T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Mar., 1914
in amount until t h e minimum is reached a t the point of complete softening. At t h e point of complete softening the methyl orange alkalinity also reaches t h e minimum while the phenolphthalein alkalinity is increasing. From the point of maximum softening all t h e constituents except t h e magnesium increase. T h e magnesium does not increase, none being added with t h e calcium lime a n d t h a t present in the magnesium-calcium lime is not dissolved. suM&I.mY-The difference between t h e action of calcium a n d magnesium-calcium limes is t h a t magnesium replaces t h e calcium until a minimum of calcium carbonate is reached. I t also reacts with magnesium acid carbonate forming magnesium carbonate. With the addition of more lime all of the magnesium is precipitated as hydroxide.
WATER SOFTENING
BY
CALCIUM
AND
MAGNESIGM-CALCIUM (DOLOMITE)
LIME
The softening of water is evidently dependent on the content of available calcium oxide and therefore all lime used for water softening should be bought o n the basis of the quantity of available calcium oxide. Moreover, i t is not advisable t o purchase lime containing magnesium because the magnesium is without value for softening a n d increases the amount of sludge to be handled. UNIVERSITY OF ILLINOIS
URBANA
~
_
_
LOWER LIMITS OF COMBUSTIBLE AND EXPLOSIVE MIXTURES OF GASES AND AIR1
191
correlation, interpretation a n d application is not a t all easy. I t is the purpose of this article t o discuss the conditions under which gaseous mixtures may become combustible or explosive, t o analyze the various factors which constitute these conditions, t o indicate a method for calculating t h e lower limits of combustible a n d explosive mixtures a n d t o compare the results of these calculations with existing experimental data. Additional d a t a bearing upon this question will be presented in a subsequent paper. A s a preliminary t o the discussion it will be necessary t o define with some care the terms which enter largely into the discussion. IGNITION TEXPERATURE
(I) The ignition temperature may be defined as the temperature to which a mixture of gas a n d air or gas a n d oxygen must be raised in order t o produce a n appreciably rapid chemical reaction. ( 2 ) T h e more usual definition of ignition temperature is the temperature to which a portion of a mixture of gas a n d air or gas and oxygen must be raised in order t h a t chemical reaction may take place throughout the entire gas. This definition implies a self-sustaining reaction a n d its application is limited t o combustible a n d explosive mixtures. I t makes no distinction between t h e ignition temperature of combustion a n d t h e ignition temperature of explosion a n d on this account it is not adequate. T h e temperature of ignition of combustion a n d t h e temperature of ignition of explosion are two distinct temperatures corresponding t o two different reaction velocities and a division into two separate definitions is desirable. (a) T h e ignition temperature of combustion is t h e temperature to which a portion of a gas mixture must be raised in order t h a t combustion may take place a n d t h e reaction be propagated throughout the entire mass of gas. ( b ) The ignition temperature of explosion is the temperature t o which a portion of a gas mixture must be raised in order t h a t a n explosion may occur a n d be propagated throughout t h e entire mass of gas. T h e first definition is of importance as defining t h e temperature necessary t o bring a b o u t a reaction. T h e latter definitions are, however, t h e ones of practical importance since they are the actual temperatures necessary t o produce a combustion or a n explosion for a given mixture under a given set of conditions.
B y E. E. SOMERMEIER Received Nov. 21, 1913
T H E I G N I T I O N TEMPERATURES O F COMBUSTIOX
T h e prevention of explosions in mines a n d t h e successful operation of many industrial a n d metallurgical processes are t o a large degree dependent upon definite information regarding the combustible a n d explosive properties of mixtures of gases a n d air. Much of t h e information on this subject found in t h e literature is indefinite, contradictory a n d incomplete or of limited application a n d the data are so scattered a n d in such unserviceable form t h a t their proper
According t o Dixon a n d Coward,l the temperatures to which various gases a n d air or gases a n d oxygen must be heated in order t o secure ignition when mixed are as follows:
1 Presented in outline to the Columbus Section of the American Chemical Society, January 23, 1913.
These values are for a pressure of one atmosphere.
GAS
Hydrogen ....................... Carbon monoxide. . . . . . . . . . . . . . . . nlethane ....................... Ethane.. .......................
1
Chem. News, 99 (1909). 139.
OXYGEN 580-590' 637-658O 536-700O 520-630'
XIR
5 80-590' 644-6.58' 650-750O 520-630°
T H E JOURNAL OF ILVDUSTRIAL A N D ENGINEERING CHENISTRY
192
Diminishing the pressure of t h e reacting gases t o onehalf atmosphere raises t h e temperature of ignition a b o u t j degrees. Increasing the pressure z atmospheres lowers t h e ignition temperature b y about 30 degrees. I n securing these values the two gases were heated in separate tubes a n d were then mixed a n d given a n opportunity t o react before any appreciable cooling had occurred. Each gas was raised t o t h e same temperature a n d under these conditions a n excess of either one or the other gas caused little difference i n t h e required ignition temperature. The values given include t h e temperatures of ignition of both rich a n d poor mixtures of gas with air a n d with oxygen. As will be seen from the table, carbon monoxide a n d hydrogen have practically the same ignition temperature with air as with oxygen. These values are the temperatures a t which active reaction takes place and correspond t o t h e first definition of ignition temperature. The experiments do not, however, take into account a t all whether or not the reaction is selfsustaining a n d on this account t h e results do not necessarily represent t h e actual ignition temperatures of combustible mixtures. They are, however, of value as a basis for the discussion of these temperatures. IGNITION TEMPERATURE O F EXPLOSION
Probably the most reliable values are those given by Falk.' These values for ignition temperatures were secured under very high pressures and hence while they probably represent the required ignition temperatures of t h e explosive wave they do not necessarily express very closely t h e temperatures necessary t o s t a r t explosive reactions and originate t h e high pressure explosive waves. Falk's values for the ignition temperatures of different explosive mixtures of hydrogen a n d oxygen a n d of carbon monoxide a n d oxygen are as follows:
+
= 514' = 540' HZ 2 0 2 = 532' Ha 4- 0 2 f 4Nz = 637'
Hz 2H2 f
+
0 2
0 2
These values are atmospheres. T h e compression of the increase in pressure ture high enough t o
2CO f Oz = 601' 4CO 0 2 = 628' C O f 0 2 = 631' 2CO f Oz f 2N2 = 685'
+
for pressures of from 30 t o 40 data were obtained by quick mixtures t o t h e point where t h e raised t h e mixture t o a temperacause ignition -and explosion.
