Znd. Eng. Chem. Res. 1995,34, 1480-1487
1480
Catalytic Ceramic Filters for Flue Gas Cleaning. 2. Catalytic Performance and Modeling Thereof Guido Saracco* and Vito Specchia Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24. 10138-Torino, Italy
In paper 1 catalytically active filters for contemporary removal of dust and gaseous pollutants (e.g., NO,, VOCs), were prepared and characterized. A y-Al203 layer was deposited on the pore walls of a tubular a-AlzO3 filter. By use of isopropyl alcohol dehydration, a reaction which is catalytically promoted by the y - A l 2 0 3 itself, the capability of these activated filters of abating chemical compounds present in flue gases flowing through them is assessed. Experimental data show t h a t nearly complete isopropyl alcohol conversion (inlet concentration 1600 ppmv) can be achieved by purely catalytic means (Le., negligible contribution of the homogeneous reaction), and for superficial velocities of industrial interest (more than 1 m3(NTPhn-2-s-1). Two models are proposed to predict the performance of such filters, based on either a monodisperse or a bidisperse pore structure (this last one possibly taking into account the effect of catalyst nonuniform distribution in the filter pores). The first model properly fits the experimental data for high catalyst loads (obtained through repeated deposition cycles), whereas the second outperforms the first when dealing with a filter prepared with a single deposition cycle. As a consequence, it might be argued t h a t a nonhomogeneous catalyst distribution is obtained after a single deposition, and that already after a second catalyst deposition this problem is overcome. On the basis of the above results, modification of the active filters is i n progress with catalysts suitable for reactions of environmental interest (e.g., V205 for NO, reduction with NH3).
Introduction In the first paper of this series the preparation and characterization of catalytic filters for contemporary abatement of particulate and gaseous pollutants were presented, and the basic concepts and application opportunities of this promising technology were outlined (Saracco and Montanaro, 1995). Porous alumina tubes were activated by insertion into their pores of a y-AlzO3 phase, forming a well-stuck layer onto the pore walls. A relevant increase of the specific surface area of the product was thereby obtained (from 0.25 up t o 7-12 m2.g-l), without a marked decrease of permeability, thus rendering the filter suitable for further activation with various catalytic principles (e.g., V205 for NO, reduction with NH3, noble metals for VOCs catalytic combustion, etc.). However, y - A l 2 0 3 itself is intrinsically active toward, e.g., dehydration reactions thanks to the acidic nature of its surface. Due t o its very high selectivity toward propylene and to the comparatively mild temperatures required (150-250 "C),isopropyl alcohol dehydration CH,CHOHCH,
-
H20
+ CH3CH=CHz
(1)
was therefore chosen as the test reaction, to assess the capability of the prepared filters of catalytically abating gaseous compounds passing through them. In paper 1 (Saracco and Montanaro, 1995)the above reaction was studied in a batch-operated recycle reactor on y - A l 2 0 3 powders derived through the same procedure employed for the preparation of the catalytic filter (i.e., urea method (Gordon et al., 1959)). A kinetic expression was worked out accounting for the dependency of the reaction rate on the main parameters affecting it (i.e., temperature, isopropyl alcohol, and water partial pressures). By use of such an expression and of differential
* Corresponding author. E-mail:
[email protected].
mass balances over the porous structure of the filter, a modeling framework is presented in this paper, based on either a monodisperse or a bidisperse pore structure. Such models are then demonstrated t o satisfactorily fit the experimental isopropyl alcohol conversion data obtained in a specific laboratory-scale pilot plant, described in the Experimental Section.
