I n d . E n g . C h e m . R e s . 1987,26,863-869
863
Studies on the Metal Poisoning and Metal Resistance of Zeolitic Cracking Catalysts Arthur W. Chester Paulsboro Research Laboratory, Mobil Research and Development Corporation, Paulsboro, New Jersey 08066
T h e effects of metal poisons (Ni, V) on zeolitic cracking catalyst have been investigated with the goal of developing a method for determining the inherent metal resistance of different catalyst compositions in the laboratory. Metal resistance is evaluated by determining activity and selectivity both before and after a synthetic poisoning procedure so that “contaminant yields”-the excess coke and hydrogen produced by metals during cracking-could be calculated. After several methods are compared, t h a t chosen involves impregnation of steam-deactivated catalysts with a gas oil doped with metal naphthenates, followed by high-temperature cracking and regeneration. With this method, i t was found that contaminant yields were dependent on the one-half power of metal content, that Ni and V do not act synergistically, and that, while Ni produces 3-4X as much hydrogen as a n equivalent amount of V, the two metals were equally active for coke formation. The effects of metal contaminants on cracking catalysts are well-known. The metals, primarily Ni, V, and Fe, are deposited during cracking and lead to catalyst deactivation and loss of selectivity (high coke and hydrogen yields and loss of liquid product). The effects of metals on the performance of amorphous cracking catalysts were extensively investigated (Mills, 1950; Duffy and Hunt, 1952; Conner et al., 1957; Grane et al., 1961; Gossett, 1960; Donaldson et al., 1961) prior to the development of zeolitic catalysts. The better activity and selectivity of the zeolitic catalysts led to apparently better metal resistance. The effects of metals on zeolitic catalysts have been examined in some detail by Cimbalo et al. (1972) and Habib et al. (1977) and more recently by Mitchell (1980),Jaras (1982),Andersson et al. (1984), Pompe et al. (1984), Masuda et al. (19851, Occelli e t al. (1985), and Wormsbecher et al. (1986). The upsurge of interest in the processing of metal-containing residual feedstocks has led to the need for a relatively rapid method of determining the characteristic behavior of different catalysts with metal poisons. Such a test would meet the needs of refiners, allowing a rapid screening of available commercial catalysts, as well as those in catalyst research, allowing the development of more effective metal resistant catalysts. The complexity of modern cracking catalysts make any fundamental study of metal poisoning very difficult. Zeolitic cracking catalysts consist in general of zeolite crystals embedded in an amorphous matrix which in itself may consist of several components including potentially active weighting agents (e.g., clay, alumina). When a metal is deposited on a catalyst during cracking, it may migrate to different elements of the catalyst during subsequent use and change its behavior accordingly. The widely varying compositions of modern cracking catalysts can, therefore, produce a variety of behaviors. As a first step in these studies, a reinvestigation of the various methods for laboratory simulation of metal poisoning was performed; this was then followed by a study of some of the fundamental parameters of metal poisoning (synergism of different metals, metal dependences of hydrogen, and coke production) and a comparison of different catalysts. The most realistic method of metal poisoning involves deposition during cracking in a cyclic, integrated pilot plant (or commercial unit) and has been used successfully by many workers, either with a natural high-metal feedstock (Duffy and Hunt, 1952; Gossett, 1960; Donaldson et al., 1961) or with a metal naphthenate doped charge stock (Cimbalo et al., 1972). The evaluation of results in such a method can be complex, due to the need, in some cases, 0888-5885/87/2626-0863$01.50/0
for continuous fresh catalyst addition, and because both cracking and metal functions age under process conditions. Although realistic, metal deposition in cyclic units is time-consuming and not readily adaptable to the rapid evaluation of a large number of catalysts. The need for a more rapid method is well illustrated by the results of Cimbalo et al. (1972) who found that two zeolitic catalysts, while more metal resistant than amorphous catalysts, differed considerably from each other. Early investigators (e.g., Mills, 1950; McIntosh, 1954) incorporated metals by aqueous impregnation of metal salts, but a method developed by Rothrock et al. (1957) involving fluid bed cracking of a metal naphthenate doped light charge stock was shown by Conner et al. (1957) to be more effective than aqueous impregnation and somewhat more effective than impregnation with metal naphthenate solutions (such as used by Habib et al. (1977)). Erickson and Keith (1966) used a variant of the aqueous impregnation method, where stable metal complexes were the metal sources. The catalysts were evaluated for both inherent selectivity and “contaminant” yields-the additional coke and hydrogen produced by the metal poisons. The need for both types of information is illustrated symbolically in Figure 1, which shows the response of four catalysts-W, X, Y , and Z-to increasing metal poison level (the abscissa). Inherent selectivity-such factors as hydrogen yield, coke selectivity (e.g., conversion/ coke), or gasoline efficiency-is represented on the ordinate. The different responses of the catalysts to metals would produce different evaluations of metal resistance a t different metal levels. Metal resistance is related to the slope of the response. Catalyst W is an ideal case-high inherent selectivity and low response t~ metals (high metal resistance). Of the three remaining catalysts, catalyst X would appear to be the most metal resistant at low metal levels-but the least metal resistant a t high metal levels. Thus, a knowledge of the response to metals-as represented by selectivity at two different metal levels-is essential to the proper evaluation of any catalyst. Experimental Section The fluid cracking catalysts used in this study were each steamed for 4 h a t 1400 O F (1033 K) and 100% steam a t atmospheric pressure (101 kPa) in a fluidized bed prior to use in order to simulate the deactivation encountered in commercial use.. Product distributions were determined by cracking a wide cut Mid-Continent gas oil (WCMCGO) a t 3 C/O, 8.3 WHSV, and 935 OF (775 K) in a fixed fluidized bed test unit before and after metal poisoning. 0 1987 American Chemical Society
864 Ind. Eng. Chem. Res., Vol. 26, No. 5, 1987 Table I. Catalyst Propertiesa catalvst
A Dore vol. cm3 of H,Ole packed density, g/cmc surface area, m2/g catalyst type a
B 0.47 0.79 328 clay
0.72 0.54 336 semisynthetic
C 0.44 0.75 169 clay-gel
D
E
F
0.52 0.64 264 semisynthetic
0.51 0.67 217 clay-gel
0.27 0.80 115 clay
All properties after calcination a t 1200 O F .
t I
"
I
-x>;>z
31
1
METAL CONTENT
I
e
Figure 1. Effect of metal content on cracking catalyst selectivity.
The methods, charge stock, and equipment used for steaming and testing have been completely described (Chester and Stover, 1977). Conversion is defined conventionally, as conversion (% vol) to materials boiling below 430 O F . In the initial screening of metal poisoning methods, the following were used. Cracking (Ni/WC). This method is similar to that of Rothrock et al. (1957). A charge stock was prepared by dissolving approximately 3000 ppm Ni as nickel naphthenate in a wide cut Mid-Continent gas oil. The steamed catalysts were poisoned by cracking this charge stock in a modified fluid bed reactor a t 980 OF (800 K, initial catalyst temperature), 3 C/O, and 8.3 WHSV. The charge stock was preheated to 375 "F (464 K) to prevent metal deposition and passed directly to the base of the fluidized catalyst bed. Sufficient dispersion nitrogen was used to maintain fluidization. During the 2.4-min. onstream time, the catalyst bed temperature drops by 150 O F (66 K), due to the low oil temperature. Microscopic observation showed carbon, and therefore metal, laydown to be reasonably homogeneous. Aqueous Impregnation (Ni/H,O). The steamed catalysts were impregnated with an aqueous nickel nitrate solution to incipient wetness, then dried (250 O F , 394 K), and calcined. Aqueous Impregnation with Chelated Metal (NiEDTA). An aqueous solution of NiEDTA (EDTA = ethylenediaminetetraacetic acid) was used as above. This method was originally proposed by Erickson and Keith (1966) and supposedly allows the chelated nickel molecule to coat the catalyst surface. Nonaqueous Impregnation (Ni/TOL). The steamed catalyst was impregnated with a solution of nickel naphthenate in toluene to incipient wetness, followed by drying (250 O F ) and calcination. This method was also used by Habib et al. (1977). The catalysts were regenerated (cracking method) or calcined in air at 1250 OF (950 K) in a fluidized bed prior to testing. In the subsequent study of fundamentals and catalyst screening, a simple impregnation-cracking method was developed to replace the cracking (Ni/WC) method. This was performed as follows. The catalysts were impregnated at ambient temperature
Table 11. Product Distribution-Steamed Catalysts catalyst A B C catalyst type semisynthetic clay clay-gel 83.2 80.2 81.5 conversion, % vol 66.5 65.1 64.8 C5+ gasoline, % vol 17.5 17.1 17.3 total C4's, % vol 7.8 7.3 6.7 dry gas, % wt 3.7 4.0 coke, % wt 4.4 hydrogen, % wt 0.03 0.05 0.06 adjusted yields (78% conv.): coke, % w t 2.4 3.3 3.3 26 HZ, scf/Bbl 11 24 ~
Table 111. Metal Poisoning by Cracking
method added Ni, ppm product distributions: conversion, 70 vol Cs+ gasoline, % vol total c i s , % vol dry gas, '70 wt coke, % wt hydrogen, % w t adjusted yield (78% conv.): coke, % wt Hz, scf/Bbl parameters: % activity loss contaminant coke, 70 contaminant H,, %
A Ni/WC 951
catalyst B Ni/WC 1100
C Ni/WC 1100
78.1 64.5 14.3 6.5 4.6 0.26
72.0 55.7 13.1 6.7 6.1 0.37
73.5 57.2 13.4 6.6 6.2 0.57
4.2 140
7.9 570
7.5 400
6 75 1100
10 140 2300
10 125 1400
with a charge stock containing an appropriate quantity of Ni and V as naphthenates diluted with xylene to a total volume sufficient to just fill the catalyst pores. The ratio of catalyst to charge stock was maintained constant at 5 wt/wt. After evaporation of the xylene at 250 O F , the impregnated samples were heated to 980 "F in a stream of nitrogen in a fixed fluidized bed and then held at 980 "F for 10 min, allowing the impregnated charge stock to crack and deposit the metals and coke on the catalyst. The coke was removed by oxidation in air at 1250 O F , and the catalysts were tested at 3 C/O and 8.33 WHSV as before. Six commercial FCC catalysts were used in these studies; some of their properties are listed in Table I. The catalyst types have been defined by Magee and Blazek (1976).
