Ind. E n g . C h e m . R e s . 1988,27,429-434
([Gnl) = S 0m E ( t ) [ G n I ~ ~dt( t )
(B.1)
The residence time distribution E ( t ) is such that E ( t ) dt is the fraction of outlet flow rate having a residence time between t and t dt. This allows the volume change caused by the chemical reaction to be properly accounted for. For ideal polycondensations, a more convenient form of the above relationship is obtained by introducing the cumulative distribution:
+
F ( t ) = 1 - S 0t E ( t f )dt'
(B.2)
Integration by parts and use the linking-group concentration u as the integration variable, owing to its finite range going from the feed value uf up to u, = min (r,l), leads to 2[Pl((u))(Gn) = 2[Pl&nf
+
I?[t(u)]
d/dt (2[P]Gnh du
2[PI((u))( G n ) = 2[P]Gnf + JUmJ'[t(u)](R,n +- Gn d ( 2 [ P ] ) / d t ) ~ du~ (B.3) Uf
with
Supplementary Material Available: Practical implementation of numerical inversion of discrete transforms with test computer program in FORTRAN 77 to carry out the procedure and table of round-off errors for the numerical inversion of Schulz-Flory CLD transform (20 pages). Ordering information is given on any current masthead page.
429
Literature Cited Abraham, W. H. Ind. Eng. Chem. Fundam. 1963,2,221-224. Abraham, W. H. Chem. Eng. Sci. 1966,21,327-336. Burkus, J.; Eckert, C. F. J. Am. Chem. SOC. 1957,80, 5948-5950. Chen, P. Y.; Spencer, J. L. Paper 31A of the 63rd National AIChE Meeting, St. Louis, Feb 1968. Cooley, J. W.; Tukey, J. W. Math. Comp. 1965,19,297-301. Costa, M. R. N.; Villermaux, J. In Polymer Reaction Engineering; Reichert, K. H.; Geiseler, W., Eds.; Huthig & Wepfi Basel, 1986; pp 205-215. Flory, P. J. Principles of Polymer Chemistry; Cornel1 University Press: Ithaca, NY, 1953. Gandhi, K. S.; Babu, S. V. AIChE J. 1979,25,266-272. Gerrens, H. Proc. ISCRE, 4th 1976,585-614. Gerrens, H. Ger. Chem. Eng. 1981,4,8-13. Gordon, M.; Temple, W. B. Makromol. Chem. 1972,152,277-289. Gupta, S. K.; Kumar, A. Chem. Eng. Commun. 1983,20,1-52. Jacobson, H.;Stockmayer, W. H. J. Chem. Phys. 1950, 18, 1600-1612. Kilkson, H. Ind. Eng. Chem. Fundam. 1964,3,281-293. Kilkson, H. Ind. Eng. Chem. Fundam. 1968,7,354-363. Mills, P. L. Chem. Eng. Sci. 1986a,41, 1045-1052. Mills, P. L. Ind. Eng. Chem. Process Des. Dev. 1986b,25,575-584. Mills, P. L. Comp. Chem. Eng. 1986c,10,399-420. Ravindranath, K.; Gandhi, K. S. Chem. Eng. Sci. 1980,35,955-962. Ray, W. H.; Laurence, R. L. In Chemical Reactor Theory. A Review; Lapidus, L., Amundson, N. R., Eds.; Prentice-Hall: Englewood Cliffs, NJ, 1977; pp 532-582. Singleton, R. C. In Collected Algorithms from CACM Wiley: New York, 1968; pp 345 P6-0, 345 P6-1. Stepto, R. F. T.; Waywell, D. R. Makromol. Chem. 1972, 152, 263-275. Stockmayer, W. H. J. Chem. Phys. 1943,11,45-55. Tadmor, Z.;Biesenberger, J. A. Ind. Eng. Chem. Fundam. 1966,5, 336-343. Truesdell, C. Ann. Math. 1945,46, 144-157. Valles, E. M.; Macosko, C. W. Macromolecules 1979,12,521-526. Villermaux, J. ACS Symp. Ser. 1983,226, 135-186. Weisskopf, K.;Meyerhoff, G. Eur. Polym. J. 1985,21,859-863.
