5’01. 2 , s o . 3
A.Yd LY T I C AL EDI TI0.V
3.14
disturbed by repeated testing, arid it is therefore possible to use the same standard for a large series of tests. The only caution to be observed in preparing the sainple\ for examination is to be certain that a representative saniplti is powdered and that all of the material is passed through thc. sieve. An agate mortar is recommended for the grinding and a sieve of a t least 80 mesh should be used. Failure to pas> any portion of coarse material through the sieve or the loss of dust particles usually materially changes the color of the test powder. The sketch illustrates the construction of the comparator and the method and precautions necessary in using it should be self-explanatory, While this method is crude and only of qualitative value, it can be used to advantage in the industrial plant for the rapid checking of products in which the perception of small color differences is somewhat difficult.
Apparatus for the Determination of Ignition Temperature of Powder Substances’ Paul W. E d w a r d s a n d Roger W. H a r r i s o n CHEMICAL
EKGIKEERING DIVISION, BUREAUOF
T
HE ignition temperature of solids has been defined in a number of ways and methods have been devised to determine some of these temperatures in accordance with the definitions. I n some cases it appears that the tempcratures obtained by the methods described and taken as ignition temperatures do not fulfil the requirements of the definitions. Some of the definitions of ignition temperature of solid combustibles found in the literature are as follow: (1) That temperature a t which substances take fire SPontaneously in air ( 4 ) . (2) Temperature of ignition in oxygen is defined as that temperature t o which fuel must be raised for its ignition t o take place without the aid of external agents of inflammation (6).
(3)
The temperature of the glow point of the fuel ( I ) . (4) The temperature zone a t which rapid self-heating begins ( 2 ) . ( 5 ) s prom a practical as u,ell as a theoretical point of view, it is sufficient to know a t what temperature, under standard conditions, different coals begin to react with oxygen so rapidly that the ultimate appearance of flame is assured (7).” (6) The temperature a t which the material, in contact with air of the same temperature, suffers such accelerated oxidation as results in a marked increase in temperature and formation of products of combustion (5).
C H E M I S T R Y A N D SOILS, W A S H I K G T O N ,
D. C.
substances as coal takes place a t normal atmospheric teinperatures. Apparatus
The apparatus used to make ignition-temperature determinations (Figure 1) consists of a multiple-unit electric crucible furnace in which is placed a Pyrex glass J-tube having a k i i m . bore. The lower end of the tube is flared funnel shape and is approximately mm. in diameter. The opposite end of the tube is passed through the cover of the furnace. Alundum cement is used to attach the tube to the cover. The end of t,he tube projecting through the cover is attackled to a calibrated manometer or flowmeter with rubber or glass tubing and the flowmeter is connected to a vacuum line. A platinum-platinum rhodium thermocouple in conjunction with a potentiometer or high-resistance millivoltmeter is used to make the temperature determinations. d rheostat is placet1 in series with the electrical circuit of the furnace. -4ir is admitted to the furnace through an opening around the cover plate.
The following definition is proposed by the authors: The ignition temperature of a solid combustible may be defined as that temperature at which the exothermic reaction predoininates to the extent that the reaction becomes rapidly accelerated and the ultimate appearance of glow or flame is assured without further addition of heat from external sources. The oxidation must accelerate rapidly for, as Wheeler has cited ( 7 ) , self-heating of such Received April 22, 1930.
