Dual-bed system

Exhaust emission standards since the 1981 model year vehicles have required the use of three-way catalysts, either alone or in combination with an oxidation catalyst. Three-way catalysts are designed to operate in a very narrow range about the stoichiometric air/fuel ratio. In this range the HC and CO are subject to oxidation and the NO, compounds undergo reduction. The downstream oxidation catalyst in a dual bed system is generally used as a "clean-up catalyst lo further control HC and CO emissions. The most common catalytic combination in three-way uses is platinum/rhodium. Current production applications use these elements in a relatively rich proportion of 5 1 lo 10 1. whereas the respective mine ratio is about 19 1. 

Softeners can be a microbial concern. A dark and moist column interior can provide a growth environment. The regeneration cycle which uses concentrated brine solution and a backwash cycle aids in reducing the bioburden. Softeners should be regenerated based on a time clock set for twice weekly regenerations and on a volumetric flow of water, whichever is shorter. Since the regeneration cycle removes the softener bed from operation, a dual bed system is often specified. 

NOx Reduction Catalyst. One of the most promising catalysts for reducing nitric oxide is ruthenium (Figure 2). It is unusual in that it catalyzes the reduction of NO with a low selectivity to ammonia which, if present, can be reoxidized in the second bed of a dual-bed system. When it is exposed at high temperatures to an oxidizing environment, however, ruthenium is lost through the formation of the volatile tetroxide. 

Over the range of operating A/F ratios of an engine, a good NO reduction catalyst gives the performance response for HC, CO, and NO as shown in Figure 6. Note that when the engine exhaust is close to the stoichiometric A/F ratio (i.e., A/F 14.65), all three pollutants could in theory be simultaneously converted and the need for a dual bed system could be eliminated. As the A/F ratio approaches the stoichiometric point, there is a narrow window where simultaneous catalytic conversion of all three pollutants occurs (Fig. 6). So, an invention... 

If the feed water contains a relatively high concentration of dissolved substances a dual-bed system is usually preferred. If the feed water contains low concentrations a mixed bed ion exchange demineralisation will be preferred. As said above for water softeners, the resins pearls in a demineralisation colunrn do not retain any other... 

The coupling between a NOx-trap catalyst and a NH3-SCR material located downstream the first one or in a double layer on the monolith, was firstly patented by Ford in 2004. In other recent studies, two main SCR catalysts are usually associated in such system, namely Cu-ZSM-5 and Fe-ZSM-5 catalysts. For instance, it is reported that using Cu-ZSM-5 material, about 15-50 % of supplementary NOx conversion can be achieved, depending on the working conditions. Whatever the zeolite used, higher performances are reported with H2 as reductants, since it enhances NH3 formation in the first NSR catalytic bed. Physical mixture of NSR and SCR catalysts gives better performances than dual bed system. It is concluded that a close proximity of both materials was required for a better use of ammonia produced during the rich pulses. 

A dual-bed system is commonly employed, so that while one bed is "onstream," the second bed can be regenerated and prepared for the adsorption step. The time period allowed for each step is typically as follows ... 

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