Results and Discussion Metal Poisoning Methods. The product distributions characteristic of the steamed (but unpoisoned) catalysts A-C are given in Table 11. For later reference, yields of coke and hydrogen, the primary products formed by metal poisons, are given at a constant conversion of 78% v01, determined by assuming a linear relationship between coke and hydrogen yields and the function [conversion/100 conversion] (other laboratory studies have verified this functionality). Product distributions for metal-poisoned catalysts are given in Tables 111-VI. In each case by reference to Table I, the effects of metal poisoning are clear: the catalyst is
Ind. Eng. Chem. Res., Vol. 26, No. 5, 1987 865 Table IV. Metal Poisoning by Nonaqueous Impregnation catalyst A B method Ni/TOL Ni/TOL added Ni, ppm 780 780 product distributions: conversion, % vol 78.5 77.1 C5+ gasoline, % vol 63.1 61.4 total C4’s, % vol 12.7 13.9 dry gas, % wt 6.5 6.8 coke, % wt 5.9 5.7 hydrogen, 90 wt 0.70 0.61 adjusted yields (78% conv.): coke, 70 wt 5.4 5.6 370 350 Hz, scf/Bbl parameters: % activity loss 6 4 contaminant coke, % 125 70 contaminant Hz, % 3150 1350
Table VI. Metal Poisoning by Aqueous Impregnation catalyst A C method NiEDTA NiEFA added Ni, ppm 1000 911 product distributions: conversion, % vol 75.5 70.6 C5+gasoline, % vol 57.4 51.7 14.9 total C4’s, % vol 13.2 dry gas, % wt 7.3 6.6 6.7 coke, % wt 8.2 0.70 hydrogen, % wt 0.89 adjusted yields (78% conv.): coke, % wt 7.2 11.7 Hz, scf/Bbl 435 720 parameters: % activity loss 9 13 contaminant coke, % 250 200 contaminant Hz, % 2650 3700
Table V. Metal Poisoning bv Aaueous ImDregnation catalyst A B method Ni/H20 Ni/HzO added Ni, ppm 860 860 product distributions: conversion, % vol 72.6 80.0 C5+ gasoline, % vol 55.2 58.6 total C1’s, % vol 11.6 16.2 6.9 7.8 dry gas, % wt 7.1 6.8 coke, % wt 0.37 0.85 hydrogen, % wt adjusted yields (78% conv.): 9.1 coke, % wt 5.8 620 Hz, scf/Bbl 370 parameters: % activity loss 0 13 75 280 contaminant coke, % 1450 5300 contaminant Hz, %
represents 3000-8000 ppm total metal, depending on the assumed make-up rate. For purposes of discussion, it may be initially assumed that the cracking method, being the closest to actual metal deposition, is the method of reference. From Table 111, catalyst A is more metal resistant than either B or C. This is particularly evident from the performance figures (adjusted yields), where the inherently superior selectivity of catalyst A is a factor, but is also apparent from the selectivity-independent contaminant parameters. Results for the nonaqueous impregnation method (Ni/TOL) are also shown in Table IV for catalysts A and B. In contrast to the cracking method, the Ni/TOL method increases metal activity for A but reverses the effect for B so that B appears to be the more metal-resistant catalyst. Nonaqueous impregnation is expected to produce an even coating of metal on all catalyst surfwes, but this is not necessarily what actually occurs during cracking. Metal deposition would be expected to occur initially where coke is deposited, a phenomenon more accurately reproduced in the cracking method. The results of the aqueous impregnation methods, Tables V and VI, show even larger differences. In both methods-Ni/H,O and NiEDTA-catalyst A produces much more contaminant coke and hydrogen than for the cracking method, as well as showing higher cracking activity loss. Catalyst B again shows better metal resistance (Ni/H,O), but C shows even higher sensitivity than with the cracking method. For the aqueous methods, ion-exchange phenomena, which are much more important for zeolitic than for amorphous catalysts, are expected to dominant the results; the added metals will associate with zeolite rather than with other portions of the catalyst. In spite of the use of a stable chelate with poor (zeolite) exchange properties in the NiEDTA method, sufficient free metal ion must exist in solution, particularly during lowtemperature drying of the impregnated catalysts, to produce exchange. Thus, two impregnation methods-aqueous and nonaqueous-suggest that B is more metal resistant than A, while the more realistic cracking method indicates A to be more metal resistant than B. Since no extraneous effects-ion exchange, dispersion-are present in the cracking method, A is, therefore, more resistant than B. While there is little direct commercial data, particularly at high metal levels, to verify this choice, available data a t low metal levels do suggest that the cracking method is qualitatively correct. The results for zeolitic catalysts differ from those for amxphous catalysts. Conner et al. (1957) found that a
