Catalytic hydrotreatment of coal-derived residua

Fuel Processing Technology, 11 (1985) 273--287

273

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

CATALYTIC HYDROTREATMENT OF COAL-DERIVED RESIDUA

D A N I E L L. CILLO, G A R Y J. STIEGEL*, R I C H A R D E. T I S C H E R and N A N D K. N A R A I N

Pittsburgh Energy Technology Center, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, PA 15236 (U.S.A.) (Received October 23th, 1984; accepted May 28th, 1985)

ABSTRACT An investigation was conducted to determine the activities of various coal-liquefaction residua during catalytic hydrotreatment. Residua produced at low- and highseverity coal-liquefaction conditions were employed, as well as a nondeashed residuum produced at low-severity conditions. All experimental runs were performed in a continuous-flow hydrotreating unit using Shell 324M catalyst. Except for hydrogenation activity, catalyst activity declined with a typical S-shaped deactivation curve. The properties of the spent catalysts do not depend significantly upon prior processing o f the feedstock; however, the prior processing history of the feedstocks affected their reactivity and the rate of catalyst deactivation. The results indicate beneficial effects o f conducting coal liquefaction at low-severity conditions and of product deashing prior to catalytic hydrotreatment.

INTRODUCTION

The technology for the direct liquefaction of coal to liquid and/or solid fuels has undergone considerable change in recent years. The first processes to proceed to pilot-plant scale involved the thermal liquefaction of coal at high reactor temperatures (i.e., > 840°F) and pressures (i.e., > 1500 psig) and at long residence times (i.e., 1 hour or longer). The Solvent-Refined Coal process (SRC-I) was such a process and produced a solid material as its product. Modifications to this technology involved the recycle of the heavy vacuum-tower bottoms containing coal ash (SRC-II process) and the hydrogenation of the coal-liquefaction solvent in an auxiliary catalyst bed (Exxon Donor Solvent Process). The H-Coal process attempted to perform the solvent-hydrogenation and coal-dissolution reactions in a single reactor. In all of the these processes, the common feature was the use of a single liquefaction reactor to produce the coal-derived product. The concept of two-stage coal liquefaction was first developed in Germany before World War II. This process scheme employs thermal liquefaction *To whom correspondence should be addressed.

0378-3820/85/$03.30

© 1985 Elsevier Science Publishers B.V.

274

similar to that of the SRC-I process, followed by catalytic hydroprocessing of distillate. This technology was further developed by Consolidation Coal; however, operation on a pilot-plant scale was unsuccessful. The revitalization of this technology following the Arab oil embargo in 1973--74 occurred as a result of interest in upgrading primary coal-liquefaction products. In subsequent investigations, it was found that the severity (i.e., reaction temperature, pressure, residence time, etc.) in the liquefaction reactor could be reduced because the removal of heteroatoms could be performed more efficiently in a catalytic hydroprocessing reactor. The products produced in the liquefaction reactor under the lower-severity conditions were more reactive in the catalytic hydroprocessing reactor than products produced at higher severity [ 1 ]. Modifications to the two-stage liquefaction concept involve the placement of the hydrotreater directly after the liquefaction reactor, with product deashing occurring after hydrotreatment [2]. Results from tests employing this scheme have shown improved operability and efficiency of the deashing process, while catalyst deactivation was reported to be no more severe than during operations with interstage deashing. Neuworth and Moroni [3] have presented an excellent review of the development of two-stage coal-liquefaction technology. In the present investigation, a direct comparison is made among the reactivities of deashed residua obtained from the SRC-I process (highseverity operation) and from the Wilsonville Advanced Coal Liquefaction Test Facility (low-severity operation), and the nondeashed residuum, from the Wilsonville facility. These three feedstocks permit the comparison of liquefaction severity on the upgradability of the residuum and the determination of the impact of interstage deashing on product quality. Since the quality of the feedstock and the process configuration may affect catalyst activity, particular attention was paid to the rate of catalyst deactivation. EXPERIMENTAL

Feedstocks and catalyst Three residuum feedstocks were employed in the present investigation; their properties are summarized in Table 1. The SRC-I thermal residuum was obtained from C.E. Lummus. Although the exact history of this material is not known, it was produced at the Fort Lewis Solvent-RefinedCoal pilot plant during 1977 using Western Kentucky 9/14 coal. Typical high temperatures and long residence times, as discussed by Moschitto [4], were probably used during its production. The Wilsonville deashed and nondeashed residua were both obtained during Run 242, which used Illinois No. 6, Burning Star coal. The reaction temperature and residence time were 860°F and ~ 15 min, respectively, which are less severe condi-

