Study of the passage of alternating electric current through a caustic soda solution
Abstract Generated abstract
This study examines the passage of industrial-frequency alternating current through sodium hydroxide solutions in the context of direct electric heating for caustic soda evaporation. Using a measuring cell with plane-parallel graphite electrodes, the authors measured total cell voltage and solution voltage drops for 25 to 55 percent NaOH at several temperatures and current densities, then calculated the specific boundary resistance at the electrode-solution interface. The results show that this resistance decreases as temperature and current density increase, while its dependence on concentration has a temperature-dependent minimum that shifts toward higher NaOH concentrations with increasing temperature. The work provides experimental resistance data relevant to designing efficient electrode heating systems for caustic soda solutions.
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PROCEEDINGS
OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION
AND ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV
Vol. 259 1975
STUDY OF THE PASSAGE OF ALTERNATING ELECTRIC CURRENT THROUGH A CAUSTIC SODA SOLUTION. COMMUNICATION 1
V. P. PISHCHULIN, N. I. KOZLOVA
(Presented by the scientific-methodological seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
Caustic soda, a valuable product of the chemical industry, is at present obtained by electrolysis of a sodium chloride solution, followed by evaporation of the solution either to a concentration of not less than 42% NaOH, or until solid alkali containing 92–94% NaOH is obtained [1]. For the evaporation of caustic soda we used direct electric heating, that is, heating of the solution by passing an alternating electric current of industrial frequency directly through the solution being evaporated, using electrodes. The use of direct electric heating makes it possible rapidly, with a high efficiency (95–98%), to supply the necessary amount of heat, to eliminate the complex and bulky steam heating chamber and heat transfer through a wall, and to increase substantially the productivity and the utilization coefficient of the apparatus volume. With direct electric heating, Joule heat is released, determined by the magnitude of the current and the resistance of the system. The electrical resistance of the system consists of the resistance of the electrodes, the solution, and two boundary resistances. The resistance of the solution depends on the specific electrical conductivity of the solution, the area, shape, and arrangement of the electrodes.
The studies we carried out to determine the specific electrical conductivity for sodium hydroxide solutions with concentrations of 10; 23; 25; 28.6; 30; 35; 40; 45; 50 and 55% (by mass) at temperatures of 50, 75, 100°C and at the boiling temperature showed satisfactory agreement with the data obtained by I. I. Maksimova and V. F. Yushkevich [1, 2].
Boundary resistances account for up to 20–40%, and in some cases more than 50%, of the total resistance of the system electrode—caustic soda solution—electrode. The boundary resistance is characterized by the specific boundary resistance and by the surface of the electrode. The specific boundary resistance, determined by the resistance of the double electric layer arising at the electrode—solution interface owing to the difference in the mechanism of electrical conductivity in the electrodes and in the solution, and by chemical reactions on the surface of the electrodes, depends on the electrode material, the composition, concentration, and temperature of the solution, the current density, and is determined experimentally.
