ANALYTICAL EDITION
May, 1945
reactions involved in the production and purification of the makeup gas, the ratio of nitrogen to argon in the make-up gas is assumed to be 83.0, the same as that in the atmosphere (6). Thus, the synthesis efficiency, E, is expressed by the relationship:
E -
83.0
- (Nt/A) 83.0
where S 2 / A is the ratio of nitrogen to argon in the gas from the converter. Introducing R as the argon ratio, A / ( N s A ) ,
+
-N&
=
1
-
1/R
(2)
From Equations 1 and 2,
Equation 3 was used t o obtain the curve in Figure 1, showing the relationship between synthesis efficiency, with respect to nitrogen, and argon ratio in gas from the converter. Figure 1 s h o w that as the argon ratio increases, it becomes a more sensitive indication of synthesis efficiency. DESCRIPTION OF APPARATUS
The apparatus for the determination of argon consists of a train for the continuous removal of gases other than argon and nitrogen from the synthesis gas, and a thermal conductivity analyzer for recording the argon ratio in the purified mixture. Since the Orsat analyses regularly performed in the laboratory furnish the sum of argon and nitrogen, the argon ratio enables calculation of both argon and nitrogen. A diagram of the purification train is shown in Figure 2. The flow of synthesis pas into the Durification train is maintained a t about 8 liters peFhour by means of a piston-type pressure stabilizer ( 2 ) and a needle valve. Accurate control of the rate of flow is unnecessary; the rate affects the time lag and reagent consumption but not the analytical results. Ammonia is removed by 30% sulfuric acid, leaving methane, hydrogen, nitrogen, and argon. Campbell and Gray (4)stated that methane was oxidized completely by copper oxide in 7 minutes at 700" C. and that, in the presence of hydrogen, the oxidation of methane proceeded more easily. Arneil ( 1 ) found that ferric oxide catalyzed the reaction and lowered the temperature of complete oxidation of methane to 500" C. In the apparatus shown in Figure 2, both hydrogen and methane are oxidized a t 700" C. by copper oxide containing 1 to 10% iron oxide to catalyze the oxidation. The temperature is re6ulated by a Brown controlling pyrometer. Channel formation in the oxidant as a result of sintering of metallic copper is minimized by mixing crushed refractory, such as alumina brick,
287
with the copper oxide. The oxidation tubes are 90-cm. (36-inch) lengths of 3.75-cm. (1.5-inch) stainless steel tubing. Tubes made of silica or of iron are unsatisfactory for continuous use. There are two oxidation tubes in the furnace; one is used while the other is regenerated with air. At intervals of about 8 hours, the partly exhausted and the regenerated oxidizers are interchanged by turning the four three-way stopcocks (Figure 2). The gas from the oxidation tube contains mater vapor, carbon dioxide, nitrogen, and argon. Most of the water is condensed by cooling and is removed from the system through a trap. The carbon dioxide is removed by concentrated potassium hydroxide solution, and the remaining mixture of argon and nitrogen is dried with concentrated sulfuric acid. A Leeds & Northrup thermal conductivity apparatus records the argon ratio in the purified argon-nitrogen mixture. The instrument is calibrated against known mixtures or argon and nitrogen, and also against density measurements of the exit gas as made with an Edwards balance. The response of the recorder is linear in respect to the argon ratio over the calibration range of 0 to 0.33, and is accurate to ~ 0 . 0 0 5 . The concentration of argon may be calculated with an over-all accuracy of *0.15%. APPLICATION
The results given by the apparatus are used in combination with Orsat analyses to determine argon and nitrogen in the synthesis gases. Application of the argon ratio to the control of the rate of bleeding from the synthesis system facilitates maintenance of the optimum rate of ammonia production. Calculated from the argon ratio (usually 0.25 to 0.30), the synthesis efficiency of the TVA ammonia plant is 96 to 97%. ACKNOWLEDGMENT
The authors are indebted to J. G. Dely, consultant to the TVA, for proposing that this work be undertaken, and to the staff of the control laboratory, supervised by J. R. Hall, for modifying the apparatus to simplify its maintenance in continuous operation. LITERATURE CITED ( 1)
(2) (3) (4)
(5) (6)
Arneil, A . , J , SOC. Chem. Znd., 53,89-92T (1934). Brown. E. H., Cline, J. E., Felger, M.M.,and Howard, R. B., Jr., I N D . ESG.CHEM., -4NAL. ED., 17, 280 (1945). Brown, E. H., and Felger, M. M., Zbid., 17, 273 (1945). Campbell, J. R., and Gray, T., J . SOC.Chem. Znd., 49, 432-7T, 447-50T (1930). International Critical Tables, Vol. I, p. 393, New York, McGraw-Hill Book Co., 1926. Miller, A . M . , and Junkins, J. N., Chem. & Met. Eng., 50, No. 11, 119-25, 152-5 (1943).
