Energy & Fuels 2006, 20, 473-480
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Impact of Multiphase Behavior on Coke Deposition in Heavy Oils Hydroprocessing Catalysts Xiaohui Zhang and John M. Shaw* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Canada ReceiVed August 4, 2005. ReVised Manuscript ReceiVed December 14, 2005
Coke deposition in heavy oil catalytic hydroprocessing remains a serious problem. The influence of multiphase behavior on coke deposition is an important but unresolved question in this literature. A model mixture comprising Athabasca vacuum bottoms (ABVB) + decane + hydrogen which is shown to exhibit low-density liquid + vapor, high-density liquid + vapor, as well as low-density liquid + high-density liquid + vapor, phase behavior at typical hydroprocessing conditions and a commercial heavy oil hydrotreating catalyst (NiMo/ γ-Al2O3) were employed in this investigation. The influence of multiphase behavior on coke deposition was explored under catalyst coking conditions (380 °C and 2 h). The liquid-liquid-vapor region extends from ∼20% ABVB to ∼50% ABVB. Coke deposition in the high-density liquid phase was found to be greater than in the low-density liquid phase, at fixed global composition. Conventional kinetics models which do not include the impact of such phase behavior effects cannot account for local maxima in the coke deposition versus coke precursor concentration profiles that result when complex phase behavior is encountered.
Introduction Coke deposition in heavy oil catalytic hydroprocessing remains a serious problem even though extensive studies have been carried out to minimize catalyst coking, including catalyst preparation, reactor design, operating condition optimization, and processes development.1 Coke deposition mechanisms are the key theoretical basis for guidance with respect to minimizing coke deposition and optimizing hydroprocessing processes. Although coke deposition mechanisms have been investigated intensively,2 there appear to be inconsistencies between coking kinetics models and observed coke deposition phenomena, which warrant further investigation. Coke deposition mechanisms on catalysts can be classified into three simplified modes: uniform surface deposition,3 pore mouth plugging,4 and bulk phase coke formation.3 A complete description of coke deposition modes has to account for the impact of catalyst pore structure, reactant molecular size, reaction time, etc. These have been investigated widely in the past.5 Phase behavior is also a contributing factor,6,7 particularly in heavy oil hydroprocessing, but it has yet to be fully investigated. In the thermal upgrading of heavy oils, it is widely accepted that coke formation is triggered by liquid-liquid phase separation,8-10 and phase behavior has a dramatic effect on coke * Corresponding author. E-mail:
[email protected]. (1) Absi-Halabi, M.; Stanislaus, A. Appl. Catal. 1991, 72, 193-215. (2) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52, 381-495. (3) Richardson, S. M.; Nagaishi, H.; Gray, M. R. Ind. Eng. Chem. Res. 1996, 35, 3940-3950. (4) Muegge, B. D.; Massoth, F. E. Fuel Process. Technol. 1991, 29, 1930. (5) Bartholomew, C. H. Catalyst Deactivation in Hydrotreating of Residua: A review. In Catalytic Hydroprocessing of Petroleum and Distillates; Oballa, M. C., Shih, S. S., Eds.; Marcel Dekker: New York, 1994. (6) Minicucci, D.; Zou, X.-Y.; Shaw, J. M. Fluid Phase Equilib. 2002, 194-197, 353-360. (7) Zhang, X. H.; Chodakowski, M. G.; Shaw, J. M. Energy Fuels 2005, 19 (4), 1405-1411. (8) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32 (11), 2447-2454.
yield.11 The initial reactions in the thermal cracking of heavy oils involve the thermolysis of large aromatic-alkyl molecules to produce volatile species (paraffins and olefins) and nonvolatile species (primarily aromatics). In addition, the formation of liquid hydrocarbon products creates regions of instability causing the highly aromatic and highly polar refractory products to separate from the surrounding oil medium as an insoluble phase that can form coke.12 In catalytic hydroprocessing, the presence of catalyst and hydrogen combined with the thermal cracking makes the products even less stable due to the hydrogenation of the oil medium so that it becomes a poorer solvent for the aromatic and polar refractory species.13-15 As a result, the separated heavy phase (highly aromatic and highly polar compounds) may cause more coke deposition on catalysts. Gray16 hypothesized and depicted the phase behavior in hydroprocessing of heavy oils in terms of a ternary composition phase diagram with a light component, middle distillates, and heavy resids as three pseudocomponents. This ternary phase diagram indicates an unstable region (L1L2V) with two liquid phases coexisting at typical operating conditions. The L2 phase is a denser liquid that has relatively larger concentrations of polar compounds and condensed ring compounds; the L1 phase is a less dense liquid that has relatively larger concentrations of saturated and nonpolar compounds. According to this ternary phase diagram, one can speculate that in fixed bed reactors, a liquid phase that is initially homogeneous at the reactor inlet (9) Li, S.; Liu, C.; Que, G.; Liang, W.; Zhu, Y. Pet. Sci. Technol. 1999, 17 (7-8), 693-709. (10) Rahmani, S.; McCaffrey, W.; Elliott, J. A. W.; Gray, M. R. Ind. Eng. Chem. Res. 2003, 42, 4101-4108. (11) Ali, V. M.Sc. Thesis, University of Toronto, Toronto, Canada, 2002. (12) Speight, J. G. Korean J. Chem. Eng. 1998, 15 (1), 1-8. (13) Bartholdy, J.; Andersen, S. I. Energy Fuels 2000, 14, 52-55. (14) Bartholdy, J.; Lauridsen, R.; Mejlholm, M.; Andersen, S. I. Energy Fuels 2001, 15, 1059-1062. (15) Matsushita, K.; Marafi, A.; Hauser, A.; Stanislaus, A. Fuel 2004, 83, 1669-1674. (16) Gray, M. R. Upgrading Petroleum Residues and HeaVy Oils; Marcel Dekker: New York, 1994.
