Preparation and properties of ceramic foam catalyst supports

PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of HeterogeneousCatalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.

345

Preparation and properties of ceramic foam catalyst supports Martyn V. Twigga and James T. Richardson b aJohnson Matthey, Catalytic Systems Division, Royston, Herts. SG8 5HE, United Kingdom bDepartment of Chemical Engineering, University of Houston, Houston, TX, 772044792, USA Ceramic foams are preformed materials used extensively as filters, advanced burners, diffusers and mixers, but catalytic applications are now beginning to appear. These include catalytic solar receivers, partial oxidation, catalytic combustion, and diesel exhaust catalytic filters.The foams are sponge-like porous structures and are made by filling flexible open-cell organic polymer foams with slurries of ceramic particles such as alpha alumina, zirconia, silicon nitride, etc.. The plastic is burned off and the ceramic sintered to yield either a positive or negative replica of the original foam depending on exact loading procedures. Mega-pore opemngs range from 0.02 to 1.5 mm, apparent densities from 0.5 to 1.5 g c m ~, porosities from 40 to 85%, and the number ofpores per cm from 4 to 30. With appropriate moulding or machining of the plastic foam, the ceramic may be fabricated ln any shape or size. Megaporosity enhances turbulence in gases flowing through the foam and produces better mixing with lower pressure drop. The foam may be impregnated with catalytic agents, with or without an appropriate washcoat. Although pressure drop correlations follow the Ergun equation, the high porosity gives much lower pressure drop than equivalent beds of particles. Mass transfer follows standard correlations but turbulent flow is seen at much lower flow rates. Heat transfer is enhanced by the superior conductivity of the web structure. These feature combine to make ceramic foams attractive possibilities for many applications. The only disadvantage is the relatively low strength, a feature which may be controlled in some cases. 1. INTRODUCTION Successful catalysts fulfill different requirements simultaneously. In addition to prolonged good activity and appropriate selectivity, a range of physical properties are important. For example, particles in a large fixed-bed reactor must not crumble, and attrition resistance of a fluidized bed catalyst must be sufficient to minimize loss of fines from the reactor but without being so abrasive that plant equipment erodes. Many catalytic and physical properties are related; an illustration is crush strength, which depends on porosity and pore structure and in turn impacts activity and selectivity through diffusion effects. These and other considerations are important when catalyst powder is agglomerated into particles by pelleting and extrusion, and many difficulties encountered are alleviated in the alternative approach of impregnating intrinsically high activity species onto preformed supports with suitable material properties. Impregnating platinum group metals on alumina is an example, but pressure drop, heat transfer, and diffusion effects are not dramatically improved because particle sizes are usually comparable and micromeritics of agglomerated powder is similar to that of most preformed supports. In fixed-bed duties, high geometric area provided by small particles xmproves low catalyst effectiveness, but the associated high pressure drop ~s

346 unacceptable, whether the catalyst is an agglomerated powder or impregnated preformed support. It therefore appears the development of new high performance catalyst systems requires complete decoupling of catalytic and physical functions to achieve maximum effectiveness, together with the use of reactors designed to take advantage of improved activity, heat and mass transfer, etc. To a considerable extent, coated monoliths achieve this, most notably with autocatalysts where strength, vibration resistance, flow patterns and low pressure drop are provided by a monolithic structure and optimized catalysis is afforded by special coatings. Since their introduction, they have been developed into highly reliable systems whose overall performance far exceeds that of conventional systems based on pellets. A related approach could involve open cell, high porosity ceramic foams, similar to those used for filtering molten metals. When fabricated in shapes and coated with suitable ceramic formulations, they should have physical advantages similar to monoliths, with additional benefits from internal radial transfer. The resulting high effectiveness could be exploited in reactors designed for high transfer coefficient conditions and high catalyst effectiveness. Although these potential benefits have been known for some time, relatively little has been published on ceramic foams in catalytic roles. Here we review the pertinent information available and detail some recent work on fundamental properties that demonstrate specific advantages of foambased catalysts. 2. PREPARATION AND PROPERTIES