Val. 6, NO.3
a detonating or explosive wave which, traveling with great rapidity (in t h e case of hydrogen z miles a second), produces a very high pressure in the gas. The gas being raised t o the ignition temperature by this high pressure, ignites a n d furnishes further energy for continuing the propagation of the explosive wave. During the explosion t h e ignition may be considered as originating a t a great number of separate points along the explosive wave, a n d if a column of hydrogen a n d air z miles long were t o be exploded, the time of t h e ignition of the portion farthest from the point of origin would be only one second later t h a n t h a t a t the origin of t h e explosion. While a practically continuous mass of igniting gas would exist from end t o end of the column, the ignition would not be a continuous propagation from molecule to molecule; and if, in t h e column of explosive mixture, a layer of inert gas & moderate thickness exists, the explosive wave would pass through this inert gas a n d the explosion continue on the other side. The distinction between a violent explosion a n d a moderate combustion is not difficult to make b u t t h e exact line of separation between vigorous combustion and feeble explosion is not so easily drawn. F o r t h e same mixture of gas t h e ignition of a small volume may be clearly a combustion while in the ignition of a larger volume the combustion merges into a n d becomes a n explosion. Assuming the distinction between an explosion a n d a combustion to be based upon whether the reaction is the continued communication from molecule t o molecule or is propagated as a n explosive wave due t o pressure, there is, a t least theoretically, no difficulty in classification. On this basis every explosion is preceded b y a more or less brief period of initial combustion. With a large quantity of gas a combustible mixture is a n explosive mixture, since when combustion is once started one of three conditions must result. T h e reaction decreases in rapidity, is constant, or increases in rapidity. If t h e rate decreases, t h e reaction is not self-sustaining a n d t h e mixture is not combustible. Under actual conditions a combustion a t a n exactly constant velocity is not likely t o occur; accordingly, any combustible gas will have a n accelerated velocity of reaction, a n d with a considerable amount of gas present a n explosion will occur.
DIFFERENCE B E T W E E N COMBUSTIOX A S D EXPLOSION
VELOCITIES OF COMBUSTIBLE AND EXPLOSIVE REACTIOXS
T h a t t h e temperature of ignition a n d t h e temperature of explosion are two separate temperatures corresponding t o t w o different reactioh velocities is perhaps best shown b y discussing in some detail t h e difference between combustion and explosion. During combustion the ignition is communicated direct f r o m molecule t o molecule a n d t h e zone of combustion spreads as a continuous advancing flame, a n d if, during t h e comparatively slow reaction of combustion, the advancing zone of combustion meets a n appreciable layer of inert gas t h e flame is extinguished a n d combustion ceases. I n a n explosion the high pressure developed b y t h e initial explosion starts
If t h e temperature of ignition of combustion a n d the temperature of ignition of explosion are clearly recognized as corresponding more or less closely t o differences in the velocities of the reactions, i t follows t h a t , for any particular mixture of gas with air or with oxygen and under a n y given set of conditions, there must be a minimum and a maximum rate of velocity of combustion. If t h e rate of reaction is less t h a n t h e minimum t h e gas is incombustible or will not burn under t h e given conditions. If the rate of reaction reaches or becomes greater t h a n a certain maximum the pressure developed produces a temperature sufficient to start ignition throughout t h e gas a n d t h e combustion becomes a n explosion. Just what t h e velocity of t h e
1
Jour. A m . Chem. SOC.,29 (1907). 1536.
