1 N D U S T R I A I, A N D E N G I N E E R IN G C H E M I S T R Y
1642
the assumption could probably be made that seed of tobacco from each area and region, if planted in the same locality, grown, and cured under similar conditions, should give a product rather similar in chemical composition to t h a t of each of the others. Incomplete and inconclusive unpublished work of the w i t e r s indicates that characteristics of an innate nature do esist between Samsun and Sniyrna tobaccos. If such is the case these characteristics probably originated as a result of the selection of natural crosses by the growers during the period, since tobacco was first introduced into Turkey shortly after the discovery of .Imerica. T h e presence of innate characteristics may cast much doubt on the validity of any of the theoretical speculations offered. They do not, however, change the fact t h a t differencesamong thechcmical compositions of these tobaccos exist. Therefore, the practice of the trade in attempting to get tobaccos of each type to blend in making blended products is justified. The data presented here shorv t h a t the tobaccos of tlic Turliish type may vary within wide limits in chemical make-up and t h a t the tobacco from any area may not be of constant chemical composition from year to year. I t also s h o w that the tobaccos of a given main region tend to be dissimilar in chemical composition from those from othcr regions. The similar data obtaiued for the 1937 and 1938 crops, TT-hcn the analyees of all samples of thc specific crops are averaged, indicate t h a t an experienced tobacco blender could maintain a blend of rather constant chemical composition, if a sufficient supply of tobacco from s e v c d crops from many areas of the different regions \vas available. LITERATURE CITED (1) Andcrson, P. J., S ~ ~ a i i h a c lT. i , R., and Stieet, 0 . I:,, (’orin. .1gr. Ecpt. Sta. Btil2. 422, 22 (1939). ( 2 ) Andreadis, T. B. and Toole, E. J., C‘hem. Zrritr., 1934, I, 3141. (3) Ahdreadis,T. B., and Toole E. J., %. C-ntwsuch. Leiiensm.. 68, 431-6 (1934).
Vol. 39, No. 12
(4j Assoc. Officiald g r . Chein.. Methods of .Inalysis, 2nd ed., 1925. ( 5 ) Bailey, C. F., and Petre. A. W., IXD.EX. CHEM.,29, 11 (1937). (6) Darkis, F. R., Dixon, L. F., and Gross. P. hf., Ihid., 27, 1152 (1935).
(7) Darkis, F.
R.,Disor~,L. Y.,ITolf, I:. A . , and Groa5, P. 11..Ibid.,
28, 1214 (193G). ( 8 ) Ilkl., 29, 1030 (1937). (9) l ~ r n l l k e n b u r g IT. , c;., .4ilCU/L (10) G u n e r , K.IT., U. d . Dept.
(1907).
11
Entymoi., 6, 309 (1916:i. Bur. Plant I i i d . B u l l . 102, liY
I..,
r.
(11) Garner, W.IT., S.L)ept. .igr., Tech. 1 3 ~ 1 1 414, . 3 1 (1934) (12) Garner. K.\I-., Baron, C. W., and Bowling, T. D., IXD. Ex(,,. (,‘HEX., 26, 970 (1934). (18) Kadir, Gulteliin, I d ~ i s a r l a rTutun Institusit R a p o r l a r i , S o . 2 . .-lug. 1939. I141 Koscmif, E r o l , Tt~tunlcriinkBlill. 115, 2S-4S (July 1041). (1 I Laureiit, AI., “lleniorial des iiianufactures de L’etat tahnc~-alluiiietes,” J-01. 3 . P a r t 1 (1895). [ici) i r o i l r , I;., A ~ ~ R .97,335 , (1~56).
c.
