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 300xc2x0 C. Deposits that form i n the shape of long filaments approximately 1 xcexcm 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 450xc2x0 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 300xc2x0 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 650xc2x0 C. (1200xc2x0 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) xe2x80x9cTriphenyl Phosphite as a Coke Inhibitor During Naphtha Pyrolysisxe2x80x9d Ind. Eng. Chem. Res. 28, 1293-1299 and Reyniers, G. C., Froment, G. F., Kopinke, F. D., and Zimmerman, G. (1994). xe2x80x9cCoke Formation in the Thermal Cracking of Hydrocarbons. 4. Modeling of Coke Formation in Naphtha Crackingxe2x80x9d 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). xe2x80x9cInitiators for Endothermic Fuels,xe2x80x9d 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), xe2x80x9cEngineering Scale Titanium Endothermic Fuel Reactor Demonstration for a Hypersonic Scramjet Engine,xe2x80x9d 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) xe2x80x9cSurface Phenomena During Pyrolysis,xe2x80x9d 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). xe2x80x9cNature of High Temperature Deposits from n-Alkanes in Flow Reactor Tubesxe2x80x9d, 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). xe2x80x9cSodium Silicate as a Coke Inhibitor During Naphtha Pyrolysisxe2x80x9d, 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 1000xc2x0 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 xe2x80x9cadding or supplyingxe2x80x9d metalloids including boron, arsenic, antimony, bismuth, phosphorous, selenium and silicon or compounds thereof xe2x80x9cto the metallic (and non-metallic, if any) materialsxe2x80x9d which come into contact with xe2x80x9chydrocarbons at high temperaturexe2x80x9d to diminish or prevent coke and soot formation. The patent indicates that metal surfaces can be coated or treated with the substances or that xe2x80x9csmall quantities of the hydrogen compoundsxe2x80x9d 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 xe2x80x9chydrides of siliconxe2x80x9d 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 700xc2x0 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 pyrolysis furnace for up to 20 hrs prior to introduction of hydrocarbon feed to generate a passivation layer. U.S. Pat. No. 4,551,227 reports the use of tin compounds, antimony compounds or both in combination with phosphorous compounds for suppression of coke formation.
Various patents report the use of chromium, tin and antimony (U.S. Pat. No. 4,863,892), combinations of tin and silicon, antimony and silicon, or tin, antimony and silicon (U.S. Pat. No. 4,692,234), and combinations of aluminum and antimony or aluminum, antimony and tin (U.S. Pat. No. 4,686,201) as effective antifouling agents in thermal cracking processes. In all cases, a test coupon consisting of Inconel 800 was immersed in solutions containing the specific metals cited and then heated in air to convert the metal to its oxide form. The coupon was exposed to a coking environment and then heated in steam, converting the coke layer to CO, which was measured by gas chromatography. Although the binary combination of additives suppressed CO formation in some cases, subsequent cycles showed increased coke formation.
U.S. Pat. Nos. 4,555,326; 4,729,064; and 4,680,421 report the use variously of boron, boron oxides, metal borides or ammonium borate to suppress coke formation in pyrolysis furnaces. U.S. Pat. Nos. 5,093,032, 5,128,023 and 5,330,970 report the use of a combination of boron compounds and a dihydroxybenzene compound for inhibiting coke formation. Coke reduction levels of up to 86% (measured in mg coked formed compared to controls) were reported when combination of ammonium biborate and hydroquinone (250 ppm/150 ppm) was added in coker feedstock in a cracking furnace.
U. S. Pat. Nos. 2,698,512; 2,959,915; and 3,173,247 relate to thermal degradation of hydrocarbon fuels at high temperatures to form gum and coke deposits. These patents report fuel compositions more stable to decomposition at high temperature that give lower levels of deposits. U.S. Pat. No. 5,923,944 reports surface treatment, including removing surface irregularities and deposition of a coating consisting essentially of a metal oxide (e.g., Ta2O5 or SiO2) and the vapors of an organometallic compound, to avoid deposition of thermal decomposition products from hydrocarbon fuels.
