Ind. Eng. Chem. Res. 2005, 44, 2069-2077
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Multitechnique Characterization of Coke Produced during Commercial Resid FCC Operation Henrique S. Cerqueira,*,† Carsten Sievers,‡ Guy Joly,§ Patrick Magnoux,§ and Johannes A. Lercher‡ Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento do Abastecimento, Tecnologia em FCC. Ilha do Funda˜ o, Av. Jequitiba´ 950, Rio de Janeiro, 21941-598 RJ, Brazil, Institut fu¨ r Technische Chemie, Technische Universita¨ t Mu¨ nchen. Lichtenbergstrasse 4, D-85748 Garching, Germany, and Universite´ de Poitiers, UMR CNRS 6503. 40, Avenue du Recteur Pineau, 86022 Poitiers cedex, France.
The characterization of industrial coked resid fluid catalytic cracking (RFCC) catalysts is reported. The aim is to provide insight into the coke deposition on commercial resid fluid catalytic cracking catalysts sampled after the stripper of a commercial RFCC unit and to relate it to the potential process chemistry. Physicochemical techniques were used to characterize the used catalysts and the deposited coke. 95% of the coke was insoluble in CH2Cl2. This coke was located in the mesopores of the catalyst matrix. The results suggest the existence of domains of polyaromatic and heterogeneously distributed coke, in which saturated hydrocarbons are trapped. IR spectroscopy of adsorbed pyridine shows that the largest fraction of strong Brønsted acid sites is free after the catalyst has passed the stripper. The results indicate that for RFCC not the local deactivation of the acid sites but rather blocking of domains of the catalyst is the most important mode of deactivation after passing through the riser reactor. Introduction Coke formation is the main cause of temporary catalyst deactivation during the transformation of organic compounds. In the refining process, coke can be defined as the byproducts that are retained inside the catalyst particles after the reaction. The retention of these molecules is due to their low volatility, trapping, and adsorption on acid sites.1 Depending on the operating conditions, the coke molecules may react further even in the absence of other reactants.2 The deactivation of zeolite-based catalysts by coke is caused by poisoning of acid sites or pore blockage.3 In the former case, one coke molecule blocks one active site affecting the activity linearly, but mainly the reaction selectivity.4-8 The deactivating effect is much more pronounced in the case of pore blockage, with one coke molecule blocking the access of reactants to more than one active site.9-13 Prior to pore blockage, coke molecules can be heterogeneously distributed over the zeolite crystallites, being more concentrated at the external surface (pore mouth). If this is the case, the effective pore diameter is reduced, and the diffusion resistance of reactants in the crystallites is enhanced.14-16 For higher coke contents, the heavier coke molecules can accumulate on the outer surface and locally block the access of reactant molecules to the pore openings. In the conventional fluid catalytic cracking (FCC) process with zeolite Y-based catalysts, a significant portion of the feedstock is converted into coke. Those conventional FCC units operate in a heat balance. The * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +55 21 3865-6626. Tel.: +55 21 3865-6635. † Petrobras, Cenpes. ‡ Institut fu¨r Technische Chemie. § Universite´ de Poitiers.
heat produced by the combustion of coke is used in various ways: (i) to heat the feed to the reaction temperature, (ii) to provide energy to the endothermic cracking reactions, (iii) to heat the combustion air and (iv) the coke on the spent catalyst to the regenerator temperature, (v) to supply the heat lost from the reactor/ regenerator, and (vi) to heat the steam to exit temperature.17 Because of the new discoveries of heavy oil deposits, many FCC units are beginning to process feedstocks with a higher tendency to form coke. This is particularly true for resid FCC units, which are designed to convert 100% residue from the atmospheric distillation tower. To maximize the profitability of those resid FCC units, part of the heat produced through the combustion of coke is recovered by means of catalyst coolers that control the regenerator dense-phase temperature and produce steam. Although the understanding of catalyst deactivation by coke has seen substantial progress,18-23 detailed characterization of the deposits is rare and even moreso for commercially operated (FCC) catalyst samples. The chemical identity of such coke components can be determined through various spectroscopic techniques such as electron paramagnetic resonance (EPR),24 magic angle spinning 13C nuclear magnetic resonance (13C MAS-NMR),21,23-27 UV,28,29 and IR spectroscopy,1,28 Xray diffraction (XRD),30 and Raman spectroscopy.31 Most of those techniques are carried out under static conditions, but the deposition of carbonaceous materials can also be followed by in situ IR spectroscopy followed by on-line gas chromatography (GC).32-36 Complementary information about coke location can be obtained by 129Xe NMR37,38 and X-ray photoelectron spectroscopy.39,40 However, in most instances, these approaches require total separation of coke from the catalyst, involving the destruction of the catalyst by HF followed by the dissolution of the soluble coke compounds in an organic
10.1021/ie048963k CCC: $30.25 © 2005 American Chemical Society Published on Web 02/23/2005
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solvent.41,42 The heavy fraction corresponding to the insoluble coke is often only characterized by the average H/C ratio. As a consequence, little information on structure-property relationships exists despite the great number of physicochemical characterization tools applied. Contrary to many previous studies with model compounds that have studied the soluble coke composition in detail, in the present paper, a multitechnique approach was used in order to obtain information about the chemical identity and composition of insoluble coke compounds as well as their location on commercial resid FCC catalyst samples. Experimental Section Spent Catalyst and Feedstock Specification. The fresh catalyst used in this unit consists of a blend of fresh catalyst, specially developed for resid applications, and a low-metal “flushing” equilibrium catalyst. Details about the fresh catalyst formulation49 and the flushing50 can be found elsewhere. Two spent catalyst samples were collected from a commercial resid FCC unit after the stripper. The main difference between the two samples was the excess of oxygen in the resid FCC unit regenerator: 1.5 and 0.13 mol % for samples A and B, respectively. The feedstock processed was a residue from the crude distillation tower with 16.3 °API, 4800 ppm of nitrogen (1800 ppm of basic nitrogen), 0.65 wt % of sulfur, 19 ppm of vanadium, 11 ppm of Ni, and a Conradson carbon residue of 8.2 wt %. The feedstock consists of 1.5 wt % of compounds insoluble in nheptane, 36 wt % of saturated compounds, and 10, 11, 4, and 2 wt % of mono-, di-, tri-, and polyaromatics, respectively. Physicochemical Characterization of Spent Catalyst Samples. The spent catalysts were characterized with various physicochemical techniques. The carbon content was measured by total combustion in air at 850 °C in a LECO 244 unit. The average hydrogen to carbon (H/C) ratio was determined on the basis of the carbon content of the coke and the amount of water produced during the combustion of 12 g of spent catalyst sample in a fixed-bed microactivity test unit. The bottom of the reactor was filled with inert silica in order to retain stripped coke molecules that might be eventually stripped from the catalysts, allowing a more reliable estimate of the H/C ratio. The samples were pretreated under nitrogen flow (30 mL min-1) for 1 h at temperatures between 100 and 250 °C, followed by a plateau of 30 min at 250 °C. The combustion was performed with pure oxygen (30 mL min-1) at 580 °C for 4 h. The humidity of oxygen was taken into account. IR analysis of the spent catalysts was performed with thin wafers (10-15 mg) of previously sieved (
