Energy & Fuels 1999, 13, 1037-1045
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Coking of Hydroprocessing Catalyst by Residue Fractions of Bitumen Murray R. Gray,*,† Yingxian Zhao,† Craig M. McKnight,‡ David A. Komar,§ and J. Donald Carruthers§ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada, Syncrude Canada Ltd., 9421-17 Avenue, Edmonton, Alberta T6N 1H4, Canada, and Cytec Industries Inc., Research and Development, 1937 West Main Street, P.O. Box 60, Stamford, Connecticut 06904-0060 Received January 19, 1999
The deposition of organic material, or coke, on hydroprocessing catalyst was studied using Athabasca bitumen vacuum residue (ABVB) and narrow fractions of ABVB, prepared by supercritical fluid extraction (SCFE) with n-pentane. The feed materials were diluted in a lowsulfur gas oil and hydroprocessed over a commercial NiMo/γ-Al2O3 catalyst in a 1 L continuousstirred tank reactor at 440 °C. The coked catalysts were Soxhlet extracted with methylene chloride; then, carbon content, surface area, pore volume, and pore size were measured. Hydrodesulfurization activity was then measured using bitumen and dibenzothiophene as reactants.The SCFE fractions that contained only saturates, aromatics, and resins gave a low yield of carbon on the catalyst ( 524 °C) were prepared by supercritical fluid extraction (SCFE) with n-pentane,6 and seven of these fractions were diluted in a low-sulfur gas oil. The resulting blends were reacted in a 1 L continuous-flow stirred tank reactor (CSTR) at 440 °C and 10.3 MPa hydrogen pressure over a commercial NiMo/γAl2O3 catalyst. To determine the effects of feed concentration on coking of catalysts, whole Athabasca vacuum residue (524 °C+) and the end-fraction of this residue (with a molecular weight of 4185) were diluted with gas oil at concentrations of 15, 30, and 50 wt %. These diluted feeds were then reacted at the same conditions. The spent catalysts were extracted with methylene chloride and analyzed for carbon content, then surface areas and pore volumes were determined. The hydrodesulfurization activity of each sample was determined by using both Athabasca bitumen and dibenzothiophene (DBT) as probes. (4) Richardson, S. M.; Nagaishi, H.; Gray, M. R. Ind. Eng. Chem. Res. 1996, 35, 3940-3950. (5) Chung, K. H.; Xu, C.; Gray, M. R.; Zhao, Y.; Kotlyar, L.; Sparks, B. Rev. Process Chem. Eng. 1998, 1, 41-79. (6) Chung, K. H.; Xu, C. M.; Hu, Y. X.; Wang, R. N. Oil Gas J. 1997, 95 (1), 66-69.
6.5 825 6.3 6160 36 71 166 18.2 0
4.4 948 5.7 6800 40 90 221 21.5 0
8.0 1240 6.1 7560 40 138 347 26.3 0
40.4 4185 6.5 10500 49 339 877 48.9 88.0
5.2 7040
14.0 24.7 524+
gas oil solvent
0 25 14.9 0 0 0 0 213-516
Experimental Section Materials. The reactants for the hydroprocessing reactions were Athabasca bitumen vacuum bottoms (ABVB; 524° C+ material), supplied by Syncrude Canada, and fractions of ABVB separated by supercritical fluid extraction (SCFE) with n-pentane.6 These residue materials were diluted in a low-sulfur hydrotreated gas oil derived from Athabasca bitumen. The properties of these materials are listed in Table 1. Due to the limited quantity of the heavier extract fractions, SCFE 7, 8, and 9 were blended in proportion to their yield from separation of residue for the reactor experiments. The catalyst was a commercial Ni/Mo on γ-alumina material designed for residue service, supplied as 1 mm cylindrical extrudate. The catalyst contained 2-4% NiO and 10-15% MoO3. Surface area was 150200 m2/g by nitrogen adsorption and the BET equation, pore volume was 0.4-0.8 mL/g and the mean pore diameter was 7-15 nm, as determined by desorption of nitrogen. Catalyst activity was measured with Athabasca bitumen and dibenzothiophene (Aldrich Chemical Co., Milwaukee, WI). All other chemicals were obtained from Fisher Scientific (Mississauga, Ontario). Reactor Systems. The hydroprocessing catalysts were reacted with diluted residue materials in a continuous-flow stirred tank reactor (CSTR). This 1 L capacity reactor is described in detail elsewhere.7 The reaction conditions were as follows: catalyst, 83 g; temperature, 440 °C; H2 pressure, 10.3 MPa (1500 psig); H2 flow rate, 4.75 L/min (STP); and liquid feed rate, 330 mL/h. The catalyst was held in an annular basket, with radial outward flow of gas and liquid through the basket following the Robinson-Mahoney reactor type. Catalyst was sulfided in place by the feed, which provided a significant concentration of hydrogen sulfide. The high rate of heat transfer from the catalyst basket to the liquid prevented any temperature excursion during sulfidation. The reactor was operated for 4 h to reach steady-state liquid product composition, then operated for a further 4 h to collect product samples. The activity of the used catalysts was measured in a micro-batch reactor, fabricated from stainless steel tubing and fittings with a nominal internal volume of 15 mL. After the catalyst and reactant were added, the reactor was tested for leaks by pressurizing with nitrogen. The reactor was then pressurized with hydrogen and purged twice before adding hydrogen to obtain the desired cold pressure. The reactor was immersed in a fluidized sand bath for the desired reaction time. (7) Gray, M. R.; Ayasse, A. R.; Chan, E. W.; Veljkovic, M. Energy Fuels 1995, 9, 500-506.
