Amil. 1928
INDUSTRIAL A N D ENGINEERING CHEMISTRY
367
The Flammability of Refrigerants'.' Mixtures of Methyl and Ethyl Chlorides and Bromides G. W. Jones PITTSBURGH EXPERIMENT STATION, U. S. BUREAUOF MINES,PITTSBURGH, PA.
This report gives the results of tests made to determine the flammable limits of methyl chloride, ethyl chloride, mixtures of methyl and ethyl chloride, methyl bromide, ethyl bromide, and the extinctive action of methyl bromide on the flammability of methyl and ethyl chloride.
T
,HE Bureau of Xines is interested in the flammability of refrigerants because as American mines are developed to greater depths, attended by higher working temperatures, methods of cooling the air supplied to these deep workings will become necessary. Some of the metal mines are a t the present time developed to such depths that means for artificial cooling may soon be necessary, if the mines are extended to lower levels. Refrigerants, if used underground, should from the standpoint of safety be non-flammable and non-toxic. Investigations have been under way for some time3 in which the extinctive effects of many chloride compounds on the flammability of methane have been determined, having in view that should any particular class of compounds be found to have a marked extinctive effect on methane flames, similar compounds might be added to explosives to reduce the flame temperature. This is of importance in the development of explosives having greater safety in gassy and dusty mines. Methyl and ethyl chloride are flammable when mixed with the proper amount of air, and a review of the values obtained by different investigators is given later. Methyl and ethyl bromide have heretofore been considered as non-flammable. It was thought that by the proper blending of the bromide with the chloride of methane or ethane a non-flammable mixture would be produced when mixed with air. Such a compound would be highly desirable when used for refrigeration in connection with the cooling of air introduced into mines. A detailed study of the extinctive effects of bromides is also desirable to determine whether they have exceptional extinctive effects on flames in connection with the use of similar compounds in explosives. The toxic effects of refrigerants are the subject of a report of the U. S.Bureau of Mines which is to be published shortly. As t o the artificial cooling of deep mines, the twelfth report of the Committee on the Control of Atmospheric Conditions in Hot and Deep Mines in Great Britain contains a note on the problem of local air-conditioning underground by means of refrigeration.4 Previous Investigations
METHYLCHLORIDE-The only values found in the literature on the limits of flammability of methyl chloride are those given by the Linde Air Products Company's research l a b ~ r a t o r y . ~The tests were made in a spherical bomb (size not stated) and ignition was produced by a spark. They report 8.9 per cent as the lower and 15.5 per cent as the upper limit of flammability. ETHYLCHLoRm-In this same report5 values for ethyl chloride are given as follows: lower limit in air, 4.3; upper 1 Presented before the Division of Gas and Fuel Chemistry at the 74th Meeting of the American Chemical Society, Detroit, h'lich., September 5 to 10, 1927. 9 Published by permission of the Director, U. S Bureau of Mines (not subject to copyright). a Coward and Jones, I n d . Eng. Chem., 18, 970 (1926). 4 Read before the Institution of Mining Engineers at Birmingham on February 2, 1927; Colliery Guardian, 133, 266 (1927). 6 Edwards, Refrigerating Eng., 11, 95 (1924).
