Scale-up of Oscillatory Baffled Reactors (OBRs)

Scale-up of oscillatory helically baffled reactors (OHBRs)

Safaa M.R. Ahmed, Anh N. Phan, Adam P. Harvey

School of Chemical Engineering and Advanced Materials, Newcastle University

Newcastle upon Tyne - NE1 7RU

Mobile:+44 7774101215

E-mail:  s.m.r.ahmed@newcastle.ac.uk

Keywords: Oscillatory baffled reactor, scale-up, residence time distribution

Abstract

An 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 flows (laminar flow regime). OBRs consist of tubes with periodically spaced baffles of various designs (orifice, helical, integral etc. baffles). There is a net flow through the reactor, and a superimposed oscillatory flow. The oscillatory flow interacts with the baffles to produce flow structures (usually vortices) that provide mixing. The mixing in the OBR is therefore independent of the net flow. As a result, the OBR’s niche application is to operate “long” reactions in continuous mode. This is usually impractical in conventional tubular reactors.

Scale-up in conventional reactors, i.e. stirred tank reactors, is unpredictable due to the non-uniform mixing at large scale, leading to large variations in concentration, temperatures etc. This means that optimum conditions obtained from laboratory scales cannot be directly used at large scales, therefore process development/ product-to-market time would increase. However, scale-up of OBRs should be more predictable, as the flow structures can be reproduced across length scales.

Recent studies on oscillatory helically baffled reactors (OHBRs) at small scales (millilitre volume) found that the helical baffled design could provide high degree of plug flow across a wide operating window due to the  combined effect of vortex formation and swirl flow. It was found that the behaviour of residence time distribution remained the same at all tested scales (10mm diameter to 25mm diameter, corresponding to volume 0.078L to volume 0.834L) when the similarity in geometric and dynamic parameters was maintained. The degree of plug flow was quantified in terms of number of 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 flow and net flow (ψ).

Where,