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Ind. Eng. Chem. Res. 1998, 37, 4302-4305
Absolute Rates of Coke Formation: A Relative Measure for the Assessment of the Chemical Behavior of High-Temperature Steels of Different Sources Gerhard Zimmermann*,† and Wolfgang Zychlinski† Department of High-Temperature Reactions at the Institute of Chemical Technology, University of Leipzig, Permoserstrasse 15, D-04303 Leipzig, Germany
Harry M. Woerde and Paul van den Oosterkamp Kinetics Technology International B.V., P.O. Box 86, NL-2700 AB Zoetermeer, The Netherlands
A standardized artificial aging procedure for steel specimen is reported. It is shown that the procedure is suitable to raise the scarcely measurable absolute coking rates at the surface of fresh highly alloyed high-temperature steels during hydrocarbon pyrolyses to orders of magnitude that make the determination of reliable measures possible. The absolute coking rates are measured in 95% confidence intervals of < 20 µg cm-2 min-1. Introduction High-temperature reactions, such as in the manufacture of lower olefins by steam cracking1 or hydrogen/ carbonmonoxide mixtures by steam reforming2,3 of suitable hydrocarbon feedstocks, necessitate the use of specially alloyed, heat-resistant steels as reactor materials (see for example, ref 4). The properties of such steels deteriorate with increase of operating time. Therefore, these steels inevitably have to be replaced after more or less short periods of time, for example, in intervals of ≈3-6 years when considering furnaces for steam cracking. In this case, the effective service life depends considerably on the number of decoking procedures carried out,4 the frequencies of which are controlled by the rates of the formation of coke deposits on the inner surfaces of the outside heated reactor tubes. Therefore, the determination of reliable and, above all, reproducible absolute coking rates at high-temperature steel surfaces by a rapid-assay method would be of great interest for the selection of suitable brands from commercially available steels as well as for the development of thermostable thin layers at the inner surfaces of marketable steels, which are supposed to suppress catalytic coke formation (see for example, ref 5). Unfortunately, the determination of reliable absolute coking rates turned out to be difficult6 because the formation of coke on the surface of specimens made from fresh highly alloyed high-temperature steels in lab-scale equipment usually is so low that a reliable kinetic pursuance is almost impossible (see the plot in Figure 1 of high-temperature steels, 2). Only lower alloyed steels (e.g., X8CrNiTi18.10; cf. Figure 1) facilitate the measuring of sufficiently high coking rates. But in such cases, the absolute coking rates could not be reproduced with high precision although extreme care was taken in the adjustment and monitoring of the test parameters. Actually, reproducible results could not be * To whom any correspondence should be addressed. † Present address: Institut fu ¨ r Nichtklassische Chemie an der Universita¨t Leipzig, Permoserstrasse 15, D-04303 Leipzig, Germany.
Figure 1. Dependence of the coking rate rC on the exposure time of different nonaged specimens. Key to materials: (9) X8CrNiTi18.10; (2) other high-temperature steels (HK 40, HPmodified, Incoloy 800).
attained even if the coking rates were determined in specimens made from one and the same piece of X8CrNiTi18.10.6 (See also ref 7.) Therefore, materialdependent differences of almost reproducible coking rates become manifest only if the specimens to be compared are sufficiently aged under cracking conditions. This aging, however, necessitates exposure times of days or even weeks by mutual changes of pyrolysis and decoking conditions7 (cf. Figure 2). Because of the long time required for aging, a time-saving, artificial aging resulting in comparable surface properties of specimens made from a particular brand steel in each case should be a useful tool to determine the extent to which different blank brand steels or such ones coated with definite surface layers suppress the catalytic formation of coke during hydrocarbon pyrolyses. In this work, we report on an aging procedure that is capable of raising the level of the absolute coking rates to an order of magnitude that allows a reliable determination of such rates at the surfaces of marketable high-temperature steels.
10.1021/ie980068b CCC: $15.00 © 1998 American Chemical Society Published on Web 10/02/1998
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4303 Table 1. Composition of the Dry Cracked Gas from Isobutanea product
content (wt %)
product
content (wt %)
hydrogen methane ethane ethene acetylene propane propene
2.9 28.3 1.7 16.6 2.4 0.1 18.0
isobutane butenes buta-1,3-diene benzene toluene remainder
9.1 8.4 2.3 4.7 0.8 4.7
a
Figure 2. Dependence of rC ) f(t) on the number of the preceded coking/decoking cycles at an HP-modified specimen. Key: (number in box) number of cycles; (b) rC(60) value, determined in an independent experiment on an artificially aged specimen (HPmodified).
