1252
Ind. Eng. Chem. Res. 1992,31, 1252-1258
Wang, B. Y.; Cummings, P. T. Non-equilibrium molecular dynamics calculation of the shear viscosity of carbon dioxide/ethane mixtures. Chem. Eng. Sci. 1991. Submitted for publication. Wang, B. Y.; Cummings, P. T.; Evans. D. J. Non-Equilibrium Molecular Dynamics Study of Molecular Contributions to the Thermal Conductivity of Carbon Dioxide. Mol. Phys. 1991, in press. Wedgewood, L. E.; Bird, R. B. From molecular models to the solution of flow problems. Znd. Eng. Chem. Res. 1988,27, 1313. Williams, D. E. Nonbonded potential parameters derived from
crystalline aromatic hydrocarbons. J. Chem. Phys. 1967,47,4680. Yamada, T.; Kawasaki, K. Application of mode-coupling theory to nonlinear stress tensor in fluids. hog. Theor. Phys. 1975,53,111. Zwanzig, R. Time Correlation Functions and Transport Coefficients in Statistical Mechanics. Annu. Rev. Phys. Chem. 1965, 16,67. Received for reuiew June 12, 1991 Revised manuscript received January 13, 1992 Accepted February 1,1992
KINETICS AND CATALYSIS De-SO, Catalyst: The Role of Iron in Iron Mixed Solid Solution Spinels, Mg0*MgA12-xFex04 Jin S. Yoo,* Alak A. Bhattacharyya, and Cecelia A. Radlowski Amoco Research Center, Amoco Chemical Company, P.O. Box 3011,Naperville, Illinois 60566
John A. Karch UOP,25 E. Algonquin Road, Des Plaines, Illinois 60017-5017 The iron mixed solid solution spinels, Mg0.MgA12-,Fe,04, were prepared by the coprecipitation method, and the role of iron in these spinels was examined for the de-SO, chemistry. The iron in the mixed spinels exhibited a dual function, namely, the catalytic oxidation of SOz to SO3 and the dramatic lowering of the sulfate reduction temperature. In short, the iron mixed solid solution spinels performed much better as a de-SO, catalyst than their single solid solution counterpart without iron, Ce/Mg0.MgAl2O4. The fractional substitution of the Al atom in the spinel structure ( x I0.4) resulted in overcoming a drawback of sulfur accumulation inherent in the solid solution spinel. Sulfur tends to build u p rather quickly in the de-SO, cycle test with the solid solution spinel catalyst without iron. T h e iron mixed solid solution spinels impregnated with cerium, Ce/MgO-MgAlZ,Fe,O4, displayed an even better de-SO, function and an excellent steam stability as well.
Introduction Recently, the sulfate of the stoichiometric spinel, MgA1204,was studied using IR spectroscopy and vacuum microbalance techniques, and it was shown that the spinel is the best sulfur-transfer catalyst for reducing SO, emission (Waqif et al., 1991a,b). There have been other reports concerning the spinel phase structure (Kwestroo et al., 1959), the solid-state properties (Narasimhan and Swamy, 1980),and the catalytic activities (Narasimhan and Swamy, 1982,1976)of the iron mixed stoichiometric spinel, MgAlz,Fe,04, along with the acid-base properties of aluminum-magnesia mixed oxides (Lercher et al., 1984). Much of the literature revolves around the fractional replacement of aluminum in the spinel structure by iron. The mixed stoichiometric spinel, MgAl2-,Fe,O4, varies in an interesting manner as a function of x and forms the solid solution in two concentration ranges of x : 0 Ix 5 0.4 and 1.3 Ix I2.0 (Narasimhan and Swamy, 1980). The iron mixed stoichiometric spinels, MgAlz-,Fe,04, as well as the iron mixed solid solution spinels, MgO. MgA12-rFe,04, were prepared and evaluated for the following key reactions involved in the de-SO, process (Yo0 et al., 1991, 199Oa-c, 1988). This work is particularly focused on the sulfate reduction reaction, which is believed to be the rate-determining step in the de-SO, cycle.