2.1 Ceramic foam morphology Open-pore ceramic foams are materials with high temperature resistance, low bulk density, and tortuous flow patterns, together with high open porosity [1]. This porosity, which varies from 40 to 85%, is formed from megapores .04 to 1.5 mm in diameter. Characteristic parameters include cell size, window size and surface area, all correlated with the number of pores per centimeter. Each cell connects with adjacent cells through the windows. The megaporosity provides a tortuous path for internal gas flow, and turbulence is much enhanced. This results in forced convective flow within the structure, a feature known to produce beneficial performance over conventional pellets that allow only diffusional transport through meso- and macro-pores [2-7]. The high porosity also provides much lower pressure drop. Higher thermal conductivity is expected from the continuous web-like structure of the foam, thereby providing improved heat transfer into and throughout the foam. These features were first applied in the development of molten metal filters [8,9], and later to catalytic combustion devices [11-13]. Catalytic applications are now appearing as the combined advantages of these structures become apparent. Specifically, these advantages are: (1) Preformed fabrication that provides shapes matching the application and allowing easy reactor loading. (2) High surface to volume ratios, simulating very small particle diameters and giving high activity with low diffusion resistance. /43/High porosity, leading to low pressure drop at high flow velocities. Increased thermal conductivity and better convective heat transfer. 2.2 Preparation techniques Commercial preparation of ceramic foam starts with a foamed organic precursor having the same porosity as the desired final product [14-19]. The most common organic precursor is polyurethane, which is available in the form of flexible, open cell foams with pore sizes ranging from 4 to 30 pores cm "1 (10 to 80 pores inch-l). However, other organic plastics, such as polyolefins, are equally suitable. The pores of the organic precursor are then filled with an aqueous slurry of the desired ceramic. This typically

347 comprises a 20 wt % mixture of ceramic particles (0.1 to 10 ~ m in diameter) in water, together with appropriate amounts of wetting agents, dispersion stabilizers and viscosity modifiers. A list of ceramics successfully formed into foams is given in Table 1. Table 1 Suitable ceramics for foam production alpha-alumina alumina silicate zirconia toughened alumina stabilized zirconia (Ca, Mg, La) mullite calcium aluminate titania kaolin haematite magnetite The original plastic foam can be fabricated in a wide variety of geometric shapes and dimensions, e.g. cylinders, rings, rods, or custom-designed configurations. These are produced either during fabrication of the plastic foam or by appropriate machining or pressing from sheets of the material. These structures are immersed or soaked in the ceramic slurry, if necessary with agitation, to ensure proper filling of theplastic ~ores. Alternatively, blocks of the foam material may be filled with the slurry and then s aped to give the required geometry. Figure 1 shows examples of commercially available shapes. "lWo variations in the procedure are possible at this point, leading to ceramic foams with different characteristics. In the first, the organic foam is impregnated with a relatively low viscosity slurry and the excess slurry removed by blowing air through the foam or by compress~'ng the foam in one or more stages. The impregnated foam is then dried at 100~ or below, leaving a coating of ceramic on the plastic, and calcined at temperatures above 1000~ This last treatment removes the organic precursor through vaporization and combustion and causes the ceramic to sinter. The resulting cerarmc foam is virtually a ceramic copy or positive image of the plastic foam skeleton, with filaments havin~ a hollow core. Bulk densities are low, porosities high, but mechanical strength is relatwely low. In the second procedure, a negative image of the plastic foam is created by using thixiotropic ceramic slurries with increased viscosity, e.g. through addition of thickening agents [19]. The foam is shaken or vibrated to remove excess slurry at the external surface of the structure, but no air blowing or compression is used so that the organic pores remain substantially filled with the ceramic. Upon calcination, the plastic is removed, leaving pores that correspond to the original organic material. Pore diameters are smaller than the previous method, bulk densities are higher and porosities lower, but mechanical strength is higher. For applications where high surface area is not important, the ceramic foam may be loaded with catalytic agents through single or multiple impregnation of suitable salts, followed by heat-treatment at moderate temperatures. Alternatively, a washcoat may be added with the same procedures used for monolithic substrates [20]. Surface areas increase from less than 1 m 2g 1 to above 30 m 2g-,1 depending on the amount of washcoat added. In this way, washcoated foams have been loaded with metals and oxides [18,19], zeolites [20] and carbon [21]. Other methods, e.g. chemical vapor deposition, have also been suggested [23,24].