Figure 3. Synoptic scheme of the apparatus: (1) microelectrobalance; (2) quartz tubular reactor; (3, 4) parts of the electric heating system; (5) specimen; (6) counter weight; (7) vaporizer; (8) dosing pump; (9) water cooler; (10) electrostatic precipitator.
Experimental Equipment and Procedures Detailed descriptions of the experimental equipment and procedure used for the determination of absolute coking rates were given in refs 8-10. The methods in the latter references are directed at the determination of TLE fouling rates in the same setup. The experimental setup used in this work is depicted schematically in Figure 3. The reaction chamber is a quartz tubular reactor (2) jacketed with two parts of an electric heating system (3, 4). Inside the reactor (2), a specimen (5) with dimensions of 30 × 7 × 2 mm and a small hole at one end (to make the attachment of the specimen at the weighing chamber (1) of an electronic device (Sartorius Micro M25D-V) possible by means of a thin quartz rod) is positioned. n-Heptane and isobutane were used as hydrocarbon feedstocks. Although n-heptane was exclusively used in the frame of the aging procedure, isobutane served for the generation of cracked gas in which the coke formation was carried out, and the absolute coking rates
T ) 850 °C; τ ) 0.7 s.
were determined at the specimen surfaces. The standard pyrolysis conditions for the determination of the coking rates (T ) 850 °C; residence time, 0.7 s; total pressure, 0.1 MPa; dilution ratio N2/isobutane, 0.5 g/g; cracking severity (as C2H4/C3H6 wt ratio in the cracked gas), ≈0.9) differ from those prevailing in industrial reactors by the substitution of steam with oxygen-free nitrogen. A representative composition of the resulting cracked gas is listed in Table 1. The exclusive use of nitrogen as diluent was intentionally made to avoid the well-known decrease of the otherwise undesired coke formation by steam.1 After the aging of the specimen surface at which the coking rates should be determined, the reactor was thoroughly purged with nitrogen, the test parameters were adjusted, and the experiment was started for the determination of the coking rates by addition of isobutane. The coke formation was displayed and the increase of the specimen weight (reasoned by coke deposits) was continuously recorded. From these results, the absolute coking rates in the dimension of µg cm-2 min-1 can be calculated for each time within the total run time of the experiment. For the reasons mentioned later (see Results and Discussion), we used the coking rates after a run time of 60 min (so-called rC(60) value) as a quantitative measure of the extent to which hightemperature steels of different sources suppress the catalytic coke formation from cracked gases. After leaving the reactor, the mixture of cracked gas and nitrogen passes a water cooler (9; to separate liquid from gaseous products) coupled to an electrostatic separator (10; to remove tar droplets). Between 2 and 9, the nitrogen-containing cracked products were sampled by closeable sampling columns and the wt ratio of ethylene and propylene (considered to be an indirect measure of the cracking severity) as well as the conversion of the feed were determined by gas chromatography.11 At the end of each run, the reactor was cooled to room temperature in the presence of nitrogen, the studied specimen was taken out, and the apparatus was prepared for the following experiment by positioning the next specimen into the reactor chamber. To obtain specimens with surface properties that approximately correspond to those resulting from a longtime exposure under pyrolysis/decoking conditions, a standardized aging procedure was derived from the results of a wealth of screening experiments in which the following parameters were determined in the temperature range of the specimens pretreated with N2 and H2: the oxygen content in the nitrogen during decoking, the number of the applied coking/decoking cycles and their temporal prolongation, cauterization by acids, etc. This procedure includes the following sequential steps: ‚purging the reactor with the corresponding specimen with nitrogen (6 L/h)
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‚heating the reactor to 970 °C, while maintaining the nitrogen flow ‚introduction of 0.6 L/h air into the nitrogen flow upon reaching 970 °C, and leaving the experimental setup under these conditions for 5 h ‚cutting off the air and reduction of the gas-phase temperature to 850 °C ‚substitution of the nitrogen by a hydrogen flow (6 L/h) and leaving the setup under these conditions for 1 h ‚resubstitution of hydrogen by nitrogen (6 L/h) and reduction of the gas-phase temperature to 830 °C ‚formation of coke deposits on the specimen surface by n-heptane pyrolysis (T ) 830 °C; N2/n-heptane wt ratio, 0.5; residence time, 0.7 s; run time, 15 min), followed by the removal of the cracked gas by nitrogen flushing and gasification of the deposited coke by introduction of air (20 L/h) at 830 °C ‚repetition of the above-described coking/decoking procedure for ≈10 times sequentially. To compare the surface changes reached by the standardized aging procedure with those resulting from a long-term exposure of steel specimen in the presence of alternating pyrolysis and decoking conditions, the dependence of coking rates of an HP-modified specimen on the number of pyrolysis/decoking cycles was determined. The results are depicted in Figure 2 together with the rC(60) values measured in parallel for an artificially aged, HP-modified specimen. Clearly, the aging procedure results in surface properties that correspond with those obtained after ≈40 pyrolysis (60 min)/decoking cycles according to rC(60) values of ≈75 ( 8 µg cm-2 min-1.
