Chlorination of Methane' E. T. McBee, H. B. Has, C. M. Neher, and H. Strickland
Purdue University and Purdue Research Foundation, Lafayette, Ind.
0th the abundance of methane and the comparative cheapness of chlorine favor the industrial utilization of chlorination for the preparation OF the chloromethanes. In most of the prior processes i t was necessary t o use some economically undesirable feature t o avoid an explosive o r burning reaction w h e n high concentrations OF chlorine w e r e employed. The present w o r k was undertaken in an effort t o apply the Hass-McBee chlorination technique t o methane. A hydrocarbon i s passed through a reactor w h i l e chlorine i s introduced through jets suitably spaced along the reactor. By this arrangement the momentary concentrations of chlorine are maintained b e l o w those w h i c h yield explosive mixtures, and the over-all mole ratio of reactants may b e any desired value-for example, 4 of chlorine t o I of methane t o produce carbon tetrachloride. Graphs of temperature gradients w e r e helpful i n selecting the most suitable bath temperature, spacings of chlorine jets, space velocity, etc. Most of the data w e r e obtained at 440" C. The ratios of the four chloromethanes in the product can b e varied from nearly 100 per cent methyl chloride t o carbon tetrachloride exclusively.
B
formed as the main products of reaction. According to Jones, Allison, and Meighan ( B ) , "if the temperature of the reaction chamber rises above 500" C., the reaction proceeds according to the formula: CH, 2CI2 -+ C 4HC1". Further, they state that "the heat control of the reaction is the important phase of the problem that must be solved before the work can be placed upon a plant basis". Many different conditions have been used to chlorinate methane. In general, chlorinations with chlorine have been thermal, catalytic, and photochemical. The rate of chlorination of methane increases with an increase in temperature. Thermal chlorinations have been conducted at temperatures as low as 250' C. (8) where the reaction is very slon, and as high as 1400" C. (7) vhere the reaction occurs extremely fast. At the higher temperatures where the reaction occurs rapidly, the heat of reaction may be sufficient to raise the reaction temperature very high, particularly if a large excess of methane is not used. Hence, many procedures have been used in an attempt to inoderate the reaction. Catalysts for this reaction have frequently been overemphasized. I n general, they have been used at elevated temperatures where the reaction occurs nithout them to yield the four chloromethanes. Also, many different sources of light have been used to effect the chlorination of methane. The reaction occurs smoothly even a t teinperatures well below ordinary room temperature if the concentration of chlorine is not too high. Blue light seems to be most effective. Since there are several reviews of methane chlorination (2, S), no further discussion of the literature nil1 be presented in this paper. I n most of the processes reported it is necessary to use large quantities of methane and, hence, recycling, a reactor filled with material of large surface, an inert diluent, or some other economically undesirable feature to avoid a burning or explosire reaction. Generally, all of the chloromethane.: are reported. The present investigation was undertaken to develop the Hass-hlcBee chlorination process (4) a5 it applies to methane. This process coiisists in passing a preheated hydrocarbon or partially chlorinated hydrocarbon through a reactor maintained a t a suitable temperature and then introducing chlorine. The chlorine is introduced a t high velocity to avoid a flame. This may be accomplished by passing the chlorine through a jet across xvhich there is a considerable pressure drop. If the velocity of the chlorine stream is sufficiently high, the gas enters the methane at a speed in excess of the speed of flame propagation, and the turbulent flow produces intimate mixing which disperses the reactants uniformly in each other before any considerable amount of chlorination takes place. Hence, the reaction yielding carbon and hydrogen chloride does not occur. Chlorine may be introduced a t any suitable number of jets, properly spaced along the reactor tube, into the material being chlorinated and maintained within the desired range of reaction temperature. By such an arrangement the momentary concentration of chlorine may be kept well beloTy those which yield an explosive mixture. The over-all mole ratio of reactants may be that required to effect any desired degree of chlorination in a single operation without recycling.
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HE four chloromethanes are important commercial T substances and are used in relatively large quantities. The supply of methane froni petroleum is enormous, and during peacetime chlorine is readily available a t a relatively lorn cost. These factors favor the chlorination of methane for the commercial preparation of all the chloromethanes. The large number of patents and other literature references in the field indicate extensive interest in this problem. The chlorination reaction is highly exothermic and requires careful control for successful operation. If the concentration of chlorine is within certain limits, there is danger of a violent and even explosive reaction. The approximate figures for the heats of reaction for the chlorination of methane at 400" C., calculated from the heats of combustion given by Bichomsky and Rossini, are as follows: CHh + Clz +. CHZCl + HC1 + 25 Cal. CHa + 2C12 +. CH2C12+ 2HC1 + 48 Cal. CH4 + 3c12 +. CHC13 + 3HCl + 72 Cal. CH4 + 4C12 +. CClr 4HC1 + 96 Cal.