Continuous Determination of Carbon Monoxide in Concentrations Up to 3.5 Per Cent by Electroconductivity EARL H. BROWN, MAURICE M. FELGER, AND R. BOWEN HOWARD, JR.
T
H E production of the hydrogen-nitrogen mixture for the synthesis of ammonia in the TVA plant was described by Miller and Junkins (4). I n one step of the process, semi-water gas is enriched with hydrogen by catalytic oxidation of the carbon monoxide with steam. Most of the resulting carbon dioxide is removed from the gas by pressure scrubbing with water, and the remainder, together with residual carbon monoxide from the catalytic oxidation step, is removed by pressure scrubbing with an ammoniacal copper solution. A continuous indication and record of the unreacted Carbon monoxide in the converted semi-water gas are desir*le for plant control. The continuous indication, along with the rate of gas flow, gives the carbon monoxide load on the copper scrubber and thus aids in both the operation of the scrubber and the re-
generation of the copper solution. An indication of high concentrations of carbon monoxide warns the plant operators of unsatisfactory operation in the catalytic oxidation step. Data taken from the continuous record are used in the preparation of material balances. In actual practice the gas leaving the water scrubber is analyzed, because the excess steam and most of the carbon dioxide present in the gas leaving the converters have been removed a t this stage of the process. The carbon monoxide concentration of this gas is about 2%. The use of neither iodine pentoxide for selective oxidation of the carbon monoxide nor of other solid oxidants, such as copper oxide, for the direct oxidation of both the carbon monoxide and the hydrogen was considered practicable in the continuous analysis of this gas because of the inordinate amount of time required
INDUSTRIAL AND ENGINEERING CHEMISTRY
288
in the frequent reactivation of the oxidant. -4method was devised, therefore, in which the gas is diluted with air in a predetermined ratio, the carbon monoxide and hydrogen are oxidized over a hot copper oxide catalyst, and the resulting carbon dioxide is determined by an electroronductometric method. DESCRIPTION
Small portions of tlie gas undergoing analysis are injected intermittently into a stream of air by means of a sampling stopcock (1). The stopcock (Figure 1) has a stainless steel barrel and a tapered brass plug. The plug encloses two coaxial, cylindrical compartments of equal volume; each compartment communicate> 13 ith either of two pairs of diametric openings in the barrel through oblique bores that meet the periphery of the plug on a ronimoii i * i r r ~ i ~ n f c ~ r ~ Tlrt n r ( * gn. containing rarhon monoxide
GAS INLET
Vol. 17, No. 5
A n analyzer for the continuous determination of carbon monoxide in the range 0 to 3.5% and in the presence of a large concentration of hydrogen is described. The sample i s diluted with air, the carbon monoxide oxidized to carbon dioxide over hot copper oxide, and the carbon dioxide determined by an electroconductometric method.
