Cell density, monoliths

In another case of controlled oxidation, the application of a variable-cell-density monolith bed was demonstrated, utilizing a segmented bed of three 2.54-cm-long sections. The result combined monoliths with cell sizes ranging from 6.35 mm (inlet) to 4.76 mm... 

The direct way to improve the volumetric activity of monolithic catalysts is to increase its cell density, by which the volumetric surface is tremendously enlarged [15,34], Figure 9.8 shows that an increase in cell density from 300 to 400 cpsi is more effective on the DeNO performance than an increase of the vanadia concentration from 1.9 to 3%, which is, moreover, accompanied by the above mentioned problems. 

Extruded SCR catalysts based on V205/W03—Ti02 with cell densities of 300cpsi are commercialized and 400 cpsi prototypes are available. Extruded SCR catalyst means that the whole monolith consists of the active mass V205/W03—Ti02, which is enforced by glass fibers and binders. 

Konstandopoulos, A. G., and Kladopoulou, E. The optimum cell density for wall-flow monolithic filters Effects of filter permeability, soot cake structure and ash loading. SAE Technical Paper No. 2004-01-1133 (2004). 

In order to improve the channel wall irrigation, modified channel geometries have been developed. Figure 8.10 shows the top view of different monoliths and the corresponding MRI-picture of one channel. The cell density for all three samples is 25 cells per square inch (cpsi) the diameter is 43 mm. Some details of the monoliths... 

Two basic trends can be determined. By comparing the 25 cpsi monoliths, channel rounding clearly leads to better mass transfer performance. The slightly rounded channels (RC) show a small increase, but a significant improvement is found for the more rounded channels (MRC). As expected, a higher S/V ratio or cell density also... 

For three-way catalysts in converters for Otto engines Pt/Rh supported on La-stabilized alumina are used. Promoters are present e.g., Ce02. The alumina is usually situated as a washcoat layer in a monolith (typically with a cell density of 400 cells/in2) consisting of cordierite. In practice, the catalysts appear to be very stable. In fact, the durability of the electronic control system (for the regulation of the X value in the converter) is more critical than the catalyst itself. 

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. 

Monoliths of low cell densities (with openings ranging from 3 to 6 mm) arc applied for deNOxification of ga.ses at coal-fired power plants. This is due to the high content of dust in the gases. If dust particles were retained in the monolith, the pressure drop would increase greatly. Because of the abrasive action of the dust particles, incoiporated-type catalysts are preferred. Often, WO3 and V2O3 are incorporated into T1O2 in the anatasc... 

Typical metallic monoliths in use today, illustrated in Fig. 10, have cell densities in the range 15-78 cells cm (100-500 cells inch ), corresponding to individual cell dimensions of 1.1-2.5 mm. To minimize additional back pressure due to the catalyst formulation, it is essential that the eoating be applied in a controlled manner. It is difficult to define a typical thickness of the catalyst layer (often referred to as washcoat layer ), since this inevitably differs from one catalyst manufacturer to another, depending upon the precise processing techniques used and the application for the finished catalyst. However, usually it is about as thick as the metal foil, typically 0.04-0.05 mm. 

Figure 11 Linear variation of geometric surface area and nonlinear pressure drop across a metal monolith as a function of metal foil thickness for three cell densities 200,400, and 600 cells inch. ...

Table 2 Geometric Surface Area (GSA) as a Function of Cell Density for Uncoated Metallic Monoliths ...

Coated metal monoliths have a similar structure, but in this case the support is represented by thin metal foils they are characterized by large cell densities and are used mostly in dust-free applications. 

Models may be one-, two-, or three-dimensional Of course, more advanced models describe the flow in the channels more accurately, but this is not required in all types of investigations. Section 2 in Chapter 8 provides an in-depth discussion of relevant flow phenomena that have to be taken into account when modeling monoliths. One-dimensional models can provide interesting information on mass and heat transfer effects in the washcoat [54]. However, if the development of a laminar flow in the monolith channels IS to be simulated, a two-dimensional model is needed. This is the case if multimonolith combustors are investigated [67]. This specific design, discussed in detail in Section IV.A, has a number of monoliths in senes, which enables vanation of monolith materials and cell density. Moreover, it leads to improved mass transfer by induced turbulence at the entrance of each monolith segment. 

Crynes et al. [41] continued the study of Kim et al. [40]. The novel monolithic froth reactor, with a monolithic section 0.42 m long and 5 cm in diameter, was used. Cordierite monoliths with a cell density of 62 cells/cm were stacked, one on top of another, to provide a structure 0.33 m long. The monoliths, washcoated with 7-alumina and impregnated with CuO, were tested at 383-423 K and 0.48-1.65 MPa. The liquid flow rate was varied from 0.4 to 3.5 cm sec", and the gas flow rate ranged from 15.8 to 50 cm sec". Phenol in a concentration of 5000 ppm was typically oxidized with air. The reaction rate versus the liquid flow rate showed a distinct maximum of approximately 2 mol g"t sec" at about 1.7 cm sec", while the dependence of the reaction rate on the gas flow rate was rather weak, with a tendency to decrease as the flow rate increased. 

Kobe Steel Co. [42] has patented a monolithic process for oxidation of Fe to Fe in acidic aqueous solutions using monolith made of carbon. The monoliths were prepared by mixing active carbon with a binder, extrusion, and thermal treatment. Slices 150 mm in diameter of cell density 20-60 cells/cm were tested. Monolithic slices 30 mm thick were stacked and separated one from another with turbulizers. The reactor was operated in the countercurrent mode with the gas flowing upward. The liquid was recirculated. The liquid flow rate was varied from 250 to 333 cm sec and the gas flow rate ranged from 83 to 250 cm sec . Pressure was up to 0.31 MPa. The oxidation efficiency was 34-80% at circulation time of 1800 sec, and rose to 89-93% at 3600 sec. 

In this equation, it is assumed that both the gas and the liquid move in slugs at the same velocity through the orifice. The contribution of the orifice effects to the total pressure drop is small and is of significance only in monolith blocks with high cell density and at high liquid flow rates. The frictional and static pressure drops prevail. The orifice effects may arise if blocks of monoliths stacked on top of each other are used. The contribution of these effects will depend on the extent of obstruction and the length of individual blocks. 

The present monoliths that are developed for emissions control in cars are not optimal for chemical three-phase reactors. Development of new monoliths, both metallic and ceramic, with higher cell densities and other geometries at much lower prices, can be expected when the market starts to grow. 

---