Characterisation of solid-liquid flow in a continuous oscillatory baffled reactor using computational fluid dynamics

Abstract

Continuous Oscillatory Baffled Reactors (COBRs) have been proven a viable alternative to traditional batch reactors for organic synthesis and crystallization processes. This thesis investigates the behaviour of solids in liquid in a COBR using CFD. Firstly, CFD is used to analyse the validity of two existing models for the estimation of power density in this type of reactors, the “quasi-steady” model (QSM) and the “eddy enhancement” model. By using a revised power law dependency on the number-of-baffles term (nx) in both models, an appropriate orifice discharge coefficient (CD) in the QSM and a proposed empirical correlation estimate EEM’s “mixing length”, both models were successfully validated. Secondly, energy losses experienced by both liquid and solid phases in COBRs are analysed; for the former, temporal pressure drop profiles and power dissipation rates along the length of a COBR are monitored for a wide range of operating and geometric conditions. The results provide detailed insights into the relationship between power dissipation and pressure drop profiles and reveals that geometries that are perfectly symmetric in the axial direction, i.e. periodically repeatable, do not present signs of energy losses. On the other hand, geometric events such as sections missing one or multiple baffle constrictions led to a decrement in power dissipation rates and velocities, caused by the eddy shedding phenomenon within the missing baffle sections. And sections with a reduced cross-sectional area of the baffle constriction and bend joints do not yield energy losses in the device; instead, they require a higher power density for the flow to overcome these constraints. A multiphase (S-L) Eulerian- Lagrangian model was employed to simulate the presence of solid particles suspended in a continuous liquid phase in a COBR. The behaviour of these particles was monitored with time as they travelled downstream the device for particles of different sizes; results unveiled that as particles increases in size they experience dampening in oscillatory velocity, translating into smaller axial dispersion, longer residence times and a reduction of particles’ suspension. For the determination of axial dispersion, both perfect and imperfect pulse methods were employed, the latter providing more reliable results. Thirdly, this research introduces an alternative Lagrangian based methodology, i.e. the Smoothed-Particle Hydrodynamics (SPH), for the simulation of fluid flow in an OBR. The results from a bespoke SPH solver are compared with those from Eulerian modelling, i.e. Finite Volume (FV) method, displaying a high degree of agreement. SPH was able to capture the expected flow characteristics in OBR as clearly and equally as its Eulerian counterpart. Making full use of SPH’s capabilities and its Lagrangian feature, two new indexes for the assessment of mixing and plug flow efficiency have also been proposed.