OPERATION
The operating oonditions for the determination of carbon inoiioxide in concentrations up to 3.*50/,are: Klectiol~te k,lectrolyte flow ratc Cell teniperature Temperature of coppel oxitlc rlir flow rate Volume of clach .:unplr injrction E requency of i~ijectioiiBridge slide-wire resistance Bridge end-coil resistance
0.04 -\' *odium hydroxide 15 nil. pcr minute ('onqtant; between 28" and 32" C. 300" to 350' C. 10 11tc.1- per hour 4.8 I'e 6 per rniiiute 60 ohms 600 ohni-
Altliough -perific rattss of flun of a11 mid gas aaniplc through :tnnlyzer arc not required, the ratch should be constant and iiiu\t be sufficient to replace entirely the gas in the compartment\ oi the baxnpling stopcoch in the lO-\econti intervals betwrcnii sampling. lh(2
111btarting the analyzer, the air and electrolyte flows are fint acljuclted to their proper rates. The distance between the men.-
Table I. Comparison of Carbon Monoxide Concentratidns Determined by Orsat and Electroconductometric Analyses GAS
Figure 1.
OUTLET
Time
(IO liy Ors:it (Fi-hrrm
Houi.8
%
5%
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
2.4 2.5 2.1 2.1 2.2 2.2 2.3 2.3 2.2 2.3 2.5 2.2 2.1 2.1 2.2 2.3 2.3 2.4 2.3 2.1 2.2
2.45 2.45 2.05
Sampling Stopcock of Carbon Monoxide Analyzer
flow through one conipartnicliit, while the air flo~vsthrough the other. The larger projecting end of the plug has four studs equally spaced on its circumference. By engagement of these studs with a rotating crank geared to a synchronous motor, the plug is turned through 90" a t regular intervals, and the gas being analyzed thus is injected int,o the air stream in successive portions of equal volume. A uniform mass for the successive port'ions is ensured by the inrorporation of a temperature and pressure compensator in the gas sample line to compensate for changes in room temperature and atmospheric pressure. The sample metering and preparation system in which the gas is prepared for introduction int,o the absorDtion-conductivitv cell is'shown in Figure 2. Air enters the-system through a piston-type pressure stabilizer ( 2 ) and flows through groowd stopcock SI, a flowmeter, and the sampling stopcock, where it picks up the rarbon monoxide sample. The diluted sample then passes through a scrubher containing potassium hydroxide solution, which removes carbon dioxide; through a heated tube containing copper oxide for the catalytic oxidation of the carbon monoxide and hydrogen; through a condensate trap where water produced by the oxidation of hydrogen is removed; and finally to an absorption-conductivity cell where the carbon dioxide produced by the oxidation of carbon monoxide is measured (3). The gas from the sample line enters the system throu h a pistontype pressure stabilizer and flows Lfrough the sampling stopcock, a temperature and pressure compensator, a gas presiure safety trap, and grooved stopcock SSfor the regulation of the line pressure. The principle and operation of the absorptionconductivity cell, as well as the electrical circuit, have been described (3).
CO by Analyzer
3ATCI 1
TRAP
Figure 9.
Sample Metering and Preparation System
2.05
2.25 2.25 2.20 2.15 2.20 2.20 2.25 2.25 2.20 2.25 2.25 2.30
2:30 2.30 2.20 2.15
ANALYTICAL EDITION
May, 1945
iiririg electrodes is then adjusted so that the recorder indicates zero. The gas flow from the sample line is started, and grooved stopcock 8 1 is adjusted so that the top of the mercury column of the temperature and pressure compensator is a t the calibration mark. The sampling stopcock is started and the analyzer is in operation. The analyzer is calibrated with gas mixtures of known composition. The accuracy of the calibration was checked during regular plant operation by comparison with analyses obtained by means of a Fisher precision gas analysis unit over a 3-day period (Table I). There is good reason for helieving that the accuracy of the electroconductometric method is better than that of the Orsat method. MAINTENANCE
The sariiple metering and preparation train is so mounted that the pressure stabilizers, flowmeter, and compensator are readily visible. The gas sample and air flows seldom require adjustment, but the pressure in the gas sample line must be adjusted at Sn (Figure 2 ) when significant changes in temperature or atmobpheric pressure occur. The rate of flow of electrolyte and the 7 ~ 1 ' 0qetting of the recorder are checked daily. The wro hetting
289
of the recorder is obtaiiietl by stoppiug the rotation of the . m i pling stopcock for sufficient time for all the carbon dioxide to be flushed from the train leading to the absorption-conductivity cell. The solution in thc potassium hydroxide scrubber is changed once each week, and the sampling stopcock is lubricated as required. The catalytic umt in n-hich the carbon monoxide and hydrogen are oxidized requires no attention other than the maintenance of its temperature. The original charge of copper oxide catalyst has shown no decrease in activity in more than a year of continuous use. ACKNOWLEDGMENT
The authors express their appreciation to the staff of the m i monia plant control laboratory for their cooperation in the inqtallation and initial operation of the analyzer. LITERATURE CITED
(1) Brown, E.H.,Isu. ENG.CHEM.,ANAL.ED.,14,551 (1949). (2) Brown, E.H., Cline, J. E., Felger, M.M., and Howard, R. E., Jr., Ibid., 17,280 (1945). (3) Brown, E. H., and Felger, M. M., Ibid., 17,277 (1945). (4) Miller, A.M.,and Junkins. J. N..C'hnn. & -Wet. Eng., 50, No. 1 1 . 119-25,152-5 (1943).
