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Ind. Eng. Chem. Res. 2004, 43, 4602-4611
Monolithic Catalysts for the Chemical Industry Thorsten Boger* Corning GmbH, Abraham-Lincoln-Strasse 30, D-65189 Wiesbaden, Germany
Achim K. Heibel and Charles M. Sorensen Process Technologies, Corning Incorporated, MP-HQ-W2-21, Corning, New York 14831
Honeycomb-shaped monolithic catalysts are the standard catalyst shape in most environmental applications. In the processes of the chemical industry, however, their current use is very limited. In this paper, the current status of the monolith technology for applications in the chemical industry is reviewed. Application areas in which monolithic catalysts have superior performance characteristics are discussed. Especially pre- and postreactors as well as replacement concepts for slurry reactors are expected to have a very good near-term potential for application in commercial operations. In addition, it is expected that several adiabatic fixed-bed processes, especially those constrained by pressure drop or mass transfer, can benefit from the use of monolithic catalysts because of their excellent pressure drop to mass transfer ratio. A new and promising area for monoliths with a high thermal conductivity is their use in various gas-phase processes where multitubular reactors are employed. Introduction Since their introduction in the mid-1970s, monolithic catalysts have become the standard catalyst shape in most environmental applications. The most prominent examples are the automotive catalytic converter and the DeNOx catalyst used in selective catalytic reduction (SCR) units used, for example, at power plants and incinerators. Annually more than 100 000 m3 of monolithic catalysts and catalyst supports are produced worldwide to meet the demand of these and other related applications.48 The term monolithic catalyst is used for a number of different structures, but we will limit the scope of this contribution to those commonly used in the abovementioned applications. Figure 1 shows a photo of a monolith composed of a large number of straight and parallel channels that extend throughout the body. These structures can be made from a range of materials.1 They are characterized by the size and geometry of the channel openings (dh), the void fraction () or open frontal area (OFA), and the cell density usually characterized by cells per square inch (cpsi) or per square centimeter. Table 1 gives some examples of structures and materials available commercially or that were made within our R&D laboratories. One of the key reasons for the success of monoliths in environmental applications is clearly their excellent ratio of pressure drop to geometric surface area. This is shown in Figure 2. Compared to any other catalyst structure such as pellets or foams, a significantly lower pressure drop is observed at a given geometric surface area. This is especially important in environmental applications because large volumetric flow rates often have to be handled. Energy consumption is an important part of the overall operating costs because there is no commercial value created and the feedstock is a * To whom correspondence should be addressed. Tel.: +49 611 7366 168. Fax: +49 611 7366 143. E-mail: BogerT@ corning.com.
waste stream. Another benefit, especially important in SCR reactors, is that the open-channel structure is capable of processing streams having high loads of particulates and dust. For automotive catalytic converters, mechanical integrity and durability are key attributes because of the thermal and mechanical stresses that occur during the many rapid changes in operating conditions. Additional important attributes for the success of monoliths in mobile applications are the low weight and low thermal mass, which is essential for a fast lightoff of the catalyst. Finally, another key factor for the relatively fast introduction of monolithic catalysts into these applications is that they were always driven by regulations. As catalysts and catalyst supports for the conversion and synthesis processes of the chemical industry, monoliths have found very limited commercial use so far. Public information indicates that monoliths are used in the production of hydrogen peroxide2 and in a postreactor for the synthesis of pththalic anhydride.3 The first example represents a selective multiphase hydrogenation reaction whereas the second one is a selective gas-phase oxidation. The limited use of monoliths in the chemical industry can be attributed to several reasons, and on the basis of our experience, the following items are some of the most important ones: (a) Today a large number of different catalysts, often with tailored properties, are employed in the many processes of the chemical industry. Often the annual volume of each catalyst is small, and it is often difficult to justify dedicated research and development efforts as well as capital investment to achieve a monolithic product with the same intrinsic catalyst properties. During the selection and evaluation of candidate monolithic catalysts, one needs to consider how much catalyst development work will be required. In the case of transferring previously developed pellet technology to monoliths, it will in many cases be required either that the benefits that come from the monolithic catalyst shape are significant enough to allow for some short-
10.1021/ie030730q CCC: $27.50 © 2004 American Chemical Society Published on Web 03/05/2004
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Figure 1. Photograph of monolithic catalyst supports with different cell densities.
