December, 1930
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Studies in the Development of Dakota Lignite 111-Drying of Lignite without Disintegration' Irvin Lavine,2 A. W. Gauger,' a n d C. A. M a n n 4 UXIVERSITY OF NORTH DAKOTA, GRAND FORKS, N . DAE.,AND UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MI".
Preliminary work indicated that N o r t h Dakota lignite is well s u i t e d for d r y i n g by the Fleissner method. T h e present paper reports t h e r e s u l t s of an extended s t u d y o n the s t e a m d r y i n g of this lignite. The work consisted in subjecting lignite to the following stages: (1) a preheating period w i t h s a t u r a t e d s t e a m directly i n c o n t a c t w i t h the coal; (2) a heating period, in which the coal mass is b r o u g h t to t h e t e m p e r a t u r e of the s t e a m ; (3) a release period, in which the s t e a m pressure is b r o u g h t to a t m o s pheric; a n d ( 4 ) an a e r a t i o n period, in which a blast of air is passed over the coal mass. T h e r e s u l t s indicate that N o r t h Dakota lignite can be processed successfully by the Fleissner m e t h o d u n d e r t h e following conditions: (1) s a t u r a t e d s t e a m pressures of n o t lower than 13 a t m o s p h e r e s ; (2)a preheating period of approximately 40 m i n u t e s in which the pressure is gradually increased to a m a x i m u m ; condensate should be removed d u r i n g this period u n t i l the first appearance of s t e a m , a f t e r which the condensate valve s h o u l d be closed;
(3) a h e a t i n g period of approximately 90 m i n u t e s ; cond e n s a t e should not b e removed d u r i n g this period: (4) a release period of approximately 30 m i n u t e s ; (5) an aerat i o n period of ll/z hours using 312 liters of air per minute, the a i r being preheated t o 80" C. before use. Under these conditions the m o i s t u r e c o n t e n t c a n be reduced f r o m a b o u t 36 t o approximately 16 p e r cent. T h e h e a t i n g value is correspondingly increased. This reduction in m o i s t u r e c o n t e n t represents a decrease of approximately 20 per cent i n the weight of the raw coal. It is evident that the freight h a n d i c a p can be materially reduced. Physical t e s t s conducted w i t h t h e dried m a t e r i a l indic a t e that it is capable of w i t h s t a n d i n g general conditions of h a n d l i n g w i t h o u t excessive breakage a n d that it has good weathering qualities. Tests have also s h o w n that s t e a m drying does n o t materially increase t h e reactivity of t h i s m a t e r i a l w i t h oxygen, indicating that s t e a m d r y i n g does not increase the tendency towards s p o n t a n e o u s combustion.
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T HAS already been pointed out (9) that the following
factors have greatly handicapped the development of the lignite fields in Korth Dakota: (1) the necessity of paying freight on large amounts of moisture in the fuel; (2) the weight loss and disintegration due to uncontrolled drying; and (3) the tendency to slack in the f i e . The problem of briquetting the lignite in an effort to overcome these disadvantages has received considerable attention, but in general this method has not been developed to a point that would warrant profitable manufacture owing largely to present-day conditions. P a t e n t s on Drying of Lignite
The possibility of drying lignite to a condition where the freight handicap is materially reduced has also received attention. I n searching the literature it was found that most of the information pertaining to this work existed in the form of patents; therefore, a survey was made of the available patent literature. A partial list of the representative patents in these arts follows: (1) A process of drying brown coal or lignite characterized by effecting a preheating in cylinders or shafts containing mechanically driven arms. Wendel (17). (2) A method of drying lignite in which the material is carried along by a current of hot gases and then separated from the gas when dry. The temperature of the gases a t completion of drying is maintained between 75" and 140O F. in order that even drying shall take place in spite of temperature changes, changes in moisture content of the raw material, etc. This is accomplished either by regulating the rate of feed, by-passing some of the gas, or by 1 Received August 11, 1930. Presented before the Division of Gas and Fuel Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930. From a thesis presented by Irvin Lavine in partial fulfilment of the requirements for the degree of doctor of philosophy, University of Minnesota. * Assistant professor of chemical engineering, University of North Dakota. Director, Division of Mines and Mining Experiments, University of North Dakota. 4 Chief, Division of Chemical Engineering, School of Chemistry, University of Minnesota.