Mar., 1914
T H E JOUR.VAL O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
reaction will be a t a n y given instant for a n y given mixture or for a n y given set of conditions is dependent upon a number of factors, t h e effects of which will be considered in detail. Some of these factors are as follows: ( I ) The thermal conductivity of t h e gases. ( 2 ) The initial temperature of t h e reacting mixture. (3) The initial pressure of t h e reacting mixture. (4) The thermal capacity of t h e container. (j) T h e amount of excess of either reacting gas over t h e ratio giving t h e maximum speed of reaction. (6) T h e amount of inert gases or products of combustion present, such as nitrogen, carbon dioxide, a n d water vapor. (;) T h e heat of t h e reaction. (8) T h e volume of t h e reacting mixture. ( 9 ) T h e length of time t h a t t h e reaction has been in progress. Nos. I , 2 , 3 a n d 4 are effective as long as t h e velocity of t h e reaction is t h a t of combustion, b u t during an actual explosion they are of little effect on t h e ignition temperature of explosion after t h e explosion is actually under way. They have, however, a very decided effect in aiding or preventing t h e occurrence of t h e explosive reaction. .Xn excess of either gas or t h e presence of inert gases retards t h e velocity of reaction and hence raises t h e ignition temperature whether i t be of combustion or of explosion. A n increase i n t h e initial temperature of the mixture lessens t h e radiation a n d conduction losses a n d hence lowers t h e actual temperature required for combustion. I n a n explosion a n increase in t h e initial temperature of t h e mixture does not lower t h e actual ignition temperatures of explosion, but since it lessens t h e additional heat required t o bring about t h e occurrence of t h e reaction, i t lowers t h e limit of t h e amount of combustible gas necessary t o be present a n d makes t h e gas correspondingly more explosive. For example, a given mixture of gas of a composition such t h a t at o o t h e mixture does not undergo combustion, may, upon a n increase in temperature, become not only combustible b u t actually explosive, because a n increase in initial temperature is equivalent t o so much more heat generated b y t h e reaction, a n d m a y so accelerate t h e rate of reaction as t o change t h e gas from a n incombustible mixture into a combustible a n d explosive one. On t h e other hand, a diminished initial temperature increases t h e conduction a n d radiation losses a n d also increases t h e temperature required for combustion, since with a large radiation a n d conduction loss the rate of reaction must be accelerated, or t h e mixtme will not support combustion. I n a n explosion t h e pressure developed is so high t h a t small differences in t h e initial pressure are of little effect if a n explosion is once started, b u t a diminished initial pressure may effectively prevent t h e explosion from beginning since the increased conduction a n d radiation losses a t diminished pressure may prevent a combustion reaction from accelerating into a n cxplosive one.
I93
E F F E C T O F RADIATION AKD CONDUCTION LOSSES
During t h e initial stage when t h e speed of combustion is slow, radiation a n d conduction losses are correspondingly large. For t h e same temperature conditions the radiation a n d conduction losses for equal intervals of time may be represented b y t h e spherical surface of the advancing combustion zone, which surface increases with t h e square of t h e radius; a n d if radiation a n d conduction losses be represented for t h e first second b y c, for t h e second second they will be 4c, for t h e third second 16c, etc. Meanwhile the gas burned, or the total heat generated, is as the cube of t h e distance t h e combustion zone has advanced, and at t h e end of t h e first second, if i t be represented b y d , a t t h e end of t h e second second i t will be 8d, a t t h e end of t h e third second 64d, etc. From these values, i t is apparent t h a t for equal temperature conditions, t h e radiation and conduction loss, if jO per cent during t h e first second, will be only about 2 j per cent during t h e second second and about I 2 ' / 2 per cent during t h e third second, etc. This diminishing of t h e percentage of radiation a n d conduction losses leaves more heat available for further raising t h e temperature of t h e reacting gases, a n d hence for an acceleration of t h e combustion reactions above t h e initial rate. This raising of t h e temperature of t h e reacting gases above t h e initial ignition temperature, is accompanied also b y increased radiation a n d conduction losses and t h e actual percentage decrease of these losses is not quite as rapid as figured above. E F F E C T OF P R E S S U R E ON RATE OF REACTIOK
A rapid increase i n t h e rate of t h e combustion reaction is accompanied b y a n increase in pressure of t h e reacting gas which in t u r n further accelerates t h e rate of reaction. Dixon a n d Coward1 show t h a t a n increase in pressure from one atmosphere t o two atmospheres lowers the required ignition temperature for combustion about 30°, from which, if t h e temperature of t h e reacting gases remains the same, doubling t h e pressure is equivalent in its effect on the rate of combustion reactions t o raising t h e temperature 30 O , which as will be shown later corresponds t o a n eightfold increase in t h e rate of reaction. E F F E C T O F DUST O N REACTION VELOCITY OR O N I G S I T I O N TEMPERATURE
A11 gas reactions are accelerated b y contact with solids a n d t h e effect on t h e ignition temperature of exceedingly fine dust particles in t h e gas should be taken into account. It is well known t h a t hydrogen and oxygen in contact with fine palladium will rapidly unite a t temperatures below 100' a n d if extremely fine particles of palladium were t o be in suspension throughout a mixture of oxygen a n d hydrogen, i t might be explosive a t ordinary temperatures. i-ery fine dust of a n y kind has t h e same effect t o a greater or less degree and t h e effect of dust in coal mines is threefold: ( I ) The very fine particles m a y accelerate t h e ignition reaction a n d hence lower the tempcratures of ignition. ( 2 ) T h e coal particles, being themselves combustible, increase t h e thermal value of the gas 1
Chem S e w , 99 (1909), 139.
194
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
a n d correspondingly less methane or other combustible gas is required t o form a n explosive mixture. (3) T h e particles, in being raised t o t h e ignition temperat u r e of t h e gas, absorb heat a n d in t h e case of t h e coarser dust this absorption of heat may more t h a n counteract a n y acceleration effect due t o surface effect upon t h e gas reaction. I n t h e case of non-combustible dust as rock dust, t h e retarding effect may so far exceed the accelerating influence as t o render gases less combustible or explosive, while if t h e dust is exceedingly fine the conditions may be reversed a n d t h e mixture may become more explosive. Experimental results by Abel' on t h e effect of fine magnesium oxide upon t h e explosive properties of mixtures of methane a n d air showed a n appreciable increase in t h e explosive quality of t h e gas. T h e Explosions in Mines Committee2 in their third report disagree with Abel's results a n d state t h a t as t h e result of a n extended series of experiments i t found no appreciable effect produced b y fine incombustible dust a n d t h a t t h e actual effect of the presence of such incombustible dust is t o render a gas a n d air mixture less explosive. Experiments b y Dixon a n d Campbell3 show errors in Abel's work a n d their results agree with t h e Explosion in Mines Committee's report, t h a t t h e presence of incombustible dust does not make a gas mixture more explosive. T h e findings of t h e Explosion in Mines Committee a n d t h e results obtained b y Dixon a n d Campbell apparently disprove Abel's conclusions a n d indicate t h a t t h e danger from rock dust has been greatly exaggerated. These experiments, however, do not necessarily disprove t h e general statement t h a t t h e presence of exceedingly fine dust particles does accelerate gas reactions a n d i t is entirely possible t h a t their experiments if repeated on gas mixtures containing exceedingly fine dust particles of other kinds might show a n increase in explosive properties due t o t h e presence of inert dust. If t h e dust itself is combustible t h e increase in t h e explosive properties of t h e gas due t o t h e presence of t h e dust is unquestioned.