limit, 14.5 per cent. Other values for limits of flammability of ethyl chloride are given by DeissI6who made tests in an explosion pipet over mercury and found when the mixtures were ignited by a spark, 3.6 and 11.2 per cent, respectively, as the lower and upper limits in air. When the mixtures were tested in a eudiometer tube and ignited by a flame, 6.4 and 11.2 per cent were obtained for the lower and upper limits, showing that the gas was more difficult to ignite with a flame than by a spark. METHYLAND ETHYL BROMIDE-NOindications were found in the literature that either methyl or ethyl bromide are flammable when mixed with air. Tanaka and Nagai,' who studied the effect of ethyl bromide on the flammability of hydrogen and other combustible gases, found that ethyl bromide vapors failed to propagate flame when mixed with air in any proportions. These tests were made in a 2.5-cm. tube 65 cm. long, the flame being propagated in a downward direction and ignition produced by means of a spark. Investigation of Flammability
The methyl and ethyl bromide and methyl and ethyl chloride used in the Bureau of Mines tests were purified products obtained from the Dow Chemical Co., Midland, Mich., and were reported as containing only traces of impurities. ANALYSISOF hIlXTURES-BeCauSe chlorides and bromides of methane and ethane are very soluble in most solutions and rubber, and also because little information is available regarding their proper analysis, analysis of the mixtures used for tests presented a difficult problem. As all the vapors are somewhat heavier than air, the density method was tried for these analyses. Some of the prepared mixture was withdrawn from the test apparatus into an evacuated bulb, weighed, and then corrected for standard conditions. These values, when compared with the density of the pure gas and air enabled the percentage to be calculated. It was found that duplicate determinations could not be checked closer than about 0.5 per cent, a value too large for accurate work. Absorbents were next tried for removing the vapors from the mixtures and obtaining the percentage in this manner. Glacial acetic acid was tried with methyl chloride-air mixtures, the absorptions being made in a Bone and Wheeler gas-analysis apparatus. This apparatus is free from rubber connections and uses mercury as the confining liquid. Tests showed the absorption to be incomplete, even when three fresh portions of the acetic acid were added and shaken 3 minutes or more during each fresh addition. METHODADOPTED-The following method proved very satisfactory, and was finally adopted for all the chlorides and bromides. If either the chloride or bromide of methane or ethane is exploded with excess of oxygen, the reaction should take place as follows: 6
Z. Elcklrochem., 29, 586 (1923). Proc. Im. Acod. ( T o k y o ) , 2, 280 (1926); Nagai, Ibid. p. 284.
INDUSTRIAL B S D E*VGINEERING CHEMISTRY
3 68
++ H20 + HC1 H2O + HBr ++ 302 = 2COa + 2Hz0 + HCl 302 = 2C02 + ZHZO+ HBr
++ l l / z
CHsCl CHaBr CzHEC1 CzHbBr
1'/2
0 2
0 2
= CO2
= COZ
(1)
cedure and these factors were used in the analysis of all the test mixtures given in this report.
(4)
Investigation showed that if the test mixtures were diluted with a 50: 50 oxygen-air mixture and then electrolytic gas (2H2 02)were added to bring the mixture well within the flammable range, and exploded, combustion was complete and all the reactions took place largely as given. After explosion, the contraction of volume was read and then the carbon dioxide and remaining hydrochloric acid were absorbed by caustic potash. The contraction due to explosion and that due to absorption by potash were then added together, and this value was used for determining the percentage of combustible present in the test mixture. The analyses were made in a Bone and Wheeler gasanalysis apparatus. To test the accuracy of this method, pure gases of the different combustibles tested were accurately measured in the apparatus, oxygen-air mixture and electrolytic gas added, and the analysis carried out as described. If the method of analysis was correct, the volumes determined by analysis according to the above equations should check the volumes of the pure gas added for analysis. This was found to be true in the case of methyl chloride, but for ethyl chloride and methyl bromide the volumes given by analysis were slightly higher than the volumes of pure gas added. To correct for this error, several analyses were made with each compound and then the proper correction factor was used to give the correct percentage of gas present. From the equations given, it is seen that for methyl chloride or bromide the correct percentage should be obtained by i i v i d i n g the sum of the contraction and potash absorption by 2.5 and for ethyl chloride or bromide by dividing by 4.0. The factors actually necessary to give the correct percentage of gas present are g i v e n in Table I. These variations may be due to deviations of these gases from the gas laws, to small amounts of impurities in the compounds, or to the fact that the reactions on explosion are not entirely as given in equations (1)to (4). I n the case of the bromides tested there was always a slight amount of free bromine vapor formed on explosion, showing that all the bromine did not combine to form hydrobromic acid, or that there was some oxidation of hydrobromic acid to bromine and water. This was most marked in the case of ethyl bromide. This pro-
Table I-Factors in Calculating Gas COMBUSTIBLE GAS FACTORS USED THEORETICAL FACTOR Methyl chloride 2.50 2.50 Methyl bromide 2 60 2.50 Ethyl chloride 4 08 4 00 Ethyl bromide 4 25 4.00
+
1 . 1
Figure 1-Apparatus for Determining t h e Explosibility of Gases
Vol. 20. No. 4
