56
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
Deactivation of Hydrodesulfurization Catalysts under Coal Liquids. 1. Loss of Hydrogenation Activity Due to Carbonaceous Deposits Aquiles Ocampo and J. Thomas Schrodt" Department of Chemical Engineering, University of Kentucky, Lexington, Kentucky 40506
Stephen M. Kovach Research and Engineering, Ashland Oil lnc., Ashland, Kentucky 4 110 1
A technique has been developed for measuring the hydrogenation activities of hydrodesulfurization catalysts before, during, and after the liquefaction of coal. Using this technique, rapid and severe deactivation of catalysts occurs within the first few hours of on-stream coal processing. This is a nonpermanent type clearly caused by the deposition of carbonaceous materials in the pore structure of the catalysts. Catalysts can be fully regenerated after short processing times to nearly their virgin active states. After their initial period of deactivation, the catalysts attain a level of activity that strongly depends upon their chemical composition, surface area, and the distribution of surface area with the pore structure. Catalysts with a preponderance of surface area in pores with r > 6.0 nm show a higher level of activity. The early stage of deactivation is related to carbon laydown and not the ash content of the coal; however, coals with high hydrogen contents are slightly less prone to this type of aging. Atomic absorption analysis reveals the presence of small quantities of metallics which are not responsible for the early stage of deactivation, but slowly lead to a permanent decline in activity.
Introduction Conversion of coal to hydrocarbon feedstocks suitable for refining requires the development of new processing technologies as well as new classes of hydrodesulfurization (HDS) catalysts that can maintain their activity under the harmful environment of coal liquids. Evaluation of the deactivation of commercially available HDS catalysts under coal liquefaction conditions can provide considerable insight into the aging mechanisms of such catalysts thereby generating information needed to design improved catalysts. Coal Liquefaction In a hydrogen donor solvent, such as tetralin, the matrix of finely divided coal swells, becomes further divided, and finally solvates exposing reactive sites. This is favored by the addition of thermal energy, particularly at temperatures from 600 to 800 K. True liquefaction results from the addition of hydrogen, desulfurization, removal of heteroatoms of S, N, and 0, and further cracking and hydrogenation of the products. Some researchers postulate that all hydrogenation takes place by hydrogen transfer from the donor solvent, which is then rehydrogenated. This requires processing at high pressures, 1-2 X 107 P a (100-200 atm), so that ample hydrogen can be retained in the liquids. This theory forms the basis of multistage processes, wherein coal liquids and solvent are catalytically hydrogenated in a reactor following removal of the undissolved coal and minerals. Cusumano et al. (1976) report that the Coalcon and Coed processes follow this procedure. The advantage here is that the catalysts are not exposed to inorganic materials which might poison the catalysts. I t is probable that coal liquefaction is greatly enhanced by the 0019-7890/78/1217-0056$01.00/0
direct infusion of hydrogen into the coal system with a catalyst present. This theory is the basis of the H-Coal and Synthoil processes. Catalyst Aging. Much of the research on HDS catalysts, -y-AlzO, and SiOpalumina supported COO-MOOS,Ni-Moos, and Ni-W, has dealt with their activity under petroleum feedstocks, model compounds such as thiophene, or pure unsaturated hydrocarbons. A survey of this research is found in Schuman and Shalit (1970) and Schuit and Gates (1973). Definitive explanations and models for the site activity of these catalysts have been advanced by DeBeer et al. (1974), Linsen (1970), Lipsch and Schuit (1969a,b), Ozawa and Bishoff (1968), and Ratnasamy et al. (1975). Under coal liquids laden with heavy aromatics, organometallic compounds, mineral matter, and undissolved coal, rapid and severe catalyst aging can be expected. Stanulonis et al. (1976) and Holloway et al. (1976) have all reported a rapid, within several hours of use, and severe, more than 5096, loss of activity. This is generally attributed to the deposition within the catalyst's pores of carbonaceous materials formed from cracking the heavy aromatics and rejecting hydrogen. Blockage at the pore mouths will drastically reduce intraparticle diffusion and hence reduce the rate of hydrodesulfurization. Thick deposits on the pore walls will cause a loss in the intrinsic activity. Thin deposits may be advantageous in that they may screen our the large organometallic compounds that permanently poison catalysts. Fortunately, deactivation by coking is not permanent, and controlled high-temperature oxidation of these deposits usually causes activity recovery. Permanent loss of activity results from residual deposits of metallic compounds emanating from the coal's mineral matter. During regeneration these are converted