Vol. 6 , KO. 3
velocity of combustion must be sufficient t o keep t h e reaction self-sustaining and, since the requirements for being self-sustaining are dependent upon the combined influences of all t h e factors mentioned, i t is evident t h a t t h e actual velocity a n d hence the actual ignition temperature are different for every different set of conditions. The maximum velocity of combustion a n d the ignition temperature corresponding t o this velocity are likewise different for each particular set of conditions,. T h e maximum velocity of combustion a n d t h e corresponding ignition temperature m a y be considered as t h e minimum velocity and t h e minimum temperature required to produce a n explosion. It follows, therefore, t h a t a n y particular temperature of ignition of combustion a n d a n y particular temperature of ignition of explosion can be accurately stated only for some definite mixture under some definite set of conditions. S U M MARY
I n order t o be adequate, statements regarding t h e ignition temperature of a gas mixture must be explicit as t o whether t h e temperature of combustion or the temperature of explosion is meant. Also t h e particular mixture of t h e gas with oxygen or with air must be stated as well as d a t a as t o the presence of inert gases a n d t h e initial temperature a n d t h e initial pressure. THERMAL
REQUIREMENTS
FOR
COMBUSTION
AND
EXPLOSION AND T H E CALCULATION O F T H E COMPOSITIONS O F COMBUSTIBLE AND EXPLOSIVE MIXTURES
While the values for ignition temperatures v a r y with differences in conditions, t h e experimental d a t a available are sufficient t o serve as t h e basis for t h e theoretical calculations of t h e composition of combustible a n d explosive mixtures. This calculation involves t h e heat of combustion, t h e temperature of ignition a n d the thermal capacity of the reacting gases. A brief discussion of this last factor is advisable. THERMAL CAPACITY OF GASES
EFFECT
OF INCREASE I N
TEMPERATURE
ON
RATE
OF
REACTION
According t o Ostwald, in many chemical reactions, t h e rate of reaction is doubled by a n increase of I O ' . If this rate of increase holds true throughout a range of IOO', t h e reaction velocity of a gas having a n initial ignition temperature of 7 0 0 ° , if raised t o 800' would increase approximately a thousandfold. T h e radiation a n d conduction loss a t 800° as compared with t h a t a t 700' is not more t h a n in t h e ratio of a b o u t 5 t o 3. Hence, . i n comparison with t h e increase in the speed of reaction, t h e increase in radiation a n d conduction loss is of minor importance. T h e minimum velocities of reaction of combustion a n d of explosion a n d t h e corresponding ignition temperatures of combustion a n d of explosion, are different for different mixtures a n d different conditions. T h e 1 Report t o t h e Secretary of S t a t e for t h e Home Department, March 2 3 , 1881. F o r review of this see Colliery Guardian, 106 (1913). 812. 2 Colliery Guardian, 105 (1913). 848 3 Jour. SOC. Chem. I n d , 32 (1913), 684-687
T h e thermal capacity of a gas for a n y given t e m perature range is t h e amount of heat required t o raise it through this range, a n d is the product of its mean specific heat, the number of degrees of temperature through which i t is raised, a n d t h e number of unit quantities heated. Tables of thermal capacity afford a n easy a n d ready means for determining the sensible heat contained in t h e different reacting gases.or in t h e products of combustion for different temperature intervals. T h e tables may be calculated t o a n y basis desired, as for example t h e thermal capacity of t h e gas per gram molecular volume, or t h e thermal capacity of t h e gas per gram, or per pound, or t h e thermal capacity per gram or per pound of one of t h e constituents. T h e thermal capacity of some of the common gases from 0' t o temperature t o is given in the following table. The values are in small calories per gram molecular volume of gas under constant pressure, a n d are derived from t h e general formulas given by Lewis
T H E J O U R i V A L O F I N D U S T R I A L A LVD E -VGI N E E RI N G C H E M I S T R Y
hlar., I914
a n d Randall.' These formulas are based upon t h e . work of Holborn a n d Austen, Holborn a n d Henning and Pier. They are t h e best d a t a available on t h e specific heats a n d thermal capacities of gases. 'CABLE OF
Temperature
C. 0 50 100
150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000
c02
0 2 ,
Nz, C O 0000 340 682 1027 1375 1725 2077 2432 2789 3148 3512 3876 4243 4613 4986 5360 5738 6116 6501 6885 7273
THERMAL CAPACITIES
H2
0000 338 679 1022 1367 1715 2064 2416 2770 3125 3485 3845 4209 4573 4942 5311 5685 6058 6436 6814 7196
so2
HzO
0000 448 910 1386 1877 2380 2897 3426 3966 4512 5081 5650 6239 6828 7436 8043 8667 9290 9929 10567 11219
0000 42 2 843 1263 1684 2104 2526 2950 3377 3804 4237 467 1 5112 5556 6008 6462 6927 7394 7875 8357 8857
CHc 0000 504 1038 1602 2196 2820 3474 4158 4872 5601 6390 7179 8028 8877 9786 10695 11664 12633 13662 14691 15780
21 calories 0000 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1 700 1800 1900 2000
Column 7 gives t h e value for 2t calories which is the amount b y which t h e thermal value a t constant pressure must be diminished t o obtain t h e thermal value a t constant volume. For example, t h e thermal capacity of one molecular volume of air a t 600' at constant pressure is 4243. A t constant volume i t is ) 3043. 4243 - ~ ( 6 0 0 or LIMITS O F COMBUSTIBLE MIXTURES
