Selective Rhenium Recovery from Spent Reforming Catalysts

After deactivation the catalysts are treated for the recovery of Pt and Re. The aim of this study was to simplify the conventional recovery process by...
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Ind. Eng. Chem. Res. 1999, 38, 1830-1836

Selective Rhenium Recovery from Spent Reforming Catalysts T. N. Angelidis,* D. Rosopoulou, and V. Tzitzios Laboratory of General and Inorganic Chemical Technology, Department of Chemistry (Box 114), Aristotle University, GR 54006, Thessaloniki, Greece

Reforming catalysts (0.3% Pt, 0.3% Re) are used extensively in industry for the improvement of the octane number of fuels. After deactivation the catalysts are treated for the recovery of Pt and Re. The aim of this study was to simplify the conventional recovery process by the application of a slight alkali solution (NaHCO3) for the selective dissolution of Re. The process was tested in batch and packed column reactors with crushed and uncrushed catalyst extrudes. The influence on the reaction yield of NaHCO3 concentration, pH, temperature and space velocity was examined and the optimum conditions were determined. The Re dissolution process seemed to follow firstorder kinetics, and the recovery yield was up to 97% for crushed and up to 87% for uncrushed catalyst samples. 1. Introduction Byproduct molybdenite or tungsten molybdenite concentrates from copper mines are the most important source of rhenium.1,2 The rhenium content is low, generally in the few hundred parts per million range. Almost all these concentrates are roasted to technical grade molybdic oxide, the most common molybdenum product sold commercially.2 Rhenium oxide is volatilized at high roasting temperatures and condensed downstream at lower temperatures, eventually reparting to the scrubber liquor as a dilute solution.1,2 Worldwide rhenium production is about 50 tons per year.3 Rhenium existed as a curiosity with little practical application for its first 50 years. (It was discovered in Berlin in 1925.)4,5 Use was limited to a few specialty and high-temperature alloy applications. In the early 1960s interest in the potential for catalytic use of rhenium developed, and by the early 1970s significant catalytic use had began. Currently, the main use of rhenium is in catalysts (90%), nickel-cobalt base superalloys, and rhenium-molybdenum and rheniumtungsten alloys.3 The major catalytic use of rhenium is in petroleumreforming catalysts used to produce high-octane hydrocarbons for low-lead, lead-free gasoline. Bimetallic platinum-rhenium catalysts have replaced many of the platinum catalysts used previously. Rhenium-containing reforming catalysts tolerate greater amounts of carbon formation and make it possible to operate at lower pressures and higher temperatures, which leads to improved yields and octane ratings.6 Reforming catalysts are dual functional in that they promote simultaneously reactions that are specific functions of the metal (platinum) and/or acidic properties of the catalyst. The metal provides the hydrogenationdehydrogenation activity. The acid activity of the catalysts provides all the other reforming reactions (production or breakage of carbon bonds) and is supplied by the addition of halogens (usually chlorine) to the catalyst base. Rhenium acts as a promoter, as mentioned above, and does not catalyze any specific reaction. * Corresponding author: E-mail: [email protected].

The metal and acidic components are supported on an alumina base (γ- or η-alumina) with a high active surface.7-9 Reforming catalysts remain in use for many years and can be regenerated in situ and reactivated by heat and chemical additives (chlorides). These procedures invert platinum particles agglomeration (sintering) and acidic components losses. When the catalyst loses activity permanently resulting in marked loss of yield, it is then ready for recycling. This loss is generally caused by slow deterioration of substrate, agglomeration of active constituents (metallic clusters), poisoning by coke, sulfur, or metallic compounds, or overheating and loss of surface area and active reaction sites of the alumina substrate.6-10 The processing of spent reforming catalysts does not only involve chemical and metallurgical activity, it also encompasses commercial and accounting controls because of the high monetary value of the material. The most common precious metals loading is 0.3% Pt and 0.3% Re. At 1997 market prices, this material had a recoverable value of about $32 per kg of which $1.55 was due to Re.6 At the end of the catalyst life cycle, it is discharged from the reactors and shipped to the reclamation facility. The feed material to the processing stream contains the original catalyst components in addition to coke, base metals, and hydrocarbon residuals. Coke and hydrocarbon materials are normally removed in situ before shipping by heating and burning. Rhenium is recovered from the spent catalysts by pyrometallurgical or hydrometallurgical processes. Pyrometallurgical processes involve chlorination at high temperatures (1173-1223 K) for recovery of rhenium along with platinum as volatile chlorides;10,11 heating at high temperatures (1073 K) in a gas flow containing water. The rhenium oxide dissolved in water, precipitates on the walls;10 annealing with Na2CO3 or annealing in oxidation atmosphere.10,11 Pyrometallurgical processing is not so attractive for Re-bearing materials because the Re would not be economically recoverable from the resultant slags and losses of volatile rhenium oxides might occur.6