348

Figure 1. Examples of available shapes in preformed ceramic foams (courtesy Hi-Tech Ceramics, Inc., Alfrea, N.Y., U.S.A.) 2.3 Properties of the ceramic foams Typical properties of positive-image, non-washcoated foams are given in Table 2. Pore diameters were determined by scanning optical micrographs. Bulk densities are low and porosities remarkably high, even with a wide variation of pore size. A comparison between a positive- and negative-image foam is given in Table 3. The positive image foam is more porous and has half the bulk density of the negative-image, but the crushing strength is lower by a factor of twenty. The effect of adding a washcoat is demonstrated in Table 4. The pore diameter decreases slightly and the porosity is sufficiently lower, suggesting some of the smallest pores may be totally blocked with washcoat. As expected, the surface area is drastically increased. The stability of the washcoat is demonstrated by the surface area after a thermal treatment at 1000~ for 4 h, showing only a 13% decrease. In another test, the washcoated ceramic foam was loaded with 0.7 wt% Rh by impregnation with RhC13. 3H20 solution. The catalyst was sintered at various temperatures for 2 h and ttie results are given in Table 5. Dispersion of the metal is fairly constant until above 600~ dropping considerably at 1000~ These results demonstrate that ceramic foams may be effectively washcoated with efficient stabilization of supported catalysts. 2.4 Characteristic length Particles in packed beds are usually characterized in terms of the equivalent diameter, dp, i.e. the diameter of a sphere with an equal surface to volume ratio as the

349 Table 2 Typical properties of ceramic foams a Ceramic: 92% ~-A1203, 18% mullite Positive-image No washeoat Pores cm 1

Pore diameter mm

Bulk density g c m "3

Porosity %

4

1.52

0.51

87

8

0.94

0.61

85

12

0.75

0.66

83

18

0.42

0.65

84

26

0.29

0.70

81

31

0.21

0.67

83

a Courtesy Hi-Tech Ceramics, Inc.

Table 3 Comparison of positive- and negative-image ceramic foams a Form: cylinders, 1.12 cm diameter, 1.30 cm length Ceramic: 0t -A]203 Original pore size: 7 pores cm 1 Property

Positive-image

Negative-image

Bulk density, g cm3:

0.75

1.54

Helium density, g cm3:

3.97

3.98

Porosity, %:

81

61

Horizontal crushing strength, kg:

11

230

a Reference 19

350 Table 4 Effect of adding the washcoat a Form: cylinders, 1.27 cm diameter, 2.54 cm length Ceramic: 92% a-AL,O,, 18% mullite Original pore size: 1~2~ores cm "1 Washcoat: 6 wt% hydrated alumina Property

No washcoat

With washcoat

Pore diameter, mm

0.759

0.734

Bulk density, g r

0.66

0.70

Helium density, g cm "3

3.45

3.45

Porosity, %

83

78

Surface area, m2g-1 Fresh 1000~ 4 h

1.0 -

4.6 4.0

"3

a Reference 25 particle. This term is then used in calculations of the Reynolds number, pressure drop, mass transfer coefficient, heat transfer parameters, and effectiveness factor. The most obvious length to use for ceramic foams is the average pore or cell size, d c, which is usually determined by examining enlarged photographs of cross-sections of foam pellets. The pores have a distribution of sizes and shapes, are interconnected and tortuous. Nevertheless, it is convenient to assume that d c represents the cylindrical form of the hydraulic diameter: de = 4 x wetted surface/wetted perimeter