I n calculating t h e composition of a mixture t h a t will support combustion, t h e thermal capacity of a mixture should be calculated at constant pressure, since in combustion without explosion t h e gases have time t o expand. I n calculating t h e lower limit of a combustible mixture t h e heat which must be supplied b y t h e combustion m a y be considered as equal t o t h e quantity lost b y radiation a n d conduction plus t h e thermal capacity of t h e mixture of gas and air a t cons t a n t pressure when raised from t h e initial temperat u r e ( t ) t o t h e ignition temperature ( t ' ) . The higher ( t ) t h e less t h e loss due t o radiation and conduction a t t h e temperature ( t ' ) , and hence ( t ' ) decreases as ( t ) increases. The difference, ( t ' - t ) being smaller, t h e thermal capacity of t h e products of combustion is smaller and less combustible gas is required t o be present in t h e mixture. I n general, a n increase in t h e initial temperature ( t ) lowers t h e ignition ternperature ( t ' ) , decreases t h e difference (t' - t ) , lowers t h e thermal requirements for combustion conditions and as a result lowers t h e limit of combustible gas required in order t o have a combustible mixture. LIm'rs
O F EXPLOSIVE
MIXTURES
I n calculating t h e composition of a n explosive mixture the thermal capacity of t h e mixture should be taken as at constant volume, since with a n explosion traveling a t a high velocity t h e gases have little opportunity t o expand. Loss of heat b y conduction 1
Jour. A m . Chem. S O L , 34 (1912). 1128.
195
is a small factor during a n explosion b u t t h e quantity of heat generated must be sufficient not only t o sustain b u t t o greatly increase t h e ordinary rate of combustion. For example, in t h e combustion of a six per cent mixture of methane in air t h e heat generated b y t h e combustion of t h e first volume of t h e mixture is sufficient t o heat about three volumes of additional mixture t o t h e ignition temperature of combustion and t h e heat generated by t h e combustion of these three volumes is sufficient t o raise 9 volumes of mixture t o this ignition temperature. The increase being a geometrical one, t h e rate of t h e reaction quickly changes t h e combustion into a n explosion. I n a n explosive reaction t h e heat developed b y t h e explosion cannot be less a n d as a matter of fact considerably exceeds t h e thermal capacity of t h e gas or products of combustion heated from t h e initial temperature ( t ) t o t h e ignition temperature of t h e explosive mixture ( t ' ) . I n determining t h e lower limit of an explosive mixture, assuming one volume of mixture, t h e equation may be written as follows: Heat developed b y combustion of X volume of gas equals t h e radiation and conduction loss plus t h e thermal capacity of X volumes of gas plus ( I - X ) volumes of air from temperature ( t ) t o temperature ( t ' ) , X being t h e unknown amount of combustible gas necessary t o be present, a n d t' t h e ignition temperature. With radiation a n d conduction losses quite small a n d with a known value for t ' . t h e value of X is readily calculated. For example, assuming t equals 0' a n d t ' , t h e temperature of a n explosive mixture of methane a n d air, as 8 j o o , t h e value of X is determined as follows: X molecular volumes of methane on combustion gives X(191,200) calories. This equals the thermal capacity of X molecular volumes of methane and ( I - X ) molecular volumes of air from o t o 8 j o ' a t constant ( I - S) volume, or X(191,200) equals X(10,933) (4416). Therefore, X = 2.4 per cent. Assuming 7 j o o as t h e ignition temperature of combustion, a 2 . 4 per cent mixture of methane i n air will not support combustion as shown b y t h e following considerations : 0.024 molecular volumes of methane on combustion gives 4589 calories. The thermal capacity of 0.024 molecular volumes of methane plus 0.976 molecular volumes of air from o t o 7 j o o a t constant pressure o . g 7 6 ( 5,360) = j488 calories, equals 0.024(10,69j) or a deficiency in heat of 900 calories. The theoretical amount of methane required for combustion a t 750' is calculated as follows: X molecular volumes of methane on combustion give X(191,200) calories. .This equals t h e thermal capacity of X molecular volumes of methane plus ( I - X ) molecular volumes of air from o t o 7 j o o a t constant pressure; or X(191,200) equals X(10,695) ( I - X) (j360), from which i t follows t h a t X equals 2.9 per cent. With a n initial temperature of 30' (86' F.) about l j 2 5 less of methane will be required or about 2.3 per cent for a n explosive mixture a n d 2.8 per cent for a combustible mixture. The experimental values for
+
+
+
T H E JOURNAL OF INDEISTRIAL A N D ENGINEERING CHEMISTRY
196
t h e lower limits of a n explosive or combustible mixture of methane a n d air are usually given as f r o m j.5 t o 6 per cent of methane, Assuming t h e value j . j , amount required the gas Over the for combustion is 5.5 - 2.8 = 2 . 7 per cent, a n d the excess gas over t h e calculated amount required for explosion is 5.5 - 2 . 3 = 3.2 per cent. These values indicate t h a t during combustion a n d for t h e temperature assumed, 7 j 0 ° , 50 per cent of t h e heat liberated is available for radiation a n d conduction losses a n d for accelerating t h e reaction; while during a n explosion for t h e temperature assumed, 8 j o o , with radiation a n d conduction losses practically negligible, nearly 60 per cent of t h e entire heat liberated is expended in accelerating t h e reaction or in increasing t h e pressure of t h e gas. CALCULATIOK OF LOWER LIMITS OF COMBUSTIBLE AXD EXPLOSIVE MIXTURES OF HYDROGEN AND AIR
