INVESTIGATION OF THE DECOMPOSITION PRODUCTS OF HYDROCARBONS IN A HIGH-VOLTAGE PULSED DISCHARGE

I. I. Vishnevetskii, L. A. Bykova, L. F. Kotlova, B. V. Semkin,
S. I. Smolyaninov

(Submitted by the scientific-methodological seminar of the organic departments
of the Chemical-Technological Faculty)

At present, searches are being conducted for new progressive routes for obtaining acetylene and low-molecular olefins, with thermo-oxidative and plasma pyrolysis of hydrocarbon feedstocks being the most promising [1, 2].

One of the most promising methods is the process of decomposition of liquid hydrocarbons in pulsed discharges, which make it possible to vary over wide limits the ratios of the products obtained under very favorable quenching conditions [3].

As a source of high-voltage pulsed discharges ensuring breakdown of the liquid, we used a pulse-voltage generator. Paraffinic hydrocarbons of normal and iso structure, cycloparaffinic hydrocarbons, and aromatic hydrocarbons were subjected to decomposition. Gas analysis was carried out by gas-adsorption chromatography using molecular sieves and alumina as adsorbents [4].

The mixture of liquid and soot after the experiment was weighed; the soot was filtered off from the liquid residue, and both fractions were analyzed.

The gaseous products are represented by hydrogen and lower saturated and unsaturated hydrocarbons. The principal product formed from any hydrocarbon is hydrogen. Its concentrations range from 50% by volume (from paraffinic hydrocarbons) to 80% by volume (from aromatic hydrocarbons) (table).

The amount of acetylene in the gas ranges within 30–57% by weight. Ethylene is formed in sufficient quantities from paraffinic hydrocarbons of normal structure and from cycloparaffins (30% by weight). The methane content in the gas is determined by the number of methyl groups in the molecule of the initial hydrocarbon. In addition to the compounds listed above, the gaseous products contain ethane (up to 1.5% by volume), propane (up to 0.3% by volume), propylene (up to 2.5% by volume), methylacetylene (up to 1% by volume), and other homologues of acetylene.

The liquid remaining after decomposition of the initial feedstock has a reddish-brown color and a sharp unpleasant odor; on prolonged standing a precipitate separates from it. Its density, viscosity, and molecular weight are slightly higher in comparison with the initial feedstock. Analysis showed that the newly formed products repre-

are unsaturated compounds—dienes, polyenes, or acetylene compounds.

Table

Composition of the decomposition gas of individual hydrocarbons in a pulsed discharge

Spectral analysis (determination of IR spectra) showed that all the residues obtained after distillation of any initial feedstock are identical. Absorption bands in the regions \(620\text{–}800\ \mathrm{cm}^{-1}\), \(920\ \mathrm{cm}^{-1}\), and \(330\ \mathrm{cm}^{-1}\), corresponding to the valence vibrations of groups with double and triple bonds, indicate the presence of unsaturated compounds. Hydrogenation of the liquid products made it possible to establish that these are acetylene derivatives of hydrocarbons \(\mathrm{C_5}\), \(\mathrm{C_6}\). In light, they polymerize to compounds with molecular mass \(200\text{–}250\ \mathrm{g/mol}\).

The soot obtained as a result of electrocracking of hydrocarbons has the following characteristics:

specific geometric surface area — \(132.7\ \mathrm{m^2/g}\);
specific adsorption surface area — \(133.5\ \mathrm{m^2/g}\);
oil number — \(60\ \mathrm{ml}/100\ \mathrm{g}\);
optical density of the benzene extract — 1.15;
elemental composition — 98.8% C; 0.8% H.

Conclusions

1. The decomposition products of hydrocarbons in a high-voltage pulsed discharge have been investigated. As a result of decomposition, gas and soot are formed. The initial liquid changes in its physicochemical properties.

2. The cracking gas contains 50–80 vol. % \(\mathrm{H_2}\), 30–57 wt. % \(\mathrm{C_2H_2}\), 30 vol. % \(\mathrm{C_2H_4}\).

3. The soot formed as a result of decomposition has a large adsorption surface area — \(133\ \mathrm{m^2/g}\).

References

1. D. N. Andreev. Organic Synthesis in Electric Discharges. Moscow–Leningrad, Publishing House of the USSR Academy of Sciences, 1953.

2. Chemical Reactions of Organic Products in Electric Discharges. Moscow, “Nauka,” 1966.

3. S. Z. Roginskii, A. B. Shekhter. ZhPZh, No. 3, 1937, pp. 473–485.

4. S. I. Smol’yaninov, A. V. Kravtsov, D. F. Kotlova, I. I. Vishnevetskii, L. F. Danilova. Izvestiya TPI, vol. 268, Tomsk, TSU Publishing House, 1976.