high-temperature sulfidation-regeneration of cu0...

I n d . Eng . Chem. Res . 1989,28,931-940 93 1

Massaldi, H. A.; Maymo, J. A. Error In Handling Finite Conversion Rosenberg, H. L.; Engdahl, R. B.; Oxley, J. H.; Genco, J. H. The Reactor Data by the Differential Method. J. Catal. 1968, 14, Status of SOz Control Systems. Chem. Eng. h o g . 1975, 71, 66. 61-68. Sarlis, J.; Berk, D. Reduction of Sulfur Dioxide with Methane over

McMillan, D. Process for Reduction of SOz with Hydrocarbon Vapor. Activated Alumina. Ind. Eng. Chem. Res. 1988, 27, 1951-54. US Patent 3615221, 1971.

Mulligan, D. J. Reduction of Sulphur Dioxide Over Transition Metal Received for review August 5, 1988 Sulphides. M. Eng. Thesis, McGill University, Montreal, Canada, Revised manuscript received January 23, 1989 1988. Accepted February 19, 1989

High-Temperature Sulfidation-Regeneration of Cu0-A1203 Sorbents

V. Patrickt and G. R. Gavalas* Department o f Chemical Engineering, California Insti tute of Technology, Pasadena, California 91 125

M. Flytzani-Stephanopoulos and K. Jothimurugesan Department o f Chemical Engineering, Massachusetts Insti tute o f Technology, Cambridge, Massachusetts 02139

Thermogravimetric analysis (TGA) and flow-reactor experiments were used to study sulfidation- regeneration of highly porous Cu0-A1203 sorbents. In TGA studies, sulfidation of reduced sorbents produced a high-temperature form of digenite ( C U ~ + ~ S ~ ) as the major crystalline product. When a platinum pan was used in the TGA, significant sulfur chemisorption on alumina occurred. Sulfur chemisorption on alumina was eliminated by use of a quartz sample pan. Reaction of the mixed-oxide sorbents with a mixture of H2S, H2, H20, and N2 in a packed-bed microreactor, a t temperatures between 550 and 800 "C, yielded prebreakthrough outlet-H2S levels considerably lower than those predicted by the sulfidation equilibrium of metallic copper. Independent reduction experiments confirmed tha t alumina stabilizes CuO against complete reduction to Cu. With this mechanism a t work, low prebreakthrough H2S levels are attributed to sulfidation reactions of copper a t oxidation states of +1 or +2. The sulfided sorbents were completely regenerable in air/N2 mixtures with no deterioration of subsequent sulfidation performance.

Among the most promising new technologies of power generation from coal are gasification integrated with combined-cycle power generation (gas turbine in series with a steam turbine), and coal gasification followed by oxidation in a molten carbonate fuel cell. Both technologies require removal of H2S (and COS) from the fuel gas prior to combustion. While this removal can be carried out a t ambient temperatures by established technology, removal a t high temperatures offers considerable improvement in process economics (Marqueen et al., 1986). In the original research on hot-gas desulfurization, pure metal oxides were tested, including those of zinc and iron (Morgantown Energy Technology Center, 1978). Recent work has fo- cused on mixed metal oxides, especially zinc ferrite (Zn- Fe,O,). Above about 600 "C, however, sorbents containing zinc oxide lose zinc by reduction to the volatile metal. Iron and copper oxides are not subject to this limitation but do not possess sufficiently large sulfidation equilibrium constants to provide the required level of sulfur removal. In the case of copper oxide, the equilibria

(1)

C U ~ O + H,S CUZS + HzO (2) are very favorable. In the fuel gas atmosphere, however, the reaction

(3) progresses rapidly so that sulfidation proceeds by reaction with metallic copper.

(4)

2CuO + H2S + Hz = CuzS + 2Hz0

CUO + H2 = CU + H2O

2Cu + H2S = CuzS + Hz

Present address: CR&DS, Monsanto Company, Creve Coeur, MO 63167.

Table I. Equilibrium Constants for Sulfidation Reactions 1,2. and 4 from Data b s Robie et al. (1984)

tema. K reaction 1 reaction 2 reaction 4 7 00 21.3 11.0 4.1 800 19.2 9.9 3.7 900 17.6 9.0 3.4

1000 16.3 8.3 3.2 1100 15.2 7.8 3.0

As shown in Table I, the equilibrium constant of reaction 4 is much lower than those of reactions 1 and 2. The constant of reaction 4 is such that sulfidation of metallic copper generally does not provide adequate H2S removal for the aforementioned power generation applications.

We have recently found that when copper oxide is employed in association with aluminum oxide, or iron oxide, or both, the level of sulfur removal is much higher than that obtained with pure copper oxide (Tamhankar e t al., 1986). Such behavior was attributed to the retardation of copper reduction to the metallic form engendered by the association with iron or aluminum oxides.

Previous work by Flytzani-Stephanopoulos et al. (1985) on the copper-containing mixed-oxide sorbents involved measurements of breakthrough curves in a packed-bed reactor and X-ray diffraction (XRD) analysis of fresh, sulfided, and regenerated sorbents to determine the crystalline phases present. The purpose of the present study was to obtain detailed mechanistic and qualitative kinetic information about the sulfidation and regeneration of the binary Cu0-A1203 sorbent. To this end, we have used extensively thermogravimetric analysis (in isothermal or temperature-programmed mode) combined with XRD and packed-bed reactor experiments. Some thermo-

0888-5885/89/2628-0931$01.50/0 0 1989 American Chemical Society

932

gravimetric sulfidation and regeneration experiments of a copper-aluminum oxide sorbent were presented by Gangwal et al. (1989).

Experimental Section 1 Sorbent Preparation. A well-known, complexation

method developed by Marcilly et al. (1970) was used to synthesize the mixed-oxide sorbents in a highly dispersed state The complexing agent (citric acid), equimolar to total metal cations, is added to an aqueous solution of metal nitrates of the desired stoichiometric proportions. An amorphous citrate precursor is prepared by evaporation of this solution. This evaporation proceeds first rapidly in a rotary evaporator under vacuum a t 70 "C, until a marked increase in viscosity of the solution is observed, and then for several hours (3-24) in a vacuum oven at 70 "C. until an amorphous solid foam forms. The foam is carefully broken up and calcined a t the desired temperature (between 550 and 900 "C) in air either under static conditions or under flow, to produce the final mixed oxide.

