Ind. Eng. Chem. Process Des. Dev. 1984, 23, 505-514 Frayer, J. A.; Lese, H. K.; McKlnney, J. D.; Metzger, K. J.; Paraskos, J. A. U.S. Patent 4116819, 1978. Gleser, M. B.; Llchtensteln, I. AIChE J . 1963, 9 . 30. Hochman, J. M.; Effron, E. Ind. €ng. Chem. fundem. 1969, 8 , 63. Jensen, R. H. U.S. Patent 4039430, 1977. Kassarjlan, J. R. U.S. Patent 3732078, 1973. Lerou, J. J.; Glasser, D.; Luss, D. Ind. Eng. Chem. fundam. 1980. 79, 66.
505
Scott, N. H. U S . Patent 4 138327, 1979. Snow, A. I.; Grosball, M. P. OilGas J . 1977, 75(21), 61. Trasl. P.; Khang, S. J. Ind. Eng. Chem. fundam. 1979, 78, 256. Van Kllnken. J.; Van Dongen, R. H. Chem. Eng. Sci. 1980, 35, 59. Wakao, N.; Tanaka, K. J . Chem. Eng. Jpn. 1973, 6 , 338.
Received for review March 18, 1983 Revised manuscript received August 26, 1983 Accepted September 8,1983
Levenspiel, 0. “Chemlcal Reaction Englneerlng”; Wlky: New York, 1962; p 282. Ruether, J. A. Canadian Patent 1042021, 1978.
.
Hydrodemetalation of Nickel and Vanadium Porphyrins. 1 Intrinsic Kinetics Rakesh Agrawalt and James Wel’ Department of Chemical €ngineering, Massachusetts Insflrute of Technology, Cambridge, Massachusetts 02 739
Experimental and theoretical kinetics were studied on the hydrodemetalation reaction of nickel(I I) and vanadyl(IV) etioporphyrin I over Co0-Mo03/A1203 catalyst in a high-pressure liquid-phase flow reactor. Consecutive kinetics with three major kinetic steps are involved. The first step is hydrogenation to reactlon intermediates with a fwst-order dependence on the metal etioporphyrin concentration in the solution and a first-order dependence on hydrogen pressure. The second reactlon is the reversible dehydrogenation of the intermediates back to the metal etioporphyrins, which Is fkst order to the Intermediates concentrations in solution and zero order to hydrogen pressure. The third reaction is the irreversible hydrogenolysis of the intermediates and demetalation, which is first order to the intermediate concentrations and second order to hydrogen pressure. The reaction intermediates for both the metal compounds have been Isolated; the intermediate for nickel etioprophyrin demetalation has been identified to be nickel etiochlorln.
Introduction In a refinery the bottom of the crude oil fractionation column, also known as residuum, contains a high proportion of sulfur, nitrogen, and metal contaminants. A world-wide shift to heavier, more contaminated crude oils has greatly increased the level of hydroprocessing of these high molecular weight residues to form gasoline and diesel fuels. In the crude oils, nearly one half of the elements in the periodic table have been identified as trace elements (Smith et al., 1959). Of these the most abundant and problematic metals are vanadium and nickel. Depending on the origin of the crude oil, the concentration of vanadium commonly varies from 8 parts per million (by weight) to 1200 ppm, while that of nickel commonly varies from 4 to 150 ppm. Vanadium and nickel compounds cause major problems in the hydrodesulfurization (HDS) and the catalytic cracking units. These metal compounds deposit on and deactivate the hydrodesulfurization catalysts, which have to be replaced. In the catalytic cracking unit, these metal contaminanta change the activity-selectivity rating of the unit’s catalyst inventory (Nelson, 1976). A number of pretreatment processes to remove these metal compounds from the crude residuum have been developed. Various investigators have studied the kinetics of the metal removal from the major crude oils, in the presence of hydrogen and over typical HDS catalysts which are Moo3) supcobalt oxides and molebdenum oxides (COO, ported on alumina (A1203). The kinetic order, with respect to the total metal concentration in the crude oil, has been
reported as second order (Beuther and Schmid, 1963; Oleck and Sherry, 1977) and first order (Larson andd Beuther, 1966; Chang and Silvestri, 1976; Riley, 1978). Cecil et al. (1968) have pointed out that if the total metals content consisted of two major fractions reacting with greatly different rates, each following first-order kinetics, then the total metal disappearance would look like a second-order kinetics. Since the metals in crude oils exist in both porphyrinic and nonporphyrinic compounds (Yen, 1975), which have wide difference in the reactivity, this hypothesis seems to be a plausible explanation for the observed first- and second-order hydrodemetalation (HDM) kinetics seen by the various investigators (Beuther and Schmid, 1963; Oleck and Sherry, 1977). However, van Dongen et al. (1979 and 1980) speculated that the probable order of the individual vanadium-bearing species should be less than 1.0. Indeed, Hung and Wei (1980) in their batch autoclave studies of pure nickel and vanadyl porphyrins observed fractional order kinetics. The value of the reaction order centered at 0.5 and varied with temperature and hydrogen pressure of the operation. The objectives of this paper are to study the kinetics in a continuous flow reactor and to explain the observed firstand the fractional-order dependence on the total metal concentrations of the HDM kinetics. Dependence of HDM kinetics on the hydrogen pressure has also been investigated. For an HDS catalyst, Audibert and Duhaut (1970) reported first-order dependence, while Chang and Silvestri (1976) and Oleck and Sherry (1977) reported the dependence to be greater than first order. This study deals with the intrinsic hydrodemetalation kinetics of model compounds nickel and vanadyl etioporphyrins with pulverized catalysts in a continuous flow
+AirProducts and Chemical Inc., Allentown, PA 18105. 0198-4305/84/1123-0505$01.50/0
0
1984
American Chemical Society
506
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984
Table I. Chemical and Physical Properties of the HDS-16A Catalysta (1)Chemical, wt % dry basis
coo MOO, Na,O Fe base (2) Physical apparent bulk density (poured), g/mL average diameter, cm average length, cm fines, wt % pore volume (H,O), mL/g pre volume (Nujol), mL/g pore volume (Hg), mL/g suraface area, mz/g density of particle, g/mL pore diameter corresponding to 50% of the total pore volume, angstrom a
5.7 12.2 0.03 0.04 alumina 0.737 0.152 in.) 0.432 0.2 (-16 mesh) 0.4331 ?: 0.0044 0.4389 i 0.0060 0.43 176 1.49 80.4 (by mercury porosimeter )
Source: American Cyanamid Co.