Figure 2. Pressure drop vs specific geometric surface area of monolithic structures, spheres, and rings. Data are for air at 20 °C and 1 bar and a superficial gas velocity of 1 m/s (at STP). Monolithic data are for commercial and developmental products (the notation is cell density in cpsi and wall thickness in 1/1000 in.). Table 1. Examples of Monolithic Structures, Materials, and Their Usea application
material
automotive
cordierite
stationary emissions (mainly oxidation and support for SCR)
cordierite
DeNOx/SCR
TiO2/V2O5
R&D
γ-Al2O3 carbon Fe2O3/K2O3
a
cell density/cpsi 400 600 900 >1000 64 100 230 400 8-35 15-60 35-200 25-600 100-500 25-400
dh/mm
tw/mm
1 1 0.8
0.10-0.18 0.08-0.10 0.05-0.08
2.9 2.2 1.5 1 3.5-8 2.8-5.5 1.5-3.5 0.5-4
0.30 0.38 0.18 0.18
1-4
0.2-1.5
0.1-1.0
or OFA/% 70-80 80-85 83-88 82 72 80 74 ∼75 ∼75 70-80 30-80 50-75 40-80
comment commercial commercial commercial R&D commercial commercial commercial commercial commercial, coal commercial, oil commercial, gas R&D, BET 200-280 m2/g R&D, coated or extruded R&D
Notation: hydraulic diameter dh, wall thickness tw, and void fraction or OFA.
comings in the intrinsic properties (at least to get started) or that standard materials can be used as supports. (b) Manufacturing of monolithic catalysts is inherently more costly than that of other shapes such as pellets or powders. Therefore, it has to be demonstrated that the economic benefit that arises from the use of monoliths exceeds the higher catalyst cost.
(c) Monolithic catalysts are provided in the form of blocks or assemblies. This requires different reactor loading, packaging, sealing, and unloading methods compared to conventional pellet methods such as sock loading and gravity-assisted unloading via dump nozzles. Although some commercial experience with monolithic handling exists, mainly from stationary environmental installations, some further work on scale-up is required.
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An analysis of how far monolithic catalysts offer economic benefits depends on the chemical application and on the design and operation of specific reactors. In this paper, we will describe the current status of monolithic technology in chemical processing from our perspective as producers of extruded honeycomb structures. We will also discuss areas and applications within the chemical industry where we believe monolithic catalysts would make a difference compared to current technology. Our focus is on areas where an easy retrofit of the existing equipment into a monolith-based solution appears feasible. This limits the economic risk to a user and in many cases enables a cost-effective expansion of the existing capacity. Review of the Status and Research on Monolithic Catalysts for Chemical Applications The status of monolithic catalyst based technology in chemical applications has been reviewed by several authors, e.g., refs 4-7. Our intention is not to reiterate what these authors already published but to try to briefly summarize the status and to draw some generic conclusions relevant to the topic of this paper. The discussion is separated into the three main areas: (a) materials, (b) engineering and design rules, and (c) application data. In the first section, materials and engineering issues will be discussed in general terms to give an overview of the technology. Specific applications and processes for which monolithic catalysts appear attractive will be discussed in the next section. (a) Materials. With respect to materials, one has to distinguish between catalysts in which the active phase is applied in the form of a coating (e.g., a washcoat) onto an inert support and catalysts in which the whole monolithic structure is made out of the active material, e.g., bulk catalysts. Coated monoliths are usually considered when the intrinsic reaction rates are fast compared to diffusion and mass transport within the catalyst structure and the performance is dependent on the geometric surface area rather than the catalyst mass. Another purpose for using an inert monolithic backbone is to provide structural integrity to a lowstrength catalyst system. This might become especially important for very porous or weak catalyst materials. For such catalysts, the standard support material can be cordierite, a magnesium aluminosilicate (2MgO2Al2O3-5SiO2), because it is available commercially today (see Table 1). The active phase is usually applied together with a high surface area support material, e.g., γ-alumina, to achieve good dispersion of the catalyst