TESTSOF SIXGLEGams-Flammability tests were first conducted on the single gases by preparing the mixtures in large containers over a 50:50 glycerol-water mixture, then introducing into a perpendicular evacuated glass tube 1.8 meters (6 feet) long and 5 em. ( 2 inches) in diameter sealed a t the lower end by a ground-glass plate and mercury seal. Samples of the mixtures for analysis were taken over mercury from the stream of gas entering the explosion tube, so that the mixture analyzed was the same as that tested for flammability. This was necessary because of the solubility of the vapors in the glycerol-water mixture and tubing connected t o the explosion tube. This apparatus was the same as that previously used for testing the explosibility of saturated hydrocarbons.x The gas mixtures in the apparatus were tested for propagation by sliding the ground-glass plate from the bottom of the tube and igniting the mixture by a flame from an alcohol lamp. The results obtained with this apparatus are shown in Table 11, the gases being roughly dried by passing over calcium chloride. Table 11-Flammable
GAS iMethyl chloride Ethyl chloride Methyl bromide Ethyl bromide
Limits of Single Gases in Air LOWER UPPER Per cent Per cent 10.75 4.25
17.40 14.35
Nan-flammable Nan-flammable
These tests indicated that a flame was not a satisfactory means of ignition for this class of compounds, especially for the lower-limit tests. Mixtures near the lower-limit values either did not propagate a t all or, if once started, they traveled rapidly and vibrated badly near the top of the tube. TESTSOF MIXTURES-AS the investigation contemplated the study of flammability of two of the compounds together, the apparatus used for conducting the tests just described could not be used for the following reasons: Owing to the great solubility of these compounds in liquids and rubber tubing, only glass and mercury should be used in contact with the test mixtures; it was desirable to ignite the mixtures by means of sparks, and to have an apparatus in which the effect of turbulence could be studied. To meet these conditions an apparatus of the design given in Figure 1was constructed. A bell jar, a, having a capacity of 2650 cc. a t the zero mark was assembled as shown. This rested in another inverted bell containing mercury. Through the bottom passed a glass capillary tube, c, and electrodes sealed in glass. These electrodes were brass and the terminals were 8 mm. apart for the tests given in this report. I n the top of bell jar a was a paraffin-coated stopper through which passed the shaft of a fan, d, made gas-tight by a mercury rotating seal, e. The fan was connected to a variable-speed motor; it was used to test the gases in motion, to mix the introduced gas-air mixtures, and to free the bell of products of combustion after each test. The upper bell and motor were supported separately, so that this part of the apparatus could be raised from the lower bell. When the fan was operated, gases from the previous test were swept from the bell and air was introduced for the next test. An electric fan was kept blowing across the bottom 8
Coward, Jones, Dunkle, and Hess, Carnegic Inst. Tech., Bull. 80,
(1926).
April, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the bell jar during this operation to insure the introduction of normal room air. The bell was then lowered into the mercury to the zero mark and clamped in position. The number of cubic centimeters of the concentrated gas necessary to give a desired mixture in the apparatus was then calculated and this amount of air, minus 75 cc. used for analysis, was withdrawn through cock f by means of a mercury buret. This was necessary in each case so that, after the gas had been added and the sample for analysis withdrawn, the level would be a t the zero mark and the gas a t atmospheric pressure. The desired volume of gas was then added into the apparatus through cock f and the fan started. It was operated 5 minutes, during which time the gas mixture was passed back and forth from the buret to the apparatus. At the end of this time 7 5 cc. of the mixture were withdrawn from the apparatus into the buret, transferred into test tubes previously filled with mercury, and stored for analysis. The mixture in the apparatus was then tested for flammability by passing a series of sparks across the terminals. The electrodes were connected t o a 15-cm. (6-inch) coil, energized by a 6-volt, 13-plate storage battery. The lead connecting the battery to the spark coil was in series with an ammeter and variable-resistance unit. Various amounts of current in the primary were tried in the preliminary tests, and it was found that this factor greatly affected the results, especially on the lower-limit mixtures of bromides. For all the values reported the ammeter reading showed approximately 4.5 amperes in the primary with vibrator of the coil operating, and 9.0 amperes when closed and short-circuited. For a current strength appreciably less than this it was impossible to obtain limits for methyl or ethyl bromide. It may also be said that the electrodes in the apparatus became amalgamated with mercury, and when a spark was passed there appeared to be considerable volatilization of mercury. This action may have produced so-called "activated mercury atoms," which may have been instrumental in promoting ignition. DETERMINaTION OF PROPAGATIO?; O F FLAME-A mixture was considered to have propagated if the flame traveled to the top of the bell jar and spread out in mushroom shape. A flame which striated a few centimeters above the electrodes was considered non-flammable. Lower-limit mixtures always showed these caps when just below the limit and then, when the concentration was slightly raised, went t o completion blowing mercury from the seal with considerable violence. With upper-limit mixtures the distinction was not so sharp. Flames which passed to the top and then spread out were very gentle, and seldom did more than force some gas out under the bottom of the seal with very little violence. Tests conducted with the apparatus as described gave the results in Table 111. Table 111-Flammable COMBUSTIBLE Methyl chloride Ethyl chloride Methyl bromide Ethyl bromide
L i m i t s of Mixtures in Air LOWER UPPER Per cent Per cent 8.25 18.70 4.00 14.80 13.50 14.50 6.75 11.25
Both methyl and ethyl bromide have flammable limits when tested under the conditions given. For each of the bromides tested, mixtures could be taken with air in which the reaction was quite violent, causing mercury to be blown several feet about the room and the reaction appearing to be complete throughout the entire mass of gas under test. The range of flammability of methyl bromide, however, is very narrow and in only one mixture containing approximately 14.0 per cent was this violence noticed.
