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VOl. 1, No. 3
Determination of the Moisture Content of Coal and Similar Substances' Manfred Mannheimerz STERLING CHEMISTRY LABORATORY, YALE UNIVERSITY, NEWHAVEN,CONN.
ARIOUS methods have been published for the determination of the moisture content of coal and similar substances (1, 2, 4).* The variety of these methods shows that no one of them is completely satisfactory and preferable t o the others. If the moisture content is determined by different methods on the same sample of coal, the values found will differ somewhat. In general, the values found when the same method is applied several times check fairly well with one another but differ from those found by other methods. This is particularly true with brown coal, lignite, and sub-bituminous coal. A new method, which permits an analysis of moisture within 5 minutes, has been worked out, which consists in the extraction of the coal by a hygroscopic liquid and the subsequent determination of the water content of this liquid. The liquid to be used should have the following properties: It should allow an extraction in the shortest possible time; it should extract practically all of the water and only water and not other substances which the coal might contain, for otherwise it would not be possible to make a simple determination of the water content of the liquid and assume it to be identical to that of the coal. Of the liquids which have been considered-namely, sulfuric acid, acetic acid, acetone, methanol, and ethyl alcoholmethanol approaches most closely to the ideal. While some of the liquids give yellow or brown solutions due to extraction of material from the coal, the extract with the two alcohols shows a t most a very weak yellowish color when viewed in a thick layer. Methanol is preferable to ethyl alcohol because of its greater affinity for water and its lower price. The other favorable properties of methanol are shown by the following experimental results, but it is not improbable that eventually an even more suitable liquid may be found, taking into consideration the chemical composition of the various coals or other substances t o be tested. For determining the moisture content of the methanol the following methods were tried: electrical conductivity of dilute salt or acid solutions in methanol; change of critical solution temperature of hydrocarbons; catalysis constant of a dissolved ester; determination of density. The last method is the most convenient one, and a method was worked out which can generally be used for measuring changes of density as small as 0.01 per cent in any liquid. A floater of glass having the same density as absolute methanol a t a certain temperature will sink a t a higher temperature and will rise a t a lower temperature. If the alcohol contains some water, its density is greater; therefore, the floater will rise. By raising the temperature it is possible to adjust the density of the liquid so that the floater will just float. The change of temperature necessary for this compensation is a measure for the water content of the alcohol.
V
Description and Handling of Apparatus
The extraction apparatus is shown in the accompanying illustration. The weighed sample of coal is put into the Received February 25, 1929. Yale University. Italic numbers in parenthesis refer to literature cited at end of article.
* Sterling Research Fellow,
*
funnel a t the top of A; then the extraction liquid from the storage bottle, D, is poured over it by means of pipet, C. Because of the hygroscopicity of methanol the pipet should not be handled with the mouth, but should be filled by heating the upper bulb, 1, with the upper stopcock open, allowing air to expand, then closing the stopcock and allowing the bulb t o cool, thus creating a diminished pressure. Then the pipet is inserted in the methanol bottle and the lower stopcock is gently opened to fill the pipet. The pipet is dried after use by heating bulb 1 over a flame with both of the stopcocks closed; the lower stopcock is then opened for a moment, so that the air can rush out. Bulb 2 is heated with the upper stopcock open and the lower one closed. Air is drawn into bulb 1 during this cooling period. If this procedure is repeated two or three times the pipet may be dried as quickly as by drawing air through it by means of pump. (Time can be saved by using an automatically adjusting pipet as described by Orth, 3.) The storage bottle is closed with a stopcock in order to save sealing the stopper with paraffin after each opening; the stopcock can be greased with a mixture of solid and liquid paraffin (white mineral oil) or with pure vaseline. A bottle that was half filled with methanol showed an increase of water content of 0.03 per cent after the bottle had been opened many times during a week. This is below the limit of error of the practical determination of water content of coal. However, it might be advisable to check the water content of the alcohol once every 2 weeks. A slight enlargement of the upper end of the stem of the extraction funnel serves to receive a roll of filter paper for the retention of the