I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
July, 194s
before (Table IV). There appears to be a relation between this effect and a decrease in adsorptive power for alcohol (as measwed in vapor phase). This suggests that some carbons in their original form (i.e., before grinding) adsorb alcohol so strongly as to form a blanket through which it is difficult for dye molecules to penetrate and find the room to occupy active centers on which they otherwiae would be adsorbed. In conclusion we quote a comment of Robinson (S),made in connection with work on the vitamin Bt complex: “At the present time, our knowledge is rather like a partially complekd jigsaw puzzle, in which the final picture is still invisible, though parts are beginning to assume recognizable shapes. One day, prasumably, all the pieces will fit together and it will be possible to see the complete picture, though one can only guess at it at the moment.”
. . .. .
649
ACKNOWLEDGMENT
The authors express appreciation to George B. Hughey and Daniel 0. Adams, West Virginia Pulp & Paper Company, for the preparation of the electron photomicrographs. LITERATURE CITED
*
(1) Alexander, Jerome, “Colloid Chemistry”, Vol. I, p. 675, New York, Chemical Catalog Co., 1926. (2) Freundlich, H., “Colloid and Capiltary Chemistry”, pp. 186-7, London, Methuen & Co., 1926. (3) Robinson, F. A,, Chemistry & Industry, Not 45, 386-9 (1944). (4) Water Purification Div., J . Am. Water Worke Aasoo., 30, No. 7, 1169 (1938). PRE~ENTED before the Eleventh Annual Chemical Eniineering Sympoeium (Adsorption and Ion Exchange), held under the auspices of the Division of Industrial and Engineering Chemistry, AVSRICANCHEMICAL SOCIETY. at Columbia University, New York, N. Y.
WATER ADSORPTION MEASUREMENTS ON SILICA GEL Rty4K. THE DAVlSON CHEMICAL CORPORATION, BALTIMORE 3, MD.
DSORFTION measurements for water on silica gel have been made, by both static and dynamic methods, over a range of relative humidities in the neighborhood of room temperature (6,8). Over the limited range of temperature the sdaorption a t a given P / P o value appeared to be virtually independent of temperature. The present work was undertaken to obtain data over a wider range of temperatures; and it was hoped that 8ome simple relatioa might be found to hold, at least approximately, between temperature and pressure for a given ratio of water to gel. As it has been considered that the presence of permanent gases t the cause of adsorption-desorption hysteresis (7,8, II), it was decided t o make measurements by the static method in the absence of air, using all-glass apparatus with no stopcocks exposed to the evacuated system (Figure 1). The sample was oontdned in bulb A, provided with a thermoFouple well. The dead space above the sample was small, the volume between the top of the sample and the mercury surface in the connecting tube being 6-10oJo of the (apparent) volume occupied by the gel. T w o tungsten contacts were sealed into the tube connecting A with the rest of the apparatus, so that when mercury completed an electric circuit between them, its surface waa in a reproducible position. The sample tube was surrounded by a glass jacket, with asbestos packed over the top and into the bottom; the jscket, in turn, was surrounded by a tube furnace, which could be regulated to give the desired temperature. During a run a slow atream of air was led downward through the jacket to equalize the temperature. Pregeuras were measured on a closed-end manometer, 23, provided with a scale divided into millimeters. Upper l i t of the scale was about 900 mm. The manometer was 80 arranged that
A
I n the absence of air, pressure-temperature measuremenb for silica gel-water have been made, up to about 900 mm. pressure, with 1 to 30% added water. The data give a straight-line relation on a
the space above the mercury could be freed of gas by raising the mercury and sweeping the gas through capillary trap at the top. A modified McLeod gage, C, was used both for measurement of low pressures of air or water vapor and for low concentrations of air in water vapor (9.The comparison tube was closed, and the relative volumes were such that if water vapor was present in e x m of about 20% of its vapor pressure, condensation occurred both in the gage and in the comparison tube when the mercury was raised to the reference marks; accordingly, the pressure of water vapor canceled out. The reading of pressure of permanent gas was taken just as if water vapor were absent. Unit D provided measurement ofathe quantities of water transferred to and from the sample. By cooling, water could be condenaed in either of two calibrated tubes for measurement; and in cum a portion of water was being removed, it-might then be diatilled into one of the small bulbs and sealed off, for a gravimetric check. The unit providing air-free water is shown in Figure 2 (I, IO). It was essentially a fractionating column; permanent gases accumulated a t the top and were removed by opening the unit periodically t o vacuum. The water in the reservoir was circulated by a small flame beneath the double-wall side tube. As the water in the side tube w m e d up, increasing vapor pressure forced it up the outlet tube; it could not return t o the reservoir through the lower connection because of the floating spherical check valve. It h a l l y blew over, spreading in a thin film on the wall of the reservoir; thereupon a fresh portion entered through the check valve, and the cycle was repeated. Passage of cool tap water through the condenser at the top ensured the continuous flow of water vapor up the column.