reactor. Part 2 of the study will deal with diffusional effects with catalyst pellets in a packed bed reactor (Agrawal and Wei, 1984). Experimental Methods Crude oils contain many metal compounds that are not very well characterized,as well as many sulfur and nitrogen compounds (Massagutov et al., 1967). Thus, when the crude oils were reacted, it was difficult to uncouple the hydrodemetalation reaction from the rest of the complex chemical reactions involving hydrodenitrogenation and hydrodesulfurization. Therefore, in this study, model metal compounds were used, which are reasonable representatives of the metal compounds found in the crude oil. These model metal compounds were dissolved in clean oil free of sulfur and nitrogen compounds. A. The Catalyst, Solvent and Model Compounds. Hydrodemetalation reactions occur mainly in hydrodesulfurization reactors. The HDS catalyst used in this research was AERO HDS16A supplied by American Cynamid Co., Linden, NJ. It was a CoO-MoO3/Al2O3catalyst supplied as 1.6 mm (1/16in.) extrudates. Chemical and physical properties of the catalyst are listed in Table I. The procedure used to measure the pore volumes listed in this table is described by Agrawal (1980). A mineral oil (Nujol) of relatively high viscosity and boiling point and free of sulfur, nitrogen, and any other metal compounds, was used as a solvent. The typical properties of this oil as supplied by the manufacturer (Plough, Inc., Memphis, TN) are listed in Table 11. The mineral oil is a mixture of liquid hydrocarbons obtained from petroleum and is a naphthenic type of white oil. This oil is chosen because it does not crack under reaction conditions, but it shows some mild cracking activity at about 375-400 O C . Costantinides and Guido (1963) studied nine different crudes and reported that the percentage of the total vanadium and nickel as porphyrins varied from about 12% to 44%. Dean and Whitehead (1963) also found that for several crudes, the percentage of metals as metalloporphyrins ranged from 5% to 34%. Sugihara et al. (1970) and Yen (1975) have concluded that the two most important types of metalloporphyrins found in petroleum are deoxophylloerythroetioporphyrin (DPEP) and etioporphyrin (Etio). As the age of petroleum increases, the petroporphyrins evolve from DPEP to Etio. Saracen0 et al. (1961) have pointed out that all the vanadium in crude oil appears in the vanadyl(+4) state, and nickel exclusively in the +2 valence state. In this study nickel etioporphyrin
Table 11. Nujol Specificationsa 0.8750 to 0.8850 360 to 390 SSU 54 SSU (typical) 216 "C (typical) -32 "C (typical) 1.48 (typical) 0.100maximum
specific gravity at 25 "C viscosity at 37.8 "C viscosity at 100 "C flash point, Pensky-Martin open cup pour point refractive index at 20 "C net optical density UV absorption at (typical) 275 pm 295/299 pm 299 p m up distillation at atmospheric pressure (typical) IBP 50% 90% 95% FBP distillation at 1 0 mm pressure (typical) IBP 5%
0.075 0.150 0.100
358 "C 429 "C 470 "C 484 "C 497 "C 208 "C 240 "C 252 "C 274 "C 305 "C 317 "C 329 "C
10% 50% 90%
95% FBP range of carbon atoms average formula average molecular weight a
cm-c,
:-r
C,,H*, 417
Source: Plough Inc., Memphis, TN.
\
"
\'
21 \
NI-Ftio ( I ) C32 N A H j e N I
M W. = 535 3 8
A Ethyl
- Methyl
Group Group
Figure 1. Nickel and vanadyl etioporphyrins.
I (Ni" Etio I) and vanadyl etioporphyrin I (VO'" Etio I) have been used as the model metal compounds. Their structures are shown in Figure 1. The compounds were supplied by Man-Win Chemicals, through Professor Peter Hambright of Howard University, Washington, DC. The purities of the compounds used were 97-99.9%. B. Reactor System and its Operation. The tricklebed reactor is the most important hydrodesulfurization reactor used in the petroleum industry (Weekman, 1976). As the flow and mixing patterns of hydrogen and oil in trickle-bed reactors are poorly characterized, a feedstream of oil saturated with hydrogen into a continuous flow reactor packed with the catalyst was employed. The liquid solution is saturated with hydrogen gas at the room temperature (-25 "C) and the desired hydrogen pressure. It has been estimated that under typical operating conditions, the approximate ratio of the moles of the dissolved hydrogen to the moles of metal would be 200-450 based on the solubilities given by Chao and Seader (1961). The excess of hydrogen in the reactor is also confirmed experimentally. In paraffins and naphthenes, the solubility of hydrogen is known to increase with temperature (Cukor and Prausnitz, 1972; Prather et al., 1977). Thus at the
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 3, 1984
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KEY I He C y nd@r 2 H~ C Y I n a e r 3 t u 8 z n o x des 4 P t I AlpO, 5 M O e c u 0 , 8 eves 6 C h o i g "9 P O T , 7 TWO L t r r 4 Y t O C O V @
6 From 9 L
"0
I t r e A.utOCIOvI f tor
I O SOmD P V O V I I 8IDDrDtOr I2 s o c k P T e I L U r P a O g u l 0 t O r 3 SOmp e C Y n d P r
Figure 2. Flow diagram of reactor system.
