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Energy & Fuels 1994,8, 426434
Characterization of Coke on Spent Hydroprocessing Catalysts by Optical Microscopy Vincente A. Munoz,t Suhas V. Ghorpadkar,*and Murray R. Gray'J Department of Chemical Engineering, University of Alberta, Edmonton, AB T6G 2G6, Canada, and Western Research Centre, CANMET, P.O. Bag 1280, Devon, AB TOC IEO, Canada Received November 3, 1993. Revised Manuscript Received December 21, 1 9 9 9
Polished cross sections of a series of spent catalysts used in processing of Athabasca bitumen and other feeds were examined by optical microscopy. Some of the samples contained distinct domains of high reflectance, indicating the presenceof coke of higher aromaticity than the surrounding matrix. These domains had a mean diameter of ca. 26 pm. Fluorescence due to adsorbed feed components and anisotropy due to mesophase formation were found in some samples but not in all. The observations on the series of samples were consistent with the adsorption of basic compounds on acidic domains in the ?-alumina, followed by coking of these domains. In samples of catalysts used to process residual feed, the background matrix eventually developed aromatic coke deposits, and the heterogeneity was no longer visible.
Introduction Alumina-based hydroprocessing catalysts tend to accumulate coke while in service, where coke is defined as a carbon-rich organic deposit originating from the hydrocarbon feed. The amount of coke tends to increase with heavier feeds, e.g. residues, and with the operating temperature.' Coke tends to accumulate during the first few hours of service, and then stay relatively constant, as measured by carbon content of the catalyst. The coke seems to accumulate on the alumina support, and not on the Ni/Mo or Co/Mo crystallites.2 The reduction in catalyst activity, therefore, is likely due to the cokeblocking the edges of the metal crystallites. At high coke levels, pore blockage would also reduce catalyst activity. Coke deposition on Ni/Mo and Co/Mo y-alumina catalysts is usually attributed to the acidity of the alumina s u ~ p o r t . This ~ interpretation is supported by many observations,includingthe effect of Lewisbases in reducing coke levels4and the high levels of organic nitrogen (from organic bases) deposited in ~ o k e .Coke ~ ~ ~is usually assumed to deposit uniformly in the catalyst pellets when treating distillates. With residual feeds, interactions between diffusion and reaction can give concentration gradients of carbon deposited within catalyst pellets.6 These gradients change with time and position within the reactor, so that coke in the periphery of the pellet can be either higher or lower than in the interior. Interpretation of these profiles is complicated by the development of gradients in the concentration of deposited nickel and vanadium. Coke deposits in catalysts tend to be at least as aromatic as the feed oil and may be almost entirely aromatic when * Address correepondence to this author.
CANMET. University of Alberta. Abstract published in Aduance ACS Abstracts, February 1, 1994. (1) Thnkur, D.S.;Tho-, M. G. Appl. Catal. 1986,16,197. (2) Diez, F.;Gatm, B. C.; Miller, J. T.; Sajkowski, D. J.; Kukea, S. G. Ind. Eng. Chem. Res. 1990,29,1999. (3) Furimky, E.AIChE J. 1979,26,306. (4) Masuyama, T.; Kageyama, Y.; Kawai, S. Fuel 19S0,60, 246. (6) Choi, J. H. K.; Gray, M. R.Ind. Eng. Chem. Res. 1988,27, 1687. (6) T m , P. W.; Harnsberger,H. F.; Bridge, A. G. Ind. Eng. Chem. Process Des. Dev. 1981,20, 262. t
t
,
the operating temperature is high or the hydrogen partial pressure is OW.^*^ Bulk petroleum and coal-derivedcokes show variations in optical reflectance depending on aromaticity; reflectance increases with the fraction of aromatic carbon in a coke ample.^^^ Under appropriate conditions,graphitized carbon materialscan undergophase separation to form mesophase, a nematic liquid crystal consisting of layers of planar aromatic molecules.1°-12This mesophase is visible under polarized light. The aromatic compounds in hydroprocessing feeds often exhibit fluorescence. As the moleculesundergo condensationreactions to form coke or mesophase, this fluorescence is gradually 10st.'~J~ The objective of this study was to examine the coke deposits in hydroprocessing catalysts using optical microscopy. Spent catalyststhat had been exposed to a range of feeds, from naphtha through to residue, were examined by reflectance microscopy. Examination by fluorescence microscopy and under polarized light gave additional information on the characteristics of the coke deposits.
