Ind. Eng. Chem. Res. 1987,26, 2466-2473
2466
Imperial Oil Ltd. is gratefully acknowledged. Nomenclature B = BHEP concentration, kmol/m3 BHEP = NJV-bis(hydroxyethy1)piperazine DEA = diethanolamine Hco = Henry's law constant, kPa/kmol/m3 K1-k5 = equilibrium constants Kbl = equilibrium constant for BHEP MDEA = methyldiethanolamine [MDEA] = total MDEA concentration, kmol/m3 MEA = monoethanolamine Pco, = C 0 2 partial pressure, kPa R = CPHdOH T = temperature, K Greek Symbols LY
= C 0 2 loading, mol of C02/mol of solvent
[ ] = molar concentration of species, kmol/m3 Registry No. MDEA, 105-59-9;BHEP,122-96-3;COz, 124-
38-9.
Blanc, C.; Elgue, J.; Lallemand, F. J. Hydrocarbon Process. 1981, 60(8),111. Chakma, A.; Meisen, A. Proceedings of the 35th Canadian Chemical Engineering Conference, Calgary, Alberta, 1985, Vol. 1, p 37. Crolet, J. L.; Bonis, M. R. Corrosion 1983, 39(2), 39. Deshmukh, R. D.; Mather, A. E. Chem. Eng. Sci. 1982,36(2), 365. Frazier, H. D.; Kohl, A. L. Znd. Eng. Chem. 1950, 42(11), 2288. Guggenheim, E. A. Phil. Mag. 1935,19, 588. Jou, F.; Mather, A. E.; Otto, F. D. Znd. Eng. Chem. Process Des. Deu. 1982, 21, 539. Kennard, M. L.; Meisen, A. Znd. Eng. Chem. Fundam. 1985,24,129. Kent, R. L.; Eisenberg, B. Hydrocarbon Process. 1976, 55(2), 87. Kohl, A. L.; Riesenfeld, F. C. Gas Purification, 4th ed.; Gulf Publishing: Houston, 1985. Maddox, R. N.; Bhairi, A.; Mains, G. J.; Shariat, A. Acid and Sour Gas Treating Processes; Newman, S. A., Ed.; Gulf Publishing: Houston, 1985; p 212. Peng, D. Y.; Robinson, D. B. Znd. Eng. Chem. Fundam. 1976,15(1), 59.
Polderman, L. D.; Steele, A. B. Oil Gas J. 1956, 54(65), 206. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw Hill: New York, 1977. Smith, R. F.; Younger, A. H. Hydrocarbon Process. 1972,51(7), 98. Vidaurri, F. C.; Kahre, L. C. Hydrocarbon Process. 1977,56(11),333.
Literature Cited
Received for review October 6, 1986 Revised manuscript received July 15, 1987 Accepted August 9, 1987
Benedict, M.; Webb, G . B.; Rubin, L. C. Chem. Eng. Prog. 1951,47,
419.
A Comparative Study of Catalyst Deactivation in Integrated Two-Stage Direct Coal Liquefaction Processes Frances V. Stohl* and Howard P. Stephens Sandia National Laboratories, Organization 6254, Albuquerque, New Mexico 87185
Catalyst samples from Wilsonville runs 242, 246,247, and 248 have been characterized and tested for hydrodesulfurization activity in order to determine the effects of different process configurations and coals on the causes and rates of deactivation. Initial rapid decreases in activity were caused by the accumulation of carbonaceous deposits within the catalyst. Variations in the amount of deposition were due to the process configuration but not the coal type. The process configuration with the most preasphaltenes and unconverted coal in the hydrotreater feed yielded the greatest buildup of carbonaceous deposits and the most deactivation. After the initial high losses, the activities continued to decrease with catalyst age due to the accumulation of contaminant metals in the catalyst. The rates of contaminant metals buildup varied significantly from run to run. Deactivation by carbonaceous deposits and contaminant metals was due to both poisoning of active sites and decreases in effective diffusivities. In addition, sintering of the active metals was observed in the aged catalysts.
