Ind. Eng. Chem. Res. 1996, 35, 113-116
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External Mass Transfer Coefficients for Monolith Catalysts Mohit Uberoi and Carmo J. Pereira* Research Division, W. R. Grace & Co.sConn., 7379 Route 32, Columbia, Maryland 21044
Monolith catalyst reactors are used for controlling emissions of automotive and industrial pollutants. Various external mass transfer correlations are available for small-pitch catalysts typical of automotive applications; however, little information is available for catalysts having larger pitch and length that are more typical of power plant applications. The present paper proposes a new correlation Sh ) 2.696[1 + 0.139ScRe(d/L)]0.81 based on data for the mass transfer limited performance for the CO oxidation reaction in square-channeled titania-silica honeycomb catalysts. The correlation is suitable for describing the performance of squarechanneled honeycomb catalysts used for the selective catalytic removal of nitric oxide from power plant effluents. The differences between the present correlation and other literature correlations are discussed. Introduction Monolith catalysts, originally developed for the catalytic converter in automobiles, are also finding widespread use for controlling pollutant emissions from chemical and power plants (Boer et al., 1990). Catalyst properties are selected to meet the performance specifications of the application. The chemistry can be noble metal-based for oxidation catalysts, or noble/base metal oxide-based for nitric oxide reduction using ammonia. Substrate properties, such as channel shape/size and wall thickness, are also tailored to meet requirements: larger channel size (or low cell density) substrates are typically used for treating particulate-laden exhausts, while smaller channel sizes (or higher cell density) substrates are used for clean exhausts. The performance limit for pollutant destruction of these catalysts is determined by the rate of external mass transfer from the bulk gas to the catalyst surface. Several correlations for the external mass transfer coefficient of monolith catalysts have been reported in the literature. Using analytical solutions presented by Kays and London (1964) for fully developed laminar flow and limited experimental data, Hawthorne (1974) proposed the correlation:
Sh ) B[1 + C(d/L)(Re)(Sc)]0.45
(1)
where Sh is the Sherwood number, B is a constant depending on the channel geometry, d is the hydraulic diameter defined as 4 times the channel open area divided by the wetted perimeter, L is the length, Re is the Reynolds number, Sc is the Schmidt number, and C is the surface roughness constant. Young and Finlayson (1976) used orthogonal collocation to solve the heat and mass transfer problem for monolith reactors and calculated asymptotic Nusselt and Sherwood numbers for various channel geometries. For square-channeled monoliths, they estimate B to be 2.976. C of 0.078 has been used for smooth surfaces, while a C of 0.095 has been used for automobile monolith catalysts. Votruba et al. (1975) used experimental data obtained from vaporization of water and hydrocarbons from porous monolith supports to develop the empirical correlation:
Sh ) 0.705(Re)(d/L)0.43Sc0.56
(2)
In contrast with the Hawthorne correlation, Sh for this correlation does not approach a limiting value at * Author to whom correspondence should be addressed.
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fully developed laminar flow. Further, Sh is approximately 3 times lower than that predicted by the Hawthorne correlation. Recently, Bennett et al. (1991) determined the reaction rates for oxidation of propane over a monolithic catalyst under mass transfer-controlled conditions. Their experimental data is a factor of 10 lower than that predicted using Hawthorne; however, the functional form of their equation was similar to the Hawthorne correlation. Ullah et al. (1992) conducted CO oxidation experiments on platinum group metal catalysts and found their experimental data close to that of Votruba et al.; further, their experimental data did not indicate an asymptotic value. Thus, depending on the correlation used, there can be a considerable variation in the predicted value of the external mass transfer coefficient. Most external mass transfer correlations have been obtained on automotive exhaust catalysts under conditions typical of that application. Performance requirements, however, can be different for power plant and chemical applications. As a result, lower cell density (between 1.4 and 7.6 channels cm2 versus 30.6 to 61.2 cells cm2) and longer (between 0.6 and 1 m versus 0.1 to 0.3 m) catalyst lengths are used. This paper presents experimental data on external mass transfer coefficients under conditions typically found in reactors used for the selective catalytic reduction (SCR) of nitric oxide using ammonia. As discussed in Gulian et al. (1991), CO oxidation was used as a probe reaction to develop a mass transfer correlation for SCR catalysts. The correlation developed was used for predicting overall activity of SCR catalysts. The experimental data and model predictions compare well. Possible reasons for differences in mass transfer coefficients calculated using the various available correlations are discussed. Experimental Section CO Oxidation Experiments. The square-channeled 7.4-mm- and 4.2-mm-pitch (defined as the sum of channel opening and wall thickness) ceramic TiO2SiO2 substrates were obtained from W. R. Grace & Co.Conn. The substrates were impregnated with Pd using a Pd(NO3)2 solution to obtain 3 wt % Pd on the catalyst. The catalysts were calcined in air to 420 °C prior to testing. Catalyst properties are given in Table 1. The feed stream consists of 200 ppm CO, 4% O2, and balance N2. The carbon monoxide concentration was © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996
Figure 1. CO conversion as a function of temperature (linear velocity ) 4.58 m(NTP)/s.