10.1021/ie9806242 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/03/1999

Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1831

Figure 2. Proposed simplified rhenium recovery process.

Figure 1. The conventional rhenium recovery process from spent reforming catalysts. Rhenium recovery from the leaching solution is accomplished by ion exchange.6

The spent-catalyst hydrometallurgical processing comprises several unit operations aimed at the recovery of the various components of the spent catalyst, alumina, platinum, and rhenium. The first task is to reduce the bulk by dissolution of alumina. Alumina is dissolved with both acids and alkali to form soluble salts.6,10,12 During the 1960s, 1970s, and early 1980s, caustic dissolution was used as the primary technique at many processing facilities. However difficulties in Re recovery from the caustic medium and residues resulted in abandonment of this process in favor of the sulfuric acid dissolution.6,13 The conventional sulfuric acid dissolution process is shown in Figure 1. During sulfuric acid dissolution alumina and Re are almost completely dissolved as well as a part of the fine particle’s size Pt (up to 10%). After reduction to remove any contained Pt the slurry is transferred to settling and filtered. The remaining solids are transferred to Pt-processing facilities, whereas the filtrate containing aluminum sulfate and Re is filtered repeatedly to remove all fine insoluble matter and then is directed to the Re recovery facilities. Re is recovered by the application of either liquid ion exchange (LIX) or solid ion exchange (SIX). In both cases the active compounds are organic amines. Other separation techniques such as pressure reduction by hydrogen or sulfur dioxide,14 ion exchange applying various active materials,12,13,15 and solvent extraction13 may be used. After elution the elute is neutralized by ammonia gas or ammonium hydroxide and then is evaporated to super-saturated solutions, cooled, and allowed to crys-

tallize. After multistage dissolution and crystallization steps, pure NH4ReO4 crystals are obtained.6,12 The main aim of the present research is to examine to what extent a slight alkali (produced by a common and cheap reagent, such as NaHCO3) leaching process may dissolve selectively Re causing a simplification to the whole recovery procedure. The expected result is shown in Figure 2, where the simplified recovery process is shown. Compared with the conventional process (Figure 1), the fundamental difference is the substitution of the complicate and expensive ion-exchange process for Re separation. 2. Thermodynamic Consideration The exact nature of rhenium compounds on spent catalysts is not completely known and may vary with the type of catalyst. Researchers have suggested that the rhenium is present in a metallic state and possibly alloyed with platinum or in an oxide state, in mixtures, etc.6 The reforming process is taking place at high pressure, in temperatures from 733 to 798 K, and in the presence of hydrogen (hydrogen to hydrocarbon feed mole ratios from 3 to 10).8 Under these reducing conditions the presence of rhenium in metallic form seems to prevail, because the main rhenium oxides are reduced to metallic rhenium according to the following thermodynamic consideration (thermodynamic data from HRC16 at 773 K; H2 pressure, 1 atm):

ReO2 + 2H2 f Re + 2H2O ReO3 + 3H2 f Re + 3H2O

∆G ) -26 kcal/mol (1) ∆G ) -56 kcal/mol (2)

Re2O7 + 7H2 f 2Re + 7H2O ∆G ) -143.6 kcal/mol (3) When the catalyst is removed from the reactor, metallic rhenium is unstable at atmospheric condition and is readily oxidized according to the reactions (thermody-

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Figure 3. Potential pH equilibrium for rhenium in aqueous solution at 298 K (rhenium concentration, 10-3 M). Full lines represent equilibrium between solid phases and liquid-solid phases, thin dotted lines represent the equilibrium between liquid phases, the double dotted lines determine the leaching working area, and the double arrow line represents the active working area.