Coking of Hydroprocessing Catalyst
When Athabasca bitumen was used to measure desulfurization activity, the reaction conditions were as follows: feed (g)/catalyst (g) 6.45, 430 °C, H2 pressure of 10 MPa at reaction temperature, and a reaction time of 1 h. The second set of experiments used pure dibenzothiophene (DBT) as a model compound to measure catalyst activity. A solution of 3.5% DBT in gas oil solution was reacted under the following conditions: feed solution (g)/catalyst (g) 6.45, 350 °C, H2 pressure of 5.9 MPa at reaction temperature, and a reaction time of 1 h. Analytical Techniques. Gas products from CSTR reactions were analyzed by an on-line GC (HewlettPackard 5840A Gas Chromatograph) using a flame ionization detector. Analysis of liquid feed and products was performed at Syncrude Research (Edmonton, AB). The contents of carbon and hydrogen were determined using a Leco analyzer. Sulfur content was determined by combustion followed by fluorescence detection. Nitrogen analysis used combustion followed by chemiluminescent detection. Analysis of microcarbon residue used an Alcor MCR analyzer following the appropriate ASTM methods. Molecular weights of the feed fractions were determined by vapor-pressure osmometry in odicholorobenzene at the University of Petroleum Biejing, and University of Alberta Micro-Analytical Laboratory. The results were consistent, and the former are reported here following Chung et al.5 Elemental analysis of the spent catalysts was performed on a Carlo Erba Stumentazione Elemental Analyzer at the Micro Analytical Laboratory, University of Alberta. Prior to the analysis, the catalyst samples were Soxhlet extracted with methylene chloride for 24 h, and then vacuum-dried at 65°C and 10 kPa for 2 h. To ensure representative sampling, a 100-mg portion of the catalyst was crushed to a powder and vacuum heated at 110 °C for 1.5 h, and a 2-mg sample was used for analysis. Analysis of surface area and porosity for the coked catalysts was done at Cytec Industries, Inc. Nitrogen porosimetry analyses were obtained using a Quantachrome Autosorb Gas Porosimeter. Porosity calculation involved a BJH analysis using the data from the nitrogen adsorption-desorption isotherms. Mercury porosimetry analyses were performed using a Quantachrome Autoscan mercury porosimeter using approximately 1.5 g of catalyst. A mercury contact angle of 140° and a surface tension of 480 erg cm2 (dynes/cm) were used to calculate pore sizes from the mercury intrusionextrusion curves. Surfaces of spent catalysts were examined by scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) analysis at the Syncrude Research Center. Catalyst pellets were encased in a mold in epoxy resin. Typically, five or six pellets could be placed in the same mold, but one with the best outer geometric surface was examined. For each sample, an image of exterior surface, an X-ray spectrum of particle exterior, and a set of linescans along a crossed-axis on the surface were obtained to identify the surface composition of the sample. Concentration of dibenzothiophene (DBT) in reactant and product solutions was determined using a Hewlett-
Energy & Fuels, Vol. 13, No. 5, 1999 1039
Packard 5890 Gas Chromatograph with a DB1 capillary column and a FID detector. The injected samples were eluted at 40 °C followed by a temperature program of 20 °C/min up to 180 °C. After 10 min, the column temperature was raised at 50 °C/min from 180 to 310 °C. Calculation Formulas. Hydrogen consumption under steady-state conditions was defined as
YH2(sL/kg feed) ) [fH2, into(g/h) - fH2, out(g/h)] × 22.414 (sL/mol) 2(g/mol) × ffeed(g/h) ×
1 kg 1000 g
(1)
The conversion of species j (sulfur, nitrogen, and MCR) was calculated as follows:
Xj )
xinto ‚Finto-‚xout j j ‚Fout xinto ‚Finto j
× 100%
(2)
was the mass fraction of species j in the where xinto j the mass fraction in the liquid product, and feed, xout j Finto and Fout were in the inlet and outlet mass flows, respectively. The sulfur removal (XS) in hydroprocessing of the whole bitumen, to measure catalyst activity, was calculated by the following equation:
XS )
xfS‚mj + xcS‚mc-xpS‚mp-xsS‚mS xfS‚mf + xcS‚mc
× 100%
(3)
where m was the mass of feed (f), liquid product (p), initial catalyst (c) or final catalyst + solid (s), and xS was the mass fraction of sulfur. In eq 3, sulfur removal was defined as removal from the liquid and solid to give hydrogen sulfide. The DBT conversion (XDBT) in microbatch reactions for testing catalyst activity was defined as