01978 American Chemical Society
Ind. Eng. Chern. Prod. Res. Dev., Vol. 17,No. 1, 1978
to oxides which are then permanently bound to the catalyst’s surface. Other permanent deactivation will occur when organometallic compounds, such as those of Ti and V, found in the coal’s matrix react to form inorganics. These likewise become permanently bound to the surface sites. Aging associated with these probably takes place over a much longer time span than the deactivation due to coking. Unfortunately, it is generally of an irreversible character. The inference here is that the loss of catalyst activity in coal liquids is a combination of two factors: permanent poisoning due to the irreversible adsorption of inorganics and the reversible deposition of heavy organics, or coking. To further study the effect of these two factors on catalytic activity during coal liquefaction, a divided two-phase program of catalyst aging due to carbon deposition will be examined, and in the following paper the effects of permanent inorganic poisons on catalyst aging will be investigated. In the first series of studies, the activity levels of different commercially available HDS catalysts were periodically measured during the liquefaction of several coals in a highpressure, bench-scale autoclave charged with coal, solvent, hydrogen, and individual catalysts. A novel technique of evaluating the activity was developed that relied upon leaving the catalyst in place within the reactor and hydrogenating to a specified end point a standard reference feedstock. Periodically catalysts were removed from the reactor and pore characterizations and carbon deposition rates were evaluated. Atomic absorption was used to quantify the intraparticle deposition of alien metals from the matrix and mineral matter of the coal. The objective of this paper is to present the techniques used as well as the findings and conclusions and to offer suggestions for designing improved catalysts for use under coal liquefaction conditions.
Experimental Section Apparatus. All liquefaction-catalyst deactivation experiments were performed in a commercially available magnedrive 2-L autoclave, standard as received except that the cooling coil was removed, and a 3/8-in. drain valve was added to the bottom to provide rapid dumping of the product. The reactor was fitted for easy and rapid addition of any liquid or gas feed. Liquid product was collected after passage through a cooling coil, pressure was controlled by a regulating valve, and all gases passed through an H2S scrubber and wet test meter before venting. Catalyst samples were suspended in a wire-mesh basket attached to a baffle plate. A schematic of this system is shown in Figure 1. Materials. The compositions of the 18 catalysts are given in Table I. The cobalt-molybdate on alumina catalyst, designated as CM5, was selected as a baseline or control catalyst to which the activities of all the others were compared. Three coals, Kentucky No. 11 and Wyodak and Illinois No. 6, were selected, with the No. 11 picked as the baseline coal. These were crushed, sieved, and quartered to pass a 200-mesh screen, stored in plastic bags in air-tight cans, and vacuum dried at 373 K prior to use. The reference standard liquid used to measure catalyst activity was a naphthalene-alkylnaphthalene mixture. The properties of this are given by Kovach et al. (1976). Procedures. For each run 25 g of catalyst was suspended in the autoclave which was sealed, pressure checked a t 1.73 X lo7 Pa, and then the catalyst was presulfided with H2S at 644 K and 1.83 X lo7 P a for 3 h. Presulfidation greatly minimized the chance for catalyst, temperature run-away. Following this, 1000 cm3 of pure tetralin and 0.3 kg of coal were added a t 394 K, and stirring at 1200 rpm commenced. The reactor was repressurized a t 1.39 X 107 P a and heated to 630 K to initiate liquefaction. Count of the process time began
57
Tetralin and Reference Feed Tanks
Figure 1. High-pressure catalyst test system.