Theory Based on a pseudo-homogeneous approach, two models for mass transfer accompanied by catalytic reaction through tubular porous media are presented in this section. Both models are isothermal, which is a fairly reasonable approximation due to the low isopropyl alcohol feed concentration employed in the experimental study (1600ppmv; see the Experimental Section) and the consequently low amount of heat generated by reaction. Monodisperse Pore Structure. The first model presented assumes that the performance of the filter, in terms of either its transport properties or the attainable conversion, can be predicted considering a single type of passing-through pores, which is taken as representative of the entire distribution of pore sizes across the filter itself (Figure la). Local mass conservation under unsteady-state conditions for each component in the radial direction of a cylindrical porous structure can be expressed for each single component,j , as follows:
For the consistency of mole fractions it should be
The system of the n - 1 eqs 2 and of eq 3 was solved with the following set of boundary conditions, typically
0888-5885/95/2634-1480$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1481 Table 1. List of Preexponential Factors, of the Activation Energy E,., and of the Heats of Adsorption of the Various Equilibrium Constants (Saracco and Montanaro, 1995) Kr KA,~ KB,i
KB,~
2.46 x 1O1O mol-kg-l-s-l 3.45 x 1013Pa 1.92 X loi5Pa 6.66 x loll Pa
1.252 1.005 0.996 0.774
The above expression is valid provided that the efficiency factor of the catalytically active phase is equal to unity, a fairly tolerable approximationdue to the very small thickness of the deposited y - A l 2 0 3 layer (i.e., a few microns). The kinetic constant Kr and the desorption constants KA,~, KB,i, and K B ,can ~ be expressed through Arrheniustype and Van't Hoff-type relationships:
N'
Kr = ee-EJRT, KA,i= pA,ie-whi'RT, etc.
N'" +
(9)
The values of all preexponential factors, the activation energy E,, and the various heats of adsorption derived in paper 1, are listed in Table 1. The mole flux Nj can be expressed as a combination of a Fickian diffusive flux and a convective flux based on a d'Arcy formulation:
Figure 1. The two modellistic approaches proposed: (a) monodisperse pore structure; (b) bidisperse pore structure.
employed in the case of fixed-bed reactors (Froment and Bishoff, 1979): inlet (inner diameter, tube side, V t f 0):
M(XF-,x;
= --
(Danckwerts-type) (4)
outlet (outer diameter, shell inside, V t
* 0):
At the initial time (t = 0) a fictitious concentration profile across the filter was assumed for each component. The above set of equations can be solved once the inlet the ,inlet concentrations (x?), and the outlet flux (iW) pressure ( P u t ) are known. The local consumption or production rate Rj is obviously zero for the inert compound. For those compounds which take place in the reaction, a suitable expression for Rj can be derived from the kinetics rate expression experimentally derived in paper 1:
Rj = @"Vj
Pi 1+-+KB,i
Pw KB,w
Gas mixture viscosity, p, was evaluated through the van Wilke method from pure gas viscosities of each component, calculated on the basis of the ChapmanEnskog theory as a function of the operating temperature (see Reid et al., 1987). As concerns the diffusive transport, in the investigated range of pore dimensions (r, < 7 pm) Knudsen diffusion (governed by molecule-pore wall interactions) is not negligible compared with bulk diffusion (controlled by molecule-molecule interactions). The transport resistances of these two regimes are assumed to be in series, according to the Bosanquet formula:
The effective bulk diffision coefficients can be determined from the diffusivities of binary gaseous mixtures through Blanc's law, which was corrected so as to account for the effect of the porosity and the tortuosity of the porous structure:
J
where xz is the mole fraction of the generic z component of the gas phase. Estimates of the binary gas-phase diffusivities, Dg were derived from the Fuller-Schettler-Giddings equation reported by Reid et al. (1987). Conversely, the effective Knudsen diffision coefficient can be calculated according to
1482 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995
(13)
Three parameters used in the above equations, namely KO,BO,and z, need to be known a priori, and must be evaluated independently via preliminary characterization methods or assumptions. KOand Bo can be measured through simple permeation runs with pure inert gases (e.g., Nz, He). At stationary conditions and in the absence of reaction, the flux expression 10 reduces, for a pure gas, t o
Assuming all the physical parameters are constant across the tubular filter, the substitution of eq 14 in eq 2 leads, after integration, to
tion (i.e., some pores are more active than others) should in fact lead to a decrease in the attainable conversion since those pores which received more catalyst during the deposition treatment are less permeable than the others. Some further model assumptions are listed hereafter: the two classes have an equal number of pores; the pores are cylindrical; all pores are placed in parallel; the catalyst is well stuck on the pore walls forming a layer of uniform thickness, which is, of course, different for the two classes of pores. A segregation parameter, accounting for the presence of a different amount of catalyst in the first of the two pore types compared with a homogeneous distribution of catalyst in each pore (monodisperse pore structure), can be defined as follows:
a=
Bo* - B,' Bo* - Bo
(19)
q,
Varying Pt and P",estimates of KOand BOcan be easily obtained, as presented under Results. Assuming that the passing-through pores can be approximated as cylindrical and have a constant radius, the following expressions for KOand BO can be deduced as shown by Mason and Malinauskas (1983):
B -fk O-z
8
(16) (17)
E and rp can be measured by Hg porosimetry (see Experimental Section), whereas KO and Bo can be evaluated from permeation runs, so that z can be derived, as the only unknown parameter, from either eq (16) or eq (17). However, since the Bo estimate is much more reliable than the KOone, eq 16 is preferable. The solution of the system of equations was obtained numerically through a Pascal program based on the finite differences method, running on a VMS VAX station. The accumulation term in eq 2 had to be taken into account in order t o overcome stiffness problems arising when comparatively fast kinetics were considered (high temperatures). Time was therefore used as a relaxation parameter and a Baker-Oliphant discretization scheme was employed (Baker and Oliphant, 1960). Each stationary solution of the system was thus attained as the asymptotic value of a series of solutions, calculated a t progressive time steps. On the basis of the obtained results, the predicted value of isopropyl alcohol conversion through the filter was thus derived as follows:
where Bo* refers to the virgin filter (before any deposition), and BO is the experimentally determined d'Arcy factor after one deposition factor (see Results). When a is varied between 0 and 1,the entire variety of conditions from a perfectly segregated system (the first type of pores has no catalyst inside) to a monodisperse pore structure (both pore classes are activated with the same specific amount of catalyst) can be represented. Once a certain value of a has been chosen, Bo' can be easily derived as the only unknown parameter in eq 19. Hence, based on simple catalyst balances, and assuming that eqs 16 and 17 hold for both pores of the bidisperse structure; the volume fraction of the catalyst is equal to the weight fraction o;the tortuosity remains constant after catalyst deposition, i.e., z' = I!'= z* = 4.84; and E* and rp* are known (from Hg porosimetry measurements), the structural parameters of interest for the bidisperse pore structure can be evaluated as follows: rP
' = ( ~ T * B ~ ' ( ~ , * ) ~ / E * ) ~ ' (20) ~
(22)
(23)
(18) Bidisperse Pore Structure. The second model presented assumes that the transport and reaction phenomena across the catalytic filter can be well interpreted by means of a bidisperse pore structure (Figure lb). The existence of two types of pores is introduced to possibly take into account catalyst maldistribution effects. Nonhomogeneous catalyst deposi-
(25) (26)
-1
Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1483 from mass-flow meters
Inlet
n
On the basis of eqs 27 and 28 the reaction rate expression 8 can be suitably modified for both types of pores. Solution of the model based on the bidisperse pore structure was then attained as follows: 1. A tentative feed mole flux, N' in, through the pores of the first class is hypothesized. 2. The flux through the pores of the second class is thus easily derived at a known experimental Ninvalue:
3. The system of eqs related to each class of pores is solved according to the same calculation routines adopted for the monodisperse pore structure model, using its specific boundary conditions and structural parameters. 4. The values of the inlet pressures P' and P" t, calculated for the two pore classes, are then compared, and if they are different, the value of N' in is changed according to a Newton-Raphson scheme. The above routine is repeated until P' = P" t. Convergence is normally obtained after few iterations. The pressure difference across the filter, derived via the model, was in good agreement with the experimental value (deviations always lower than f5%). 5 . The performance of the filter is then easily calculated as a linear combination of those of the two structures. For instance, the overall isopropyl alcohol conversion is derived from
By least-squares fitting of the experimental data the optimum a value can be easily worked out minimizing the function:
Experimental Section A scheme of the module for the reactive and permeation runs performed is presented in Figure 2. The gas flows (isopropyl alcohoVHe, N2, He) were fed to the module at the tube side of the filter, through a number of mass-flow meters (Brooks) capable of dosing flow rates in the range from 10 to 15 000 cm3*min-l.The isopropyl alcohol-in-He concentration has been kept, in all reactive runs, equal to 1600ppmv. This concentration was conservatively chosen as representative of high concentration levels of pollutants present in flue gases (e.g., NO,., VOCs, etc.). The absolute pressure a t the outlet of the module was measured via a pressure transducer (Transinstruments),whereas the pressure drop across the filter was evaluated through a waterfilled U tube. A back pressure regulator (Tescom) placed at the outlet of the module was employed, only in the permeation runs for Bo and KOdetermination, so as to raise the average operating pressure in the module.