Figure 4. Comparison of the dependence of coking rates (rC) on the exposure time of three artificially aged specimens made from different steels. Key: (9) HK 40; (b) HP-modified; (2) Incoloy 800.
Results and Discussion Whenever an attempt was made to determine absolute coking rates at the surface of fresh specimens from high-temperature steels, the resulting rC(60) values were very low (e5 µg cm-2 min-1), except for those obtained for specimens of the steel type X8CrNiTi18.10 (rC(60) > 300 µg cm-2 min-1). In addition, the pattern of coking rates versus run time exhibited pronounced maxima after 5-10 min when X8CrNiTi18.10 specimens were used (Figure 1), whereas in the cases of all other hightemperature steels, such a pattern becomes manifest in aged specimens only (cf. Figure 2). Behind the maxima, the coking rates tend gently downward to reach almost constant levels after ≈60 min. Therefore, the rC(60) values appear to be a useful measure for evaluation of steels in terms of their ability to suppress coke formation. To reach an rC(60) value of ≈75 µg cm-2 min-1 for the steel HP-modified by repeated coking (pyrolysis)/ decoking in a lab-scale setup, as was measured at the surface of a specimen after precedingly carried out standardized aging, one needs almost 2 weeks. In contrast, the artificial aging takes only 2 days. A further advantage of the artificial aging is that subsequent determinations of rC(60) values lead to insignificant deviations with specimens made from one and the same steel but to significant differences when differently composed high-temperature steels are investigated. In Figure 4 such an example of the dependence of the absolute coking rate on the run time is demonstrated for eight artificially aged specimen made from the steels HK 40, HP-modified, and Incoloy 800. The rC(60) values amounted to 170 ( 12 (HK 40), 75 ( 8 (HP-modified), and 25 ( 5 (Incoloy 800) µg cm-2 min-1. The bars
Figure 5. Comparison of rC(60) values determined for different coated and blank HP-modified specimens (after artificially aging in each case). Key: (right-hatched box) blank (uncoated); (lefthatched box) pretreated in the presence of hexamethyldisilathiane,12 (checkered box) coated by Cr-plating and laser remelting;14 (vertical-lined box) coated by annealing in the presence of aluminum;13 (solid box) error bars.