Continuous Determination of Methyl Bromide in the Atmosphere DWIGHT WILLIAMS, GEORGE S. HAINES, A N D F. D. HEINDEL Research Department, Westvaco Chlorine Products Corporation, South Charleston, W. V a .
A method and apparatus for the continuous determination of the concentration of methyl bromide at several points in the atmosphere are described. The methyl bromide is converted to hydrogen bromide b y adding hydrogen to the sample and passing the mixture through a quartz tube which is maintained at a temperature above 800" C. The hydrogen bromide is continuously absorbed in a stream of water and the electrical conductivity of the water solution measured b y means of a recording conductivity meter. Concentrations as
T
Hkl determinatioii of lo\\. concentrittions of methyl bromide in the atmosphere by combustion on passage through a quartz tube followed by titration of the bromine by the KolthoffYutzy ( 1 ) procedure has been described (6). This method has numerous disadvantages when considering the safety of personnel exposed to the vapors. The most serious of these is the time lag between the sampling operation and the completion of the analysis. Since the methyl bromide wa5 converted on combustion either to hydrogen bromide or to free bromine, which could presumably be readily converted to hydrogen bromide, it appeared that electrical conductivity offered a ready means for the continuous determination of methyl bromide. The possibility of determining carbon tetrachloride in air continuously and automatically by measuring the electrical conductivity has been suggested by Olsen, Smyth, Ferguson, and Scheflan ( 2 ) . Thomas, Ivie, Abersold, and Hendricks ( 4 ) have described an automatic apparatus suitable for determining volatile chlorine compounds by combustion and conductivity measurements. However, they abborbed the combustion product- hatch\$-ise and obtained their quantitative results from the rate of change of conductivity. It appeared that this procedure rould he improved if the combus-
low as 1 part per million may b e detected and concentrations up to
600 parts per million have been determined with a precision of *5% of the amount present. The method is not specific for methyl bromide but gives the total chlorine and bromine compounds in the air. A similar apparatus in which the hydrogen is omitted and the air stream is saturated with water vapor is used for the determination of chlorinated hydrocarbons when brominated hydrocarbons are absent.
tion products could be absorbed ill a coiitinuous f l u x of water. This would involve the continuous passage of a constant stream of the air sample through t>hefurnace and the continuous pumping of a constant stream of water through the absorber. Obviously this method is not specific for bromide, as is the KolthoffYutzy procedure, but it should prove useful where methyl bromide is the most probable atmospheric contaminant and will in any case give results on the high side, which is in tho interest of safety. A consideration of tlie coiiceutration of methyl bromide which it was desired to detect and of the probable optimum air and liquid flows indicated that a conduct.ivity cell having a cell (*onstant of about 0.1 reciprocal centimeter was required. The conductivity cell was designed to permit the complete absorption of the combustion products, reduce holdup to a minimum, aiicl permit the compact assembly of the apparatus. Low and erratic recoveries of methyl bromide were obtairicd in early tests based on the assumption that the methyl bromidc was quantitatively converted to hydrogen bromide. Thi. was traced to the formation of free bromine. The addition of hydrogen to the air stream eliminated this sourw of error.
