Wall-Flow Monoliths

W. Addy Majewski

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• Introduction
• Monolith Properties
• Materials
• Pressure Drop
• Canning

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 wall-flow monoliths.

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

Wall-flow filter walls have a distribution of fine pores that have to be carefully controlled in the manufacturing process. Total material porosity is typically between 45 and 50% or higher, while the mean pore size (MPS) usually ranges from 10 to 20 µm. Filtration on monolith wall-flow filters occurs through 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) by mass. Higher efficiencies are observed for solid PM fractions—elemental carbon and metal ash—and for solid particle numbers (PN).

Wall-flow monoliths are typically extrusions made from porous ceramic materials. Materials most commonly used in commercial filters include cordierite and silicon carbide (SiC). Cordierite is a synthetic ceramic 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 such as semiconductors, abrasives, or high temperature/molten metal contact materials. In the mid-2000s, SiC was introduced as a filter material for diesel passenger cars and remains common in light-duty applications. Another commercial filter material, aluminum titanate, is also used primarily for light-duty diesel vehicles.

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