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Atomistry » Oxygen » Chemical Properties » Ignition Temperatures » Flash-point » |
Ignition Temperatures
The temperature at which rapid combustion becomes independent of external supplies of heat is termed the ignition temperature. Wheeler defines it as " the lowest temperature to which a mixture of a combustible gas with air or oxygen must be raised in order that the chemical action between the gas and the oxygen can become so rapid as to produce flame." A clear conception of this phenomenon may be obtained by supposing a combustible mixture of gases, such as that of air and the vapour of carbon disulphide, to issue through an orifice into an indifferent atmosphere. If the orifice is surrounded by a ring of platinum wire, which is gradually heated up by a current of electricity, a flame will gradually make its appearance. If, as soon as this is observed, the heating of the wire by the current be discontinued, the flame will disappear; it is, in fact, not self-supporting, but depends on the accessory supply of heat through the electrically heated wire. If now the ring is raised to a higher temperature a brighter flame results, owing to an increased rate of chemical action, and at last we shall reach a point where it is possible to cut off the electric current without causing at the same time the extinction of the flame. This is the true temperature of ignition, the temperature at which the reaction proceeds at a rate just sufficient to overbalance the loss of heat by radiation, conduction, and convection from the burning layer of gases, so that the next layer is put in the same state, and steady combustion proceeds.
The minimum temperature at which the reaction in a combustible mixture of gases becomes self-supporting is termed the sub-ignition temperature. This may not correspond to ordinary ignition. In many cases an ordinary flame, causing more or less complete and rapid combustion, cannot be obtained by heating a gas to its sub-ignition temperature, and in those cases in which such a flame appears it is only produced through the intermediary of a cool flame. In the case of a few substances the ignition temperature lies at or below that of the atmosphere. Liquid phosphoretted hydrogen, P2H4, and certain metallic alkyl derivatives are cases in point. These are spontaneously inflammable. Pure gaseous hydrogen phosphide, PH3, may be ignited by a tube of boiling water, and carbon bisulphide vapour by a gently heated glass rod, the ignition temperature in this latter case being about 120° C. An interesting case of ignition is afforded by ordinary ethyl ether. Its vapour ignites when mixed with air and allowed to rush into a partly exhausted tube. The conversion of the translational energy of the mixture into heat as the gases enter the tube effecting a rise in temperature sufficient to attain to the ignition point. In 1816 Davy gave the results of the first systematic attempts to determine the ignition temperatures of the more common combustible gases. The importance of the subject, particularly in connection with hydrocarbons, will be evident when its bearing upon explosions in coal mines is remembered. Of modern methods of determining ignition temperatures, the following deserve consideration: Experiments of Meyer and Munch
A stream of the combustible gases mixed with air is passed through a tube, the temperature of which is raised until the gases inflame.
In the experiments of Meyer and Munch the mixture of combustible gases and air (or oxygen) was passed through a capillary tube to the base of a small glass vessel, in which the ignition was destined to take place, and which was inserted in the bulb of an air thermometer. When the mixture inflamed, the temperature of the gases was calculated from the volume of the gas in the air thermometer. This method is simple and possesses the advantage of yielding results at atmospheric pressures. But the results are liable to be influenced by the catalytic activity of the walls of the tube. Some of the results obtained in this manner are given in the table. It is important to remember that these results refer to the ignition temperatures of the gases when in motion. These are not quite the same as when the gases are at rest. Thus, it has been observed that the ignition temperature of detonating gas at 150 mm. pressure falls as the velocity rises to a maximum, after which further increase in the velocity has but little influence. This is well shown by the following data:
The ignition temperature rises with the pressure (vide infra). The diameter of the tube is without influence between the range 3.6 to 11 mm. With tubes of diameter less than 0.5 mm. no definite ignition temperature has been observed. Ignition Temperatures as obtained by Method I
Method devised by Dixon
Experiments carried out with hydrogen and oxygen at pressures ranging from 428 mm. to 1460 mm. yielded interesting results, some of which were as follow: Ignition Temperature of Hydrogen and Oxygen at various pressures
The ignition temperature is seen to rise but slowly as the pressure falls to almost half an atmosphere; but increase of pressure above one atmosphere effects a considerable depression of the ignition temperature. The rate of depression, whilst well marked up to one and a half atmospheres is much slower afterwards, and at high pressures the ignition temperature would probably become fairly constant at about 550° to 560° C. The main results obtained at atmospheric pressure were as follow: Ignition Temperatures
It is interesting to note that the ignition temperature of hydrogen in oxygen was found the same as for hydrogen in air. The same is true generally for carbon monoxide and ethane; but not for methane or ethylene. Cyanogen and hydrogen sulphide ignite at temperatures in oxygen considerably lower than in air. In the latter case, indeed, there is a difference of 140° C. Adiabatic Compression
It is a matter of common knowledge that when a gas is rapidly compressed, a rise in temperature takes place, the relationship between the initial and final temperatures being given by the expression
when T is the absolute temperature and y the ratio of the gaseous specific heats at constant pressure and volume. It is clear that if a mixture of a combustible gas and air were employed and the compression were sufficiently great, the temperature might rise to the ignition-point and rapid combustion ensue. This was suggested many years ago by Nernst, and carried into effect by Falk 1 in 1906 and by Dixon eight years later. Falk's method, which may be regarded as of a pioneering character, is open to criticism and has led to untrustworthy results.
