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ed-iron catalysts. Int. Chem. Eng. 1984,24 (4),710-717. Krebs, H. J.; Bonzel, H. P.; Schwarting, W. Microreactor and electron spectroscopy studies of Fischer-Tropsch synthesis of magnetite. J. Catal. 1981,72, 199-209. Kuivila, C. S.; Stair, P. C.; Butt, J. B. Compositional aspects of Iron Fischer-Tropsch catalysts: an XPS/Reaction study. J. Catal. 1989,118,175-191. Laine, J.; Brito, J.; Gallardo, J.; Severino, F. The role of nickel in the initial transformations of hydrodesulfurization catalysts. J. Catal. 1985,91,64-68. Leary, K. J.; Michaelis, J. N.; Stacy, A. M. Carbon and oxygen atom mobility during activation of Mo,C catalysts. J. Catal. 1986,101, 301-313. Lin, C.; Park, S. W.; Hatcher, W. J. Zeolite catalyst deactivation by coking. Ind. Eng. Chem. Process Des. Dev. 1983,22, 609-614. McDaniel, M. P.; Johnson, M. M. A comparison of Cr/SiOz and Cr/AlP04 polymerization catalysts. J. Catal. 1986,101,446-457. Montes, M.; Penneman de Bosscheyde, Ch.; Hodnett, B. K.; Delaunay, F.; Grange, P.; Delmon, B. Influence of metal-support interactions on the dispersion, distribution, reductibility and catalytic activity of Ni/SiOz catalysts. Appl. Catal. 1984,12,309-330. Nix, R . M.; Judd, R. W.; Lambert, R. M.; Jennings, J. R.; Owen, G. High-activity methanol synthesis catalysts derived from rareearth/Copper precursors: Genesis and deactivation of the catalytic system. J. Catal. 1989,118,175-191. Noelke, C. J.; Rase, H. F. Improved hydrodechlorination catalysis: chloroform over Platinum-Alumina with special treatments. Ind. Eng. Chem. Prod. Res. Deu. 1979,18,325-328.
Olazar, M.; Aguayo, A. T.; Arandes, J. M.; Azkoiti, M.J.; Bilbao, J. Mecanismos de desactivacidn de un-catalizador de d i c e alumina en la polimerizacidn de alcohol bencilico gas. h o c . 11th Iberoam. Symp. Catal., Guanajuato 1988,I , 897. Olazar, M.; Aguayo, A. T.; Arandes, J. M.; Bilbao, J. Polymerization of gaseous benzyl alcohol. 3. Deactivation mechanisms of a Si0,/A1,03 catalyst. Ind. Eng. Chem. Res. 1989,28, 1752. Perti, D.; Kabel, R. L. Kinetics of CO oxidation over Co3O4/~-AlZO3 AIChE J . 1985,31,1420-1426. Pijolat, M.; Perrichon, V.; Bussiere, P. Study of the carbonization of an iron catalyst during the Fischer-Tropsch synthesis: Influence on its catalytic activity. J. Catal. 1987,107,82-91. Prada Silvy, R.; Deprez, P.; Berge, P. C.; Martin, M. A.; Grange, P.; Delmon, B. Activacidn de catalizadores de hidrodesulfuracidn: Influencia del coque en la etapa-inicial de redqccidn-sulfuracidn sobre las propiedades fisico-quimicas y cataliticas. Proc. I1 th Iberoam. Symp. Catal., Guanajuato 1988,2,1051. SzBpe, S.; Levenspiel, 0. Catalyst deactivation. Proceedings of the 4th European Symposium on Chemical Reaction Engineering. Brussels, September 1968, Pergamon Press: Oxford, 1971. Tau, L. M.; Bennet, C. 0. The oxidation effect on the CO/Hp reaction over Titania-supported Fe catalysts. J. Catal. 1985, 96, 408-419. Received for review April 6 , 1990 Revised manuscript received July 30, 1990 Accepted August 10,1990
Camet Oxidation Catalyst for Cogeneration Applications Franklin J. Gulian, Jeffery S. Rieck, and Carmo J. Pereira* W. R. Grace & Company, Research Division, 7379 Route 32, Columbia, Maryland 21044
Environmental pollution control regulations have required the use of oxidation catalysts to reduce emissions of carbon monoxide and unburned hydrocarbons from cogeneration plants. Oxidation catalyst performance requirements include low pressure drop and high conversion efficiency. The conversion limit is controlled by external mass-transfer limitations determined by the properties of the monolith substrate. Conventional monoliths contain identical channels that pass straight through the monolith and are parallel to the flow direction. This paper discusses the conversion and pressure drop performance of a new metal monolith substrate based on a folded metal foil that is corrugated in a herringbone pattern. The external mass-transfer pressure drop performance of this type of metal monolith catalyst is compared with conventional ceramic substrate based catalysts.