2. Wet Chemical Analysis. Hot nitric acid extraction, coupled with atomic absorption spectroscopy (AAS), was used to determine quantitatively the amounts of CuA1204, CuO, and A1,03 present in a given sample. Paulsson and Ros6n (1973) have reported that copper in the form of CuA120, cannot be extracted by hot nitric acid; however, copper present as a pure oxide is easily extracted. This wet chemistry has been confirmed in our laboratories. The concentration of the copper ions in solution was easily and accurately determihned by a modified Varian Techtron Model AA5 atomic absorption spectrometer.

3. X-ray Diffraction. A Siemens D500 step-scanning diffractometer employing Ni-filtered Cu Ka radiation (1.54056 a) was used for qualitative chemical analysis of the polycrystalline components present in a sample. The X-ray tube was operated at 40 kV and 30 mA. X-ray powder diffractograms in a 28 range of 25-60" (or 30-50") were obtained that could detect the presence of CuO, CuA1,04, Cu20, CuAlO,, Cu, crystalline transition alu- minas, Cu2S, CuS, and various other copper sulfides. Samples were finely ground and spread out evenly, so as to avoid preferred orientations, over a piece of double-stick tape adhered to a glass slide. Diffractograms were scanned at 0.1" intervals (in 28) for 60 s per interval.

4. Scanning Electron Microscopy. The samples were examined by an ETEC Corporation scanning electron microscope operating a t 20 kV with a resolution of 70 A. The sample was carefully ground and sprinkled on a metal stub containing a light coat of silver paste. This metal stub was then coated with a gold-palladium film 100 A in thickness prior to observation.

5. Thermogravimetric Analysis. A Du Pont 951 thermogravimetric analyzer interfaced through an ana- log-to-digital converter to a microcomputer served to measure the sample weight continuously. The quartz housing and flow path of the TGA were modified according to the system used by Ruth et al. (1972) so that corrosive gases, such as H2S, could be accommodated. A temperature programmer enabled either isothermal operation or operation under a linear temperature profile.

It was verified experimentally that the rate of reduction of ultrapure CuO (99.999% purity) decreased when the particle size exceeded 25 mesh (707 pm), when the gas flow rate was less than 60 cm3/min or greater than 120 cm3/ min, or when the sample size exceeded 50 mg. On the basis of these findings, a 30-mg sample of particles, 120-170 mesh (88-125 km), was typically employed to minimize internal mass-transfer effects (e.g., intra- and inter-particle diffusion), and the gas flow rate was fixed at 80 cm3/min

Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989

to minimize external mass-transfer effects (e.g., gas-film diffusion and particle entrainment). A flow rate of 80 cm3/min was also used for the stream of N2 serving as the protective backflow gas.

For temperature-programmed reduction, a reactant gas containing 5% H2 in N2 was used and the temperature was ramped from 233 to 912 "C a t a rate of 1.6 "C/min. For experiments other than temperature-programmed reduction, the reaction-gas mixtures were 5% H2 in N2 for the reduction runs, 4.2% H2S in N, for the sulfidation runs, and air for the regeneration runs.

6. Packed-Bed Microreactor Experiments. The experimental system will be described here very briefly, for it has been described in detail by Tamhankar et al. (1986). The reactor consists of a quartz tube of 1-cm i.d. and 41-cm length loaded to a bed height of 4-6 cm with a mixture of sorbent granules (-20 to +35 mesh) and inert alumina particles of low surface area (Alcoa T-64, -28 to +48 mesh). The sorbent bed was supported by a fritted quartz disk on one end and packed with quartz wool at the opposite end. The reactor tube was mounted vertically inside an electric furnace, and the bed temperature was monitored by a K-type thermocouple moving inside a quartz thermowell (0.3-cm i.d.) concentric to the reactor tube. Different gases from cylinders passed through pu- rifiers and calibrated flowmeters into a common gas line. The desired gas mixture flowed either upward (sulfidation) or downward (regeneration) through the sorbent bed. The lines leading to the reactor tube were insulated and heated. Nitrogen bubbling through water maintained at a constant temperature in a three-neck flask assembly was used to introduce known amounts of water vapor into the gaseous feed stream. Temperatures at various locations in the reactor system were monitored by K-type thermocouples connected to a multichannel digital readout. The reactor pressure in all cases was slightly above atmospheric.

For a sulfidation run, fresh or sulfur-free sorbent was exposed to a feed gas containing H2 (15-20%), H20 (7-25%), H2S (0.2-1%), and N2 balance a t a constant temperature in the range 550-800 "C. Sulfided sorbents were regenerated using a mixture of 90 mol % N2 and 10 mol 9% air at temperatures between 550 and 800 "C. Feed gas rates of - 200 cm3/min (STP) were typically used, and the gas hourly space velocity was -2000 h-' (STP) in most tests. The product gas was analyzed for H2S in sulfidation runs and SO, in regeneration runs by a HP-5830A gas chromatograph equipped with a flame photometric de- tector.

The TGA experiments and the packed-bed microreactor experiments each have distinct advantages and disadvan- tages and provide complementary information. The packed-bed reactor experiments are suitable for obtaining overall sorbent performance under conditions similar to those of an industrial reactor. These experiments can provide information about sulfur-removal efficiency and can generate large samples for solid analysis. However, they are not well suited to kinetic studies because the inherent gradients of gas and solid composition along the reactor make the calculation of reaction rates mathe- matically tedious. TGA runs, on the other hand, involve uniform gas and solid composition and are better suited for kinetic investigations. However, the small sample size that must be used to avoid mass-transfer limitations may be insufficient for certain analytical procedures.

