Acetylene, the Principles of Its Generation and Use Part 13

THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE

It will only be necessary for the purpose of this book to indicate the more important chemical and physical properties of acetylene, and, in particular, those which have any bearing on the application of acetylene for lighting purposes. Moreover, it has been found convenient to discuss fully in other chapters certain properties of acetylene, and in regard to such properties the reader is referred to the chapters mentioned.

PHYSICAL PROPERTIES.--Acetylene is a gas at ordinary temperatures, colourless, and, when pure, having a not unpleasant, so-called "ethereal"

odour. Its density, or specific gravity, referred to air as unity, has been found experimentally by Leduc to be 0.9056. It is customary to adopt the value 0.91 for calculations into which the density of the gas enters (_vide_ Chapter VII.). The density of a gas is important not only for the determination of the size of mains needed to convey it at a given rate of flow under a given pressure, as explained in Chapter VII., but also because the volume of gas which will pa.s.s through small orifices in a given time depends on its density. According to Graham's well-known law of the effusion of gases, the velocity with which a gas effuses varies directly as the square root of the difference of pressure on the two sides of the opening, and inversely as the square root of the density of the gas. Hence it follows that the volume of gas which escapes through a porous pipe, an imperfect joint, or a burner orifice is, provided the pressure in the gas-pipe is the same, a function of the square root of the density of the gas. Hence this density has to be taken into consideration in the construction of burners, i.e., a burner required to pa.s.s a gas of high density must have a larger orifice than one for a gas of low density, if the rate of flow of gas is to be the same under the same pressure. This, however, is a question for the burner manufacturers, who already make special burners for gases of different densities, and it need not trouble the consumer of acetylene, who should always use burners devised for the consumption of that gas. But the Law of effusion indicates that the volume of acetylene which can escape from a leaky supply-pipe will be less than the volume of a gas of lower density, _e.g._, coal-gas, if the pressure in the pipe is the same for both.

This implies that on an extensive distributing system, in which for practical reasons leakage is not wholly avoidable, the loss of gas through leakage will be less for acetylene than for coal-gas, given the same distributing pressure. If _v_ = the loss of acetylene from a distributing system and _v'_ = the loss of coal-gas from a similar system worked at the same pressure, both losses being expressed in volumes (cubic feet) per hour, and the coal-gas being a.s.sumed to have a density of 0.04, then

(1) (_v_/_v'_) = (0.40 / 0.91)^(1/2) = 0.663

or, _v_ = 0.663_v'_,

which signifies that the loss of acetylene by leakage under the same conditions of pressure, &c., will be only 0.663 times that of the loss of coal-gas. In practice, however, the pressures at which the gases are usually sent through mains are not identical, being greater in the case of acetylene than in that of coal-gas. Formula (1) therefore requires correction whenever the pressures are different, and calling the pressure at which the acetylene exists in the main _p_, and the corresponding pressure of the coal-gas _p'_, the relative losses by leakage are--

(2) (_v_/_v'_) = (0.40 / 0.91)^(1/2) x (_p_/_p'_)^(1/2)

_v_ = 0.663_v'_ x (_p_/_p'_)^(1/2)

It will be evident that whenever the value of the fraction (_p_/_p'_)^(1/2), is less than 1.5, _i.e._, whenever the pressure of the acetylene does not exceed double that of the coal-gas present in pipes of given porosity or unsoundness, the loss of acetylene will be less than that of coal-gas. This is important, especially in the case of large village acetylene installations, where after a time it would be impossible to avoid some imperfect joints, fractured pipes, &c., throughout the extensive distributing mains. The same loss of gas by leakage would represent a far higher pecuniary value with acetylene than with coal-gas, because the former must always be more costly per unit of volume than the latter. Hence it is important to recognise that the rate of leakage, _c?teris paribus_, is less with acetylene, and it is also important to observe the economical advantage, at least in terms of gas or calcium carbide, of sending the acetylene into the mains at as low a pressure as is compatible with the length of those mains and the character of the consumers' burners. As follows from what will be said in Chapter VII., a high initial pressure makes for economy in the prime cost of, and in the expense of laying, the mains, by enabling the diameter of those mains to be diminished; but the purchase and erection of the distributing system are capital expenses, while a constant expenditure upon carbide to meet loss by leakage falls upon revenue.

