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Ind. Eng. Chem. Res. 2005, 44, 302-308
Monolithic Supports with Unique Geometries and Enhanced Mass Transfer Robert M. Ferrizz, John N. Stuecker, Joseph Cesarano, III, and James E. Miller* Sandia National Laboratories, Albuquerque, New Mexico 87185-1349
Novel monolithic catalyst supports with regular three-dimensional structure and channel-tochannel interconnectivity have been fabricated using a direct ceramic fabrication technique known as “robocasting”. Using the oxidation of CO over a Pt/γ-Al2O3 catalyst as a probe reaction, we have quantified the mass transfer over several new geometries and compared them to traditional straight-channel monolithic supports. A geometry of alternating rods that presents no line-of-sight flow paths and about 45% void volume increases the dimensionless Sherwood number by a factor of 3 over that of traditional honeycomb supports. However, the resulting pressure drop is similar to that of a packed bed (up to a 1000-fold increase). A similar robocast structure with 74% void volume improves the Sherwood number by a factor of about 1.5 relative to the honeycomb geometry but only increases the pressure drop by a factor of 4. The results illustrate that robocasting technology affords an unprecedented degree of freedom, allowing optimization of ceramic monoliths for specific applications. 1. Introduction Monolithic catalytic reactors, widely utilized for environmental applications (automotive and stationary emissions control), are currently being considered for a large array of additional applications.1,2 Monoliths offer several advantages over traditional packed-bed systems. Perhaps the most significant of these is a high surfaceto-volume ratio, similar to that of very small particles, but with a significantly lower pressure drop. The large geometric surface areas provided by monoliths can reduce pore diffusion limitations and improve reaction selectivities and yields. However, for highly active, highthroughput systems, the standard honeycomb monolith structure of adjacent straight-through channels can also introduce bulk gas-solid mass-transfer limitations.1,3,4 This can be a concern for applications such as catalytic combustion, a process that could eliminate NOx emissions from gas-fired turbines without requiring posttreatment.3,4,7-10 For this application, 100% conversion is required, and space velocities can be as high as 106 h-1. Another limitation of honeycomb-type monoliths is that the separation of the channels from one another can severely restrict heat transfer into and out of the system. This limits their usefulness for highly endothermic or exothermic reactions such as steam reforming. New monolith geometries with three-dimensional complexity, including ceramic foams5,6 and lattices,11 can help overcome these effects by allowing channelto-channel mixing and promoting turbulent flow through the tortuous pathways. In this paper, experimental results are presented that quantify the mass transfer over several novel three-dimensional lattice geometries. The results are compared to those of traditional honeycomb monoliths, and the pressure drop penalty for improved mass transfer is evaluated. 2. Theory: Quantifying Mass Transfer There is a spectrum of kinetic regimes for monolithic reactors that is similar to packed-bed reactors. Surface kinetics, pore diffusion, gas-solid mass transfer, or even * To whom correspondence should addressed. Tel.: (505) 272-7626. Fax: (505) 272-7336. E-mail:
[email protected].
Figure 1. Idealized plot of reaction rate (or conversion) versus temperature exhibiting the possible rate-controlling regimes for monolithic reactors.
homogeneous reaction rates may control the overall rate of reaction. A common representation1,3,4,12 of these effects is shown in Figure 1, which plots the reaction rate (i.e., conversion) versus temperature in a catalytic monolith. At low temperatures, the reaction rate is controlled by surface kinetics. The Arrhenius equation (eq 1), wherein the kinetic rate constant increases exponentially with inverse temperature, applies under these conditions.
