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Comments on “Kinetic Modeling of Coke Formation during Steam Cracking” Lyle F. Albright School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Sir: Obtaining a reliable kinetic model for coke formation in both industrial and laboratory furnaces producing ethylene (or ethene) is clearly a desired objective. Wauters and Marin1 made such an effort using what they report to be a mechanistic model. Unfortunately, their model is incorrect and incomplete, as will be discussed next. Overview to Steam Cracking The main products formed when feedstocks such as ethane, propane, naphthas, or gas oils are pyrolyzed at temperatures of about 700-1000 °C include ethylene, propylene, butadiene, and aromatics. Coke is a highly undesired byproduct. Coke precursors and/or coke collect to some extent on the inner walls of the pyrolysis coils, and some are transferred out of the coils. Once out of the coils, they enter the quench cooler, often referred to as the transferline exchanger (TLX); part of the precursors or coke deposit in the TLX, and some are further transferred to the quench tower. Both the pyrolysis furnace and TLXs need to be shut down at intervals for cleaning (to remove coke deposited in each). Up to about 5-10 years ago, the times between decokings were often 20-40 days. Since then, as will be discussed later, improved coils (referred here as new coils) have been used in a rather large number of commercial ethylene units. Times between decokings have as a result been much extended. Various decoking techniques are employed, but usually a combination of steam and air is used to gasify the coke-forming carbon oxides. Basic Chemistry and Phenomena Occurring as Coke Is Formed Numerous investigators have reported key information in the last 40 years. Unfortunately, Wauters and Marin1 failed to reference several papers which I consider to be most important. In the Introduction of their paper, they mention with essentially no details that coke is produced by three distinct mechanisms; they then ignored two. Albright and Marek2 obtained experimental results; using those results plus those of several previous investigators, they then discussed these three coking mechanisms, designated here as mechanisms 1-3. Mechanism 1 often produces coke filaments with nickel and iron acting as catalysts. Mechanism 2 involves the formation of tar droplets in the gas phase; these droplets form as a result of both condensation and chemical reactions. Some, but not all of these droplets, collect on the inner metal walls or on the coke that has already formed on the inner walls of the coil. Once the droplets collect, they dehydrogenate to form amorphous coke with free radicals on the coke surfaces. Mechanism 3 consists of the reactions between free radicals on the solid coke or tar droplets and various gaseous entities
including free radicals, olefins, acetylenics, etc. Wauters and Marin1 claimed, based on the results of Reynier and Froment,3 that mechanism 3 is the predominant, and apparently exclusive, mechanism for coke formation in the coil once “the metal surface is covered with coke” (formed during the initial stages of a pyrolysis run). Considerable literature does not support their postulate, and plant data indicate that it is incorrect, as will be discussed shortly. Relative Importance of Mechanisms 1-3 Albright and Marek2 and Albright4,5 analyzed several coke samples produced in different industrial ethylene furnaces. Their findings include the following: 1. Significant fractions of the industrial cokes were produced by mechanisms 1 and 2, as indicated by scanning electron photomicrographs. Filamenteous structures and spherical (or semispherical) structures were frequently detected over the entire cross section of the coke samples indicating these types of coke formed from the initial to the final phases of the run. 2. Energy-dispersive X-ray analysis indicated that nickel, iron, and chromium particles were distributed throughout the coke samples. Higher metal concentrations were usually found in the layer of coke that had been next to wall of the coil, i.e., the coke formed at the start of the pyrolysis run. Metal particles were also detected in some photomicrographs. The presence of metal in the coke explains the filamentous coke structures found in the photomicrographs. 