1226
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 22, No. 11
Studies in the Development of Dakota Lignite' I-Aqueous Tension of the Moisture in Lignite Irvin Lavine2and A. W. Gaugers UPIWERSITY
OF
NORTH DAKOTA, GRANDFORKS, N. DAK.
The desorption and adsorption of water vapor by three CCORDING to the istics of this material with a different lignites mined in North Dakota have been U n i t e d S t a t e s Geoview to working out a method studied. logical Survey, North of improving its heating value The vapor pressure decreases as the moisture content Dakota alone possesses about a t a profit. is reduced and depends on whether the lignite is being 600 billion tons of lignite. The working out of a satisdehydrated or hydrated. When one adds to this the factory commercial drying The vapor-pressure lowering, during dehydration as 316 billion tons in Montana process is a chemical engiwell as during hydration, varies but little with the differand South Dakota, it beneering problem of great coment Dakota lignites. comes apparent that this vast plexity involving many inIncreasing the temperature shifts the dehydration northern Great Plains deposit dividual problems of more or curve to the region of lower water content. represents a large proportion less difficulty. O b v i o u s l y , The radii of the capillaries in lignite have been found of the total fuel reserve of the first logical step is a study by calculation to vary between 56.73 X 10-7 cm. near the United States and any of the fundamental charactersaturation to 0.3 X lo-' cm. near the dry condition. s t u d i e s l e a d i n g to the istics of water in l i g n i t e , The average value for the latent heat of vaporizad e v e l o p m e n t and b e t t e r and one feature of this study tion (20" to 40' C.) has been calculated to be 609.1 caloutilization of this fuel are has been the determination ries per gram. of importance to this counof the aqueous t e n s i o n of try. l i g n i t e s of different water The Division of Mines of the University of h'orth Dakota contents. The results of this study, together with some has been engaged in the work of developing 'the resources of conclusions as to the colloidal nature of lignite, are presented this state. The results of certain phases of this work will be in this paper. reported in this and in subsequent papers. Table I-Proximate Analysis of Fuels Tables I and I1 show the average proximate and ultimate BITUMINOUS ANIHRACITR compositions of Dakota lignite and other fuels. A study of LIGNITE COAL COAL Per cent Per cent these tables indicates that lignite is characterized (1) by high Per cent Fixed carbon 30.0 .m n xn -. n moisture content and consequently low heat value as mined, Volatile matter 27.0 35.0 1.5 Moisture 36.0 5.0 3.5 and ( 2 ) by high oxygen content in proportion to carbon. Aqh 7 n i_n- .nii n . .-.As a result of their composition lignites have certain characB. t . u. per Ib. 600&%00 14,000 teristics that are of importance when methods of utilization Table 11-Ultimate Analysis of Fuels (Moisture- a n d Ash-Free Basis) are considered. BITFMIVOUS ANTHRACITE North Dakota lignite, upon exposure to the atmosphere, PEAT LIGNITE COAL COAL Per cent Per cent Per cent loses moisture to an extent that depends upon the humidity Per cent Carbon 55 0 68 0 85 0 94 0 and temperature. I n this drying process, however, there Hydrogen 6 0 5 5 5 0 2 5 occurs considerable checking and cracking which results in Oxygen 36 5 24 5 7 0 1 5 Nitrogen 1 5 1 0 1 4 1 0 the formation of much slack. I n consequence of its high Sulfur 1 0 1 0 1 6 1 0 moisture content and this weathering process, the profitable Previous Work shipping radius of lignite is limited and transportation must take place in closed box cars. Thiessen's (14) and later Gauger and Iverson's (4) microFurthermore, in spite of its relatively high volatile content, scopic studies of thin sections of Dakota lignite have shown lignite lacks the properties of fusing and caking in the fire that bituminous coals possess. As a result of this tendency to that the largest proportion of the materials composing lignite slack in the fire special grate constructions or firing methods is colloidal in nature. It might be expected, therefore, that in are