Chemical Engineering Science, 59, N° 17, 3601-3618, (2004) doi:10.1016/j.ces.2004.05.021 RESIDENCE TIME DISTRIBUTION OF SOLID PARTICLES IN A CONTINUOUS, HIGH-ASPECT RATIO MULTIPLE-IMPELLER STIRRED VESSEL F. Scargiali*, F. Grisafi*, J. Cermakova**, V. Machon** and A. Brucato* * Dipartimento di Ingegneria Chimica dei Processi e dei Materiali - Viale delle Scienze – 90128 Palermo (Italy) ** Prague Institute of Chemical Technology - Technickà 5, 16628 Praha 6 (Czech Republic) ABSTRACT In this paper experimental information on the retention time distribution (RTD) of solid particles in a high aspect-ratio vessel, stirred by three equally spaced Rushton turbines, is presented. The relevant data were obtained by a special technique named Twin System Approach (TSA), that greatly simplifies the handling of particle laden streams and is therefore particularly suited for investigating particle RTD in flow systems. The technique fundamentals are first summarised, together with the data analysis procedure. This last requires a numerical deconvolution operation that is easily performed with the help of Z-transforms. Two different approaches for excluding the spurious contributions of the external piping required for the experimentation are tested and discussed. Particle tracing was performed by an effective particle-coating/optical-detection technique that allows particles recovery and reuse after each experimental run. The RTD data obtained indicate that a cascade of ideally mixed tanks with backflow results into very good agreement with experiment, with practically any number of tanks in series but one, provided that the recirculation rate parameter is chosen accordingly. In all cases, the backflow is large enough for the resulting flow model to be quite close to a single perfectly stirred vessel. Keywords: Retention Time Distribution (RTD), Twin Systems Approach (TSA), particle tracing, particle RTD, stirred vessels, multiple-impeller, flow model Corresponding author: Alberto Brucato, brucato@unipa.it, tel. +39 091 6567216, fax +390916567280