Application of Direct Electric Heating for Concentrating Sulfuric Acid
Abstract Generated abstract
This paper examines direct electric heating as an alternative to furnace gas heating for concentrating sulfuric acid, with attention to reducing decomposition, entrainment, corrosion, and equipment bulk. It reports experimental measurements of specific boundary resistance in sulfuric acid solutions using plane parallel electrodes made of carbon, graphite, and two steels, varying acid concentration, temperature, and current density. The results indicate that carbon and graphite electrodes have much lower boundary resistance than steel electrodes, with values generally not exceeding 1.1 ohm cm² under the studied conditions, and that increasing current density initially reduces this resistance. Temperature and concentration effects are also characterized, supporting the suitability of carbon and graphite electrodes for direct electric heating in sulfuric acid concentration equipment.
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APPLICATION OF DIRECT ELECTRIC HEATING FOR CONCENTRATING SULFURIC ACID. COMMUNICATION 1
V. P. PISHCHULIN
(Presented by the scientific-methodological seminar of the Department of PMAChP KhTF)
The concentration of sulfuric acid in industry is currently carried out mainly in bubbling concentrators [1]. The required amount of heat is supplied by furnace gases. The enormous overconsumption of furnace gases, the large temperature difference between the furnace gases and the solution being concentrated, which leads to decomposition of the sulfuric acid, the impossibility of obtaining sulfuric acid with concentrations above 94–95%, the considerable entrainment of sulfuric acid, the significant pressure of the furnace gases, and the bulkiness of low-productivity equipment subject to severe corrosion have served as the reason for many studies in this field. To eliminate these shortcomings, we have proposed direct electric heating in an apparatus of the dephlegmation type.
The use of direct electric heating makes it possible rapidly and with a high coefficient of useful energy utilization (96–98%) to supply the required amount of heat by passing an alternating electric current of industrial frequency (50 hertz) through the sulfuric acid solution by means of plane-parallel electrodes made of carbon or graphite; to practically eliminate corrosion of the equipment through the use of nonmetallic corrosion-resistant materials; to sharply increase the volumetric productivity of the concentrator; and to eliminate decomposition and entrainment of sulfuric acid.
When an electric current passes, heat is released directly in proportion to its resistance according to the Joule–Lenz law. In our case, the electrical resistance of the system consists of the resistance of the electrodes, which possess electronic conductivity, the resistance of the solution with ionic conductivity, and the resistance at the electrode–solution boundary, due to the resistance of the electrical double layer and chemical reactions on the surface of the electrodes [2]. The resistance of the electrode is a relatively small quantity; therefore the heat released will be determined by the resistance of the solution and the boundary resistance, with the resistance of the solution depending on the specific electrical conductivity of the solution, the shape, surface, and arrangement of the electrodes; the magnitude of the boundary resistance is determined by the specific boundary resistance and the working surface area of the electrodes. The specific boundary resistance depends on the electrode material, the composition and concentration of the solution, temperature, and current density, and is determined experimentally.
This communication presents experimental data from a study of the specific boundary resistance as a function of the concentration of the sulfuric acid solution, temperature, and current density for
electrodes made of electrode carbon, electrode graphite, and steels Kh18N10T and EI943.
A certain volume of the solution under study was poured into a measuring cell with plane-parallel electrodes. Alternating electric current was supplied to the solution by means of the electrodes through an ammeter, voltmeter, and autotransformer. After a specified current density and solution temperature had been established, the total voltage drop across the cell was measured; in addition, with the aid of a probe and a high-resistance voltage meter, the voltage drop of the electric current in the solution between the electrodes was determined. The difference between the values of the total voltage drop across the cell and the voltage drop in the solution, divided in half, represents the boundary voltage drop at one electrode. The specific boundary resistance was calculated as the quotient obtained by dividing the boundary voltage drop by the current density.
The results of the investigation are given in Table 1.