The critical temperature of acetylene, _i.e._, the temperature below which an abrupt change from the gaseous to the liquid state takes place if the pressure is sufficiently high, is 37 C., and the critical pressure, _i.e._, the pressure under which that change takes place at that temperature, is nearly 68 atmospheres. Below the critical temperature, a lower pressure than this effects liquefaction of the gas, _i.e._, at 13.5 C. a pressure of 32.77 atmospheres, at 0 C., 21.53 atmospheres (Ansdell, _cf._ Chapter XI.). These data are of comparatively little practical importance, owing to the fact that, as explained in Chapter XI., liquefied acetylene cannot be safely utilised.

The mean coefficient of expansion of gaseous acetylene between 0 C. and 100 C., is, under constant pressure, 0.003738; under constant volume, 0.003724. This means that, if the pressure is constant, 0.003738 represents the increase in volume of a given ma.s.s of gaseous acetylene when its temperature is raised one degree (C.), divided by the volume of the same ma.s.s at 0 C. The coefficients of expansion of air are: under constant pressure, 0.003671; under constant volume, 0.003665; and those of the simple gases (nitrogen, hydrogen, oxygen) are very nearly the same. Strictly speaking the table given in Chapter XIV., for facilitating the correction of the volume of gas measured over water, is not quite correct for acetylene, owing to the difference in the coefficients of expansion of acetylene and the simple gases for which the table was drawn up, but practically no appreciable error can ensue from its use. It is, however, for the correction of volumes of gases measured at different temperatures to one (normal) temperature, and, broadly, for determining the change of volume which a given ma.s.s of the gas will undergo with change of temperature, that the coefficient of expansion of a gas becomes an important factor industrially.

Ansdell has found the density of liquid acetylene to range from 0.460 at -7 C. to 0.364 at +35.8 C., being 0.451 at 0 C. Taking the volume of the liquid at -7 as unity, it becomes 1.264 at 35.8, and thence Ansdell infers that the mean coefficient of expansion per degree is 0.00489 for the total range of pressure." a.s.suming that the liquid was under the same pressure at the two temperatures, the coefficient of expansion per degree Centigrade would be 0.00605, which agrees more nearly with the figure 0.007 which is quoted, by Fouche As mentioned before, data referring to liquid (_i.e._, liquefied) acetylene are of no practical importance, because the substance is too dangerous to use. They are, however, interesting in so far as they indicate the differences in properties between acetylene converted into the liquid state by great pressure, and acetylene dissolved in acetone under less pressure; which differences make the solution fit for employment. It may be observed that as the solution of acetylene in acetone is a liquid, the acetylene must exist therein as a liquid; it is, in fact, liquid acetylene in a state of dilution, the diluent being an exothermic and comparatively stable body.

The specific heat of acetylene is given by M. A. Morel at 0.310, though he has not stated by whom the value was determined. For the purpose of a calculation in Chapter III. the specific heat at constant pressure was a.s.sumed to be 0.25, which, in the absence of precise information, appears somewhat more probable as an approximation to the truth. The ratio (_k_ or C_p/C_v ) of the specific heat at constant pressure to that at constant volume has been found by Maneuvrier and Fournier to be 1.26; but they did not measure the specific heat itself. [Footnote: The ratio 1.26 _k_ or (C_p/C_v) has been given in many text-books as the value of the specific heat of acetylene, whereas this value should obviously be only about one-fourth or one-fifth of 1.26.

By employing the ordinary gas laws it is possible approximately to calculate the specific heat of acetylene from Maneuvrier and Fournier's ratio. Taking the molecular weight of acetylene as 26, we have

26 C_p - 26 C_v = 2 cal.,

and

C_p = 1.26 C_v.

