Honeycomb structure, monolithic converters

Honeycomb structures offered to the market have a similar cell density as the ceramic ones. After welding inlet and outlet cones to the outer shell, the metallic monolithic converter can be inserted directly into the exhaust gas pipe, which means that the canning procedure used for the ceramic monoliths is not needed anymore. 

Catalytic converters are composed of a monolithic support surrounded by a mat and tightly packed into a stainless steel housing. The monolith has a honeycomb structure traversed by straight, square channels and consists of a ceramic or sometimes metallic material, coated with the so called washcoaL To carry out the catalytic reactions, PGE are dispersed throughout the washcoat, which typically has a thickness of about 10-30 pm at the walls and 100-150 pm at the comers of the support channels and contains as major constituents alumina and other oxides. 

Monolithic catalysts (or honeycombs) have received much attention ever since they were first applied in automotive catalytic converters [1]. An increasing interest in the use of monolithic reactors for other applications has also been noticed during recent years [2]. One application which particularly profits from the opportunities offered by the honeycomb structure is catalytic combustion for use in advanced gas turbines [3]. In a catalytic combustor, a premixed lean fuel-air mixture is ignited by the catalyst which results in complete combustion at maximum temperatures far lower than possible in conventional gas-phase combustors. Hence, the thermal formation of nitrogen oxides can almost completely be circumvented. This fact has promoted large R D programs in catalytic combustion during recent years. 

A billion cars and coimting, himdreds of millions of them with catalytic converters—this application is a landmark success of catalytic science and technology. Automobile catalytic converters are mostly monoliths— like ceramic honeycombs with porous catalyst layers on their inner wall surfaces. These monoliths are the most widely used structured reactors, the topic addressed by Moulijn, Kreutzer, Nijhuis, and Kapteijn. In contrast to the classical reactors containing discrete particles of catalyst and characterized by random and chaotic behavior, structured reactors are characterized by regular structures and predictable laminar flow. Structured reactors can be designed in full detail up to the local surroimdings of the... 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

Honeycomb monoliths (Fig. 6.1) are structured catalyst supports consisting of parallel straight capillary channels. Nowadays, they are widely used for the catalytic exhaust converter in the automobile industry and end-of-pipe gas cleaning. The gas-only application of monoliths stems from the fact that the pressure drop is low using the surface area of the catalyst as a criterion, the pressure drop in a monolith is an order of magnitude lower than in randomly packed beds. The channels are about a millimeter in diameter, and on the wall (-100 pm) a wash-coat of catalytic material (-50 pm) is applied. 

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