XDBT )
xfDBT‚mf-xpDBT‚mp xfDBT‚mf
× 100%
(4)
where m was the mass of feed (f) and liquid product (p) and xDBT was the concentration (wt %) of DBT in feed solution (f) and product solution (p), respectively. Results and Discussion Overall Mass Balance and Conversions. The overall mass balance on feeds and products from the experiments was in the range of 97-102%. Liquid yields were in the range of 84-88% of total products, while gas yield was 11-14.5% (including hydrogen). The reactivity of the SCFE fractions has been previously discussed in detail by Chung et al.5 therefore, only a summary of the conversion results are presented here. The data of Table 2 show the conversion of S, N, and MCR. Conversion of residue could not be determined accurately due to the high conversions and the use of a gas oil diluent. Conversion of sulfur was high for all fractions, in the range of 92-96%. Nitrogen conversion was over 85% for SCFE 1 through SCFE 6, then it dropped slightly for SCFE 6+7+8, then decreased significantly to 69% for SCFE 10. Similarly, the conver-
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Table 2. Conversion of Residue Fractions in a CSTR at 440 °C and 10.3 MPaa
feed SCFE 1 SCFE 2 SCFE 4 SCFE 5 SCFE 6 SCFE 7+8+9 SCFE 10 ABVB a
yield of coke on hydrogen conversion residue in feed consumption, S, N, MCR, L/kg residue reactor, catalyst, % % % in feed wt % wt % 94 96 92 96 94 95 92 96
88 90 93 87 85 81 69 88
91 94 96 98 97 99 81 99
60 62 71 78 91 103 159 76
0.45 0.47 0.49 0.51 0.50 0.64 6.47 2.39
0.84 0.83 0.97 1.10 1.03 1.11 2.56 1.18
All feeds 30% residue material, 70% gas oil.
sion of MCR material was high except for SCFE 10, where the catalytic activity was less effective. Under the steady-state reaction conditions, ABVB gave a sulfur removal of ∼95% and a nitrogen removal of ∼85%. Both results were insensitive to ABVB concentration in the range of 15∼50%. An increase in the concentration of SCFE 10 to 50 wt %, however, caused a decline of sulfur removal to 85% and a significant decrease of nitrogen removal to 57%. These results suggest that the catalyst activity was more deactivated by higher concentrations of SCFE 10. The consumption of hydrogen increased for the series SCFE 1 through SCFE 10, consistent with the increase in aromatic carbon content from 26 to 49% and the increase in overall sulfur content. As in the conversion results, SCFE 10 was unusual in its high hydrogen consumption relative to the other narrow fractions. The hydrogen consumption for reaction of ABVB was comparable to the weighted mean of the SCFE fractions. Various reactions take place simultaneously during hydroprocessing of bitumen residues. The cracking reaction is primarily thermal, hydrogenation (HG) and hydrodesulfurization (HDS) are mostly catalytic, while hydrodenitrogenation (HDN) is completely catalytic.9 Investigation with some model compounds has also indicated the presence of two distinctive types of active sites on NiMo/Al2O3 catalysts: one is responsible for HG, and the other is responsible for hydrogenolysis of the heteroatom.10 Removal of heterocyclic sulfur from compounds such a dibenzothiophene can proceed by two catalytic pathssdirect removal of the sulfur from the thiophenic ring, and prior hydrogenation of the adjacent rings followed by sulfur removal. In contrast, hydrogenation of the pyrrolic or pyridinic aromatic rings that contain nitrogen is a necessary prelude to hydrogenolysis of C-N bonds.11 Therefore, it is not surprising that hydroprocessing of residue gave a lower nitrogen removal than sulfur removal at the same conditions, as in Table 2. What was surprising was the independence of sulfur removal on the concentration of sulfur in feeds. This result would be expected if the sulfur removal were actually controlled by thermal cracking, to generate (8) de Jong, K. P. Ind. Eng. Chem. Res. 1994, 33, 821-824. (9) Miki, Y.; Yamadaya, S.; Oba, M.; Sugimoto, Y. J. Catal. 1983, 83, 371-383. (10) Moreau, C.; Geneste, P. in Theoretical Aspects of Heterogeneous Catalysis; Moffat, J. B., Ed.; Van Nostrand Reinhold: New York, 1992; p 256. (11) Marafi, M.; Stanislaus, A. Symp. Removal of Aromatics, Sulfur Olefins Gasoline Diesel, Orlando, FL, August 25-29, 1996; pp 604608.