10.1021/ef0502498 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/31/2006
474 Energy & Fuels, Vol. 20, No. 2, 2006
Zhang and Shaw
Figure 1. Pressure-composition diagrams for the heavy oil hydroprocessing model system at 380 °C: (a) ABVB + decane; (b) ABVB + decane + hydrogen (the mass ratio of hydrogen/ABVB + decane is 0.0057:1).
can develop a second liquid phase as the reactions progress. Phase splitting would be most likely to occur at high conversion levels, where heavy fractions become increasingly dissimilar to the bulk composition. Consequently, in batch hydroprocessing reactors and CSTRs, phase behavior may vary with conversion; in trickle beds, phase behavior may vary by location. Mixtures including significant concentrations of heavy oil are opaque to visible light, so few multiphase equilibrium data are available for them in the literature. The X-ray view-cell technique17 allows one to observe the phase behavior of such opaque mixtures. There has been speculation that phase behavior affects coke deposition under typical catalytic hydroprocessing conditions,6,16,18 but no direct experimental proof has yet been presented. The polar compounds and condensed ring compounds present in the L2 phase are coke precursors, and L2 drops can adhere to catalyst surfaces. Also, the solubility of hydrogen in the L2 phase is much lower than in the L1 phase.19-21 As liquid-liquid mass transfer rates are low, these drops can quickly become hydrogen deficient, as reactions progress, which enhances condensation and aromatization reactions in the L2 phase that lead to coke formation. In the present work, the impact of phase behavior on coke deposition on a commercial catalyst used for heavy oil hydroprocessing was investigated. More specifically, the coke deposition in different phases and different phase behavior regions was studied at fixed reaction temperature and reaction time. The principal objective of this study is to evaluate the significance of the relationship between phase behavior and coke deposition in catalytic heavy oil hydroprocessing. Experimental Section Materials. Athabasca vacuum bottoms, ABVB, is the 525 °C+ boiling fraction of Athabasca bitumen, comprising approximately 32 wt % asphaltenes (SARA analysis). A detailed composition analysis for ABVB is available elsewhere.22 Decane, carbon disulfide, and toluene were supplied by Aldrich with 99.5+%, 99.9+%, and 99.8+% purity, respectively. Hydrogen was supplied by Praxair with 99.999% ultrahigh purity. The heavy oil hydrotreat(17) Abedi, S. J.; Seyfaie, S.; Shaw, J. M. Pet. Sci. Technol. 1998, 16 (3-4), 209-226. (18) Ternan, M.; Rahimi, P. M.; Clugston, D. M.; Dettman, H. D. Energy Fuels 1994, 8 (3), 518-530. (19) Dukhedin-Lalla, L.; Sun, Y. S.; Shaw, J. M. Fluid Phase Equilib. 1989, 53, 415-422. (20) Shaw, J. M. Can. J. Chem. Eng. 1987, 65, 293-298. (21) Shaw, J. M.; Gaikwad, R. P.; Stowe, D. A. Fuel 1988, 67, 15541559. (22) Zou, X. Y.; Dukhedin-Lalla, L.; Zhang, X. H.; Shaw, J. M. Ind. Eng. Chem. Res. 2004, 43, 7103-7112.