Assuming a n ignition temperature of 600' t h e calculation for a combustible mixture is as follows: I n one volume of mixture let X equal the volume of hydrogen a n d I - X the volume of air. Then X(58,100), t h e calories of heat developed, equals t h e thermal capacity at constant pressure from 0 ' t o 600" of X volumes of hydrogen plus ( I - X) volumes of air, which equals X ( 4 z o g ) X(4243). F r o m this it follows t h a t X equals 7.3 per cent. With a n initial temperature of air a n d gas a t 30' (86' F.) t h e thermal capacity of t h e mixture is about ~ / Z O less t h a n this value; therefore, l/20 less of hydrogen or only 6.9 per cent will be required t o furnish t h e necessary heat units. Assuming a n ignition temperature for explosion of 7 0 0 " C. a n d neglecting radiation a n d conduction losses, t h e lower limit of a n explosive mixture of hydrogen a n d air is obtained b y taking t h e thermal capacity of the gas a t constant volume a n d is found as follows: Let X equal the hydrogen present in one volume of t h e mixture; then the heat produced b y t h e combustion of t h e hydrogen, equals the thermal capacity a t constant volume of X volumes of hydrogen, plus I - X volumes of air heated from o t o 7 0 0 " . I n figures this is as follows:
+
+
X(58,100) = X ( 3 , 5 4 2 ) ( I - X) (3,586), in which t h e value of X = 6.2 per cent. With a n initial temperature of air a n d gas a t 30' t h e thermal capacity of the mixture is about l / * j less t h a n this value; therefore, less hydrogen or only 6 per cent will be required t o furnish t h e necessary heat units. T h e calculated values, 6.9 per cent a n d 6.0 per cent, are slightly too high, owing t o the fact t h a t the thermal capacity of the products of the reaction, Hz air ( l / ~ 0 1.9 Nz)= H z O 1.9 Nz, is lower t h a n t h e capacity of t h e unburned gas, hence after t h e reaction a n d after the products have been raised t o t h e temperature of ignition there is a surplus of heat which is available for heating a small additional a m o u n t of unburned mixture or for heating t h e products t o a temperature higher t h a n t h e ignition temperature. . F r o m the table of thermal capacity t h e capacities
+
+
+
Vol. 6, No. 3
from 0' to 600' before and after t h e combustion of one molecular volume of hydrogen with oxygen t o form water vapor are as follows:
+
Capacity of unburned gas, H2 1/2 0 2 = 4209 Capacity of water vapor formed Excess
+ 1/2
(4243) = 6331 = 5112 = 1219
This excess (1219 calories) is a little over 2 per cent of t h e total heat liberated by the combustion of one molecular volume of hydrogen ( j 8 ,I 00 calories). Therefore, the calculated values of combustible a n d explosive mixtures, 6.9 a n d 6.0 per cent, are correspondingly 2 per cent, or 1 / 5 0 too high. Making this correction t h e values are about 6.8 a n d j.9 per cent. T h e thermal calculations on methane a n d air are subject t o a similar correction, which, however, amounts t o only 470 calories per molecular volume of methane burned or 191,200 calories, which is less t h a n '/4 per cent of t h e total heat produced a n d is more of theoretical interest t h a n of practical importance. T E U P E R A T U R E REQUIRED FOR IGNITION FOR COMBUSTION COMPARED WITH TEMPERATURE R E Q U I R E D FOR IGNITION FOR EXPLOSIOX
I n t h e foregoing calculations t h e ignition temperature of combustion for a mixture of hydrogen in air is assumed as 600' a n d t h e ignition temperature for combustion for a mixture of methane in air is assumed as 7 50". These values are approximately Dixon a n d Coward's highest values for ignition temperatures, which for hydrogen a n d air are j 8 0 " to 590' a n d for T h e ignition temperamethane a n d air 650' t o 750'. tures of explosion of hydrogen a n d air a n d methane a n d air are assumed as 100" higher t h a n t h e ignition temperatures of combustion. These values are not t o be regarded as exact b u t serve merely as a basis of calculation a n d for discussion of explosive conditions. The value 700' assumed for hydrogen is 186O higher t h a n Falk's value for the ignition temperature of explosion for a 50 per cent mixture of hydrogen in air a n d 67" higher t h a n his value for the ignition temperature of a 16 per cent mixture of hydrogen in air. If temperature alone were the cause for t h e greater rapidity of a n initial explosion reaction compared with a combustion reaction, the temperature required t o s t a r t a n explosion necessarily would be very considerably higher t h a n t h a t required t o sustain combustion. However, as has been already shown, t h e high pressure during a n explosion is a n important factor in producing ignition of the gas a t independent points throughout t h e mass. During combustion the large radiation a n d conduction losses, which losses are practically absent during a n explosion, necessarily raise the actual temperature a t which the combustion reaction is self-sustaining considerably above t h a t which would be required if these losses are absent. Taking t h e influences of these factors into account it is conceivable t h a t the temperature necessary t o s t a r t a n d sustain a continuous combustion reaction actually may be as high or higher than the required ignition temperature of a n explosion reaction after t h e explosive wave is once started.