fine coal particles. By means of a piece of rubber tubing the funnel is fastened t o a stopcock and this to a thin-walled test tube closed by a rubber stopper. From the test tube, which contains the floater, a second tube with a stopcock leads to a calcium chloride drying tube connected to a water-jet pump. After the methanol has been shaken with the coal, it is drawn into the test tube by the pump, then air is allowed to reenter through the drying tube, the funnel and the drying tube are taken off, and the test tube is transferred to apparatus B. I n order to avoid change of concentration by evaporation of methanol, the stopcock on the right side in A must be kept closed most of the time while pumping. If the suction becomes too weak, it may be re-opened for a moment. Shortly before the upper stopcock is closed, the contents of the test tube are allowed to boil by warming the test tube with the hand, thus securing uniform distribution of the water. After use, the test tube with the floater is washed out with methanol and then exhausted by the pump, and the rest of the liquid in the glass is evaporated by warming it with the hand. The apparatus B consists essentially of a transparent water bath with an outlet, containing also a stirrer and a thermometer reading to tenths of a degree. The temperature of the water in the bath is regulated by letting in hot water from the beaker on the left side or ice water from the beaker on the right side; in the same way it is possible to regulate the rate of change of temperature by dropping in hot or cold
I N D U S T R I A L A N D EiVGINEERING CHEMISTRY
July 15, 1929
water. Thus, the equilibrium temperature of the floater can be determined in less than 2 minutes. Analytical Procedure The ratio of coal and methanol should be chosen so that the final water concentration in the methanol will not exceed 2 per cent if accuracy is required. The coal, ground to pass a 60-mesh screen, is weighed and shaken for a t least half a minute in the funnel A with methanol which has been taken from'the storage bottle D by the pipet C; then the methanol is sucked through the filter into the test tube with the floater. The equilibrium temperature of the floater in the aqueous methanol is determined in the apparatus B. From the equilibrium temperature, the amount of coal, and the quantity of methanol, the water content of the coal can be determined simply. After some practice the whole procedure may take less than 5 minutes. Experimental Results For the floater the following calibration table was found by adding known quantities of water to absolute methanol: WATER I N
METHANOL b y wt.
0 0.844 2.257 5.446 7.809
10.453
DIPF.I % by
wt.
0.844 1.413 3.189 2.363 2,644
EQUILIBRIUM TEMPERATUREDIFF. I 1
= c.
21.80 24.42
28.81 38.82 46.32
54.52
=
c.
155
more, it does not seem to extract non-aqueous matter from the coal to a measurable degree a t room temperature in a short time. It is not accidental that the longer extraction gives a slightly lower value for the water content. It must be considered that extraction is a rather coniplicated process wherein the water adsorbed by the coal is gradually displaced by alcohol, so the first part of the alcohol vhich gets in contact with the coal extracts more water than the last parts. If the alcohol is removed immediately, there is no time to reach equilibrium of adsorption, and the last part of the alcohol meets an almost water-free coal, so that more alcohol will remain adsorbed in the coal than after shaking for a certain time. Therefore, in the first case, the concentration of the water in the alcohol is found a little higher than in the second case, and as alcohol is relatively more adsorbed in coal than is water, the concentration is found too high in any case. So it can be understood why the extraction method gives a value about 0.03 per cent higher on the average for the water content than Marcusson's xylene method (9)which was applied t o the
DIFF. I1 DIFF. I
2.62
3.10
4.39
3.11
10.01
3.14
7.50
3.17
8.20
3.10
This table makes it possible t o find the water content with more than sufficient accuracy from the observed equilibrium temperature very easily by interpolation. The change of temperature is practically proportional to the water content. The equilibrium temperature can easily be determined as closely as 0.01' C. but in practice a reading of 0.1' C. is close enough, since the method is very sensitive. Because of the proportionality the methanol need not be absolutely water-free, but about every 2 weeks it should be checked with the floater, since it might have absorbed moisture from the air, and the initial equilibrium temperature thereby changed. Different types of coal with a great variation of moisture content and volatile matter were selected. The new method mas found t o give excellently checking results for the water content of any sample. Small deviations up t o 0.1 per cent were observed sometimes; but these decreased if larger quantities of coal were used or if the coal contained a high percentage of moisture. The data obtained with three samples of coal of widely different properties will now be discussed.