necessary to prevent excessive loss of unburned fuel in its behavior towards water vapor lignite should show the characteristic colloidal property of adsorption. the ash pit. One of the very early investigations on the adsorption of To develop successfully the Dakota deposit it will be water vapor by a solid adsorbent was made by van Bemnecessary t o overcome the threefold handicap of paying freight on the large amount of water in the fuel, the weight melen (S), who carried out a series of extensive and careful loss and disintegration due to uncontrolled drying, and the experiments with silica gel. I n this work the adsorption and tendency t o slack in the fire. It is not only of interest, but desorption of water by the gel was followed by observing the also of practical importance, to study the drying character- variation of the aqueous tension of the gel with varying water content. The gels were kept in desiccators in which 1 Received August 6, 1930. Presented by I. Lavine and A. W. Gauger definite vapor pressures were maintained by means of mixbefore the Division of Gas and Fuel Chemistry at the 78th Meeting of the tures of sulfuric acid and water of known concentrations. American Chemical Society, Minneapolis, Minn., September 9 to 13, 1929. Loss or gain of water was determined by weighing until An abstract from a thesis presented by Irvin Lavine in partial fulfilment equilibrium was attained. of the requirements for the degree of doctor of philosophy, University of Minnesota. Zsigmondy and his co-workers ( 1 6 ) extended this work. 2 Assistant professor of chemical engineering, University of North Instead of working with desiccators, they considerably reDakota. duced the time required to reach equilibrium and avoided the 8 Director, Division of Mines and Mining Experiments, University of influence of air by carrying out the experiments in vacuum. North Dakota.
A
Kovember, 1930
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Both van Bemmelen and Zsigmondy found that certain regions of their vapor-pressure curves were not reversiblethat is, the adsorption process did not reverse itself on desorption. Lowry and Hulett (IO), in studying the adsorption of water vapor by various charcoals, came to the conclusion that water is not adsorbed by charcoal, but is held by capillary action. Katz ( 7 ) made an exhaustive study of the adsorption of water vapor by solid adsorbents. He regards the desiccator method as well suited for adsorption studies. Allmand, Hand, and their co-workers ( I ) studied the system activated charcoal-water vapor and found hysteresis to occur whether the sorption process was carried out in absence or presence of foreign gases. Porter and Ralston ( I S ) studied the system coal-water vapor to determine the vapor pressure of the inherent water in coal. They worked with samples from Wyoming, Illinois, and Pennsylvania, and found hysteresis with each material. They used the desiccator method in this study. Kreulin and Ongkiehong (8) have recently calculated the absolute pore size of brown coal from the results of dehydration experiments. They believe that 70 per cent of the tibound water" in brown coal is held in pores with a radius less than 5.6 mp. Pidgeon and Maass ( 1 2 ) studied the adsorption of water by wood. They found hysteresis to occur with wood and from the nature of the sorption curves they conclude that wood is a swelling gel. Salt Solutions Used to Maintain Constant H u m i d i t i e s a t 20' C. RELATIVE VAPOR VAPOR DESICCATOR SOLUTION PRESSURE PRESSURE Per cent Cm. Hg 100.00 Water 1.754 1.719 98.0 Pb (Nod2 90.0 1.579 ZnSOi.7HzO 84.0 KBr 1.473 79.2 1,389 NH4C1 76.0 1,333 NaCl 66.0 1.158 NaNOz 58.0 1.017 NaBr.2HzO 52.0 0.9121 NaH.SOcHz0 9 0.7893 45,O KN02 10 32.3 CaCh6Hz0 11 0.5665 12 0.3508 20.0 KCzHaOz HzSO4 (sp. gr. 1 . 6 0 3 ) 0,0737 4.2 13 0.0410 2.34 14 HzSOi (sp. gr. 1 . 6 5 7 ) 0.0037 0.21 15 H2SO4 (sp. gr. 1 . 7 7 )
Table 111-Saturated
Table IV-Sulfuric DESICCATOR A B C D E F G H I
I( L
M N
Acid Solutions Used t o Maintain Constant Humidities a t 40'C. SP. GR.OF RELATIVE VAPOR VAPOR ACID SOLN. PRESSURE PRESSURE Per cent Cm. Hg 1 . 0 0 0 (water) 100.00 5.532 1.0648 94.72 5.24 1.1354 86.40 4.78 80.62 4.46 1.1753 1,2109 74.83 4.14 1.2443 66.88 3.69 1.2970 55.13 3.05 1.3412 44.46 2.46 1.3911 33,26 1.84 22.05 1.4492 1.22 1.4815 16.99 0.94 1.5390 9.76 0.54 1.5989 0.25 4.55 1.6643 1.81 0.10