* T o whom correspondence should be addressed. 0888-5885/92/2631-1252$03.00/0
SO2 + O2 SO3 MgO
+
* SO3
-+
(1)
MgS04 (2) MgSO4 + H2 MgO H2S + H2O (3) The single solid solution spinel with cerium, Ce/ Mg0.MgAl2O4,tends to accumulate sulfur for each cycle due to insufficient sulfur removal in the sulfate reduction half cycle (Bhattacharyya et al., 1988). However, the iron mixed solid solution spinels were able to overcome this drawback and exhibited an excellent catalyst rejuvenation by significantly lowering the temperature required for the sulfate reduction. These observations prompted us to explore the effectiveness of these iron mixed solid solution spinels as a de-SO, catalyst. -+
Experimental Section The spinel materials studied in this work can be classified into the following four types: stoichiometric spinel, MgA1204;iron mixed stoichiometric spinel, MgA12,Fe,04; single solid solution spinel, MgO-MgA1204;iron mixed solid solution spinel, Mg0.MgA12,Fe,04. The mixed stoichiometricspinels and the mixed solution spinels in which A1 atom was fractionally substituted by iron were prepared according to the coprecipitationmethod described for MgAb,,Fel.404below. A magnesium nitrate solution was prepared by dissolving 512.8 g (2.0 mol) of Mg(N03)2.6H20in about 100 mL of 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No 5,1992 1253 Table I. Elemental Analysis of Mixed Spinels atomic ratio Mg/Al/Fe nominal formula calcd analysis 1.0/1.8/0.2 %A1.8Fe0.204 1.0/1.8/0.2 1.0/1.3/0.6 Mg~1.1Fe0.B04 1.0/1.4/0.6 1.0/0.6/1.3 Mg&.BFe1,404 1.0/0.6/1.4 MgAlFeO, 1.0/1.0/1.0 1.0/1.0/1.0 2.0/1.9/0.5 Mg0.MgA11,BFeo.404 2.0f1.6f0.4 2.0/1.7/0.2 Mg0.MgA11,8Feo,204 2.0/1.8/0.2 2.0/1.9/0.1 Mg0-MgAll,&eo.104 2.0/1.9/0.1 2.0f0.6f1.3 Mg0.MgAb,BFe1,404 2.0/0.6/1.4
i I
Theor sald
Ce/MgAI,O,*
-@-
yMgO
Virgin
MgAli sFeoaO4. yMgO
+Steamed CelMgAlzOn. YMQO
b- Virgin Ce/MgAIzOr. yMgO
Figure 2. Saturated sulfur level on Ce/MgA11,6Feo.404.yMg0 and Ce/MgAl,O,.yMgO (virgin and steamed).
q , ,
,
,
,
,
0
0
M g M &eonon. YMQO
1
350 f 315
-
10
20
30 40 50 Angles (in degrees)
60
1 70
Figure 1. XRD pattern, Ce/Mg0~MgA11,~eo.104.
water, to which 8.2 mL (1.3 mol) of concentrated nitric acid was added slowly. The total volume was made to 1400 mL with water. The sodium aluminate solution was prepared with 131.8 g of Nalco sodium aluminate (0.60 mol of A1203, 0.66 mol of Na20) and 127.2 g (3.2 mol) of sodium hydroxide in about 800 mL of water. This solution was filtered through Whatman No. 1 filter paper and then brought to a volume of 1400 mL with additional water. A ferric nitrate solution was made by dissolving 1131 g (2.8 mol) of ferric nitrate nanohydrate in enough water to bring the final volume to 1400 mL. Three hundred fifty grams (8.85 mol) of sodium hydroxide was dissolved in sufficient water to bring the total volume to 1400 mL. The magnesium nitrate/nitric acid solution was added to a heel of 1700 mL of water. The sodium aluminate and ferric nitrate solutions were metered into the heel at the adjusted rates over the course of 1 h. During the addition the slurry was stirred and the pH was monitored. After the addition was complete, the slurry was stirred for an additional 1 h, during which time the final pH was adjusted to 9.5 by adding dropwise a small amount of 20 wt 9% NaOH solution. The slurry was aged quiescently for 16 h. The slurry was then filtered and washed with 12 L of water. The filter cake was dried at 127 "C in a forced-air oven for 16 h. The dried product was ground to pass through a 60-mesh screen. The ground material was gradually heated to 732 "C over 4 h and maintained at that temperature for 3 h under flowing air. The elemental analyses of the iron mixed spinels were carried out and the results are summarized in Table I. The XRD of a representative iron mixed solid solution spinel impregnated with cerium, Ce/Mg0.MgA11.&?eo.104 calcined at 550 OC, is shown in Figure 1. Two discrete phases of Ce02and spinel were clearly present in the XRD pattern. The SO, Pickup Half Cycle Test in a Batch Reactor. Approximately 15-25 g of the spinel material blended in the equilibrium fluid cracking catalyst (FCC) obtained from ARC0 Watson refinery, Watson, CA, in 1.5 wt %, was heated to 732 "C under a flow of nitrogen. At this temperature (732 "C), the nitrogen flow was discontinued,
