOH 2.0 3.5 6.5 Soda lime Coconut charcoal, highly 9.5 22.8 28.3 Activated charcoal ~. activated with steam Fuller’s earth Florida clay 4.54 7.5 ’ Groub 7-Various Forms o_f Carbon Connellsville coke Domestic coke 0.33 0.60 1.02 Best grade for rubber trade Carbon black 2.48 3.42 3.85 Coconut charcoal, highly steam-activated 9.5 22.8 28.3 Activated charcoal G r o w 8-Finely Divided Inorganic Substances Asbestos fiber Finely divided, organic free 0.22 0.26 0.40 “Celite” Kieselguhr 0.50 0.88 1.40 0.17 Best grade for rubber trade Zinc oxide 0.29 0.36 Florida Kaolin 0.30 0.60 0.92 Glass, wool 0.09 0.09 0.17 1 M T Schloesing Jr Bull soc encour. ind nat 1898. 2 Mi. Atkinson’s r&ull‘s were obt‘ained a t No;the&ern University under the direction of the writers, and
...
institution. 8 Muller and Haussner “Der Herstellung und PrUfung des Papiers,” p. 1642. 4 C. H. Bailey, THISJ~URNAI,, 12 (1920),1102.
It is interesting to note that practically all of the curves have the same general shape, the water content rising comparatively rapidly at low and again at high humidities, and flattening out in the intermediate range between 20 and 60 per cent relative humidity. Under Group 1, natura2 textiEe Jibers (Fig. 3)) it will be noted that wool, jute, and silk have the greatest affinity for moisture, while cotton and especially linen have the least. Absorbent cotton is, however, higher than any of the untreated fibers, presumably as a result of the effect of the severe treatment with alkali, etc., which changes the colloidal properties of the fibers. The results on Group 2 , artijicial textile Jibers, shown in Fig. 4, show a surprisingly close agreement between the equilibrium curves for bhe viscose and nitrocellulose silks and natural silk. Cellulose acetate, however, holds far less moisture than the others, which accords with its markedly different behavior on dyeing, etc. This should make it possible to use the acetate silk and also cellulose acetate films for certain purposes, where the other cellulose products would not be suitable. Group 3, covering pulp and paper Jibers of different types (Fig. 5)) shows a very good concordance between the different
6 6
Fuwa R L. A. C. Sch1o;sio’gl Atkinsoqz R. L. A. C. Storch,.R. I,. A. C. Schloesing Schloesing Fuwa R. L. A. C. Fuwa: R. I,. A. C.
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10.2 10.1 13.0 (at 80%)
...
0.62 2.00 0.41 1.06 0.23
DATA
5.26
30.0
1.48 4.31 30.0
SOURCE OR
32.7
Mtiller and Haussner, Fuwa, R . L. A. C. Abrams, R. L. A. C.
Atkinson, R. I ,. A. C. Bailey4 Atkinson, R. I,. A. C. Phelpss Fuwa, R. . ,I A. C. Atkinson, ,R. L. A. C. Fuwa R L.A.C. Pores; Pioducts Lab. Fuwa, R. L. A. C. Lord, R. L. A. C. Fuwa, R . I,. A. C. Wilson, R . I,. A. C. Fuwa, R . L. A. C. Selvig and Kaplan’ Fuwa, R. L. A. C.
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Fuwa, R. L. A. C.
formed the basis of a Bachelor’s thesis at t h a t
C. Phelps M. I. T. Thesis 1919. W. A. S e k g and B. B. d p l a n , THISJOURNAL, 12 (1920),783.
types of material. The two highest curves are for materials which contain large amounts of lignin (mechanical pulp and wall-board), while the fibers which are primarily pure cellulose check up very well. The kraft and bond papers will, of course, vary, depending upon the amount and nature of sizing and filling materials used. Group 4,foodstufls, shown in Fig. 6, as a class has a fairly high humidity equilibrium, macaroni being the highest, and flour, starch, bread, and crackers agreeing well among themselves. Group 5, other organic colloids, shown in Fig. 7, shows the highest humidity equilibria of any class of substances, cigarette tobacco having the highest of any substance studied, and leather the next. The curve for glue is slightly higher than that for gelatin in Fig. 6. Rubber (solid tire stock) is very much lower than any other material in this group, but the equilibrium water content is nevertheless quite appreciable. Group 6, covering various types of absorbents (Fig. 8)) shows a very interesting series of curves with considerable variation in their general shape. Evidently, on a weight basis, activated charcoal holds a far greater amount of moisture than any other absorbent, with unheated silica gel running second.
THE JOURNAL OF INDUISTRIAL A N D ENGINEERING CHEMISTRY
Oct., 1922
In the case of silica gel and fuller’s earth, the same equilibrium was not readily attained when approached from opposite sides, there being quite a discrepancy between the moisture contents held even after equilibriating for periods as long as 3 or 4 days. This effect is shown clearly in Fig. 9, I
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because of its very low apparent density. By making the comparison on a volume basis, as in Fig. 10, it will be noted that there is a fairly close agreement between the curves for the three principal adsorbents, charcoal, silica gel, and ferric hydroxide gel, though their shape is markedly different.
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which shows the values obtained under the two sets of conditions. It also brings out clearly the effect of heating these types of absorbents even to 150°, which makes a marked and apparently permanent change in the amount of moisture which they hold a t different humidities. In comparing different absorbent materials, it is frequently the efficiency per unit volume rather than per unit weight which determines their comparative value for many uses. Charcoal, for example, shows up well on a weight basis
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It should also be emphasized that the affinity for water is not a true measure of adsorptive efficiency for other types of gases or vapors and that, in general, silica gel shows up less favorably, and charcoal more favorably, on organic solvents, ferric hydroxide gel having intermediate properties. A study of the data for Group 7, various forms of carbon, shown in Fig. 11, indicates the enormous difference in the absorptive capacity of a single material, depending upon its state of dispersion. The average individual would assume that carbon black would expose much more surface than activated charcoal and this may be true; but, actually, either its amount or its effectiveness must be much less than that of an equal weight of activated charcoal. Lampblack and carbon black average about the same, though different samples vary considerably.