apparently deactivated, giving much lower conversion of charge stock. Even at the lower conversions, both coke and hydrogen yields are significantly higher than for the unpoisoned catalysts; other yields adjust accordingly. In each case, adjusted yields a t a constant 78% vol conversion are given for coke and hydrogen. Estimates of gasoline yields are difficult to make, due to the nonlinearity of gasoline-conversion relationships and the changes in these relationships produced by metal poisons (Cimbalo et al., 1972; Habib et al., 1977). Increased coke yields generally result in lower gasoline yields; thus coke and hydrogen yields will be the basis of catalyst comparison. The adjusted yields allow a comparison of catalyst performance with metals, but this performance is a function of both inherent catalyst selectivity and metal resistance. The adjusted yields may be used to determine contaminant coke and hydrogen parameters, which are the amount of each product produced by the metal function. These parameters are given in Tables 111-VI as a percentage of catalytic coke and hydrogen (the amounts produced by the steamed catalysts before metal poisoning a t 78% vol conversion) and allow a comparison of metal resistance independent of inherent selectivity. In addition, activity loss is given in each case, normalized to the activity of the unpoisoned catalysts. In each of these methods, all of the added metal poison is assumed to be active. A level of approximately 1000 ppm active metal was chosen so that the effects could be clearly observed. In commercial use, metal activity ages with time; the proportion of the total metal on catalyst that is active is a function of fresh catalyst make-up rate (Cimbalo et al., 1972). The 1000 ppm active metal level
866 Ind. Eng. Chem. Res., Vol. 26, No. 5, 1987
E
200
m Y
$
150
E
&0 1 0 0 ij z 50
rJ 0
25
100
250
500
750
0
Added Metal, ppm (As 1 / 2 Power)
Figure 2. Effect of Ni (0)and V (0) on hydrogen yields (adjusted to 75% vol conversion). Abscissa is scaled as one-half power.
cracking method produced higher metal activities than either aqueous or nonaqueous impregnation methods for different amorphous catalysts (high and low activity, high and low alumina). While these observations hold for B, the opposite is true for A. Thus, the greater complexity of zeolitic catalysts makes substitute metal-poisoning methods unreliable in assessing relative metal resistance. Metal Dependence and Synergism. The dependence of product distribution on metal level and the possibility of synergism between Ni and V were investigated for catalyst D. In this and the subsequent studies to be discussed, the impregnation-cracking method was used. Nonlinear metal dependences, particularly at higher metal levels, have been observed in the past (Conner et al., 1957; Cimbalo et al., 1972; Habib et al., 1977), and possible synergism between metal contaminants has been observed by McIntosh (1954) and Conner et al. (1957) for amorphous catalysts and recently by Jaras (1982) for zeolite catalysts. In these experiments, steamed catalyst D was poisoned with Ni and V alone, as well as with combinations of varying ratios of Ni and V from -50 to 600 ppm total metal. The actual product yields obtained in the FCC bench test (3 C/O, 8.3 WHSV, WCMCGO, 935 OF) were adjusted to a constant conversion of 75% vol in order to determine dependences. The poisoned samples prepared are listed in Table VII, along with the actual product distributions obtained and coke and hydrogen yields adjusted to a constant 75% vol conversion. The loss of conversion and gasoline yield and increased coke and hydrogen yields are most obvious a t the higher metal poison levels. The adjusted yields were used for coke and hydrogen dependences. When the adjusted hydrogen yields for the catalysts with Ni or V alone are plotted against metal, the plots are nonlinear (particularly Ni); however, when metal to the one-half power