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I n d . Eng. C h e m . Res. 1993, 32, 56-61
Relative Rates of Coke Formation from Hydrocarbons in Steam Cracking of Naphtha. 2. Paraffins, Naphthenes, Mono-, Di-, and Cycloolefins, and Acetylenes Frank-Dieter Kopinke*?+ Section of Remediation Research, Centre of Environmental Research Leipzig-Halle GmbH, Permoserstrasse 15, D 7050 ( 0 )Leipzig, Germany
Gerhard Zimmermannt WIP-Research Group "Organic High Temperature Chemistry", Permoserstrasse 15, D 7050 (0)Leipzig, Germany
Geerd C. Reyniers and Gilbert F. Froment Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Belgium
Relative rate constants of coke formation from saturated hydrocarbons, olefins, and acetylenes during steam cracking of naphtha at 810 "C were determined by application of 14C-labeledcompounds. The range of hydrocarbons investigated comprises 20 representatives of paraffins, from methane to hexadecane, and of naphthenes, from cyclopentane to hydroanthracene, and 20 olefins from ethene to styrene, including cyclo- and diolefins, and five acetylenes. Carbonaceous deposits in pyrolysis reactors from steel and quartz as well as in the transfer line exchanger (TLE) section were determined separately. The formation of carbonaceous deposits in olefins production by steam cracking of hydrocarbons is a very undesirable phenomenon with considerable effects on the operation of the plant. Therefore, it would be of great help to have a model for the prediction of the extent of coke formation. The development of accurate fundamental models has been hampered by the lack of reliable information on coking rates from individual hydrocarbons. An experimental method for measuring the contribution of certain carbon atoms of individual hydrocarbons in the coke formation by application of the radio tracer technique was described in part 1of the current series (Kopinke et al., 1988a). Beyond the literature survey given in part 1some recent papers on coke formation should be mentioned here: Froment (1990) mainly dealt with the kinetics of coke formation and the influence of anticoking additives, while Albright and Marek (1988a) reviewed the characterization of coke deposited in industrial reactor tubes (Albright, 1988) and in laboratory experiments (Albright and Marek, 1988b). Kunzru and co-workers published a series of experimental studies directed to the kinetics of coke formation (Kumar and Kunzru, 1985; Pramanik and Kunzru, 1985; Kumar and Kunzru, 1987; Sahu and Kunzru, 19881, its inhibition (Gosh and Kunzru, 1988; Vaish and Kunzru, 1989), and the gasification of coke deposits (Mandal and Kunzru, 1986). They concluded that ethene and aromatic hydrocarbons are the main coke precursors in the pyrolysis of n-hexane and naphtha, respectively. The rate of coke formation was found to be proportional to the square of the concentration of aromatics in the cracked product. All aromatic hydrocarbons were treated as a lump. Benzene and toluene were found to affect the coking rate to the same degree. Zou et al. (1987) found ethene to be the dominant coke precursor in propane pyrolysis. According to their model the relative rates of coke formation from ethene and propene are 264:l at 850 "C, which is an unlikely high ratio. The investigations have been performed partially in the former Department for Basic Organic Materials, Central Institute for Organic Chemistry, in Leipzig. 0888-5885/93/2632-0056$04.00/0
Chaverot et al. (1986) assign an important role to acenaphthylene as a coke precursor. They used 20 wt % mixtures of the investigated compounds in the n-decane feed. This procedure is comparable to that applied in the present paper, but it does not avoid a significant effect of the additive on the pyrolysis. In his review on the catalytic mechanism for the growth of carbon filaments, Baker (1989) came to the conclusion that the available theories on this phenomenon fail to account for all aspects of the experimental results. Carbon filament formation is very helpful in understanding the influence of reactor materials on the rate of carbon formation and the morphology of the deposits, but the actual contribution of this mechanism to coke formation in industrial steam crackers is still a matter of debate. Starting from kinetic equations for the pyrolysis and the coke formation, Plehiers et al. (1990) successfully predicted the run length of an industrial ethane cracker. The fouling of the transfer line exchanger (TLE),which is important in industrial steam crackers, is not represented adequately in the recent scientific literature. Relevant papers were published by Kaiser et al. (1984), Horak and Baranek (19861, Zander (1989),Hundrots et al. (1989),and Fernandez-Baujin et al. (1990). Only very few data are available because of the difficulties involved in performing laboratory scale experiments which are representative of industrial TLE conditions. For the purpose of a better understanding of the data discussed in this paper, a brief presentation of our views on the mechanism of coke formation, explained in more detail in part 1 already, is given here: (1)Coke deposits in the cracking coil are mainly formed by reactions of gas-phase species (radicals and unsaturated molecules) with the growing coke, which is in reality a macroradical. (2) Carbonaceous deposits in the quench zone result from the physical condensation of high boiling components of the cracked gas on the cooled walls. These deposits are subject to further transformations like dehydrogenation and chemical condensation. The objective of this study is to come to a comprehensive insight into structure-reactivity relations for the coking tendency of hydrocarbons. For this purpose a wide range 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 57 Temperature I ° C I Naphtha, Steam LOO 600 800 \/,Quartz
200
Table I. Relative Rate Constants of Coke Formation from Paraffins and Naphthenes in the Cracking Tube and the TLE tracer hydrocarbon Dosition of "C MCC) MTLE-C) methane u 0.085 0.05 0.21 u ethane 0.14 n-hexane 0.64 1 0.58 0.61 0.43 1 n-octane 0.61 1 0.56 n-hexadecane 0.67 0.50 3-methylpentane 1 0.72 2,2,4-trimethylpentane 5 0.64
Reactor
Coil-Coke Tube from Cr-N i -Steel
Temperature Profile
;/i
o f Deposits
mgth
- TLE -Coke 11'
Cracked Gas
Figure 1. Deposition of coke in the laboratory pyrolysis reactor.
of positionally 14C-labeledhydrocarbons from methane to chrysene were tested. These are typical feedstock constituents and pyrolysis intermediates and final products. The results obtained with aromatic hydrocarbons will be presented in a further paper of this series.
Experimental Equipment and Procedure A detailed description of the experimental equipment and procedure was given in part 1 of this series and by Kopinke (1985). The pyrolysis experiments were conducted in tubular reactors made from quartz (Figure 1) or Cr-Ni-ateel. The feedstock was a straight-run naphtha with a boiling range of 50-180 "C. The standard reaction conditions (2' = 810 OC; residence time, 0.4 s; total pressure, 0.1 MPa; dilution ratio HzO/naphtha, 0.7 g/g; C,H,/C& weight ratio in the cracked gas, 2.5) are similar to those in industrial pyrolysis reactors. A small amount (
