REGENERATION OF ACIDIC EFFLUENTS

V. P. PISHCHULIN, S. A. BABENKO

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

In the production of aromatic sulfochlorides, obtained as a result of the interaction of aromatic compounds with chlorosulfonic acid, the technology requires a fourfold excess of chlorosulfonic acid; the latter decomposes during the reaction process, forming acidic aqueous solutions containing about 8% \(H_2SO_4\), 6% \(HCl\), and a certain amount of impurities of aromatic compounds. These solutions constitute the main quantity of wastewater discharged into the sewer system. Thus, only 20% of the valuable chemical raw material is used directly for the reaction, while the remaining 80% is lost uselessly, polluting natural bodies of water.

Analysis of research and practice in the processing of acidic effluents indicates that the neutralization of acidic effluents can be carried out with lime, and their utilization by separating sulfuric and hydrochloric acids and water—by rectification.

Neutralization of acidic effluents with lime leads to the formation of calcium sulfate, whose solubility in water is low and amounts to approximately 0.2%, and of a calcium chloride solution. Calcium sulfate can subsequently be used as a binding construction material, while the calcium chloride solution must be subjected to evaporation. Thus, for the detoxification and utilization of sulfuric and hydrochloric acids from the acidic effluents of aromatic sulfochloride production with lime, significant capital expenditures are required, associated with the construction of a lime-preparation section, a neutralization section, a filtration section, an evaporation section, as well as high energy consumption for evaporating the calcium chloride solution.

It is most expedient to utilize acidic effluents by separating sulfuric and hydrochloric acids and water—by rectification. In this case, to organize the process it is possible to limit the installation to two rectification columns. At the same time, capital expenditures are reduced, operation is simplified, and much more valuable products are obtained—sulfuric and hydrochloric acids—at lower energy consumption.

The essential problems in the utilization of acidic effluents by rectification are the choice of construction materials, owing to the high aggressiveness of solutions of sulfuric and hydrochloric acids, and also the choice of heat-transfer medium.

For carrying out the process of rectification of acidic effluents, we have adopted a highly effective method of concentrating sulfuric solutions.

acid by means of direct electric heating in electrode-type apparatuses [1].

Under laboratory conditions, an apparatus was built analogous to that described in communication [2], with carbon electrodes and a packing made of fluoroplastic-4 shavings. As a result of studies carried out on a rectification unit with direct electric heating, the following were obtained: the bottom residue of the first column, consisting of an aqueous solution of sulfuric acid with a concentration from 80 to 95% and containing up to 5% of oxidized organic compounds as an impurity; the bottom residue of the second column, consisting of a hydrochloric acid solution with a concentration of 15–19% and a boiling temperature of about 108°C; and the distillate of the second column, representing pure distilled water. All the data necessary for calculating the industrial process of rectification of acidic effluents were determined. Analysis of the resulting sulfuric acid solution from the first column indicated complete stripping of hydrochloric acid into the vapor phase, which is consistent with data on equilibrium in the system HCl—H₂SO₄—H₂O [3].

On the basis of the data obtained, a technological scheme was proposed for carrying out the process of rectification of acidic effluents, consisting of two rectification columns. According to this scheme, the initial mixture, containing 8% H₂SO₄ and 6% HCl, preheated by the heat of condensation of the water vapors from the second column, is fed as reflux to the upper part of the first column, flows downward over the packing through the column, and is gradually enriched in sulfuric acid. From the bottom of the column there flows a technical solution of sulfuric acid with a concentration of up to 97%, containing only impurities of oxidized organic compounds, which can be purified if necessary. As a result of mass transfer in the column, hydrogen chloride and water pass completely into the vapor phase and enter the middle part of the second column for separation. In the second column, a hydrochloric acid solution with a concentration of 15–19% HCl is obtained as the bottom residue, while water leaves as vapor from the upper part of the second column. Part of the water is returned as reflux; the remaining part can be used for the technical needs of the enterprise.

An enlarged economic calculation showed the advisability of using the rectification process with direct electric heating for the regeneration of sulfuric and hydrochloric acids from acidic effluents that are currently discharged into the sewer system.

REFERENCES

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

2. V. P. Pishchulin, V. I. Kosintsev, B. B. Evteev. Concentration of sulfuric acid in a dephlegmation-type apparatus with direct electric heating (present collection).

3. A. B. Zdanovskii, E. I. Lyakhovskii, R. E. Shleimovich. Handbook on the Solubility of Salt Systems. Vol. 1, Moscow, Goskhimizdat, 1953.