Cor chart. In theabsence of air, no hysteresis is detected for changes of temperature or of composition. In the presence of 10 mm. partial pressure of air, no hystereais i s detected for changes of temperature.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 37, No. 7
cooled while open to this measured portion of water. After it had been adsorbed, mercury was brought to and kept at the reference contact, and pressure readings were taken at various temperatures. As the dead space between the sample and the mercury surface was small, the graph obtained may be regarded as an isostere. Successive measured portions of water were added in the same way, to obtain isosteres for various water contents. Water could also be removed from the sample; it was collected in a measuring tube and then distilled into one of the small bulbs and sealed off for gravimetric determination. At the completion of tests on a given sample, dry air was admitted and the sample tube was cut from the apparatus to permit determination of the final water content of the Figure 1. All-Glarr Apparatus for Measuring Adsorption sample; from this and from the measurements of auantiThe various units were connected to the manifold through Yties of water added and subtracted, the ratio of water to sample traps with floating spherical check valves (Figure 3). By suitable corresponding to the various isosteres was computed. Values thus manipulation of the stopcocks, the mercury was brought to and obtained could be compared with those calculated from the temheld at t,he desired position. perature and pressure of activation at the beginning of the series. Temperatures were taken by thermocouple, which was calibrated against a National Bureau of Standards certified thermomMETHOD OF OPERATION eter. Additional check was obtained by taking pressure and A weighed sample of silica gel, of known water content, was temperature readings with liquid water in the sample bulb; introduced into the sample tube and the latter was sealed. temperatures from vapor pressure of water checked thermocouple Pumping was then commenced and continued until the partial indication within 1" C. pressure of air was low. As a general rule, in pumping either the At the time these experiments were made, equipment for the sample or the water unit, direct communication was not made precise automatic control of temperature was not available, and between the unit and the pump; rather, the manifold and gage manual control was used. Temperature variation probably were opened alternately to the pump and to the unit. Air was represents the major source of error in the data; low thermal conremoved from the sample by alternate adsorption, and desorption ductivity of the sample was a complication as the thermocouple at elevated temperature, of water, and required considerable was at the center and did not immediately reflect temperature time. I n the earlier stages the desorbed water was condensed in changes of the environment. Technique finally adopted was to one of the measuring tubes, the vapor phase being pumped off bring pressure to a chosen value by manipulation of heating and whenever the partial pressure ot air built up enough to interfere to hold it there until drift of thermocouple reading apparently with dist$illation. Later, when air removal was more nearly ceased. complete, the water was distilled directly into the water unit; I n some of the accompanying data, pressures with ascending most of the air remained above the top plate and was transferred and descending temperatures do not agree. At first sight this to the evacuated gage for measurement almost completely by a might be taken as evidence of hysteresis; but as the higher pressingle expansion. sures are observed with rising temperatures and hysteresis would During the course of air removal, it was observed that when produce the opposite effect, the explanation must be sought elsewater was removed from the sample by progressive increase of where. Incomplete attainment of thermal equilibrium, in them temperature, the last portions at the highest temperatures carried cases, appears a plausible cause for the disagreement; with rising the highest concentrations of air; the first fraction might show temperature and temperature gradient from surrounding3 to virtually none. This is not in accord with the view that adsorbed thermocouple, the thermocouple at the center of the gel would gases should be displaced from the silica gel by the preferential show a reading lower than the average temperature of the sample. adsorption of water. Pressure measurements are subject to correction for vapor When the air had been removed to the extent that at no stage pressure and for lower density of mercury in the manometer leg did its partial pressure amount to more than a few thousandths of connected to the sample when it is at elevated temperature. a millimeter, the sample was brought to the chosen activation These corrections were not applied; they are opposite in sign and, temperature and held there for several hours while in communicain most instances, probably of no greater magnitude than varia-tion with the water unit, the pressure being noted. This defined tions caused by temperature fluctuations. Data obtained are. the initial state of the sample. An appropriate quantity of water presented in Table I. was tken condensed in a measuring tube, and the sample was
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1948
Table
T O
d.
c
94.5 111 149 167 181 207 221 232 P
40.5 57 62 71 81 89 100.5 121.5 125 132.5 127 119 117 116 114 111
105.5 101 96 86
P.
