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Ind. Eng. Chem. Res. 1998, 37, 388-390
Design of a Monolith Catalyst for Fouling Resistance James C. Fu† and Carmo J. Pereira*,‡ Research Division, W. R. Grace & Co.-Conn., 7379 Route 32, Columbia, Maryland 21044
Monolithic reactors are used in a number of environmental applications including for automobile exhaust control and for the reduction of organic compounds and nitrogen oxides from stationary sources. Typically, such catalysts are designed for high initial activity and poisoning resistance. In some cases, the catalysts may have to operate in particulate-laden environments. The present paper calls attention to the design of monolith catalysts for improved fouling resistance. A simple model is used to illustrate the design of monolith catalysts for the selective catalytic reduction of nitrogen oxides. Introduction Monolith catalysts are used in automobile converters for emission control. With the enactment of federal and state air pollution control regulations, they are also being used for controlling pollutant emissions from stationary sources such as industrial and power plants. These sources can emit pollutants such as carbon monoxide, unburned hydrocarbons (including air toxics), and nitrogen oxides. Catalysts are typically designed to meet the performance requirements of each application over the guaranteed life. Examples of catalyst performance requirements are 100 000 mi of in-service use for automobiles and 2-3 years of operation for stationary applications. The design of monolith catalysts for higher initial activity and improved poisoning resistance has been discussed in the literature (e.g., Oh and Cavendish, 1983; Pereira et al., 1988; Buzanowski and Yang, 1990; Gulian et al., 1991; Beeckman and Hegedus, 1991). The studies have examined catalyst design in particulatefree exhausts. When exhaust gases contain particulate matter, however, substrate design considerations will include the deposition of solids. Oh et al. (1981) have studied the continuous deposition of diesel particulates in fibrous filters. Foulant deposition can occur continuously, as a result of local flow maldistribution or as a result of transient reactor operation. As discussed by Oh et al. (1981), continuous deposition can occur via a number of different mechanisms. Larger particles may be collected by impaction or interception; smaller particles, on the other hand, can deposit as a result of Brownian diffusion. Local flow maldistribution in portions of the exhaust duct can cause the linear velocity in some channels in the monolith to be much lower than the design linear velocity. Such conditions may cause particles to drop out of the gas and eventually block channels. Transient conditions experienced during reactor operation can sometimes cause the gas phase temperature to drop below the dew point, resulting in condensation on the external surfaces of the monolith channels. These wet, and sometimes sticky, condensates can nucleate deposit buildup. * To whom all correspondence should be addressed. † Present address: Engelhard Corp., 101 Wood Avenue, Iselin, NJ 08830. ‡ Present address: DuPont Engineering, 1007 Market Street, N6527B, Wilmington, DE 19898.
The nature and size distribution of particulate foulants can vary widely depending on the application. Inks used in the printing industry may contain silicones which can be converted to submicron-size silica particles during the drying process. The particles can foul catalysts used for the complete oxidation of volatile organic compounds. Another example of catalysts operated in dusty environments is selective catalytic reduction (SCR) catalysts that control the emissions of nitrogen oxides in coal-fired power plants. To combat fouling, these reactors are designed at linear velocities that are high enough to prevent deposition but not so high as to cause abrasion. Typical linear velocities range between 5 and 7 m/s at operating conditions of approximately 350 °C. For high dust loadings, soot blowers that periodically clean the catalyst are installed. In such cases, the design of the catalyst will depend not only on activity and poisoning resistance but on foulant type and loading as well. The purpose of the present paper is to call attention to the need for fundamental studies on foulant deposition in monoliths and to illustrate the potential impact of dust deposition on catalyst selection. Basic Equations For simplicity, the mechanism for foulant buildup is assumed to be due to the continuous deposition of foulant by the diffusion of submicron particles in the exhaust to the walls of the catalyst. The transport of the foulant to the external surface of the monolith channel is described by an external mass-transfer coefficient, km,f. Thus, km,f describes the net transport of particles from the bulk gas to the walls of the catalyst. The driving force for foulant buildup is the concentration of particles in the gas phase, Cf. For a square-channel ceramic monolith catalyst having a channel opening of b and a wall thickness of a (Figure 1), the foulant conservation equation in the case of plug flow is
d [v(b - 2w)2 Cf] ) -4km,f(b - 2w)Cf dx
(1)
with inlet condition:
Cf(0) ) Cof
(2)
where w is the foulant thickness, v is the velocity in
S0888-5885(96)00441-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/01/1998
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 389 Table 1. Properties of Catalyst and Foulant Layer Catalyst type porosity (cm3/cm3) pore diameter (cm) intrinsic activity (1/s) (@ temp. ) 573 K)
V2O5/TiO2 monolith 0.45 0.154 × 10-5 1420
Foulant Layer bulk density (g/cm3) porosity (cm3/cm3) pore diameter (cm)
0.92 0.63 8.5 × 10-4
Table 2. Operating Conditions Used in Calculations temperature (K) GHSV @ STP (1/h) flue gas composition
Figure 1. Monolith channel geometry.