(1)

which gives dc = 4 e / a

(2)

where e is the porosity and a the surface area per unit volume. Typical values of a based on the data in "I~able 2 ~re given in Table 6. The inherent appeal of the foam i~ apparent. For example, the 12 pore cm 1 foam has an equivalent diameter of 1.4 mm, yet it would be impossible to pack a bed ofparticles this small with a void fraction of 0.83. Particles this small typically exhibit a voidfraction of about 0.30-0.35 rather than the 0.83 for the foams. Pressure drop, which is dependent on porosity, will be higher. Thus the foam offers high porosity with all the inherent advantages of small diameter particles.

351 Table 5 Stability of the ceramic foam loaded with Rh a Form: cylinders, 1.27 cm diameter, 2.54 cm length Ceramic: 92% a-A1,O,, 18% mullite Original pore size: ~2 ~ores cm "1 Washcoat: 6 wt% hydrated alumina Wt% Rh: 0.7 b Calcination Temperature,~

Dispersion c %

400

26.3

600

24.5

800

15.5

1000

7.2

a Reference 25 b Measured by oxygen uptake of the reduced catalyst at 500~ c Measured by hydrogen uptake at 25~ Table 6 Surface area per unit volume for typical ceramic foams Ceramic: 92% a-ALOe, 18% mullite Positive-image, no ffas~acoat Pores cm 1

Pore diameter

a

mm

cm 2 cm 3

4

1.52

22.8

8

0.94

36.2

12

0.75

44.2

18

0.42

80.0

26

0.28

112

31

0.21

158

352 3. PROVEN APPLICATIONS From the above discussion, foam catalysts are expected to display maximum advantages in reactions which are chemically fast but suffer significant diffusion limitations. Compared with conventional porous catalysts in such situations, selectivity is also likely to be markedly improved with foam catalysts that minimize secondary reactions leading to by-products. In the following section, most of the published information on foam catalysts is reviewed. The common theme that emerges vindicates these suggestions; the only exception concerns catalyzed diesel particulate traps, which use the filtering properties of foams. 3.1 Ammonia oxidation

Selective oxidation of ammonia to nitric oxide, a key step in nitric acid manufacture, is conventionally carried out over a pad of Pt/Rh wire gauze from 800-1100~ with extremely short contact times to give high selectivity. This situation should be well suited for foam-based catalysts, and such novel systems have recently been reported [26]. Examples include Pt (about 10 wt%) on mullite foam. As predicted, the foam-based catalyst works well with the claimed advantages of using less than 15% of the amount of t~ineCOn~entiona~gau~es and emler~afiOneOf'~hot SPrOotS~ttho~t~o behaerCOnSeesqUee~egf ry P P g 9 p ' g Y "g p P and higher thermal mass, which helps dampen out hot spots. It will be interesting to see how this approach develops, one practical limitation might be migration of catalyst poisons (impurities) from the foam material caused by high temperature operation. 3.2 Catalytic combustion

Foam-based catalytic combustion was one of the earliest applications, and a number of novel designs have been suggested [27-30]. These include combustion of hydrogen [27], light hydrocarbons [28, 29], and natural gas [30]. Ceramic foams are attractive for combustion devices because of improved turbulence and mixing, together with preformed shapes configured to suit specific equipment. 3.3 Partial oxidation