reactor temperatures, which are higher than the saturation temperature, no hydrogen gas would leave the solution phase. A schematic flow diagram of the reactor system is shown in Figure 2, designed on the basis of information supplied by Eliezer et al. (1977). The reactor was designed to work up to 13.8 MPa of hydrogen pressure and 400 "C temperature, so safety was a concern. The whole equipment was mounted a t the back of a portable stainless steel barricade. The back of the barricade was closed by a thin flexible sheet; in the event of a mishap, most of the pressure would be released from the back. A 2-L autoclave (Autoclave Engineers, Erie, PA) was used as the saturator. The gases were supplied to the autoclave through a filter (Water Associates, Milford, MA) which acted as a sparger. Ultrahigh-purity hydrogen, in 15.17-MPa cylinders, was used, with concentration of oxygen less than 3 ppm and hydrogen a t least 99.999%. Helium, in 24.13-MPa cylinders, of 99.995% purity was used. Both the gases were purchased from Matheson Gas Products (Gloucester, MA). Another 1-L autoclave, shown by point 8 in Figure 2, was connected to the apparatus. Two autoclaves gave much more flexibility in varying the solution feed conditions to the reactor and also removed the time limitation, as when one autoclave was about to be empty, the flow could be switched to the other one. The fresh metal solution in the autoclave was first purged with helium and then saturated with hydrogen at the desired reaction pressure. The saturated feed from the autoclave was filtered through a 0.5-pm filter (Millipore Corp. Bedford, MA) and supplied to a high pressure feed delivery pump (Water Associates Inc., Milford, MA). A frequency synthesizer was connected to the control unit of the pump, so that the solution could be pumped to the reactor at flow rates as low as 1-2 cm3/h, and the flow rates could be varied in 0.1 cm3/h step sizes. The microreactor had a length of 45.1 cm and inside diameter of 0.52 cm. m e whole reactor tube was placed snugly in a 316 SS tube of 3.2 cm o.d., which provided uniform heat distribution to the reactor tube and held two thermocouples against the external surface of the reactor tube. The reactor tube arid the external jacket were mounted inside a Lindberg compact tube furnace (Scientific Products, Columbia, MD). By using a microreactor, the necessary quantity of the very expensive metal porphyrins was minimized. The physical dimensions of the reactor and the catalyst were chosen to minimize mass and heat transfer limitations, based on the works of Meam (1971a,b). The details
507
are given in Agrawal's doctoral thesis (1980). The 1.6-mm catalyst extrudates were crushed to the size range of 0.088-0.074 mm (170-200 mesh). There was no sulfur in the system, and the catalyst was not presulfided. At the end of the run, the deposited metal profiles inside the catalyst particle were fairly uniform. To produce negligible axial effects, the length of the catalyst bed should be greater than 1.2 cm. The length of the catalyst bed actually used ranged from 6 cm to 15 cm. The catalyst bed was diluted with inerts to avoid radical temperature gradients. The ratio of diluent to catalyst volume ratio ranged from 1to 2. Crushed Pyrex glass of the size range 0.074-0.088 mm was used as the diluent. The amount of catalyst used varied from 0.3 to 1.5 g. Calculations also showed that there would not be any significant heat or mass transfer resistances between the liquid and the solid. Thus the real kinetics would not be disguised by any transport limitations. At the bottom and top of the catalyst bed, the reactor was packed with crushed inert Pyrex glass to provide preand post-heating zones for the solution. Once the reactor was leak-tight, it was heated in 3 h with a step increase of about 60-80 "C/30min to a temperature of 370-390 "C. For the next 3 h the reactor was purged with helium at the rate of 2.5 L/min. Helium purging of 3 h at 370 "C removed all the air from the reactor system, and it removed the moisture of the catalyst. Once the reactor had been purged and the temperature and the hydrogen pressure brought to the desired value, the metal solution was pumped through it. The solution coming out of the reactor passed through a cooling jacket and was cooled to near room temperature by a cold water stream. The sampling valve was a high pressure liquid chromatographic valve (Valco Instruments, Houston, TX). About 2 mL of the samples were periodically collected from the valve. C. Analysis of Liquid Samples. A weighed quantity (0.3-0.75 g) of solution was transferred to a 10-mL volumetric flask, the xylene was added to bring the total volume to 10 mL. Two techniques of analysis were used: atomic absorption spectrophotometry and the visible spectrophotometry. In atomic absorption spectrophotometry, the solution was burnt in the suitable flame to ionize the metal atoms. Thus atomic absorption gives the total metal concentration (Hofstader et al., 1976) irrespective of the chemical form of the metal in the solution phase. A Perkin-Elmer 403 (Norwalk, CT) atomic absorption spectrophotometer was used. Both nickel and vanadyl etioporphyrins are red in color and have intensive absorption peaks in the visible band. The spectra of nickel and vanadyl etio were scanned in the visible range on the Cary-14 spectrophotometer and are given in Figure 3. In both figures the breaks in the curves around 420 nm are due to scale changes on the absorbance scale. Both the spectra have high absorbance peaks around 400 nm, called the Soret bands, which resulted from high conjugation and resonance (Smith, 1975). Two other peaks in the spectra are characteristic of the side chains. Either of these two peaks can be chosen and the concentration of the metal prophyrin can be calculated by Beer's law. The solutions prepared for the atomic absorption were also used for the visible spectrophotometry, in a Coleman 111 (Perkin-Elmer, Norwalk, CT) visible spectrophotometer. For Ni-Etio and VO-Etio analysis, peaks at 517 and 534 nm were chosen, respectively. In order to prepare a calibration curve, standard solutions of pure metal etio prophyrin in xylene were prepared and their total metal concentrations were measured by both atomic absorption
508
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984
-1
WAVELENGTH
4117
,,0
42:
*ALELEIIT-
5 ' 3
7c F
-
1
n i \n
Waar
"3
( nm
00
3
450 500 WAVELENGTH
4 00
350
T
Figure 3. The visible spectrophotometer absorptions for pure vanadyl and nickel etioporphyrins.