Thermocouples*-
to bubble-flow meters and Figure 2. Scheme of the reactor.
Thermal control was assured by a PID-regulated oven (van Wilgen bv), in which the entire module was contained. The filter temperature was measured by three K-type thermocouples touching the shell-side surface of the filter in three different positions along the axial direction (see Figure 2). The maximum temperature difference registered between these three measurements remained always lower than 8 "C. In any case, the average filter temperature was assumed for model calculations. The virgin filter tubes (supplied by S.C.T., Bazet, France; length 250 mm; rt = 3.5 mm; rS = 5 mm; E* = 0.245; e* = 2.45 x lo3 kgmF3)were enamel-coated at both ends for a length of about 20 mm so that the effective length of the porous filters remained ca. 210 mm (available inlet surface at the tube side St = 46.2 cm2). The coating was performed in order to allow a proper sealing into the module via compressed Teflon rings (capable of withstanding the temperatures of interest for the reaction runs: 150-250 "C). Copper gaskets were used to allow gas-tight sealing at both ends of the stainless steel module (internal shell radius 10 mm) by compression between opposite flanges after the filter had been fixed into the module itself. Two filters, which underwent one and two deposition cycles respectively, were studied. Further deposition cycles would promote frequent pore obstruction in the filter, leading to unacceptably high pressure drops across it, as described by Saracco and Montanaro (1995). The catalyst loads, a,in these two filters were 1.45 and 2.59%; the porosities, E, 0.215 and 0.144; the densities, e, 2.49 x lo3 and 2.52 x lo3 kgm-3, respectively. The outlet flow rates were measured through a set of bubble-flow meters. By these means, the mass-flow meters were calibrated as well. The chemical composition of inlet and outlet flow rates was determined by gas-chromatographic analysis (HP 5890 Series I1 by Hewlett Packard equipped with a Porapak Q column and a thermal conductivity detector). By these means overall mass balances of each components a t stationary conditions could be verified with a deviation always lower than 4 4 % .
Results The results of the permeation runs for the determination of Bo and & are plotted in Figure 3 according to
1484 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
PS-Pt (m2s-'
0.5 0 0.8
Figure 3. Results of permeation runs. Virgin filter symbols, Nz; empty symbols, He.
1
1.2
1.4
1.6
(m, 0);after one deposition
1.8
2
2.2
2.4
cycle (A, A); aker two deposition cycles (0,0). Full
Table 2. Results of Permeation Runs virgin filter Bo* x lOI3 (m2)
KO*x IO7 (m) permeating gas He
N2 average values
2.14 3.20 2.67
after one deposition Bo x 1013(m2)
KO x lo7(m)
3.32 3.80 3.56
1.79 1.94 1.87
2.66 2.86 2.76
1 00
100
80
80
3 60
W
&
40
20
20
n
0
Figure 4. Accordance of monodisperse (-) and bidisperse (- -, optimized a = 0.4) models to experimental isopropyl alcohol conversion data obtained with the once-deposited filter. Inlet fluxes x lo2 (m3(NTP)m-2 s-l): (+I 0.013;(a)0.049;(A)0.12;( 0 )0.24; (e)0.69;( x ) 1.67.
