Counter-current operation of a structured catalytically packed-bed reactor:: liquid phase mixing and mass transfer

Abstract

The liquid-phase residence time distribution has been measured in two structured packed column configurations, of 0.1 and 0.24 m diameter, in which the catalyst particles are enclosed within wire gauze envelopes ("sandwiches"). In order to interpret these results Computational Fluid Dynamics (CFD) has been used to model the liquid flow within the packed sandwich structures. A representative sandwich structure, containing catalyst particles, is modeled as a set of triangular tubes ("Toblerones"), intersecting at 90° angles. The liquid flowing in a tube has the possibility of maintaining its flow direction or taking a sharp 90° turn. Using CFD, the dispersion characteristics of the "cross-over" junction can be determined as a sum of two components: straight-through and 90°-turn flow. The dispersion characteristics of the entire sandwich can be estimated reasonably well from information on the number of cross-over junctions along the flow direction. Comparison of the liquid-phase RTD measured in the two columns with those determined from CFD lead to the conclusion that there is channeling of liquid through the open channels, with good interchange of liquid between the open and packed channels. Liquid-phase mass transfer within the packed channels is studied also by means of CFD techniques. Due to the "upheaval" caused by the flow splitting at the crossovers, the mass transfer coefficient is about 2-3 times larger than for fully developed laminar flow in a circular tube.