and 5.9% O2in nitrogen was allowed to fluidize the catalyst bed for 4 min. A gas mixture containing 1.5% SO2 and 5.9% O2 in N2was then introduced for 15 min at 732 "C. Approximately 105-115 mg of SO2 was delivered during this cycle, and unreacted SOz in the reactor effluent was collected in a trap containing hydrogen peroxide. The flow of the gas mixture containing oxygen but not SO2 continued on for an additional 4 min. The oxygen flow was then stopped, and the system was flushed with nitrogen for 10 min at 732 "C. The peroxide trap was disconnected and the collected sulfur was analyzed as per the EPA Test No. 6 method. The sulfate collected in the trap was titrated with the barium perchlorate solution using Thorin as an indicator. The sulfur retained on the catalyst sample was analyzed by the Leco and/or peroxide method whenever it was deemed necessary. The Sulfate Reduction Half Cycle Test in a Batch Reactor. The sulfated blend was flushed with nitrogen after the SO, pickup half cycle and brought to the designated reduction temperature. At the reduction temperature, the nitrogen was cut off, and the catalyst was subjected to 100% hydrogen or propane for 5 min. The reactor effluent was collected in a trap containing 1 M solution of sodium hydroxide. The reactor was flushed with nitrogen at this temperature for 10 min and then cooled under a flow of nitrogen. The amount of sulfur removed with each reduction cycle was determined from the analysis of sulfur in the trap.
Results and Discussion The SO, Pickup Activity of Mixed Spinels. The sulfur level on the f d y sulfated iron mixed spinel samples resulted from the SO, pickup half cycle run was examined and compared with the sulfur level on the fully sulfated single magnesia excess solid solution spinels with cerium, Ce/MgA1204.yMg0, in Figure 2 and Table 11. The theoretical sulfur levels were calculated on the basis of the assumption that all Mg and Ce atoms or only Mg atoms in the spinel are completely accessible to form sulfates such as MgS04 and Cez(S04)3during the SO, pickup half cycle. The results are listed in columns A and B in Table 11. Neither A1 nor Fe was included in this calculation because of the anticipated thermal instability of their sulfates, A12(S04)2and Fe2(SO4I2,under the SO, pickup test conditions. The reason for excluding Ce in column B was based on the observation that only one sulfate peak corresponding to MgS04 was detected, and the Ce2(S04)3peak was absent in the XRD of the fully sulfated samples such as Ce/MgA1204-(0.54)Mg04 as shown in Figure 3. Within the entire range of single solid solution spinel, y = 0-1 in Ce/MgA1204.yMg04,the SO, pickup efficiency
1254 Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992 Table 11. Sulfur Content of Fully Sulfated Spinelsa sulfur. % ~~
~
theor
SO, pickup effic, actual/ theor CIB D/B
actual
A Mg, Ce
B Mg alone
C virgin
D steamed
14.7
13.2 15.6 16.5 17.1 18.8 20.6 18.1 13.7 25.4
6.5 11.8 10.5 13.5 10.5 8.5 14.6 11.2 15.3
4.8
Ce(lO%) / MgAl2O4-yMg0 y = o
y = 0.47 y = 0.72 y = 1.0 y = 1.63 y = 3.0 Mg0*MgA11.8Fe0.204 MgA11.8e0.4O4 Ce(lO%).MgO
17.7 18.2 19.7 21.4 25.8
0.49 0.76 0.64 0.79 0.56 0.41 0.81 0.82 0.60
13.0 14.0 11.5 10.0 7.8
0.36 0.79 0.85 0.61 0.49 0.31
"A, calculated on the basis of an assumption that all Mg and Ce atoms are available for S pickup; B, calculated on the basis of an assumption that only Mg atoms are available for S pickup. 100
I
" 10 100
I
I
37.5
65
I
100 I
92.5
120
92.5
120
I
80
0 '
10
37.5
65
I
I
0
1
2 9 degrees
Figure 3. XRD patterns of sulfated and H2-reducedsteamed Ce/ MgAl204.0.54MgO.