Introduction During the past few years, standards that limit the emissions of carbon monoxide, hydrocarbons, and nitrogen oxides from stationary sources have been established in the US.and in several other nations. Facilities governed by these standards include those operated by independent power producers ranging in size from 1 MW to hundreds of megawatts. Cogeneration is often the preferred power generation option as it offers a high rate of return on investment. These plants typically consist of a natural gas or oil-fired turbine for generating electricity and a Heat Recovery Steam Generation (HRSG) system. The steam generated in the HRSG can be used as process steam or to drive a steam turbine. Cogeneration plants that are out of compliance on carbon monoxide emissions usually require an oxidation catalyst that removes at least 90% of the carbon monoxide. A low pressure drop through the catalyst (4in. of water or less) is a necessity since every 4 in. of water backpressure causes a 0.4-1.5 per cent loss of turbine power output (Jung and Becker, 1987). Oxidation catalyst systems are therefore placed as “curtain walls” just before the HRSG system.
* Author to whom correspondence is t o be addressed.
An example of a curtain wall reactor is shown in Figure 1. Typical reactor dimensions in such a case are 40 f t X 35 ft X 3.5 in. The catalytic reactor contains a number of 2 ft X 2 f t X 3.5 in. catalyst modules. Carbon monoxide and hydrocarbon emissions are most easily reduced by passing the turbine effluent over a noble metal containing oxidation catalyst. Noble metal oxidation catalyst technology was originally developed nearly 20 years ago for automotive exhaust emission control. Jung and Becker (1987)and Cordonna et al. (1989)have discussed the catalytic chemistry required for the oxidation of carbon monoxide and hydrocarbons in cogeneration effluents. The oxidation conversion limit is controlled by external mass-transfer limitations that are determined by the properties of the monolithic substrate. Thus, there is considerable incentive for developing substrates having improved external mass-transfer characteristics. Ceramic or metal monolith-supported noble metal catalyst systems can be designed to provide the high conversion efficiency and low pressure drop required in cogeneration applications. Conventional monoliths contain identical channels that pass straight through the monolith and are parallel to the flow direction. The mass-transfer and pressure drop characteristics of such monoliths have been quantified by Hegedus (1973).
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Figure 1. Commercial oxidation reactor for cogeneration (courtesy of W. R. Grace & Co.-Conn.).
This paper discusses the performance of a novel metal monolith substrate, called the Camet metal monolith, which consists of a folded stainless steel foil that is corrugated in a herringbone pattern and coated with a washcoat containing noble metals. The properties of these metal monoliths and their commercial use have been discussed by Pereira et al. (1988). The use of herringbone metal monoliths having a wound rather than folded corrugated foil for the reduction of nitrogen oxides from automobile exhausts has been discussed by Mondt (1976). Correlations for the external mass-transfer and pressure drop characteristics of Camet monolith based catalysts have been developed. The momentum-mass-transfer analogy was found not to apply for such catalysts. The external mass-transfer-limited performance and pressure drop characteristics of these metal monoliths are compared to that of conventional ceramic substrates.