275

tions than those used for the production of the SRC-I residuum. In R u n 242, a major portion of the recycle solvent was hydrotreated, whereas in the SRC-I process, the solvent is not hydrotreated. The differences in the properties :of the three residua can be seen in Table 1. The SRC-I residuum contains more nitrogen and preasphaltenes, and less oils and asphaltenes than the other residua. The Wilsonville deashed residuum contains more hydrogen, which can be attributed to the hydrogenation of the recycle solvent; if a correction for ash is considered, the Wilsonville nondeashed residuum would have a hydrogen concentration similar to that of the deashed residuum. Because of the large a m o u n t of ash in the nondeashed residuum, an analysis of sulfur forms was performed to determine the a m o u n t of organic and inorganic sulfur contained in the residuum. From the results shown in Table 1, the sulfur content of TABLE1 Properties of residuum feedstock SRC-I thermal residuum

Wilsonville deashed residuum

Wilsonville nondeashed residuum

Ultimate analysis, wt.% Carbon Hydrogen Nitrogen Sulfur

85.8 5.9 2.1 0.7

88.3 6.5 1.15 0.56

81.2 5.9 1.46 1.13

Atomic hydrogen-to-carbon ratio Water, wt.% Ash, wt.% Specific gravity

0.82 0.6 <0.1 1.30

0.88 0.06 <0.05 1.23

0.87 <0.03 7.1 1.25

Solvent analysis, wt.% Preasphaltenes Asphaltenes Oils (by diff.)

46.8 36.0 17.2

17.2 41.8 41.0

28.2 a 38.4 26.3

Insolubles by filtration, wt.% Ethyl acetate Cyclohexane

48.3 98.5

23.1 87.5

35.2 93.2

Sulfur forms, wt.% Inorganic

--

Organic

--

(ASTM D-1160) IBP, °F 5%, °F 10%, °F End point, °F Recovery, vol.%

620 792 900 940 18

0.04 0.52

0.13 1.00

Distillation

aDoes not include ash.

827 903 931 980 23

737 860 -921 13.5

276 t h e WilsonviUe d e a s h e d r e s i d u u m was a b o u t 93 wt.% organic sulfur c o m p a r e d t o 88 wt.% in t h e n o n d e a s h e d m a t e r i a l . N o analyses f o r sulfur f o r m s w e r e p e r f o r m e d o n t h e S R C - I r e s i d u u m . F o r all t h r e e f e e d s t o c k s , a p p r o x i m a t e l y 10 wt.% o f t h e r e s i d u u m boils b e l o w 900°F. T h e s o l v e n t u s e d in t h e p r e s e n t investigation was a h y d r o g e n a t e d creos o t e oil o b t a i n e d f r o m C.E. L u m m u s , B l o o m f i e l d , NJ. T h e analyses o f t w o d i f f e r e n t d r u m s o f m a t e r i a l u s e d in this investigation are s h o w n in T a b l e 2. N o significant d i f f e r e n c e s in t h e c h e m i c a l c o n s t i t u t i o n o f t h e m a t e r i a l s f r o m t h e s e d r u m s are a p p a r e n t ; t h e r e f o r e , a n y d i f f e r e n c e s in r e a c t i v i t y c a n n o t be a t t r i b u t e d t o t h e solvent. T h e f e e d s t o c k c o m p r i s e d an e q u a l p o r t i o n , b y weight, o f r e s i d u u m a n d s o l v e n t in o r d e r t o s i m u l a t e a t y p i c a l c o m p o s i t i o n o f t h e f e e d s t o c k to TABLE 2

Properties of creosote oil feedstock Drum No. 1

Drum No. 2

Ultimate analysis, wt.% Carbon Hydrogen Nitrogen Sulfur

90.6 8.5 0.20 0.18

90.5 8.5 0.24 0.17

Atomic hydrogen-to-carbon ratio Water, wt.% Ash, wt.% Specific gravity, 60/60 Kinematic viscosity, cS at 140°F

1.12 <0.03 0.01 1.067 21.8

1.12 <0.03 0.01 1.085 23.1

Solvent analysis, wt.% Preasphaltenes Asphaltenes Oils (by diff.)

