Determination of Argon in Ammonia-Synthesis Gases EARL H. BROWN AND JAMES E. CLINE pressure in the system; if their concentration reaches a certain percentage, it is economical to prevent a further rise by bleeding part of the circulating gas to the atmosphere. The optimum percentage of inerts depends on the activity of the synthesis catalyst; on the rate of production, purification, and compression of the make-up gas; on the rate of circulation of the synthesis gas; and on the optimum operating pressure of the system. Once the optimum concentration of inerts has been determined by observation of plant operation, the argon and methane determinations can be used in control of the bleeding rate. The third value of the argon determination is that it enables a direct calculation of synthesis efficiency, which is defined as the ratio of the amount of reacting component (nitrogen or hydrogen) removed from the system as ammonia to the amount of the component entering the system during the same period. Since argon is unchanged in the synthesis process, a steady state is reached in which the amount of argon entering the system in the make-up gas equals the amount of argon leaving the system through leakage, solution in liquid ammonia, and bleeding. By calculating the ratios of reacting component to argon (parts per part argon) in both make-up gas and gas from the converter, the ratio of the amount of reacting component entering the system to that leaving the system in unsynthesized condition may be estimated. With normal gas compositions in the system, it was found that an error of less than 1% would be introduced into the calculation if the loss of gas by leakage occurred at a stage in the synthesis cycle other than that from which the sample was taken for analysis. Errors caused by differential solubility of the gases in liquid ammonia were considered negligible. The synthesis efficiency can thus be estimated from purely analytical data with no reference to production rates or gas flows. For calculation of the synthesis efficiency with respect to hydrogen, determinations of the concentration of hydrogen, the sum of the concentrations of nitrogen and argon, and the argon ratio are required. (Argon ratio is defined here as the argon content divided by the sum of the argon and nitrogen contents.) For the synthesis efficiency with respect to nitrogen, however, only the argon ratio in the gas from the converter is needed. As neither the nitrogen nor the argon enter into any of the chemical
A n apparatus for the continuous determination of the argon-nitrogen ratio in ammonia-synthesis gas is described. The gas is scrubbed free of ammonia, the hydrogen and methane are oxidized at 700" C. with a copper oxide-iron oxide mixture, the oxidation products are removed, and the purified argon-nitrogen mixture is passed through a commercial thermal conductivity instrument which continuously records the argon content.
T
HE purified gas used for synthesis of ammonia at the T V A plant contains hydrogen, nitrogen, argon, and methane (3, 6). Other noble gases and hydrocarbons are present in negligible concentrations. I n the synthesis system the gas is circulated through the ammonia-synthesis converter and sufficient make-up gas is added to the circulating system to replace the gas removed as ammonia and that lost from the system through leaks, through solution in the liquid ammonia, and through bleeding to the atmosphere. The concentration of inerts (primarily argon and methane) in the Circulating gas depends on their concentration in the make-up gas and on the fraction of circulating gas leaving the system. The determination of argon in the circulating synthesis gas is desirable for three reasons. In the first place, the determination of the percentage of nitrogen in the gas by the usual Orsat analysis is impossible without a simultaneous determination of argon. The hydrogen-nitrogen ratio, which must be kept close to 3.0 for optimum ammonia production, is indeterminate to the extent to which the argon concentration is uncertain. The determination of argon is necessary also for calculating the total percentage of inerts in the synthesis gas. The inerts act as a diluent and have approximately the same effect as a decrease of 1.01
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0.6 Figure 1. Relationship between Synthesis Efficiency, with Respect to Nitrogen, and A r g o n Ratio in Gas from Converter
Figure 2.
Train for Preparation of Argon-Nitrogen Mixture from Synthesis Gases
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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 at 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 at 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 at 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