has steadily increased and reached a maximum for the virgin and steamed samde, - . when N becomes 1, Le., Ce/ MiO.MgA1204. To our surprise, the SO, Rickup efficiency of the virgin materials in ihe entire range of y-in the MgO excess sdid solution spinel, Ce/MgA1204*yMg0,was improved despite the severe steam treatment, 100% steam at 760 "C for 6 h, except y = 0, Ce/MgA1204,and y = m , Ce/MgO (Yo0 et al., 1990b). At the maximum SO, efficiency,approximately 79435% of the theoretically available Mg absorption sites in Ce/ Mg0.MgA1204was utilized, but the efficiency precipitously dropped as MgO increased in the region of y > 1. On the other hand, both the virgin iron mixed stoichiometric spinel and the virgin iron mixed solid solution spinel showed an excellent SO, pickup efficiency of 0.81 and 0.82, respectively, in the MgO excess solid solution region, where A1 is partially substituted with Fe. These results indicate that the SO, pickup efficiency is remarkably improved by incorporating iron into the spinel structure, and that the iron in the spinel structure functions as an active oxidation catalyst for converting SO2to SO3. One can speculate that both Ce and Fe function as an oxidant for the oxidation of SO2to SO3 in a synergistic manner, but neither of them participates in the sulfate formation in the SO, pickup half cycle. It is interesting to note that, in the subsequent sulfate reduction half cycle, the iron again functions as a promoter for the sulfate reduction, but cerium remains neutral. The Sulfate Reduction Temperature Profile Study. A t the early stage of the de-SO, catalyst development, 25 g of the single solid solution spinel impregnated with cerium, Ce/Mg0.MgA1204,blended (1.5 w t %) in the FCC equilibrium catalyst was subjected to both the SO, pickup and the sulfate reduction half cycle tests in the batch
2
3
4
5
Number of cycles
Figure 4. Variation of sulfur level on Ce/Mg0.MgAl2O4with number of de-SO, cycle. 60 Ce/MgOMgAI2O4
50 40 )1 + ._ > ._
2
30
X
a
20 10
-
'
677'C
Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992 1255 Table 111. The Sulfate Reduction Temperature Profiles of Mixed Spinelsa SO, activity at reduction temp, "C spinel composition 732 691 677 649 621 593 MnO.MgA1, 31 33 30 29 - - .-nFen -..,O, . .
566 19
538 16
MgAlFeO, Fe(2% )/Ce( 10%)/Mg0-MgAl2O4,double impreg Fe(lO%)/Ma0 I
-
(10) 31
(10) 19
16
Ce(10%) / Mg0.MgA120, Ce(10%)/MgA1204 Ce(l0% ) /MgA1204(prototype) Ce(10%)/MgA1204-yMg00, = 0.53) ( x = 0.18) Ce(1O%)/MgAl2O4.nAl2O3 a
Values in parentheses are data for the steamed counterpart.