3. The coke often contained voids or porous regions. Hence, the surface areas available for mechanism 3 type reactions are greater than those on coke samples with smooth surfaces. Wauters and Marin1 report on p 2388, column 2, of their paper that they use “a unique factor” to account for the “surface area of the cokes”, but they did not explain this “unique factor”. They do report, however, that it “does not coincide with the geometric surface area”. One must question the reliability of their surface areas. It should be emphasized that formation and growth of tar droplets in the gas phase involve both condensation and chemical steps. There are obviously reactions between the tar droplets (which have free radicals at the surfaces) and gaseous free radicals, olefins, and acetylenics. It has been experimentally demonstrated that these tar droplets, when hitting the surface, either can collect on the surface or can bounce off much like a ping pong ball. These droplets have various compositions; they vary from light (rather low viscosity) tars, to heavier (higher viscosity) tars, and finally to at least semisolids. Dehydrogenation steps forming free radicals on carbon atoms obviously occur in the droplets. Coke formation is affected by the axial position in the coil and the average residence time of the gases in the coil. Neither of these were considered by Wauters and Marin.1 First, coke deposits vary with axial position and
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are often restricted to near the exit end of the coil. Second, commercial furnaces currently operate at very high gas velocities and with residence times varying from about 0.1 to 0.6 s in the coils. Higher gas and metal temperatures are usually employed at lower residence times. Formation and especially growth of tar droplets likely differ significantly as the residence time varies. Importance of the Inner Surface of the Coil The nature of the inner surface of the coils has been recognized for many years as being of importance relative to the amount and type of coke that forms or collects there. Numerous investigators have discussed this importance, and several6-8 are listed as examples. Yet, Wauters and Marin1 failed to consider the importance of the surfaces. Reducing the levels of nickel and iron on the inner surfaces results in several benefits; such levels sometimes differ greatly in current ethylene furnaces. First, the formation of filamenteous coke, via mechanism 1, is reduced or maybe even completely eliminated in certain coils. Mechanism 1 during especially the initial phases of a pyrolysis run corrodes and roughens the inner surface of the coils. Metal fines often result; many are transferred by the high-velocity gases. Considerable fines often become incorporated in the coke that collects in the commercial coils and in the coke that collects in the TLX. The incorporated nickel or iron catalyzes the formation of more coke. In laboratory or pilot-plant units, the gas velocities tend to be much lower so transfer of metal fines is usually of little or no importance. Second, the filaments formed in the initial stages of a pyrolysis run are often strongly attached to the metal surfaces. Such high adhesion is undesired, as discussed later. Third, filamenteous coke is a highly effective collection site for coke formed via mechanism 3 and for tar droplets produced via mechanism 2. Attempts have been made to develop suitable coils whose inner surfaces contained little or no nickel and iron. In the last several years, several coils have been developed and are designated here as new coils, in which the inner surfaces have low or perhaps even zero concentrations of nickel and iron. Wauters and Marin1 did not reference such developments. Such coils have now been installed in over 30 industrial furnaces. The coils of Alon Surface Technologies, Inc., and of Westaim have extended the lengths of time before decoking is needed by factors of 2 or even higher.9,10 Hence, the rate of coke buildup on the surface was (is) reduced by two or more. Benum11 has recently reported even larger reductions in coke collection in several coils in commercial ethylene furnaces of Nova Chemical Co. Direct comparisons were made between a conventional coil and a new coil developed by Nova. In one case, the time between decokings was extended from 33 to 516 days. These results indicate that the rate of coke collection in this new coil had decreased by a factor of about 1516. The model developed by Wauters and Marin1 does not explain such differences. Obviously the level of coke formed by mechanism 1 was much reduced in many tests with new coils, but presumably the production of tar droplets in the gas phase by mechanism 2 is not. The following factors may occur and may explain the differences: 1. The rates of collection of tar, semitar, etc., droplets on the inner coke surfaces in new coils are much reduced. Benum11 reported that the coke formed in the