Mm.Hg 1% HIO4.0 19.3 67 134.3 213 353 467 650 862
157 HzOf5.4 37.8 50.3 84
128 184 802 612 697 a37 731 538 509 491 468 396 306 271 220 139
PIP0 0.0064 0.0126 0.0193 0.0243 0.0277 0.0322 0.0347 0.0366 0.0396
T 69 97 111 126 138 149 157 162 170
--
P
PlPO
-3% HnO10.5 48 92.5 165 243.5 346.5 465 560 687
20% HIO20.3 29.5 56 48.4 64 74.8 73 120 83 180 94.5 284 407 103 528 110 575 111 613 113 809 122 905 125 126 895 772 122 886 100 41 97
26 49 73 93 109 119
I.
6.6
81.7 115 269 530 735
ISOSTERES
0.0469 0.0704 0.0831 0.0832 0.0850 0.0998 0.1082 0.1149 0.1158 0.348 0.371 0.391 0.416 0.461 0.449 0.456 0.482 0.491 0.517 0.516 0.610 0.519 0.499 0.487 0.441 0.262 0.860 0.482 0.457 0.510 0.509
Water Adsorption Measurements
T 2 % 114 149 168 191
P 60
Fi ure 2. Unit for Prod i n g Air-Free Water
PIP0 Rz0-
218.5 887 713
r
29 29 39 47 54 60 67 73 84
01
94.5 95 103 108 1 IO 118 116.5 103 100
13.1 18.3 23.3 37.9 56.6 77.6 111 151 236 316 353 367 514 637 647 869 834 468 416
0.0489 0.0630 0.0884
0.0741
-25% 0.436 0.444 0.445 0.476 0.501 0.520 0.542 0.568 0.566 0.579 0.568 0.580 0.608 0.634 0.601 0.622 0.625 0.554 0.548
-T
P
-5% Ha07.8 14.3 24 36.8 84 54 94 86 106.6 140 117 216 128 334 138 447 146 587 154.5 756 50 59 67.5 75.5
H:O 45 59 71 81 86 100 110
P/Po
0.0865 0.0990 0.1148 0.1248 0.1310 0.1387 0.1618 0.1595 0.1752 0.1744 0.1832 0.1906
32.5 70.7 132 207 263 427 630
0.462 0.496 0.541 0.660 0.582 0.562 0.580
12.9 28.7 39.8 58 68 111 170 276 434 662
-30% 0.575 0.609 0.639 0.660 0.700 0.677 0.726 0.717 0.710 0.706
c -
24 37 42 49 61 62 70 82 94 106
-
T -10% 53 60 67 68.5 75 86 94.6 104 115 132 137 143 139 138 131 124
P
P/Po
Ht0-
18 28 41.6 45.4 04.2 108.4 148 230 346 602 699 869 751 725 570 455
0.168 0.187 0.203 0.207 0.222 0.240 0.239 0.263 0.273 0.280 0.281 0.294 0.285 0.283 0.273 0.270
The same data are presented in another form (Figure 5 ) , as isosteres on a graph of relative humidity us. temperature. This plot gives somewhat greater separation of the isosteres. It is very sensitive to errors in temperature readings. Over the range of measurements, the relation appears to be linear; but linearity must obviously fail at sufficiently high or sufficiently low temperatures and pressures. I t may be speculated that the approach to the 0 and 1 values of P / P o would be asymptotic. The data are typical of those obtained with this apparatus. The concentrations of water given are subject to some uncertainty; accidental admission of air to the sample on one or two occasions necessitated reactivation, and it was found late in the investigation that apparently the activation procedure was not sufficiently prolonged to bring the water content to the same value in all cases. Most of the data presented here are selfconsistent in that they are from the same series of measurements without reactivation, and the differences in water content are known from direct measurement. In some of the runs after reactivation, the water content may have been different by as much as 2% from the supposed value. I s all cases the data for given isostem gave straight lines on the Cox chart, but the straight lines were not always coincident with others for supposedly the same water content. For this reason some of the later data are omitted, although the P-T values are probably better from improved AlR technique and give smoother graphs. This unfortunate lack of precise definition of VACUUM water content, however, does not in any way alter the general pattern of converging straight lines on the Cox chart. The point of convergence is approximately 3.5 X 106 mm., 1000° C. Silica gel of another type was tested in the same way and gave similar results; spacing between the isosteres and the point of convergence were slightly different Figure 3. Y-Tra with Sphere from those shown here. ice1 Floating Qheck Valve
Plotted on a Cox chart (Figure 4), a graph of log P 08. modified 1/T (a, 3, 4), the points lie reasonably close to straight lines; greatest deviations are at the lower pressures, where pressure readings are proportionately less accurate. A similar graph is shown elsewhere (6),but with no information as to source of data or experimental details.