adding cooled exhaust gas to the gas stream. Rigby and Testrup (13). (3) A double continuously operating centrifuge for dehydrating lignite, brown coal, etc. Lambrecht (8). (4) A method of drying lignite on the surface and then coating this surface with crude petroleum to prevent further evaporation and disintegration. Sterne (16). ( 5 ) The lignite is dried in powdered form, the first stage of the drying being effected while the particles are suspended in a current of hot flue gas; similar to the inventor's British patent (13). Rigby (12). (6) Lignite is dried in two or more stages, first at a low and then at a higher temperature. Drying is carried on in a vertical shaft or hopper through 'which the material travels. The shaft is provided with superposed rows of hot air channels. Jacobs ( 6 ) . (7) Lignite is heated in contact with a hydrocarbon oil such as gas oil or ''engine distillate" to dehydrate it. The lignite is then separated from the excess oil which it has not adsorbed. Schoch claims that disintegration is greatly minimized and that the heating value of the lignite is materially increased by the addition of oil. Schoch ( 1 4 ) . (8) Lignite is heated in contact with a mixed gas oil and fuel oil or other hydrocarbon oil to drive off the moisture and to impregnate the lignite with the oil. The unadsorbed oil is then separated while a portion of the adsorbed oil can be removed by use of superheated steam. Schoch (15). (9) Methods for the steam drying of lignite to prevent disintegration. The details of these patents will be discussed later. Fleissner ( 4 ) . (10) Method for the drying of lump lignite, particularly North Dakota lignite, in a specially constructed drum drier. Evesmith ( 3 ) .
It is evident that many ideas have been advanced for the drying of lignite. As far as the writers have been able to determine, only two of the above-mentioned processes, those of Fleissner and Schoch, have been carried to semi-commercial stages. A consideration of the principles involved in the drying of a material like lignite may be of interest in ascertaining the probable success of any process. Principles Involved
Carrier and Stacey (2) have pointed out that in drying hygroscopic materials two critical points with respect t o
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INDUSTRIAL A N D EATCINEERING CHEMISTRY
relative humidity must be considered. I n the first stage free or non-hygroscopic moisture is removed, in which case 8 high relative humidity must be maintained to prevent hardening and shrinkage of the exterior. For many materials a relative humidity of at least 80 per cent is required for this purpose. In the second stage only hygroscopic moisture is removed and frequently a certain amount of this moisture must be retained to prevent physical injury. This maybe accomplished by maintaining a definite vapor-pressure gradient betnreen the moisture in the material and the moisturein the drying medium. This vapor-pressure gradient will be dekrrnined to a larKe e*nt by the of the materiel being dried.
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water from the interior to the exterior. In order to obtain the ideal drying condition which minimizes degradation, i t will be necessary, therefore, either to slow down the rate of evaporation of moisture from the surface or speed up the rate of transfer of moisture to the surface. Controlled Humidity Drying The first process will have to be accomplished by accurately controlling the humidity of the drying medium, and may therefore be termed "humidity drying." The possibility of utilizing this method of processing lignite on a commercial scale is being investigated a t the University of North Dakota, where a commercial-sized drier has been installed. Waste stack gas from the burning of lignite in the university power plant is being utilized, because this gas is exceedingly rich in moisture. The humidity of the drying gas can be controlled by mixing definite portions of the entering gas with portions of the exit gas from the drier. The waste stack gas leaves the stack a t a percentage humidity of 52.5 corresponding to a drybulb temperature of 6OO" F. and a dew point of 125' F. The gas leaves the drier practically saturated. Lavine and Sutherland (10) have calculated that, in drying lignite from a moisture content of 36 per cent down to 16 per cent using stack gas of the above condition, the temperature of evaporation is 147.9" F.; 39.2 cubic feet of gas saturated a t this temperature are removed from the drier; and 35.78 pounds of bone-dry gas are required to supply the heat and carry away the moisture. If the entering gas is to be maintained at a humidity of $3.2 per cent by circulating saturated drier gas, then 681 cubic feet of such gas must be used. Fleissner Process