Results and Discussion TGA results are discussed in terms of normalized weight

(W,) versus time, while microreactor results are given in terms of the mole fraction of H,S in the product gas versus

Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 933

Table 11. Fresh Sorbents Used in TGA Studies BET

mole % surface sample CuO:CuA1,04:A19On area, m2/g

72-1-CA 47.5547.5 120 33-1-CA6 2:96:2 2.72 11-1-CA3 1oo:o:o 1.11

normalized time (t/t*). Normalized weight is the ratio of the instantaneous weight change to the maximum weight loss as follows:

where m, mo, and mf are the instantaneous, initial, and final mass (at complete reduction to Cu), respectively. For example, following complete reduction of pure CuO to Cu, the final normalized weight is WN = 0. Subsequent complete sulfidation of this Cu to Cu2S results in WN = 1. Normalized time is the ratio of the actual run time, t , over the time that would be required for complete conversion of the sorbent to the postulated sulfide product, t*.

1. Mechanistic and Kinetic Studies. a. Sorbent Characterization. Two amorphous citrate precursors were prepared and exposed to 4-h calcinations. One precursor was calcined a t 900 "C to yield predominantly compound oxide (CuA1204, sample 33-1-CA6), while the other was calcined a t 550 "C to yield a mixture of oxides (CuO and A1203, sample 72-1-CA). For comparison, a sample of pure CuO was prepared by the same procedure and calcined in air a t 550 "C for 4 h (sample 11-1-CA3). Components present in crystalline form were identified by XRD analysis, while quantitative chemical compositions (Table 11) were determined by atomic absorption spectroscopy (AAS). SEM analysis showed the markedly different microstructures possessed by these three materials (Figure 1). Note that the micrographs display both a primary porous structure and a superimposed secondary texture. The secondary texture is smooth in the case of CuA1204 (sample 33-1-CA6), homogeneous and coarse for pure CuO (sample ll-l-CA3), and inhomogeneous with granules for a mixture of oxides (sample 72-1-CA). Con- sequently, the presence of free alumina creates the ap- pearance of crystallites on the surface of fresh sorbents.

Figure 2 shows the results of temperature-programmed reduction (TPR) of the three sorbents. The major observation is that the reduction rate of CuA1204 (sample 33-1-CA6) is more than 1 order of magnitude lower than that of pure CuO (sample 11-1-CA3) proceeding through CuA102 and Cu20 as intermediates. Furthermore, independent TGA experiments revealed that, for a sorbent containing a mixture of CuA1204, CuO and inert A1203, the major fraction of the pure oxide, CuO, were reduced rapidly, while a smaller fraction of the CuO and the compound oxide, CuA1204, was reduced very slowly. For sample 72- 1-CA, 37 mass ?& of the CuO was observed to reduce more slowly than pure CuO. The association of CuO with A1203 was believed to be responsible for retarding reduction of this species (Patrick and Gavalas, 1989). This phenome- non has been reported for NiO/7-A1203 (Puxley et al., 1983) and CuO/y-A1203 prepared by impregnation (Friedman and Freeman, 1978; Strohmeier e t al., 1985).

The type of transition alumina present in fresh samples could not be ascertained by XRD. Since no amorphous halo was evident in X-ray diffractograms, it was assumed that the alumina in fresh samples existed in a microcrystalline form. Furthermore, both the high BET surface areas measured for fresh samples (Table 11) and the association between CuO and alumina, which was compa-

(a) 72-1-CA (1,) 33-I-CAB

( c ) 1 l - l - C A 3

Figure 1. Scanning electron micrographs of fresh oxides prepared by the citrate process. All cases a t 2750X: (a) CuO, A120s (72-1-CA); (b) CuA1204 (33-1-CA6); (c) CuO (11-1-CA3); markers are 1 pm.

1

0 . 8

0 . 6

z 3

0 . 4

0 . 2

0 .

0 . 100 200

TIFIE ( b l I N )

300

Figure 2. TPR of samples (a) 11-1-CA3, (b) 72-1-CA, and (c) 33-1- CA6. Temperature increased by 1.6 OC/min starting from 233 "C.

rable to that reported for materials containing CuO and 7-A1203, indicated that some fraction of the alumina in fresh samples was 7-A1203.

b. Isothermal Experiments in the TGA. Desulfur- ization of coal gas by CuO or Cu0-A1203 involves simultaneous reduction and sulfidation of the sorbent (reactions

934 Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989

z 3

a

2 . 5

2

1 . 5

1

0 - 5

0 .

0 . 25 50 75

TIME (MIN) Figure 3. Reduction, sulfidation, and air regeneration in series for sample 72-1-CA at (a) 600 "C, (b) 700 "C, (c) 800 "C, and (d) 900 "C. In all cases, regeneration commenced at 43 min from the origin of time.

1-4). For the purpose of mapping out the reaction network and measuring reaction rates, however, it is useful to separate reduction and sulfidation. Therefore, in the TGA experiments, reduction and sulfidation were carried out consecutively.

Isothermal reduction and sulfidation of sample 72-1-CA were performed in the TGA system for temperatures of 600,700,800, and 900 "C (Figure 3). Consistent data were obtained by careful reproduction of sorbent heating procedures. Reduction was carried out with 5% H2 in N2 for the first 5 min, followed by sulfidation with 4.2% H2S in N2 for the next 35 min. XRD analysis of a sample following sulfidation a t 700 "C identified CulbS (digenite) as the dominant crystalline phase, while SEM analysis of the same sample revealed large crystals (5-12.5 pm) com- posed of interlocking malformed octahedra. In the ma- jority of the octahedral building blocks, two of the eight faces predominated, creating flat hexagonal plates. This same crystal morphology was observed by Donnay et al. (1958) in scanning electron micrographs of digenite crystals grown from mixtures containing copper-to-sulfur ratios between 9:5 and 2:l and grown a t temperatures between 500 and 775 "C. Since for both Donnay's study and this study SEM was performed on samples cooled to room temperature, the observed phase was a low-temperature form of digenite, low digenite. There exists a high-temperature form of digenite, high digenite, that forms from low digenite a t temperatures greater than approximately 83 "C (Craig, 1974).

