Static Sorption Isotherm for
Beta-Soluble Anhydrite and Humid Air STANLEY H. JURY1 AND WILLIAM LICHT, JR. University of Cincinnati, Cincinnati, Ohio
I
N CONNECTION with a study of the adsorption kinetics in fixed beds, the results of which are being published elsewhere, it was necessary to know the adsorption isotherm for the system, P-soluble anhydrite-moist air. Both Hammond and Withrow ( 7 ) and Wheat (16) have reported isotherms. The isotherms do not include the low humidity range which was important in the study of the kinetics mechanism in fixed beds. It was necessary, therefore, t o investigate the matter in detail. The results o,f this investigation are reported herein. The term “static” is used t o imply t h a t there is no gas flow a s such during the course of testing. The desiccant under static test is exposed t o moisture in a closed container. The humidity in the container may be such as t o cause the desiccant t o adsorb water, in which case the test is referred t o as the “in” test. The “out” test is distinguished by the fact that the humidity is such as t o cause desorption of water from the desiccant. Both the in and the out static adsorption isotherms were determined for the p-soluble anhydrite, which appears on the market under the trade name Drierite. The method employed is similar in some respects t o t h a t used by Hammond and Withrow (7).
mainder was successively ground and sized to obtain stocks of 4 to 5 mesh and 42 to 48 mesh. Another stock of 4 to 5-mesh material was screened from the used 3l/2- to 6 m e s h lot. The balance, along with the 10- t o 20-mesh used lot, was reduced to stocks of 10 t o 12 mesh and 20 to 24 mesh. The history, then, of the stocks upon which the present work is based can be summarized as follows: 2 - 2 1 / 2 t 0 441 to 0 312 inch new c 0.441 to 0 312 inch: new 4-6 c 3’/z to 6 mesh, used 4-5 10-12 t 10 to 20 mesh used 10-12 t 31/2 to 6 mesh’, used 20-24 t 10 to 20 mesh used 20-24 t 31/2 to 6 mesh: used 42-48 c 0 441 to 0 312 inch, new
where the arrow indicates the origin of the present stocks. I n accord with the recommendations of Kelley et al. (12) and the Drierite Co. all stocks were regenerated a t 400’ F. for 2 hours. Higher tem eratures and longer regeneration periods are permissible ancftend t o increase the drying efficienc . These conditions are not used normally because they t e n 2 t o reduce the drying capacity of the desiccant. T h e material was spread in I/%- t o 1-inch layers in clean porcelain pans and these were placed in an electric furnace which was controlled t o within 1 2 5 ” F. by a Leeds and Northrup Micromax controller. Each pan was supsorted in the furnace by 11/2-inch angle iron skids t o prevent irect contact between pan and furnace floor. The latter was a n obvious precaution taken t o avoid overheating of lower layers in the bed being regenerated. After regeneration t h e stocks were removed and bottled hot so as t o effect a vacuum packaging. Samples of about 5 t o 10 grams of t h e Drierite stocks were placed in tared ground glass-covered weighing dishes. These were weighed and then uncovered and placed in glass desiccators, which had previously been charged with 1000 grams of sulfuric acid solutions to control the humidity when the glass cover was sealed t o t h e desiccator with ordinary stopcock grease. The large quantity of sulfuric acid employed was sufficient so t h a t changes in concentration during test were so small as t o not change t h e humidity a preciably. All samples were started a t low humidity and rafually transferred t o higher humidity in order t o determine t8e in curve. This procedure was followed b y the successive return t o the lower humidities t o determine the out curve. T h e concentration of the acid solutions was based on the room temperature (72” F.) data of Wilson (1’7) and the International Critical Tables (5, 11). Reports in the literature (7) indicate that 1 week is sufficient time for equilibrium t o be established at a given humidity. This point was checked b y allowing the early samples t o be run over a 2-week period with frequent weighing (I- t o 2-day intervals). I n some cases small amounts of moisture were still being taken up a t the end of 2 weeks and, consequently, subsequent samples were run 3 weeks with less frequent weighing in order t o minimize the disturbing influences on the process taking place in the desiccator. T h e rate of moisture adsor tion at the end of 3 weeks was negligible for the most part. On tRis basis t h e entire procedure consumed t h e better part of 1 year. As a final step in all of t h e above work an absolute dry base weight was obtained by transferring the samples t o tared covered porcelain crucibles which were dried overnight a t 750” t o 800” F. in the same electric furnace previously mentioned. The samples were weighed hot followed by immediately emptying the crucibles wiping clean, and reweighing t o determine loss in weight by the