Received for review March 9, 1987 Revised manuscript received October 28, 1987 Accepted November 12, 1987
Reduction of Nickel-Alumina Catalysts Ienwhei Chen* and Dar-Woei Shiue Department o f Chemical Engineering, Tatung Institute of Technology, Taipei, Taiwan, R.O.C.
A study of the chemistry involved in the reduction of NiO supported on yAl,O, has been investigated. T h e surface area, the extent of reduction to nickel metal, dispersion of nickel, and particle size of nickel as a function of reduction temperature and time are reported. Nickel aluminate is detected by X-ray diffraction. An empirical reaction rate equation is obtained with a differential method of analysis. The activation energy is found to be 26.4 kJ/mol, while the reaction order with respect to nickel oxide is 2.0. Reaction mechanisms have been proposed and discussed. The experimental results of reduction are well described by the shrinking unreacted-core model with chemical reaction control. Nickel catalysts find widespread industrial application in hydrogenation, hydrotreating, and steam-reforming reactions. The chemical and physical structure of supported nickel catalysts has been the subject of many investigations. Yet there is little quantitative information in the literature dealing with the reduction kinetics, nickel surface areas, dispersion of the nickel, particle size of the nickel, and the extent of reduction on alumina-supported nickel catalysts. In the recent literature, metal surface areas have been measured by chemisorption of hydrogen, oxygen, and carbon monoxide over a range of temperatures and pres0888-5885/88/2627-0429$01.50/0
sures. The adsorption stoichiometries remained to be confirmed under well-defined conditions (Farrauto, 1974). Hydrogen adsorption was found to be well defined at rmm temperature over the pressure range 100-400 Torr (1Torr = 133.3 Nm-2), and the H/Ni,,, ratio was found to be 1.0. Carbon monoxide adsorption, however, was determined to be considerably more complex, with the stoichiometry depending greatly upon temperature and pressure (Pannell et al., 1977). In laboratory or commercial preparation of catalysts, metal salts are impregnated or coprecipitated on oxide supports. The salts are converted to oxide by calcination 0 1988 American Chemical Society
430 Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988
in air at 673-873 K. This is followed by reduction in flowing hydrogen at 673-973 K to give well-dispersedmetal catalysts. But previous studies (Martin et al., 1973) indicate that nickel supported on alumina is not completely reduced under typical reducing conditions to the metallic state because of a strong oxide support reaction. This reaction depends on nickel loading and calcination temperature and has been attributed to the formation of nickel aluminate spinel (Shalvoy et al., 1980) or to nickel ions in tetrahedral and octahedral sites of y-alumina (Wu and Hercules, 1979) or to a “modification” of the electronic properties of nickel oxide due to reaction with alumina (Vedrine et al., 1978). The strong oxide support reaction has been found to have an important effect on oxide reducibility (Ross et al., 1978) and on catalytic activity (Cimino and Schiavello, 1971). The surface area of the catalyst, the extent of reduction to the metal, dispersion of metal on the support, and particle size of the nickel catalyst have important effects on catalytic activity, too. The purpose of this study is to investigate nickel surface area, the extent of reduction to nickel metal, dispersion of nickel, particle size of nickel at various reduction temperatures and times, the kinetics of nickel oxide reduction at 673-973 K, and the effect of strong oxide support reaction in the alumina-supported nickel catalysts. Experimental Section Materials. Analytically pure Ni(N0&6H20 (Hayashi) was used in the TGA experiments and in preparation of supported nickel catalysts. Solutions of this salt were impregnated by y-alumina (Merck). After impregnation, the samples were dried in air at 333 K followed by calcination at 873 K for 2.5 h and subsequent reduction in hydrogen at temperatures ranging from 673 to 973 K for 0.5-9.0 h. The compositions of the catalysts were 15 wt % Ni/y-A1203. Apparatus and Procedure. Adsorption measurements were performed by using a conventional Pyrex glass volumetric adsorption apparatus with digital pressure gauge (Unitec). The catalyst sample was placed in a flowthrough cell to permit reduction in flowing hydrogen prior to the adsorption measurements. Following reduction of catalyst samples in flowing hydrogen (80 cm3/(g of catalysbmin) at temperatures ranging from 673 to 973 K and subsequent evacuation, nickel surface areas were measured by using hydrogen chemisorption at room temperature (Farrauto, 1974). The uptake due to hydrogen chemisorption was found by extrapolating the straight-line portion of the isothermal above the saturation pressure to zero pressure. Catalyst surface areas were calculated assuming H/Ni(s) = 1and a surface area of 6.77 A2 per nickel atom (10 A = 1nm) based on an equal distribution of the three lowest index planes of nickel (fcc) (Bartholomew, 1976; Bartholomew and Pannell, 1980). The extent of reduction to nickel metal was determined after hydrogen chemisorption by evacuating the sample at 723 K and pressures of 10-4-10-5 Torr followed by measuring the uptake of pure oxygen at 673-703 K (Bartholomew, 1976). It was assumed that in the reduced catalyst all unreacted nickel was in the form of NiO and that at 673-703 K in the presence of oxygen all nickel in the metallic form would be oxidized to NiO. Comparison of the actual oxygen uptake with the amount of oxygen needed if all of the nickel was assumed to be initially in the metallic state constituted the basis for calculating the percentage reduction to the metal. The amount of nickel contained in the nickel oxide form was determined by dissolving the catalyst in 5 N HC1. It had been shown that nickel contained as NiA1204is not
4 56
656
856
1056
Temperature ( K )
Figure 1. Thermal gravimetric analysis of Ni(N03)2-6H20.