Materials and Methods A series of spent commercial catalysts from industrial,pilotscale, and laboratory reactors were examined. The composition and service histories of each catalyst are listed in Tables 1 and 2. The metal contents and properties were not analyzed. From the manufacturer's data sheets, the y-alumina Catalysts were typical examples of commercial materials,with 12-20 wt 9% Mo and 2-4 % Ni or Co promoter. Pore volumes were all in the range 0.4-0.8 mL/g, and surface areas were in the range 160-300 m2/g. ~
~~~~
(7) Egiebor, N. 0.; Gray, M. R.; Cyr, N. Appl. Catal. 1989,66, 81. (8)Stach, E.; Mackomky, M. T.; Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmuller, D. Coal Petrology, 3rd ed.; Gebruder Borntraeger, Berlin, 1982; p 319. (9) Weinberg, V. A,; Yen, T. F. Fuel 1982,61, 383. (10) Brooks, J. D.; Taylor, G. H. Carbon 1966,9,186. (11) Brooks,J. D.;Taylor, G. H. In Chemistry ana' Physics of Carbon; Walker, P. L., Ed.;Marcel Dekker, New York, I=, Vol. 4, p 243. (12) Dubob, J.; Agache, C.; White, J. L. Metallography 1970,9,337. (13) White, C. E.;Argauer, R. J. Fluorescence Analysis: A Practical Approach; Marcel Dekker: New York, 1970. (14) Muqoz, V. A.; Lam, W. W.; Payetta, C.; Mikula, R. J. Proc. 49th Electron Mgcroscopy SOC. Am. 25th Microbeam Aml. Soc. Conf., San Jose, CA 1991.
0 1994 American Chemical Society
Characterization of Coke on Spent Catalysts
Energy & Fuels, Vol. 8, No. 2, 1994 427
Table 1. Origin of Catalyrt Samples catalyst feed type” time in service, mo Naphtha A coker naphtha Ni/Mo 12-14 Naphtha B coker naphtha Ni/Mo 12-14 Gas oil A coker gas oil Ni/Mo 1 Gas oil B coker gas oil Ni/Mo 12 Gas oil C coker gas oil + gas oil (23 ratio) Ni/Mo 24 Athabasca bitumen Co/Mo 2.5 wk Bitumen A Bitumen B Athabasca bitumen Co/Mo 3 wk MD-A coker middle dietillate Ni/Mo 18 unwed Ni/Mo 0 CSTR 1 CSTR 2 Athabasca bitumen Ni/Mo 2h CSTR 3 Athabasca bitumen Ni/Mo 15.5 h a All catalysts were supported on y-alumina, and supplied as extruded pellets. * Not available. Table 2. Carbon and Ash Content of Selected Catalysts catalyst C,wt % ash,* % Naphtha A 8.2 77.6 Naphtha B 12.6 73.6 Gas oil A 16.3 63.3 Gas oil B 20.3 66.1 Gas oil C 6.3 9.6 CSTR 2 9.4 nd CSTR 3 12.8 nd Bitumen A 15.0 nd Bitumen B 10.7 12.7 MD-A 5.7 4.7 Microscopy of Catalyst Samples. For initial examination of the interior of catalyst materials, the extrudates were embedded in epoxy resin. The resin mold was f i i t polished on a duo belt wet surfacer to remove any plasticine and other impurities on the surface. This mold was then consecutivelypolished on silicon carbide 240 grit, 320 grit, 400grit, and 600grit polishing surfaces with cold water as the medium. Final polishing used a 0.3-cm alumina solution (0.3-wm particle size y-polishing alumina suspended in distilledwater) and then a 0.05pm alumhasolution. Samples for quantitative reflectance measurements and for fluorescence microscopy were prepared by affixing extrudates horizontally to a thin layer of epoxy, to avoid contaminating the samplewith resin. The pellets were then ground to give a polished cross section. Reflectance microscopy was carried out using a Carl Zeiss research microscopephotometer with an incident light system. A halogen lamp with an average brightness of 1750 cd/cm2 provided the white light. Reflectance measurements were done under bright field illumination (polarizer out) polarized light with 600X magnification at 545 nm using an oil immersion objective. The photometric system of the microscope was calibrated using a glass standard (1.68% reflectance value) and it was recalibrated every 30 min. The procedure for calibration was similar to the one recommended for the microscopical determination of the reflectance of c0al.6~~’’The reflectance values were obtained from clearly distinguishable surface features by averaging at least 50 measurements. Fluorescence was observed using a high-pressure mercury lamp with an average brightness of 170 OOO cd/cm2. The selection of the wavelength of the incident beam was accomplished with a combination of filters which provided a range from 450 to 490 nm (blue light). The separation of the emitted fluorescence from the incident light was achieved with a 515-nm barrier filter. Photographs of the polished catalyst cross sections for image analysis were taken wing a Carl Zeiss Ultraphot I11 optical microscope with a tungsten light of average brightness 1200 cd/ m* using bright-field illumination with nonpolarized light at a magnification of 200X. (15) Davis, A. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic: New York, 1978; Vol. 1,p 27. (16) Piller, H. Microecope Photometry; Springer-Verlag: New York, 1977. (17) American Society for Testing and Materials (ASTM),D 2798-88 ‘Microscopic determination of the reflectanceof the organic components in a polished specimen of coal”, ASTM, Philadelphia, 1991.
temp, O C
press., MPa
280-330 330-400 36o-400 360-400
5.5-6.9 5.5-6.9 9.6-11 9.6-11
nab
na
430 410-430
11.5-14 11.614
na
na
400
13.9 13.9
400
Scanning electron micrographs and energy-dispersive X-ray analysis was otained using a Hitachi S-2700 scanning electron microscope with a Link eXL energy-dispersive X-ray analyser. The accelerating voltage was kept at 20 kV. The magnification and the resolution used were changed depending on the size of the coke deposita and type of specimen. The catalyst samples were prepared for observation by coating with carbon. Copper foil was used to provide a registration mark for the field of view in optical and electron microscopy.
Rssults Microscopy of Distillate Catalyst Samples. Five of the catalysts used to hydroprocess distillates showed dramatic heterogeneity when polished cross sections were examinedby reflectance microscopy: Naphtha A, Naphtha B, Gas oil A, Gas oil C, and MD-A. Figure 1 shows a typical view of a Naphtha A catalyst pellet at 300X magnification under bright field illumination using an oil immersion objective. Carbonaceous deposits are indicated as bright domains (B)which are distributed in a dark matrix (D)formed by deposited organics of low reflectance. Figure 2, showing a portion of Figure 1 at 19OOX magnification, reveals more details of the morphology of bright areas which clearly show inclusions of dark areas within the bright domains. The reflectance of the deposit marked (B)was 1.61% compared to 0.75% for the surrounding dark matrix. Mean random reflectance values for distillate Catalysts increased with increasing carbon content, from 0.52 % for MD A to 0.64% for Naphtha A and 0.87% for Naphtha B. Figures 3 and 4 show the distribution of reflectance values measured in Naphtha A and Naphtha B. These data include measurements of both the dark and bright areas found in the catalyst pellets and are averages from a number of pellets. The bright and dark areas of the Naphtha A catalyst had mean random reflectance values of 0.86 and 0.46%, respectively, while the corresponding features in the Naphtha B catalyst had values of 1.18 and 0.55 % , respectively. These results indicated that the carbonaceous deposits in the Naphtha A catalyst were less aromatic than those found in the Naphtha B catalyst, in agreement with l3C NMR measurements on these catalysts which average over the entire pellet volume.’ When the field of view in Figure 1was examined under cross-polarized light, many of the high-reflectance areas exhibited anisotropy, indicating the formation of distinct mesophase domainswithin the catalyst extrudate. Under blue light (440-450 nm) the Naphtha A catalyst exhibited yellow fluorescence from the dark matrix, shown in another field of view in Figure 5. Samples Naphtha B and MD-A did not exhibit fluorescence. The bright domains in these catalysts were also observed in scanning electron microscopy. The domains appeared
428 Energy & fiels, Vol. 8, No.2,1994
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