High and low digenite both have sulfur atoms in approximate cubic close packing and are solid solutions of formula Cug+,S5. On the other hand, the composition range of low digenite (-0.166 < x < 0.125) is narrower than that of high digenite (-0.35 < x < 1.00), and low digenite has been isolated in the form of various polymorphs or superstructures of high digenite (Roseboom, 1966). For example, high digenite has a cubic cell of lattice parameter 5.56 A, while low digenite has a pseudocubic cell with lattice parameter (5.56 &N, where N has been found to

( c ) Sulfidcd (d) S I J I f i d C d

Figure 4. Scanning electron micrographs (all cases at 2750X) of sample 72-1-CA (a) fresh, (b) reduced at 700 "C, (c) sulfided at 700 "C in a platinum pan, and (d) sulfided at 700 "C in a quartz pan (markers are 1 pm).

take on both integral (i.e., N = 5,6) and nonintegral (i.e., N = 5.2,5.5,5.7,5.8) values (Morimoto and Gyobu, 1971; Morimoto and Koto, 1970; Morimoto and Kullerud, 1963). While the method of powder XRD enables the distinction between high and low digenite, the method of single-crystal XRD is required for identification of a particular super- structure of low digenite.

The succession of scanning electron micrographs shown in Figure 4 exhibits the marked structural changes ac- companying consecutive reduction and sulfidation a t 700 "C. The fresh 72-10! having inhomogeneous grainy texture is converted upon reduction to a dispersion of spherical copper particles on alumina. Subsequent sulfidation produces a bimodal dispersion of large (2-6-pm), octahedral crystals and small (<0.3-pm), irregular crystals of digenite. The weight gain during sulfidation of 72-1-CA suggests that the amount of nonstoichiometric sulfur increases with reaction temperature. Since a value WN = 1 corresponds to total conversion of copper to stoichiometric Cu2S, nonstoichiometric sulfur produces weight gain WN > 1. However, sulfidation a t 900 "C exhibits unexpected behavior, suggesting decomposition of the nonstoichiometric sulfides that have formed.

Regeneration of sulfided sorbents in air was investigated to test the sorbent performance in repeated sulfidation- regeneration cycles. Regeneration was carried out in air for 40 min following sulfidation. The regeneration of the sorbent nominally proceeds by the reactions

(6) cu2s + 20, = 2CuO + so2 19 + x

2 CUg+xS5 + -02 = (9 + X)CUO 5SO2 (7)

followed by the solid-state reaction


3 . 5

3

2 . 5

z P 2

1 . 5

1

0 . 5

0 10 20 30 4 0

TIbE ( M I N )

Figure 5. Reduction followed by sulfidation of pure A120, using a platinum pan.

the extent of which depends on the regeneration temperature. However, the production of SOz in creactions 6 and 7 can lead to the simultaneous formation of sulfate by the following side reaction:

(9)

Isothermal regeneration of sulfided 72-1-CA at 600, 700, 800, and 900 "C is included in Figure 3. Regeneration at 600 "C is rapid and yields both CuO and a significant fraction of CuSO, (corresponding to about one-third of the copper). The slow weight loss during the remainder of regeneration suggests the presence of other sulfur-containing species since CuSO, does not decompose below 650 "C. It is possible that a portion of the weight gain during regeneration is due to the adsorption of SOz on alumina or to the formation of aluminum sulfate. The presence of CuS04 was confirmed by XRD, while the presence of other sulfur-containing species could not be detected directly. A t 700 "C, the sulfate formed (about 8% of the copper) and decomposed at appreciable rates. At 800 and 900 "C, there is no evidence of sulfate formation during regeneration. XRD analysis shows a higher content of CuA1204 in the regenerated sorbent as compared to the fresh sorbent, in keeping with the fact that the regeneration temperatures are greater than the calcination temperature (550 "C) used in sorbent preparation. The sample regenerated a t 900 "C contained some crystalline CuA102 in disagree- ment with the thermodynamic data of Jacob and Alcock (1975). The implication is that a fraction of CuAIOz remained unreacted during both sulfidation and regeneration at 900 "C. A possible explanation for this finding is the formation of a layer of sintered CuAlZ0, surrounding CuAIOz during reduction, causing mass-transfer resistance to the sulfidation and regeneration of CuA102.

c . Sulfur Chemisorption on Alumina. The sulfidation curves of Figure 3 suggest a contradiction: Weight gain exceeds W, = 1.1, which corresponds to complete conversion of copper to Cuss, (digenite), the most sul-

CUO + so2 + 1/02 = c u s o ,

Table 111. Normalized Weight, WN, Following Reduction, Sulfidation, and Regeneration in the TGA

sample T," "C panb 72-1-CA 700 P

Q

Q

Q

800 P

11-1-CA3 700 P

800 P Q

33-1-CA6 700 P Q

Q 800 P

reduction 0.03 0.07 0.06 0.03 0.00 0.00 0.12 0.00 0.24 0.28 0.03 0.03

sulfidation 1.27 0.96 1.47 0.99 1.10 1.06 1.11 1.05 1.26 0.93 1.51 1.00

regeneration 1.10 1.03 1.11 0.99 1.04 1.00 1.02 1.02 1.12 1.06 1.07 1.02

"Temperature measured about 1 cm above the sample pan. b P is platinum; Q is quartz.

fur-rich form of sulfide identified by XRD. One possible explanation is that the alumina component of the sorbent contributes directly to sulfur retention. To test this possibility, a sample of pure A1203 (prepared by the citrate process using calcination a t 550 "C) was exposed to isothermal reduction and sulfidation at different temperatures (Figure 5 ) , as in the case of sample 72-1-CA (Figure 4). The final weight (W,) used in the expression for WN in Figure 5 was estimated as 0.05Wi based on a separate TPR experiment (Thomas et al., 1981). During sulfidation, the alumina showed significant weight gain, which increased with temperature. Reduction and sulfidation in succession of an empty platinum pan at 700 and 800 "C revealed no weight change; thus, chemisorption of sulfur on the platinum pan or formation of platinum sulfides was negligible. Since alumina is inert to sulfidation, these observations indicate the adsorption of sulfur-containing species on alumina.

d. Experiments using a Quartz Pan. In an attempt to eliminate any role of 'platinum on sulfur species chemisorption on the alumina in the sorbent, reduction and sulfidation experiments a t 700 and 800 "C were repeated using a quartz boat in the TGA. While the basic trends of the weight cycle were similar to those observed with the platinum pan, the weight excursion above W, = 1 was drastically reduced (Table 111). Note that this weight excursion was significant only for the alumina-containing samples. With both the quartz and the platinum pans, the weight gain following sulfidation increased with temperature. The difference in the weight excursion above W , = 1 between the experiments with the platinum and quartz pans suggests that the platinum pan catalyzed the decomposition of H2S to elemental sulfur, which, in turn, chemisorbed readily on alumina. This conclusion is supported by the well-known, catalytic activity of platinum in HzS decomposition (Fukuda et al., 1978; Wore11 and Kaplan, 1979; Bartholomew and Agrawal, 1982).