I. Introduction During the past several years, direct coal liquefaction processes have evolved from single-stage processes to two-stage processes (Neuworth and Moroni, 1984; Schindler et al., 1982; Rao et al., 1983). Dissolution of the coal occurs in the first stage, and catalytic upgrading takes place in the second stage. Separation of the two stages enables each to be operated under suitable conditions for the reactions that occur in that stage. Current two-stage processes are operated in the integrated mode in which a portion of the second-stage product is recycled back to the first stage as a solvent to be mixed with the coal. Researchers at the Wilsonville Advanced Coal Liquefaction R & D Facility have evaluated the use of three integrated two-stage process configurations for direct liquefaction of two coals (Lamb et al., 1985; Moniz and Nalitham, 1985; Moniz et al., 1983, 1984; Gough et al., 1985). Results showed that resid (material that is nondistillable at 316 "C and 0.1 mmHg) conversion, which was used as an indicator of catalyst performance, decreased 0888-5885/87/2626-2466$01.50/0
with time for all process configurations and coals. However, the rates of decrease in resid conversion varied among runs (Moniz and Nalitham, 1985). The greatest rates of decrease in resid conversion were early in all the runs. Most studies of aged coal liquefaction catalysts have been of catalysts derived from single-stage or nonintegrated two-stage processes (Thakur and Thomas, 1984; Cable et al., 1981, 1985; Stohl et al., 1987). There have only been a few studies of catalysts from integrated two-stage processes (Stiegel et al., 1985; Freeman et al., 1985). The objective of our work was to determine the causes for the differences in catalyst deactivation in the Wilsonville runs. To accomplish our objective, we characterized and tested catalyst samples from four Wilsonville liquefaction runs: 242, 246, 247, and 248. Catalyst deactivation (Ocampo et al., 1978; Kovach et al., 1978; Furimsky, 1979; Thakur and Thomas, 1984) is caused by the accumulation of carbonaceous deposits and contaminant metals on coal liquefaction catalysts. The buildup of carbonaceous deposits is very rapid, whereas 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987 2467 ITSL (Integrated Two-Stage Llquefactlon) PRODUCT
DIST SOLVENT
T
RECYCLE SOLVENT
HYDROOEN
MTSL (Double ITSL) PRODUCT
6
DlST SOLVENT
I
* I ,
6
II
TR
"1 I
HYDROOEN
LTR
RECYCLE SOLVENT
I
I .
RITSL (Reconfigured ITSL)
Table I. Selected Run Parameters hydrotreater feed" heaviest resid components* run coal' configuration resid PA UC ash 242 IL#6 ITSL 50-55 4 0 eo.1 246 Wyodak DITSL (to aged = 300) 24 0.3 0 0 ITSL (remainder of run) 35 247 IL#6 RITSL 4 8 8 3 4 248 IL #6 DITSL (to age = 100) 30-40 "Results in w t %. *Selected components. PA = preasphaltenes; UC = unconverted coal. Moniz and Nalitham, 1985. "L #6 = Illinois No. 6 Burning Star; Wyodak = Clovis Point Mine. dCatalyst age = l b of resid/lb of catalyst.
HVDROOEN PRUCUCT
______
-
Run 2 4 2
Run 2 4 6
--
Run 247
Run 2 4 8
400
Figure 1. Process configurations. TLU = thermal liquefaction unit; CSD = Kerr-McGee critical solvent deasher; HTR = hydrotreater; Bot = separator bottoms (approximately = 510+ "C); AC = ash concentrate; TR = thermal resid; LTR = light thermal resid; Dist = distillate.
the accumulation of contaminant metals is slower. These two contaminants eliminate active sites by fouling, poisoning, and pore-mouth blockage. They can also cause decreases in the effective diffusivities of the catalysts (Prasher et al., 1978). In addition, deactivation can be due to changes in the structure of the active phase, such as caused by sintering. Our study was performed by characterizing the catalysts with regard to changes in chemical composition, in distributions of contaminants, and in physical properties and then comparing these changes to catalyst hydrodesulfurization activity measurements determined in the laboratory using thiophene as the model compound. The results were then compared to the Wilsonville run conditions to determine which ones had the greatest impact on deactivation.