PREPARATION OF IMATERIALS AND STATIC TEST PROCEDURE
The work of Wheat (16) indicated that the equilibrium moisture content (in curve) of Drierite might be a function of nominal granule size. In fact he investigated three different s i z e s 4 . 4 4 1 t o 0.312 inch, 31/2 to 6 mesh, and 10 t o 20 mesh-and found t h a t on the average the medium size picked up about 1% more water than did the larger and smaller sizes. Hammond and Withrow ( 7 ) , who determined the in curve for what corresponds to 6- t o 8-mesh material, gave data which suggests t h a t the equilibrium charge lies between t h a t of the smaller samples investigated by Wheat. Consequently it was considered advisable t o investigate a range of sizes which for this and other purposes seemed to be well covered by the selection: 2 t o 21/2 mesh, 4 t o 5 mesh, 20 to 24 mesh, and 42 to 48 mesh (Tyler Standard mesh). Actually the Drierite on hand was that left over from the work by Wheat. It consisted of some new 0.441- to 0.312-inch granular Drierite and two lots, 31/2 to 6 mesh and 10 to 20 mesh, which were marked “used,” indicating that they had undergone regeneration one or more times and were to be regenerated again. I n subsequent sizing for the present experiments the new and used lots were never mixed. A preliminary screening of all three lots indicated that sizes marked on the cans meant no more than t o indicate approximate maximum size t o be expected. The size of the actual granules in each lot ranged from the maximum down to a dust, with appreciable fractions belonging t o the lower range. The 2- to 21/2-mesh stock was screened from the new 0.441- to 0.312-inch lot in accordance with standard practice. The re1 Present address, Department of Chemical Engineering, University of Tennessee, Knoxville, Tenn.
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I n an effort to prevent relating this effect to particle size a new sample of 10- to 20-mesh prodTemperature, 65Oto 90' F. uct obtained from the Drierite Co. was tested. A 10- t o 12-mesh fraction was screened from this Water Adsorption, yo by Weight Abs. Dry Basis sample t o make the results most convincing in Out In Out In Out In Out In Out the light of the adsorption isotherm already de0.426 0.341 0.324 0.449 termined for used 10- to 12-mesh (see Figure 3). 0.741 0.615 0.558 0.802 6.12 5.97 3.97 4.27 Regeneration for this fraction was the same as 6.16 6.01 4.09 4.38 6.02 3.962 4.28 5.93 that employed with all previous samples. The 6.3 6.18 4.22 4.51 6.02 sample was run a t 25% relative humidity to mini6.25 4.21 4.51 6.13 6.53 6.4 4.52 4.79 6.28 mize the effect of variation in room temperature. 6.5 4.54 4.82 6.41 7.08 6.89 4.84 5.11 6.76 The water adsorption on the absolute dry basis was 7.74 7.31 7.42 5.04 5.21 5.37 6.53 7.05 7.42 found to be 6.04% by weight. This, of course, com7.83 5,.41 5.74 7.68 pares favorably with the new samples, the data for which are plotted in Figures 1, 2, and 5. It far exceeds corresponding values shown either in Figure 3 or 4. The question as to whether the used samples picked up less water because they were less pure or because they had been repeatedly regenerated was investigated further. For purposes of investigation a new 4- t o 5-mesh sample and a used 20- to 24-mesh sample were selected as subjects. They were annealed 12 days a t 212" F. and then regenerated for the customary 2 hours. In equi1ibriu.m with 357, relative humidity their absolute moisture contents were found to be 6.32% for the 4 to 5 mesh and 4.617, €or the 20 to 24 mesh. In both cases the adsorption was decreased by about O.2ql, compared with previous determinations on the same materials.
TABLEI. EQUILIBRIUM 1vOIsTURE CONTENT Per Cent Relative Humidity, p/pa.lOO 0.0313 0.409 1 2 3.63 5 10 25 35 40 50
65
In 0.439 0.735
6.2 6.42 6.66
7.57 8.1
Vol. 44, No. 3
OF
DRIERITE SAMPLES
:'it
DISCUSSION
4
ib
I REL%YE
I
""QPOITY
I
,PE4PCENT, 5 0
Figure 1. Adsorption Isotherms of New 2- to 21/pMesh Drierite
Throughout the range of about 5 t o 30% relative hbmidity the square-shaped isotherm, except possibly for the knee, is exactly that expected for a typical crystal hydrate. Both above and below the range the crystal certainly resembles a zeolite. !A7ithregard to the lower range this conclusion is in accord with that of Ilelley et al. ( I d ) .