Experimental Section The metal monolith catalysts were obtained from Camet Co., a subsidiary of the Davison Chemical Company, a division of W. R. Grace & Co.-Conn. Each catalyst was 3 in. 0.d. and 3.5 in. long. The ceramic substrates used in this study were square-channeled cordierite monoliths obtained from Corning Glass Works. The washcoat thickness on each monolith was approximately 0.001 in. and consisted of 25 wt % bulk ceria and 75 wt % stabilized alumina (containing 4 wt % La203on an alumina basis). The noble metal loading was 30 g of Pt per cubic foot of monolith. Catalysts were calcined a t 600 "C for 1h prior to testing. Catalyst properties are shown in Table I. The 3-in.-i.d. reactor was machined so that the catalysts would fit snugly inside. Six-inch-long metal sheaths with an inside diameter of 2.5 in. were placed against the cat-
Table I. Properties of Monolith Oxidation Catalysts4 Camet metal ceramic monolith monolithb A B C D cell density, channels/in.2 132c 18Y 100 200 geometric surface area, l/in. 43.4 50.6 31.6 43.8 void fraction 0.859 0.836 0.624 0.600 wall thickness, in. 0.0025 0.0025 0.017 0.012 0.080 0.055 channel size, in. corrugation height, in. 0.058 0.050 corrugation length, in. 0.130 0.108 With a Monolith substrate coated with a 0.001-in. washcoat. square channels. Calculated as 1/ [(corrugation length)(corrugation height)].
alysts both upstream and downstream to minimize bypassing and to block out edge distortions around the perimeter of the ceramic and metal monoliths. The inlet gas contained 100 ppmv carbon monoxide in air. The gases were heated to 1000 O F and passed over the catalyst. A carbon monoxide light-off curve for catalyst A in Table I is shown in Figure 2. Carbon monoxide was found to light off a t temperatures below 400 O F . Thus, under the experimental conditions of this study, each catalyst operates a t the mass-transfer limit. Mass-transfer-limited carbon monoxide conversions and pressure drops were measured over a space velocity range of 15000-170000 h-' (NTP). T o obtain higher space velocities (up to 1300000 h-l), washers having inside diameters of 2, 1.5, or 1 in. were placed in front of and behind the monolith to force the gas through the smaller exposed area. For a given space velocity, identical pressure drops were measured independent of the size of the washer covering the monolith, indicating that restricting the
124 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 "
100
80
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e r
8 I
0
"
"
"
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6ol/I CO Inlet Concentration = 100 vppm
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I
I
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I
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'
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Figure 2. Carbon monoxide light-off curve for catalyst A at space velocity = 105000 h-*at STP.
monolith volume in this manner did not result in flow bypassing. The carbon monoxide concentration was measured by using a Beckman Model 865 infrared analyzer. The pressure drop across the catalyst was measured with a magnehelic pressure guage.
Results and Discussion Table I shows that the corrugated metal monolith has a higher geometric surface area and void fraction than conventional ceramic monoliths. This difference is primarily due to the smaller thickness of the metal foil (0.0025 in. compared to the 0.012-in. wall thicknesses for ceramic catalysts). These differences have a profound effect on the performance of the metal monoliths versus ceramic monoliths. In the development of pressure drop and mass-transfer correlations for the metal substrate based catalysts, the hydraulic diameter, Dh,was defined as 4 times the open cross-sectional area divided by the wetted perimeter. A schematic diagram of the washcoated metal foil is shown in Figure 3. D h depends on the corrugation height, the distance between successivecorrugations, and the washcoat thickness. Although the folded metal foil may not necessarily have discrete cells, the equivalent cell densities listed in Table I are defined according to the geometry shown in Figure 3 for purposes of comparison with ceramic monoliths. The Reynolds number for the metal monolith is Re = D h U p / C p (1) where u is the superficial velocity of the gas, p is the gas density, c is the void fraction of the monolith, and p is the gas viscosity. Since the hydraulic diameter can be expressed as D h = (2) where a is the geometric surface area per volume of catalyst, eq 1 can be rewritten as Re = 4 u p / a p (3) The friction factor, f, and the Colburn factor for mass transfer, j D , are f = 2C3Ap/pU2LcU ~ J J (k,/U)(p/pDab)o'68 (4) where AI' is the pressure drop, L is the monolith length, k, is the mass-transfer coefficient,and D a b is the gas-phase diffusivity of CO. For externally .mass-transfer-limited reactions, FE, is obtained from k, = -SV In (1- %)/a (5) where SV is the space velocity at reaction conditions, and
Figure 3. Schematic diagram of the washcoated metal foil (inset: corrugated metal foil).