An important application has recently appeared for foam catalysts in partial oxidation, both for synthesis gas production and oxydehydrogenation [30-34]. Here the emphasis is on very short contact times that allow highly selective reactions with good temperature control and efficient mixing. These examples demonstrate the selectivity provided by foam catalysts in partial oxidation. There should be many more similar applications in the future. 3.4 Steam reforming Steam reforming of natural gas and light hydrocarbons to synthesis gas is another highly diffusion-lima'ted reaction operating up to 1000~ In addition, the process is endothermic and heat transfer limited, with consequential pressure drop problems in the large array of parallel tube reactors. It is expected that foam-based catalysts will have decided advantages over conventional large-diameter particles. This has been shown to be the case in an application in which foam catalyst particles were of comparable size to conventional catal),sts, unlike the larger foam structures previously discussed [19]. This takes advantage otthe forced convection in the foamed pellets to provide higher activity and lower pressure drop. For example, overall heat transfer was increased by about 10% and pressure drop reduced by 25%. With diffusion limitations removed, the catalyst was

353 better heat transfer through the ceramic solid structure. These features are still being explored. Nevertheless, it is clear ceramic foam supports are superior for reactions with high activity and low effectiveness factors. I

1500

1

I

1000 r

o

E I-tv"

500

-

(K 0 -500

600

Z

Pellets

700

800

TEMPERATURE,

900

1000

~

Figure 2. Comparison of catalytic CO2-CH 4 reforming on Rh-loaded ceramic foams and pellets 4.2 Pressure drop measurements

Pressure drop through the catalyst bed is an important factor in reactor design. This is especially important for heat-transfer limited reactions, such as steam reforming, for which long, narrow reactor tubes are required. Ceramic foams, with their large porosities, promise substantially lower pressure drop. Although this has in fact been shown, very little fundamental investigation on pressure drop correlations has been reported. As part of the University of Houston program, pressure drop-flow rate measurements were made for a large number of ceramic foams with varying pore size. Typical results are shown in Figure 3. Pressure drop was measured with a water manometer across the length of a single pellet 2.5 cm in length and 1.25 cm in diameter, packed tightly into a quartz tube. Measurements were made using air at 25~ The curvature of the data in Figure 2 shows that flow is non-Darcian and most probably turbulent. The Forcheimer dependence found by Philipse and Schram is confirmed [52]. We fitted the data with the Ergun equation [53] expressed in terms of the hydraulic or pore diameter: DP/L = [6.667(1-e )vU/d c + 0.1167dgU2]/e dc

(3)

where DP/L is the pressure drop (Pa cml), e the porosity, v the viscosity,, d the gas density de the pore size and U the gas velocity. The best fit to Equation 3 was fobnd with constants of 7.227 and 0.1378 respectively instead of the Ergun values. This may have been due to uncertainties in the values of ~ and d. These studies reveal the classical Ergun equation is an adequate representation for pressure drop estimations through bulk ceramic foam structures. However, it should be

354 more active at higher temperatures, but this full potential may not be realized due to heat delivery limitations of conventional reformers. 3.5 Auto and diesel exhaust Among the first reports (more than 30 years agol) of foam catalysts was the oxidation of residual hydrocarbons in vehicle engine exhaust gas by a vanadized ceramic foam catalyst [14]. The autocatalyst area has developed remarkably since those pioneering days of simple oxidation catalysts, and work on foam catalysts have appeared [35,36]. Three-way autocatalyst formulations have been applied to ceramic foams with apparently acceptable results [15, 37-39], and it is not obvious from published work why foams have not competed successfully with monolithic catalysts. Perhaps this is due to physical considerations such as strength and vibration resistance. Catalyzed ceramic foam as a catalytic diesel particulate filter is an application different from other systems discussed in this paper. Here high temperature resistance and low bulk density are important, but tortuous flow paths together with open porosity are key features. As with conventional monolithic catalysts, cordierite foams have been used because of the very low thermal coefficient of expansion [40-45] 3.6 Solar processes An interesting application was a recent test in the solar CAESAR project, a joint U.S.-German program [46-50]. A parabolic foam volumetric receiver was fabricated, 65 cm in diameter and 5 cm thick and mounted in a quartz reactor situated at the focal point of a solar furnace. The foam, loaded with 0.2 wt% Rh, was then used to absorb solar energy to drive the CH.-CO. reaction as a means of storing solar energy. The foam was a very good absorber" of ~olar energy, a fact consistent with excellent foam performance in radiant heaters, and satisfactory results were obtained proving the feasibility of solar applications. This feature could be important in conventional uses in which a foam-based catalyst structure is heated by radiant infra-red energy.

