Residence times and mixing of a novel continuous oscillatory flow meso reactor

摘要

A novel meso reactor based on oscillatory flow technology (Harvey et al., 2001) has beenrecently presented in Harvey et al. (2003) as a new technology for reaction engineering andparticle suspension applications. Due to the demonstrated enhanced performances for fluid micromixing and suspension of catalyst beads and to the small volume of the reactor, this novelminiature reactor is suitable for applications at specialist chemical manufacture and highthroughput screening. Furthermore, a high control of environment conditions (e.g. mixingintensity, temperature) coupled with an online monitoring turns this reactor suitable for smallscaleapplications to the bioengineering field, such as for fast parallel bioprocessing tasks.This work concerns with the fluid dynamics characterisation of a novel miniature reactor.Experimental results using state-of-art fibre-optic technology is used in order to demonstrate thatan accurate control of the residence time distribution (RTD) of liquid and solid phases can beachieved within this reactor as well as enhanced (oxygen) mass transfer rates. Furthermore,numerical simulations using Fluent ® software will be presented where simulated RTDs agreeswith the experimental results.The meso reactor unit consists of 4.4 mm internal diameter and 35 cm long jacketed glass tubes,with a unit volume of 4.5 ml and provided with smooth periodic constrictions (SPCs), with anaverage baffle spacing of 13 mm. The internal diameter at the constricted zone (baffle internaldiameter) is 1.6 mm, leading to a reduction of the baffle free are of 87 %. This unit is able tosupport batch or continuous operations mode, simply by configuring the tubes in parallel or inseries, according to the intended application. Mixing is achieved by oscillating the fluid at thebottom or the top of the reactor by means of a piston pump, using oscillation amplitudes andfrequencies ranging from 0 to 4 mm centre-to-peak and 0 to 25 Hz, respectively.Experimental studies using the Particle Image Velocimetry (PIV) technique (Harvey et al., 2003)showed that different fluid mechanics are originated at different oscillation conditions(oscillation amplitudes and frequencies). A plug flow or a stirred tank behaviour can be obtainedjust by controlling the oscillation conditions. At low oscillatory Reynolds numbers (Reo), e.g. 10to 100, the formation of axisymmetric eddies detached from the constrictions is coupled with lowaxial velocities and makes it possible to continuously operate the reactor in a plug flow mode.Increasing the Reo to values higher than 100, the eddy symmetry is broken and a completemixing state is achieved inside the meso reactor. Low oscillation amplitudes must be used ifaxial dispersion is intended to be minimized, namely at plug flow setup.Through an overall oscillation cycle, changes of the location of the main flow stream from nearthe wall to the centre of each cavity and vice-versa was observed and is expected to lead to highmass and heat transfer rates (Perry, 2002). Due to the observed high radial velocities, narrowresidence times distributions are expected to be obtained (Perry, 2002). Also high axialcirculation rates were also observed at high Reos (above 100) and it was proved to lead to anenhanced performance on catalyst beads suspension. The relation of this fluid mechanics withthe real performance of this novel meso reactor will be demonstrated.Tracer injection technique is applied to perform RTD studies inside a single SPC tube of themeso reactor. Spectroscopy UV/VIS technique is used to measure the concentration of acoloured tracer at the inlet and outlet (at continuous mode) or at the bottom and the top of thetube (at batch mode). A fibre optic apparatus is employed in order to obtain highly accurateonline measurements of the UV/VIS absorbance. Mixing times are calculated for experiments atbatch mode. Different flow rates are used to determine the effect of the flow rate over the RTD atcontinuous operation and axial dispersion is presented by the Bodenstein number, Bo.Determination of KL.a values is achieved by online measurement of the oxygen concentrationusing a special fibre optic probe. The working tip of the probe was dip-coated with a rutheniumcomplex immobilised in a sol-gel matrix. This complex is excited to fluorescence by a blue led(470 nm outpuk peak) and the level of the fluorescence is inversely related to the concentrationof the oxygen through the Stern-Volmer equation (Wang et al., 1999), which is measured by thefibre-optic apparatus. Retention of solid phases (e.g. catalyst beads and yeast cells) inside themeso reactor will also be tested.Further studies using the Computation Fluid Dynamics (CFD) technique will be presented whereaccurate prediction of the distribution of residence times is achieved. The use of the distributionfunctionspermits to classify the flow behaviour inside this novel meso reactor patterns and tocalculate mixing efficiencies and axial dispersion coefficients (expressed by the Bo number) atdifferent oscillation conditions.A simple 2-D axisymmetric laminar model showed good agreement with flow patternsvisualisations using PIV for Reo below 100 but a 3-D model with a very fine mesh was requiredto simulate breakage of axisymmetry. Consequently, 3-D models based on laminar and LargeEddy Simulations (LES) will be used to maximize the matching of RTD at higher oscillationconditions. Main intended application of CFDs to this novel meso reactor is the design of a mesoreactor unit, which could operate at the best oscillation conditions and flow rate for cell culturesand biocatalyst applications.