Mar., 1914 VALUES
T H E J O U R N A L O F I N D L - S T R I A L A N D ENG1NEERIil-G C H E M I S T R Y
OBTAISED
BY
THERNAL
CALCULATIOKS
COJI-
P A R E D W I T H ACTUAL VALUES
I n ordinary combustion, as has already been stated, t h e products of t h e reactions are necessarily heated considerably above t h e temperature actually required t o produce t h e reaction. Otherwise t h e conduction and radiation of heat t o adjacent molecules of unburned gas a n d air mould not be sufficient t o raise any of them t o t h e actual combining temperature, a n d in thermal calculations for combustion where t h e heat evolved is assumed as equal t o t h e thermal capacity of t h e reacting gases raised t o t h e ignition temperature, t h e calculated value for t h e amount of combustible gas required must be lower t h a n t h e amount actually required. If on t h e other hand, t h e amount of combustible gas actually required t o produce a n explosive mixture is made t h e basis of a thermal calculation, t h e value obtained for ignition temperature of t h e mixture is higher t h a n t h e actual temperature required t o cause chemical reaction t o t a k e place in t h e mixture. T h e effect of radiation a n d conduction losses is t o raise t h e required ignition temperature a n d t o increase t h e amount of combustible gas required t o produce a self-sustaining combustion. I n general, a n y thermal equation based on t h e temperature a t which two gases will unite if raised t o t h a t temperature will give a result for t h e amount of combustible gas required lower t h a n t h a t which will be obtained experimentally. Likewise, a n y thermal equation for combustion which balances if it is based om experimental d a t a actually obtained on gas mixtures t h a t support combustion must give a value for t h e ignition temperature higher t h a n t h e temperature a t which combustion takes place. POTENTIAL EXPLOSIVE PROPERTIES
OF
GAS
ULIXTURES
The results of thermal calculations based upon t h e temperature a t which actual chemical reaction takes place fixes t h e lowest limits of t h e amount of combustible gas required for a given set of conditions. Any increase i n amount of combustible gas over this lower limit or a n y change of conditions may make t h e mixture a t least potentially combustible or explosive. T h e theoretical value of 5.9 per cent as t h e lower limit of a n explosive mixture of hydrogen in air a n d t h e value of 2 . 4 per cent as t h e lower limit of a n explosive mixture of methane in air are based on ignition temperatures of 700' for hydrogen a n d 8 j o o for methane. Falk's value for t h e ignition temperature of explosion of a 16 per cent mixture of hydrogen i n air is only 637O, from which i t appears t h a t t h e assumption of 7 0 0 O as t h e ignition temperature with approximately a 6 per cent mixture is probably not too low. h-atural gas and gas in coal mines frequently contain I O t o 1 5 per cent ethane which has a n ignition temperature of combustion as low as 520' a n d has about twice t h e heating value of methane volume for volume, which makes a 2 per cent mixture of natural gas a t least as explosive as a 2 . 4 per cent mixture of methane. From these considerations i t is apparent t h a t a n y mixture of over 6 per cent of hydrogen in
I97
air and of over 2 per cent of natural gas with air may be a t least potentially explosive. The results obtained in t h e laboratory with explosive mixtures are presumably obtained with practically dust-free gas and hence with gas having a high ignition temperature. Furthermore, in laboratory tests t h e results are usually obtained with mixtures rich enough t o ignite a n d explode without a n y accelerating influence analogous t o blow-out shots, local explosions of rich mixtures, etc. I n mines a n d factories t h e mixtures of gas a n d air are very liable t o contain appreciable quantities of fine combustible dust a n d therefore may have appreciably lower ignition temperatures and be correspondingly more explosive. Any mixture of gas which is potentially explosive is t o be regarded as dangerous, as i n mines or in other localities there is always the possibility of conditions changing t o such an extent t h a t a potential explosion may become a n actual one. S U h.121A RY
The d a t a in t h e literature regarding t h e explosive properties of gas mixtures are often incomplete a n d misleading. Definite knowledge is of importance in securing safety i n mining and other industrial operations. This knowledge can be made more usable if t h e various factors bearing upon t h e question are collected a n d their effects analyzed. I n order t o discuss t h e problems involved i t is desirable t o define with some exactness some of t h e factors a n d t o consider in detail t h e difference between combustion a n d explosion. T h e ignition temperature of a mixture is t h e temperature t o which i t must be raised t o bring about a n appreciably rapid chemical reaction. Ignition temperature of combustion or explosion is the temperature t o which a portion of t h e mixture must be raised i n order t h a t a combustion or a n explosion may be propagated throughout t h e entire mixture. Ignition temperature of combustion a n d ignition temperature of explosion are two distinct temperatures corresponding t o two different reaction velocities. The minimum velocity of a combustion reaction is the lowest 7-elocity a t which t h e combustion is sustained a n d below which t h e combustion flame is extinguished. The maxim u m velocity is t h e highest velocity for combustion beyond which t h e reaction becomes explosive. Combustion is produced b y direct transfer of energy from molecule t o molecule. I n a n explosion t h e heat produced b y a rapid initial reaction raises t h e pressure high enough t o start an explosion wave, the pressure of which raises the gas t o its ignition temperature a n d t h e reaction takes place throughout the entire mixture. T h e exact value for ignition temperature of combustion and for ignition temperature of explosion is affected b y changes in initial temperature of t h e gas, b y changes in the initial pressure, and by radiation a n d conduction losses, and is different for every mixture a n d for every set of conditions. I n order t h a t a gas mixture may support combustion or be explosive t h e heat produced b y t h e reaction must be more t h a n sufficient t o raise t h e products of combustion a n d any
198