<0.2 0.6 99.5

<0.2 1.2 98.9

0.1

0.7

Cyclohexane insolubles, wt.% Distillation (ASTM D-1160) IBP °F 5% °F 10% °F 20% °F 30% °F

40% °F 50% °F 60% °F 70% °F 80% °F 90% °F 95% °F End point, °F Recovery, vol.%

419 560 608 656 68O 693 730 759 784 807 847 868 1005 98

434 575 605 657 685 712 744 763 779 822 857 907 945 96

277 t h e Wilsonville h y d r o t r e a t e r . T h e d a t a p r e s e n t e d in T a b l e 3 are an average o f t h e f e e d s t o c k s a m p l e s a n a l y z e d f o r a p a r t i c u l a r run. In m o s t cases, t h e s e values can be c a l c u l a t e d d i r e c t l y f r o m t h e d a t a p r e s e n t e d in T a b l e s 1 a n d 2. I t can be seen in T a b l e 3 t h a t a p p r o x i m a t e l y 50 wt.% o f t h e f e e d s t o c k boils a b o v e 8 5 0 ° F . T h e c a t a l y s t used in this investigation was Shell 3 2 4 M ( N i - - M o / A l : O 3 ) a n d was o b t a i n e d f r o m t h e Wilsonville A d v a n c e d Coal L i q u e f a c t i o n T e s t Facility. This c a t a l y s t has a u n i m o d a l p o r e d i s t r i b u t i o n w i t h an average p o r e d i a m e t e r o f a b o u t 115 /~. T h e c a t a l y s t also c o n t a i n s a p p r o x i m a t e l y 24 wt.% active m e t a l s . A s u m m a r y o f t h e p h y s i c a l a n d c h e m i c a l characteristics o f t h e c a t a l y s t is p r e s e n t e d in T a b l e 4. A fresh s a m p l e o f c a t a l y s t was u s e d in e a c h e x p e r i m e n t a l run.

TABLE 3 Average feedstock properties SRC-I

Wilsonville

Wilsonville

thermal residuum

deashed residuum

nondeashed residuum

88.9 7.3 1.15 0.46

89.6 7.6 0.69 0.36

86.4 7.3 0.80 0.65

0.98 <0.03 0.15 1.103

1.02 <0.1 <0.1 1.138

1.01 <0.03 3.4 1.164

21.3 20.2 58.5

8.5 19.9 71.6

11.0a 20.7 64.9

25.9 43.3

7.8 26.0

15.5 33.0

Ultimate analysis, wt.%

Carbon Hydrogen Nitrogen

Sulfur Atomic hydrogen-to-carbon ratio

Water, wt.% Ash, wt.% Specific gravity, 60/60 Solvent analysis, wt.% Preasphaltenes Asphaltenes

Oils (by diff.) Insolubles by filtration, wt.% Ethyl acetate Cyclohexane Distillation (ASTM D-1160) IBP, °F 5%, °F 10%, °F 20%, °F 30%, °F 40%, °F 50%, °F 60%, °F aDoes not include ash.

428 605 647 708 748 789 830 --

494 611 663 725 776 824 873 --

494 602 653 710 759 808 851 932

278 TABLE 4 Physical and chemical properties of Shell 324M Bulk density, g/cm 3 Pellet density, g/cm 3 Pore volume, cm3/g Surface area, m2/g Average pore diameter, ~ NiO, wt.% MoO 3, wt.% A1203, wt.% Na20, wt.% SiO 2, wt.% P205, wt.%

