Studies in the Development of Dakota Lignite VII. Effect of Temperature and Pressure on Sorption of Wate-r Vapor by Lignite MOSESGORDON, IRVIN LAVINE,AND L. C. HARRINGTON School of Mines, University of Xorth Dakota, Grand Forks, V . D. The sorption process for Dakota lignite has EXPERIXIENTAL NEIHOD~ R E V I O U S publications from this d e p a r t m e n t been followed at 50" C. Hysteresis is present at The sorption of a vapor by a (9-4) have Shown that 50" C., but the hysteresis area is smaller than at solid adsorbent may be studied the moisture in lignite possesses either in the absence Or presence 40" and 20" C. Hysteresis is practically elimic e r t a i n abnormal p r o p e r t i e s of s o m e f o r e i g n gas such as which may be attributed to the nated with lignite that is processed at elelsated air. If carried out in the prescolloidal character of this fuel. temperatures with saturated steam. This is ence of air, the studies fo1ioIT7 These studies have i n d i c a t e d attributed to a setting of the hydrogel. either (1) a dynamic or streanithat the moisture in lignite is Lignite can be dried successfully by the method ing m e t h o d , or (2) a static or to the physical of controlled humidity, providing the temperature d e s i c c a t 0 r method. Lavine structure of this material and and Gauger (3) adopted the Of the drying medium is suficiently high to modify further that the colloidal strucd e s i c c a t o r m e t h o d for the the structure of fhe material. At [OW temperature of lignite can be modified Tyork with lignite primarily beby subjecting the fuel t o certures this method has been found to yield an cause this method was rapid tain treatments with saturated inferiorproduct. and gave r e s u l t s t h a t mere s t e a m a t e l e v a t e d temperaapplicable to i n d u s t r i a l use. tures. Seborg and Stamm (5) reached This paper deals with the effect of temperature and pressure a similar conclusion in adopting a streaming method for on the sorption Processes (dehydration +hydration) for the studying the sorption of water vapor by paper-making masystem lignite-water vapor. I n the earlier report it was shown terials. Experiments now being carried on with lignite by that an increase in temperature from 20" to 40" c.causes a shift the present authors a t this laboratory indicate that the of the sorption CUWS to the region oflo~+'ermoisture content- streaming and static methods yield results that agree closely. This work has been extended to 50" C. for freshly mined lig- It is reasonable to assume that a steady state is attained in nite, and these results, together with similar results for steam- either method. dried lignite, are presented here. The sorption studies with the steam-dried samples were made a t 20" C.
P
USED STUDIES AT 50" C. The samples were received in sealed mason jars from the Truax-Traer Coal Company, Kincaid, h-. D., and from the hliller Coal Company, Burlington, N. D. The analyses of these samples on the as-received basis are as follows: SOURCES O F S.4MPLEs
BURLIXGTON
KINCAID Moisture Volatile matter Fixed carbon Ash Sulfur B. t. u per pound
%
%
36 2 26 3 31 6 5 9 0 2 6982
36 5 24 9 31 6 7.0 0.2 6584
STUDIESAT 20" C. WITH STEAM-DRIED LIGNITE. The samples were obtained from the investigation of Lavine, Gauger, and Mann (4)in which Dakota lignite was dried with saturated steam a t elevated pressures. These samples were air-dried immediately after being processed and were then pulverized and placed in well-stoppered bottles until needed. Data for the samples chosen for the Sorption exPeriments are as follows: Sample
16
19
17
7
Mine Lehigh Velva Velva Velva Original moisture content, yo 41 21 36 69 34 86 33 49 Moisture content, air-driedbasla, % 17 11 13 88 13.48 13 72 Moisture content a t i o 98 i o 88 9.75 9.52 time of work, % Steam pressure used, atm. 5 o 7 o 9 o 10 5