extracted by this method (Gavalas et al., 1984). The amount of nickel in solution was determined by atomic absorption spectrophotometry. Hence, the difference in nickel content before and after high-temperature calcination provided the amount of nickel in NiAI2O4. The NiA1204 could be detected from X-ray diffraction. X-ray diffraction was performed with Fe K a radiation. In the calculation of metal dispersion (or the fraction of metal atoms exposed) (Bartholomew, 1980), the metal loading was multiplied by the fraction of nickel reduced to the metallic state based on the assumption that unreacted nickel is present in a separate dispersed phase in intimate contact with the support. Thus, the equation used to calculate dispersion was % D = 1.17G/ Wf (1) where G = H2 uptake in moles per gram of catalyst, W = the weight percentage of nickel, and f = the fraction of nickel reduced to the metal. Average crystallite diameters were calculated from % D assuming spherical metal crystallites with the same size d. Thus, d = 971/(% D) (2) Results and Discussion Chemisorption of Hydrogen. The results of thermal gravimetric analysis (TGA) of nickel nitrate are shown in Figure 1. Nickel nitrate decomposes to nickel oxide at 673 K via analysis of weight loss. This result is also identified with X-ray diffraction and discussed in a later section on the strong oxide support reaction. Catalyst is calcined in air at 873 K for 2.5 h and then reduced in flowing hydrogen at temperatures ranging from 673 to 973 K for 0.5-9.0 h. The effects of various reduction temperatures and reduction times are demonstrated by hydrogen chemisorption at 298 K in Figure 2. It is observed that hydrogen chemisorption increases with increasing reduction time at constant reduction temperature and decreases with increasing reduction temperature at the same reduction time. A possible explanation is that sintering occurs at the high reduction temperatures. Hydrogen chemisorption is greater at the initial period of reduction but increases little at the final period of reduction. It appears that nickel oxide can be easily reduced at the initial period of reduction, and strong metal support interaction occurs as most of the nickel oxide has been reduced. Therefore, hydrogen chemisorption increases little at the final period of reduction. Data presented in Figure 3 illustrate the effects of various reduction temperatures and various reduction times on the extent of reduction to nickel metal. It is
Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988 431 \
-
="
2or I
2
&
15
I
6
8
Reductm Time (hounl
3.
Figure 2. Effect of reduction temperature and reduction time on H2 chemisorption of 15 wt % Ni/y-A1203: (0) 673 K, (0) 773 K, (0) 873 K, (A)973 K. 2
4
Reduction
6
8
Time (hours)
Figure 4. Effect of reduction temperature and reduction time on nickel dispersion of 15 wt % Ni/y-A1203: (0) 673 K, (0) 773 K, (0) 873 K, (A) 973 K.