kKIN ) A exp(-EACT/RT)
(1)
As the temperature is increased, a point can be reached at which surface kinetics (which may be adjusted for pore diffusion using an effectiveness factor approach13-15) exceeds the rate for bulk mass transfer, causing the overall reaction rate to become masstransfer-limited. Under these conditions, the reactant molecules diffuse through the boundary layer at the gas-solid interface to reach the catalyst with roughly a T 3/2 rate dependence.6
kMT ) BT3/2
(2)
Because the mass-transfer coefficient (i.e., diffusion rate) increases relatively slowly with temperature, an apparent “plateau” in the reaction rate (conversion) is observed in the region where bulk mass transfer is ratecontrolling. Finally, homogeneous reaction, with its
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relatively large activation energy, becomes favorable at high temperature. At this point, the reaction rate rapidly increases with temperature and complete conversion is reached. Considering surface reaction, mass transfer, and homogeneous reaction using a series/parallel approach, the overall rate equation for an isothermal, first-order reaction can be written as follows:
-r ) ν
dC ) -kAPPC dz
(3)
where
kAPP ) 1/(1/SkMT + 1/ηkKIN) + kHOMO
(4)
When ηkKIN > SkMT (and kHOMO ∼ 0), the reaction is mass-transfer-limited, and the rate equation simplifies to the following:
ν
dC ) -SkMTC dz
(5)
This expression can be further simplified1,6,13,16 to
ln(CO/CI) ) ln(1 - X) ) -SkMTτ ) -SkMT/SV
(6)
This equation allows the mass-transfer coefficient, kMT(T), to be determined experimentally by obtaining temperature/conversion data at a given flow rate for monolithic samples of known geometric properties. Note that the space velocity (SV) is defined as the volumetric flow rate per “volume catalyst”. In the case of monoliths, the “volume catalyst” is simply the monolith volume (including void space), which can be calculated from the overall monolith dimensions as follows:
VMONO ) πd2L/4
(7)
Using eq 6 to solve for kMT assumes that the reaction is indeed controlled solely by bulk mass transfer.1,16 Given that there will be a transition region where both reaction kinetics and mass transfer are important, a more precise method of determining kMT from experimental data incorporates the following equation, used in conjunction with eq 4:
ln(1 - X) ) -kAPP/SV
(8)
This assumes that an accurate value for ηkKIN(T) is known and that the homogeneous reaction is insignificant. Alternatively, kAPP(T) can be experimentally determined using eq 8, and then eqs 1, 2, and 4 can be applied to solve for kKIN and kMT simultaneously.6 Favorable experimental conditions for operating in the mass-transfer-controlled regime include high temperatures, short residence times (large SV), and an extremely active catalyst.1 Experimentally, this is achieved through the use of high flow rates coupled with small monolith dimensions. In this paper, CO oxidation over a Pt/γ-Al2O3 catalyst is used as the probe reaction to quantify mass transfer in monolithic samples. Under the correct (and readily attainable) experimental conditions, this reaction is sufficiently rapid to induce masstransfer limitations. The predominant metric for quantifying mass transfer found in the literature1,12,13,15-19 is the Sherwood number, NSH, which is a dimensionless concentration gradi-
ent.20 The Sherwood number is defined as
NSH ) kMTdH/D
(9)
Diffusivity, D, is dependent on the system temperature, pressure, and particular gas-phase molecules. For CO in air, the following empirical correlation (calculated from Fuller et al.21) has been reported:22
D ) 9.2635 × 10-5T1.75/P
(10)
The hydraulic (characteristic) diameter, dH, is dependent solely on the physical dimensions and geometry that best characterizes the system in question. For packed beds, the hydraulic diameter is defined as the pellet diameter; for flow in pipes, dH is the tube diameter. For straight-channel monoliths, dH is the diameter of an individual channel. A common expression6,16,19,23 to determine dH for unusual geometries (such as those investigated here) is
dH ) 4/S
(11)