17) Piatnitzki, S. AI., State Inst. Tobacco Inuest. (U.S.S.K.) Bull. 38 (1027j. (1s) Piatnitxki, l I . , ILid., Bull. 5 1 (1929). j 19) Pyriki, Constatitin, Z . Cnlersuch Lebensm., 73, 199 ( 1 9 3 ) . [1$lA1)Pj-riki, C o n ~ t n n r i n .ICid., . 77, 157 (1039,. ( 2 0 ) llichnids, 11. SI.: C’aiiicgielnst. 11-ash. Pub., 209 (1915). (,21I Siiiii.nov, A I., niicl Izvoakihov, V. P., S t n t e ’ l n s t . Tobacco Inced. (r,7.S.S.R.)Bull. 7 1 (1930). dniucli, A , , Irist. Tohacco Iucest., Krasiiodar LT.S.S.R. BidZ. 33 i
(1!)?7J, (?;-err of metal foil and paper has been employed for drying the tissue. The interrelation of the physical phenomena of moisture sorption and the practical problems involved in its removal by vacuum pumping systems are presented. Drying may be supplanted at high temperatures by what appears to be a decomposition of some paper constituent.
P
APER exposed to room conditions noimally contains a t least 570 water sorbed throughout its structure. The
exact amount to be found on any particular type of paper depends on the relative humidity of the surrounding atmosphere and also on the previous history of the paper sample, the temperature, and the pulp composition (4,5, 14,15, 16, 22, 26, SO, 3 f ) . This moisture exerts a marked effect on the physical properties of papers, although its exact role is not known (1, 2, 3, 6, 9). The serious decrease of electrical insulating quality with increasing moisture content is of importance in the numerous uses of dielectric paper (7, 8, 11, 24, S I ) . Tissue paper (23, 27) used in the manufacture of rolled electrical capacitors was examined in the present investigation.
I:vacuation under heat has been the method used to dry radio parts containing paper. This process presents several problemb, but the two major factors considered are the extent of the drying that wsults from various times of treatment under factory conditions \\ ith varying loads in commercial vacuum tanks, and the extent of damage attending exposure to elevated temperatures. The removal of water from materials with large surface area is usually achieved by lo^ ering the relative humidity or partial pressure of water in the gas surrounding the sorptive material The effectiveness of various reductions in humidity has not been tho~oughlystudied hut will probably depend upon temperature and the partial pressure of water vapor. Inert gas pressure should be of importance only as it affects the rate of drying (11, 13). I n vacuum drying these same considerations hold. The rate oi water removal and the limiting desiccation attained depend upon the temperature and the atmosphere within the system. The vacuum tank atmosphere, in turn, is dependent upon the load, the leaks in the system, and the pump speed. I n order t o obtain data relative t o paper drying and decomposition, sorption isotherms for kraft paper a t the full range of temperatures were studied by successively evacuating and reestablishing equilibrium within a large volume connected to paper samples through a large vacuum stopcock. Details of this
1643
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1947
niethod of estimating both the composition of the sorptive system and t,he partial pressures of water a t the temperatures of testing are presented later. Since the ultimate drying conditions seemed t o present thti most serious drying problem, the sorption isotherms at' ion-er partial pressures were determined most carcfully. I n most esperiments the paper was first held a t room conditions and then subjected t o progressively higher temperatures and lower pressures, so t h a t the highest temperature isotherms of about 150' C. were determined after a history of lower temperature measurements. Furthermore, the equilibria established at, the highel, temperatures occur in the presence of products of decompositioii (it' the paper and, for this reason, are uncertain. T h e nature,
8-
1 7-
1
2
3
4
5
PeRCEnT WATER
Figure 1. Desorption Isotherm of Kraft Capacitor Tissue at 27' C.
extent, and over-all effect of this decomposition have not t m s n established ( 7 , 8, 18, 24, 25, 88). Equilibrium moisture curves for paper at room conditions arc 01 the typical sigmoid shape (5, 17). Figure 1 represents the loivei portion of the desorption isotherm of kraft capacitor tissue. It zhows the amounts of water t o be removed in drying such paper and the limits t o which the paper may be dried under various conditions at room temperature. The portion for lower water contents is of significance in these applications a n d indicates, for euample, t h a t for zero humidity the equilibrium water content will be zero. However, the problem of approaching complete dryness presents considerable difficulty, since the rate of approaching this condition falls off toward zero for any given drying condition as the water content decreases. The drying rate will vanish for water contents having partial pressures of water equal to t h a t of the desiccant used or, in the case of vacuum drying, equal to the system vapor pressure. I n t h e case of paper at room temperature exposed t o fresh calcium chloride, equilibrium would correspond t o more than 1% water. I n order t o obtain electrically insulating paper of low water content, severe drying conditions must be employed (27). MOISTURE SORPTION BY PAPER
In order t o determine the factors influencing both the equilibria and the rate of drying radio capacitors, specimen parts consisting of tightly wound layers of papcr and aluminum foil were placed in large Pyrex test tubes, which were then drawn don-n for sealing to a n all-glass high vacuum system. I n several experiments samples of paper only were placed in t h e sample tubes. After the samples were sealed onto the system, they were usually allowed t o come t o equilibrium with the room atmosphere.
