Doory, Layla Kim; (1992) Development of catalytic reactor designs for enhanced CO oxidation. Doctoral thesis (Ph.D.), University College London.

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Abstract

The catalytic removal of pollutants including nitrogen oxides (NOx), hydrocarbons (HC's) and carbon monoxide (CO) in the exhaust of automobiles is generally performed by using monolithic supports coated with noble metal catalysts, notably platinum and rhodium (Pt/Rh) adsorbed onto a washcoat. This is typically achieved to within 90-98% conversion efficiency for average entry conditions (50<Re<400 at actual conditions), with pressure drops not exceeding 1.25-2 kN/m2. The monolith is a honeycomb structure, essentially composed of many parallel channels of square cross section. This therefore acts as a high surface area reactor. One of its drawbacks lies in the amount of precious metal requirements. With the increasing demand and price of these it is likely that in the future they may contribute to the cost of manufacture even more significantly. Detailed analysis shows that the overall rate of reaction of the monolith reactor is usually mass transfer rather than kinetically limited. Thus any boost in the mass transfer rate should increase conversion. Conversely for there to be any reduction in overall surface area or precious metal content there would have to be an increase in mass transfer rate. The effect of increasing mass transfer was studied by two methods namely by axially segmenting the ceramic monolith core sample (consisting of 62 cells/cm 2 of1.04 mm channels) and secondly by inserting static mixers into a catalyst coated pipe (ie."Active Transport Catalytic Reactor" (ATCR)). This was carried out for carbon monoxide oxidation over a commercially prepared catalyst supplied by Johnson Matthey. The intrinsic kinetics of this reaction were determined experimentally in a differential reactor. Conversions and pressure drops were measured for each system for varying Reynolds numbers from 73-440 (S.T.P.) in the channel and 160-2140 (S.T.P.) in the pipe, under stoichiometric reactant concentrations, and for steady state fully warmed up reactor conditions ranging from 250°C to 4(X)°C. A one dimensional model is presented and its predictions compared to the experimental data for conversion and outlet gas temperature. Good agreement between experimental and theoretical data for the ATCR was found using the one-dimensional model for the conditions investigated. Also the model was found to be sufficiently accurate in predicting monolith conversions (ie. less than 10% difference between experiment and theory) and exit gas temperatures (ie. average of 4% difference) for high temperatures of 371°C and above. Pressure drops were also successfully predicted for both segmented monoliths as well as ATCR systems. Monolith segmentation was found to be successful in both enhancing CO oxidation as well as reducing the total catalyst requirements with the result that up to 30% saving of catalyst was possible. A simple optimization process using the theoretical data for the ATCR showed that up to 65% saving in reactor surface area (and hence catalyst requirements) is possible. Thus the novel idea of carrying out heterogeneous reactions within an ATCR shows promising results and indeed there is much scope for future research and possible applications.

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