Figure 4. γ-Alumina solubility in aqueous solution at 298 K. Full lines represent equilibrium between solid and liquid phases, thin line represents equilibrium between liquid phases, the double dotted lines determine the leaching working area, and the double arrow line represents the active working area.

namic data from HRC16 at 298 K and O2 pressure, 1 atm):

•Rhenium dioxide disproportionates in alkali environment to form perrhenic ions and rhenium sesquioxide

Re + 3.5O2 f Re2O7 Re + O2 f ReO2 Re + 1.5O2 f ReO3

∆G ) -260.3 kcal/mol ∆G ) 93.5 kcal/mol ∆G ) -121.2 kcal/mol

(4)

3ReO2 + 2OH- f ReO4- + Re2O3 + H2O (10)

(5)

and in an acid environment to form perrhenic ions and rhenous ions

(6)

2ReO2 + 2H+ f Re3+ + ReO3 + H2O

To predict the behavior of rhenium during a dissolution hydrometallurgical process thermodynamic data were applied for the more common rhenium forms (metallic rhenium and main rhenium oxides and dissolved species in aqueous solution).17 In Figure 3 an E-pH diagram is shown, depicting the thermodynamic stability regions of the various solid and soluble Re species in aqueous solution (without complexing agents). According to Figure 3 the following conclusions are obtained: •It is almost impossible for Re metal to exist in the catalyst, because it is readily oxidized to reactions 4, 5, and 6 •Perrhenic anhydrite is readily soluble in pure water to form perrhenic acid:

Re2O7 + H2O f

2ReO4-

+

+ 2H

(7)

•Rhenium trioxide disproportionates in an alkali environment to form perrhenic ions and rhenium dioxide

3ReO3 + 2OH- f 2ReO4- + ReO2 + H2O

(8)

and in an acid environment to form perrhenic ions and rhenous ions

3ReO3 + 2H+ f 2ReO4- + Re3+ + H2O

(9)

(11)

•Rhenium sesquioxide disproportionates in alkali environment to form perrhenic ions and rhenides ions

Re2O3 + 2OH- f ReO4- + Re- + H2O

(12)

and in acid environment to form rhenous ions

Re2O3 + 6H+ f 2Re3+ + 3H2O

(13)

According to the sequence of reactions above the final products in an alkali environment are perrhenic ions and rhenides ions, whereas in an acidic environment, perrhenic ions and rhenous ions. In practice, rhenide ions are not stable at atmospheric conditions and are readily oxidized to perrhenic ions:

Re- + 2O2 f ReO4-

(14)

The rhenous ions are oxidized to perrhenic ions during the acid-leaching procedure, because of the mild oxidative action of the concentrate sulfuric acid applied in this case. So, in both cases the final product is a perrhenic ion solution. In alkali leaching, alumina is dissolved by strong alkali (NaOH) in autoclaves at high pressure and temperature. The application of a weak alkali (i.e., NaHCO3) may cause a selective rhenium dissolution at atmospheric conditions, because alumina is not expected to dissolve considerably in the slight alkali environment, pH 6-9, provided by NaHCO3 solutions (Figure 4).

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Although other weak alkalis may be applied, because the procedure depends on pH and not on the nature of the alkali, NaHCO3 was selected as a common and inexpensive reagent. The main metallic contaminants of the reforming catalysts (iron and copper)7,8 are also slightly soluble in this pH region. So, the leachate produced under this condition will be rather pure, containing only perrhenic ions; and a less expensive and elaborate procedure is required.

