Diesel Catalysts: Catalyst Construction
M-Series Diesel Oxidation Catalyst
The M-Series line of Nett® diesel exhaust purifiers utilizes metallic monolith catalyst supports. The supports are made with a corrugated, high temperature stainless steel foil. Packages of several foil layers are fitted in stainless steel housings and secured in place by stainless steel rings. A special herring-bone foil corrugation pattern creates a mixed flow cell structure. Exhaust gases are forced into the turbulent flow regime resulting in better contact between gas and catalyst, enhanced mass-transfer conditions, and higher conversion efficiency. Selected physical properties of Nett® metallic monoliths are listed in Table 1.
The catalyst is deposited onto the foil prior to forming the substrate. A special foil washcoating process provides unequaled control of washcoat uniformity, adhesion, and efficient catalyst use. Thick washcoat concentrations in cell corners which are inherent for other designs of metallic substrates completely disappear with the precoated foil technology.
| Metallic | Ceramic | |||||
|---|---|---|---|---|---|---|
| Cell Density, cpsi | 160 | 240 | 320 | 200 | 300 | 400 |
| Wall Thickness, mm | 0.05 | 0.05 | 0.05 | 0.30 | 0.21 | 0.18 |
| Geometric Surface Area, cm2/cm3 | 19 | 23 | 26 | 18.5 | 23.6 | 27.2 |
| Open Frontal Area, % | 94 | 92 | 91 | 69 | 74 | 74 |
| Bulk Density, g/cm3 | 0.43 | 0.53 | 0.59 | 0.53 | 0.45 | 0.45 |
D-Series Catalyst
The D-Series catalysts are available on ceramic substrates. Round cordierite substrates with square cell geometry are used in all D-series catalysts. Selected properties of the substrates are listed in Table 1.
Catalyzed substrates are wrapped in special packaging mat and canned into a steel container using the tourniquet packaging technology. Tourniquet is known as the best catalytic converter canning technology, producing the most rugged and durable converters.
Ceramic substrates produce somewhat higher pressure drop than metallic substrates of the same dimensions, due to their thicker walls. However, for most applications the D-series catalysts have to be sized larger than the standard oxidation catalysts, in order to provide sufficient volume of the HC trap. By using D-series catalyst substrates of larger diameter and larger frontal area, it is possible to achieve comparable pressure losses for the standard catalyst and the D-series catalyst installed on the same engine.
Most pressure drop comparisons between the ceramic and metal catalyst supports are based on bare (uncoated) substrates. While the uncoated ceramic supports do have thicker walls (Table 1), the difference in wall thickness decreases after the catalyst coating is applied. This is explained by the inherent porosity of ceramic substrates, which "soak in" a portion of the catalyst coating into the wall pores. Since metallic substrates are not porous, the entire catalyst coating stays at their surface. Therefore, when the same loading of catalyst material is applied to a ceramic and a metallic substrate, it produces a thicker coating layer and more flow restriction in the metallic support.
Several advantages of Nett® catalytic converters make them the superior choice as a generic, retrofitting converter for all diesel engine applications:
- High mechanical durability. Metallic substrates will not crack or disintegrate even under harsh operating conditions and offer best thermal durability. The ceramic substrates are canned using the tourniquet technology, which produces the most rugged and durable catalytic converters.
- Easy Maintenance. The cell densities of the catalytic monoliths are carefully selected for each application. There is very little risk of clogging by diesel particulates. Most applications are virtually maintenance-free.
- Low Pressure Drop. Due to thin walls and large frontal area the catalyst substrates have low pressure drop and do not cause any fuel penalty.