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When high reaction temperatures are attained, as by high concentration of chlorine, carbon and hydrogen chloride are 1 This paper 1s t h e fifteenth in a series on the subject of syntheses from T h e others appeared in INDUSTRIALAND ENCInatural gas hydrocarbons N E E R I N Q CHEMISTRY, 23, 352 (1931); 27, 1190 (1936): 28, 333, 339, 1178 (1936); 29, 1335 (1937): 30, 67 (1938); 31, 118, 648 (1939); 32, 34, 427 (1940); 33, 178, 181, 185 (1941)
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INDUSTRIAL AND ENGINEERING CHEMISTRY APPARATUS AND PROCEDURE
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34 a t the desired rate of flow. A uniform flow was maintained by a constant water head, 37. C h l o r i n e v a l v e s 12-18 w e r e opened, and valve 8 was regulated to build up a chlorine pressure of 50 pounds per square inch as shown by gage 9. This pressure was maintained throughout each run. The amount of chlorine introduced into the reactor a t jets 27-33 was controlled by the needle valve adjustment in each jet, as previously described, and measured by flowmeters 20-26. The gases from the reactor passed from the apparatus through 45 until experimental conditions were regulated. A chlorination experiment was essentially a sampling procedure. After conditions for the experiment had been suitably regulated, valves 45 and 8 were closed, and simultaneously valves 44, 7, and 5 were opened. Valve 5 was adjusted to maintain the 50 pounds chlorine pressure. The amount of chlorine used during an experiment was determined by differencein weight of chlorine container 4. Tnis was filled with liquid chlorine by application of nitrogen pressure on the chlorine in container 3, which was charged from a commercial cylinder. Products of reaction passed to a solid carbon dioxide condenser, 39, two water scrubbers, 40 and 41, a solid carbon dioxide condenser, 42, and a liquid air condenser, 43.
Good mixing of chlorine and methane should be attained in order to avoid concentrations of chlorine which result in burning. Chilton and Genereaux (1) show pictures of mixing obtained by adding titanium tetrachloride to air a t a number of angles and using various velocity ratios. They concluded that best mixing results from introducing the added gas upstream a t a 45" angle to the main stream, but that good mixing is obtained with a T-tube, or perpendicular mixing, if the mass velocity of the added stream is two or three times that of the main stream. Many of the experiments reported here were conducted by introducing chlorine along the line of flow of the material being chlorinated. This type i f mixFigure Jet ing was used because of the ANALYSIS AND IDENTIFICATION 1, '/*-inch union convenience in introducing 2. '/r-inch pipe chlorine into a U-reactor Receiver 39 was warmed to expel dissolved hydrogen chlo3. Cotton packins 4. '/&-inch street ell through a tee. Later work ride. Organic material which vaporized with the hydrogen showed that comparatively chloride passed through the scrubbers and condensed in re5. '/&inch nickel rod 6. Iron packing nut little more chlorine could ceivers 42 and 43. The amount of hydrogen chloride was 7. Nickel packing gldnd be introduced a t any one determined by titration. 8 . Durd Pldrtic packing 9. '/&-inch nickel tee point with perpendicular Contents of receivers 39 and 42 were combined and rectified 10. '/&-inch nickel pipe mixing. for analysis. A column (Figure 4) was constructed so that 11. N o . 1 6 Chrome1 w i r e The chlorine jet (Figure the analysis could be conducted in a single operation. The 19. W e l d e d and threaded 1) is similar in design to the column head, 1, was cooled with solid carbon dioxide for the 13. No. 5 8 drill hole nozzle of a garden hose. separation of methyl chloride. Condensate passed through a The flow of chlorine through side tube, 2, which was in a box, 3, cooled by solid carbon dioxide. Some of the condensate returned to column 4 for reeach jet was controlled by turning the l/g-inch nickel rod flux, and some of it was removed through stopcock 5 t o a which, in turn, closed or opened the small jet hole. By this procedure the amount of chlorine delivered at each jet could be varied a t will from almost nothing to the maximum capacity of the drilled hole. The chlorine pressure in the line t o the jet was arbitrarily set a t 50 pounds per square inch. Since the gas stream in the reactor mas about atmospheric pressure, the pressure drop across the jet itself was about 50 pounds. This type of jet is very satisfactory and especially suitable for experimental work because of flexibility. Figure 2 is a flow diagram of the chlorination apparatus, and Figure 3 is a photograph of the actual equipment. Prior to starting a run, reactor 34 (Figure 5 2) was lowered by a chain hoist into molten sodium nitrate maintained a t a suitable bath temperature by a large natural gas burner. The reactor was made of '/(-inch iron pipe size (0.92-cm. inside diameter) nickel tubing in the form of several bends connected by means of tees and street ells. Methane was then passed from Figure 2. Flow Diagram for Chlorination of Methane tank 36 through flowmeter 38 and reactor
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
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OF THE RATIOOF CHLORINE TO METHANE ON TABLE I. EFFECT
Run No. 35 34 31 30 29 32 33 36
Bath Temp., O
C
440 440 440 440 440 440 440 460
CIP/CHI
Clz Used Mole;
0.50 1.10 1.68 1.98 2.28 3.02 3.31 3.88
6.97 6.80 7.51 7.54 7.72 6.34 6.02 6.49
htole Ratio,
THE
MOLEFRACTION OF CHLOROMETHANES Clz
% of Original Cll in: CHsCl
CHzCIz
18.40 8.60 3.69 1.85 0.89 0.40
17.80 18.80 16.80 12.00 9.95 4.28 1.48
... ...