Figure 1. Yield of coke in the reactor and on the catalyst versus fraction of asphaltenes in the feed. Reaction was at 440 °C and 10.3 MPa hydrogen pressure in a continuous-feed stirred reactor for 8 h. Yield of coke was calculated on the basis of the amount of asphaltene (either diluted in ABVB or SCFE 10).
lighter fractions which could readily diffuse through the catalyst pore. However, it is generally accepted that the sulfur removal in residue hydroprocessing depends on various reaction variables such as temperature, pressure, H2/oil and oil/catalyst ratios, and feed concentration. Since our experiments were conducted under high severity conditions, it is likely that a very high sulfur removal level and the experimental errors concealed the true dependence of sulfur removal on feed concentration. Coke Formation in Reactor and Coke Deposition on Catalyst. The data of Table 2 show the yields of coke in the reactor and on the catalyst. For SCFE 1 through SCFE 7+8+9, total coke yield was 1.3-1.7% of the feed residue fraction. ABVB gave a higher yield of 3.6% total coke, while SCFE 10 gave over 9% total coke. Considering that the front-end fractions (SCFE 1 through SCFE 7+8+9) are asphaltene-free materials, the asphaltenes seem to be the key component causing thermal coke. The yield of coke on the catalyst was in the range 0.8-1.2 wt % except for SCFE 10, therefore, coking on catalyst was much less sensitive to feed properties than the coke deposition in the reactor. Coke on the catalyst is reported as the increase in catalyst mass during the reaction experiment. The data of Figure 1 show the dependence of coke deposition in the reactor and on the catalyst on the concentration of asphaltenes in the feed. At a given concentration of aspahltenes in the feed, both ABVB and SCFE 10 gave equivalent yields of coke in the reactor and on the catalyst. Both feeds produced more coke in the reactor than on the catalyst. As the concentration of the asphaltenes from either feed increased, the yield of coke in the reactor remained in the range 5.5-7%, except for the highest concentration of SCFE (50% in gas oil). The yield of coke on the catalyst decreased with increasing asphaltene concentration. From a kinetic point of view, the coke formation in reactor was consistent with an accumulation process, while the coke deposition on catalyst reached a limiting level with respect to the liquid-phase concentration. Such different behaviors suggest that coke formation in reactor and
Coking of Hydroprocessing Catalyst
Figure 2. Carbon on spent catalyst as a function of concentration of asphaltenes in feed. Reaction at 440 °C and 10.3 MPa hydrogen pressure in a continuous-feed stirred reactor for 8 h.
