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Comments on “Surface-Enhanced Light Olefin Yields during Steam Cracking” Lyle F. Albright School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283
Sir: The paper authored by Golombok et al.1 reports methods to obtain enhanced yields of ethene (ethylene) and/or lower amounts of coke on the solid surfaces. The reasons for these findings were postulated, but in my opinion the most probable explanation for reduced levels of coke was apparently not considered. The key findings as reported in this paper appear to be as follows. First, increased ethene yields were obtained when different reactor tubes were packed with one of the following: granular alumina, silica solids, or Fecralloy gaze and fibers. Second, when the surface of the packing material was coated with KVO3, ZrO2/ MnO2, boric acid, etc., the ethene yields were decreased, but the amount of coke formed on the solid surfaces during the pyrolysis reactions decreased. The authors’ conclusion 1 on p 290 states the following: “(1) No catalytic effect is observed with either KVO3 or the Zn/MnO2 catalyst. The function of potassium in the catalyst seems to be mainly coke suppression, and we have established that this is a catalytic effect.” This conclusion is confusing, first stating that the surface coating is not catalytic and then immediately stating that it is catalytic. Also, how can KVO3 and Zn/MnO2 be called catalysts and said to be noncatalytic or are the authors differentiating in some way between KVO3 and potassium? It seems obvious that neither pure KVO3 nor pure potassium exists on the solid surfaces. Furthermore, the claim that potassium is suppressing coke (formation) seems unlikely and was not clearly explained. Catalysts increase the kinetics of specific reactions. Increased kinetics seems contradictory to suppressed coke formation. The authors indicate that potassium is not catalyzing reforming reactions between hydrocarbons and steam (see particularly p 290, about 65% down column 1). To support this conclusion, they state that “The route to coke runs via methane...”. This apparent claim that methane is a precursor for coke is thought to be incorrect, or can they provide evidence to support their claim? Extensive coking literature including Albright and Marek2 indicates that major coke precursors during conventional pyrolyses for the production of ethylene include acetylene, dienes including butadiene, and aromatics. None of these precursors are readily produced from methane in ethylene units. Acetylene (and other acetylenic compounds) is produced in ethylene units mainly from ethylene (and other olefins). Also on the basis of theoretical considerations, the formation of coke from methane would be unexpected because methane contains only primary C-H bonds; such bonds are relatively strong. It should be emphasized that I am not questioning, however, their conclusion that gaseous hydrocarbons and steam do not react to any significant extent. The authors though make no mention of reactions between steam and coke deposited on the coated surfaces. Potassium salts (which probably convert to K2O) are known to be excellent catalysts for gasification of
coke (or carbon) to produce CO, CO2, and hydrogen. Patents using potassium nitrate3 and potassium acetate4 report techniques to be used in ethylene furnaces to oxidize and hence remove much if not all of the coke deposits as they form. Vanadium oxides may also form on the surfaces and then act as oxidizing catalysts. The main role of the catalysts in the investigation of Golombok et al.1 may be to promote (or catalyze) coke gasification reactions rather than to supress coke formation. The amounts of carbon oxides produced will likely indicate the amounts of coke being gasified. Because the authors analyzed the effluent streams using gas chromatography, they may still be able to determine the amounts of these compounds in the effluent gas streams for experiments with and without catalysts. To obtain a better understanding of the mechanisms of coke formation and removal from the solid surfaces, the following information would be most helpful. (1) Electron microscopy results of the coke in contact with the inner surfaces of the reactor tube, coke exposed to gas phase, and coke at cross sections. These results would provide information on the types of coke formed. During conventional pyrolyses in unpacked tubes, three types of coke are produced (2). Both nickel and iron catalyze the formation of filamenteous coke (type 1 coke). Tar droplets formed in the gas phase by noncatalytic reactions are the starting point of so-called type 2 coke. Type 2 coke frequently accounts for a major fraction of the total coke formed in unpacked reactor tubes. These tar droplets suspended in the flowing gases agglomerate and convert with time to larger, semicoke particles. Packed surfaces obviously serve as much better collection sites for the droplets as compared to an unpacked reactor tube. Filamenteous coke also serves as collection sites for the droplets. When the droplets collect on the solid surfaces, carbon-hydrogen bonds continue to break and the tar or semicoke deposits on the surface convert to coke, which is often semispherical in shape. Both the coke filaments and semispherical deposits grow in size as a result of type 3 coke formation. Other microscopic information of importance would be as follows: the location and thickness of coke in the reactor and in the packing. Almost certainly the coke will not be distributed uniformly on the packing. Further, evidence may be obtained as to whether the coke is strongly bonded to the solid surfaces. In my laboratory, coke adhered poorly to some surfaces. This evidence suggests that surfaces may be developed in the future to which no coke collects or adheres. (2) Analyses of metals in coke. In conventional industrial furnaces using regular unpacked coils, the coke always contained iron, nickel, chromium, and other metals. Were vanadium, manganese, etc., present in the coke formed in the current investigation? Several analytical procedures are available for such measurements, including EDAX (energy dispersive x-ray analyzer) and X-ray photoelectron spectroscopy.
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(3) Results obtained by decoking the reactor and packing. The authors report that a loss of weight of the packing and/or reactor occurred during pyrolysis using “potassium” as the catalyst. They postulated that “zero coke” was not formed but that a major loss of weight was caused by “evaporation of K2O” (see p 290, column 1). Decoking experiments would likely determine both the weight of coke on the surface and the loss of weight due to “corrosion” and/or evaporation from the surfaces of the quartz reactor tubes. (4) Electron microscopy and EDAX (or comparable) analysis of the packed material before and after a cracking run would provide information on surface changes and on surface corrosion. Is the weight loss of the packing due to K2O losses as was postulated, corrosion of the surface, or both? The standard kinetic program SPYRO was used by the current investigators to predict both ethene and methane yields. This program is thought to be based on the assumption that only gas-phase reactions occur. When surface reactions are important, the model would obviously be less reliable. It is not clear how or if the SPYRO model explains or predicts the change in ethene/
methane ratios that occurred in the packed-bed reactors. These packed beds with much higher surface-to-volume ratios than present in conventional unpacked coils would affect the formation and termination of free radicals and the levels of different radicals in the gas phase. Literature Cited (1) Golombok, M.; Kornegoor, M.; van den Brook, P.; Diercikx, J.; Grotenbreg, R. Surface-Enhanced Light Olefin Yields during Steam Cracking. Ind. Eng. Chem. Res. 2000, 39, 285-291. (2) Albright, L. F.; Marek, J. C. (a) Coke Formation during Pyrolysis: Role of Residence Time, Reactor Geometry, and Time of Operation. (b) Analyses of Coke Produced in Ethylene Units: Insights on Process Improvements. (c) Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene. Ind. Eng. Chem. Res. 1988, 27, 743-759. (3) Kohfeldt, W. C.; Hebert, F. J. Removing and Preventing Coke Formation in Tubular Furnaces by Use of Potassium Nitrate. U.S. Patent 2,843,941, July 7, 1959. (4) Forester, D. R. Methods for Retarding Coke Formation During Pyrolytic Hydrocarbon Processing. U.S. Patent 4,889,614, Dec 26, 1989.
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