Oscillatory Baffled Reactors (OBRs)

Fig. 1 - Vortex formation observed via simulation in a 2D domain containing orifice baffles

The oscillatory baffled reactor (OBR), consists of a tube fitted with equally spaced baffles presented transversely to an oscillatory flow. The baffles disrupt the boundary layer at the tube wall, whilst oscillation results in improved mixing through the formation of vortices (see Figure 1). If a net flow is also superimposed on to this oscillatory motion, then the reactor behaves as many well mixed tanks-in-series. Thus, the OBR is able to achieve good plug flow behaviour where the plug flow/mixing is decoupled from the net flow. Their niche application is the accommodation of long residence time processes with L/D ratios several orders of magnitude smaller than conventional tubular plug flow reactors.

Continuous processing tends to be more efficient than batch processing, and tends to be used at larger scales: it is an economy of scale. However it is seldom used for long reactions as conventional continuous reactor designs have certain drawbacks:

CSTRs: Broad residence time distribution (leading to product variability, e.g. large particle size distributions)

PFRs: Require great lengths of narrow tubing, causing: control, operability, footprint and pumping problems

CSTRs-in-Series: Tend to be expensive (requiring multiple reactors and control systems)

The OBR provides a solution to these problems. Specific applications include:

• Saponification

• Biodiesel production (an esterification reaction)

• Fermentations, including beer/bioethanol production 

• Wastewater treatment

• Screening (at mesoscales)

• Liquid-liquid reactions

• Liquid-gas reactions (e.g. bioreactions)

• Liquid-solid reactions (i.e. heterogeneous catalysis)

• Reactive extraction

A fairly recent development is the "mesoscale" (or millimetre scale) OBR first presented in 2003 (Fig. 5). This tubular reactor has a characteristic inner diameter of 4.4 - 5 mm and offers the potential to exploit the benefits of flow chemistry for screening at  µL/min to mL/min throughputs. Figures 2-5 show various examples of OBRs used within the group, whilst active research areas in the group involving OBRs are summarised in the following list.

• Process screening (at mesoscale):

  • Imine synthesis

  • Biodiesel production

  • Terpene epoxidation

• Scale up of helical and multi-orifice baffles (from 10 mm to 25 mm and 50 mm i.d.'s)

• Mass transfer (DO)

• Heat transfer using water and water-glycerol mixtures (Fig. 4)

• Pressure drop analysis

• Fluid mechanics analysis of mesoscale helical baffles via CFD and PIV

• Further intensification of mesoscale OBRs:

  • Hybridisation with a heat pipe

  • Microwave heating

  • External ultrasonic mixing

Fig. 2 - Pilot scale jacketed OBR (25 mm i.d.) containing orifice baffles	Fig. 3 - Batch operated OBR (50 mm i.d.) used for bioethanol production
Fig. 4 - Jacketed OBR used to study heat transfer enhancements	Fig. 5 - Mesoscale (5 mm i.d.) baffle geometries

Useful Review Papers