2
4
6
8
Reduction Time (hours)
Figure 3. Effect of reduction temperature and reduction time on percentage of reduction of 15 wt % Ni/y-AlzOB: (0) 673 K, (0) 773 K, (0)873 K, (A) 973 K.
observed that reduction to nickel metal increases much more with increasing reduction time at the initial period of reduction but increases little with increasing reduction time at the final period of reduction. Hydrogen chemisorption is not increased when the extent of reduction to nickel metal is increased at 673-973 K at the same reduction time. It has been reported that nickel sintering is more serious at high reduction temperatures (Bartholomew, 1976). Furthermore, the metal surface area is largest at 673 K when the reduction temperatures range from 673 to 973 K. The dispersion and particle size of nickel measured as a function of reduction temperature and time are shown in Figures 4 and 5. Dispersion decreases with increasing reduction temperature at the same reduction time and decreases with increasing reduction time at the same reduction temperature. The particle size of nickel is opposite to this result. The reason is that the degree of sintering increases with increasing reduction temperatures and times. Dispersion of nickel is more changeable at low reduction temperature and is little changeable at high reduction temperature. The possible reason is that the sintering is more significant at high reduction temperature. Strong Oxide Support Reaction. From Figure 3, the extent of reduction to nickel metal increases with increasing reduction temperature and reduction time, but it cannot be reached completely. The reason for the low extent of reduction to nickel metal may be found to be the metal-support interaction using X-ray diffraction (XRD).
2
4
6
8
Reduction Time (hours)
Figure 5. Effect of reduction temperature and reduction time on particle size of nickel crystallite of 15 wt % Ni/y-A1203: (0) 673 K, (0) 773 K, (0) 873 K, (A) 973 K.
According to the "Fink Index to the Powder Diffraction File", 20 of nickel oxides (lll),(200), and (220) are 47.4', 55.3', and 82.0°, 20 of y-aluminas (311), (400), and (440) are 47.8', 58.7', and 88.0°,and 20 of nickel aluminates (311), (400), and (440) are 47.1', 57.5', and 85.8'. XRD spectra of NiO, y-A1203,and NiO/r-A1203 as well as NiO/y-A1203following an HC1 wash, shown in Figure 6, indicate that nickel aluminate exists in the calcined NiO/A1203 catalyst. It has been shown (Bartholomew, 1976) that nickel aluminate has an inhibiting effect on NiO reduction. The NiO/Al,O, catalyst was dissolved in 5 N HCl and the nickel concentration was determined by atomic absorption spectrophotometry. Results show that 64 wt % nickel is contained in NiO and 36 wt % exists as nickel aluminate, indicating that nickel oxide cannot be
432 Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988 @
A
d
-52p\ 0
53
73
01
33
I
0
- 5.61 X
~i
2
1
-~
5.:
73
?^
~~
-~
@I
-601
----
,
i -
7r
0-
30
I
5:
Deg :e
5'
30
26
Figure 6. X-ray diffraction spectra of (A) unsupported NiO, (B) y-A1203,(C) calcined 15 wt 70Ni/y-Alz03, and (D)calcined 15 wt % Ni/y-Alz03followed by 5 N HC1 wash.
- 6.4
\,
IL
0
13
1.1
10
-3
/T
( K
1.5 -1
1
Figure 8. Plot of In k vs 1/T. mI
4.04 X at 973 K. For each at 873 K, and 5.30 X reaction, the reaction rate constant is defined by the Arrhenius' law,
-
0
u
L.
k = kOe-E/RT
12-
i
Taking logarithms of both sides, In k = In ko - E / R * ( I / T )
1 NiO 1
2
Figure 7. Plot of -rNio vs [Ni0I2: (0) 673 K, (0) 773 K, (0) 873 K, (A)973 K.