11. Wilsonville Processing Parameters A. Process Configurations. Figure 1 illustrates the three different process configurations employed in runs 242,246,247, and 248: the Integrated Two-Stage Liquefaction (ITSL) process, the Double-Integrated Two-Stage Liquefaction (DITSL) process, and the Reconfigured Integrated Two-Stage Liquefaction (RITSL) process. In the ITSL and DITSL modes, deashing is performed between the first and second stages. Deashing not only removes most of the mineral matter but also rejects a portion of the organic matter (mainly the preasphaltenes and unconverted coal) (Moniz and Nalitham, 1985). This deashing is a three-step procedure. In the ITSL configuration, all of the deashed material goes to the second-stage ebullated-bed hydrotreater, whereas with DITSL processing only the material from the third deashing step, referred to as light thermal resid, goes to the hydrotreater and the material from the second deashing step, the thermal resid, is recycled back to the first-stage thermal reactor. In the RITSL mode, the deashing is performed after the second stage so that all the material from the first stage goes to the second stage. B. Run Conditions. Table I shows the process configuration, the coal, and the composition of the hydrotreater feed for each run. The space velocities for the hydrotreater in runs 242, 246, and 247 were similar and ranged from 0.9 to 1.1 lb of resid/(h.lb of catalyst), whereas
300 100
200
300
400
SO0
800
I
700
CATALYST AGE (Ib re8ldllb cat)
Figure 2. Average daily hydrotreater temperature for each run.
DITSL processing in run 248 had a space velocity of 0.6 lb of resid/(h.lb of catalyst). The amounts of resid input into the hydrotreater, as a function of time, in runs 242 and 247 were similar because the space velocities and amounts of resid in the hydrotreater feeds were similar. However, the feed in run 247 was heavier than that for run 242 and the other runs, because the resid contained a greater amount of preasphaltenes and unconverted coal. Run 246 deashed subbituminous hydrotreater feed from DITSL processing contained about 0.3 wt % preasphaltenes and no ash or unconverted coal. It also had lower sulfur and nitrogen contents and a higher hydrogen content than the deashed or nondeashed Illinois No. 6 coal (Moniz and Nalitham, 1985). Therefore, DITSL processing in run 246 had the lightest hydrotreater feed. When the configuration in run 246 was changed from the DITSL to the ITSL mode, the resid content of the hydrotreater feed increased about 50%, to 35 wt % of the feed, as a result of the different configuration. Run 248 used DITSL processing up to an age of about 100 lb of resid/lb of catalyst. The resid content of the hydrotreater feed was about 30-40 wt % . However, because the space velocity was only 0.6 lb of resid/(h-lb of catalyst), the total resid input to the hydrotreater as a function of time was similar to DITSL processing in run 246. DITSL processing of Illinois coal did not yield good resid conversion (Gough et al., 1986). Figure 2 shows the average daily temperatures in the hydrotreater for all the runs as a function of catalyst age. Most temperature increases during these runs were made to compensate for decreased resid conversion. Run 242 had a 50-min temperature excursion to 471 "C,which was caused by a power outage at a catalyst age of about 90 lb of resid/lb of catalyst. This excursion is only shown in Figure 2 as a small spike because this figure shows average daily temperatures.
2468 Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987
All four runs used Shell 324M catalyst in the second stage. The catalyst for run 242 was presulfided, prior to coal processing, using dimethyl disulfide added to a coal-derived solvent. Catalyst for other runs was presulfided using No. 2 fuel oil in place of the coal-derived solvent. Catalyst samples, typically 1 or 2 lb out of the hydrotreater inventory of 400-500 lb, were periodically withdrawn from the hydrotreater during the runs. These samples were replaced with presulfided catalyst to maintain a constant inventory. Run 246 had 2% Fe203and 1% dimethyl disulfide (on a moisture-free (mf) coal basis) input to the first stage to produce pyrrhotite in situ, which would function as a disposable catalyst to enhance conversion (Gir and Rhodes, 1983; Risbud and Nalitham, 1984). The other runs had no added catalyst in the first stage. 111. Experimental Section
