Ind. Eng. Chem. Res. 1993,32, 2620-2625
2620
Relative Rates of Coke Formation from Hydrocarbons in Steam Cracking of Naphtha. 3. Aromatic Hydrocarbons ' Frank-Dieter
Kopinke'J
Centre for Environmental Research Leipzig-Halle GmbH, Section of Remediation Research, Permoserstrasse 15, D 04318 Leipzig, Germany
Gerhard Zimmermannt WIP-Research Group "Organic High Temperature Chemistry", Permoserstrasse 15, D 04303 Leipzig, Germany
Geerd C . Reyniers and Gilbert F. Froment Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Belgium
Relative rate constants of coke formation (12) from 18 aromatic hydrocarbons during steam cracking of naphtha a t 810 "C were determined by application of 14C-labeledcompounds. Benzene is a poor coke precursor (k= 0.3), whereas polycyclic structures like acenaphthylene,anthracene, and chrysene have a high coking potential in the pyrolysis reactor (k = 4.5-6) as well as in the TLE section (k = 12-30). The relation between structure and coke formation rate of aromatic hydrocarbons can be interpreted on the basis of their reactivity in radical reactions. Constituents of the fuel fraction (LC9) derived from nonaromatic feed components are more efficient in the TLE fouling than those stemming from benzene derivatives. The deposition of carbon-rich solid products on the reactor walls, heat exchangers, and transfer lines, which are in contact with hot hydrocarbon streams, is a widely encountered, but undesired phenomenon in petrochemical processes. Although, such coke-like deposits are minor side products in steam cracking of hydrocarbons (yield < 0.1 wt %),theiraccumulation in the pyrolysis reactor and in the transfer line exchanger (TLE) strongly affects the optimum operation conditions. Besidesthe literature cited in parta 1and 2 of the present series (Kopinke et al., 1988 and 19931, recently a review was presented by Dmitriev (1991). From the scientific point of view an understanding of the relationship between structure of hydrocarbons and their tendencies to form carbonaceous deposits can be of great help in developing accurate fundamental models for the prediction of the extent of coke formation. An experimental method to obtain reliable information on the contribution to coke formation of individual hydrocarbons as constituents of complex steam cracker feedstocks by application of the radiotracer technique on the basis of 14C-labeledcompounds was described in part 1of the current series (Kopinke et al., 1988). The results of its application to nonaromatic hydrocarbons-paraffins, naphthenes, olefins, and acetylenes-were described in part 2 (Kopinke et al., 1993). Aromatic hydrocarbons, from benzene to chrysene, form the subject of this paper. They are representatives of both feedstock components and of cracked products. Some partially hydrogenated aromatics were involved as possible constituents of hydrotreated feedstocks. All of the 1%-hydrocarbons are labeled positionally, with the exception of benzene which is labeled uniformly.
Experimental Section A detailed description of the experimental procedure and of the data treatment was given elsewhere (Kopinke t The investigations have been performed partially in the former Central Institute for Organic Chemistry, Department for Basic Organic Materials in Leipzig, Germany.
0SSS-5SS5/93/2632-2620$04.00/0
et al., 1988 and 1993). Therefore, only a brief description shall be given in this paper: a straight-run naphtha, to which a small amount (