species, e.g., precious metals, nickel, cobalt, etc. Cordierite is generally utilized in applications where the operating temperature is below 1200 °C, although short temperature excursions in excess are probably acceptable (the melting point is about 1465 °C). In this temperature range, a key feature of cordierite is its very low coefficient of thermal expansion, which allows for rapid temperature cycles and steep temperature gradients. Cordierite has some limitations in its chemical durability when contacted with alkali and alkalineearth contaminants at temperatures in excess of about 700 °C.8 With respect to acids, no degradation was observed in gas-phase applications as long as no condensation occurs.8 In the liquid phase, the use of cordierite is limited to a pH range of roughly pH 4-9. For applications that fall outside the range of conditions described above, modified support materials may be
needed. One possible alternative is to apply protective coatings 9 on cordierite. The key is to ensure that no uncoated areas remain because they could result in localized chemical attack and potential disintegration of the cordierite structure. Another option is to select other monolithic materials as the support. At Corning Incorporated, we have developed carbon and graphite monoliths10,11 for use in a highly corrosive environment and mullite monoliths12 (3Al2O3-2SiO2) as a substrate for ceramic membranes. Coating monolithic support structures with high surface area materials and metals was recently reviewed by Nijhuis et al.13 Typically, coatings with high surface area materials such as γ-alumina, silica, and zeolites are applied at an average thickness of 15-100 µm. For many types of catalysts and materials, the coating and impregnation can be viewed as state of the art. Most of the well-known catalyst companies have the capability of producing coated monolithic catalysts for chemical applications because they are also in the business of producing coated monolithic catalysts for the automotive and stationary environmental applications in large volumes. Bulk catalysts are required when the reaction rate is slow compared to the rate of mass transport to and within the catalyst structure. In this case, the productivity is directly related to the amount of active catalyst, e.g., the mass, within the reactor. Generally, the manufacturing cost of a bulk extruded honeycomb is less compared to that of a coated monolith. For bulk catalysts, one has to distinguish between those that are made directly from the active species and others that need to be impregnated with the active components. In the first case, the whole monolithic body is made out of the catalytic material, whereas in the second case, the catalyst may or may not be homogeneously distributed throughout the body of the part. Two examples of materials that fall into these categories are being developed in our laboratories. One is γ-alumina monolith14,15 with specific surface areas up to 280 m2/g [measured by the Brunauer-Emmett-Teller (BET) method] and pore volumes of 0.4-0.7 cm3/g. This material is a well-known catalyst support that can be impregnated with many different catalyzing solutions. Large γ-alumina monolithic blocks were extruded in a wide range of cell densities and with good strength (see Table 1). The other example is an extruded potassiumpromoted iron oxide catalyst monolith, a bulk catalyst material used in the dehydrogenation of ethylbenzene to styrene monomer16,17 (see Table 1). The extruded bulk catalyst also meets the requirements for physical properties and shows good catalytic activity and selectivity.16,17 In addition to the examples mentioned, we can state that the knowledge of producing monoliths from materials different from those commercially available today has made significant progress throughout the last years. It can therefore be expected that for applications with a clear economic benefit to the user as well as the producer the required monolithic materials can be made available. (b) Engineering Know-How and Design Correlations. For single-fluid-phase applications, the design of the catalyst and reactors utilizing monolithic catalysts is well understood. Under most conditions of practical interest, the fluid flow inside the channels is laminar and the engineering correlations developed initially for flow in tubes and capillaries can be employed.5 A key
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Figure 3. Photograph of gas and liquid flow in a 50-mm-diameter monolith with 400 cpsi (the channel diameter is 1 mm).