is used, reasonable linearity results, as shown in Figure 2. From the slopes in Figure 2, the activity ratio Ni’/2/V1/2 3.4 for hydrogen production. An activity relationship in widespread use is given by (Ni + V/4) in ppm, based on published data by Cimbalo e t al. (1972) for H2 production. Figure 3 shows adjusted hydrogen yield against this function to the one-half power for all of the poisoned catalysts. The reasonable linearity obtained suggests that the literature value of 4 for the Ni/V activity ratio for hydrogen production is approximately correct. It is also apparent that no synergism between Ni and V occurs, as samples with both metals fall on the same line as those with only Ni or V. The dependences of adjusted coke yield on metal content (one-half power; direct plots against metal content show distinct nonlinearity) for the Ni-only and V-only
-
Ni
+ V14,
ppm (As 1 1 2 Power)
Figure 3. Effect on adjusted hydrogen yields of catalysts with Ni V only (A), both Ni and V (V),and no metals (0). Abscissa only (O), scaled
5 E
6
Y
8
0
Added Metal, ppm (As 1 1 2 Power)
Figure 4. Effect of Ni ( 0 )and V (0)on coke yields (adjusted to 75% vol conversion). Abscissa is scaled as one-half power. 5 5
0 5 -
0
100
25
Ni
+ V,
250
500
750
ppm (As 1 1 2 Power)
Figure 5. Effect on adjusted coke yield of catalysts with Ni only (O), V only ( A ) , both Ni and V (V),and no metals (0). Abscissa scaled as one-half power.
catalysts are shown in Figure 4. Unlike hydrogen production, Ni and V show approximately equal activity for (contaminant) coke yield. A plot of coke yield against the one-half power of total metal (Ni + V) for all the catalysts is shown in Figure 5. The fit is excellent; the lack of synergism between Ni and V, this time for coke yields, is also effectively demonstrated. The conclusion that Ni and V do not act synergistically is in apparent disagreement with the recent results of Jaras (1982) but is not necessarily inconsistent, as his observed synergism relates primarily to activity loss due to metals a t high metal levels (500010 000 ppm). The results show a clear correlation of metal activity with the one-half power of metal content. Such a relationship is reasonable for a laboratory method in that metal surface area would have such a dependence on crystallite size if the same number of sites are involved when larger quantities of metal are added. Whether this
Ind. Eng. Chem. Res., Vol. 26, No. 5, 1987 867 Table VII. Effect of Contaminant Metal Level on Yields (Catalyst Do) adjusted to 75% conv.
actual product distributionb added metal poisons (i) base (no metals) (ii) Ni only
Ni, ppm
V, ppm
47 126 303 601
(iii) V only
(iv) Ni
+V
48 54 231 455 86 35 99 136 263
47 107 56 177 312
conv., % vol 79.8 81.3 80.6 80.2 76.7 79.7 80.7 77.6 77.7 77.9 79.0 78.7 78.5 77.2
C6+ gasoline,
coke,
% wt
% wt
% wt
63.8 66.6 64.0 62.8 60.8 63.3 63.4 61.6 61.0 61.0 60.9 62.6 61.7 60.0
4.2 4.5 5.3 6.2 5.7 5.1 5.2 4.6 5.5 4.7 5.4 4.9 5.3 5.5
0.02 0.08 0.19 0.38 0.41 0.05 0.06 0.08 0.14 0.06 0.19 0.10 0.24 0.30
3.3 3.2 3.9 4.7 5.2 3.9 3.8 4.1 4.8 4.1 4.3 4.0 4.4 4.9
% vol
HZ9
coke,
HZI scf/Bbl 10 32 76 152 203 22 22 39 65 30 85 43 107 147
"Steamed 4 h a t 1400 O F , 0 psig, 100% steam. b 3 C/O, 8.3 WHSV, WCMCGO (935 OF).
Table VIII. Comparative Metal Resistance of Cracking Catalysts" catalyst
A product distributions:b,c conversion, % vol C6+ gasoline, % vol total C i s , % vol dry gas, 70 wt coke, % wt Hz, % wt adjusted yields (75% conv.): coke, % wt Hz,scf/Bbl parameters: % activity loss contaminant coke, '70 contaminant Hz, 70
base 78.2 68.2 14.8 6.1 2.9 0.02
+Ni, V 73.8 59.9 13.6 6.3 5.6 0.60
B base +Ni, V 75.0 65.4 65.0 54.4 14.1 10.7 5.7 5.5 3.0 4.8 0.03 0.69
2.5 7
5.9 352
3.0 16
6 140 4760
7.4 601 13 147 3570
C
D
F
E
base 80.3 66.7 15.8 7.0 3.6 0.04
+Ni, V 65.7 52.3 9.8 6,4 6.3 0.77
base 80.2 63.3 18.3 8.1 4.4 0.02
+Ni, V 76.6 58.5 15.5 8.0 6.8 0.50
base 81.6 66.4 18.9 7.8 4.4 0.05
+Ni, V 72.9 56.2 13.1 7.1 7.2 0.79
base 77.1 63.3 15.9 6.7 3.5 0.03
+Ni, V 72.7 59.3 14.3 6.5 4.2 0.20
2.7 15
9.7 656
3.3 8
6.3 252
3.1 19
8.0 482
3.2 17
4.7 126
18 259 4420
6 90 2910
11
158 2420
4 48 655
"Base catalysts steamed 4 h a t 1400 O F , 0 . psig, - 100% steam before metal incorporation. b 3 C/O, 8.3 WHSV, WCMCGO (935 OF). 'Added metals, 340 ppm each Ni and V.