and by visible spectrometry. Atomic absorption spectrophotometry gave the total metal concentration, and the visible spectrometry gave the concentration of the original metal etioporphyrin in the solution. D. Preparation of Nickel and Vanadyl Etioporphyrin I Solution in the White Oil. Both nickel and vanadyl etioporphyrins are powders at room temperature. In order to dissolve them in the white oil, the mixture had to be heated to about 200 "C. Even when oil and powder were heated in the inert gas, helium or nitrogen, 10-15 percent of the total metal present in the resulting solution became nonporphyrinic. The oxygen dissolved in the white oil was sufficient to convert a significant fraction of the metalloporphyrin to nonporphyrinic form. Therefore, the first step in the preparation of the solution was to filter the oil through a 5-pm filter paper under vacuum. The filtered oil with the required quantity of porphyrin was heated for about 1 h under vacuum at the temperature of about 80 " C to remove dissolved gases. At the end of 1 h, the vacuum pump was disconnected, and helium at about 10 psig was introduced in the flask. Under this inert environment, the solution was heated at 300 "C for 4 h and left under the helium environment to cool down to room temperature. The cooled solution was filtered through 0.5-pm filter papers to remove all the undissolved metal porphyrin. In this filtered solution, the total metal and the metalloporphyrin concentrations agreed reasonably well. The solubility limit of both Ni and VO Etio is about 40 ppm. E. Separation and Identification of Reaction Intermediates. The visible spectra of the samples collected from the reactor outlet for VO-Etio and Ni-Etio are given in Figures 4a and 5a, respectively. In comparison with Figure 3, there are new peaks at 631 and 616 nm. In most runs,the concentrations of the total metal were higher than the metal etioporphyrin concentrations by 10-3070. If it is assumed that there is only-one important reaction intermediate, then its concentration can be calculated by the difference in the concentrations given by atomic absorption spectrophotometry and visible spectrophotometry. The calculated intermediate concentrations have a range of 0.5 ppm, and the concentration values of metal etioporphyrins from the visible spectrophotometer have an uncertainity of 1.5 ppm. We isolated the reaction intermediates by "dry column" chromatography (Loev and Goodman, 1970). A 2.5-cm diameter Pyrex glass tube, with a fritted disk at the bot-
600
550
650
1 nm ;
Figure 4. The spectra of (a) total liquid product and (b) isolated intermediates in vanadyl etio experiments.
z 0 c
a
5:
: L
330
I
-
370
410
450
490
WAVELENGTH
1.0
-
1
610
I
650
(nm) I
I
N I - E l 13 l ~ t ? r ~ ? a ~ o ! ~
0.9 0.8-
0,
570
530
CGr7on
:JI>-!
A
-
0.4 0.3
0.2 0.1
0 330
370
410
450
490
530
570
610
650
WAVELENGTH
Figure 5. The spectra of (a) total liquid product and (b) isolated intermediates in nickel etio experiments.
tom, was packed to 8 cm height with A-540 adsorption alumina (Fisher Scientific, Fair Lawn, NJ) of the size 80-200 mesh; 200 mL of the liquid product from the reactor was diluted with 200 mL of xylene and allowed to trickle down the alumina column. As the liquid trickled down, the metal compounds were adsorbed at various heights and a colorless metal-free solution came out of the column. The unreacted metal-etio was adsorbed at the top as a dark red band, while a gree band appeared at the bottom. The green color of the reaction intermediates was not a surprise. From Figures 4a and 5a, the peaks for the major reaction intermediates are a t wavelengths greater than 600 nm, and the complementary color for wavelengths greater than 600 nm is green (Evans, 1948). The adsorbed column of alumina was taken out of the Pyrex glass tube, and the green band was separated. The glass tube was again packed with 10 cm of pure alumina, and the green adsorbed alumina was placed at the top. About 200 mL of pure xylene was allowed to trickle down. A band of red porphyrins remained at the top, and a more pure green band traveled down. The alumina was cleaned
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984
I-
509
NI-Etio
VO-Chlorin ( Etio tYPQ
Ni - C h l o r i n ( E t i o t y p e )
Figure 6. Vanadyl and nickel etiochlorins.