26 32 Garrison Velva 34 35
34 Telva
36 73 35 00
10 00
8 56
9 28
io
68
7 56
8 95
13
o
13 0
15 0
LIGNITEOVER CONFIGURE 1. RATEOF DRYING CENTRATED
SULFURIC ACID
The procedure used in the present investigation has been described by Lavine and Gauger (3). hlention should be made that the cycle, dehydrationhydration, was followed for all samples studied in the vork described in this paper. RATEO F LOSS O F hIOISTURE FROM L I G ~ I TAT E 50" c. Lavine and Gauger (3) found that the time required to attain equilibrium a t 20" C. by the desiccator method was
928
I N D U S T R I A L A N D E N G I N E E K I N G C H E 41 I S T R Y
August, 1932
929
any influence on the nature of the sorption process due to the air-drying might be noted. A portion of this freshly mined lignite was steam-dried, and a sample of the product was then air-dried (referred to in tables as sample 34 dry) simultaneously with the sample of freshly mined material. The sorption studies (dehydration +hydration) with these materials were conducted a t 20" C. by the method des c r i b e d . The results for two series VAPOR PRESSURE OF ?*IOISTURE1 s LIGNITEAT 50°C. of experiments are The results of the experiments showing the effect of dehydra- given in Tables I1 tion and subsequent hydration on the vaDor Dressure a t 50" C. and 111. S o m e of of the moisture- in two samples t h e s e r e s u l t s are of Dakota l i g n i t e a r e given in plotted in Figure 3. Table I. The average results are 4 consideration plotted in Figure 2. of the curves shown A consideration of F i g u r e 2 in Figure 3 reveals shows that the vapor pressure de- an interesting phecreases as the moisture content nomenon-name1 y, is reduced and also that hysteresis that hysteresis has is present a t this higher tempera- b e e n p r a c t i c a l l y ture. When t h e s e r e s u l t s are eliminated i n t h e compared with the results of the case of steam-dried earlier study a t lower tempera- lignite. The accutures, it is found that the charac- racy of the desiccateristics of the sorption curves are tor method does not essentially similar. However, the warrant the stateregion of hysteresis is decidedly ment that hysteresis smaller a t 50" C., and this indi- is e l i m i n a t e d encates that the increase in tem- t i r e l y , since some perature has altered the physical d i s c r e p a n c y w a s FIGURE2. S O R P T I O N structure of the lignite. Further found in the regions CURVE FOR FRESHLY M I N E D L I G N I T EA T reference to this phenomenon will of very low moisbe made later in this paper. 50" c. ture contents (Tables I1 and 111). EQUILIBRIUM AT 50" C. TABLEI. VAPORPRESSURE-MOISTURE However, it is the MOISTURE, DRYBasis o p i n i o n of t h e RELATIVE -DEHYDR.&TION---HYDRATIOXa u t h o r s that this DESICCATOR HUMIDITY Kincaid Miller Av. Kincald Miller Av. % % % % % % % ._ discrepancy is due 3 1 . 2 0 4 4 . 5 1 30.58 31.82 40.75 1 100.0 4g.-27 mainly to the 27.60 27.56 37.60 27.64 39.46 35.74 95.8 21.47 21.25 21.69 25.48 25.76 26.05 85.0 method rather than 18.88 18.43 19.34 23.20 23.43 23.31 80.0 17.27 17.08 17.46 19.39 20,97 20.18 74.1 to t h e m a t e r i a l . 15.55 15.65 15.44 17.08 16.97 17.02 67.3 The lower portions FIGURE3. SORPTION CURVES FOR LIG12.20 12.23 12.17 12.67 12.79 12.91 48.4 9.43 9.49 9.46 9.45 9.65 9.26 37.2 s9 NITE DRIEDWITH SATURATED STEAM of these curves are 6 . 1 0 7 . 3 9 6 . 7 5 7 . 2 4 7 . 4 7 28.3 7.02 AT VARIOUSPRESSURES 5,83 4.85 5.34 6.02 6.03 6.01 19.7 being studied a t this 10 3.13 3.15 3.18 3.24 3.21 11 7.2 3.19 u n i v e r s i t y by the 1.65 1.54 1.44 1.74 1.63 2.3 1.53 12 vacuum method, since the latter allows a greater degree of accuracy. For all practical purposes, the present study indiT' ~ P O RPRESSURE OF ~ I O I S T UIN RE STEIU-DRIED cates a complete reversibility. LIGKITE The sorption curves for lignite processed a t pressures of 11ention has been made previously that the samples for i . 