4. RESEARCH AT THE UNIVERSITY OF HOUSTON Research at the University of Houston has been devoted for a number of years to catalytic applications of ceramic foams. These include solar receivers, steam- and CO 2reforming of methane, and catalytic conversion of potentially hazardous wastes to useftil products. The following examples have been selected to demonstrate the relevent properties of ceramic foams as catalyst supports. 4.1 CH4-CO 2 reforming experiments Rhodium has been shown to be an effective catalyst for CH.-CO. reforming without carbon formation [51]. This is similar in many ways to ste~am ~eforming and low effectiveness factors, and heat-transfer limitations are usually encountered. A comparison between a Rh-loaded ceramic foam and a conventional pellet catalyst with the same amount of metal was made and the results are shown in Figure 2. The ceramic foam described in Table 5 was cut into small segments and used in a differential reactor to measure the rate of reforming at different temperatures. The same procedures were carried out with a 3-mm "egg-shell" pellet containing 0.5 wt% Rh/A120 3. Both catalysts were heated at 1000~ before the measurements. Dispersions were almost identical at 7.2% and 11.0% respectively. The CO./CH. ratio was one. Figure 2 shows a large difference between the two supports. Fo~ example, at 900~ the foam has a rate of 12~0 mol h 1g R h ,1 whereas the perle.ted catalyst is 100 mol h-1g R h .1 Expressed as turnover numbers, this corresponds to 497 and 27 molecules s 1 site-1 respectively, a factor of 18 different! An obvious explanation for this difference is the foam has a very high effectiveness factor compared to the pellet, but there could also be an enhancement in rate due to

355 remembered that particles with the same equivalent diameter would not pack into a reactor bed at the same voidage as the foam, so that the pressure drop is substantially higher. 1400

1200 E o

1000

o ra

o. 0

o~ =) o3 o~

'"

o~ o.

800

600

400

200

IV"

,•r"•lwllr'•l

0

-

0

100

I

200

I

300

I

400

I

500

I,

600

700

VELOCITY, c r n / s

Figure 3. Pressure drop in a 12 pores cm1 ceramic foam 4.3 Mass transfer correlations

Predicting mass transfer coefficients is important in designing ceramic foam applications, yet no systematic investigation of these parameters has been reported. We adopted the procedure of measuring catalytic conversions under conditions deliberately selected to ensure mass transfer limitations [54]. The reaction used was the oxidation of carbon monoxide over platinum catalysts. The washcoated foam described in Table 4 was loaded with 5-10 wt% Pt using chloroplatinic acid impregnation procedures. A thin section (0.318 cm) was wrapped in quartz wool, tightly fitted into a quartz tube and surrounded on each side with quartz wool packing. The reactor was operated differentially with 5 vol% CO in oxygen in the temperature range 200-600~ Tests confirmed the system was operating in an external diffusion-controlled regime. Rate data were taken at 550 C for increasing velocities, and the mass transfer coefficient, kc, calculated assuming first order dependence. Fluid properties were used to find the mass transfer factor, Jd, which was then correlated with the Reynolds number, Re., based on the pore diameter. Results are shown in Figure 4. The data correlated" well with Equation (4), which is within accepted range of precision for the hydraulic equivalent of e Jd = 0"326R% "~

(4)

the popular Satterfield equation (0.487R%~ This agreement indicates that standard correlations for mass transfer coefficients are acceptable for ceramic foams.

356 4.4 Heat transfer correlations Heat transfer into foams is expected to be higher than packed particles because of added conduction through the struts and forced convection into the pores due to their larger size [55, 56]. This was tested in a series of experiments in which four pellets (each 10

~

10

0

|

i

i

|

|.