- - - - Ceramic
t
t
0.001' 10
'
' """'
' 100
' """' Re
' 1000
1-I I o
l
l
l
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Figure 4. Variation of f and jD for square channel ceramic and Camet catalysts.
x is the conversion. The experimentally determined carbon
monoxide conversions were used to determine 12, and to develop a jDversus Re correlation. Observed AP data were used to obtain an f versus Re correlation. Figure 4 shows the dependence of the friction and Colbum factors on the Reynolds number for both ceramic and metal monolith catalysts. For ceramic catalysts, the friction and Colburn factors were found to be proportional to Re-l-O,which agrees with literature correlations for fully developed laminar flow inside square channels (Hegedus, 1973). This also shows that entrance and exit effects are negligible for this system. For the metal monoliths, however, the friction factor was proportional to over the entire range of flow conditions. This indicates that the herringbone pattern of the metal monolith introduces irregularities in the flow patterns at Reynolds numbers where laminar flow through the channels would be expected. Similar results were found previously for experiments in tubes with periodic axial diameter variations (Deiber and Schowalter, 1979). In these experiments,
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 125 Preseure Drop
Conversion
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8,
I 7-8
8-
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/
-
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'
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-Camet
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Figure 5. Mass-transfer-limited activity versus pressure drop comparison of Camet and square channel ceramic catalysts at temperature = lo00 O F and space velocity = 105000 h-' (Pereira et al., 1988).
Reynolds number exponents having absolute values less than 1.0 were measured and attributed to vortex behavior that begins a t Reynolds numbers much lower than that required for turbulent flow. Values of the Colburn factor determined from carbon monoxide conversion data of the metal monolith catalysts over the entire were found to be proportional to range of flow conditions. Thus, the momentum-masstransfer analogy does not apply for the corrugated metal monolith. This is to be expected, since the herringbone corrugation pattern of the metal monolith introduces a high degree of form drag into the system. A similar dependence of the Colburn factor for heat transfer j,, on the Reynolds number has been observed for flow through wavy fins in heat exchangers (Kays and London, 1964). Since the Colbum heat- and mass-transfer analogies hold in the presence of form drag, the observed dependence of the Colburn factor for mass transfer on the Reynolds number for the metal monoliths is not surprising. As shown in Table I, changes in monolith cell density (and wall thickness, in the case of ceramic monoliths) are manifested as large changes in geometric surface area and small changes in void fraction. Since eqs 3-5 show that the performance of a monolith catalyst is a strong function of these two parameters, the relationships between the friction and Colburn factors and the Reynolds number given in Figure 4 were used to generate a correlation between mass-transfer-limited activity and pressure drop for both ceramic and metal monolith oxidation catalysts using cell density as the independent variable. The relationship between mass-transfer-limited activity (defined as kma/SV or -[ln (1- x ) ] ) and pressure drop, shown in Figure 5, demonstrates that, over the pressure drop range of interest for most commercial applications (0.7-2.0 in. of water), the corrugated metal monolith has significantly higher activity relative to a ceramic monolith. Catalyst volumes were held constant (3 in. diameter X 3 in. length), and the space velocity was maintained at 105000 h-l. For these conditions, the Reynolds number is between 100 and 300 for both monolith substrates. Figure 4 shows that the friction and Colburn factors are similar for the two substrates over this range of Reynolds numbers. As shown in Table I, for a given cell density, a metal monolith will have a much higher geometric surface area and void fraction relative to a ceramic monolith. Therefore, the dependence of pressure drop on and the proportionality between activity and a shown in eqs 4 and 5 result in the higher activity for the metal monolith system a t a given pressure drop.