0 , 9.0, 10.5, and 15.0 atmospheres are found to be practically this study were obtained from the work of Lavine, Gauger, identical. This indicates that the effect of pressure as reand hIann (4;. These investigators found that lignite dried lated t o the sorption process is constant through this range. with saturated steam a t elevated pressures ret,ains its lump form very well. A consideration of their data led them to EFFECTOF AIR-DRYING O N XATUREOF SORPTIOS conclude that steam-drying produces a material change in PROCESS the colloidal structure of the lignite. Thus it was found The effect of a lowered moisture content on the nature of that a sample of steam-dried lignite will dry to a much lon-er moisture content than will a corresponding sample of freshly the sorption process can be seen from Figure 4 in which are mined lignite rl-hen both are subjected to identical air-drying plotted sorption data a t 20" C. for (1) a sample of freshly conditions. This phenomenon was of sufficient interest mined T'elva lignite, ( 2 ) a sample of air-dried T'elva lignite, to warrant further study. It was decided, therefore, to and (3) a sample of air-dried processed Velva lignite. The determine the nature of the dehydration and hydration proc- data for the freshly mined sample were obtained from the work of Lavine and Gauger ( 3 ) . Emphasis is again laid on the esses for the system processed lignite-water vapor. Data concerning the steam-processed samples after air- fact that samples 2 and 3, as above, were air-dried simuldrying have been given above. These samples were studied taneously. A consideration of Figure 4 shows very clearly that airsimultaneously with a sample of freshly mined T'elva lignite that was first air-dried to a moisture content of 15.02 per drying does not alter materially the nature of the sorption cent (referred t o in tables as sample 34 wet) in order that process. Hysteresis is very marked in the case of the air-
about 40 days. A t a temperature of 50" C. it was found by the present authors that the time was shortened to approximately 8 days, owing probably to the increased rate of diffusion of the moisture. Figure 1 represents graphically the rate of loss of moisture a t 20" and 50" C. from a sample of freshly mined lignite of 8&100 mesh when placed over concentrated sulfuric acid.
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930
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 24, No. 8
TABLE11. SORPTION DATA FOR STEAM-DRIED LIGXITE,SERIES AT 20' HCMIDITY RELATIVE
Sample 26 Bb
c.
Aa
% 28.26 19.32 17.87 15.32 13.57 11.89 10.63 8.41 6.13
% 98.0 84.0 76.0 67.0 55.4 45.0 33.2 19.2 11.3
% 31.08" 19.97 17.94 15.25 13.46 11.66 9.65 7.31 5.25
Sampie 32 A B
% 28.94 19.91 18.26 15.80 13.71 11.80 10.50 8.33 6 13
% 31.08C 20.16 18.28 15.88 13.66 11.84 9.76 7.34 5.29
. MOISTURE,DRYBASIS Sample 34 dry 4 B
%
70 27.47 19.08 17.64 15.15 13.54 11.96 10.79 8.52 6.23
31.11C 19.96 17.81 15.22 13.52 11.78 9.90 7.29 5.34
A
Average
B
%
% 28.25 19.43 17.92 15.42 13.61 11.88 10.64 8.42 6.16
31.09C 20.03 18.01 15.42 13.55 11.76 9.81 7.31 5.29
Sample 34 n e t A B
% 36.25 27.06 25.79 18.56 18.29 15.22 11.93 8.55 6.59
% 36.05C 22.50 20.51 14.99 14.86 13.20 10.52 7.54 5,99
DATA C O S C E R N I N G COALS
Mine Processing pressure, atm. Moisture a t end of processing, % Moisture of samples used in sorption expts., % a A , dehydration equilibrium moisture. b B , hydration equilibrium moisture. c Relative humidity, 98.5%
Garrison 13 0
Veiva 13.0
15.59
10.68
Velva
16.95
15.0 13.10
7.56
8.95
Velva Unprocesaed
.....