The sample was shut off from the remaining volumes of' the system, Ivhich were evacuated with a diffusion pump. This permitted a n adjustment of the residual air pressure throughout the whole system. Any leaks in the system would be indicated a t this time by a n increasing partial pressure of air. It was usually possible t o conduct a whole series of rate and equilibrium measurements at essentially the same air pressure, in the micron range. By the use of suitable dry ice or liquid air cold traps n i t h intervening vacuum stopcocks, water could be pumped away from thc' paper, measured, and condensed in a reservoir trap. If necessary this accumulated water could be readmitted t o the paper or it could be evacuated and eliminated from the system by direct pumping on the reservoir section. ii few measurements of adsorption equilibria were made by readmitting portions of this accumulated water t o the paper. Figure 2 is a sketch of portions of the vacuum system. Paper sealed in d may be exposed to cold traps in sections B , C, or D, and evacuated through E. The volumes of sections A , B , C, and D are 85 ml., 490 ml., 300 ml., and 1350 nil., respectively. Vacuum measurements (10, 19, 20, 89) in such a system t h a t contains condensable vapors are subject tci very large errors unless special precautions are taken. The partial pressure of air Fyas determined by two suitably trapped LIcLcod gages. These were constructed in different sizes with overlapping ranges and were found to agree when they were used t o measure dry gases. The water partial pressure was estimated from nieasurements of the total pressure. Two gages of overlapping ranges were used in this case also. The first was useful for pressures up to 30 mm. of mercury and consisted of a n inverted hollow floating metal tube such as that described by Germann and Gagos (12). The second consisted of a differential oil manonieter suitable u p t o pressures of about 10 nim. of mercury; when employed against a high vacuum utilizing a cat hetometer, it n a s useful down t o 20 microns pressure. The errors inherent
- .
E -
E
Figure 2.
A
Outline of Vacuum System
in these gages seem t o iange iiom a fractlon of a millimeter of the oil manometer scale reading u p t o about 1 mm. of mercury at the highest pressures with the floating barometer gage. The use of short scale hIcLeod gages cannot be recommended for measuring total piessure, since adsorption may occur on the gage walls and precede condensation so t h a t low readings result. This effect is present mith gages having surface films of any nature but is particularly troublesome with gages cleaned with strong solutions t h a t leave a coating of silica gel on the glass walls. The pressures of the water vapor in the system, together with the volume of the chambers, permitted calculation of the weights
Vol. 39, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
1644
TABLE I. DEHYDRATIOX OF PAPER AT 27 C. Rate of K a t e r V i r h d r a w a l . Cc./.\lin. Water, %
P , hlm.
0.40 n:m. air
4.1 3.4 2.9 2.i 1.9 1.6 1.5 1.3 1.2 1.0 0.7 0.6 0.4 0.2
5.0 3.3 2.2 1.0 0.8 0 6
7100 6800 481)O
0.02
inin.