Nova investigation was much harder. Perhaps it is more graphitic. In this regard, Wauters and Marin1 claimed that the coke produced, in presumably conventional coils, was mainly or exclusively graphitic. This claim is with high certainty incorrect. As already discussed, coke formed by mechanism 2 is amorphous. Furthermore, the type of coke formed by mechanism 3 likely depends significantly on the gaseous entities that react with the surface free radicals. Additional evidence that much of the coke formed in conventional coils is amorphous is provided by Baker and associates.12,13 Perhaps the decreased rates of collection in the recent tests with new coils are due at least partially to smoother coke and/or surfaces with lower levels of adhesion. 2. The metal surfaces of the new coils may be smoother or exhibit low levels of adherence between the metal surface and the coke. If so, the coke that does collect on the surface may be removed to a high degree by the high-velocity gases in the coil. The new coil of Nova (and named “ANK-400”) is produced using two gaseous treatments of a conventional coil.14,15 Similar gas pretreatments of high-alloy steels at Purdue University16,17 resulted in surfaces with very low levels of nickel and iron. Certain Purdue pretreatments also resulted in surfaces having low levels of adherence to coke formed during pyrolysis. 3. A sulfur additive was used in the tests at the Nova Chemical Co. plant. Sulfur compounds tend to form metal sulfides on the metal surfaces or particles. This additive may contribute to the decreased level of coke collection. Tan and Baker13 have recently found that the specific additive, level of additive employed, and operating conditions used all have a large effect on the amount and type of coke produced. How to incorporate the role(s) of sulfur additives in coking models is a challenge for the future. 4. The levels of erosion and oxidation of the coke deposited in the coil may also vary in new coils as compared to those in conventional coils. Such erosion and oxidation are caused by the high-velocity gas flow. Conclusions The model developed by Wauters and Marin1 fails to explain both industrial and laboratory results. Features not considered in this model include coke formed by mechanisms 1 and/or 2, collection on solid surfaces of tar droplets formed by mechanism 2, erosion and oxidation of coke deposits in coils of industrial furnaces, and effective (or available) surface areas of coke (when modeling mechanism 3). Perhaps their current model can be incorporated in future models. In addition, modeling of coke formation in TLXs is needed; quite different parameters will be needed in this latter model. Literature Cited (1) Wauters, S.; Marin, G. B. Kinetic Modeling of Coke Formation during Steam Cracking. Ind. Eng. Chem. Res. 2002, 41, 23792391. (2) Albright, L. F.; Marek, J. C. Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene. Ind. Eng. Chem. Res. 1988, 27, 755-759. (3) Reynier, 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. (4) Albright, L. F. Special Analyses Reveal Coke-Deposit Structure. Oil Gas J. 1988, Aug 1, 35-40. (5) Albright, L. F. Coke from Small Diameter Tubes Analyzed. Oil Gas J. 1988, Aug 29, 44-48.
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(6) Baker, R. T. K.; Chludzinski, J. J. Filamenteous Carbon Growth on Nickel-Iron Surfaces: The Effect of Various Oxide Additives. J. Catal. 1980, 60, 464-478. (7) Brown, D. E.; Clark, J. T. K.; Foster, A. I.; McCarroll, J. J.; Sims, M. L. In Inhibition of Coke Formation in Ethylene Steam Cracking in Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC: 1982; pp 23-43. (8) Albright, L. F.; McGill, W. A. Aluminized Ethylene Furnace Tubes Extend Operating Life. Oil Gas J. 1987, Aug 31, 46-50. (9) Kuriekar, A.; Bayer, G. T. Enhance Furnace Tube Resistance to Carburization and Coke Formation. Hydrocarbon Process. 2001, 81 (No. 1), 80-84. (10) Petrone, S.; Mandyam, R.; Wysiekierski, A.; Tzatzov, K.; Chen, Y. A Carbon-Like Coating for Improved Resistance in Pyrolysis Furnaces, National Meeting of American Institute of Chemical Engineers. Session on Ethylene Plant Technology, New Orleans, LA, Mar 1998. (11) Benum, L. Achieving Longer Runs at Nova Chemicals. Spring 2002 Meeting of the American Institute of Chemical Engineers, New Orleans, LA, March 10-14, 2002.
(12) Rodriguez, N. M.; Kim, R.; Fortin, F.; Mochida, I.; Baker, R. T. K. Carbon Deposition on Iron-Nickel Alloy Particles. Appl. Catal. A 1997, 148, 265-282. (13) Tan, C. D.; Baker, R. T. K. The Effect of Various Sulfides on Carbon Deposition on Nickel-Iron Particles. Catal. Today 2000, 63, 3-20. (14) Benum, L. W.; Wong, W.; Oballa, M. C. Treatment of Furnace Tubes. U.S. Patent 5,630,887, May 20, 1997. (15) Benum, L. W.; Wong, W.; Oballa, M. C. Surface Treatment of Stainless Steel. Canadian Patent Appl. 2,164,021, May 30, 1997. (16) Luan, T. C. Reduction of Coke Deposition in Ethylene Furnace. Ph.D. Thesis, Purdue University, West Lafayette, IN, 1993. (17) Szechy, G.; Luan, T. C.; Albright, L. F. Pretreatment of High Alloy Steels to Minimize Coking in Ethylene Furnaces. In Novel Production Methods for Ethylene, Light Hydrocarbons and Aromatics; Albright, L. F., Crynes, B. L., Nowak, S., Eds.; Marcel Dekker: New York, 1993; Chapter 18, pp 341-360.
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