eq 15. From the slope and the intercept of the lines in this figure, the above constants can be easily derived. Table 2 lists the obtained data. The Bo and KOaverage values between those derived from He and N2 permeation measurements have been used in model calculations. Figures 4 and 5 plot the experimentally obtained isopropyl alcohol conversion as a function of temperature, a t varying the gas fluxthrough the filter and for both the once- and the twice-deposited filters, respectively. The predictions of the monodisperse pore structure model are reported as well in both figures. Further, Figure 4 also reports the behavior of the bidispersepore-structure model, for which an optimum value of the segregation parameter (a= 0.4, obtained by leastsquares fitting of the experimental data) was used. By the way, this optimal a value implies that the 38% of the entire catalyst amount is placed in the pores of the
0.99 1.18 1.09
1.22 1.95 1.59
60
LF40
ul10 130 150 170 190 210 230 250 T ("C)
after two depositions Bo x 1013(m2)
KO x lo7 (m)
110 130 150 170 190 210 230 250
T ("C) Figure 5. Accordance of the monodisperse model to the experimental isopropyl alcohol conversion data obtained with the twicedeposited filter. Inlet fluxes x lo2 (m3(NTP)m-2 s-l): (+) 0.013; (a)0.12;(A)0.24;( 0 )0.69;(e)1.42.
first class and the remaining part in those of the second class, as can be easily calculated on the basis of the eqs listed under Bidisperse Pore Structure. Discussion Catalytic filters should possess a few basic properties in order to outperform on an economic basis alternative catalytic converters; technologies (e.g., dust filters Saracco and Montanaro, 1995): 1. The filter media should remove particulate effectively so as to allow a very low dust penetration in the filter structure. In fact this could eventually lead to catalyst deactivation, or, a t least, to pore obstruction in the filter. 2. The catalyst load should be high enough t o catalytically abate the chemical pollutants passing through the active filters, without increasing unacceptably the pressure drop through the filters themselves.
+
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1485
3. The above properties should be verified for sufficiently high superficial velocities. As concerns point 1,the literature available on hightemperature filtration (see, e.g., Clift and Seville, 1993) confirms that, for the pore size of the filters used in this study, most of the dusts of industrial interest would give negligible penetration in the filter. In particular from data presented by Zievers et al. (1991)one may predict that the penetration of fly ash, generally considered as one of the toughest dusts to be filtered due to its comparatively low size (50%cut diameters = 1-5 pm), the penetration in the filter structure should be lower than ca. 50 pm (i.e., about 1/30 of the entire filter thickness). In fact, a cake-filtration mechanism controls the dust removal efficiency of such filters. A dust cake forms on the surface of the filter, and acts as the true filter media thus preventing dust from further penetrating the filter. After jet-pulse cleaning of these filters (Le., when the pressure drop is unacceptably high, a strong countercurrent pulse is forced through the filter to promote cake detachment), a thin dust layer still remains attached and governs the separation. On the basis of the above evidence, the topic of dust filtration was not considered in the present study, which was thus focused on the catalytic removal of pollutants inside the filter. Concerning point 2, as above underlined, only two consecutive y-Al203 deposition cycles could be performed by the urea method because of the occurrence of pore obstruction which leads to an unacceptable increase of the pressure drop. This limits the amount of catalyst that can be deposited in the filter. As concerns the twice-deposited filter, carrying the higher specific y-Al203 load (2.59% by weight), nearly complete isopropyl alcohol conversion could be reached for Superficial velocities lower than m3(NTP)m-2 s-l, and up to 90% conversion in the case of a 1.42 x m3(NTP)m-2 s-l could be achieved a t temperatures lower than 250 "C. At such temperatures isopropyl alcohol dehydration is almost entirely controlled by catalysis and the effect of homogeneous reaction is negligible, hence the adequacy of this reaction to simulate other catalytic processes (e.g., NO, reduction with NH3). Coming to point 3, it has to be underlined how the above superficial velocities are frequently met in literature (Kudlac et al., 1992;Clift and Seville, 19931,though several times higher values are met in the common practice of high-temperature filtration (up to lo-' m3(NTP)m-2 s-l). It is quite clear that if, wishing to combine the catalytic abatement of pollutants with high temperature dust filtration, one has to reduce the superficial velocity so as to attain suitable conversion, a proportional increase in the available filtration area will have to be set up. This would reduce a t least part of the advantages of the combination of separation and reaction, depending on the required increase of the filter section. It has to be considered though that the laboratory-scale filters employed in this work were rather thin (i.e., 1.5 mm). The world's leading manufacturers of industrialscale ceramic filters (BWF, Cerafil, Schumacher, Industrial Filters and Pumps, etc.) generally produce relatively long filter candles (usually longer than 1 m) having a 10-20 mm thickness in order to achieve proper structural and mechanical resistance. Provided the urea method gives equal results when applied t o such large-scale filters (which is likely