air
2000 1000
0.5
700
0.4 0.3 0.2 0.1 0.08 0.05 0.02
50U
400 400 400 400 400
4300 2'300 2260 2100 2000 200u 2000 2000
TABLE 11. D B S O R P ~ IPRESSURE ON OF SY~TI.\I Iixirr P ~ P E R \yATbK
TVater,
>'
Temp., C. 52 89 93 82 10% 62 78 102 82 105 122 104 122 131 102 120 145
1.2 1.2 1.2 0.9 0.9 0.8 0.8 0.8 0.35 0.35 0.35 0.1 0.1 0.1 0.07 0.07
0.07
2.1~0.46
2I-
5
2-
t-
P, ,\Im
5
1.2 13 21 8 20 1.2 4.1 17 2.0 6.5 1: 1.4 3.2 5.0 0.35 1.0 3 0
8a
1-
1
1
1
I K 1 1
0.5
1
5
1
PRESSURE
mm.ny
Figure 3. Water Desorption Isotherm Illustrating Freundlich Relation
of water removed. *Lt convenient intervals during nicasurements of the rate of water removal, tlie sample chamber !vas allowed t o come t o equilibrium for the mcasuremcnt of the apparent equilibrium partial pressure of Ivater. After repc~ntcd steps in this process, the quantities of n a t e r rc,muved bixcoine smaller, and the partial pressures diminish also. By this mean? the system pressure and composition werr established; tliia lcft only the determiriatioii of tile quantity of residual i v a t r i ' a n d pa pe r . T h e criteria for dryness of such niat(~riiLlsarc ti(w-sarily arbitrary. It has betln suggested ( 2 4 ) t h t tlilring direct c'vacuatioii thc evolution of gas of Co1i;talit coiiipojitiori indic,atei the completion of drying. Vincent and Yitiions (80)ertr:iyJhted the square root of the equilibrium vapor pressure against t h e weight of n-ater removcil. They statcil that all thc. nioisiuic in t,he pnpcr contributes tu tlie vapor pressure and irlterseetion at zero pres>ure t o represent drync,s.5, of dryrice adopted here is essentially t h a t of \7iiieeiit atid Siiiioii,*, except that i i l s t c ~ d(Jf thP 0.5 pO\\-Cr of the Vapor J>l'v>3IIt~(',t i l e esponent used \vas t h a t found from the Frcuiiillich equutioii. From thc' ineasurenients made at room teiiiperaturc, thc, erporient derived \\-as 0.46 but increased a t higher teiiipc'r:iturc>q. -At th(, conclusioii uf cach cxperimclnt the thoroughly ~ ~ v a c u t i t e d eampk was scaled off under vacuum nnd n-ciplied (Iiuoyaricy correctcd). The \wight of dry paper \vas detcrmiued l)y removing tlic, paper and xvcighing the remainder of the saniplc tube. Having this n-eight permitted the calculation of tlie percentage composition throughout each experiment. Table I li obtained in this n a y for typical esperiiiients at 27" desorption pressure; may be expressed by the equation:
w
sure. Substitution of a liquid air t r a p slightly increased the rate of withdrawal. A fivefold increase in the ra.te of water n-ithdraxal was achieved by decreasing the foreign gas pressure t o one tn-entieth of its former value. The data of Table I are pwsented graphically in Figures 1, 3, and 4.
The data obtained a t elevated trmperatures are listed in Table 11. Thcsc are plotted against a scale of the reciprocal of a l m l u t e temperature in Figure 5 . From the slopes one obtaiiia tin wtiniate of the heat of the desorption reaction (16). Tliix is 14 kilocalories per mole of water for average Xvater content, approximntely the value found in the literature ( I O , 1 6 ) . Ihoivledge of the vapor pressure-compositiuri relations a t t h e tenipcratures employed in the vacuuin drying is important in accounting for the result? of rarious vacuum treatments. \Then large quantities of papcr are to be dried in a vacuum tank, the composition and presbiire of oariuus parts n-ill \Tar>- with their tcniperature. Under temperature differences the various parts \vi11 teiid to liavc different compositions such t h a t their partial pressure of n-ater n-ill be the same. I n addition, the volumes of vapor n-hich tlie vacuum pump must withdraw in o d e r to produce any given change in composition n-ill vary n.ith the tenipt~ratureand the pressure a t which the n-ater vapor is rc111cJJ'ed. Iinoi\-lcdgc oi the water vapor pressure in equilibrium with paper is of tiicrmodynsinic interest as evidence of the state of the sorbet1 witer (3,.5, l O , l / i , 20,21,26). It is of practicalsignifi-