\'
0
of adsorbed oil. The bottom green alumina was taken out, and the adsorbed porphyrins were extracted with reagent grade chloroform. After chloroform was evaporated under vacuum, one was left with the green compound. The visible spectrum of the green compound was scanned to see if the purification step with xylenes needed repeating. The visible spectra of the isolated green compounds for VO- and Ni-Etio are shown in Figures 4b and 5b, respectively. For VO-Etio intermediates, the peak at 631 nm has shifted during the process of separation to 627.5 nm, which may be due to matrix interference. The absorbance at 631 nm is about 95% of the absorbance at 627.5 nm. For both nickel and vanadium, the isolated compound has one major peak, as well as other minor peaks in the spectra, which raises the possibility of the presence of more than one intermediate. Indeed, when the separation over alumina with xylene was repeated a few times, the solution could be separated into two major fractions-one was yellowish green while the other was bluish green. For these two fractions, the ratios of the absorbances at various peaks were not identical. An attempt to identify the nickel intermediates by mass spectrometry (Baker, 1966; Baker et al., 1967) was made. Ni-Etio chlorin, formed by the addition of two hydrogen atoms to the porphyrin ring, was the dominant intermediate compound (Figure 6). Even though higher hydrogenated products were not observed, their presence cannot be totally ruled out. Fuhrhop (1970) synthesized nickel octaethyl chlorin and found that the ratio of the absorbance of the Soret peak to the major peak (395 to 613 cm) was 2. The structure difference between the nickel octaethyl chlorin and nickel etiochlorin is that the former has eight ethyl groups while the latter has four ethyl and four methyl groups. Substitution of four methyl groups for the four ethyl groups would not alter the visible spectra significantly. Indeed for our intermediate compound, the ratio of the absorbances at Soret band (397 nm) and at 616 nm was about 2. This further confirms that nickel etiochlorin is the dominant reaction intermediate. Unfortunately, the vanadium intermediate solution was highly unstable and became colorless in about 12 to 24 h. This did not allow sufficient time to run the sample on the mass spectrometer. For the quantitative analysis of VO-Etio and Ni-Etio intermediates, visible spectral peaks at 631 and 616 nm
02
04
06 WJQ
08
(g
12
1
Cat
14
16
Hr
cm3
Figure 7. Outlet concentrations vs. Contact time. Run N5 (IV), T = 315 OC, P = 9.65 MPa.
were used. Total metal concentration of the isolated green solution was found by atomic absorption spectrophotometry and was correlated with the corresponding absorbance at the proper wavelength on the visible spectrometer. The absorption constants (absorbance/metal concentration in ppm) for vanadium and nickel were 0.61 and 0.72, respectively. The concentrations of the reaction intermediates, thus measured, will be referred to as the concentrations of the VO-Etiochlorin and Ni-Etiochlorin, although these are actually lumped reaction intermediate concentrations with chlorin as the major component. Results A. Transient Behavior and Catalyst. For both commercial and this experimental hydroprocessing unit, fresh catalysts go through a transient activity period before reaching stability. In this system, the transient period may last up to 3 e 4 0 h. Figure 7 compares the kinetic results during the transient and stable activity periods of the run. In this run, the flow rate through the reactor was varied to measure the conversions at various contact times. As time on stream progressed, there was a drop in the total metal concentration at various contact times, and an increase in the concentrations of the nickel chlorin. Identical transient catalytic behavior was observed for vanadyl Etio I runs. All the subsequent kinetic analyses were performed for the stable catalyst operation. The metal deposited during the transient period varies from 0.2 to 0.5 wt. %. In this study, the maximum metal deposited on the catalyst in any single run never exceeded 10 wt % and up to this metal loading no significant change in the catalyst activity was observed. Thus once the catalyst achieved its steady-state activity, it was maintained for a fairly long period of time to give stable kinetic data. B. Kinetic Runs for Stable Catalysts. For the first run, the reactor was packed with Pyrex glass only. The run was made at 343 "C and 9.65 MPa (1400 psig) of hydrogen pressure. The reactor outlet concentration was monitored over a period of 18 h. There was no difference between the inlet and the outlet solution concentrations,
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Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 3, 1984
Table 111. Kinetic Experiments
run no.
N-3 (111) N-5 (11) N-5 (111) N-5 (IV) N-8 (111) N-8 (IV) N-13 (11) N-13 (111) N-13 (IV) N-14 (111) N-14 (IV) N-14 (V) v-2 v - 3 (11) v-3 (111) V-6 (11) v - 7 (11) v-8 (111) v - 9 (111) v - 9 (V)
24
pressure, inlet concn, temp, "C MPa PPm Nickel Etioporphyrin 27.0 343 9.65 288 9.65 25.0 302 9.65 25.0 25.0 315 9.65 27.8 343 6.89 26.0 329 6.89 315 4.14 26.5 27.0 343 4.14 26.0 329 4.14 16.3 343 6.89 15.0 329 6.89 14.5 31 5 6.89 Vanadyl Etioporphyrin 343 9.65 288 9.65 302 9.65 343 4.14 343 9.65 288 9.65 315 9.65 34 3 6.89
Vanadyl Etio
2
o
A
o ;a ,tl A lntermediote Mltal
: -
Consocutivo Kinetics Model Result
16
0
.02
.04
WIO T*343'C,
25.0 28.0 23.5 22.0 22.5 15.9 24.0 25.0
.06
1
0.1
0.12
P . 6 8 9 MPa . V - 9
(V)
.00
(-gym"' Hr)
Figure 9. Experimental and model results with vanadyl etio feed.
Ni E t i o - I
0.2
o
s
o
V
r\
0
cB/cM
01
Run# N 5 ( I V ) T z315-C Pz965MPa
t 0 0
.02
.04
.06
.OB
0.1
0.12
0.14
0.16
0
t.
