Study of the Passage of Alternating Electric Current Through a Sodium Hydroxide Solution
Abstract Generated abstract
This study examines the specific boundary resistance of carbon electrodes during the passage of alternating current through concentrated sodium hydroxide solutions under conditions relevant to direct electric heating in electrode evaporators. Measurements were made in a fluoroplastic cell with plane parallel carbon electrodes for NaOH concentrations of 45 to 80 percent by mass, over a range of temperatures and current densities. The results indicate that carbon electrodes have lower boundary resistance than previously studied graphite electrodes, that the phase electrode shows somewhat higher resistance than the neutral electrode, and that resistance generally increases with solution concentration, especially above about 70 percent NaOH, while decreasing with increasing temperature and current density.
Full Text
INVESTIGATION OF THE PASSAGE OF ALTERNATING ELECTRIC CURRENT THROUGH A SODIUM HYDROXIDE SOLUTION
Communication 2
V. P. Pishchulin, A. P. Marchenko
(Presented by the scientific-methodological seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
Studies of the process of evaporating a sodium hydroxide solution with direct electric heating [1] showed that, from an economic point of view, the use of electrode-type evaporators is most expedient in the third stage of the evaporation process, i.e., in evaporating caustic soda solutions with concentrations of 42% NaOH and higher. The results of corrosion tests of electrodes made of various materials [2] made it possible to recommend electrode carbon as the electrode material for the design of electrode evaporators. This communication presents the results of a study of the specific boundary resistance of carbon electrodes in sodium hydroxide solutions with concentrations of 45, 50, 55, 60, 65, 70, 75, and 80% (by mass) at various temperatures and electric-current densities.
The studies of specific boundary resistances were carried out according to the method described earlier [3], in a measuring cell made of fluoroplastic-4 with plane-parallel carbon electrodes having a working surface area of 4 cm² and a distance between the electrodes of 54 mm.
The obtained averaged values of the specific boundary resistances in the system carbon electrode—sodium hydroxide solution—carbon electrode at various concentrations and temperatures of the solution and at various electric-current densities are given in Table 1.
The results of the study showed that the values of the specific boundary resistances for carbon electrodes are lower than those for graphite electrodes at equal values of the concentration and temperature of the solution and of the electric-current density, obtained earlier [3]. In addition, it was noted that the specific boundary resistance of the “phase” electrode is somewhat higher than the specific boundary resistance of the “zero” electrode.
The increase in the specific boundary resistance for graphite electrodes can be explained by the greater corrosion resistance of carbon under conditions of direct electric heating in sodium hydroxide solutions, since graphite electrodes are partially destroyed as a result of the probable electrolytic oxidation of carbon and impregnating resins, which increases the electrical resistance of the boundary layer by oxidation products; moreover, at the “phase” electrode the process of electrolytic oxidation proceeds more intensively.
An increase in the concentration of the sodium hydroxide solution leads to a nonuniform increase in the specific boundary resistance. Thus, with an increas-
Table 1
| No. | Temperature, °C | Current density, A/cm² | 45 | 50 | 55 | 60 | 65 | 70 | 75 | 80 |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 90 | 1,25 | 0,64 | — | — | — | — | — | — | — |
| 2 | 90 | 1,87 | 0,54 | — | — | — | — | — | — | — |
| 3 | 90 | 2,50 | 0,48 | — | — | — | — | — | — | — |
| 4 | 90 | 3,12 | 0,45 | — | — | — | — | — | — | — |
| 5 | 100 | 1,25 | 0,48 | 0,64 | 0,69 | 0,72 | 0,76 | 0,80 | — | — |
| 6 | 100 | 1,87 | 0,42 | 0,54 | 0,56 | 0,58 | 0,60 | 0,62 | — | — |
| 7 | 100 | 2,50 | 0,36 | 0,45 | 0,47 | 0,50 | 0,50 | 0,52 | — | — |
| 8 | 100 | 3,12 | 0,35 | 0,42 | 0,43 | 0,44 | 0,46 | 0,48 | — | — |
| 9 | 110 | 1,25 | 0,40 | 0,54 | 0,60 | 0,62 | 0,64 | 0,68 | — | — |
| 10 | 110 | 1,87 | 0,36 | 0,45 | 0,47 | 0,48 | 0,50 | 0,51 | — | — |
| 11 | 110 | 2,50 | 0,33 | 0,39 | 0,41 | 0,42 | 0,43 | 0,45 | — | — |
| 12 | 110 | 3,12 | 0,32 | 0,36 | 0,37 | 0,38 | 0,40 | 0,43 | — | — |
| 13 | 120 | 1,25 | 0,36 | 0,44 | 0,50 | 0,53 | 0,54 | 0,58 | 0,78 | 2,40 |
| 14 | 120 | 1,87 | 0,31 | 0,37 | 0,40 | 0,43 | 0,44 | 0,44 | 0,64 | 2,36 |
| 15 | 120 | 2,50 | 0,30 | 0,33 | 0,36 | 0,37 | 0,38 | 0,40 | 0,56 | 2,20 |
| 16 | 120 | 3,12 | 0,30 | 0,32 | 0,33 | 0,34 | 0,35 | 0,38 | 0,53 | 2,00 |
| 17 | 130 | 1,25 | 0,32 | 0,38 | 0,42 | 0,44 | 0,46 | 0,52 | 0,62 | — |
| 18 | 130 | 1,87 | 0,29 | 0,31 | 0,36 | 0,39 | 0,40 | 0,42 | 0,54 | — |
| 19 | 130 | 2,50 | 0,28 | 0,28 | 0,32 | 0,34 | 0,35 | 0,36 | 0,48 | — |