-1

-2

I

10 0

!

n

I

I

I

!

t

I

10 1

~

t

~

t

t

t

n

I

10 2

Re h Figure 4. Mass transfer factor correlation for a 12 pores cm "1 ceramic foam 12 pores cm 1 maintained at temperatures transfer into coefficient, h and flow rate h

and 2.54 cm in length) were loaded into a 1.25 cm diameter quartz tube a constant temperature between 500~ and 850~ and inlet and outlet were measured at increasing flow rates. Using a 1-D model for heat the foam, the functional form of the wall convective heat transfer , was adjusted until the best overall fit was obtained over the temperature range. This relationship is given by Equation (5) where kg is the thermal

= 0.755kgReh~176

(5)

conductivity of the gas. Comparison of Equation (5) with similar expressions for packed beds is difficult since particles with similar diameters to the foams have much lower porosities. This was done more effectively in simulation calculations described in the next section. 4.5 Model comparisons Precise comparisons between packed beds and ceramic foam structures are complex since many factors - activity, effectiveness factors, mass and heat transfer, and pressure drop - are all interdependent. We have simulated the performance of a conventional steam reformer and compared it to one containing a ceramic foam cartridge loaded to achieve equivalent intrinsic activity per gram of catalyst. A 1-D model developed and tested previously for heat-pipe reformers with isothermal walls was used [57]. Pressure drop, mass transfer and heat transfer correlations for the packed bed were known to be accurate for commercial catalysts; those used for the foam were determined in the studies described above. Process conditions and results are given in Table 7. The most dramatic result is a decrease in the required length of the reformer tube by about a factor of two. This is a consequence of the higher effectiveness factor and heat transfer properties of the foam. The higher porosity gives a decrease in the pressure drop

357 for the same tube length of a factor of three. The smaller bed required for the foam is an added advantage, decreasing pressure drop by almost a factor of ten. These advantages promise substantial reduction in reformer size, capital costs, and operating conditions. It must be emphasized, however, that these benefits may only be realized with reformers having higher heat transfer coefficients than conventional radiant or convective systems. The impact of lower mechanical strength for the foam remains to be addressed.

Table 7 Simulation comparison between a conventional packed bed reformer and a ceramic foam cartridge. Tube diameter, cm: 10 CH. flow, mol hX: 1500 H.t)/CH. ratio: 3 W~all temperature, ~ 800 Inlet temperature, ~ 550 Pressure, atm: 20 Convention catalyst: multi-hole cylinders Ceramic foam: 1:~pores cm"1 Property

Conventional

Foam

Tube length for reaction, cm:

916

439

Effectiveness factor at outlet:

0.05

1.00

Average heat flux kW m':

31.2

67.1

Pressure drop atm:

1.21

0.14 (0.40) a

a for the same tube length as the Conventional Reformer 5. FUTURE DIRECTIONS The potential benefits of foam-based catalysts have been adequately demonstrated. The most important attributes are decreased diffusion limitations, lower pressure drop, increased heat transfer, improved mixing, and prefabrication of special shapes. Ideal processes are highly exo- and endothermic reactions and those requiring good selectivity control. Other novel applications, e.g. in trickle bed reactors, will no doubt appear. The main disadvantage is the relative weakness of ceramic foams. Although some work on the elastic and mechanical properties has appeared [58-60] very little attention has been given to improving these properties. Possible alternatives are negative-image foams [19] and incorporation of additives into the ceramic [61].

358 6. ACKNOWLEDGMENTS Research at the University of Houston reported in this paper was supported by Sandia National Laboratories, Albuquerque, N.M., U.S.A. under contract No: 55-4032 and by the Texas Higher Educational Coordinating Board ATP Program, Grant No: 003652121 ATP. We are grateful for the contributions of M. Garrait, D. Remue, and J-K Hung. REFERENCES

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