coke deposition on catalysts involves different mechanisms. de John8 reported that coke in heavy oil processing might be formed by two parallel routes: thermal coking involving aromatic radicals and catalytic coking involving dehydrogenation. The catalytic coke likely tends to cover the catalyst surface so that the maximum of the coke formed on the catalyst surface is equal to monolayer coverage, as suggested by Richardson et al.4 Composition of Spent Catalyst. The carbon content of spent catalysts has been plotted versus the asphaltene concentration in the ABVB and SCFE 10 feeds in Figure 2. A blank experiment was also conducted with the gas oil diluent, but the lack of sulfur in the feed gave a catalyst with completely different properties than the rest of the samples, therefore, the coke content cannot be compared. When the gas oil was presulfided in a 15 mL micro-batch reactor at 430 °C and 10 MPa H2 pressure with CS2, the gas oil diluent gave a carbon content of 6.4 wt %. Consequently, carbon levels above this level can be considered to be significant. Carbon content was linearly proportional to the weight increase of the catalyst during the reaction (coke on catalyst as reported above); therefore, carbon content was an appropriate and convenient measure of the organic material deposited on the catalyst. The data of Figure 2 show that the carbon content on catalyst was proportional to the concentration of asphaltenes, for both ABVB and SCFE 10 feed blends. Clearly, the increasing carbon content with concentration does not follow a saturation phenomenon, where a monolayer of organic material is established on the catalyst as observed by Richardson et al.4 The data in Figure 3 show the carbon content in spent catalysts from hydroprocessing the series of SCFE fractions at a concentration of 30 wt %. The carbon content on catalysts ranged from 6 to 8% for SCFE 1 through SCFE 7+8+9, while SCFE 10 gave twice as much carbon content as the other fractions. Clearly, SCFE 1 through 9 (with a range of molecular weight from 500 to 1500) had a similar propensity to form coke on catalyst, while the heavier SCFE 10 (consisting mainly of asphaltenes, with a molecular weight ca. 4000) had much higher coking propensity. The hydrogen-to-carbon atomic ratio in catalysts can serve as an indicator of aromatization of carbonaceous compounds on a catalyst. The data of Table 3 show that
Energy & Fuels, Vol. 13, No. 5, 1999 1041
Figure 3. Carbon content of catalyst from hydroprocessing of SCFE fractions. Reaction of a 30 wt % solution of residue fraction in gas oil at 440 °C and 10.3 MPa hydrogen pressure in a continuous-feed stirred reactor. Table 3. Molar Ratios of Hydrogen, Carbon, and Nitrogen in Feeds and Spent Catalysts molar H/C ratio feed SCFE 1 SCFE 2 SCFE 4 SCFE 5 SCFE 6 SCFE 7+8+9 SCFE 10 ABVB
molar N/C ratio
enrichment
residue spent residue spent (N/C)catalyst/ feed catalyst feed catalyst (N/C)feed 1.663 1.602 1.507 1.482 1.436 1.403 1.223 1.369
1.762 1.748 1.445 1.297 1.340 1.280 0.695 0.830
0.0031 0.0042 0.0052 0.0064 0.0069 0.0078 0.0115 0.0073
0.0378 0.0606 0.0534 0.0380 0.0538 0.0553 0.0322 0.0265
12 14 10 6 8 7 3 4
the H/C ratios of spent catalysts are comparable to the residue portion of the feeds for SCFE 1 through SCFE 4. The coke on the spent catalyst was slightly more deficient in hydrogen than the feed for SCFE 5 through SCFE 7+8+9. These observations are consistent with formation of the “coke” deposit by material similar to the feed oil. In contrast, the spent catalysts from SCFE 10 and ABVB have significantly lower H/C ratios than the respective feeds, indicating that surface reactions have continued after deposition of carbonaceous compounds from the liquid phase onto the catalyst. These surface reactions harden the “coke precursor” by dehydrogenation and cause a decrease of H/C ratio. The exact nature of the interaction between “coke precursors” and surface sites is unknown, but it is most likely an acidbase or electron donor-acceptor interaction. If so, the site strength will be a factor affecting such interaction. Overall, the above data suggest that both extent (indicated by carbon content) and nature (indicated by H/C ratio) of the coke on catalyst depend on the composition of the residue components in the liquid phase. Asphaltenic material in the feed promoted a higher carbon content on the catalyst and a more aromatic coke, based on H/C ratio. The data of Table 3 also allow comparison of the N/C molar ratios of the feeds and the spent catalysts. The enrichment of the coke on the catalyst was calculated as the ratio of N/C on the catalyst versus N/C in the residue portion of the feed. The N/C ratio on the catalyst was highest SCFE 2 through SCFE 7+8+9. The ratio was markedly lower for SCFE 10 and ABVB. The
