Cells per square inch

Honeycomb with 200 cells per square inch (CPSl)... 

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... 

For the control of carbon monoxide, hydrocarbon, and nitrogen oxide emissions from automobiles, oval-shaped extruded cordierite or metal monolith catalysts are wrapped in ceramic wool and placed inside a stainless steel casing (Fig. 19-18a). The catalytic metals are Pt-Rh or Pd-Rh, or combinations. Cell sizes typically ranges between 400 and 600 cells per square inch. The catalysts achieve over 90 percent reduction in all three pollutants. 

Monolith catalysts are used for the control of carbon monoxide and hydrocarbon (known as volatile organic compounds or VOCs) emissions from chemical plants and cogeneration facilities. In this case, square bricks are stacked on top of one another in a wall perpendicular to the flow of exhaust gases at the appropriate temperature location within the heat recovery boiler. The size of the brick can vary from 6 in (ceramic) to 21 ft (metal). Pt and Pd catalysts are used at operating temperatures between 600 and 1200°F. Cell sizes typically range between 100 and 400 cells per square inch. Typical pressure drop requirements for monoliths are less than 2 in of water. 

FIGURE 2 Typical examples of monoliths with different numbers of cells per square inch the numbers below are the geometric surface areas (/Tigeometric/rr7reactor)-... 

The two promising candidates are adsorbent monoliths and adsorbent sheets. The fabrication of activated carbon and zeolite monoliths are reported in the literature. Zeolite monoliths have also been tested for air separation application by PSA.50 51 However, the use of monoliths for use in H2 PSA is not known to the authors. Monoliths having very high cell density (several hundred to thousand cells per square inch) will be necessary in order to have fast adsorption kinetics as well as reasonable bulk density for a PSA application. Manufacture of such monoliths is complex, and they are not yet commercially available. Gas channeling through the monoliths can also be a problem.52 Adsorbent sheets have been used for air separation by RPSA.53 54 The thickness of the adsorbent sheets and the space between the... 

On first introduction in 1974 cerarriic substrates had a density of 200 cells per square inch of cross section or cpsi (31 cells/square cm.) and a wall thickness of 0.012 inch or 12 mil (0.305 mm). By the end of the 1970 s the cell density had increased through 300 to 400 cpsi and wall thickness had been reduced by 50% to 6 mil. Now 400 and 600 cpsi substrates are commercially available and wall thickness has been reduced to 4 mil - a mere 0.1 mm. Further increases in cell density to 900 cpsi and reductions in wall thickness to 2 mil are promised for 2000 and beyond. 

The ceU density (N) is defined as the number of cells or channels per unit of cross-sectional area perpendicular to the axis of the channel. This is usually expressed in units of cells per square inch, and abbreviated cpsi. The open frontal area (OFA) is equal to the open area of an individual channel multiphed by the cell density and is usually expressed as a percent. The geometric surface area (GSA) of a cellular structure is derived by establishing the surface area per unit length of an individual channel that is then multiphed by the cell density. This represents a surface area per unit volume and is expressed as cm /cm, m /hter, or some other appropriate set of units. The total surface area (TSA) of a structure is then the geometric surface area multiphed by the volume (V) of the structure under consideration. 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

Ceramic monoliths have been produced in various cell densities and geometries. Table 1 is a comparison of the geometric properties of those utilized for automotive application. Automotive industry standard is mostly 400/6 sq. shaped cells and some 236/12 triangular shapes. The initial designs of 1975 were largely 200/12 but process developments over that period of time have allowed for increasing number of cells per square inch and thinner walls. These bring with the associated benefits of increased GSA, lower density and lower back pressure. 

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. 

Honeycombs are monoliths with a multiplicity of parallel, straight channels. Like in natural honeycombs, the shape of the channel cross-sections, the so-called cells, may be hexagonal, but square, triangular, and sinusoidal cell shapes are more common in fabricated materials. Geometric features used to characterize honeycombs are the cell density expressed as cells per square inch (cpsi, in. ), and the wall thickness (mm). 

Kreutzer et al. investigated the hydrogenation of a-methylstyrene to cumene in monolith reactors of varying cell density [41]. They found that at low to moderate cell densities (200-400 cells per square inch) the reaction was mass transfer limited at temperatures above 393 K. At a cell density of 600 cpsi, however, the external mass-transfer rate was sufficiently high at temperatures above 413 K that the reaction was not fully mass transfer limited. 

Alumina foams have been directly impregnated for propane CPO and OSR [13, 14, 40] to yield 0.01 wt.% rhodium. The catalyst on the foam body, which was 15 mm in diameter, 7 mm long and contained 400 cells per square inch (84% porosity), showed optimum performance at an oven temperature of700 °C and good stability under CPO conditions (C O = 0.8), even though a remaining hot spot of more than 200 K was observed in the foam. Under OSR conditions (C 0 = 0.5 and steam to carbon ratio = 1) only a 150 K hot spot was observed. However, the catalyst deactivated more rapidly, maybe due to the increase in byproduct formation. Complete homogeneous conversion was observed at an oven temperature of 800 °C... 

The dominant catalyst support for the auto exhaust catalyst is a monolith or honeycomb structure. (For some early history on the use of bead catalyst, see Reference (6).) The monolith can be thought of as a series of parallel tubes, with a cell density ranging from 300 to 1200 cpsi (cells per square inch). Advances in monolith technology, catalyst-mounting methods, flexibility in reactor design, low pressure drop, and high heat transfer and mass transfer rates are the main reasons the monolithic support dominates the entire market as the preferred catalyst support. 

The fabrication of ceramic monoliths is performed mainly via extrusion techniques [9] resulting in usually elliptic or squareshaped monoliths. Cell densities as high as 1600 cells per square inch (cpsi) are achieved [9], which corresponds to a channel width of about 500 pm of the mostly rectangular or hexagonal channels. Typical key features are 75% porosity and a geometric surface area of 2.8 m /1 for 400 cpsi for ceramic monoliths [10]. The thermal shock resistance amounts to 800°C and higher. 

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