completely reduced in NiO/A1203. Kinetics of Reduction. The differential method of analysis (Levenspiel, 1983) for the conversion of reduction to nickel metal versus reduction time at different reduction temperatures is applied to determine the rate equation for the reduction of nickel oxide. The conversion data for the reduction of nickel oxide are found to fit a second-degree polynomial equation via regression analysis. The rate of reduction can be determined from the slope of these conversion curves. For conditions of excess hydrogen, as used in these experiments, the empirical rate equation in a power-law form for conversion of nickel oxide can be expressed as (3) The values of k and n obtained at each reaction condition and n = 2.09 at 673 K, k = 1.95 X are k = 1.35 X and n = 1.87 at 773 K, k = 3.78 X and n = 2.04 at 873 K, and K = 5.06 X and n = 2.11 at 973 K. It was observed that all these values of n are located around the value of 2.0. Thus,the reaction order with respect to nickel oxide is proposed to be 2.0. Straight-line plots for the rate of NiO reduction vs [Ni0I2 at different temperatures, shown in Figure 7, indicate the empirical rate equation to be -rNiO
-rNjO
= k[Ni0ln
= k[Ni0I2.O
(4)
The slopes of these straight lines give the following at 673 K, 2.00 X lom3at 773 K, values for k = 1.28 X
(5)
(6)
The plot of In k vs 1/T shown in Figure 8, resulting in a straight line including all four points, is considered to be reasonably accepted. The least-squares method was used to determined the activation energy ( E )and In ko. The value for E is 26.4 kJ/mol, and KO is 5.8 X loF3. Therefore, the empirical rate equation for the reduction of nickel oxide can be expressed as -rNiO = 5.8 x 10-3e-26.4(kJ/mol)IRT[Ni0]2.0 (7)
Reaction Mechanisms. The reaction mechanisms of reduction of nickel oxide can be proposed as follows: H2 + 2s
& 2HS
+ HS 2NiOH + S 2NiOH A NiO + H 2 0 + Ni NiO
(8) (9) (10)
Nickel oxide reacts with adsorbed hydrogen to form nickel hydroxide after the hydrogen molecule is dissociated and adsorbed on the active sites, S. Nickel hydroxide reacts with nickel hydroxide in the rate-determining step. A Langmuir treatment can be applied in the above reaction mechanisms, and this method leads to the following rate equation: -~Nio
= K3KB2KA[H2][Ni0I2
(11)
Assuming excess H2, the reaction rate can be reduced to - r N i o = k [NiOJ (12) Second order with respect to nickel oxide is obtained from the above reaction mechanisms. This is matchable with the value of second order shown above in the empirical rate equation of the reduction of nickel oxide. In other words, these reaction mechanisms reveal that nickel hydroxide reacting with nickel hydroxide controls the whole reduction reaction.
Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988 433
1
2
Figure 9. Reaction model of reduction: (A) the interior structure of the catalyst; (B)shrinking unreacted-core model with chemical reaction control.
Reaction Model of Reduction. The interior structure of the catalyst is shown in the left half of Figure 9. Nickel oxide is adsorbed on alumina. Reaction occurs first at the outer skin of the particle (NiO). The zone of the reaction then moves into the solid may leave behind completely converted material and inert solid (Ni) which is referred as “ash”. Thus, at any time there exists an unreacted core of material which shrinks in size during reaction. Fluid A (H2)is considered to react with a solid (NiO) to produce a fluid product (H20)plus a porous solid product layer or ash layer (Ni). The unreacted core model (Wen, 1968; Levenspiel, 1983) is that unreacted solid which is impervious to fluid A because it is densely packed. On the other hand, the solid product layer is quite porous so that reactant fluid A can diffuse in and product fluid can diffuse out. A spherical particle having an initial radius R is considered to be reacted by fluid A. At fiist, the reaction takes place a t the outside surface of the particle, but as the reaction proceeds, the surface of the reaction will move into the interior of the solid to leave behind inert ashes. During the process, the reaction surface moves inward to form an unreacted core which shrinks with time, but the external radius of the particle still remains the same, R, assuming no deformation of the ash layer that has been formed. In order for fluid A to reach the surface of the unreacted core, it must move through various layers of resistances in series, the fluid film around the surface of particle, the porous ash layer, and then the reaction surface at the core. It is assumed to be neglected resistances of fluid film and the porous ash layer that are shown in the right half of Figure 9. R and r, are the radius of the particle and radius of the unreacted core, respectively. Assuming excess H2, the reaction rate equation, -rNiO = k[Ni0I2, is valid from previous study. The overall density of the nickel oxide particle is assumed to be constant during the period of reduction. If we let X = 1- r,3/R3,the extent of reduction to nickel metal can be expressed in terms of reduction time as X = 1 - l / ( k t + 1) (13)
Comparing the data obtained from this unreacted core model with experimental results as shown in Figure 10 at different reduction temperatures it is observed that the experimental data are in good agreement with those obtained from this model. In other words, the shrinking unreacted-core model with chemical reaction control can be applied to describe the reduction process of nickel oxide in y-alumina-supported nickel catalyst. Conclusions Data for the reduction reaction of nickel oxide have been collected at atmospheric pressure and four levels of re-
4
6
8
Reduction Time (hours 1
Radial position
Figure 10. Comparison of conversion data obtained from experiments with those, shown as solid line, obtained from the shrinking unreacted-core model at different reduction temperatures: (0) 673 K, (0)773 K, (0)873 K, (A)973 K.
