Catalyst extruded honeycomb

The ceramic wall-flow filter is an innovative extension of the extruded honeycomb catalyst support described in Chapter 2. This filter concept, shown in Fig. 5. involves having the alternate cell openings on one end of the unit plugged in checkerboard fashion. The... 

Zeolite catalysts, either in extruded honeycombs or washcoated on honeycomb substrates, show good catalytic activity for methanol conversion. 

Tronconi and co-workers (98,116) have validated against experiment a more complex, heterogeneous, transient model, accovmting also for diffusion and reaction of NO and NH3 inside the porous walls of extruded honeycomb SCR catalysts. The model equations are presented in Table 5 x and z are the intraporous and axial coordinate, respectively is the ammonia adsorption capacity of the catalyst 6 is the NH3 surface coverage is the effective intraporous diffiisiv-ity s is the monolith wall half-thickness i is the gas velocity in the monolith channels are gas-solid mass transfer coefficients and dh is the hydraulic diameter of the monolith channels. Notably, a pseudo-steady-state assumption... 

A mathematical model for numerical simulation of onboard SCR monolithic converters has been developed in our group in cooperation with Daimler, as outlined in References (144-148). It represents an evolution of the fully transient, heterogeneous model of a single monolith channel (98,116), whose equations are summarized in Table 5. As opposite to many of the other literature models, this is a spatially 2D (ID -I- ID) model, accounting also for diffusion and reaction of reactants and products inside the porous walls of extruded honeycomb SCR catalysts. It should be emphasized that simplified surface reaction models omitting to describe the complex, rate-controlling dynamics of NH3 intraporous dif-fusion/adsorption/reaction may yield significant errors under certain conditions, especially in the case of extruded monolith catalysts. 

Fig. 1. Prediction of the model for 10 and 100 CPSI honeycomb reactors extruded with the ViOs/sulfated Xi02 catalyst. (—, prediction with the parameters estimated from the experimental data over a packed-bed flow reactor —, prediction with the parameters estimated from the experimental data over a honeycomb reactor).

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

Tronconi et al. [46] developed a fully transient two-phase 1D + 1D mathematical model of an SCR honeycomb monolith reactor, where the intrinsic kinetics determined over the powdered SCR catalyst were incorporated, and which also accounts for intra-porous diffusion within the catalyst substrate. Accordingly, the model is able to simulate both coated and bulk extruded catalysts. The model was validated successfully against laboratory data obtained over SCR monolith catalyst samples during transients associated with start-up (ammonia injection), shut-down (ammonia... 

Cordierite [12182-53-5/, Mg4Al4Si5018, is a ceramic made from talc (25%), kaolin (65%), and A Oj (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and more recently for automotive exhaust catalyst substrates. In the latter, the cordierite raw materials are mixed as a wet paste, extruded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

Types of SCR Catalysts. The catalysts used in the SCR were initially formed into spherical shapes that were placed either in fixed-bed reactors for clean gas applications or moving-bed reactors where dust was present. The moving-bed reactors added complexity to the design and in some applications resulted in unacceptable catalyst abrasion. As of 1993 most SCR catalysts are either supported on a ceramic or metallic honeycomb or are directly extruded as a honeycomb (1). A typical honeycomb block has face dimensions of 150 by 150 mm and can be as long as one meter. The number of cells per block varies... 

There are three important outstanding issues that need concerted R D. The first issue is the further development of materials with thermal stability under combustion condition at the high temperatures prevailing in a gas turbine combustion chamber. Promising materials have been developed, but none fulfills the demand of a lifespan of at least 1 year. Besides, the most promising materials, such as the family of the hexa-aluminates, must be available in a honeycomb monolith shape, either as washcoat or directly extruded. Much work still needs to be done to optimize the preparation of monolithic thermostable catalysts. 

Tlie conversion and product selectivity is optimized when the gas mixing is improved, which reduces the boundary layer thickness at the catalyst surface and increases the mass transfer coefficient for a given channel dimension. The laminar flow through the honeycomb channels in the extrudate monolith results in a relatively thick boundary layer... 

Catalysts in thin-wall honeycomb form offer the advantages of low pressure drop, high geometric surface area, and short diffusion distance as compared to conventional pellets and beads in fixed bed reactors (1). Active zeolite catalysts may be extruded in the form of a honeycomb structure or they may be washcoated on ceramic honeycomb substrates. The latter technique has been widely used in automotive emissions control (2), woodstove combustors (3), control of volatile organic emissions from organic solvents (4), ozone abatement in jet aircraft passenger cabins (5), and N0x abatement... 

Catalytic reactor, Figure 7, with catalyst substrates made of a ceramic honeycomb of an extruded oxide ceramic material inside the catalytic section envelope made of silicon nitride material. 

