Scale up of oscillatory helically baffled reactors

The Oscillatory Baffled Reactor (OBR) is an intensified design of continuous plug flow reactor (PFR) in which plug flow behaviour can be achieved at very low net flow rates (laminar flow regime). OBRs consist of tubes with periodically spaced baffles of various designs (orifice, helical, integral, etc). There is a net flow through the reactor, and a superimposed oscillatory flow. Resultantly, the OBR’s niche application is to operate “long” reactions in continuous mode. This is usually impractical in conventional tubular reactors.

Scale-up of conventional reactors, e.g. stirred tank reactors, is unpredictable due to the non-uniform mixing at large scale, leading to variations in the concentrations and temperatures. This means that optimum conditions obtained from laboratory scales cannot be directly used at large scales without re-optimisation. Therefore, process development time and product-to-market times would increase. However, scale-up of OBRs should be more predictable, as the flow structures produced at 5 mm scales (diameter) can be reproduced at larger scales (25–200 mm diameters).

Recent studies on oscillatory helically baffled reactors (OHBRs) at small scales (millilitre volumes) found that the helical baffle design could provide high degrees of plug flow across a broad operating window due to the combined effect of vortex formation and swirling. In this study, the scale-up characteristics of helically baffled OBRs are being explored using the residence time distribution (RTD).

It has been found that the behaviour of RTD is the same at all tested scales (10–25 mm diameters, with respective reactor volumes of 0.078 L and 0.834 L) when the geometric and dynamic parameters are maintained. The degree of plug flow was quantified in terms of the number of equivalent tanks-in-series (N). At a fixed geometry, a scale-up correlation was established and validated over a range of operating conditions such as oscillatory conditions (Strouhal number, St, and oscillatory Reynolds number, Re_o) and the velocity ratio of oscillatory to net flow (ψ). This correlation has also been successfully extrapolated and validated at 50 mm diameters.

Current work is focussed on achieving scale-up through mass transfer (using multi-orifice baffles) and heat transfer (using regular orifice baffles), as these will have implications on other types of process (e.g. biological processes).