While considerable efforts have been made toward identifying methods and additives for reducing coke formation from hydrocarbons under pyrolysis conditions, there is still a significant need in the art for reliable additives which provide high levels of coke suppression (about 90% or more) at low additive concentrations (less than about 100 ppm) and which are particularly effective for suppression of filamentous coke formation.
This invention relates to the reduction or prevention of coke formation and deposition on metal surfaces during hydrocarbon processing at high temperature. More specifically, the invention provides selenium additives and methods of using such additives for the reduction of filamentous carbon formation in furnaces or reactors for processing hydrocarbons or in engines that employ hydrocarbon fuels. The invention is particularly useful for reducing filamentous coke formation on iron and/or nickle-containing metal and/or alloy surfaces. The invention also relates to a method of pretreating metal surfaces to inhibit or prevent filamentous coke formation by contacting an appropriately heated metal surface with an additive of this invention. The invention is based on the identification of selenium additives, including organoselenium compounds, that inhibit or prevent coke formation and particularly inhibit filamentous coke formation. The organoselenium additives are believed to inhibit or prevent metal carbide formation, such carbides are intermediates in the formation of coke, particularly filamentous coke. The invention specifically relates to organoselenium additives that prevent or inhibit the formation of nickel and iron carbides which can be formed on furnaces, reactors and/or engine surfaces having steel parts (including, for example, carbon steel and stainless steel). Additives of this invention are believed to react with metal components in furnace, reactor or engine walls or parts thereof to generate metal compounds that are sufficiently stable that they do not undergo further reaction with fuel to form metal carbides.
Preferred selenium compounds of this invention are organoselenium compounds including without limitation, organoselenides (Rxe2x80x94Sexe2x80x94Rxe2x80x2), organodiselenides (Rxe2x80x94Sexe2x80x94Sexe2x80x94Rxe2x80x2), and organoselenols (Rxe2x80x94Sexe2x80x94H), where R and Rxe2x80x2, may be the same or different, and are selected from aliphatic or aryl groups which may contain one or more heteroatoms.
The invention also provides improved hydrocarbon feedstock and hydrocarbons fuels which contain from about 0.01 ppm selenium to about 1000 ppm selenium as an organoselenium additive that is an inhibitor of filamentous coke formation. Compositions of this invention can comprise less than or equal to about 100 ppm selenium. More preferred feedstock or fuel compositions comprise levels of organoselenium inhibitors ranging from about 1 ppm to about 50 ppm. The improved feedstock and fuel compositions exhibit improvements in reduction or prevention of coke formation and particularly in reduction of filamentous coke formation. Feedstock and fuel compositions may include additional additives that are known to affect coke formation, and particularly any additional additives that reduce and/or inhibit non-filamentous coke formation.
The method and additives of this invention can be applied in any hydrocarbon processing system or engine where coke formation, particularly filamentous coke formation, occurs. Filamentous coke formation can be a significant problem for hydrocarbon processing under pyrolysis conditions (e.g. at high temperatures of 300xc2x0 C. or more in the substantial absence of oxygen, i.e. at most about 0.1 atm. partial pressure of oxygen). The method and additives are useful in systems that are operated at ambient pressures or at pressures above ambient. The methods and additives are particularly well suited for use in pyrolysis furnaces (steam crackers or ethylene furnaces) used for hydrocarbon cracking, e.g., for the production of ethylene, and in engines or propulsion systems in which fuel can reach temperatures of 300xc2x0 C. or more, e.g., in hypersonic aircraft engines. The method and additives of this invention are useful when the metal surface is about 650xc2x0 C. as well as when the temperature of the hydrocarbon is about 800xc2x0 C or more.
The additives described herein represent a significant improvement over the prior art. The additive is more effective for inhibition of filamentous coke formation at lower concentrations than other additives described previously. In addition, discontinuing additive injection, does not lead to increases in carbon deposition.