...
CHCL 6.23 13.00 19.45 23.60 26.30 22.40 16.75 1.30
calibrated receiver, 6. Water cooling was substituted for solid carbon dioxide in the column head after methyl chloride had been removed. This procedure gave good separation of the four chloromethanes. Receiver 43 (Figure 2), cooled with liquid air, usually contained some methyl chloride and unreacted methane. The contents were weighed, and an aliquot was obtained for analysis by a modification of the procedure given by Martinek and Marti (6). This mixture of methane and methyl chloride was mixed with an excess of air and passed through a clay combustion tube a t 1000° C. The gaseous products were then passed through two wash bottles filled with a sodium carbonate-sodium arsenite solution, and chloride was determined by Mohr's method. The data reported in Table I and Figure 5 were obtained with 99 per cent methane. Many other runs (not reported) were made with material which was 92 per cent methane, 2 per cent ethane, 5.2 per cent nitrogen, and 0.8 per cent oxygen. Natural gas was also used, which analyzed 74.3 per cent methane, 14.5 per cent ethane, 11.1 per cent nitrogen, and 0.1 per cent oxygen. The four chloromethanes are common substances, and the nature of the starting material, together with the well known physical constants, was considered sufficient for identification.
CCL 1.18 2.38 3.46 6.33 9.35 16.50 26.00 41.60
Vol. 34, No. 3
Recovery, HC1 50.46 50.50 50.30 51.10 50.10 49.90 51.50 51.70
%
94.07 93.28 93.70 94.88 95.59 93.48 95.73 94.60
Mole Fraction of Product CHaCl CHzClz CHCli CCL 0.620 0.374 0,190 0.107 0.053 0.027
... ...
...
0.070 0.189 0.334 0.455 0.517 0,529 0.435 0.040
0.010 0.026 0.044 0.091 0.137 0.292 0.509 0.960
SPACING AND CAPAClTY OF JETS
A reactor was built stepwise for the chlorination of methane to carbon tetrachloride. The reactor tube was 1/4-inch i. p. s. nickel with one U-bend between each jet. A bath temperature of 440' C. and a methane flow of 150 liters per hour were a d o p t e d . It was necessary to determine approximately the amount of chlorine which, under these conditions, could be introduced a t each jet and the minimum length of Figure
Figure 3.
0,300 0.410 0.431 0.348 0.292 0.151 0.057
Equipment for Chlorination of Methane
4.
Rectifying Column
each U-bend between jets. These determinations were made by the aid of several factors. The chlorination reaction occurs rapidly a t a bath temperature of 440" C.; a t considerably lower temperatures the reaction is sluggish. The capacity of the first jet was determined by making several runs with various rates of chlorine flow and titrating the hydrogen chloride obtained from a weighed amount of chlorine. When the moles of hydrogen chloride exceeded moles of chlorine used, it was assumed that pyrolysis and/or burning had occurred. Fifty liters of chlorine per hour was adopted for the first jet because 60 liters gave excess hydrogen chloride. The length of the reactor between the first and second jets was determined by the amount of unreacted chlorine and by temperature gradient diagrams. Figure 6 shows the temperature gradients for 10, 20, and 30 liters of chlorine per
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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be used, and when high yields of methyl chloride and methylene chloride are desired, a large number of jets is to be preferred to maintain a low momentary concentration of chlorine at all times. EFFECT O F M O L E R A T I O
M O L E R A T I O OF CHLORINE TO 99% M E T H A N E
Figure
5.