From this it follows that C_p, _i.e._, the specific heat at constant pressure of acetylene, should be 0.373.] It will be seen that this value for _k_ differs considerably from the corresponding ratio in the case of air and many common gases, where it is usually 1.41; the figure approaches more closely that given for nitrous oxide. For the specific heat of calcium carbide Carlson quotes the following figures:

0 1000 1500 2000 2500 3000 3500 0.247 0.271 0.296 0.325 0.344 0.363 0.381

The molecular volume of acetylene is 0.8132 (oxygen = 1).

According to the international atomic weights adopted in 1908, the molecular weight of acetylene is 26.016 if O = 16; in round numbers, as ordinarily used, it is 26. Employing the latest data for the weight of 1 litre of dry hydrogen and of dry normal air containing 0.04 per cent. of carbon dioxide at a temperature of 0 C. and a barometric pressure of 760 mm. in the lat.i.tude of London, viz., 0.089916 and 1.29395 grammes respectively (Castell-Evans), it now becomes possible to give the weight of a known volume of dry or moist acetylene as measured under stated conditions with some degree of accuracy. Using 26.016 as the molecular weight of the gas (O = 16), 1 litre of dry acetylene at 0 C. and 760 mm.

weighs 1.16963 grammes, or 1 gramme measures 0.854973 litre. From this it follows that the theoretical specific gravity of the gas at 0/0 C. is 0.9039 (air = 1), a figure which may be compared with Leduc's experimental value of 0.9056. Taking as the coefficient of expansion at constant pressure the figure already given, viz., 0.003738, the weights and measures of dry and moist acetylene observed under British conditions (60 F. and 30 inches of mercury) become approximately:

Dry. Saturated.

1 litre . . . 1.108 grm. . . 1.102 grm.

1 gramme . . . 0.902 litre. . . 0.907 litre.

1000 cubic feet . 69.18 lb. . . . 68.83 lb.

It should be remembered that unless the gas has been pa.s.sed through a chemical drier, it is always saturated with aqueous vapour, the amount of water present being governed by the temperature and pressure. The 1 litre of moist acetylene which weighs 1.102 gramme at 60 F. and 30 inches of mercury, contains 0.013 gramme of water vapour; and therefore the weight of dry acetylene in the 1 litre of moist gas is 1.089 gramme. Similarly, the 68.83 pounds which const.i.tute the weight of 1000 cubic feet of moist acetylene, as measured under British standard conditions, are composed of almost exactly 68 pounds of dry acetylene and 0.83 pound of water vapour.

The data required in calculating the ma.s.s of vapour in a known volume of a saturated gas at any observed temperature and pressure, _i.e._, in reducing the figures to those which represent the dry gas at any other (standard) temperature and pressure, will be found in the text-books of physical chemistry. It is necessary to recollect that since coal-gas is measured wet, the factors given in the table quoted in Chapter XIV. from the "Notification of the Gas Referees" simply serve to convert the volume of a wet gas observed under stated conditions to the equivalent volume of the same wet gas at the standard conditions mentioned.

HEAT OF COMBUSTION, &C--Based on Berthelot and Matignon's value for the heat of combustion which is given on a subsequent page, viz., 315.7 large calories per molecular weight of 26.016 grammes, the calorific power of acetylene under different conditions is shown in the following table:

Dry. Dry. Saturated.

0 C. & 760 mm. 60 F & 30 ins. 60 F. & 30 ins.

1 gramme 12.14 cals. 12.14 cals. 12.0 cals.