In this communication the results are presented of a study of the specific boundary resistance of electrodes made of electrode graphite in a caustic soda solution with concentrations of 25, 30, 35, 40, 45, 50 and 55% (by mass—
Table 1
Values of specific resistance for graphite electrodes in a caustic soda solution at various concentrations, solution temperature, and current density
General voltage drop across the cell \((U)\) and specific resistance \((R_{\text{sp. gr.}})\) for graphite electrodes at solution concentrations (mass percent)
| Temperature, °C | Current density, A/cm² | 25: \(U\), V | 25: \(R_{\text{sp. gr.}}\), Ω·cm² | 30: \(U\), V | 30: \(R_{\text{sp. gr.}}\), Ω·cm² | 35: \(U\), V | 35: \(R_{\text{sp. gr.}}\), Ω·cm² | 40: \(U\), V | 40: \(R_{\text{sp. gr.}}\), Ω·cm² | 45: \(U\), V | 45: \(R_{\text{sp. gr.}}\), Ω·cm² | 50: \(U\), V | 50: \(R_{\text{sp. gr.}}\), Ω·cm² | 55: \(U\), V | 55: \(R_{\text{sp. gr.}}\), Ω·cm² |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50 | 0,5 | — | — | 5,6 | 1,30 | 6,7 | 1,60 | 5,6 | 1,80 | 7,5 | 2,10 | 6,8 | 2,30 | — | — |
| 50 | 1,0 | 9,5 | 1,10 | 10,4 | 1,25 | 12,5 | 1,27 | 10,7 | 1,30 | 13,0 | 1,40 | 12,0 | 1,50 | — | — |
| 50 | 2,0 | 16,0 | 0,75 | — | — | 21,3 | 0,80 | 18,3 | 0,80 | — | — | — | — | — | — |
| 75 | 1,0 | 6,4 | 0,85 | 7,0 | 0,95 | 7,5 | 1,00 | 6,9 | 1,20 | 8,5 | 1,30 | 8,2 | 1,40 | — | — |
| 75 | 2,0 | 11,6 | 0,65 | 13,0 | 0,67 | 14,2 | 0,70 | 12,5 | 0,75 | 15,0 | 0,95 | 13,4 | 1,00 | — | — |
| 75 | 4,0 | 20,0 | 0,32 | 24,0 | 0,35 | 23,8 | 0,50 | 24,2 | 0,55 | — | — | — | — | — | — |
| 100 | 1,0 | 5,1 | 0,80 | 4,9 | 0,70 | 5,2 | 0,75 | 5,0 | 0,90 | 5,0 | 1,00 | 5,6 | 1,2 | — | — |
| 100 | 2,0 | 9,4 | 0,72 | 9,2 | 0,55 | 9,8 | 0,68 | 9,4 | 0,70 | 10,0 | 0,80 | 9,8 | 0,85 | 10,3 | 0,90 |
| 100 | 4,0 | 18,2 | 0,61 | 17,8 | 0,35 | 18,7 | 0,50 | 16,8 | 0,53 | 19,0 | 0,60 | 16,8 | 0,70 | 18,0 | 0,75 |
| Room temperature | 1,0 | — | — | — | — | 4,5 | 0,70 | 4,0 | 0,85 | 4,8 | 0,80 | — | — | — | — |
| Room temperature | 2,0 | 9,0 | 0,6 | 6,6 | 0,52 | 8,1 | 0,65 | 7,6 | 0,68 | 7,6 | 0,70 | 6,8 | 0,75 | 6,5 | 0,78 |
| Room temperature | 4,0 | 18,0 | 0,49 | 12,0 | 0,35 | 15,7 | 0,50 | 13,0 | 0,51 | 15,0 | 0,52 | 12,5 | 0,55 | 12,0 | 0,57 |
soda) at temperatures of 50, 75, 100°C and at boiling temperatures.
The investigations were carried out in a measuring cell with plane-parallel electrodes. Electrical energy was supplied to the solution by means of an autotransformer through an ammeter and a voltmeter. A definite volume of the solution under study was poured into the cell. At the established temperature of the solution and a specified current density, the total voltage drop across the cell and the voltage drop of the electric current in the solution were measured by means of a probe and a high-impedance IV-4 output meter. From the difference between the values of the total voltage drop across the cell and the voltage drop across the solution, the limiting voltage drop at the two electrodes was determined. The ratio of the limiting voltage drop at one electrode to the electric-current density on the electrode surface was used to calculate the specific limiting resistance in ohms per square centimeter.
Table 1 gives the experimentally obtained values of the total voltage drop across the cell and the calculated values of the specific limiting resistances in the system graphite electrode—NaOH solution—graphite electrode.
The results of the investigation showed that the specific limiting resistance for all concentrations of the NaOH solution decreases with increasing temperature and current density. The dependence of the specific limiting resistance on the concentration of caustic soda has its own minimum for each temperature; moreover, with increasing temperature the minimum shifts toward higher concentrations of the caustic-soda solution. The curve of the dependence of the specific limiting resistance on concentration is analogous to that for the specific electrical resistance of the NaOH solution.
REFERENCES
- N. P. Fedot’ev [et al.], Applied Electrochemistry. Moscow, “Khimiya,” 1967.
- I. N. Maksimova, V. F. Yushkevich, Zh. Fiz. Khim., vol. 37, p. 903 (1963), “Elektrokhimiya,” 3, 1967.