1 . . . 0.0
0
w/o
Figure 8. Ratio of metal chlorin to total metal concentrations vs. contact time.
which proved that the crushed Pyrex glass is suitable as an inert. The catalytic runs were made at the hydrogen pressures of 4.14, 6.89, and 9.65 MPa; the temperatures were 288, 302, 315, 329, and 343 "C;and the reactor inlet concentrations were 27 ppm and 15 ppm of nickel metal as nickel etioporphyrin I. A list of the runs is given in Table 111. Figure 7 shows the outlet concentrations as a function of the contact time for the stable activity period in experiment N-5(IV). At very short contact times, the chlorin concentration increased to a maximum and the total metal concentration declined slowly. At longer contact times, the chlorin concentration dropped and the total metal concentration dropped rapidly. The ratio of the chlorin to the total concentration vs. the contact time is given in Figure 8. Since there was no chlorin in the feed, the initial ratio was zero. The ratio rapidly increased at short contact times and then remained constant for the reactions at higher contact times. This behavior was observed for all runs. Vanadyl Etio I runs were made at the pressures of 4.14, 6.89, and 9.65 MPa and a t temperatures of 288,302,315, 329, and 343 "C, and for two different reactor inlet concentrations. The list of the runs for vanadyl etioporphyrin is also given in Table 111. The result of run V-9(V) is given in Figure 9. Once again the intermediate concentration increased rapidly at short contact times and then dropped. Under identical operating conditions, the maximum con-
x 10
Figure 10. Firsborder plot for nickel etio. Run N3 (1111, CM in ppm and W / Q in g of cat. h/cm3; T = 343 "C; P = 9.65 MPa. centration of the reaction intermediate formed was higher for vanadium than for nickel. Similar to Figure 8, the ratio of the vanadyl intermediate to the total metal concentration remained nearly constant for most contact times.
Discussion A. Kinetics of Nickel Etio Porphyrin I. For the disappearance of total metal in the solution, simple fractional order, first-order, and second-order kinetics have been proposed. All of these simple kinetics do not take an intermediate into account and are not appropriate. For the run N3-(111), the log of total metal concentration vs. time is plotted in Figure 10, and the square root of total metal concentration vs. time is shown in Figure 11. Neither is an adequate representation. Various kinetic models with reaction intermediate concentrations were considered. The following was the most successful k
A &k2B - c
k3
where A is the original metal etioporphyrin, B is the reaction intermediate, and C is metal deposited on the catalyst; k , is the hydrogenation rate constant, k , is the dehydrogenation rate constant, and k3 is the rate constant for the final metal removal step. Prior to the metal deposition on the catalyst, B might very rapidly go through a sequence of species, and k , might be a lumped rate constant.
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984 511
Etio-I
Ni
In k
4.0 3.0
* :0 ' 5
2.0 -
-
1.0 1 .o
0
0.0
1.6 0
W l c l x 10
1
I
1.6
Figure 11. Half-order plot, same run as in Figure 10. C, is in ppm.
I
I
I
I
I
I
1.7
I
Ni E t i o - I
5.0 t-
w 2
00
:+L,,
I
1.8
12
5
8
3 2
In I(
0
Figure 12. Experimental and model results with nickel etio. Run N3 (111), T = 343 "C P = 9.65 MPa.
All the steps in the model were assumed to be first order in metal concentrations. The rate expressions for this model were easily solved analytically (Wei and Prater, 1962). The rate constants were evaluated by either the nonlinear least-square or the Himmelblau-Jones-Bishoff (Himmelblau et al., 1967) methods. For run N5(IV), the three rate constants are 21.1, 60.0, and 33.4 mL of solution/g of cat. h. In figure 7, the solid lines are the calculation results of this model. Figure 12 shows the model results for run N3(III) shown in Figures 10 and 11. This kinetic model fits the total metal concentration as well as the metallochlorin concentration to a remarkable accuracy. The same observation is made with regard to the other runs. Total metal removal is slow until the intermediate concentration reaches a maximum. The total metal removal rate starts slowly and then accelerates later, resembling a fractional order kinetics. Figure 13 shows the Arrhenius plot for the rate constants k,, k2,and k3 at a hydrogen pressure of 9.65 MPa. Similar Arrhenius plots were obtained for runs at pressures 6.89 and 4.14 MPa. Activation energies for the rate constants k l , k2,and k3 are 17.1 f 1.1, 22.9 f 1.25, and 32.6 f 1.5 kcal/g-mol, respectively. The activation energies for the hydrogenation rate constant k1 and dehydrogenation rate constant k2 are similar to those reported in the literature of hydrodenitrogenation and hydrodesulfurization (Bhinde, 1979). Figure 14 shows the plots of the log of the rate constants k l , k2,and k3 vs. the log of hydrogen pressures at 329 OC.
2'ol 1.o
1.4
1.6
1.8
In
2.2
20 P
Figure 14. Relationship between rate constants kz,and k3 with hydrogen pressure at 329 "C. P is in MPa. All the rate constants are in mL/g-h.
Similar plots were obtained for the runs at other temperatures. The dependence of kl on hydrogen pressure is first order, which is in agreement with the molecular mechanism of: metal etioporphyrin H2 metal etiochlorin. From Figure 14, k z is independent of hydrogen pressure. It is found that the dependence of k3 on the hydrogen pressure at 315,329, and 343 OC is 1.7,1.8, and 2.0 order, respectively. It may be concluded that the dependence of kl, k2,and k3 on the hydrogen pressure are first, zero, and second order, respectively. The second reaction is the reversal of the first reaction. The third reaction is in agreement with the molecular mechanism of metal etio chlorin + 2Hz ring opening and metal deposition. No metal-free porphyrins were detected in the solution in agreement with Hung (1980). The rate constants are summarized in Table IV. Having a suitable kinetic model, we can address the question of the constant ratio of metal chlorin to total metal concentration shown in Figure 8. It can be readily shown that outlet solution concentrations with respect to
+
-
-
512
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984
Table IV. Rate Constants
reaction 1 2 3
281
hydrogen pressure dependence, n
E
Ink,
I
-
1
-
Vancdyl Etlo
-
0 Total M u t a l A lnturmudiatu C m i u c u t i v a Ruaction Modal Singlo l r r a v a r s i b l a l S t O r d e r Mod01
For Nickel Etio Hydrodemetalation 17.1 1 15.38 i 0.06 22.9 0 23.6 f 0.12 32.6 1.83 27.41 i 0.11 21.08 i 0.14 32.6 2 In h = In k, - ( E / R T ) t n In P
-
-
-
-
-
-
For Vanadyl Etio Hydrodemetalation 16.96 i 0.03 18.3 1 2 17.88i 0.02 16.5 0 3 21.22 i 0.08 24.9 1.82 20.84 i 0.11 24.9 2 1
00
0.2
01
gm VO(l11),315'C,
03
Cat. H r
965MPa
Figure 15. Comparison of experimental results, consecutive kinetic model, and the results from a single irreversible first-order model.