Figure 2. CO conversion as a function of linear velocity (temperature ) 320 °C).
Table 1. Properties of Monoliths catalyst
A
B
pitch (mm) geometric surface area (m2/m3) channel opening (mm) wall thickness (mm) length (cm)
7.4 430 6.05 1.35 25, 21, 18
4.2 750 3.45 0.78 30, 20, 9
measured using a Beckman Model 865 infrared analyzer. The oxygen concentration was measured using the Beckman Model 755 oxygen analyzer. The reactor system consists of a tubular reactor, furnace, preheaters, mass flow controllers, thermocouples, pressure gauges, and gas analysis equipment. Experiments were conducted with different catalyst lengths and gas flow rates. Since two different catalyst pitch were used, the catalyst pitch/catalyst length ratio (d/L) could be varied by varying d and L independently. To ensure that the catalyst was operating in the mass transfer controlled regime, experiments were conducted to determine the light-off temperature. These experiments were conducted with the shortest catalyst length. This way it was ensured that the experiments with longer length would also be external mass transfer limited. The light-off curves for these catalysts are shown in Figure 1. CO was found to light-off at temperatures below 310 °C. Therefore under the experimental conditions of this study (T ) 320 °C), all data on CO oxidation were obtained under external mass transfer limited conditions. SCR Experiments. The SCR experiments were conducted with a feed stream containing 400 ppm NO, 400 ppm NH3, 800 ppm SO2, 4% O2, and 10% H2O. The reactor temperature was maintained at 350 °C. Other experimental details are given in a previous publication (Fu and Solar, 1990). Results and Discussion The overall monolith catalyst activity for an isothermal first order reaction is (Gulian et al., 1991)
K)-
SV ln(1 - X) Ap
(3)
where SV is the space velocity, Ap is the geometric surface area per unit volume, and X is the fractional CO conversion. K is related to the external mass transfer coefficient, km, by the relationship
1/K ) 1/km + c(T)
(4)
where c(T) is a term that only depends on the surface reaction and internal mass transfer resistances.
Figure 3. Sh versus Re(d/L) for CO oxidation catalysts of varying pitch, length, and linear velocity (temperature ) 320 °C; Sc ) 0.93).
External Mass Transfer Coefficient. Figure 2 shows CO conversion for a 4.2-mm-pitch catalyst, 9 cm in length, as a function of linear velocity at 320 °C. Similar data were obtained by varying catalyst length and pitch. Since the reaction is externally mass transfer limited, km ()K) can be obtained from eq 3. For the range of experimental conditions Sc was calculated to be 0.93. Using this value of Sc, Sh is calculated and plotted against Re(d/L) as shown in Figure 3. Using least squares regression, the following correlation was obtained:
Sh ) 2.696(1 + 0.139ScRe(d/L))0.81
(5)
The external mass transfer correlation developed was used to predict the effect of variables such as linear velocity and catalyst length on the overall activity of a V2O5/TiO2 catalyst for the selective catalytic reduction (SCR) of NO using ammonia. This reaction has been found to be first order in NO and zero order in ammonia under our present experimental conditions (Beeckman and Hegedus, 1991). Thus, at a constant temperature, changes in K with linear velocity are entirely due to changes in the external mass transfer rate. Experimental data on K as a function of linear velocity were obtained at 350 °C for both 7.4- and 4.2-mm-pitch catalysts. For each pitch, the experimentally-obtained K at a fixed linear velocity (9.23 m/s for 7.4-mm-pitch and 7.9 m/s for 4.2-mm-pitch catalysts) was used together with the value of km obtained using eq 5 to determine c(T) in eq 4. c(T) was then held constant in the model predictions. The effect of linear velocity on
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 115 Table 2. Experimental Parameters Used in Various Studies Votruba Bennett Ullah present work
Figure 4. Effect of linear velocity on predicted K (defined in eq 5) for 7.4-mm-pitch catalyst.
Figure 5. Effect of length on predicted K (defined in eq 5) for 4.2-mm-pitch catalyst.
d (cm)
Re(d/L)
L (cm)
0.1-1 0.1 0.1 0.345-0.605
2-400 1-55 1-100 2-30
1.2-4.0
