CONCENTRATION OF SULFURIC ACID IN A DEPHLEGMATION-TYPE APPARATUS WITH DIRECT ELECTRIC HEATING

V. P. Pishchulin, V. I. Kosintsev, B. B. Evteev

(Presented by the scientific-methodological seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)

In the laboratory of the Department of Processes and Apparatuses of Chemical Technology of the Chemical-Technological Faculty of TPI, an effective method has been proposed for concentrating sulfuric acid using direct electric heating [1]. The theoretical studies necessary for the calculation and design of industrial apparatuses with direct electric heating for concentrating sulfuric acid have been carried out [1, 2].

In the present communication a description is given of one design of an electrode apparatus of the dephlegmation type for concentrating sulfuric acid, as well as the results of tests on a laboratory model.

Figure 1 shows the general view of the apparatus. The dephlegmation-type electrode apparatus consists of a closed steel still (retort) 1 and a dephlegmation packed column 2. The column was equipped with nozzles 3, 4 for removing water vapor and feeding diluted sulfuric acid; the still of the apparatus was equipped with a nozzle 5 for removing concentrated sulfuric acid and with two plane-parallel electrodes 6. In the first variant, the electrodes were attached to the still cover 7 by means of holders 8. Current was supplied to the electrodes by means of current-conducting bolts 9 and electrode holders electrically insulated from the body by bushings 10 made of an electrical-insulating material—fluoroplastic 4; the still of the apparatus was lined on the inside with diabase tile 11.

Fig. 1. Electrode apparatus of the dephlegmation type for concentrating sulfuric acid solutions

The concentration process was carried out as follows. The initial dilute solution of sulfuric acid entered the column through nozzle 4 and a distributing device and gradually flowed down over the packing into the column still. In the still, intensive boiling of the concentrated sulfuric acid occurred; acid vapors entered the lower part of column 2. On contact with the dilute acid, the vapors in the lower part of the packing partially condensed with the release of heat, necessary—

suitable for the concentration process; that is, heat and mass exchange occurred between the liquid and vapor phases. The concentrated acid flowed into the still and was then continuously withdrawn from boiler 1 through fitting 5. Only water vapor left the upper part of the packing. The amount of heat required for the sulfuric-acid concentration process was released as an alternating electric current passed through the sulfuric-acid solution between the electrodes. The sulfuric-acid solution circulated intensively in the still of the column through the interelectrode space, partially equalizing the temperature field and the sulfuric-acid concentration in the still.

The first tests of an electrode apparatus of the dephlegmation type showed that the process of concentrating sulfuric acid proceeded very intensively; however, the design of the electrode holders did not satisfy the requirements of the process: as a result of severe corrosion and the formation of a nonconductive film of nickel and iron sulfates on the surface of the holders, the electrical contact between holder and electrode was disrupted, causing local overheating and electrochemical and electrical corrosion. Subsequently the current-lead design was changed. The upper parts of the electrodes were brought directly out through the cover of the still, and current-carrying busbars were attached to them. The tightness of the still at the points where the electrodes emerged was ensured by means of stuffing boxes. This current-lead design proved workable and made it possible to study the process of concentrating sulfuric acid from an initial concentration of 65–72% to 96–98%.

As a result of tests of the laboratory model of the electrode apparatus of the dephlegmation type, the following was established:

1. The sulfuric-acid concentration process proceeded satisfactorily at an electric-current density of \(3\ \mathrm{A/cm^2}\) and intensified appreciably with increasing current density.

2. The coefficient of useful utilization of electrical energy was 95–98%.

3. The productivity per unit volume of the apparatus increased by more than a factor of 25 compared with a dephlegmation-type apparatus with still heating by furnace gases.

4. The circulation rate of the sulfuric-acid solution at current densities up to \(3\ \mathrm{A/cm^2}\) did not ensure equalization of the temperature field in the column still. The circulation rate depended to a considerable extent on the current density and reached the required value at current densities on the order of \(5\text{–}6\ \mathrm{A/cm^2}\) for this design of the apparatus still.

5. Corrosion studies of electrode materials in sulfuric acid of concentrations 66.3; 75; 93; 96; 97.5; and 98.5% at temperatures of 100, 150°C and at the boiling temperature showed that metal electrodes made of steels Kh18N10T and EI943 are completely unsuitable for conducting the process at any electric-current density. Electrode carbon proved to be the best material for the electrodes. Carbon electrodes retained their mechanical strength and underwent practically no destruction in the investigated range of temperatures and concentrations at electric-current densities of 3, 5, 7, and \(10\ \mathrm{A/cm^2}\). Short-term operation at current densities up to \(30\ \mathrm{A/cm^2}\) also caused no changes in the mechanical strength of the electrodes.

As packing, crushed quartz and shavings of fluoroplastic-4 showed good corrosion resistance and satisfactory performance. On the basis of the data obtained, a technological scheme for concentrating sulfuric-acid solutions was proposed, and a calculation was carried out for a dephlegmation-type apparatus with three pairs of plane-parallel electrodes installed in the still. Current supply through three pairs of plane-parallel electrodes creates uniform loading of all three

of the phases of the electric current, which is especially necessary in large-scale production, ensures uniformity of the electric field between the electrodes and a high rate of circulation of the solution in the still of the apparatus.

Conclusions

1. A design has been developed for an electrode apparatus of the dephlegmation type for concentrating sulfuric acid solutions.

2. Tests have been carried out on a laboratory model of the electrode apparatus.

3. The optimal conditions for carrying out the process of concentrating sulfuric acid solutions have been found.

4. The best corrosion-resistant materials for manufacturing the electrodes and the column packing have been established.

Literature

1. V. P. Pishchulin. Application of direct electric heating for concentrating sulfuric acid. Communication I. Izvestiya TPI, vol. 259, Tomsk, Tomsk University Press (in press).

2. V. P. Pishchulin, B. B. Evteev, A. P. Pishchulin, L. I. Kupriyanova. Concentration of sulfuric acid. Report at the jubilee conference of the D. I. Mendeleev All-Union Chemical Society, Tomsk, 1971.