represent 95% confidence intervals and an overlap of the error bars of the different brand steels can be excluded. These results indicate that the rC(60) values, determined in the above-described way, are a useful measure to evaluate marketable high-temperature steels in terms of their ability to suppress catalytic coke formation. Moreover, the artificial aging procedure makes it possible to find out if the application, like the plating or coating of surface layers on high-temperature steels, leads to a significant suppression of the coke formation compared with the blank steel in question. Figure 5 gives an impression of such an application. Four fresh HP-modified specimens were pretreated in each case with hexamethyldisilathiane,12 by aluminum diffusion annealing,13 and by chromium plating/laser remelting.14 Then, the coated and four more blank specimens were subjected to the artificial aging procedure. The rC(60) values were subsequently measured at the surface of these aged specimen. The results are compiled in Figure 5. It is obvious that the surface layers have a higher ability to suppress coke formation than those of the uncoated HP-modified steel. Although metallographic studies of the artificially aged surface are not
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available, we assume that the applied aging procedure causes a similar change of the surface properties as was observed in industrially used cracking coils15,16 (see Figure 2). Conclusions The results presented in this paper show that absolute coking rates for original high-temperature steels can be determined with an adequate reproducibility for the purpose of comparison of the tendencies of different brand steels to suppress the catalytic coke formation during the thermal cracking of hydrocarbons whenever the specimens are aged in advance by means of a special procedure. This procedure seems to be useful, too, to evaluate the suitability of thin layers on the surfaces of high-temperature steels to suppress coke formation even after a more or less long period of use under conditions close to industrial practice. Literature Cited (1) Grantom, R. L. Ethylene from Pyrolysis of Hydrocarbons. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz, W., Ed.; VCH: Weinheim, 1987; Vol. A10, pp 45-76. (2) Ha¨ussinger, P.; Lohmu¨ller, R.; Watson, A. M. Catalytic Reforming of Hydrocarbons. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz, W., Ed.; VCH: Weinheim, 1987; Vol. A13, pp 319-328. (3) Renner, H.-J.; Marschner, F. Catalytic Reforming of Natural Gas and other Hydrocarbons. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz, W., Ed.; VCH: Weinheim, 1987; Vol. A12, pp 186-204. (4) Parks, St. B.; Schillmoller, C. M. Update in Alloy Selection for Ethylene Furnaces. In Proceedings of the 7th Ethylene Producers Conference, American Institute of Chemical Engineers, New York, 1995; pp 435-442. (5) Albright, L. F. Metal Samples from Small-diameter Furnace Tubes Examinated in SEM and EDAX Analyses. Oil Gas J. 1988, Sept 19, 90-96. (6) Kopinke, F.-D.; Zimmermann, G.; Nowak, S. On the Mechanism of Coke Formation in Steam Cracking - Conclusions from Results Obtained by Tracer Experiments. Carbon 1988, 26 (6), 117-124.
(7) Zimmermann, G.; Zychlinski, W. Unpublished results, Leipzig, 1993. (8) Kopinke, F.-D. Struktur-Reaktivita¨ts-Beziehungen fu¨r die Koksbildung unter den Bedingungen der Mitteltemperaturpyrolyse von Kohlenwasserstoffen. Dissertation B, Academy of Sciences of the GDR, Leipzig, 1985. (9) Kopinke, F.-D.; Bach, G.; Zimmermann, G. New Results about the Mechanism of TLE Fouling in Steam Crackers. J. Anal. Appl. Pyrolysis 1993, 27, 45-55. (10) Bach, G.; Kopinke, F.-D.; Zimmermann, G.; Zychlinski, W. Chemical Reaction Engineering Aspects of Lab-scale Studies of the Coke Formation under the Conditions of Steam Cracking. Chem.-Ing.-Tech. 1994, 66, 1086-1091. (11) Struppe, H. G.; Ahlheim, J.; Luther, U.; Ondruschka, B. Conversion of Mixtures of Pyrolysis Feedstocks and Thermally Pretreated Plastics. Simplified Balancing of Lab-scale Experiments by Means of GC-Analyses using Closeable Sampling Columns. Chem. Tech. 1995, 47, 179-189. (12) Zimmermann, G.; Zychlinski, W. Process to Reduce Coke Formation at Heat Transfer Surfaces. DE 4 405 884 C1, 1995; Applicants: K. T. I. Group B. V. (13) Woerde, H. M.; Zimmermann, G.; Steurbaut, C.; van Buren, F. R.; Gommans, R. J. N.; Jones, J. J. Process for Reducing the Formation of Carbon Deposits. WO 97/16507, 1997; Applicants: K. T. I. Group B. V., DSM N. V., DOW Benelux N. V., Paralloy Ltd. (14) Montgomery, R. S. Laser Treatment of Chromium Plated Steels. Wear 1979, 56, 155-166. (15) Tsai, T. Ch.-H.; Albright, L. F. Surface Reactions Occurring during Pyrolysis of Light Paraffins. In Industrial and Laboratory Pyrolysis; Albright, L. F., Crynes, B., Eds.; ACS Symposium Series 32; American Chemical Society: Washington, DC, 1976; pp 274295. (16) Albright, L. F.; Tsai, T. Ch.-H. Importance of Surface Reactions in Pyrolysis Units. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B.; Cocoran, W. H., Eds.; Academic: New York, 1983.
Received for review February 3, 1998 Revised manuscript received June 29, 1998 Accepted July 9, 1998 IE980068B