Scope of Present Work
A consideration of previous adsorption studies led the present authors to conclude that the desiccator method would prove most practical for the investigation with lignite. It was apparent that the results so obtained would serve for theoretical considerations as well as for industrial applications. It was decided, however, to alter the usual method of procedure. Whereas the majority of previous investigators studied the cycle hydration +dehydration, the present work is on the cycle dehydration -+ hydration. The work consisted in determining the vapor pressure of the moisture in samples of North Dakota lignite a t tempera-
1227
tures of 20" and 40" C. This was acc,omplished by keeping samples of three different lignites in desiccators where constant humidities were maintained. I n the experiments a t 20" C. the humidities were controlled by means of saturated salt solutions in contact with an excess of the solid salt. Sulfuric acid solutions were used to maintain the required humidities a t 40" C. Tables I11 and I V give the vapor pressures of the various solutions used in this work (6). Lignite Samples Used
The samples used in these experiments were received in sealed mason jars directly from the following mines: Sample A, from the Truax-Traer Coal Company, Velva Station, Ward County, N. Dak. Sample B from the Rupp Coal Company, Garrison Station, McLean County, N. Dak. Sample C from the Truax-Traer Coal Company, Columbus Station, Burke County, N. Dak.
The average proximate analysis on as-received basis of ten samples taken from each of the above mentioned mines is given as follows:
Moisture Volatile matter Fixed carbon Ash Sulfur B. t. u. per lb. Fusion temperature of ash
VELVA MINE Per cent 38.94 26.59 30.15 4.32 0.28 6691 2340' F. (1282' C.)
GARRISON MINE Per cent 39.12 26.84 29.98 3.97 0.45 6749 2370' F. (1299" C.)
COLUMBUS MINE
Per cent 33.11 26.22 33.26 7.41 0.34 7343 2100' F. (1149" C.)
Experimental Procedure
The method of sampling was essentially the same as that described by the United States Bureau of Mines in Technical Papers 1 and 76. A face as nearly vertical as possible for the full thickness of the seam mined was first selected. A strip about a foot wide was then cleaned from floor to roof with a pick, thus exposing a fresh surface. The floor was immediately cleared a t the base of the strip and covered with a piece of oil cloth. A cut about an inch deep and 3 or 4 inches wide was then made in the fresh face from the top to the bottom of the seam and these cuttings were caught on the oil cloth. The larger lumps were then broken and the sample was thoroughly mixed. A representative 2-quart sample was then obtained by quartering and the sample so obtained was placed in a glass jar, which was then sealed airtight. Mixing and sealing of the sample were done immediately after cutting. The jars were then shipped to the laboratory. Upon reaching the laboratory the samples were immediately crushed in a small jaw crusher and then pulverized by hand in an agate mortar. It was found advisable to pulverize the crushed material in small fractions. A quantity of about 10 grams would be pulverized in the mortar for about 5 minutes and then sized. The quantity between 80 and 100 mesh was then placed in a small glass jar having a well-fitting cover and the remainder discarded. This process was repeated until a sufficient amount of the desired size was obtained. Samples of approximately 1 gram each of the three lignites investigated were then accurately weighed into small weighing bottles having well-fitting covers and placed in each of the desiccators listed in Tables I11 and IV, where they were allowed to remain for 42 days. I n the first series the temperature of the cabinet containing the desiccators was held a t approximately 20" C., varying between 19' and 22" C. I n the second series the temperature was maintained a t 40" * 1O C. by means of a Cenco thermoregulator. At the end of the 42 days the weighing bottles were re
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1228 100
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90
