Mathematical Modelling in Catalytic Automotive Pollution Control

Doktorsavhandling, 1995

Design and control of honeycomb monolith reactors for catalytic automotive pollution control by mathematical modelling of the reactor has been studied. One aspect of the design has been that the use of metallic monolithic catalysts for automotive emission control raises certain optimization possibilities. Creating obstacles in the channels by disfiguring the channel wall in the manufacturing process can give rise to an increased transport rate to the catalyst wall and an increased reaction rate in the washcoat. In the present mathematical simulation study, the decrease in the residual carbon monoxide at the outlet and the accompaning pressure drop increase were calculated for different designs of the metallic monolithic catalyst. A standard unsegmented monolith without obstacles was compared with monoliths with two obstacles, monoliths with changed dimensions of the channels and with monoliths segmented into three separate pieces in series. The use of small (due to space limitations), transient operated reactors for selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia, has been studied in order to unravel its use for cleaning exhausts from trucks and buses with Diesel engines. A comparison was made between a coated honeycomb catalyst with incorporated honeycomb catalysts. The study of a structured mathematical model (based on physical principles) for control purposes has been done on SCR-reactors for heavy-duty vehicles. The mathematical model predicts the momentary state of the reactor which is used for controling the ammonia injection. The model predicts the measurable quantities with very small deviations from the measured values in real-time on a low cost CPU (Intel 80486 DX2). It has been shown that the control of ammonia injection based on the mathematical model of the process enhance the reactor performance with increasing NOx reduction and decreasing ammonia slip. Ammonia stored on the catalyst has been shown to be a good control variable. Experimental results that support the theorethical values of Nusselt and Sherwood numbers for laminar flows in quadratic (and quadratic with rounded corners) ducts are presented. Furthermore the thesis reviews briefly the catalytic automotive pollution control with emphasis on mathematical modelling. A review of mathematical modelling of two-phase honeycomb reactors is also included in the thesis.