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Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 206-213
Catalyst Deactivation in Two-Stage Coal Liquefaction Gary J. Stlegel," Rlchard E. Tlscher, Danlel L. Clllo, and Nand K. Naraln Pittsburgh Energy Technology Center, U.S.Department of Energy, Pittsburgh, Pennsylvania 15236
An investigation was conducted to assess the performanceand deactivation characteristics of catalysts used in the hydroprocessing reactor at the Wilsonville Advanced Coal Liquefaction Test Facility. Spent catalyst samples were collected periodically from the reactor throughout three runs. The samples were characterized by various analytical technlques and subjected to a batch acthri test. All catalyst samples showed similar declines in specific pore volume, specific surface area, and average pore diameter. Although the carbon concentration reaches equilibrium very quickly, the atomic hydrogen-to-carbon ratio decreased throughout the runs. The deposition of trace metals on the catalyst increased continuously throughout the runs but at a decreasing rate.
Introduction The concept of two-stage coal liquefaction has been under investigation for several years (Schindler et al., 1983; Technical Progress Reports, 1983,1984a,b), and the various processes have shown considerable improvements in operability and product yields over previous technologies. These processes consist of three primary steps: liquefaction, solids separation, and catalytic hydroprocessing. The hydroprocessing step is a vital part of this operation, since it not only upgrades the product by improving its hydrogen content and reducing the concentration of heteroatoms, but it also provides a high quality solvent for coal dissolution. The catalyst is the key component of the hydroprocessing stage because changes in product composition can be affected by the use of different catalysts. Because of its importance to the entire liquefaction scheme, investigations directed at assessing the effects of chemical and physical properties of catalysts on their activity, selectivity, and longevity can have profound impact on the economics of the overall process. Catalysts used for the hydroprocessing of heavy petroleum and synthetic fossil fuel feedstocks all experience a loss of activity and selectivity with time (Stiegel et al., 1983). The loss of activity can result from coverage of active sites by coke and by trace metals, increased diffusional restrictions due to pore filling or blocking by the deposition of coke and trace metals, poisoning of active sites by the reversible and irreversible adsorption of heterocyclic compounds, and sintering of the active components, thereby reducing the number of active catalytic sites. Unless hot spots develop in the reactor due to poor distribution of the liquid phase, sintering of the sulfided crystallites will not usually occur at typical hydroprocessing conditions. Catalyst deactivation by coking has been studied quite extensively for petroleum hydroprocessing and other hydrocarbon reactions. Two reviews have recently been published on catalyst deactivation by coking (Wolf and Alfani, 1982; Trimm, 1983). Investigations have also been conducted to determine the mechanism of coke formation and its structure on supported metal oxide catalysts (Baker, 1980; Beuther et al., 1980; Sanders et al., 1983). Coke deposits have been found to consist of mono- and polycyclic aromatic rings connected by aliphatic and alicyclic fragments, the relative proportions of each group being dependent on the reaction-catalyst system under investigation. The use of additives to reduce the growth rate of filamentous carbon has also been investigated (Baker, 1980). In a recent laboratory investigation of the hvdraprocessing of solvent-refined coal, it was ohserved This article not subject to U S Copyright.
that coke deposition causes a severe reduction in the pore size distribution, resulting in reduced catalyst efficiency (Chang et al., 1982). Hydrogenation and denitrogenation activity of the catalyst can be related to the coke content of the catalyst. Mathematical modeling of catalyst deactivation by coking has also been studied (Beeckman and Froment, 1979; Corella and Asua, 1981). Deactivation of hydroprocessing catalysts by trace metals is becoming more important as the use of heavier petroleum feedstocks increases and the production of synthetic fuels from coal, oil shale, and tar sands nears commercialization. The deposition of trace metals on the catalyst surface generally occurs slowly over the life of the catalyst, but its effect, like that of coke, is an increase in the intraparticle diffusion resistance caused by pore filling or blockage (Prasher et al., 1978). For hydroprocessing coal liquids, several metals commonly found in coal caused a permanent loss of catalyst activity; however, the extent of deactivation was a function of the particular metal (Kovach et al., 1978). Iron, sodium, calcium, and magnesium in their oxide states caused the greatest degree of deactivation. Titanium present as an oxide did not cause any loss in catalyst activity; however, when present as an organometallic salt, it adsorbed easily on the catalyst surface and caused significant deactivation. Liquids derived from coal have high concentrations of polyaromatic compounds and significant quantities of trace metals, especially iron and titanium. In two-stage liquefaction processes, the catalyst in the hydroprocessing reactor is subjected to an environment containing all ingredients necessary for severe catalyst deactivation. To obtain insight into the deactivation phenomena in twostage coal liquefaction, catalyst samples were collected periodically from the hydrotreater at the Wilsonville Advanced Coal Liquefaction Test Facility during runs 242, 243, and 244. Differences in catalyst activity among the three runs were also observed. This paper presents and discusses the results obtained from various analyses of the catalyst samples. Experimental Section Process Conditions. The catalysts used in the present investigation were obtained from the H-Oil reactor at the Wilsonville Advanced Coal Liquefaction Test Facility. Catalysts were withdrawn from the reactor at various times during runs 242,243, and 244. During these runs, the plant was operated in the integrated mode, in which the product from the liquefaction reactor was subjected to critical solvent deashing. The overflow from the deasher was then hydrotreated. Table I presents some of the operating conditions of the liquefaction and hydrotreating reactors Published 1985 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 207
Table I. Process Conditions for the Wilsonville Runs (Coal: Illinois No. 6) run 243 run242 partA part B run244 Liquefaction Conditions coal 36 36 36 36 concentration, w t % 27 57 45 coal space 95 rate, lb/(h ft3) 9.2 4.42 4.4 reactor 2.1 volume, ft3
reactor temperature, O F reactor pressure, Psig gas feed rate, scfk
840
785, 800
810,825
810
2400
1400
2400
1500-2400
3500
3500
3500
3500
residuum concentration, wt % LHSV reactor pressure, PSig gas feed rate, scf/lb of resid hydrogen purity, mol % catalyst total weight of catalyst, lb
50-55
Hydrotreater Conditions 50-55 50-55
50-55
0.79-1.13 2700-2800
0.7-1.4
0.7-1.4
0.97-1.22
20
20
20
20
84-97
97-99
90-99
90-94
Shell 324M Shell 324M Shell 324M Shell 324M 411 410 410 410
Table 11. Properties of Shell 324M bulk density, g/cm3 pellet density, g/cm3 pore volume, cm3/g surface area, m2/g average pore diameter, A NiO, w t % Moo3, w t % A1203,w t % Na20, wt % SOz,wt % PZO,, wt %
0.78 1.38 0.43 180 115 3.4 19.3 62.5 0.1
