A new technique for the determination of contact time distribution (CTD) from tracers experiments in heterogeneous systems

Abstract

The concept of residence time distribution (RTD) has been widely used in the case of homogeneous isothermal reactors (Danckwerts, 1953). For isothermal heterogeneous catalytic reactors, the chemical reaction occurs only when the fluid phase gets into contact with catalyst. In order to take this phenomenon into account, the concept of contact time and its distribution (contact time distribution or CTD) has been defined in the literature (Orcutt et al. 1962; Nauman and Collinge, 1968a,b; Shinnar et al., 1972). After a comparative study of the concept as defined in Nauman and Collinge (1968a,b) and Shinnar et al., 1972, an algorithm devoted to calculate the CTD as defined by Shinnar and co-workers is proposed. Within the framework of this theory, the reactor is considered as the combination of an extra granular volume (the mobile fluid phase or chemically inactive zone) and of an intra granular volume (the pores within the pellets or stationary phase which is chemically active). The residence time distribution within this active zone has been defined as the CTD. The method is illustrated through simulated and experimental data. To apply the proposed technique, one has to perform two tracer experiments. The first one is dedicated to the measurement of the global time distribution (GTD) in the whole system (the distribution of the time spent in the extra and in the intra granular volumes). The second one is devoted to the measurement of the fluid time distribution (FTD) in the extra granular volume. The extra granular FTD can be measured for example by using a non-porous solid instead of the porous one.