relationship would hold in commercial units, where metal is deposited and aged continuously, rather than deposited in one cycle, remains to be determined. The differing activity ratios for formation of contaminant hydrogen and coke by Ni and V are striking and have not previously been reported. Comparative Catalyst Metal Resistance. Six different commercial catalysb were tested at a constant metal level of 340 ppm each Ni and V (the doped charge stock contained 1700 ppm each Ni and V naphthenates) using the impregnation-cracking method. Product distributions before and after poisoning are presented in Table VIII, as are adjusted coke and hydrogen yields a t 75% vol conversion. From the loss of conversion due to the metals, and from adjusted yields, the data can be converted to parameters: activity loss, normalized to the base conversion, and contaminant coke and hydrogen, the additional coke and hydrogen produced by the metal functions, given as a percentage of the corresponding base (steamed but not poisoned) catalyst yields. The results in Table VI11 clearly separate catalysts by their response to metals. Catalysts B, C , and E show major activity and selectivity losses with metals and would require very high fresh catalyst make-up rates to maintain reasonable activity in the presence of metals; catalysts A, D, and F show much more moderate activity loss. Catalysts D and F show the lowest contaminant coke yields. Note, however, that catalyst A actually produces less total coke than D because of its superior inherent (metal-free) selectivity. Of the six catalysts reported in Table VIII,
catalyst F is clearly the most metal resistant. Effect of Steaming. Since contact with steam represents the primary mechanism of catalyst deactivation in FCC units, the effect of steaming on the metal-poisoned catalysts A-F was investigated. Both Mitchell (1980) and Jaras (1982) have used steam treatment after metal poisoning in their evaluations, and Jaras concluded that the ultimate results were independent of steaming (although his results were a t high fresh metal levels), although Wormsbecher et al. (1986) have shown that steam is required to produce deactivation of the zeolite by vanadium. For this study, the metal-poisoned catalysts above were regenerated in air for 1 h and then steamed with 100% steam for 4 h, a t 1200 and 1250 O F , respectively, and atmospheric pressure. Such conditions are sufficiently mild to preclude significant catalyst deactivation of the zeolite function by the interaction of deposited V with the zeolite as shown by, e.g., Pompe et al. (1984). Catalyst testing was done as above, and the adjusted yields, activity loss, and contaminant coke and hydrogen calculated relative to the base catalysts are given in Table IX. Also shown are calculated metal aging rates for coke and hydrogen production, obtained from the contaminant yields of Tables VI11 and IX. In terms of activity loss, the steam treatment results in fewer differences, with losses relative to fresh unpoisoned catalyst ranging from 3% to 8%,than found for the freshly poisoned catalysts, where losses ranged from 4% to 18%. In all cases, the controlled steaming actually increased catalyst activity relative to the freshly poisoned catalyst
868 Ind. Eng. Chem. Res., Vol. 26, No. 5, 1987 Table IX. Effect of Steaming on Metal Poisoned Catalysts catalyst A B C D E F adjusted yields (75% conv.): 5.2 coke, % wt 4.2 4.9 5.4 5.0 4.4 hydrogen, scf / B bl 237 161 142 150 153 117 parameters: % activity loss 4 4 8 3 6 4 contaminant coke, % 110 42 79 63 63 39 contaminant H,,% 3170 880 880 1690 700 600 metal aging rates: coke, % 21 72 69 30 60 18 33 75 80 42 71 8 H,,% Table X. Effect of Oxidation Promoter on Metal Resistance' catalyst A promoted A adjusted yields (75% conv.): base +Ni, V base +Ni, V 2.5 5.9 2.5 5.8 coke, % wt H,,scf/Bbl 7 352 10 381 parameters: % activity loss 6 6 contaminant coke, % 140 131 contaminant H,,% 4760 3660 Catalysts steamed 4 h at 1400 O F , 0 psig, 100% steam and then poisoned with 340 ppm each Ni and V. Test conditions as in Table 111.