Reduction with propane instead of HO 40
Table IV. The Sulfate Reduction Temperature Profiles of Iron Mixed Solid Solution Spinels. M a 0 MaAl,,Fe,O," sulfate reduction temp, "C n 732 691 677 621 566 510 454 0 37 30 13 30 29 19 14 10 0.4 40 (21) (15) 0.6 (10) (13) 1.0 (27) (25) 1.4 (19) (25) 1.8
(Wb
(17)
la
(22)b
(21) (23)b
18
16
12
4
n
oValues in parentheses are data for the steamed counterpart. Reduction with propane instead of HO
conclusion that the catalyst deactivated rapidly when the sulfate reduction was performed at 677 "C. The temperature employed in the sulfate reduction half cycle played a key role for rejuvenation of the sulfated catalyst to establish an effective de-SO, cycle. Also data points illustrated with a dotted line in Figure 5 show that the catalyst deactivated by the successive sulfur accumulation in five cycles run at 677 O C was almost completely rejuvenated in a single reduction run by raising the temperature to 732 "C. In order to study how to overcome this drawback, i.e., sulfur accumulation, the iron mixed spinels samples were fully sulfated to a saturation point in the SO, pickup half cycle. This reduction temperature profile study was started with this fully sulfated blend. The fully sulfated sample was subjected to a cycle of sulfate reduction half cycle a t a designated temperature followed by the SO, pickup half cycle for a maximum SO, uptake. The same cycle was repeated at various reduction temperatures in the range of 400-732 "C. The SO, activity was determined at each designated temperature by determining the amount of SO, uptake subsequent to the reduction. The results of the iron mixed stoichiometric spinel without cerium were compared against those of its stoichiometric spinel counterpart without iron, Ce/MgA120r, in Figure 6a. The corresponding solid solution case is shown in Figure 6b. All these data were combined in Tables 111-V and plotted together in Figure 6c. It is anticipated from the earlier reports (Waqif et al., 1991a,b; Bensitel et al., 1989) that a t least two types of sulfate species, surface and subsurface bulk sulfate, exist on the fully sulfated MgO-derived spinels, and there is no significant difference in the ease with which any of the sulfate species can be reduced with hydrogen. We also thought that, in the presence of oxygen, the iron in the mixed spinels performs the initial step of catalytic oxidation of SO2 to SO3, which then forms a stable surface
b 50
-
450
500
550
600
650
Reduction temperature,
700
750
'C
?
2ol 10
0'
400
450
500
550
600
650
700
750
Reduction temperature, ' C
Figure 6. (a) Sulfate reduction temperature profiles of iron mixed stoichiometric spinel and single stoichiometric spinel. (b) Sulfate reduction temperature profiles of iron mixed solid solution spinel and single solid solution spinel. (c) Reduction temperature profiies of iron mixed spinels.
sulfate at either Mg2+,AI3+, or Fe3+,and most likely sulfate species in interaction with Mg2+,A13+,and/or Fe3+sites as in the case of copper aluminate case (Waqif et al., 1991a,b).
1256 Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992 Table V. The Sulfate Reduction TemDerature Profiles of Iron Mixed Stoichiometric SDinels. MaAL.Fe.0, reduction temp, "C X 732 691 677 649 621 593 566 538 482 421 0 21 23 15 8 4 0.2 33 (8) (8) 0.4 35 25 25 23 24 20 16 9 (14) 0.6 36 24 24 10 96 1.0 38 (13) 1.3 37 5b [19] (5-7) 2.0 19 Ib [51 nValues in parentheses are data for the steamed counterpart. Without steam stripping; brackets indicate with steam stripping at 593 "C. Table VI. Critical Reduction Temperature catalyst Tc, "C MgA11.6Fe0.404 440 M ~ O . M ~ A ~ I . B F ~ O . ~ O455 ~ Ce/Mg0.MgAl,04 640 Ce/MgAlz04 620 660 Ce/MgO
TM,OC 732 690 710 690 >I32
A SO, activity of approximately 10 was arbitrarily chosen as an acceptable catalyst performance level of the de-SO, catalyst in the commercial FCC inventory. Therefore, the appropriate sulfate reduction temperature range, within which a satisfactory de-SO, activity could be maintained, was found from the plots in Figure 6. The minimum temperature required to obtain a SO, activity of 10 from the highly sulfated system is defined as the critical reduction temperature, T,; the temperature above which there is no further improvement in SO, activity is defined as the maximum temperature, T M The temperatures beyond 732 "C are excluded from this work because they are far above the temperature range expected even in a commercial high temperature mode operation FCC unit. T, and TMwere obtained from these two figures and are listed in Table VI. The data bring out a dramatic lowering of T,by the incorporation of iron in the mixed spinels. The T, was lowered from 620 to 440 "C and from 640 to 455 "C for the iron mixed stoichiometric and the iron mixed single solid solution spinels, respectively. The fact that the iron in the mixed spinel structures promotes these remarkable lowerings of the critical temperatures for the sulfate reduction half cycle is in good agreement with the earlier reports (Tagawa, 1984; Kwestroo, 1959). Effect of Iron Content in Mixed Spinels on the SO, Activity. The effect of iron content in the steamed stoichiometric and steamed iron mixed solid solution spinels on the SO, pickup activity is illustrated in Figure 7. The SO, activity increases as the fraction of substituted iron ( x ) in Mg0.MgA12,Fe,04 increases in the lower solid solution range ( x = 0-0.4). Beyond this range the activity assumes a broad ill-defined plateau between x = 0.4 and x = 1.3. There is a definite downturn in the activity as the full replacement of aluminum with iron is approached, as in MgO.MgFe,O,. The stoichiometric iron spinel, MgFe204,assumes an "invert" spinel structure, Fe[MgFe]04, in contrast to the "normal" spinel structure, Mg[A12]04, known for MgA120, (Cimino, 1974). Considering a probable adverse effect of the high-iron-containing materials on the cracking reaction because of its coke-forming property, the spinels with a rather limited iron substitution, 0.03 Ix I0.4, would be a more realistic candidate for the commercial application of the de-SO, catalyst. The Batch de-SO, Cycle Study. The batch de-SO, cycle study was carried out with iron mixed solid solution
\I 0.5
1 1.5 Fraction of iron substitution (x)
"0
2
Figure 7. Effect of iron content on SO, activity.