(1)
where w = percentage of water p = pressure, mm. of Hg Table I gives not only the desorption isotherm but also the rates at which it was possible t o withdraw mater from the edge of the paper roll. These values represent the time rate of removal of water vapor when t h e sample was opened t o an adjacent dry ice trap. T h e rate was nearly- constant a t each pres-
Figure 4. Drying Rates for 6.5-Gram Roll of Paper a t 0.4 3Im. Hg Air Pressure (lower curve) and 0.02 RIm. Hg Air Pressure (upper curve)
Figure 5 . Equilibrium Desorption Pressures for hloist Kraft Paper a t Elevated Temperatures
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1947
cance in defining the limits nithin nhich dehydration b y exhaustion may be accomplished. VACUUhI PUMP LOAD 13' SORPTIVE SYSTE3IS
I n ordcr to reduce tlie pressure in a n all-gas system, only a portion of the gas must be removed. I n a sorptive system with condensed vapor prcsent, however, a portion of this condensed vapor must be removed. The amounts of this sorbed water vapor n-hich must be removed and the pressurcs existing nithin the systcm during its removal are related t o the sorption isotherm for the temperature of drying. One may obtain t,he changcs in composition required t o produce given reductions in the pressure of the system from the sorption isotherm. For 100" C. this equation is
w = 0.107~','~
1645
The rate of regain of water by a dehydrated sample unit when opened t o room conditions is shown in Figure 7. When the vacuum-sealed unit weighing 6.5 grams n-as openod, air filled the interstices, and the s1ow gain in w i g h t resultcd from the diffusion of ivater into the layered roll. A sudden inc~reawin weight was noted ivhen the unit was unrolled.
(2 )
so t h a t dw
0.070
&=jp
(3)
whcre w = percentage of water p = pressure, mm. of H g
For estimating t h e changing role played b y the sorbed water a s tlie pressure is reduced, this rate of change of water content with change in pressure may be compared m-ith a volume of vapor of such a size t h a t its rate of change of water content with change in pressure would be the samc. This volume is
where V = volume, liters R = gas constant, liter-mm./mole degree '1' = absolute temperature
* dP
= molal change in water content with pressure
Since it is customary t o state the speed of vacuum pumps in terms of a volume per unit time, this apparcnt volume is a convenient way of measuring the v:iriable load on the pumps. If the load x e r e not variable h u t consisted of 1 pound of water t o be removed a t 0.1 mm. nit>rcuryhy x commercial pump having a, specd of 100 cubic feet per minute, the exhaustion would require about 35 hours. IIoncvcr, with the sorptive system the pressure will fall as the dr?-ing procceds, and thc apparent volume presented to the pump by thc wsirlu,zl \rater n d l vary. For the case under consideration
n here (z = m i g h t of papcr Jf = molccular neiglit of water so t h a t thc apparcnt volume of the sorptive system is
.
This equation and t h a t for pump speed give tlie minimum exhaustion required t o reduce the moisture content of paper tfJ t h a t corresponding t o any givcri pressure a t 100' C. The curves of Figure 6 give t h r apparent volumes calculated by this equation for 1800 grami of paper containing initiallv l.Snc natcr. DIFFUSION AND DECO?vIPOSITION RATES
T h e actual drSing rates will be l o w r than those predicted from these equations. This is because of slowness in diffusion and also the influt of heat ncccssary to maintain uniform tempcraturo a t sites of desorption.
