Wall thickness, monoliths

Two ways to reduce the diffusion length in TBRs are 1) use of smaller catalyst particles, or 2) use of an egg-shell catalyst. The first remedy, however, will increase pressure drop until it becomes unacceptable, and the second reduces the catalyst load in the reaction zone, making the loads of the TBR and the MR comparable. For instance, the volumetric catalyst load for a bed of 1 mm spherical particles with a 0.1 mm thick layer of active material is 0.27. The corresponding load for a monolithic catalyst made from a commercial cordierite structure (square cells, 400 cpsi, wall thickness 0.15 mm), also with a 0.1 mm thick layer of active material, is 0.25. 

A first data set was measured on a heavy-duty diesel engine test bench. Extruded monoliths with 300 cpsi, a wall thickness of 0.32 mm and a diameter of 144 mm were used in these runs. By varying the number of monolith catalyst samples it was possible to test different catalyst volumes (25, 32 and 43 L). The complete SCR catalyst setup resulted in SVs ranging from 21,000 to... 

Initially, packed beds were also used. They, however, were no success, and at present monoliths are applied exclusively. This should not be misunderstood. Monolith means literally a single stone. However, metal-based analogues are also included in the definition of monolith. In fact, for catalytic converters in cars, in addition to ceramics, metal-based monoliths have been and still are used. A major advantage of metal was the thin wall thickness that could be achieved. Later, industry succeeded in manufacturing ceramic structures of comparable wall thickness. In view of their higher resistance against corrosion, ceramic monoliths are now more generally applied than metal ones. 

If the aim of the catalytic process is to optimize yield and selectivity, one can distinguish two extremes fast reactions and slow reactions (Figure 25). In slow reactions, the intrinsic reaction kinetics control the process, so the catalyst inventory should be as high as possible. Increasing the wall thickness of a monolith can have the desired effect. In fact the degree of variation in this way is virtually from 10-90 volume %, whereas a packed bed will always yield an inventory of around 60% or lower if hollow catalyst particles are used. 

Figure 14 Temperature profiles of gas and catalysts in a regenerative process for hydrogen cyanide manufacture at the start and finish of the reaction cycle. Cycle duration is 4 minutes and the monolithic catalyst used has the following dimensions wall thickness 4 mm, channel width 20 mm, length 2000 mm.

Monoliths are mainly produced by extrusion, although other methods are applied, in particular for the production of metal monoliths from thin corrugated sheets. The size of the channels and the wall thickness can be varied independently, and the optimal values depend on the particular application. Therefore, an optimum can be established between the amount of the solid phase (catalyst loading), the void space in the monolith, and the wall thickness. As a consequence of the extrusion process and the use of plasticizers, the channel walls are not completely dense but possess a macroscopic porosity, t)q)ically 30-40%. Thus, the thermal expansion properties can also be adjusted. 

It is no surprise that monoliths are applied in many morphologies (cell sizes, wall thicknesses, channel shapes, materials of construction, microstructures (texture of the coating)) and overall dimensions. Monoliths are flexible to operate. They are well suited to optimal semi-batch, batch, continuous, and transient processing. Catalytic conversion can be... 

All the monolith composites were prepared at a 1 1 ratio between the magnesium silicate clay binder and the AC or alumina. After premixing of the dry powders by careful addition of water a dough was formed. This dough was extruded as honeycomb monolithic structures with parallel channels of square section at a cell density of 8 cells cm and a wall thickness of 0.9 mm using a Bonnot single screw extruder. 

The dynamic adsorption capacity of activated carbon containing monoliths has been shown to be equivalent to the micropore volume. However, this condition can only be met when the external area is above c. 100 m g" and the threshold diameter wide. In systems with no micropore volume or poor internal diffusion due to a low external surface area and narrow threshold diameter the breakthrough point is reached when c. 9% of the external area is covered. Future work will concentrate on using higher linear velocities and adsorption temperatures md different monolith geometries (wall thickness and channel width) in order to study the internal diffusion limitations of these types of adsorption units. 

Technical constraints are often imposed on the design of the monolith geometry by the extrusion process, as well as by the mechanical properties of the extrudate the specific SCR application (e g., high-dust vs. low-dust) is also crucial for the definition of the catalyst geometrical features. Here, attention is paid to the influence that the monolith parameters (wall thickness, channel size, channel shape) have on both DeNOx reaction and SO2 oxidation in order to advance guidelines for optimization of the catalyst geometry. 

It is well known that though NO conversion is unaffected by the thickness of the monolith wall beyond a small critical value, SO2 conversion increases linearly with increasing wall thickness. This is indicated in Fig. 9 such trends reflect the different influence of internal diffusional resistances on DeNOx reaction and SO2 oxidation, which, as discussed previously, are respectively confined to a superficial layer of the catalyst and active inside the whole wall. Consequently, the design of SCR monoliths should pursue the realization of very thin catalytic walls Fig. 9, for example, shows that reducing the catalyst half-thickness from 0.7 mm to 0.2 mm does not alter the DeNOxing performance but causes a decrease of SO2 oxidation as significant as 78%. 

A variety of monoliths are offered to the market which differ in the size of the channels and in the thickness of the walls. Together, these parameters fix what is called the cell density, the number of cells per unit frontal surface area. With ceramic monoliths, these two parameters can be varied to some extent independently. As of today, the most commonly used supports have about 62 channels per square centimeter, which corresponds to a channel width of about 1 mm and a wall thickness of about 0.15 mm. The bulk density of such a support is about 420kgm . The supports currently used have, within the production variance, homogeneous channel... 

Table 11. Typical value of the key catalyst reaction conditions as a function of the design parameters of the monolithic support for operating conditions relative to a monolith with 62 cells per square centimeter and wall thickness 0.17 mm.

The cell density and the wall thickness of the monolith mainly affect the pressure drop over the monolith (eq 43). 

Metallic monoliths have, in contrast to ceramic monoliths, a high heat conductivity. Metallic monoliths have a lower pressure drop than ceramic monoliths due to a small wall thickness. 

Beeckman and Hegedus [50] determined the intrinsic kinetics over two commercial vanadia on titania catalysts. A mathematical model was proposed to compute NO and SO2 conversions and the model was validated by experimental values. Slab-shaped cutouts of the monolith and powdered monolith material were used in a differential reactor. The cutouts contained nine channels with a length of 15 cm and with a channel opening and wall thickness of 0.60 and 0.13 cm, respectively. The SCR reaction over a 0.8 wt% V2O5 on titania catalyst was first-order in NO and zero-order in NH3. 

For the preparation of the three-way catalysts several procedures were used, which are summarized in Table 1. The reference catalyst samples were prepared by coating monolithic ceramic substrates with a cell density of 400 cpsi and a wall thickness of 6.5 mil. After drying and calcination of the coated monoliths, they were impregnated Avith the precious metals, precious metal loadings and precious metal ratios of choice. This method will be referred to as preparation method A. The novel catalyst technologies were prepared by placing the precious metals directly onto the washcoat. To do so, several methods were used. Preparation method C was used to apply the precious metals on all the washcoat components. The methods B and D were used to apply the precious metals selectively on one or more of the washcoat components. Details of the catalysts are given in Table 2. 

The catalysts were prepared as monolithic structures of parallel channels of square section with a density of 8 cells/cm and wall thickness of 0.90 mm. All the monolithic supports were subsequently heat treated at 500°C, if not otherwise stated, for 4 hours in an air atmosphere. 

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