1 litre 14.l9 " 13.45 " 13.22 "

1 cubic foot 40.19 " 380.8 " 374.4 "

The figures in the last column refer to the dry acetylene in the gas, no correction having been made for the heat absorbed by the water vapour present. As will appear in Chapter X., the average of actual determinations of the calorific value of ordinary acetylene is 363 large calories or 1440 B.Th.U. per cubic foot. The temperature of ignition of acetylene has been generally stated to be about 480 C. V. Meyer and Munch in 1893 found that a mixture of acetylene and oxygen ignited between 509 and 515 C. Recent (1909) investigations by H. B. Dixon and H. F. Coward show, however, that the ignition temperature in neat oxygen is between 416 and 440 (mean 428 C.) and in air between 406 and 440, with a mean of 429 C. The corresponding mean temperature of ignition found by the same investigators for other gases are: hydrogen, 585; carbon monoxide, moist 664, dry 692; ethylene, in oxygen 510, in air 543; and methane, in oxygen between 550 and 700, and in air, between 650 and 750 C.

Numerous experiments have been performed to determine the temperature of the acetylene flame. According to an exhaustive research by L. Nichols, when the gas burns in air it attains a maximum temperature of 1900 C.

20, which is 120 higher than the temperature he found by a similar method of observation for the coal-gas flame (fish-tail burner). Le Chatelier had previously a.s.signed to the acetylene flame a temperature between 2100 and 2400, while Lewes had found for the dark zone 459, for the luminous zone 1410, and for the tip 1517 C, Fery and Mahler have also made measurements of the temperatures afforded by acetylene and other fuels, some of their results being quoted below. Fery employed his optical method of estimating the temperature, Mahler a process devised by Mallard and Le Chatelier. Mahler's figures all relate to flames supplied with air at a temperature of 0 C. and a constant pressure of 760 mm.

Hydrogen . . . . . . . . . . . 1900 1960 Carbon monoxide . . . . . . . . . -- 2100 Methane . . . . . . . . . . . -- _ 1850 Coal-gas (luminous) . . . . . . . . 1712 | " (atmospheric, with deficient supply of air) . 1812 | 1950 " (atmospheric, with full supply of air) . . 1871 _| Water-gas . . . . . . . . . . -- 2000 Oxy-coal-gas blowpipe . . . . . . . 2200 -- Oxy-hydrogen blowpipe . . . . . . . 2420 -- Acetylene . . . . . . . . . . 2548 2350 Alcohol . . . . . . . . . . . 1705 1700 Alcohol (in Denayrouze Bunsen) . . . . . 1862 -- Alcohol and petrol in equal parts . . . . 2053 -- Crude petroleum (American) . . . . . . -- 2000 Petroleum spirit " . . . . . . . -- 1920 Petroleum oil " . . . . . . . -- 1660

Catani has published the following determinations of the temperature yielded by acetylene when burnt with cold and hot air and also with oxygen:

Acetylene and cold air . . . . . . 2568 C.

" air at 500 C . . . . 2780 C.

" air at 1000 C . . . . 3000 C.

" oxygen . . . . . . 4160 C.

EXPLOSIVE LIMITS.--The range of explosibility of mixtures of acetylene and air has been determined by various observers. Eitner's figures for the lower and upper explosive limits, when the mixture, at 62.6 F., is in a tube 19 mm. in diameter, and contains 1.9 per cent. of aqueous vapour, are 3.35 and 52.3 per cent. of acetylene (_cf._ Chapter X.).

In this case the mixture was fired by electric spark. In wider vessels, the upper explosive limit, when the mixture was fired by a Bunsen flame, was found to be as high as 75 per cent. of acetylene. Eitner also found that when 13 of the 21 volumes of oxygen in air are displaced by carbon dioxide, a mixture of such "carbon dioxide air" with acetylene is inexplosive in all proportions. Also that when carbon dioxide is added to a mixture of acetylene and air, an explosion no longer occurs when the carbon dioxide amounts to 46 volumes or more to every 54 volumes of air, whatever may be the proportion of acetylene in the mixture. [Footnote: According to Caro, if acetylene is added to a mixture composed of 55 per cent. by volume of air and 45 per cent. of carbon dioxide, the whole is only explosive when the proportion of acetylene lies between 5.0 and 5.8 per cent. Caro has also quoted the effect of various inflammable vapours upon the explosive limits of acetylene, his results being referred to in Chapter X.] These figures are valuable in connexion with the prevention of the formation of explosive mixtures of air and acetylene when new mains or plant are being brought into operation (_cf._ Chapter VII.). Eitner has also shown, by direct investigation on mixtures of other combustible gases and air, that the range of explosibility is greatly reduced by increase in the proportion of aqueous vapour present.