Table V. Ni Etio 1. Ratio of Chlorin to Total Metal Concentrations ~
run
T,"C
N3 (111) N5 (11) N5 (IV) N5 (111) N8 (111) N8 (IV) N13 (11) N13 (111) N13 (IV)
34 3 288 315 302 343 329 315 343 329
iCB/
P, MPa CMlexp 9.65 0.127 0.193 9.65 0.187 9.65 9.65 0.218 6.89 0.119 6.89 0.138 4.14 0.123 4.14 0.118 4.14 0.122
[ C B / C M ] e x p = experimental value. calculated from eq 4.
[CB/ CMli
0.124 0.226 0.195 0.224 0.111 0.145 0.140 0.102 0.125
[c B / c M ] =
the contact time t are given by following equations (Wei and Prater, 1962)
I'[ c1 = -
eA*t -
[ ~ +] h+Ck k,
A+, A- =
-Ck
f
[(Ck)2 - 4klk3]1/2 2
In run N5(IV), the three rate constants are 21.1,60.0, and 33.4 mL/g-h. The eigenvalues computed from eq 3 are X+ = -6.53 and X- = -107.9. Thus by eq 2 exp(-6.53t) - exp(-107.9t) CB -_ 5.12 exp(-6.53t) - 0.313 exp(-107.9t) CM when t is above 0.02, the ratio CB/CMrapidly approaches the asymptotic value of 0.195. A greater ratio of h / X + would lead to a more rapid approach. The calculated values of CBand CM by the above equation along with the experimental values are summarized in Table V. The calculated values agree reasonably well with the experimental values. At short contact times, the concentration of nickel etiochlorin increases and attains a dynamic
equilibrium with respect to the nickel etioporphyrin. It should be noted that this equilibrium is not a thermodynamic equilibrium. We can summarize the final model as k,'Pn*
A
F=
k2
-
kSIPH21
B
metal deposition on catalyst and metal-free product in solution
For the runs N3 to N13 the inlet concentrations of the solution were maintained at a constant value of about 27 ppm. Run N14 was made with the inlet solution nickel concentrations of about 15 ppm. For this run when the inlet concentration of nickel was reduced to 15 ppm, there was no change in the activation energy and the dependence on hydrogen pressure; however, the values of the rate constants all doubled. This observation is in agreement with Hung (1979) in his batch autoclave studies. A Langmuir isotherm would be consistent with these observations for all species (nickel etioporphyrin, nickel chlorin, metal-free final products) if all absorb equally strongly on the catalyst surface, so that the concentration of available free catalytic sties is inversely proportional to the initial nickel etioporphyrin concentration. A critical test of this kinetics scheme would depend on experiments with a wider range of initial concentrations. At the present, the maximum concentration is limited by solubility and the minimum concentration is limited by analytical accuracies. B. Kinetics of Vanadyl Etioporphyrin I. The proposed model of consecutive reactions, the formation of an intermediate followed by metal deposition on the catalyst, also works well for the vanadium runs. The experimental and calculated results from run V9(V) are given in Figure 9 and the results from run VS(II1) are given in Figure 15. For the solution concentration of about 4 ppm, the uncertainty in vanadium analysis may be as high as 1.0 ppm. This uncertainty is reflected in relatively poor fit at the high conversions. The one-step irreversible first-order kinetics did not give a good fit with the vanadium kinetics data. Arrhenius plots for the rate constants kl, k2,and k , are given in Figure 16. The activation energies for the rate constants kl, k2,and k3 are 18.3, 16.5, and 24.9 kcal/g-mol, respectively. The activation energies for the hydrogenation and dehydrogenation steps fall in the range of the reported hydrogenation and dehydrogenation energies in the literature (Bhinde, 1979; Giiltekin, 1980). Dependence of the rate constants k , , k2, and k3 on the hydrogen saturation pressure, for the reaction temperature of 343 O C , are plotted in Figure 17. As expected, k , and
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984
513
Table VI. Vanadyl Etio I. Ratio of the Reaction Intermediate to the Total Metal Concentrations
vg(vj V7 (11) V9 (111) V3 (111) V3 (11)
2.0
343 343 315 302 288
6.89 9.65 9.65 9.65 9.65
0.270 0.305 0.313 0.312 0.323
0.270 0.283 0.314 0.317 0.349
a [C,/C, Iexp = experimental value. b [ c B / c M ] , = calculated from eq 4.
t
Table VII. Ratio of the Two Eigenvalues hJh+a for Some of the Nickel and Vanadium Runs
t
T;"C 1.7
1.8
IIT
343 343 343 315 302 288
1.8
x 1 ~ (3K - ' )
Figure 16. Arrhenius plots for rate constants kl,kz, and k3 at a hydrogen pressure of 9.65 MPa. All the rate constants are in mL/ g-h. a
P. MPa 4.14 6.89 9.65 9.65 9.65 9.65
Ni h / A
V h /A.