difference with conventional packed beds is that there is no radial transport of mass and axial dispersion takes place almost exclusively by molecular diffusion. The initial distribution of fluid is more stringent to achieve good reactor utilization. With respect to the use of monoliths in multiphase applications, e.g., gas and a liquid reactant, significant knowledge was built during the past decade. A major focus area was the operation of monolithic catalysts under conditions where the gas and liquid flow rates are on the same order of magnitude, e.g., gas-to-liquid ratio of ∼0.2-2. In this case, a so-called bubble train or slug flow is achieved within the channels of the monolith. This flow regime offers exceptionally high masstransfer characteristics at low-pressure drop.4-6,18-21 The high mass-transfer rate is based on two key features: (a) a very thin liquid film that separates the gas phase (a bubble) from the solid catalyst surface and (b) a rapid circulation of the liquid within the liquid slugs trapped between two bubbles.22 This ensures continuous refreshment of the liquid in contact with the solid and gas. A good distribution of the liquid and gas throughout all of the channels was achieved without major effort. Figure 3 shows a photo of the gas and liquid flow within a 50-mm-diameter monolith taken by high-speed video. Clearly, liquid and gas is welldistributed into all channels. We have also successfully operated cold-flow pilot units with diameters of up to 600 mm without problems for several months. A good
distribution of gas and liquid was confirmed by tomography measurements. Until recently,23-25 limited research was done on the multiphase operation of monoliths where the liquid flows as a liquid film along the channel walls (Figure 4).24 This mode of operation can be done in cocurrent or countercurrent flow of gas and liquid. For cocurrent flow, the operating conditions are similar to those of the trickle-bed operation today, e.g., at volumetric gas flow rates well in excess of the liquid flow. In this regime, the distribution of the liquid requires more sophisticated technologies and probably still some further development and scale-up work. For the countercurrent operation, care has to be taken with respect to the onset of flooding, especially at the outlet of the monolith. However, if designed appropriately,25 we have found that monolithic packings can be operated in countercurrent mode with structures comprising geometric surface areas as high as 1850 m2/m3. This is shown in Figures 5 and 6.50 This hydrodynamic behavior, combined with the excellent wetting characteristics and the ability to have the monolith made entirely out of an active catalyst, makes them an attractive catalyst support for applications such as reactive distillation or reactive stripping. Attractive Applications and Processes for Monolithic Catalysts Research was done on many different applications. In this paper, we want to limit the discussion to a few key technology areas of interest where we believe monoliths offer benefits that will make them economically attractive to users. As mentioned earlier, we will focus on areas in which an easy retrofit into existing equipment appears feasible. Multiphase Reactors. For multiphase applications, significant work was done, especially on various hydrogenation reactions.4,5,26-30 This was driven by the outstanding mass-transfer characteristics of monoliths and the fact that these reactions often are constrained by mass transfer. In many cases, it was found that the
Figure 4. Gas/liquid distribution for different monolithic channel geometries under film flow conditions.
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Figure 5. Flooding curves for different monolithic packings operated under countercurrent gas/liquid flow. Symbols: (b) 200 cpsi, 1849 m2/m3, 31% solid fraction, hydraulic diameter 1.49 mm; (9) 100 cpsi, 1354 m2/m3, 26% solid fraction, hydraulic diameter 2.18 mm; ([) 50 cpsi, 964 m2/m3, 25% solid fraction, hydraulic diameter 3.11 mm. The system is water/air at ambient conditions.
Figure 6. Comparison of flooding curves for monoliths and other conventional packings operated under countercurrent gas/liquid flow. The system n-decane/air is at ambient conditions.
volumetric activity of the monolithic catalyst exceeds that of conventional catalysts by a factor larger than 3. This, combined with the low pressure drop, allows operation of fixed-bed units at relatively high space velocities. In most cases, the hydrodynamic regime was slug flow. This regime requires superficial liquid velocities typically in excess of ∼0.1 m/s compared to only 0.02 m/s or less for conventional trickle-bed reactors. Although this higher liquid velocity can be a benefit for new reactors because it allows for smaller diameter reactor vessels, it makes retrofitting monolithic technology into existing trickle-bed reactors difficult. For a given volumetric throughput, the reactor cross section needs to be smaller to achieve the required liquid velocity. Pumps and compressor requirements are not increased by the higher velocity because of the low pressure drop nature of the monolithic structures. Another requirement that has to be considered for new units and revamps is that the gas-to-liquid ratio for the slug-flow regime is quite different from the one under trickle-flow operation; e.g., there is less gas per unit volume of liquid. In the case of hydrogenations, there usually is an excess of hydrogen provided by the gas phase. Under slug-flow operation, however, the gas-toliquid ratio is usually only about 10% of the ratio in trickle-flow operation. Although the excellent mass
