Methods for suppression of filamentous coke formation

Details Methods for suppression of filamentous coke formation Methods for suppression of filamentous coke formation

2000-08-01

2002-11-19

Preisch, Nadine (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

With prevention or removal of deleterious carbon...

C208S04800Q, C585S950000

Reexamination Certificate

active

06482311

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to the inhibition or prevention of coke formation on metal surfaces in contact with hydrocarbons at high temperatures. Such conditions can occur in hydrocarbon cracking processes and in certain types of engine systems in which hydrocarbon fuels reach very high temperatures. The invention more specifically relates to suppression of filamentous coke formation.
Carbon deposits (coke) can result from an interaction of the hydrocarbon processing stream and the metals contained in the walls and heat exchangers of reactors at temperatures above about 300° C. Deposits that form i n the shape of long filaments approximately 1 &mgr;m in diameter are referred to as filamentous coke. Non-filamentous coke can also form under pyrolysis conditions by several different mechanism. Filamentous coke is typically more abundant at higher temperatures (greater than about 450° C.), is hard and can be difficult to remove.
Coke formation is generally detrimental to the productivity and efficiency of the operation of a given system, causing fouling of lines and erosion of surfaces which increase operation down-time for cleaning and maintenance.
Filamentous coke formation is observed in naphtha cracking and ethylene production operations. The formation of coke in ethylene and naphtha reactors lowers product yield, heat transfer and reactor life along with the increased cost of time and money for decoking operations Froment, G. F., Reyniers, G. C., Kopinke, F., Zimmermann, G. (1994).
Ind. Eng. Chem. Res
., V. 33, 2584 ). Much research on the formation of coke catalyzed by metal surfaces is based on attempts to solve these problems.
Coke formation is also a significant problem in engine systems in which the hydrocarbon fuel temperatures can reach levels greater than about 300° C. For example, hypersonic aircraft employ fuel to cool ramjet/scramjet propulsion system. In these systems, sensible heating and endothermic reactions can be used to provide the required heat sink, but in the process, the fuel temperature can reach 650° C. (1200° F.) or more. When fuel reaches these temperatures, carbonaceous deposits (coke), including filamentous coke, form on the walls of the heat exchangers. These deposits can inhibit fuel flow and reduce heat transfer across the heat exchanger surface.
Filamentous coke formation is sensitive to the type of metal used in reactor walls. Nickel and iron present on the metal surface, as occurs in nickel and/or iron alloys and various types of steel, for example, are believed to catalyze the formation of filamentous coke through the formation of metal carbides that decompose (Vaish, S. and D. Kunzru (1989) “Triphenyl Phosphite as a Coke Inhibitor During Naphtha Pyrolysis”
Ind. Eng. Chem. Res
. 28, 1293-1299 and Reyniers, G. C., Froment, G. F., Kopinke, F. D., and Zimmerman, G. (1994). “Coke Formation in the Thermal Cracking of Hydrocarbons. 4. Modeling of Coke Formation in Naphtha Cracking”
Ind. Eng Chem Res
., 33, pp 2584-2590). Filamentous coke does not form in copper-lined reactors (Wickham, D. T., J. V. Atria, J. R. Engel, B. D. Hitch and M. E. Karpuk (1997). “Initiators for Endothermic Fuels,” 10/97 JANNAF Combustion/JSM Meeting) and titanium metal is resistant to filamentous coke formation (Chen, F. F., Karpuk, M. E., Hitch, B. D., and Edwards, J. T. (1998), “Engineering Scale Titanium Endothermic Fuel Reactor Demonstration for a Hypersonic Scramjet Engine,” presented at the 35th JANNAF Joint Combustion, Airbreathing Propulsion, and Propulsion Systems Hazards Subcommittees Meeting, Tuscon Ariz., December 7-11).
Significant effort has been expended to identify ways to passivate metal surfaces under high temperature pyrolysis conditions. The formation of metal oxide layers on alloys is reported to passivate the surface and reduce coking. One method is to oxidize the metal alloy with oxygen or steam to create an oxide layer such as chromia which is more resistant to carbon diffusion (Albright, L. F. and Marek, J. C. (1982) “Surface Phenomena During Pyrolysis,” in
