Ind. Eng. Chem. Res. 2002, 41, 6213-6214
6213
Reply to Comments on “Kinetic Modeling of Coke Formation during Steam Cracking” Marie-Franc¸ oise S. G. Reyniers, Sandra Wauters, and Guy B. Marin* Laboratorium voor Petrochemische Techniek, Universiteit Ghent, Krijgslaan 281 S5, B-9000 Ghent, Belgium
Sir: We appreciate the interesting comments of Dr. Albright. His comments are mainly related to the claim that the presented model is incomplete and fails to include not only some important aspects of the coke formation process but also some important characteristics of the coke formed during thermal cracking of hydrocarbons. As clearly pointed out by Dr. Albright, coke formation in thermal cracking of hydrocarbons is a complex phenomenon and it is generally accepted that three mechanisms, as described in his comments, contribute to coke formation. As mentioned in the Introduction, the paper commented by Dr. Albright is aimed at modeling coke formation by one of these mechanisms, namely, mechanism 3. The model aims to describe further coke growth on an already existing coke layer. Perhaps it was not sufficiently clearly expressed that this already existing coke layer is formed through mechanism 1. As Dr. Albright quite rightly points out, the properties of the metal surface of the reactor wall play an important role in mechanism 1 and coke formation by this mechanism can indeed be strongly influenced by coating of the reactor surface1 or by using additives.2 Kinetic data3,4 indicate that initially the coking rate is high but that it rapidly declines to reach a constant value. The high initial coking rate is mainly associated with mechanism 1. Data concerning morphology and physical and chemical analysis5-7 of various coke samples obtained from industrial units indicate that the coke formed by this catalytic mechanism consists of whiskers with metal particles at the top and is concentrated near the reactor wall. Once the metal particles are covered with coke, the importance of mechanism 1 decreases and the contribution of mechanisms 2 and 3 gains in importance. However, the contribution of mechanism 2 is less important for light feedstocks, in particular for ethane cracking, and at temperatures lower than 900 °C.8 In our model, the active centers for further growth of coke by mechanism 3 are situated on the coke surface. This model hypothesis is corroborated by experimental data reported by Kopinke et al.,9 who observed no differences in the relative rate constants of coke formation from a particular hydrocarbon on different wall materials. The coke layer can be represented by an aromatic structure. This choice is, among other things, based on the observation that X-ray diffraction5 spectra of coke formed on the reactor wall and of coke formed at the coke-gas interface in an industrial unit show a single broad peak at 12.9°, characteristic of the (002) plane of graphite. Of course, the intensity of this peak, and thus the degree of graphitization, is far higher for the coke formed on the reactor wall. These observations support the idea that the coke is aromatic in nature and has, at * To whom correspondence should be sent. E-mail: Guy.
[email protected]. Fax: 0032/9/2644999. Tel: 0032/9/2644516.