transfer in the monolithic channels can help to better utilize the hydrogen supplied into the unit, one has to ensure that there is sufficient hydrogen available for the reaction and to manage catalyst deactivation by coking. Examples where the benefits of monoliths can be utilized and where the constraints are no problem are reactions in which only a minor fraction of the components is hydrogenated, for example, a selective hydrogenation of multiply unsaturated compounds in an olefin-containing product stream. Examples of such reactions are given in refs 26 and 42. The first example26 shows experimental data for the selective hydrogenation of olefins in an aromatic stream. The second example42 is related to a problem common in the production of styrene by dehydrogenation of ethylbenzene. In this application, the undesired byproduct phenylacetylene needs to be removed from the crude product by selective hydrogenation to styrene. In these applications, where monoliths are employed in polishing units, the high catalytic efficiency, combined with the ability to operate at very high liquid hourly space velocities, e.g., well in excess of 10 h-1, at low pressure drop, can be utilized to build very small and compact reactors. Related areas in which monolithic catalysts are proposed for multiphase reactions are concepts offering an alternative to processes based on slurry reactor technology.31-34 In this approach, a monolithic catalyst replaces the powder catalyst used today, and separation of the solid catalyst from the liquid product is straightforward and simple. These concepts also allow easy retrofits into existing equipment, and hence the required capital cost and related financial risk for the revamp are low. Furthermore, the fixation of the catalyst to the monolith prevents loss of catalyst during the entire cycle from charging to cleaning of the equipment, which can be in some cases quite substantial (economically as well as ecologically) in the case of the slurry system. Overall, significant savings in operating cost and unit availability are expected. A schematic showing three such revamp designs is shown in Figure 7. The design shown in Figure 7a is the so-called monolithic stirrer.34 Here the existing slurry reactor stirrer is replaced by a stirrer to which the monolithic catalyst is mounted. Because of its open structure, liquid is passed through the monolithic channels when the stirrer is rotating.43 Practical limitations of this design are probably the amount of catalyst that can be installed. A second approach shown in Figure 7b is based on the concept of adding a (small) fixed-bed reactor external to the existing reactor vessel.32,33 The liquid and gas is recirculated from the vessel to the add-on reactor through an external loop, and this concept is sometimes called monolith loop reactor. Some of the key advantages of a monolithic catalyst in an add-on reactor are the excellent performance and mass-transfer characteristics. As mentioned above, it was observed that for fast reactions the performance of monolithic catalysts is several times better than that for standard catalyst pellets when compared on a reactor volume basis. This keeps the size and cost of such an add-on unit relatively small. The capability of operating at high velocities without excessive pressure drop results in low operating cost for the required pumps. Under certain operating conditions, free gas suction is feasible, not requiring any gas pumping devices.32,33 Another attractive feature of this concept is that it is very flexible with respect to catalyst change-out (a simple exchange of the module), process
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4607
Figure 7. Options to retrofit slurry reactors with design concepts based on monolithic catalyst: (a) monolith stirrer; (b) external monolith loop reactor; (c) internal monolith loop reactor.
Figure 8. Alternative designs for the internal monolith loop reactor.
safety, heat management, and reduced yield losses as a result of improved catalyst contact throughout the process cycle from charging to filtration.49 A third approach, shown in Figure 7c, is to install the monolithic catalyst within a basket positioned inside the existing reactor vessel.31 An internal circulation through the monolith and the surrounding space is achieved by gas addition or mechanical agitation (internal monolith loop reactor). This concept is also very flexible in terms of its realization. Some examples of possible variations are shown in Figure 8. Many others are obviously possible as well. This concept requires only minimal modifications to existing hardware. Moreover, it might even allow operation without any mechanical stirring with
the help of a circulated gas only. The performance of the internal monolithic reactor design is essentially identical with that of an ideal stirred tank as a result of the high recirculation rates. The monolithic packing itself operates as an adiabatic plug-flow reactor. Because of the very short residence time the accumulation of heat and the conversion per pass are very low. Finally, a feature that is common to the last two slurry replacement concepts is that the immobilization of the catalyst on the monolith allows one to stage different catalyst functions in series for a controlled consecutive reaction scheme. This is a fundamental difference from the slurry system, with its chaotic flow behavior of the catalyst in the vessel.