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enrichment factors clearly show that the catalyst preferentially accumulated nitrogen compounds from the feed in all cases, but that this accumulation was most significant for the asphaltene-free SCFE fractions. Accumulation of nitrogen species is commonly observed in hydrotreating catalysts,12 but the lack of enrichment for the feeds with the most nitrogen present (SCFE 10 and ABVB) suggests a shift in the mechanism of coke formation away from adsorption of basic species onto acidic surface sites. Concentration Profiles across Pellets of Spent Catalysts. Scans across the cross section of the spent catalysts by SEM-EDX showed similar results for most of the samples. The metals K, Ca, Fe, Ni, and V were uniformly distributed across the interior of the pellets. The catalyst exposed to 50% SCFE 10 in gas oil exhibited the same uniform distribution of K, Ca, and Ni. The concentrations of Fe and V were higher overall, with a 2-fold decrease in concentration from the exterior of the pellets to the center (based on arbitrary units from EDX scan). Similar analysis conducted by Carruthers et al.13 showed a “skin” of iron and calcium deposited on the external surface of pellets of spent hydroprocessing catalysts, which may or may not cause some deactivation of the catalyst. The uniformity of metal distributions on the spent catalysts from this study, excluding spent catalyst from 50% SCFE 10, can be attributed to the lower metal concentration in the feeds and shorter time-on-stream (TOS) of catalysts. If a longer TOS were used, a skin of Fe and Ca would be likely be formed on the exterior surface of spent catalysts, as found on the commercial spent catalysts. EDX line scans for carbon distribution across spent catalyst pellets from 50% ABVB and 50% SCFE 10 gave weak signals that were above the detectable limit. Both catalysts showed flat concentration profiles of carbon across the catalyst pellets. These results show that even the heaviest feed fractions were efficiently transported across the diameter of the catalyst pellets. Comparable results for carbon distributions have been reported previously;1 in general the carbonaceous species are much less likely to show strong profiles than vanadium. Change in Catalyst Properties with Coke Deposition. The pore volume of catalysts, measured from the nitrogen adsorption isotherms, decreased with the increase of coke concentration as shown in Figure 4. The pore volumes were adjusted to the equivalent mass of coke-free catalyst, then normalized by the initial pore volume of the fresh catalyst. Data from mercury intrusion showed the same trend in pore volume versus coke content. The data in Figure 5 show the surface area as a function of carbon content on the spent catalysts, while the data in Figure 6 show the median pore diameter. Both the surface area and pore diameter were normalized to the values measured for the fresh catalyst. In all three cases, the change in catalyst properties with carbon content was gradual over the range of carbon content of 5-17 wt %. The single catalyst sample
with 24% carbon (from 50% SCFE 10) had sharply reduced pore volume, with only one-third of the original pore volume remaining. The surface area and pore diameter were less sensitive to the deposition of organic material, retaining approximately 60% of the surface area and 75% of the pore diameter. The data of Figures 3-6 should all give a y-axis intercept of 1.0, because the data were normalized to the fresh catalyst with 0.0% carbon content. None of the data would give this intercept upon extrapolation, therefore, the pore volume, surface area, and pore diameter were more sensitive to carbonaceous deposits in the range 0-5% carbon on the catalyst. Two modes of coke deposition have been observed within catalyst pellets: uniform surface deposition4 and pore plugging or blocking.14,15 If coke deposition occurred uniformly through the catalyst structure, pores would
(12) Choi, J. H. K.; Gray, M. R. Ind. Eng. Chem. Res. 27, 15871595, 1988. (13) Carruthers, J. D.; Brinen, J. S.; Komar, D. A.; Greenhouse, S. Chem. Ind. 1994, 58, 175-201.
(14) Ternan, M.; Furimsky, E.; Parsons, B. Fuel Process. Technol. 1979, 2, 45-55. (15) Muegge, B. D.; Massoth, F. E. Fuel Process. Technol. 1991, 29, 19-30.
Figure 4. Pore volume of catalysts after hydroprocessing of SCFE fractions and ABVB blends as a function of carbon content. Pore volumes are given relative to the pore volume of the fresh catalyst.
Figure 5. Surface area of catalysts after hydroprocessing of SCFE fractions and ABVB blends. Surface areas are relative to the surface area of the fresh catalyst.
Coking of Hydroprocessing Catalyst
Figure 6. Median pore diameter of catalysts vs carbon content of catalysts. The pore diameter is relative to the pore diameter of the fresh catalyst.
be expected to narrow, shifting medium pore size to smaller value. This process of pore narrowing would gradually reduce pore volume, by occupying interior space, and reduce internal surface. The gradual shift in properties illustrated in Figures 4-6 were consistent with the model of uniform deposition, for carbon contents above 5%. Assuming a simple pore geometry, pore volume would decrease with the square of the thickness of the carbonaceous deposit, while the surface area and pore diameter would decrease linearly with the thickness of deposits. In contrast, pore mouth plugging would give a dramatic loss in pore volume and surface area with small increases in the amount of coke. Such sensitivity was observed only for