90
80
0
70
70
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50
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Vol. 22, No. 11
40 I-
2
42 I0
er
L 10
WATER, GRAMS PER 100 GRAMS OF DRY LIGNITE
moved, covered, accurately weighed, and then replaced in their respective desiccators for 4 to 6 days longer. A second weighing was then made and the process repeated until successive weighings checked within b0.5 mg. Upon reaching constant weight the samples were placed in desiccators over pure sulfuric acid (sp. gr., 1.84). Weighings were made every 10 days until constant weight had been attained. I n general about 40 days were required for this process. Hydration was accomplished by replacing the samples in their original desiccators. I n the experiments at 40" C. the old acid was replaced by fresh acid of identical concentration as used in the dehydration process. Weighings were then made after 40 days and check-weighed as before to +0.5 mg. The moisture contents of the three lignites at different vapor pressures were then calculated, on the basis of the dry coal, from the final weights of the samples, both for dehydration and hydration a t 20" and 40" C., respectively. Discussion of Results
The results of these experiments are tabulated in the accompanying tables and graphs. RATE OF ATTAININGEQuILIBRIuM-The authors clearly realize that drying lignite over pure sulfuric acid for 42 days does not remove all the moisture. Even with the very best dehydrating agents it is impossible to remove the films of adsorbed water which are a few molecules in thickness. However, for a standard of reference in this work it will be assumed that drying over pure sulfuric acid for a sufficient length of time removes all the moisture from lignite. The results of a preliminary experiment in drying pulverized lignite over sulfuric acid of 1.84 sp. gr. a t 20" C. to determine the time required to attain equilibrium are represented graphically in Figure 3. The samples were pulverized between 80 and 100 mesh, weighed into glass-stoppered weighing bottles, and permitted to stand in a desiccator over the pure acid. From Figure 3 we find that the rate of loss of moisture is a maximum a t first, falling off rapidly as the adsorbed water has been reduced to a vapor pressure about equal to that of the acid. I n general about 40 days are required to
reach equilibrium. Similar results were obtained in drying pulverized lignites over various saturated salt solutions. DESORPTION OR DEHYDRATION PROCESS-A consideration of the dehydration curves (indicated by downward arrows) in Figure 1 shows that a close relationship exists between the three lignites. I n every case the loss of moisture is accompanied by a decrease in the vapor pressure. I n general this decrease is very marked and can be divided into three stages-(1) a gradual decrease in vapor pressure from 100 to 80 per cent; (2) a rapid decrease from 80 to 15 per cent; and (3) a gradual change again from 15 to 0 per cent. It is evident that Dakota lignite upon being exposed to the atmosphere will dry to a condition where the vapor pressure of the moisture in the lignite is in equilibrium with the vapor pressure of the moisture in the atmosphere. According to the reports of the United States Weather Bureau ( I l ) , the relative humidity of the state varies from about 60 to 80 per cent. I n drying for storage, therefore, it is not practicable to reduce the moisture content below approximately 16 per cent unless the colloidal structure of the lignite is so changed that the moisture-equilibrium condition over this range of humidity is reduced. Curve D in Figure 1 represents the desorption of wood and is taken from data by Hawley and Wise ( 5 ) . It becomes apparent from a consideration of this curve that the desorption processes for lignite and wood are identical in their general characteristics. Pidgeon and Maass (la) in a later study found hysteresis to be present with spruce and pine woods, while Larian, Lavine, Mann, and Gauger ( 9 ) found this phenomenon to exist with birch wood. The close relationship between lignite and wood becomes evident from these investigations. ADSORPTION OR HYDRATIOX PRocEss-The adsorption curves for the three lignites (Figure 1, upward arrows) are essentially similar and possess the following characteristics : (1) the lower portions are reversible with the corresponding desorption curves. As moisture is adsorbed the vapor pressure over this range increases but gradually; (2) a second portion rises very abruptly, indicating that over this range of adsorption the vapor pressure increases very rapidly; and (3) as saturation is reached the curves again flatten out.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November, 1930
