(16b) Scale-up of Oscillatory Helically Baffled Reactors (OHBRs)

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

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).

N=25 Str-0.3  ψe-0.13ψ+30.2