after the 50 f 5 min heat-up period. After either 6 or 8 h, stirring was stopped, hydrogen pressure interrupted, and liquid product dumped and collected while the gases were passed through a cooling coil to the scrubber. All heavy hydrocarbons were removed from the catalyst and vessel by washing for 15 min each, once with tetralin and then with the reference standard. Charges of the latter were then used to evaluate the catalyst’s hydrogenation activity. Other researchers have determined activity by either measuring the amount of unconverted coal and the liquid product distribution or the amount of sulfur removed. In these methods the gross overall process tends to mask the true activation mechanism. A more sensitive method of evaluation is to periodically determine the residual catalyst activity by a carefully controlled hydrogenation of a standard aromatic hydrocarbon. Holloway et al. (1976) have also recommended this technique. Such a test method is very sensitive to catalyst deactivation by carbon deposition and permanent inorganic poisons. Since the coal process used tetralin as the donor solvent, the methyl naphthalene concentrate was ideally suited inasmuch as catalyst activity measured as the time required to hydrogenate the concentrate to 50% tetralin avoided complications resulting from hydrogenation of tetralin to decalin. The activity was normalized by carrying out a series of hydrogenation tests using the virgin presulfided control catalyst and reference concentrate to determine the time to reach the 50% conversion. The fresh concentrate has a refractive index (RI) of 1.5877 and a t 50% conversion RI = 1.5700. Use of RI as a rapid measure of the extent of conversion further simplified the evaluation procedure. For the CM5 control catalyst, 171 min was required to reach RI = 1.5700. Successive tests indicated negligible aging of the catalysts when additional concentrate was hydrogenated. The equation activity = 171/O(min) to RI = 1.5700 gave the activities of the catalysts at their periodic levels of performance ranging from 1 to the lower limit of 0. For catalysts having inherent activities better than the control catalyst the normalized initial values exceeded 1.
Results Based upon the reports of Akhtar et al. (1971) and Yavorsky et al. (1972), it was anticipated that 50-100 kg of coal/kg of catalyst could be processed before any significant decline in activity would be measured. This was never realized, and in fact, as is shown in Figure 2, deactivation occurred upon first exposure of the No. 11coal to the CM5 control catalyst at 630 K. I t is known that HDS catalysts experience slight declines in activity upon initial exposure to heavy aromatic feedstock;
58
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
fresh
! \ C
I 0
I
1
I
I
I
I
12
24
36
48
60
72
'
0 2.0
A
regen
0
used
4
\
'
I
f
i
'
l
l
l
4.0 5.0 6.07.08.09.010.0 Pore Radius, nm
3.0
84
200
30.0 40.0 50.0 60.0
Figure 4. Pore volume distributions of CM5 catalyst.
kg Coal / kg Catalyst
Figure 2. Deactivation of CM5 catalyst under coal liquefaction conditions.
Fresh
B 200
A
Regen
1
Used
AI
I Pore
R a d i u s , nrn
Figure 3. Surface area distributions of CM5 catalyst.
however, under coal liquids the effect is obviously more severe as was fully confirmed in subsequent tests. At 658 and 687 K, temperatures that should crack and enhance hydrogenation of the heavier fractions, initial activity improved, as shown in Figure 2, but again rapidly decreased to the same asymptotic level of activity. The cause of the rapid decline was not immediately apparent. Analyses of the catalyst by x-ray diffraction and atomic absorption (AA) showed only trace levels, less than 0.1 wt % each, of the coal minerals that had been identified in the No. 11coal, and these were not considered to be sufficient to account for the great loss of activity. It was then felt that the rapid loss of activity was due to deposition of carbonaceous materials. Specimens of unsulfided, sulfided, and used catalysts as well as specimens that had been regenerated in air at 770 K were then examined for their surface area, pore volume, and carbon-hydrogen deposits. The used catalysts samples recovered from the reactor were first covered with tetralin and then cooled to room temperature. All specimens were degassed a t Torr before pore charac373 K for a minimum of 8 h at terization was attempted. Surface areas and pore volumes and the distribution of these characteristics with pore radii were evaluated by Ocampo (1976) from nitrogen absorption-desorption isotherms at 77 K. The pore volume distributions results were confirmed by mercury intrusion. Surface area and pore volume distribution curves for the control catalyst, presented in Figures 3 and 4, clearly show that a significant decline in these properties, particularly in the pore size range