Lower aurve is in curve, u p p e r is out curve
crucible itself. The latter allowed correction for traces of adsorbed moisture on the crucibles, if necessary. Other samples of Drierite were runat isolated humidities tocheck certain points of interest. I n each case the procedure was the same as that described. I n one case, t o determine t h e effect of annealing (4,6,18-16) a new 4- t o 5-mesh sample and a used 20- t o 24-mesh sample from 10 t o 20 mesh were annealed 12 days in the electric furnace a t 212" F., folloxed by t h e usual regeneration. RESULTS OF STATIC TESTS
The static experiments included all of those in which ,9-soluble anhydrite was exposed for some period of time to a vapor laden atmosphere enclosed in a conventional ground glass-grease-sealed desiccator. The major part of this work had as its objective the determination of both the in and out adsorption isotherm for various sizes of stock. A summary of these results is given in Table I. The same data are plotted in Figures 1 to 5. The lower curves are the in curves and the upper are the out curves. All static data were corrected to the absolute dry basis, which means t h a t they were computed based on the dry weight of sample as determined after heating at 700"t o 800" F. for a minimum of 24 hours. Thus it was possible t o compute the water content of the samples after they were regenerated preparatory to running the adsorption isotherm experiments. These water contents on the absolute dry basis were found t o be as follows: 2 t o 2l/, mesh new, 0.282%; 4 to 5 mesh new, 0.1840/,; 10 to 12 mesh used, 0.234%; 20 to 24 mesh used, 0.237%; and 42 to 48 mesh new, 0.512%. Figures 1 t o 5 demonstrate the fact t h a t used material has a definitely lesser tendency to pick up water than new material.
ob
ib
$0
3T!o-
R E L A T I V E HUMIDITY (PER CENT1
Figure 2. Adsorption Isotherms of New 4- to 5-Mesh Drierite Lower curve is in curve, upper is out curve
A comparison of adsorbed water concentrations along the flat portions of Figures 1, 2, and 5 shows a gradual decrease with nominal particle size as follows: 2 to 21/2 mesh, 6.53%; 4 to 5 mesh, 6.4%; and 42 to 48 mesh, 6.25%. The effect is definite even though it is small. The actual reduction in capacity from the largest size to the smallest is 4.37,. Whether the reduction i s due to heating during grinding (and consequently, the formation
INDUSTRIAL AND ENGINEERING CHEMISTRY
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of insoluble anhydrite), a sorting process during screening or some other cannot be decided without further experiments. There is no relationship t o regeneration because the samples were all new. Figures 3 and 4 and the tests of new and annealed samples show t h a t heating during regeneration or for any other purpose causes degeneration of the material as a subsequent desiccation agent. This effect is apparently independent of particle size and in this respect serves only to confuse the major issue. These findings are
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sites of 12CaS04.(6)Hz0are being filled, and according t o the basic theory of Hey these would be completely filled only a t infinite pressure. At about 3Oy0relative humidity the much more volatile or “forbidden” 2 sites start t o fill t o give the 12CaS04. (6 2 ) H z 0 proposed by Bunn. On this basis investigation a t humidities exceeding 650/, should show a maximum in the adsorption isotherm a t about 8.8%.