Coke Formation on Metal Surfaces
, ACS Symposium Series 202, 123). The use of alumina and silica coatings are also reported to create a barrier to carbon diffusion and reduced coke filament formation on metal surfaces (Albright, L. F. and Marek, J. C. (1982); Atria, J. V, H. H. Schobert, and W. Cermignani (1996). “Nature of High Temperature Deposits from n-Alkanes in Flow Reactor Tubes”,
ACS Preprints, Petroleum Chemistry
, pp. 493-497; Baker, R. T. K. and Chludzinski, J. J. (1980). J Catal., V. 64, 464; Ghosh, K. K, and D. Kunzry (1992). “Sodium Silicate as a Coke Inhibitor During Naphtha Pyrolysis”,
Canadian Journal of Chemical Engineering
, 70, pp. 394-397). The preparation of alumina coatings is difficult and requires the use of aluminum-containing metal alloys in processing equipment or engines. For example, an inert alumina surface layer can be formed on aluminum containing alloys such as Incoloy 800 by treating the alloy at temperatures above 1000° C. in a hydrogen atmosphere with a low partial pressure of water. Silica coatings are not very effective. Atria et al. (1996) observed cracking of silica layers allowing filaments to grow. Ghosh and Kunzru (1992) found that passivation with sodium silicate initially reduced the coke formation rate by about 50%, but that the beneficial effect was reduced each time a decoking step was employed. Further, repeated oxidation or sulfiding of metal surfaces or repeated decoking applications roughens metal surface increasing the surface area and leading to formation of larger amounts (Albright and Marek 1982). Polishing of alloy and metal surfaces has been indicated to help reduce coke formation.
Various additives have been reported to reduce coke formation. U.S. Pat. No. 1,847,095 reports “adding or supplying” metalloids including boron, arsenic, antimony, bismuth, phosphorous, selenium and silicon or compounds thereof “to the metallic (and non-metallic, if any) materials” which come into contact with “hydrocarbons at high temperature” to diminish or prevent coke and soot formation. The patent indicates that metal surfaces can be coated or treated with the substances or that “small quantities of the hydrogen compounds” of the metalloids may be added to hydrocarbons. The hydride of selenium, among others, is reported to be of high utility in this process. The patent specifically reports addition of 0.01%-0.05% of “hydrides of silicon” to an ethylene-hydrogen-carbon dioxide mixture. GB patents 275,662 and 296,752 relate to the same or similar processes.
Trimethyl- or triphenylphosphite and benzyldiethylphosphate are reported to decompose at 700° C. to form phosphorous compounds which passivate metal surfaces (Kunzru, D. and Chowdhury, S. N. (1993)
Can. J Chem. Eng
., V. 71,873; Kunzru, D. and Vaish, S. (1989)
Ind. Eng. Chem. Res
., V. 28, 1293; Vaish, S. and D. Kunzru (1989)
Ind. Eng. Chem. Res
. 28, 1293-1299). However, reductions in coke deposition of only 10-30% were reported. In addition, Vaish and Kunzru (1989) reported that high concentrations (up to 1000 ppm) of trimethyl- and triphenyl phosphites were required to achieve good (approximately 90%) reduction in coke formation. Further, when the additive was discontinued, the rate of coke formation increased and approached the rate measured when no additive was present.
U.S. Pat. No. 4,116,812 reports the use of organo-sulfur compounds to inhibit fouling at elevated temperatures in pyrolysis furnaces used to produce ethylene. U.S. Pat. Nos. 3,531,394; 4,024,050; 4,024,05; 4,105,540; 4,542,253; 4,835,332; 5,354,450; and 5,360,531 report the use of various phosphorous-compounds for coke suppression. U.S. Pat. No. 3,531,394 reports the use of bismuth-containing compounds for coke suppression. Tong and Poindexter U.S. Pat. No. 5,954,943 report that a mixture of sulfur and phosphorous compounds having a sulfur to phosphorous atomic ratio of at least 5:1 can be used to reduce coke formation. The mixture of compounds is used to pretreat the surfaces of a pyrolys

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