16.02
TABLE111. SORPTIOX DATAFOR STEAM-DRIED LIGNITE,SERIESB MOISTURE, DRYBASIB Sampie 17 A B
c
RELATIVE HUMIDITP A T 20'
Sample 16 Aa Bb
c.
%
%
%
98.5 92.0 85.0 76.0 67.0 52.0 45.0 33.0 22.0 11.0
31.60 26.61 22.24 17.82 16.87 15.06 14.20 12.60 9.15 6.85
27:ig 22.71 18.54 16.86 13.70 13.32 10.42 7.88 5.82
Sample 19 A B
% 28.40 24.14 20.09 17.51 13.68 13.57 13.07 11.28 8.00 6.28
%
%
%
24:i3 20.72 17.39 13.68 12.44 12.23 9.34 7.40 5.36
29.26 24.39 22.17 18.56 15.01 13.07 12.42 11.14 8.45 6.39
24:45 20.83 17.61 15.42 12.35 12.20 9.42 7.78 5.45
B %
A
% 28.03 23.60 19.46 16.86 14.69 12.83 12.16 10.71 8.88 5.90
23:Qi
20.00 17.08 14.80 12.03 11.76 9.31 7.80 5.20
Sample 34 A B
% 28.70 24.14 20.18 17.00 14.38 12.28 11.44 10.01 7.72 5.73
% 2i:k 20.44 17.08 14.65 11.87 11.49 9.12 7.35 5.01
DATA C O N C E R S I N G COALS
Mine Processing pressure atm. Moisture a t end of processing, % Moisture of samples used in sorption expts., % a A , dehydration equilibrium moisture. b B , hydration equilibrium moisture.
Velva
30.52
7.0 22.72
Velva 9.0 19.21
10.5 21.14
Velva 15.0 13.10
10.98
10.88
9.75
9.52
8.95
Lehigh 5.0
dried sample. A direct comparison cannot be made, since these samples were not obtained simultaneously, although they were obtained from the same mine. It is evident, however, that the air-drying of the steam-processed sample cannot account for the elimination of hysteresis with this material.
EFFECTOF TEMPERATURE ON SORPTIOX PROCESS The effect of temperature is seen from Figure 5 in which are plotted sorption data for lignite-water vapor a t temperatures of 20", 40", and 50" C., and also the sorption data a t 20" C. for steam-processed lignite. In general, it is found that the characteristics of the sorption curves are similar in inany respects. However, it is seen that the region of hysteresis is reduced as the temperature is increased from 20" C. The included hysteresis areas were measured by a planimeter and the following values obtained: TEMP.
c.
20
40
HYSTERESIS AREA Sq. cm.
34.31 20.12
TEMP.
c. 50 20 (steam-dried lignite)
HYSTERESIS AREA sq. c f f l .
14.25 00 00
HYSTERESIS PHESOMENOK. The phenomenon of hysteresis has engaged the attention of many investigators who have studied a variety of materials. The direct consequence has been the formulation of many theories to explain its existence. A general theory to explain all known cases of hysteresis is not yet available. In the light of the present-day knowledge concerning this phenomenon, it is not unreasonable to believe that hysteresis is a function of specific properties which are characteristic only for the material in question. The work a t this university has indicated that, in the case of lignite, hysteresis is a function of shrinkage with subsequent capillary collapse and also of difficulty of wetting the capillary surface due t o adsorbed films of foreign gases after complete dehydration. When moisture is removed from lignite, it shrinks and
Velva
thereby causes the collapse of certain capillaries which constitute part of its colloidal structure. Obviously, on rehydration the dried material will no longer reach the original moisture content. Furthermore, it is probable that foreign gases, such as nitrogen, oxy100 gen, etc., r e p l a c e t h e capillary bound w a t e r when l i g n i t e is dehy90 drated completely; this en makes the replacement of m o i s t u r e o n t h e c a p i l l a r y walls more 70 difficult. PORESIZE AND DIS60 TRIBUTIOS. The r e l a $ tive distribution of capil$ 50 lary sizes in l i g n i t e a t ol 20",40",and50"C.have 4 been calculated from the $ corresponding sorption y, data a c c o r d i n g t o the Thompson equation (6). 20 The results of these calculations are tabulated in Table IV and plotted in Figure Figure 6. 6 shows that 60 0 &am6 C+ Water Per 100 Grams brq Liqnite 5,
Dm . -
per cent Of the capillary FIGURE 4. SORPTION CURVES FOR bound water in lignite is TREATED AND FRESHLY MINED retained as follows: (1) LIGNITEAT 20" C . at 20" C. capillaries less than 1.75 mp in radius; ( 2 ) a t 40" C. in capillaries less than 1.71 mp in radius; (3) a t 50" C. in capillaries less than 2.45 mp in radius; and (4) in steam-dried lignite a t 20" C. in capillaries less than 1.40 mp in radius.