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Monolithic structures have also been proposed as a suitable catalyst packing for countercurrent operation.44 In this area monoliths do not necessarily provide a superior separation performance as a column packing but do offer a good combination of separation, hydrodynamic, and reactive characteristics. Other catalytic packings, although commercially used for applications such as reactive distillation,45 have some shortcomings with respect to how the catalyst is presented to the fluid phases and how effectively the catalyst is brought into contact with the gas and liquid phases. In the case of the monolithic structures, an improved contact between all three phases and enhanced catalyst utilization can be achieved. The high geometric surface area and catalyst fraction, the thin liquid films surrounding the catalyst combined with a good flooding performance, and a low pressure drop25 create an attractive package for the use of monoliths in reactive countercurrent applications. The wall thickness, void fraction, and channel geometry can be tailored to the needs of the individual application. Examples of applications where monoliths have been discussed or studied are the reactive distillation of methyl acetate to acetic acid and methanol46 and the reactive stripping of water to enhance the esterification of an alcohol with a carboxylic acid.47 It has to be mentioned that these studies were somewhat hampered by the use of coated monolithic structures only. Significantly higher catalyst loadings per reactor volume (monoliths with a 50% solid fraction have been evaluated) can be achieved if, for example, extruded alumina monoliths would be used onto which the active catalyst is applied. Recently, multiphase applications of monolith-type structures have been extended to liquid/liquid and liquid/liquid/gas systems. Dummann et al.53 uses a capillary tube with an inner diameter of 0.5-1 mm to study the nitration of a singlering aromatic as an example of a liquid/liquid reaction. The capillary size is in the same range as that of conventional monolithic catalysts, therefore representing a single channel. The authors show that within the capillary the two immiscible liquids, the aromatic and the nitration acid, flow in a mode that resembles the slug or bubble-train flow described previously for gas/ liquid systems. Very good mass-transfer characteristics and a near-plug-flow behavior were observed. In the case of the partial hydrogenation of benzene to cyclohexene, it has been shown that the addition of water can significantly improve the selectivity, acting as a diffusion barrier, which allows the reactant to diffuse faster, and therefore slowing down readsorption of the product on the catalyst.51 This liquid/liquid/gas system can be implemented in a more defined manner using a monolithic catalyst, allowing one to balance activity and selectivity with a minimum addition of water, because of a very thin aqueous barrier around the solid interface of the monolith. Recent experiments52 with a model reaction system indicated the easy formation of a liquid layer in the case of a small addition of water to the organic hydrogenation reaction using a monolith. Gas/Solid Reactions. Significant work was also done in the area of gas/solid reactions in which monoliths are studied as an alternative to conventional random packings. The production of synthesis gas (CO and H2) is one area where monoliths are proposed as suitable catalyst supports.35,36 Another area where monolithic
Figure 9. Replacement of a radial-flow bed (a) by a monolithic bed operated in axial flow (b and c).
catalysts are very attractive are a postreactor or polishing reactor downstream of a main conversion reactor. This type of application provides some significant economic benefit to a user at low risk and cost. These reactors are usually operated at high space velocities to ensure the required selectivity. The low pressure drop nature of monoliths allows the integration of such reactors into existing units without creating the need to increase the hydraulic capacity of the compressors. There is also the advantage that monolithic catalysts can be operated in up, down, and horizontal flow directions. The latter may be a desirable configuration to reduce changes to existing hardware and piping but is hardly possible with random packings because of settling of the bed, which creates bypass routes around the catalyst. Examples of applications where monoliths are attractive are selective hydrogenations of C2-C4 streams, e.g., from steam crackers, or postreactors in selective oxidation processes. For phthalic anhydride, two successful postreactor installations utilizing coated cordierite monoliths exist in India.3 In these units, the monolithic reactor is integrated into the heat exchanger equipment downstream of the reactor. The main economic driver for this application is to extend the life of the main catalyst because underoxidized byproducts, which result from a deactivated main catalyst, are converted in the postreactor and the product is kept within specifications. Furthermore, the postcatalyst helps to dampen the effect of fluctuations introduced, for example, by process or feed changes. The overall economic benefit of the postreactor is quite significant because downtime for a catalyst change-out including the start-up phase is reduced and the cost to replace the main catalyst is spread over a longer time. The use of a monolith-type catalyst in a postreactor for formaldehyde production is proposed in a patent assigned to Haldor Topsoe A/S.37 In this application, the adiabatic postreactor reduces the amount of formic acid and dimethyl ether in the product. It is claimed that an adiabatic pre- or postconverter allows for an increase in capacity of 25%.38 Interesting concepts were also presented for the dehydrogenation of ethylbenzene,16,17,39 where the low pressure drop of monoliths allows one to convert the radial-flow reactors into much simpler and more spaceefficient axial-flow reactors. This is shown generically in Figure 9. In one retrofit concept, the existing reactor shell is maintained while the radial-flow internals are
