Determination of Methane by Catalytic Oxidation I. F. WALKER AND B. E. CHRISTENSEN, Oregon State College, Corvallis, Ore. HE danger of explosion constitutes a serious objection
T
ing 1.2 grams of Cu(N03)2.3Hz0 and 14 grams of Co(hTO3)z6H20 on 40 grams of 10-mesh unglazed porcelain particles. Attempts to coat this oxide on copper oxide wire resulted in an inferior catalyst.
to the use of the slow-combustion method for the determination of methane when carried out by one who infrequently uses this procedure. Attempts have been made to increase the safety factor by modifying the pipet, but Steacie's method (6) is not suited to use in standard gas analysis, while Kobe's modification (4) lacks the necessary mechanical stability. The Jager method (a), besides necessitating the use of a quartz; combustion tube, requires such prolonged heating a t high temperature that the process becomes too laborious. This method does not meet with the approval of some authorities (2). The results of the recent work of Yant and Hawk (6) indicate that methane a t comparatively low temperatures and space velocities can be completely oxidized in the presence of suitable catalysts. The possibility of adapting these reactions to analytical work offers a problem worthy of investigation. It is the purpose of this paper to show that catalytic oxidation of methane provides a safe and reliable method of analysis.
TABLE I. ADSORPTIONTESTS QAMPLBI
AMOUNTO P CATALYST
c.c~. .
TOTALCOBALT R ~ C O V E RAT ED 600' C. c.c...
15
1.0 0.20 2 7.0 0.05 2 3.6 0.00 The porcelain particles of sample 1 carried a much heavier coating of oxide.
OPTIMUMCONDITIONS OF CATALYSIS In order to determine the optimum conditions for the oxidation of methane, the effect of space velocity on catalytic efficiency was first studied. Results given in Table I1 are from runs made over 3.5 cc. of catalyst at 500" C., using a mixture of 15 per c e n t of m e t h a n e I and 85 per cent of o x y g e n . Since a rate of 20 to 25 cc. per minute is essential for rapid analysis, these results in2.6 dicate that the use of a smaller volume U z of catalyst w o u l d not be a d v i s a b l e . In all s u b s e q u e n t tests 3.5 cc. of cata- w lyst were employed. 8 As shown in Figure 1, temperature exhibits a very marked influence on e f f i c i e n c y below 250 350 450 330 500" C. These TEMPERATUFE 'C tests were made at FIGURE 1. INFLUENCE OF TEMPERATURE ON EFFICIENCY OF CATALYST 20 cc. per minute, using a mixture of 15 per cent of methane and 85 per cent of oxygen. The points given in the curve were taken from a single run in the order of descending temperatures. Points on the upper part of the curve were readily duplicated with different batches of caialyst. In the lower portions of the curve where conditions were obviously unfavorable, only qualitative agreement was obtained, but the curve represents the general trend of all such observations. TABLE11. EFFICIENCY O F CATALYST
APPARATUS A Burrell Bureau of Mines gas-analvsis apparatus (I) was used in this work. A pipet equipped with a three-way stopcock, mounted next to the slow-combustion chamber, made it easier t o flush the system and provided an additional gas reservoir. The original furnace was replaced by one which could be maintained at a confitant temperature through a range of GOO" C. A platinum-platinum-rhodium thermocouple incased in a Pyrex glass tube was mounted closely adjacent to the catalyst chamber with the junction at such a point that it accurately measured the temperature t o which the catalyst was subjected. The construction of the furnace was such that the temperature was constant within 1 5 " C. over a range of 8 cm. Two types of combustion tubes (simple and compound) were used. The compound tube was made by replacing one arm of an ordinary copper oxide tube with a catalyst chamber. One side then contained the copper oxide and the other the catalyst. The chamber finally adopted was 7.5 cm. in length and 3.5 cc. in volume. The sim le combustion tube consisted of merely the catalyst chamber, tge copper oxide arm being replaced by capillary tubing.
E
PREPARATION OF THE CATALYST Previous investigation (6) indicates that cobalt oxide under suitable conditions might serve as a catalyst for the oxidation of methane. Though cobalt oxide preparations gave high efficiencies, it was found that adsorption of carbon dioxide a t 300" C.made them unsatisfactory for analytical purposes. This difficulty was overcome, however, by mounting a thin layer of the oxide on unglazed porcelain particles. As shown in Table I, the adsorption was in this manner rendered negligible. The tests for adsorption were made by filling the combustion tube with carbon dioxide a t 300" C. The tube a t this temperature was carefully flushed out with nitrogen until it was apparently free of carbon dioxide. The temperature was then raised to 600" C., the tube was again flushed, and the carbon dioxide determined. Previous work had shown no adsorption a t 600" C. The catalyst finally adopted was made by the evaporation and subsequent thermal decomposition of a solution contain-
RATEI
SPACBVBLOCITY
10 30
40 60 90 120
EFFICIBNCY~
%
Cdmin.