least on a microscale, graphitic character. Mechanism 3 further implies that the number of radical sites on the coke surface not only depends on the gas-phase composition but also on the characteristics, e.g., surface area, porosity, C/H ratio, etc., of the coke layer formed during mechanism 1. For convenience, experimentally determined rates of coke formation and rate equations for coke formation are usually based on the geometric surface area of the reactor wall. However, the surface area of the coke formed during mechanism 1, i.e., the surface area on which coke formation by mechanism 3 occurs, is certainly largely different from the geometric surface area of the reactor. Therefore, a factor accounting for the coke surface area was introduced in order to be able to compare our simulated results with experimental data obtained by others.10 As reported, coke formation data during ethane cracking could be described over a broad range of conditions by using a unique value for this factor. In our view, this indicates that the characteristics of the coke layer, in particular its surface area, formed by mechanism 1 are largely determined by the catalytic properties of the metal surface and are relatively independent of the cracking conditions. That “coke formation is affected by the axial position in the coil and the average residence time of the gas in the coil” is an obvious consequence of the dependence of the coking rate on the gas-phase composition and the temperature and is accounted for in the reported modeling effort. In our opinion, an adjustment of the applied model parameters, as proposed by Dr. Albright, would not provide an accurate description of coking in the transfer line exchanger (TLE). At the low temperatures prevailing in the TLE, the coke growth by mechanism 3 will probably not contribute substantially to TLE coking. Although the mechanisms that contribute to coke formation in the TLE are still under debate, we believe that TLE coking cannot be regarded as a simple extrapolation of coil coking at lower temperatures.2 Physical condensation of the high boiling cracked products on the colder metal wall has often been claimed to explain coke formation in the TLE. Ranzi et al.,7 however, reported that, in a properly designed TLE, the wall temperature remained higher than the dew point. Experimental data reported by Kopinke et al.11,12 provide evidence for a catalytic route to TLE coking. In summary, the presented model does not claim to include coke formation by mechanisms 1 and 2. Models to describe coke formation by those mechanisms would indeed be very welcome. In fact, the presented model combined with one that describes coke formation by mechanism 1 would indeed allow a more accurate description of the coke formation process in the reactor coil during thermal cracking of light hydrocarbon feedstocks by providing, among others, information on the
10.1021/ie020778o CCC: $22.00 © 2002 American Chemical Society Published on Web 11/05/2002
6214
Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002
surface area of the coke layer on which further coke growth by mechanism 3 occurs. Literature Cited (1) Broutin, P.; Ropital, F.; Reyniers, M.-F.; Froment, G. F. Anticoking coatings for high-temperature petrochemical reactors. Oil Gas Sci. Technol. 1999, 54, 375-385. (2) Dhuyvetter, I.; Reyniers, M.-F.; Froment, G. F.; Marin, G. B. The influence of dimethyl disulfide on naphtha steam cracking. Ind. Eng. Chem. Res. 2001, 40, 4353-4362. (3) Froment, G. F. Coke formation in the thermal cracking of hydrocarbons. Rev. Chem. Eng. 1991, 6 (4), 295-328. (4) Reyniers, M.-F.; Froment, G. F. Influence of metal surface and sulfur addition on coke deposition in the thermal cracking of hydrocarbons. Ind. Eng. Chem. Res. 1995, 34, 773-785. (5) Schools, E. Kinetiek en simulatie van de ontkoling van reactoren voor de thermische kraking van koolwaterstoffen. Ph.D. Thesis, Ghent University, Ghent, Belgium, 2002. (6) Bennet, M. J.; Price, J. B. A physical and chemical examination of an ethylene steam cracker coke and of the underlying pyrolysis tube. J. Mater. Sci. 1981, 16 (1), 170-188.
(7) Ranzi, E.; Dente, M.; Pierucci, S.; Barendregt, S.; Cronin, P. Coking simulation aids on-stream time. Oil Gas J. 1985, 4952. (8) Herndon, W. C. Thermal reactivities of polynuclear aromatic hydrocarbons and alkyl derivatives. Tetrahedron 1982, 10 (1), 1389-1396. (9) Kopinke, F.-D.; Zimmerman, G.; Novak, S. On the mechanism of coke formation in steam crackingsconclusions from results obtained by tracer experiments. Carbon 1988, 2, 117-124. (10) Reyniers, G. C.; Froment, G. F.; Kopinke, F.-D.; Zimmerman, G. Coke modeling in the thermal cracking of hydrocarbons. 4. Modeling of coke formation in naphtha cracking. Ind. Eng. Chem. Res. 1994, 33, 2584-2590. (11) Kopinke, F.-D.; Bach, G.; Zimmerman, G. New results about the mechanism of TLE fouling in steam crackers. J. Anal. Appl. Pyrolysis 1993, 27, 45-55. (12) Bach, G.; Zimmermann, G.; Kopinke, F.-D.; Barendregt, S.; van den Oosterkamp, P.; Woerde, H. Transfer-line heat exchanger fouling during pyrolysis of hydrocarbons. 1. Deposits from dry cracked gases. Ind. Eng. Chem. Res. 1995, 34, 1132-1139.
IE020778O