The rates of reduction for samples 72-1-CA, 33-1-CA6, and 11-1-CA3 revealed that the maximum rates of both reactions were highest for sample 72-1-CA and lowest for sample 11-1-CA3. This difference is attributed to sintering of copper, which occurred in sample 11-1-CA3 but not in 72-1-CA, in agreement with independent experiments (Patrick and Gavalas, 1989). The intermediate reduction and sulfidation rates exhibited by sample 33-1-CA6 were due to the slower reduction and sulfidation kinetics of CuAlZO4 as compared to CuO. These trends were observed to be even more drastic a t 800 "C than at 700 "C.

Low digenite (Cus+,S5) was again identified by XRD in cooled samples, suggesting that high digenite was the major sulfidation product a t 700 and 800 "C (Table IV) for all


Table IV. XRD Analysis of Sulfided Sorbents cu9+*s5 CU$, wt % sorbent sulfidation temp, "C crystallinity$ % CuA1204 y-Al203, wt %

CAT" 800 50-60 none 45 55 none CA;I" 72-1-CAb 72-1-CAE 72-1-CAC 11-1-CA3b 11-1-CA3' 11-1-CA3' 33-1-CA6b 33-1-CA6' 33-1-CA6'

800 700 700 800 700 700 800 700 700 800

50-60 N A N A N A N A N A N A N A N A N A

none trace trace trace none none none major major mediui m

29 none none none none none none none none none

71 none none none none none none none none none

none major major major major major major medium small major

' " From microreactor experiments. T G A experiments using the platinum pan. TGA experiments using the quartz pan. NA: not available.

three samples. In addition, a large quantity of unreacted CuA1204 was identified in sample 33-1-CA6 after sulfidation a t 700 "C. This observation was consistent with the extent of reduction to metallic copper, which at 700 "C was nearly 100% for 72-1-CA and 11-1-CA3 but only 70% for 33-1-CA6 (Table 111). Copper oxide, CuO, and products of reduction Cu20, CuA102, and Cu were not identified a t the end of the sulfidation. Evidently, these compounds reacted more rapidly than CuA1204 to form the sulfide

In a study by Chung and Massoth (1980), i t was found that CoA1204 could not be sulfided a t 400 "C, and conse- quently, i t was proposed that tetrahedrally coordinated C O + ~ was very stable. Both CoA1204 and CuA1204 possess spinel structures; however, the distribution of cations in each compound is different. CoA1204 is a normal spinel with Co2+ occupying exclusively tetrahedral sites and A13+ occupying exclusively octahedral sites (Navrotsky and Kleppa, 1968), while CuA1204 is a partially inverse spinel with Cu2+ occupying both tetrahedral (tet) and octahedral (oct) sites such that x = 0.4 in the formula [Cul-,A1,],,- [Cu,A12-,lod04. I t is conceivable that the 60% Cu2+ that occupies tetrahedral sites in CuA1204 was responsible for the slow sulfidation kinetics observed for sample 33-1-CA6 at 700 and 800 "C.

The XRD patterns of sulfided 33-1-CA6 reveal that the peaks of CuA1204 are about twice as intense as those of Cug+,S5. In view of the sharpness of these peaks, the intensities are approximately proportional to the content of the crystalline compounds. However, comparing the weight loss during reduction with the weight gain during sulfidation (Table 111) shows that the content of the sulfide is actually higher than the content of unreacted CuA1204. Thus, the XRD intensity relations can be explained only by assuming that the sulfide is predominantly in an amorphous state form. Furthermore, the XRD intensities for Cug+,S5 formed by sulfidation of 11-1-CA3 are 2-3 times greater than those for Cug+,S5 formed by sulfidation of alumina-containing samples, suggesting that a high fraction of copper sulfided in the presence of alumina forms amorphous or microcrystalline sulfide.

The scanning electron micrographs (Figures 4 and 6) of sulfided alumina-containing samples (sulfided 72-1-CA and 33-1-CA6) show large (2-6-pm), octahedral crystals and small (<0.3-pm), irregular crystallites of C U ~ + ~ S ~ . The scanning electron micrographs (Figure 7) of sulfided CuO (sulfided ll-l-CA3), however, show exclusively large (8- 16-pm) crystals of sulfide. Apparently, alumina hinders crystallite growth of digenite (Cug+,S5). Furthermore, the bimodal dispersion of Cug+,S5 in alumina-containing samples is consistent with the previous conclusion that the sulfide is present in both amorphous and crystalline forms. The sulfided CuO sample, on the other hand, is well

cu9+xs5.

(a) Frrsh

(c) Srilfidrd

f

((1) Sulfidcd

Figure 6. Scanning electron micrographs (all cases a t 2750X) of 33-1-CA6 (a) fresh, (b) reduced a t 700 "C, (c) sulfided a t 700 "C in a platinum pan, and (d) sulfided a t 700 "C in a quartz pan (markers are 1 pm).

(a) Fresh (b) Sulfidcd

Figure 7. Scanning electron micrographs (both cases a t 2750X) of 11-1-CA3 (a) fresh and (b) sulfided a t 700 "C in a platinum pan (markers are 10 pm).


Table V. Propert ies of Fresh Sorbents Used in Microreactor Experiments crystalline phases XRD,

wt 70 calcination BET bulk temp ("C), pore vel,* surface density, porosity,b crystallinity,

sorbent" time (h) cm3/g area, m2/g g/cm3 70 % CuO CuA1204 a-A1203

CAI1 850, 4 0.37 5.4 1.72 58 85 13 87 none 550, 3 0.39 99 1.32 68 C 95 none 49. CAI

"Calcined in static air, -20 to +35 mesh granules. *Based on Hg porosimetry data only (pore diameter larger than 60 A). "road reflections.

crystallized as evidenced both by sharp high-intensity peaks in XRD patterns and large crystals in scanning electron micrographs.