168 600 666 1000 1600 2000
98 97 94 89 84 76
4000 63 240 a Effiaienoy means the percentage of methane oxidized in one passage over the catalyst.
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Vol. 7, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
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To determine the effect of concentration, samples containing varying percentages of methane in methane-oxygen mixtures were passed over the catalyst a t 20 cc. a minute and 500" C. Table I11 summarizes the results of this study, which indicate that up to concentrations approaching one part of methane to three of oxygen high efficiencies are maintained. TABLE111. EFFECTOF CONCENTRATION METHANE IN METHANE-OXYG~N MIXTURE %
EFFICIENCY
%
414 7.4 14.1 22.1 23.4
98.0 98.7 98.5 96.8 97.1
Life tests made on the catalyst showed that after passage of 8 liters of a mixture of 15 per cent of methane and 85 per cent of oxygen (equivalent to eighty determinations in ordinary gas analysis) the efficiency had not changed. These results were duplicated using a 20 per cent methane mixture. The data of Table 111, which show a combustion of 97 per cent of the methane in the methane-oxygen mixture with only one passage over the catalyst, indicate that twice over the catalyst a t suitable conditions, followed by flushing a t 550" C., should insure complete oxidation. DETERMINAT~ON OF METHANE IN METHANE RESIDUES In standard gas analysis, methane is determined by the slow combustion of an aliquot portion of the gaseous residue which remains after the other components (carbon dioxide, carbon monoxide, hydrogen, oxygen, and illuminants) have been removed. These residues are usually mixtures of nitrogen and hydrocarbons (methane). Since ordinary methane residues contain nitrogen, which is not considered in Table 111, the effect of nitrogen as a diluent was studied. Preliminary work with methane-air mixtures had already shown under favorable conditions (low space velocities a t 550" C.) a high degree of combustion. A summary of this work is given in Table W . These results confirm the work of other investigators (6). TABLEIV. DETERMINATION OF ME THAN^ RXSIDUES
IN
METHANE IN METHANBI-AIR MIXTURE
SAMPLE 1 2 3 4
EFFICIENCY
%
3.8 6.2 6.8 9.9
95 99 99 95
Since nitrogen even in high concentrations apparently has no effect on efficiency of combustion, catalytic oxidation suggests itself as another method for the determination of methane in methane residues. TABLEV. COMPARATIVE TESTS METHANE I N METRANII RESIDUE
SAMPLBI 1 2 3 4 5 6 7 8 9
10
MODIFICATIONS OF PROCEDURE To adapt this method to standard gas analysis apparatus two modifications present themselves: 1. One may employ two furnaces, one operating at 300' C. with a copper oxide tube for the determination of carbon monoxide and hydrogen, and the other operating at 550' C. with the simple catalyst tube for the determination of methane in the gas residue. 2. One may employ a single furnace capable of operating at variable temperatures, using only a compound combustion tube containing both the copper oxide and the catalyst. Since it previously had been found that methane did not reduce the catalyst at the temperature of use and that carbon dioxide was not adsorbed at 300' C., the use of one furnace involves only a slight modification in procedure. In this case after the oxidation of carbon monoxide and hydrogen, the combustion tube should be flushed and the carbon dioxide absorbed. Then the system should be cooled to room temperature and the hydrogen determined by contraction. (When the compound tube is used, it is necessary t o reoxidize the copper oxide before proceeding with the analysis of the methane residues.)
To show that the compound combustion tube gave reliable results for the determination of carbon monoxide and hydrogen in the presence of methane, comparative tests were made with an ordinary copper oxide tube. These analyses (Table VI) were made on samples containing a mixture of carbon dioxide, carbon monoxide, methane, oxygen, hydrogen, and nitrogen. TABLEVI. COMPARATIVE TESTS SAMPLE1
CUO
+
CuO
oatalyat
%
%
SAMPLE2
CuO %
SAMPLE3
CUO f catalyst
CuO
CUO f catalyst
%
%
%
METHANE
%
Slow combustion
60 to 80 cc. of oxygen for each combustion. Although in most cases this would be more oxygen than is necessary, it is well to provide sufficient gas for flushing. Results are tabulated in Table V and show complete agreement a t all concentrations of methane ordinarily encountered in gas analysis.