Selective Catalytic Reduction (SCR) using ammonia as the reductant provides NOx reduction levels of greater than 80%. Three types of catalyst systems have been deployed commercially noble metal, base metal and zeolites. Noble metals are typically washcoated on inert ceramic or metal monoliths and used for particulate-free, low sulfur exhausts. They function at the lower end of the SCR temperature range (460-520°F) and are susceptible to inhibition by SOx [14]. Base metal vanadia-titania catalysts may either be washcoated or extruded into honeycombs [11]. Typically washcoated catalysts are only used for treating particulate-free, clean gas exhausts. Extruded monoliths are used in particulate-laden coal and oil-fired applications. The temperature window for these catalysts is 600-750°F. Zeolites may also be washcoated or extruded into honeycombs. They function at relatively high temperatures of 650-940°F [15]. Zeolites may be loaded with metal cations (such as Fe, Cu) to broaden the temperature window [16]. 

As an example for technical ceramic compounds the figures 17, 18, 19 offer the same grafics for a honeycomb compound to produce catalysts with 300 cells per square inch on a extruder with auger diameter 180 mm (throughput 2801/h, pressure 150 bar). Hence of the 150 bar at the pressure head the compound is very stiff. The yield point calculates to 0.3 [bar], which is double to the brick compound and the Bingham viscocity with 2.2 [bar s] is more then triple than the brick compound. 

Typical ceramic materials produced on a co-rotating twin screw extruder are for example catalyst carriers. They are commonly shaped into granules for use as bulk material in reactors in the chemical industry or into honeycombs for catalytic converters in automobiles exhaust systems (Fig. 12). After extrusion, the catalyst carriers are cut oversized in the lineal direction, dried and then cut to the proper length. Afterwards the binder is removed and the carriers are calcinated or sintered. Finally, to provide them with catalytic properties, they are impregnated with an active film in a bath [Fri76]. 

Fig. 18 Extruder cascade for producing honeycomb catalyst carriers...

The fundamental characteristic of a piston extruder which distinguishes it from a screw extruder is the intermittent mode of operation, from which all essential benefits but also disadvantages derive. There are clear benefits for particular applications such as in the laboratory or in producing large honeycomb catalyst converters in other fields the piston extruder has to be ruled out as its disadvantages are of too greater significance for those applications. 

Extrusion is also used to produce the alumina shells for sodium vapor lamps and the honeycomb-shaped catalyst supports for automotive emission-control devices (see Chapter 37). The catalyst supports are designed to give a high surface area and can consist of hundreds of open cells per square centimeter with wall thicknesses <100 pna. To produce these shapes, cordierite ceramic powder is mixed with a hydraulic-setting polyurethane resin. The mix is extruded into a water bath at a rate that matches the rate of cure of the polyurethane (about 2mm/s). It is then fired to produce the final ceramic. 

Honeycomb monoliths are obtained by extruding a paste made by catalytic material, whereas plate catalysts are made by depositing the catalytic material onto a stainless steel net or a perforated metal plate. Composite ceramic monolith catalysts, consisting of a monolith matrix made of cordierite coated with metal oxide SCR material, are also offered. However, they may suffer adhesion problems in the presence of dust and their use may be preferably limited to clean environment. Coated metal monoliths are constructed of thin metal foil and are characterized by large cell densities. In view of this, they are used exclusively in dust-free applications. 

Inter- and Intraphase Mass Transfer Limitations in the DeNOx Reaction. It is well established that in both laboratory and power plant conditions, extruded monolithic SCR catalysts work under combined intraparticle and external diffusion control because of the high reaction rate and of the laminar flow regime prevailing in the channels of the monolith catalysts. As an example. Figure 12 points out that, for the same reaction conditions, different extents of NO reduction are observed over SCR honeycomb catalysts with identical composition but different channel openings. 

Effect of Monolith Geometry. Technical constraints are often imposed on the design of the monolith honeycomb geometry by the extrusion process, as well as by the mechanical properties of the extrudate the specific SCR application (eg, high dust vs low dust) is also crucial for the definition of the catalyst geometric features. Here, attention is paid to the influence that the monolith parameters (channel shape, wall thickness, and channel size) exert on both the DeNO reaction and the SO2 oxidation to advance guidelines for optimization of the catalyst geometry. 

Figure 13.1.4 shows the thermal expansion of extruded cordierite from 40 to 800°C, in comparison with other ceramics. No material is equal to cordierite in thermal expansion properties. Hence, extruded cordierite honeycombs having extremely low thermal expansion, are widely used as automotive catalyst substrates, diesel particulate filters, and heat exchangers, where high thermal shock resistance is required. 

Due to durability requirements of catalysts for long-term vehicle use such as 120 000 miles or so, extruded cordierite honeycombs with low thermal... 

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