The presence of alumina also plays a role in the weight attained after sulfidation at 700 "C (Table 111). While 11-1-CA3 sulfided at 700 "C approaches a final weight W, = 1.1, the alumina-containing samples (72-1-CA and 33- 1-CA6) approach final weights WN C 1. These final weights imply that reduction and sulfidation of 11-1-CA3 at 700 "C result in nearly complete conversion to digenite (Cug+*S5), while for 72-1-CA and 33-1-CA6 conversion is partial, leaving behind the unreacted reduction intermediates Cu, Cu20, and CuA102. These reduction intermediates are not identified by X-ray diffraction, suggesting that they are present as a fine dispersion (C50-A crystallites). An attempt was made to identify these reduction intermediates by X-ray photoelectron spectroscopy. However, extensive charging caused by alumina rendered the attempt futile. I t is possible that the reduction intermediates are hindered against sulfidation because of association with alumina. A similar effect was observed for a certain fraction of CuO during reduction of these sorbents.

e. Regeneration Experiments. Sulfate formation during regeneration was found to depend on the alumina content of the sorbents. Regeneration of sorbent 11-1-CA3 (no alumina) yielded sulfate at 600 "C (14% of the copper) but not a t 700 "C. By contrast, regeneration of the alumina-containing sorbents 72-1-CA and 33-1-CA6 yielded sulfate a t both 600 and 700 OC, and the amount of sulfate was higher for sorbent 72-1-CA, which had the higher content of free alumina.

A related observation concerning the role of alumina can be made by comparing sulfate formation in experiments using a quartz pan to sulfate formation in experiments using a platinum pan. For both sorbents, 72-1-CA and 33-1-CA6, more sulfate is formed with the quartz pan. We have no convincing explanation for this behavior although it is possible that the sulfur chemisorbed on alumina during sulfidation in the presence of the platinum pan may hinder the catalytic role of alumina in sulfate formation. Alternatively, the weight gain attributed to copper sulfate formation may instead be due to either SO2 chemisorption on alumina or the formation of surface aluminum sulfate, as mentioned earlier.

The ratio of CuO to CuA1204 in the regenerated 33-1- CA6 and 72-1-CA materials depends, as expected, on the regeneration temperature. In the case of 33-1-CA6, the content of CuAl,04 is less than that in the original sample, while for sample 72-1-CA the content of CuA1204 is higher than in the original sample. The difference in chemical composition between regenerated and fresh materials is easily explained by the difference between calcination and regeneration temperatures.

A sample of sulfided 11-1-CA3 was cooled during regeneration to identify the chemical species present in the sharp dip of the regeneration weight loss curve. XRD identified the presence of Cu, Cu20, CuO, and digenite in

samples cooled in this manner. The isolation of Cu and Cu20 during regeneration suggests stepwise oxidation:

(10)

(11)

(12)

2. Packed-Bed Studies. Packed-bed reactor experiments were carried out with two sorbents prepared as before by the citrate method. The first sorbent (CAI) was prepared by calcining the citrate precursor for 4 h a t 550 "C and contained a mixture of the pure oxides CuO and Al,O,. The second sorbent (CAI,) was prepared by 4-h calcination at 850 "C and contained mainly the compound oxide CuA1204. Table V lists the properties of the two sorbents. The compositions listed were determined by XRD analysis and refer to the crystalline phases. Broad reflections in the XRD pattern of CAI indicated that the material was microcrystalline, while narrow peaks in CAI1 showed a well-crystallized solid. The higher BET surface area of CAI, calcined a t 550 "C, is due predominantly to the presence of the free alumina. In terms of pore volume and macroporosity, the two sorbents were similar, but CAI1 was made up of larger grains and had lower microporosity. In scanning electron micrographs of sorbent CAI, one can clearly see the macropores and grains contained in the sorbent (Figure 8).

The sulfidation performance of sorbents CAI and CAI1 in the packed-bed microreactor was examined at temperatures between 550 and 800 "C. The feed gas contained H2S, hydrogen, water, and nitrogen. The results have been summarized in Figures 9 and 10. At all temperatures, the prebreakthrough H2S levels remained below those corresponding to sulfidation equilibrium for metallic copper (Table I). The low outlet-H2S levels were due to the sta- bilization of copper in oxidation states of +2 or +1 when in association with alumina, as compared to bulk CuO, which is rapidly reduced to metallic copper. The sulfidation equilibria for cupric oxide or cuprous oxide are much more favorable than those of copper metal.

The temperature of regeneration has an important effect on sulfate formation, as noted earlier. Another somewhat less tangible effect is evident in the relative performance of fresh and regenerated sorbents. When the regeneration temperature is higher than the calcination temperature, the ratio of compound oxide (CuA1204) to mixed oxide (CuO and A1203) in the regenerated sorbent would be higher than in the fresh sorbent and vice versa. This trend is manifested in a comparison of the breakthrough curves for sulfidation of CAI and CAD as a function of regeneration history (Figures 9 and 10). For example, CAI1 exhibits markedly lower conversion than CAI following sulfidation of the fresh sorbents a t 550 "C (cycle 1). This difference exists in part because fresh CAI contains no CuA1204, while fresh CAI1 contains mostly CuA1204, and in part because CuA1204 is reduced and sulfided rather slowly. After two sulfidation-regeneration cycles a t 550 "C, the sulfidation curves (cycle 3) for the two sorbents are nearly identical

C U ~ S ~ + 502 = ~ C U + 5S02

4cu + 02 = 2cu20

2Cu20 + 02 = 4CuO


Table VI. Physical Properties of Sorbents Reaction in the Microreactor BET surface

sorbent reaction reaction temp, "C pore vo1: cm3/g area, m2/g bulk density, g/cm3 porosity: ?% CAI sulfidation 800 0.35 13.0 1.50 52 CAI1 sulfidation CAI regeneration CAI1 regeneration

800 800 800

0.34 0.40 0.35

a Based on Hg porosimetry data only (pore diameter larger than 60 A).