August, 1932
INDUSTRIAL AND E S G I K E E R I S G CHEMISTRY
931
Certain conclusions may be drawn in regard to the effect of temperature and pressure on the physical structure of lignite. Figure 6 shows that heat treatment causes a collapse of capil-
FIGURE 5. SORPTION CURVESFOR LIGNITEAT VARIOUSTEVPERATURES
laries of all size. Increasing the temperature from 20" t o 40" C., the effect is small and the distribution of destroyed pores is in proportion to their occurrence in the original sample. Increasing the temperature to 50" C., however, causes the collapse of an increased number of the smaller capillaries as manifested by the fact that 60 per cent of the bound water is held a t this temperature in capillaries whose average radius is greater than that a t 20" C., or 40" C., for a corresponding quantity of water. A similar comparison shows that steam-drying affects, in the main, the larger capillaries. When lignite is heated in an atmosphere of saturated steam a t elevated pressures, its structure is definitely modified. The elimination of hysteresis with this material indicates that steam-drying causes the hydrogel structure to "set," with the result that the further removal of moisture from this L I G S I T E AT I I I G N I T E 4T LIGNITEA T STEAM-DRIED lignite is not accompanied by shrinkage. In the case of 50' C. LIGNITE 40' C 20' C RELATIVE Radius Radius Radius Radius freshly mined lignite heated to 50" C., the structure is HUMIDITY modified but not set, and the further dehydration of this ma31.11 1 0 . 3 1 24.26 22.73 8.83 90.0 1 0 . 3 1 38.50 9 . 2 8 terial is accompanied by shrinkage which is responsible largely 42.22 6.20 34.10 32.05 5 . 3 1 84.0 6.20 44.34 5.5R for hysteresis. 53.33 3.93 42.95 42.05 3 . 3 6 76.0 3.93 49.36 3 . 5 4 2 . 2 3 63.11 2.60 49.83 52.27 5 6 . 0 2.60 54.05 2.34 RELATIONOF MOISTUREIN LIGNITETO ITS PHYSICAL 02.0 1.65 57.70 62.07 1 . 4 1 21.33 1.65 61.75 1.48 1 . 1 6 ( 4 . 6 7 1 . 3 5 60.33 1.22 66.36 4 5 . 0 1.35 66.64 STRUCTURE.The results of the above experiments indicate 80.67 0.97 67.21 73.41 0 . 8 3 33.0 0.97 75.86 0.87 that the moisture in lignite must be considered as part of its 86.67 80.45 0 . 5 7 0.67 73.77 20.0 0.67 81.75 0.60 92.22 0.45 82.62 87.05 0 . 3 8 9.2 0.45 89.59 0 . 4 0 physical structure. The removal of this moisture by ordinary 95.56 0.32 91.80 9 3 . 6 4 0.27 3.5 0.32 92.52 0.29 0.00 100.00 0.0 0.00 100.00 0.00 100.00 0 . 0 0 100.00 means involves a destruction of the lump forrn and inflicts, thereby, a severe disadvantage in the use of this fuel. Per cent bound water = X 100, where T = moisture a t 100 per Various methods for the successful dehydration of lignite cent relative humidity, and E = moisture content in any other humidity.
7
932
1N
L)
I T S 'I' L I I A I .