Experimental Section 3.1. Materials. The spent catalyst used for the rhenium recovery tests was obtained from the naphtha reforming facilities of the EKO refinery (Thessaloniki, Greece). The catalyst has the commercial name KX-120 (Cyanamid-Ketjen) and contains platinum and rhenium as active constituents. The chemical composition and the physical characteristics of the fresh catalyst (dry base) were as follows: Pt ) 0.3% w/w, Re ) 0.3% w/w, Cl ) 0.91% w/w, Fe ) 0.02% w/w, Cu ) 24 ppm; specific surface, 193 m2/ g; density, 0.7 g L-1; mean diameter, 1.63 mm; mean length, 4.6 mm; shape, extrudes. The samples of spent catalysts were derived from the naphtha reforming reactor after decoking by air heating and were analyzed after dissolution in aqua-regia for the active constituents and the main metallic contaminants. Pt, Cu, and Fe were analyzed by atomic absorption spectroscopy (AAS) (Perkin-Elmer 2380), Re by the spectrophotometric R-furildioxime method,19 using a Hitachi U-2000 spectrophotometer. Chemical analysis of selected samples gave the following results (dry base): Re, 0.290% w/w (SD ) 0.006, n ) 5); Pt, 0.287% w/w (SD ) 0.005, n ) 5); Fe, 0.03% w/w; Cu, 30 ppm. The specific surface after the decoking procedure was 162 m2 g-1. Differences from the nominal values are attributed to changes and losses during use, regeneration cycles and the decoking procedure. 3.2. Batch Experiments. Selective rhenium dissolution was initially studied by batch experiments. Of the leaching solution (NaHCO3) 50 mL were placed with 1 g of crushed or uncrushed catalyst in 100-mL conical flasks. The flasks were agitated for 2 h in an agitated temperature-controlled water bath. Preliminary tests show that the reaction reaches equilibrium after 90 min, so the time of reaction was fixed at 2 h. At the end of the experiment the solids were washed with deionized water, and the concentration of rhenium was determined both in the liquids as well as in the solid phase by the R-furildioxime method. The main metallic impurities, iron and copper, were also determined in the liquid phase by AAS. Crushed-catalyst samples were produced by crushing the long extrudes to pieces with lengths from 1 to 2 mm. The standard condition applied at the batch experiments were: reaction time, 2 h; NaHCO3, 0.1 M; temperature, 313 K; solution, 50 mL; solids, 1 g. 3.3. Packed Column Experiments. The NaHCO3 leaching procedure was carried out using a fixed-bed laboratory glass reactor with internal diameter, 1.8 mm. Five grams of catalyst were used to form a column 4.5 cm in height. Two beds of glass balls (diameter, 3 mm; bed height, 5 cm each) were placed before and after the catalyst bed to normalize the liquid flow through the

Figure 5. Rhenium recovery yield as a function of the NaHCO3 concentration (standard conditions). The vertical lines represent the stoichiometrically required concentration of NaHCO3 for Re2O3 and ReO2 dissolution.

catalytic bed. The leaching solution was pumped continually through the catalyst bed from the bottom of the reactor (upflow) by a peristaltic pump. Aliquots were derived at certain times at the outlet of the reactor and analyzed for rhenium. At the end of every experimental run rhenium concentration was determined in the bulk of the leaching solution collected at the outlet of the reactor. The standard condition applied at the packed column experiments, if not otherwise stated, were: NaHCO3 concentration, 0.05 M; temperature, 298 K; catalyst weight, 5 g; sample volume, 20 mL; sampling interval, 5 min. 4. Results and Discussion 4.1. Batch Experiments. A series of experiments was carried out to study the effect of the initial NaHCO3 concentration on the yield of rhenium recovery. The results are shown in Figure 5 for crushed and uncrushed catalyst samples. Even without NaHCO3 deionized water (pH ) 6) dissolves a quantity of rhenium (about 17% w/w). (This experiment is not shown in Figure 5 because of the logarithmic scale of the concentration axis.) This indicates that a part of rhenium exists on the catalyst in the form of Re2O7 and produces HReO4 during dissolution by deionized water according to reaction 7. As shown in Figure 5, as the concentration of NaHCO3 increases, the recovery yield follows a sharp increase and then becomes almost constant at NaHCO3 concentrations greater than 0.01 M. This behavior is expected from the thermodynamic consideration because the increase of the NaHCO3 concentration is followed by a respective pH increase caused by hydrolysis:20

HCO3- + H2O f H2CO3 + OH-

(15)

In Figure 6 the measured initial pH values are determined as a function of the initial NaHCO3 concentration. The stabilization of the recovery yield follows a

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Figure 6. Initial pH values as a function of the concentration of NaHCO3.