60.0 21.4 17.8 16.5 17.6 23.7
19.5 9.1 6.4 7.2 6.9 8.4
X, and A - are calculated from eq 3.
conditions, the rate of hydrodemetalation for vanadyl etioporphyrin is higher than that for nickel etioporphyrin. For nickel etioporphyrin in the range of concentrations studied, the three rate constants were doubled when the inlet solution concentration was halved; for vanadyl etioporphyrin, the rate constants remained invariant. The relative ratios of the eigen values A+ and A are quite different between nickel and vanadium. Depending on the ratio of A+ and A, at a value o f t greater than some tl, the second term on the right-hand side of eq 2 can be neglected and the following approximate relation holds
In k
14
16
18
20
22
2.4
In P
Figure 17. Relationship between kl, kz, and k3 and hydrogen pressure at 343 "C. P is in MPa. All the rate constants are in mL/g-h.
k2 have the order of dependence of one and zero, respectively; k3 has kinetic order of 1.82, which is again close to 2. The rate constants for vanadyl etioporphyrin I are also summarized in Table IV. For vanadium the calculated values of C B / C M from eq 4 and the experimental values are summarized in Table VI. Once again the calculated values agree reasonably well with the experimental ones. When the inlet concentration of vanadyl porphyrin was changed from 28 ppm in run V-3(11) to 16 ppm in run V8(III), the three rate constants changed little from 16.0, 22.0,20.2 to 17.0,20.0, 18.0. This observation is consistent with a Langmuir isotherm where the adsorption of vanadyl porphyrin and reaction products are weak. Thus kinetics of vanadyl etioporphyrin can be adequately described in terms of bulk rate constants kl,k2,and k3. C. Differences Between Nickel and Vanadium. A comparison of Tables V and VI showed that the C B / C M ratio is higher for vanadium than for nickel. In the temperature and pressure range studied, vanadium has twice the hydrogenation rate constant and the same dehydrogenation and hydrogenolysis rate constants. Thus at a given total metal concentration and identical operating
For both nickel and vanadium the ratio of the two eigen values L / X + for some of the runs are listed in Table VII. It is seen that for nickel the ratios are more than twice that of vanadium. Therefore for nickel, eq 5 becomes a good approximation at a much lower conversion than it does for vanadium. Because of the much higher ratios of A-/X+, the nickel removal can be better approximated by single first-order kinetics. The dynamic equilibrium in Table V for nickel was attained at much lower conversions than for vanadium in Table VI. D. The Transient Behavior of the Catalyst. The initial catalyst activity was exceedingly high, which was followed by a period of rapid deactivation, a period of activation, and finally stabilization for a long period of time. The rapid deactivation may be due to rapid coke buildup on the catalyst. The spent catalyst from the reactor contained coke concentrations as high as 3-4%. a time period of 15 h may be sufficient to build up most of the coke formed during the total course of the reactor operation. Beuther and Schmid (1963) observed that of the total carbon deposited on the HDS catalyst in 16 days of operation, one-half was deposited in the first 12 h. The second phase of increase in activity may be due to metal deposition in the catalyst. From our model, the metal etioporphyrin first went through hydrogenation and then the demetalation step. In Figure 7, as the catalyst activity increases, the concentration of the hydrogenated intermediate in the solution also increased. Bridge and
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Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 3, 1984
Green (1979) also reported that the metal deposit is catalytically active for hydrogenation and demetalation, and for hydrogenolysis.
Conclusions Metal removal from the etioporphyrin ring was observed to occur by consecutive reactions with a hydrogenated reaction intermediate. For both the nickel and vanadium runs, the intermediates were isolated, and the corresponding peaks in the visible spectrum were identified. The reaction intermediate for the nickel etio runs was identified by mass spectrometry to be nickel etiochlorin. For both nickel and vanadium hydrodemetalation, the mechanism consists of three major kinetic steps. The first step is hydrogenation of metal porphyrin to the reaction intermediate metal chlorin; the second step is the reversible dehydrogenation of the intermediate; the thud step is the demetalation of the reaction intermediate. The kinetics is consistent with first-order dependency on metal compound concentrations with Langmuir isotherms; the adsorption coefficients are large for nickel, but small for vanadium. For vanadyl etioporphyrin, the hydrogenation step is first order with respect to hydrogen pressure; the dehydrogenation rate is independent of hydrogen pressure; and the hydrogenolysis step is second order with respect to hydrogen pressure. Acknowledgment The authors are grateful to the National Science Foundation for support of the work under Grant No. EMG 75-16456. R.A. is also grateful to Raymond R. Cwiklinski of Massachusetts Institute of Technology for useful discussions.
Nomenclature A = original metal etioporphyrin B = reaction intermediate CA = concentration of model metal etioporphyrin in solution (parts per million by weight of the metal), ppm C R = concentration of reaction intermediate in solution, .. ppm c&= C A + C* = total metal concentration in the inlet solution to the -.reactor E = activation energy, kcal/g-mol k = rate constant mL of solution/(g of cat. h) k , = first-order rate constant for hydrogenation step k2 = first-order rate constant for dehydrogenation step k3 = first-order rate constant for the demetalation step n = order of dependence on hydrogen pressure P = hydrogen pressure, MPa Q = solution flow rate to reactor, mL/h R = universal gas constant, kcal/(g-mol K) T = reaction temperature, K t = W / Q , contact time W = weight of catalyst in the reactor, g X+,X.. = eigenvalues given by eq 3 Abbreviations
HDN = hydrodenitrogenation HDM = hydrodemetalation HDS = hydrodesulfurization
ppm = concentration of metal in solution in parts per million by weight Registry No. Ni-Chlorin, 89922-68-9; VO-chlorin, 89922-69-0; COO, 1307-96-6; MOOS, 1313-27-5.
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Received for review March 28, 1983 Accepted August 4, 1983