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4609 Table 2. Qualitative Rating of the Technical Benefits of Monolithic Catalysts in Different Processes gas/solid and liquid/solida type of reaction hydrodynamics
reactive performance
structural
fast
low pressure drop low fouling potential catalyst bed uniformity high countercurrent hydraulic capacity better solid irrigation/utilization high catalyst load/pressure drop ratio high mass transfer/pressure drop ratio good dynamic response better temperature control (conductive monolith) enables egg-shell/thin thickness catalyst system good structural integrity low catalyst attrition
++ +++
slow +++ ++ ++ N/A N/A
++
+++b/+c
fast
slow +++ + ++ +++
+++ +
++ +++ +++
gas/liquid/solida
++ ++ +++
+++ +++ + ++ ++
N/A +
+++ N/A +++b/+c ++ +
+: small benefit. ++: solid benefit. +++: very significant benefit. b Catalyst supported on an inert monolithic backbone. c Bulk monolithic catalyst. a
removed and replaced by a support grid and a monolithic catalyst bed. Converting the bed to an axial-flow arrangement frees up approximately 30-40% extra reactor space, which can be utilized for installing extra catalyst volume or, as proposed by Liu et al.,39 installing an additional heat exchanger into the reactor vessel. This is especially efficient because the heat that is available for the endothermic reaction limits the overall performance of styrene reactors. With the additional heat exchanger, an increase in the styrene yield by more than 10% is predicted. An economic study showed that for a 200 ktpa unit the additional annual margin could be on the order of $1-1.7 million, with the higher cost for a monolithic catalyst and estimates for the capital cost for a revamp already being considered. For new grassroots units, the capital costs will also be lower because the reactor section can be built smaller. In some cases, a very compact axial-flow design might even allow the installation of only one reactor vessel with multiple beds and interstage reheating instead of current process designs with two reactors in series. The use of monoliths in an area that was so far not considered to be addressable for monolithic catalysts is described by Groppi and Tronconi40 and in a patent application by EVC.41 These authors show that monoliths with a high thermal conductivity can be used to significantly improve the operation of highly exothermic partial oxidations and oxychlorinations performed in the gas phase in multitubular reactors. The basic idea is to fundamentally change the mechanism for radial heat transport. In current tubular reactors, most of the radial heat transfer occurs through the gas phase, mainly as a result of the radial flow within the packed bed. The conduction within the solid phase of the pellets is almost negligible because only point contact exists between the pellets. The only practical method to enhance the heat transfer is to increase the flow rate. The effect of this is limited by space velocity and pressure drop, especially as the pressure drop increases nonlinearly with the flow rate. By using monolithic honeycomb structures as catalyst elements, no radial transfer of gas exists, but because the walls are connected through the whole monolithic block, the thermal conduction through the solid phase (the walls) is high if appropriate materials are used. Tronconi et al. show that significantly more heat can be removed by this mechanism than the convection processes in the case of conventional reactors. As a result, a more isothermal operation is possible, and the performance improves as a result of enhanced selectivity to the desired products. For the production of formaldehyde from methanol, a gain in yield by more
than 3.5% is predicted40 at a pressure drop that is 10 times lower compared to that of the current bead catalyst. In the case of the oxychlorination of ethylene to ethylene dichloride, the low pressure drop is especially attractive.41 This process is already operated with a very high selectivity of 98.2-98.5%. While a further selectivity improvement by 0.3% is demonstrated for a monolithic catalyst in the patent application, the lowpressure drop benefits operating costs for the significant recycle of unconverted ethylene. For new units, further advantages come from designing in smaller recycle compressors. A practical issue in this new approach that has not been addressed in the public literature so far is the heat transfer from the monolith to the inner tube wall. In heat-transfer experiments performed by Eigenberger et al.54 on various structured catalysts (Sulzer type, ceramic monolith and corrugated and wrapped metal monolith), this was found to be a bottleneck for the overall radial heat transfer. In our recent research, we have addressed these concerns, and promising results demonstrating that practical solutions exist will be published in a separate paper.55 Summary Although the commercial use of monolithic catalysts is limited today, we believe that developments that have taken place during the past few years have delivered significant new insights into how such catalyst structures can be used. On the material side, monolithic catalysts can be made with a wide range of different coated catalyst materials today, some on a commercial level. New monolithic materials are at the horizon and will be developed and commercialized as needed. The engineering knowledge for monolithic catalysts in gasphase applications has been developed over many years. Scale-up appears to be more straightforward now, especially because recent commercial applications as well as stationary environmental applications have been successfully employed in reactors ranging from several cubic meters up to 1000 m3 bed volume. Reliable engineering correlations and data are now available for multiphase applications when operated in the slug-flow regime. In the case of monolithic catalysts operating in trickle (film flow) or countercurrent mode, some data are available, but in these technology areas, additional developmental efforts are required. Before implementation of multiphase monolithic reactors on a large scale, several items deserve further attention, including the distribution of gas and liquid, loading, packaging, sealing, and unloading.