Figure 1-Ignition
Temperature Apparatus for Dusts in Static Condition
INDUSTRIAL A N D E S G I S E E R I N G CHEMISTRY
July 15, 1930
Determination
Some asbest'os fibers are placed in the neck of the J-tube to prevent the sample from being drawn through the tube. The thermocouple is placed on the asbestos and the funnel filled with the sample. When testing fine dust, the 200-mesh fraction and smaller, less is used than when the dust is coarser, to prevent restricting the flow of air through the tube. The J-tube is inserted in a porcelain crucible placed in the bottom of the furnace to protect the sample from radiant heat from the furnace walls. The flow of air through the apparatus is adjusted to approximately 400 cc. per minute. A greater or less quantity of air between the limits 300 and 500 cc. per minute may be used without affecting the ignition-temperature determination except that it changes the slope of the time-temperature curve. The current passing through the furnace is regulated so that an increase of temperature from about 25" C. to 300" C. is reached in approximately 8 minutes. A time-tempera-ture curve of a blank determination, with the furnace heating a t this rate, is almost a straight line. After the sample has been placed in the J-tube and the flow of air adjusted, the furnace current is turned on, and time and temperature readings are taken simultaneously for each 10 degrees rise in temperature until the temperature of the sample approaches its ignition temperature. Readings are then taken with each increase of temperature of 5 degrees. The data obtained are plotted as a time-temperature curve (Figure 2) and the point where the curve breaks from a straight line determines the uncorrected ignition temperature of the sample. Calibration corrections for millivoltmeter or potentiometer and thermocouple are applied to give the ignition temperature of the dust. Check determinations are always made by duplicating this procedure except that no time or temperature readings are taken, and the power supplied to the furnace is turned off when the temperature of the sample approaches its predetermined ignition point. I n every test made in this way, within * 10 degrees of the predetermined ignition temperature, the temperature continued to rise rapidly and glowing or flame resulted. Ignition T e m p e r a t u r e s of Several D u s t s
The following table gives the ignition temperatures, within =+=lo"C., of several dusts as determined by the method described above: Cocoa dust Pyrethrum flowers Ground cloves Tobacco dust TVheat smut Tvheat elevator dust Corn elevator dust Ground oat hulls
0 c . 237 252 231i '33 253 263 26i 272
"c. Tvood-pulp dust Wood flour Leather fertilizer d u s t Cornstarch Hard-wheat flour Paper dust Cellulose
272 277 324 3'4 327 337 354
Discussion
Kitli the apparatus described, the sample i t heated at a uniform rate with air heated in the furnace. Since the thermocouple is in the neck of the J-tube, on the asbestos and under the sample, all the gases passing through the sample must go over the thermocouple and any accelerated exothermic reaction in the sample will cause the temperature of the gases passing through the sample to rise sufficiently t o cause an abrupt break in the time-temperature curve. Preliminary tests indicated that the size of the dust particles within the range studied-i. e., from about 60 to 1000 microns in diameter-did not affect the ignition temperature of the dust. The rate of air flow through the sample between the limits
345
300 to 500 cc. per minute did not affect the ignition teniperature, but it did affect the slope of the time-temperature curve. For that reason it is desirable to maintain the flow of air constant during each determination. Very fine dust and materials, such as starch, that intumesce, tend to clog the tube and restrict the flow of air, thus making a determination of their ignition temperatures more difficult.
TIME
IN SECONDS
The platinum-platinum rhodium thermocouple was calibrated a number of times. It was not affected over the temperature range used. -4 copper-constantan couple is not satisfactory. It fails after two or three determinations even if removed from the J-tube before the temperature exceeds 500" C., which is, of course, well above the ignition temperatures of the materials tested. The ignition temperatures of wood and cellulose have been determined by other investigators. Hawley and Wise (S) report the ignition temperature of wood as 275" C. The method used in making this determination is not given. Holm ( 4 ) gives 360" C. as the ignition temperature of cellulose. Both these values agree very well with those reported in this paper of 277" and 354" C. for wood flour and cellulose, respectively. The apparatus and method described give results that can be duplicated, and the determination fulfils the requirements of the definition for ignition temperature. Acknowledgment
The authors Trish to acknowledge the helpful assistance rendered by Roswell Ewart, formerly of this bureau. L i t e r a t u r e Cited (1) Arms, University of Illinois Eng. Expt. Sta., Bull. 128. ( 2 ) Bunte and Kdlmel, Gas Tt'asserfach, 65, 592 (1922).
(3) (4) (5) (6) (7)
Hawley and Wise, "Chemistry of Wood," p . 187, Chemical Catalog Holm, Z . angew. Chem., 26, 273 (1913). Planz, Gas T17asserfach, 65, 478 (1929). Sinnatt and Moore, J . SOC.Chem. I n d . , 39, 72 (1920). Wheeler, J . Chem. Soc., 113, 949 (1918).
Denmark Changes Duties on Methanol and Fusel Oil The duty on fusel oil entering Dmmark for use in certain manufactures has been removed in order to protect the domestic nitrocellulose industry. Furthermore, methanol, which before was dutiable a t 2 cents per kilogram, has been placed on the free list when imported in such form that it cannot be used for the preparation of materials, perfumes, and other toiletries. Otherwise i t is dutiable a t 12 cents per liter, and subject to a heavy internal alcohol tax.
