Continuous Determination OF Carbon Dioxide by Electroconductivity In Concentrations Up to
2
Per Cent
EARL H. BROWN AND MAURICE M. FELGER' A n electroconductometric analyzer for the continuous determination of the carbon dioxide concentration in ammonia plant gases is described. The application of the results given b y the analyzer to the operation of both the water and copper scrubbers of the gaspurification system of the TVA synthetic ammonia plant is given. The range of the analyzer described is 0 to 2%, but the analyzer is adaptable to other ranges.
R,/R, ratios ( 2 ) can be converted to recorder indications for any similar bridge by the relation which is shown herewith :
where R,
= resistance of electrolyte between measuring elec-
trodes
R, = resistance of electrolyte between reference elec-
T
HE converted semi-water gas entering the purification sys-
of A end coil recorder indication, scale divisions = resistance of slide-wire per scale division = resistance of B end coil
= resistance
c
=
R, Rb
tem of the TVA ammonia plant consists of hydrogen and nitrogen in the ratio of 3 to 1, together with about 29% carbon dioxide, about 2% carbon monoxide,' and some inert gases (3). About 98% of the carbon dioxide is removed by scrubbing the gas with water, and the concentrations of both carbon dioxide and carbon monoxide are reduced to about 5 p.p.m. by scrubbing with ammoniacal copper solution. The analyzer described in this paper gives a continuous indication of the carbon dioxide content of the gas leaving the water scrubber.
trodes
R.
?EL
DESCRIPTION AND OPERATION
KUi-ASCARTE
The analyzer consists of a gas-control train, an absorptionconductivity cell, and a recording alternating current Wheatstone bridge. The gas-control train is shown in Figure 1. The pressure stabilizer ( 1 ) gives a uniform operating pressure; the grooved stopcock and flowmeter are used to regulate the rate of gas flow; and the potassium hydroxide-Ascarite tube removes carbon dioxide from the gas when the recorder zero is being checked. The principle and operation of the absorption-conductivity cell and recording alternating current Wheatstone bridge have been described ( 2 ) . The operating conditions for the determination of carbon dioxide in the range of 0 to 27, are: Electrolyte Electrolyte flow rate Gas flow rate Cell temperature
Figure 1.
Gas-Control Train for Carbon Dioxide Analyzei
During the early operation of this analyzer, frequent gravimetric determinations were made of the carbon dioxide content of the gas being analyzed. It was established that the analyzer gave results that were accurate to about *0.0370.
0.04 S sodium hydroxide 15 ml. per minute 8 liters per hour Constant; between 28" and 32" C.
APPLICATION
During normal operation the concentration of carbon dioxide in the gas leaving the water scrubber depends largely on the temperature and quantity of water pumped to the scrubber. This residual carbon dioxide is removed in the copper scrubber. Since there is no control of the temperature of the process water, t,he concent,ration of carbon dioxide in the effluent gas and, consequently, the carbon dioxide load on the copper scrubber, must be controlled by regulation of the quantity of water pumped to the water scrubber. This control is important because an overload on the copper scrubbcr resulting from insufficient removal of carbon dioxide in the water scrubber would lead to the presence of carbon dioxide in the synthesis system in concentrations harmful to the catalyst. On the other hand, the practical limit to which the carbon dioxide concentration may be decreased by increased flows of scrubbing water is fixed by the loss of hydrogen tlirougli solution in the water. The results given by this carbon dioxide analyzer and the ammonia activity analyzer ( 1 ) are used to control the flow of water to the v,-ater scrubber. The carbon dioxide analyzer gives the load on the copper scrubber, and the ammonia activity ana-
The analyzer is placed in operation by first adjusting the gas and electrolyte flows to their respective rates. During this adjustment the gas sample is passed through the potassium hydroxide-Ascarite tube. The distance between the measuring electrodes is then adjusted so that the recorder indicates zero. The 3-way stopcock is turned 180" to the position shown in Figure 1, which places the analyzer in operation. The analyzer requires very little attention during operation. The combination of gas pressure stabilizer and grooved stopcock give:: reliable control of the gas flow rate. The recorder zero and clectrolyte flow rate are checked once each day. The potassium hydroxide-Ascarite tube is rrfilled after several months' use. CALIBRATION
The original calibration was made empirically using gas: mixtures of known carbon dioxide content. The relation between concentration and recorder indication is specific for the given operating conditions and the bridge circuit used-that is, a circuit with 220-ohm end coils and a 66.s-ohm slide-xire. The
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