Effect of M o l e Ratio of Reactants o n the M o l e Fraction of Chloromethanes
hour through the first jet. The methane flow was 150 liters per hour. It was passed into a l/r-inch i. p. 8. nickel pipe in a molten salt bath maintained at 390' C. and was preheated to 320' C. before chlorine was iptroduced. The reaction was slow, and several inches of reactor were necessary for its completion. Figure 7 shows temperature gradients for
The data in Table I are plotted graphically in Figure 5. The latter gives a clear picture of the effect of mole ratio of reactants on the ratio of the four chloromethanes. As would be expected, the mole fraction of methyl chloride approaches 1 as the mole ratio of chlorine to methane decreases. Also, the mole fraction of carbon tetrachloride approaches 1 as the mole ratio of chlorine to methane increases to 4. No partially chlorinated products are obtained at higher mole ratios of reactants, provided sufficient reaction time is allowed for all hydrogens to be substituted. It is interesting to note that the highest mole fraction of methylene chloride is obtained with a mole ratio of chlorine to methane of about 1.4. Further, the mole fractions of methyl chloride and chloroform are substantially the same a t this ratio. The mole fractions of products corresponding to a mole ratio of reactants of 1.4 are approximately: CHaCI, 0.26; CH&12,0.45; CHCl,, 0.26; C C 4 ,0.04. Also, the highest mole fraction of chloroform is obtained with a mole ratio of reactants of about 2.66. Again, two of the products (methylene chloride and carbon tetrachloride) have the same mole fraction. The mole fractions of products corresponding to a mole ratio of reactants of 2.66 are approximately: CH8C1,0.02; CH2C12,0.20; CHCI,, 0.58; CCl,, 0.20.
T E M P E R A T U R E GRADIENT DIAGRAM FOR CflLORlNATlON O F M E T H A N E $-"NICKEL TUBING JET I
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METHANE 150 L/HR. BATH T E M P E R A T U R E 390'c. M I X I N G T E M P E R A T U R E 320'C. CONSIDERABLE UNREACTED CHLORINE I N E X I T GASES
4
8
IC
INCHES
figure 6. Temperature Gradient Diagram for Chlorination of Methane at 390' Figures on curves dre chlorine flows in liters per hour.
c.
the same conditions except that the bath temperature was 440" C. and the methane was preheated to 360" C. The reaction was much faster than before. On the basis of Figure 6 and the small amount of unreacted chlorine, the second jet was placed in the reactor 14 inches from the first. I n this way the reactor was lengthened and more jets were added. A reactor was constructed for the chlorination of methane to carbon tetrachloride in which jets 1 to 7 delivered 50, 50, 60, 60, 60, 90, and 120 liters of chlorine per hour, respectively; the lengths of '/4-inch reactor following jets 1to 7 were 14, 17, 17, 20, 24, 25, and 29 inches, respectively. EFFECT O F N U M B E R O F JETS
It is important to have sufficient jets for a given set of conditions so the ratio of methane to the chlorine introduced per jet is well above that which gives burning and/or pyrolysis. A larger number of jets has little or no effect on yields or ratio of products. There is no upper limit to the number which may