369
Tests were conducted to determine the flammability of mixtures of methyl and ethyl chloride and to determine whether mixtures of these gases were flammable in all proportions and obeyed LeChatelier's law. This law as modified by Coward, Carpenter, and Paymang is stated as follows:
where L is the limit of the mixture which it is desired to c a l c u late, Ll and L?are the limits for each pure constituent of t h a t mixture in air, and p1 and p2 the percentage volumes of the constituents on an airfree basis.
The upper and lower l i m i t s of flammability of various mixt u r e s of t h e t w o chlorides were determined and the res u l t s a r e given in Table IV and graphically in Figure 2.
+
CHzCI 0 40 60 eo 20 C ~ H S C I100 EO 60 40 LOMBL'STIBLE IN ORIGISAL MIXTURE, PER CENT
J 0
comparison. It is seen that the calculated and observed upper-limit values agree fairly ivell; however, the lower-limit values may be in error by amounts equal to almost 1.0 per cent. In fact, a lomerlimit mixture containing 25 per cent of methyl chloride and 75 per cent of ethyl chloride a t the start was prepared which, according to the law, should just be flammable, byt on ignition the explosion was so violent as to almost completely wreck the apparatus. of F l a m m a b i l i t y of Methyl a n d Ethyl Chloride Mixtures COMPOSITIOX OF TESTMIXTURE% FLAMXABLE LIMIT Methyl chloride Ethyl chloride Determined Calculated Per cent by volume Per cent b y volume
Table IV-Limits
LOWER
24.0 52.0 74.0 100.0
0.0
100.0 76.0 48.0 26.0 0.0
4.00 4 , 15 4 . t55 6.15 8.25
0.0 25.0 50.0 75.0 100.0
100.0 75.0 50.0 25.0
14.80 15.65 16.25 17.15 18.70
...
4.55 5.45 6.50 UPPER
15.60 16.50 17.55
DISCUSSIO?;OF RESULTS-The results of these tests (Table IV) show that mixtures of methyl and ethyl chloride are flammable in all proportions when mixed with the proper amount of air, that the upper limits of mixtures of the two chlorides may be calculated fairly accurately by LeChatelier's law, and that for lower-limit mixtures the law only holds approximately. Extinctive Action of Bromide and Chloride-Air Mixtures
The remainder of the investigation comprised a study of the extinctive action of methyl bromide on methyl and ethyl chloride-air mixtures, and the results of these tests are shown in Figures 3 and 4. Figure 3 shows what was obtained with mixtures of methyl 8
J . Chcm. SOL.(London), 116, 27 (1919).
370
INDUXTRIAL A N D ENGINEERING CHEMISTRY
chloride and methyl bromide. With reference to upperlimit mixtures, the action of the bromide is comparable to that of two combustible gases and no extraordinary extinctive action of the bromide is apparent. I n the case of lower-limit mixtures, when the bromide is increased to about 2 per cent, the lower limit of methyl chloride is raised slightly, showing that the bromide does not take part in the reaction to any appreciable extent and largely acts as an inert substance; however, when more than 2 per cent is added the bromide begins to take part in the reaction and continues more so with greater additions. As a result the lower limit for methyl chloride is continually lowered, until a t 13.50 per cent methyl bromide addition propagation is obtained with no methyl chloride present. Inspection of the curve shows that addition of methyl bromide narrows the flammable range; however, it is impossible to produce a non-flammable mixture by the blending of these two compounds.