As the proportion of aqueous vapour in gas standing over water increases with the temperature the range of explosibility of mixtures of a combustible gas and air is naturally and automatically reduced when the temperature rises, provided the mixture is in contact with water. Thus at 17.0 C., mixtures of hydrogen, air, and aqueous vapour containing from 9.3 to 65.0 per cent, of hydrogen are explosive, whereas at 78.1 C., provided the mixture is saturated with aqueous vapour, explosion occurs only when the percentage of hydrogen in the mixture is between 11.2 and 21.9. The range of explosibility of mixtures of acetylene and air is similarly reduced by the addition of aqueous vapour (though the exact figures have not been experimentally ascertained); and hence it follows that when the temperature in an acetylene generator in which water is in excess, or in a gasholder, rises, the risk of explosion, if air is mixed with the gas, is automatically reduced with the rise in temperature by reason of the higher proportion of aqueous vapour which the gas will retain at the higher temperature. This fact is alluded to in Chapter II.

Acetone vapour also acts similarly in lowering the upper explosive limit of acetylene (_cf._ Chapter XI.).

It may perhaps be well to indicate briefly the practical significance of the range of explosibility of a mixture of air and a combustible gas, such as acetylene. The lower explosive limit is the lowest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in less than that percentage explosion is impossible. The upper explosive limit is the highest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in more than that percentage explosion is impossible. Mixtures, however, in which the percentage of combustible gas lies between these two limits will explode when a light or spark is applied to them; and the comprehensive term "range of explosibility" is used to cover all lying between the two explosive limits. If, then, a naked light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is below the lower limit of explosibility, the gas will not take fire, but the light will continue to burn, deriving its necessary oxygen from the excess of air present. On the other hand, if a light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is above the upper limit of explosibility, the light will be extinguished, and within the vessel the gaseous mixture will not burn; but it may burn at the open mouth of the vessel as it comes in contact with the surrounding air, until by diffusion, &c., sufficient air has entered the vessel to form, with the remaining gas, a mixture lying within the explosive limits, when an explosion will occur. Again, if a gaseous mixture containing less of its combustible const.i.tuent than is necessary to attain the lower explosive limit escapes from an open-ended pipe and a light is applied to it, the mixture will not burn as a useful compact flame (if, indeed, it fires at all); if the mixture contains more of its combustible const.i.tuent than is required to attain the upper explosive limit, that mixture will burn quietly at the mouth of the pipe and will be free from any tendency to fire back into the pipe--a.s.suming, of course, that the gaseous mixture within the pipe is constantly travelling towards the open end. If, however, a gaseous mixture containing a proportion of its combustible const.i.tuent which lies between the lower and the upper explosive limit of that const.i.tuent escapes from an open- ended pipe and a light is applied, the mixture will fire and the flame will pa.s.s back into the pipe, there to produce an explosion, unless the orifice of the said pipe is so small as to prevent the explosive wave pa.s.sing (as is the case with a proper acetylene burner), or unless the pipe itself is so narrow as appreciably to alter the range of explosibility by lowering the upper explosive limit from its normal value.

By far the most potent factor in altering the range of explosibility of any gas when mixed with air is the diameter of the vessel containing or delivering such mixture. Le Chatelier has investigated this point in the case of acetylene, and his values are reproduced overleaf; they are comparable among themselves, although it will be observed that his absolute results differ somewhat from those obtained by Eitner which are quoted later:

_Explosive Limits of Acetylene mixed with Air._--(Le Chatelier.)