0
5
10
15 20 25 30 35 Linear Velocity, (Ftlsec)
40
45
50
Figure 6. Effect of superficial velocity on pressure drop and mass-transfer-limited oxidation performance at temperature = lo00 OF. Table 11. Effect of Monolith Catalyst Properties monolith properties performance case u, ft/min CD" L,in. z apb -[ln (1 - x ) ] 4.61 0.882 1.03 88 4.5 1 1390 4.61 0.861 1.16 129 3.5 2 1390 4.61 185 2.76 0.836 1.37 3 1390
(AI')
"Cell density (CD) has units of channels/in.*. *Pressure drop has units of in. of water.
Equations 3-5 show that changes in space velocity or superficial velocity will also strongly impact catalyst performance. According to Figure 4, as the Reynolds number is increased over 300, the ratio of the Colburn factors for the metal monolith system and the ceramic system increases more rapidly than the corresponding ratio of the friction factors. This suggests that as the space or linear velocity is increased, thereby increasing the Reynolds number, the activity advantage of the metal monolith system shown in Figure 5 will become more pronounced. This is demonstrated in Figure 6 for 3-in.-diameter X 3.5-in.-long metal (132 cells per square inch) and ceramic (100 cells per square inch) monoliths. These cell densities were selected so that the pressure drops of the two monoliths would be similar over the range of superficial velocities studied. For a superficial velocity of 40 ft/s (180000 h-' NTP), the conversions shown in Figure 6 of 97% for the metal monolith catalyst and 88% for the ceramic monolith correspond to a 60% higher masstransfer-limited activity for the metal substrate. This activity advantage could be important in cases of flow maldistribution through the catalyst. A potential advantage of the metal monolith system over the ceramic system is the design flexibility in selecting monolith cell density and dimensions. While the cell density of a ceramic substrate is fixed by the extruder die employed, the cell density of a corrugated metal monolith can easily be changed by altering the corrugation height and/or length. Thus, while ceramic monoliths are available with a fixed number of cell densities, metal monoliths can be fabricated to provide an infinite number of cell densities with little additional cost. The gains in performance provided by this design flexibility are illustrated in Table 11. A fixed activity level can be maintained while decreasing the monolith cell density by increasing the monolith length. Table I1 shows that this allows operation with lower pressure drops, thereby reducing energy losses due to the introduction of the catalyst system. Thus, for a given design situation, the corrugated metal monolith
Ind. Eng. Chem. Res. 1991,30, 126-129
126
offers greater potential for tailoring the catalyst properties to fit the application relative to the ceramic system. Conclusions The performance attributes of catalysts used for the oxidation of carbon monoxide from cogeneration plants have been discussed. Mass-transfer and pressure drop correlations are developed for a Camet metal monolith catalyst. The performance of such catalysts is compared to that of conventional ceramic catalysts. Camet metal monolith catalysts were found to have a higher masstransfer-limited activity than conventional ceramic catalysts at a fixed pressure drop. The design flexibility of metal monolith catalysts is discussed. Nomenclature Dab = bulk diffusion coefficient, ft2/s Dh= hydraulic diameter, in. f = friction factor h = corrugation height, in. jD = Colburn factor k, = mass-transfer coefficient, ft/s 1 = corrugation length, in. L = monolith length, in. SV = space velocity, l / h u = superficial velocity, ft/s x = conversion Greek L e t t e r s (Y
= geometric surface area per volume, l/ft
gas density, lb/ft3 gas viscosity, Ib/(ft.s) t = void fraction aP = pressure drop, in. of water Registry No. CO, 630-08-0; Pt, 7440-06-4; stainless steel,
p = p =
12597-68-1; ceria, 1306-38-3.