VOl. 20, N o . 4
sis, source of ignition, and other conditions which affect the results. These values are for upward propagation of flame in rather large apparatus. Table V-Value
for F l a m m a b l e L i m i t s of Refrigerating C o m p o u n d s MOL.
BOILING POINT
FLAMMABLEREFLIMITSIN AIR ER-
(1 atmos.) Lower Upper C. Per cent bv volume Ammonia 17.03 -33.35 N Ha 16.1 26.4 Butane 58.10 .~ 1.86 8.41 0.6 C4HlO Carbon dioxide 44.00 coz ... h70n-f3ammable Ethane 30.06 -88.3 CzHa 3.22 12.45 Ethyl bromide CzHaBr 109.0 38.4 6.75 11.25 Ethyl chloride CzHKl 64.51 13.1 4.00 14.80 Methyl bromide CHaBr 96.00 13.50 14.50 4.5 CHaCl Methyl chloride 50.49 -24.09 8.25 18.70 Propane 44.08 -44.5 CsHs 2.37 9.50 Sulfur dioxide 64.06 -10.0 SOa Non-flammable a White J . Chem. SOC.(London), 121, 1244 1688 2561 (1922). REFRIGERANTSYMBOL
WT.
ONCE
~
b Coward, Jones, Dunkle, and Hess, Min'ing ahd Metallurgical Investigation, Carnegze I n s l . 01 Tech., Bull. SO (1926). c This publication.
Conclusions
1-The substitution of a chlorine or bromine atom for hydrogen in a methane or ethane molecule causes the resultant gas, when mixed with the proper proportions of air, to be more difficult of ignition. 2-Flammable mixtures of methyl or ethyl chloride with air are more easily ignited by means of a strong electric spark than by means of a flame. 3-Mixtures of methyl or ethyl bromide with air were found to be non-flammable when ignition was attempted by means of a flame, but with an intense spark ignition limits were found for these compounds with air. 4-The upper limits of flammability of mixtures of methyl chloride and ethyl chloride may be calculated fairly accurately by LeChatelier's law; however, lower-limit mixtures do not follow the law closely and the observed values were almost 1.0 per cent lower than the calculated values. 1
c
METHYL BROMIDE IN ATYOSPHERE. PER CENT
Figure 3-Limits of I n f l a m m a b i l i t y of Mixtures of Methyl Chloride P l u s M e t h y l Bromide i n Air
I m1 c u '
E 1
The results for methyl bromide on ethyl chloride-air mixtures (Figure 4) are quite similar to those for methyl chloride and methyl bromide. The upper-limit curve is quite like those obtained for any two combustible gases, and no exceptional extinctive effect of methyl bromide occurs. I n the case of lower-limit values the addition of methyl bromide up to about 2.5 per cent causes a slight rise in the lower flammable limit for ethyl chloride; the methyl bromide over this range acts largely as an inert substance and the slight rise can be largely so accounted as due to the heat capacity of the added methyl bromide. Above about 2.5 per cent the methyl bromide begins to react on ignition and the limit for ethyl chloride is lowered; this lowering continues as the methyl bromide is increased until, when the amount of methyl bromide added equals 13.5 per cent, flammation occurs without any ethyl chloride being present. The flammable limits of these mixtures are appreciably narrowed by the addition of methyl bromide; however, it is impossible to produce a non-flammable mixture by the blending of the two compounds. Flammability of Refrigerants
Table V7 lists values for the flammable limits of various compounds used as refrigerating materials. Consideration has been given to the size of the apparatus in which the tests were made, direction of flame propagation, method of analy-
E
B1 z
8
4E *
B METHYL BROMIDE IN ATMOSPHERE, PER CENT
Figure 4-Limits of I n f l a m m a b i l i t y of Mixtures L of E t h y l Chloride a n d Methyl Bromide in Air
5-All mixtures of methyl bromide with either methyl chloride or ethyl chloride were found to be flammable when mixed with the proper proportions of air, although the flammable limits were quite appreciably narrowed by the addition of methyl bromide. 6-Methyl bromide does not exert any unusual extinctive action on either methyl or ethyl chloride mixtures with air, provided the ignition source is sufficiently strong. The limits of flammability of the four compounds tested individually with air, the tests being conducted in an apparatus of 2650 cc. capacity, propagation of flame upward, and ignition by a very intense electric spark, gave the results shown in Table 111.