The Fleissner process of drying Austrian brown coal seems to fulfil the conditions required in the second case, whore diffusion of moisture t o the surface is increased. In this process the raw material is brought in direct contact with saturated steam and the pressure of the steam atmosphere gradually increased to 13 atmospheres or more. After re. maining in steam at the maximum pressure for a short time, the pressure is gradually diminished to atmospheric and then dry air is blown through the autoclave containing the coal. This treatment heats the lump throughout without perniitting any evaporation, since the surrounding steam atmosphere is saturated. Drying is begun by rcleasing the steam pressure ana completed by blowing air through the coal mass. As a result drying may t&e place rapidly without the usual degradation of the lump. The Fleissrier process has been tried out on a large scale in Austria and is rcported to be operating succesvfully ( 5 ) . I t was decided, therefore, to investigate the suitability of this process for use with North Dakota lignite. Through thc courtesy of F. A. Oetken, teclinical director of the Lurgi Gesellschaft fur Wirme Tcchnik, a small Fleissner retort was loaned to the Cnivcrsity of North Dakota for the purpose of making the tests. The experiments were started at the University of North Dakota and latcr continued at the University of Minnesota under a coiiperative arrangement between the Division of Mines of the former and the School of Chemistry of the latter. The present paper presents the results of a11 the experimental work to date. EXPERIMENTAL
of a collbidalmaterial such as lignite is that of the transfer of
Exclusive of the purely mechanical phases of Bling and emptying the autoclave, the Fleissner process may be divided into four distinct periods: (1) the preheating period, in which the steam Dressure is eraduallv built to a maximum: (2) the heating ieriod, in wbch the maximum pressure ii
INDUSTRIAL AND ENGI.NEERIN(: CIIEMISTRY
December, 1930
maintained for a definite period; (3) the release period, in which the pressure is gradually reduced to atmospheric; (4) the aeration period, in which air is blown over the coal mass. The process is therefore a discontinuous one. The Fleissner process is characterized by the use of a saturated vapor. It is important to realize that the use of superheated steam would cause disintegration rather than prevent it. Raw Lignite Lignite from three different mines in North Dakota was investigated: (1) Velva lignite from Velva, N. Dak.; (2) Lehigh lignite from Lehigh, N. Dak.; and (3) Rupp lignite from Garrison, N. Dak. Arrangements were made to receive the lignite as it was needed. Seven shipments were received, as follows: (1) Velva lignite shipped from Grand Forks in wood barrels, well covered and insulated with tar paper. (2) Velva lignite shipped from Grand Forks in a manner just described. ( 3 ) Velva lignite shipped from Grand Forks. (4) Arrived directly from the mine at Lehirh. _ . N. Dak.. in a closed steel drum. . (5) Shipped direct from the mine at Velva, N. Dak., and arrived in steel drums with welded heads to prevent drying during transportation. ( G ) Rupp lignite shipped from Grand Forks. (7) Second shipment from the mine at Velva, N. Dak., in steel drums with welded heads.
The moisture contents of these samples upon their arrival a t the laboratory are tabulated in Table
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valve, Lz; and water drain valve, La. The construction of this trap is shown in Figure 4. Steam leaving trap L passed through valve L, into coil condenser, M ,and the liquid so formed drained into collector bottle, N,which was placed on a small platform scale. The weight of the steam separated from the condensate could therefore be obtained easily. Non-condensable gases accompanying the condensate passed through bottle N into the calibrated gas holder, P. Samples for analysis were taken either from the holder a t completion of a run or during a run by means of sampling device 0. Air coming from the compressor tank (not pictured in Figure 3) a t approximately 4 atmospheres was reduced to 1 atmosphere or less by means of reducing regulator U and measured by orifice meter V , the volume being read by means of manometer W. It was then passed through preheater X and its temperature ascertained by thermometer Y. It next passed through the coal mass and out through valve R, into coil condenser R, which served to cool the air and condense out much of its moisture. A sample of the air for analysis could be obtained by means of sampling device Q. From the condenser the air passed through tower S, which was filled with about 23 kg. of anhydrous calcium chloride and as a further precaution passed through two additional glass towers, T, which were also filled with calcium chloride. Preliminary tests conducted to determine the moisture content of the air after having passed through these towers showed that the air was completely dried. It can Le seen from Figure 3 that the calciurn chloride towers were mounted on a platform scale and therefore the total weight of the moisture rcrnovcd by the air in passing through the coal mass was
XIII. The construction of the autoclave used in the present work limited the size of lumps to a maximum of 10 om. However, three different sizes were employed, as follows: (I) lO