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With respect to commercial use of monoliths in chemical reactors, it does appear that a wide range of applications could benefit from the use of structured catalyst beds. This is summarized in the form of a qualitative rating with respect to the various technical benefits of monolithic catalysts in Table 2. A differentiation between processes with single and multiple fluid phases and with fast or slow reactions is made because the benefits are different for them. With respect to applications, monoliths are ideally suited for use in postand prereactors because of their excellent ratio between mass transfer and low-pressure drop. This allows for high space velocities and small reactors. In addition, these kinds of special applications tend to have lower financial risk, which is always desirable for new technologies. Similar thoughts hold for the slurry replacement concepts where we believe the reactor technology is ready, but catalyst formulations may still need further work. Results obtained with monolithic catalysts in new application areas under development such as multitubular reactors are suggesting clear technological advantages. The economic value of these advantages needs to be proven, but there appears to be good reason for optimism that we will see monolithic catalysts being employed in more processes in the future. Literature Cited (1) Williams, J. L. Monolith structures, materials, properties and uses. Catal. Today 2001, 69, 3. (2) Edvinsson Albers, R.; Nystro¨m, M.; Siverstro¨m, M.; Sellin, A.; Dellve, A.-C.; Andersson, U.; Herrmann, W.; Berglin, Th. Development of a monolith-based process for H2O2 production: from idea to large-scale implementation. Catal. Today 2001, 69, 247. (3) Eberle, H.-J.; Breimair, J.; Domes, H.; Gutermuth, T. Postreactor technology in Phthalic Anhydride plants. Pet. Technol. Q. 2000, Summer, 129. (4) Irandoust, S.; Andersson, B. Monolithic catalysts for nonautomobile applications. Catal. Rev.sSci. Eng. 1988, 30 (3), 314. (5) Cybulski, A.; Moulijn, J. A. Monoliths in heterogeneous catalysis. Catal. Rev.sSci. Eng. 1994, 36 (2), 179. (6) Kapteijn, F.; Nijhuis, T. A.; Heiszwolf, J. J.; Moulijn, J. A. New nontraditional multiphase catalytic reactors based on monolithic structures. Catal. Today 2001, 66, 133. (7) Voecks, G. E. Unconventional utilization of monolithic catalysts for gas-phase reactions. In Structured Catalysts and Reactors; Cybulski, A., Moulijn, J. A., Eds.; Marcel Dekker: New York, 1998; Chapter 7. (8) Day, J. P.; Cutler, W. A.; Boger, T. The Chemical Stability of Cordierite and Mullite as RTO Heat Exchange Materials. Proceedings of the 921st Annual Meeting of the Air & Waste Management Association, St. Louis, MO, 1999. (9) Gadkaree, K. P.; Tao, T. Metallic catalysts for nonneutral liquid media. U.S. Patent 6,455,023B1, 2000. (10) Golino, C. M.; et al. Method of making activated carbon and graphite structures. U.S. Patent 5,215,690A, 1990. (11) Gadkaree, K. P. Extruded structures from thermosetting resins. U.S. Patent 5,820,967, 1996. (12) Brundage, K. R. Production of porous mullite bodies. U.S. Patent 6,254,822B1, 1999. (13) Nijhuis, T. A.; Beers, A. E. W.; Vergunst, T.; Hoek, I.; Kapteijn, F.; Moulijn, J. A. Preparation of Monolithic catalysts. Catal. Rev. 2001, 43 (4), 345. (14) Brundage, K. R.; Swaroop, S. H. High strength/high surface area alumina ceramics. U.S. Patent 6,365,259B1, 1999. (15) DeAngelis, T. P.; Lachman, I. M. Preparation of monolithic catalyst supports having an integrated high surface area phase. U.S. Patent Re 34,853, 1990. (16) Addiego, W. P.; Liu, W. Extruded honeycomb dehydrogenation catalyst and method. U.S. Patent 6,461,995B1, 2000. (17) Addiego, W. P.; Liu, W.; Boger, T. Iron oxide-based honeycomb catalyst for the dehydrogenation of ethylbenzene to styrene. Catal. Today 2001, 69, 25.
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Received for review September 15, 2003 Revised manuscript received December 4, 2003 Accepted December 17, 2003 IE030730Q