For simple geometries, this expression reduces to 4 times the cross-sectional area per wetted perimeter, which in turn reduces to the channel diameter. This expression is often used to calculate the geometric surface area after applying microscopy to measure the channel diameter or mean pore size. However, for the case of the robocast samples, the geometric surface area is a known quantity; it is calculated from the rod length, diameter, number, and monolith dimensions. However, dH is difficult to precisely measure. Therefore, for the three-dimensional lattice geometries, bulk porosity and geometric surface area are used to solve for dH. Many empirical correlations have been derived to relate the Sherwood number to other system parameters such as fluid velocity (ν), viscosity (µ), density (F), etc. These correlations often present NSH as a function of the Reynolds (NRE), Schmidt (NSC), or Graetz (NGZ) numbers. Several relations have been reported that specifically address mass transfer in honeycomb monoliths. Uberoi and Pereira13 proposed the following model for mass transfer in square monolith channels, based on experimental results for CO oxidation:
NSH ) 2.696[1 + 0.139NSCNRE(dH/L)]0.81
(12)
Hawthorn18 derived the following semianalytical expression (using the analytical solution for fully developed laminar flow and limited experimental data) for laminar flow in a monolith channel with developing boundary layers:
NSH ) B[1 + C(dH/L)NRENSC]0.45
(13)
B and C are parameters dependent on the channel geometry and surface roughness, respectively. B is estimated to be 2.976 for square-channel monoliths, while C ranges from 0.078 to 0.095 depending on the degree of surface roughness. Holmgren and Andersson16 used experimental data for CO oxidation and fluid dynamics simulations to obtain the following expression for gas-solid mass transfer in square channels with rounded corners:
NSH ) 3.53 exp[0.0298NRE(dH/L)NSC]
(14)
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Table 1. Physical Properties of Monolith Samples material (primary phase) geometry length diameter geometric surface area wall porosity bulk porosity
honeycomb
Robo FCC and SC
Robo FCC-74
Cordierite (2MgO-2Al2O3-5SiO2) square channels, 400 cpsi (20 PPI) 10.7 mm 11.8 mm 24.5 cm2/cm3 33% 74%
R-Al2O3 FCC or SC 10.5 mm 10.7 mm 24.2 cm2/cm3 33% 43%
R-Al2O3 FCC-like 10.1 mm 10.5 mm 22.5 cm2/cm3 33% 74%
Expressions 12-14 all yield similar plots and serve as a good starting point for understanding and quantifying the mass-transfer enhancements generated by monoliths with unique geometries.
Table 2. Catalyst Loadings (mg) for the Monolith Samples γ-Al2O3 Pt
honeycomb
Robo SC
Robo FCC
Robo FCC-74
72 14
69 14
69 15
58 12
3. Experimental Section 3.1. Monolithic Supports. Honeycomb-type samples, consisting of 52 square channels arranged in a cylindrical shape, were cut from a larger extruded monolith (400 cpsi) manufactured by Corning. Additionally, threedimensional samples were prepared in-house using the “robocasting” technique. Robocasting is a novel, rapidprototyping process developed at Sandia National Laboratories that can be used to fabricate structures with controlled geometries from ceramics, polymers, metals, and combinations thereof. This process has been described in detail elsewhere.24-26 Three differently structured robocast samples, based on a series of interconnected rods, were evaluated during this study. The first has a face-centered-cubic-like (FCC) geometry of alternating rods with no line-of-sight pathways. The second has a simple-cubic-like (SC) structure with line-of-sight pathways. In all other respects, the FCC and SC samples are identical; thus, any differences in performance are the direct result of the different geometries. The third sample has a “modified” FCC structure (FCC74), wherein select rods are eliminated to obtain a bulk porosity similar to that of an extruded honeycomb. The physical properties of the monolithic samples are outlined in Table 1. Schematic representations of the robocast geometries are presented in Figure 2. In all cases, the rod dimensions and fabrication conditions were chosen so that geometric surface areas per unit volume and the wall porosities of the robocast monoliths are similar to the honeycomb sample. 3.2. Washcoating. A γ-Al2O3 washcoat was applied to the monoliths using a procedure similar to that detailed by Torncrona et al.27 The washcoat slurry (1835 wt % solids) was prepared by mixing the “solids” [25 wt % boehmite (Condea); balance γ-Al2O3 (Condea)] with the appropriate weight of deionized water. Darvan 821A (2 wt % based on solids) and methocel (6 wt % based on solids) were also added to enhance dispersion. An HNO3 solution (1.0 N) was added to the slurry to adjust the pH to 4.0. The monolith sample was immersed in the well-dispersed washcoat slurry for approximately 10 s. After waiting an additional 30 s, excess slurry was removed by gently blowing air through the monolith.
Figure 2. Schematic side-view representation of the FCC, SC, and FCC-74 geometries. Flow is from the top.
