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Oscillatory baffled reactors (OBRs)

Oscillatory baffled reactors (OBRs)

The oscillatory baffled reactor consists of a tube fitted with equally spaced baffles. The baffles are transverse to an oscillatory flow. They disrupt the boundary layer at the tube wall.

Oscillation results in improved mixing through the formation of vortices. If a net flow is also superimposed on to this oscillatory motion, the reactor behaves like 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. They provide L/D ratios several orders of magnitude smaller than conventional tubular plug flow reactors.

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

Continuous stirred tank reactors (CSTRs) have broad residence time distribution. This leads to product variability, such as large particle size distributions.

Plug flow reactors (PFRs) need great lengths of narrow tubing. This causes control, operability, footprint and pumping problems.

CSTRs-in-series tend to be expensive. This is because they need 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, ie bioreactions
  • liquid-solid reactions, ie heterogeneous catalysis
  • reactive extraction

A recent development is the mesoscale (or millimetre scale) OBR, first presented in 2003. This tubular reactor has a characteristic inner diameter of 4.4-5mm. It offers the potential to exploit the benefits of flow chemistry for screening at µL/min to mL/min throughputs.

Active research areas in the group involving OBRs include:

  • process screening at mesoscale
  • imine synthesis
  • biodiesel production
  • terpene epoxidation
  • scale-up of helical and multi-orifice baffles, from 10mm to 25mm and 50mm id
  • mass transfer (DO)
  • heat transfer using water and water-glycerol mixtures
  • 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
Useful review papers