Literature Cited Cordonna, G. W.; Kosanovich, M.; Becker, E. R. Gas Turbine Emission Control. Platinum and Platinum-palladium Catalysta for Carbon Monoxide and Hydrocarbon Oxidation. Platinum Met. Reu. 1989, 33, 46-54. Deiber, J. A.; Schowalter, W. R. Flow Through Tubes with Sinusoidal Axial Variations in Diameter. AIChE J. 1979,25,638-645. Hegedus, L. L. Effects of Channel Geometry on the Performance of Catalytic Monoliths. Prepr.-Am. Chem. SOC., Diu. Pet. Chem. 1973, 18, 487-502. Jung, H. J.; Becker, E. R. Emission Control for Gas Turbines. Platinum-Rhodium Catalysts for Carbon Monoxide and Hydrocarbon Removal. Platinum Met. Rev. 1987, 31, 162-170. Kays, W. M.; London, A. L. Compact Heat Exchangers; McGrawHill: New York, 1964; pp 109-110. Mondt, J. R. NOx Catalyst Performance Comparison of 304 Stainless Steel, Inconel, and Monel in a 10'-herringbone Foil Configuration. AIChE Symp. Ser. 1976, 73,169-177. Pereira, C. J.; Plumlee, K. W.; Evans, M.Camet Metal Monolith Catalyst System for Cogen Applications. In 1988 ASME Cogen Turbo; Serovy, G. K., Fransson, T. H., Eds.; ASME: New York, 1988; pp 131-136. Receiued for review November 3, 1989 Revised manuscript received July 6, 1990 Accepted July 25, 1990
MATERIALS AND INTERFACES Scale-up Studies on an Alumina Aerogel Catalyst Support A n t h o n y J. Fanell&*$+S a t y a j i t Verma,* Ted Engelmann,$ and Joan V. B u r l e w t Allied-Signal Inc., Research & Technology, Morristown, New Jersey, 07960-1021, a n d Allied-Signal Inc., Engineered Materials Sector, Polyolefins Plant, Baton Rouge, Louisiana 70805
Attempts to prepare alumina aerogel in a 215-gal (814-L) pressure reactor are described. The material had lower specific surface area and altered surface morphology compared to alumina aerogel prepared in the laboratory. The altered properties are believed to have been caused by the shearing stresses imparted by the fixed rate, high-speed agitator in the reactor. Introduction Aerogels are a unique class of catalyst supports which provide exceptionally high pore volume and surface area (Ayen and Iacobucci, 1988: Armor and Carlson, 1987). However, in spite of their unique physical properties, they have enjoyed limited commercial success. This is largely due to the fact that most aerogel preparations have been conducted in small laboratory-scale reactors for research purposes. A continuous process for preparing silica aerogel is described in a 1959 patent (Sargent and Davis, 1959). Although the purpose of the process appears to be pro-
'* Allied-Signal Inc., Morristown, NJ. Allied-Signal Inc., Baton Rouge, LA.
duction of silica aerogel on a commercial scale, the venture was apparently discontinued. A continuous process capable of producing 2 kg/day in a pilot-scale reactor has recently been reported (Yamanis, 1989; Haig and Yamanis, 1989). The process for producing slabs of silica aerogel intended for insulation appears to be the largest current aerogel manufacturing process (VonDardel et al., 1983; Henning, 1986). The slabs are made in a 98-L reactor in a batch-type process. This paper summarizes the results of three runs carried out in a 215-gal (814-L) pressure vessel and the physical properties of the alumina aerogel product. To the authors' knowledge, these runs represent the largest batch-type production scale-up ever attempted for an aerogel material.
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