A factorial approach to understanding the effect of inner geometry of baffled meso-scale tubes on solids suspension and axial dispersion in continuous, oscillatory liquid–solid plug flows

Abstract

Oscillatory flow reactors (OFRs) are a new generation of tubular mixing and reaction equipment uniquely capable of combining continuous near plug flow with homogeneous particle suspension, yet the design of OFRs for liquid–solid and multi-phase flow processes relies on rules established during the past two decades from single, liquid-phase studies. A Design of Experiment (DoE) approach was herein implemented for establishing the relationship between four key geometrical parameters of the inner tube baffles and both the suspension of particles and the axial dispersion for liquid–solid continuous flows in 10mminternal diameter (d) meso-scale tubes with periodic baffles. The parameters evaluated were the orifice open diameter, do=0.35d–0.50d; the open cross section, α=0.12d–0.25d, constriction spacing, l=1.5d–3.0d, and baffle shape (sharp vs smooth edged). A total of ten tubes were tested, five consisting of smooth periodic constrictions (SPC) and the other five of sharp edged periodic constrictions (SEPC) according to a complete 2×2 factorial design with 1 central point. Each tube was experimentally evaluated via optical imaging of suspended monodispersed polyvinyl chloride (PVC) particles. Both SPC and SEPC meso-tubes were capable of delivering a near plug flow behaviour and the values of axial dispersion coefficient (Dc) estimated for the solids were in the range of 1.0–2.2×10−4m2s−1. In contrast, the minimum (critical) fluid oscillation conditions required for full suspension of particles varied significantly, in general with the SPC tubes requiring up to 50% lower amplitude for full particles suspension. Overall, α revealed the dominant parameter in controlling solids backmixing, and the inner tube geometry requiring the lowest energy input for homogenous particle suspension and minimum Dc (i.e. sharpest residence time distribution) presented a l/d=3, do=0.35d, α=12% and SPC design. This study is believed to support the future design of optimised meso-scale OFR systems for continuous screening and manufacturing of value-added liquid–solid and multi-phase systems, such as catalytic and crystallisation processes.