Figure 8. Scanning electron micrographs of (a) the small grains (at 550X) and (b) the macropores of fresh sorbent CAI calcined at 550 "C (at 11OOOX).

I I 0 c - I 0 c - 7

" c - 3 * c - 9 ,c.7 A c - 2 A C - 8

o C - 4 C-io v c - 5 0 c - I I x C - 6 .t C- 12 lli:n 1

6 0

2 0

80

H20 : 19% . H2 = 13%. l lzS: 1%. N2: 6 7 %

TREGENf650'C IC-3 T O C - 5 ) 550°C ( C - I , C - 2 )

- :7OO0C ( C - 6 . C- 7 ) ~ 7 5 0 ° C (C-8. C - 9 1 - 8 0 0 ° C (C-IO TOC-121

(WITH N2 /A IR = 90/10

C = C Y C L E NO

8 0 0 ° C

750.C

7 0 0 ' C 650.C

, 5 5 0 . C

NORMALIZED ABSORPTION T IME ( t /I*)

Figure 9. Breakthrough curves in successive sulfidation cycles of sorbent CAI at various temperatures.

7.6 9.9 3.3

1.62 1.29 1.03

59 67 60

I O l o l m ; - - 7 - - - - - - 2 80 0 c - l 0 c - 7

c - 2 A c - 8 n c - 3 7 c - 9 0 c-4 c - I O 0 c - 5 0 c - l l Y C - 6 + C - 1 2

TREGEN :55O"C (C - I , C - 2 1 = 6 5 0 " C ( C - 3 T O C - 5 ) = 7 0 0 T ( C - 6 . C - 7 ) =75O'C ( C - 8 . C-9)

C - C Y C L E NO.

0 0.2 0.4 0.6 0.8 1.0 NORMALIZED ABSORPTION TIME ( t / t * )

Figure 10. Breakthrough curves in the sulfidation of sorbent CAI*. Conditions are the same as in Figure 10.

because the compound oxide content of CAn has decreased as a result of the transformation of Cu2S-A1203 to CuO- A1203 upon regeneration a t temperatures lower than the calcination temperature of the original CAI1 sorbent. At higher temperatures, sulfidation of the two sorbents is very similar because the compound oxide content of both samples prior to sulfidation is determined by the most recent regeneration temperature. The H2S concentration in the outlet gas shown in Figures 9 and 10 increases with temperature as expected from the decreasing equilibrium constant of sulfidation. An additional process responsible for the increase with temperature of the outlet H,S level is the faster reduction of copper oxides to metallic copper.

Along with increasing the level of H2S in the outlet gas, higher temperatures also result in higher sorbent conversion at breakthrough. Two factors seem to be at work here: a faster reduction of CuA1204 to more readily sulfidable phases and enhanced reaction and diffusion rates for the processes involved in sulfidation. Assuming that the regeneration temperature is high enough to produce a significant fraction of the compound oxide, CuA1204, the subsequent sulfidation temperature involves a tradeoff between the level of outlet H2S and the sorbent conversion. The temperature must be sufficiently high to convert CuA1204 to the more readily sulfidable phases Cu20 and C d 0 2 but not so high as to cause rapid reduction of these phases to metallic copper.

BET and XRD analyses of CAI and CAI1 samples after the 12th sulfidation-regeneration cycle are presented in Table VI. The overall surface area of the sulfided solid is higher than that of the regenerated solid due to the generation of free alumina during sulfidation. In addition, the ratio of amorphous to crystalline phases in sulfided samples is large as in the TGA experiments. Apparently, redispersion from a crystalline aluminate phase to a dispersed Cu2S-A1203 phase takes place during sulfidation. This redispersion endows the mixed-oxide sorbent with an unexpected stability over successive sulfidation-regeneration cycles. The XRD analyses of sulfided CAI and CAI1 listed in Table I11 confirm the presence of dispersed y- A1203 and Cu2S. In addition, the XRD analysis of CAI1

Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989 939

Table VII. XRD Analysis of Regenerated Sorbents sorbent regeneration gas, mol % temp, OC crystallinity,d 70 CuO, wt % CuA1204, wt % CuA102, wt %

CAI" 90 N2/10 air 800 75-80 none 69.0 31.0 CAII" 90 N2/10 air 800 NA 41.0 59.0 none 72-1-CAb 100 air 700 NA medium major none 72-1-CAc 100 air 800 NA medium major small 33-1-CA6b 100 air 700 NA medium major none 33-1-CA6' 100 air 800 NA small major none

OFrom microreactor experiments. See Figure 7. See Figure 8. dNA: not available.

[H2Slegl 2Cu t H2S -.+ Cu2 S t H21 _ - _ _ _ _ _ _ - _ _ _ _ _ - _ _ _ - - - - - - - -

0 0 2 0 4 0 6 0.8 1.0 NORMALIZED ABSORPTION T I M E ( t / t * )

Figure 11. Sulfidation of sorbent CAI1 a t 650 "C with Hz:H20 = 20:25.

regenerated at 800 "C by a mixture containing 90 mol % air and 10 mol 9'0 Nz identified copper aluminate and CuO in an approximate 3:2 mass ratio (Table VII). Sorbent CAI was left overnight under a nitrogen purge a t 800 "C, allowing complete conversion to CuA1204 followed by partial decomposition to CuA102, as indicated by the data of Table VI.

The effect of changing the H2/H20 and H2/H2S ratios was examined in another series of experiments with CAI and CAI1 a t 650 "C. A gas containing 20% Hz, 25% H20, 1% HzS, and 54% N2 was used instead of the mixture 13% Ha, 19% HzO, 1% HzS, and 67% N2 used in the runs of Figures 9 and 10. Sorbent CAI1 was more stable than CAI in this more reducing gas. The data obtained for CAI1 have been reproduced in Figure 11. The prebreakthrough H2S level remained below 50 ppm up to 100% conversion. With sorbent CAI, the prebreakthrough levels were higher, finally reaching the value of 90 ppm (equilibrium H2S concentration for reaction 4) a t breakthrough. Apparently, the difference in outlet H2S levels between CAI and CAI1 was due to the difference in compound oxide content between the samples.