50
MgOeMgAI, 6Feo404 II
10
-
01 0
41
I
1
I
I
3 Number of cycles
2
1
I
4
5
Figure 8. Batch de-SO, cycle test of Mg0-MgA11,6Feo,101.
spinels, MgO*MgA12-,Fe,0,, blended in Watson equilibrium FCC catalyst. The results are summarized in Tables VI1 and VIII, and compared in Figure 8. The initial SO, activity of the single solid solution spinel, Ce/MgOMgAl,04, was higher than that of the mixed spinel a t the reduction temperature of 677 "C, but it declined much faster as the cycle progressed. After the fifth cycle, the SO, activity of mixed spinel was more than twice that of the single solid solution counterpart, 30 vs 12. With the steamed mixed solid solution spinels containing a high percentage of iron, the reducibility of the sulfated system was examined with hydrogen and propane. As shown in Mg0-AI,,$'e,,O4 shows a SO, activity of 25 with Table E, hydrogen and 22 with propane, while MgO.MgAlFe0, yields a SO, activity of 25 with hydrogen and 28 with propane. Although there are some variations in SO, activity between these two reducing gases, a clear-cut trend is not discernible. The mixed spinel containing high percentage iron required steam stripping t~ remove the metal sulfide, mostly likely FeS, resulted in the reduction of sulfate when the
Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992 1257 Table VII. SO, Activity Cycle Test of Iron Mixed Stoichiometric Spinels cycle SO, mixed spinel virgin steamed no. activity 33 MgNl.8e0.204 1 21 2 20 X 4 1 7 38 MgA11.6Fe0.404 1 30 732 "C 2 30 732 "C 3 26 732 "C 4 27 732 OC X; 732 "C, 6 h 8 1 15 2 14 3 13 X; 732 "C, 12 h 5 1 732 "C 10 732 "C 2 10 36 Mg~l.4Fe0.604 1 15 677 "C 2 19 677 "C 24 677 OC 3 4 24 593 "C + steam strip 5 19 538 "C steam strip X 10 1 13 677 "C 2 13 677 "C 37 MgAb.7Fe1.304 1 677 "C 5" 4 2 677 "C X 14 1 17 732 "C 2 732 "C 15 3 17 732 "C MgFe204 X 19 1 0 677 "C 2 1 677 "C 5 3 732 "C + steam strip X; 732 "C, 6 h 7 1 5 38 MgAlFeO, X 1 96 677 OC X 10 1 13b 677 "C 2 13b 677 "C
+
Table VIII. SO, Activity of Batch Cycle Test of Iron Mixed Solid Solution Spinels cycle SO, mixed spinel virgin steamed no. activity Mg0.MgA11,6Fe0,404 X 15 1 21" 3Mg0.MgA11.6FeO.404 X 11 1 14' MgO-MgAlFeO, X; 732 "C, 6 h 27 1 29 2 25" 16 X; 732 "C, 12 h 19 X; 732 "C, 6 h Mg0.MgAb.6Fei.404 1 25" MgO.MgAb.zFei.sO4 X; 732 6C, 6 h 18 1 23" 10 mol % Fe/MgO X 31 1 13' 2 1w 3 19b 4 16* 5 15b
" Sulfate reduction at 677 "C. lowed by steam stripping.
