I n d . E n g . Chem. Res. 1988, 27, 751-755
and significant radial gradients occurred. The concentrations of tar droplets in the gas stream varied greatly as the geometry of the boat varied or as obstructions are positioned in flowing gases. Since the morphology of coke often changes appreciably during the course of a pyrolysis run,the coking mechanism also changes as the run progresses. With clean stainless steel coupons, numerous metal-catalyzed reactions occurred as coke was formed at the start of a run. Such reactions are however of lesser importance as some (but not all) metal particles are covered with coke. When the catalytic activity decreases, the coke is produced to a greater degree by noncatalytic mechanisms. At high temperatures (above about 900 "C),one coking mechanism seems to be predominant since filamentous coke and spherical coke grow or thicken rather uniformly in diameter. Geometrical factors were apparently in such cases of little or no importance. Acknowledgment Financial support for this investigation was provided by Gulf Research Foundation, Alon Processing, Inc., and Purdue Research Foundation. Literature Cited Albright, L. F.; Tsai, T. C. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 10, pp 233-254. Albright, L. F.; McConnell, C. F.; Welther, K. In Thermal Hydrocarbon Chemistry; ACS Symposium Series 183; American Chemical Society: Washington, D.C., 1979 Chapter 10, pp 176-191. Baker, R. T. K.; Chludzinski, J. J. J. Catal. 1980, 64,464.
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Baker, R. T. K.; Harris, P. S. In Chemistry and Physics of Carbon; Walker, P. L., Thrower, P. A., Eds.; Marcel Dekker: New York, 1978; Vol. 1, Chapter 1. Bennet, M. J., personal communication, March 1981. Brown, D. E.; Clark, J. T. K.; Foster, A. I.; McCarroll, J. J.; Sims, M. L. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 2, pp 23-42. Cleland, F. A.; Wilhelm, R. H., AIChE J. 1956,2, 489. Dente, M. E.; Ranzi, E.; Barendregt, Ir. S.; Tsai, F. W., "Ethylene Cracker Transferline Exchanger Fouling". Presented at the National Meeting of the American Institute of Chemical Engineers, Houston, TX, March 1983. Goosen, A. G.; Dente, M. E.; Ranzi, E. Hydrocarbon Process. 1977, Nou., 34-39. Graff, M. J.; Albright, L. F. Carbon 1982,20, 391. LaCava, A. I.; Fernandez-Raone, E. D.; Caraballo, M. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; ACS Chemical Society: Washington, D.C., 1982; Chapter 5, pp 89-107. Lahaye, J.; Badie, P.; Ducret, J. Carbon 1977, 15, 87. Marek, J. C.; Albright, L. F. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 7, pp 123-149. Mol, A. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 18, pp 451-471. Siklos, P.; Izsaki, Z.; Varga, T. A.; Kalman, J. Erdol Kohle-ErdgasPetrochem. Brenstoff-Cham. 1986, 39, 243. Tibbets, G. G. GMR-4518 Report, Oct 21, 1983; General Motors Research Laboratories, Warren, MI. Trimm, D. L., In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 9, pp 203-232.
Received for review September 19, 1986 Revised manuscript received December 8, 1987 Accepted December 21, 1987
Analysis of Coke Produced in Ethylene Furnaces: Insights on Process Improvements Lyle F. Albright* and James C. M a r e k t School of Chemical Engineering, Purdue University, W e s t Lafayette, Indiana 47907
Six industrial coke samples were analyzed by using a scanning-electron microscope and EDAX. These samples were from the pyrolysis coils, transfer lines, and inlet cones of transfer-line exchangers of the ethylene units. All coke samples contained iron, nickel, and chromium that had been abstracted from a metal wall or had been transferred through the coil and then incorporated in the coke. The coke which was formed catalytically contained metals and served as a collection site for noncatalytic coke. T h e catalytic coke was most prevalent on and near stainless steel surfaces. The coke near these surfaces was porous and obviously had high resistances to heat transfer. Methods of reducing coke formation are proposed. Although information on coke production has been reported for numerous laboratory units, relatively little are available, at least to the general public, on cokes produced in industrial furnaces. Some industrial companies consider data that they have obtained as proprietary. Other companies have, however, made little effort to obtain much information since they do not yet realize its importance. Since thermal contractions of the coke and metal differ substantially, cooling of a coil containing coke often damages the coil. Hence, coke samples from the coil of an ethylene furnace are obtained only after unscheduled shutdowns of a furnace. 'Present address: E. 1. d u Pont de Nemours & Co., Inc., Aiken, SC 29801.
Several industrial coke samples were obtained for use in the present investigation. The results obtained clarify features of coke formation so that operating modifications can be recommended. Results Six coke samples from three industrial ethylene units were analyzed by using a scanning-electron microscope (SEM) and accompanying EDAX (to determine the metal content of the coke). In addition, coke samples obtained when atactic polypropylene was thermally cracked were investigated. Coke Samples from Ethylene Units. Coke samples were obtained from three pyrolysis units, all of which used ethane or mixtures of ethane and propane as feedstocks
0888-~8~5/88/2627-0751$01.50/0 0 1988 American Chemical Society
752 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 Table I. Coke Samples from Ethylene Units thickness coke source of of coke, sample cake mm comments c-1 coil 5.3-6.4 section of coke in coil; 1-2 year old C-2 coil 4.2 unit shutdown because of hurricane TL-1 transfer line 10.1 physically removed transfer line 1.75-1.85 physically removed TL-2 TLX-1 cone of TLX 10.+11.6 physically removed physically removed TLX-2 cone of TLX 6.1-8.6
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