to some extent (Chester and Stover, 1981), in contrast to Jaras' (1982) recent results for high metal levels. The activity compensation is really due to more rapid aging of the metal functions than of the acid (cracking) function by steam,resulting in considerably lower contaminant coke yields. The least metal resistant catalysts-B, C and E-show the highest rates of metal function aging by steam, indicating that the metals on these catalysts are chemically-as well as catalytically-more active than on the more resistant catalysts. The results for freshly poisoned catalysts (Table VIII) are more representative of the state of circulating catalysts for several hours after contact with metals and are a better guide to relative metal resistance. Effect of Oxidation Promoter on Metal Resistance. The impregnation-cracking technique is useful in determining the effects of compositional, structural, or preparative variations for a series of catalysts on metal resistance. Recently, promoter-catalyst systems for CO combustion during regeneration have been in widespread use (Chester et al., 1981; Rheaume et al., 1976). Promoted catalysts, for example, are prepared by addition of a platinum group metal promoter in minute quantities during the manufacture of existing catalysts. The possibility of a synergistic interaction between the promoter and the base metal poisons, therefore, exists. The effect of promoting catalyst A on its metal resistance is shown in Table X in terms of both adjusted yields and parameters; the promoted version appears somewhat more metal resistant. Effect of Metal Passivation Additives. Recently, several additives have been proposed that passivate metal poisons. Preferred additives are antimony (Johnson and Tabler, 1973; McKay, 1977; Dale and McKay, 1977) and bismuth and/or manganese (Readal et al., 1976). The additives may be incorporated in the catalyst or preferably added continuously as a soluble compound in the charge stock. The impregnation-cracking technique of incorporating metals provides an ideal method for testing the effects of such additives. The data in Table XI show such experi-
Table XI. Effect of Passivation Additives on a Commercial Equilibrium Catalyst catalyst equilib. poisoned +Sb +Bi/Mn added Ni, ppm 220 220 220 220 220 added V, ppm 220 additive, ppm 2200 8801220 product distributions:" conversion, % vol 75.4 72.4 75.1 75.3 C,* gasoline, % vol 62.1 56.5 60.3 58.8 total C4's, % vol 13.7 14.3 15.8 14.2 dry gas, % wt 6.7 6.6 7.1 7.1 coke, % wt 3.8 5.0 3.6 5.2 hydrogen, % wt 0.07 0.49 0.17 0.56
* 3 C/O, 8.3 WHSV, WCMCGO (935 OF).
ments in which a commercial equilibrium catalyst (which contains CO oxidation promoter) was poisoned with 220 ppm each Ni and V in the presence of Sb (dissolved in the charge stock as triphenylantimony) and of a combination of Bi and Mn (as triphenylbismuth and manganese naphthenate), as well as in their absence. The amounts of additive were as suggested in the original patents. The Bi-Mn combination had little effect on metal activity but prevented activity loss. The antimony was quite effective, preventing activity loss and reducing coke and hydrogen yields significantly, although gasoline yield showed less response.
Conclusion The impregnation-cracking technique of metal poisoning provides a rapid method for determining the metal resistance of a series of cracking catalysts. The method is useful for determining the effects of changes in a closely related series of catalyst or in evaluating a wide spectrum of commercially available catalysts. Contaminant yield parameters allow determination of metal resistance independently of other catalyst properties. Actual catalyst performance, as indicated by the adjusted yields, is determined by both metal resistance characteristics and inherent selectivity. The results obtained in these tests are, however, relative and are not quantitatively translatable to commercial performance, nor do they address the deactivation of the zeolite component by vanadium. The different relative metal (Ni and V) activities for coke and hydrogen production are of importance in unit design for high metal feedstocks. In commercial FCC units, conversion or throughput is limited by either coke (regenerator metallurgy and air rate) or hydrogen yields (gas plant and compressor capacities). Thus units designed for increased contaminant yields based on a Ni/V activity ratio of 4 may well be underdesigned for coke. A knowledge of catalyst metal resistance, as determined here, coupled with feedstock properties, should allow more efficient designs for cracking processes for high metal residual feedstocks. R e g i s t r y No. Ni, 7440-02-0; V, 7440-62-2.