Table IX. Reduction of Solid Solution Spinels Containing High Percent Iron with H2 o r Propane reduction at 677 "C catalyst Hz propane SO, activity steamed Mg0.MgAb.6Fel.404 18-19 X 25 X 22 steamed Mg0.MgAb.zFe1,804 17-18 X 21 X 23 steamed Mg0.MgAlFe04 26-27 X 25 X 28 SO, pickup cycle at 732'C Reduction cycle at 677'C Steamed CelMgO*MgAI,.,Fe, O4 X=0.4
.r" 40h\
:'-
X=O.l
Steamed Ce/MgO-MgAI,O,
"Sulfate reduction at 677 "C followed by steam stripping. Sulfate reduction at 677 "C. v,
Fe/Al atomic ratio is 21. However, when the Fe/Al ratio is less than 1,steam stripping is not necessary because the sufficient amount of aluminum is present in the spinel structure to sustain the catalytic hydrolysis of FeS into H2S under the partial steam pressure present in the FCC cracking unit. The Catalyst Stability Test in an Automated Cycle Unit. An automated cycle test reactor was constructed to repeat a de-SO, cycle consisting of the SO, half cycle and the subsequent sulfate reduction half cycle for a prolonged time. The catalyst stability test was conducted in an automated continuous unattended cycle reactor to evaluate the catalyst stability of these materials. The steamed iron mixed solid solution spinels impregnated with cerium, Ce/Mg0.MgAlkFe,04 where x is 0.4,0.1,and 0.03, and the steamed counterpart without cerium, MgO-MgA11.9Fe0.104,were subjected to the continuous cyclic test, and the results are compared with the single solid solution spinel, Ce/Mg0.MgA1204, in Figure 9. The Ce/MgO. MgA11.6Fe0.404 exhibited the best steam stability among these materials including the prototype solid solution
Sulfate reduction at 677 "C fol-
(prototype)
Steamed MgO-MgAI, ,Fe, ,04
0
0
10
20
,
,
30
40
; 50
Number of cycles
Figure 9. Stability study of iron mixed solid solution spinels: continuous cycle runs with steamed Ce/MgO~MgAlz-=Fe1Ol.
catalyst, Ce/Mg0.MgA1204,prepared by Katalisticks, Inc. The results also brought out the distinctive advantage of a combination of cerium with iron over cerium alone. However, this advantage of incorporating cerium with iron was diminished as the iron content in the iron mixed spinels increased. Iron was impregnated by the incipient wetness technique on the Ce/MgA1204matrix to prepare the catalyst Fe(2%)/Ce/MgA120,. I t showed a low SO, activity of 10 when steamed (see Table 111). It shows that the way of incorporating iron into the spinel structure by coprecipitation is better than the incipient wetness impregnation method. Even after these samples were subjected to more than 45 de-SO, cycles in the automated continuous reactor, all of these iron mixed spinels with impregnated cerium maintained a good SO, activity of
1258 Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992
25-32, while the steamed iron solid solution counterpart without cerium showed a lower SO, activity of 11. The same prolonged cycle test also showed that the mixed solid solution spinel containing low iron showed a better physical property than that of the single solid solution spinel counterpart, Ce/Mg0.MgA120,. h particular, the attrition characteristic was quite compatible with that of the commercial FCC catalyst. In addition to the de-SO, activity, the iron mixed spinels as well as the single solid solution spinel with cerium exhibited a significant catalytic activity for the reduction of NO, in the presence of carbon monoxide (Yo0 et al., 1988). NO + CO f/,N2 + CO2 +
In short, these spinel materials are capable of achieving a simultaneous removal of SO, and NO, in the emission from the FCC regeneration unit.
Conclusions The role of iron in iron mixed spinels for the de-SO, chemistry was defined. Iron catalyzes the oxidation of SO2 to SO3 in the initial step for the SO, pickup half cycle and, in particular, promotes the sulfate reduction in the subsequent half cycle. The critical temperature required for the sulfate reduction was dramatically lowered by the substitution of iron spinels. However, it seems that the iron does not participate in the formation of s sulfate in the SO, pickup half cycle. The iron mixed solid solution spinels, MgO. MgAl2_,Fe,O4 ( x = 0.03-0.4),were found to be excellent de-SO, catalysts, and the same spinels impregnated with cerium exhibited an even better de-SO, activity and a better steam stability. In other words, a proper combination of iron and cerium catalyzes the oxidation of SO2 to SO3 in a synergistic way and results in a remarkable improvement in the catalyst stability. The method of incorporating iron into the mixed spinel structure appears to be an important factor in controlling the catalyst performance. The coprecipitation method is superior to the incipient wetness impregnation technique. The iron incorporation also improved physical properties such as attrition characteristics. The physical compatibility of the iron mixed solid solution spinels with the FCC equilibrium catalyst was demonstrated in the automated cycle reactor in the laboratory as well as in the pilot-plant unit. Acknowledgment This work was carried out a t Harvey Research Center, ARC0 Petroleum Product Co., Harvey, L.Currently this
technology is owned by UOP/Katalistiks Inc. We appreciate UOP,Des Plaines, IL, for granting permission to publish this work. Special thanks go to Amoco Chemical Company for providing us the opportunity to present this work.
