Energy & Fuels 2007, 21, 927-940
927
Delayed Coker Coke Morphology Fundamentals: Mechanistic Implications Based on XPS Analysis of the Composition of Vanadium- and Nickel-Containing Additives during Coke Formation S. R. Kelemen,* M. Siskin,* M. L. Gorbaty, D. T. Ferrughelli, P. J. Kwiatek, L. D. Brown, C. P. Eppig, and R. J. Kennedy ExxonMobil Research and Engineering Company, Corporate Strategic Research Laboratories, 1545 Route 22 East, Clinton Township, Annandale, New Jersey 08801 ReceiVed October 4, 2006. ReVised Manuscript ReceiVed January 25, 2007
The present study was carried out using the microcarbon residue (MCR) test to investigate the mechanism by which vanadium- and nickel-containing additives cause a dramatic reduction in mosaic size in coke from delayed coker feeds. Since a fine mosaic microtexture is one of the key characteristics of shot coke, these additives have the potential to steer the morphology of the coke produced in a delayed coker drum to freeflowing shot coke. Midcontinent U.S. (MCUS) vacuum resid was selected because of its low metals content and its tendency to produce exclusively sponge coke in delayed coking. This allows us to easily observe changes in its shot-coke-forming tendency by monitoring the reduction in the microscopic domain size of the MCR coke using polarized light optical microscopy. The concentrations and chemical states of vanadium porphyrin, acetylacetonate and naphthenate, and nickel porphyrin were quantified using X-ray photoelectron spectroscopy before and after coking. The surface concentration is depressed for vanadium and nickel porphyrins added from 1000 to 10 000 atomic parts per million carbon atoms to MCUS vacuum resids. This observation is consistent with the behavior of native porphyrins in resids. The surface concentration of native vanadium and nickel porphyrins is depressed relative to the bulk in both petroleum residua and asphaltenes to the same degree as in feeds with additives. Following the coking of MCUS with vanadium and nickel porphyrin additives in the MCR test, the surface concentration gets closer to the bulk average. No evidence was found for the decomposition of any of the added vanadium and nickel porphyrins following coking. For resids, the surface concentration of vanadium acetylacetonate was severely depressed relative to the bulk; however, the surface concentration of vanadium naphthenate was enhanced relative to the bulk. Both vanadium acetylacetonate and vanadium naphthenate transformed into V2O5 following coking, and the surface concentration of vanadium was comparable to the predicted bulk average. All of the above-mentioned vanadium and nickel additives produced a coke having a fine mosaic domain structure characteristic of shot coke. The observation of similar coke morphology for soluble vanadium additives with very different chemical end products in the coke argues that additives have similar effects on the major hydrocarbon decomposition pathways. The disparity of vanadium and nickel concentrations at the surface is a new observation that confirms that petroleum resid is inhomogeneous at the microscopic level and asphaltenes self-associate via a mechanism involving secondary bonding interactions.
I. Introduction It is important to understand the factors that control the morphology of the coke produced in the delayed coking processing of petroleum resid. The ability to avoid the production of “transition coke” (mixture of shot and sponge coke) is motivated by safety concerns during coke cutting. Additionally, the ability to reliably produce free-flowing shot coke could lead to a delayed coking process with a reduced cycle time associated with not having to cut the coke before emptying the coke drum. Considerable recent work has been conducted to identify the factors that control coke morphology. It is known that both physical processing conditions and the chemical composition of the coker feed influence the tendency to produce either sponge or shot coke.1-3 Previous work has found that nitrogen, sulfur, and metal contents of the coker feed are important parameters * Authors to whom correspondence should be addressed. Tel.: (908) 730-2430. Fax: (908) 730-3323. E-mail:
[email protected] (M.S.). Tel.: (908) 730-2389. Fax: (908) 730-3323. E-mail:
[email protected]. (S.R.K.). (1) Marsh, H.; Calvert, C.; Bacha, J. J. Mater. Sci. 1985, 20, 289-302.
related to delayed coke morphology.4-7 High nitrogen and sulfur content has been related to higher solubility-parameter polynuclear aromatic (PNA) cores in the feed asphaltenes, which increase the likelihood of their phase separation from the cracked aliphatic-rich products produced during coking.4 The further coking of the PNA cores in this segregated environment is believed to result in shot coke formation.4 Following up on this, oxidation of the feed prior to coking raises both the amount and the oxygen content of asphaltenes, further favoring higher (2) Eser, S.; Jenkins, R. G.; Malladi, M.; Derbyshire, F. J. Carbon 1986, 24, 77-82. (3) Derbyshire, F. J.; Karsner, G. G.; Eser, S. Carbon 1987, 25, 54-55. (4) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227-1234. (5) (A) Siskin, M.; Ferrughelli, D. T.; Gorbaty, M. L.; Kelemen, S. R.; Brown, L. D. U.S. Patent Appl. 2003102250, June 6, 2005. (B) Siskin, M.; Kelemen, S. R.; Gorbaty, M. L.; Ferrughelli, D. T.; Brown, L. D.; Eppig, C. P.; Kennedy, R. J. Energy Fuels 2006, 20, 2117-2124. (6) Siskin, M.; Eppig, C. P.; Gorbaty, M. L.; Brown, L. D.; Kelemen, S. R.; Ferrughelli, D. T.; Bernatz, F. A. W02004104139, December 12, 2004. (7) Eppig, C. P.; Siskin, M.; Bernatz, F. A.; Mart, C. A. W02005113708, December 1, 2005.
10.1021/ef060493e CCC: $37.00 © 2007 American Chemical Society Published on Web 03/02/2007
928 Energy & Fuels, Vol. 21, No. 2, 2007
Kelemen et al.
Table 1. List of Additives additive
wt % metal
vanadyl acetylacetonate vanadyl naphthenate V(IV) mesotetraphenylporphine Ni mesotetraphenylporphine
19.2 3.0 7.5 8.7
Table 2. Surface and Bulk Nickel and Vanadium Concentrations for Selected Resids nickel (wppm)
Bachaquero Maya Cold Lake Cerro Negroa MCUS a
vanadium (wppm)
bulk (ICPES)
surface (XPS)
bulk (ICPES)
surface (XPS)
110 141 267 131 37
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