predictable) and that the attainable conversion is proportional to the residence time in the filter, nearly complete conversion would be attained for superficial velocities up t o 1order of magnitude higher than those tested in this work, thus covering almost all the industrial application opportunities. A further goal of this work was to set up a model capable of properly predicting the performance of the catalytically-activated filters, thus allowing performance of project calculations with confidence. The model based on a monodisperse pore structure (see under Theory) properly fits the experimental conversion data obtained with the twice-activated filter (Figure 5). This is particularly satisfactory since this model approach does not use any fitting parameter (i.e., all structural and kinetics parameters are determined with different and independent measurements). However this model fails, though not dramatically, when dealing with the once-deposited filter. Experimental conversions are in this case generally lower than model predictions (Figure 4). A possible explanation of this can lie in a certain degree of catalyst bypassing. In paper 1 (Saracco and Montanaro, 1995)catalytic runs performed on powdered filter samples lead to the conclusion that the filters are homogeneously activated at a macroscopic level along their entire length. However, a t a microscopic level, aRer a single deposition cycle, different pores may carry different amounts of catalyst. Hg porosigrams, reported by Saracco and Montanaro (19951,seem to confirm this hypothesis; they show that the pore size distribution, after y-Al2O3 deposition, is wider than that of the virgin filter. This is probably a sign that some of the original pores were filled with more catalyst than others. Pores which carry a lower amount of catalyst should obviously be less active but more permeable, leading to lower overall conversions. A model was set up which lumps into two pore types, the one more active than the other, the actual structure of the filter in order t o take into account this phenomenon (see Bidisperse Pore Structure). This model turned out to allow a much better agreement with the experimental conversion data obtained with the onceactivated filter, compared with the monodisperse pore structure model (Figure 4). Ultimately, this is a consequence of the use of a fitting parameter, namely the segregation parameter a, whose optimum value, 0.4, was obtained by the least-squares method applied to the entire collection of data derived a t different superficial velocities. As represented in Figure 6, the minimum of the square deviation function, referred to the square deviation for a = 1,is well-defined, which is an indirect sign of the reliability of the basic model assumptions. The accordance of the bidisperse pore structure model to the experimental data is also rather good. However, a slight though rather general tendency to overestimate ti at low conversion levels is perceivable. Figure 7 reporting the model predictions vs experimental data at varying a, for the highest superficial velocity tested, clearly represents this behavior. This can be possibly overcome by elaborating more sophisticated models, based on, e.g., networks of pores with different catalyst loads, placed either in series or in parallel. Such models would provide a closer representation of reality than those given by the two models outlined under Theory. However, the authors feel that the eventual advantages of this model enhancement would not compensate the disadvantage arising from a
1486 Ind. Eng. Chem. Res., Vol. 34,No.4,1995
3.5 3.0 2.5
1.o
0.5
0
0 . 1 .2 .3 .4 .5 .6 .7 .8 .9 1
a Figure 6. Fitting of experimental data of the once-activated filter with the bidisperse model: optimization of the a value by the leastsquares method.
100
a
90
one, more simple, is however capable of properly predicting the performance of the filter which underwent two consecutive activation steps through the above method. The second, more complicated and requiring higher computational time, is however needed to attain a sufficiently accurate fitting of the experimental data obtained testing the filter which underwent a single deposition step. This is a likely sign that after a single deposition treatment the catalyst distribution is not uniform in all pores of the filter. Such a problem seems to be overcome after a second deposition cycle. On the basis of the above rather promising results, further work is currently in progress concerning the impregnation of the y-A1003 layer, deposited onto the pore walls of the filters, with V205, so as to get suitable catalysts for NO, reduction with NH3. Such novel filters may find application in the treatment of flue gases which need to be treated by removal of both particulate and nitrogen oxides (e.g., from pressurized fluidized bed coal combustors, boilers, waste incinerators, etc.).
Acknowledgment The financial support of C.N.R. (Consiglio Nazionale delle Ricerche, Rome, Italy) is gratefully acknowledged.