Regeneration of the sulfided sorbent with air produced SOz (reaction 6). In the presence of 02, SOz is known to combine with CuO to form copper sulfate (reaction 9), as long as pso$o,1/2 exceeds the equilibrium constant. It is also known that under similar conditions SOz and O2 combine with the alumina to form a surface aluminum sulfate, the bulk aluminum sulfate being unstable a t the temperatures of interest. Sulfate formation is undesirable because during subsequent sulfidation it is reduced with the elution of SO2. We did not attempt to quantify the amount of sulfate formed during this series of regeneration experiments. However, the formation of sulfate could be qualitatively tested by monitoring the concentration of SOz in the gas phase. Upon completion of a typical regeneration run, the product gas contained a few parts per million SO2 slowly desorbing from the alumina surface. If at that point the flow of air was replaced by flow of nitrogen, a drastic increase of SO2 indicated decomposition of sulfate,

previously restrained by oxygen.

Conclusions Copper-aluminum oxide sorbents prepared by the ci-

trate process consist of varying amounts of compound oxide, CuA1204, and mixed oxides, CuO and A1203, with the fraction of the compound oxide increasing with the temperature of calcination. Temperature-programmed reduction of CuA120, was slower by 1 order of magnitude compared to that of CuO in the mixed form. XRD analysis showed that reduction of the compound oxide proceeded with CuA10, and Cu20 as intermediates.

Sulfidation of the sorbents produced digenite ( C U ~ + ~ S ~ ) as the only crystalline sulfide phase. The sulfidation rate was much slower for those sorbents that consisted mainly of the compound oxide, CuA1204, compared to those that contained mostly mixed oxides (CuO and Alz03).

The outlet H2S concentrations measured in the packed-bed sulfidation experiments were substantially below the equilibrium values corresponding to the sulfidation of metallic copper. Evidently, the slowdown of copper reduction engendered by the alumina preserves some copper at oxidation state +2 or +1 for the duration of sulfidation. The level of H2S in the product gas was insensitive to sorbent composition (compound versus mixed oxide) and regeneration history, provided regeneration was complete. During regeneration, alumina en- hances the formation of sulfate, copper sulfate, or surface aluminum sulfate. This undesirable reaction can be sup- pressed by keeping the regeneration temperature above 750 "C.

Acknowledgment

This work was funded by the Department of Energy, Morgantown Energy Technology Center, Contract DE- FC21-85MC22193, and by the Synthetic Fuels Center of the MIT Energy Laboratory.

Registry No. H,S, 7783-06-4; CuO, 1317-38-0; AlzO,, 1344-28-1.

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Chung, K. S.; Massoth, F. E. Studies on Molybdenum-Alumina Catalysts: VIII. Effect of Cobalt on Catalyst Sulfiding. J . Catal.

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Flytzani-Stephanopoulos, M.; Tamhankar, S. S.; Gavalas, G. R.; Bagajewicz, M. J.; Sharma, P. K. High-Temperature Regenerative Removal of H,S by Porous Mixed Oxide Sorbents. Prepr. Pap. -Am. Chem. SOC., Diu. Fuel Chem. 1985,30(4), 16-25.

Friedman, R. M.; Freeman, J. J. Characterization of Cu/Alz03 Cat- alysts. J . Catal. 1978, 55, 10-28.

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Received for reuieu June 29, 1988 Revised manuscript receiued February 24, 1989

Accepted March 26, 1989

Influence of Support on the Performance of Coal Liquid Hydrotreating Catalysts

Robert L. McCormick, Julia A. King, Todd R. King, and Henry W. Haynes, Jr.* Department of Chemical Engineering, Uniuersity of Wyoming, P.O. Box 3295, University Station, Laramie, W.yoming 82071

A variety of supports for Co- or Ni-promoted molybdenum sulfide were used in hydrotreating coal liquid materials a t high-severity conditions. Both distillate- and residuum-containing feedstocks were employed in runs lasting up to 500 h. The supports studied were alumina, titania, silicated alumina, titania-alumina, magnesia-alumina, chromia-alumina, activated carbon, and nitrided activated carbon. The lined out catalyst activities for both hydrogen uptake and hydrodenitrogenation correlated with either pore volume in 60-200-A-diameter pores or relative number of acid sites per unit mass. However, it was not possible to determine which independent variable had the pre- dominant effect, on activity. The silicated alumina catalyst exhibited a markedly reduced coking tendency relative to all other materials studied. It is hypothesized that this is due to the presence of Bronsted acid sites on the surface of this catalyst in the sulfided state. Cobalt sintering was observed for the alumina and nitrided activated carbon supported catalysts. All catalysts exhibited some level of initial deactivation, but activity maintenance was excellent for several of the catalysts despite the high-severity conditions employed.

The research described in this paper was undertaken with the goal of developing improved catalysts for hydrotreating coal liquids. In a general sense, this type of catalyst has three potential applications: as a coal liquefaction catalyst, as a catalyst used to produce hydrogen- donor solvent for coal liquefaction, and as a catalyst for upgrading coal liquids in an initial refining step. Each of these applications may require a catalyst with somewhat different properties in order to achieve good activity maintenance and an optimum product slate.

The conventional catalyst employed in these processes is molybdenum or tungsten sulfide promoted by Co or Ni and supported on a high surface area, porous alumina.

*To whom correspondence should be addressed.

O888-5885/89 /2628-0940$01.50/0

When used in coal liquefaction applications, these catalysts experience rapid coking and a consequent decrease in activity (Thakur and Thomas, 1984; Stohl and Stephens, 1987). Metals deposition (Stanulonis et al., 1976; Stiegel et al., 1982) and active phase sintering (Freeman et al., 1986; Sajkowski et al., 1988) may also contribute to catalyst activity decline.

Conventional catalysts catalyze hydrogenation as well as hydrocracking, hydroisomerization, hydro- desulfurization, hydrodenitrogenation, and hydro- deoxygenation (Gates et al., 1979). If the purpose of the hydrotreatment is to produce a low heteroatom refinery feedstock, then all of these reactions are desirable as long as hydrogen consumption is not excessive and light gas yields are low. But in coal liquefaction applications, cat-

8 1989 American Chemical Society

high-temperature sulfidation-regeneration of cu0...

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