similar behavior with the pH values. It is obvious that at higher pH values the other rhenium oxides (ReO2, Re2O3, and ReO3) starts to dissolve according to the thermodynamic consideration (reactions 8, 10, and 12). The vertical lines in Figure 5 represent the required concentration of NaHCO3 for the complete dissolution of rhenium at the standard experimental conditions according to the stoichiometry of the respective overall dissolution reactions:

1. ReO2 dissolution: 3ReO2 + 4HCO3- + 2H2O + 2O2 f 3ReO4- + 4H2CO3 (16) The stoichiometric ratio [Re]/[NaHCO3] is 3/4.

2. Re2O3 dissolution: Re2O3 + 2HCO3- + H2O + 2O2 f 2ReO4- + 2H2CO3 (17) The stoichiometric ratio [Re]/[NaHCO3] is 1/1. According to the consideration above the relative NaHCO3 concentrations are 0.0032 and 0.0043 M. The position of the vertical lines in Figure 5 shows that up to the yield stabilization the overall quantity of the added NaHCO3 is consumed by the dissolution reaction. In the stabilization region there is an excess quantity of NaHCO3. Acid titration of the solution at the end of the dissolution experiments in this region show that the sum of the consumed and the remaining quantity of NaHCO3 is approximately equal to the initial one. This means that a small quantity of NaHCO3 is consumed by side reactions (i.e., alumina dissolution). The difference (10-15%) may be attributed mainly to NaHCO3 remaining adsorbed on the catalyst species and to losses by CO2 formation:20

HCO3-

-

f CO2 +OH

(18)

As shown in Figure 5 the behavior of crushed and uncrushed catalyst samples differ. Crushed catalyst

Figure 7. Rhenium recovery yield as a function of temperature.

samples present better recovery yields (up to 97%) than uncrushed (up to 87%). The difference is attributed to the presence of rhenium in clogged pores. These rhenium species are not reached by the leaching solution. After crushing a quantity of these clogged pores open and the relative rhenium species are available to dissolution. It is also possible that the system is slightly kinetically controlled by diffusion through the catalyst pores or by the dissolution reaction and the stabilization region is not really in equilibrium. The later is proved by measurements at increasing temperatures (Figure 7) which show a slight increase of the recovery yield at higher temperatures. 4.2. Packed Bed Experiments. The application of a batch process prevails problems in industrial practice (e.g., mixing problems for uncrushed samples and restrictions on the processed catalyst quantity). For this reason the slight alkali process was tested in a packedbed reactor that overcomes these disadvantages. The packed-bed experiments were carried out with continuous flow of the leaching solution to predict the kinetic behavior of the system. Atmospheric conditions (temperature, 298 K) and uncrushed catalyst samples were applied. The mass balance of the reactor for rhenium is given by:

Q dCr ) rRe dVr

(19)

where Q ) the volumetric flow, mL min-1; Cr ) the rhenium concentration in the reactor, mg L-1; rRe ) the rate of rhenium dissolution, mg L-1 min-1; Vr ) the active volume of the reactor (the part that is not covered by the catalyst extrudes), mL. For first-order kinetics:

rRe ) -kCr

(20)

where k ) a kinetic constant, min-1. Because (dVr/Q) ) dt (t is the time of reaction in minutes), eq 19 becomes:

(dCr/dt) ) -kCr

(21)

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Figure 9. The Co value as a function of volumetric flow. Figure 8. Rhenium concentration at the outlet of the packed bed as a function of time for various volumetric flows.

The rhenium recovery yield for the packed bed experiments, based on the experimental results, was calculated as follows:

Table 1. Rhenium Recovery Yield (R%), the Constant Co, the Kinetic Constant (k), and the R2 Values of the Simulation Procedure for Various Values of Volumetric Flow (Space Velocity) volumetric flow, mL s-1

space velocity,a s-1

Co, mg L-1

0.30 0.81 1.49 1.67

0.07 0.20 0.35 0.39

42.5 24.5 9.8 8.3

k,

min-1

0.085 0.095 0.068 0.077

R2

R, %

0.994 0.992 0.967 0.991

75.6 82.8 76.5 77.4

a Space velocity is calculated by the ratio Q/V where Q is the v volumetric flow and Vv is the void fraction of the catalyst bed.