+
I
2
E
5 5
I* L
53
e I
‘0
10
PO
30
40
?O
RELATIVE HUMIDITY (PER CENT1
60
Figure 3. Adsorption Isotherms of Used 10- to 12-Mesh Drierite
Figure 4. Adsorption Isotherms of Used 20- to 24-Mesh Drierite
Lower curve is in curve, upper is out curve
Lower curve is in curve, upper is out curve
This explanation would appear to be complete were it not for the apparent hysteresis which appears t o be most pronounced for the 42- to 4&mesh sample. Moreover, above 35% relative humidity the out curves tend toward linearity for the two large sizes and the smallest, whereas the in curves show marked curvature which decreases with particle size to linearity for the 42- to 48-mesh sample. The latter fact suggests t h a t a t smaller particle size the gas pressure tends t o behave according t o the ideal gas law. Placing the 2- to 21/~-meshcurve over the 42- t o 48-mesh curve shows t h a t the out curve is parallel, whereas the in curve of the 2 to 21/2 mesh tends to bend over to eventually intersect the 42- t o &mesh curve. From a purely physical chemical point of view the isotherms should be investigated in detail beyond 35% relative humidity. T o obtain small sizes and at the same time eliminate heat effects it may be necessary t o ball mill the material mixed with dry ice, for example. It may be t h a t with more data the in isotherms may all be linear or all curved. The exact nature and extent of the hysteresis loop should be determined. Then, too, the relationship of this entire portion of the plot t o t h a t 7. for the dihydrate should be investigated. P 2 4 After all, according t o the data of B 9 Kelley (19)one would expect dihydrate above about 22% relative humidity. Without further data i t is only possible to suggest t h a t during grinding of large granules to small the larger intercrystallite spaces are destroyed first, because they cause the granule t o be weakest a t these points. As grinding continues RELeTIVE HUMIDITY (PER CENT1 t o produce smaller granules the next smaller size intercrystallite space is deFigure 5. Adsorption Isotherms of stroyed and so on, until eventually at New 42- to 48-Mesh Drierite a sufficiently,small Bize, there remains Lower curve is in curve, upper is out curve
in accord with the opinion of Hougen and Dodge (10)and Kelley et al. ( 1 2 ) . It appears t h a t heating causes formation of insoluble anhydrite nuclei which probably grow with more extensive heating until eventually the entire granule would be consumed. The effect is the same as if nonwater-adsorbing sand particles had been injected into the granule. The effect is even noticeable at temperatures as low as 212” F. as compared with the regeneration temperature of 400’ F. The fact that regenerated new desiccant contains 0.184 to 0,512y0 by weight (absolute dry basis) moisture agrees in general with the report by Kelley that this material contains 0.16 to 0.27% by weight (wet basis) and in some cases even more water. The vertical portion of the isotherm lies between 0.081 and 0.198 mm. of mercury. For large installations the interest of the chemical engineer in the adsorption 8 isotherm for @-soluble anhydrite is generally confined to the lower regions of 7 humidity, because of the comparative cost of Drierite and the fact t h a t it is a 6 good finish drier of low capacity as indi0 cated by the isotherm already presented. Z’ In this regard the knee represents about
-
i’.5Y0 reduction in capacity and one naturally is curious as t o how this might come about. The theory of Damerall (16) seems to give a most plausable explanation. A more general explanation of the complete isotherm is probably to be based and on the combined theory of Hey (8,9) Bunn ( 3 ) ,as distinguished from that of Harkins and Jura (1) and Brunauer, Emmett, and Teller (9)which is primarily concerned with solid surfaces. Up to about 30~orelativehumiditythesix
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for the most part a powder, the particles of which are individual crystallites or groups of them with sufficiently small intercrystallite cracks as to be undetectable under the conditions of a particular experiment. On this basis one would expect a linear rise of pressure as water molecules filled more and more forbidden sites in the crystallites of small granules. The same process would be expected of a larger granule but the effect would be obscured by superimposition of another effect due t o the tendency of intercrystallite pore ~ a l l to s act something like solid surfaces. This explanation leaves the apparent hysteresis to be associated with the properties of the dihydrate. The latter seems plausible in the light of a statement by Kelley ( 1 2 ) that p-hemihydrate “probably does not exist as such in the presence of H,O ( I ) or in the presence of the implied equilibrium pressure of HzO (Q).” LITERATURE CITED
(1) Alexander, J., “Colloid Chemistry,” Vol. VI, pp. 1-76, New York, Reinhold Publishing Corp., 1946. (2) Brunauer, S., “The Adsorption of Gases and Vapors,” 1st ed.,
Princeton, K.J., Plinceton University Press, 1943.