the channel, and x is the distance from the monolith inlet. Catalyst pitch is defined in millimeters as a + b. The change in the foulant thickness, w, is
dw/dt ) km,fCf/Ff
(3)
with initial condition:
w(0) ) 0
(4)
where Ff is the foulant density and t is the time. The implicit assumption in eq 3 is that the foulant particles are larger than the pores within the catalyst. The main reactant, A, diffuses to the outside surface of the foulant layer, through the foulant layer, and reacts inside the catalyst wall. The isothermal conservation equation for the reactant A, having the pseudofirst-order kinetic rate, is
4Ac dCA )dx (a + b)2Q
(b - 2w)CA w 2 1 + + km,A De,f kηa
{
}
(5)
with inlet conditions:
CA(0) ) CoA
(6)
where Q is the flow rate of exhaust, Ac is the reactor cross-sectional area, De,f is the effective diffusion coefficient, φ [)(a/2)xk/De,c] is the Thiele modulus, k is the reaction rate constant, and η [)(tanh φ)/φ] is the effectiveness factor. Conversion of reactant, χ, is given as:
ln(1 - χ) ) -
4Ac (a + b)2Q
∫0L
(b - 2w) dx w 2 + + km,A De,f kηa
{
1
}
(7)
Results and Discussion The above equations represent an oversimplification of the fouling process. The external mass-transfer coefficient, km,f, is not based on a detailed description of particle transport in channel flow. It does not include the dynamics of the deposition process, whereby particle transport and deposition can be affected by the structure of the layer formed by previously deposited particles. The model is more likely to be valid in the case of diffusive transport of particles to the surface, rather
573 10 000
density (g/cm3) molecular diffusivity of NO (cm2/s) effective diffusivity of NO (cm2/s) through foulant layer through monolith foulant (for particles < 1 × 10-4 cm) concentration (g/cm3) avg. particle diameter (cm) km,f (cm/s)
400 ppm NO, 400 ppm NH3, 1000 ppm SO2, 4% O2, 10% H2O, and balance N2 0.58 × 10-3 0.63 0.22 6.28 × 10-3 1.46 × 10-6 0.7 × 10-4 2.46 × 10-3
than in cases where buildup by particle interception is also important. Further, km,f is assumed to be a constant. In reality, km,f will vary with monolith properties and operating conditions, and this relationship will affect the fouling behavior of the catalyst. The properties of the catalyst and the foulant layer are given in Table 1. Catalyst properties are typical of fresh extruded vanadia-titania catalysts used for the selective catalytic reduction (SCR) of nitrogen oxides using ammonia (e.g., see Beeckman and Hegedus, 1991). The catalyst has a unimodal pore structure with an average pore diameter of 154 Å. A sample of the foulant layer was obtained from a catalyst that had been aged in coal-fired service. The foulant layer was found to have a unimodal pore structure with a much larger pore diameter of 8500 Å. Both catalyst and foulant pore properties were obtained by Hg porosimetry. Operating conditions are typical for the first layer of catalyst in commercial NOX reduction reactors (Table 2). The effective diffusivity of NO through the catalyst and foulant layers was estimated using the method of Wakao and Smith. The much larger value of the effective diffusion coefficient in the foulant layer is due to the presence of much larger pores in this layer. In the present calculations, the concentration of particles responsible for foulant buildup was limited to particles of less than 1 µm in size. The rationale for this assumption was that such particles are more likely to deposit via diffusive transport. The average size of particles of less than 1 µm is approximately 7000 Å. In contrast, as discussed above, the pore size in the foulant layer is 8500 Å. Possible reasons for the larger pores may include the presence of larger particles and the packing characteristics of particles in the foulant layer. The void fraction of the monolith catalyst at t ) 0 is assumed to be 0.645. This is also typical for NOX removal catalysts across a range of cell sizes. Thus, for constant void fraction,
(b/(a + b))2 ) 0.645
(8)
The conversion of NO with time for catalysts with pitch ranging from 2 to 10 mm is shown in Figure 2.