”-.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Jo
Eo0
Vol. 37, No. 7
IS3
fQ0
z
D
r, ‘C Figure 5.
Figure 4.
Cox Chart
Per cent water is amount added to activdtrd (525’ F.) gel, based on weinht of activatrd eel.
The utility of the Cox chart lies in the possibility of locating an isostere with only two experimental points or with only one if the point of convergence is known. Considerable attention has been given to the phenomenon of hysteresis in adsorption. Commonly, however, the isothermb have been involved; and it should be pointed out that the following observations involving temperature changes are not directly applicable to isotherms followed in the usual way. No consistent trend was detected between readings taken with ascending and those taken with descending temperatures. Furthermore, after a series of isosteres had been run with successive additions of water, the same quantities were successively removed and check runs were made at 30, 25, and 20% water; the second set of isosteres was coincident with the first set. Points from both sets are plotted together on the isosteres. With 6% and again with 30% added water, air was admitted to the extent of about 10 mm. partial pressure, and P-T measurements were taken. In both cases data for ascending and descending temperatures were coincident; at the higher temperatures, where the 10 mm. of air pressure was a small fraction of the total pressure, the data checked those taken in the absence of air. Thus in this work no evidence of hysteresis in the absence of air was found; and the presence of 10 mm. partial pressure of air did not cause it, in changes of pressure with temperature. Aside from questions of precision and accuracy of the data here submitted, an explicit statement should be made as to the range of applicability. Silica gel is not a substance of absolutely fixed
lsorterer of Relative Humidity
VI.
Temperature
composition and properties; rattier, by suitable changes in methods of preparation, corresponding variations over an appreciable range can be produced in such properties as adsorption characteristics and apparent density. Accordingly, the above data, or adsorption isotherms obtainable from them by cross plotting, are representative only of silica gel of the specific type employed; and the same is necessarily true of any pressuretemperature-composition data on such systems. It is, then, only the type of relationship (such ai3 the converging straight-line pattern on the Cox chart) that possesses any degree of generality. ACKNOWLEDGMENT
Thanks are due Earl K. Seybert for the preparation of drawinga. LITERATURE CITED
(1) Bruun, J. H., IND. ENO.CHEM.,ANAL.ED., 1,212 (1929). (2) Calingaert, G., and Davis, D . S..
[email protected].,17, 1287 (1935). (3) Cox, E. R., Ibid., 15, 592 (1923). (4) Davis, D. S., Ibid., 17, 735 (1925). (6) Davison Chemical Corp., Research Dept., unpublished data. (6) Hougen, 0. A., and Watson, K. M.. “Chemical Process Principles”, Part 1, pp. 159, 160. New York, John Riley & Sons, 1943. (7) MPGavack, J. M., Jr., and Patrick, W . 1., .I. Am. Chem. SOC., 42, 946 (1920). (8) Patrick, W. A., Colloid Syrnposic~mMonograph, 7, 192 (1930). (9) Taylor, R . K., J. Am. Chem. Soc., 50. 2937 (1928). (10) Ibid., 52, 3576 (1930). (11) Zsigmondy, R., 2.anorg. allyern (’hew.. 71, 356 (1911). before the Eleventh hnuP.1 Chemical Engineering Syrnposiuni (Adsorption and Ion Exchange) held under the auspices of the Division of Industrial and Engineering Chemistry, AMERICAN CHEMICAL SOCIETY, at Columbia University, New York. N. Y.
PREeENTED