___________________________________________________________ | | | | | | Explosive Limits. | | | Diameter of Tube |_______________________| Range of | | in Millimetres. | | | Explosibility. | | | Lower. | Upper. | | |__________________|___________|___________|________________| | | | | | | | Per Cent. | Per Cent. | Per Cent. | | 40 | 2.9 | 64 | 61.1 | | 30 | 3.1 | 62 | 58.9 | | 20 | 3.5 | 55 | 51.5 | | 6 | 4.0 | 40 | 36.0 | | 4 | 4.5 | 25 | 20.5 | | 2 | 5.0 | 15 | 10.0 | | 0.8 | 7.7 | 10 | 2.3 | | 0.5 | ... | ... | ... | |__________________|___________|___________|________________|

Thus it appears that past an orifice or constriction 0.5 mm. in diameter no explosion of acetylene can proceed, whatever may be the proportions between the gas and the air in the mixture present.

With every gas the explosive limits and the range of explosibility are also influenced by various circ.u.mstances, such as the manner of ignition, the pressure, and other minor conditions; but the following figures for mixtures of air and different combustible gases were obtained by Eitner under similar conditions, and are therefore strictly comparable one with another. The conditions were that the mixture was contained in a tube 19 mm. (3/4-inch) wide, was at about 60 to 65 F., was saturated with aqueous vapour, and was fired by electric spark.

_Table giving the Percentage by volume of Combustible Gas in a Mixture of that Gas and Air corresponding with the Explosive Limits of such a Mixture._--(Eitner.)

____________________________________________________________________ | | | | | | Description of | Lower | Upper | Difference between the | | Combustible Gas. | Explosive | Explosive | Lower and Upper Limits, | | | Limit. | Limit. | showing the range | | | | | covered by the | | | | | Explosive Mixtures. | |__________________|___________|___________|_________________________| | | | | | | | Per Cent. | Per Cent. | Per Cent. | | Carbon monoxide | 16.50 | 74.95 | 58.45 | | Hydrogen | 9.45 | 66.40 | 57.95 | | Water-gas | | | | | (uncarburetted) | 12.40 | 66.75 | 54.35 | | ACETYLENE | 3.35 | 52.30 | 48.95 | | Coal-gas | 7.90 | 19.10 | 11.20 | | Ethylene | 4.10 | 14.60 | 10.50 | | Methane | 6.10 | 12.80 | 6.70 | | Benzene (vapour) | 2.65 | 6.50 | 3.85 | | Pentane " | 2.40 | 4.90 | 2.50 | | Benzoline " | 2.40 | 4.90 | 2.50 | |__________________|___________|___________|_________________________|

These figures are of great practical significance. They indicate that a mixture of acetylene and air becomes explosive (_i.e._, will explode if a light is applied to it) when only 3.35 per cent. of the mixture is acetylene, while a similar mixture of coal-gas and air is not explosive until the coal-gas reaches 7.9 per cent. of the mixture. And again, air may be added to coal-gas, and it does not become explosive until the coal-gas is reduced to 19.1 per cent. of the mixture, while, on the contrary, if air is added to acetylene, the mixture becomes explosive as soon as the acetylene has fallen to 52.3 per cent. Hence the immense importance of taking precautions to avoid, on the one hand, the escape of acetylene into the air of a room, and, on the other hand, the admixture of air with the acetylene in any vessel containing it or any pipe through which it pa.s.ses. These precautions are far more essential with acetylene than with coal-gas. The table shows further how great is the danger of explosion if benzene, benzoline, or other similar highly volatile hydrocarbons [Footnote: The nomenclature of the different volatile spirits is apt to be very confusing. "Benzene" is the proper name for the most volatile hydrocarbon derived from coal-tar, whose formula is C_6H_6.

Commercially, benzene is often known as "benzol" or "benzole"; but it would be generally advantageous if those latter words were only used to mean imperfectly rectified benzene, _i.e._, mixtures of benzene with toluene, &c., such as are more explicitly understood by the terms "90.s benzol" and "50.s benzol." "Gasoline," "carburine," "petroleum ether,"