As the investigations showed, the specific boundary resistance depends to a considerable extent on the electrode material. For carbon and graphite it is the lowest, and its value does not exceed 1.1 ohm·cm² over the investigated range of temperatures, concentrations, and current densities; moreover, the values
Table 1
Effect of current density on the specific boundary resistance for graphite and EI943 steel electrodes at a temperature of 100°C in sulfuric acid of 94% concentration
| No. | Current density, A/cm² | Graphite electrodes: voltage drop across the cell, V | Graphite electrodes: boundary voltage drop, V | Graphite electrodes: specific boundary resistance, ohm·cm² | EI943 steel electrodes: voltage drop across the cell, V | EI943 steel electrodes: boundary voltage drop, V | EI943 steel electrodes: specific boundary resistance, ohm·cm² |
|---|---|---|---|---|---|---|---|
| 1 | 0.50 | 6.75 | 0.55 | 1.10 | 28.6 | 13.0 | 26 |
| 2 | 0.75 | 10.20 | 0.75 | 1.00 | 32.8 | 14.0 | 18.7 |
| 3 | 1.00 | 13.10 | 0.96 | 0.96 | 36.00 | 14.6 | 14.6 |
| 4 | 1.50 | 18.00 | 1.30 | 0.87 | 41.2 | 16.0 | 10.7 |
| 5 | 2.00 | 23.40 | 1.50 | 0.75 | 48.4 | 16.4 | 8.2 |
| 6 | 2.50 | 27.40 | 1.65 | 0.60 | 56.0 | 20.4 | 8.16 |
of the specific boundary resistance for carbon and graphite are identical under equal conditions. For electrodes made of steels Kh18N10T and EI943, the value of the specific boundary resistance is approximately an order of magnitude greater.
An increase in current density up to 3–5 A/cm² with all electrode materials leads to a significant decrease in the specific boundary resistance; with a further increase in current density, the values of the specific boundary resistance practically do not change.
With an increase in the concentration of sulfuric acid to approximately 83%, for electrodes made of electrode graphite there is at first an increase in the specific boundary resistance, and then, with a further increase in concentration to ~93%, the values of the specific boundary resistance decrease to a minimum value, followed by subsequent growth. The curve of the change in specific boundary resistance as a function of sulfuric acid concentration is analogous to that for the reciprocal of the specific electrical conductivity, i.e., the specific electrical resistance.
With an increase in the temperature of the sulfuric acid solution, carbon and graphite are characterized by a smooth decrease in the specific boundary resistance, which is explained by the greater mobility of ions as the temperature rises. For steel Kh18N10T, as the temperature increases to 150°C
Table 2
Dependence of the specific limiting resistance on the temperature of sulfuric acid of 94% concentration at a current density of 1.5 A/cm² for electrodes made of graphite, carbon, steel Kh18N10T, and EI943
| No. | Temperature, °C | Specific limiting resistance, Ω·cm², for electrodes made of graphite | Specific limiting resistance, Ω·cm², for electrodes made of carbon | Specific limiting resistance, Ω·cm², for electrodes made of steel Kh18N10T | Specific limiting resistance, Ω·cm², for electrodes made of steel EI943 |
|---|---|---|---|---|---|
| 1 | 75 | 0.93 | 0.93 | 12.90 | 11.94 |
| 2 | 100 | 0.87 | 0.75 | 14.35 | 10.70 |
| 3 | 150 | 0.56 | 0.55 | 19.50 | 10.70 |
| 4 | 200 | 0.50 | 0.51 | 6.27 | 8.34 |
| 5 | 298 | 0.47 | 0.49 | 3.40 | 2.54 |
Table 3
Effect of sulfuric acid concentration on the specific limiting resistance for graphite electrodes at a temperature of 100°C and a current density of 1.5 A/cm²
| 65 | 73.2 | 78 | 83.3 | 93 | 97.5 | |
|---|---|---|---|---|---|---|
| Sulfuric acid concentration, % | 65 | 73.2 | 78 | 83.3 | 93 | 97.5 |
| Specific limiting resistance, Ω·cm² | 0.72 | 0.83 | 0.87 | 1.0 | 0.87 | 1.2 |
an increase in the specific limiting resistance is observed up to a value of 19.5 Ω·cm² at a current density of 1.5 A/cm². With a further rise in temperature, a sharp decrease in the specific limiting resistance occurs, down to 6.27 Ω·cm² at 200°C. This dependence of the specific limiting resistance on temperature is explained by the formation of a sulfate film on the electrode at temperatures below 150°C; this film has high electrical resistance and, with a further increase in temperature, passes into a soluble state, as a result of which the specific limiting resistance decreases.
A similar, but more smoothed-out, dependence of the specific limiting resistance on temperature is found for steel EI943, whose solubility as electrodes in sulfuric acid is considerably greater than that of steel Kh18N10T.
REFERENCES
- A. G. Amelin. Production of Sulfuric Acid. “Khimiya,” 1967.
- Studies in the Field of Industrial Electroheating. Proceedings of VNIIETO. Issue 2. Edited by A. P. Altgauzen and A. E. Nikolsky. “Energiya,” 1967.