Wall-Flow Monoliths

W. Addy Majewski

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Abstract: Wall-flow monoliths became the most popular diesel filter design. They are derived from flow-through catalyst supports where channel ends are alternatively plugged to force the gas flow through porous walls acting as filters. Wall flow monoliths are made of specialized ceramic materials such as cordierite and silicon carbide. A number of mechanical and thermal properties have been defined to characterize and compare different monoliths. Filters of different sizes have been developed and are available as standard products.

Introduction

Ceramic wall-flow monoliths, which are derived from the flow-through cellular supports used for catalytic converters, became the most common type of diesel filter substrate. They are distinguished, among other diesel filter designs, by high surface area per unit volume and by high filtration efficiencies. Monolithic diesel filters consist of many small parallel channels, typically of square cross-section, running axially through the part. Diesel filter monoliths are obtained from the flow-through monoliths by plugging channels as shown in Figure 1. Adjacent channels are alternatively plugged at each end in order to force the diesel aerosol through the porous substrate walls which act as a mechanical filter. To reflect this flow pattern, the substrates are referred to as the wall-flow monoliths.

[flow pattern image]

Figure 1. Gas Flow in a Monolith Filter

(Courtesy of Corning Inc.)

Wall-flow monoliths are most commonly available in cylindrical shapes, as shown in Figure 2, although oval cross-section parts are also possible for space constrained applications.

[photo]

Figure 2. Cylindrical Wall-Flow Monoliths

Left: silicon carbide; Right: cordierite

(Courtesy of NGK)

Wall-flow filter walls have a distribution of fine pores which have to be carefully controlled in the manufacturing process. Total material porosity is typically between 45 and 50% or higher, while the medium pore sizes range usually from 10 to 20 µm. Filtration mechanism on monolith wall-flow filters is a combination of cake and depth filtration. Depth filtration is the dominant mechanism on a clean filter as the particulates are deposited in the pore network inside the wall material. As the soot load increases, a particulate layer develops along the wall surface in inlet channels and cake filtration becomes the prevailing mechanism. Typically, monolith filters have filtration efficiencies between about 70 and 95% of total particulate matter (TPM). Higher efficiencies are observed for solid PM fractions—elemental carbon and metal ash (as discussed earlier, particulate filters may be ineffective in controlling SOF and sulfate particulates).

Wall-flow monoliths are typically extrusions made from porous ceramic materials. Two materials most commonly used in commercial filters include cordierite and silicon carbide (SiC). Cordierite is a synthetic ceramics developed for flow-through catalyst substrates and subsequently adapted for the filter application. Cordierite filters have been used mostly in heavy-duty engine applications. Silicon carbide has been used for a long time in a number of industries for such applications as semiconductors, abrasives, or high temperature/molten metal contact materials. More recently, it was introduced as a filter material in diesel passenger cars. The newest commercial filter monolith material is aluminum titanate, also introduced for the passenger car application.

Table 1
Commercial Wall-Flow Monolith Materials
Material Formula Monolith Suppliers
Cordierite 2MgO-2Al2O3-5SiO2 Corning, NGK, Denso, Hitachi Metals
Silicon carbide SiC Ibiden, NGK, Saint-Gobain, LiqTech
Aluminum titanate Al2TiO5 Corning

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