0.78 1.38 0.43 180 115 3.4 19.3 62.5 0.1 0.3 5.7

Experimental procedures All experimental data were obtained from a continuous-flow stirredtank reactor; a detailed description of the apparatus and procedures, as well as data reproducibility, can be found elsewhere [5]. The runs using the SRC-I and nondeashed residua lasted for 100 h. The run using the Wilsonville deashed residuum lasted for 1200 h. A summary of the catalyst presulfiding and reaction conditions employed in this investigation is presented in Table 5. Although the catalyst presulfiding temperatures were slightly different, no effects have been observed over this small temperature range in other experiments. For these tests, overall material balance closures using the raw data were usually better than 95%. Closures were usually improved to better than 98% if carbon was forced to 100% by adjusting the feed rate. Because of some inaccuracies in the measurement of the feed rate and some leakage from the feed pumps, adjusting the feed rate to improve the closure is believed to be more appropriate. The activity of the catalyst in each run was monitored by collecting several control samples. The control samples were subjected to ultimate analysis, vacuum distillation, and rapid microfiltration using ethyl acetate and cyclohexane. Material soluble in ethyl acetate is considered to be oils and asphaltenes, whereas that soluble in cyclohexane is considered to be oils. Details of the rapid microfiltration procedure and its advantage over the conventional Soxhlet m e t h o d have been discussed elsewhere [6]. At least three material balance periods were performed during each run. During these periods, one sample of feedstock, three of tall gas, three of flash gas, and one of heavy oil product were collected. Very little light oil was produced during these runs. All gas samples were analyzed by gas chromatography. The feedstock and heavy oil samples were subjected to the following analyses: (1) ultimate analysis, (2) ash, (3) water content, (4) specific gravity, (5) vacuum distillation (ASTM D-1160), and (6) rapid microfiltration. The activity of the catalyst and reactivity of the feedstocks were

279

determined by calculating the conversion of cyclohexane- and ethyl acetateinsolubles, the extent of desulfurization and denitrogenation, and the percentage change in the hydrogen concentration of the feedstock due to reaction. At the end of each run, the spent catalyst was collected and extracted with tetrahydrofuran {THF) in a Soxhlet extractor to remove any excess oils from the catalyst surface. The extracted catalyst pellets were analyzed by mercury porosimetry and nitrogen BET in order to determine the pore size and pore volume distributions, and surface area of the aged catalyst. The extracted catalyst samples were also analyzed for carbon, hydrogen, nitrogen, and sulfur, and for deposited metals, such as titanium, iron, and calcium. TABLE 5 Experimental run conditions

Catalyst presulfiding Temperature, °C Pressure, psig Gas flow rate, L/rain Time, h H2S/H 2 concentration, tool.% Reaction conditions Temperature, °C Pressure, psig Hydrogen flow rate, scfh Residuum concentration, % Slurry feed rate, g/h Volume hourly space velocity, cm 3 residuum/h/cm 3 catalyst Catalyst volume, cm 3 Catalyst weight, g Agitator speed, rpm

400 a 50 1 5 10

415 2000 8 50 120 0.9 80 67 1600

aSulfiding temperature of 350°C was used for the run with SRC-I thermal residuum.

DISCUSSION OF RESULTS

Thermal base-line results

Noncatalytic base-line runs were performed using the SRC-I thermal residuum and the Wilsonville deashed residuum at the same reaction conditions as the catalytic runs. The reaction conditions are outlined in Table 5. A summary of the results from the thermal runs is presented in Table 6. As shown, the conversions of ethyl acetate- and cyclohexane-insolubles are only marginally different between the t w o feedstocks. S o m e desulfurization is observed, but no denitrogenation occurs under these conditions.

280 In both cases, the feedstock was found to be dehydrogenated under the present reaction conditions. Even though a noncatalytic experimental run using the nondeashed residuum from Wilsonville was n o t performed, the results would be expected to show trends similar to those obtained with the deashed residuum. TABLE6 Thermal conversion of SRC-I and Wilsonvilledeashed residua

z~H/C, % Desulfurization, wt.% Denitrogenation, wt.% Ethyl acetate-insoluble conversion, wt.% Cyclohexane-insoluble conversion, wt.%