A 3 I)
E 'i ( G 1 ? 4 1 l i b : I t I I I I ;
I lie phj-sics1 apirearctnce oi tlir Iuniiis during ttie drying jrocess was ohserved tiirougli a glass door in tlie drier. A further indication of the pliysical strengili of tlie dried coal was obtained from drop tests performed with 10-pound (4.fikg.) samples of tire fuel before and after each experiment. ,\
I : l l l ~ M l s ' ~ l ~ Y
V d 2,1, No. 8
eacli of the three planes, it is uiiifvnii ilk any m e of these planes. It is becaose of t,liis latter fact that wood may lie dried siiceessfully by this method.
Evidence obtained frnrn microst.rocture studies indicates tliat, in tlie transformation of wood to lignite, there occnrs more or less destruction of the tracheids arid middle lamellae of the nriginal wood substance. The coalification pnicess also involves some destruction of tlie cementing niaterial iii tlic cell walls of the woody subsbncc. At the Sam: tinie, uneven deposition of t.he original plant material, as well as disturbances during the coalification process, causes an irregularity in the physieal integration of the lignite. In wood the cells are built, up in regular order and definitely oricnted througliout. In lignite the orientation is lost and the arrangement chaotic. As a result, when moisture is lost from lignite, nileveti shrinkage takes place in each of tlie three planes and the unequal forces so set up are sufficient to cause disintegration of the lump nraterial, On the basis of this theory, the successful drying of lignite vcitliout disintegration shonld he possible only wlien the pliysical structure of the material is altered in such a way as to prevent uneven shrinkage in any one plane. Contrdlcd liuniidity drying should prove successful with lignite if ttie temperature of the drying irredium is sufficiently high to cause a modification of tlie structure as outlined above. Experiments conducted recently at this university have shown tliat lignite can he dried successfully from a moisture content of approxiiiiately Xi to 20 per cent by tire w e of waste stack gas from the hurnng of lignite. This gas leaves t.lie st,acli at a percentage Iiriinidity of 525, corresponding to IL dry-bulb tenipcrature of 315" C. (600" P.) and a dew point of 51.5' C. (125O F.). Tiic humidity of the drying gas was controlled by mixing definite portions of the entering gas witti portions of the exit as from the drier. Tlie success of t.he process in this < isdue, proliahly, to the liiglier tenipcrature.
FlGUtlE
This test cmisists in dropping the lignite froni a height of 7 feet (2.1 nictersj on to a concrete floor. Tlie cycle is performed five times, after wlricli the coal is sized arid weiglicd. The ratio of t;he iiercentage retained 1111 a i.R-iiich (3.8-mn.) screen for tlie dried and freshly mined lignite e m be taken then as an index of tlre pliysical strcngth of t i l e product. 5-cx.
7.5-ccw.
Lavsa
IO-la.
I.UXPJ
Lll>>P*
9h "/I 1.n-i.clr screes. I * K iili,um ltetsiaed on I.,&iiich semen, dried l i g n i t e
iistaillcd
I,*dex
tiidox (or *leani-dried lignite
85.0 10.5
0.124 0.400
9% 82.6 24.4 0.2!14 0.450
70 x1.7 17.9
0.21!, O.tl3
Il"W8
11.75
10.25
II0irra
2i:&
1.00
29.00
8.75
35.75
1P.00
47.79
15.25
57.75 73.00
moo 5.00
78.00
c. " c.
%
40.0 40.0
88.6
38.0
8'2 90
.. . , Cbechirri: Dniaiiel tu brddiilr
40.0 40,O 40.0
87.5 38.7
117 RP
Chcckiw cleadyevidont MsLeiisi badly eiicckcd
e
40.0
36.1 35.3
40.0
34.2
40.0
82.8
80 74 87
pisite 8Larl.ed
60
...... ...... ...... ......
ACKXOWLEDGXEXT The autllors desire to express t.lieir appreciat.ion to Ikxi Ikrg,er for providing tlie fellowship which made the study possdile. They desire also to thank A. W. Gauger for his suggestions during tlic conrsc of this work.
, ,..
~ l l i l R . 4 T l ; R C (;lTE!>
