Heterogeneously catalyzed reactions in the presence of irreducible transport phenomena

In view of the tendency towards the intensification of chemical processes, the importance of fast reactions is growing. Chemists have developed heterogeneous catalysts which allow industrially very important reactions such as catalytic cracking of gas oil or the removal of NOx from stack gas to proceed very rapidly indeed. It is well known that transport phenomena do limit the obtainable rates, however. When designing or optimizing a reactor, the catalytic reaction engineer takes these transport limitations into account by writing down and solving the corresponding conservation equations of mass and energy, in which both transport and intrinsic chemical kinetics are accounted for in an appropriate way. The textbook approach consists in obtaining the latter separately, i.e. at conditions where the effects of transport phenomena can be neglected. There are more and more cases, however, where such a separation is not possible without loosing essential features of the catalyzed reaction. In his book ‘Kinetics of Chemical Processes’ M. Boudart spends a chapter on this category with as title: ‘Irreducible Transport Phenomena’. The present set of papers provides some examples of irreducible transport phenomena, linked to catalyst deactivation during catalytic cracking, reduction of NO and oxidative dehydrogenation of light alkanes. Some contributions focus on dedicated experimental set-ups. One of the messages is that the involved transport phenomena should be amenable to a quantitative treatment without adjustable parameters, e.g. by the use of structured reactors and by maintaining laminar rather than turbulent flow. Others provide illustrations as to how important reaction variables, like the catalyst surface temperature, can be measured. In order to remind the reader of more ‘exotic’ consequences of irreducible transport phenomena, a contribution on the simulation of autonomous oscillations caused by relatively fast surface reactions has also been added.