ducing temperatures from 673 to 973 K. From these results, it is possible to draw the following conclusions. Hydrogen chemisorption increases with increasing reduction time at the same reduction temperature; however, it decreases with increasing reduction temperature at the same reduction time. The extent of reduction to nickel metal increases with increasing reduction time at the same reduction temperature; however, it increases with increasing reduction temperature at the same reduction time. The dispersion of nickel decreases with increasing reduction time at the same reduction temperature, and it decreases with increasing reduction temperature at the same reduction time. The particle size of nickel increases with increasing reduction time at the same reduction temperature, and it increases with increasing reduction temperature at the same reduction time. Strong oxide support reaction can be seen with X-ray diffraction (XRD). By use of atomic absorption spectrophotometry, 64 wt % nickel is found to be contained in nickel oxide and 36 wt % nickel is contained in nickel aluminate for calcined 15 wt % Ni/A120,. Conversion data were found to be correlated with a quadratic equation, and the rate of reaction was obtained from the slope of these curves. A differential method of analysis was used to determine the empirical rate equation. The activation energy was found to be 26.4 kJ/mol, and the reaction order with respect to nickel oxide is 2.0. The reaction mechanisms of reduction were proposed based on nickel hydroxide reacting with nickel hydroxide in the rate-determining step. The experimental data are fitted well with those obtained from the shrinking unreacted-core model with chemical reaction control for the reduction of nickel oxide in the alumina-supported nickel catalyst. Acknowledgment We thank the Tatung Company and the Chinese National Science Council for financial aid. Nomenclature D = dispersion of nickel metal d = average diameter of nickel crystallite, A E = activation energy, kJ/mol ’ f = fraction of nickel reduced to the metal G = H2uptake, mol/g of catalyst KA, KB = equilibrium constants ko = preexponential factor of the Arrhenius equation k,, k = reaction constant, l/min n = reaction order R = ideal gas law constant, kJ/(mol K) r = rate of reaction, w t %/min T = temperature, K t = time, min W = weight percentage of nickel X = conversion
Ind. Eng. Chem. Res. 1988,27,434-439
434
Registry No. NiO, 1313-99-1; H2, 1333-74-0.
Pannell, R. B.; Chung, K. S.; Bartholomew, C. H. J. Catal. 1977,46, 340. Ross, J. R. H.; Steel, M. C. F.; Zeini-Isfahani, A. J . Catal. 1978,52, 280.
Literature Cited Bartholomew, C. H. J . Catal. 1976,45, 41. Bartholomew, C. H.; Pannell, R. B. J . Catal. 1980, 65, 390. Cimino, A.; Shiavello, M. J. Catal. 1971,20, 202. Farrauto, R. J. AZChE Symp. Ser. 1974, 70, 9. Gavalas, G. R.; Phichitkul, C.; Voecks, G. E. J . Catal. 1984,88, 54. Levenspiel, 0. Chemical Reaction Engineering; Wiley: New York, 1983; Chapter 12, pp 367-371. Martin, G . A.; Ceaphalan, N.; Moutgolfier, P. d. J. Chim. Phys. 1973, 70. 1422.
Shalvoy, R. B.; Reucroft, P. J.; Davis, B. H. J . Vac. Sci. Technol. 1980, 17, 209. Vedrine, J. C.; Hollinger, G.; Minh, 0. T. J . Phys. Chem. 1978, 82, 1515.
Wen, C. Y. Ind. Eng. Chem. 1968, 60, 34. Wu, M.; Hercules, D. M. J . Phys. Chem. 1979, 783, 2003.