I
150.C. 10
4
i i
I
20
30
PRESSURE mm. Hg
Figure 6. Volumetric Loads Presented to Vacuuni System by Water Present on 1800 Grams of Paper in 25-Liter Tank
The thermal insulation of the sample when held in a vacuum requires t h a t considerable time be taken for the adjustmcnt of test temperatures. Figure 8 illustrates this and several other points of interest in t h e work. A capacitor with 6.43 grams paper n-hich had been reduced t o 0.027, \Tator content \vas heated by a thermal bath a t 144' C. Within 1 hour thc prcssure n-as increasing rapidly. One of the t\vo McLeod gages einploycd gave readings of pressure t h a t exhibited a sharp brc:ik ut 0.063 mm. This corrcsponds closely with the ca1cul:ited condcn~ation point for the condensation of water in t h e McT,eod capi1l:try. The pressures given by the manometer gages (broken lirict) fit smoothly n-ith the first curve. The presence of gaseous dccomposition products is indicated by the increasing values aftcr thc "break" corresponding t o condensation in t!ie lleLcod gage. The second McLeod gage had walls adsorptive to water and measured only the other gases being liberated. In this case decomposition (or rather the. liberation of othcr gases) had been first dctectcd at about 0 . 3 5 x a t c r conteut and 122" C. Further drying had been effected a t lower tt,niperntures without added evidence of decomposition. At higher tcmperatures and as the quantity of water removed bccanie smaller, the presence of the gaseous deconiposition products became rnore noticeable. Upon continued evacuation t h e composition of the gases evolved bccame more constant, and, as estimated from the condensation at low temperatures, using baths of liquid air and dry ice in a mannt:r similar to t h a t of \Iurphy (24), approximated equal portions of carbon dioxide and. carbon monoxide Ivith water and smaller amounts of other gases. In general, as the tempcr:ttiire of such paper is raised, sonic paper constituent undergoes decomposition (8, 18, 24, 25, 28, Sf ). This iJ presumably the cellulose, and gives gaseous decomposition products of carbon dioxide, carbon monoxide, and water, together with small amounts of other gases and condensed products. .didsorption of some of the products on the remaining paper surfaces masks their evolution and makes difficult the direct study of this reaction. T h e complicated chemistry of cellulose does
INDUSTRIAL AND ENGINEERING CHEMISTRY
1646
Vol. 39, No. 12
Measurements of the rate of evolution of these gascs ut impregnation temperatures and a t pressures from 0.001 t o 10.0 mm. mercury exhibit the temperature variation of the rate and are listed in Table 111. The decomposition rate per gram may be reprrsented hy
>---1
".I _
_
20
I
40
80
minu-rEs Figure 8. Illustrating Simultaneous Evolution of Water and Noncondensable Gases at 154" C. and 0.02yc Water Content Tim€
20
40
60
HOURS
Figure i. Water Uptake at Room Conditions by 6.5Gram Roll of Dried Paper Before and .ifter Unrolling
I n K = 33
,
60
where T
K
= =
14710 - __ T
(7)
absolute temperature rate in liter-mm./day
Figure 9 gives these data plottrd on the scale of the reciprocal of abaolute temperature. The activation energy of this procem as given by these results is 29 kilocalories, which is 10 kilocalories lower than the value obtained by X u r p h y (24). . CONCLU SION S
not permit even an estimate of the extent to which hydrocc.llulu~i~ or oxidized celluloses are present and partake in reactions. Th(: heats of hydration of several celluloses mwe determined by Lauer et al. (.%!I), who examined material from many sources and found differences between cellulose and hydrocellulose nhich they relate t o different crystal lattices (14). Kickerson and Habrle ($6) attribute distinctive hygroscopic and hydrolytic behavior t o amorphous components of cellulose. An attempt t o study this decomposition as made by observing the rate of pressure increase in a system having the paper samples under heat. After thorough and repeated evacuation, t.his rate of pressure increase was reproducible a t any temperature. The natures of the reactions giving rise t o these gases and of whatever residues remain in the paper are not knova. A liquid of very low vapor pressure accumulated outside the heated zone. This is eit,her a product of repolymerization of decomposition products or a distillate from the paper, The slowest process in their evolution seems to be one of constant rate a t fixed temperature, although the rate may fall off slightly as the process rraches higher pressures.
Figure 9. Rate of Decomposition of Paper at Elevated Temperatures as Rleasured by Gas Evolution
TABLE
OF GASESFRO31 6.5 GRAXSOF 111. RATEO F EVOLUTIOP; HEATED PAPER
Temp.,
C.
Liter-Mm./Dau
Terng., C.
Liter-hlrn./Dag
The sorption of water by kraft paper and the decomposition of the paper were studied under conditions comparable to those present in impregnation processes. Decomposition accompanies severe drying and at high temperatures supplants the final drying. The interrelation of these processes and the effect of these processes on the electrical insulating qualities of paper are only imperfectly understood. A method of estimating the minimum load presented to a vacuum pump by such sorptive systems is given. LITERATURE CITED (1) .Idalria, D. O., Paper TradeJ., 122, 43-52 (Feb. 14, 1946). (2) -Irgue, G . H., and Maass, O., Can. J . Research. 12, 564-74 (1935). (3) Ibid., 13B, 156-66 (1935). (A! Assaf, -1.G.. HaaJ, I