Catalytic oxidation
%
%
3.5 8.6 13.9 15.4 15.3 27.9 40.5 56.0 69.6 89.3
3.4 8.3 13.6 15.3 15.4 28.2 40.3 55.7 69.8 89.3
As a final substantiation ten methane residues (methanenitrogen) containing from 3.5 to 89.3 per cent of methane were made up. Comparative tests with the slow-combustion method were made, employing 20-cc. samples, and using
COMMENTS KO difficulty was experienced in reproducing the catalyst. Three separate preparations were used throughout this work. Later experiments have shown that the amount of copper oxide (up to 50 per cent) incorporated in the catalyst had no effect on its activity. Gaseous hydrocarbons such as Flamo (principally butane), a by-product of the Standard Oil Company of California, were completely oxidized under the conditions specified as suitable for methane. To insure no change in catalytic efficiency when oxidizing butane, lower space velocities should be used. It is evident that the presence of ethane and propane would have no effect on the catalysis. Though the presence of moisture does not affect efficiencies, wetting the catalysts with other confining liquids renders them unfit for further use. Traces of sulfur (1 part in 100,000) were found to poison the catalysis slowly, Since it is highly improbable that the gas after being subjected to the reagents used in standard gas analysis preliminary to the methane determination would contain the ordinary catalytic poisons, this phase of the subject has not been thoroughly investigated. SUMMARY Catalytic oxidation offers a rapid, satisfactory, and safe method for the determination of methane in gas residues.
ANALYTICAL EDITION
January 15, 1935
Methane in methane residues was quantitatively oxidized when twice passed over 3.5 cc. of cobalt oxide catalyst a t a rate of 20 to 25 cc. per minute and 550" C., followed by flushing a t this temperature, provided the ratio of oxygen to methane was at least 3 to 1. Comparisons made between the slow-combustion and catalytic oxidation methods show complete agreement a t all concentrations of methane. Two procedures are given for the use of this catalyst: the utilization of a single furnace with a compound combustion tube, and the use of two furnaces employing separate
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combustion tubes with a modified manifold. The authors recommend the latter. LITERATURE CITED (1) Braun-Knecht-Heimann Co., Catalog, p. 382 (1934). (2) Burrell and Oberfell, J. ID. ENQ.CHEM.,8,228 (1916). (3) Dennis, L. M.,"Gas Analysis," 2nd ed., p. 198, New York, Macmillan Co., 1913. (4)Kobe, IND. ENQ. CHEM.,Anal.Ed., 3, 159 (1931). (5) Steacie, J. Am. Chem. Soc., 52,2811 (1930). (6) Yant and Hawk, Ibid., 49,1457 (1927). RECEIVED September 6, 1934.
,Molecular Weight of Cracked Distillates OGDENFITZSIMONS AND E. W. THIELE, Standard Oil Co. (Ind.), Whiting, Ind.
M
OLECULAR weights are an important property of petroleum products, finding extensive use in the design of refinery equipment. However, the routine determination of molecular weights is not practicable, because of the care and time required in this type of work. Some means of estimating molecular weights from other more easily determined properties is therefore desirable. For straight-run stocks this need was met by a previous study (S),and in this paper the results of an extension of this work to cracked stocks are given. Some modifications have also been made in the method of making the determination.
The results of a series of determinations, by the cryoscopic method, of the molecular weights of various cracked stocks, all f r o m Midcontinent gas oil, and of a few Midcontinent pressure-still charging stocks are given. Some novelties in the cryoscopic determination are presented. The molecular weight results are correlated with boiling point, viscosity, and density to simplifv the estimation of the molecular weight of any given cracked stock.
PREPARATION OF SAMPLES The cracked stocks used in this work were all prepared in pilot-plant cracking equipment. One set of samples was prepared in this plant from a Midcontinent gas oil by simulating conditions of a commercial liquid-phase process in which the product boiling above naphtha end point and below a heavy tar is recycled. A product of 450' F. (232" C.)
BECKMANN
end point on the A. 8. T. M. distillation was produced. The distillate and cycle stocks were cut into 5 per cent cuts in a Hempel-type column and the samples thus obtained were studied. (These samples are numbered from 200 up in the tables and data which follow.) Another set of samples was obtained by the successive once-through vapor-phase cracking of a Midcontinent gas oil. In this process no stock was recycled with the fresh feed, but the product from the first once-through cracking was fractionated to eliminate gasoline and heavy tar, and the residue was fed through the still in a separate operation. This was repeated five times. Selected cuts of the various cycle stocks were chosen for study. Certain Midcontinent straight-run cuts were also examined. APPARATUS AND PROCEDURE
ATINUM STIRRER LINE
, TEST
TUBE FLASK
[VACUUM RELEASED]
DOUBLE WALLED BRASS CAN
ICE 6 WATER MIXTURE
PERFORATED
BRASS
FIGURE 1
MOLECULAR WEIGHTS. The apparatus used is illustrated in Figure 1, which will require little explanation. Attention is called to the stirring device which lifts the stirrer slowly and after a pause drops it suddenly, thereby producing the maximum of agitation with a minimum and uniform heat development in the solution. It also prevents the mercury thread from sticking and reduces supercooling of the solution. The Dewar flask with vacuum released was found to give the right amount of cooling of the solution for convenient reading when the temperature difference between the bath and solution was 4" to 5" C. (7.2' to 9" F.). The same results have been obtained by other investigators by maintaining a smaller temperature differential between the bath and solution, but for accurate determinations this requires means for varying the bath temperature in order to keep the temperature difference constant. In making a determination, the solvent was measured out with a pipet, the temperature being noted. Two grams