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Ind. Eng. C h e m . R e s . 1987,26,869-874 Dale, G. H.; McKay, D. L. Hydrocarbon Process. 1977, 56(9), 97. Donaldson, R. E.; Rice, T.; Murphy, J. R. Ind. Eng. Chem. 1961,53, 721. Duffy, B. J., Jr.; Hunt, H. M. Chem. Eng. Prog. 1952,48, 344. Erickson, H.; Keith, C. D. US Patent 3 234 119, 1966. Gossett, E. C. Pet. Refin. 1960, 39(6), 177. Grane, H. R.; Conner, J. E., Jr.; Masologites, G. P. Petrol. Refin. 1961, 40(5), 168. Habib, E. T., Jr.; Owen, H.; Snyder, P. W., Jr.; Streed, C. W.; Venuto, P. B. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 291. Jaras, S. Appl. Catal. 1982, 2, 207. Johnson, M. M.; Tabler, D. C. US Patent 3 711 422, 1973. Magee, J. S.; Blazek, J. J. In Zeolite Chemistry and Catalysis; ACS Monograph 171; Rabo, J. A., Ed.; American Chemical Society: Washington, DC 1976; p 615. McIntosh, C. H. “Effect of Metal Oxides on Cracking Catalyst Activity and Selectivity,” Presented at the 126th National Meeting of the American Chemical Society, Sept 1954.
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Received f o r review August 8, 1986 Accepted December 22, 1986
Carbon Monoxide Hydrogenation Using Manganese Oxide Based Catalysts: Effect of Operating Conditions on Alkene Selectivity Richard G. Copperthwaite, Graham J. Hutchings,* Mark van der Riet, and Jeremy Woodhouse Department of Chemistry, University of the Witwatersrand, Johannesburg 2001, Republic of South Africa
A comparative study of non-alkali-promoted iron/ and cobalt/manganese oxide matrix catalysts for CO hydrogenation is reported. Iron/manganese oxide is less active than the cobalt/manganese oxide catalyst and also gives appreciably higher methane and decreased alkene yields. T h e alkene/alkane ratio is dependent on space time and conversion, and a t constant conversion/space time the effect of reaction pressure is also found to be significant. The results are explained in terms of primary alkene selectivity and subsequent secondary alkene hydrogenation. The high secondary hydrogenation activity of these catalysts is confirmed by using model ethene hydrogenation studies. I n addition, a distinct b u t short-lived catalytic hydrogenation function for pure MnO is reported which is rationalized in terms of the defect structure of MnO. In recent years there has been considerable interest in the study of carbon monoxide hydrogenation catalysts that demonstrate high selectivities for the production of Cz-C4 alkenes. Particular interest has centered on catalysts exhibiting strong metal support interactions for metals supported on, or in solid solution with, a partially reducible oxide, e.g., MnO (Kugler, 1980). For a number of years it has been known that Fe/Mn oxide catalysts can give high alkene selectivity with low methane selectivity (Kolbel and Tillmetz, 1976; Bussemeier et al., 1976). These catalysts are known to contain ironjmanganese oxide solid solutions (Hutchings and Boeyens, 1986) and have been termed matrix catalysts (Schulz, 1985). Cornils et al. (1984) have shown that the yield of C2-C4alkenes with this type of catalyst is highly dependent on the reaction conditions. By modeling the reaction conditions, it was shown that increases in (a) conversion, (b) catalyst packing height, and (c) reactor tube diameter all decreased the C2-C4 selectivity. In addition, Schulz and Gokcebay (1984) have shown that for matrix catalysts of this type, variation in the CO/H2 ratio has little effect on the alkene selectivity. Barrault and co-workers (1983a,b: 1984; 1985) reported very high yields of C2-C4alkenes for low iron concentration matrix catalysts which were maintained for ca. 100 h; however, these results were obtained a t atmospheric
* To whom correspondence should be addressed.
pressure, and the catalyst activity was very low. The majority of published data on these matrix catalysts have been obtained a t atmospheric pressure, and the activities quoted require considerable improvement if these catalysts are to have potential industrial interest. A typical procedure to secure such an improvement is to utilize an increased reaction pressure, but the effect of this parameter on alkene selectivity with these catalysts has received scant attention. We have recently reported (van der Riet et al., 1986) a stable cobalt/manganese oxide catalyst that demonstrates high alkene selectivity at elevated pressures. In this paper, we now wish to report our findings on the effect of reaction conditions on alkene selectivity of the manganese oxide matrix catalyst containing comparable amounts of iron and cobalt.
Experimental Section Catalyst Preparation. Coprecipitated cobalt/manganese catalyst (Co:Mn = 1:l by mass) and iron/manganese catalyst (Fe:Mn = 1:l by mass) were prepared by continuous coprecipitation a t pH 8.3 and 70 OC by using the method of Maiti et al. (1983). The precipitate was collected by filtration, washed with distilled water, and dried a t 110 OC and 10 kPa. Catalysts were pelleted, ground, and sieved to give particles (0.5-1.1%” diameter) which were calcined in air a t 500 “C, 24 h, and then loaded into the reactor. Catalysts were reduced in situ in
0888-5885/ 87/ 2626-0869$01.50/ 0 0 1987 American Chemical Society