SRC-I thermal residuum

Wilsonville deashed residuum

-2.75 9.7 -4.4 42.9 16.7

-2.23 23.7 -1.3 34.0 21.2

Catalytic results The results obtained from the 1200 h run using deashed residuum are presented in Fig. 1. The hydrogenation reactivity of the feedstock was determined by calculating the change in the atomic hydrogen-to-carbon ratio of the feedstock due to catalytic hydrogenation. Desulfurization and denitrogenation were calculated directly from the elemental analyses of the feedstocks and products, while conversion based on solubility of the feedstocks and products in cyclohexane were calculated from the microfiltration data. As shown in Fig. 1, desulfurization, denitrogenation, and conversion of cyclohexane-insolubles appear to follow a typical S-shaped deactivation curve although the data near the end of the run are inconclusive. Similar results have been reported by Schindler et al. [7] and Nalitham et al. [8] for catalyst deactivation in the L u m m u s and Wilsonville two-stage coal liquefaction processes. The hydrogenation acitivity of the catalyst, however, does not show an initial loss of activity but rather remains constant or declines slowly until the end of the run. For comparison of the reactivity of the residua, only those data obtained during the first one hundred hours of this run will be used. Although the data to be used for the comparisons from all runs are f r o m the initial period of rapid catalyst deactivation, differences in feedstock reactivity and the effect of the feedstock on the rate of catalyst deactivation could have implications on the final lined-out activity and life of the catalyst. However, to test all feedstocks and catalysts for extended periods of time such as that in Fig. 1 is both expensive and time consuming. A comparison of the hydrogenation results obtained using the three residuum feedstocks is presented in Fig. 2. No differences are observed

281

in feedstock reactivity or rate of catalyst deactivation during the first 100 h of operation. For the nondeashed residuum at longer reaction times, the hydrogenation activity of the catalyst has been observed to decline because of enhanced pore blockage owing to trace metals deposition [9,10]. If the trace metals are capable of diffusing into the catalyst pores, it would be expected that the loss of hydrogenation activity would occur sooner for the nondeashed residuum because of its higher concentrations of ash and trace metals together with additional coke deposition, which would ultimately plug the catalyst pores. These points will be discussed later in reference to the spent catalyst and data in Table 7. ~o

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282

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The desulfurization data obtained for the three feedstocks are presented in Fig. 3. Although the sulfur concentrations in the feedstocks are low and approximately the same, the Wilsonville deashed residuum is much more reactive than the other feedstocks. Because of the large a m o u n t of ash contained in t h e nondeashed residuum and its higher concentrations of inorganic sulfur, the data for the nondeashed residuum were corrected to account for the inorganic sulfur, which was assumed to be nonreactive. I00

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283

Regardless of whether or n o t the correction was made, the nondeashed residuum possessed the lowest desulfurization catalyst activity. Very little loss of catalyst activity was observed for the deashed and SRC-I residua; however, the nondeashed feedstock showed a substantial loss of activity. It would therefore be expected that desulfurization activity of the catalyst would be lost sooner with the nondeashed feedstock than with the deashed material. The results for catalyst denitrogenation activity are presented in Fig. 4. As shown, the denitrogenation reactivity of the SRC-I residuum and that of the nondeashed residuum are comparable, whereas that for the deashed residuum is slightly higher. The SRC-I residuum and nondeashed residuum had higher concentrations of nitrogen, and because all three feedstocks had comparable hydrogenation reactivities, the nitrogen compounds in the deashed residuum appear to be less refractory, hence more reactive, than those in the other feedstocks. It is likely that some of the more refractory c o m p o n e n t s are removed by the critical-solvent deasher used at Wilsonville. The rates of deactivation observed in the SRC-I-residuum and nondeashed-residuum runs are comparable to each other. The rate of catalyst deactivation for the deashed residuum is slightly less than that for the other feedstocks. 80

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Conversions based on insolubles in the feed and product samples, as determined by rapid microfiltration using ethyl acetate and cyclohexane as solvents, are shown in Figs. 5 and 6. In all cases, these data were corrected for the amount of ash contained in the feedstock. In Fig. 5, the deashed residuum again had the highest reactivity for the conversion of preasphaltenes, whereas the nondeashed residuum possessed the lowest reactivity. Rates of catalyst deactivation for the deashed and nondeashed

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285

residua appeared to be comparable, whereas only slight deactivation was observed for the SRC-I residuum. Because of the fairly low concentration of ethyl acetate-insolubles in the deashed residuum compared to the SRC-I residuum, a comparison of ethyl acetate-insoluble conversion is not very meaningful. A much more meaningful comparison of insoluble conversion is that based on cyclohexane-insolubles, since over 85 wt.% of the residuum is insoluble in cyclohexane. As presented in Fig. 6, the deashed residuum is considerably more reactive than the other t w o feedstocks, which show comparable reactivities. The rates of deactivation, however, appear to be comparable for the three feedstocks. No major changes in the physical properties of the feedstocks due to reaction were observed. The analyses of the spent catalysts obtained from these three runs are presented in Table 7. The catalyst from the run using the Wilsonville deashed TABLE 7 Analyses of spent catalyst SRC-I thermal residuum