The samples were then dried and calcined in air for 2 h at 600 °C. Finally, air was blown through the monolith to remove any poorly adhered γ-Al2O3. The entire procedure was repeated as necessary to produce the desired washcoat loading. The loadings were determined by weight difference. Monolith cross sections show relatively uniform washcoat layers with no significant channel blockage. The monolith samples were impregnated with Pt by washing with a chloroplatinic acid (H2PtCl6) solution. Prior to application of the H2PtCl6 solution, the samples were dipped in deionized water and blown with air to minimize the deposition of Pt in difficult to access interior portions of the monolith walls. The samples were then dried and calcined in air for 2 h at 600 °C, and the process repeated until the desired weight loading of Pt was achieved, as determined by weight difference. Finally, the samples were calcined at 500 °C in an H2/argon environment to ensure complete reduction of the Pt to metal. The loadings of γ-Al2O3 and Pt for each of the samples examined are given in Table 2. Similar loadings of ∼3 wt % Pt (relative to the cordierite sample) were utilized for each sample. 3.3. Activity Measurements. The oxidation of CO (1% in air) over Pt was chosen as the probe reaction for mass transfer based on the high rate of reaction achievable at reasonable temperatures. Conversion was measured as a function of temperature at flow rates of 500, 10 000, and 13 000 sccm in a conventional tubular quartz laboratory reactor. Monolith samples were suspended tightly in the reactor tube by wrapping quartz fibers around the sample exterior. Gas chromatography (Poraplot Q and molecular sieve 5A columns) was utilized to quantify the reactor effluent composition. Two thermocouples were placed in contact with the monolithic sample to measure the inlet and outlet temperatures. A weighted average of the temperature16 (TCALC ) 1/3TIN + 2/3TOUT) was used for all figures and calculations presented in this work. Reactor pressures were measured upstream of the monolith with a mechanical gauge. Pressure drops across the monoliths were small compared to the reactor operating pressures resulting from flow resistance in the downstream tubing. 3.4. Pressure Drop Measurements. The pressure drop across the monolith samples was measured as a function of the flow rate in a system equipped with a differential capacitance manometer (MKS). Each monolith sample (without washcoat) was suspended in a quartz tube and tested over air flow rates ranging from 3000 to 11 000 sccm. The results were corrected for the pressure loss in an “empty” tube.
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Figure 3. CO conversion (500 sccm) over the Robo FCC (0), Robo SC (4), and honeycomb (O) samples.
Figure 5. CO conversion (13 000 sccm) over the Robo FCC (0), Robo SC (4), and honeycomb (O) samples.
Figure 4. CO conversion (10 000 sccm) over the Robo FCC (0), Robo SC (4), and honeycomb (O) samples.
Figure 6. Pressure drop across the Robo FCC (0), Robo SC (4), Robo FCC-74 (]), and honeycomb (O) samples.
4. Results and Discussion Figure 3 presents the conversion results obtained at a flow rate of 500 sccm, which corresponds to a SV of approximately 30 000 h-1. At this low flow rate, reaction kinetics should be the rate-controlling step. Thus, given the similar catalyst loadings, the results should be nearly identical for each sample. The expected behavior is verified in Figure 3; the different curves deviate no more than about 10 °C from one another. Figure 4 shows the results for these same samples at a flow rate of 10 000 sccm (SV ∼575 000 h-1). As for the lower flow, the samples all light off at a common temperature, in this case at ∼170 °C. The conversion curves also track each other as the temperature is increased, indicating kinetic rate control. However, as the temperature is increased above about 250 °C, differences that can be attributed to mass transfer become apparent. The SC geometry exhibits improved mass transfer relative to the honeycomb, as evidenced
by the higher conversion at the mass-transfer plateau. There is arguably no mass-transfer limit observed for the FCC sample; only at 99% conversion does a slight “tailing” in conversion emerge. The results for a flow rate of 13 000 sccm (SV ∼ 750 000 h-1) are presented in Figure 5. At this high flow rate, mass transfer limits the conversion over all three geometries. The improvement in the mass-transfer effectiveness of the Robo FCC and Robo SC samples is still clearly present. However, improvements in mass transfer generally come at the cost of an increase in the pressure drop. Figure 6 plainly shows that the FCC and SC geometries result in a pressure drop that is up to 1000 times greater than that of the honeycomb geometry. This is an unacceptable penalty for most applications. (A packed bed is generally