A. Catalyst Samples. Shell 324M catalyst contains 12.4 wt % Mo, 2.8 w t % Ni, and 2.7 wt % P on an alumina support. This catalyst is in the form of cylindrical extrudates about 4 mm in length and 0.8 mm in diameter. Catalyst samples from run 242 were shipped from Wilsonville in toluene; samples from the other runs were sent in process solvent. Upon receipt, all samples were filtered, Soxhlet extracted with tetrahydrofuran (to remove as much soluble material as possible prior to analysis), and then dried under vacuum at 100 "C overnight. These catalysts are referred to as aged samples. A portion of each sample was regenerated by heating it to 500 "C a t a rate of 2 "C/min in air in a muffle furnace and leaving it at 500 "C overnight. Regeneration removed the carbonaceous deposits and oxidized the metal sulfides but did not remove the contaminant metals. Portions of both the aged and regenerated catalysts were ground to -200 mesh to eliminate the effects of intraparticle diffusion. The catalyst samples in this paper are designated by the run number followed by either the catalyst age in lb of resid/lb of catalyst or PS for presulfided catalyst. Results for runs 242,246, and 247 are reported up to catalyst ages of about 600 lb of resid/lb of catalyst. Results could not be compared beyond this age because there were major changes in process conditions in some of the runs,and large quantities of catalyst were replaced with fresh catalyst during one of the runs. Because of the configuration change at 100 lb of resid/lb of catalyst, only two catalyst samples from run 248 were used to determine the impact of DITSL processing of Illinois No. 6 coal on the catalyst. B. Characterization Methods. Carbon was determined by combustion of the aged catalysts in oxygen followed by coulometric determinations of C02. Distributions of carbon in the aged extrudatea were analyzed by using a Physical Electronics Model 595 Scanning Auger Microprobe (SAM). Catalyst extrudates for the SAM analysis were broken about halfway down their length and mounted in indium foil. Analyzed regions were sputtered before analysis to remove contaminants adsorbed from the air and were sputtered during data collection to prevent hydrocarbon deposition on the sample during analysis. The Mo, Ni, Fe, and Ti contents of aged catalysts were quantitatively determined by atomic absorption. Distributions of active and contaminant metals were obtained with a Cameca MBX electron microprobe. These microprobe analyses were performed on polished cross sections about halfway down the length of the aged extrudates. X-ray diffraction and electron spectroscopy for chemical analysis (ESCA) were used to determine if sintering of the active metals occurred during processing (Cable et al., 1985). X-ray diffraction was performed on a Siemens
-0
Run 2 4 2
0 Run 2 4 6
-.-.-
0 Run 2 4 7 A Run 2 4 8
' 0
100
200
300
400
500
600
7 0
CATALYST AGE (Ib residllb cat)
Figure 3. Catalyst carbon contents as a function of catalyst age.
diffractometer, and ESCA studies were performed on a Physical Electronics Model 548 instrument. Both analyses were carried out on ground samples. ESCA studies were done on fresh and regenerated samples, which were excited with A1 X-rays produced at 10 kV and 35 mA. Intensities were measured as areas under the A1 2p, Mo 3d3 t Mo 3d6 2, and Ni 2p3 peaks. Results, reported as t i e ratio of tke active m e d to Al, are directly proportional to active metal surface areas. Total catalyst surface areas were obtained by the BET method with nitrogen; total pore volumes and pore volume distributions were evaluated from the nitrogen desorption isotherms. Good reproducibility was achieved by degassing the samples in vacuum at ambient temperature prior to analysis. C. Activity Testing. Catalyst hydrodesulfurization (HDS) activity was determined by measuring the conversion of thiophene. Testing was performed with 0.2 g of catalyst at 350 "C and at atmospheric pressure in a fixed-bed flow reactor (Trudell, 1984; Stohl et al., 1987). Prior to testing, all catalyst samples were presulfided at 400 "C with a 10 mol % H2S in H2mixture flowing at 60 mL/min for 2 h. Each model compound was introduced into the reactor by bubbling hydrogen at a flow rate of 60 mL/min through two bubblers, in series, containing the model compound. The first bubbler was at room temperature and the second was maintained at 0 "C. Thiophene conversion was measured after attaining steady-state conversion, which took about 5 h. Concentrations of reactant and products were determined online with a Hewlett-Packard 5880A gas chromatograph equipped with a dual-flame ionization detector. The reaction products were n-butenes and n-butane. Results were compared to those of fresh gas-phase presulfided catalyst. D. Calculations. The surface areas, pore volumes, and activity testing results were corrected to a fresh catalyst weight basis, which enabled results for aged and regenerated catalysts to be directly compared to fresh catalyst, using a previously reported normalization method (Qader, 1986; Stohl et al., 1987).
IV. Results and Discussion A. Characterization. 1. Carbonaceous Deposits. Carbon analyses are used as an indication of the amount of carbonaceous deposits in the catalyst rather than the type of deposits that are present. Carbon contents of the catalysts from the four runs (Figure 3) increased rapidly as soon as coal processing began. Runs 242,247, and 248 all used Illinois No. 6 coal, but the catalysts from these runs had different carbon accumulations. Of these three runs, the greatest initial (