Literature Cited Bensitel, M.; Waqif, M.; Saur, 0.; Lavalley, J. C. The structure of sulfate species on magnesium oxide. J. Phys. Chem. 1989, 93, 6581. Bhattacharyya, A. A.; Woltermann, G. M.; Yoo, J. S.; Karch, J. A.; Cormier, W. E. Catalytic SO, Abatement: The Role of Magnesium Aluminate Spinel in the Removal of SO, from FCC Flue Gas. Ind. Eng. Chem. Res. 1988,27, 1356. Cimino, A. Catalytic activity of transition metal ions in an oxide matrix. Chem. Ind. 1974,56 (l),27. Kwestroo, W. Spinel phase in the system Mg0-FezO3-Al2O3.J. Znorg. Nucl. Chem. 1959, 9, 65. Lercher, J. A.; Columbier, C.; Noller, H. Acid-base properties of alumina-magnesia oxides, part 4.-Infrared study of adsorption of carbon dioxide. J. Chem. SOC.,Faraday Trans. 1 1984,80,949. Narasimhan, C. S.; Swamy, C. S. Catalytic decomposition of isopropyl alcohol on MgAlz-zFe,Or. Curr. Sci. 1976,45, 795. Narasimhan, C. S.; Swamy, C. S. Studies on the solid state properties of the solid solution system MgAl,.Fe,O,. Stat. Sol. (a) - _ _ _ . Phys. . 1980, 59, 817. Narasimhan. C. S.: Swamv. C. S. Studies on the decomDosition of isopropyl 'alcohol on MgA12,Fe,0,. Appl. Catal. 1982, 2, 315. Tagawa, H. Thermal Decomposition Temperatures of metal sulfates. Thermochim. Acta 1984,80, 23. Waqif, M.; Saur, 0.; Lavalley, J. C.; Wang, Y.; Morrow, B. A. Evaluation of magnesium aluminate spinel as a sulfur dioxide transfer catalyst. Appl. Catal. 1991a, 71, 319. Waqif, M.; Saw, 0.; Lavalley, J. C.; Perathoner, S.; Centi, G. Nature and Mechanism of formation of sulfate species on copperlalumina sorbent-catalysts for SO2 removal. J. Phys. Chem. 1991b, 95, 4051. Yoo, J. S.; Radlowski, C. A.; Karch, J. A,; Bhattacharyya, A. A. U.S. Patent 4 790 982,1988. Yoo, J. S.; Bhattacharyya, A. A.; Radlowski, C. A. Magnesium Aluminate Spinel and Ita Solid Solutions For FCC SO, Abatement. Presented at the Material Research Society: Fall Meeting, Boston, MA, Nov 26-Dec 1, 1990a, paper 56.7. Yoo, J. S.; Karch, J. A.; Radlowski, C. A.; Bhattacharyya, A. A. Desox Process-SO, Emission Control from Fluid Catalytic Cracking (FCC) Regenerator. Presented at The Catalyst Society of Japan: First Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, Japan, July 1-5; 1990b, paper 0-16, p 111. Yoo, J. S.; Radlowski, C. A.; Karch, J. A.; Bhattacharyya, A. A. U.S. Patents 4 963 520, 4 957 718, 199Oc. Yoo, J. S.; Bhattacharyya, A. A.; Radlowski, C. A.; Karch, J. A. de-SO, Catalyst: An XRD Study of Magnesium Aluminate Spinel and Ita Solid Solutions. Ind. Eng. Chem. Res. 1991, 30, 1444. Received for review October 3, 1991 Revised manuscript received January 16, 1992 Accepted February 24, 1992