Received for review March 20, 1987 Revised manuscript received October 26, 1987 Accepted November 14, 1987
Sorption of SO2 on Metal Oxides in a Fluidized Bed Bekir Zuhtu Uysal,*+Inci Aksahin,*and Hayrettin Yucelf Chemical Engineering Department, Jordan University of Science and Technology, Zrbid, Jordan, and Chemical Engineering Department, Middle East Technical University, Ankara, Turkey
A preliminary experimental study is described in which a low concentration of SO2 (0.2% v/v) is removed from air at temperatures between 358 and 640 K by reaction with bauxite and red mud, which consist of several metal oxides in a lab-size fluidized bed reactor of 5-cm diameter. CuO on activated alumina, a well-known sorbent for SOz,was also tested in the same system for comparison. A model based on the two-phase theory of fluidization was used to simulate the sorption process. It was shown that a kinetic rate expression for sorption of SO2 on a single metal oxide to yield a metal sulfate can also be used for mixtures of metal oxides by defining an overall apparent rate coefficient and an overall consumption coefficient for active sites. The work indicated that bauxite and red mud could also be considered as potential sorbents like CuO for SO2sorption in a typical range of stack gas temperatures. I t is generally accepted that emission of sulfur oxides from combustion of fossil fuels causes a serious air pollution problem. The control of sulfur dioxide emissions becomes more important due to the trend of using increasing amounts of high sulfur coal to meet energy requirements. A great number of processes have been proposed for removal of sulfur oxides from flue gases, and they are summarized in several references (Slack, 1971; Kohl and Riesenfield, 1979). All of the proposed processes for flue gas desulfurization can be classified into two basic categories: throwaway processes and recovery processes. In each category, the processes can be further classified into wet or dry processes. Wet throwaway processes such as alkali or limelimestone scrubbing have been preferentially developed on a commercial scale. However, dry recovery processes are thought to be more advantageous over wet systems since they may permit flue gas treatment a t elevated temperatures, avoiding flue gas reheating which is usually required for a wet process to maintain plume buoyancy. On the other hand, the dry recovery processes might have the usual problems of large-scale handling of regenerable solids, mainly high investment costs due to the complexities of the regeneration system and excessive losses of sorbent due to attrition. Recently, a significant amount of work has been done to develop dry recovery processes based on sorption of sulfur oxides on metal oxides (Bienstock et al., 1961; Thomas et al., 1969; Lowell et al., 1971). In a study by Tracor Corporation (Thomas et al., 1969; Lowell et al., 1971) the oxides of 48 metals were screened to determine which were best suited for the removal of sulfur oxides from flue gases by chemical reactions. The screening was t
Jordan University of Science and Technology. Middle East Technical University. 0888-5885/88/2627-0434$01.50/0
based on the thermodynamic requirement for efficient SO2 removal and product regeneration. Oxides of Ti, Zr, Hf, V, Cr, Fe, Co, Ni, Cu, Zn, Al, Sn, Bi, Ce, Th, and U were selected as a result of this screening process. Further screening of metal oxides by consideration of their reaction rates with SO2in a flue gas atmosphere showed that oxides of Cu, Cr, Fe, Ni, Co, and Ce would have economically feasible reaction rates with SO2. After further evaluation of factors such as sorption reaction stoichiometry, formation of product layers which affect the reaction rate, and SO, partial pressure over the sorption product, oxides of copper and iron were selected as the most promising. Of the potential metal oxide sorbents for SO2 removal, CuO received the most attention. Laboratory-scale work on copper oxide impregnated into porous alumina has been conducted by the US. Bureau of Mines (McCrea et al., 1970). The results showed that sulfate formation goes essentially to completion at temperatures of about 450 "C. The regeneration by the reducing gases such as hydrogen and methane could be accomplished at much lower temperatures. Work on a copper oxide process has also been conducted by Shell in the Netherlands (Dautzenberg and Nader, 1971). This process, which has been named the shell flue gas desulfurization (SFGD) process, also uses CuO on an alumina support. Two fixed bed units are used in the process. One is used for gas purification and the other undergoes regeneration by use of hydrogen, carbon monoxide, or methane. Both steps are accomplished at about 400 "C. The main object of this study was to test the possibility of using bauxite and red mud for SO2 sorption. Bauxite mineral and red mud-a byproduct of aluminum plants-contain oxides of metals such as iron, titanium, and aluminum which can be considered to be potential sorbents for SOz. Copper impregnated on activated alu1988 American Chemical Society