After integration for the limiting conditions, t ) 0, Cr ) Co, where Co is the outlet concentration after the first pass of the solution through the reactor when the time measurement begins during the experiments. The rhenium concentration at the outlet at any time t is given by:

C ) Co exp(-kt)

(22)

The experimental results are shown in Figure 8, where the outlet concentration of rhenium is plotted as a function of time for various volumetric flows of the leaching solution. The fitting of eq 22 on the experimental data was tested statistically, and the respective constants were calculated through the linear transformation of eq 22:

LNC ) (LNCo) - kt

(23)

The results are shown in Table 1. The fitting procedure was satisfactory and the reaction rate seems to follow first-order kinetics concerning the rhenium concentration. The kinetic constant seems to be independent of volumetric flow (mean value ) 0.081, n ) 4; SD ) 0.011), whereas Co is a linear function of volumetric flow as shown in Figure 9 (Co ) A + BQ, A ) 47.8; B ) -24.9; and RSQ ) 0.973). The result above confirms the conclusion that the Knudsen’s diffusion through the pores of the catalyst or the dissolution reaction is the rate-controlling step.

R% ) (mo- mr)*100/mo

(24)

where R% is the recovery yield, mo is the initial mass of rhenium in the packed bed (mg), and mr is the recovered mass of rhenium at the end of each experimental run. mo was measured by analysis of the catalyst extrudes before treatment. mr was calculated from the experimental results taking into account the removal of a significant quantity of rhenium by the sampling procedure as follows:

mr ) VfCf + Σ(Cnv)

(25)

where Vf is the total volume collected at the end of each experimental run (L), Cf is the rhenium concentration in this volume (mg L-1), Cn is rhenium concentration in every sample (mg L-1), and v is the volume of each retrieved sample (L). The calculated recovery yields are shown in Table 1. The rhenium recovery yield may be also calculated by eq 24 through the calculation of mr by integration of eq 22 for the limiting conditions t ) 0 and t: t {exp(-kt)} dt ) ∫t)0

mr ) QCo

QCo(1/k)[1 - exp(-kt)] (26)

Because Co ) A + BQ, eq 26 becomes:

mr ) Q(A + BQ)(1/k)[1 - exp(-kt)]

(27)

and the recovery yield can be calculated for any value of the volumetric flow. The recovery yield as a function of volumetric flow for t ) 50 min is shown in Figure 10. The maximum recovery yield (95%) is achieved at volumetric flow 0.96 mL s-1 (space velocity, 0.242 s-1). As shown in Figure 10 the behavior of recovery yield changes presents two separate regions. At low values of volumetric flow the recovery yield increases and at high values increases. It possible that at the first region the dissolution reaction is under external diffusion control, whereas in the second one is under pore

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Literature Cited

Figure 10. The calculated recovery yield as a function of volumetric flow. Dotted line represents the optimum conditions and separates the regions of external and pore diffusion.

diffusion control. The experimental results are in agreement with the second region as shown in Figure 8. 5. Conclusion The laboratory experiments carried out in batch and packed column reactors show that rhenium may be recovered satisfactorily from spent reforming catalysts by dissolution with a slight alkali leaching solution (NaHCO3). Recovery yields up to 97% were achieved for crushed catalyst samples. The application of the abovementioned rhenium recovery procedure may simplify considerably the conventional applied procedure, because of the substitution of the expensive and complicated separation of rhenium from the product of the complete dissolution of the catalyst mass by a rather simple and inexpensive process (Figures 1 and 2). The dissolution reaction seems to follow first-order kinetics, and the rate-controlling step seems to be Knudsen’s diffusion through the catalyst’s pores or the dissolution reaction. Analysis of the produced liquids showed that they contain only rhenium, because no copper or iron was detected in measurable amounts. The abovementioned kinetic analysis refers to the specific catalyst and the obtained results must be applied only qualitatively for other type of catalysts, because the catalyst composition, the working conditions and the decoking procedure may cause significant differences. Acknowledgment The authors thank the administration of the EKO refinery (Thessaloniki, Greece) for the provision of the reforming catalyst samples.

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Received for review September 30, 1998 Revised manuscript received December 29, 1998 Accepted February 5, 1999 IE9806242