390 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998
calculations show that, for the case of continuous foulant deposition, in addition to considerations of maximum initial activity and poisoning resistance, monolith catalysts may also have to be designed to impart fouling resistance when used in particulate-laden exhausts. Nomenclature
Figure 2. NO conversion versus time for varying catalyst pitch.
a ) monolith wall thickness, cm Ac ) catalyst cross-sectional area, cm2 b ) channel opening, cm CA ) concentration of the reactant, ppm Cf ) concentration of the foulant, g/cm3 De ) effective diffusivity, cm2/s k ) pseudo-first-order rate constant, 1/s km ) external mass-transfer coefficient, cm/s Q ) exhaust gas flow rate, cm3/s t ) time, s v ) gas velocity in the monolith channel, cm/s w ) foulant layer thickness, cm x ) distance from the monolith inlet, cm Greek Symbols F ) density, g/cm3 η ) effectiveness factor, defined in text φ ) Thiele modulus, defined in text χ ) conversion Subscripts
Figure 3. Time to reach NO conversion of 40% for varying catalyst pitch.
f ) foulant layer A ) main reactant, NO c ) catalyst Superscript
The conversion of NOX at a given time will depend on the correlation used to determine km,f. Figure 2 shows that small-pitch catalysts have a high initial conversion but foul rapidly while larger-pitch catalysts have a lower initial conversion but also a lower deactivation rate. Some of our assumptions will, of course, break down as the channels get completely blocked (at t ) Ffb/2 km,fCof ). In typical industrial operations, at least a 40% NOX conversion is required over the first layer. An overall NOX conversion of 80 or 90% can then be achieved across 3 or 4 layers. Figure 3 plots the time on stream at which the conversion in Figure 2 drops below the required 40% NOX conversion as a function of pitch. As seen in Figure 3, a maximum time on stream is obtained for an optimum catalyst pitch of 5 mm at a GHSV of 10 000/h. Catalyst pitches to the left of the maximum have a lower time on stream due to channel blockage by deposits. Catalyst pitches to the right of the maximum have lower initial conversion and a lower fouling rate. These combined effects result in a time on stream to reach 40% conversion that is lower than the maximum. A solution to particulate fouling is, therefore, to provide a larger volume of larger-pitch catalyst. This approach, however, increases the cost of the catalyst system.
o ) inlet value
Literature Cited Beeckman, J. W.; Hegedus, L. L. Design of Monolith Catalysts for Power Plant NOx Emission Control. Ind. Eng. Chem. Res. 1991, 30, 969-978. Buzanowski, M. A.; Yang, R. T. Simple Design of Monolith Reactor for Selective Catalytic Reduction of NO for Power Plant Emission Control. Ind. Eng. Chem. Res. 1990, 29, 2074-2078. Gulian, F. J.; Rieck, J. S.; Pereira, C. J. Camet Oxidation Catalyst for Cogeneration Applications. Ind. Eng. Chem. Res. 1991, 30, 122-126. Oh, S. H.; Cavendish, J. C. Design Aspects of Poison-resistant Automobile Catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 509-518. Oh, S. H.; MacDonald, J. S.; Vaneman, G. L.; Hegedus, L. L. Mathematical Modeling of Fibrous Filters for Diesel ParticulatessTheory and Experiment. Presented at the SAE Congress and Exposition, Detroit, MI, 1981; SAE Paper 810113. Pereira, C. J.; Kubsh, J. E.; Hegedus, L. L. Computer-aided Design of Catalytic Monoliths for Automobile Emission Control. Chem. Eng. Sci. 1988, 43, 2087-2094.
Received for review July 24, 1996 Revised manuscript received May 16, 1997 Accepted October 18, 1997X IE960441K
Conclusions The present paper calls attention to the design of monolith catalyst for optimum fouling resistance. The
X Abstract published in Advance ACS Abstracts, December 15, 1997.