4
Figure
8 INCflES
12
16
7. Temperature Gradient Diagram for Chlorinat i o n of M e t h a n e at 440' c.
Figures on curves are chlorine flows in liters per hour.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The maximum for chloroform is more than for methylene chloride because the former is more difficultly chlorinated. Four other mole ratios of reactants a t which two of the products have the same mole fraction are: 0.96, 1.83, 2.1, 3.3. EFFECT O F TEMPERATURE
Bath temperatures substantially lower than 440" C. may be used; but the reaction becomes increasingly sluggish as the temperature is decreased, and more and more reactor volume is required. Much higher temperatures may be uscd, provided the momentary concentration of chlorine is maintained low; that is, a large number of jets is used and the exposure time is sufficiently short. There is not much difference in the relative amounts of chloromethanes obtained a t 390", 440", 500", and 550" C., but considerable excess hydrogen chloride was obtained a t 550" C., even with four times the number of jets used a t 440" C. A bath temperature of 440" C., with other conditions as employed in this investigation, is not necessarily a critical point but is within the range of most suitable temperatures. DISCUSSION
The process is very flexible in that almost any desired ratio of chloromethanes may be obtained. It may be made to yield relatively large proportions of the two more costly compounds, methyl chloride and methylene chloride. It has the advantage that any ratio of chlorine to methane may be used in a single pass through a given reactor, and if desired, carbon tetrachloride only can be obtained without the expense of recycling. 'jTable I shows that substantially 95 per cent of the chlorine was accounted for. It is believed that most of the other 5 per
cent can be accounted for as holdup in the apparatus and rectifying column, and as loss in transferring from the receivers to the column. The methane balance was determined in several runs, and in all cases more than 90 per cent was accounted for. Therefore, particularly since there is substantially no carbonaceous material or by-products, it is believed that on a large scale where small losses are less significant the yield of cliloromethanes based on methane would be substantially 100 per cent and based on chlorine, 50 per cent. All of the products of the reaction have important industrial uses, but an outlet for the hydrochloric acid must be assured for the process t o be practical. ACKNOWLEDGMENT
The authors are indebted t o the Ethyl Gasoline Corporation and the Purdue Research Foundation for defraying the expense of this investigation. LITERATURE CITED (1) Chilton and Genereaux, Chem. & M e t . Eng., 37, 755 (1930). (2) Egloff, Schaad, and Lowry, Chem. Rev., 8 , 1 (1931). (3) Ellis, "Chemistry of Petroleum Derivatives", Vol. 1, p. 686 (1934); Vol. 2, p. 726 (1937). (4) Ham and McBee, E. 5.Patent 2,004,072 (1935) ; Canadian Patent 374,241 (1938). (5) Jones, Allison, and Meighan, U. S. Bur. Mines, Tech. Paper 255 (1921). (6) Martinek and Marti, IND. ENG.CHEM., ANAL.ED.,3,408 (1931). (7) Mason and Wheeler, J. Chem. SOC.,1931, 2282. (8) Pease and Walz, J . Am. Chem. Soc., 53,382, 3728 (1931). THIS paper contains material abstracted from B thesis submitted t o the faculty of Purdue University b y C. M. Neher in partial fulfillment of t h e requirements for t h e degree of doctor of philosophy.
Nitration of Methane'. natural gas, freed of a l l hydrocarbons except methane, was nitrated a t atmospheric pressure i n the vapor phase w i t h 67 p e r cent nitric acid at temperatures ranging From 375" to 600" The most favorable conditions found, using a ratio of 9 moles of methane to 1 of nitric acid, are a temperature of 475' C. and an exposure time of 0.18 second. The optimum conversion of nitric acid to nitromethane was 1 3 per cent per pass. The reaction has an activation energy of 52 Calories per mole.
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Vol. 34, No. 3
.. Thomas Boyd* and
H. B. Hass
Purdue University and Purdue Research Foundation, Lafayette, Ind.
C.
LTHOUGH nitromethane has been known since A and has been readily available for many years as a laboratory product, it first appeared as an industrial chemical 1872
(7)
in May, 1940. That now produced is obtained as a by-product of the nitration of propane ( 3 , 4) a t the Peoria plant of Commercial Solvents Corporation. Because of the long ac1 This paper is the sixteenth in a series on t h e subject of synthesrs from A N D ENGInatural gas hydrocarbons. T h e others appeared in INDUSTRIAL N E E R I N G C H E M I S T R Y , 23, 352 (1931); 27, 1190 (1935); 28, 333, 339, 1178 (1036); 29, 1335 (1937); 30, 67 (1938); 31, 118, 648 (1939); 32, 34, 427 (1940); 33, 176, 181, 185 (1941); 34, 296 (1942). 2 Prerent address, Monsanto Chemical Company, Springfield, Mass.
cessibility of nitromethane, the many researches which have been carried out on it, and its great reactivity, there are many actual and potential commercial outlets for its derivatives. It seemed advisable, therefore, to study possible means of augmenting the supply of nitromethane by a direct attack upon its parent hydrocarbon. The preliminary attempt t o nitrate methane in this laboratory met with failure ( 3 ) because the exposure time was too short for the temperature used. Landon's excellent work on the vapor-phase nitration of methane is disclosed in two patents (6) which describe the process in a ferrous or nonferrous reactor a t exposure times between 1 and 0.005 second under pressures from 1 t o 50 atmospheres. Since the processes of Landon have not so far resulted in commercial success, the subject was re-investigated by the authors. ANALYSIS O F METHANE
Purified natural gas was passed into a flask cooled by liquid air. d sensitive flowmeter on the exhaust side of the receiver showed that practically all of the gas passed into the receiver