COALFROM MOSHANNON MIKE,CLEARFIELD COUNTY, PA.This coal had the following proximate analysis: moisture 2.2, volatile matter 23.2, fixed carbon 66.2, ash 8.4 per cent. This case must be considered as unfavorable for the application of the extraction method. The water content is small and therefore it is to be expected that it cannot be totally extracted within a short time; on the other hand, if the extraction is extended too long, the methanol will perhaps dissolve some non-aqueous matter and the whole analysis will be erroneous. For these reasons, in one analysis the coal was shaken 15 minutes with methanol, in another it was shaken only once and the methanol was removed immediately. Results of 2.54 and 2.61 per cent, respectively, were found for the water content of the coal. Obviously, methanol is a fast-working extracting substance for water, perhaps aided by its good wetting qualities (low surface tension). Furt,her-
A floafer
C
same coal. The xylene method gave for Moshannon PtIine coal a water content of 2.15 per cent. After drying for 11/2 hours a t 108' C. in a Freas oven the same coal shows a loss of weight of 1.84 per cent, and 1.90 per cent on a duplicate sample. The drying was not carried out in strict accordance with the A. s. T. M. method, the xylene method being used for a strict control. COKE
FROM
SUTCLIFFE
LOWTEMPERATURE CSRBONI-
PRoCEss-This sample had the following proximate analysis: moisture 6.5, volatile matter 3.1, fixed carbon 84.6, ash 5.8 per cent. This must also be considered as an unfavorable substance for testing, since it may be considered as an adsorptive coke. For instance, 1.5 grams, previously dried a t 108' C. in a Freas oven, increased its weight by 50 mg. in 6 hours and by 130 mg. in 23 hours on standing in a covered dish. Nevertheless, the extraction method gave the values of 6.89 and 6.73 per cent of water in two analyses, and the xylene method gave 6.53 per cent. The different quantities of methanol were taken so that the final concentration of water in the alcohol was 6.293 per cent in the first case, and 2.257 per cent in the second. Here, too, as with different length of time of extraction, the first case gives a higher value for water content, because the amount of alcohol adsorbed in the coke affects the result more than in the second case with lower concentration. If the discrepancy had been due to dissolved nonaqueous matter, the effect would have been the opposite. zA'rIoN
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156
LIGNITEFROM DICKINSON,N. D.-This sample had the following proximate analysis: moisture 30.9, volatile matter 47.3, fixed carbon 13.1, ash 8.7 per cent. The results obtained by different methods were as follows: Per cent 3 1 . 8 and 3 0 . 9 3 0 . 5 and 3 0 . 8 3 2 . 5 and 3 0 . 5
Drying a t 108' C. Xylene Methanol extraction
Conclusion
The extraction method shows a small but fairly constant higher Value for the water Content than that obtained by extraction by xylene, this difference changing only very slightly with the length of time of extraction, the amount of final water content of the methanol, and the species of coal. The values obtained check one another a t least as well as
Vol. 1, No. 3
those obtained by any other method. The essential advantages found in the new method are fast working, easy cleaning of the apparatus, and the possibility of working a t room temperature. ' Of course, before this new method could be considered as a substitute for the A. S. T. M. method, it should be applied to a considerable number of different types of coal and other substances, though the widely different species of coal to which it already has been applied suggest that it will work in any case. Literature Cited Am. SOC. Testing Materials, Standards, 1924, pp. 901 and 1016. Marcusson, Mitt. kgZ. Versuchsanstalten, Bei,lin, aa, 48; Chem. Zentv., 75 11, 962 (1904) ; Mitt.kgl. Matevialpriifungsaml Gyoss-Liclzleufelde West, as', 58; Chem. Zentu., 77 I, 289 (1906). (3) Orth, z , angeru. Chem,, 33, 492 (1926). (4) Piatschek, Buaunkohle, 27, 49 (1928). (1) (2)
An Improved, Air-Gas Ratio Apparatus' Crandall Z. Rosecrans LEEDSA N D NORTHRUP COMPANY, PHILADELPHIA, PA.