1.0
I 170 180 190 200 210 220 230 240 250 260
T ("C) Figure 7. Effect of variation of segregation parameter a on accordance of the bidisperse model to the experimental conversion m3(NTP) m-2 data (once-deposited filter; inlet flux: 1.67 x 5-1).
much higher computational time. They believe that the proposed models would be accurate enough for design purposes, at varying the catalyst load of the filter and/ or the number of activation cycles. Further, on the basis of the above evidence, one may guess that after a single deposition step the catalyst does not coat uniformly all pores, causing a certain decrease of the attainable conversion. However, after the second deposition cycle, adding more catalyst to the filter, a more homogeneous coverage of the pore walls is obtained and catalyst bypass phenomena are almost negligible.
Conclusions Two major conclusions can be drawn out of the presented work: 1. The activated filters prepared via the urea method as described in paper 1 of this series (Saracco and Montanaro, 1995) allow nearly complete catalytic dehydration of isopropyl alcohol fed a t 1600 ppmv in a mixture with helium and at superficial velocities of industrial interest (up to ca. 1.5 x m3(NTP) m-2 S-1).
2. Two models were proposed based on either a monodisperse or a bidisperse pore structure. The first
Nomenclature Bo = d'Arcy factor (m2) D = diffusion coefficient (m2 s-l) E = activation energy (Jemol-I) K = desorption constant (Pa) KO = Knudsen factor (m) K, = reaction constant (molokg-ls-l) M = molecular weight (kgmol-l) n = number of components N = mole flux (mol.m-2s-1) p = partial pressure (Pa) P = absolute pressure (Pa) Q = mole flow rate (moles-') r = radius (m) R = ideal gas constant = 8.314J*mol-lK-l R = reaction term (m~l.m-~s-l) S = surface (m2) t = time (s) T = temperature (K) W = heat of adsorption (Jmol-') x = mole fraction Greek Letters
a = segregation factor E = porosity 5 = conversion p = viscosity (Paos) Y = stoichiometric coefficient 8 = density of the catalytic filter (kgm-3) u = deviation t = tortuosity w = catalyst weight fraction Superscripts
* = virgin filter ' = first class of pores in the bidisperse-pore-structure model " = second class of pores in the bidisperse-pore-structure model 0 = preexponential constant e = effective exp = experimental g = gas phase in = reactor inlet mod = modellistic
Ind. Eng.'Chem. Res., Vol. 34, No. 4, 1995 1487 out = reactor outlet s = shell side of the filter t = tube side of the filter
Subscripts A, B = adsorption sites b = bulk i = isopropyl alcohol j = generic component k = Knudsen p = pore r = reaction w = water z = generic component
Gordon, L.; Salutaky, M. L.; Willard, H. H. Precipitation from homogeneous solutions; Chapman & Hall: New York, 1959. Kudlac, G. A,; Farthing, G. A.; Szymanski, T.; Corbett, R. SNRB catalytic baghouse laboratory pilot testing. Environ. Prog. 1992, 11,33-38. Mason, E. A.; Malinauskas, A. P. Gas transport in porous media: the Dusty-Gas-Model; Chemical Engineering Monographs 17; Elsevier: Amsterdam, 1983. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The properties ofgases and liquids, 4th ed.; McGraw-Hill: New York, 1987. Saracco, G.; Montanaro, L. Catalytic Ceramic Filters for Flue Gas Cleaning. 1.Preparation and Characterization. Znd. Eng. Chem. Res. 1996,34, 1471-1479. Zievers, J. F.; Eggerstedt, P.; Zievers, E. C. Porous Ceramics for Gas Filtration. Ceram. Bull. 1991,70, 108-111.
Literature Cited Baker, G. A.; Oliphant, T. A. An implicit numerical method for solving the two dimensional heat equation. Q.Appl. Math. 1960, 17,361-373. Clifi, R.; Seville, J. P. K. Gas Cleaning at High Temperatures; Chapman & Hall: London, 1993. Froment, G. F.; Bishoff, K. B. Chemical reactor analysis and design; J. Wiley and Sons: New York, 1979.
Received for review September 16,1994 Accepted January 17,1995
IE940548C ~~~
~
~~~
* Abstract published in Advance A C S Abstracts, March 15, 1995.