ing catalyst is a commercial 1 mm diameter cylindrical extrudate NiMo/γ-Al2O3 catalyst with 10-15 wt % MoO3 and 2-4% NiO. The catalyst has a surface area of 220 m2/g and a pore volume of 0.59 cm3/g. ABVB + decane + hydrogen mixtures were used as the model system for hydroprocessing. Phase Behavior Experiments. To ensure that the catalyst remained in the targeted phase, preliminary phase behavior experiments were performed prior to catalyst coking experiments in a variable volume X-ray view cell.23 The detailed phase behavior experimental strategy is described briefly here and in more detail elsewhere.7 First, the phase behavior for the mixture, ABVB + decane was evaluated using the synthetic method over the pressure range from 1 to 10 MPa at 380 °C. A total mass of 60 g of ABVB + decane was added to the view cell at each composition. At each examined condition an X-ray transmission video still image was obtained. On the basis of the observation of a series of images and the systematic analysis of phase density and volume, the pressurecomposition diagram shown in Figure 1a was constructed. Thermal cracking occurs at temperatures above 340 °C.24,25 As a result, the phase diagram at 380 °C is only approximate. During phase behavior measurements, the samples were exposed to temperatures above 340 °C for less than 1 h. The phase behavior evaluation was repeated for the mixture ABVB + decane + hydrogen, where the mass ratio of hydrogen to ABVB + decane was fixed at 0.0057. The phase behavior experiments with hydrogen were conducted for 10-40 wt % ABVB, and a pressure-composition diagram at 380 °C is shown in Figure 1b. Again exposure times to temperatures above 340 °C were limited to less than 1 h. Catalyst Preparation. Commercial hydrotreating catalyst samples were presulfidized in 15 mL microbatch reactors constructed from stainless steel tubing and Swagelok fittings.26 Prior to presulfidation, 350 mg of catalyst was desiccated in an oven at 200 °C for 2 h. Then, the catalyst was transferred to a microbatch reactor, followed by injection of 70 µL of liquid carbon disulfide, approximately twice the amount required to sulfidize the catalyst. After purging several times with hydrogen, the sealed microbatch reactor was pressurized to 750 kPa at room temperature with hydrogen. Then, the reactor was placed in an air-fluidized sand bath at 350 °C for 2 h to convert any remaining carbon disulfide to hydrogen sulfide. After catalyst presulfidation, the reactor was removed from the sand bath and quenched in a water bath. Coking Experiments. The catalyst coking experiments were performed in a variable volume X-ray view cell23 equipped with a magnetically coupled mixer, and a variable position catalyst holder, allowing one to control the phase to which the catalyst is exposed. (23) Abedi, S. J.; Cai, H. Y.; Seyfaie, S.; Shaw, J. M. Fluid Phase Equilib. 1999, 158-160, 775-781. (24) Cai, H.-Y.; Shaw, J. M.; Chung, K. H. Fuel 2001, 80, 1055-1063. (25) Cai, H.-Y.; Shaw, J. M.; Chung, K. H. Fuel 2001, 80, 1065-1077. (26) Kanda, W.; Siu, I.; Adjaye, J.; Nelson, A. E.; Gray, M. R. Energy Fuels 2004, 18, 539-546.
Coke Deposition in Hydroprocessing Catalysts
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Figure 2. X-ray transmission video image and schematic representation (a) showing the catalyst held in the L1 and L2 phases and (b) showing the catalyst held in the L1 phase with the L2 phase dispersed. Table 1. Repeatability of Coking Experiment Results at 30 wt % ABVB L1 carbon (wt %) surface area (m2/g) pore volume (cm3/g) a
L2
No. 1
No. 2
No. 3
errora
No. 1
No. 2
No. 3
errora
14.64 155.5 0.34
14.35 164.5 0.36
14.42 166.3 0.34
0.17 7 0.01
16.35 151.3 0.31
16.40 160.0 0.33
16.32 158.3 0.32
0.05 5 0.01
The error is the maximum deviation of the experimental values from the average.
All catalyst coking experiments were conducted at 380 °Csa typical heavy oil hydrotreating temperature. The catalyst charge was 0.3 g. The liquid charge was 60 g. To ensure that the catalyst was exposed to only one phase, great care was taken when placing the catalyst in the view cell and the mixing rate was controlled to avoid vortex formation. A detailed description of the catalyst placement procedure is available elsewhere.7 X-ray transmission video stills taken during two experiments, one for catalyst placed in both L1 and L2 phases, another for catalyst placed in L1 + L2 dispersed and shown in Figure 2, exemplify the general arrangements. For clarification, schematic representations are also shown in Figure 2. In Figure 2a, catalyst pellets were placed in both the L1 and the L2 phases. In Figure 2b, catalyst pellets were placed in the continuous L1 phase with the L2 phase dispersed in the L1 phase. The examples in Figure 2 are only for the case with L1L2V phase behavior. If there is no L2 phase at the experimental conditions (>50 wt % ABVB), the catalyst placement shown in Figure 2b was applied. At low ABVB composition (