HE apparatus to be deA gas-analysis apparatus of the thermal conductivity pend on the thermal conducscribed is for the detertype is described, particularly designed for measuretivity of the gas. By makmination of the volume ments of ratios of air to fuel gas, where the fuel gas may ing the wire of platinum, or percentage of any one gas or be of any composition. The apparatus is also adaptable other material with a submixture of gases in air or in for general gas laboratory work, where concentrations s t a n t i a l temperature coeffiof known gases are to be determined. I t consists of a cient of electrical resistance, any single gas. It operates thermal conductivity cell mounted in a constantit is possible to measure the on the well-known thermal conductivity principle, detemperature block controlled by a bimetallic thermowire temperature by electrical scribed by Palmer a n d stat, and an improved Wheatstone bridge gas-analysis means. Hence the thermal circuit. conductivity of the gas surWeaver ( I ) , * and is a modirounding the wire can be defication and improvement of the original Bureau of Standards device. While the thermal termined in relation to some standard gas, such as air. conductivity method for gas analysis is quite widely used, a Description of Apparatus brief description of the fundamental theory will be given here. A thermal gas-analysis cell is shown in Figure 1. It conGases differ markedly in their ability to conduct heat. If the conductivity of air is taken as unity, conductivities of sists of a brass block in which a 3/&ch (0.95-cm.) hole is other -gases range - from that of carbon disulfide vapor a t drilled. Mounted axially in this hole i i a platinum wire, 0.284 to that of hydrogen 0.002 inch (0.005 mm.), which is kept stretched by a small btcr nda rd Tube S c A n a , y s ; n g Tube a t 6.95. Gaseous mix- helical spring a t the lower end. The wire is electrically \r t u r e s . of course, have connected to the block a t the conductivities which de- lower end and is insulated a t pend on their constitu- the top by a glass-platinum ents and the relative seal. The wire is heated to a a m o u n t s of each con- temperature between 100 and stituent. Now, if a fine 200' C. by electrical energy; wire is supported in a substantially all the heat loss tube and heated by an from the wire takes place by electrical current, practi- conduction through the gas. A complete gas cell consists Meos u r n g W i r as cally all of the heat will b e l o s t b y conduction of two such tubes, both drilled A through the gas. Thus in the same block. One tube is the equilibrium tempera- sealed up with dry air (over ture of the wire will de- P,OJ as a standard. The other tube has connections through W k
T
c
O
i
l Received April l l , 1929. Presented before the Division of Gas and Fuel Chemistry a t the 77th Meeting of the American Chemical Society, Columbus, Ohio, April 29 to M a y 3, 1939. Italic numbers in parenthesis refer to literature cited Gas-Analysis Cell a t end of article.
*
F i g u r e 1-Thermal
which the unknown gas to be F i g u r e 2-Gas-Analysis C i r c u i t analvzed can be admitted. T i e simple gas-analysis circuit in Figure 2 illustrates the method of measurement. S is the standard (sealed) wire and X the analyzing wire. A and B are resistances of zero temperature coefficient. K is a slide wire which can be adjusted to balance the Wheatstone bridge, the balance point being shown by the galvanometer G. A 4.5-volt battery supplies