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
inert gases or a n y excess air present to the ignition temperature of the mixture. Having given the heat of the reaction a n d t h e ignition temperature, the calculation of t h e lower limits of combustible a n d explosive mixtures of gas is comparatively easy. These theoretical calculations, if based on the temperature a t which a reaction will take place, give results for the amounts of combustible gas required lower t h a n t h e results f o u n d b y experimental means, as the experimental results necessarily include enough additional combustible gas t o overcome t h e effect of radiation a n d conduction losses. T h e values by theoretical calculations while lower t h a n actual values are useful in t h a t they show t h e potentially explosive properties of a mixture a n d a n y potentially explosive gas is t o be regarded as dangerous. Assuming ignition temperatures of combustion of 600 a n d 7 5 0 ’ a n d ignition temperatures of explosion of 7 0 0 a n d 8 j o o for mixtures of hydrogen in air a n d of methane in air a n d determining t h e amount of each required t o satisfy t h e thermal requirements of t h e reaction for these temperatures, a n d assuming combustion as occurring a t constant pressure a n d explosion as occurring a t constant volume, t h e theoretical lower limit of a combustible mixture of hydrogen in air is 6.8 per cent a n d of methane in air is 2.8 per cent a n d the theoretical lower limit of a n explosive mixture of hydrogen in air is j . 9 a n d of methane in air is 2.4 per cent. I n a succeeding paper entitled “Partial a n d Intermittent Combustion of Gas,” the writer gives t h e results of some experiments undertaken in order t o obtain further experimental data upon these limits a n d also t o t r y t o harmonize the different experimental values found in t h e literature, which for air a n d hydrogen range from j t o I O per cent a n d for air a n d methane range from 3.2 t o 6 per cent. T h e writer desires t o express appreciation a n d indebtedness t o Dr. W. E . Henderson of the Department of Physical Chemistry for advice a n d sugiestions. DEPARTMENT O F METALLURGY
OHIOSTATE UNIVERSITY COLUMBWS
A NEW METHOD FOR DETERMINING THE VALUE OF DISINFECTANTS By C. A. DUYSERAND W. K. LEWIS Received Dec. 26, 1913
Since the chief function of a disinfectant is to kill bacteria or other micro-organic growth, its commercial value may be measured in terms of either of two quantities--first, the time required for a disinfectant of definite dilution t o destroy a predetermined bacterial culture; second, t h a t certain dilution necessary t o kill t h e bacteria of this culture in a definite interval of time. B u t bacteria are living organisms having more or less a n individuality. Not only are there many different strains or types of each organism, b u t the same culture of bacteria may differ in many of its characteristics from d a y t o day. Hence i t ‘is impossible to employ as a n analytical standard for determining the killing power of disinfectants a n organism which m a y vary in its vitality, or a culture which
Vol. 6, N O . 3
m a y be heterogeneous as t o the vitality of the individual bacteria present. The other alternative is t o agree upon a certain chemical compound of known composition, a n d which may be obtained with ease, as the standard disinfectant, a n d t o measure all other disinfectants in terms of t h e killing power of this standard. Phenol is t h e substance more generally employed for the purpose, a n d t h e ratio of t h e ability of a disinfectant t o kill t h e bacteria of a certain culture t o the ability of phenol t o kill the same bacteria under absolutely t h e identical conditions is called t h e “Phenol-coefficient.” There are a t present three methods for measuring t h e bactericidal value of disinfectants, all using t h e above principles, b u t differing in details of manipulation; these are t h e Rideal-Walker, the Lancet a n d t h e Hygienic Laboratory Methods. Each of these, however, gives unsatisfactory results, not only when carried on by different experimenters in different laboratories, but by the same operators when carrying on his work in duplicate. Blythe’ has called attention t o this fact most forcibly, a n d has made a strong appeal for a more chemical method for testing disinfecting materials. We believe t h a t t h e methods now in use are not sound for the following reasons: T h e mechanism of t h e reaction b y which a disinfectant kills bacteria is not definitely known, but it is generally conceded t h a t t h e concentration of t h e disinfecting solution falls in proportion as the number of living bacteria present is decreased. It is possible t h a t the number of molecules of disinfectant is so great in proportion t o t h e number of bacteria present t h a t the change in t h e concentration of t h e disinfectant as t h e living bacteria disappear is negligible. B u t i t is generally true in carrying out these methods t h a t so large a number of bacteria is used t h a t t h e strength of t h e disinfectant is materially changed as t h e killing of the bacteria proceeds a n d before t h e last, or more hardy individuals are killed, the disinfectant is appreciably exhausted. Misleading results are therefore obtained. Each of the above methods provides for removing what is supposed t o be a perfectly constant volume of the culture from a tube or flask b y means of t h e so-called standard loop. This is a circular loop made b y winding a platinum wire of determined size around a rod having a definite diameter. There are so many physical conditions which apparently would influence t h e volume of liquid such a loop would carry t h a t i t was thought of interest t o determine this volume. A strong solution of iodine of known strength served as the liquid t o be transferred, a n d a dilute solution of thiosulfate was used t o measure the amount of iodine contained in each loopful. A number of loopfuls were withdrawn with great care a n d placed in cold distilled water. T h e weight of iodine in each was then determined by titration, and from this d a t a t h e volume of the loopfuls calculated. When great precautions were used, t h e volume of one loop varied a s much as 30 per cent from the mean of 2 j loopfuls, a n d when hurriedly done the variation rose as high as 80 per cent. T h e volume transferred by the standard 1
Orig. Communications, Intern. Cong. Applied Chem., 1909.
