COMBUSTION EXPERIMENTS with VAPORS, GASES, and DUSTS FELIX ZEMAN State Commercial Academy, Chrudim, Czechoslovakia
k...%-...7'
OMBUSTION experiments performed in the top part of a Kipp apparatus, described by Ohmann in his excellent manual ( I ) , have not received the attention they deserve. This is partly attributable to the cumbrous form and difficult cleaning of the apparatus, and partly to the mistrust and aversion of teach- . ers for experiments with fast-burning mixtures. The combustion vessel illustrated in Figure 1, operated with remote heat-control, makes such experiments easy, safe, and accurate. A few drops of combustible liquid are measured into the combustion vessel by means of a capillary pipet; then both ends are stoppered and equilibrium is attained by rotating the vessel for 10 to 20 seconds. The vessel is then placed in an inclined position, stem upwards, on a straw ring. The upper cork is removed and a short paper tube is inserted into the stem (Figure 3). Explosion is induced by means of a short piece of electrically heated nichrome wire or by a sp&k from a Ford coil. The shape of the vessel permits quick removal of exhaust gases by swinging, and the experiments can follow in rapid succession. The boiling points, flash points, vapor pressures, and other relevant properties of common laboratory solvents are readily ascertainable from any handbook. The following explosion limits, expressed in terms of ml. of solvent introduced into a 2000-ml. vessel a t 20°C., may serve as a convenient guide to the prospective experimenter, however.
C
Sduenf
Lmn
Ethanol Methanol
0.17 0.26 0.10 0.11 0.12 0.11 0.38 0.20
mhei Gasoline
B~~~~~~ Carbon d i i l f i d e Ethyl acetate ~~~t~~
~ x p l o s i o n~inrifr UPPn OPfimum 1.3 1.5 0.9 0.4 0.7 1.8 0.9 0.6
0.37 0.39 0.39 0.27 0.31 0.48 0.52 0.30
&
k m...........
Froun~1 a, rubber tube.
h, cable with nichrome wire.
The easy control of heating permits the demonstration of differences in kindling temperatures and' the influence of such catalysts as palladium asbestos, and finely divided oxides of cobalt, manganese, and copper. Decomposition of large molecules preceding ignition
is shown by a strip of celluloid fastened with a drop of collodion to the heating wire. The temperature is raised slowly until the strip is vaporized and the vapors are exploded (2). The rdle sometimes played by solid combustibles of low kindling temperature in initiating the combustion of vapors may be suggested by attaching a bit of sulfur to the heating wire. The influence of ionization on the limits and the velocity of combustion can be demonstrated by vaporizing in the vessel, filled with an explosive mixture, a bit of celluloid by means of a condensed spark from a Ford coil. The coil gives a highly ionized flame of constant length
All kinds of combustible dusts (e. g., sugar, starch, coal) can be exploded when properly dried, by a matchhead glued t o the wire. The experiment with coal dust shows in a striking manner the danger of lighting a match in a coal mine. Otherwise inexplosive coal dust may become explosive by absorbing combustible gases (3).
The filling of the vessel for experiments with lighting gas is shown in Figure 2. The filling device consists of a two-hole stopper fitted with an inlet tube for gas and an outlet protected by an absorption tube packed with steel wool. The vertical tube (1 cm. X 60 cm.), fitted into the stem by means of a one-hole stopper, is corked a t the upper end. When the effluent gas burns a t the outlet with a bright flame the filling is finished. The filling device is then replaced by a one-hole stopper with a 'i-mm. bore and the gas is lighted a t the top of the uncorked vertical t.ube: The oscillatory character of the explosion (4) manifests itself in the deep musical tone of the flame which descends with increasing velocity and starts the explosion when it reaches the combustion vessel. The principle of the Davy safety lamp may be illustrated by means of steel shavings inserted into the stem of the vessel. Many other experiments may be devised ( 5 ) with this simple and inexpensive, but efficient, apparatus. The author has had such an apparatus in constant use for five years. The experiments are not only attractive, forming a welcome visual support to lessons on safe handling of combustibles (G), but also constitute an introduction to the discussion of the kinetics of chemical reactions between gases and the possibilities of technical application of recent discoveries concerning the mechanism of combustion. Ionization by charged metal particles may be effected by short-circuiting a strip of metal foil acrois a 6-A,,110-v. line. Inhibition can be shown by adding iodine to the vaporized liquids. When trichloroethylene is used for this purpose the phosgene danger should be kept in mind. I t is best to remove poisonous exhaust gases quickly by means of a water pump. Detonation of gasoline vapor can be induced by a trace of a fulminate fastened to the wire.
(1) OHMANN, OTTO,"Unfallverhiitung." Winckelmann, Berlin, 1928, pp. 1 2 9 4 7 . ( 2 ) "Celluloid fires," The Fireman, 55, 188 (1932). ( 3 ) Glens, W. E., "The dust hazard in industry," Ernest Benn. Lld., London. 1925; BENGERSD~RFER, "Stauhenplosionen," Steinkopf, Dresden, 1925. (4) MAXWELL, C. B. AND WHEELER, R . V., Petroleum Times, 21, 948 (1929); I d Eng. Chsm.:,ZO, 1044 (1928); Schildwichter. "Verhrennungsverlauf, Z. angelu. Chew.. 46, 587 (1933). (5) LANGHANS, "Explosionen," Dr. Schrimpf, Miinchen, 1930. (6) JORDAN, 0170, "Chemische Technologic der Lihungsmlttel," Julius Springer, Berlin. 1933.