Wilsonville deashed residuum

Wilsonville nondeashed residuum

Catalyst age, lb residuum/lb catalyst

90

Ultimate analysis, wt.% Carbon Hydrogen Nitrogen Sulfur

12.8 0.99 0.5 6.8

14.3 0.85 0.5 6.7

12.1 0.91 0.5 6.7

1100

90

Atomic hydrogen-to-carbon ratio

0.92

0.71

0.90

Metal analysis wt.% Titanium Iron Calcium Nickel Molybdenum

0.4 0.3 0.1 1.8 9.0

1.3 0.9 0.3 2.4 9.4

0.4 0.1 0.1 1.9 9.4

Atomic sulfur-to-molybdenum ratio

2.26

2.13

2.13

Pellet density, g/cm 3

1.90

1.78

1.73

Pore volume, cm3/g Mercury BET

0.15 0.20

0.15 0.17

0.16 0.21

Average pore diameter,,~ Mercury BET Surface area, m2/g Mercury BET

60 70

45 68

56 73

107 118

108 98

110 124

286

residuum had higher concentrations of carbon and trace metals, and smaller average pore diameter; however, this particular catalyst was processed to an age of approximately 1100 lb residuum/lb catalyst. The observed differences in activity could not be attributed to differences in the effectiveness of the sulfiding procedure because the atomic sulfur-to-molybdenum ratios are about the same. The concentrations of trace metals in the catalyst for runs using the deashed residuum are usually similar to those for the runs using SRC-I residuum at comparable catalyst ages. For the nondeashed run, the concentrations of titanium, iron, and calcium on the catalyst are again comparable to those for the SRC-I residuum. Also presented in Table 7 are pore size distribution data obtained from BET and mercury porosimetry analyses. For fresh and slightly aged catalysts, these two techniques yield similar results. However, for aged catalysts, the mercury porosimetry technique measures the pore mouth, which skews the pore volume distribution towards smaller pore sizes. Surface areas that are calculated b y integrating the mercury porosimetric pressure--volume data are therefore overestimated. In the adsorption mode, the BET technique is not hindered by the pore-mouth blockage; therefore, nitrogen BET measures an unbiased average pore size. A comparison of the results obtained from the t w o techniques indicates that pore-mouth blockage is occurring. Since the chemical and physical analyses of the spent catalysts obtained from these runs are not significantly different, the differences in feedstock reactivity and the effect of the feedstock on the rate of catalyst deactivation must be attributed to the reactivity of the components constituting these feedstocks. Hence, the performance of the catalytic hydroprocessing reactor is dependent upon the prior extent and severity of the processing of the residuum. CONCLUSIONS

The following conclusions can be made from the experimental results: (1} Deashing of the feedstock appears to be required for higher initial catalyst activity and slower rates of deactivation. (2) ~Deashed residuum produced at lower reaction severity is more reactive than that produced at higher severities. The properties of the spent catalyst appeared not to be significantly dependent upon the prior processing of the feedstock whereas catalyst activity and deactivation are dependent upon the past processing history. Based on the results from these experimental tests, a preferred two-stage coal liquefaction process should consist of low-severity coal liquefaction, followed by product deashing and then catalytic hydrotreating. This, however, does not mean that close coupling of the liquefaction and hydrotreating reactors is not desired because the chemistry of the free radicals and the occurrence of different types of retrograde reactions upon cooling could have affected the reactivities of these feedstocks.

287 ACKNOWLEDGEMENTS T h e a u t h o r s w o u l d like t o a c k n o w l e d g e Mr. R . F . H i c k e y ( f o r m e r l y w i t h DOE}, Dr. J. C h e n ( f o r m e r l y w i t h C.E. L u m m u s ) , a n d Dr. M. M o n i z {Catalytic, Inc.) f o r s u p p l y i n g t h e c a t a l y s t a n d f e e d s t o c k s used in t h e presen~ investigation. T h e assistance o f P E T C ' s A n a l y t i c a l C h e m i s t r y B r a n c h and, in p a r t i c u l a r , o f Mrs. M.J. M i m a a n d Dr. S.S. Pollack is v e r y m u c h appreciated. DISCLAIMER: Reference in this report to any specific commercial facility, product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy.

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