Figure 1 shows that the desorption and adsorption processes are reversible over only a very short moisture range, approximately 1 to 5 per cent, but above this the two curves diverge very rapidly and reach a maximum a t complete saturation. I
.-
1229
are essentially similar t o the corresponding curves a t 20" C. Here also a great similarity exists with the three lignites investigated, indicating that they are closely related in colloidal structure. The effect of temperature on the dehydration process can be more easily determined from Figure 4,in which the curve at 20" C. is represented by the solid line, while the broken line represents the results at 40" C. This figure shows that increasing the temperature displaces the dehydration curve towards the region of lower water content; that is, under a given humidity Dakota lignite vi11 tend to dry t o lower moisture content at 40" than a t 20" C. This does not hold true, however, a t the lower moisture contents, where the two curves become identical. Table V-Vapor
Pressure-Moisture E q u i l i b r i u m Data MOISTURE-DRY BASIS
RELATIVE DESIC- VAPOR CATOR PRESSURE
5
0
15
IO
TIME
20
25
35
30
40
Adsorption studies with other materials have indicated that hysteresis disappears a t saturation. In the present investigation, however, the two curves are found t o diverge rather widely a t this point. The explanation for this apparent discrepancy lies in the fact that other investigators performed the cycle adsorption +desorption, starting with a previously dried material. The two curves, therefore, have common points a t 0 and 100 per cent relative vapor pressures. The reverse cycle was followed in the present investigation with lignite, and the fact that the two processes are not reversible emphasizes the fact that the true condition for Dakota lignite is represented by the desorption curve. EFFECT O F TEhlPERATURE--\ study of Figure 2 shows that the dehydration, as well as the hydration, curves at 40" C. 100
100
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10 11
12
RP
R2
45 32.3 20 4.2 2.34 0.21
13 14 15 A t 40' C .
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HYDRATIOS
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Per cent Per cent Per cent
Per cent At 200 c. 1 100 2 98 3 90 4 84 $7 79.2 6 76 7 6fi
IN DAYS
DEHYDRATION
50
WATER, GRAMS PER 100 GRAMS OF DRYLIGNITE
55.69 50.47 41.77 34.18 30.41 28.48 23.66 19.17 19.21 17.00 12.24 9.31 4.52 2.51 0.41
45.64 41.45 35.69 29.49 28.87 25.50 22.42 18.42 18.31 16.14 11.74 9.14 4.57 2.45 0.496
44.38 40.84 36.40 31.46 28.90 26.47 22.60 19.91 17.85 15.70 10.74 8.13 3.7s
49.16 43.09 35.63 30.29 25.47 22.98 16.54 14.44 12.10 9.53 7.80 5.86 3.55 1.69
43.75 38.54 31.84 27.10 22.64 21.52 15.50 14.39 11.97 9.16 7.62 5.21 3.27 1.59
Per cent Per cent Per cent
0.32
41.16 32.99 26.99 22.77 22.06 20.33 17.33 15.55 14.65 13.55 10.56 7.99 3.95 2.11 0.27
37.40 32.22 25.80 22.30 21.25 19 84 16.99 15 21 14.16 13.24 10.22 7.85 4.07 2.14 0.368
37.63 30.45 24.39 21.00 20.26 18.55 15.82 14.07 13.11 12.27 9.34 7.16 3.51 1.97 0.32
39.04 35.21 29.64 26.61 21.30 20.05 14.22 13.10 10.26 7.84 6.54 4.47 2.58 1.50
36.04 29.66 25,43 24.02 21.63 18.69 15.31 12.70 10.35 8.44 7.41 4.86 2.94 1,57
32.16 29.54 23.24 22.01 20.17 18.43 14.69 12.77 10 72 8.12 7.15 4.50 2.83 1.42
33.05 29.78 22.35 21,94 18.12 16.57 13.18 11.69 8.82 6.97 6.10 3.76 2.43 1.39
1.86
INDUSTRIAL AND ENGINEERING CHEMIXTRY
1230
AVERAGERESULTS-The average results for the three lignites investigated a t 20" and 40" C. are plotted in Figures 5 and 6, respectively. These curves represent more closely the vapor pressure-moisture content relationships a t 20 " and 40; C. for lignite as mined in North Dakota.*
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