The descent, of the cylinder was centred by the steel collar, and hard chrome steel plates, cut with a slot, could be placed on this collar to stop the piston-head at any point in its descent. The cylinder was held by an iron frame, which rested upon a large concrete bed. It was surrounded by a brass water-jacket, not shown in the figure, for regulating the temperature. The compression was effected by allowing a mass of iron, weighing 76 kilograms (2.5 cwt.), to fall from a given height, usually 1.5 metres (5 feet) on to H. Lanoline was employed as lubricant. The value for γ was taken as 1.40, and the assumption made that there was no loss of heat during the compression of the gases in the cylinder. Actually that was not quite the case, but experiment showed that the compression lasted only about 0-06 second in the case of electrolytic gas, so that the loss of heat would be small. The effect would be to raise the calculated temperature of ignition. The experiments were carried out by the method of trial and error, by regulating the number of steel collars until an explosion just took place. The initial and final volumes were thus known, and these data, coupled with the initial temperature, enabled the ignition temperature to be calculated from the equation given above. The main results obtained are given in the table. A study was made of the influence of the initial temperature upon the ignition, but variation between the normal temperature of the room and 100° C. made no appreciable difference. Increase of pressure likewise appeared without effect, although reduction to half an atmosphere raised the ignition temperature from 526° to 549° C. Addition of excess of oxygen beyond that required for complete combustion resulted in a gradual depression of the ignition temperature until, when the gases were in the proportions 2H2 + 32O2, no explosion would take place. The ignition temperatures of Hydrogen and Oxygen as determine by Adiabatic compression
Ignition temperatures of mixtures of electrolytic gas and argon
Addition of excess of hydrogen, on the other hand, served to raise the ignition temperature steadily, with practically linear precision, so that the temperature of ignition in the presence of x molecules of hydrogen, within the limits x = 0 and 13, could be calculated from the expression (2H2 + O2 + xH2) explodes at (526 + 18x) °C. Nitrogen behaved similarly, the corresponding expression between the limits x = 0 and 14 being (2H2 + O2 + xN2) explodes at (526 + 11x)° C. Fiesel's method
The apparatus consisted of an iron globe C (fig.), in which combustion took place. Into this was fixed a thin-walled glass bulb G, attached to a manometer M, and to the oxygen supply. A small piece of metal is hung in the rubber-pressure tubing at B, and can be released at will. C rests on an iron pillar A, and is kept in position by the iron tube D, which is pressed on to it by screwing up nuts E1, E2. The apparatus was placed in an electric furnace and G filled with oxygen. The temperature was now raised, and hydrogen introduced into C. A steady temperature being attained, as registered by two thermo-couples in C (not shown in the figure), the pressure of the oxygen in G was slightly increased, and the metal piece in B allowed to fall and break G, thus causing the oxygen to pour into the hydrogen. If combustion took place, a variation in pressure occurred which was registered either by the manometer or by a delicate membrane attached thereto. The following results were obtained: Ignition temperatures of mixtures of oxygen and hydrogen
The minimum ignition temperature was found to occur with 3 volumes of hydrogen to 2 of oxygen, both in the dry and when moist. Further addition of hydrogen raised the ignition temperature, as was observed by Dixon and Crofts, but the results obtained by the latter investigators were very much higher. With the moist gases the rate of combination suggested a bimolecular reaction, which might proceed through the formation of hydrogen peroxide. For the dry gases the reaction was found to be trimolecular, as is to be expected from the equation: 2H2 + O2 = 2H2O. The results for acetylene were not altogether satisfactory, the ignition temperature of a mixture of acetylene and air appeared to be about 390° C. Hot-wire Ignition
In 1917 McDavid suggested that ignition temperatures might be determined by allowing the mixture of combustible gases and air to inflate soap bubbles and causing these to impinge upon a heated platinum wire. The short time of contact with the wire should reduce to a minimum any surface action such as that detailed above. The results obtained, however, are of uncertain value. |
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