VoI. 44, No. 3
(3) Bunn, C. W., “Chemical Crystallography,” Cambridge, England, Oxford University Press, 1946. (4) Garner and Tanner, J . Chem. Soc., 1930, 47. (5) Greenewalt, IND. ENG.CHEM.,17, 522 (1925). (6) Growther and Coutts, Proc. Rou. SOC.( L o n d o n ) , 106,215 (1924). (7) Hammond, W. A., and Withrow, J . R., IND. ENG.CHEX.,25, 653-9 (1933). (8) Hey, M. H., M i n e d o g . Mag., 24, 99 (1935); 24, 227 (1936). (9) Hey, M. H., PhL-Z.Mall., 22, 492 (1936). (10) Hougen, 0. A., and Dodge, F. Ti’., “The Drying of Gases,” Ann Arbor, Mich., Edward Brothers, Inc., 1947. (11) “International Critical Tables,” Vol. 111, pp. 302-3, New York, McGraw-Hill Book Co., Inc , 1929. (12) Kelley, X. K., Southard, J. C., and Anderson, C. T., U. S.Bur. Mines, Tech. Paper 625 (1841). (13) Kohlschutter and Nitschmann, 2. p h y s z k . Chem., 494 (1931). (14) Rae, J . Chem. S O L ,1916, 109, 1230. (15) Weiser, H. B., “Colloid Symposium-1935,’’ p p ~143-52, Baltimore, htd., Williams 8: Wilkins Go. (1936). (16) Wheat, T. C., M.S. thesis, University of Cincinnati, 1948. (17) Wilson, R. E., IND.Eva. CHEM., 13, 326 (1921). RECEIVED for review August 11, 1950.
ACCEPTP~D September 20, 1081.
Catalytic Vapor-Phase Oxidation
Hydrocarbons J
R . H. BRETTON, SHEN-WU WAN, AND B. F. DODGE Yale University, New Haven, Conn.
A
LTHOCGH the catalj tic vapor-phase oxidation of hydro-
carbons, using air ab the oxidizing agent, is quite attractive economically, relatively fev hl-drocarbons have been oxidized successfully in this manner on a commercial scale. The oxidation of naphthalene and o-xylene to phthalic anhydride, benzene to maleic acid, and ethylene to ethylene oxide may be cited as typical examples of successful industrial applications. Associated with the above process are such problems as heat removal a t relatively high temperature levels, temperature control, and recovery of product from a gas stream diluted by the large excess of air usually required. These problems must be solved if the process is to be successful. Of equal importance in determining whether or not a hydrocarbon mag be processed successfully in the above manner are such chemical factors as the stability of the product toward further oxidation and the thermal stability of both hydrocarbon and product. All of the above factors are influenced by the specific or nonsprcific nature of the catalyst employed. With the view of extending the range of application of the above process, it would be extremely desirable to have quantitative information on the effect of hydrocarbon structure, catalyst type and structure, and operating conditions on the types and amounts of products formed. In addition, it would be desirable to have information on the behavior of the intermediate products themselves when subjected to conditions similar to those used for the hydrocarbons. From such information as that outlined above, a t least for a particular catalyst, it might be possible to formulate a scheme of oxidation from which results for other hydrocai bons can be predicted. With this point of view in mind, it is evident that the information supplied by any experimental investigation must necessarily be complete in regards to both the qualitative
analysis and identification of products, and to the quantitative determination of each of these products in the presence of the others. Very often investigators in this field have circumvented the analytical difficulties by reporting only products occurring in preponderate amounts and promising to be of special interest. In the present investigation, however, efforts were made to identify all constituents in the product, and to develop satisfa,ctory recovery and analytical procedures for quantitative det,ermination. Such procedures are believed to be valuable to anyone engaged in similar research and therefore are described in this paper. The purpose of the present investigation is t,o furnish information such as that’ outlined above for several four-carbon hydrocarbons over a few selected catalysts. The hydrocarbons selected for study were n-butane, 1-butene, %butene, isobutylene, and 1,3-butadiene. Several silver and silver oxide catalysts and one vanadium pentoxide catalyst have been studied in this investigation. Silver and silver oxide catalysts have been successfully used in the oxidation of ethylene t o ethylene oxide and it was thought that such catalysts might prove to be effective in the oxidation of the four-carbon olefins. Vanadium pentoxide is an effective cat’alyst for a large number of oxidation processes and this was the basis for its selection. The major portion of this investigation represents a study of the vapor-phase oxidation of isobutylene, 1-butene, 2-butene, and butadiene over t,he silver, silver oxide, and vanadium pentoxide catalysts. But#aneproved to be quite resistant to oxidation and for this reason only a fev.7 runs werc made with this hydrocarbon. Mainly for the purpose of checking equipment performance and catalyst activity, the oxidation of ethylene over a silver oxide catalyst was also investigated. In addition to the above, a few