| 20 | 130 | 3,12 | 0,28 | 0,27 | 0,30 | 0,32 | 0,33 | 0,34 | 0,46 | — |
| 21 | 140 | 1,25 | — | 0,32 | 0,36 | 0,40 | 0,43 | 0,46 | 0,56 | 1,41 |
| 22 | 140 | 1,87 | — | 0,27 | 0,32 | 0,35 | 0,36 | 0,37 | 0,48 | 1,20 |
| 23 | 140 | 2,50 | — | 0,25 | 0,30 | 0,32 | 0,33 | 0,34 | 0,44 | 1,00 |
| 24 | 140 | 3,12 | — | — | 0,30 | 0,29 | 0,30 | 0,31 | 0,42 | 0,95 |
| 25 | 150 | 1,25 | — | — | 0,34 | 0,34 | 0,34 | 0,40 | 0,52 | — |
| 26 | 150 | 1,87 | — | — | 0,31 | 0,30 | 0,29 | 0,33 | 0,44 | — |
| 27 | 150 | 2,50 | — | — | 0,30 | 0,30 | 0,27 | 0,30 | 0,40 | — |
| 28 | 150 | 3,12 | — | — | — | 0,27 | 0,27 | 0,28 | 0,38 | — |
| 29 | 160 | 1,25 | — | — | — | — | 0,30 | 0,32 | 0,48 | 0,72 |
| 30 | 160 | 1,87 | — | — | — | — | 0,25 | 0,30 | 0,42 | 0,66 |
| 31 | 160 | 2,50 | — | — | — | — | 0,24 | 0,26 | 0,36 | 0,66 |
| 32 | 160 | 3,12 | — | — | — | — | 0,24 | 0,25 | 0,35 | 0,64 |
| 33 | 170 | 1,25 | — | — | — | — | — | — | 0,44 | — |
| 34 | 170 | 1,87 | — | — | — | — | — | — | 0,37 | — |
| 35 | 170 | 2,50 | — | — | — | — | — | — | 0,33 | — |
| 36 | 170 | 3,12 | — | — | — | — | — | — | 0,31 | — |
| 37 | 180 | 1,25 | — | — | — | — | — | — | 0,40 | 0,48 |
| 38 | 180 | 1,87 | — | — | — | — | — | — | 0,33 | 0,47 |
Specific limiting resistance in ohm·cm² for carbon electrodes at a caustic-soda solution concentration (mass percent).
Continuation of Table 1
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
|---|---|---|---|---|---|---|---|---|---|---|
| 39 | 2.50 | — | — | — | — | — | — | 0.30 | 0.44 | |
| 40 | 3.12 | — | — | — | — | — | — | 0.29 | 0.43 | |
| 41 | 190 | 1.25 | — | — | — | — | — | — | 0.36 | — |
| 42 | 190 | 1.87 | — | — | — | — | — | — | 0.31 | — |
| 43 | 190 | 2.50 | — | — | — | — | — | — | 0.28 | — |
| 44 | 190 | 3.12 | — | — | — | — | — | — | 0.28 | — |
| 45 | 200 | 1.25 | — | — | — | — | — | — | — | 0.40 |
| 46 | 200 | 1.87 | — | — | — | — | — | — | — | 0.38 |
| 47 | 200 | 2.50 | — | — | — | — | — | — | — | 0.36 |
| 48 | 200 | 3.12 | — | — | — | — | — | — | — | 0.35 |
When the solution concentration is changed from 45 to 70% NaOH, the specific limiting resistance increases by a factor of 1.5–1.7; with a further increase in concentration from 70 to 80%, there is a sharp increase in the specific limiting resistance by a factor of 3–4, which is associated with the transition of the sodium hydroxide solution to a less dissociated state, corresponding to the composition NaOH · H₂O (69% NaOH).
An increase in current density leads to a decrease in the specific limiting resistance, which is explained by the greater mobility of the ions and the acceleration of electrode reactions with increasing alternating-current density; moreover, at higher temperatures the effect of the decrease is reduced. For a sodium hydroxide solution with a concentration of 60%, at a temperature of 100°C, the specific limiting resistance decreases by a factor of 1.64 as the current density increases from 1.25 to 3.12 A/cm²; at 130°C, by a factor of 1.38; and at 150°C, already only by a factor of 1.26.
As the temperature of the sodium hydroxide solution increases, there is a smooth decrease in the specific limiting resistance, associated with greater ion mobility as temperature rises. For lower current densities, the decrease in specific limiting resistance is more pronounced. Thus, for a sodium hydroxide solution with a concentration of 60%, at an electric-current density of 1.25 A/cm² the specific resistance decreased by a factor of 2.12 when the solution temperature was increased from 100 to 150°C, while at a current density of 3.12 A/cm² it decreased only by a factor of 1.63.
Conclusions
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Studies have been carried out of the specific limiting resistance of carbon electrodes in sodium hydroxide solutions under conditions of direct electric heating.
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It has been established that the specific limiting resistance depends on the electrode material and on the electrode connection scheme: for the “phase” electrode the specific limiting resistance is somewhat higher than for the “neutral” electrode; the specific limiting resistance increases with the concentration of the NaOH solution over the investigated interval, and decreases with increasing temperature and current density.
References
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N. I. Kozlova, V. P. Pishchulin. Application of direct electric heating for evaporating a NaOH solution. Proceedings of Young Scientists. Tomsk, 1971.
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V. P. Pishchulin, V. I. Kosintsev. Corrosion of electrode and construction materials during evaporation of sodium hydroxide solutions in apparatuses with direct electric heating. Communication 1 (present collection).
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V. P. Pishchulin, N. I. Kozlova. Investigation of the passage of alternating electric current through a sodium hydroxide solution. Proceedings of TPI, vol. 259. Tomsk, Tomsk University Press (in press).